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EP2697831A1 - Hemt transistors consisting of (iii-b)-n wide bandgap semiconductors comprising boron - Google Patents

Hemt transistors consisting of (iii-b)-n wide bandgap semiconductors comprising boron

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Publication number
EP2697831A1
EP2697831A1 EP12717088.4A EP12717088A EP2697831A1 EP 2697831 A1 EP2697831 A1 EP 2697831A1 EP 12717088 A EP12717088 A EP 12717088A EP 2697831 A1 EP2697831 A1 EP 2697831A1
Authority
EP
European Patent Office
Prior art keywords
layer
bgan
structure according
channel
electronic structure
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
EP12717088.4A
Other languages
German (de)
French (fr)
Inventor
Abdallah Ougazzaden
Marie-Antoinette Poisson
Vinod RAVINDRAN
Ali Soltani
Jean-Claude DE JAEGER
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Centre National de la Recherche Scientifique CNRS
Universite de Lille 1 Sciences et Technologies
Thales SA
Alcatel Lucent SAS
Georgia Institute of Technology
Original Assignee
Centre National de la Recherche Scientifique CNRS
Universite de Lille 1 Sciences et Technologies
Thales SA
Alcatel Lucent SAS
Georgia Institute of Technology
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Centre National de la Recherche Scientifique CNRS, Universite de Lille 1 Sciences et Technologies, Thales SA, Alcatel Lucent SAS, Georgia Institute of Technology filed Critical Centre National de la Recherche Scientifique CNRS
Publication of EP2697831A1 publication Critical patent/EP2697831A1/en
Ceased legal-status Critical Current

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    • H01L29/778—Field effect transistors with two-dimensional charge carrier gas channel, e.g. HEMT ; with two-dimensional charge-carrier layer formed at a heterojunction interface
    • H01L29/7786—Field effect transistors with two-dimensional charge carrier gas channel, e.g. HEMT ; with two-dimensional charge-carrier layer formed at a heterojunction interface with direct single heterostructure, i.e. with wide bandgap layer formed on top of active layer, e.g. direct single heterostructure MIS-like HEMT
    • H01L29/7787—Field effect transistors with two-dimensional charge carrier gas channel, e.g. HEMT ; with two-dimensional charge-carrier layer formed at a heterojunction interface with direct single heterostructure, i.e. with wide bandgap layer formed on top of active layer, e.g. direct single heterostructure MIS-like HEMT with wide bandgap charge-carrier supplying layer, e.g. direct single heterostructure MODFET
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    • H01L29/778—Field effect transistors with two-dimensional charge carrier gas channel, e.g. HEMT ; with two-dimensional charge-carrier layer formed at a heterojunction interface
    • H01L29/7782—Field effect transistors with two-dimensional charge carrier gas channel, e.g. HEMT ; with two-dimensional charge-carrier layer formed at a heterojunction interface with confinement of carriers by at least two heterojunctions, e.g. DHHEMT, quantum well HEMT, DHMODFET
    • H01L29/7783—Field effect transistors with two-dimensional charge carrier gas channel, e.g. HEMT ; with two-dimensional charge-carrier layer formed at a heterojunction interface with confinement of carriers by at least two heterojunctions, e.g. DHHEMT, quantum well HEMT, DHMODFET using III-V semiconductor material
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    • H01L29/107—Substrate region of field-effect devices
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Definitions

  • the invention relates to a heterojunction field effect transistor electronic structure, said HEMT (High Electron Mobility Transis) transistor based on heterostructures formed by wide bandgap semiconductor materials, called large gap materials.
  • HEMT High Electron Mobility Transis
  • semiconductor materials that have a bandgap greater than about 2 eV, which corresponds to the range of micron wavelengths, from near infrared to deep UV. They typically include element III nitrides, but also diamond and oxides such as zinc oxide.
  • Element III nitride is a composition of one or more elements of column III, for example, B, Al, Ga, In, which form an alloy with nitrogen N (element V).
  • column III for example, B, Al, Ga, In, which form an alloy with nitrogen N (element V).
  • element V an alloy with nitrogen N
  • binary compositions such as GaN, AlN, BN; or ternary alloys, two III elements such as Al x Gay -x N, In x Al -x N, B x Ga -x N, X B -X N quaternary AI 3 section III B x Al y Ga].
  • x . y N even
  • HEMT transistors made from structures formed of a stack of nitrides of elements III, and more generally of large gap semiconductor materials, have properties of great interest for microwave applications and / or requiring power. These structures use in a known manner different compositions III-N, in stacked layers. Each composition is chosen for its particular electronic properties, for example the effective mass of the electrons, their mobility or the width of the band gap. Considerations on mesh parameters are also taken into account in the choice of compositions, since they determine the possibilities of growth of materials with good structural qualities. Stacking materials leads to an electronic structure that is characterized in particular by the corresponding energy band diagram. The choice of materials III-N and their compositions for producing an electronic structure of HEMT transistor thus meets considerations on bandgap widths, depending on the desired properties and performance and on the mesh agreements that condition the obtaining of layers. materials with less structural defects.
  • a known electronic structure of high bandwidth semiconductor materials is a heterostructure comprising the superposition of a layer of a first semiconductor material to wide bandgap (the barrier zone) on a layer of a second semiconductor material also wide bandgap (the active area) but in which the first material at a gap greater than that of the second material.
  • the term "material”, used alone, is to be understood as being a broad bandgap semiconductor material. in the rest of the presentation.
  • an electronic structure of HEMT transistor consists essentially of three materials:
  • This structure allows the formation and the circulation of a two-dimensional electron gas 2DEG in a channel C formed in the M1 material with smaller gap Eg- ⁇ , at the interface 10 (or interface M2-M1), between the two M2 / M1 materials of the heterojunction. As illustrated in FIG. 3, this channel corresponds to a confinement of the electrons in a QW quantum well which is formed at the interface 10 (or interface M2-M1) between the two materials M2 / M1.
  • heterojunction structures based on a stack of high bandwidth semiconductor materials offer particularly interesting prospects for obtaining high performance HEMT (High Electron Mobility Transistor) fast transistors for microwave power applications (ranging from 2 GHz to 100 GHz and beyond), and are the subject of much research in order to obtain the most favorable structures that combine high gas density two-dimensional electron n s a mobility of the higher carrier possible, in order to obtain transistors with high drain current, a necessary condition for an amplification in effective power.
  • HEMT High Electron Mobility Transistor
  • heterojunction M2 / M1 An important property of the heterojunction M2 / M1, is the good confinement of the electrons in the quantum well QW, crucial for the efficiency of the electronic transport of the transistor.
  • the present invention provides a novel way to improve the confinement of two-dimensional electron gas in the channel.
  • BGaN Since boron incorporation is uniform in volume, the thickness of the BGaN layer can be very thin or thick (from a few tenths of nanometers to a few microns). Moreover, BGaN offers good characteristics in terms of mesh correspondence with the usual growth substrates (Al 2 O 3 , SiC (4H-6H), Si (1 1 1, 100, 1 10), GaN (monocrystalline), composite substrates, or with a large gap such as ⁇ or poly- or monocrystalline diamond) which have good thermal conductivity.
  • the BGaN ternary has a lower bandgap than that of the GaN binary, for low levels of boron incorporation, as well as an electronic polarization of the important material, like InGaN.
  • the invention therefore relates to a HEMT transistor structure, comprising
  • a BGaN material with an average boron concentration of at least 0.1%, semi-insulating is inserted into the buffer layer in the form of at least one layer under the channel layer, modifying the diagram of the energy bands by creating an electrostatic potential barrier favoring the confinement of the two-dimensional electron gas.
  • This BGaN layer may be formed as a layer of BGaN in the buffer layer, under the channel, which has a uniform boron concentration throughout the thickness; or which has a graduated concentration or stair step on the thickness, starting from a zero concentration, and increasing with the thickness, towards the channel.
  • BGaN clusters can be made directly in the buffer layer.
  • This confinement layer may also be in the form of a super network of very thin layers successively alternating BGaN and GaN or ⁇ .
  • the invention also relates to the use of other BGaN layers for the purpose of improving the electronic structure of the HEMT transistor.
  • the structure comprises a layer of BGaN as a nucleation layer, making it possible to improve the structural quality of the second layer obtained by growth of material from this nucleation layer.
  • BGaN the structural qualities of BGaN that are exploited.
  • the structure comprises a layer of BGaN or BN as a surface passivation layer, to minimize the influence of potential traps on the surface. It is here the resistive properties of BGaN or BN that are advantageously exploited.
  • FIG. 1 schematically illustrates an electronic structure for a HEMT transistor, according to the state of the art
  • FIGS. 2 and 3 respectively illustrate an electronic structure for a HEMT transistor in a first exemplary implementation of the invention, and a corresponding diagram. energy bands with the use of a thin layer of BGaN and the formation of an electrostatic barrier;
  • FIGS. 4 and 5 respectively illustrate an electronic structure for a HEMT transistor in a second exemplary implementation of the invention, and a corresponding diagram of the energy bands with the use of a graded layer of boron composition and the formation of an electrostatic barrier, the top of which is at the gas end;
  • FIGS. 6 and 7 respectively illustrate an electronic structure for a HEMT transistor in a third example of implementation of the invention, and a corresponding diagram of the energy bands with the use of a thick layer of BGaN and the formation a wider electrostatic barrier;
  • Fig. 8 shows a super-array type BGaN layer structure, which may be used in the structures illustrated in Figs. 2, 4 and 6;
  • FIG. 9 illustrates another BGaN layer structure of localized incorporation type by volume
  • FIG. 10 illustrates a structure comprising improvements according to the invention
  • FIGS. 11 to 13 illustrate three practical examples of an AIGaN / GaN type structure with an insertion of a layer of a BGaN material according to the invention, respectively in a thin layer with a uniform Bore concentration, in one layer. Thickness at uniform concentration of boron and in a thick layer with a concentration gradient of boron;
  • FIG. 14 illustrates the curves obtained by simulation of the BGaN thin-film structure of FIG. 11, with in an upper window (a) the energy level curve of the conduction band of the structure along the axis. Y corresponding to the thickness of the structure, starting from the surface towards the substrate, in the lower window (b), the concentration curve of the carriers in the structure along this axis Y;
  • FIG. 15 illustrates these same power level curves of the conduction band, and carrier concentration, but obtained by simulation of the BGaN thick film structure, with uniform concentration of boron illustrated in FIG. 12, and that with gradual concentration of boron illustrated in Figure 13;
  • FIG. 16 shows on the same diagram the curves of the energy levels of the conduction band for the three structures of FIGS. 11, 12 and 13;
  • FIG. 17 shows on the same diagram the carrier concentration curves for each of the three structures of FIGS. 1 to 13.
  • AIGaN is the M2 material of the barrier layer having a gap Eg 2 greater than that, Eg of the first material M1 of the buffer layer, which is GaN.
  • the structure comprises a BGaN layer in the buffer layer, under the channel.
  • FIG. 2 A first example of an electronic structure according to the invention is illustrated in FIG. 2. It comprises the following stack of layers in the order of growth (stacking):
  • substrates Different types are commonly used: low cost substrates such as Si, with crystalline orientations (1 1 1), (100), (1 10), Al 2 O 3 monocrystalline, SiC (4H, 6H) whose cost is high.
  • Composite substrates such as SopSiC (polycrystalline silicon-oxide-SiC), SiCopSiC (monocrystalline SiC-polysilicon-SiC), polycrystalline diamond; ZnO substrates, SiC substrates (to a lesser extent) and monocrystalline diamond substrates are materials that exhibit good properties for heat dissipation.
  • substrates known as "pseudo substrates" of GaN, AlN, ZnO or else flexible substrates, such as Kapton, PTFE (polytetrafluoroethylene) on which the epimaterial has been reported.
  • Kapton polytetrafluoroethylene
  • PTFE polytetrafluoroethylene
  • this substrate may be a temporary substrate, used for the realization of epitaxy, by growth of materials. It can then be removed by any known technique to transfer the structure thus detached from its growth substrate to another substrate, for example a glass, a flexible substrate or a substrate having a good thermal conductivity. An electronic structure can thus be provisionally free of substrate, or have a final substrate, in the component, which is not the growth substrate.
  • a buffer layer 2 ("template” or “buffer” in the Anglo-Saxon literature) of a nitride, in the GaN example, generally composed of a GaN 2a first layer which serves in a known manner, of base material of good crystallographic quality, for the crystallographic growth of a second layer 2b of GaN having excellent structural qualities. Indeed, the two-dimensional electron gas will form in this layer close to the heterojunction.
