JP6978241B2 - GaN substrate - Google Patents

Description

The present invention relates to a GaN substrate.

When manufacturing a nitride semiconductor self-supporting substrate or when manufacturing a semiconductor device such as a light emitting device or a high-speed transistor, for example, a substrate (GaN substrate) made of gallium nitride (GaN) crystals is prepared and placed on the surface of this substrate. In some cases, a process of epitaxially growing a crystal may be performed (see, for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2011-016680

When the above-mentioned crystal growth treatment is performed under temperature conditions of, for example, 1200 ° C. or higher, the GaN substrate may disappear due to thermal etching.

An object of the present invention is to provide a GaN substrate that is not easily subjected to thermal etching even when it is used for a crystal growth treatment performed under a temperature condition of 1200 ° C. or higher, for example.

According to one aspect of the invention
At least each of the front surface and the back surface is composed of a GaN single crystal, and the etching rates of the front surface and the back surface are respectively when heated to a temperature of 1200 to 1400 ° C. under an inert gas atmosphere at a pressure of 1 to 105 kPa. A GaN substrate is provided in which is less than 20 μm / hr.

According to the present invention, it is possible to provide a GaN substrate that is difficult to be thermally etched even when it is used for a crystal growth treatment performed under a temperature condition of 1200 ° C. or higher, for example.

It is sectional drawing of the GaN substrate which concerns on 1st Embodiment of this invention. It is a schematic diagram of the vapor phase growth apparatus used when growing a crystal. It is sectional drawing of the GaN substrate which concerns on 2nd Embodiment of this invention, (a) is the figure which shows the appearance that the bowl-like warp occurred in the surface-side substrate, (b) is the figure which shows the appearance that the surface-side substrate was dome-shaped. It is a figure which shows the state which the warp occurred. It is sectional drawing of the GaN substrate which concerns on the modification of 2nd Embodiment of this invention, (a) is the figure which shows the appearance that the bowl-like warp occurred in the surface side substrate, (b) is the figure which shows the state that the surface side substrate warped. It is a figure which shows the appearance that the dome-shaped warp occurred in. It is sectional drawing of the GaN substrate which concerns on other modification of 2nd Embodiment of this invention, and is the figure which shows the appearance which the dome-shaped warp occurred on the surface side substrate. (A) and (b) are schematic views of the GaN substrate according to the third embodiment of the present invention, respectively.

<First Embodiment>
Hereinafter, the configuration and manufacturing method of the GaN substrate 10 according to the first embodiment of the present invention will be described with reference to FIG.

(1) Configuration of GaN substrate As shown in FIG. 1, the GaN substrate 10 is a front surface side substrate 11 (hereinafter, abbreviated as substrate 11) made of a GaN single crystal and a back surface side substrate made of a GaN single crystal. 12 (hereinafter, abbreviated as substrate 12).

The substrates 11 and 12 can be manufactured, for example, by epitaxially growing GaN crystals on a base substrate such as a sapphire substrate, cutting out the grown crystals from the base substrate, and polishing the surface thereof. As the crystal growth method, any known method can be used regardless of the vapor phase growth method or the liquid phase growth method. The substrates 11 and 12 can be, for example, in the shape of a disk having a diameter of 2 to 6 inches. The thickness of the substrates 11 and 12 is preferably made as thin as possible in order to reduce the manufacturing cost of the GaN substrate 10, but each of them can stand on its own in order to carry out the bonding between the substrate 11 and the substrate 12 described later with a high yield. It is preferable to have a thick thickness. From these facts, it is preferable that the thicknesses of the substrates 11 and 12 are, for example, 0.4 to 1.0 mm, respectively.

Both main surfaces (front and back surfaces) of the substrates 11 and 12 are the polar planes of the wurtzite structure (hexagonal crystal) of the GaN crystal, that is, the N polar planes (-c plane, (000-1) plane) and the (000-1) plane, respectively. , Ga polar plane (+ c plane, (0001) plane). Of these two polar surfaces, the N-polar surface has the characteristics of having higher thermal decomposition resistance and less thermal etching than the Ga polar surface.

The thermal etching rate of the Ga polar surface of the GaN crystal (hereinafter, also simply referred to as the etching rate) is, for example _{, 20 μm / under the conditions of a nitrogen (N 2} ) gas atmosphere with a pressure of 1 to 105 kPa and a temperature of 1200 to 1400 ° C. It may reach a size of hr or more, 1000 μm / hr or more depending on the conditions, and 2000 μm / hr in some cases. Even when the inert gas constituting the atmosphere is composed of a rare gas such as argon (Ar) or _{a mixed gas of N 2} gas and rare gas, the thermal etching rate of the Ga polar surface is N ₂ gas. Is the same as that when the atmosphere gas is used. Further, when the atmosphere of the GaN crystal contains a crystal growth atmosphere described later, that is, a halide such as _{gallium trichloride (GaCl 3} ), hydrogen nitride such as _{ammonia (NH 3 ), hydrogen (H 2} ) and the like. The thermal etching rate of the Ga polar surface may be even higher than the above-mentioned etching rate.

