JP2008303138A - GaN SINGLE-CRYSTAL SUBSTRATE, NITRIDE TYPE SEMICONDUCTOR EPITAXIAL SUBSTRATE AND NITRIDE TYPE SEMICONDUCTOR DEVICE - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a GaN single crystal substrate having a flat surface, a nitride-based semiconductor epitaxial substrate, a nitride-based semiconductor device and method for manufacturing them. <P>SOLUTION: The GaN single-crystal substrate 11 has a polished surface subjected to heat treatment for at least 10 minutes at a substrate temperature of ≥1,020°C in a mixed gas atmosphere containing at least an NH<SB>3</SB>gas. As a consequence, an atomic rearrangement is effected in the surface of the substrate 11 in which a large number of minute defects are formed by polishing, so as to flatten the surface of the substrate 11. Therefore, the surface of the GaN single crystal substrate 11 exhibits a root-mean-square roughness of ≤0.2 nm and has steps and terraces corresponding to one atomic layer, such that the surface of a GaN epitaxial layer 12 formed on the substrate 11 can be made flat. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a GaN single crystal substrate, a nitride semiconductor epitaxial substrate, and a nitride semiconductor element for use in a light emitting device or the like.

  In recent years, light-emitting devices using nitride-based compound semiconductors have attracted attention because they can emit light with a short wavelength, such as ultraviolet to blue-green. These devices such as light-emitting diodes and laser diodes are expected as illumination, display devices, and light sources for next-generation DVDs. As a substrate used in these light emitting devices, it is preferable to use a GaN single crystal substrate having a lattice constant that is the same as that of the main nitride compound semiconductor layer. However, conventionally, it has been considered difficult to manufacture a GaN single crystal substrate.

Therefore, normally, a sapphire substrate having a lattice constant close to that of GaN and chemically stable is used. As a method for epitaxially growing a GaN layer on such a sapphire substrate, the OMVPE method is usually used. In this OMVPE method, the surface temperature of the sapphire substrate is maintained at about 1050 ° C. in an H ₂ gas atmosphere, and then a buffer layer of GaN or AlN is grown at a substrate temperature of about 450 to 600 ° C. The GaN layer is grown at a high temperature of 1000 ° C. or higher.

  However, the use of a sapphire substrate has the following problems. First, since the sapphire substrate has a lattice constant similar to that of the GaN layer but does not match, a large number of defects such as dislocations due to lattice mismatch are introduced at the interface between the sapphire substrate and the GaN layer. This defect extends in the growth direction and appears as a large number of through defects on the surface of the epitaxial layer, and remarkably deteriorates the characteristics and lifetime of a light emitting device such as a laser diode. In addition, since the thermal expansion coefficient of the sapphire substrate and the thermal expansion coefficient of the GaN layer are greatly different, a large warp occurs in the substrate after epitaxial growth. Furthermore, since the sapphire substrate does not have a cleavage property, it is extremely difficult to manufacture a laser diode having a cleavage surface as a reflection surface.

In view of such a situation, a single crystal GaN substrate suitable for forming a nitride-based compound semiconductor layer has been realized (International Publication No. WO99 / 23693). According to this method, a GaN substrate can be obtained by forming a mask having a stripe or a circular shape on a GaAs substrate, vapor-depositing a GaN layer thereon, and then removing the GaAs substrate. Further, by this method, a GaN layer is further grown on the GaN substrate to produce an ingot, and the GaN substrate is cut out from the ingot, whereby the GaN substrate can be mass-produced. That is, this new method enables mass production of GaN single crystal substrates.
International Publication Number WO99 / 23693