  • the barrier layer may comprise a plurality of elementary layers (not shown), in particular a doped layer, called a donor layer which supplies the free electrons that will participate in forming the two-dimensional electron gas in the buffer layer, and a non-layer intentionally doped, called spacer, between the doped layer and the buffer layer, which promotes the mobility of electrons in the two-dimensional electron gas transport channel. No doping is envisaged in the nitride structures ... doping is often unnecessary, the electrons coming essentially from the surface by piezoelectric polarization effect and spontaneous generation.
  • a passivation layer 4 ("cap layer” in the Anglo-Saxon literature) as illustrated in FIG. 1 may be provided (not shown in FIG. 2), formed in a material having a lower bandgap width than the M2 material of the barrier layer, and which is strongly n-type doped, to allow the realization of ohmic source and drain contacts (not shown) of the HEMT transistor.
  • This is for example a n-type strongly doped GaN layer.
  • the passivation layer will be little used when the structural quality of the layer is good. It mainly prevents the oxidation of aluminum in the barrier layer. If passivation there is, a possible doping can be realized under the contacts exclusively.
  • the structure further comprises a layer 5 of BGaN in buffer layer 2, under channel C.
  • the BGaN layer is inserted between the GaN layer 2a and GaN layer 2b.
  • BGaN layer or BGaN material is to be understood throughout the description as encompassing both the BGaN ternary and higher order alloys, that is to say that it may also be a quarter BlnGaN, BAIGaN, or a quarter BAlInGaN. This remark applies to the other materials of the structure.
  • the BGaN layer is a thin layer, of thickness of the order of 1 nanometer, with a uniform concentration of boron.
  • the BGaN material is a ternary, with a boron concentration of the order of 1 to 4%, which is written as: B 0, OiGa 0 , 96N, B 0 , o4Ga 0 , 96N respectively.
  • the modeled energy band diagram for this structure is shown in Figure 3. It shows the levels of energy, in electron volts, of the valence band BV and the conduction band BC obtained (left vertical axis), as well as the distribution of the density of electrons in the structure (in cm “3 ) (vertical axis right), on the height of the structure along the transverse axis Y (nanometers)
  • the origin Y 0, corresponds to the surface of layer 3 (figure 2), and shows the formation of the potential well.
  • the presence of the BGaN layer 5 under the channel C of the structure according to the invention is also reflected in the band diagram by the creation of two energy peaks 1 1 which correspond to the valence and conduction bands of the BGaN These peaks form an electrostatic barrier that makes it more difficult for the electrons to leak out of the well. The confinement of the electrons in the potential well QW at the interface 10 is thus improved.
  • This barrier is in this example rather narrow, corresponding to the small thickness, 1 nm in the example, of the BGaN 5 layer.
  • the BGaN layer has another effect, that of increasing the resistivity of the structure under the channel, preventing the leakage of electrons to the substrate.
  • the BGaN layer has two effects, each of which tends to improve the confinement of the two-dimensional electron gas: first, because the BGaN layer modifies, improving it, the energy band diagram; and secondly because the BGaN layer increases the resistivity of the structure under the channel, preventing electron leakage from the channel to the substrate.
  • FIGS. 4 and 6 show two other examples of structure according to the invention, and FIGS. 5 and 7 their respective energy band diagrams. These figures show that depending on the concentration and thickness of the BGaN layer, the electrostatic barrier can be increased and / or enlarged. created by the BGaN layer, improving the confinement of two-dimensional electron gas.
  • the BGaN layer is thicker, of the order of 50 nm (compared with 1 nm in the example illustrated in FIG. 2), but with a boron concentration which is steep, or graduated in steps : the boron concentration is zero at the interface with the layer 2b, and increases towards the channel (in the layer 2a), for example up to 4%.
  • Figure 5 of the corresponding band diagram shows an enhanced and wider electrostatic barrier effect. The use of a boron concentration gradient over a greater layer thickness thus makes it possible to form a more distinctive electrostatic barrier which will further limit the movement of electrons out of the potential well.
  • the BGaN layer is even thicker, of the order of 100 nm, but with a very low concentration of boron, of the order of 1% (B 0, oiGao, 9gN).
  • FIG. 7 of the corresponding band diagram shows that an even larger electrostatic barrier 13 is obtained in connection with the greater thickness of the layer. This structure is very interesting because it is easily known to produce such a layer with a low concentration of boron. And even at these low boron concentrations, electrostatic barrier effects and increased resistivity of the structure under the channel are observed.
  • the BGaN layers used according to the invention are characterized by a mean boron concentration of at least 0.1%.
  • the thickness of the layer is preferably from 1 nanometer to several hundred nanometers approximately.
  • the invention which has just been described in an example of heterojunction AIGaN / GaN structure, thus provides for the insertion of a BGaN layer in the buffer layer, under the channel, to obtain a double effect of favorable modification of the bands with formation of an electrostatic barrier even wider than the BGaN layer is wide, and increasing the resistivity of the structure under the channel.
  • the invention applies in particular more generally to all heterojunction structures obtained with layers chosen from the III element nitride binaries, that is to say AIN, GaN, InN, BN, and the ternary, quaternary or quintenary formed from these binaries. It more generally applies to HEMT transistor structures based on wide-bandgap semiconductor materials, including semiconductor materials III-V, diamond or zinc oxide (and any other material cited above). upper).
  • the first material M1 will preferably be a nitride of elements III, in binary form, typically AIN, or a ternary or quaternary alloy formed from a binary of the following list: AIN, GaN, InN, BN. It can also be diamond or zinc oxide ZnO.
  • the second material M2 may be an element III nitride, and in particular a binary (AlN, GaN, InN, BN), or a ternary or quaternary alloy formed from a binary of the AlN list, GaN, InN, BN.
  • the BGaN layer on the buffer layer 2a can be obtained in different ways, using the range of growth techniques of this material available at present, that is to say typically: molecular beam (MBE) or vapor phase epitaxy; organo-metallic (MOCVD) or hybrid (HVPE) technique; boron implantation techniques in a GaN layer, and diffusion techniques, with deposition and annealing phases.
  • MBE molecular beam
  • MOCVD organo-metallic
  • HVPE hybrid
  • the BGaN layer may be formed with a homogeneous, uniform concentration of boron in the volume, as in the example illustrated in FIG.
  • the layer BGaN can also be formed with a steep or graduated concentration in stair steps, starting from 0, and increasing towards the channel, to a higher concentration, for example 4%, as schematically illustrated in FIG. 4.
  • the BGaN layer can also be produced in the form of a super lattice (super lattice in English) formed of an alternation of very thin layers, for example an alternation of BGaN and GaN layers, as schematically illustrated in the structure of FIG. 8, on a determined structure thickness, in the example 50 nm, to reach an equivalent average concentration. It is also possible to envisage an alternation of BGaN and AIN layers.
  • the BGaN layer can be further produced by forming a GaN or BGaN layer, with localized bulk incorporation of BGaN more concentrated in boron, forming small volumes or "clusters" in the surrounding buffer layer, as schematically illustrated in the figure 9.
  • the thickness of the surrounding layer and the density of the clusters and the respective concentrations of the surrounding layer and clusters are determined to obtain the desired average concentration.
  • buffer layer 2a is a layer of GaN (GaN or GaN alloy with other elements of column III)
  • these clusters can be made directly in the buffer layer.
  • the surrounding layer in which the BGaN clusters are made can also be a layer inserted in the buffer layer 2 of the structure.
  • the invention is furthermore proposed to improve the HEMT transistor structure described above, by using the electrical properties, in particular the resistive properties, and the structural qualities of the BGaN layers at other levels in the structure, further improving the electrical performance of the HEMT transistor.
  • FIG. 10 illustrates these improvements in an example of AIGaN / GaN heterojunction structure.
  • a first improvement is to use a low boron BGaN layer at the interface between the substrate and the buffer layer for use as a nucleation layer 6 for growth of the buffer layer.
  • this nucleation layer 6 will preferably be produced by the cluster technique shown in FIG. 9.
  • a second improvement consists in using a BGaN layer with a low boron concentration, to produce the surface passivation layer 4 for its resistive properties, the passivation layer having the function of reducing any surface traps of the structure.
  • this passivation layer BGaN 4 will preferably be made with a uniform concentration of boron, or superlattice.
  • BN can also be used resistive properties as interesting for this layer 4 of surface passivation.
  • a third improvement is to use a BGaN or BN layer, to promote the heat dissipation of the HEMT structure.
  • BGaN and BN are good thermal conductors, in particular they are better thermal conductors than the SiN or SiO 2 commonly used for layer 4, for the passivation of the structure.
  • BGaN or BN layer on the surface of the structure, in order to reduce the thermal bridge with a possible radiator placed above the structure. Since we previously saw that such a BGaN or BN layer could also be used for passivation, two alternative embodiments are possible:
  • a cooling down of the structure and to make a BGaN or BN layer under the buffer layer, such as the layer 6 illustrated in FIG. 10. It is then necessary to provide a transfer of the structure on a suitable substrate 1 (ex : SiC, diamond, with thermally compatible interface and / or bonding) to improve thermal conductivity in volume and total thermal resistance.
  • a suitable substrate 1 ex : SiC, diamond, with thermally compatible interface and / or bonding
  • Layer 6 can then serve as a nucleation layer in the process of fabricating the structure, and then as a layer promoting heat dissipation, after transfer to a suitable substrate.
  • FIGS. 11 to 17 show the simulation results obtained for three structures formed according to the invention, and illustrate the effects of confinement of the charge carriers at the barrier layer / buffer layer interface of a transistor structure HEMT with a layer of BGaN inserted in the buffer layer according to the invention, and increasing the resistivity under the channel. They make it possible to show that these effects are remarkable even with a low concentration of boron, which in the example of the simulation is 1%, as well as the notable evolution of these effects with the layer thickness BGaN inserted.
  • the three simulated HEMT structures are AIGaN / GaN structures comprising a BGaN material inserted according to the invention.
  • the barrier layer 3 is an AIGaN layer, chosen with an Al concentration of 32% and a thickness of 13 nanometers.
  • the BGaN layer 5 is inserted according to the invention in the GaN buffer layer, so that a part 2b of the buffer layer is found between the barrier layer AIGaN 3 and the BGaN layer 5.
  • this layer part 2b buffer has a thickness of 40 nanometers.
  • the BGaN layer 5 is thin, with a thickness of 5 nanometers, and has a uniform boron concentration of 1% in the example.
  • its concentration of boron is uniform, 1% In that of FIG. 13, it is at a concentration gradient, starting from 0%, at the limit with the part 2a of the buffer layer under the BGaN layer, in the representation of the figure where the barrier layer 3 is located at above the buffer layer, up to 1% at the limit with the part 2b of the buffer layer above the BGaN layer
  • the buffer "layer” according to the invention is thus formed in the structure by the GaN sequence 2b / BGaN 5 / GaN 2a.
  • the position of the layers in their succession in the structure along the Y axis ie: AIGaN / GaN / BGaN / GaN.
  • the level of fermi, denoted NF is also represented.
  • the upper window (a) of FIG. 14 illustrates the energy level curve (in electro-volt "eV") of the structure, indicated by the symbol fb.
  • FIG. 15 represents the corresponding curves, but obtained:
  • FIG. 16 compares the different conduction band energy level curves of all these structures, and similarly, FIG. 17 compares the different carrier concentration curves of all these structures, and the induced effects.
  • the inserted BGaN layer according to the invention improvement of the confinement by the electrostatic barrier effect, reduction of electron leakage to the substrate by the resistive barrier effect.
  • the peak of energy in the conduction band, at the GaN / BGaN interface denoted respectively E-fb for the structure of FIG. 11, E-ub, for the structure of FIG. 12 and E-gb, for the structure of FIG. 13 thus has an amplitude that is all the greater, and the electrostatic barrier induced is all the greater, as the BGaN layer is thicker.
  • the amplitude of the peak and the electrostatic barrier are greater for a uniform concentration at 1% boron (curve “ub”, peak E-ub) than for a gradient 0% -1% concentration (curve " gb ", peak E-gb).
  • the width of the base of the triangular potential well at the AIGaN / GaN interface respectively denoted W-fb, W-ub and W-gb, is also dependent on the boron concentration and the thickness of the BGaN layer, as very well shown in figure 17: the narrowest for the curve ub, the widest for the curve fb.
  • the invention which has just been described makes it possible to produce high performance HEMT transistors with improved electrical properties.