On the other hand, the etching rate of the N-polar surface of the GaN crystal is smaller than that of the Ga polar surface, for example, _{under the conditions of N 2} gas atmosphere with a pressure of 1 to 105 kPa and a temperature of 1200 to 1400 ° C., 20 μm /. The size is less than hr, preferably less than 10 μm / hr. _{Under an N 2} gas atmosphere with a pressure of 1 to 105 kPa, the etching rate of the N polar surface of the GaN crystal is less than 0.5 μm / hr at a temperature of 1200 ° C or higher and lower than 1300 ° C, and at a temperature of 1300 ° C or higher and 1350 ° C or lower. It has been confirmed that the size is less than 4 μm / hr and less than 10 μm / hr at a temperature of more than 1350 ° C and 1400 ° C or less. Even when the inert gas constituting the atmosphere is composed of a rare gas such as Ar or a _{mixed gas of N 2} gas and rare gas, the thermal etching rate of the N polar surface uses N ₂ gas as the atmospheric gas. It is the same as that in the case of. The atmosphere of the GaN crystal, crystal growth atmosphere to be described later even if a (halide, hydrogen nitriding atmosphere containing H _2, etc.), thermal etching rate of the N-polar surface, for example less than 40 [mu] m / hr, It has been confirmed that the size is much smaller than the thermal etching rate of the Ga polar surface, such as preferably having a size of less than 20 μm / hr.

The substrates 11 and 12 have Ga polar surfaces as joint surfaces, and these surfaces are joined so as to be in contact with each other. By joining the substrates 11 and 12 via the Ga polar surfaces of each other in this way, the front surface 10a (non-bonded surface of the substrate 11) and the back surface 10b (non-bonded surface of the substrate 12) of the GaN substrate 10 are formed. Each is composed of an N-polar surface having a relatively small etching rate. Further, the Ga polar surface having a relatively large etching rate is not exposed over the entire front and back surfaces of the GaN substrate 10. In the crystal growth treatment described later, the front surface 10a of the GaN substrate 10 is used as a crystal growth surface, and the back surface 10b is used as a supported surface (amorphous growth surface) that abuts on the heated susceptor 208.

By making the GaN substrate 10 have the above-mentioned configuration, in the crystal growth treatment described later, the surface 10a and the surface 10a when the GaN substrate 10 is heated to a temperature of 1200 to 1400 ° C. under an inert gas atmosphere at a pressure of 1 to 105 kPa. The respective etching rates of the back surface 10b can be, for example, less than 20 μm / hr, preferably less than 10 μm / hr. Furthermore, the GaN substrate 10, described later crystal growth atmosphere when heated to the temperature conditions mentioned above under (a halide, hydrogen nitride, an atmosphere containing H _2, etc.), each of the etching rate of the surface 10a and the back surface 10b, For example, the size can be less than 40 μm / hr, preferably less than 20 μm / hr.

The off-angles of the crystals constituting the front surface 10a and the back surface 10b of the GaN substrate 10 can be set to any orientation and any size, respectively. Here, the "off angle" means an angle formed by an axis orthogonal to the main surface (front surface 10a or back surface 10b) of the GaN substrate 10 and the c-axis of the GaN crystal constituting the main surface. _{By setting the off angle θ 1} on the surface 10a to an angle within the range of 0.5 to 5 °, it is possible to properly secure the crystal step that triggers the crystal growth on the surface 10a, which is the crystal growth surface. , It becomes easy to set the growth rate of the GaN crystal to a practical size. _{On the other hand, by setting the off angle θ 2} of the back surface 10b to an angle smaller than the off angle θ _{1 of the} front surface 10a, for example, an angle within ± 0.1 °, thermal etching is performed on the back surface 10b which is an amorphous growth surface. It is possible to reduce the number of crystal steps that trigger the process, and to further improve the thermal etching resistance of the back surface 10b, which is originally good.

(2) Method for Manufacturing a GaN Substrate Next, a case where the substrates 11 and 12 are joined to manufacture the GaN substrate 10 will be described by the procedure shown below.

First, the substrates 11 and 12 configured as described above are prepared, respectively. When the joining method described later is used, it is preferable that the Ga polar surfaces of the substrates 11 and 12 to be the joining surfaces are mirrored by polishing or the like, respectively, from the viewpoint of increasing the joining strength of the substrates 11 and 12.

Subsequently, plasma of a rare gas such as Ar gas is irradiated (plasma treatment) to each of the Ga polar surfaces (joint surfaces) of the substrates 11 and 12 in a vacuum (decompression) atmosphere. As a result, the entire surface of the Ga polar surface of the substrates 11 and 12 is removed while removing the natural oxide film formed on the Ga polar surface of the substrates 11 and 12 and impurities adhering to these Ga polar surfaces. Can be activated. Specifically, it is possible to generate dangling bonds (unbonded hands) of gallium (Ga) atoms on the Ga polar planes of the substrates 11 and 12. The activation of the Ga polar planes of the substrates 11 and 12 may be performed by ion treatment, chemical treatment with a chemical solution, cleaning treatment, CMP treatment, or the like, in addition to the plasma treatment described above. Then, the substrates 11 and 12 are exposed to the atmosphere. As a result, at least a part of the unbonded hands of Ga atoms existing on the Ga polar planes of the substrates 11 and 12 reacts with moisture and the like contained in the atmosphere and is terminated by a hydroxy group (−OH group). Then, for example, at a predetermined temperature within the range of 20 to 30 ° C., for example, 300 K (Kelvin), about 27 ° C.) to 400 ° C., the Ga polar surfaces of the substrate 11 and the substrate 12 abut against each other. (Pressure welding). Subsequently, the temperature of the laminate of the substrate 11 and the substrate 12 is further raised to, for example, 400 to 1000 ° C., an annealing treatment (heat treatment) is performed, and a dehydration reaction is caused at the joint surface between the substrate 11 and the substrate 12. By going through these steps, the substrates 11 and 12 are firmly bonded directly via the Ga polar plane, that is, at the atomic level, and the above-mentioned GaN substrate 10 is obtained.