  However, the conventional GaN substrate described above has the following problems. That is, in order to form an epitaxial layer on the produced GaN single crystal substrate, it is necessary to mechanically polish and flatten the substrate surface, but the GaN single crystal substrate is chemically very stable. It is difficult to polish by chemical mechanical polishing (CMP) used in other semiconductor substrates. Therefore, it is difficult to flatten the substrate surface after mechanical polishing so as to be suitable for epitaxial growth, and Rms (average square root roughness) of a typical substrate surface after mechanical polishing is about 1.0 nm. As described above, when an epitaxial layer is formed on a substrate having a rough surface, three-dimensional growth due to random nucleation in the concavo-convex portion occurs, so that it is difficult to obtain a flat surface. Also, in such a growth mode, when the generated growth nuclei coalesce, crystal defects such as dislocations are generated due to the slight misorientation of crystals existing between the nuclei, which causes deterioration of crystallinity. It becomes. That is, in order to form a higher quality semiconductor device on a GaN single crystal substrate, it is necessary to remove defects (for example, scratches, strains, etc.) associated with surface processing.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a GaN single crystal substrate, a nitride-based semiconductor epitaxial substrate, a nitride-based semiconductor element, and a method for manufacturing the same, having a planarized surface. And

The GaN single crystal substrate according to the present invention is characterized in that the polished surface is heat-treated at a substrate temperature of 1020 ° C. or more for 10 minutes or more in a mixed gas atmosphere containing at least NH ₃ gas, and the surface is flattened. And

In this GaN single crystal substrate, in a NH ₃ gas atmosphere, the substrate temperature is set to 1020 ° C. or higher and a predetermined heat treatment is performed for 10 minutes or more, whereby atoms on the substrate surface on which many fine defects are formed by polishing. Rearrangement is performed and the substrate surface is flattened. Therefore, the surface of the epitaxial layer formed on this substrate can be flattened.

  Moreover, the average square root roughness of the surface is 0.2 nm or less by the heat treatment. Thus, when the mean square root roughness of the substrate surface is 0.2 nm or less, the substrate is sufficiently flat to form a good-quality epitaxial layer.

  A nitride semiconductor epitaxial substrate according to the present invention includes the GaN single crystal substrate and a nitride compound semiconductor layer epitaxially grown on the GaN single crystal substrate.

In this nitride semiconductor epitaxial substrate, a nitride compound semiconductor layer is formed on a GaN single crystal substrate that has been subjected to a predetermined heat treatment for 10 minutes or more in an NH ₃ gas atmosphere at a substrate temperature of 1020 ° C. or higher. ing. That is, since the nitride compound semiconductor layer is epitaxially grown on a substrate that is sufficiently flat to form an epitaxial layer, a nitride compound semiconductor layer having a flat surface and good crystallinity can be obtained. . Further, since the surface of the semiconductor layer laminated on the nitride-based compound semiconductor layer is also flat and has good crystallinity, a light-emitting device or a semiconductor element such as a transistor using the nitride-based semiconductor epitaxial substrate is used. High performance and high yield can be achieved.

Further, the nitride compound semiconductor layer is preferably Al _x Ga _y In _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1).

  The nitride compound semiconductor layer is preferably GaN. In this case, since there is no mismatch with the substrate, generation of defects at the interface between the substrate and the epitaxial layer can be suppressed.

  Further, the mean square root roughness of the surface of the nitride-based compound semiconductor layer epitaxially grown on the GaN single crystal substrate is preferably 0.2 nm or less. In this case, a nitride compound semiconductor layer having a flat surface and good crystallinity can be obtained. Further, when a desired semiconductor is further epitaxially grown on the nitride-based compound semiconductor layer, the epitaxial growth can be performed in a state where the steepness and crystallinity of the stacked structure are good. Thereby, a semiconductor layer having a flat surface can be formed.

  Moreover, it is preferable that the half width of the X-ray diffraction of the nitride-based compound semiconductor layer is 100 seconds or less. That is, the half width of the X-ray diffraction shows the fluctuation (mosaic property) of the crystal axis of the epitaxial layer. If this is 100 seconds or less, the nitride compound semiconductor layer having a flat surface and good crystallinity Can be obtained. Further, the crystallinity of the semiconductor layer further stacked on the epitaxial layer is improved.