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Abstract

The invention relates to an electronic structure of an HEMT transistor, including: a heterojunction consisting of a first so-called buffer layer (2), a first wide bandgap semiconductor material (M1), and a second layer (3) consisting of a second wide bandgap semiconductor material (M2), the width of the bandgap Eg2 of which is greater than that Eg1 of the first material; and a two-dimensional electron gas (2DEG) flowing in a channel (C) defined in the first layer (2) under the interface (10) of the heterojunction. The first layer also includes a layer (5) of a BGaN material under the channel (C), the mean boron concentration of which is at least 0.1%, thereby improving the electrical performance of the transistor. The invention can be used in microwave power components.

Description

TRANSISTORS HEMTS COMPOSES DE SEMI - CONDUCTEURS BORES A LARGE BANDE INTERDITE (III-B) -N HAND TRANSISTORS COMPOUNDES OF BANDWIDTH PROHIBITED BANDED SEMICONDUCTORS (III-B) -N
DOMAINE DE L'INVENTION FIELD OF THE INVENTION
L'invention concerne une structure électronique de transistor à effet de champ à hétérojonction, dit transistor HEMT (pour High Electron 5 Mobility Transis^ à base d'hétérostructures formées par des matériaux semi-conducteurs à large bande interdite, dits matériaux grands gaps.  The invention relates to a heterojunction field effect transistor electronic structure, said HEMT (High Electron Mobility Transis) transistor based on heterostructures formed by wide bandgap semiconductor materials, called large gap materials.
DESCRIPTION DE L'ÉTAT DE L'ART DESCRIPTION OF THE STATE OF ART
Les matériaux semi-conducteurs à large bande interdite sont des Broad bandgap semiconductor materials are
10 matériaux semi-conducteurs qui présentent une largeur de bande interdite supérieure à environ 2 eV, ce qui correspond au domaine des longueurs d'onde micrométriques, du proche infrarouge à l'UV profond. Ils comprennent typiquement les nitrures d'élément III, mais aussi le diamant et les oxydes comme l'oxyde de zinc. 10 semiconductor materials that have a bandgap greater than about 2 eV, which corresponds to the range of micron wavelengths, from near infrared to deep UV. They typically include element III nitrides, but also diamond and oxides such as zinc oxide.
15 Un nitrure d'élément III est une composition d'un ou plusieurs éléments de la colonne III, par exemple, B, Al, Ga, In, qui forment un alliage avec l'azote N (élément V). On a des compositions binaires, tel que GaN, AIN, BN ; ou des alliages ternaires, à deux éléments III tel que AlxGai-xN, lnxAI -xN, BxGa -xN, BXAI -XN quaternaires à 3 éléments III BxAlyGa].x.yN, voireElement III nitride is a composition of one or more elements of column III, for example, B, Al, Ga, In, which form an alloy with nitrogen N (element V). There are binary compositions, such as GaN, AlN, BN; or ternary alloys, two III elements such as Al x Gay -x N, In x Al -x N, B x Ga -x N, X B -X N quaternary AI 3 section III B x Al y Ga]. x . y N, even
20 quinquénaires. Ces alliages sont réalisés par substitution partielle de l'un des éléments III par un autre élément de la même colonne III. Dans ces écritures de composition des matériaux, x et y sont des fractions comprises entre 0 et 1 . 20 years old. These alloys are made by partial substitution of one of the elements III by another element of the same column III. In these compositional writing materials, x and y are fractions between 0 and 1.
Les transistors HEMT réalisés à partir de structures formées d'un 25 empilement de nitrures d'éléments III, et plus généralement de matériaux semi-conducteurs grand gap, présentent des propriétés très intéressantes pour les applications hyperfréquence et/ou nécessitant de la puissance. Ces structures utilisent de manière connue différentes compositions lll-N, en couches empilées. Chaque composition est choisie pour ses propriétés 30 électroniques particulières, par exemple la masse effective des électrons, leur mobilité ou encore la largeur de la bande interdite. Des considérations sur les paramètres de maille sont aussi prises en compte dans le choix des compositions, puisqu'ils déterminent les possibilités de croissance de matériaux avec des bonnes qualités structurales. L'empilement des matériaux conduit à une structure électronique qui se caractérise notamment par le diagramme des bandes d'énergie correspondant. Le choix des matériaux lll-N et leurs compositions pour réaliser une structure électronique de transistor HEMT répond ainsi à des considérations sur les largeurs de bande interdite, en fonction des propriétés et performances recherchées et sur les accords de maille qui conditionnent l'obtention de couches de matériaux à moindre défauts structuraux. HEMT transistors made from structures formed of a stack of nitrides of elements III, and more generally of large gap semiconductor materials, have properties of great interest for microwave applications and / or requiring power. These structures use in a known manner different compositions III-N, in stacked layers. Each composition is chosen for its particular electronic properties, for example the effective mass of the electrons, their mobility or the width of the band gap. Considerations on mesh parameters are also taken into account in the choice of compositions, since they determine the possibilities of growth of materials with good structural qualities. Stacking materials leads to an electronic structure that is characterized in particular by the corresponding energy band diagram. The choice of materials III-N and their compositions for producing an electronic structure of HEMT transistor thus meets considerations on bandgap widths, depending on the desired properties and performance and on the mesh agreements that condition the obtaining of layers. materials with less structural defects.
En particulier, pour la conception de transistor à effet de champ de type HEMT, une structure électronique connue, de matériaux semi- conducteurs à grande largeur de bande interdite, est une hétérostructure comprenant la superposition d'une couche d'un premier matériau semiconducteur à large bande interdite (la zone de barrière) sur une couche d'un deuxième matériau semi-conducteur aussi à large bande interdite (la zone active) mais dans laquelle le premier matériau à un gap supérieur à celui du deuxième matériau.  In particular, for the HEMT field effect transistor design, a known electronic structure of high bandwidth semiconductor materials is a heterostructure comprising the superposition of a layer of a first semiconductor material to wide bandgap (the barrier zone) on a layer of a second semiconductor material also wide bandgap (the active area) but in which the first material at a gap greater than that of the second material.
Le contexte de l'invention étant lié à ces hétérostructures formées d'empilement de couches de matériaux semi-conducteurs à large bande interdite le terme "matériau", utilisé seul, est à comprendre comme étant un matériau semi-conducteur à large bande interdite Eg dans la suite de l'exposé.  Since the context of the invention is related to these heterostructures formed by stacking layers of semiconductor materials with a wide bandgap, the term "material", used alone, is to be understood as being a broad bandgap semiconductor material. in the rest of the presentation.
Comme illustré schématiquement sur la figure 1 , une structure électronique de transistor HEMT est constituée essentiellement de trois matériaux :  As illustrated schematically in FIG. 1, an electronic structure of HEMT transistor consists essentially of three materials:
• un substrat 1 ,  • a substrate 1,
· une couche 2 dite couche tampon, d'un matériau M présentant un gap Eg-i ,  A so-called buffer layer 2 of a material M having an Eg-i gap,
• une couche 3 dite couche barrière, d'un matériau M2 présentant un gap Eg2, où Eg est inférieur à Eg2. Cette structure permet la formation et la circulation d'un gaz bidimensionnel d'électrons 2DEG dans un canal C formé dans le matériau M1 à plus petit gap Eg-ι , à l'interface 10 (ou interface M2-M1 ), entre les deux matériaux M2/M1 de l'hétérojonction. Comme illustré sur la figure 3, ce canal correspond à un confinement des électrons dans un puits quantique QW qui se forme à l'interface 10 (ou interface M2-M1 ) entre les deux matériaux M2/M1 . A layer 3 called barrier layer, of a material M 2 having a gap Eg 2 , where Eg is smaller than Eg 2 . This structure allows the formation and the circulation of a two-dimensional electron gas 2DEG in a channel C formed in the M1 material with smaller gap Eg-ι, at the interface 10 (or interface M2-M1), between the two M2 / M1 materials of the heterojunction. As illustrated in FIG. 3, this channel corresponds to a confinement of the electrons in a QW quantum well which is formed at the interface 10 (or interface M2-M1) between the two materials M2 / M1.
Ces structures à hétérojonction à base d'un empilement de matériaux semi-conducteurs à grande largeur de bande interdite offrent des perspectives particulièrement intéressantes quant à l'obtention de transistors rapides HEMT (High Electron Mobility Transistor) performants pour les applications hyperfréquence de puissance (allant de 2 GHz à 100 GHz voire au-delà), et font l'objet de nombreuses recherches afin d'obtenir les structures les plus favorables qui associent une forte densité de gaz d'électrons bidimensionnel ns à une mobilité des porteurs la plus élevée possible, dans le but d'obtenir des transistors à fort courant de drain, condition nécessaire pour une amplification en puissance efficace. These heterojunction structures based on a stack of high bandwidth semiconductor materials offer particularly interesting prospects for obtaining high performance HEMT (High Electron Mobility Transistor) fast transistors for microwave power applications (ranging from 2 GHz to 100 GHz and beyond), and are the subject of much research in order to obtain the most favorable structures that combine high gas density two-dimensional electron n s a mobility of the higher carrier possible, in order to obtain transistors with high drain current, a necessary condition for an amplification in effective power.
Une propriété importante de l'hétérojonction M2/M1 , est le bon confinement des électrons dans le puits quantique QW, crucial pour l'efficacité du transport électronique du transistor.  An important property of the heterojunction M2 / M1, is the good confinement of the electrons in the quantum well QW, crucial for the efficiency of the electronic transport of the transistor.
Pour améliorer ce confinement, on cherche généralement à augmenter la résistivité du matériau M1 de la couche tampon, pour éviter les fuites d'électrons depuis le canal C vers le substrat qui créent une conduction parallèle. Il est en effet difficile d'obtenir un matériau lll-V naturellement résistif. Dans ce contexte, on a proposé des hétérostructures insérant sous la couche canal dans le matériau M1 , une couche d'un autre matériau avec un gap plus élevé que celui du matériau M1 , et éventuellement dopé Fe. Ces hétérostructures se montrent en pratique décevantes, lors de l'utilisation en hyperfréquence, du fait d'une augmentation significative de la quantité d'impuretés dans la structure, créant de manière irréversible des pièges, sources de dégradation des performances du transistor. Celles-ci sont observées sur la caractéristique Ids(Vds) par une dégradation du courant.  To improve this confinement, it is generally sought to increase the resistivity of the material M1 of the buffer layer, to avoid electron leakage from the channel C to the substrate which create a parallel conduction. It is indeed difficult to obtain a naturally resistive lll-V material. In this context, it has been proposed heterostructures inserting under the channel layer in the material M1, a layer of another material with a gap greater than that of the material M1, and possibly doped Fe. These heterostructures are in practice disappointing, during use in the microwave, because of a significant increase in the amount of impurities in the structure, creating irreversibly traps, sources of degradation of the performance of the transistor. These are observed on the characteristic Ids (Vds) by a degradation of the current.
Une autre voie d'amélioration du confinement du gaz bidimensionnel d'électrons pour les structures à hétérojonction M2/M1 , avec AIGaN/GaN, a été proposée dans la publication IEEE Electron Device Letters Vol.27, N °.1 , Janvier 2006, "AIGaN/GaN High Electron Mobility Transistors with an InGaN Back-Barriers" de T. Palacios et al. Elle consiste en l'insertion d'une couche mince d'InGaN sous la couche GaN tampon de la structure HEMT conventionnelle AIGaN/GaN. Cette publication montre que l'alliage InGaN, bien qu'ayant un plus petit gap que le GaN, accroît le niveau de la bande de conduction de la structure, grâce à des effets importants de polarisation électrostatique dans ce type de matériau. La couche InGaN forme ainsi une barrière électrostatique qui permet un confinement plus efficace du gaz d'électrons bidimensionnel dans le canal de GaN. Another way of improving the confinement of two-dimensional electron gas for M2 / M1 heterojunction structures, with AIGaN / GaN, has been proposed in the publication IEEE Electron Device Letters Vol.27, No. 1, January 2006, "AIGaN / GaN High Electron Mobility Transistors with an InGaN Back-Barriers" by T. Palacios et al. It consists of the insertion of a thin layer of InGaN under the GaN buffer layer of the conventional AIGaN / GaN HEMT structure. This publication shows that the InGaN alloy, although having a smaller gap than GaN, increases the level of conduction band of the structure, thanks to significant effects of electrostatic polarization in this type of material. The InGaN layer thus forms an electrostatic barrier that allows more efficient confinement of two-dimensional electron gas in the GaN channel.
