Method of producing gallium nitride (GaN) independent substrate, method of producing GaN crystal body, and method of producing GaN substrate

A method of producing a separated GaN crystal body grown by vapor phase epitaxy on a substrate made of material different from GaN is provided. In this method, a nitride deposit is formed during the growth on a periphery of the substrate and GaN crystal body. The present method comprises the steps of: processing the periphery of the substrate to remove the nitride deposit; and, after the peripheral processing, separating the substrate from the GaN crystal body to make the substrate and the GaN crystal body independent of each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of producing a GaN independent substrate, a method of producing a GaN crystal body, and a method of producing a GaN substrate.

2. Related Background Art

Document 1 (Japanese Patent Application Laid-Open No. 2000-12900) describes a GaN single-crystal substrate. A mask having zigzag windows or stripe windows is formed on a GaAs (111) substrate, a GaN buffer layer is formed at low temperature by HVPE or by MOC, a thick GaN epitaxial layer is grown at high temperature by HVPE, and the GaAs substrate is then removed. By using the self-sustained films of GaN as seed crystals, a thick GaN is grown by HVPE to form a GaN ingot. This ingot is sliced with a slicer to obtain a transparent and colorless GaN wafer with little warp.

Document 2 (Japanese Patent Application Laid-Open No. 2002-284600) describes a method of producing a GaN crystal substrate, and a GaN crystal substrate. A metal film is deposited on a starting base which is one of the following: a single-crystal sapphire substrate; a substrate obtained by growing a single-crystal GaN film on a sapphire substrate; and a single-crystal semiconductor crystal substrate. A GaN film is then deposited on the metal film to form a laminated substrate, which facilitates separation of the grown GaN film and the starting base.

Document 3 (Japanese Patent Application Laid-Open No. 2003-168820) describes a method of separating a crystal layer formed on a substrate by exposing the crystal layer and the substrate to light. This separation method is characterized by irradiating the layer with a line pattern of light. At this time, the irradiation width with the light is approximately equal to or smaller than the thickness of the crystal layer, whereby the crystal layer can be separated from the substrate, without occurrence of crack.

SUMMARY OF THE INVENTION

In the fabrication of the GaN independent substrate, the GaN film is grown on the GaAs substrate and thereafter GaAs is removed by etching to form the self-sustained GaN crystal body. During the growth of the GaN crystal body on the GaAs substrate, a nitride deposit is grown on the periphery of the GaN crystal body and the GaAs substrate. Even if the GaAs substrate is removed by wet etching after the growth of the GaN crystal body, there remain deposit projecting outward from the periphery of the GaN crystal body. Therefore, the GaN crystal body cannot be shaped before the removal of the projections. However, in order to remove this projecting deposit, it is necessary to perform substantially manual processing. What is needed is to efficiently remove the nitride deposit from the periphery of the GaN crystal body and the GaAs substrate and thereby achieving further improvement in productivity.

As described above, Document 2 discloses that the GaN film is grown through the aluminum film evaporated on the sapphire substrate and that the aluminum evaporated film is dissolved by etching (HCl+H2O2) to obtain the self-sustained GaN film. Document 3 discloses that the GaN film is grown on the sapphire substrate and that the sapphire substrate is irradiated with the linear pattern of laser beam to separate the GaN film from the substrate, whereby stress is relaxed to prevent the GaN film from cracking. These techniques disclosed in the above documents are different from the removal of the nitride deposit from the periphery of the GaN crystal body and the GaAs substrate.

The present invention has been accomplished in view of the above-discussed matter and an object of the invention is to provide a method of producing a GaN independent substrate, a method of producing a GaN crystal body, and a method of producing a GaN substrate, which relate to the efficient removal of the nitride deposit formed on the periphery of the GaN crystal body and the GaAs substrate.

