HK1024099B - Gan single crystal substrate - Google Patents

Description

GaN single crystal substrate

Technical Field

The present invention relates to a GaN single crystal substrate for a blue light emitting element such as a Light Emitting Diode (LED) or a Laser Diode (LD) using a group III-V nitride semiconductor (GaN series), and a method for producing the same.

Background

Fig. 1 shows the ratio of the lattice constant and the thermal expansion coefficient of GaN to a material that can be used as a substrate for GaN growth. For sapphire (Al)₂O₃) SiC, Si, GaAs, ZnO, etc. A nitride-based semiconductor light emitting device or a GaN-based light emitting device has been conventionally produced by epitaxially growing a GaN thin film or the like on a sapphire substrate. Sapphire (Al)₂O₃) The substrate is chemically stable and also heat resistant. By forming a buffer layer of a substance whose lattice constant differs from that of GaN by about 16%, GaN is epitaxially grown on the buffer layer. With such an advantage, a sapphire substrate is used. The sapphire substrate with a thin film of GaN or the like added thereto can be used as an LED or LD as it is. That is to say a composite element of sapphire and GaN. It is a practical element, and GaN series LEDs on sapphire substrates are commercially available. Further, GaN-series LDs are available on the market soon.

The lattice constants of sapphire and GaN are not uniform. Nevertheless, practical GaN elements are grown on sapphire substrates. This is because the relaxation of the lattice constant is gradually caused. Fig. 2 is a graph showing the relationship between the thickness of the GaN film on sapphire and the change in lattice constant. The lattice constant gradually changes with the change in film thickness. Sapphire is also currently the best substrate. All the materials produced in mass production at present have GaN/Al₂O₃And (5) structure. Such structures are described, for example, in the following documents:

① JP 5-183189A

② Japanese patent application laid-open No. Hei 6-260680.

However, sapphire substrates also have problems. The defect density of the GaN epitaxial layer on the sapphire substrate is extremely high. This is presumably caused by lattice mismatch. How the defect density is10⁹cm^-2. The defect can be said to be loose. However, the GaN LEDs nevertheless have a short lifetime. Is really an incredible material. Therefore, it is thought that the high density defects are a crystallographic problem, but the LED has not actually had a large problem. However, there is a high possibility that a problem may occur as the LD.

But there are several mechanical difficulties with sapphire. Sapphire (Al)₂O₃) Is chemically stable and has high hardness. Chemically stable, but it may seem that this is not actually the case. Only the substrate is etched away, leaving behind the GaN. The most difficult is cleavage. It is too hard. When the GaN/sapphire substrate is divided into LED chips, the division process is difficult. Since the separation is not natural, destructive cutting is performed with a tool such as a knife. The product yield is low due to breakage. In addition, sapphire was removed from the GaN/sapphire substrate with a polishing agent, and examples of conducting experiments were reported.

③ S.Nakamura et al, J.J.appl.Phys.vol.37, Part2, No.3B (1988), ppL309-L312 are currently in experimental stage.

For easy dicing, a material having cleavage properties such as SiC is considered as a substrate. For example, in the case of a car,

④ A.Kuramate et al, J.J.appl.Phys.vol.36, Part2 No.9A/B (1997), PP L1130-L1132 propose SiC substrate GaN elements, but SiC is also problematic.

In any case, the conventional GaN element is formed by growing GaN on a different kind of substrate, and is still accompanied with sapphire since the substrate is not removed. Is a composite device.

In order to epitaxially grow GaN on a substrate, the substrate must be heated to a high temperature of 1000 ℃ or higher. If such a high temperature is not reached, a gas phase reaction cannot be caused. When the temperature is lowered after the growth of the epitaxial layer of GaN or the like, the influence is caused by the difference in thermal expansion coefficient between the thin film and the substrate. The coefficient of thermal expansion is a function of temperature and not a constant. Therefore, the following results can be obtained by simply performing a brief comparison. Assuming that the thermal expansion coefficient of GaN is 1, the thermal expansion coefficient of GaAs is about 1.08 times, that of SiC is about 0.87 times, and that of sapphire is about 1.36 times.

The first problem caused by the difference in thermal expansion coefficient between the film and the substrate is the generation of thermal stress on the GaN film and the occurrence of minute cracks in the GaN film. A second problem arising from the difference in thermal expansion coefficient is the occurrence of bowing upon cooling. The entire wafer is deformed by the influence of the bending. These problems are particularly significant in the case of growing the thickness of GaN, and as described above, even if the sapphire substrate is removed by polishing, a GaN substrate of a large area cannot be obtained due to bending. Only GaN substrates with tops many a few mm square have been reported. It is not industrially available in any way.

In the past, a test was also conducted in which GaN was grown using GaAs as a substrate. However, GaAs substrates have drawbacks. As evaporates from the GaAs surface in a high temperature atmosphere during growth. GaAs is reacted with ammonia. For this reason, it is impossible to produce a GaN crystal of good quality on a GaAs substrate. There is little hope for GaN growth on GaAs substrates.

Now only the GaN/sapphire element remains. Several developments that may make the sapphire substrate process more pure are possible. The LED may have a long lifetime regardless of the dislocation density, and may have a long lifetime if the dislocation density is low. Wherein the lifetime of blue LD has not been satisfactory so far. This may be due to the high density of defects present. GaN with lower defects grown on sapphire substrates was further tested.

⑤ journal of society for electronic information and communications, journal C-II, vol. J81-C-II, p 58-64

It is obtained by forming a striped mask on a sapphire substrate and growing a thick film of GaN on the striped mask. GaN growth soon interconnects across the stripes from the planes laterally separated by the stripes. There have been reports on a large reduction in defect density due to such streak growth. Defect density reduction is a result. However, the stripe growth method on a sapphire substrate has little effect on solving other problems. Always on sapphire, still with the sapphire substrate. The problem of being strong and difficult to split is not solved. Since cleavage is difficult, the cutting process is difficult and the yield is low. Since sapphire is still carried, there is a difference in thermal expansion coefficient, which causes dislocations and many minute cracks in the GaN single crystal. In addition, the curvature cannot be ignored. There is also a problem that it is not suitable for wafer processing due to the bending.

The use of materials with different coefficients of thermal expansion and different lattice constants is often limited. The most desirable substrate is a GaN substrate. However, the GaN substrate is preferable. However, there is no large GaN substrate. The wafer diameter suitable for a semiconductor manufacturing process is required to be one inch or more, preferably two inches or more. However not so large GaN substrates.

In order to grow large crystals, although there are the czochralski method and the brilliant method, they are all solidified from a raw material melt to a solid. Since the starting point of the melt is possible, large single crystals can be produced. However, GaN can be melted only by heating. Subliming into a gas. A small amount of GaN is added to Ga and heated at an ultrahigh pressure of several tens of thousands of atmospheres to form a Ga-GaN melt. However, the space capable of exhibiting an ultrahigh pressure is extremely narrow. It is impossible to make a large crystal in a narrow space. If a large-scale ultrahigh-pressure device is manufactured, the cost is too high to be practical. Since the method for manufacturing a large crystal is not applicable, a large GaN cannot be manufactured so far, and a GaN substrate does not exist.

And manufacturing the GaN film by using a film growth method. Either method is a reaction from the gas phase to the solid phase. The following 4 methods for growing a GaN thin film on a sapphire substrate are known.