Cependant, la mise en œuvre pratique, industrielle, de cette solution s'avère difficile, compte-tenu des températures très différentes à utiliser pour la croissance des différents matériaux de cette structure. Plus précisément, la croissance d'InGaN s'effectue à une température autour de 700 °C, bien inférieure à celles du GaN ou de l'AIGaN, qui se situent autour de 1000 °C et 1300 °C respectivement.  However, the practical, industrial implementation of this solution proves difficult, given the very different temperatures to be used for the growth of the different materials of this structure. Specifically, the growth of InGaN occurs at a temperature around 700 ° C, much lower than those of GaN or AIGaN, which are around 1000 ° C and 1300 ° C respectively.
Or il n'est pas possible d'envisager d'abaisser la température de croissance du GaN, car cela conduirait à réduire ses qualités structurales et électroniques. Aussi, l'incorporation d'aluminium pour former la couche AIGaN impose de toute façon d'aller-au-delà de 1000°C.  However it is not possible to consider lowering the growth temperature of GaN, as this would lead to reduce its structural and electronic qualities. Also, the incorporation of aluminum to form the AIGaN layer requires in any case to go beyond 1000 ° C.
II n'est pas possible non plus d'envisager de passer, en quelques fractions de seconde, à l'interface InGaN/GaN, de 700°C à 1000°C : cela aurait des effets très néfastes sur les propriétés électroniques du matériau GaN et structurales du matériau InGaN avec des risques de cassures notamment.  It is also not possible to envisage passing, in fractions of a second, to the InGaN / GaN interface, from 700 ° C. to 1000 ° C. This would have very detrimental effects on the electronic properties of the GaN material. and structural InGaN material with risks of breakage in particular.
La présente invention propose une nouvelle voie pour améliorer le confinement du gaz bidimensionnel d'électrons dans le canal.  The present invention provides a novel way to improve the confinement of two-dimensional electron gas in the channel.
Dans l'invention on s'est intéressé aux études rapportées dans la publication de A. Ougazzaden et al, "Progress on new wide bandgap materials BGaN, BgaAIN and their potential applications", Proc. Of SPIE Vol. 6479 (2007) conduites sur les qualités électriques et structurales de minces couches de BGaN. Il ressort de ces études que l'incorporation de bore jusqu'à 2% augmentait significativement la résistivité et la mobilité des porteurs de charge par rapport au matériau GaN. Ces deux propriétés électriques sont corrélées à une très bonne qualité cristalline de la structure des matériaux BGaN. Cette publication montre qu'avec une composition de bore d'au moins 1 %, la couche BGaN peut être caractérisée comme semi- isolante (>102 ohms. cm) et peut donc être utilisée comme couche tampon dans une structure HEMT. Comme l'incorporation du bore est uniforme en volume, l'épaisseur de la couche BGaN peut être très mince ou épaisse (de quelques dixièmes de nanomètres à quelques microns). Par ailleurs, le BGaN offre de bonnes caractéristiques en terme de correspondance de maille avec les substrats de croissance usuels (AI2O3, SiC (4H-6H), Si (1 1 1 , 100, 1 10), GaN (monocristallin), substrats composites, ou à grand gap comme ΑΙΝ ou le diamant poly- ou monocristallin) qui présentent une bonne conductivité thermique. In the present invention, the studies reported in A. Ougazzaden et al., "Progress on new wide bandgap materials BGaN, BgaAIN and their potential applications", Proc. Of SPIE Vol. 6479 (2007) conducted on the electrical and structural qualities of thin layers of BGaN. These studies show that boron incorporation up to 2% significantly increased the resistivity and mobility of charge carriers relative to GaN material. These two electrical properties are correlated with a very good crystalline quality of the structure of BGaN materials. This publication shows that with a boron composition of at least 1%, the BGaN layer can be characterized as semi-insulating (> 10 2 ohm.cm) and can therefore be used as a buffer layer in a HEMT structure. Since boron incorporation is uniform in volume, the thickness of the BGaN layer can be very thin or thick (from a few tenths of nanometers to a few microns). Moreover, BGaN offers good characteristics in terms of mesh correspondence with the usual growth substrates (Al 2 O 3 , SiC (4H-6H), Si (1 1 1, 100, 1 10), GaN (monocrystalline), composite substrates, or with a large gap such as ΑΙΝ or poly- or monocrystalline diamond) which have good thermal conductivity.
En outre, comme détaillé dans la publication "Bandgap bowing in BGaN thin films" de A.Ougazzaden et al, Applied Physics letters 93, 0831 18 (2008), le ternaire BGaN possède une largeur de bande interdite inférieure à celle du binaire GaN, pour de faibles taux d'incorporation de bore, ainsi qu'une polarisation électronique du matériau importante, à l'image de l'InGaN.  Furthermore, as detailed in the publication "Bandgap bowing in BGaN Thin Films" by A.Ougazzaden et al., Applied Physics Letters 93, 0831 18 (2008), the BGaN ternary has a lower bandgap than that of the GaN binary, for low levels of boron incorporation, as well as an electronic polarization of the important material, like InGaN.
RESUME DE L'INVENTION SUMMARY OF THE INVENTION
Dans l'invention, on a ainsi eu l'idée d'utiliser une couche BGaN semi-isolante, comme barrière électrostatique, plutôt que l'InGaN sous le canal. On bénéficie alors d'un double effet, de barrière de potentiel favorisant le confinement des électrons dans le puits de potentiel, par la forte polarisation électronique de la couche BGaN, et d'augmentation de la résistivité de la structure sous le canal, empêchant la fuite d'électrons vers le substrat, par le caractère résistif de cette couche.  In the invention, it was thus the idea to use a semi-insulating BGaN layer, as an electrostatic barrier, rather than InGaN under the channel. There is then a double effect, a potential barrier favoring the confinement of the electrons in the potential well, by the strong electronic polarization of the BGaN layer, and of increasing the resistivity of the structure under the channel, preventing the electron leakage to the substrate, by the resistive nature of this layer.
Ces deux effets sont obtenus pour de faibles quantités de bore, à partir de 0,1 %, permettant une mise en œuvre aisée d'une telle structure avec les techniques de l'état de l'art.  These two effects are obtained for small amounts of boron, from 0.1%, allowing easy implementation of such a structure with the techniques of the state of the art.
L'invention concerne donc une structure de transistor HEMT, comprenant  The invention therefore relates to a HEMT transistor structure, comprising
-au moins une première couche dite tampon, d'un premier matériau semi-conducteur à large bande interdite Eg-i , et une deuxième couche d'un deuxième matériau semi-conducteur à large bande interdite Eg2, avec une largeur de bande interdite Eg2 supérieure à celle Eg-ι , et at least one first so-called buffer layer, of a first wide bandgap semiconductor material Eg-i, and a second layer of a second wide bandgap semiconductor material Eg 2 , with a forbidden bandgap width Eg 2 greater than that Eg-ι, and
-un gaz bidimensionnel d'électrons qui circule dans un canal délimité dans la première couche à l'interface entre la première couche et la deuxième couche.  a two-dimensional gas of electrons circulating in a channel delimited in the first layer at the interface between the first layer and the second layer.
Selon l'invention, un matériau BGaN avec une concentration moyenne en bore d'au moins 0,1 %, semi-isolant, est inséré dans la couche tampon sous forme d'au moins une couche sous la couche canal, modifiant le diagramme des bandes d'énergie par création d'une barrière de potentiel électrostatique favorisant le confinement du gaz bidimensionnel d'électrons. According to the invention, a BGaN material with an average boron concentration of at least 0.1%, semi-insulating, is inserted into the buffer layer in the form of at least one layer under the channel layer, modifying the diagram of the energy bands by creating an electrostatic potential barrier favoring the confinement of the two-dimensional electron gas.
Cette couche BGaN peut être réalisée sous forme d'une couche de BGaN dans la couche tampon, sous le canal, qui présente une concentration uniforme de bore sur toute l'épaisseur ; ou qui présente une concentration graduée ou en marches d'escalier sur l'épaisseur, partant d'une concentration nulle, et croissante avec l'épaisseur, vers le canal.  This BGaN layer may be formed as a layer of BGaN in the buffer layer, under the channel, which has a uniform boron concentration throughout the thickness; or which has a graduated concentration or stair step on the thickness, starting from a zero concentration, and increasing with the thickness, towards the channel.
Quand la couche tampon est une couche du binaire GaN, ou d'un alliage de GaN, des clusters de BGaN peuvent être réalisés directement dans la couche tampon.  When the buffer layer is a layer of the GaN binary, or a GaN alloy, BGaN clusters can be made directly in the buffer layer.
Cette couche de confinement peut encore se présenter sous forme d'un super réseau de couches très fines alternant successivement du BGaN et du GaN ou de ΙΆΙΝ.  This confinement layer may also be in the form of a super network of very thin layers successively alternating BGaN and GaN or ΙΆΙΝ.
L'invention concerne aussi une utilisation d'autres couches de BGaN à des fins de perfectionnements de la structure électronique du transistor HEMT.  The invention also relates to the use of other BGaN layers for the purpose of improving the electronic structure of the HEMT transistor.
Dans un premier perfectionnement, la structure comprend une couche de BGaN comme couche de nucléation, permettant d'améliorer la qualité structurale de la deuxième couche obtenue par croissance de matériau à partir de cette couche de nucléation. Ce sont ici les qualités structurales du BGaN qui sont exploitées.  In a first improvement, the structure comprises a layer of BGaN as a nucleation layer, making it possible to improve the structural quality of the second layer obtained by growth of material from this nucleation layer. These are the structural qualities of BGaN that are exploited.
Dans un autre perfectionnement, la structure comprend une couche de BGaN ou de BN comme couche de passivation en surface, pour minimiser l'influence des pièges éventuels en surface. Ce sont ici les propriétés résistives du BGaN ou du BN qui sont avantageusement exploitées.  In another improvement, the structure comprises a layer of BGaN or BN as a surface passivation layer, to minimize the influence of potential traps on the surface. It is here the resistive properties of BGaN or BN that are advantageously exploited.
D'autres avantages et caractéristiques de l'invention sont détaillés par la description de plusieurs modes de réalisation de l'invention, et en référence aux dessins annexés, dans lesquels : Other advantages and features of the invention are detailed by the description of several embodiments of the invention, and with reference to the appended drawings, in which:
- la figure 1 illustre schématiquement une structure électronique pour un transistor HEMT, selon l'état de l'art;  - Figure 1 schematically illustrates an electronic structure for a HEMT transistor, according to the state of the art;
- les figures 2 et 3 illustrent respectivement, une structure électronique pour transistor HEMT dans un premier exemple de mise en œuvre de l'invention, et un diagramme correspondant des bandes d'énergie avec l'emploi d'une couche mince de BGaN et la formation d'une barrière électrostatique; FIGS. 2 and 3 respectively illustrate an electronic structure for a HEMT transistor in a first exemplary implementation of the invention, and a corresponding diagram. energy bands with the use of a thin layer of BGaN and the formation of an electrostatic barrier;
les figures 4 et 5 illustrent respectivement, une structure électronique pour transistor HEMT dans un second exemple de mise en œuvre de l'invention, et un diagramme correspondant des bandes d'énergie avec l'emploi d'une couche graduée en composition de bore et la formation d'une barrière électrostatique dont le sommet se trouve à l'extrémité côté gaz; les figures 6 et 7 illustrent respectivement, une structure électronique pour transistor HEMT dans un troisième exemple de mise en œuvre de l'invention, et un diagramme correspondant des bandes d'énergie avec l'emploi d'une couche épaisse de BGaN et la formation d'une barrière électrostatique plus large; FIGS. 4 and 5 respectively illustrate an electronic structure for a HEMT transistor in a second exemplary implementation of the invention, and a corresponding diagram of the energy bands with the use of a graded layer of boron composition and the formation of an electrostatic barrier, the top of which is at the gas end; FIGS. 6 and 7 respectively illustrate an electronic structure for a HEMT transistor in a third example of implementation of the invention, and a corresponding diagram of the energy bands with the use of a thick layer of BGaN and the formation a wider electrostatic barrier;
la figure 8 représente une structure de couche BGaN de type super-réseaux, qui peut être utilisée dans les structures illustrées aux figures 2, 4 et 6; Fig. 8 shows a super-array type BGaN layer structure, which may be used in the structures illustrated in Figs. 2, 4 and 6;
la figure 9 illustre une autre structure de couche BGaN du type à incorporations localisées en volume; FIG. 9 illustrates another BGaN layer structure of localized incorporation type by volume;
la figure 10 illustre une structure comprenant des perfectionnements selon l'invention ; FIG. 10 illustrates a structure comprising improvements according to the invention;
les figures 1 1 à 13 illustrent trois exemples pratiques d'une structure de type AIGaN/GaN avec une insertion d'une couche d'un matériau BGaN selon l'invention, respectivement en une couche fine à concentration uniforme de Bore, en une couche épaisse à concentration uniforme de Bore et en une couche épaisse avec un gradient de concentration de Bore; FIGS. 11 to 13 illustrate three practical examples of an AIGaN / GaN type structure with an insertion of a layer of a BGaN material according to the invention, respectively in a thin layer with a uniform Bore concentration, in one layer. Thickness at uniform concentration of boron and in a thick layer with a concentration gradient of boron;
la figure 14 illustre les courbes obtenues par simulation de la structure à couche fine de BGaN de la figure 1 1 , avec dans une fenêtre supérieure (a) la courbe de niveau d'énergie de la bande de conduction de la structure suivant l'axe Y correspondant à l'épaisseur de la structure, en partant de la surface vers le substrat, dans la fenêtre inférieure (b), la courbe de concentration des porteurs dans la structure suivant cet axe Y; - la figure 15 illustre ces mêmes courbes de niveau d'énergie de la bande de conduction, et de concentration des porteurs, mais obtenues par simulation de la structure à couche épaisse de BGaN, avec concentration uniforme de bore illustrée à la figure 12, et celle avec concentration graduelle de bore illustrée à la figure 13; FIG. 14 illustrates the curves obtained by simulation of the BGaN thin-film structure of FIG. 11, with in an upper window (a) the energy level curve of the conduction band of the structure along the axis. Y corresponding to the thickness of the structure, starting from the surface towards the substrate, in the lower window (b), the concentration curve of the carriers in the structure along this axis Y; FIG. 15 illustrates these same power level curves of the conduction band, and carrier concentration, but obtained by simulation of the BGaN thick film structure, with uniform concentration of boron illustrated in FIG. 12, and that with gradual concentration of boron illustrated in Figure 13;
- la figure 16 reprend sur le même schéma les courbes des niveaux d'énergie de la bande de conduction pour les trois structures des figures 1 1 , 12 et 13; et  FIG. 16 shows on the same diagram the curves of the energy levels of the conduction band for the three structures of FIGS. 11, 12 and 13; and
- la figure 17 reprend sur le même schéma les courbes de concentration de porteurs pour chacune des trois structures des figures 1 1 à 13.  FIG. 17 shows on the same diagram the carrier concentration curves for each of the three structures of FIGS. 1 to 13.