The method of joining the substrate 11 and the substrate 12 is not limited to the above-mentioned method. For example, a high-pressure joining method in which the Ga polar surfaces (joining surfaces) of the substrates 11 and 12 are cleaned with a chemical solution and pure water and then the substrates 11 and 12 are joined by applying a high pressure of about 0.1 to 10 MPa. A high-vacuum joining method in which the Ga polar surfaces (joining surfaces) of the substrates 11 and 12 are cleaned with a chemical solution and pure water, and then the substrates 11 and 12 are joined in a high vacuum atmosphere of about ^10-6 to ^{10-3 Pa.} Can be preferably used because it is not necessary to use an adhesive as in the above-mentioned method.

(3) Crystal Growth Process Next, an example of a process of epitaxially growing a GaN crystal on the surface 10a of the above-mentioned GaN substrate 10 in the N-polar plane direction (−c axis direction) will be described. This process can be performed, for example, by using the vapor phase growth apparatus 200 shown in FIG. 2.

The vapor phase growth apparatus 200 includes an airtight container 203 made of a heat-resistant material such as quartz and having a growth chamber 201 inside. In the growth chamber 201, a susceptor 208 as a support member for holding the GaN substrate 10 is provided. The susceptor 208 is connected to the rotation shaft 215 of the rotation mechanism 216 and is rotatably configured.

A gas supply pipe 232a for supplying _{GaCl 3} gas, a gas supply pipe 232b for supplying _{NH 3} gas, and a gas supply pipe 232c for supplying _{nitrogen (N 2} ) gas are connected to one end of the airtight container 203, respectively. A gas supply pipe 232d for supplying _{hydrogen (H 2} ) gas is connected to the gas supply pipe 232c. On the downstream side of the gas supply pipes 232a to 232c, nozzles 249a to 249c that supply various gases supplied from these gas supply pipes toward the GaN substrate 10 held on the susceptor 208 are connected, respectively. ..

The gas supply pipe 232a is provided with a gas generator 233a and a valve 243a in this order from the upstream side of the gas flow. Inside the gas generator 233a, a solid raw material (solid GaCl ₃ ) that is solid at room temperature is housed. A heater 207a for heating a solid raw material _{to obtain a vaporized gas (GaCl 3 gas) is provided outside the gas generator 233a.} The gas generator 233a, a gas supply pipe 232e is connected for supplying the _{N 2} gas as a carrier gas. _{The GaCl 3} gas generated in the gas generator 233a is carried into the growth chamber 201 by the carrier gas supplied from the gas supply pipe 232e. The gas supply pipes 232a and 232e are provided with pipe heaters (not shown) for preventing liquefaction and solidification of _{GaCl 3 gas.}

The gas supply pipes 232b to 232e are provided with flow rate controllers 241b to 241e and valves 243b to 243e in this order from the upstream side of the gas flow. A bypass pipe 232f is connected between the upstream side of the gas supply pipe 232e from the valve 243e and the downstream side of the gas supply pipe 243a from the valve 243a. The bypass pipe 232f is provided with a valve 243f. Valve 243 e, with closed 243a, and opening the valve 243 f, it is possible to supply _{N 2} gas to bypass the gas generator 233a growth chamber 201.

At the other end of the airtight container 203, an exhaust pipe 230 for exhausting the inside of the growth chamber 201 is provided. The exhaust pipe 230 is provided with a pump 231. A zone heater 207 for heating the GaN substrate 10 held on the susceptor 208 to a desired temperature is provided on the outer periphery of the airtight container 203. In the airtight container 203, a temperature sensor 209 for measuring the temperature in the growth chamber 201 is provided. Each member included in the vapor phase growth apparatus 200 is connected to a controller 280 configured as a computer, and is configured so that a processing procedure and processing conditions described later are controlled by a program executed on the controller 280. There is.

