JP2013230971A - Group iii nitride semiconductor substrate for ld and group iii nitride semiconductor epitaxial substrate for ld using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a group III nitride semiconductor substrate for laser diodes (LD's) in which a flat cleavage plane having little irregularity such as uneven levels can be obtained, and a group III nitride semiconductor epitaxial substrate for LD's using the same.SOLUTION: A group III nitride semiconductor substrate 6 for LD's has a diameter of not less than 25 mm and a thickness of not less than 250 μm. In an outer periphery within 5 mm from the outer edge of the group III nitride semiconductor substrate 6 for LD's, at least a portion at the outer edge side exhibits a tensile stress as a stress within the main surface of the group III nitride semiconductor substrate 6 for LD's, and the tensile stress is relatively larger than a stress in a portion nearer to the central side than the portion at the outer edge side of the group III nitride semiconductor substrate 6 for LD's.

Description

  The present invention relates to a group III nitride semiconductor substrate for LD and a group III nitride semiconductor epitaxial substrate for LD using the same, and more particularly to a group III nitride semiconductor substrate for LD with improved cleavage and LD III using the same. The present invention relates to a group nitride semiconductor epitaxial substrate.

  Group III nitride semiconductors such as gallium nitride (GaN), indium gallium nitride (InGaN), and gallium aluminum nitride (AlGaN) are attracting attention as blue light-emitting diode (LED) and laser diode (LD) materials. ing. Furthermore, Group III nitride semiconductors have begun to be applied to electronic devices by taking advantage of their good heat resistance and environmental resistance.

  A substrate for GaN growth that is currently in wide use is sapphire, and a method of epitaxially growing GaN on a single crystal sapphire substrate by metal organic vapor phase epitaxy (MOVPE method) is generally used. Hereinafter, GaN will be described as a representative example of a group III nitride semiconductor.

  Since the sapphire substrate has a lattice constant different from that of GaN, a single crystal film cannot be grown by directly growing GaN on the sapphire substrate. For this reason, a method has been devised in which an AlN or GaN nitride buffer layer is once grown on a sapphire substrate at a low temperature, lattice strain is relaxed by this low temperature growth nitride buffer layer, and then GaN is grown thereon. ing. Using this low-temperature grown nitride layer as a buffer layer, GaN single crystal epitaxial growth has become possible. However, even with this method, the lattice displacement between the sapphire substrate and the GaN crystal is still difficult, and the grown GaN has countless crystal defects. This crystal defect may be an obstacle in manufacturing a GaN-based LD or a high-intensity LED.

  For the reasons described above, the appearance of a GaN free-standing substrate has been strongly desired. Since GaN is difficult to pull up a large ingot from a melt like Si and GaAs, various methods such as an ultra-high temperature and high pressure method, a Na flux method, and a hydride vapor phase growth method (HVPE method) have been tried. ing. Development of the HVPE GaN substrate is the most advanced among these methods. Distribution to the market has begun, and there are great expectations not only for LD applications but also for high-brightness LEDs.

  Usually, the end face mirror of an LD resonator is formed by cleavage. In order to know whether the GaN single crystal substrate has good cleaving property, the strain value is measured by the photoelastic effect by applying monochromatic light to the entire surface of the GaN single crystal substrate, and the maximum value in the plane of the measured strain value is below a predetermined value. Has been proposed (see Patent Document 1).

Japanese Patent Application Laid-Open No. 2002-299741

  As described above, although a GaN substrate by the HVPE method has been put into practical use, the characteristics of the GaN substrate still leave much room for improvement. The problem here is the cleaving property of the GaN substrate. Usually, the end face mirror of an LD resonator is formed by cleavage. In principle, the cleavage plane has atomic level flatness and should be ideal as a mirror. However, the actual cleavage plane is disturbed (level difference) due to various factors, which causes the yield of LD to decrease. FIG. 5 shows a differential interference image obtained by observing a cleavage plane obtained by cleaving a conventional GaN substrate manufactured by the HVPE method, using a differential interference microscope (BX11 manufactured by Olympus). A stepped portion is seen on the cleavage plane surrounded by the broken line in FIG.

  An object of the present invention is to provide a group III nitride semiconductor substrate for LD and a group III nitride semiconductor epitaxial substrate for LD using the same, which can obtain a flat cleaved surface with less disturbance such as steps.