One aspect of the present invention is a method of producing a GaN independent substrate from a GaN crystal body grown by vapor phase epitaxy on a substrate made of material different from GaN, and during the growth, a nitride deposit is formed on a periphery of the substrate and GaN crystal body. The method comprises the steps of: processing the periphery of the substrate and GaN crystal body to remove the nitride deposit; and after the peripheral processing, separating the substrate from the GaN crystal body to form a GaN independent substrate by making the substrate and the GaN crystal body independent of each other.

In this method according to the present invention, the step of processing the periphery of the substrate and GaN crystal body to remove the nitride deposit comprises removing the nitride deposit with a grinding stone while rotating the GaN crystal body about a predetermined axis.

In the method according to the present invention, the step of processing the periphery of the substrate and GaN crystal body to remove the nitride deposit comprises the steps of: processing the nitride deposit with a first grinding wheel having a first stiffness; and, after processing the nitride deposit with the first grinding wheel, processing the nitride deposit with a second grinding wheel having a second stiffness. The first stiffness is greater than the second stiffness.

In the method according to the present invention, in each of the processing steps with the first and second grindings, the nitride deposit is removed by the peripheral processing while oscillating one of the GaN crystal body and the grinding wheel relative to the other in a direction the predetermined axis.

In the method according to the present invention, the step of removing the nitride deposit and GaN crystal body comprises an initial step and a finish step, and a feed speed of the grinding wheel in the initial step is different from a feed speed of the grinding wheel in the finish step.

Another aspect of the present invention is a method of producing a GaN crystal body for forming a GaN substrate. This GaN crystal body is grown by vapor phase epitaxy on a substrate made of material different from GaN, and as the result of the growth, a nitride deposit is formed on a periphery of the substrate and GaN crystal body. The method comprises the step of: grinding a peripheral part of the nitride deposit by machining to remove a first outer periphery part of the nitride deposit on the side face of the GaN crystal body and a second outer periphery part of the nitride deposit on the side face of the substrate. In this step, a first inner periphery part of the nitride deposit is left on the side face of the GaN crystal body and a second inner periphery part of the nitride deposit is left on the side face of the substrate. The method further comprises the steps of: after the grinding by machining, removing the substrate by etching; after removing the substrate, removing the second inner periphery part of the nitride deposit; and, after the removal of the second inner periphery part, removing the first inner periphery part of the nitride deposit by machining to form the GaN crystal body.

In this method according to the present invention, after the peripheral part of the nitride deposit has been ground, the inner periphery part of the nitride deposit and the GaN crystal body have a size within a circular cylinder of a predetermined diameter, the predetermined diameter is larger than a diameter of the substrate, and the difference between the predetermined diameter and the substrate diameter is equal to or more than one millimeter and equal to or less than four millimeters.

In the method according to the present invention, after the peripheral part of the nitride deposit has been ground, the inner periphery part of the nitride deposit and the GaN crystal body have a size within a circular cylinder of a predetermined diameter. The predetermined diameter is larger than a diameter of the substrate, and the difference between the predetermined diameter and the substrate diameter is equal to or more than one millimeter and equal to or less than three millimeters.

In the method according to the present invention, the grinding by machining comprises removing the nitride deposit by use of a grinding stone while rotating the GaN crystal body about a predetermined axis.

Still another aspect of the present invention is a method of producing a GaN substrate from a GaN crystal body grown by vapor phase epitaxy on a substrate made of material different from GaN, and in the growth, a nitride deposit is formed on a periphery of the substrate and GaN crystal body. The method comprises the step of: grinding a peripheral part of the nitride deposit by machining to remove a first outer periphery part of the nitride deposit on the side face of the GaN crystal body and a second outer periphery part of the nitride deposit on the side face of the substrate. In this step, a first inner periphery part of the nitride deposit is left on the side face of the GaN crystal body and a second inner periphery part of the nitride deposit is left on the side face of the substrate. The method further comprises the steps of: after the grinding by machining, removing the substrate by etching; after the removal of the substrate, removing the second inner periphery part of the nitride deposit; after the removal of the second inner periphery part, removing the first inner periphery part of the nitride deposit by machining to form the GaN crystal body; and fabricating one or more GaN substrates from the GaN crystal body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The teaching of the present invention can be readily understood in view of the following detailed description with reference to the accompanying drawings presented by way of example. Subsequently, embodiments of a method of producing a GaN independent substrate according to the present invention will be described with reference to the accompanying drawings. This method is to produce a GaN independent substrate from a GaN crystal body grown by vapor phase epitaxy on a substrate made material different from GaN. The same portions will be denoted by the same reference symbols as much as possible.