Further, the threading dislocation density of the nitride-based compound semiconductor layer is preferably 1 × 10 ⁶ cm ⁻² or less. That is, when the threading dislocation density of the nitride-based compound semiconductor layer is 1 × 10 ⁶ cm ⁻² or less, the threading dislocation in the semiconductor layer further stacked on the nitride-based compound semiconductor layer (epitaxial layer). Density can be suppressed.

In the nitride-based semiconductor device according to the present invention, the GaN single crystal substrate has n-type conductivity, and n is made of Al _x Ga _1-x N (0 <x <1) on the substrate. A type cladding layer is laminated, an active layer is laminated on the cladding layer, and a p-type cladding layer made of Al _x Ga _1-x N (0 <x <1) is further laminated on the active layer, A p-type GaN layer is laminated on the p-type cladding layer.

  In this nitride-based semiconductor device, the crystallinity is improved by laminating a clad layer and an active layer on a GaN single crystal substrate having a flat surface. Therefore, a laser diode having high luminous efficiency and long life. An element can be obtained.

The nitride-based semiconductor device according to the present invention is represented by Al _x Ga _y In _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1) on the GaN single crystal substrate. And a plurality of nitride-based semiconductor layers.

  In this nitride-based semiconductor element, a collector layer, a base layer, and an emitter layer made of a nitride-based semiconductor layer represented by, for example, AlGaInN are sequentially formed on a GaN single crystal substrate having a flat surface. The crystallinity is improved, so that a transistor with a high current gain can be obtained.

  The GaN single crystal substrate according to the present invention is characterized in that the mean square root roughness of the surface is 0.2 nm or less.

  Thus, when the mean square root roughness of the substrate surface is 0.2 nm or less, the substrate is sufficiently flat to form a good-quality epitaxial layer.

The method for producing a GaN single crystal substrate according to the present invention includes subjecting a GaN single crystal substrate having a polished surface to heat treatment in a mixed gas atmosphere containing at least NH ₃ gas at a substrate temperature of 1020 ° C. or more for 10 minutes or more. The average square root roughness of the surface of the single crystal substrate is 0.2 nm or less.

In this method for manufacturing a GaN single crystal substrate, the GaN single crystal substrate is subjected to a predetermined heat treatment at a substrate temperature of 1020 ° C. or higher in an NH ₃ gas atmosphere. Thereby, atomic rearrangement is performed on the substrate surface on which many fine defects are formed by polishing, and the substrate surface is flattened. Therefore, the surface of the epitaxial layer formed on this substrate can be flattened. Further, if a single crystal layer different from this is grown on such a flat epitaxial surface and a heterojunction is formed, this junction interface becomes flat, and the element formed by such a junction becomes a junction with a non-flat interface. Its characteristics are higher than those of the element having

The mixed gas preferably contains H ₂ gas. That is, when the H ₂ gas generated by the decomposition of the NH ₃ gas is insufficient, the insufficient H ₂ gas is replenished.

  The method for producing a nitride-based semiconductor epitaxial substrate according to the present invention provides a nitride-based compound on the GaN single-crystal substrate without oxidizing the surface of the GaN single-crystal substrate obtained by the method for producing a GaN single-crystal substrate. The semiconductor layer is epitaxially grown.

  In this method for manufacturing a nitride-based semiconductor epitaxial substrate, since the GaN single crystal substrate having a flat surface is not oxidized, it is not necessary to perform reprocessing such as heat treatment when forming an epitaxial layer on the substrate. Thereby, the manufacturing process of an epitaxial substrate can be simplified.

  The nitride compound semiconductor layer is preferably n-type. In this way, when the epitaxial layer is n-type, if the substrate is n-type, a light-emitting element in which an n-type semiconductor, an active layer, and a p-type semiconductor are stacked in order, an npn-type bipolar transistor element, and the like can be manufactured. .

