JP2012104722A - Method of manufacturing nitride compound semiconductor element and nitride compound semiconductor element - Google Patents

Abstract

A method for manufacturing a nitride-based compound semiconductor device with reduced leakage current and high withstand voltage and a nitride-based compound semiconductor device are provided.
A growth step of epitaxially growing a nitride compound semiconductor layer comprising group III atoms including at least gallium atoms and nitrogen atoms on a substrate, and a laser beam applied to the nitride compound semiconductor layer before forming an element structure Or a decomposition step of irradiating with ionizing radiation to decompose a complex of group III vacancies and hydrogen atoms in the nitride-based compound semiconductor layer.
[Selection] Figure 2

Description

  The present invention relates to a method for manufacturing a nitride compound semiconductor device and a nitride compound semiconductor device.

  Nitride-based compound semiconductors such as gallium nitride (GaN) -based semiconductors have higher band gap energy and higher breakdown voltage than silicon-based materials. Can be produced. For this reason, GaN-based semiconductors are expected as materials for power devices such as inverters and converters that replace silicon-based materials.

  For power devices, a high off-breakdown voltage is an important parameter that determines the maximum output of a transistor. In order to obtain a high off breakdown voltage, it is necessary to realize a high buffer breakdown voltage, that is, to reduce a leakage current (leakage current).

Since a GaN-based semiconductor is usually heteroepitaxially grown on a substrate made of a material different from that of a GaN-based semiconductor, it contains many point defects such as nitrogen vacancies and gallium vacancies (V _Ga ) and lattice defects such as dislocations. There is a problem. In particular, when a heterogeneous material substrate such as a silicon substrate is used as the growth substrate, the lattice constant difference from GaN (approximately 17% in the case of silicon) and the thermal expansion coefficient difference (approximately 56% in the case of silicon) are large. High-density dislocations exceeding 10 ¹⁰ cm ⁻² may be introduced. A GaN-based semiconductor element in which high-density dislocations are introduced in this way has a large leakage current and a low withstand voltage.

  To increase the breakdown voltage, there is a method of increasing the resistance of the buffer layer formed immediately above the substrate. In order to increase the resistance of the buffer layer, an auto-doping method using carbon contained in an organic metal as a raw material as an additive has been proposed when using metal organic chemical vapor deposition (MOCVD) (see Patent Document 1). ).

JP 2007-251144 A

  However, there is a demand for a nitride-based compound semiconductor device with a high withstand voltage and a reduced leakage current, compared to a device in which carbon is doped and the resistance of the buffer layer is increased as disclosed in Patent Document 1. ing.

  The present invention has been made in view of the above, and an object of the present invention is to provide a method for manufacturing a nitride-based compound semiconductor device with reduced leakage current and high breakdown voltage, and a nitride-based compound semiconductor device. .

  In order to solve the above-described problems and achieve the object, a nitride-based compound semiconductor device manufacturing method according to the present invention includes a nitride-based compound comprising a group III atom containing at least a gallium atom and a nitrogen atom on a substrate. A growth step of epitaxially growing a semiconductor layer, and before the device structure is formed, the nitride compound semiconductor layer is irradiated with laser light or ionizing radiation, and a group III vacancy and hydrogen atoms in the nitride compound semiconductor layer And a decomposition step of decomposing the composite.

  The method for manufacturing a nitride-based compound semiconductor device according to the present invention is characterized in that, in the above invention, the laser beam is a laser beam having a wavelength of 380 nm or less.

  In the method of manufacturing a nitride-based compound semiconductor device according to the present invention, the laser beam is a pulsed laser beam having a pulse width of 100 microseconds or less.

In the method for manufacturing a nitride compound semiconductor element according to the present invention, in the above invention, the light intensity at the outermost surface of the nitride compound semiconductor layer is 1 W / cm ² or more and 1000 W / cm ² or less in terms of CW. It is characterized by irradiating with pulsed laser light.

  In the method for manufacturing a nitride-based compound semiconductor device according to the present invention, the ionizing radiation is synchrotron radiation.

  The method for producing a nitride-based compound semiconductor device according to the present invention is characterized in that, in the above invention, the ionizing radiation is a neutron beam.

  In the method of manufacturing a nitride-based compound semiconductor device according to the present invention, the neutron beam is an epithermal neutron beam or a thermal neutron beam having an energy of 1 eV or less.

  In the method of manufacturing a nitride-based compound semiconductor element according to the present invention, the substrate is made of silicon, sapphire, silicon carbide, or zinc oxide.

The nitride-based compound semiconductor device according to the present invention is a nitride-based compound semiconductor device manufactured by the manufacturing method of the present invention, wherein a group III vacancy concentration in the nitride-based compound semiconductor layer is 5 × 10 5. ^{It is 16} to 1 × 10 ¹⁸ cm ⁻³ .

  According to the present invention, it is possible to reduce the generation of leakage current due to the complex of group III vacancies and hydrogen, so that it is possible to realize a nitride-based compound semiconductor device having a small leakage current and high withstand voltage. .

FIG. 1 is a diagram illustrating calculation results of the state density of electrons in a system including V _Ga —H (broken line) and a system including only V _Ga (solid line). FIG. 2 is a schematic cross-sectional view of the HFET according to the first embodiment. FIG. 3 is a schematic diagram illustrating a laser beam irradiation method. FIG. 4 is a diagram showing photoluminescence (PL) spectra of the epitaxial substrate before and after laser light irradiation in the manufacturing process of the HFET of Example 1. FIG. 5 is a diagram showing the leakage characteristics of the HFETs of Example 1 and Comparative Example 1. FIG. 6 is a diagram showing the relationship between the laser beam irradiation time and the leakage current of the HFETs of Example 1, Examples 1-1 to 1-4, and Comparative Example 1 when the source-drain voltage is 500V. FIG. 7 is a diagram showing the change over time in the resistance value (relative value) between the source and drain of the HFETs of Example 1 and Comparative Example 1. FIG. FIG. 8 is a schematic cross-sectional view of the SBD according to the second embodiment. FIG. 9 is a schematic plan view of the SBD according to the second embodiment. FIG. 10 is a schematic diagram for explaining a method of irradiating the epitaxial substrate with white light of synchrotron radiation. FIG. 11 is a schematic diagram for explaining a method of irradiating monochromatic light obtained by separating synchrotron radiation light onto an epitaxial substrate. FIG. 12 is a diagram showing the leakage currents of the SBDs of Examples 2 and 3 and Comparative Example 2 when the source-drain voltage is 500V. FIG. 13 is a schematic cross-sectional view of an HFET according to the third embodiment. FIG. 14 is a diagram showing a change with time of the resistance value between the source and the drain of the HFET of Comparative Example 3 and the HFET irradiated with the thermal neutron beam.

