JP5531983B2 - Method for manufacturing group III-V nitride semiconductor substrate - Google Patents

Description

The present invention relates to a method for producing a group III-V nitride semiconductor substrate having a low dislocation density and a substantially uniform carrier concentration distribution on the surface.

  Nitride semiconductor materials have a sufficiently large forbidden band width and a direct transition type between band transitions. Therefore, application to short-wavelength light emitting devices has been actively studied. In addition, application to electronic devices is also expected due to the high saturation drift velocity of electrons and the use of two-dimensional carrier gas by heterojunction.

  The nitride semiconductor layer constituting these elements is formed by using a vapor phase growth method such as metal organic chemical vapor deposition (MOVPE), molecular beam vapor phase epitaxy (MBE), or hydride vapor phase epitaxy (HVPE). It is obtained by performing epitaxial growth on the substrate. However, since there is no underlying substrate having a lattice constant matching that of the nitride semiconductor layer, it is difficult to obtain a high-quality growth layer, and the obtained nitride semiconductor layer contains many crystal defects. Since crystal defects are a factor that hinders improvement in device characteristics, studies to reduce crystal defects in nitride semiconductor layers have been actively conducted so far.

  As a method for obtaining a group III element nitride-based crystal with relatively few crystal defects, a method of forming a low temperature deposition buffer layer (buffer layer) on a dissimilar substrate such as sapphire and forming an epitaxial growth layer thereon is known. Yes. In the crystal growth method using a low-temperature deposition buffer layer, AlN or GaN is first deposited on a substrate such as sapphire at around 500 ° C. to form an amorphous film or a continuous film partially including polycrystal. By raising the temperature to around 1000 ° C., a part thereof is evaporated or crystallized to form high-density crystal nuclei. Using this as a growth nucleus, a GaN film with relatively good crystallinity can be obtained. However, even if a method for forming a low temperature deposition buffer layer is used, crystal defects such as threading dislocations and vacancies are present in the obtained substrate, which is insufficient to obtain a high-performance device that is currently desired. Met.

  In view of the above circumstances, a method of using a GaN substrate as a crystal growth substrate and forming a semiconductor multilayer film constituting an element portion thereon has been actively studied. In this specification, a GaN substrate for crystal growth is referred to as a self-standing GaN substrate (GaN free-standing substrate). An ELO (Epitaxial Lateral Overgrowth) technique is known as a method for obtaining a GaN free-standing substrate. The ELO method is a technique for obtaining a GaN layer with few dislocations by forming a mask having an opening on a base substrate and laterally growing from the opening. JP-A-11-251253 proposes that after forming a GaN layer on a sapphire substrate using this ELO method, the sapphire substrate is removed by etching or the like to obtain a GaN free-standing substrate.

  The FELO (Facet-Initiated Epitaxial Lateral Overgrowth) method (A. Usui, et al., Jpn. J. Appl. Phys. Vol. 36 (1997) pp. L.899-L .902) was developed. The FIELO method is common to the ELO method in that selective growth is performed using a silicon oxide mask, but differs in that facets are formed in the mask opening during selective growth. By forming facets, the propagation direction of dislocations is changed, and threading dislocations reaching the upper surface of the epitaxial growth layer are reduced. If a thick GaN layer is grown on a base substrate such as sapphire using the FIELO method, and then the base substrate is removed, a high-quality GaN free-standing substrate with relatively few crystal defects can be obtained.

  A DEEP (Dislocation Elimination by the Epi-growth with Inverted-Pyramidal Pits) method has been developed as a method for obtaining a low-dislocation GaN free-standing substrate (K. Motoki et. Al., Jpn. J. Appl. Phys. Vol. 40, Japanese Patent Laid-Open No. 2003-165799). In the DEEP method, GaN is grown using a mask made of silicon nitride or the like patterned on a GaAs substrate to form a plurality of pits intentionally surrounded by facet surfaces on the crystal surface, and dislocations are formed at the bottom of the pits. By accumulating, other regions are lowered in dislocation.

  A GaN substrate obtained by ELO or DEEP usually has morphology such as pits and hillocks on its surface in an as-grown state, and it is difficult to grow an epitaxial layer for device fabrication. For this reason, the substrate surface is generally polished to a mirror finish and then used for device fabrication.

  Under such circumstances, Japanese Patent Laid-Open No. 2003-178984 (Patent Document 1) discloses a first group III nitride semiconductor layer on a base material as a method for producing a group III nitride semiconductor substrate having a low dislocation density. A metal film is formed on the base substrate or the base substrate made of the first group III nitride semiconductor, and the base substrate is heat-treated in an atmosphere containing hydrogen gas or a hydrogen-containing compound gas. A method has been proposed in which voids are formed in the group III nitride semiconductor layer and a second group III nitride semiconductor layer is formed on the metal film. In Example 14 and FIG. 16 of Patent Document 1, in the fluorescent microscope image of the cross section, the black band-like streak disappears, and a black shadow is observed in a substantially uniform state in the vicinity of the separation surface from the sapphire substrate. A substrate is described. Regarding this phenomenon, Patent Document 1 describes that the defect is suppressed from being taken over to the surface by increasing the amount of hydrogen mixed in the carrier gas.

Japanese Patent Laid-Open No. 2003-178984

  Although the GaN free-standing substrate produced by such a method has a reduced dislocation density, it has been found that the carrier concentration on the substrate surface varies. In the first place, the carrier concentration distribution in the surface of the substrate was a problem that could not be achieved by the manufacturing method of a conventional semiconductor substrate such as Si or GaAs. Since it is used as a substrate, a region having a nonuniform carrier concentration may locally exist in the substrate. If crystal growth is performed while faceting the growth interface with the aim of lowering the dislocation of the GaN free-standing substrate, there will be a difference in the crystal growth rate between the facet and other surfaces, so effective segregation of impurities between the two A difference occurs in the coefficient, and the impurity distribution, that is, the carrier concentration varies. The regions having different carrier concentrations appear as a history of the facet grown region, and are distributed in a form extending in the crystal growth direction. If regions with different carrier concentrations reach the substrate surface, inevitably variations in carrier concentration occur on the substrate surface.

  It has been found that if a region with a non-uniform carrier concentration exists on the surface of the GaN substrate, surface irregularities are likely to occur in the GaN epitaxial layer grown thereon. In other words, it has been found that even when the underlying GaN substrate is mirror-polished, the epitaxial layer surface becomes rough. If a GaN epitaxial layer having a uniform surface morphology is not formed, it may cause deterioration or dispersion of characteristics when a device is formed thereon.

  When a crystal is grown while a pit surrounded by a facet surface is formed at the crystal growth interface, dislocations accumulate at the bottom of the pit. Not all the accumulated dislocations are united, but forms a high dislocation region that spreads out. In the region where dislocations are gathered, it is considered that a region where the carrier concentration is locally non-uniform is formed by impurity diffusion.

  Even in a GaN crystal in which the number of dislocations accumulated at the bottom of the pits is reduced, a non-uniform distribution of carrier concentration may occur on the surface. When a GaN epitaxial layer is grown on such a GaN crystal substrate, an uneven morphology appears on the surface. The degree of surface irregularity is not much different from that of a GaN substrate having a region where many dislocations are accumulated. From this, it is considered that the unevenness appearing on the epitaxial surface is caused by local distribution of carrier concentration, not dislocation density.