  The nitride-based compound semiconductor layer is preferably p-type. Thus, when the epitaxial layer is p-type, if the substrate is made p-type, a light-emitting element, a pnp-type bipolar transistor element, or the like in which a p-type semiconductor, an active layer, and an n-type semiconductor are stacked in this order can be manufactured. .

  In addition, it is preferable to use one of the OMVPE method (metal organic vapor phase epitaxy), the HVPE method (hydride vapor phase epitaxy), and the MBE method (molecular beam epitaxy method) for the epitaxial growth. In this case, a good epitaxial layer can be formed on the substrate.

  Further, it is preferable to perform both the heat treatment and epitaxial growth of the GaN single crystal substrate in an apparatus for performing epitaxial growth. In this case, an epitaxial layer with good crystallinity can be grown with the substrate surface kept clean.

In the method for manufacturing a nitride semiconductor device according to the present invention, the GaN single crystal substrate obtained by the method for manufacturing a GaN single crystal substrate has n-type conductivity, and Al _x Ga _{1 is formed} on the substrate. _An n-type cladding layer made of -xN (0 <x <1) is stacked, an active layer is stacked on the cladding layer, and Al _x Ga _1-x N (0 <x <1) is further stacked on the active layer. P-type cladding layer is laminated, and a p-type GaN layer is laminated on the p-type cladding layer.

  In this method of manufacturing a nitride-based semiconductor device, the crystallinity is improved by laminating a clad layer and an active layer on a GaN single crystal substrate having a flat surface, so that the light emission efficiency is high and the lifetime is long. A certain laser diode element can be obtained.

The method for manufacturing a nitride-based semiconductor device according to the present invention includes Al _x Ga _y In _1-xy N (0 ≦ x ≦ 1) on a GaN single crystal substrate obtained by the method for manufacturing a GaN single crystal substrate. , 0 ≦ y ≦ 1, x + y ≦ 1), and a plurality of nitride-based semiconductor layers are stacked.

  In this nitride semiconductor device manufacturing method, a collector layer, a base layer, and an emitter layer made of a nitride semiconductor layer represented by, for example, AlGaInN are sequentially formed on a GaN single crystal substrate having a flat surface. This improves the crystallinity and improves the flatness of the heterojunction interface. Therefore, a transistor with a high current gain can be obtained.

  According to the present invention, the surface roughness of a GaN single crystal can be reduced, and a GaN single crystal substrate suitable for epitaxial growth can be provided. Further, a nitride compound semiconductor element exhibiting good characteristics can be formed by epitaxially growing a nitride compound semiconductor layer on the GaN single crystal substrate of the present invention.

  Hereinafter, preferred embodiments of a GaN single crystal substrate, a nitride-based semiconductor epitaxial substrate, a nitride-based semiconductor device, and a manufacturing method thereof according to the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a cross-sectional view of a nitride-based semiconductor epitaxial substrate according to this embodiment. This nitride-based semiconductor epitaxial substrate 10 includes a nitride-based compound semiconductor layer 12 that is epitaxially grown on a GaN single crystal substrate 11 by OMVPE, HVPE, MBE, or the like. The nitride-based semiconductor epitaxial substrate 10 is an intermediate for manufacturing light-emitting devices such as light-emitting diodes and laser diodes, and forms an appropriate pn junction, more preferably a double heterojunction, and even more preferably a quantum well structure. A light emitting device is completed by attaching an electrode for supplying current.

The material of the nitride-based compound semiconductor layer 12 is a 2-quaternary compound represented by Al _x Ga _y In _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1). Selected from semiconductors. Among them, since GaN can be directly homoepitaxially grown on the GaN single crystal substrate 11 and there is no mismatch with the GaN single crystal substrate 11, generation of defects at the interface between the substrate 11 and the nitride-based compound semiconductor layer 12 can be suppressed. Most preferred in terms.

