JPH0754806B2 - Method for growing compound semiconductor single crystal film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Use of the Invention] The present invention relates to vapor phase epitaxial growth of a compound semiconductor GaN single crystal film using sapphire (Al ₂ O ₃ ) as a substrate.
The present invention relates to a method for growing a high-quality GaN film with few misfit dislocations.

[Conventional technology]

Since a large bulk crystal cannot be grown on GaN, a method of vapor phase epitaxial growth on a sapphire substrate is generally used. However, the lattice mismatch between sapphire and GaN is large, for example, sapphire C-plane ((0001)
In the case of GaN growth on (plane), the C-axis ((000
1) A lattice mismatch of about 13.8% occurs with the (axis) -oriented GaN film. Due to this lattice mismatch, high-density misfit dislocations occur in the GaN film, and the film quality deteriorates.

Misfit dislocations decrease with an increase in the GaN film thickness, so the film quality can be improved by growing a thick film. However, if the thickness exceeds a certain level due to a large difference in thermal expansion coefficient between sapphire and GaN, cracking and further peeling occur, and it is practically impossible to grow a thicker film.
Therefore, there is a limit to improving the quality of GaN by thick film growth.

In heteroepitaxial growth with a large lattice mismatch, the quality of the epitaxial film can be improved by inserting a material with a physical constant intermediate between the substrate and the epitaxial film as a buffer layer. It is effective as a buffer layer. Metal Ga and NH ₃
In the reactive molecular beam epitaxy method using as a raw material, prior to GaN growth, a buffer layer of AlN is grown using metallic Al and NH ₃ as raw materials, so that the electrical characteristics (electron concentration, electron mobility) / optical Characteristics (light emission intensity of photoluminescence) can be improved. In addition, trimethylgallium (TM
Metal-organic vapor was an organic Ga compound and NH ₃ of Ga), or the like as a raw material phase epitaxy (MOVPE: Metal Organic Vapor Phase Epitaxy (Metal Organic vapor ph
ase epitaxy)) method, an AlN film is grown from an organic Al compound such as trimethylaluminum (TMAl) and NH ₃ as a raw material prior to the growth of GaN. The characteristics (light emission intensity of photoluminescence) can be improved. This is because AlN has a lattice constant intermediate between GaN and sapphire, so that lattice mismatch relaxation is more effective than when GaN is directly grown on sapphire.

However, even if the lattice constant of AlN takes an intermediate value between that of GaN and sapphire, that value is much closer to that of GaN, and therefore the lattice mismatch between sapphire and AlN is large. For example, in the case of growth on a sapphire C-plane substrate, a lattice mismatch of about 11.7% occurs between sapphire and AlN. Therefore, with this method, sapphire / AlN
The occurrence of misfit dislocations at the film interface cannot be suppressed,
The dislocations generated there propagate to the GaN film and deteriorate the quality of the GaN film.

Also, comparing the coefficients of thermal expansion in the C plane, GaN5.59 × 10
^-6 K ^-1 , sapphire 7.3 × 10 ^-6 K ^-1, whereas AlN is 5.3 × 10
^-6 K ^-1 and the intermediate value of both are not taken. Therefore, in the case of growth on the C-face of sapphire, the AlN buffer layer has no effect of relaxing thermal strain.

Thus, the quality improvement of the GaN film by the AlN buffer layer is also insufficient.

[Problems to be solved by the invention]

As described in detail above, when an attempt is made to vapor-phase epitaxially grow a compound semiconductor GaN single crystal film on a sapphire substrate, since the lattice mismatch between the two is large, high density misfit dislocations occur in the GaN film, The film quality deteriorates. To alleviate this lattice mismatch, an AlN buffer layer with an intermediate lattice constant is inserted between the two, but the lattice constant of AlN is much closer to that of GaN and is A lattice mismatch still occurs with AlN. Further, the thermal expansion coefficient of AlN does not take an intermediate value between that of sapphire and GaN, so the AlN buffer layer cannot relax the thermal strain. Thus, according to the conventional technology, high-quality compound semiconductor GaN is formed on the sapphire substrate.
There is a problem that a single crystal film cannot be formed.

An object of the present invention is to provide a method for forming a high-quality GaN film by relaxing the lattice mismatch between sapphire and GaN and thermal strain in order to solve the above problems.

[Means for solving problems]

In order to solve the above problems, the compound semiconductor single crystal film growth method of the present invention forms a surface nitride layer by heat-treating a sapphire substrate surface in an N source gas atmosphere as a pretreatment step of the GaN film growth step. The greatest feature is to include the step of performing. More specifically, the present invention relates to a method for vapor phase epitaxial growth between compound semiconductor GaN single crystals on a sapphire substrate by heat treating the sapphire substrate in a source gas atmosphere containing N to form a nitride layer on the surface of the sapphire substrate. Is formed, and a second step of growing a GaN film on the nitride layer by a reaction of a source gas containing Ga and a source gas containing N is included.

