Nitride crystal substrate and manufacturing method of nitride crystal substrate

Abstract

To stably provide a nitride crystal substrate containing AlGaN.SOLUTION: A nitride crystal substrate is composed of a crystal represented by a compositional formula of AlxGa1-xN. An Al composition ratio x in the compositional formula is more than 0 and 1 or less. The Al composition ratio x cyclically changes in a thickness direction of the nitride crystal substrate.SELECTED DRAWING: Figure 12

Description

Technical Field

[0001] The present disclosure relates to a nitride crystal substrate and a method for manufacturing the nitride crystal substrate.

Background Art

[0002] As a method for obtaining a nitride crystal substrate made of a group III nitride crystal, various methods have been disclosed (for example, Patent Document 1 and Non-Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of the present disclosure is to stably obtain a nitride crystal substrate containing AlGaN.

Means for Solving the Problems

[0006] According to an aspect of the present disclosure, a nitride crystal substrate, Al x Ga 1-x composed of a crystal represented by the composition formula of N, wherein the Al composition ratio x in the composition formula is greater than 0 and less than or equal to 1, and the Al composition ratio x changes periodically in the thickness direction of the nitride crystal substrate is provided.

[0007] According to another aspect of the present disclosure, (a) preparing a lower base substrate; (b) forming an intermediate layer containing an n-type group III nitride crystal above the lower base substrate; (c) forming a cover layer containing a group III nitride crystal having a carrier concentration lower than that of the intermediate layer on the intermediate layer; (d) making the intermediate layer porous through dislocations in the cover layer while maintaining the surface state of the cover layer by electrochemical treatment; (e) epitaxially growing a regrowth layer made of a group III nitride crystal on the cover layer; (f) peeling off the regrowth layer from the substrate with at least a part of the porous intermediate layer as a boundary; and comprising In (e), as the regrowth layer, a crystal represented by the composition formula of Al x Ga 1-x N with an Al composition ratio x greater than 0 and less than or equal to 1 is grown, and the Al composition ratio x is periodically changed in the thickness direction of the regrowth layer A method for manufacturing a nitride crystal substrate is provided.

Advantages of the Invention

[0008] According to the present disclosure, a nitride crystal substrate containing AlGaN can be stably obtained.

Brief Description of the Drawings

[0009]

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Mode for Carrying Out the Invention

[0010] <Findings Obtained by the Inventors, etc.> Light-emitting diodes (LEDs) and laser diodes (LDs) with wavelengths ranging from about 365 nm to 530 nm (green) are realized by using a substrate made of gallium nitride (GaN) crystals (hereinafter also referred to as a GaN crystal substrate).

[0011] On the other hand, for ultraviolet LEDs and ultraviolet LDs with wavelengths shorter than 365 nm, it is necessary to grow a layer made of aluminum gallium nitride (AlGaN) crystals containing aluminum (Al) (hereinafter also referred to as an AlGaN layer). However, when using the above-mentioned GaN crystal substrate, due to the lattice mismatch between the AlGaN layer and the GaN crystal substrate, it is difficult to realize a high-quality device. Also, in other cases, there may be a case where an AlGaN layer is grown on an AlN crystal substrate manufactured by the sublimation method as described in Non-Patent Document 1. However, also in this case, due to the lattice mismatch between the AlGaN layer and the AlN crystal substrate, it is difficult to realize a high-quality device. For this reason, the realization of a high-quality AlGaN crystal substrate is desired.

[0012] Here, the GaN crystal substrate can be obtained, for example, by the VAS (Void-Assisted Separation) method described in Patent Document 1 above. In the VAS method, first, a template having a GaN layer having voids and a mesh-like titanium nitride (TiN layer) is prepared on a sapphire substrate in this order. Next, a thick GaN bulk growth layer is grown on the GaN layer and the TiN layer in this state. Then, with the GaN layer having voids as a boundary, the GaN bulk growth layer is peeled off from the substrate. As a result, a GaN crystal substrate can be obtained from the peeled GaN bulk growth layer.