  If the amount of hydrogen mixed in the carrier gas is increased as in Patent Document 1 or facet growth is terminated by means such as changing the crystal growth conditions during crystal growth, the crystal growth interface is flattened, and the surface carrier concentration Distribution may be uniform. However, since there has never been a technical idea to control the carrier concentration distribution on the substrate surface almost uniformly, the region of the carrier concentration distribution is scraped off by polishing the substrate surface. Often, carrier concentrations vary widely. The thickness of the surface layer with a uniform carrier concentration distribution has not been studied at all so far, so even if a GaN substrate having a surface layer with a uniform carrier concentration distribution is prepared. In many cases, the surface layer is almost lost or becomes too thin by mirror finishing. As described above, it has been impossible to stably produce a GaN substrate that has low dislocations and a small variation in carrier concentration on the surface and does not cause defects in devices formed thereon.

Accordingly, an object of the present invention is to provide a method for producing a free-standing substrate of a III-V nitride semiconductor having a sufficient thickness of a surface layer having a low dislocation density and a small variation in carrier concentration. .

As a result of diligent research in view of the above object, the present inventor has found that (a) a dislocation density is uniformly reduced in order to form a light emitting device with uniform characteristics on a III-V nitride semiconductor substrate with a high yield. In addition, it is important that the in-plane uniformity of the carrier concentration is good. (B) The carrier concentration in the vicinity of the surface of the III-V nitride semiconductor substrate (at least up to a depth of 100 μm) If the distribution is substantially uniform, the surface morphology and the uniformity of the characteristics of the GaN-based epitaxial layer grown thereon are not disturbed, and conversely (c) a surface with a substantially uniform carrier concentration. It was discovered that when the layer is thinner than 100 μm, when a GaN-based epitaxial layer is grown on it, surface morphology that reflects the carrier concentration distribution of the substrate, and mixed crystal composition non-uniformity occur. .

  By deliberately appearing facets at the growth interface in the early stage of III-V nitride semiconductor substrate growth, the dislocation propagation direction is bent to reduce the number of dislocations reaching the surface of the substrate and to grow during the crystal growth process. If the interface is flattened, a substrate having a uniform carrier concentration distribution on the surface can be grown without increasing the dislocation density (while maintaining a low dislocation density). As a condition for flattening the growth interface, it is effective to increase the hydrogen partial pressure in the carrier gas during vapor phase growth, but the hydrogen partial pressure and GaCl partial pressure are somewhat high from the beginning of crystal growth. In some cases, the growth interface can be planarized without changing the growth conditions during the crystal growth process. In addition, the growth interface can be planarized also by a method of adding an impurity (such as Mg) that promotes the lateral growth of the III-V nitride semiconductor.

The present invention has been made on the basis of the above discovery, and since it has low dislocations and a uniform carrier concentration distribution on the surface, it can grow a uniform GaN-based epitaxial layer with good crystallinity. A method for manufacturing a nitride-based semiconductor substrate is provided.

The self-supporting group III-V nitride semiconductor substrate manufacturing method according to the first aspect of the present invention has a plurality of irregularities at the crystal growth interface at the initial stage or in the middle of the group III-V nitride semiconductor crystal growth. Crystal growth is performed while the crystal is grown, and then the crystal growth interface is flattened to flatten the crystal growth interface, and the III-V group nitride semiconductor layer is maintained while maintaining the flattened crystal growth interface shape. Crystal growth continues until the thickness reaches 400 μm or more and 1 mm or less, and the boundary between the high-lightness region and the low-lightness region exists in the cross-sectional fluorescence microscopic image of the surface layer from the surface to a depth of at least 100 μm after the crystal growth is completed. The substrate surface is polished so as not to occur.

The self-supporting group III-V nitride semiconductor substrate manufacturing method according to the second aspect of the present invention includes forming a group III-V nitride semiconductor layer on an upper surface of a heterogeneous substrate by epitaxial growth, and then the group III-V group. A method of manufacturing a self-supporting group III-V nitride semiconductor substrate including a step of separating a nitride semiconductor layer and the heterogeneous substrate, wherein the growth of the group III-V nitride semiconductor layer is in the initial stage or in the middle In this stage, the crystal growth is performed while projecting a plurality of irregularities on the crystal growth interface, and then the crystal growth is performed so as to fill the irregularities, thereby flattening the crystal growth interface and maintaining the flattened crystal growth interface shape. Crystal growth is continued until the group III-V nitride semiconductor layer has a thickness of 400 μm or more and 1 mm or less. After the crystal growth is completed, a fluorescence microscope image of a cross section of the surface layer from the surface to a depth of at least 100 μm is highlighted. Degree region and low brightness region Wherein the boundary of polishing the substrate surface so as not to exist.

After the crystal growth is completed, at least a part or all of the grown region may be removed while projecting a plurality of irregularities on the growth interface. The shape of the concave and convex recesses formed at the crystal growth interface in the initial or middle stage of crystal growth in a cross section parallel to the growth direction may be a V shape or an inverted trapezoidal shape surrounded by facet surfaces. The concave and convex recesses formed at the crystal growth interface at the initial stage or in the middle of the crystal growth may have a mortar shape surrounded by facets.

At least part of the crystal growth may be performed by the HVPE method. In order to fill the unevenness of the crystal growth interface during the crystal growth, the hydrogen concentration of the growth atmosphere gas and the partial pressure of the group III material may be made higher than before.

  The surface of the III-V nitride semiconductor substrate of the present invention is preferably a (0001) group III surface.

  In the group III-V nitride semiconductor substrate of the present invention, the dislocation density on the front surface is preferably smaller than the dislocation density on the back surface.

  The III-V nitride semiconductor substrate of the present invention preferably includes a layer made of GaN or AlGaN. It is preferable that the group III-V nitride semiconductor crystal is doped with impurities.

  In the group III-V nitride semiconductor substrate of the present invention, it is preferable that at least part of the group III-V nitride semiconductor crystal is grown by the HVPE method.

  According to the present invention, a self-supporting group III-V nitride semiconductor substrate having a low dislocation density and a substantially uniform carrier concentration on the surface can be stably obtained. By using the self-supporting group III-V nitride semiconductor substrate of the present invention, it becomes possible to manufacture devices such as light emitting elements and electronic elements as designed with high yield.

It is a schematic sectional drawing which shows the structure of the GaN self-supporting substrate of this invention. It is a fluorescence micrograph which shows the cross section of the GaN self-supporting substrate of this invention ( reference example 1). It is a fluorescence micrograph which shows the cross section of the GaN self-supporting substrate of a prior art example (comparative example 1). It is the schematic which shows an example (Example 2) of the manufacturing process of the GaN self-supporting substrate of this invention. It is the schematic which shows the other example (Example 3) of the manufacturing process of the GaN self-supporting substrate of this invention. It is a graph which shows carrier concentration distribution in the surface of the GaN self-supporting substrate of this invention. It is a graph which shows carrier concentration distribution in the back surface of the GaN self-supporting substrate of this invention. It is the schematic which shows the further another example (Example 4) of the manufacturing process of the GaN self-supporting substrate of this invention. It is the schematic which shows the further another example (Example 5) of the manufacturing process of the GaN self-supporting substrate of this invention. It is a fluorescence micrograph which shows the cross section of the GaN self-supporting substrate of this invention (Example 5). It is the schematic which shows the further another example (Example 6) of the manufacturing process of the GaN self-supporting substrate of this invention.