HVPE method (Hydride Vapor phase growth method: Hydride Vapor phase Epitaxy)

MOC method (Metal organic chloride vapor phase growth method)

MOCVD method (Metal organic chemical vapor deposition)

4. Sublimation process

The MOC method is a method in which an organic metal of Ga such as trimethylgallium TMG is reacted with HCl gas in a hot-wall furnace, and when GaCl is grown, NH gas flowing near a substrate is reacted with the resulting reaction product₃And reacting to grow a GaN film on the heated substrate. Hydrogen is actually used as a carrier gas, and an organic metal gas or HCl gas is supplied. Since an organic metal is used as the Ga raw material, carbon is mixed into GaN as an impurity. Yellow due to the action of carbon. It is difficult to obtain colorless transparent GaN crystals. Due to the action of carbon, the carrier concentration (free electrons) increases and the electron mobility decreases. The electrical characteristics deteriorate due to the action of carbon. Although the gas phase process of organo-metal chlorides is a good process, it has such disadvantages.

The MOCVD method is most used as a GaN thin film growth method. In a cold wall type reaction furnace, an organic metal of Ga such as TMG and ammonia NH are added₃Along with hydrogen gas, onto a heated substrate. TMG and NH on a substrate₃And reacting to generate the GaN film.The method has low recovery rate of raw material gas due to the use of a large amount of gas. When a thin layer such as a light-emitting layer is formed, it is not known that this is a major problem. However, as described in the present application, assuming that a thick GaN substrate is fabricated, this problem becomes a great difficulty. In addition, there is a problem of carbon contamination as in the MOCmethod. It is yellow. Carbon becomes an n-type impurity and emits electrons. Thus, the mobility is low. The electrical characteristics are deteriorated. There are such difficulties.

The HVPE method uses metallic Ga as the Ga raw material. A container for storing Ga is provided in the hot wall type furnace, and Ga metal is put in the container. Ga has a low melting point and becomes a melt at 30 ℃ or higher. If hydrogen gas or HCl gas is injected into the reactor, gallium chloride is generated. Using a carrier gas H₂GaCl is delivered near the substrate and reacts with ammonia, depositing GaN on the substrate surface. The method is used inThe metal Ga raw material does not contain carbon. Carbon is not incorporated into the film and is therefore not dyed. Also has the advantages that the electron mobility is not reduced, and the like.

GaN single crystals are most suitable for use as substrates for GaN light-emitting devices. There is no large GaN substrate to date.

Disclosure of Invention

It is a first object of the present invention to provide a freestanding GaN crystal with a large area that can be made into a substrate. A second object of the present invention is to provide a GaN substrate that is not bent. A third object of the present invention is to provide a colorless and transparent GaN substrate containing no carbon. However, depending on the growth conditions, As having a large vapor pressure is incorporated into GaN single crystal and may have a light brown color. It is a fourth object of the present invention to provide a method of manufacturing a GaN single crystal ingot, slicing it into thin slices, and manufacturing a plurality of wafers.

Representative embodiments of the invention can be summarized as follows:

1. a GaN single crystal substrate characterized in that: the substrate has a diameter of 20mm or more and a thickness of 0.07mm or more, an inherent internal stress of 7MPa or less, a curvature radius of bending of 600mm or more, a deflection amount at the center of the substrate having a diameter of 2 inches of 0.55mm or less, and an angle between the normal direction of the substrate surface and the normal direction of a low-index crystal plane having the highest parallelism to the substrate surface of 3 DEG or less in the substrate.

2. The GaN single crystal substrate according to 1, characterized in that: the absorption coefficient of light with the wavelength of 400 nm-600 nm is 120cm^-1The following.

3. The GaN single crystal substrate according to 1, characterized in that: the substrate contains 10 of As¹⁷One cm^-3The following.

4. The GaN single crystal substrate according to 1, characterized in that: the substrate does not contain carbon.

5. The GaN single crystal substrate according to 1, characterized in that: the substrate surface and the back surface are ground.

6. The GaN single crystal substrate according to claim 5, characterized in that: the dispersion of the normal line of the low-index crystal plane having the highest parallelism to the substrate surface in the substrate is 4 DEG or less.

7. The GaN single crystal substrate according to claim 5, characterized in that: the warpage of the polished substrate is 200 μm or less in terms of a wafer having a diameter of 2 inches.

8. The GaN single crystal substrate according to claim 5, characterized in that: the surface of the substrate is a GaN (0001) plane.

A mask having windows distributed at equal intervals in the [11-2](note: 2, the same applies hereinafter) direction and at equal intervals in the [ 110]direction is formed on a GaAs (111) single crystal substrate, a GaN buffer layer is grown on the window portion on the mask at a low temperature, then the temperature is raised to a high temperature, and the GaN layer is epitaxially grown on the buffer layer and on the mask by an HVPE method, and the GaAs substrate is removed to obtain a GaN single crystal substrate. This is a method of making a substrate. Or using the single crystal substrate as a seed crystal, forming a thick GaN epitaxial layer thereon as a GaN ingot having a thickness of at least 10mm, and cutting or cleaving it to produce a plurality of GaN substrates. This is the method for manufacturing the GaN substrate of the present invention. And etching the GaAs substrate by using aqua regia. And grinding the GaN surface to be smooth. A large crystal is produced by the epitaxial growth method as a method for producing a thin film.

The GaN crystal of the present invention is most characterized by its size. In the present invention, when the GaN substrate is circular, the diameter is 1 inch or more, preferably 2 inches or more. In order to industrially produce a light-emitting element such as an LED at low cost, a GaN substrate is preferably large. When the GaN substrate is square, it is preferably 20mm square or more. If the GaAs substrate as a starting material is large, a GaN crystal of a large area can be produced.

The thickness of the GaN substrate must be such that it can be handled independently. Therefore, it is preferably 50 μm to 1 mm. Even if it exceeds 1mm, a semiconductor can be produced, but the cost of the wafer itself becomes high. When the thickness is 50 μm or less, the shape itself cannot be maintained. The thickness is preferably 70 μm to 1 mm.

The GaN substrate fabricated by these methods is bent. Bending occursin a GaN substrate single crystal due to internal stress. Bowing can be a significant obstacle in the processing of wafers to produce devices. It is necessary to reduce the bending of the substrate. The biggest challenge in GaN fabrication by these methods is "bending reduction". To reduce bowing, the inventors have improved the growth process and have newly proposed polishing the substrate.

(1) Improving lateral growth of the growth process … over and under the mask shape.

(2) The lapping … can planarize by lapping, if it is somewhat thick, even if it is curved.

(3) The surface polishing … enables the surface to be shifted from a predetermined crystal orientation by polishing the bend. In order to correct the crystal orientation shift, it is also necessary to perform a surface mask. When the surface roughness is large, surface grinding is also performed. After the above-described processing, the present inventors specified the deviation of crystal orientation of the polished surface in a state where the surface was slightly bent, and clarified the deviation of crystal orientation to be possessed as a GaN single crystal substrate.