  The present invention contemplates a method by which the present inventors have investigated the mechanism of generation of leakage current generated in a nitride-based compound semiconductor device and, based on the knowledge obtained thereby, eliminates one of the causes of leakage current generation. , Completed.

  In the following, first, consideration will be given to the mechanism of leak current generation performed by the present inventors. Next, the present invention completed based on the knowledge thus obtained will be described with reference to an embodiment thereof.

<Prediction of defect level created by V _Ga -H by first-principles electronic state calculation>
The present inventors use ammonia containing hydrogen as a source gas when growing a nitride-based compound semiconductor element, so that hydrogen decomposed during growth causes vacancy defects in the nitride-based compound semiconductor crystal. And a complex was formed, which was considered to supply carriers contributing to leakage current.

In view of this, the present inventor confirmed the influence of the defect level produced by a complex of a Ga vacancy, which is a group III vacancy defect, and hydrogen (V _Ga -H) on the electrical characteristics of the GaN crystal in the GaN crystal. In order to do this, first-principles electronic state calculation (simulation) was performed.

For this simulation, Advance / PHASE made by Advance Software Co., Ltd. was used. In addition, a Vanderbilt ultrasoft pseudopotential was used for the calculation. The exchange interaction was calculated within the range of generalized gradient approximation. Further, the calculation conditions were as follows.
-Atomic model: Supercell consisting of 32 atoms (15 Ga atoms, 16 nitrogen atoms, 1 hydrogen atom)-Cutoff energy: 25 Ry and 230 Ry in wave function and charge density distribution, respectively
・ K point sample: 3 × 3 × 4
-Calculated number of bands: 100

FIG. 1 is a diagram illustrating calculation results of electron density of states (DOS) of a system containing V _Ga —H (broken line) and a system containing only V _Ga (solid line). The horizontal axis represents the energy of electrons, and the position where the energy is 0 represents the Fermi level. Further, FIG. 1 shows only DOS near the valence band.

In FIG. 1, the defect level produced by V _Ga -H corresponding to the peak P1 and the defect level produced by V _Ga corresponding to the peak P2 appear at 46 meV and 128 meV positions from the valence band, respectively.

Thus, it can be seen that V _Ga -H forms a relatively shallow acceptor level.
For this reason, by irradiating visible light to a GaN crystal containing V _Ga -H, electrons in the valence band are excited to the acceptor level and holes are formed in the valence band. It is expected to be detected higher than the value (light response). In addition, since the level is relatively shallow, electrons are thermally excited even at room temperature and carriers are supplied, which is considered to cause a leakage current.

On the other hand, isolated V _Ga forms a deep level, so that no optical response is caused by visible light. In addition, since the deep level formed by V _Ga can compensate for donor-type carriers, an effect of reducing leakage current can be expected.

Furthermore, when the binding energy of V _Ga -H was similarly calculated by the first principle electronic state calculation, it was about 3 eV. This binding energy can be decomposed by irradiation with laser light having a wavelength from visible to ultraviolet, ionizing radiation such as synchrotron radiation or thermal neutron radiation.

From the above results, the inventors of the present invention have succeeded in forming V _Ga -H contained in a crystal-grown nitride-based compound semiconductor by irradiating it with laser light or ionizing radiation to form isolated V _Ga , thereby forming the isolated V _Ga. It was conceived that the leakage current of the device to be reduced can be reduced.

<Embodiment>
Embodiments of a nitride compound semiconductor device manufacturing method and a nitride compound semiconductor device according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments. Also, it should be noted that the drawings are schematic, and the thicknesses and ratios of the layers are different from the actual ones. Moreover, the part from which the relationship and ratio of a mutual dimension differ also in between drawings is contained.

(Embodiment 1)
FIG. 2 is a schematic cross-sectional view of a heterojunction field effect transistor (HFET) that is a nitride-based compound semiconductor device according to the first embodiment of the present invention. As shown in FIG. 2, the HFET 10 is made of a silicon substrate 11 whose main surface is a (111) plane, which is a substrate made of a material different from a nitride compound semiconductor, and GaN formed on the silicon substrate 11 sequentially. A low-temperature buffer layer 12 made of carbon, a buffer layer 13 made of carbon (C) -doped GaN, an electron transit layer 14 made of GaN having a C concentration lower than that of the buffer layer, an electron supply layer 15 made of AlGaN, and an electron supply layer 15 A gate electrode 16, a source electrode 17, and a drain electrode 18 formed thereon are provided. That is, the HFET 10 is an AlGaN / GaN-HFET having an AlGaN / GaN heterojunction.

  In the HFET 10, the buffer layer 13 is doped with C to reduce the leakage current and increase the resistance of the buffer layer 13, and the leakage current is reduced and the breakdown voltage is increased. Moreover, the electron transit layer 14 maintains a high electron mobility by setting the C concentration sufficiently low.

Furthermore, since this HFET 10 is obtained by crystallizing a nitride-based compound semiconductor layer and then irradiating a laser beam to decompose V _Ga -H in the crystal into an isolated V _Ga , the leakage current is further reduced. It will be a thing.