A first aspect of the present invention is a group III nitride semiconductor substrate for LD in which an end face mirror of an LD resonator is formed by cleavage, and the group III nitride semiconductor substrate for LD has a diameter of 25 mm or more and a thickness of 250 μm. In the above, at least the outer edge side portion of the outer peripheral portion within 5 mm from the outer edge of the group III nitride semiconductor substrate for LD is tensile stress in the main surface of the group III nitride semiconductor substrate for LD. And a group III nitride semiconductor substrate for LD, wherein the group III nitride semiconductor substrate for LD is relatively larger in tensile stress than the portion on the outer edge side of the group III nitride semiconductor substrate for LD. (However, the oxygen concentration in the central portion is 1 × 10 ¹⁶ cm ⁻³ or less and the oxygen concentration in the outer peripheral portion is 1 × 10 ¹⁸ cm ⁻³ is excluded).

According to a second aspect of the present invention, in the group III nitride semiconductor substrate for LD according to the first aspect, the oxygen concentration in the outer edge portion is 1 × 10 ¹⁸ cm ⁻³ or more and 5 × 10 ²⁰ cm ⁻³ or less. And the oxygen concentration in the central portion is 2 × 10 ¹⁶ cm ⁻³ or less.

  According to a third aspect of the present invention, the group III nitride semiconductor substrate for LD according to the first or second aspect is formed of GaN, AlN, or AlGaN.

  A fourth aspect of the present invention is a group III nitride semiconductor epitaxial substrate for LD in which an epitaxial layer having an LD structure is formed on the group III nitride semiconductor substrate for LD of any one of the first to third aspects. is there.

  ADVANTAGE OF THE INVENTION According to this invention, the group III nitride semiconductor substrate for LD which can obtain the flat cleavage surface with few disturbances, such as a level | step difference, and the group III nitride semiconductor epitaxial substrate for LD using the same can be provided.

It is process drawing which shows the process of the manufacturing method of the group III nitride semiconductor substrate which concerns on one Example of this invention. It is a graph which shows the correlation with an outer periphery tensile stress and the oxygen concentration of an inclination growth part. It is a graph which shows the relationship between an outer periphery tensile stress, the level | step difference density of a cleavage surface, and a crack generation rate. It is a graph which shows the relationship between a periphery tensile stress and the yield of LD. It is a differential interference image which shows the level | step difference of a cleavage plane by a differential interference microscope. FIG. 6A is a plan view and FIG. 6B is a side view showing details of an as-grown GaN layer obtained in a crystal growth process of a group III nitride semiconductor layer in one embodiment of the present invention. FIG. 6C is an enlarged cross-sectional view of a part of the inclined surface (conical surface portion) of the GaN layer.

  Prior to the description of one embodiment of a group III nitride semiconductor substrate and a method for manufacturing the same according to the present invention, when a substrate of a group III nitride semiconductor such as GaN is cleaved, a main step is considered to cause a step on the cleavage plane. The factors are listed and explained below.

(A) Incomplete cleavage due to crystal structure The stable crystal structure of III-nitride semiconductors including GaN is hexagonal, and the cleavage of hexagonal crystal is not as clear as the cleavage of cubic. Absent. Therefore, the group III nitride semiconductor originally tends to cause a step in the cleavage plane.
(B) Local change in internal stress In most cases, a group III nitride semiconductor crystal such as GaN is produced by heteroepitaxial growth using a heterogeneous substrate such as sapphire or GaAs as a base. Since the heteroepitaxial growth has a large degree of lattice mismatch, high-density crystal defects (dislocations) occur at the interface between the epitaxial layer and the substrate. In order to reduce the dislocation density, for example, a technique of obtaining a GaN layer with few dislocations by forming a mask having an opening on a base substrate and laterally growing a GaN layer from the opening, so-called ELO (Epitaxial Lateral Overgrowth) technique Is often used. However, although the dislocation density can be reduced, a region having a low dislocation density and a region having a high dislocation density may be formed, and the density of the dislocation density may be increased. The density of dislocation density is thought to cause local changes in internal stress through the stress field of dislocations, resulting in a step in the cleavage cross section. The local change in internal stress can occur not only in the in-plane direction of the substrate but also in the thickness direction of the substrate.
(C) How to apply force at the time of cleavage A general method for cleaving a GaN substrate is to first make a small sharp flaw along the cleavage direction at the edge of the substrate. At this time, a so-called diamond pen or scriber is used. Thereafter, when a force is applied to widen the scratches at the end portions of the substrate, the cracks extend to the opposite side of the substrate starting from the scratches at the end portions, and the cleavage of the substrate is completed. At this time, if the direction and strength of the force to be applied are not appropriate, a step on the cleavage plane is likely to occur particularly in the vicinity of a scratch attached to the end.