For the crystal growth treatment, for example, the following treatment procedure is carried out by the above-mentioned vapor phase growth apparatus 200. First, the GaN substrate 10 is charged (carried in) into the airtight container 203 and held on the susceptor 208. Further, the solid raw material is housed in the gas generator 233a and heated by the heater 207a to be vaporized to generate _{GaCl 3 gas.} Further, the gas supply pipes 232a and 232e are heated to a desired temperature. Then, while heating and exhausting the inside of the growth chamber 201, the valves 243c, 243d, 243f are appropriately opened with the valves 243a, 243e, 243b closed, and H ₂ gas or H _{2 is introduced into the growth chamber 201.} supplying a mixed gas of gas and N ₂ gas. Then, with the inside of the growth chamber 201 reaching the desired treatment temperature and treatment pressure and the atmosphere inside the growth chamber 201 being the desired atmosphere, the valve 243f is closed and the valves 243a, 243e, 243b are opened. _{, GaCl 3} gas and NH ₃ gas are supplied to the surface 10a of the GaN substrate 10. As a result, the GaN crystal grows epitaxially on the surface 10a of the GaN substrate 10 in the −c axis direction. The supply of NH ₃ gas may be started before the supply of _{GaCl 3 gas.} In order to improve the in-plane uniformity of the GaN crystal grown on the surface 10a, it is preferable to carry out the crystal growth treatment in a state where the susceptor 208 is rotated. When the crystal growth is completed, the valves 243e, 243a, and 243b are closed, the valves 243f are opened, and _{the supply of GaCl 3} gas and NH ₃ gas into the growth chamber 201 is stopped. Thereafter, the temperature is lowered the growth chamber deposition chamber 201 while continuing supply of N ₂ gas into the 201, the GaN substrate 10 after the processing is unloaded from the airtight vessel 203, and ends the crystal growth process of this embodiment ..

The following are examples as processing conditions.
Temperature of gas generator 233a: 90-110 ° C
Temperature of gas supply pipes 232a and 232e: 200 to 210 ° C.
Carrier gas flow rate: 80-120 sccm
_{Partial pressure of NH 3} gas / partial pressure of GaCl ₃ gas in the growth chamber 201: 18-22
Processing temperature (temperature of GaN substrate 10): 1200 to 1400 ° C, preferably 1250 to 1300 ° C.
Processing pressure (pressure in growth chamber 201): 90 to 105 kPa, preferably 90 to 95 kPa.

As described above, the crystal growth performed here is the growth of the GaN crystal in the −c axis direction. Growth in this direction is difficult to proceed under the temperature conditions adopted for growth toward the Ga polar plane direction (+ c-axis direction), that is, temperature conditions of 900 to 1100 ° C. Therefore, in the present embodiment, by setting the temperature condition high as described above, the crystal growth in the −c axis direction is allowed to proceed at a practical rate, for example, at a rate of 80 to 90 μm / hr. I have to.

However, under the above-mentioned temperature conditions, if a GaN substrate having an N-polar surface on the front surface and a Ga polar surface on the back surface is used as the seed crystal substrate, the thermal etching on the back surface proceeds excessively and the seed crystal substrate disappears. I will invite you. Since each of the front surface 10a and the back surface 10b of the GaN substrate 10 of the present embodiment has excellent thermal etching resistance, it is possible to avoid disappearance even if the temperature conditions are set high as described above. .. As described above, the etching rates of the front surface 10a and the back surface 10b composed of the N-polar surfaces are less than 20 μm / hr, preferably 10 μm under the inert gas atmosphere of 1 to 105 kPa and the temperature condition of 1200 to 1400 ° C. It is less than / hr. The etching rate of the surface 10a and rear surface 10b which is constituted by N-polar surface, as described above, crystal growth atmosphere (halides, hydrogen nitriding atmosphere containing _{H 2,} etc.), the temperature condition of 1200 to 1400 ° C. , 40 μm / hr, preferably less than 20 μm / hr. Since the substrate 12 constituting the back surface 10b of the GaN substrate 10 has a thickness of, for example, 0.4 to 1.0 mm, it will not disappear for at least 10 hours, and in some cases 20 hours, under the above-mentioned temperature conditions. Become. Unless the substrate 12 disappears, the junction surface of the substrate 11, that is, the Ga polar surface that is easily subjected to thermal etching is not exposed. Therefore, the GaN substrate 10 in the present embodiment is suitable as a seed crystal substrate under the above-mentioned temperature conditions. It is possible to continue using it.

When a GaN crystal is grown on the surface 10a, the substrate 11 may be warped. This is because the off-angle of the substrate 11 has a predetermined in-plane distribution (for example, in the case of a substrate having a diameter of about 2 inches, the in-plane distribution of the off-angle of about ± 0.3 ° in the main surface thereof). Since it exists, compressive stress or tensile stress is applied to the growth surface (N-polar plane) of the crystal grown on the surface 10a (N-polar plane of the substrate 11) in the above-mentioned crystal growth treatment in the in-plane direction. become. These stresses gradually increase as the crystal growth progresses, which may cause bowl-shaped (concave) or dome-shaped (convex) warpage of the substrate 11. For example, when the substrate 11 is manufactured by the VAS (Void-Assisted Separation) method, the crystals grow on the surface 10a in the above-mentioned crystal growth treatment, so that the substrate 11 is likely to be warped in a bowl shape. In the present embodiment, since the substrates 11 and 12 are directly bonded with a very high bonding strength, even if the substrate 11 is warped, the bonding surface of the substrates 11 and 12, that is, the Ga polar surface is formed. Not exposed. Therefore, it is possible to more reliably avoid the disappearance of the GaN substrate 10 due to thermal etching.

