JPH04294547A - Inalas/ingaas heterojunction structure semiconductor device and field effect transistor using it - Google Patents

Abstract

PURPOSE:To achieve a high mobility by increasing In composition ratio of InGaAs channel layer and then enable reduction in electron density accompanying it to be suppressed. CONSTITUTION:In a heterojunction structure of InGaAs 4 and InAlAs 5.6, each in composition ratio is 0.8 and 0.65 and is larger at InGaAs layer 4 where a two dimensional electron gas 4a is formed, thus achieving a high mobility of the InGaAs layer 4 and suppressing reduction in an amount of band discontinuity and reduction in density of electrons. With layers 5, 6, and 7 which are formed on the InGaAs layer 4 with the In composition ratio of 0.65, the total film thickness is set to a critical film thickness for preventing generation of transformation accompanying a lattice mismatching with the InGaAs layer 4 and the lattice mismatching between the InGaAs layer 4 and the semi-insulation GaAs substrate 1 is absorbed and relaxed by an InGaAs graded buffer layer 2.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to InAlAs/InGa
Regarding As heterojunction structure semiconductor devices, it is particularly suitable for use in improving high carrier mobility of field effect transistors.

0002

[Prior Art] Si is used as a material for field effect transistors.
However, compound semiconductors are materials that have higher carrier mobility than Si and improve transistor performance. Among them, H
The EMT element has excellent high speed and low noise, and is compatible with 12 GHZ.
It has been put into practical use in satellite broadcast receivers. However, in recent years,
There is a strong demand for the use of higher frequency bands, and InGaAs is attracting attention as a material that has higher mobility than GaAs and enables higher-speed operation, especially for semi-insulating I
A HEMT device using an InAlAs/InGaAs heterojunction grown on an nP substrate is an AlGaAs/Ga
It has better mobility, saturated electron velocity, and sheet carrier concentration than those using As heterojunctions, and is attracting attention as a high frequency/OEIC material.

[0003] Here, InGaAs is InAs and GaA
This is a material made by growing a mixed crystal of s. Comparing the lattice constants of InAs and GaAs, InAs has a lattice constant of 6.06 Å, G
Since aAs is different from 5.65 Å, InP having a lattice constant of 5.87 Å between these is used as a substrate for epitaxial growth of InGaAs.

0004

[Problem to be solved by the invention] By the way, this InA
In the lAs/InGaAs heterojunction system, as shown in FIG. 2, the greater the In composition ratio x, the greater the mobility required for high-speed operation. Note that FIG. 2 is a characteristic diagram showing the relationship between the In composition ratio x, lattice constant, and mobility at room temperature of InGaAs in which the two-dimensional electron gas layer is formed.

However, as shown in FIG. 2, the lattice constant also changes depending on the In composition ratio. Here, InGaAs,
Due to its composition ratio, InAlAs is
Since it has a lattice mismatch rate of -3.7 to +3.2%, in order to grow InGaAs and InAlAs with good crystallinity on an InP substrate, the In composition ratio x should be changed to InX Ga1-x.
0.53 for As, 0.5 for InX Ga1-x As
2, it is necessary to precisely control the lattice constant to match that of the substrate. This is because when grown with lattice mismatch, distortion occurs due to the difference in lattice constant between the substrate and the grown layer, and crystallinity deteriorates due to the influence of the distortion or dislocations generated by the distortion.
This is because electrical properties (particularly mobility) are significantly reduced.

That is, the mobility of a system using InGaAs is limited to that when the In composition ratio is 0.53 due to the necessity of lattice matching the InGaAs to the substrate, and it is difficult to fully utilize the high mobility effect of this system. Can not do it.

[0007] In view of the above circumstances, the purpose of the present invention is to
The present invention aims to review the AlAs/InGaAs heterojunction structure semiconductor device and provide a field effect transistor that can fully utilize the high mobility effect of this system.