For each of the factors listed above, the following mitigation measures can be considered.
First, before cleaving, the substrate is cut from the back side to make the substrate thinner. By making the thickness approximately 200 μm or less, the step on the cleavage plane is considerably reduced. This is considered to be due to the fact that the change in stress in the thickness direction of the substrate becomes smaller by making the substrate thinner.
It is also effective to use a uniform substrate without dislocation density. For producing a substrate having a uniform dislocation density, for example, Ti is vapor-deposited on the surface of the GaN thin film on the sapphire substrate, and this is heat-treated to form a void structure in the GaN thin film. A VAS method (Void-Assisted Separation Method) that grows and peels the sapphire substrate from the void structure portion is preferably used.
However, even after these measures are taken, the step on the cleavage plane near the edge of the substrate due to the factor (c) still remains likely to occur.

The present inventor found that the cleavage plane is disturbed by the factor (c) because of the unclear cleavage nature inherent in the group III nitride semiconductor crystal described in the factor (a). When cleaving with a large amount of force, and when cleaving with a large amount of force, steps are likely to occur depending on the direction and strength of the force applied, and if it can be cleaved with a lighter force Thought that this problem could be solved.
Although it can be easily imagined that the thickness of the substrate can be reduced by making the substrate thinner, it is possible that the substrate will be damaged at the time of handling. It is impossible to further improve the cleavage by the method of thinning the substrate.

  Therefore, as a result of intensive studies, the present inventor has come up with the idea that a tensile stress should be included in the outer peripheral portion (outer edge portion) of the substrate that is the starting point of cleavage. If tensile stress is inherent in the outer peripheral part (outer edge part) of the substrate, cleavage will start spontaneously with a slight light force applied. The idea is that the occurrence is suppressed.

  As a specific method for making tensile stress inherent in the outer peripheral portion (outer edge portion) of the substrate, the present inventor has found a method of performing crystal growth while providing a region having a high doping concentration on the outer peripheral portion of the substrate. That is, a region having a high doping concentration is provided in the outer peripheral portion of the growth region, and tensile stress is generated due to physical mismatch due to doping. However, the width of the region having a tensile stress is preferably within 5 mm from the outer edge of the substrate having a diameter of 25 mm or more, for example. This is because if the tensile stress region is wider than 5 mm, the entire substrate may be warped. In order to generate sufficient tensile stress even if the width of the tensile stress region is narrow, the boundary of the region having the tensile stress must be steep. However, it is usually difficult to supply the dopant raw material without being limited to a narrow region during the crystal growth process.

  In view of this, the present inventor has devised a method that utilizes the difference in dopant incorporation efficiency depending on the crystal plane index. For example, even if grown under the same conditions, GaN grown on the (10-11) plane or the (11-22) plane has nearly 1000 times more oxygen than GaN grown on the (0001) plane (C plane). May be captured. That is, for example, an inclined facet plane (for example, a plane composed of (10-11) plane, (11-22) plane, etc.) is formed on the outer periphery of the crystal to be grown with the C plane as the main plane, Crystal growth is performed while supplying oxygen or an oxygen compound. As a result, two regions with greatly different doping concentrations are separated by a steep interface such that the oxygen doping concentration in the outer peripheral portion growing on the inclined facet surface is high and the oxygen doping concentration in the central side portion growing on the C plane is low. Can be formed.

  However, excessive tensile stress causes unintended cracks in the substrate due to minor triggers during handling and epitaxial growth. Moreover, even if the tensile stress is too small, the effect of improving the cleavage property does not appear. It is necessary to adjust the tensile stress to an appropriate range. In addition to providing the substrate with a doping concentration difference, the tensile stress can be adjusted by reducing the width of the tensile stress generating portion by grinding the outer peripheral portion of the substrate.

  Hereinafter, an embodiment of the group III nitride semiconductor substrate and the method for manufacturing the same of the present invention will be described.

  The group III nitride semiconductor substrate according to the present embodiment is a group III nitride semiconductor substrate having a diameter of 25 mm or more and a thickness of 250 μm or more, and at least the outer periphery within 5 mm from the outer edge of the group III nitride semiconductor substrate. In the outer edge portion, the stress in the main surface of the group III nitride semiconductor substrate is a tensile stress, and relative to the portion on the center side relative to the outer edge side portion of the group III nitride semiconductor substrate. The tensile stress is large.

  Promoting the spontaneous propagation of cracks during cleavage by providing a portion with tensile stress in the outer periphery (particularly the outer edge (outer edge)) within 5 mm from the outer edge of the group III nitride semiconductor substrate. And the occurrence of a step on the cleavage plane can be suppressed.