FIG. 1shows a deposition apparatus for vapor phase epitaxy such as HVPE used for epitaxial growth in an embodiment according to the present invention. A vertical reactor1is surrounded by a cylindrical heater2. Source gas inlets3,4are located in the top part of the reactor1. Mixed gas G1of hydrogen chloride (HCl) as a source gas and hydrogen (H2) as a carrier gas is introduced through the gas inlet3into the reactor1. A gallium (Ga) source5is opposed to the inlet3. Metal gallium is charged in the source board and, when heated by the heater2, it turns into a Ga melt6because the melting point of metal gallium is low. When HCl arrives at the Ga melt, the reaction of 2Ga+6HCl→2GaCl3+3H2occurs and gallium chloride (GaCl3) is produced thereat. Mixed gas G2of gallium chloride and carrier gas H2is provided in the space inside the reactor1. Mixed gas G3of ammonia (NH3)+hydrogen (H2) is introduced through the inlet4into the reactor1. Through the reaction of GaCl3and NH3in the source gas, gallium nitride (GaN) is deposited on a substrate9.

A susceptor7is supported by a shaft8so as to allow free rotation and up-and-down motion. The substrate, such as a GaAs, substrate9is placed on the susceptor7. The GaAs substrate9is of inch size and is of an approximate disk shape having the diameter of two inches, for example. A product GaN is deposited on the principal surface of the substrate9. Mixed gas G4of the remainder of the source gas and the reaction product gas is discharged through a waste gas outlet10. A GaN crystal body made by HVPE demonstrates the n-conductivity type even when undoped. The carrier concentration of the GaN crystal body is, for example, approximately 1×1016cm−3. After the temperature of the deposition apparatus is lowered to room temperature, the complex of the GaAs substrate9and the GaN crystal body grown thereon is taken out of the apparatus. The thickness of the GaN crystal body is larger than the thickness of the substrate9.

Part (A) inFIG. 2shows a peripheral processor for carrying out a step of removing a nitride deposit by peripheral processing. A complex20includes a GaAs substrate9, a GaN crystal body24, and a nitride deposit26. For example, the GaN crystal body24of a cylinder shape is deposited on the substrate9of the disk shape. In conjunction with the growth of the GaN crystal body24, the nitride deposit26is inevitably formed on each of the side faces9a,24aof the substrate9and the GaN crystal body24. The nitride deposit26has a projection located outside a virtual cylindrical shape having the diameter of the substrate9. The complex20is mounted on the peripheral processor22so as to be rotatable about a predetermined axis Ax. The peripheral processor22has, for example, a grinding stone28as a grinding tool for machining the periphery of the complex20. The complex20has the dimension represented by symbol D1.

Part (B) inFIG. 2is a view showing a peripheral processing step. While the complex20is rotated about the predetermined axis Ax, the grinding wheel28is brought into contact with the periphery of the complex20. As the grinding wheel28is moved, for example reciprocated, at an appropriate feed speed, the periphery of the complex20is gradually ground. Namely, the nitride deposit26is gradually removed. Referring to part (B) inFIG. 2, the grinding changes the nitride deposit26on the periphery of the complex20to a nitride deposit26a, and the dimension D2of the complex20is made smaller than the diameter “D1.” The periphery of the complex20is ground using the peripheral processor22to the diameter of 50 millimeters, for example. This grinding results in removing projecting portions. In the complex20ashown in part (B) inFIG. 2, a small amount of the nitride deposit is left on the side faces of the substrate9and the GaN crystal body24, and the nitride deposit is completely removed.

