JP2001015442A - GaN film and GaN film forming method - Google Patents

Abstract

(57) Abstract: A cubic GaN film is easily formed. SOLUTION: AsH ₃ and TMG are introduced to form a GaAs epitaxial layer on a GaAs substrate (process A), and DMHy and TMG are introduced to form a GaN buffer layer (process B). In the process of heating and growing the GaN buffer layer, AsH ₃ is introduced (Process C), and then DMHy and TMG are introduced to form a GaN film. By introducing a gas containing argon in the process C, formation of stable hexagonal GaN is suppressed, and metastable cubic GaN is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a GaN film and a method for forming the same, and more particularly, to the formation of a cubic GaN film.

[0002]

2. Description of the Related Art Heretofore, a GaN film has been epitaxially grown on a substrate and used for an electrically conductive device such as a power FET or an optical device such as a light emitting element. In order to grow a GaN film, a method of growing a crystal on a sapphire substrate to form a hexagonal (urzite-type) GaN film is often used. That is, an AIN phase or a GaN layer is formed on a sapphire substrate by several tens n using MOVPE (metal organic chemical vapor deposition) or MBE (molecular beam epitaxy).
A method of growing the GaN layer by about m and then growing the GaN layer by several μm is adopted.

[0003] On the other hand, cubic GaN has more threading dislocations (stacking hold) than hexagonal GaN, and high mobility of carriers can be expected.
Crystal growth of N is being sought.

FIG. 9 shows a cubic GaN crystal growth diagram by the conventional MOVPE method. In the figure, the horizontal axis represents time, and the vertical axis represents temperature. Below the graph, the introduction / introduction of arsine gas (AsH ₃ ), dimethylhydrazine (DMHy), and trimethylgallium (TMG) /
The state of the stop is shown in a timing chart. First, a GaAs epitaxial layer was formed on a GaAs substrate from which oxide was removed by etching at a temperature of about 600 to 750 ° C. for about 1 hour.
It is grown to about 00 to 200 nm (process α). In this GaAs layer forming process, for example, 10% of AsH ₃ and TMG are used as reaction gases. GaAs
When the layer is formed, a GaN low-temperature buffer layer of about 10 to 20 nm is formed on the GaAs layer at a temperature of 400 to 600 ° C.
It is formed of MG and DMHy (process β). After the formation of the GaN low-temperature buffer layer, the GaN buffer layer is grown by heating and raising the temperature of the reaction tube again (process γ). Ga
At the time of heating the N buffer layer, DMHy is introduced. Then, the reaction tube temperature is adjusted to a predetermined GaN growth temperature (800 to 95).
0 ° C.), TMG is introduced in addition to DMHy,
A GaN layer is grown (process δ).

[0005]

Conventionally, using such a method, the pressure in the reaction tube and the V / II
By optimizing parameters such as group I ratio and reaction tube temperature, the mixing ratio of hexagonal GaN is reduced.

However, the hexagonal crystal is basically lower in energy and more stable than the cubic crystal, and the hexagonal crystal easily grows from facets (planes where the same kind of atoms are arranged). Of hexagonal GaN is 50
%, There is a problem that it is difficult to obtain a lower mixing ratio.

The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object to control a formation method and a cubic crystal capable of easily reducing the hexagonal GaN mixing ratio in cubic GaN. Another object is to provide a GaN film having a characteristic.

[0008]

In order to achieve the above object, the present invention provides a method for forming a GaN thin film on a semiconductor substrate, the method comprising introducing a group V source gas and a group III source gas. Forming a GaN buffer layer on the substrate; heating the GaN buffer layer to stop the introduction of the group III source gas;
s-containing gas is introduced to introduce Ga on the GaN buffer layer.
A heating step of raising the temperature to an optimum temperature for growing the GaN epitaxial layer to form an N epitaxial layer; and introducing the group V source gas and the group III source gas after the heating step to form a GaN epitaxial layer on the GaN buffer layer. Forming a GaN layer.