[0008]

[Process of devising the invention] Therefore, in order to achieve the above object, the present inventors provided a buffer layer therebetween to absorb the lattice mismatch between the substrate and the InGaAs layer, and a two-dimensional electron gas layer was formed. We focused on realizing high mobility by increasing the In composition ratio of this InGaAs layer. In this case, In
I of the InAlAs layer forming a heterojunction with the GaAs layer
The n composition ratio needs to be controlled to match that of the InGaAs layer in order to match the lattice constant with that of the InGaAs layer.

However, as a result of repeated consideration, the n-type I
In the nX Al1-x As/InX Ga1-x As heterojunction, increasing the In composition ratio x certainly improves the mobility of the two-dimensional electron gas layer, but on the other hand, the amount of band discontinuity at the hetero interface is small. It has become clear that the sheet carrier concentration, that is, the electron concentration, in the two-dimensional electron gas layer decreases.

FIG. 3 shows the I
The band gaps of nGaAs and InAlAs are shown. n
The amount of band discontinuity at the hetero interface of the type InAlAs/InGaAs heterojunction is between the n-type InAlAs layer and the InGaAs layer.
It is proportional to the difference in the band gaps of the As layers. Therefore, Figure 3
As is clearer, the lattice constant of the InAlAs layer is
If the In composition ratio is increased to match that of the GaAs layer, the amount of band discontinuity will decrease accordingly. As shown from the relationship between the amount of band discontinuity and the electron concentration in FIG. 4, as the amount of band discontinuity becomes smaller, the electron concentration of the two-dimensional electron gas layer decreases.

In other words, considering that the high frequency performance of a field effect transistor is approximately proportional to the product of mobility and electron concentration, even if the mobility is improved by increasing the In composition ratio, the electron concentration will decrease. This means that overall the performance of the transistor has not been improved. In addition, in FIG. 4, the doping concentration of the n-type dopant to the n-type InAlAs layer is 2×1018 cm-3, the donor level is 4 meV,
5 non-doped InAlAs spacer layers at the hetero interface.
The characteristics of a film formed at 0 Å are shown as an example.

0012

SUMMARY OF THE INVENTION Therefore, in order to achieve the above object while improving the high frequency characteristics of the transistor as a whole, the field effect transistor to which the present invention is applied is made of InAlAs.
The In composition ratio is smaller than that of InGaAs, and the amount of band discontinuity of InGaAs and InAlAs is made larger than when the In composition ratio is the same, and
The film thickness of s is set to be less than or equal to a critical film thickness corresponding to the difference in In composition ratio in order to suppress the generation of dislocations due to lattice mismatch caused by the difference in In composition ratio with InGaAs.

According to the experimental considerations of the present inventors, even if the growth involves lattice mismatch, the crystal lattice absorbs the energy of the lattice strain, and the maximum film thickness that can be grown without generating misfit dislocations has been determined. There is a (critical film thickness), and if the film is not grown beyond this thickness, it is metastable (pseudomorp).
hic) state was realized, and it was confirmed that good electrical properties were obtained without deterioration of crystallinity.

That is, for example, InAlAs/InGaAs
At the heterojunction, a two-dimensional electron gas layer is formed.
In order to suppress the decrease in electron concentration caused by increasing the In composition ratio of the nGaAs layer to achieve high mobility, that is, to increase the amount of band discontinuity, as shown in FIG. If the In composition ratio of the InAlAs layer is shifted to be smaller than the In composition ratio of the InGaAs layer, and the thickness of the InAlAs layer is set below the critical film thickness according to the In composition ratio, the lattice defect at the heterojunction interface is reduced. This means that the occurrence of dislocation defects due to alignment can be absorbed.

As described above, according to the present invention, it is possible to significantly improve mobility while suppressing a decrease in electron concentration, which is advantageous for high-speed operation, and is suitable for amplifiers in the ultra-high frequency region above microwaves. An optimal basic structure can be obtained.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained below based on embodiments shown in the drawings. In addition, as an example, InAlAs/InGa
This will be explained by taking an As heterojunction field effect transistor as an example.