As shown in part (C) inFIG. 2, after the peripheral processing, the substrate9in the complex20bis mechanically separated from the GaN crystal body24. This separation results in making the substrate9and the GaN crystal body24independent of each other. The separation can be carried out, for example, as follows. Since the nitride deposit is removed, a border between the GaN crystal body24and the substrate9is exposed. When a force is applied to this border, the complex20bis separated into the substrate9and the GaN crystal body24. This provides the GaN crystal body24for formation of GaN wafers.

Since this method is arranged to remove the nitride deposit26prior to the separation of the substrate9from the GaN crystal body24, the nitride deposit26can be removed by peripheral processing.

Part (A) inFIG. 3shows the GaN crystal body that the nitride deposit has been removed and has been separated from the substrate9to be independent. The GaN crystal body24has been processed so as to have a desired diameter, and, as shown in part (B) inFIG. 3, the GaN crystal body24is sliced and polished to obtain one or more GaN wafers24a-24d.

A GaN film about 3 mm thick was deposited on a GaAs substrate having the diameter of 50 mm to form a complex. The outside diameter of the complex was 58 mm because of the nitride deposit. The periphery of the complex was ground at the peripheral speed of 2000 mm/min with a resin bonded grinding wheel (which is a grinding wheel obtained by binding diamond grains with resin) #600, and, after grinding the complex from its outside diameter of 58 mm to 49.5 mm, the GaN crystal body was separated from the GaAs substrate. The time required for the grinding was 80 minutes.

Another complex having the outside diameter of 58 mm because of the nitride deposit was processed using a straight type of metal bonded diamond wheel (which is a grinding wheel obtained by sintering diamond grains with metal powder). The time required for the grinding was 80 minutes. No crack was observed in the initial stage of processing, but several cracks were observed around the substrate at the time of completion of the processing. In general, metal bonded grinding wheels have greater wear resistance and are thus more advantageous in terms of lifetime than soft resin bonded grinding wheels. But, the metal bonded grinding wheels cause greater damage to products because of its high stiffness than the soft resin bonded grinding wheels. Vitrified grinding wheels (which are a grinding wheel obtained by sintering diamond grains with alumina) have much the same properties as the foregoing metal bonded grinding wheels.

Part (A) and part (B) inFIG. 4show an example of the peripheral processing step. As shown in part (A) ofFIG. 4, the peripheral processing step is to process the complex20with a first grinding wheel29ahaving a first stiffness. This processing results in grinding the nitride deposit26of the complex20to nitride deposit26b. Then the nitride deposit26bis processed with a second grinding wheel29bhaving a second stiffness. This processing results in grinding the nitride deposit26bof the complex20to nitride deposit26c, as shown in part (B) inFIG. 4. The first stiffness is greater than the second stiffness. The inner peripheral region of the nitride deposit, different from the outer peripheral region, is adjacent to the GaN crystal body that will be processed into products. Therefore, the inner peripheral region and the outer peripheral region are ground using the respective grinding wheels so as to reduce damage to products, if necessary.

A GaN film about 3 mm thick was deposited on a GaAs substrate having the diameter of 50 mm to prepare a complex. The outside diameter of the complex was 58 mm because of the nitride deposit. First, the portion, 80%, of the grinding process was conducted at the peripheral speed of 2000 mm/min with a vitrified grinding wheel. Then, the rest, 20%, of the grinding process was ground at the peripheral speed of 2000 mm/min with a resin bonded grinding wheel. The time required for the grinding was 80 minutes. No crack was observed. After grinding the complex to the desired dimension to form the GaN crystal body, the GaN crystal body was separated from the GaAs substrate.