The term “self-supporting substrate” in this specification refers to a substrate that not only can retain its shape but also has a strength that does not cause inconvenience in handling. In order to have such strength, it is preferable that the thickness of the free-standing substrate is 400 μm or more. In consideration of easiness of cleavage after element formation, etc., the thickness of the self-supporting substrate is preferably 1 mm or less. If the self-supporting substrate is too thick, it becomes difficult to cleave, and the cleaved surface is uneven. As a result, when applied to, for example, a semiconductor laser, deterioration of device characteristics due to loss of reflection becomes a problem.

The group III-V nitride semiconductor to which the present invention can be applied has a general formula: In _x Ga _y Al _1-xy N (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, and 0 ≦ x + y ≦ 1) Can be expressed as Among these, semiconductors such as GaN and AlGaN are particularly preferable from the viewpoint of satisfying characteristics required for the substrate material such as strength and manufacturing stability.

  A region having a different carrier concentration on the crystal surface cannot be discriminated visually, but can be easily detected by applying ultraviolet light to the surface and utilizing the photoluminescence of the crystal. In a hexagonal crystal grown in the C-axis direction, the pits surrounded by the facet plane are hexagonal or dodecagonal when viewed from the C-axis direction. Therefore, the growth history is a hexagonal column or a dodecagonal column, but since the size of the pit is small at the initial stage of crystal growth, it is generally a hexagonal pyramid or a dodecagonal pyramid that spreads toward the substrate surface. If the area surrounded by facets is not a pit but a stripe shape due to the growth of an ELO mask such as FIELO, the history is that the cross-sectional shape perpendicular to the stripe direction is an inverted triangular wedge shape, an inverted trapezoidal flat plate shape, or Close shape.

  Since regions having different carrier concentrations have a clear boundary with the surroundings, they can be easily detected from the contrast of the image using a fluorescence microscope. The depth that can be detected varies depending on the wavelength and intensity of the ultraviolet light used as the excitation light, but whether or not the detected image is visible on the surface of the sample can be easily determined from the position where the image is focused. Note that detection of regions having different carrier concentrations can be easily performed with a normal scanning electron microscope (SEM) or cathodoluminescence (CL).

  In the III-V nitride semiconductor substrate of the present invention, the thickness of each region having a different carrier concentration is preferably 1 mm or less. This is because the chip size of a device (for example, a laser diode or a light emitting diode) manufactured using such a substrate is 1 mm or less. If the thickness of the region with different carrier concentration is more than 1 mm, when the chip is manufactured on the entire surface of the substrate, the probability that the boundary of the region with different carrier concentration is applied to the chip is increased, and the yield of the device is greatly reduced. Cause it. Of course, even if the thickness of the regions with different carrier concentrations is more than 1 mm, the uniformity of the carrier concentration on the surface is better and does not hinder the effectiveness of the present invention.

  The surface of the substrate of the present invention is preferably a (0001) group III surface. This is because GaN-based crystals are more polar, the group III surface is more chemically and thermally stable than the group V surface (nitrogen surface), and the device is easy to fabricate.

  Since the present invention provides a substrate that reduces dislocations propagating to the crystal surface during crystal growth and has a uniform carrier concentration on the crystal surface, the dislocation density of the obtained substrate is higher on the surface than on the back surface. It has the feature of few. For example, when the dislocation density is measured by the etch pit method or the like, it is desirable that the dislocation density on the front surface is 1/2 or less of the dislocation density on the back surface.

  As a means for growing the III-V nitride semiconductor substrate of the present invention, it is desirable to use HVPE (hydride vapor phase epitaxy). This is because the HVPE method has a high crystal growth rate and is suitable for manufacturing a substrate.

Since the absolute value of the carrier concentration of the group III-V nitride semiconductor substrate should be appropriately controlled according to the target device, it cannot be determined uniformly. Accordingly, the magnitude of the carrier concentration variation should also be changed according to the absolute value of the carrier concentration, and cannot be defined uniformly. For example, when the carrier concentration of the target Si-doped n-type GaN substrate is about 1 × 10 ¹⁷ cm ⁻³ , the carrier concentration variation on the substrate surface is preferably within ± 25%, and the carrier concentration is When it is about 5 × 10 ¹⁷ cm ⁻³ , the carrier concentration variation on the substrate surface is preferably within ± 15%, and when the carrier concentration is about 5 × 10 ¹⁸ cm ⁻³ , the carrier on the substrate surface is The concentration variation is preferably within ± 10%. When the carrier concentration of the target Si-doped n-type GaN substrate is less than 1 × 10 ¹⁷ cm ⁻³ , the carrier concentration variation on the substrate surface is preferably within ± 100%. Thus, the preferable carrier concentration variation varies depending on the carrier concentration of the substrate because the lower the carrier concentration of the substrate, the smaller the influence exerted by the larger variation.

  The conductivity type of the substrate of the present invention should be appropriately controlled according to the target device, and cannot be determined uniformly. As the conductivity type of the substrate of the present invention, for example, n-type doped with Si, S, O, etc., p-type doped with Mg, Zn, etc., Fe-Cr, etc. doped or n-type and p-type dopants Semi-insulating properties such as doping at the same time.

The surface of the III-V nitride semiconductor substrate (eg, GaN substrate) of the present invention is preferably mirror-polished. In general, the surface of the as-grown GaN-based epitaxial layer has a large number of large irregularities such as hillocks and minute irregularities that appear to be caused by step bunching. These are not only factors that make the morphology, film thickness, composition, etc. of the epitaxial layer grown on it uneven, but also factors that reduce the exposure accuracy of the photolithography process in the device fabrication process. Become. Therefore, the substrate surface is preferably a flat mirror surface. In order to obtain a mirror surface by polishing, it is necessary to scrape several μm to several hundred μm from the surface of the crystal. In the present invention, it is necessary to leave a layer having a substantially uniform carrier concentration to a thickness of 100 μm or more even after the surface layer is scraped off by polishing. Therefore, when polishing the substrate surface, it is necessary to allow a layer having a uniform carrier concentration to grow thickly in advance during crystal growth in anticipation of the polishing allowance.

  It is desirable that the back surface of the III-V nitride semiconductor substrate of the present invention be polished flat. In general, a free-standing substrate of a group III-V nitride semiconductor (such as GaN) is often obtained by heteroepitaxial growth on a different type of base substrate and then peeling off. For this reason, the back surface of the substrate that has been peeled off is often roughened in a satin state, or a part of the base substrate is often attached. Further, it may not be flat due to warpage of the substrate. These cause nonuniformity of the temperature distribution of the substrate when the epitaxial layer is grown on the substrate, and as a result, the uniformity of the epitaxial layer is lowered or the reproducibility is deteriorated.