The magnitude of the tensile stress that the outer edge of the substrate has is preferably 30 MPa or more and 150 MPa or less. This is because when the tensile stress is set to 30 MPa or more, the generation of a step on the cleavage plane can be effectively suppressed, and when the tensile stress exceeds 150 MPa, the frequency of the substrate being cracked irregularly during handling or the like increases. Because it will end up. A more preferable magnitude of tensile stress is 50 MPa or more and 120 MPa or less.
The stress (stress distribution) in the main surface of the group III nitride semiconductor substrate can be determined by, for example, photoelasticity measurement. Here, photoelasticity measurement is a method of measuring the amount of deviation of the phase of the light transmitted through the sample due to birefringence. The amount of phase deviation and the stress of the sample are correlated, so the stress of the sample is measured. can do.

  The tensile stress is relatively larger at the outer edge portion of the substrate than at the central side portion when the central portion of the substrate has a tensile stress smaller than the tensile stress at the outer edge portion. This includes the case where the central portion of the substrate is zero stress, or the case where the central portion of the substrate is compressive stress.

  The main surface of the group III nitride semiconductor substrate is preferably a C plane or an inclined plane inclined from the C plane. If the main surface of the group III nitride semiconductor substrate is a C plane or an inclined plane close to the C plane, it is suitable for growing an element structure such as a light emitting device on the group III nitride semiconductor substrate (self-standing substrate). It is. The main surface of the group III nitride semiconductor substrate may be an A surface, an M surface, or an inclined surface inclined from the A surface, the M surface, etc. in addition to the C surface or the inclined surface inclined from the C surface. Good. Here, the inclined surface inclined from the C surface or the like is a surface inclined with respect to the C surface or the like within an angle range of 10 ° or less.

  The group III nitride semiconductor substrate is preferably a self-supporting substrate. The “self-supporting substrate” refers to a substrate that can not only maintain 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 250 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.

  The group III nitride semiconductor substrate is preferably a free-standing substrate having a diameter of 25 mm or more. The diameter of the group III nitride semiconductor substrate depends on the diameter of the base substrate (seed crystal substrate) used at the time of manufacture. By using the base substrate having a large diameter, a large-diameter self-standing substrate can be obtained. For example, since a sapphire substrate having a diameter of 6 inches (152.4 mm) is commercially available, a GaN seed crystal substrate having a diameter of 6 inches is manufactured using the sapphire substrate, and further using this GaN seed crystal substrate, the diameter is about 6 mm. A GaN free-standing substrate according to the present invention having an inch or less can be manufactured.

  Next, a method for manufacturing a group III nitride semiconductor substrate according to an embodiment of the present invention will be described.

  The Group III nitride semiconductor substrate manufacturing method of the present embodiment is a Group III nitride semiconductor substrate manufacturing method including a step of crystal growth of a Group III nitride semiconductor layer on a seed crystal substrate. In the step of crystal growth of the layer, crystal growth is performed while forming a growth surface inclined at an angle larger than 0 ° and smaller than 90 ° with respect to the main surface of the seed crystal substrate in the outer peripheral portion of the group III nitride semiconductor layer And the growth concentration of the group III nitride semiconductor layer in the outer peripheral portion having the inclined growth surface is set higher than the doping concentration of the group III nitride semiconductor layer in the center side of the outer peripheral portion. I do.

In order to perform crystal growth while forming an inclined growth surface on the outer peripheral portion of the group III nitride semiconductor layer, for example, the outer peripheral portion of the seed crystal substrate surface is covered with an annular mask, and the seed crystal substrate on the inner side of the mask is covered. There is a method of using a growth condition in which an inclined facet is formed on the outer edge of the group III nitride semiconductor layer when growing the group III nitride semiconductor layer.
For the inclined growth surface, a surface in which a dopant such as oxygen is easily taken in is selected. For example, the {10-11} plane which is equivalent to the (10-11) plane including the (10-11) plane, or (11 The crystal grows by forming an inclined facet plane composed of a {11-22} plane that is equivalent to the (11-22) plane including the (-22) plane.

  As the seed crystal substrate, a group III nitride semiconductor substrate such as a GaN substrate, a sapphire substrate, or a GaAs substrate is used. Examples of the group III nitride semiconductor substrate manufactured from the group III nitride semiconductor layer include a GaN substrate, an AlN substrate, and an AlGaN substrate.