Part (A) and part (B) inFIG. 5show another example of the peripheral processing step. The peripheral processing step removes the nitride deposit26by peripheral processing while oscillating one of the GaN crystal body24and the grinding wheel28relative to the other during a part or the whole of the peripheral processing step. The grinding wheel28is supported on an oscillating device30so as to be movable in the direction of the predetermined axis Ax.

In the grinding wheel28supported on the oscillating device30, as shown in part (A) inFIG. 5, a first area28bof grinding surface28ais first used. The first area28bof the grinding wheel28is in contact with the nitride deposit26d, and the grinding wheel28moves in a direction indicated by arrow M1, so that the second area28cof the grinding wheel28is in contact with the nitride deposit26d. When the oscillating device30moves the grinding wheel28to a limit point, the grinding wheel28then moves in the opposite direction indicated by an arrow M2, as shown in part (B) inFIG. 5. Almost the whole of the grinding surface28ais used through such movement. This method uniformly abrades the entire surface of the grinding wheel by oscillating the grinding wheel because the thickness T1of the grinding wheel28is larger than the thickness T2of the GaN crystal body.

A GaN film about 3 mm thick was deposited on a GaAs substrate having the diameter of 50 mm to prepare complex. The outside diameter of the complex was 58 mm because of the nitride deposit. The complex was ground at the peripheral speed of 2000 mm/min with a straight type of diamond grinding wheel (resin bonded grinding wheel) #600. The grinding wheel was oscillated in the thickness direction of the complex, whereby the grinding wheel became uniformly worn.

Part (A) and part (B) inFIG. 6show still another example of the peripheral processing step. The peripheral processing step includes an initial step shown in part (A) ofFIG. 6and a finish step shown in part (B) ofFIG. 6. Feed speed of the grinding wheel28for grinding of nitride deposit26f,26gcan be changed by a feeding device32installed on the peripheral processor22. The feed speed V1of the grinding wheel28in the initial step is different from the feed speed V2of the grinding wheel28in the finish step. In a preferred example, the feed speed V2is smaller than the feed speed V1. For example, the feed speed V1is preferably equal to or more than 3 mm/Hr and equal to or less than 7 mm/Hr, and the feed speed V2is preferably equal to or more than 1 mm/Hr and equal to or less than 4 mm/Hr.

According to this method, the inner peripheral region of the nitride deposit is closer to the GaN crystal body that will be processed into products, as compared to the outer peripheral region of the nitride deposit. Therefore, the inner peripheral region and the outer peripheral region are ground at the respective feed speeds changed on that way, so as to reduce damage to products.

The peripheral processing of a complex having the diameter of 58 mm was first conducted at the processing feed speed of 5 mm/hr and, when the diameter is decreased to 52 mm in the processing, the processing feed speed was changed to 2 mm/hr and the processing was continued to the diameter of 50 mm. No crack was observed.

In the first to fourth examples described above, since the nitride deposit26was removed prior to the separation of the substrate9from the GaN crystal body24, the removal of the nitride deposit26can be carried out in the peripheral processing as above.

Comparative Example 1

When a GaN thick film42was grown on a GaAs substrate40, deposit44was inevitably grown on the periphery of a GaN crystal body having a shape of cylinder of the diameter of 50 mm, so that the diameter of the complex was about 58 mm. As shown in part (A) ofFIG. 7, the grown deposit has a variety of shapes, and it had a downwardly spreading shape from the GaN crystal body42along the substrate40. This complex was subjected to etching with aqua regia or the like to remove the GaAs substrate, and the grown deposit (downwardly spreading part44a), which had been on the side face of the GaAs substrate, was left in the spreading shape as shown in part (B) inFIG. 7. This as-obtained shape does not allow us to shape an independent GaN thick film and to form wafers therefrom. Thus, as shown in part (C) inFIG. 7, it is necessary to remove the spreading portion by a preprocessing. This preprocessing is a work by human power and thus necessitates the time of about 50-80 minutes. For further improvement in productivity, it is desirable to reduce this time. The various methods described in the examples of the present invention can reduce the time for the processing and enhance productivity. By reducing the time for manual processing, it also becomes feasible to avoid breakage of wafers due to handling error or the like during the manual processing.