Incidentally, it means the "surface layer from mirror-polished surface to a depth of at least 100 [mu] m", the surface layer depth after mirror polishing of at least 100 [mu] m. " Therefore, the depth of this surface layer before mirror polishing should be at least 100 μm + mirror polishing allowance. Also, “the carrier concentration distribution is substantially uniform” does not mean that the carrier concentration distribution is completely constant regardless of the position of the substrate, but the characteristics of the device formed on the substrate are constant. This means that the carrier concentration variation is as small as possible. Therefore, for example, in the case of a Si-doped n-type GaN substrate (carrier concentration: about 5 × 10 ¹⁷ cm ⁻³ ), this means that the carrier concentration variation is within ± 15%.

  In the method for producing a group III-V nitride semiconductor substrate of the present invention, it is preferable that the front and back surfaces of the substrate cut out from the thickly grown crystal are finished by polishing. This is because the cut surface of the crystal generally has irregularities such as saw marks introduced at the time of cutting, and it is difficult to perform good epitaxial growth as it is. For cutting the crystal, an outer peripheral blade slicer, an inner peripheral blade slicer, a wire saw, or the like can be used. Among these, it is preferable to use a wire saw.

  Although the present invention is applied to a III-V nitride semiconductor (GaN, etc.) free-standing substrate, the technical idea of the present invention can also be applied to a GaN epitaxial substrate (template) with a base substrate attached. is there.

  The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto.

Reference example 1
By growing a GaN epitaxial layer on the sapphire substrate and then removing the sapphire substrate, as shown in FIG. 1, there are a layer containing the region 2 having a different carrier concentration and a layer having a substantially uniform carrier concentration. A GaN free-standing substrate 1 was produced and evaluated. Hereinafter, a method of manufacturing the GaN free-standing substrate of this reference example will be described with reference to FIG.

First, using the sapphire substrate 11, the GaN epitaxial layer 12a was grown by the HVPE method. The HVPE method transports GaCl, which is a halide of a group III element, to the heated substrate surface, mixes it with NH ₃ in the substrate region, and reacts them to form a vapor phase of GaN crystals on the substrate. It is a way to grow. The source gas is flowed with a carrier gas such as H ₂ or N ₂ . The temperature of the substrate region was set to 1000 ° C. with an electric furnace. In addition, SiH was doped by supplying SiH ₂ Cl ₂ as a doping source gas to the substrate region during the growth of the GaN crystal.

The partial pressures of the raw material GaCl and NH ₃ were 5 × 10 ⁻³ atm and 0.3 atm in the substrate region, respectively. A mixed gas of 2% H ₂ and 98% N ₂ was used as a carrier gas. Under these conditions, the GaN crystal 12a nuclei were generated in a three-dimensional island shape on the sapphire substrate 11, and then facet surfaces appeared on the side walls of the crystal nuclei, and crystal growth proceeded (step (b)). . This state was confirmed by observing the substrate surface and cross-section taken out of the furnace with a microscope under a different growth time.

As the growth time was lengthened, the top of the GaN crystal 12a was flattened (step (c)), and then the crystals grew laterally and joined to each other, and the surface flattened. However, the growth interface was not completely flattened, and crystal growth proceeded with many pits 13 on the surface (step (d)). The pit 13 was substantially circular with a diameter of about several μm to several tens of μm when viewed from directly above. When a fluorescent microscopic image of the cross section of the sample corresponding to (d) was observed, a dark region 14 was observed that was connected from the interface of the sapphire substrate to the bottom of the pits 13 present on the surface of the GaN. This region 14 is considered to have a small amount of dopant incorporation and have a lower carrier concentration than the surroundings. Actually, SIMS analysis was performed aiming at the dark area in the fluorescence microscope image, and when compared with the surrounding area, the concentration of Si was 3 × 10 ¹⁷ cm ^-3 in the dark area, whereas in the surrounding area, The concentration of Si was 7 × 10 ¹⁷ cm ⁻³ or more.

After the GaN crystal 12a was grown to the state of (d), the growth of the GaN crystal was continued by switching the carrier gas only to the mixed gas of 10% H ₂ and 90% N ₂ with the raw material gas flow rate unchanged. . As a result, the growth interface 12c of the GaN crystal 12a tended to be flattened (step (e)). After the growth interface of the GaN crystal 12a was flattened, the GaN crystal 12b was grown to a thickness of 100 μm or more. When the cross section of the region 12b grown after the growth interface was flattened was observed with a fluorescence microscope, it was found that a region having a different brightness was not newly generated. That is, it was confirmed that the region 14 with different brightness terminated in the middle of the GaN crystal 12 (growth interface 12c) (step (f)) and did not reach the outermost surface of the GaN crystal 12. A measured fluorescence microscope image of the GaN crystal 12 is shown in FIG. Some pits still reach the surface without terminating, but the majority of the pits terminate in the middle of the GaN crystal 12, and the brightness of the fluorescence microscope image in the depth range of at least 10 μm from the crystal surface is almost It was confirmed that it was uniform.

  In this way, a GaN crystal 12 having a total thickness of 250 μm was grown on the sapphire substrate 11. The average growth rate of the GaN crystal 12 was about 50 μm / h.

  The sapphire substrate 11 on which the GaN epitaxial layer 12 was formed as described above was taken out of the reaction tube, the sapphire substrate 11 was removed, and a GaN free-standing substrate 15 was obtained. As a method of removing the sapphire substrate, the sapphire substrate is transmitted, but GaN laser is irradiated with high-power ultraviolet laser light with a wavelength that is absorbed by GaN, and the vicinity of the interface of the GaN crystal is melted and removed. The so-called laser lift-off method was used. In addition to this, it is also possible to remove the sapphire substrate by, for example, mechanical polishing or etching with a strong alkali or strong acid chemical. Moreover, you may perform the physical etching by a charged beam or a neutral beam for the removal of a sapphire substrate.

  The flatness was improved by removing the surface and the back surface of the GaN free-standing substrate 15 thus obtained by 10 μm each and performing mirror polishing. The final thickness of the GaN free-standing substrate 15 was 230 μm, and the strength was sufficient to withstand handling using tweezers. By observing the cross section of the GaN free-standing substrate 15 with a fluorescence microscope, it was confirmed that there were almost no regions with different carrier concentrations near the surface of the GaN free-standing substrate 15 (up to a depth of at least 10 μm).

The carrier concentration distribution on the front and back surfaces of the GaN free-standing substrate 15 was measured by a van der Pauw method at intervals of 5 mm in the diameter direction of the substrate. The results are shown in FIGS. As shown in FIG. 6, the carrier concentration on the surface of the GaN free-standing substrate 15 is in the range of 6.9 × 10 ¹⁷ cm ^{−3 to} 7.6 × 10 ¹⁷ cm ⁻³ , and was confirmed to be sufficiently uniform. On the other hand, as shown in FIG. 7, it has been found that the carrier concentration on the back surface of the GaN free-standing substrate 15 varies greatly from 2.7 × 10 ¹⁷ cm ^{−3 to} 7.1 × 10 ¹⁷ cm ⁻³ .

  A GaN epitaxial film was grown by 1 μm on the GaN free-standing substrate 15 using the MOVPE method, and its surface morphology was examined. It was confirmed that the entire surface of the substrate was in a uniform mirror state.