The HVPE process is used in order to not contain carbon in the raw material. GaN contains no carbon and therefore is hardly yellow. The electron mobility is not reduced by the action of carbon and the electron as a carrier. Since carbon does not enter, GaN is a colorless transparent wafer. In fact, if the GaN wafer of the present invention is placed on the characters, the characters on the bottom layer are transparently visible. As if it were glass. There was no incorporation of material. However, As or the like evaporated from the GaAs substrate side may be mixed with yellowish, light brown, or dark gray. However, since the mask layer and the buffer layer prevent evaporation of As from the GaAs substrate, the amount of As mixed into GaN is extremely small. The amount is 10¹⁷One cm^-2The following. If the amount of As is large, the crystallinity of GaN is not good. In the case of the LED, the characteristics of As are deteriorated. Can be used forIf the HVPE method is adopted, such a fear is not necessary.

Drawings

Fig. 1 is a graph showing differences in thermal expansion coefficient and lattice constant between a substrate material for GaN crystal growth and GaN on x and y coordinates.

Fig. 2 is a graph showing that when GaN is epitaxially grown on a sapphire substrate, the lattice constant is smoothly changed by changing the thickness of the GaN film.

Fig. 3 is a plan view of a staggered dot window mask fixed to the GaAs (111) a plane.

Fig. 4 is a plan view of a state where a GaN buffer layer is epitaxially grown on a portion exposed from the mask window.

Fig. 5 is a plan view showing a state where GaN is subjected to epitaxial growth on the mask and buffer layers and then to crystal from an adjacent window.

Fig. 6(1) is a process diagram showing a process of mounting a mask on a GaAs substrate, growing a GaN buffer layer and a GaN epitaxial layer, and etching and removing the GaAs substrate, and a process diagram of forming a mask on a GaAs (111) substrate.

Fig. 6(2) is a process diagram showing a process of growing a GaN buffer layer and a GaN epitaxial layer by mounting a mask on a GaAs substrate, and etching and removing the GaAs substrate, and a process diagram showing a process of growing a buffer layer on a portion not covered with the mask.

FIG. 6(3) is a flowchart showing steps of mounting a mask on a GaAs substrate, growing a GaN buffer layer and a GaN epitaxial layer, and etching away the GaAs substrate, and a flowchart showing steps of growing a GaN epitaxial layer on the buffer layer and the mask.

Fig. 6(4) is a process diagram showing steps of mounting a mask on a GaAs substrate, growing a GaN buffer layer and a GaN epitaxial layer, and etching and removing the GaAs substrate, and shows a state in which the GaAs substrate is removed to form a GaN independent film.

Fig. 7(1) is a view showing a process of growing a GaN ingot on a GaN substrate to a thickness of GaN, and slicing the GaN ingot to form a wafer, and a GaN ingot thickened on the GaN substrate.

Fig. 7(2) is a view showing the steps of growing a GaN ingot on a GaN substrate to a GaN thickness, slicing the GaN ingot to form wafers, and slicing the GaN ingot into sliced wafers by an inner peripheral blade slicer.

Fig. 7(3) is a view showing wafers cut out from a process diagram in which a GaN ingot is grown on a GaN substrate to a GaN thickness, and the ingot is cut into wafers.

FIG. 8 is a schematic cross-sectional view of an HVPE apparatus.

Fig. 9 is a cross-sectional view showing a state in which a composite substrate formed by growing GaN on a GaAs substrate is bent by thermal stress.

FIG. 10 is a cross-sectional view showing a flat state of GaN after removing GaAs if the internal stress is 0.

FIG. 11 is a cross-sectional view showing a case where internal stress is present in GaN itself and even after GaAs is removed, the GaN is deformed.

Fig. 12 shows a view of definition of the warpage of the GaN wafer. The maximum rise of the central portion of the wafer having a diameter of 50mm is represented by H.

FIG. 13(A) is a graph showing measured values of a group of film thicknesses and internal stresses at a gas flow rate of 1800sccm to 900sccm and a Ga partial pressure of 1kPa, and (B) is a graph showing measured values of a group of film thicknesses and internal stresses at a gas flow rate of 900sccm and a Ga partial pressure of 2 kPa. The black dots are the B groups with rough surfaces. White dots are surface-smoothed group a.

Fig. 14 shows the distributions of the film thickness and the radius of curvature of the group a samples (6) and the group B samples (5) described above. The black dots are group B, and the white dots are group A.

Fig. 15(a) is a view showing the definition of the substrate surface normal and the crystal surface normal in a GaN wafer having a warp, and shows a case where the substrate surface normal and the crystal surface normal are coincident in a warped state.

FIG. 15(b) is a view showing the definition of the normal line of the substrate surface and the normal line of the crystal surface in a GaN wafer having a curvature, in which the normal line of the substrate surface becomes parallel when the convex surface is polished to be flat, and the normal line of the crystal surface becomes a fan shape as it is.

Fig. 15(c) is a view showing the definition of the normal to the substrate surface and the normal to the crystal surface in a GaN wafer having a warp, and the definition of the normal to the crystal surface oscillating within the substrate surface.

FIG. 16 is a graph showing the relationship between the bending angle ⊙ of the end of a wafer having a 2-inch diameter and the radius of curvature of the bend, with the radius of curvature (mm) on the horizontal axis and the bending angle of the end on the vertical axis.

Fig. 17 is a graph showing the measurement results of the anisotropy of the internal stress when a striped mask and a dotted mask are used.

Detailed Description

The GaN of the present invention is fabricated starting from a GaAs substrate. No sapphire is used. The sapphire substrate is difficult to remove later. However, GaAs can be removed by aqua regia, which takes a certain time to remove. As mentioned earlier, the use of GaAs substrates was tried since 1970 and was abandoned for the time being without any success. Why is this method adopted again now? Because the present inventors have newly found that the lateral growth method, which is an advantageous thin film growth method, is employed on the GaAs substrate. This method is a method of covering a GaAs substrate with a mask having staggered windows, and growing GaN independently from the independent windows. The method developed by the present inventors is to realize GaN on a GaAs substrate. This is described in Japanese patent application No. Hei 10-078333.

GaN is a hexagonal system. (0001) The face has six degrees of symmetry. GaAs is cubic, so the (100) or (110) plane does not have cubic symmetry. Therefore, the GaAs (111) A-plane or B-plane is used as a substrate. This is the plane orthogonal to the axis of cubic symmetry. The "A" plane refers to a plane where Ga atoms are exposed. The B face is a face where As atoms are exposed.

Fig. 3 shows a portion of a mask used in lateral growth. The mask may be Si without direct GaN₃N₄Or SiO₂And the like. The mask thickness is 100nm to several hundred nm. Is a mask with equally spaced windows. The window is in the shape of a small square. Is a small window of a few microns. It may be circular, triangular, elliptical, hexagonal, etc. Since the machining is fine, a circleor an ellipse is easily machined. The arrangement is important. Along [11-2]]The directions are arranged in columns. The interval is L. Along a line [ -110]orthogonal thereto]The columns adjacent in the direction are staggered by half an interval. The distance between adjacent columns is d. Preferably d is 3^1/2L/2. That is, the window is preferably disposed at the vertex of the regular triangle. For example, the windows are made as squares of 2 microns on each side, the window pitch L is 6 microns,the column spacing d is 5 microns. The reason why such a window in a regular triangular distribution is good is that: as shown in fig. 5, GaN grown from adjacent windows are simultaneously connected at the boundary. However, d or L may deviate somewhat from the above formula. The arrangement of such individual windows in parallel rows of dots is referred to as dot type or dot type.