FIG. 1 shows a cross-sectional structural diagram of a first embodiment of the present invention. In FIG. 1, on a semi-insulating GaAs substrate 1,
An InGaAs graded buffer layer 2 is formed in which the In composition is gradually changed from approximately zero to the composition of the InX Al1-x As barrier layer 3 (x=0.8 in the example). This graded buffer layer 2 has a lattice constant of GaAs (5.654 Å) and
Lattice constant of lAs barrier layer 3 (5.8 at In composition 0.5
6 Å and 5.979 Å with an In composition of 0.8), the purpose is to alleviate and absorb the difference of 5.979 Å with an In composition of 0.8, and to suppress the generation of dislocation defects due to lattice mismatch. Note that I of the graded buffer layer 2
The n composition change rate is set to 0.4 μm −1 in this example. That is, I per 1 μm of thickness of the graded buffer layer 2
The purpose is to change the n composition ratio by 0.4, and in this example, since the composition is changed from 0 to 0.8, the buffer layer is formed to have a thickness of 2 μm.

The upper layer of the graded buffer layer 2 contains InX Al1-x to prevent characteristic deterioration during high electric fields.
An As barrier layer 3 is formed with a thickness of about 2000 Å, and on top of this, an In
000 Å. Here, InX Al1-x
The In composition ratios of the As barrier layer 3 and the InX Ga1-x As channel layer 4 are both set to the same value x (0.8 in this example).

An Iny Al1-y As spacer layer 5 having a thickness of 30 Å is formed on the InX Ga1-x As channel layer 4. This spacer layer 5 is made of n-type InA
Its role is to increase the spatial separation between the n-type impurity added to the lAs-doped layer 6 and the two-dimensional electron gas 4a, thereby increasing the mobility.

Further, in the upper layer, there is an n-type InyAl1-yAs doped layer 6 doped with Si as an n-type dopant in order to supply electrons to the InGaAs channel layer 4.
00 Å, and the n-type InyGa1-yAs cap layer 7 that prevents this InAlAs doped layer 6 from oxidizing is 1
00 Å is formed.

Here, the layers labeled 5, 6, and 7 have the same In composition y (0.65 in this example).
The two In composition ratios x and y have a relationship of x>y (for example, x=0.8, y=0.65). Note that the above structures are sequentially formed by the MBE method (molecular beam epitaxial growth method) or the MOCVD method.

In order to operate this structure as a field effect transistor, a drain electrode 8, a source electrode 10, which are ohmic electrodes, and a gate electrode 9, which is a Schottky electrode, are formed on the outermost surface. Note that conventionally known materials are used for these electrode materials.

In such a heterojunction field effect transistor, the carriers doped in the InAlAs doped layer 6 are accumulated at the heterojunction interface, producing a two-dimensional carrier gas 4a on the InGaAs channel layer 4 side. In a field effect transistor using a heterojunction, an electric field is applied in parallel to the two-dimensional carrier gas formed on the channel layer 4 side of the heterojunction interface via the source and drain electrodes 10 and 8, and the gas layer 4a is used as an active layer, and by controlling the carriers induced in this active layer by the electric field applied from the gate electrode 9, the current flowing between the source and drain electrodes is controlled to perform transistor operation. Therefore, the electrical characteristics (particularly the mobility) of the InGaAs channel layer 4 through which the carrier gas actually travels have a large effect on the high-speed operation of the transistor.

Conventionally, a two-dimensional carrier gas runs on an In
The In composition ratio of the GaAs channel layer 4 was set to 0.53 to lattice match the substrate to prevent dislocations from occurring with the substrate. With such a composition, the high mobility feature of InAs cannot be fully exhibited.

On the other hand, in the structure of this embodiment, InGa
The In composition ratio of the As channel layer 4 is controlled to 0.8 to increase the InAs composition, which increases the mobility, and at the same time
GaA
An InGaAs graded buffer layer 2 is formed as a lattice mismatch buffer layer between the s-substrate 1 and the InAlAs barrier layer 3.
This prevents the formation of dislocation defects.