DESCRIPTION DÉTAILLÉE DETAILED DESCRIPTION
À titre liminaire on remarquera que les figures illustrant les empilements de couches de la structure électronique ne sont pas à l'échelle. Notamment, les épaisseurs ne sont pas représentées de manière proportionnelle. Par ailleurs, dans un souci de simplification des références, les éléments communs à toutes les structures portent les mêmes références.  As a preliminary, it will be noted that the figures illustrating the stacks of layers of the electronic structure are not to scale. In particular, the thicknesses are not represented proportionally. Moreover, for the sake of simplifying the references, the elements common to all the structures bear the same references.
L'invention va être plus particulièrement décrite dans un exemple non limitatif d'application à une structure électronique pour transistor HEMT, à base de nitrures d'éléments III, et plus particulièrement, à hétérojonction AIGaN/GaN. AIGaN est le matériau M2 de la couche barrière présentant un gap Eg2 plus grand que celui, Eg du premier matériau M1 de la couche tampon, qui est le GaN. The invention will be more particularly described in a nonlimiting example of application to an electronic structure for HEMT transistor, based on nitrides of elements III, and more particularly, heterojunction AIGaN / GaN. AIGaN is the M2 material of the barrier layer having a gap Eg 2 greater than that, Eg of the first material M1 of the buffer layer, which is GaN.
Selon l'invention, la structure comprend une couche BGaN dans la couche tampon, sous le canal.  According to the invention, the structure comprises a BGaN layer in the buffer layer, under the channel.
Un premier exemple d'une structure électronique selon l'invention est illustré sur la figure 2. Elle comprend l'empilement de couches suivant dans l'ordre de croissance (d'empilement) :  A first example of an electronic structure according to the invention is illustrated in FIG. 2. It comprises the following stack of layers in the order of growth (stacking):
- un substrat 1 , semi-isolant, spécifique de la filière, c'est-à-dire accordé ou partiellement accordé en maille avec les matériaux formant l'hétérostructure et obtenus par croissance cristallographique à partir de ce substrat. Différents types de substrats sont couramment utilisés : des substrats bas coût tels que Si, avec des orientations cristallines (1 1 1 ), (100), (1 10), AI2O3 monocristallin, le SiC (4H, 6H) dont le coût est élevé. Les substrats composites tels que SopSiC (Silicium-Oxyde-SiC polycristallin), SiCopSiC (SiC monocristallin- Oxyde-SiC polycristallin), le diamant polycristallin ; les substrats ZnO, les substrats SiC (dans une moindre mesure) et les substrats en diamant monocristallin sont des matériaux qui présentent de bonnes propriétés pour la dissipation thermique. On peut également citer les substrats dits "pseudo substrats" de GaN, AIN, ZnO ou encore les substrats souples, tels que le Kapton, PTFE (polytétrafluoroéthylène) sur lequel on a reporté l'épimatériau. La liste des substrats ne se prétend pas exhaustive. Le choix du substrat est étroitement lié au cahier des charges de l'application, en tenant compte du coût, des performances attendues, ainsi que du paramètre de maille des couches de matériaux de l'hétérostructure envisagée. - A substrate 1, semi-insulating, specific to the die, that is to say, or partially tuned in mesh with the materials forming the heterostructure and obtained by crystallographic growth from this substrate. Different types of substrates are commonly used: low cost substrates such as Si, with crystalline orientations (1 1 1), (100), (1 10), Al 2 O 3 monocrystalline, SiC (4H, 6H) whose cost is high. Composite substrates such as SopSiC (polycrystalline silicon-oxide-SiC), SiCopSiC (monocrystalline SiC-polysilicon-SiC), polycrystalline diamond; ZnO substrates, SiC substrates (to a lesser extent) and monocrystalline diamond substrates are materials that exhibit good properties for heat dissipation. There may also be mentioned substrates known as "pseudo substrates" of GaN, AlN, ZnO or else flexible substrates, such as Kapton, PTFE (polytetrafluoroethylene) on which the epimaterial has been reported. The list of substrates does not claim to be exhaustive. The choice of the substrate is closely related to the specifications of the application, taking into account the cost, the expected performance, as well as the mesh parameter of the material layers of the heterostructure envisaged.
On notera que ce substrat peut-être un substrat temporaire, utilisé pour la réalisation de l'épitaxie, par croissance de matériaux. Il peut ensuite être enlevé par toute technique connue, pour reporter la structure ainsi détachée de son substrat de croissance, sur un autre substrat, par exemple un verre, un substrat souple ou un substrat ayant une bonne conductivité thermique. Une structure électronique peut ainsi être provisoirement sans substrat, ou avoir un substrat final, dans le composant, qui n'est pas le substrat de croissance.  It will be noted that this substrate may be a temporary substrate, used for the realization of epitaxy, by growth of materials. It can then be removed by any known technique to transfer the structure thus detached from its growth substrate to another substrate, for example a glass, a flexible substrate or a substrate having a good thermal conductivity. An electronic structure can thus be provisionally free of substrate, or have a final substrate, in the component, which is not the growth substrate.
- une couche tampon 2 ("template" ou "buffer" dans la littérature anglo-saxonne) d'un nitrure, dans l'exemple GaN, composée généralement d'une première couche GaN 2a qui sert de manière connue, de matériau de base de bonne qualité cristallographique, pour la croissance cristallographique d'une deuxième couche 2b de GaN ayant des qualités structurales excellentes. En effet, le gaz bidimensionnel d'électrons va se former dans cette couche proche de l'hétérojonction.  a buffer layer 2 ("template" or "buffer" in the Anglo-Saxon literature) of a nitride, in the GaN example, generally composed of a GaN 2a first layer which serves in a known manner, of base material of good crystallographic quality, for the crystallographic growth of a second layer 2b of GaN having excellent structural qualities. Indeed, the two-dimensional electron gas will form in this layer close to the heterojunction.
- une couche barrière 3 formée d'un matériau à plus grand gap. Dans l'exemple cette couche est une composition grand gap d'AIGaN ou d'InAIN, telle que AI0,32Ga0,68 (x=0,32). Ce pourrait être aussi une couche d'AIN. En pratique la couche barrière peut comprendre plusieurs couches élémentaires (non illustré), notamment une couche dopée, appelée couche donneuse qui fournit les électrons libres qui vont participer à former le gaz bidimensionnel d'électrons dans la couche tampon, et une couche non intentionnellement dopée, appelée espaceur, entre la couche dopée et la couche tampon, qui favorise la mobilité des électrons dans le canal de transport du gaz bidimensionnel d'électrons. On n'envisage pas de dopage dans les structures nitrures... le dopage est souvent inutile, les électrons venant essentiellement de la surface par effet de polarisation piézoélectrique et de génération spontanée. a barrier layer 3 formed of a larger gap material. In the example, this layer is a large gap composition of AIGaN or InAIN, such that AI 0 , 32Ga 0 , 68 (x = 0.32). It could also be a layer of AIN. In practice, the barrier layer may comprise a plurality of elementary layers (not shown), in particular a doped layer, called a donor layer which supplies the free electrons that will participate in forming the two-dimensional electron gas in the buffer layer, and a non-layer intentionally doped, called spacer, between the doped layer and the buffer layer, which promotes the mobility of electrons in the two-dimensional electron gas transport channel. No doping is envisaged in the nitride structures ... doping is often unnecessary, the electrons coming essentially from the surface by piezoelectric polarization effect and spontaneous generation.
-une couche de passivation 4 ("cap layer" dans la littérature anglo- saxonne) comme illustrée sur la figure 1 peut être prévue (non illustrée sur la figure 2), formée dans un matériau présentant une largeur de bande interdite plus faible que le matériau M2 de la couche barrière, et qui est fortement dopée type n, pour permettre la réalisation des contacts ohmiques de source et drain (non représentés) du transistor HEMT. C'est par exemple une couche de GaN fortement dopée type n. La couche de passivation sera peu utilisée lorsque la qualité structurale de la couche est bonne. Elle permet surtout d'empêcher l'oxydation de l'Aluminium dans la couche barrière. Si passivation il y a, un dopage éventuel pourra être réalisé sous les contacts exclusivement.  a passivation layer 4 ("cap layer" in the Anglo-Saxon literature) as illustrated in FIG. 1 may be provided (not shown in FIG. 2), formed in a material having a lower bandgap width than the M2 material of the barrier layer, and which is strongly n-type doped, to allow the realization of ohmic source and drain contacts (not shown) of the HEMT transistor. This is for example a n-type strongly doped GaN layer. The passivation layer will be little used when the structural quality of the layer is good. It mainly prevents the oxidation of aluminum in the barrier layer. If passivation there is, a possible doping can be realized under the contacts exclusively.
Selon l'invention, la structure comprend en outre une couche 5 de BGaN dans la couche tampon 2, sous le canal C.  According to the invention, the structure further comprises a layer 5 of BGaN in buffer layer 2, under channel C.
Dans l'exemple illustré d'une couche GaN 2a contenant le canal In the illustrated example of a GaN 2a layer containing the channel
C, obtenue par recroissance d'une couche GaN 2b, comme expliqué ci- dessus, la couche BGaN est insérée entre la couche GaN 2a et la couche GaN 2b. C, obtained by regrowth of a GaN 2b layer, as explained above, the BGaN layer is inserted between the GaN layer 2a and GaN layer 2b.
L'expression couche BGaN ou matériau BGaN est à comprendre dans toute la description comme englobant aussi bien le ternaire BGaN, que des alliages d'ordres plus élevés, c'est-à-dire que cela peut aussi être un quartenaire BlnGaN, BAIGaN, ou un quintenaire BAlInGaN. Cette remarque s'applique aux autres matériaux de la structure.  The expression BGaN layer or BGaN material is to be understood throughout the description as encompassing both the BGaN ternary and higher order alloys, that is to say that it may also be a quarter BlnGaN, BAIGaN, or a quarter BAlInGaN. This remark applies to the other materials of the structure.