Further, GaN can be grown even with a mask having a striped window with parallel continuous windows. If a striped window is used, it has the advantage that the time to reach coverage of the entire mask surface with GaN is short. But still has a tendency to have many defects and large bending.

The lateral growth method of growing GaN over the mask with the window has the following meaning. Since the mask is not directly bonded to the GaN, the GaAs on the substrate and the thin film GaN are bonded only at the window portion. In the case of GaN growth, generally, many nuclei are grown on the buffer layer, being grown closely in combination with each other. This results in a number of disadvantages. However, as described in the present invention, in the case of the mask, the portion that grows beyond the mask in the lateral direction is not obstructed. Since it is not a hindrance, it is considered that the growth is almost performed without defects. Since the connection area is small, thermal stress can be relaxed even if the temperature is lowered after growth at a high temperature. The thermal stress of the laterally grown layers in relation to the window becomes very small compared to the case of full area bonding. So how the windows are arranged and distributed is possible. Furthermore, as shown in fig. 5, the windows are preferably arranged such that the regular hexagons are simultaneously in contact, and then can be grown with a uniform thickness. The regular hexagon in fig. 4 and 5 represents the bottom shape of the hexagonal pyramidal crystal.

In order to add a mask, a mask material is applied to all GaAs substrates, and windows are opened at equal intervals by photolithography. This state is shown in the sectional view in fig. 6 (1).

Then, a GaN buffer layer with a thickness of several tens of nm to 100nm is formed by HVPE at a relatively low temperature of 450 ℃ to 500 ℃. Being thinner than the mask, the buffer layer is independently present within the window. Fig. 2 shows this state.

The temperature is raised to 800-1050 ℃, and a GaN epitaxial layer is formed by an HVPE method. At this time, the buffer layer is crystallized. As shown in fig. 4, GaN crystals that generate nuclei in the isolated windows generally form a hexagonal pyramid. After the nucleus occurs, the hexagonal cone gradually grows along the height direction and the bottom side direction. The bottom surface is expanded in a hexagonal shape to cover the window. Soon the GaN spreads over the mask. It continues to maintain the shape of the hexagonal pyramid. As shown in fig. 5, crystals from adjacent windows grow upward upon contact. The size of the substrate crystal is determined by the thickness of the epitaxially grown layer. One wafer may have a thickness of 70 micrometers to 1 mm. It takes the state shown in fig. 6 (3). Due to the above growth process, the growth surface is in a roughened glass surface state. Grinding is necessary for transparency.

The GaAs part is etched away by aqua regia. The mask portions are ground away. In the state shown in fig. 6. This is a GaN crystal. Transparent and independent. If only one wafer is to be fabricated, this is finished.

If it is desired to reproduce a plurality of wafers, the substrate is used as a seed crystal and then subjected to epitaxial growth. Fig. 7(1) shows epitaxial growth of thick GaN on a GaN substrate by HVPE. Into a cylindrical GaN ingot. The thickness is more than 10 mm. By fixing the support member to the side surface, the wafers are cut one by an inner peripheral blade dicing machine or the like. This is shown in FIG. 7 (2). After the wafer cut by the cutter was ground, a transparent and smooth GaN wafer was produced as shown in fig. 7 (3). In this case, As does not mix into the GaN crystal.

The HVPE method used in the epitaxial growth in the present invention is described with reference to FIG. 8. A cylindrical heater 2 is wound around the longitudinally long reaction furnace 1. The top of the reaction furnace 1 has raw material gas inlets 3 and 4. Introducing a raw material gas HCl + H from a raw material gas introducing port 3₂。H₂Is a carrier gas. Directly below it there is a Ga container 5. Metal Ga is stored here. The Ga melt 6 is obtained by heating because of its low melting point. Introduction of HCl into the Ga melt causesTo produce gallium chloride GaCl. The GaCl and the carrier gas H2 were carried to the lower side in the space of the reaction furnace. The raw material gas inlet 4 is open further downward. Ammonia NH₃+ H₂From where the mixed gas of (2) is introduced into the reaction furnace. From GaCl and NH₃Cause toThe reaction of (1).

The base 7 is provided tobe rotatable up and down by a shaft 8. A GaAs substrate 9 or GaN substrate is placed on the susceptor 7. The substrate is heated, and GaN, a product of the gas phase reaction, is attached to the substrate. The discharged gas is discharged from the gas discharge port 10. The HVPE method uses Ga metal as a raw material. GaCl was used as an intermediate. This is one of its features.

Epitaxial growth requires that the source material be gaseous, but that no Ga-containing gas be present. Ga itself is liquid at 30 ℃ or higher. Examples of the method using an organic metal for the gas include a MOC method and an MOCVD method. Although it turns into gas, carbon is mixed as an impurity into the GaN crystal because it contains carbon. In contrast, the HVPE method heats Ga to turn it into a liquid, which reacts with HCl to produce GaCl. GaCl is carried by a large amount of hydrogen as a gas. Since no organic metal is used, carbon does not enter the crystal as an impurity, which is an advantage thereof.

The GaN single crystal substrate produced by the present invention is undoped, but it is n-type. Carrier concentration of 1X 10¹⁶cm^-3. The present inventors have found that a material having n-type conductivity contains a trace amount of oxygen in the raw material gas. Using the prepared high-purity raw material, the HVPE method can control the carrier concentration to 1X 10 by controlling the oxygen partial pressure in the furnace¹⁶cm^-3～1×10²⁰cm^-3Within the range of (1). By controlling the oxygen partial pressure, the electron mobility can be adjusted to80cm²/Vs～800cm²In the range of/Vs. The specific resistance can be controlled at 1 × 10^-4Omega cm to 1 x 10 omega cm. When a low-purity material is used without controlling the oxygen partial pressure, the carrier concentration increases and the electron mobility decreases even without doping. The carrier concentration varies depending on the growth conditions, but is preferably 1X 10¹⁷cm^-3～1×10¹⁹cm^-3Within the range of (1). The specific resistance is preferably 5X 10^-2In the range of not more than Ω cm.

The GaN substrate thus fabricated had good characteristics. Is a large independent film and has the properties of transparency, no color and the like. However, the color of the medium sometimes appears yellow, light brown or dark gray depending on the growth conditions. As a substrate for an optical device, it is an important condition that absorption of light is small. Therefore, it is required to be colorless and transparent. This is important as a GaN substrate. According to the Lambert-Beer rule, the transparency of a substrate is given by:

I＝I₀exp(-αx)

in the formula I₀Is the intensity of incident light, I is the intensity of observed light, α is the absorption coefficient, x is the thickness of the substrate, the smaller α is the greater the light transmittance is, the more transparent the GaN substrate obtained by the present invention has the absorption coefficient between 400nm and 600nm, the smaller the GaN substrate has the higher transparency to light of these wavelengths^-1The following substrates. In addition, the absorption coefficient of 80cm is manufactured^-1The following substrates. As a substrate for an optical device, the smaller the absorption coefficient, the better. However, this is not sufficient and still problematic. Where the problem is? In the problem of deformation and internal stresses. The internal stress is large, and the warpage is large, which provides an obstacle to wafer processing such as photolithography.