Further, according to the present invention, the semiconductor substrate is GaAs.
It is an s substrate.

Further, the present invention includes a GaAs layer forming step of epitaxially growing a GaAs layer on the semiconductor substrate prior to the buffer layer forming step, wherein the GaN buffer layer is formed on the GaAs layer. Features.

Further, the present invention is characterized in that the gas containing As is arsine gas.

Further, according to the present invention, the arsine gas contains 5%
From 20% to 20%.

Further, the present invention relates to a GaN film formed on a GaAs substrate, wherein the value of ω of hexagonal crystal is about 10.790 degrees in X-ray reciprocal lattice mapping, and the cubic crystal is And dominant.

As described above, in the present invention, a gas containing As, for example, an arsine gas is introduced at an initial stage of growing the GaN buffer layer and the GaN epitaxial layer. Hexagonal GaN is known to grow easily on the (111) B plane. That is, when the (111) B plane is formed by facets (similar atoms are aligned), kinks, and steps (misalignment of the growth plane) generated during growth, stable hexagonal GaN is formed from the plane. . GaAs (1
11) In the step formed on the surface, a single bond As is formed on the step surface in the [011] direction,
Triple bond As on step surface in [0-11] direction
Is formed. Since many facets are generated on the step plane in the [0-11] direction, a gas containing As is introduced to terminate As on the step plane, and a step in which hexagonal GaN that should be generated in the [0-11] direction should be generated. GaAs preferentially on surface
To suppress the formation of hexagonal GaN. In the heating process of the buffer layer, GaAs
As in the metal is dissociated and replaced with N to form GaN. However, since GaN is formed with the crystal structure of GaAs formed initially as a nucleus, the formation of hexagonal crystals is suppressed continuously.

When a GaN film is formed on a GaAs substrate, a GaAs epitaxial layer is first formed on a GaAs substrate, and a GaN buffer layer is formed on the GaAs epitaxial layer. In the prior art, only dimethylhydrazine is introduced when heating to grow a GaN buffer layer, and no arsine gas is introduced (see FIG. 9). For this reason, As in the GaAs epitaxial layer is dissociated, causing a structural defect such as a pit, and a GaN buffer layer or a GaN layer formed thereon is distorted to form a stacking fault (stacking fault). When a stacking fault occurs, stable hexagonal GaN is formed therefrom. In the present invention, by introducing a gas containing As in the initial process of growing a GaN buffer layer formed on a GaAs epitaxial layer, dissociation of As from the GaAs epitaxial layer is suppressed, and stacking faults are prevented. The formation of crystalline GaN can be suppressed.

As a gas containing As, arsine gas (AsH ₃ ) is preferable, but its concentration is 5% to 20%.
Is desirable. If the concentration is too low, the dissociation of As from the GaAs epitaxial layer cannot be effectively prevented. If the concentration is too high, the bonding between Ga and N is inhibited and the quality of the GaN film is reduced, so that a so-called mirror surface cannot be obtained. This is because the electrical characteristics or optical characteristics of the device deteriorate. Also, Ga
Since the N buffer layer grows with the crystal structure of the underlayer as a nucleus, the A buffer is formed at an initial stage of growing the GaN buffer layer.
It is necessary to introduce a gas containing s. The initial stage refers to a time period that can be regarded as simultaneous or substantially simultaneous when the GaN buffer layer is heated.

In the initial stage of the growth of the GaN buffer layer, a gas containing As is introduced to suppress the formation of hexagonal GaN, so that cubic GaN in a metastable state is dominant.
An N film can be obtained. "Dominant" means hexagonal Ga
This means that the amount of cubic GaN is larger than that of N.
By forming the GaN buffer layer in a gas containing As, the value of ω in the X-ray reciprocal lattice mapping of hexagonal GaN also differs from the conventional one. That is, the ω value of the conventional GaN film is about 10.010 degrees, and the ω value of the GaN film of the present invention is 1
0.790 degrees.