The electron mobility I of the InGaAs channel layer 4 whose lattice mismatch with the substrate is alleviated by the buffer layer 2
FIG. 7 shows the n composition ratio dependence. Here, FIG. 7 shows the mobility when the temperature is set to room temperature (300°K), taking into account phonon scattering, alloy scattering, and ionized impurity scattering. In addition, the impurity concentration is 2×1015 cm
A high purity case of -3 is assumed, and the contribution of ionized impurity scattering is small and does not appear in the figure. As shown in FIG. 7, by setting the In composition ratio of the InGaAs channel layer 4 to 0.8 in this embodiment from 0.53 in the conventional case, the mobility is increased to 10000 cm2 at room temperature compared to the conventional case.
/Vs can be increased by about 40% to 14000 cm2 /Vs, and a field effect transistor useful for high-speed operation can be obtained.

Furthermore, since the lattice mismatch between the InGaAs layer 4 and the substrate 1 is absorbed by the buffer layer 2, there is no need to insist on using an InP substrate as the substrate as in the past, and instead a GaAs substrate, which is inexpensive and has stable characteristics, can be used. can be used and also S
It also becomes possible to use an i-board.

Next, the InGaAs channel layer 4 and the n-type I
The respective In composition ratios with respect to the nAlAs doped layer 6 will be explained. As mentioned above, if the In composition ratio is increased to approach 1 in order to increase the mobility, the In composition ratio of the InAlAs doped layer 6 will also approach 1 in order to achieve lattice matching with the channel layer 4. . That is, since both of them approach the InAs layer, the amount of band discontinuity decreases as shown in FIG. 3, and the electron concentration Ns of the two-dimensional electron gas 4a decreases.

Therefore, in this example, InX Ga1
-x Iny A from the In composition ratio x of the As channel layer 4
l1-y The In composition ratio y of the As-doped layer 6 is made small. That is, as is clear from FIG. 3, by setting the relationship x>y, it is possible to obtain a larger amount of band discontinuity than when x=y, and as a result, it is possible to prevent a decrease in electron concentration. becomes.

In order to deal with the lattice mismatch and the accompanying generation of dislocation defects caused by making the In composition ratio different from x>y, in this embodiment, the In composition ratio is set to y (=0.
65) (InAlAs spacer layer 5, n-type In
AlAs doped layer 6, n-type InGaAs cap layer 7)
Set the sum of the thicknesses to be smaller than the critical film thickness,
The crystal lattice is elastically distorted to prevent dislocation defects from occurring. FIG. 5 shows the relationship between the deviation of the In composition ratio and the critical film thickness according to the energy balance model.

In this embodiment, in order to supply sufficient electrons, the InAlAs spacer layer 5 is made of 30 Å, n-type InA.
The lAs doped layer 6 is 400 Å, the n-type InGaAs cap layer 7 is 100 Å, and the total film thickness on the carrier supply layer side is 530 Å. Note that it is generally said that a film thickness of this level is required in order to supply sufficient electrons. Here, the critical film thickness does not always match the value shown in Fig. 5, and may vary within a certain range depending on the conditions, so in this example, a safety factor of 2 is assumed for the critical film thickness value shown in Fig. 5. , it is considered that the film thickness is set to 1/2 or less of the critical film thickness value shown in FIG. 5 to reliably prevent the occurrence of dislocations.

That is, in this embodiment, an InAlAs spacer layer 5, an n-type InAlAs doped layer 6, an n-type InGaAs
Since the total film thickness of the cap layer 7 is set to 530 Å, if this total film thickness is considered as an allowable film thickness, the critical film thickness in FIG. According to FIG. 5, the optimum In composition ratio deviation is 0.15, and the In composition ratio y is set to 0.65.

Note that, as shown in FIG. 3, if the deviation of the In composition ratio is made larger, the amount of band discontinuity increases and the electron concentration can be further improved, but as shown in FIG. 5, the critical film thickness becomes smaller. . In that case, in order to supply sufficient electrons to the two-dimensional electron gas 4a, a sufficient amount of dopant may be introduced into the carrier supply layer, which becomes thinner due to the critical thickness.