Dans ce premier exemple de structure, la couche BGaN est une couche mince, d'épaisseur de l'ordre de 1 nanomètre, avec une concentration uniforme en bore. Le matériau BGaN est un ternaire, avec une concentration en bore de l'ordre de 1 â 4%, ce qui s'écrit : B0,oiGa0,96N ,B0,o4Ga0,96N respectivement. In this first example of structure, the BGaN layer is a thin layer, of thickness of the order of 1 nanometer, with a uniform concentration of boron. The BGaN material is a ternary, with a boron concentration of the order of 1 to 4%, which is written as: B 0, OiGa 0 , 96N, B 0 , o4Ga 0 , 96N respectively.
Le diagramme des bandes d'énergie obtenu par modélisation, pour cette structure, est illustré sur la figure 3. Il montre les niveaux d'énergie, en électrons volts, de la bande de valence BV et la bande de conduction BC obtenus (axe vertical de gauche), ainsi que la répartition de la densité d'électrons dans la structure (en cm"3) (axe vertical de droite), sur la hauteur de la structure suivant l'axe transversal Y (nanomètres). L'origine Y=0, correspond à la surface de la couche 3 (figure 2). Il met en évidence la formation du puits de potentiel triangulaire QW à l'interface 10 entre les deux matériaux AIGaN (M2) et GaN (M1 ). Les courbes d'énergie des bandes de valence et de conduction présentent une décroissance suivie d'une recroissance très marquées à l'endroit de l'interface 10, correspondant à la formation du puits de potentiel QW. Ce puits de potentiel confine le gaz bidimensionnel d'électrons 2DEG à l'interface, comme illustré par la courbe de répartition de la densité d'électrons représentée en pointillé sur la figure. Dans ce puits, la densité d'électrons ns y est maximum. C'est le principe du gaz 2D lié à l'hétérojonction. The modeled energy band diagram for this structure is shown in Figure 3. It shows the levels of energy, in electron volts, of the valence band BV and the conduction band BC obtained (left vertical axis), as well as the distribution of the density of electrons in the structure (in cm "3 ) (vertical axis right), on the height of the structure along the transverse axis Y (nanometers) The origin Y = 0, corresponds to the surface of layer 3 (figure 2), and shows the formation of the potential well. triangular QW at the interface 10 between the two materials AIGaN (M2) and GaN (M1) The energy curves of the valence and conduction bands show a very marked decay followed by a regrowth at the location of the interface 10, corresponding to the formation of the potential well QW This potential well confines the two-dimensional electron gas 2DEG to the interface, as illustrated by the electron density distribution curve shown in dashed lines in the figure. in this well, the electron density n s is the maximum. C'e st the principle of 2D gas linked to the heterojunction.
La présence de la couche BGaN 5 sous le canal C de la structure selon l'invention, se traduit en outre dans le diagramme de bandes par la création de deux pics d'énergie 1 1 qui correspondent aux bandes de valence et de conduction du BGaN : ces pics forment une barrière électrostatique qui rend plus difficile la fuite des électrons en dehors du puits. Le confinement des électrons dans le puits de potentiel QW à l'interface 10 est ainsi amélioré. Cette barrière est dans cet exemple assez étroite, correspondant à la faible épaisseur, 1 nm dans l'exemple, de la couche BGaN 5.  The presence of the BGaN layer 5 under the channel C of the structure according to the invention is also reflected in the band diagram by the creation of two energy peaks 1 1 which correspond to the valence and conduction bands of the BGaN These peaks form an electrostatic barrier that makes it more difficult for the electrons to leak out of the well. The confinement of the electrons in the potential well QW at the interface 10 is thus improved. This barrier is in this example rather narrow, corresponding to the small thickness, 1 nm in the example, of the BGaN 5 layer.
La couche BGaN a un autre effet, celui d'augmenter la résistivité de la structure sous le canal, empêchant la fuite des électrons vers le substrat.  The BGaN layer has another effect, that of increasing the resistivity of the structure under the channel, preventing the leakage of electrons to the substrate.
Ainsi, la couche BGaN a deux effets qui tendent chacun à améliorer le confinement du gaz bidimensionnel d'électrons : d'une part parce que la couche BGaN modifie, en l'améliorant, le diagramme de bandes d'énergie ; et d'autre part parce que la couche BGaN augmente la résistivité de la structure sous le canal, empêchant les fuites d'électrons du canal vers le substrat.  Thus, the BGaN layer has two effects, each of which tends to improve the confinement of the two-dimensional electron gas: first, because the BGaN layer modifies, improving it, the energy band diagram; and secondly because the BGaN layer increases the resistivity of the structure under the channel, preventing electron leakage from the channel to the substrate.
Les figures 4 et 6 représentent deux autres exemples de structure selon l'invention, et les figures 5 et 7, leurs diagrammes de bandes d'énergie respectifs. Ces figures montrent que selon la concentration et l'épaisseur de la couche BGaN, on peut augmenter et/ou élargir la barrière électrostatique créée par la couche BGaN, améliorant le confinement du gaz bidimensionnel d'électrons. FIGS. 4 and 6 show two other examples of structure according to the invention, and FIGS. 5 and 7 their respective energy band diagrams. These figures show that depending on the concentration and thickness of the BGaN layer, the electrostatic barrier can be increased and / or enlarged. created by the BGaN layer, improving the confinement of two-dimensional electron gas.
Sur la figure 4, la couche BGaN est plus épaisse, de l'ordre de 50nm (contre 1 nm dans l'exemple illustré sur la figure 2), mais avec une concentration en bore qui est abrupte, ou graduée en marches d'escalier : la concentration en bore est nulle à l'interface avec la couche 2b, et va croissante vers le canal (dans la couche 2a), par exemple jusqu'à 4%. La figure 5 du diagramme des bandes correspondant montre un effet barrière électrostatique 12 accentué et plus large. L'utilisation d'un gradient de concentration de bore sur une plus grande épaisseur de couche permet ainsi de former une barrière électrostatique plus distinctive qui va limiter davantage le mouvement des électrons en dehors du puits de potentiel.  In FIG. 4, the BGaN layer is thicker, of the order of 50 nm (compared with 1 nm in the example illustrated in FIG. 2), but with a boron concentration which is steep, or graduated in steps : the boron concentration is zero at the interface with the layer 2b, and increases towards the channel (in the layer 2a), for example up to 4%. Figure 5 of the corresponding band diagram shows an enhanced and wider electrostatic barrier effect. The use of a boron concentration gradient over a greater layer thickness thus makes it possible to form a more distinctive electrostatic barrier which will further limit the movement of electrons out of the potential well.
Sur la figure 6, la couche BGaN est encore plus épaisse, de l'ordre de 100nm, mais avec une très faible concentration en bore, de l'ordre de 1 % (B0,oiGao,9gN). La figure 7 du diagramme des bandes correspondant montre que l'on obtient une barrière électrostatique 13 encore plus large, en lien avec la plus grande épaisseur de la couche. Cette structure est très intéressante car on sait facilement produire une telle couche à faible concentration de bore. Et même à ces faibles concentrations de bore, les effets de barrière électrostatique et d'augmentation de la résistivité de la structure sous le canal sont observés. In Figure 6, the BGaN layer is even thicker, of the order of 100 nm, but with a very low concentration of boron, of the order of 1% (B 0, oiGao, 9gN). FIG. 7 of the corresponding band diagram shows that an even larger electrostatic barrier 13 is obtained in connection with the greater thickness of the layer. This structure is very interesting because it is easily known to produce such a layer with a low concentration of boron. And even at these low boron concentrations, electrostatic barrier effects and increased resistivity of the structure under the channel are observed.
En pratique, les couches de BGaN utilisées conformément à l'invention sont caractérisées par une concentration moyenne en Bore, d'au moins 0,1 %.  In practice, the BGaN layers used according to the invention are characterized by a mean boron concentration of at least 0.1%.
L'épaisseur de la couche va de préférence de 1 nanomètre à plusieurs centaines de nanomètres environ.  The thickness of the layer is preferably from 1 nanometer to several hundred nanometers approximately.
L'invention qui vient d'être décrite dans un exemple de structure à hétérojonction AIGaN/GaN, prévoit ainsi l'insertion d'une couche BGaN dans la couche tampon, sous le canal, pour obtenir un double effet de modification favorable du diagramme de bandes avec formation d'une barrière électrostatique d'autant plus large que la couche BGaN est large, et d'augmentation de la résistivité de la structure sous le canal.  The invention which has just been described in an example of heterojunction AIGaN / GaN structure, thus provides for the insertion of a BGaN layer in the buffer layer, under the channel, to obtain a double effect of favorable modification of the bands with formation of an electrostatic barrier even wider than the BGaN layer is wide, and increasing the resistivity of the structure under the channel.
L'invention s'applique notamment plus généralement à toutes les structures à hétérojonction obtenues avec des couches choisies parmi les binaires de nitrures d'éléments III, c'est-à-dire AIN, GaN, InN, BN, et les ternaires, quaternaires ou quintenaires formés à partir de ces binaires. Elle s'applique plus généralement à des structures de transistor HEMT à base de matériaux semi-conducteur à grande largeur de bande interdite, comprenant les matériaux semi-conducteur lll-V, le diamant ou l'oxyde de zinc (et tout autre matériau cité plus haut). Le premier matériau M1 sera de préférence un nitrure d'éléments III, sous forme binaire, typiquement AIN, ou d'un alliage ternaire ou quaternaire formé à partir d'un binaire de la liste suivante : AIN, GaN, InN, BN. Ce peut aussi être du diamant ou un oxyde de zinc ZnO. Le deuxième matériau M2 peut être un nitrure d'élément III, et notamment un binaire (AIN, GaN, InN, BN), ou un alliage ternaire ou un quaternaire formé à partir d'un binaire de la liste AIN, GaN, InN, BN. The invention applies in particular more generally to all heterojunction structures obtained with layers chosen from the III element nitride binaries, that is to say AIN, GaN, InN, BN, and the ternary, quaternary or quintenary formed from these binaries. It more generally applies to HEMT transistor structures based on wide-bandgap semiconductor materials, including semiconductor materials III-V, diamond or zinc oxide (and any other material cited above). upper). The first material M1 will preferably be a nitride of elements III, in binary form, typically AIN, or a ternary or quaternary alloy formed from a binary of the following list: AIN, GaN, InN, BN. It can also be diamond or zinc oxide ZnO. The second material M2 may be an element III nitride, and in particular a binary (AlN, GaN, InN, BN), or a ternary or quaternary alloy formed from a binary of the AlN list, GaN, InN, BN.
En pratique, la couche BGaN sur la couche tampon 2a peut être obtenue de différentes manières, en utilisant l'éventail des techniques de croissance de ce matériau disponible à l'heure actuelle, c'est-à-dire typiquement : les techniques d'épitaxie par jet moléculaire (MBE), ou en phase vapeur ; la technique des organo-métalliques (MOCVD) ou hybride (HVPE) ; les techniques d'implantation de bore dans une couche de GaN, et les techniques de diffusion, avec phases de dépôt et recuit. Ces techniques permettent en outre de manière connue de former la couche BGaN de différentes manières. Notamment :  In practice, the BGaN layer on the buffer layer 2a can be obtained in different ways, using the range of growth techniques of this material available at present, that is to say typically: molecular beam (MBE) or vapor phase epitaxy; organo-metallic (MOCVD) or hybrid (HVPE) technique; boron implantation techniques in a GaN layer, and diffusion techniques, with deposition and annealing phases. These techniques also make it possible in known manner to form the BGaN layer in different ways. Especially :
-la couche BGaN peut être formée avec une concentration homogène, uniforme, de bore dans le volume, comme dans l'exemple illustré sur la figure 1 .  the BGaN layer may be formed with a homogeneous, uniform concentration of boron in the volume, as in the example illustrated in FIG.
-la couche BGaN peut aussi être formée avec une concentration abrupte ou graduée en marches d'escalier, partant de 0, et croissant en allant vers le canal, jusqu'à une concentration supérieure, par exemple 4%, comme schématiquement illustré sur la figure 4.  the layer BGaN can also be formed with a steep or graduated concentration in stair steps, starting from 0, and increasing towards the channel, to a higher concentration, for example 4%, as schematically illustrated in FIG. 4.