Comparative Example 1
Reference Example 1 except that the partial pressures of the raw material GaCl and NH ₃ were 5 × 10 ⁻³ atm and 0.3 atm in the substrate region, respectively, and a mixed gas of 2% H ₂ and 98% N ₂ was used as the carrier gas. In the same manner, a GaN thick film crystal was grown on the sapphire substrate. As a result, many pits on the surface remained unfilled until the GaN thickness reached 300 μm.

  The substrate was taken out from the reaction tube, and the sapphire substrate was removed using the laser lift-off method described above to obtain a GaN free-standing substrate. Flatness was improved by mirror-polishing both the front and back sides of the GaN free-standing substrate to a depth of 30 μm and 10 μm, respectively. Due to the mirror polishing, most of the pits remaining on the surface of the substrate disappeared. The final thickness of the GaN free-standing substrate was 260 μm.

  When the cross section of the GaN free-standing substrate was observed using a fluorescence microscope, as shown in FIG. 3, there were many wedge-shaped regions connecting the front surface and the back surface and having different brightness from the surroundings. I understood.

The carrier concentration distribution on the front and back surfaces of this GaN free-standing substrate was measured at 5 mm intervals in the diameter direction of the substrate by the van der Pauw method. As a result, the carrier concentration on the substrate surface varies greatly from 2.4 × 10 ¹⁷ cm ^{−3 to} 7.7 × 10 ¹⁷ cm ^−3, and the carrier concentration variation on the back surface (2.6 × 10 ¹⁷ cm ^{−3 to} 8.1 × 10 ¹⁷ cm ^-3 ) and no significant difference.

  On this GaN free-standing substrate, a GaN epitaxial film was grown by 1 μm using the MOVPE method, and when the surface morphology was examined, many terrace-shaped irregularities with a diameter of about 10 to 60 μm were generated on the entire surface of the substrate. It was confirmed that These irregularities are expected to be obstacles when actually manufacturing a device.

Example 2
A GaN free-standing substrate shown in FIG. 1 is obtained by growing a GaN epitaxial layer on the sapphire substrate and then removing the sapphire substrate in substantially the same manner as in Reference Example 1 except that the crystal growth conditions of the HVPE method are slightly changed. Prepared and evaluated. Hereinafter, a method of manufacturing the GaN free-standing substrate of this example will be described with reference to FIG.

First, using a sapphire C-plane substrate 11, a GaN epitaxial layer 12a was grown by the same HVPE method as in Reference Example 1. The temperature of the substrate region was set to 1050 ° C. in an electric furnace. The partial pressures of GaCl and NH ₃ as raw materials were 6 × 10 ⁻³ atm and 0.4 atm, respectively, in the substrate region, and a mixed gas of 10% H ₂ and 90% N ₂ was used as a carrier gas from the beginning. . During the growth process of the GaN crystal, doping was performed by supplying SiH ₂ Cl ₂ as a doping source gas to the substrate region, and the GaN crystal was doped with Si.

  First, nuclei of the GaN crystal 12a were generated in a three-dimensional island shape on the sapphire substrate 11, and then facet surfaces appeared on the side walls of the crystal nucleus 12a, and crystal growth proceeded (step (b)). This state was confirmed by observing the substrate surface and cross-section taken out of the furnace with a microscope under a different growth time. As the growth time is lengthened, the top of the GaN crystal 12a is flattened with the upper surface as the (0001) Ga plane, and then the crystals grow laterally and bond to each other, and the flattening of the surface proceeds (step (c) )). Furthermore, when crystal growth was continued under the same conditions, the pits at the growth interface of the GaN crystal 12a terminated spontaneously and a tendency to flatten was observed (step (e)). Thus, even after the growth interface 12c of the GaN crystal 12a was flattened, the growth of the GaN crystal 12b was continued to a thickness of 100 μm or more.

  In the region 12b grown after the growth interface was naturally flattened, it was confirmed by observation of the cross section with a fluorescence microscope that no new region having different brightness was generated. That is, the region 14 with different brightness terminated in the middle of the GaN crystal 12 (step (f)) and did not reach the outermost surface of the crystal.

  In this way, a GaN crystal 12 having a total thickness of 550 μm was grown on the sapphire substrate 11. The average growth rate of the GaN crystal 12 was about 65 μm / h.

  This substrate was taken out from the reaction tube, and the sapphire substrate 11 was removed by using the laser lift-off method described above to obtain a GaN free-standing substrate 15. The front and back surfaces of the GaN free-standing substrate 15 were mirror-polished to remove 30 μm on the front surface and 90 μm on the back surface, thereby improving the flatness. By mirror polishing, the final thickness of the GaN free-standing substrate was 430 μm.

The carrier concentration distribution on the front and back surfaces of the GaN free-standing substrate 15 was measured by a van der Pauw method at intervals of 5 mm in the diameter direction of the substrate. As a result, the carrier concentration on the substrate surface was in the range of 0.9 × 10 ¹⁸ cm ^{−3 to} 1.6 × 10 ¹⁸ cm ⁻³ and was confirmed to be sufficiently uniform. On the other hand, the carrier concentration on the back surface of the substrate was found to vary greatly from 4.7 × 10 ¹⁷ cm ^{−3 to} 13.1 × 10 ¹⁷ cm ⁻³ .

  When the cross section of the obtained GaN free-standing substrate 15 was observed with a fluorescence microscope, it was confirmed that there were no regions having different brightness over a region having a depth of 100 μm or more from the substrate surface.

Example 3
A GaN epitaxial layer was grown on a sapphire substrate by using a void-assisted separation method (VAS method), and then the sapphire substrate was removed to prepare and evaluate a GaN free-standing substrate. The details of the VAS method are described in Japanese Patent Application No. 2002-64345. To put it simply, a method of crystal growth by sandwiching a thin film of titanium nitride having a network structure between a sapphire substrate and a GaN growth layer. It is. Hereinafter, a method of manufacturing the GaN free-standing substrate of this example will be described with reference to FIG.

An undoped GaN layer 22 was grown to a thickness of 300 nm using trimethylgallium (TMG) and NH ₃ as raw materials on a single-crystal sapphire C-plane substrate 21 with a diameter of 2 inches (process (b)). . Next, a metal Ti film 23 is deposited on the GaN epitaxial substrate to a thickness of 20 nm (step (c)) and placed in an electric furnace to mix 20% NH ₃ and 80% H ₂ . Heat treatment at 1050 ° C. × 20 min was performed in a gas stream. As a result, a part of the GaN layer 22 is etched to change to a layer 24 in which high-density voids are generated, and the metal Ti film 23 is nitrided to form fine submicron holes on the surface with high density. Changed to TiN layer 25. As a result, a substrate having the structure shown in (d) was obtained.

This substrate was put into an HVPE furnace, and a GaN crystal 26 was deposited to a total thickness of 400 μm. First, the raw materials used for the growth of the GaN crystal 26a were NH ₃ and GaCl, and a mixed gas of 5% H ₂ and 95% N ₂ was used as a carrier gas. The growth conditions were normal pressure and a substrate temperature of 1040 ° C. The partial pressures of GaCl and NH ₃ in the feed gas were 8 × 10 ⁻³ atm and 5.6 × 10 ⁻² atm, respectively, at the start of growth, and the V / III ratio was 7. In addition, Si was doped by supplying SiH ₂ Cl ₂ as a doping source gas to the substrate region during the growth process of the GaN crystal 26a.