[0013] Alternatively, in order to obtain a GaN crystal substrate, there may be a case where a GaN bulk growth layer is grown on a template covering the surface of the GaN layer with a mask having an opening partially.

[0014] In any of the above cases, at the initial stage of the growth of the GaN bulk growth layer, GaN crystals grow in an island shape. Then, when thick-film GaN crystals grow, the island-shaped GaN crystals coalesce to obtain a GaN bulk growth layer as a continuous film. In the process of such coalescence of the island-shaped GaN crystals, the crystal strain introduced during the growth of the thick-film GaN crystals is significantly relaxed. As a result, even on a heterogeneous substrate, it becomes possible to grow a crack-free thick-film GaN bulk growth layer.

[0015] An aluminum nitride (AlN) crystal substrate can be obtained, for example, by the sublimation method as described in Non-Patent Document 1. In this case, first, in the initial stage, an AlN layer is grown on a heterogeneous substrate such as a silicon carbide (SiC) substrate. Next, the AlN layer is thickened by the sublimation method using the AlN layer as a seed crystal. Then, the heterogeneous substrate is peeled off from the AlN layer to obtain an AlN crystal substrate. Once the AlN crystal substrate is obtained, an AlN ingot can be obtained by growing an AlN layer again by the sublimation method using the AlN crystal substrate as a seed crystal. Then, the AlN ingot is sliced to obtain an AlN crystal substrate.

[0016] On the other hand, the method of obtaining an AlGaN crystal substrate is more difficult than the method of obtaining a GaN crystal substrate or an AlN crystal substrate. First, since the vapor pressures of Al and Ga are different, it is extremely difficult to obtain an AlGaN crystal with a desired Al composition by the sublimation method. For this reason, growing an AlGaN bulk growth layer on a template such as the above-described VAS method is a practical method. However, in that case, since Al atoms are extremely difficult to move on the growth surface, it is difficult to realize a growth process including island growth, coalescence of island crystals, and flattening of the crystals as in the case of GaN.

[0017] In particular, when the island-shaped AlGaN crystals grow three-dimensionally, the movement of Al atoms decreases as described above, so that a portion with a high Al composition ratio and a portion with a low Al composition ratio are formed in the plane. For this reason, crystal strain is likely to occur in the AlGaN bulk growth layer. As a result, cracks may occur in the AlGaN bulk growth layer.

[0018] Therefore, in order to obtain an AlGaN crystal substrate from an AlGaN bulk growth layer, a method of first growing a thick GaN layer, flattening the surface of the GaN layer, and then growing a thick-film AlGaN bulk growth layer can be considered. However, even with this method, due to the lattice mismatch between the GaN layer and the AlGaN bulk growth layer, it is difficult to obtain a crack-free AlGaN bulk growth layer.

[0019] Therefore, the inventors considered the following method as a method for manufacturing an AlGaN crystal substrate. First, an intermediate layer and a cover layer containing a group III nitride crystal were grown on a base substrate in this order, and an electrochemical treatment was performed on the laminate. As a result, a seed substrate in which the intermediate layer was made porous while maintaining the flatness of the cover layer was produced. Next, using the seed substrate, a regrown layer made of an AlGaN crystal as a continuous film was regrown on a flat cover layer. At this time, the Al composition ratio was periodically changed in the thickness direction of the regrown layer. By such a method, the above-described problems can be solved, and a crack-free AlGaN crystal substrate was successfully obtained from the regrown layer.

[0020] The following disclosure is based on the above-mentioned new findings found by the inventors.

[0021] <Details of Embodiments of the Present Disclosure> Next, an embodiment of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these examples, and is intended to be indicated by the claims and to include all modifications within the meaning and scope equivalent to the claims.

[0022] <One Embodiment of the Present Disclosure> Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings.

[0023] (1) Method for Manufacturing a Nitride Crystal Substrate Referring to FIGS. 1 to 10, a method for manufacturing a nitride crystal substrate according to the present embodiment will be described. In FIGS. 2A to 4, FIG. 6, FIG. 7, FIG. 9, and FIG. 10, hatching of some cross-sections is omitted. In FIGS. 7, 9, and 10, as will be described later, the gray portion in the second regrowth layer 540 indicates a region where the Al composition ratio x is relatively large.