Next, FIG. 6 shows the relationship between the In composition ratio x of the InGaAs channel layer 4 and the electron concentration Ns, using the deviation of the In composition ratio as a parameter. Note that the characteristics in FIG. 6 show that the doping concentration to the n-type InAlAs doped layer 6 is 4×10 18
cm-3, the thickness of the InAlAs spacer layer 5 is 30 Å,
The temperature is 300°K.

Compared to the case where the deviation in the In composition ratio is 0, that is, x=y, the larger the deviation in the In composition ratio, the more the decrease in electron concentration is suppressed in the region with a high In composition ratio where high mobility can be expected. I can see that you are there.

Here, using the mobility values in FIG. 7, In
FIG. 8 shows the relationship between the In composition ratio x of the GaAs channel layer 4 and the high frequency characteristics of the transistor, that is, the product of electron concentration and mobility.

As is clear from FIG. 8, the In composition ratio difference (
The larger the deviation), the larger the value of the product of mobility and electron concentration, and the larger the In composition ratio that takes the maximum value (the composition ratio considered to be optimal in terms of performance). That is, in this example in which the In composition ratio difference is 0.15, strictly speaking, InGaAs
The In composition ratio x of the channel layer 4 is 0.83, while the InAl
The In composition ratio y of the As-doped layer 6 is optimally controlled to 0.68. Furthermore, from FIG. 8, compared to the conventional structure where x=y, the maximum value of the product, that is, the high frequency characteristics of the field effect transistor can be improved by about 1.6 times.

As described above, the InGaAs channel layer 4
In addition to increasing the In composition ratio x of the InGaAs channel layer 4 to achieve high mobility, the I
By setting the In composition y of the nAlAs layer 6 to the relationship y<x, it is possible to increase the band gap difference, suppress a decrease in electron concentration, and improve the overall performance of the transistor. Can be done. Also, InG
The effect of lattice mismatch caused by the difference in In composition ratio between the aAs layer 4 and the InAlAs layer 6 can be resolved by making the InAlAs layer less than the critical film thickness corresponding to the difference in In composition ratio, resulting in excellent high-speed operation. A basic structure suitable for ultra-high frequency devices higher than microwaves can be obtained.

Next, a second embodiment of the present invention will be explained. In the first embodiment, the In composition ratio y of the InAlAs spacer layer 5 in FIG. 1 was set to be the same as the In composition ratio y of the n-type InAlAs doped layer 6. The In composition ratio of the layer 5 is set to the same composition ratio x as that of the InGaAs channel layer 4.

When each layer is grown using an MBE growth apparatus, there is no problem with the structure of the first embodiment as long as it has two In evaporation sources, but if there is only one In evaporation source, In the structure of the first embodiment, the InGaAs channel layer 4 and the InGaAs channel layer 4 are
Since the In composition ratio of the AlAs spacer layer 5 is different, it is necessary to once interrupt the crystal growth and change the temperature of the In evaporation source. The interface that interrupts this growth is the two-dimensional electron gas 4
Since a is an important heterointerface that travels, if residual impurities in the vacuum are attached due to interruption of growth, it becomes a factor that causes a decrease in mobility. Therefore, as in the second embodiment, InA
If the In composition of the lAs spacer layer 5 is the same as that of the InGaAs channel layer 4, there is no need to interrupt the growth at this interface. Note that the InAlAs spacer layer 5 and the n-type InA
Although it is necessary to interrupt the growth between the lAs-doped layers 6, the probability of the existence of two-dimensional electron gas at this interface is almost zero, and this does not cause a decrease in mobility. Therefore, according to the second embodiment, when there is only one In evaporation source, it is possible to effectively prevent a decrease in mobility.

[0041] InX, which is a crystal with different lattice constants,
Ga1-x As channel layer 4 and n-type Iny Al1
In the bonding of the −y As doped layer 6, it is thought that the thinner n-type Iny Al1-y As doped layer 6 side is basically distorted, but the thicker InX Ga1-x As
It is presumed that the vicinity of the interface of the channel layer 4 is also slightly distorted. Strain causes mobility reduction. However, book 2
In the structure of the example, since the In composition is switched at the interface between the InAlAs spacer layer 5 and the n-type InAlAs doped layer 6, the InAlAs spacer layer 5 and
The strain in the InGaAs channel layer 4 through which the two-dimensional electron gas 4a travels is reduced. In this respect as well, the second embodiment is advantageous in suppressing a decrease in mobility.