The HVPE method is preferably used for crystal growth of the group III nitride semiconductor layer. For example, the growth of GaN by the HVPE method is performed as follows. HCl gas is supplied to a container containing molten Ga to generate GaCl gas, and this GaCl gas and NH ₃ gas introduced separately are supplied to the seed crystal substrate heated in the growth furnace of the HVPE apparatus. As a result, GaCl and NH ₃ react with each other on the surface of the seed crystal substrate to grow a GaN crystal.
By performing crystal growth of a group III nitride semiconductor such as GaN in a growth furnace in an atmosphere where oxygen gas or oxygen compound gas is present, the group III nitride semiconductor crystal is doped with oxygen. Oxygen gas or oxygen compound gas is supplied from the outside of the growth furnace in which crystal growth is performed, or oxygen gas generated by the reaction between a quartz member such as a reaction tube constituting the growth furnace and the atmospheric gas in the growth furnace, etc. Supply.

Although the detailed mechanism by which oxygen gas is generated by the reaction between the quartz member such as the reaction tube constituting the growth furnace and the atmospheric gas in the growth furnace is not well understood, the HVPE method is a hot wall system, and the quartz reaction tube Are heated by an external heater and become a high temperature, and in the HVPE method, a chloride raw material (HCl gas or the like) is used. Therefore, when the chloride raw material and the quartz member come into contact with each other at a high temperature, quartz is decomposed and Si ( It is considered that a gas containing silicon) and O (oxygen) is generated. For this reason, it is considered that GaN grown by the HVPE method is more likely to be doped with Si and O as compared to the MOVPE method which is a cold wall method and does not use a chloride raw material.
In order to adjust the amount of oxygen gas generated by the reaction between the quartz member such as a reaction tube and the atmospheric gas in the growth furnace, for example, the following may be performed. Increasing the surface area of the quartz member (installed in the high temperature region of the growth furnace) that comes into contact with the source gas increases the amount of oxygen. If the high temperature region of the growth furnace is made of, for example, graphite, the oxygen amount decreases. Also, the growth conditions can be adjusted, and when the growth rate or growth temperature is increased, oxygen incorporation into the group III nitride semiconductor crystal tends to increase.

When the group III nitride semiconductor layer is doped with oxygen, the oxygen concentration in the group III nitride semiconductor layer in the outer peripheral portion having an inclined growth surface is 1 × 10 ¹⁸ cm ⁻³ or more and 5 × 10 ²⁰ cm ⁻³ or less. Is preferable. By setting the oxygen concentration within this range, it is possible to apply an appropriate tensile stress that can suppress the occurrence of a step during cleavage at the outer edge of the group III nitride semiconductor substrate. It is preferable that the oxygen concentration in the group III nitride semiconductor layer at the center side with respect to the outer peripheral portion is not more than the lower limit of detection (2 × 10 ¹⁶ cm ⁻³ ) of a general SIMS analyzer.

You may make it implement the process of grinding the outer peripheral part of a group III nitride semiconductor layer after the process of crystal-growing a group III nitride semiconductor layer. The tensile stress is applied and adjusted by providing a doping concentration difference in the group III nitride semiconductor substrate. In addition to providing this doping concentration difference, the outer periphery of the group III nitride semiconductor substrate is ground. By applying, it is possible to adjust the width of the tensile stress portion to be small.
As shown in the examples described later, when a tensile stress is applied by adding oxygen to the outer peripheral portion having an inclined growth surface at a high concentration, a group III nitride semiconductor having a low oxygen concentration at the center side from the outer peripheral portion. Since tensile stress remains also in the layer region, a group III nitride semiconductor substrate excellent in cleavage is obtained even when all of the outer peripheral portion having an inclined growth surface is removed by grinding.

  Next, examples of the present invention will be described.

Example 1
In Example 1, a GaN substrate having a tensile stress portion on the outer periphery of the substrate was produced. The manufacturing process of the GaN substrate of Example 1 and the cleavage of the obtained GaN substrate will be described with reference to FIG.

  First, a disc-shaped GaN free-standing substrate (seed crystal substrate) 1 having a diameter of 60 mm and a thickness of 400 μm with a C surface (Ga surface) as a main surface was prepared (FIG. 1A). It was confirmed by photoelasticity measurement that the stress distribution of the seed crystal substrate 1 was substantially uniform. 1A to 1E, the upper part is a plan view, the lower part is a cross-sectional view, and FIG. 1F is a perspective view of the GaN substrate cleaved in a bar shape.