[0024] Hereinafter, in a group III nitride crystal having a wurtzite structure, the <0001> axis (for example, the

[0001] axis) is referred to as the "c-axis", and the (0001) plane is referred to as the "c-plane". The (0001) plane may be referred to as the "+c plane (group III element polar plane)", and the (000-1) plane may be referred to as the "-c plane (nitrogen (N) polar plane)". The <1-100> axis is referred to as the "m-axis", and the <11-20> axis is referred to as the "a-axis". The "carrier concentration" as used in the present disclosure means the free carrier concentration at room temperature (22°C).

[0025] As shown in FIG. 1, the method for manufacturing a nitride crystal substrate according to the present embodiment includes, for example, a lower substrate preparation step S10, a lower layer formation step S20, an intermediate layer formation step S30, a cover layer formation step S40, a porous step S50, a regrowth step S60, a peeling step S70, and a post-treatment step S80.

[0026] (S10: Lower Substrate Preparation Step) First, as shown in FIG. 2A, a lower substrate 100 is prepared.

[0027] In the present embodiment, for example, a lower substrate 100 made of a material different from that of the group III nitride is prepared. Specifically, examples of the lower substrate 100 include a sapphire substrate, a SiC substrate, a silicon (Si) substrate, and a gallium arsenide (GaAs) substrate. The lower substrate 100 may be insulating or conductive. Here, the lower substrate 100 is, for example, a sapphire substrate.

[0028] The diameter of the underlying substrate 100 is, for example, 1 inch (25 mm) or more, or may be 2 inches (50 mm) or more, or may be 4 inches (100 mm) or more. Thereby, a large-area regrowth layer 500 described later can be grown.

[0029] The thickness of the underlying substrate 100 is, for example, 150 μm or more and 3 mm or less.

[0030] The underlying substrate 100 has, for example, a main surface 120 that becomes the growth surface. When the underlying substrate 100 is a sapphire substrate or a SiC substrate, the low-index crystal plane closest to the main surface 120 is, for example, the c-plane (+c plane). When the underlying substrate 100 is a Si substrate or a GaAs substrate, the low-index crystal plane closest to the main surface 120 is, for example, the (001) plane or the (111) plane.

[0031] In the present embodiment, the c-plane of the underlying substrate 100 may be inclined with respect to the main surface 120. That is, the c-axis of the underlying substrate 100 may be inclined at a predetermined off-angle with respect to the normal of the main surface 120. The off-angle of the underlying substrate 100 is, for example, 0° or more and 5° or less.

[0032] The arithmetic mean roughness (Ra) of the main surface 120 of the underlying substrate 100 is, for example, less than 0.3 nm.

[0033] (S20: Underlayer formation step) After preparing the underlying substrate 100, as shown in FIG. 2A, for example, an underlayer 200 containing a group III nitride crystal is formed on the underlying substrate 100 by a vapor phase growth method (an underlayer 200 made of a group III nitride crystal is formed).

[0034] Specifically, for example, by the hydride vapor phase epitaxy (HVPE) method, an aluminum nitride (AlN) buffer layer is grown by supplying aluminum chloride (AlCl3) gas and ammonia (NH3) gas to a lower base substrate 100 heated to a predetermined growth temperature. Next, a gallium nitride (GaN) layer is grown by supplying gallium chloride (GaCl) gas and NH3 gas to the lower base substrate 100 heated to a predetermined growth temperature. Note that the growth temperature of each layer is, for example, 900°C or higher and 1100°C or lower. As described above, an AlN buffer layer and a GaN layer are formed in this order as the lower layer 200 on the main surface 120 of the lower base substrate 100. However, the lower layer 200 may not have an AlN buffer layer.

[0035] In this embodiment, the lower layer 200 is, for example, an n-type. Specifically, when growing the GaN layer as the lower layer 200, a dichlorosilane (SiH2Cl2) gas as an n-type dopant gas is further supplied to grow an Si-doped GaN layer.