-la couche de BGaN peut être aussi réalisée sous forme d'un superréseau {super lattice en anglais) formé d'une alternance de couches très fines, par exemple une alternance de couches BGaN et GaN, comme illustré schématiquement dans la structure de la figure 8, sur une épaisseur de structure déterminée, dans l'exemple 50 nm, pour atteindre une concentration moyenne équivalente. On peut aussi envisager une alternance de couches BGaN et AIN. -la couche BGaN peut encore être réalisée en formant une couche GaN ou BGaN, avec des incorporations localisées en volume de BGaN plus concentrées en bore, formant des petits volumes ou "clusters" 20 dans la couche tampon environnante, comme schématiquement illustré sur la figure 9. L'épaisseur de la couche environnante et la densité des clusters et les concentrations respectives de la couche environnante et des clusters 20 sont déterminées pour obtenir la concentration moyenne recherchée. Lorsque la couche tampon 2a est une couche de GaN (GaN ou alliage de GaN avec d'autres éléments de la colonne III), ces clusters peuvent être réalisés directement dans la couche tampon. La couche environnante dans laquelle sont réalisés les clusters BGaN peut aussi être une couche insérée dans la couche tampon 2 de la structure. the BGaN layer can also be produced in the form of a super lattice (super lattice in English) formed of an alternation of very thin layers, for example an alternation of BGaN and GaN layers, as schematically illustrated in the structure of FIG. 8, on a determined structure thickness, in the example 50 nm, to reach an equivalent average concentration. It is also possible to envisage an alternation of BGaN and AIN layers. the BGaN layer can be further produced by forming a GaN or BGaN layer, with localized bulk incorporation of BGaN more concentrated in boron, forming small volumes or "clusters" in the surrounding buffer layer, as schematically illustrated in the figure 9. The thickness of the surrounding layer and the density of the clusters and the respective concentrations of the surrounding layer and clusters are determined to obtain the desired average concentration. When buffer layer 2a is a layer of GaN (GaN or GaN alloy with other elements of column III), these clusters can be made directly in the buffer layer. The surrounding layer in which the BGaN clusters are made can also be a layer inserted in the buffer layer 2 of the structure.
Dans l'invention, on propose en outre de perfectionner la structure de transistor HEMT décrite précédemment, en utilisant les propriétés électriques, notamment résistives, et les qualités structurales des couches BGaN à d'autres niveaux dans la structure, améliorant encore les performances électriques du transistor HEMT.  In the invention, it is furthermore proposed to improve the HEMT transistor structure described above, by using the electrical properties, in particular the resistive properties, and the structural qualities of the BGaN layers at other levels in the structure, further improving the electrical performance of the HEMT transistor.
La figure 10 illustre ces perfectionnements dans un exemple de structure à hétérojonction AIGaN/GaN.  FIG. 10 illustrates these improvements in an example of AIGaN / GaN heterojunction structure.
Un premier perfectionnement consiste à utiliser une couche BGaN à faible concentration de bore à l'interface entre le substrat et la couche tampon, pour une utilisation comme couche de nucléation 6 pour la croissance de la couche tampon. Cette couche de BGaN déposée sur le substrat 1 , dans une épaisseur pouvant aller jusqu'à 2μιη, agit alors comme filtre de dislocation favorable pour obtenir une couche tampon 2 ayant de très bonnes qualités structurales. Dans ce cas, cette couche 6 de nucléation sera de préférence réalisée par la technique des clusters présentée à la figure 9.  A first improvement is to use a low boron BGaN layer at the interface between the substrate and the buffer layer for use as a nucleation layer 6 for growth of the buffer layer. This layer of BGaN deposited on the substrate 1, in a thickness of up to 2μιη, then acts as a favorable dislocation filter to obtain a buffer layer 2 having very good structural qualities. In this case, this nucleation layer 6 will preferably be produced by the cluster technique shown in FIG. 9.
Un deuxième perfectionnement consiste à utiliser une couche BGaN à faible concentration de bore, pour réaliser la couche 4 de passivation de surface pour ses propriétés résistives, la couche de passivation ayant pour fonction de réduire les éventuels pièges en surface de la structure. Dans ce cas, cette couche BGaN 4 de passivation sera de préférence réalisée avec une concentration uniforme en bore, ou en superréseau. En alternative au BGaN, on peut aussi utiliser du BN, qui offre des propriétés résistives aussi intéressantes, pour cette couche 4 de passivation de surface. A second improvement consists in using a BGaN layer with a low boron concentration, to produce the surface passivation layer 4 for its resistive properties, the passivation layer having the function of reducing any surface traps of the structure. In this case, this passivation layer BGaN 4 will preferably be made with a uniform concentration of boron, or superlattice. As an alternative to BGaN, BN can also be used resistive properties as interesting for this layer 4 of surface passivation.
Un troisième perfectionnement consiste à utiliser une couche BGaN ou BN, pour favoriser la dissipation thermique de la structure HEMT. Favoriser la dissipation thermique dans la structure est en effet un aspect important dans toutes les applications de puissance. Or le BGaN et le BN sont de bons conducteurs thermiques, notamment ils sont meilleurs conducteurs thermiques que le SiN ou le SiO2 utilisés couramment pour la couche 4, pour la passivation de la structure. A third improvement is to use a BGaN or BN layer, to promote the heat dissipation of the HEMT structure. Favoring heat dissipation in the structure is indeed an important aspect in all power applications. However, BGaN and BN are good thermal conductors, in particular they are better thermal conductors than the SiN or SiO 2 commonly used for layer 4, for the passivation of the structure.
On propose ainsi avantageusement de réaliser une couche BGaN ou BN, en surface de la structure, dans le but de réduire le pont thermique avec un radiateur éventuel placé au-dessus de la structure. Puisqu'on a vu précédemment qu'une telle couche BGaN ou BN pouvait aussi être utilisée pour la passivation, deux variantes de réalisation sont envisageables :  Advantageously, it is proposed to make a BGaN or BN layer, on the surface of the structure, in order to reduce the thermal bridge with a possible radiator placed above the structure. Since we previously saw that such a BGaN or BN layer could also be used for passivation, two alternative embodiments are possible:
-réaliser une couche BGaN ou BN en surface de la structure pour former la couche de passivation 4, et ainsi à la fois réaliser la passivation de la structure et réduire le pont thermique entre la zone active en dessous d'elle, et un radiateur éventuel placé au-dessus, -réaliser une couche BGaN ou BN sur une couche de passivation 4, par exemple SiN. On a alors la superposition couche de passivation realize a BGaN or BN layer on the surface of the structure to form the passivation layer 4, and thus both passivate the structure and reduce the thermal bridge between the active zone below it, and a radiator if any placed above, -realize a BGaN or BN layer on a passivation layer 4, for example SiN. We then have the superposition passivation layer
4/couche 7 BGaN ou BN comme illustré sur la figure 10. 4 / layer 7 BGaN or BN as illustrated in FIG.
On peut aussi envisager un refroidissement par le bas de la structure et réaliser une couche BGaN ou BN sous la couche tampon, telle la couche 6 illustrée sur la figure 10. Il faut alors prévoir un report de la structure sur un substrat 1 adéquat (ex : SiC, diamant, avec une interface et/ou un collage compatible thermiquement) pour améliorer la conductivité thermique dans le volume et la résistance thermique totale. La couche 6 peut alors servir de couche de nucléation dans le processus de fabrication de la structure, puis de couche favorisant la dissipation thermique, après report sur un substrat adéquat. It is also possible to envisage a cooling down of the structure and to make a BGaN or BN layer under the buffer layer, such as the layer 6 illustrated in FIG. 10. It is then necessary to provide a transfer of the structure on a suitable substrate 1 (ex : SiC, diamond, with thermally compatible interface and / or bonding) to improve thermal conductivity in volume and total thermal resistance. Layer 6 can then serve as a nucleation layer in the process of fabricating the structure, and then as a layer promoting heat dissipation, after transfer to a suitable substrate.
Les différents perfectionnements décrits peuvent être utilisés séparément ou en combinaison, selon les qualités et performances recherchées pour le transistor HEMT réalisé avec cette structure. Les figures 1 1 à 17 mettent en évidence les résultats de simulation obtenus pour trois structures formées selon l'invention, et d'illustrer les effets de confinement des porteurs de charge à l'interface couche barrière/couche tampon d'une structure de transistor HEMT avec une couche de BGaN insérée dans la couche tampon selon l'invention, et d'augmentation de la résistivité sous le canal. Elles permettent de montrer que ces effets sont remarquables même avec une faible concentration de bore, qui dans l'exemple de la simulation est de 1 %, ainsi que l'évolution notable de ces effets avec l'épaisseur de couche BGaN insérée. The various improvements described can be used separately or in combination, depending on the qualities and performance sought for the HEMT transistor produced with this structure. FIGS. 11 to 17 show the simulation results obtained for three structures formed according to the invention, and illustrate the effects of confinement of the charge carriers at the barrier layer / buffer layer interface of a transistor structure HEMT with a layer of BGaN inserted in the buffer layer according to the invention, and increasing the resistivity under the channel. They make it possible to show that these effects are remarkable even with a low concentration of boron, which in the example of the simulation is 1%, as well as the notable evolution of these effects with the layer thickness BGaN inserted.
Plus précisément, les trois structures HEMT simulées sont des structures AIGaN/GaN comprenant un matériau BGaN inséré selon l'invention. Dans ces structures, la couche barrière 3 est une couche AIGaN, choisie avec une concentration en Al de 32% et une épaisseur de 13 nanomètres. La couche BGaN 5 est insérée selon l'invention dans la couche tampon GaN, en sorte qu'une partie 2b de la couche tampon se retrouve entre la couche barrière AIGaN 3 et la couche BGaN 5. Dans l'exemple cette partie 2b de couche tampon a une épaisseur de 40 nanomètres.  More precisely, the three simulated HEMT structures are AIGaN / GaN structures comprising a BGaN material inserted according to the invention. In these structures, the barrier layer 3 is an AIGaN layer, chosen with an Al concentration of 32% and a thickness of 13 nanometers. The BGaN layer 5 is inserted according to the invention in the GaN buffer layer, so that a part 2b of the buffer layer is found between the barrier layer AIGaN 3 and the BGaN layer 5. In the example, this layer part 2b buffer has a thickness of 40 nanometers.
Dans la structure représentée schématiquement sur la figure 1 1 , la couche BGaN 5 est fine, avec une épaisseur de 5 nanomètres, et est à concentration uniforme de bore, de 1 % dans l'exemple.  In the structure shown diagrammatically in FIG. 11, the BGaN layer 5 is thin, with a thickness of 5 nanometers, and has a uniform boron concentration of 1% in the example.
Dans celles des figures 12 et 13, la couche BGaN 5 est plus épaisse, avec une épaisseur de 80 nanomètres (1 nm=10"9m). Dans la structure de la figure 12, sa concentration en bore est uniforme, de 1 %. Dans celle de la figure 13, elle est à gradient de concentration, partant de 0%, à la limite avec la partie 2a de la couche tampon sous la couche BGaN, dans la représentation de la figure où la couche barrière 3 est située au-dessus de la couche tampon, jusqu'à 1 % à la limite avec la partie 2b de la couche tampon au-dessus de la couche BGaN. La "couche" tampon selon l'invention est ainsi formée dans la structure par la séquence GaN 2b/ BGaN 5/ GaN 2a. In those of Figures 12 and 13, the BGaN layer 5 is thicker, with a thickness of 80 nanometers (1 nm = 10 -9 m) .In the structure of Figure 12, its concentration of boron is uniform, 1% In that of FIG. 13, it is at a concentration gradient, starting from 0%, at the limit with the part 2a of the buffer layer under the BGaN layer, in the representation of the figure where the barrier layer 3 is located at above the buffer layer, up to 1% at the limit with the part 2b of the buffer layer above the BGaN layer The buffer "layer" according to the invention is thus formed in the structure by the GaN sequence 2b / BGaN 5 / GaN 2a.