(Production method)
An example of a method for manufacturing the HFET 10 according to the first embodiment will be described with reference to FIG. Note that the flow rate of raw materials, the thickness of each layer, the growth temperature, and the like are examples, and are not particularly limited.

First, trimethylgallium (TMGa) and ammonia (NH ₃ ) are flown at a flow rate of 14 μmol / min and 12 L / min, respectively, in a metal organic chemical vapor deposition (MOCVD) apparatus in which a silicon substrate 11 having a thickness of 500 μm is installed. The low temperature buffer layer 12 made of GaN having a layer thickness of 30 nm is epitaxially grown on the silicon substrate 11 at a growth temperature of 550 ° C.

Next, TMGa and NH ₃ are introduced at flow rates of 58 μmol / min and 12 L / min, respectively, and the buffer layer 13 made of GaN having a thickness of 600 nm is epitaxially grown on the low temperature buffer layer 12 at a growth temperature of 1050 ° C. . By adjusting the growth pressure at this time, the carbon concentration doped in the buffer layer 13 can be set to 1 × 10 ¹⁸ cm ⁻³ or more, and the resistance can be increased and the leakage current can be reduced.

Next, TMGa and NH ₃ are introduced at flow rates of 19 μmol / min and 12 L / min, respectively, and the electron transit layer 14 made of GaN having a layer thickness of 100 nm is epitaxially grown on the buffer layer 13 at a growth temperature of 1050 ° C. . By adjusting the growth pressure at this time, the carbon concentration doped in the electron transit layer 14 can be reduced (for example, 1 × 10 ¹⁷ cm ⁻³ or less), and high electron mobility can be maintained.

Subsequently, after the growth of the buffer layer 13 and the electron transit layer 14, trimethylaluminum (TMAl), TMGa, and NH ₃ were introduced at flow rates of 100 μmol / min, 19 μmol / min, and 12 L / min, respectively, and a growth temperature of 1050 ° C. Then, an electron supply layer 15 made of AlGaN having a thickness of 30 nm is epitaxially grown on the electron transit layer 14.

Next, a pulse laser beam of the fourth harmonic of the YAG laser (wavelength 266 nm, hereinafter referred to as the fourth harmonic of YAG) is applied to the epitaxial substrate obtained by epitaxially growing each semiconductor layer on the silicon substrate 11 as described above. Irradiate. The pulse width of this laser beam is 1 nm, and the light intensity is about 3 W / cm ² in terms of CW (Continuous Wave). The repetition frequency of the pulsed light is 5 kHz. However, the pulse width, light intensity, and repetition frequency are not limited to these values.

  Further, since the beam diameter of the laser beam is usually smaller than the area of the epitaxial substrate, it is necessary to scan the laser beam in order to irradiate the entire epitaxial substrate. FIG. 3 is a schematic diagram illustrating a laser beam irradiation method. Here, as shown in FIG. 3, the epitaxial substrate is placed on the XY stage, and the surface of the electron supply layer 15 is irradiated with the laser beam L1 of the beam spot B1, and the XY stage is moved to move the beam spot B1. To scan.

By this laser light irradiation, hydrogen atoms are separated from V _Ga -H in the crystal, and become isolated V _Ga . Note that the penetration length (the distance at which the intensity becomes 1/10) of the fourth harmonic of YAG into the GaN crystal is about 100 nm, and the effect of laser irradiation can be sufficiently obtained in the electron transit layer 14.

The effect of reducing the leakage current is greater when the buffer layer 13 is also irradiated with laser light. Therefore, after irradiation with the fourth harmonic of YAG (or before or simultaneously with irradiation), pulsed laser light of the third harmonic of YAG (wavelength 355 nm) may be irradiated. Since the penetration length into the GaN crystal becomes longer at this wavelength, the laser light reaches the silicon substrate 11. When irradiating the third harmonic of YAG, the pulse width of the laser light is 1 nm, and the light intensity is about 30 W / cm ² in terms of CW. The repetition frequency of the pulsed light is 10 kHz. However, the pulse width, light intensity, and repetition frequency are not limited to these values. The irradiation method is the same as that of the YAG fourth harmonic pulse laser beam.

Next, in order to form an element structure, titanium (Ti) and Al are vapor-deposited in this order on the electron supply layer 15 to form the source electrode 17 and the drain electrode 18 which are ohmic electrodes. Next, nickel (Ni) and gold (Au) are deposited in this order between the source electrode 17 and the drain electrode 18 to form the gate electrode 16 that is a Schottky electrode. Note that, after the source electrode 17 and the drain electrode 18 are deposited, a good ohmic characteristic can be obtained by performing a heat treatment at 700 ° C. for 30 minutes. Further, in this heat treatment, hydrogen atoms have been disconnected from the V _Ga by laser beam irradiation diffuses, it is moved out of the epitaxial substrate.
The HFET 10 according to the first embodiment can be manufactured by the above manufacturing method.

The HFET 10 according to the first embodiment manufactured in this way has a reduced leakage current because V _Ga -H in the crystal is decomposed into isolated V _Ga .

(Example 1, Comparative Example 1)
As Example 1 of the present invention, the HFET 10 according to the first embodiment is irradiated with the YAG fourth harmonic pulse laser beam followed by the YAG third harmonic pulse laser beam in the manufacturing method described above. An HFET having the following structure was manufactured. The Al composition of the electron supply layer made of AlGaN was 0.23 according to the evaluation by the X-ray diffraction method. As for the size of the HFET, the gate length was 2 μm, the gate width was 0.2 mm, and the source-drain distance was 15 μm.

  Further, as Comparative Example 1, a HEFT having the same structure as that of the HFET of Example 1 was manufactured except that laser light was not irradiated.

  Since the beam diameter of the 3rd harmonic and 4th harmonic pulse laser beam of YAG used was 0.3 mm, the epitaxial substrate was placed on the XY stage and the stage was moved 0.3 mm per second. Moved and scanned the beam spot. That is, the laser beam is irradiated for 1 second to all the regions on the epitaxial substrate.