Next, the manufacturing process of the GaN single crystal substrate and the nitride semiconductor epitaxial substrate 10 will be described.
(A) First, the GaN single crystal substrate 11 is manufactured, the surface of the manufactured GaN single crystal substrate 11 is polished using an abrasive, and liquid cleaning is performed using pure water or the like (single crystal substrate manufacturing process).
(B) Next, the GaN single crystal substrate 11 is placed in an atmosphere of a predetermined mixed gas G1 containing NH ₃ gas and heated at the substrate temperature T1 for a time t1 (surface heat treatment step).
(C) Thereafter, the raw material G2 of the nitride compound semiconductor layer 12 is supplied to the surface in an overheated state at the substrate temperature T2, and the nitride compound semiconductor layer 12 is epitaxially grown on the GaN single crystal substrate 11 (epitaxial growth). Process).

  The details will be described below.

  The manufactured GaN single crystal substrate 11 is subjected to surface polishing using an abrasive and is subjected to liquid cleaning using pure water or the like. For this liquid cleaning, an organic solvent, an acid, or an alkaline solution may be used in addition to pure water. A work-affected layer due to mechanical polishing damage exists on the surface of the manufactured GaN single crystal substrate 11, and this is removed by a suitable surface treatment. At this point, the surface of the GaN single crystal substrate 11 is flattened and is in a mirror state, but when observed with a microscope, fine scratches due to mechanical polishing were confirmed on the surface of the substrate 11. As a representative example, a surface image of the polished GaN single crystal substrate 11 observed with an atomic force microscope is shown in FIG. As shown in the figure, many fine defects due to polishing are observed on the surface of the substrate 11. The average square root roughness (Rms) of this surface was about 1.0 nm.

  Therefore, when the nitride-based compound semiconductor layer 12 is directly epitaxially grown on the surface of the substrate 11 having such a roughness, a large number of random crystal nuclei are generated at a stepped portion such as a scratch, resulting in three-dimensional crystal growth. Therefore, it is difficult to obtain the nitride compound semiconductor layer 12 having a flat surface.

  Next, the surface heat treatment step will be described.

On the surface of the substrate 11, it is considered that the reduction of the surface roughness by the heat treatment proceeds in the following process. That is, as schematically shown in FIG. 3A, first, when NH ₃ gas is supplied onto the substrate 11, NH ₃ is decomposed into N ₂ and H ₂ . The decomposed H ₂ reacts with GaN to generate Ga atoms. These reactions are represented by the following formulas (1) and (2).

Then, Ga atoms migrate on the surface of the substrate 11 at a high temperature and gather in the recesses so as to lower the surface energy. Thereafter, as shown in FIG. 3B, Ga atoms react with NH ₃ to form GaN. By such atomic rearrangement, the recesses on the surface of the substrate 11 are filled, and a flat surface is obtained (see FIG. 3C). This reaction is represented by the following formula (3).

  FIG. 4 shows a typical surface observation image of a GaN single crystal substrate flattened by the heat treatment as described above. The substrate 11 shown in FIG. 4 is a substrate that has been heat-treated in a mixed gas G1 atmosphere under conditions of a substrate temperature (T1) of 1020 ° C. and a time (t1) of 10 minutes. Rms on the substrate surface was about 0.19 nm, and a step and terrace corresponding to one atomic layer were observed. That is, by such heat treatment, Rms on the surface of the GaN single crystal substrate 11 could be reduced to 0.2 nm or less.

In the above-described process, it is important that GaN is decomposed in the presence of H ₂ to form Ga atoms as shown in the formula (2). However, the reaction rate of the formula (1) is very slow, and even at 1000 ° C., only a few percent of NH ₃ is decomposed into H ₂ and N ₂ . Since this amount of H ₂ is not sufficient to cause the surface heat treatment process to proceed, it is preferable to supplement the mixed gas G1 by adding H ₂ gas. Therefore, the surface heat treatment step is preferably performed in a mixed gas atmosphere of NH ₃ and H ₂ gas.