[0018]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a conceptual diagram of an experimental apparatus for forming a GaN film in the present embodiment. A gas inlet 10a is formed at one end of the reaction tube 10, and a gas outlet 10b is formed at the other end. An induction coil 12 for heating is wound around the reaction tube 10.
Are connected to an AC power supply (not shown). A slope 14 for controlling the path of the inflow gas is disposed in the reaction tube 10, and a susceptor 16 is disposed on the slope 14. The susceptor 16 is formed of, for example, SiC / carbon, and generates heat by an alternating current flowing through the induction coil 12. A semiconductor substrate, for example, a GaAs substrate 18 is mounted on the susceptor 16.

In such a configuration, the gas inlet 10
Introducing H2 from a, AsH ₃ , TMG, D
MHy is introduced at a predetermined timing, and the temperature of the reaction tube 10 (more precisely, the temperature of the susceptor 16) is controlled by adjusting the alternating current supplied to the induction coil 12, and the GaAs epitaxial layer is sequentially formed on the GaAs substrate 18. , A GaN buffer layer and a GaN film are laminated to form a semiconductor device.

FIG. 2 shows a semiconductor device formed by the method of the present embodiment. A GaAs epitaxial layer 20 having a thickness of 100 to 200 nm is formed on a GaAs substrate 18, and a GaAs epitaxial layer 20 having a thickness of 10 to 200 nm is formed on the GaAs epitaxial layer 20.
A 0 nm GaN buffer layer (low temperature buffer layer) 22 is formed. Then, a 2 μm GaN film is formed on the GaN buffer layer 22. The semiconductor device of the present embodiment can also be used as a power FET or an optical device as in the related art, and can be incorporated in an OEIC (Opto-Electronic Integrated Circuit).
However, as described later, the GaN film 2 in this embodiment is
No. 4 is a cubic crystal having a very low hexagonal mixture ratio, so that it has excellent carrier mobility and can exhibit electric and optical characteristics higher than those of the prior art.

FIG. 3 shows the MOVPE in this embodiment.
A cubic GaN crystal growth diagram by the method is shown. First, 10% AsH ₃ was introduced, the temperature of the reaction tube was heated to 600 to 750 ° C., and AsH ₃ was introduced.
By introducing TMG of a group-based reaction gas, a GaAs epitaxial layer 20 is grown to 100 to 200 nm on the GaAs substrate 18 from which oxides have been removed by etching (process A). When the GaAs epitaxial layer 20 is formed, T
Stop introduction of MG and cool the reaction tube. And As
The introduction of H ₃ was stopped, and DMHy and T
Introducing MG, 10-20 at a temperature of 400-600 ° C
A GaN buffer layer (low temperature buffer layer) 22 of nm is formed (process B). After the GaN buffer layer 22 is formed, the introduction of TMG is stopped, and the temperature of the reaction tube is increased by heating the reaction tube. At the same time as heating the reaction tube, the introduction of TMG is stopped, and 10% of AsH ₃ is immediately introduced to raise the temperature to the growth temperature of the GaN epitaxial layer while holding the GaN buffer layer 22 (process C). Therefore, in the growth process of the GaN buffer layer 22, particularly in the initial stage, not only DMHy but also AsH ₃ exists as in the prior art, and in the presence environment of AsH ₃ , the GaN buffer layer 22 becomes a GaN epitaxial layer. The temperature is raised to the growth temperature.

The temperature of the reaction tube 10 rises, and a predetermined GaN
When the film growth temperature (800-950 ° C.) is reached, AsH ₃
Is stopped, TMG is introduced and reacted with DMHy to form a GaN film 24 having a thickness of 2 μm on the GaN buffer layer 22 (process D). If AsH ₃ gas remains when the GaN film 24 is formed, GaH may be generated when the GaN film is formed.
Since the NAs film is formed transiently and causes peeling of the GaN film 24, it is preferable to form the GaN film 24 after confirming that the AsH ₃ gas has been sufficiently exhausted. GaN film 24
After the formation of, the introduction of TMG and DMHy is stopped, and the reaction tube is cooled.