GaN was grown on the heated GaAs substrate, and the composite was taken out of the apparatus at room temperature. Due to the different coefficients of thermal expansion, different deformations are caused by the cooling. As shown in fig. 9, the GaN/GaAs complex is bent. Stress occurs in GaN. Stress in the opposite direction also occurs in GaAs. There are two types of stress. They are thermal and intrinsic stresses. The thermal stress is a stress caused by a temperature change when two materials having different properties and different thermal expansion coefficients are stuck.

If it is only thermal stress, the thermal stress disappears after the GaAs substrate is removed. Therefore, as shown in fig. 10, GaN should be flat. The situation is different if the stresses are intrinsic. After the GaAs substrate is removed, there is still stress on the GaN. Thus, as shown in fig. 11, GaN itself is deformed. This bending is independent of GaAs, and the stress on the surface and the back surface are different due to the inclination of the stress in the thickness direction.

GaN has not been grown well on GaAs substrates in the past, because of the large intrinsic stress. The internal stress including the thermal stress is too large, resulting in many defects of GaN and peeling. The method of reducing intrinsic internal stress is in fact the lateral growth method using a mask as described previously. A number of individual windows are formed in the mask from which a GaN buffer layer is grown, followed by epitaxial layer growth. The cause of the internal stress is considered to be defects such as dislocations. In the lateral growth method, since the dislocation is separated from the mask, partial defects grown on the mask are reduced, and thus the internal stress of GaN can be reduced.

It is true that there is still internal stress. The GaN substrate is still bent. If the warpage is large, wafer processing cannot be performed. The bending must be evaluated to determine the upper limit of allowed bending.

FIG. 12 shows a definition of the bending assay or representation method. A wafer having a constant diameter is manufactured, and the center protrusion H is measured on a flat stage. The center bulge H is obtained by converting the diameter of the wafer to 2 inches, for example. H is a measurement method, namely a representation method.

The curvature may be defined by the curvature ξ or radius of curvature R of the wafer bend, which can be expressed as R ═ D²8H or ξ ═ 8H/D²Conversion is performed. D is a wafer diameter, and in the case of a 2-inch wafer, D is 50 mm.

The bending or deflection is externally exhibited and can be directly measured. Internal stresses are potentially internal and cannot be easily determined.

The curvature of the disk is δ and the internal stress upon deflection is given by

(formula 1)

σ is the internal stress, E is the elastic modulus, ν is the poisson's ratio, b is the thickness of the substrate, I is the substrate diameter, δ is the deflection (equivalent to H). When I is 50mm, δ is H in the above definition. This is the Stoney equation for calculating the internal stress of a film from deflection. As long as it is made into

Thin films (since GaN is a single layer), provided that d is b

(formula 2)

From the deflection δ, σ is calculated using this equation. This stress value σ can be interpreted as the value of the internal stress after planarization of the curved substrate. The relationship of bending, radius of curvature, and internal stress is as follows. The greater the internal stress, the greater the bend, and the smaller the radius of curvature for a given substrate thickness. The thicker the substrate thickness, the smaller the bend, and the larger the radius of curvature for a given internal stress. The present inventors have proposed the ease of processing devices on a GaN substrate and the substrate strength, and studied the allowable ranges of bending, radius of curvature, and internal stress.

Depending on the thickness of the wafer, to an appropriate value, but in general,

1. radius of curvature R: 600mm or more (curvature of 1.67X 10)^-3mm^-1The following)

2. Bend H (50 mm diameter): 0.55mm or less

3. Internal stress σ: less than 7MPa

That is, the present inventors set the conditions for the wafer to: r is more than or equal to 600mm, H is less than or equal to 0.55mm, and sigma is less than or equal to 7 Mpa. The internal stress σ is preferably 3MPa orless. The radius of curvature is preferably 750mm or more.

In the present invention, in addition to the method of manufacturing one GaN substrate, a method of manufacturing a single crystal ingot by using a GaN substrate as a seed crystal and performing epitaxial growth of GaN thickness thereon is employed. At this time, the thickness is 10mm or more, and several tens of wafers can be cut. The bending is small because the ingot is thick. Since the bending is small, a slice with high accuracy can be manufactured. Since the thickness is large, dislocations are small. The wafer sliced out as a slice has less dislocation. And thus the bending is small.

In the above-described substrate having a warp, a polishing step is further added, and the warp of the substrate itself can be greatly reduced. However, if polishing is performed, as shown in fig. 15, the crystal orientation of the substrate itself is oscillated, and the substrate is directly polished to be planarized. Fig. 15(a) shows a state before polishing, which is a GaN substrate having a convex curvature on the upper side. Due to the curvature, the substrate normals are not parallel and fan-shaped. The normal lines of the crystal faces are also distributed in a fan shape. The normal of the crystal plane is consistent with the normal of the surface of the substrate. If it is ground, as shown in FIG. 15 (b). Only the upper surface becomes flat. Even if the distribution is flat, the fan-shaped distribution in the normal direction of the crystal plane is not changed. However, after the substrate surface is planarized, the normals become parallel. The substrate normal and the crystal plane normal coincide at the central portion. However, the normal of the crystal plane is deviated from the normal of the substrate in the peripheral portion.

In the case of the simple convex deformation shown in fig. 15(a), the distance from the center is x, the substrate diameter is L, and when x is ± L/2, θ has the minimum value and the maximum value, and the value is ± ⊙, that is, when x is — L/2, the value is- ⊙, and when x is + L/2, the value is + ⊙, when a circular disk having a radius of curvature R is bent, the diameter is D, the bending angle of the end is ± ⊙, and 2R ⊙ is D, and the diameter of the substrate is naturally various, but here, the diameter of the substrate is 2 inches, and the relationship between the radius of curvature R and the bending angle ⊙ of the end is determined, and if the angle is ⊙, the above equation can be expressed as pi R ⊙, 90, 2, D.D, 50mm, if the angle is ⊙, the relationship is expressed as 2/14316.

The following description will be made for the case of normalization to 2 inches, and in a 2-inch wafer, in order to make the radius of curvature 600mm or more, it is necessary to make the offset of the normal line of the crystal plane at the end portion within a range of ± 2 °, that is, it is necessary to make the total of the offset angles of the crystal plane and the substrate flat plane 4 ° or less.

The end deviation angle ⊙ is adjusted to a precision of + -1 DEG when polished, and must be + -3 DEG or less according to the condition that the radius of curvature is 600mm or more as described above and + -2 DEG in addition, that is, the deviation between the normal of the main crystal plane and the normal of the substrate surface is within + -3 DEG, which is the bending condition of the substrate.

In addition, as described above, the polished surface should be a mirror surface in principle and should be flat. However, this is not always the case, and the bending may occur again after the polishing. This can be considered to be caused by internal stress within the GaN substrate. The results of various studies show that: the warpage H (fig. 12) after polishing can be suppressed to 0.2mm or less (converted to 2 inches), which is an allowable warpage amount in device processing for forming a fine pattern. As described above, R ═ D is the relationship between the radius of curvature and the bend H²8H, so that a limit of 0.2mm corresponds toAnd R is 1563mm or so. In addition, if considering the adaptability of device processing, the bending amount is preferably 0.1mm or less. In this case, the same value of H-0.1 mm corresponds to R-3125 mm.