[Action]

According to the above configuration of the present invention, N (nitrogen) atoms diffuse into the substrate during the heat treatment performed before the GaN film growth step, and O (oxygen) atoms diffuse from the substrate, so that Al ₂ O
A surface nitriding layer with a continuous composition change from ₃ (sapphire) to AlN can be formed. Due to this continuous compositional change, generation of dislocations is suppressed, and therefore propagation of dislocations to the GaN film can be significantly reduced.

Further, in this surface nitrided layer, the coefficient of thermal expansion also continuously changes between the values of sapphire and AlN. Since the coefficient of thermal expansion of AlN is much closer to that of GaN than that of sapphire, the coefficient of thermal expansion of the surface nitrided layer takes an intermediate value between that of sapphire and GaN. Therefore, it is also effective in alleviating thermal strain.

〔Example〕

Example 1 FIG. 1 is a diagram showing an example of an apparatus for carrying out the growth method of the present invention when an organic gallium compound is used as a Ga source gas. In the figure, 1 is a sapphire substrate, 2
Is a SiC coated carbon susceptor, 3 is a quartz reaction tube, 4
Is a high frequency induction coil, 5 is a thermocouple, 6 is a Ga raw material gas introduction pipe, 7 is a N raw material gas introduction pipe, 8 is a H ₂ gas introduction pipe, and 9 is an exhaust port.

In order to grow a GaN film using this apparatus, first, the inside of the quartz reaction tube 3 is made a high vacuum of 10 ⁻⁶ Torr or less by a vacuum evacuation device (not shown) such as a turbo molecular pump. Next, for the purpose of cleaning the surface of the substrate 1, by energizing the high frequency induction coil 4, the carbon susceptor 2
Is heated to 1100 to 1200 ° C. and kept for 10 to 30 minutes in the H ₂ gas atmosphere introduced from the introduction pipe 8 (H ₂ gas is continuously introduced from the introduction pipe 8 and exhausted from the exhaust port 9). . Then, with the susceptor temperature kept constant, the introduction of H ₂ gas was stopped and the NH
₃ gas is introduced through the inlet pipe 7 and held for 2 to 10 minutes (NH ₃
Gas is continuously introduced through the introduction pipe 7 and exhausted through the exhaust port 9. ) Thereby nitriding the sapphire surface. After the above substrate pretreatment, the susceptor temperature is set to a growth temperature of 900 to 1000 ° C. 2-10cc generated by bubbling TMGa liquid kept at -12 ℃ in addition to 1.0-2.5 / min NH ₃ flow.
/ min Ga gas is mixed with 1.0 to 2.0 / min N ₂ gas and introduced through the introduction pipe 6, thereby growing a GaN single crystal film. During the growth, a rotary pump or the like is used and the inside of the reaction tube 3 is kept under a reduced pressure of 0.1 atm.

FIG. 2 shows the electron concentration n of the GaN film thus obtained.
Is plotted against the film thickness d.
The figure shows the result of plotting the electron mobility μ of the GaN film thus obtained against the film thickness d. In these figures, ○ and ● are the sapphire C-plane substrate and R, respectively.
The result when a plane ((012) plane) substrate is used is shown. 2 and 3, the results on the sapphire C-face and R-face when surface nitriding is not performed together are shown by Δ and ▲ marks for comparison.

When the surface nitriding is not performed, the electron concentration n and the electron mobility μ of the GaN film show almost no film thickness dependence.
When surface nitriding is performed, the electron concentration n sharply decreases and the electron mobility μ sharply increases as the film thickness increases. About 5μ
When compared with a relatively thin film thickness of m, n obtained by the present invention is about 6 × 10 ¹⁸ cm ⁻³ (on the C plane), 2.5 × 10 ¹⁸ cm ⁻³ (R).
(On the surface), μ is about 110 cm ² / Vs (on the C surface), and 70 cm ² / Vs (on the R surface) exceed the results reported so far, including growth through the AlN buffer layer. It is a thing.

The origin of the donor in the GaN film is N at the growth temperature.
These are N vacancies caused by high vapor pressure. 2 and 3
The results shown in the figure show that N vacancies decrease with an increase in film thickness, that is, relaxation of lattice mismatch, and that this effect is more remarkable when surface nitriding is performed. . This is due to the presence of the sapphire surface nitride layer.
This is because the dislocation propagation to the N film is greatly suppressed.

In this example, trimethylgallium was used as the Ga source gas, but triethylgallium may be used instead of this, and similar results are obtained. Also, instead of NH ₃ as N source gas, N
Similar results are obtained with ₂ H ₄ . Further, in this example, the N ₂ carrier gas was used, but if the initial nucleation stage growth is performed in the N ₂ carrier gas, the same effect can be obtained even if the subsequent process is switched to the H ₂ carrier gas.