As described above, in the present embodiment, in the process C in the process of growing the GaN buffer layer 22, A containing As
By introducing the sH ₃ gas, the cubic GaN film 24 can be formed while suppressing the formation of hexagonal crystals in the GaN film 24. Note that the processes A, B, and D may be the same as the processes α, β, and δ of the related art, but in this embodiment, GaN
Since the formation of hexagonal crystals and the formation of stacking faults in the buffer layer 22 are already suppressed by AsH ₃ , especially in the process D, the reaction tube temperature and the ratio of the V / III group of the reactive gas are strictly different from those of the prior art. Need not be controlled, and the cubic GaN film 24 can be formed relatively easily.

FIG. 4 shows A introduced in process C.
The change in the hexagonal crystal mixture rate of the GaN film 24 when the concentration of sH ₃ is changed is shown. In the figure, the horizontal axis is A
The sH ₃ concentration (%) is shown, and the vertical axis is the hexagonal mixture ratio (%) of the GaN film 24. In the region where the concentration of AsH ₃ is low (0 to 5%), the mixture ratio is as large as 30% or more.
In the region where the concentration of AsH ₃ is high (20% or more), the mixture ratio is increased. And, the concentration of AsH ₃ is 5% to 20%.
%, The mixture ratio is suppressed to as low as 30% or less. In particular, when the concentration of AsH ₃ is 10%, a mixture ratio of 20% is obtained. As described above, the mixing ratio of hexagonal crystals depends on the concentration of AsH _{3. If} the concentration is too low, the dissociation of As from the GaAs epitaxial layer cannot be effectively prevented. Probably because it would hinder.
Therefore, the concentration of AsH ₃ is 5% to 20%,
More preferably, it is around 10%.

FIG. 5 shows A introduced in process C.
The change of the hexagonal mixture ratio of the GaN film 24 when the flow rate of sH ₃ is changed is shown. In the figure, the horizontal axis is 10
% Of AsH ₃ (sccm), and the vertical axis represents GaN.
This is the hexagonal mixture ratio (%) of the film 24. Note that process D
Of DMHy at 50 sccm, 10%
Flow rate 20sccm of AsH ₃ is, of 100% AsH ₃ 2s
ccm. As can be seen from the figure, 10% AsH
_At a flow rate of 100 to 250 sccm (corresponding to 1 to 2.5 sccm in 100% AsH ₃ ), a mixture rate of 30% or less is obtained. Therefore, AsH ₃ in process C
Is preferably set within the range shown in FIG.

FIG. 6 shows the time for introducing AsH ₃ in the process C and the change in the hexagonal mixture ratio of the GaN film 24. In the figure, the horizontal axis represents the introduction duration time (sec) when AsH ₃ is introduced immediately after heating, and the vertical axis represents the hexagonal mixture ratio (%) of the GaN film 24. If AsH ₃ is continued for about 100 sec to 250 sec, the hexagonal mixture ratio can be suppressed to 30% or less, and the introduction time of AsH ₃ is shorter than 100 sec or the introduction time is 250 sec.
If it is longer than sec, the hexagonal mixture ratio increases to 30% or more. The reason why the minimum mixing ratio is obtained when the introduction duration of AsH ₃ is about 150 sec is considered to be that it took about this time for 10% of AsH ₃ to sufficiently cover the surface of the GaN buffer layer 22.