Next, an annular high-purity carbon mask 2 having a circular opening having a diameter of 55 mm is overlaid on the seed crystal substrate 1 (FIG. 1B), and the laminated state is placed in the growth furnace of the HVPE apparatus. GaN homoepitaxial growth was performed using the C plane as the growth plane. GaCl gas and NH ₃ gas were used as raw materials for GaN growth. GaCl gas was generated by a reaction between Ga melt and HCl gas installed in the upstream region of the growth furnace. The partial pressures of GaCl gas and NH ₃ gas were 0.8 kPa and 5 kPa, respectively. A mixed gas of H ₂ and N ₂ was used as the carrier gas. Further, oxygen gas of 5 Pa was added and supplied into the growth furnace. The pressure in the growth furnace was atmospheric pressure, and the growth temperature was 1060 ° C. At this time, the growth rate was about 120 μm / h. By growing for 5 hours, a GaN layer 3 of 600 μm was obtained (FIG. 1C).
An inclined surface 4 composed of a {10-11} plane and a {11-22} plane was formed on the outer peripheral portion of the GaN layer 3 (portion of about 350 μm radially inward from the outer edge). As a result of the SIMS analysis, the oxygen concentration in the C-plane growth portion grown on the inner side of the inclined surface 4 of the GaN layer 3 is not more than the detection lower limit (2 × 10 ¹⁶ cm ⁻³ ). The oxygen concentration was 1 × 10 ¹⁹ cm ⁻³ , and it was confirmed that the oxygen concentration was greatly different.

Details of the as-grown GaN layer 3 will be further described with reference to FIG. 6A is a plan view (top view) of the GaN layer 3 in the as-grown state, FIG. 6B is a side view of the GaN layer 3, and FIG. 6C is a part of the inclined surface 4 of the GaN layer 3 ( It is sectional drawing to which the conical surface part 4b) was expanded.
As shown in FIGS. 6A and 6B, the GaN layer 3 has a truncated cone shape as a whole. The upper surface 3a of the GaN layer 3 is a C plane, and the inclined surface 4 which is the side surface of the GaN layer 3 is composed of a flat portion 4a and a conical surface portion 4b. The flat portions 4a are {10-11} planes and appear at six locations every 60 degrees along the outer periphery of the truncated GaN layer 3. The conical surface portion 4b between the flat portions 4a and 4a appears to be a conical surface with the naked eye, but when enlarged and observed with a microscope or the like, as shown in FIG. 6C, the {10-11} surface and the {11-22 surface } The surface is a jagged surface with fine and alternating surfaces. In addition, the chain line in FIG.6 (c) shows the outline R of the cross section of the conical surface considered as a smooth surface by visual observation.

  After the GaN growth was completed, the back surface side (seed crystal substrate 1 side) of this substrate was ground by 500 μm, and after removing the seed crystal substrate 1 completely, mirror polishing was performed. Furthermore, the surface side (GaN layer 3 side) of the substrate was also ground and polished to obtain a GaN substrate 5 having an inclined growth portion having a thickness of 400 μm and a diameter of 55 mm (FIG. 1D). When the photoelasticity measurement is performed again on the GaN substrate 5, concentric tensile stress (hereinafter referred to as “outer peripheral stress”) is generated in the outer peripheral portion within about 3 mm from the edge (outer edge) of the GaN substrate 5. It was confirmed that

  Furthermore, the outer peripheral part including the inclined growth part of this GaN substrate 5 was ground, and the diameter was adjusted to 2 inches (50.8 mm) to obtain a disk-shaped GaN substrate 6 having a diameter of 2 inches (FIG. 1 (e)). ). Even in the GaN substrate 6 in which the outer peripheral portion including the inclined growth portion was ground, it was found that when the photoelasticity measurement was performed, an outer peripheral stress of about 50 MPa remained. Although the high oxygen concentration region (gradient growth part) that caused the generation of tensile stress has been completely removed, the tensile stress remains because of the crystal growth under the influence of the tensile stress at the outer periphery. As a result, the distribution of defects in the C-plane growth portion on the center side changes, and it is assumed that the stress distribution changes as a result.

The GaN substrate 6 obtained by the above process was set in an MOVPE apparatus, and an epitaxial layer having an LD (laser diode) structure was grown on the GaN substrate 6. TMG (trimethylgallium), TMA (trimethylaluminum), TMI (trimethylindium), and NH ₃ were used as raw materials. An n-type AlGaN cladding layer, a GaN barrier layer / InGaN well layer multiple quantum well structure (MQW) active layer, a p-type AlGaN cladding layer, and a p-type GaN contact layer are formed on the GaN substrate 6 as an LD structure epitaxial layer. Laminated sequentially. Thereafter, the back surface of the epitaxial substrate on the GaN substrate 6 side was ground and processed until the total thickness became 200 μm.

  A ruled line (scribe line) having a length of 1 mm along the M plane was applied to the edge portion on the back surface side (GaN substrate 6 side) of the obtained epitaxial substrate using a diamond scribe device. Next, when both sides of the crease line were sandwiched with flat tweezers and a very slight force was applied to open the scratches (notch) by the crease, the cleavage was easily completed.