  Moreover, in the manufacturing process of the HFET of Example 1, the photoluminescence (PL) spectrum of the epitaxial substrate before and after the laser beam irradiation was measured. This measurement was performed under weak excitation conditions using an ultraviolet light source.

FIG. 4 is a diagram showing photoluminescence (PL) spectra of the epitaxial substrate before and after laser light irradiation in the HFET of Example 1. The spectrum S1 is the PL spectrum of Comparative Example 1, and the spectrum S2 is the PL spectrum of Example 1. As shown in FIG. 4, it can be seen that the spectrum S2 of Example 1 has a reduced intensity of broad light emission (so-called yellow light emission) near 2.2 eV compared to the spectrum S1 of Comparative Example 1. This result means that V _Ga -H was decomposed by laser light irradiation, and the residual carriers in the semiconductor layer were reduced.

Next, when the characteristics of the HFET of Example 1 were measured, the mobility of the two-dimensional electron gas was 1100 cm ² / Vs, and the sheet carrier concentration was 8 × 10 ¹² cm ⁻² . Also, the mobility and sheet carrier concentration of the HFET of Comparative Example 1 were comparable to those of the HFET of Example 1, and did not depend on the presence or absence of laser light irradiation.

  Next, the leakage characteristics of the HFETs of Example 1 and Comparative Example 1 were measured. Here, a leakage current was measured when a voltage of −10 V was applied between the source and the gate, and a voltage of 0 V to 650 V was gradually applied between the source and the drain.

FIG. 5 is a diagram showing the leakage characteristics of the HFETs of Example 1 and Comparative Example 1. The data point group D1 is the data of Comparative Example 1, and the data point group D2 is the data of Example 1. The horizontal axis indicates the source-drain voltage. In the value of the leakage current on the vertical axis, “E” is a symbol representing a power of 10, for example, “2.0E-04” means “2.0 × 10 ⁻⁴ ”.

As shown in FIG. 5, in Comparative Example 1, the leakage current increases from around 400 V in the source-drain voltage, whereas in Example 1, the leakage current increases up to about 500 V in the source-drain voltage. It was suppressed. This result shows that in the HFET of Example 1, V _Ga -H was decomposed by irradiating laser light, so that residual carriers in GaN were reduced and leakage current was reduced.

  Next, as Examples 1-1 to 1-4 of the present invention, HFETs having the same structure as Example 1 were manufactured. However, the time interval for moving the stage during laser light irradiation is adjusted, and the laser irradiation time per unit area of the epitaxial substrate is set to 0.1 seconds (Example 1-1), 0.3 seconds (Example). 1-2), 0.5 seconds (Example 1-3), and 0.75 seconds (Example 1-4). The leakage characteristics of the manufactured HFETs of Examples 1-1 to 1-4 were measured in the same manner as in Example 1 and Comparative Example 1.

FIG. 6 is a diagram showing the relationship between the laser beam irradiation time and the leakage current of the HFETs of Example 1, Examples 1-1 to 1-4, and Comparative Example 1 when the source-drain voltage is 500V. As shown in FIG. 6, the above-mentioned laser beam conditions (pulse width 1 nm for YAG fourth harmonic, light intensity is about 3 W / cm ^{2 in} CW conversion, repetition frequency 5 kHz, pulse width for YAG third harmonic) 1 nm, the light intensity is about 30 W / cm ^{2 in} terms of CW, and the repetition frequency is 10 kHz. If the laser irradiation time is 0.1 seconds or more, the effect of reducing the leakage current appears. It was confirmed that

  Next, the optical response characteristics of the HFETs of Example 1 and Comparative Example 1 were measured. In this measurement, the HFET of Example 1 and Comparative Example 1 was irradiated with visible light (here, the light of a fluorescent lamp), then moved to a dark place, and the change over time in the resistance value between the source and drain was measured. It is. The resistance value was measured by setting the source-gate voltage to 0 V and applying a voltage of 0.5 V between the source and drain.

  FIG. 7 is a diagram showing the change over time of the resistance value (relative value) between the source and drain of the HFETs of Example 1 and Comparative Example 1, where the horizontal axis represents the elapsed time since moving to a dark place, and the vertical axis It is the relative value of the resistance value when the resistance value at saturation is 1. As shown in FIG. 7, in Example 1, the decrease in resistance value due to optical response was about 10%, and the change was saturated in about several hours to 10 hours. However, in Comparative Example 1, the change in resistance value due to optical response was about 30%, and it took 50 hours or more to saturate.

In the HFET of Comparative Example 1 that is not irradiated with laser light, carriers are supplied from the valence band to the shallow acceptor level formed by V _Ga -H by irradiation with visible light before energization. As a result, the resistance value appears lower than the actual resistance value. On the other hand, in the HFET of Example 1 irradiated with the laser light, V _Ga -H was decomposed, the residual carriers decreased, and the shallow acceptor level also decreased.

  As described above, it was confirmed that the HFET of Example 1 of the present invention has reduced leakage current and optical response.

By the way, verification that V _Ga -H in the crystal is decomposed can be performed by a positron annihilation method. In this method, the positron lifetime is measured using NaCl containing ²² Na (˜2 MBq) which is a radioisotope as a positron source. The detector can be a BaF ₂ scintillation counter.

The lifetime of positron trapped in V _Ga is about 230 ps. On the other hand, since V _Ga -H is a shallow trap for positrons, the positron lifetime in this case is almost the same as about 160 ps, which is the lifetime in a GaN crystal containing no defects.