FIG. 7 shows the X-ray reciprocal lattice mapping of the GaN film 24 formed by the method of the present embodiment.
In the figure, orthogonal x-axis and y-axis indicate 2θ and ω, respectively, and z-axis indicates intensity. For comparison, FIG. 8 shows an X-ray reciprocal lattice mapping of a GaN film formed by a conventional method. As is clear from the comparison between the two figures, the cubic (002) peak increases in the GaN film in this embodiment,
Conversely, it can be seen that the peak of the hexagonal crystal (1-101) decreases and the cubic crystal becomes dominant. Further, in the present embodiment, the hexagonal crystal has a peak at an ω value of about 10.790 degrees, whereas the conventional technique has a peak at about 10.010 degrees, which is a marked difference. Such a deviation of the ω value is as follows.
This is because the formation of a stable hexagonal crystal was suppressed by introducing a gas containing As.

As described above, the embodiment of the present invention
Although the case of forming on a substrate has been described, a semiconductor substrate other than a GaAs substrate, for example, a ZnO substrate can also be used. When a ZnO substrate is used, GaO
An N buffer layer may be formed, and a GaN film may be formed on the GaN buffer layer.

[0030]

As described above, according to the present invention,
Cubic GaN can be easily formed by suppressing the formation of hexagonal crystals.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram illustrating a device configuration of an embodiment.

FIG. 2 is a configuration diagram of a GaN film according to the embodiment.

FIG. 3 is a diagram illustrating a GaN film forming method according to an embodiment.

FIG. 4 is a graph showing the relationship between the concentration of AsH ₃ and the hexagonal mixture ratio in the embodiment.

FIG. 5 is a graph showing a relationship between a flow rate of AsH ₃ and a hexagonal crystal mixture rate in the embodiment.

FIG. 6 is a graph showing the relationship between the introduction time of AsH ₃ and the hexagonal mixture ratio in the embodiment.

FIG. 7 is an X-ray reciprocal lattice mapping diagram of the embodiment.

FIG. 8 is a prior art X-ray reciprocal lattice mapping diagram.

FIG. 9 is a diagram showing a conventional method of forming a GaN film.

[Explanation of symbols]

10 reaction tube, 12 induction coil, 14 slope, 1
6 susceptor, 18 GaAs substrate, 20 GaAs epitaxial layer, 22 GaN buffer layer, 24 GaN
Film (GaN epitaxial layer).

Continued on front page F term (reference) 4K030 AA05 AA11 BA08 BA35 BA38 BA55 BA56 BB02 BB13 CA04 FA04 JA06 LA15 LA16 5F045 AA04 AB10 AB14 AC01 AC08 AD08 AD09 AD10 AD11 AD12 AD13 AF04 DA53 DA61 DP04 DQ06 EE12 EK02 HA16

Claims (6)

[Claims] 1. A method for forming a GaN thin film on a semiconductor substrate, comprising: forming a GaN buffer layer on the semiconductor substrate by introducing a group V source gas and a group III source gas; Heating the GaN buffer layer to stop the introduction of the group III source gas and introduce a gas containing As to raise the temperature to the GaN epitaxial layer growth optimum temperature so as to form a GaN epitaxial layer on the GaN buffer layer; Forming a GaN epitaxial layer on the GaN buffer layer by introducing the group V source gas and the group III source gas after the heating step.
A GaN film forming method, comprising: a layer forming step. 2. The method according to claim 1, wherein the semiconductor substrate is a GaAs substrate. 3. The method according to claim 2, further comprising, prior to the buffer layer forming step, a GaAs layer forming step of epitaxially growing a GaAs layer on the semiconductor substrate.
A GaN film forming method characterized by being formed on an As layer. 4. The method according to claim 1, wherein the gas containing As is an arsine gas. 5. The method according to claim 4, wherein the arsine gas has a concentration of 5% to 20%. 6. A GaN film formed on a GaAs substrate, wherein a value of ω of a hexagonal crystal is about 1 in X-ray reciprocal lattice mapping.
A GaN film, wherein the cubic crystal is 0.790 degrees and the cubic crystal is dominant to the hexagonal crystal.