In the same manner, a GaN substrate 6 having a diameter of 2 inches is cleaved into bars 7 each having a width of 5 mm (FIG. 1 (f)), and a region of the cleaved surface 8 within 20 mm from the marked edge (the mesh is applied in the figure). The density of the step formed in the above was investigated using a differential interference microscope. That is, the step density was determined by counting the steps generated on the cleavage plane 8 under a differential interference microscope and dividing the counted number by the observed width of 20 mm.
The same step density was investigated by changing the oxygen concentration in the inclined growth portion of the GaN layer 3 by changing the oxygen gas partial pressure in the atmosphere during the growth of the GaN layer 3 in the same process as described above. Various GaN substrates 6 were also tested. A photoelastic method was used to measure the peripheral stress of the GaN substrate 6, and a value at a position 1 mm from the edge of the GaN substrate 6 was measured. FIG. 2 shows the correlation between the peripheral stress (peripheral tensile stress) and the oxygen concentration in the inclined growth portion. As shown in FIG. 2, the peripheral stress tended to increase as the oxygen concentration in the inclined growth portion increased.

  In addition, FIG. 3 shows the results of investigation on the relationship between the step density of the cleavage plane and the peripheral stress (peripheral tensile stress). It was found that a large step-suppressing effect was obtained when the peripheral stress was 30 MPa or more, and that it was even more remarkable at 50 MPa or more. Further, as shown in FIG. 3, when the peripheral stress exceeds 120 MPa, the frequency (called “crack occurrence rate”) of cracking indefinitely during the MOVPE growth or after the growth or during the subsequent grinding process. It was found that the crack generation rate rapidly increased when the pressure exceeded 150 MPa. From the above results, it was found that good results were obtained by setting the peripheral stress to 30 MPa or more and 150 MPa or less, and more preferable effects were obtained by setting it to 50 MPa or more and 120 MPa or less.

  In addition, regarding the GaN substrate 6 having a small crack generation rate and an outer peripheral stress of less than 120 MPa, an LD was actually fabricated from the portion where the step density was measured, and the yield was evaluated. However, the yield was evaluated as a defective product having an LD threshold current value 20% or more higher than the normal value. The result is shown in FIG. It was found that a very good value can be obtained when the magnitude of the peripheral tensile stress is about 30 MPa or more.

(Example 2)
First, a GaN substrate 6 was produced in the same manner as in Example 1. However, the oxygen supply amount during HVPE growth was adjusted so that the oxygen concentration in the inclined growth portion of the GaN layer 3 was 5 × 10 ²⁰ cm ⁻³ . Moreover, the grinding amount of the outer periphery of the GaN substrate 5 having the inclined growth portion was increased so that the diameter of the GaN substrate 6 became 45 mm. It was confirmed by photoelasticity measurement that the outer peripheral tensile stress of 80 MPa remained on the GaN substrate 6 of Example 2 after outer periphery grinding.
An epitaxial layer having the same LD structure as that of Example 1 was grown on the GaN substrate 6 by the MOVPE method, and after grinding to a thickness of 200 μm by crushing, the step density of the cleaved surface was examined. However, a very good value of 0.08 lines / mm was obtained. At this time, the yield of LD evaluated in the same manner as in Example 1 was very good at about 97%.

(Comparative example)
In the comparative example, the cleaving property was examined using a GaN free-standing substrate having a uniform stress distribution in the main surface (C surface). First, a GaN free-standing substrate (seed crystal substrate) having a diameter of 2 inches and a thickness of 400 μm having the same C surface (Ga surface) as that of Example 1 was prepared. It was confirmed by photoelasticity that the stress distribution of the seed crystal substrate was substantially uniform.
Next, this GaN seed crystal substrate was set in an MOVPE apparatus, and an epitaxial layer having an LD structure was grown in the same manner as in Example 1. Thereafter, the back surface was ground and processed until the total thickness reached 200 μm.
A ruled line having a length of 1 mm along the M-plane was attached to the edge portion of the obtained epitaxial substrate using a diamond scribe device. The LD epitaxial substrate was cleaved when the both sides of the crease line were sandwiched with flat tweezers and a force was applied to open the wound by the crease. A stronger force was required at the time of cleavage than in Examples 1 and 2. When the step density on the cleavage plane was examined in the same manner as in Example 1, it was as high as about 1.5 lines / mm. Further, the yield of LD evaluated in the same manner as in Example 1 was very low at about 55%.