In the above embodiment, an InGaAs graded buffer layer was used to alleviate the effect of lattice mismatch between the substrate 1 and the channel layer 4, but this layer may also be made of InAlAs. Dislocation defects may be reduced by using a strained superlattice buffer layer or a combination thereof.

Furthermore, in the above embodiment, a two-dimensional electron gas is generated at the hetero interface by using the n-type InAlAs doped layer 6 doped with Si as the carrier supply layer, but it is also possible to generate a two-dimensional electron gas at the hetero interface by injecting a P-type dopant. The present invention can also be implemented in a device that forms a two-dimensional hole gas.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional structural diagram showing a field effect transistor according to a first embodiment of the present invention.

FIG. 2 is a graph showing the relationship between the In composition ratio x, lattice constant, and mobility of InGaAs.

FIG. 3 is a characteristic diagram showing the In composition ratio dependence of the band gap of InAlAs and InGaAs.

FIG. 4 is a characteristic diagram showing the relationship between the electron concentration NS of a two-dimensional electron gas layer and the amount of band discontinuity.

FIG. 5 is a characteristic diagram showing the relationship between the total critical film thickness of each layer having an In composition ratio y including an InAlAs doped layer and the In composition ratio difference.

[Figure 6] Electron concentration NS using In composition ratio difference as a parameter
FIG. 3 is a characteristic diagram showing the relationship between In composition ratio x and In composition ratio x.

FIG. 7 is a characteristic diagram showing the dependence of the electron mobility of InGaAs on the In composition ratio.

[Figure 8] Product of the mobility and electron concentration of InGaAs
FIG. 3 is a characteristic diagram showing the composition ratio x dependence using the In composition ratio difference as a parameter.

[Explanation of symbols]

1 Semi-insulating GaAs substrate 2 InGaAs graded buffer layer 3 I
nX Al1-x As barrier layer 4 InX Ga1
-x As channel layer 4a Two-dimensional electron gas 5 Iny Al1-y As spacer layer 6 n-type Iny Al1-y As doped layer 7 n-type Iny
Ga1-y As cap layer 8 Drain electrode 9 Gate electrode 10 Source electrode

Claims (2)

[Claims] [Claim 1] InAlAs/InG made of InGaAs and InAlAs with a wider energy bandwidth
In the aAs heterojunction, the In composition ratio of InAlAs is smaller than the In composition ratio of InGaAs.
The amount of band discontinuity of InAlAs and InAlAs is made larger than when the In composition ratio is the same, and the film thickness of InAlAs is
InAl is characterized in that the film thickness is set below a critical thickness according to the difference in In composition ratio in order to suppress the generation of dislocations due to lattice mismatch caused by the difference in In composition ratio with nGaAs.
As/InGaAs heterojunction structure semiconductor device. 2. A two-dimensional carrier gas formed in the InGaAs layer at the InAlAs/InGaAs heterojunction interface is used as an active layer to inject InAl into the two-dimensional carrier gas.
A field effect transistor in which carriers are supplied from source/drain electrodes via an As layer and carrier travel of the two-dimensional carrier gas is controlled by an electric field applied to a gate electrode, the InGaAs layer having an In composition ratio of It is set to a value greater than 0.53, and the InGa
The InAlAs layer that supplies carriers to the two-dimensional carrier gas formed in the As layer has an In composition ratio set to a value smaller than that of the InGaAs layer, and a film thickness that is smaller than that of the InGaAs layer. A field effect transistor characterized in that the film thickness is set to be less than or equal to a critical film thickness corresponding to the difference in In composition ratio in order to suppress the generation of dislocations due to lattice mismatch caused by the difference in In composition ratio.