Although an example of a GaN crystal growth method performed under a temperature condition of 1200 ° C. or higher has been described above, the growth method that can be used in the present embodiment is not limited to the above-mentioned method. For example, even when a GaN crystal is grown by the Tri-HVPE method in _{which a Ga raw material and a Cl 2} gas or the like are reacted inside a growth chamber where crystal growth is performed _{to generate a GaCl 3 gas, the GaN of the present embodiment is used.} The substrate 10 can be suitably used as a seed crystal substrate. Further, for example, Ga containing no halogen element as a raw material gas for the seed crystal substrate in a state where the growth chamber reaches the treatment conditions of 1 to 100 kPa and 1100 to 1400 ° C. and the atmosphere in the growth chamber becomes a desired atmosphere. Even when a _{GaN crystal is grown by a non-halogen VPE method that supplies steam and NH 3} gas, the GaN substrate 10 of this embodiment can be suitably used as a seed crystal substrate. Further, for example, in a state where the treatment conditions of 1 to 105 kPa and 1200 to 1400 ° C. are reached in the growth chamber and the atmosphere in the growth chamber becomes a desired atmosphere, trimethylgallium (TMG) gas is used as a raw material gas for the seed crystal substrate. Even when a _{GaN crystal is grown by a high-temperature MOCVD method in which and NH 3} gas are supplied, the GaN substrate 10 of this embodiment can be suitably used as a seed crystal substrate. The etching rates of the front surface 10a and the back surface 10b composed of the N-polar planes are less than 40 μm / hr, preferably less than 20 μm / hr under the crystal growth atmosphere and temperature conditions in these methods.

(4) Effects obtained by the present embodiment According to the present embodiment, one or more of the following effects can be obtained.

(A) By forming each of the front surface 10a and the back surface 10b of the GaN substrate 10 with an N-polar surface that is difficult to be thermally etched, the GaN substrate 10 is heated at 1200 to 1400 ° C. under an inert gas atmosphere at a pressure of 1 to 105 kPa. The etching rates of the front surface 10a and the back surface 10b when heated to a temperature can be set to, for example, less than 20 μm / hr, preferably less than 10 μm / hr. Furthermore, the GaN substrate 10, the crystal growth atmosphere above when heated to a temperature condition above under (halides, hydrogen nitride, an atmosphere containing H _2, etc.), each of the etching rate of the surface 10a and the back surface 10b, For example, the size can be less than 40 μm / hr, preferably less than 20 μm / hr. As a result, the GaN substrate 10 of the present embodiment is suitable as a seed crystal substrate in a crystal growth process in which the temperature condition needs to be set high as described above, for example, in a GaN crystal growth process directed in the −c axis direction. It can be used.

(B) Since the substrates 11 and 12 are directly bonded at the atomic level by the above-mentioned method, it is possible to make these bonding strengths extremely high. Therefore, even when the substrate 11 is warped in the above-mentioned crystal growth treatment, the exposure of the Ga polar surface can be prevented, and the disappearance of the GaN substrate 10 can be reliably avoided.

(C) By directly joining the substrates 11 and 12 without using an adhesive, the GaN substrate 10 can be composed of only a GaN single crystal. Therefore, in the above-mentioned crystal growth treatment, it is possible to prevent the component of the adhesive from being mixed in the crystal grown on the surface 10a, and it is possible to improve the quality of this crystal.

(D) GaN that grows on the front surface 10a in the above-mentioned crystal growth treatment while increasing the thermal etching resistance of the back surface 10b by setting the off angle θ ₂ of the back surface 10b to be smaller than the off angle θ _{1 of the front surface 10a.} It is possible to increase the growth rate of crystals. In particular, by _{setting the off angle θ 2} of the back surface 10b to, for example, within ± 0.1 °, it is possible to further improve the thermal etching resistance of the back surface 10b.

<Second Embodiment>
Next, the configuration of the GaN substrate 10A according to the second embodiment of the present invention will be described focusing on the differences from the first embodiment with reference to FIGS. 3 (a) and 3 (b).

As shown in FIGS. 3A and 3B, the thickness of the substrate 12 may be thinner than the thickness of the substrate 11. For example, the thickness of the substrate 11 can be 0.6 to 1.0 mm, and the thickness of the substrate 12 can be 0.4 to 0.5 mm.

By setting the thickness of the substrate 12 in this way, the rigidity of the substrate 12 can be appropriately reduced. Therefore, when the crystal 20 grows on the surface 10a in the above-mentioned crystal growth treatment, the substrate 11 has a bowl shape as exemplified in FIG. 3 (a) or a dome shape as exemplified in FIG. 3 (b). When the warp of the substrate 11 occurs, it becomes easy to make the substrate 12 follow the warp of the substrate 11. As a result, even when the substrate 11 is warped in the above-mentioned crystal growth treatment, the bonded state of at least the outer peripheral portion of the bonded surfaces of the substrates 11 and 12 is more reliably maintained, and the Ga polarity is easily subjected to thermal etching. The surface is even less exposed. Therefore, even when the GaN substrate 10A is used as the seed crystal substrate in the crystal growth process in which it is necessary to set a high temperature condition, it is possible to reliably avoid its disappearance.