In the surface heat treatment step, the heat treatment temperature affects the speed of each reaction described above, the migration length of Ga atoms, the desorption speed of Ga atoms, and the like. That is, since the migration distance of Ga atoms becomes longer as the substrate temperature T1 is higher, Ga atoms easily reach the polishing scratches (recesses). On the other hand, when the substrate temperature T1 is low, GaN is generated by reacting with NH ₃ before Ga atoms reach the polishing scratches, and nuclei are generated outside the recesses, so the surface is not flattened. Assuming that the surface is flattened by such a mechanism, the present inventors have intensively studied a temperature suitable for it, and as a result, the present inventors have found that the substrate temperature T1 is preferably 1020 ° C. or higher. Therefore, in the surface heat treatment step, the substrate temperature T1 is preferably 1020 ° C. or higher.

  Furthermore, the time required for planarizing the surface of the substrate 11 is the time required for the polishing scratches to be sufficiently filled by the migration of Ga atoms and the generation of GaN. This depends on the polishing state of the substrate 11, but the surface of the substrate 11 can be flattened by performing heat treatment for 10 minutes or more when the surface roughness is generally about 1.0 nm, which is finished by mechanical polishing. The present inventors have found that. That is, the heat treatment time is preferably 10 minutes or more.

  Next, the epitaxial growth process will be described.

The GaN single crystal substrate 11 whose surface has been flattened by the heat treatment as described above is made of the same kind of material as GaN, that is, Al _x Ga _y In _1-xy N (0 ≦ x ≦ 1, 0 ≦ The present invention is very suitable for directly epitaxially growing a 2-quaternary compound semiconductor represented by y ≦ 1, 0 ≦ x + y ≦ 1). As shown in FIG. 4, terraces and steps corresponding to one atomic layer are regularly arranged on the surface of the substrate 11, and if the nitride compound semiconductor layer 12 has a close lattice constant, the step flow growth can be easily performed. This is because it can be done.

  Further, it is desirable not to expose the GaN single crystal substrate 11 to the atmosphere after the heat treatment and before epitaxial growth. That is, when exposed to the atmosphere, the surface of the substrate 11 is oxidized and organic substances and other pollutants are adsorbed, which adversely affects the subsequent epitaxial growth. In this case, surface treatment for cleaning the surface of the substrate 11 again must be performed before epitaxial growth, which increases the number of processes. This means that a high-quality epitaxial substrate 10 can be manufactured by epitaxially growing the nitride-based compound semiconductor layer 12 on the GaN single crystal substrate 11 after the heat treatment. In particular, when the nitride compound semiconductor layer 12 is GaN, the lattice constant coincides with that of the GaN single crystal substrate 11, so that no misfit dislocation occurs at the interface, and the crystal of the nitride compound semiconductor layer 12 is generated. This is preferable because there is no risk of deterioration of the properties.

  As described above, when an extremely flat epitaxial substrate 11 having a surface Rms of 0.2 nm or less is manufactured as an index of high quality of the epitaxial layer 12, the surface flatness is maintained in the subsequent growth of the device structure. It is advantageous and effective in improving the yield of light emitting elements such as light emitting diodes and laser diodes, and electronic devices such as bipolar transistors and field effect transistors.

  Moreover, the crystal axis fluctuation of the epitaxial layer 12 can be reduced by performing the surface heat treatment step described above. More specifically, the full width at half maximum of X-ray diffraction of the nitride-based compound semiconductor layer 12 is 100 seconds or less. That is, the surface of the epitaxial substrate 12 becomes flat, and an epitaxial substrate with good crystallinity can be obtained.

Furthermore, the threading dislocation density of the epitaxial layer 12 can be suppressed to 1 × 10 ⁶ cm ⁻² or less by performing the surface heat treatment step described above. Thereby, the threading dislocation density in the semiconductor layer laminated on the epitaxial layer 12 is suppressed.