The present invention is by no means limited to the specific configurations disclosed in the examples of the present invention. A mask having a predetermined pattern can be provided on the substrate, prior to the formation of the GaN crystal body, as occasion may demand. The present invention can also be applied to GaN substrates including single-crystal substrates and composite substrates consisting of a plurality of single crystals.

Subsequently, a method of producing a GaN crystal body and a GaN substrate will be described with reference to the accompanying drawings. This method produces a GaN substrate from a GaN crystal body grown by vapor phase epitaxy on a substrate made of material different from GaN.

FIG. 8shows a deposition apparatus for vapor phase epitaxy, such as HVPE, used for epitaxial growth. A vertical reactor101is provided in a heater102of a cylinder shape. Source gas inlets103,104are provided in the upper wall of the reactor101. Mixed gas G1of hydrogen chloride (HCl) as a source gas and hydrogen (H2) as a carrier gas is introduced through the gas inlet103into the reactor101. A gallium (Ga) source105in which metal gallium is charged is opposed to the inlet103. Since metal gallium has a low melting point, it turns into a Ga melt106when heated by a heater102. When HCl is brought into contact with the Ga melt, the following reaction takes place to generate gallium chloride (GaCl3):2Ga+6HCl→2GaCl3+3H2. Mixed gas G2of gallium chloride and carrier gas H2is produced in the reactor101. Mixed gas G3of ammonia (NH3)+hydrogen (H2) is introduced through the inlet104into the reactor101. The reaction of GaCl3and NH3in the source gas takes place to deposit a GaN thick film for fabrication of a GaN substrate, on a substrate109.

A susceptor107is supported by a shaft108so as to enable rotation and up-and-down motion. The substrate, such as a GaAs substrate109, is placed on the susceptor107. The GaAs substrate109is of inch size and is of an approximate disk shape having the diameter of two inches, for example. Gallium nitride (GaN) is deposited on the primary surface of the substrate109. Mixed gas G4of the remainder of the source gas and the reaction product gas is evacuated through waste gas outlet110. The GaN crystal body grown by HVPE demonstrates the n-conductivity type when undoped. The carrier concentration is, for example, approximately 1×1018cm−3. After the temperature of the deposition apparatus is lowered to room temperature, the complex of the GaAs substrate109and the GaN crystal body grown on this substrate109is taken out from the deposition apparatus. The thickness of the GaN crystal body is larger than the thickness of the substrate109.

Part (A) ofFIG. 9shows a peripheral processor for performing a step of removing a nitride deposit by peripheral machining. The complex120includes a GaAs substrate109, a GaN crystal body124, and a nitride deposit126. For example, the GaN crystal body124having the shape of a cylinder is formed on the substrate109of the disk shape. In conjunction with the growth of the GaN crystal body124, the nitride deposit126is formed on each of side faces109a,124aof the substrate109and the GaN crystal body124. The nitride deposit126includes a projection located outside a virtual cylinder shape having the diameter of the substrate109. The complex120is mounted on the peripheral processor122so as to be rotatable about a predetermined axis Bx. The peripheral processor122has, for example, a grinding wheel128as a grinding tool for machining the periphery of the complex120. The complex120has a typical dimension, such as a diameter, indicated by symbol D10. The nitride deposit126has an outer periphery part126awhich will be ground by the following peripheral processing step, and an inner periphery part126bwhich remains even after this peripheral processing step.