Further, by setting the rigidity of the substrate 12 to be low, the warp of the substrate 11 is not strongly hindered by the substrate 12. That is, it is possible to appropriately relax (release) the compressive stress or the tensile stress applied to the crystal 20 growing on the surface 10a in the above-mentioned crystal growth treatment. As a result, it becomes possible to improve the quality of the crystal 20 grown on the surface 10a.

(Modification example)
This embodiment can be modified as shown in the following modifications.

[Modification 1]
As shown in FIGS. 4A and 4B, a gap 13 configured as a closed space may be provided between the substrate 11 and the substrate 12 as shown in the cross-sectional configuration diagram of the GaN substrate 10B. Then, the substrates 11 and 12 may be joined so that the Ga polar surfaces of each other are used as joint surfaces and only the outer peripheral portions of these surfaces are brought into contact with each other. The void 13 can be formed, for example, by providing a recess (drilled portion) in the inner peripheral portion of at least one of the Ga polar surfaces (joining surfaces) of the substrate 11 or the substrate 12. 4 (a) and 4 (b) exemplify a case where a recess is provided in the inner peripheral portion of the substrate 12. The width (bonding width) of the bonded portion of the substrates 11 and 12 is a width that can maintain a continuous bonded state even if the substrate 11 is warped in the above-mentioned crystal growth treatment, and the warp of the substrate 11 is maintained. The width should be as small as possible.

Also in this modification, the same effect as that of the above-mentioned second embodiment can be obtained. That is, even if the substrate 11 is warped in the above-mentioned crystal growth treatment, the bonded state of the outer peripheral portion of the bonded surface of the substrates 11 and 12 is maintained and the Ga polar surface is not exposed. Therefore, the seed crystal substrate is not exposed. Even when the GaN substrate 10B is used, it is possible to avoid its disappearance.

Further, even if the Ga polar planes of the substrates 11 and 12 are thermally etched and Ga evaporates from the Ga polar planes when the GaN substrate 10B is heated in the above-mentioned crystal growth treatment, the void 13 is configured as a closed space. , The generated Ga vapor can be contained in the void 13. Therefore, even if the heating of the GaN substrate 10B is continued as it is, the partial pressure of Ga in the void 13 reaches the saturated vapor pressure, so that further evaporation of Ga does not proceed. This makes it possible to more reliably avoid the disappearance of the GaN substrate 10B due to thermal etching.

Further, by providing the void 13, the bowl-shaped or dome-shaped warp generated in the substrate 11 in the above-mentioned crystal growth treatment is hardly hindered. As a result, the compressive stress or tensile stress applied to the crystal 20 growing on the surface 10a can be significantly relaxed, and the quality of the crystal 20 can be significantly improved.

[Modification 2]
The substrates 11 and 12 may be joined so that the joining strength on the inner peripheral side of the joining surface is lower than the joining strength on the outer peripheral side. By setting the joint strength distribution (in-plane strength distribution) in the joint plane in this way, the substrate 11 is dome-shaped in the above-mentioned crystal growth treatment, as shown in FIG. 5 as a cross-sectional configuration diagram of the GaN substrate 10C. When attempting to do so, the inner peripheral portion of one of the substrate 11 and the substrate 12 is peeled off from the inner peripheral portion of the other substrate different from the one substrate, and the inner peripheral portion of the substrate 11 is released from the substrate 12. Will be done.

Regarding the above-mentioned in-plane intensity distribution, for example, when plasma treatment is performed at the time of joining the substrates 11 and 12, a mask or the like is applied to the inner circumference of the Ga polar surface of the substrates 11 and 12 to prevent plasma irradiation. , 12 can be realized by preventing the formation of OH group terminations and the like on the inner peripheral side of the Ga polar surface.

Further, for example, the above-mentioned in-plane strength distribution can be realized by performing hydrogen embrittlement processing such as smart cut (registered trademark) on the inner peripheral side of at least one of the Ga polar surfaces of the substrate 11 or the substrate 12. can. By performing such processing on any of the substrates 11 and 12, when the substrate 11 tries to warp in a dome shape, at least one of the substrates 11 and 12 is on the inner peripheral side of the Ga polar surface, that is, The above-mentioned hydrogen embrittled portion can be intentionally broken (the inner peripheral side of the surface of the joint surface is peeled off), and the inner peripheral portion of the substrate 11 can be released from the substrate 12. When performing this processing, it is preferable to drive hydrogen ions or the like into the target substrate from the Ga polar surface side.

Also in this modification, the same effect as that of the above-mentioned second embodiment can be obtained. That is, even if the substrate 11 is warped in the above-mentioned crystal growth treatment, the bonded state of the outer peripheral portion of the bonded surface of the substrates 11 and 12 is maintained and the Ga polar surface is not exposed. Therefore, the seed crystal substrate is not exposed. Even when the GaN substrate 10C is used, it is possible to avoid its disappearance.

Further, when the substrate 11 tries to warp in a dome shape, the inner peripheral portion of the substrate 11 or the substrate 12 is peeled off, so that the compressive stress or the tensile stress applied to the crystal 20 growing on the surface 10a in the above-mentioned crystal growth treatment is applied. It can be relaxed and the quality of the crystal 20 can be improved.