  First, GaN nuclei 26a are formed in a three-dimensional island shape on the substrate 21 (step (e)), and then the crystals grow laterally and bond to each other, and the surface flattening proceeds ( Step (f)). This state was confirmed by observing the substrate surface and cross-section taken out of the furnace with different growth times under a microscope. As the growth time increased, the number of pits 27 at the growth interface of the GaN crystal 26a decreased, but it did not disappear completely, and crystal growth proceeded with many pits still existing on the surface. did. The pit 27 was almost circular or dodecagonal with a diameter of several μm to several tens of μm when viewed from directly above. In the fluorescence microscopic image of the cross section of the sample corresponding to (f), a dark region 28 that is connected from the interface of the substrate 21 to the bottom of the pit 27 existing on the GaN surface was observed. In this region 28, it is considered that the amount of dopant taken in is small and the carrier concentration is lower than the surroundings.

After growing the GaN crystal 26a to the state shown in (f), the crystal growth was continued by increasing the GaCl partial pressure in the supply gas to 12 × 10 ⁻² atm. The growth interface of 26a tended to flatten further (step (g)). After the growth interface 26c of the GaN crystal 26a was flattened, the growth of the GaN crystal 26b was further continued to a thickness of 200 μm or more. In the region 26b grown after the flattening of the growth interface, it was found by observation with a cross-sectional fluorescence microscope that a region having a different brightness was not newly generated. That is, it was confirmed that the region 28 having different brightness was terminated in the middle of the GaN crystal 26 (step (h)) and did not reach the outermost surface of the GaN crystal.

  In the process of cooling the HVPE apparatus after completion of the GaN crystal growth, the GaN layer 26 was naturally separated from the sapphire base substrate with the void layer as a boundary, and a GaN free-standing substrate 30 was obtained (step (i)). The front and back surfaces of the GaN free-standing substrate substrate 30 were mirror-polished to remove the front surface by 20 μm and the back surface by 50 μm, thereby improving the flatness. By mirror polishing, the final thickness of the GaN free-standing substrate 30 became 330 μm (step (j)).

The carrier concentration distribution on the front and back surfaces of the obtained GaN free-standing substrate 30 was measured by a van der Pauw method at intervals of 5 mm in the diameter direction of the substrate. As a result, it was confirmed that the carrier concentration on the substrate surface was in a range of 9.2 × 10 ^{17 to} 10.1 × 10 ¹⁷ cm ⁻³ and was sufficiently uniform. On the other hand, the carrier concentration on the back surface of the substrate was found to vary greatly from 2.8 × 10 ¹⁷ cm ^{−3 to} 8.8 × 10 ¹⁷ cm ⁻³ . Further, when the cross section of the GaN free-standing substrate 30 was observed with a fluorescence microscope, it was confirmed that there were no regions having different brightness over a region having a depth of 100 μm or more from the surface.

The dislocation density on the front and back surfaces of the GaN free-standing substrate 30 was measured. The dislocation density on the surface was determined by immersing the GaN free-standing substrate 30 in a heated mixed solution of phosphoric acid and sulfuric acid and counting the number of pits generated by etching. The dislocation density on the back surface was determined from a plan-view transmission electron microscope (TEM) observation image. As a result, it was found that the dislocation density on the surface of this GaN free-standing substrate 30 was 4.2 ± 1 × 10 ⁶ cm ⁻² and the dislocation density on the back surface was 7.2 ± 1 × 10 ⁸ cm ⁻² .

Example 4
In the same manner as in Example 3, a GaN epitaxial layer was grown on the sapphire substrate using the VAS method, and then the sapphire substrate was removed to prepare and evaluate a GaN free-standing substrate. Hereinafter, a method of manufacturing the GaN free-standing substrate of this example will be described with reference to FIG.

On the single crystal sapphire C-plane substrate 31 having a diameter of 2 inches, an undoped GaN layer 32 was grown to a thickness of 300 nm using TMG and NH ₃ as raw materials by the MOVPE method (step (b)). On this GaN epitaxial substrate, a metal Ti film 33 is deposited to a thickness of 20 nm (step (c)), and this is put into an electric furnace to mix a mixed gas of 20% NH ₃ and 80% H ₂ . Heat treatment at 1050 ° C. × 20 min was performed in an air stream. As a result, a part of the GaN layer 32 is etched to change to a layer 34 in which high-density voids are generated, and the Ti layer 33 is nitrided to form submicron fine holes at a high density on the surface. Changed to TiN layer 35. As a result, a substrate having the structure shown in (d) was obtained.

This substrate was placed in an HVPE furnace, and GaN crystal 36 was deposited to a thickness of 550 μm. The raw materials used for crystal growth were NH ₃ and GaCl, and a mixed gas of 5% H ₂ and 95% N ₂ was used as a carrier gas. The growth conditions were normal pressure and a substrate temperature of 1040 ° C. The partial pressures of GaCl and NH ₃ in the supply gas were 8 × 10 ⁻³ atm and 5.6 × 10 ⁻² atm, respectively, at the start of crystal growth, and the V / III ratio was 7. In addition, Si was doped by supplying SiH ₂ Cl ₂ as a doping source gas to the substrate region during the GaN crystal growth process.

  First, GaN nuclei 36a are formed in a three-dimensional island shape on the substrate 31 (step (e)), and then crystals grow laterally and bond to each other, and the surface flattening proceeds ( Step (f)). This state was confirmed by observing the substrate surface and cross-section taken out of the furnace with a microscope under a different growth time. As the growth time was increased, the number of pits 37 at the growth interface of the GaN crystal 36a decreased, but it did not disappear completely, and crystal growth proceeded with many pits still existing on the surface. did. The pit 37 was almost circular or dodecagonal with a diameter of about several μm to several tens of μm when viewed from directly above. In the fluorescence microscopic image of the cross section of the sample corresponding to (f), a dark region 38 was observed that connected from the interface of the substrate to the bottom of the pits 37 present on the GaN surface. This region 38 is considered to have a small amount of dopant incorporation and a lower carrier concentration than the surroundings.

After growing the GaN crystal 36a to the state shown in (f) and then increasing the GaCl partial pressure in the supply gas to 12 × 10 ^-2 atm, the pit 37 terminated and the growth of the GaN crystal 36a There was a tendency for the interface to further flatten (step (g)). By this time, a GaN crystal 36a having a thickness of about 80 μm had grown. After planarization of the growth interface of the GaN crystal 36a, the growth of the GaN crystal 36b was further continued to a thickness of 470 μm. In the region 36b grown after the flattening of the growth interface, no new generation of regions with different brightness was observed by observation of the cross section with a fluorescence microscope. That is, it was confirmed that the region 38 with different brightness terminated in the middle of the GaN crystal 36 (step (h)) and did not reach the outermost surface of the crystal.