  As described above, the crystallinity of the epitaxial layer 12 is improved and the crystal defects on the surface are reduced, so that the crystallinity of the semiconductor layer stacked on the epitaxial layer 12 is improved, and the semiconductor layer in the semiconductor layer Generation of crystal defects can be suppressed. Therefore, the use of the epitaxial substrate 10 on which the epitaxial layer 12 is formed means that the characteristics, reliability, and yield of a light emitting element such as a light emitting diode or a laser diode, an electronic device such as a bipolar transistor or a field effect transistor, etc. It is effective to improve.

  The conductivity of an epitaxial layer (not shown) formed on the epitaxial substrate 10 can be controlled in accordance with the structure of various conductor elements. For example, in the case of a light emitting device such as an LED or LD, an epitaxial substrate 10 in which an n-type GaN layer 12 is grown on an n-type single crystal substrate 11 is manufactured, and an n-type cladding layer, an active layer, It can be manufactured by growing a basic structure of a p-type cladding layer and a p-type contact layer. Thereafter, electrodes are formed, current terminals are connected, and if it is a laser diode, it goes without saying that the element is completed through processes such as forming a reflecting surface.

  Further, for example, in the case of an npn type bipolar transistor element, a combination of an n type GaN layer 12 and an n type GaN substrate 11 is preferable, and in the case of a field effect transistor, a semi-insulating GaN substrate is preferable. .

 The growth method used for epitaxial growth can be selected from the OMVPE method, HVPE method, MBE method and the like. Regardless of which is selected, the GaN single crystal substrate 11 is heat-treated in the growing apparatus, and then the nitride-based compound semiconductor layer 12 is epitaxially grown as it is without taking out the GaN single crystal substrate 11 to the outside. 11 The surface is not contaminated. Therefore, a surface treatment process required when the surface is oxidized or contaminated is not necessary, and a high-quality epitaxial substrate can be easily manufactured.

  The GaN substrate 11 was heat-treated under various conditions in the OMVPE apparatus. Further, the GaN layer 12 was epitaxially grown in the same apparatus. As shown in FIG. 5, the OMVPE apparatus used is a vertical growth furnace that injects a raw material gas from a direction perpendicular to the substrate surface. The growth furnace 20 includes a water-cooled outer wall 21 provided with a raw material supply port 21a and an exhaust port 21b, a sample stage 22 disposed in the water-cooled outer wall 21 and rotating a substrate 11 installed, and a sample stage from below. And a heater 23 for heating. Reference numeral 24 denotes a water-cooling jacket for cooling the source gas in order to prevent the source gas from being heated and reacting before reaching the substrate 11.

The substrate 11 is set on a SiC-coated carbon sample table 23, and the sample table 23 is rotated at a high speed of about 1000 rpm. NH ₃ at the time of heat treatment was 11 slm, and H ₂ or N ₂ was 5 slm. The conditions for the growth of GaN were a substrate temperature of 1000 ° C., ammonia 11 slm, H _{2 5} slm, trimethylgallium 180 to 400 μmol / min, and a pressure of about 27 kPa (ie, 200 Torr). As a comparative example, sapphire was used as a substrate, and a GaN / sapphire substrate on which a GaN layer had been grown in advance was grown simultaneously.

  The results of observing the surface of the substrate 11 with an atomic force microscope after the heat treatment are shown in Table 1, and the results of evaluating a sample obtained by growing the GaN layer 12 by 2 μm by an atomic force microscope and X-ray diffraction are shown in Table 2. X-ray diffraction was evaluated by the half width of the ω scan of (0002) reflection showing fluctuation of c-axis and (10-11) reflection showing fluctuation of both c-axis and a-axis.

From Table 1, even when heat treatment is performed at a substrate temperature of less than 1020 ° C., flatness and surface roughness are not improved, and when heat treatment is performed at a substrate temperature of 1020 ° C. or more and 10 minutes or more in a mixed atmosphere of NH ₃ and H ₂ , It can be seen that an excellent flat GaN single crystal substrate with low roughness can be produced. Table 2 also shows that the comparative example on sapphire has both a large surface roughness and a half-width of X-ray diffraction, but it is markedly improved when a GaN substrate is used. In particular, the surface roughness and crystal axis fluctuation of the epitaxial layer can be greatly improved by heat treatment in a mixed atmosphere of NH ₃ and H ₂ for 10 minutes at a substrate temperature of 1020 ° C. or higher.