<Third Embodiment>
Next, the configuration of the GaN substrate according to the third embodiment of the present invention will be described with reference to FIGS. 6 (a) and 6 (b), focusing on the differences from the first and second embodiments.

When joining the substrates 11 and 12, as shown in FIGS. 6 (a) and 6 (b), the crystal orientation in the main plane of the substrate 11 (for example, the direction of the M axis or the a axis) and the main of the substrate 12. By adjusting the in-plane crystal orientation (for example, the direction of the M axis or a axis) and the amount of deviation (rotation angle) when the -c axis is viewed as the rotation axis, the desired characteristics (opening characteristics and peeling strength) are adjusted. Etc.) may be obtained.

For example, as shown in the schematic diagram of the GaN substrate 10D in FIG. 6A, the crystal orientation in the main surface of the substrate 11 and the crystal orientation in the main surface of the substrate 12 may be matched. The term "match" as used herein means that the above-mentioned deviation amount (rotation angle) is 0 ° or more and less than 5 °, preferably 0 ° or more and 3 ° or less. Further, "the amount of deviation is 0 °" means that the crystal orientation in the main surface of the substrate 11 and the crystal orientation in the main surface of the substrate 12 are completely the same.

With such a configuration, the GaN substrate 10D can be made into a substrate having excellent cleavage (a substrate that can be easily cleavage along the M plane or the a plane). Further, with such a configuration, when the substrate 11 and the substrate 12 are directly bonded, it is possible to increase their bonding strength.

Further, for example, as shown in FIG. 6B as a schematic diagram of the GaN substrate 10E, when joining the substrates 11 and 12, at least one of the substrate 11 or the substrate 12 has a predetermined angle with the −c axis as the rotation axis. It may be rotated so that the crystal orientation in the main surface of the substrate 11 and the crystal orientation in the main surface of the substrate 12 do not match. FIG. 6B illustrates a case where the substrate 12 is rotated. The term "mismatch" as used herein means that the above-mentioned deviation amount (rotation angle) is 5 ° or more and 30 ° or less, preferably 25 ° or more and 30 ° or less.

With such a configuration, the GaN substrate 10E can be made into a substrate having excellent cleavage resistance (a substrate that is difficult to cleavage along the M plane or the a plane). Further, with such a configuration, when the substrate 11 and the substrate 12 are directly bonded, the bonding strength thereof can be appropriately suppressed.

<Other embodiments>
The first to third embodiments of the present invention have been specifically described above. However, the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the gist thereof.

In the above-described embodiment, the case where the front surface side substrate and the back surface side substrate are directly bonded has been described, but the present invention is not limited to this. For example, polycrystalline GaN or the like may be interposed between the front surface side substrate and the back surface side substrate.

In the above-described embodiment, the case where the front surface and the back surface of the GaN substrate are each composed of N-polar planes has been described, but the present invention is not limited thereto. For example, in the second and third embodiments described above, at least one of the front surface and the back surface of the GaN substrate may be configured with a Ga polar surface.

The present invention is not limited to GaN, and for example, nitride crystals such as aluminum nitride (AlN), aluminum gallium nitride (AlGaN), indium gallium nitride (InN), indium gallium nitride (InGaN), and aluminum indium gallium nitride (AlInGaN). That is, it is also suitably applicable when growing a Group III nitride crystal represented by the composition formula of _{Al x} In _y Ga _{1-xy N (0 ≦ x + y ≦ 1) on the surface.}

In addition, the above-described embodiments and modifications can be combined as appropriate.

<Preferable Aspect of the Present Invention>
Hereinafter, preferred embodiments of the present invention will be described.

(Appendix 1)
According to one aspect of the invention
At least each of the front surface and the back surface is composed of a GaN single crystal, and the etching rates of the front surface and the back surface are respectively when heated to a temperature of 1200 to 1400 ° C. under an inert gas atmosphere at a pressure of 1 to 105 kPa. A GaN substrate is provided in which is less than 20 μm / hr.

(Appendix 2)
The substrate of Appendix 1, preferably
The inert gas includes N ₂ gas, or at least one of noble gases.

(Appendix 3)
The substrate of Appendix 1 or 2, preferably
When heated to a temperature of 1200 to 1400 ° C. under a crystal growth atmosphere at a pressure of 1 to 105 kPa containing at least one of a halide, hydrogen nitride, and hydrogen, the etching rates of the front surface and the back surface are 40 μm / m. It is less than hr.

(Appendix 4)
The substrate is any of the above-mentioned appendices 1 to 3, preferably one of the above-mentioned substrates.
The front side substrate made of a GaN single crystal and the back side substrate made of a GaN single crystal are joined with each other's main surface (Ga polar surface) as a bonding surface.
The front surface is composed of the N-polar surface of the front surface side substrate, and the back surface is composed of the N polar surface of the back surface side substrate.

(Appendix 5)
The substrate of Appendix 4, preferably
The angle formed by the axis orthogonal to the back surface and the c-axis of the crystal constituting the back surface (off angle of the back surface) is the angle formed by the axis orthogonal to the front surface and the c-axis of the crystal constituting the front surface (front surface). Is smaller than the off angle).