Therefore, when the average lifetime of positrons was measured and the V _Ga concentration was evaluated for the epitaxial substrates of Example 1 and Comparative Example 1, the epitaxial substrate of Example 1 (that is, V _Ga -H was decomposed, and V _Ga V _Ga concentration was in the range of 1 × 10 ¹⁷ to 1 × 10 ¹⁸ cm ⁻³ . On the other hand, in the epitaxial substrate of Comparative Example _1, positron capture speed of _{V Ga} is more than an order of magnitude _{smaller, V Ga} concentration was less than 5 × ¹⁰ 16 ^{cm -3.} From this result, it is considered that V _Ga —H is decomposed when the V _Ga concentration is in the range of 5 × 10 ¹⁶ to 1 × 10 ¹⁸ cm ⁻³ .

From this result, it was confirmed that in the epitaxial substrate of Example 1 irradiated with laser light, the concentration of V _Ga -H that causes leakage current and photoresponse is low.

  In Example 1 described above, the laser beam is irradiated on the entire surface of the epitaxial substrate by scanning with a laser beam having a small beam diameter. However, the beam diameter is applied to the laser beam having a high light intensity by using a beam expander. May be enlarged so that a large area is irradiated with laser light at once.

In order to shorten the irradiation time of the laser beam, the higher the light intensity of the laser beam, the better. However, 1 W / cm ² or more is preferable. In addition, when using pulsed laser light, the nitride semiconductor surface may be modified or the nitride-based compound semiconductor layer may be destroyed if the light intensity exceeds 1000 W / cm ^{2 in} terms of CW. The light intensity is preferably 1000 W / cm ² or less in terms of CW.

In addition, the wavelength of the laser beam to be irradiated is preferably 380 nm or less in consideration of the light energy necessary for the decomposition of V _Ga -H and the absorption coefficient of the nitride compound semiconductor. Accordingly, the laser to be used may be a HeAg laser (wavelength 224.3 nm) or a NeCu laser (wavelength 248.6 nm). However, since these lasers have a wide pulse width of about 50 microseconds, the peak intensity is low. Therefore, when these lasers are used, it is necessary to make the irradiation time of the laser light about 10 times that of a laser having a pulse width of about nanoseconds. Considering the irradiation time of the laser beam in this way, the pulse width of the pulsed laser beam to be irradiated is preferably 100 microseconds or less. Further, second harmonics (wavelengths of 244 nm and 257.2 nm) of laser light from an argon ion laser that is a CW laser can also be used. In this case, the light intensity is desirably about 100 kW / cm ² .

(Embodiment 2)
Next, a Schottky barrier diode (SBD) that is a nitride-based compound semiconductor device according to the second embodiment of the present invention will be described. In the SBD according to the second embodiment, synchrotron radiation is used to decompose V _Ga —H in the crystal.

  FIG. 8 is a schematic cross-sectional view of the SBD according to the second embodiment. FIG. 9 is a schematic plan view of the SBD according to the second embodiment. As shown in FIGS. 8 and 9, the SBD 20 includes a silicon substrate 21 whose main surface is a (111) plane, a first buffer layer 22 made of AlN, and a GaN layer 23a and an AlN, which are sequentially formed on the silicon substrate 21. A second buffer layer 23 formed by alternately stacking eight layers 23b, a third buffer layer 24 made of GaN, an electron transit layer 25 made of GaN having n-type conductivity (n-GaN), and A circular Schottky electrode 26 formed on the electron transit layer 25, and an ohmic electrode 27 formed so as to surround the Schottky electrode 26 at a predetermined interval.

  Since the SBD 20 includes the second buffer layer 23 in which the GaN layers and the AlN layers are alternately stacked, the SBD 20 is controlled to suppress the occurrence of cracks in the epitaxial substrate and to reduce the amount of warpage of the epitaxial substrate. be able to.

Furthermore, since this SBD 20 is obtained by crystallizing a nitride-based compound semiconductor layer and then irradiating synchrotron radiation to decompose V _Ga -H in the crystal to form isolated V _Ga , the leakage current is further reduced. Will be. Synchrotron radiation is an electromagnetic wave emitted in the tangential direction of an orbit by bending an electron or positron moving in an ultra-high vacuum at a speed close to a high speed with a magnetic field or an electric field. This radiant light is a high-luminance light having a white spectrum ranging from visible / vacuum ultraviolet to X-rays. As a generation source of synchrotron radiation, for example, a beam line BL-15 of a high energy accelerator mechanism radiation light facility (KEK-PF) can be used.

(Production method)
An example of a method for manufacturing the SBD 20 according to the second embodiment will be described. Note that the flow rate of raw materials, the thickness of each layer, the growth temperature, and the like are examples, and are not particularly limited.

First, TMAl and NH ₃ are introduced at a flow rate of 175 μmol / min and 35 L / min, respectively, into a MOCVD apparatus in which a silicon substrate 21 having a thickness of 1 mm is installed, and the growth temperature is 1000 ° C. on the silicon substrate 21. A first buffer layer 22 made of AlN having a layer thickness of 40 nm is epitaxially grown.

Next, a pair of a GaN layer 23a having a layer thickness of 180 nm and an AlN layer 23b having a layer thickness of 20 nm is repeatedly grown eight times under the conditions of a growth temperature of 1050 ° C. and a growth pressure of 200 Torr, and the second buffer layer 23 is formed. Note that the flow rates of TMAl, TMGa, and NH ₃ during the growth of the AlN layer 23b and the GaN layer 23a are 195 μmol / min, 58 μmol / min, and 12 L / min, respectively.

Next, the third buffer layer 24 made of GaN having a layer thickness of 200 nm is epitaxially grown under conditions of a growth temperature of 1050 ° C. and a growth pressure of 50 Torr. During the growth of the third buffer layer 24, the flow rates of TMGa and NH ₃ are set to 58 μmol / min and 12 L / min.

Next, TMGa and NH ₃ are introduced at flow rates of 19 μmol / min and 12 L / min, respectively, and the electron transit layer 25 made of n-GaN having a layer thickness of 500 nm is epitaxially grown. The growth temperature is 1050 ° C. and the growth pressure is 200 Torr. In order to dope silicon (Si) that is an n-type dopant, silane (SiH ₄ ) is allowed to flow during the growth of the electron transit layer 25, and the n-type carrier concentration is 2 × 10 ¹⁶ cm ^−3. The flow rate of SiH ₄ is adjusted.