La figure 14 illustre les courbes de niveau d'énergie de la bande de conduction et de concentration de porteurs suivant l'épaisseur Y de la structure de la figure 1 1 , en partant de la couche barrière 3 vers la couche tampon, épaisseur qui est représentée par l'axe Y. L'épaisseur est donnée en angstroms (1 Â=10"10 m). Sur la figure (comme sur les figures suivantes 15 à 17) on a indiqué la position des couches dans leur succession dans la structure suivant l'axe Y, soit : AIGaN/GaN/BGaN/GaN. On a également représenté le niveau de fermi, noté NF. La fenêtre supérieure (a) de la figure 14 illustre la courbe de niveau d'énergie (en électro-volt "eV") de la structure, repérée par le symbole fb. Elle montre la courbe qui serait obtenue pour la même structure, mais sans la couche BGaN selon l'invention (toutes choses égales par ailleurs) : cette courbe est repérée par le symbole no-b. La fenêtre inférieure (b) illustre la courbe de concentration de porteurs (en cm"3): celle repérée par le symbole fb, correspondant à la structure de la figure 1 1 , et celle repérée par le symbole no-b, correspondant à cette même structure mais sans la couche BGaN 5. La figure 15 représente les courbes correspondantes, mais obtenues : FIG. 14 illustrates the energy level curves of the conduction and carrier concentration band according to the thickness Y of the structure of FIG. 11, starting from the barrier layer 3 towards the buffer layer, which thickness is The thickness is given in angstroms (1 Å = 10 -10 m) In the figure (as in the following figures 15 to 17) the position of the layers in their succession in the structure along the Y axis, ie: AIGaN / GaN / BGaN / GaN. The level of fermi, denoted NF, is also represented. The upper window (a) of FIG. 14 illustrates the energy level curve (in electro-volt "eV") of the structure, indicated by the symbol fb. It shows the curve that would be obtained for the same structure, but without the BGaN layer according to the invention (all things being equal): this curve is indicated by the symbol no-b. The lower window (b) illustrates the carrier concentration curve (in cm "3 ): that marked by the symbol fb, corresponding to the structure of FIG. 11, and that indicated by the symbol no-b, corresponding to this same structure but without the BGaN layer 5. FIG. 15 represents the corresponding curves, but obtained:
-pour la structure de la figure 12, avec une couche de BGaN épaisse, 80 nm dans l'exemple contre 5 nm dans la structure de la figure 1 1 , et une concentration uniforme de bore : les courbes correspondant à cette structure sont repérées par le symbole ub ;  for the structure of FIG. 12, with a thick BGaN layer, 80 nm in the example against 5 nm in the structure of FIG. 11, and a uniform concentration of boron: the curves corresponding to this structure are identified by the ub symbol;
-pour la structure de la figure 13, également à couche de BGaN épaisse, 80 nm dans l'exemple, mais à concentration graduelle de bore : les courbes correspondant à cette structure sont repérées par le symbole gb.  for the structure of FIG. 13, also with a thick BGaN layer, 80 nm in the example, but with a gradual concentration of boron: the curves corresponding to this structure are marked by the symbol gb.
Les courbes repérées par le symbole no-b correspondant à une structure identique mais sans couche de BGaN, sont également représentées.  The curves marked with the symbol no-b corresponding to an identical structure but without a layer of BGaN, are also represented.
La figure 16 permet de comparer les différentes courbes de niveaux d'énergie de bande de conduction de toutes ces structures, et de manière similaire, la figure 17 permet de comparer les différentes courbes de concentration de porteurs de toutes ces structures, et les effets induits par la couche BGaN insérée selon l'invention : amélioration du confinement par l'effet barrière électrostatique, réduction des fuites des électrons vers le substrat par l'effet barrière résistive.  FIG. 16 compares the different conduction band energy level curves of all these structures, and similarly, FIG. 17 compares the different carrier concentration curves of all these structures, and the induced effects. by the inserted BGaN layer according to the invention: improvement of the confinement by the electrostatic barrier effect, reduction of electron leakage to the substrate by the resistive barrier effect.
Ces différentes figures mettent clairement en évidence l'influence de la concentration en bore et de l'épaisseur de la couche BGaN insérée selon l'invention. Le pic d'énergie dans la bande de conduction, à l'interface GaN/BGaN (couche 2b/couche 5), noté respectivement E-fb pour la structure de la figure 1 1 , E-ub, pour la structure de la figure 12 et E-gb, pour la structure de la figure 13 a ainsi une amplitude d'autant plus importante et la barrière électrostatique induite est d'autant plus large, que la couche BGaN est plus épaisse. A égale épaisseur, l'amplitude du pic et la barrière électrostatique sont plus importantes pour une concentration uniforme à 1 % de bore (courbe "ub", pic E-ub) que pour un gradient 0%-1 % de concentration (courbe "gb", pic E-gb). La largeur de la base du puits de potentiel triangulaire à l'interface AIGaN/GaN, respectivement noté W-fb, W-ub et W-gb, est aussi dépendante de la concentration en bore et de l'épaisseur de la couche BGaN, comme très bien montré par la figure 17 : la plus étroite pour la courbe ub, la plus large pour la courbe fb. Ces courbes sont à rapprocher des explications déjà données dans l'exposé de l'invention en relation avec la figure 3. These different figures clearly show the influence of the boron concentration and the thickness of the inserted BGaN layer according to the invention. The peak of energy in the conduction band, at the GaN / BGaN interface (layer 2b / layer 5), denoted respectively E-fb for the structure of FIG. 11, E-ub, for the structure of FIG. 12 and E-gb, for the structure of FIG. 13 thus has an amplitude that is all the greater, and the electrostatic barrier induced is all the greater, as the BGaN layer is thicker. At equal thickness, the amplitude of the peak and the electrostatic barrier are greater for a uniform concentration at 1% boron (curve "ub", peak E-ub) than for a gradient 0% -1% concentration (curve " gb ", peak E-gb). The width of the base of the triangular potential well at the AIGaN / GaN interface, respectively denoted W-fb, W-ub and W-gb, is also dependent on the boron concentration and the thickness of the BGaN layer, as very well shown in figure 17: the narrowest for the curve ub, the widest for the curve fb. These curves are to be compared with the explanations already given in the description of the invention in relation to FIG.
L'invention qui vient d'être décrite permet de réaliser des transistors HEMT très performants, avec des propriétés électriques améliorées.  The invention which has just been described makes it possible to produce high performance HEMT transistors with improved electrical properties.

Claims

REVENDICATIONS
1 . Structure électronique de transistor HEMT, comprenant -au moins une première couche dite tampon (2), d'un premier matériau semi-conducteur à large bande interdite Eg-ι (M1 ), et une deuxième couche (3) d'un deuxième matériau semi-conducteur à large bande interdite Eg2 (M2), avec une largeur de bande interdite Eg2 supérieure à celle Eg ; et -un gaz bidimensionnel d'électrons (2DEG) qui circule dans un canal (C) délimité dans la première couche (2) à l'interface (10) entre la première couche et la deuxième couche, 1. HEMT transistor electronic structure, comprising - at least a first so-called buffer layer (2), a first Eg-ι wide bandgap semiconductor material (M1), and a second layer (3) of a second material wideband forbidden semiconductor Eg 2 (M2), with a forbidden bandwidth Eg 2 greater than Eg ; anda two-dimensional electron gas (2DEG) circulating in a channel (C) delimited in the first layer (2) at the interface (10) between the first layer and the second layer,
la structure étant caractérisée en ce qu'un matériau BGaN avec une concentration moyenne en bore d'au moins 0,1 % est inséré dans la couche tampon (5), sous forme d'au moins une couche (5) sous le canal (C), modifiant le diagramme des bandes d'énergie par création d'une barrière de potentiel électrostatique favorisant le confinement du gaz bidimensionnel d'électrons. the structure being characterized in that a BGaN material with a mean boron concentration of at least 0.1% is inserted into the buffer layer (5) as at least one layer (5) under the channel ( C), modifying the energy band diagram by creating an electrostatic potential barrier favoring the confinement of the two-dimensional electron gas.
2. Structure électronique selon la revendication 1 , dans laquelle la couche BGaN (5) sous le canal a une épaisseur comprise entre 1 nanomètre et une centaine de nanomètres.  2. An electronic structure according to claim 1, wherein the BGaN layer (5) under the channel has a thickness of between 1 nanometer and a hundred nanometers.
3. Structure électronique selon la revendication 1 ou 2, comprenant une couche BGaN (6) à l'interface entre la couche tampon (2) et un substrat (1 ) de la structure, comme couche de nucléation, formant filtre de dislocation lors de la croissance de la couche tampon (2).  An electronic structure according to claim 1 or 2, comprising a BGaN layer (6) at the interface between the buffer layer (2) and a substrate (1) of the structure, as a nucleation layer, forming a dislocation filter during the growth of the buffer layer (2).
4. Structure électronique selon la revendication 1 ou 2, comprenant une couche BGaN ou BN (6) à l'interface entre la couche tampon (2) et un substrat (1 ) de la structure pour favoriser la dissipation thermique du transistor HEMT.  4. An electronic structure according to claim 1 or 2, comprising a BGaN or BN layer (6) at the interface between the buffer layer (2) and a substrate (1) of the structure to promote the heat dissipation of the HEMT transistor.
5. Structure électronique selon l'une quelconque des revendications 1 à 4, comprenant une couche BGaN ou BN (4) en surface de la structure, sur la couche barrière (3), ladite couche BGaN ou BN servant de couche de passivation de surface, et permettant une dissipation thermique par le dessus de la structure.  5. Electronic structure according to any one of claims 1 to 4, comprising a layer BGaN or BN (4) at the surface of the structure, on the barrier layer (3), said layer BGaN or BN serving as a surface passivation layer and allowing heat dissipation from above the structure.
6. Structure électronique selon l'une quelconque des revendications 1 à 4, comprenant une couche de passivation (4) formée sur la couche barrière (3), et une couche BGaN ou BN (7) sur la couche de passivation (4) permettant une dissipation thermique par le dessus de la structure. An electronic structure according to any one of claims 1 to 4, comprising a passivation layer (4) formed on the barrier layer (3), and a BGaN or BN layer (7) on the layer of passivation (4) allowing heat dissipation from above the structure.
7. Structure électronique selon l'une quelconque des revendications 1 à 6, dans laquelle la couche BGaN (5) sous le canal est à concentration en volume uniforme de bore.  An electronic structure according to any one of claims 1 to 6, wherein the BGaN layer (5) under the channel is at a uniform boron volume concentration.
8. Structure électronique selon l'une quelconque des revendications 1 à 6, dans laquelle la couche BGaN (5) sous le canal est à concentration en bore graduée ou en marches d'escalier, croissante dans la direction du canal.  8. An electronic structure according to any one of claims 1 to 6, wherein the BGaN layer (5) under the channel is graduated boron concentration or stair steps, increasing in the direction of the channel.
9. Structure électronique selon l'une quelconque des revendications 1 à 6, dans laquelle la couche BGaN (5) sous le canal est un super-réseau alternant des couches BGaN avec des couches GaN ou avec des couches AIN.  An electronic structure according to any one of claims 1 to 6, wherein the BGaN layer (5) under the channel is a superlattice alternating BGaN layers with GaN layers or with AlN layers.
10. Structure électronique selon l'une quelconque des revendications 1 à 6, dans laquelle la couche BGaN (5) sous le canal est formée d'une couche environnante de GaN ou BGaN, incorporant localement du BGaN en volume en différentes zones (20) dites "clusters", avec une teneur en bore supérieure à celle de la couche environnante.  10. An electronic structure according to any one of claims 1 to 6, wherein the BGaN layer (5) under the channel is formed of a surrounding layer of GaN or BGaN, locally incorporating bulk BGaN in different areas (20). so-called "clusters", with a boron content higher than that of the surrounding layer.
1 1 . Structure électronique selon la revendication 3, dans laquelle la couche BGaN (6) réalisée comme couche de nucléation à l'interface entre la couche tampon (2) et le substrat (1 ) de la structure est à incorporation locale en volume de BGaN en différentes zones (20) dites "clusters".  1 1. An electronic structure according to claim 3, wherein the BGaN layer (6) formed as a nucleation layer at the interface between the buffer layer (2) and the substrate (1) of the structure is locally incorporated by volume of BGaN in different zones (20) called "clusters".
12. Structure électronique selon l'une quelconque des revendications précédentes dans laquelle lesdits premier et deuxième matériaux sont des nitrures d'élément III.  An electronic structure according to any one of the preceding claims wherein said first and second materials are element III nitrides.
13. Structure électronique selon la revendication 12, dans laquelle le premier matériau est un binaire GaN, ou un alliage de ce binaire avec un ou des éléments III ou V, et le deuxième matériau est un alliage ternaire AIGaN ou un alliage de ce ternaire avec des éléments III ou V.  An electronic structure according to claim 12, wherein the first material is a GaN binary, or an alloy thereof with one or more III or V elements, and the second material is a ternary alloy AIGaN or an alloy of this ternary with elements III or V.
14. Dispositif électronique comprenant au moins un transistor 14. Electronic device comprising at least one transistor
HEMT avec une structure électronique selon l'une quelconque des revendications précédentes. HEMT with an electronic structure according to any one of the preceding claims.
EP12717088.4A 2011-04-14 2012-04-16 Hemt transistors consisting of (iii-b)-n wide bandgap semiconductors comprising boron Ceased EP2697831A1 (en)

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