Example 1 (lateral growth by HVPE method to produce a GaN single crystal)

A GaAs (111) a substrate was set in the reaction vessel. The substrate size is a circular substrate of 30mm diameter. In an atmospheric pressure (atmospheric pressure) CVD apparatus, Si with a thickness of 0.1 μm was formed on a GaAs substrate₃N₄And (3) a layer. The layer is provided with regularly distributed windows by photolithography. 3 kinds of windows were used. The staggered dot window shown in FIG. 3,<11-2>A striped window,<1-10>The number of the stripe windows is 3.

1. As shown in FIGS. 3 to 5, the staggered dot windows … form groups of windows adjacent to each other in a line parallel to GaAs<11-2>, and are staggered by half a pitch. d 3.5 microns and L4 microns.

<11-2>stripe window … is parallel to the long window (stripe) mask in the<11-2>direction. The stripe width was 2 microns, the spacing was 2 microns, and the pitch was 4 microns.

<1-10>stripe window … is parallel to the long window (stripe) mask in the<1-10>direction. The stripe width was 2 microns, the spacing was 6 microns, and the pitch was 8 microns.

Si to be provided with such a window₃N₄As a mask, a GaN buffer layer and an epitaxial layer were grown.

(1)Formation of GaN buffer layer

The GaAs substrate was set in an HVPE apparatus, and the GaAs substrate was covered with a mask having periodic windows. The HVPE apparatus was evacuated and the GaAs substrate was heated to about 500 ℃. The Ga vessel made of quartz is heated to 850 ℃ or higher to melt Ga. Introducing hydrogen gas H from the raw material gas inlet₂And hydrogen chloride HCl into a Ga container to synthesize gallium chloride GaCl. Introducing hydrogen H from another raw material gas introduction port₂And ammonia NH₃Is caused in the vicinity of the substrate heated to 500 deg.CDepositing GaN on the GaAs substrate. Thus, a GaN buffer layer of about 70nm was formed on the GaAs substrate. Si₃N₄Has the function of inhibiting the growth of GaN on Si₃N₄No GaN is deposited on the mask. The buffer layer (70nm) is thinner than the mask (100 nm). Thus forming a GaN buffer layer only on the GaAs portion of the window.

(2) Formation of GaN epitaxial layer

The introduction of HCl was stopped. The substrate temperature is increased from 500 c to about 1000 c. Then will beHCl was introduced into the Ga vessel. Gallium chloride GaCl is synthesized by reacting Ga with HCl as in the previous procedure. Since hydrogen gas flows as carriers, GaCl also flows downward together. Ammonia NH₃And GaCl react in the vicinity of the heated substrate to produce GaN. It is epitaxially grown on the buffer layer in the window. Once the mask thickness (100nm) is exceededThe GaN crystal is expanded into a regular hexagon on the mask. But before the entire surface of the mask is covered with GaN, the GaN crystal is in a hexagonal cone shape. Fig. 4 and 5 are schematic views showing the state of the bottom surface portion of the hexagonal pyramid. The windows are located at the vertices of the regular triangle, so GaN expanded into a regular hexagon is exactly connected with crystals expanded from the neighboring windows. Because the growth speeds are equal, the crystals of the regular hexagonal cone are contacted everywhere. If the GaN crystal layer is left without covering the mask, GaN begins to deposit upward. The growth rate was 50 μm/hr. An epitaxial layer about 100 microns thick is grown. Since independent nuclei are generated from numerous small windows and crystal growth (lateral growth) is performed, internal stress in GaN can be greatly reduced. The surface is in a frosted glass shape.

(3) Removal of GaAs substrate

Next, the sample was set in an etching apparatus. And etching with aqua regia for about 10 hours. The GaAs substrate is completely removed. Only GaN crystals remain. Both surfaces were polished to produce a GaN single crystal substrate. It is a stand-alone film. The mask window size, window pitch L, and distance d to the adjacent column were varied, and 3 samples of GaN were grown under otherwise substantially the same conditions. Sample 1 was grown using a staggered dot window (window 2 micron angle, L4 micron, d 3.5 micron) mask. Sample 2 was a sample grown using a<11-2>stripe mask. Sample 3 was a sample grown using a<1-10>stripe mask.

[ Table 1]

Carrier concentration, electron mobility, and specific resistance of 3 samples having different mask shapes

Sample number	Mask window	Concentration of carriers cm-3	Degree of electron mobility cm2/Vsec	Specific resistance Ω·cm
					1	Staggered dot window	3×1018	200	8.3×10-3
2	Stripe window	6×1018	150	6×10-3
					3	Stripe window	1×1019	120	3.5×10-3

(4) Optical characteristics

Undoped, n-type electron conduction. In view of maintaining crystallinity, a low carrier concentration is preferable, and a high electron transfer rate is preferable. However, the specific resistance is preferably high. These electrical characteristics vary with growth conditions. The stripe mask is insufficient in reducing the internal stress. These samples were clear and light brown in color. The absorption coefficient of the light with the wavelength of 400 nm-600 nm is 40cm^-1～80cm^-1. As content 9X 10¹⁶cm^-3。

(5) Diffraction by X-ray

In this GaN substrate, the angular relationship between the substrate surface and the GaN (0001) plane was investigated using an X-ray diffraction apparatus. As a result, it was found that the angle formed by the normal to the substrate surface and the normal to the GaN (0001) plane was 2.5 ° in the substrate. In addition, it was found that the parameter of the normal line of the GaN (0001) plane was 3.2 ° in the substrate. The amount of warpage of the polished substrate was measured for two additional samples, with H25 microns and H48 microns for a 1 inch length (D25 mm). Since R is D²and/8H, so R3125 mm, and R1628 mm. As described above, the photolithography limit of the 2-inch wafer is 0.2mm, and R is 1563mm, but this embodiment is below this limit. Is a curve that can be patterned by photolithography.

Example 2(HVPE method, laterally grown GaN seed crystal, GaN thickening by HVPE method)

A GaAs (111) A surface having a diameter of 2 inches was used as a substrate. On which SiO is formed₂An insulating film. The window shown in fig. 3 is set using photolithography.

(1) Formation of GaN buffer layer

The GaAs substrate with the mask was set in an HVPE apparatus. The apparatus shown in FIG. 8 was used, but the Ga vessel was heated to 800 ℃. As a raw material gas, H₂+ HCl into Ga vessel, H₂+NH₃Directly onto the substrate. In thatAt a low temperature of about 500 c (substrate temperature), a GaN buffer layer is formed. The thickness of the buffer layer was 80 nm.

(2) Formation of epitaxial layers

Next, the substrate temperature was increased to 1000 ℃, and a GaN epitaxial layer having a thickness of 80 μm was formed using the same raw material gas.

(3) Removal of GaAs

The GaN/GaAs substrate was taken out of the HVPE apparatus. It was confirmed that a mirror-like continuous GaN film was formed. It was placed in aqua regia and the GaAs substrate was etched away.

(4) Thickening of GaN

It was washed thoroughly. The state is shown in FIG. 6 (4). The remaining GaN was again placed in the HVPE apparatus. The substrate temperature was set to 1020 ℃, and GaN was thickened by HVPE to obtain a GaN ingot. The state is shown in FIG. 7 (1). The central portion of the ingot is slightly concave. Is an ingot with a minimum height of about 20mm and an outer diameter of 55 mm.