Next, the synchrotron radiation is irradiated on the epitaxial substrate obtained by epitaxially growing each semiconductor layer on the silicon substrate 21 as described above at room temperature, atmospheric pressure, and air. As a result, the V _Ga —H bonds in all the regions from the electron transit layer 25 to the second buffer layer 23 can be _broken, and V _Ga and H can be separated. Therefore, the leakage current can be remarkably reduced as in the case of using the laser beam of the first embodiment.

  The synchrotron radiation may be emitted as white light. For example, the synchrotron radiation is dispersed with a monochromator made of Si (111) crystal, and monochromatic light (wavelength: 1.4; obtained by this). -1.6 angstrom (0.14-0.16 nm)) may be irradiated. In the case of using monochromatic light, since the intensity is weaker than that of white light, it is necessary to perform irradiation for a long time, but there is an advantage that X-ray topography observation can be performed while irradiating. On the other hand, when white light is used, the irradiation time can be shortened because the intensity is strong. However, since ozone may be generated from oxygen in the air and the surface of the nitride compound semiconductor may be modified, it is preferable to make the irradiation time as short as possible.

  FIG. 10 is a schematic diagram for explaining a method of irradiating the epitaxial substrate with white light of synchrotron radiation. As shown in FIG. 10, a large beam diameter is obtained in the case of white light L2. Therefore, if the diameter of the epitaxial substrate 28 is about 4 inches (about 100 mm), the entire main surface can be irradiated by irradiating the main surface perpendicularly. The white light L2 irradiation time is, for example, 1 second.

  FIG. 11 is a schematic diagram for explaining a method of irradiating monochromatic light obtained by separating synchrotron radiation light onto an epitaxial substrate. Since the monochromatic light L3 has a beam diameter as small as 1 cm × 6 cm, for example, the epitaxial substrate 29 may be inclined in order to irradiate the entire surface of the epitaxial substrate 29 with one shot. In FIG. 11, the crystal plane S3 ((0001) plane) parallel to the main surface of the epitaxial substrate 29 is set to be inclined by the angle A1 with respect to the monochromatic light L3. The angle A1 is, for example, ˜2 °. In the arrangement of FIG. 11, when the wavelength of the monochromatic light L3 is 1.6 angstroms, the diffraction lines L4 from the diffractive surface S4 which is the (1, -2, 1, 4) plane are substantially equal to the epitaxial substrate 29. It can be observed from directly above. At this time, for example, the defect in the epitaxial substrate 29 can be detected by observing the diffraction line L4 using a nuclear dry plate. The irradiation time of the monochromatic light L3 is set to 60 seconds, for example.

Next, in order to form an element structure, a Schottky electrode 26 and an ohmic electrode 27 having a concentric pattern having a diameter of 160 μm and an interelectrode distance of 10 μm are formed on the electron transit layer 25. The Schottky electrode 26 has a Ni / Au structure, and the ohmic electrode has a Ti / Al structure. In addition, good ohmic characteristics can be obtained by performing heat treatment at 700 ° C. for 30 minutes after vapor deposition of the ohmic electrode. Further, in this heat treatment, hydrogen atoms have been disconnected from the V _Ga by synchrotron radiation is expelled out of the epitaxial substrate by diffusion.
The SBD 20 according to the second embodiment can be manufactured by the above manufacturing method.

(Examples 2 and 3 and Comparative Example 2)
As Examples 2 and 3 of the present invention, an SBD having the structure of the SBD 20 according to Embodiment 2 was manufactured by the above-described manufacturing method. The SBD of Example 2 was irradiated with white light of synchrotron radiation for 1 second. The SBD of Example 3 was irradiated with monochromatic light having a wavelength of 1.6 angstroms dispersed from synchrotron radiation at an angle of 2 ° from a plane parallel to the main surface of the epitaxial substrate for 60 seconds. As Comparative Example 2, an SBD having the same structure as the SBDs of Examples 2 and 3 was manufactured except that synchrotron radiation was not applied. The leakage characteristics of the SBDs of Examples 2 and 3 and Comparative Example 2 were measured in the same manner as in Examples 1 and Comparative Example 1. The measurement was performed at room temperature.

  FIG. 12 is a diagram showing the leakage currents of the SBDs of Examples 2 and 3 and Comparative Example 2 when the source-drain voltage is 500V. As shown in FIG. 12, it was confirmed that the SBDs of Examples 2 and 3 irradiated with synchrotron radiation can reduce the leakage current by one digit or more than the SBD of Comparative Example 2 that was not irradiated. Further, the breakdown voltage of the element was about 600 V in the SBD of Comparative Example 2, whereas it was improved to about 700 V in the SBDs of Examples 2 and 3. Moreover, a significant difference was not seen between Example 2 and 3, and it was confirmed that the same effect is acquired even if it is white light or monochromatic light.

Further, when the optical response characteristics were measured by the same method as in Example 1 and Comparative Example 1, it was confirmed that the optical response was reduced in the SBDs of Examples 2 and 3. These results indicate that V _Ga -H was decomposed by irradiation with synchrotron radiation, the residual carriers were lowered, and the shallow acceptor level was also extinguished.

  As described above, it has been confirmed that the HFETs of Examples 2 and 3 of the present invention have reduced leakage current and optical response.

(Embodiment 3)
Next, an HFET that is a nitride-based compound semiconductor element according to Embodiment 3 of the present invention will be described. In the HFET according to the third embodiment, thermal neutron beams are used to decompose V _Ga -H in the crystal.