(Appendix 6)
The substrate of Appendix 4 or 5, preferably
Even when the surface-side substrate is warped due to the growth of crystals on the surface, the bonded state of at least the outer peripheral portion of the bonded surface is maintained.

(Appendix 7)
The substrate of Appendix 6, preferably
The thickness of the back surface side substrate is thinner than the thickness of the front surface side substrate.

(Appendix 8)
The substrate of Appendix 6 or 7, preferably
The front surface side substrate and the back surface side substrate are joined to each other at their outer peripheral portions.
A gap configured as a closed space is provided between the front surface side substrate and the back surface side substrate.

(Appendix 9)
The substrate of Appendix 6 or 7, preferably
Of the joint surfaces between the front surface side substrate and the back surface side substrate, the joint strength on the inner peripheral side is lower than the joint strength on the outer peripheral side.
When the front surface side substrate tends to warp in a dome shape due to the growth of crystals on the surface, the inner peripheral portion of either the front surface side substrate or the back surface side substrate becomes the one substrate. Is configured to peel off from the inner circumference of the other different substrate.

(Appendix 10)
The substrate of Appendix 4, preferably
The crystal orientation in the main surface of the front surface side substrate (direction of M axis or a axis) and the crystal orientation in the main surface of the back surface side substrate (direction of M axis or a axis) are the same.

(Appendix 11)
The substrate of Appendix 4, preferably
The crystal orientation in the main surface of the front surface side substrate (direction of M axis or a axis) and the crystal orientation in the main surface of the back surface side substrate (direction of M axis or a axis) do not match.

(Appendix 12)
According to another aspect of the invention.
A front-side substrate made of a GaN single crystal and a back-side substrate made of a GaN single crystal are joined with each other's main surface as a joining surface.
The front surface is composed of a main surface different from the joint surface of the front surface side substrate, and the back surface is composed of a main surface different from the joint surface of the back surface side substrate.
Provided is a GaN substrate in which at least the outer peripheral portion of the bonded surface is maintained in a bonded state even when the surface-side substrate is warped due to the growth of crystals on the surface.

(Appendix 13)
The substrate of Appendix 12, preferably
The thickness of the back surface side substrate is thinner than the thickness of the front surface side substrate.

(Appendix 14)
The substrate of Appendix 12 or 13, preferably
The front surface side substrate and the back surface side substrate are joined to each other at their outer peripheral portions.
A gap configured as a closed space is provided between the front surface side substrate and the back surface side substrate.

(Appendix 15)
The substrate of Appendix 12 or 13, preferably
Of the joint surfaces between the front surface side substrate and the back surface side substrate, the joint strength on the inner peripheral side is lower than the joint strength on the outer peripheral side.
When the front surface side substrate tends to warp in a dome shape due to the growth of crystals on the surface, the inner peripheral portion of either the front surface side substrate or the back surface side substrate becomes the one substrate. Is configured to peel off from the inner circumference of the other different substrate.

(Appendix 16)
According to still another aspect of the invention.
A front-side substrate made of a GaN single crystal and a back-side substrate made of a GaN single crystal are joined with each other's main surface as a joining surface.
GaN in which the crystal orientation in the main surface of the front surface side substrate (direction of M axis or a axis) and the crystal orientation in the main surface of the back surface side substrate (direction of M axis or a axis) match. A substrate is provided.

(Appendix 17)
According to still another aspect of the invention.
A front-side substrate made of a GaN single crystal and a back-side substrate made of a GaN single crystal are joined with each other's main surface as a joining surface.
The crystal orientation in the main surface of the front surface side substrate (direction of M axis or a axis) and the crystal orientation in the main surface of the back surface side substrate (direction of M axis or a axis) do not match. Is provided.

10,10A-10E GaN substrate 11 Front side substrate 12 Back side substrate

Claims (4)

A front-side substrate made of a GaN single crystal and a back-side substrate made of a GaN single crystal are joined with each other's main surface as a joining surface.
The front surface is composed of a main surface different from the joint surface of the front surface side substrate and is composed of N polar surfaces, and the back surface is composed of a main surface different from the joint surface of the back surface side substrate and N. It is composed of polar surfaces and
A GaN substrate in which the angle formed by the axis orthogonal to the back surface and the c-axis of the crystal constituting the back surface is smaller than the angle formed by the axis orthogonal to the front surface and the c-axis of the crystal constituting the front surface. The GaN substrate according to claim 1 , wherein the thickness of the back surface side substrate is thinner than the thickness of the front surface side substrate. The front surface side substrate and the back surface side substrate are joined to each other at their outer peripheral portions.
The GaN substrate according to claim 1 or 2 , wherein a gap configured as a closed space is provided between the front surface side substrate and the back surface side substrate. Of the joint surfaces between the front surface side substrate and the back surface side substrate, the joint strength on the inner peripheral side is lower than the joint strength on the outer peripheral side.
When the front surface side substrate tries to warp in a dome shape due to the growth of crystals on the front surface side substrate, the inner peripheral portion of one of the front surface side substrate and the back surface side substrate is the one. The GaN substrate according to claim 1 or 2 , which is configured to be peeled off from the inner peripheral portion of the other substrate different from the substrate.

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