(5) Wafer dicing with a microtome

The ingot was cut in a direction perpendicular to the axial direction by an inner peripheral blade slicing machine. The state is shown in FIG. 7 (2). 20 GaN single-crystal substrates having an outer diameter of about 50mm and a thickness of 350 μm were obtained. When analyzing GaN, the content of arsenic (As) and carbon (C) is close to the background. It was found that GaN contains almost no arsenic (As) and carbon (C).

(6) Grinding

And then polishing and grinding, and finishing and grinding. The transparent wafer is shown in FIG. 7 (3). No bending of the substrate occurs due to the machining.

(7) Diffraction by X-ray

The GaN substrate was examined for the angular relationship between the substrate surface and the GaN (0001) plane using an X-ray diffraction apparatus, as in example 1. It was found that the angle formed by the normal to the substrate surface and the normal to the GaN (0001) plane was 0.6 ° at maximum in the substrate. It was found that the parameter of the direction of the normal to the GaN (0001) plane was 0.5 ° in the substrate. The polished substrate was bent by about 15 μm when the substrate was 2 inches long (D: 50 mm). Since R is D²(ii)/8H, so that R is about 20000mm. The flatness thereof can be sufficiently adapted to lithography.

(8) Measurement of Electrical characteristics

The electrical characteristics of the wafer taken from the upper end (the last-stage portion of growth) of the ingot were measured. Is n-type and has a carrier concentration of 5 × 10¹⁸cm^-3. Electron mobility is 200cm²Vs. The specific resistance was 0.017. omega. cm.

The electrical characteristics of the wafer taken from the lower end (portion in the initial stage of growth) of the ingot are as follows. Is n-type and has a carrier concentration of 10¹⁸cm^-3. Electron mobility is 150cm²Vs. The specific resistance was 0.01. omega. cm. This is the electrical characteristic of both end portions thereof. The middle part is supposed to be the middle value.

(9) Measurement of light absorption

These wafers were transparent, dark grey or colorless. The absorption coefficient of the light with the wavelength of 400 nm-600 nm is 20cm^-1～40cm^-1。

(10) Fabrication of LEDs

Since a GaN substrate is produced, an LED having InGaN as a light emitting layer is produced thereon. Compared with the existing LED with a sapphire substrate, the luminous brightness is improved by about 5 times. The reason why the light emission luminance is improved is that dislocations are reduced. In the conventional sapphire substrate LED, a large number of threading dislocations exist in the active layer, but the LED of the present invention of the GaN substrate has a greatly reduced number of threading dislocations.

Example 3(MOC method, laterally grown GaN seed crystal, GaN thickening by HVPE method)

The GaAs (111) B plane was used as a substrate. Mixing SiO₂Applied to a substrate and formed with a rim [1-10]by photolithography]A directionally extending fringe window.

(1) Formation of GaN buffer layer

A GaN buffer layer having a thickness of 90nm was formed on the substrate at a low temperature of about 490 c by an organic metal chloride vapor phase growth method (MOC method).

(2) Formation of GaN epitaxial layer

In the same apparatus, the substrate temperature was increased to about 970 ℃ to form a GaN epitaxial layer having a thickness of 25 microns.

(3) Removal of GaAs substrate

The GaN/GaAs sample was taken out of the MOC apparatus. A mirror-like GaN single crystal was produced. The orientation of the striped mask is the [11-20]direction of the GaN. That is, the [11-20]direction of GaN grows along the [1-10]direction of GaAs. The GaAs substrate is dissolved and removedby aqua regia.

(4) Thickened growth of GaN

GaN of 25 microns thickness was used as a seed crystal and placed in an HVPE apparatus. The GaN film was heated to 1000 ℃ and epitaxially grown to a thickness of GaN by HVPE. A cylindrical GaN ingot with a minimum height of about 3 cm was grown.

(5) Wafer dicing with inner peripheral blade dicing machine

The ingot was sliced in a direction perpendicular to the axial direction by an inner peripheral blade slicer to a thickness of 400 μm. 25 diced wafers were cut out.

(6) Grinding

And polishing and finishing the cut wafer. As a finished product, a GaN single crystal wafer was obtained.

(7) Measurement of Electrical characteristics

The electrical characteristics of the wafer were measured. Is n-type, and has electron mobility of 250cm²Vs. The specific resistance was 0.05. omega. cm.

The GaN substrate was examined for the angular relationship between the substrate surface and the (0001) plane using an X-ray diffraction apparatus, as in example 1. The angle formed by the surface normal of the substrate and the normal of the (0001) plane is at most + -1.1 deg. in the substrate. The parameter of the direction of the normal to the GaN (0001) plane was 1.4 ° within the substrate. When the polished substrate was bent by 2 inches, H was about 45 μm. R is about 6900 mm.

In this example, GaN itself was used as a seed crystal to grow a GaN single crystal thick. Thick single crystal GaN was grown, and cut with a microtome to prepare 25 substrates at a time. The manufacturing cost is reduced by 64% compared to the case of growing one by one from GaAs. The substrate can be manufactured at low cost. The manufacturing time of each substrate including quality management can be greatly shortened. When GaN was analyzed, the content of arsenic (As) and carbon (C) was background.

The mask through which the window passes is preferably located at the apex of the regular triangle. But may also be a mask with a striped window. Thus having the effect of reducing internal stress. By the lateral growth on the mask, defects in the crystal can be further reduced, the internal stress can be reduced, and at the same time, the contact area between GaAs and GaN can be reduced, and the internal stress can be relaxed. Therefore, although the temperature change is large, the occurrence of the bending can be suppressed.

Example 4 (relationship between Ga partial pressure and surface Structure/internal stress)

Using [1-10]]Stripe mask, [11-2]]A GaN wafer was produced by an HVPE method using a stripe mask and a dot mask. The feed gas is H₂+NH₃And H₂+ HCl. If the Ga partial pressure in the raw material gas is increased, the surface structure becomes rough. However, it is considered that the internal stress tends to be reduced.

[ Table 2]

Relationship between surface structure and curvature

Test specimen Number (C)	Temperature of (℃)	Time of day (minute)	Ga Partial pressure (kPa)	NH3 Partial pressure (kPa)	Surface of Status of state	Film thickness (μm)	Curvature Radius of (mm)	Inner part Stress (MPa)
									1	1030	180	1	4	Flat and flat	35	54	26
2	1030	180	1	6	Flat and flat	40	58	28
									3	970	180	2	6	Roughness of	120	1000	4
4	970	180	1	6	Flat and flat	60	225	11
									5	970	180	1	6	Flat and flat	40	61	26
6	1020	240	2	6	Roughness of	300	10417	1.2
									7	1020	240	1	6	Flat and flat	120	167	29
8	1030	240	1	6	Flat and flat	70	125	23
									9	970	360	2	6	Roughness of	200	1488	5.1
10	970	180	2	12	Roughness of	200	1359	5.9
									11	970	180	2	24	Roughness of	200	2604	3.1

Six of 1, 2, 3, 4, 5 and 6 are groups A, and five of 7, 8, 9, 10 and 11 are groups B.