(Example 3)
Using a GaN free-standing substrate (seed crystal substrate) 1 having a diameter of 6 inches (152.4 mm) and a circular high-purity carbon mask 2 having a circular opening having a diameter of 147.4 mm, a 1200 μm GaN layer 3 is grown. Except for the above, a GaN substrate 5 having a thickness of 1000 μm, a bottom diameter of 147.4 mm, and an inclined growth portion was obtained by the same manufacturing process as in Example 1. The 6-inch diameter seed crystal substrate 1 forms a GaN thin film on a 6-inch diameter sapphire substrate, deposits a Ti layer, and heat-treats it to form a void structure in the GaN thin film. This is a substrate obtained by growing GaN thickly by the HVPE method and peeling the sapphire substrate from the void structure portion.
Also in the GaN substrate 5 of Example 3 above, as in Example 1, it was confirmed that concentric outer peripheral stress was generated in the outer peripheral portion within about 3 mm from the edge of the GaN substrate 5.
The outer peripheral portion including the inclined growth portion of the GaN substrate 5 was ground to obtain a GaN substrate 6 having a diameter of 143 mm. When the photoelasticity measurement was performed on the GaN substrate 6, an outer peripheral stress of about 50 MPa remained.
An epitaxial layer having the same LD structure as in Example 1 is grown on the GaN substrate 6 in Example 3 by the MOVPE method, and back-surface grinding is performed to a thickness of 200 μm. When the density was examined, it was a good value of 0.1 / mm. Further, the yield of LD evaluated in the same manner as in Example 1 was a favorable value of about 96%.

Example 4
A diameter of 2 inches obtained by grinding an outer peripheral portion including an inclined growth portion by the same manufacturing process as in Example 1 except that a GaN free-standing substrate (seed crystal substrate) 1 having an inclined surface inclined by 10 degrees from the C plane is used. A GaN substrate 6 having a thickness of 400 μm was obtained. The growth surface of the GaN substrate 6 of Example 4 was an inclined surface inclined by 10 degrees from the C plane. When the photoelasticity measurement was performed on the GaN substrate 6, an outer peripheral stress of about 50 MPa remained.
An epitaxial layer having the same LD structure as that of Example 1 is grown on the GaN substrate 6 of Example 4 by the MOVPE method, and back-surface grinding is performed to a thickness of 200 μm. When the density was examined, it was a favorable value of 0.09 pieces / mm. Further, the yield of LD evaluated in the same manner as in Example 1 was a favorable value of about 98%.

  Although the case of GaN has been described in the above embodiments, the present invention can be suitably used for other group III nitride semiconductors such as AlN and AlGaN in addition to GaN. In addition to the HVPE method, the present invention can be similarly applied to a solution growth such as a high-temperature and high-pressure method, a Na flux method, or an ammonothermal method.

DESCRIPTION OF SYMBOLS 1 Seed crystal substrate 2 Mask 3 GaN layer 4 Inclined surface 5 GaN substrate 6 having an inclined growth part 6 GaN substrate 7 Cleaved GaN substrate 8 Cleaved surface inspected for step density

Claims (4)

In a group III nitride semiconductor substrate for LD in which an end face mirror of an LD resonator is formed by cleavage,
The group III nitride semiconductor substrate for LD has a diameter of 25 mm or more and a thickness of 250 μm or more,
At least the outer edge side portion of the outer peripheral portion within 5 mm from the outer edge of the group III nitride semiconductor substrate for LD is tensile stress in the main surface of the group III nitride semiconductor substrate for LD, and the LD The group III nitride semiconductor substrate for LDs, wherein the group III nitride semiconductor substrate for LD is characterized in that the tensile stress is relatively larger than the portion on the central side rather than the portion on the outer edge side of the group III nitride semiconductor substrate (for the central portion) (Except for GaN substrates having an oxygen concentration of 1 × 10 ¹⁶ cm ⁻³ or less and an oxygen concentration of 1 × 10 ¹⁸ cm ⁻³ at the outer periphery). The group III nitride semiconductor substrate for LD according to claim 1,
The oxygen concentration in the outer edge portion is 1 × 10 ¹⁸ cm ⁻³ or more and 5 × 10 ²⁰ cm ⁻³ or less,
The group III nitride semiconductor substrate for LD, wherein the oxygen concentration in the central portion is 2 × 10 ¹⁶ cm ⁻³ or less.   3. The group III nitride semiconductor substrate for LD according to claim 1 or 2, wherein the group III nitride semiconductor substrate for LD is formed of any one of GaN, AlN, or AlGaN.   A group III nitride semiconductor epitaxial substrate for LD, wherein an LD layer epitaxial layer is formed on the group III nitride semiconductor substrate for LD according to claim 1.