  FIG. 13 is a schematic cross-sectional view of an HFET according to the third embodiment. As shown in FIG. 13, this HFET 30 includes a silicon substrate 31 whose main surface is a (111) plane, a first buffer layer 32 made of AlN, and a GaN layer 33a and an AlN layer 33b, which are sequentially formed on the silicon substrate 31. And 12 layers alternately stacked, a third buffer layer 34 made of GaN, an electron transit layer 35 made of GaN, an electron supply layer 36 made of AlGaN, and an electron supply layer 36 The AlGaN / GaN-HFET includes a gate electrode 37, a source electrode 38, and a drain electrode 39 formed thereon.

  Since the HFET 30 includes the second buffer layer 33 in which the GaN layer and the AlN layer are alternately stacked, the HFET 30 is controlled to suppress the occurrence of cracks in the epitaxial substrate and to reduce the amount of warpage of the epitaxial substrate. be able to.

Furthermore, since this HFET 30 is obtained by crystallizing a nitride-based compound semiconductor layer and then irradiating a thermal neutron beam to decompose V _Ga -H in the crystal into isolated V _Ga , the leakage current is further reduced. It will be.

Here, since the neutron beam has no charge, it mainly interacts only with the nucleus. In the energy region of thermal neutrons (˜25 meV), only elastic scattering with nuclei need be considered. In elastic scattering, the interaction between a neutron and a proton (hydrogen atom) having a similar mass is the largest. Therefore, by irradiating the nitride compound semiconductor layer with a thermal neutron beam, the V _Ga -H bond is broken and hydrogen atoms are knocked out.

  If the energy of the thermal neutron beam increases, the cross-sectional area of inelastic scattering with Ga and nitrogen nuclei increases, so that vacant point defects are formed, which may adversely affect the electrical characteristics of the device. There is. Therefore, the energy of the thermal neutron beam is desirably 1 eV or less, and more desirably 0.5 eV or less. Further, when the energy of the thermal neutron beam is 0.5 meV or less, the effect of knocking out hydrogen by elastic scattering is reduced. Thermal neutrons can be irradiated using a nuclear reactor such as JRR-3 (research reactor of Japan Atomic Energy Agency).

(Production method)
An example of a method for manufacturing the HFET 30 according to the third embodiment will be described. Note that the flow rate of raw materials, the thickness of each layer, the growth temperature, and the like are examples, and are not particularly limited.

First, TMAl and NH ₃ are introduced at a flow rate of 175 μmol / min and 35 L / min into an MOCVD apparatus in which a silicon substrate 31 having a thickness of 1 mm is installed, and the growth temperature is 1000 ° C. on the silicon substrate 31. A first buffer layer 32 made of AlN having a layer thickness of 40 nm is epitaxially grown.

Next, a pair of a GaN layer 33a having a layer thickness of 180 nm and an AlN layer 33b having a layer thickness of 20 nm is grown 12 times under the conditions of a growth temperature of 1050 ° C. and a growth pressure of 200 Torr, and the second buffer layer 33 is formed. Note that the flow rates of TMAl, TMGa, and NH ₃ during the growth of the AlN layer 33b and the GaN layer 33a are 195 μmol / min, 58 μmol / min, and 12 L / min, respectively.

Next, the third buffer layer 34 made of GaN having a layer thickness of 600 nm is epitaxially grown under conditions of a growth temperature of 1050 ° C. and a growth pressure of 50 Torr. During the growth of the third buffer layer 34, the flow rates of TMGa and NH ₃ are 58 μmol / min and 12 L / min.

Next, TMGa and NH ₃ are introduced at flow rates of 19 μmol / min and 12 L / min, respectively, and are made of GaN having a layer thickness of 100 nm on the third buffer layer 34 under the conditions of a growth temperature of 1050 ° C. and a growth pressure of 200 Torr. The electron transit layer 35 is epitaxially grown.

Subsequently, after the third buffer layer 34 and the electron transit layer 35 are grown, TMAl, TMGa, and NH ₃ are introduced at flow rates of 100 μmol / min, 19 μmol / min, and 12 L / min, respectively, and the growth temperature is 1050 ° C. Then, an electron supply layer 36 made of AlGaN having a thickness of 32 nm is epitaxially grown on the electron transit layer 35.

Next, a thermal neutron beam is irradiated to the epitaxial substrate obtained by epitaxially growing each semiconductor layer on the silicon substrate 31 as described above. As a thermal neutron beam irradiation condition, for example, a thermal neutron beam may be irradiated for 60 minutes under the condition of a neutron beam flux of 1 × 10 ¹⁷ cm ⁻² , but is not particularly limited.

Next, in order to form an element structure, Ti and Al are vapor-deposited in this order on the electron supply layer 36 to form a source electrode 38 and a drain electrode 39 as ohmic electrodes. Next, Ni and Au are deposited in this order between the source electrode 38 and the drain electrode 39 to form a gate electrode 37 as a Schottky electrode. In addition, a favorable ohmic characteristic is acquired by performing the heat processing for 30 minutes at 700 degreeC after vapor deposition of an ohmic electrode. Further, in this heat treatment, hydrogen atoms have been disconnected from the V _Ga by thermal neutron irradiation is expelled out of the device.
The HFET 30 according to the third embodiment can be manufactured by the above manufacturing method.

  Next, as Comparative Example 3 of the present invention, an HFET having the structure of the HFET 30 according to Embodiment 3 was manufactured by the above-described manufacturing method, except that the thermal neutron beam was not irradiated. The Al composition of the electron supply layer made of AlGaN was 0.24 according to the evaluation by the X-ray diffraction method. As for the size of the HFET, the gate length was 2 μm, the gate width was 0.2 mm, and the source-drain distance was 15 μm.

In addition, when the HFET of Comparative Example 3 was manufactured, the characteristics were calculated by simulation on the assumption that the thermal neutron beam was irradiated as in the manufacturing method described above. The thermal neutron beam was irradiated under the condition that the neutron beam flux was 1 × 10 ¹⁷ cm ⁻² for 60 minutes.