Part (B) ofFIG. 9shows the peripheral processing step. While the complex120is rotated about the predetermined axis Bx, the grinding wheel128is brought into contact with the periphery of the complex120. When the grinding wheel128is moved at an appropriate feed speed, it is grinding the periphery of the complex120. Namely, the nitride deposit126is gradually removed. In part (B) ofFIG. 9, the periphery of the complex120is ground to leave the nitride deposit126cand the diameter of the complex120is reduced to dimension D20(less than D10, D20<D10). The periphery of the complex120is ground, for example, to the diameter of 54 millimeters, using the peripheral processor122. This grinding results in removing the projecting part. In the complex120ashown ofFIG. 9(B), a small amount of the nitride deposit remains on the side faces of the GaN crystal body124and substrate109. This step removes all the outer periphery part126aof the nitride deposit126, while leaving the inner periphery part126b. For the subsequent description, the nitride deposit126aleft on the side face124aof the GaN crystal body124is referred to as first inner periphery part126d, and the nitride deposit126aleft on the side face109aof the substrate109is referred to as second inner periphery part126e.

As shown in part (C) inFIG. 9, the substrate109in the complex120ais removed from the GaN crystal body124after the peripheral processing. This removal is carried out by etching to form a complex120c. Since the substrate109is made of GaAs semiconductor in the present embodiment, the substrate109can be selectively removed by wet etching with an etchant, such as aqua regia. The complex120cincludes the first inner periphery part126dleft on the side face124aof the GaN crystal body124, and the second inner periphery part126eextending from the first inner periphery part126d. The second inner periphery part126eis a wall located along the edge of the GaN crystal body124, and the height of this wall is approximately equal to the thickness of the substrate109.

Next, the second inner periphery part126eis removed from the complex120c. This step results in eliminating the above wall, which extends from the edge of the GaN crystal body124, from the complex120c.

Part (A) ofFIG. 10shows a complex120dobtained by removing the second inner periphery part126efrom the nitride deposit126c. This complex120dis mounted on a peripheral processor132so as to be rotated about a predetermined axis Cx. The peripheral processor132has a grinding wheel138for machining the periphery of the complex120d. When the complex120dis rotated about the predetermined axis Cx, the grinding wheel138is brought into contact with the periphery of the complex120c. When the grinding wheel138is moved at an appropriate feed speed, the periphery of the complex120dis ground. The nitride deposit126is gradually removed and the first inner periphery part126dof the nitride deposit126ais completely removed at last. The periphery of the complex120is ground using the peripheral processor132, for example, to the diameter of 50 millimeters. This step forms a GaN crystal body125processed to have the predetermined diameter as shown in part (B) ofFIG. 10.

As shown in part (A) ofFIG. 11, the GaN crystal body125is processed to have a desired diameter, and then it is sliced and polished to form one or more GaN wafers125a-125d, as shown in part (B) ofFIG. 11.

GaN was grown on a GaAs substrate having the diameter of 50 mm, and a deposit was also grown on the side face of the GaN crystal body on the GaAs substrate, and the outside diameter of the complex was 58 mm. The GaAs substrate was removed by etching to form a projection on the periphery. If this projection were removed by manual processing, it would take the time of 50 to 80 minutes. This processing time should be shortened in terms of production efficiency. The deposit on the periphery grows in proportion to deposition time and is grown to a bell shape. This bell-shaped downwardly spreading part is preliminary shaved off by peripheral processing, thereby reducing the time required for subsequent processing. The grown deposit was ground to the peripheral diameter of 54 mm. The time necessary for the peripheral processing was approximately 40 minutes. Since this peripheral processing reduced the size of the bell-shaped downwardly spreading part to about half, the time of 25 minutes was taken for the processing of the projecting part after the removal of the GaAs substrate by etching. Therefore, the processing time was shortened. The finished GaN crystal was of a perfect circle, but was decentered by about 1.5 mm and was thus eccentric from the original product part.

The peripheral part was processed to the diameter pf 51 mm by a similar method. The peripheral processing took the time of about 70 minutes. After the GaAs substrate was removed, the time of about 5 minutes was taken for the manual processing of the projecting part. The decentering was about 0.9 mm.