  In the process of cooling the HVPE apparatus after completion of the crystal growth, the GaN layer 36 was naturally separated from the base substrate 31 with the void layer as a boundary, and a GaN free-standing substrate 40 was obtained (step (i)). The front and back surfaces of the GaN free-standing substrate 40 were mirror-polished to remove the front and back surfaces to a depth of 20 μm and 100 μm, respectively, thereby improving the flatness. By mirror polishing, the final thickness of the GaN free-standing substrate 40 was 430 μm.

The carrier concentration distribution on the front and back surfaces of the obtained GaN free-standing substrate 40 was measured by a van der Pauw method at intervals of 5 mm in the diameter direction of the substrate. As a result, it was confirmed that the carrier concentration on the substrate surface was in the range of 9.2 × 10 ¹⁷ cm ^{−3 to} 10.1 × 10 ¹⁷ cm ⁻³ and was sufficiently uniform. It was also found that the carrier concentration on the back surface of the substrate was 8.8 × 10 ¹⁷ cm ^{−3 to} 10.8 × 10 ¹⁷ cm ^−3, which was not significantly different from the surface. When the cross section of the GaN free-standing substrate 40 was observed with a fluorescence microscope, it was confirmed that there were no regions with different brightness within the substrate.

Example 5
Using the FIELO method (A. Usui, et al., Jpn. J. Appl. Phys. Vol. 36 (1997), pp. L.899-L.902), a GaN epitaxial layer is grown on the sapphire substrate, Thereafter, a GaN free-standing substrate was fabricated by removing the sapphire substrate and evaluated. Hereinafter, a method of manufacturing the GaN free-standing substrate of this example will be described with reference to FIG.

On the single crystal sapphire C-plane substrate 41 having a diameter of 2 inches, an undoped GaN layer 42 was grown to a thickness of 600 nm using TMG and NH ₃ as raw materials by the MOVPE method (step (b)). Next, a SiO ₂ film is deposited to a thickness of 0.5 μm on this GaN epitaxial substrate by a thermal CVD method, and a stripe-like window is opened in parallel with <11-20> on the SiO ₂ film by photolithography to obtain a GaN layer 42 Was exposed (step (c)). The width of the window was 3 μm, and the width of the SiO ₂ mask 43 was 7 μm.

This substrate was placed in an HVPE furnace, and a GaN crystal 44 was deposited to a total thickness of 500 μm. The raw materials used for crystal growth were NH ₃ and GaCl, and a mixed gas of 5% H ₂ and 95% N ₂ was used as a carrier gas. The growth conditions were normal pressure and a substrate temperature of 1040 ° C. At the start of crystal growth, the partial pressures of GaCl and NH ₃ in the supply gas were 8 × 10 ⁻³ atm and 5.6 × 10 ⁻² atm, respectively, and the V / III ratio was 7. In the GaN crystal growth process, Si was doped by supplying SiH ₂ Cl ₂ as a doping source gas to the substrate region.

  The GaN crystals 44 were first selectively grown on the underlying GaN in the window, and were arranged in stripes parallel to <11-20>. The cross section perpendicular to <11-20> was as shown schematically in (d).

When the groove of the mask was filled, the GaN crystal 44a grew in the lateral direction on the SiO ₂ mask 43 so as to cover the entire surface of the substrate. At this time, a facet surface appeared on the side surface of the GaN crystal 44a extending in a stripe shape, and a V-shaped groove 45 appeared in a region associated with the adjacent crystal (step (e)). This state was confirmed by observing the substrate surface and cross-section taken out of the furnace with a microscope under a different growth time.

In the fluorescence microscopic image of the cross section of the sample corresponding to (e), a dark region 46 connected from the interface with the SiO ₂ mask 43 to the bottom of the V-shaped groove 45 existing on the GaN surface was observed. This region 46 was a region where the amount of dopant incorporated was small and the carrier concentration was lower than the surroundings.

  If the crystal growth time is increased as it is, the crystal growth proceeds while leaving the aforementioned V-shaped grooves 45 at the growth interface, but these grooves 45 are gradually filled as the growth proceeds, and the thickness of the GaN crystal 44a increases. When the thickness exceeded 100 μm, a GaN film having a flat surface was obtained (step (f)).

  After flattening the growth interface of the GaN crystal 44a, the growth of the GaN crystal 44b was continued to a thickness of about 400 μm. As a result of observing the cross section of the GaN crystal with a fluorescence microscope, there was no new region with different brightness in the region grown after the growth interface was flattened. That is, it was observed that the regions 46 with different brightness terminated in the middle of the GaN crystal 44 (step (g)) and did not reach the outermost surface of the crystal.

  Thus, a GaN crystal 44 having a total thickness of about 500 μm was obtained. The average growth rate of the GaN crystal 44 was about 75 μm / h. The substrate was taken out from the reaction tube, and the sapphire substrate 41 was removed by the laser lift-off method described above to obtain a GaN free-standing substrate 50 (step (h)).

  The front and back surfaces of the GaN free-standing substrate 50 were mirror-polished to remove the front and back surfaces by 20 μm and 60 μm, respectively, thereby improving the flatness (step (i)). The final thickness of the GaN free-standing substrate 50 was 420 μm. As a result of observing the cross section of the substrate with a fluorescence microscope, it was found that there was no region with a different carrier concentration in most of the front side of the substrate (thickness of 380 μm). FIG. 10 shows a fluorescence microscope image of a cross section of the GaN free-standing substrate 50.

The carrier concentration distribution on the front and back surfaces of the GaN free-standing substrate 50 was measured by a van der Pauw method at intervals of 5 mm in the diameter direction of the substrate. As a result, it was confirmed that the carrier concentration on the substrate surface was in the range of 6.6 × 10 ¹⁷ cm ^{−3 to} 7.2 × 10 ¹⁷ cm ⁻³ and was sufficiently uniform. On the other hand, the carrier concentration on the back surface of the substrate was found to vary greatly from 1.7 × 10 ¹⁷ cm ^{−3 to} 7.2 × 10 ¹⁷ cm ⁻³ .

  A GaN epitaxial film was grown to a thickness of 1 μm on the GaN free-standing substrate 50 by the MOVPE method, and its surface morphology was examined. As a result, it was confirmed that the entire surface of the substrate was in a uniform mirror state.

Example 6
As shown in FIG. 11, first, a first GaN layer 12a including a region 14 having a different carrier concentration is grown on a sapphire substrate 11 having a diameter of 50 mm under the same method and conditions as in Reference Example 1 (steps (a) to (a)). (d)) Next, the growth interface 12c was flattened (step (e)) to grow a second GaN layer 12b having a uniform carrier concentration (step (f)). The difference from Reference Example 1 is that the second GaN layer 12b having a uniform carrier concentration was continuously grown to a thickness of about 20 mm.

  The second GaN layer 12b having a thickness of about 20 mm was attached to a fixing jig with the sapphire substrate 11 still attached, and was cut using a wire saw electrodeposited with diamond abrasive grains. The GaN crystal 12b was cut perpendicular to the crystal growth direction (parallel to the surface of the sapphire substrate 11) (step (g)). Thus, 19 GaN substrates 12d having a diameter of 50 mm and a thickness of 450 μm were cut out from the thickly grown second GaN layer 12b. Both the front and back surfaces of each cut GaN substrate were mirror-polished to obtain a colorless and transparent GaN free-standing substrate 12d (step (h)).