Next, when the characteristics of the HFET of Comparative Example 3 were measured, the mobility of the two-dimensional electron gas was 1300 cm ² / Vs, and the sheet carrier concentration was 9 × 10 ¹² cm ⁻² . Moreover, in the calculation results when the thermal neutron beam is irradiated, the same characteristics as those in Comparative Example 3 are obtained for these characteristics, and there is an adverse effect on the ON characteristics of the HFET due to the thermal neutral beam irradiation. Not.

Next, the leakage characteristics of the HFET of Comparative Example 3 were measured. Here, a voltage of −5 V was applied between the source and the gate, and a voltage of 200 V was applied between the source and the drain. As a result, the leakage current was 2 × 10 ⁻⁸ A / mm. Further, in the calculation result when the thermal neutron beam was irradiated, the leakage current was 3 × 10 ⁻⁹ A / mm, and the leakage current was reduced by about one digit by the thermal neutron beam irradiation.

  Next, the optical response characteristics of the HFET of Comparative Example 3 were measured in the same manner as in Example 1 and Comparative Example 1. FIG. 14 is a diagram showing a change with time of the resistance value between the source and the drain of the HFET of Comparative Example 3 and the HFET irradiated with the thermal neutron beam. As shown in FIG. 14, in the HFET of Comparative Example 3, it takes 1000 seconds or more until the resistance value becomes constant, whereas in the calculation result when the thermal neutron beam is irradiated, it is saturated within 30 seconds, As a result, the resistance value became constant.

  From the above results, it was confirmed that an HFET with reduced leakage current and optical response can be realized by thermal neutron irradiation.

As described above, by irradiating the nitride compound semiconductor layer after the epitaxial growth with laser light or ionizing radiation, the bond of V _Ga -H in the crystal can be cut. As a result, it has been clarified that a relatively shallow acceptor level due to V _Ga -H can be suppressed, and that there is an effect of reducing leakage current and suppressing optical response. By using this effect, a power device such as an inverter or a converter using a highly efficient GaN-based semiconductor having excellent long-term reliability can be realized.

In the above embodiment, a silicon substrate which is a different substrate is used as the substrate. However, the different substrate to be used is not particularly limited, and sapphire, silicon carbide (SiC), zinc oxide (ZnO), LiGaO ₂ , ( The same effect can be obtained by using Mn, Zn) Fe ₂ O ₄ or the like.

  Moreover, in the said embodiment, although an electron transit layer is GaN which doped n-GaN or C, undoped GaN may be sufficient.

  Moreover, in the said embodiment, although the thermal neutron beam is used as a neutron beam to irradiate, if an energy is 1 eV or less, you may use an epithermal neutron beam.

  Further, the nitride-based compound semiconductor element according to the present invention is not limited to HFET and SBD, and can be various elements, for example, MOSFET. The nitride compound semiconductor according to the present invention is not limited to GaN, and may be a nitride compound semiconductor containing Ga atoms and nitrogen atoms. In addition to Ga atoms, Al atoms and indium (In) atoms may be used. And one or more group III atoms selected from boron (B) atoms.

  Further, the present invention is not limited by the above embodiment. What was comprised combining each component mentioned above suitably is also contained in this invention. In addition, other embodiments, examples, operational techniques, and the like made by those skilled in the art based on the above-described embodiments are all included in the present invention.

10, 30 HFET
11, 21, 31 Silicon substrate 12 Low temperature buffer layer 13 Buffer layer 14, 25, 35 Electron travel layer 15, 36 Electron supply layer 16, 37 Gate electrode 17, 38 Source electrode 18, 39 Drain electrode 20 SBD
22, 32 First buffer layer 23, 33 Second buffer layer 23a, 33a GaN layer 23b, 33b AlN layer 24, 34 Third buffer layer 26 Schottky electrode 27 Ohmic electrode 28, 29 Epitaxial substrate B1 Beam spot D1, D2 Data Point cloud L1 Laser light L2 White light L3 Monochromatic light L4 Diffraction line P1, P2 Peak S1, S2 Spectrum S3 Crystal plane S4 Diffraction plane

Claims (9)

A growth step of epitaxially growing a nitride-based compound semiconductor layer comprising a group III atom containing at least a gallium atom and a nitrogen atom on the substrate;
A decomposition step of irradiating the nitride-based compound semiconductor layer with laser light or ionizing radiation before element structure formation to decompose a complex of group III vacancies and hydrogen atoms in the nitride-based compound semiconductor layer;
A method for manufacturing a nitride-based compound semiconductor device comprising:   2. The method of manufacturing a nitride-based compound semiconductor device according to claim 1, wherein the laser beam is a laser beam having a wavelength of 380 nm or less.   3. The method for producing a nitride-based compound semiconductor device according to claim 1, wherein the laser beam is a pulsed laser beam having a pulse width of 100 microseconds or less. The pulse laser beam is irradiated so that the light intensity at the outermost surface of the nitride-based compound semiconductor layer is 1 W / cm ² or more and 1000 W / cm ² or less in terms of CW. The manufacturing method of the nitride type compound semiconductor element as described in any one.   The method of manufacturing a nitride-based compound semiconductor device according to claim 1, wherein the ionizing radiation is synchrotron radiation.   The method for producing a nitride-based compound semiconductor device according to claim 1, wherein the ionizing radiation is a neutron beam.   The method of manufacturing a nitride-based compound semiconductor device according to claim 6, wherein the neutron beam is an epithermal neutron beam or a thermal neutron beam having an energy of 1 eV or less.   8. The method for manufacturing a nitride-based compound semiconductor element according to claim 1, wherein the substrate is made of silicon, sapphire, silicon carbide, or zinc oxide. The nitride compound semiconductor device manufactured by the manufacturing method according to claim 1, wherein a group III vacancy concentration in the nitride compound semiconductor layer is 5 × 10 ¹⁶ to 1 ×. A nitride-based compound semiconductor device having a density of 10 ¹⁸ cm ⁻³ .

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