Example 2 FIG. 4 is a diagram showing an example of an apparatus for carrying out the growth method of the present invention when gallium halide is used as a Ga source gas. In the figure, 10 is a sapphire substrate, 11 is an operating rod, 12 is a quartz reaction tube, 13 is a resistance heater, 14 is a thermocouple, 15 is a hydrogen halide introduction tube, and 16 is a metal.
Ga and 17 are N source gas introduction pipes, 18 is H ₂ gas and N ₂ gas introduction pipes, and 19 is an exhaust port.

In order to grow a GaN film using this apparatus, first, the inside of the quartz reaction tube 12 is made a high vacuum of 10 ⁻⁶ Torr or less by a vacuum evacuation device such as a turbo molecular pump. Then the substrate
For the purpose of cleaning the surface of 10, the resistance heater 13 is energized to heat the substrate to 1100 to 1200 ° C.
Hold for 10 to 30 minutes in the H ₂ gas atmosphere introduced from. Then, with the substrate temperature kept constant, the introduction of H ₂ gas was stopped, and NH ₃ was added.
A sapphire surface is nitrided by introducing gas through the introduction pipe 17 and holding it for 2 to 10 minutes. After the above substrate pretreatment,
Set the substrate temperature to 900-1000 ℃ growth temperature, 1.0-2.5
In addition to the NH ₃ flow of / min, N ₂ gas of 7 / min is supplied from the introduction pipe 18. In this state, 5 to 20cc / min of HC from the inlet pipe 15
l is introduced and reacted with Ga16 heated and held by the resistance heater 13 at 800 to 900 ° C., and the generated gallium chloride gas is supplied onto the substrate to grow a GaN single crystal film. The pressure in the reaction tube during growth is maintained at normal pressure.

In this example, gallium chloride was used as the Ga source gas,
Instead of this, the same result can be obtained by using gallium fluoride or other gallium halide such as gallium bromide. The same effect can be obtained by using N ₂ H ₄ instead of NH ₃ as the N source gas.

〔The invention's effect〕

As described above, according to the present invention, since it is possible to form a surface nitride layer whose composition continuously changes from Al ₂ O ₃ to AlN, propagation of dislocations to the GaN film is suppressed, and the GaN film is suppressed. It is possible to improve the quality of. Surface nitride layer is sapphire and GaN
In addition to having the effect of alleviating the thermal strain due to the difference in the thermal expansion coefficient of the above, the growth method of the present invention can obtain a good film with a much thinner film thickness than the conventional method, resulting in higher quality. The film can be obtained more stably and with high yield. In addition, the growth method of the present invention also has an advantage that it is not necessary to use an Al raw material that easily causes deterioration due to oxidation, unlike the conventional case, in order to form the buffer layer.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an example of an apparatus used in the method for growing a compound semiconductor single crystal film of the present invention when an organic gallium compound is used as a Ga source gas, and FIG. 2 is a case where surface nitriding is performed ( The changes in the electron concentration n of the GaN film with respect to the film thickness d are shown in the case of ○ mark: on the C plane, ● mark: on the R face) and when not performed (Δ mark: on the C face, ▲ mark: on the R face). Figures and 3 show Ga when surface nitriding was performed (○ mark; on C plane, ● mark; on R plane) and when it was not performed (△ mark; on C plane, ▲ mark; on R plane).
FIG. 4 is a diagram showing a change of the electron mobility μ of the N film with respect to the film thickness d. FIG. 4 shows an apparatus used in the method for growing a compound semiconductor single crystal film of the present invention when gallium halide is used as a Ga source gas. It is a figure which shows the structure of an example. 1, 10 ... Sapphire substrate 2 ... Carbon susceptor 3, 12 ... Quartz reaction tube 4 ... High frequency induction coil 5, 14 ... Thermocouple 6 ... Ga source gas introduction tube 7, 17 ... N source gas Introducing pipe 8 …… H ₂ gas introducing pipe 9, 19 …… Exhaust port 11 …… Operating rod 13 …… Resistance heater 15 …… Hydrogen halide introducing pipe 16 …… Metal Ga 18 …… H ₂ gas and N ₂ Gas inlet pipe

Claims (3)

[Claims] 1. A method of vapor phase epitaxially growing a compound semiconductor GaN single crystal film on a sapphire substrate, comprising:
The first step of forming a nitride layer on the surface of the sapphire substrate by heat-treating the sapphire substrate in a raw material gas atmosphere containing, and the reaction of the raw material gas containing Ga and the raw material gas containing N with the nitride layer Second GaN film growth on top
And a step of growing a compound semiconductor single crystal film. 2. The method for growing a compound semiconductor single crystal film according to claim 1, wherein the source gas containing Ga is an organic gallium compound or gallium halide. 3. The source gas containing N is NH ₃ or N ₂ H
_{4. The} method for growing a compound semiconductor single crystal film according to claim 1, wherein the method is 4.