  When arbitrary surfaces and cross sections of the GaN free-standing substrates 12d thus obtained were observed with a fluorescence microscope, no regions with different brightness were observed at all.

When the carrier concentration distribution on the surface of each GaN free-standing substrate 12d is measured at 5 mm intervals in the diameter direction of the substrate by the van der Pauw method, the carrier concentration is in the range of 6.9 × 10 ¹⁷ cm ^{−3 to} 7.4 × 10 ¹⁷ cm ⁻³ . It was confirmed that the film was sufficiently uniform.

  A GaN epitaxial film was grown to a thickness of 2 μm on each GaN free-standing substrate 12d by the MOVPE method, and when the surface morphology was examined, it was confirmed that the entire surface of the substrate was in a uniform mirror state.

The present invention has been described in detail on the basis of the embodiments. However, these are exemplifications, and various modifications can be made to combinations of the respective processes, and such modifications are also within the scope of the present invention. Will be understood by those skilled in the art. For example, in the embodiment, the MOVPE method may be combined with part of the GaN crystal growth. In addition, in order to cause the growth to occur while projecting a plurality of irregularities at the crystal growth interface in the initial or middle stage of crystal growth, it is used in combination with ELO technology using a mask such as SiO ₂ known as the prior art. Also good. In the examples, a sapphire substrate is used as the base substrate, but any substrate that has been reported as a conventional GaN-based epitaxial layer substrate such as GaAs, Si, ZrB ₂ , or ZnO can be applied.

  After removing the base substrate, the carrier concentration distribution on the surface of the GaN substrate may be made uniform by heat treatment. This utilizes the phenomenon in which atoms (or molecules) on the crystal surface are reconstructed by mass transport by holding the GaN crystal at a high temperature around 1000 ° C. However, in this method, since the depth of the surface to be modified is limited, the effect of homogenization cannot be obtained as much as in the present invention.

  In the embodiment, the method for manufacturing a GaN free-standing substrate is exemplified, but it is of course applicable to an AlGaN free-standing substrate.

1,15,30,40,50 ... GaN free-standing substrate 2,14,28,38,46 ... regions with different carrier concentrations
11, 21, 31, 41 ... Sapphire substrate
12, 26, 36, 44 ... GaN crystal
12a, 26a, 36a, 44a ... Layers that do not contain regions with different carrier concentrations
12b, 26b, 36b, 44b ... Layers with substantially uniform carrier concentration
13, 27, 37 ... Pit surrounded by facets
23, 33 ... titanium metal
GaN crystal with voids
25, 35 ... Network-structured titanium nitride
42 ・ ・ ・ MOVPE-grown GaN base crystal layer
43 ・ ・ ・ SiO ₂ mask
45 ・ ・ ・ V-shaped groove

Claims (13)

At the initial stage or in the middle of the growth of the group III-V nitride semiconductor crystal, the crystal growth is performed while projecting a plurality of irregularities on the crystal growth interface, and then the crystal growth is performed so as to fill the irregularities. Crystal growth is continued until the thickness of the group III-V nitride semiconductor layer becomes 400 μm or more and 1 mm or less while maintaining the shape of the flattened crystal growth interface, and at least from the surface after the crystal growth is completed. A self-supporting group III-V nitride semiconductor substrate characterized in that the substrate surface is polished so that the boundary between the high brightness region and the low brightness region does not exist in the fluorescence microscope image of the cross section of the surface layer up to a depth of 100 μm Manufacturing method. A self-supporting group III-V nitride including a step of separating a group III-V nitride semiconductor layer and the heterogeneous substrate after forming a group III-V nitride semiconductor layer on the upper surface of the heterogeneous substrate by epitaxial growth A method of manufacturing a semiconductor substrate, wherein crystal growth is performed while projecting a plurality of irregularities at a crystal growth interface at an initial stage or in the middle of the growth of the group III-V nitride semiconductor layer, and then the irregularities are filled. Crystal growth is performed to flatten the crystal growth interface, and the crystal growth is continued until the thickness of the III-V nitride semiconductor layer becomes 400 μm or more and 1 mm or less while maintaining the flattened crystal growth interface shape. Continue and polish the substrate surface so that the boundary between the high brightness region and the low brightness region does not exist in the fluorescence microscope image of the cross section of the surface layer from the surface to a depth of at least 100 μm after the crystal growth is completed. -Group V nitride The method of manufacturing a semiconductor substrate. 3. The method of manufacturing a group III-V nitride semiconductor substrate according to claim 1, wherein at least a part of the grown region is removed while projecting a plurality of irregularities on the growth interface after the crystal growth is completed. A method for manufacturing a group III-V nitride semiconductor substrate. 3. The method of manufacturing a group III-V nitride semiconductor substrate according to claim 1 or 2, wherein after the crystal growth is finished, all the grown regions are removed while projecting a plurality of irregularities on the growth interface. A method for manufacturing a group nitride semiconductor substrate. The method for manufacturing a group III-V nitride semiconductor substrate according to any one of claims 1 to 4, wherein the concave and convex recesses formed in the crystal growth interface at an initial stage or in the middle of the crystal growth are parallel to the growth direction. A method for producing a group III-V nitride semiconductor substrate, wherein the cross-sectional shape is a V-shape or an inverted trapezoidal shape surrounded by a facet plane. 6. The method for manufacturing a group III-V nitride semiconductor substrate according to claim 1, wherein the concave and convex concave portions formed at the crystal growth interface at the initial stage or in the middle of the crystal growth are surrounded by facet surfaces. A method for producing a group III-V nitride semiconductor substrate characterized by having a mortar shape. 7. The method for producing a group III-V nitride semiconductor substrate according to claim 1, wherein at least part of the crystal growth is performed by an HVPE method. Manufacturing method. The method of manufacturing a group III-V nitride semiconductor substrate according to any one of claims 1 to 7, wherein the hydrogen concentration of the growth atmosphere gas is set higher than before in order to fill the irregularities of the crystal growth interface during the crystal growth. A method for producing a group III-V nitride semiconductor substrate, characterized by comprising increasing the height. The method for producing a group III-V nitride semiconductor substrate according to any one of claims 1 to 8, wherein the partial pressure of the group III raw material is set higher than before in order to fill the unevenness of the crystal growth interface during the crystal growth. A method for producing a group III-V nitride semiconductor substrate, characterized by comprising increasing the height. In the claims 1-9 production method of the III-V nitride semiconductor substrate according to any one of, III-V nitride semiconductor, wherein the substrate surface is a Group III surface of (0001) A method for manufacturing a substrate. The method for producing a group III-V nitride semiconductor substrate according to any one of claims 1 to 10 , wherein the dislocation density on the front surface is lower than the dislocation density on the back surface. Manufacturing method . In the claims 1-11 production method of the III-V nitride semiconductor substrate according to any one of method of the III-V nitride semiconductor substrate, characterized in that it comprises a layer made of GaN or AlGaN . The group III-V nitride semiconductor substrate according to any one of claims 1 to 12 , wherein the group III-V nitride semiconductor crystal is doped with an impurity. A method for manufacturing a nitride semiconductor substrate.