(A) Ga partial pressure of 1kPa (10)^-2atm), GaN was grown on the buffer layer/mask at 970 ℃ for 1 hour, GaN was grown on the buffer layer/mask at 1030 ℃ for 3 hours, and epitaxial growth was performed for a total of 4 hours, the results are shown by white circles ○ in fig. 13, 6 samples (1, 2, 3, 4, 5, 6) having flat surfaces and good structures, but having large internal stress and cracks in the samples, and in fig. 13, the horizontal axis represents the film thickness (micrometer), the film thickness distribution is in the range of 30 micrometers to 120 micrometers, the vertical axis represents the internal stress (MPa), the sample represented by white circles ○ has an internal stress of 10MPa to 30MPa, and most of them exhibits an internal stress of more than 10MPa, although the internal stress is preferably 7MPa or less (7 × 10 MPa)^-3Gpa)。

(B) The Ga partial pressure was 2 kPa. GaN was epitaxially grown on the buffer layer/mask at 970 ℃ for 6 hours. The number of samples was 5. As shown in fig. 13. The film thickness is distributed between 120 microns and 300 microns. The surface of the GaN sample was rough. Rmax is about 20 microns. The GaN substrate size was 20mm × 20 mm. The surface state is not good, but the internal stress is small. In FIG. 13, the internal stress is 1MPa to 6MPa as indicated by the black circles. The target was 7MPa or less, and therefore, it was acceptable. However, under the same conditions, the film thickness variation was large (120 to 300 μm). The curvature radius R of the bending is 1000 mm-10417 mm.

(X-ray diffraction)

The relationship between the surface of the substrate and the normal line of the (0001) plane was investigated using an X-ray diffraction apparatus. The maximum angle of deflection within the substrate is ± 2.0 °. Further, the dispersion of the normal line of the GaN (0001) plane in the substrate was 2.4 °. The warpage of the substrate after polishing was 60 μm in terms of 2 inches. It becomes R5200 mm.

Example 5 (relationship of curvature radius)

In the same manner as in the above example, the relationship between the film thickness and the curvature radius was examined for 6 samples (a) and 5 samples (B). The bending was evaluated by the stylus method. The results are shown in FIG. 14. The horizontal axis represents the film thickness (μm). The vertical axis is the radius of curvature (mm). The radius of curvature is preferably 600mm or more.

(A) Sample A, which was grown at 1030 ℃ for 3 hours, 970 ℃ for 3 hours, 1020 ℃ for 4 hours, and 1030 ℃ for 4 hours, had a thin film thickness and a flat surface but had a large curvature. The curvature radius is less than 225 mm. The range of the desired radius of curvature is 600mm or more. All 6 specimens failed to reach the target.

(B) Sample B, which had been grown at 970 ℃ for 3 hours, 1020 ℃ for 4 hours, 970 ℃ for 6 hours, and 970 ℃ for 3 hours, had a thick film and a rough surface, but had a small internal stress and a small warpage. All 5B samples exceeded the desired range of 600 mm. Particularly, although no data is disclosed, the curvature radius is extremely small and the curvature is large in the maskless growth.

Example 6 (polishing)

The grinding of sample a failed. Sample B was a polished sample having a film thickness of 200 μm, an internal stress of 5.4MPa, a radius of curvature of 1488mm, and an Rmax of 20 μm. The film thickness was reduced to 80 microns by grinding. The radius of curvature increased to 3500mm after grinding. By the polishing, the surface roughness Rmax was 7.2nm, which was reduced to Ra of 2 nm. Grinding smoothes the surface and increases the radius of curvature.

(X-ray diffraction)

The relationship between the surface normal of the substrate and the normal of the (0001) plane was investigated using an X-ray diffraction apparatus. The maximum value of the angle formed by the substrate surface normal and the (0001) plane normal is ± 1.7 °. The dispersion in the normal direction of the GaN (0001) plane was 3.7 ° within the substrate. The substrate after polishing was bent by 90 μm at a length of 2 inches. The curvature radius R is 3400 mm. It is also within the lithographic limits.

Example 7 (anisotropy of internal stress in wafer)

The internal stress when GaN was grown using a striped mask was compared with that when GaN was grown using a dot mask. The graph shown in fig. 17 represents the relationship between the mask and the internal stress. The following two types of GaN were used:

(1) a stripe mask extending in the<11-2>direction was formed on a GaAs (111) plane wafer, and GaN having an average film thickness (average value was used because of dispersion in film thickness) of 120 μm was grown;

(2) GaN with an average film thickness of 160 μm was grown using the same dot mask as in example 1.

The growth conditions were all: ga partial pressure of 1X 10^-2atm(1kPa)、NH₃The partial pressure was 0.24atm (24kPa), the growth temperature was 970 ℃, and the growth time was 240 minutes. After growth, cooling is performed to remove the GaAs substrate. A GaN independent film was produced, and the internal stress of GaN was investigated with X-rays. The results are shown in fig. 17. The GaN substrate manufactured using the dot mask has extremely low anisotropy and low internal stress. In contrast, the GaN substrate manufactured using the stripe mask has anisotropy in the stripe direction and the direction orthogonal thereto. In particular, large bends occur in the direction parallel to the stripes. Up to a factor of 10 compared to the vertical. The orientation shown in fig. 17 is an orientation related to GaN crystals. Of GaAs<1-10>With GaN<11-20>Corresponding to that of GaAs<11-2>With GaN<1-100>And (7) corresponding. Of GaN<11-20>And<1-100>are orthogonal.

The invention provides a large-sized GaN single crystal wafer. Since the lateral growth method using a mask with a window is used, defects such as dislocations in the GaN crystal are reduced. Since GaN is thickened by a method not using carbon, a wafer having high transparency is obtained. Since the growth is lateral, the internal stress can be suppressed small and the bending is small. Since the substrate surface is flattened by polishing, the warpage is extremely small. Since the warpage is minute, a conventional wafer processing method such as photolithography can be used. In addition, the crystal face shaking is also within a practical range. The GaN single crystal wafer can be used as a substrate of a GaN light-emitting device. Since the thin film portion is integral with the substrate, no bending occurs. The internal stress is also small. And thus the dislocation density is also small. The luminous efficiency is high. The GaN-LED and GaN-LD have long service life.

Claims (8)

1. A GaN single crystal substrate characterized in that: the substrate has a diameter of 20mm or more and a thickness of 0.07mm or more, an inherent internal stress of 7MPa or less, a curvature radius of bending of 600mm or more, a deflection amount at the center of the substrate having a diameter of 2 inches of 0.55mm or less, and an angle between the normal direction of the substrate surface and the normal direction of a low-index crystal plane having the highest parallelism to the substrate surface of 3 DEG or less in the substrate.

2. The GaN single crystal substrate of claim 1, wherein: the absorption coefficient of light with the wavelength of 400 nm-600 nm is 120cm^-1The following.

3. The GaN single crystal substrate of claim 1, wherein: the substrate contains 10 of As¹⁷One cm^-3The following.

4. The GaN single crystal substrate of claim 1, wherein: the substrate does not contain carbon.

5. The GaN single crystal substrate of claim 1, wherein: the substrate surface and the back surface are ground.

6. The GaN single-crystal substrate of claim 5, wherein: the dispersion of the normal line of the low-index crystal plane having the highest parallelism to the substrate surface in the substrate is 4 DEG or less.

7. The GaN single-crystal substrate of claim 5, wherein: the warpage of the polished substrate is 200 μm or less in terms of a wafer having a diameter of 2 inches.

8. The GaN single-crystal substrate of claim 5, wherein: the surface of the substrate is a GaN (0001) plane.