FIG. 12is a drawing showing the eccentricity in the peripheral processing. After the crystal growth, the GaN complex is of a distorted cylinder the outside diameter of which is not always uniform throughout the entire periphery, though it is not a visible level. Therefore, it is not easy to effect accurate centering (to determine a center of a circle) in finishing the periphery to a circle. With poor centering accuracy, a complex formed for final products of a perfect circle is decentered, and a portion of the complex for the final products may be shaved off. In view of centering error, it is thus necessary to finish the complex to a dimension in the peripheral processing slightly larger than the target dimension itself. For fabricating accurate circular wafers from the product, it is desirable to adopt the following process, in terms of ensuring quality as well.

(1) To grind down the complex to a size a little larger than the final diameter in the peripheral processing.

(2) To remove the GaAs substrate by etching.

(3) To remove the remaining bell-shaped projecting part off by processing.

As shown inFIG. 12, investigation was conducted as to present decentering accuracy. An average value of decentering in twelve grinding works was 1.25 mm, and the standard deviation, “sigma,” 0.371. In the peripheral processing carried out prior to the etching of the substrate, the upper value of the one sigma range was 1.621 mm (1.25+0.371), and the upper value of the two sigma range was 1.991 mm (1.25+2×0.371).

In the peripheral grinding step carried out prior to the etching of the substrate, the diameter of the ground complex is larger than the diameter of the substrate preferably by a value in the range of equal to or more than 1 mm and equal to or less than 4 mm. If the upper limit can be 4 mm, decentering of two sigma (2*σ) can be permitted.

As a result of further improvement made in the centering method, such as a technique of determining an apparent center with the use of a circular tool circumscribed to the GaN complex, the value of two sigma was decreased to about 1.5 mm. In the peripheral grinding step carried out prior to the etching of the substrate, the diameter of the ground complex is larger than the diameter of the substrate by preferably a value in the range of not less than 1 mm nor more than 3 mm.

In either case, the lower limit is ideally zero millimeters but the lower limit was set to one millimeter. This lower limit prevents the grinding of the GaN crystal body in the peripheral grinding step, and thus the GaN crystal body can be protected from unpredictable damage.

The embodiment of the present invention described above provides a method of producing a GaN crystal body and a method of producing a GaN substrate, by the efficient removal of the nitride deposit formed on the side faces of the substrate and the GaN crystal body for fabrication of GaN substrates.

Comparative Example 2

When a GaN thick film142was grown on a GaAs substrate140, a deposit144was inevitably grown on the periphery of a GaN crystal body having the shape of a cylinder of the diameter of 50 mm. The maximum outside diameter of the complex was about 58 mm. The grown deposit can have a variety of shapes, and, as shown in part (A) ofFIG. 13, it had a downwardly spreading shape from the GaN crystal body142along the substrate140. This complex was subjected to etching with aqua regia or the like to remove the GaAs substrate, and, as shown in part (B) inFIG. 13, the grown deposit (downwardly spreading part) on the side face of the GaAs substrate remains in a projecting shape because the GaAs substrate had been removed. This shape as-obtained does not allow us to form an independent GaN thick film and further to form wafers therefrom. Thus, it is necessary to remove the projecting part by preprocessing, as shown in part (C) ofFIG. 13. This preprocessing is a work by human power and necessitates the time of about 50-80 minutes. For further improvement in productivity, it is desirable to shorten this time. The method described in the embodiment of the present invention can reduce the time for the processing and enhance the productivity. By reducing the time for the manual processing, it becomes feasible to avoid breakage of wafers due to handling error or the like during the manual processing.

The principle of the present invention has been illustrated and described in the preferred embodiments, but it is apparent to those skilled in the art that the present invention can be modified in arrangement and details without departing from the principle. The present invention is not limited to the specific configurations disclosed in the embodiments of the present invention. A mask having a predetermined pattern can be provided on the substrate prior to the formation of the GaN crystal body, if necessary. The present invention can also be applied to any one of GaN substrates as single-crystal substrates and composite substrates constituted by the arrangement of a plurality of single crystals. We therefore claim all modifications and variations coming within the spirit and scope of the following claims.