_{Incidentally,} NH heat treatment and at 3 + _{N 2 atmosphere,} or NH 3 + the time at the heat treatment in an atmosphere of _{H 2} is less than 10 minutes, the surface roughness when the substrate temperature is 1020 ° C. or less (Rms) is 0. It is not preferable because it exceeds 2 nm or the half width of X-ray diffraction exceeds 100 seconds. In Table 2, the surface roughness (Rms) after the heat treatment is not measured, but the heat treatment conditions for sufficiently reducing the surface roughness and crystal axis fluctuation of the epitaxial layer are as follows. Is consistent with the condition. That is, in order to obtain a good epitaxial layer, a GaN single crystal substrate having a surface roughness of 0.2 nm or less must be used.

Further, when the threading dislocation density was actually determined from the pit-like defects observed with an atomic force microscope image, the epitaxial layer grown on the sapphire substrate was about 10 ⁸ to 10 ⁹ cm ^−2. The epitaxial layer 12 grown on the GaN substrate 11 was as good as 10 ⁶ cm ⁻² or less in many samples.

It is a schematic diagram of the epitaxial substrate which concerns on embodiment of this invention. 4 is an atomic force micrograph of the surface of a GaN single crystal substrate after mechanical polishing. It is a figure which shows the planarization process of the substrate surface in a surface heat treatment process. It is an atomic force microscope photograph of the surface of a GaN single crystal substrate after heat treatment. It is the schematic diagram of the growth furnace of the OMVPE apparatus used for the Example of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Epitaxial substrate, 11 ... GaN single crystal substrate, 12 ... Epitaxial layer, 20 ... OMVPE apparatus.

Claims (9)

The polished surface is a GaN single crystal substrate flattened by being heat-treated at a substrate temperature of 1020 ° C. or more for 10 minutes or more in a mixed gas atmosphere containing at least NH ₃ gas,
The mean square root roughness of the surface is 0.2 nm or less,
The surface has steps and terraces corresponding to one atomic layer;
GaN single crystal substrate. GaN single crystal substrate according to claim 1,
A nitride compound semiconductor layer epitaxially grown on the GaN single crystal substrate;
A nitride semiconductor epitaxial substrate comprising: 3. The nitride compound semiconductor layer according to claim 2, wherein the nitride compound semiconductor layer is Al _x Ga _y In _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1). Nitride semiconductor epitaxial substrate.   The nitride-based semiconductor epitaxial substrate according to claim 2, wherein the nitride-based compound semiconductor layer is GaN.   5. The mean square root roughness of the surface of the nitride-based compound semiconductor layer epitaxially grown on the GaN single crystal substrate is 0.2 nm or less, according to claim 2. Nitride semiconductor epitaxial substrate.   The nitride semiconductor epitaxial substrate according to any one of claims 2 to 5, wherein a half width of X-ray diffraction of the nitride compound semiconductor layer is 100 seconds or less. 7. The nitride semiconductor epitaxial substrate according to claim 2, wherein a threading dislocation density of the nitride compound semiconductor layer is 1 × 10 ⁶ cm ⁻² or less. The GaN single crystal substrate according to claim 1 has n-type conductivity, and an n-type cladding layer made of Al _x Ga _1-x N (0 <x <1) is laminated on the substrate. An active layer is stacked on the cladding layer, and a p-type cladding layer made of Al _x Ga _1-x N (0 <x <1) is stacked on the active layer. A nitride-based semiconductor device, wherein a p-type GaN layer is stacked. A nitride-based semiconductor layer represented by Al _x Ga _y In _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1) on the GaN single crystal substrate according to claim 1. A nitride semiconductor device characterized in that a plurality of layers are stacked.

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