[0036] In this embodiment, the carrier concentration in the GaN layer of the lower layer 200 is made lower than, for example, the carrier concentration of the intermediate layer 300. In other words, the n-type impurity concentration in the GaN layer of the lower layer 200 is made lower than, for example, the n-type impurity concentration of the intermediate layer 300. Specifically, the carrier concentration (and n-type impurity concentration) in the lower layer 200 is, for example, 2×10 18 cm -3 or less, or 1×10 18 cm -3 or less. Thereby, in the porous process S50 of making the intermediate layer 300 described later porous by electrochemical treatment, the lower layer 200 can function as an etching stopper on the lower side of the intermediate layer 300.

[0037] The lower limit value of the carrier concentration in the GaN layer of the lower layer 200 is not limited. However, the carrier concentration in the GaN layer of the lower layer 200 is, for example, 1×10 16 cm -3The above may be sufficient. In this way, since the underlying layer 200 itself also has some conductivity, in the subsequent porous process S50, the etching of the intermediate layer 300 can proceed toward the lower part of the intermediate layer 300 by electrochemical treatment.

[0038] The thickness of the underlying layer 200 is not particularly limited. However, the thickness of the underlying layer 200 may be, for example, more than 0 nm and 5 μm or less. By setting the thickness of the underlying layer 200 to 5 μm or less, the total thickness of the stack on the underlying substrate 100 can be adjusted to, for example, 15 μm or less. Thereby, cracks caused by the difference in the linear expansion coefficients between the underlying substrate 100 and each layer such as the underlying layer 200 can be suppressed.

[0039] In the present embodiment, the low-index crystal plane closest to the surface of the underlying layer 200 is the c-plane of the GaN layer. By previously forming a flat surface of the underlying layer 200 having such a crystal plane, a highly crystalline intermediate layer 300 and a cover layer 400 can be grown on the surface of the underlying layer 200.

[0040] (S30: Intermediate layer formation step) After forming the underlying layer 200, as shown in FIG. 2B, for example, by a vapor phase growth method, an intermediate layer 300 containing an n-type group III nitride crystal is formed on the underlying layer 200 located above the underlying substrate 100. (In this step, an intermediate layer 300 composed of a group III nitride crystal without voids is formed.)

[0041] Specifically, for example, by the HVPE method, GaCl gas, NH3 gas, and SiH2Cl2 gas as an n-type dopant gas are supplied to the underlying substrate 100 heated to a predetermined growth temperature, and a Si-doped GaN layer is grown as the intermediate layer 300 on the underlying layer 200. Note that the intermediate layer 300 grows with the +c plane as the growth plane.

[0042] In this embodiment, the carrier concentration in the intermediate layer 300 is made higher than, for example, the carrier concentration of the underlying layer 200 and the carrier concentration of the cover layer 400. In other words, the n-type impurity concentration in the intermediate layer 300 is made higher than, for example, the n-type impurity concentration of the underlying layer 200 and the n-type impurity concentration of the cover layer 400. Specifically, the carrier concentration (and the n-type impurity concentration) in the intermediate layer 300 is set to, for example, 3×10 18 cm -3 or more, or may be set to 1×10 19 cm -3 or more. Thereby, the intermediate layer 300 can be selectively made porous in the porous process S50 described later.

[0043] Note that the upper limit value of the carrier concentration in the intermediate layer 300 is not limited. However, the carrier concentration in the intermediate layer 300 may be set to, for example, 1×10 20 cm -3 or less, or may be set to 5×10 19 cm -3 or less. Thereby, a decrease in the crystallinity of the intermediate layer 300 can be suppressed.

[0044] In this embodiment, the thickness of the intermediate layer 300 is set to be more than 100 nm, for example, or may be 500 nm or more, or may be 1 μm or more. Thereby, in the porous process S50 described later, large voids can be formed in the intermediate layer 300. Thereby, in the regrowth process S60 described later, the voids 360 of the intermediate layer 300 can be maintained. As a result, in the peeling process S70 described later, at least a part of the intermediate layer 300 maintained in a porous state can be easily and stably peeled from the underlying substrate 100 with the regrowth layer 500 as a boundary.

[0045] The upper limit value of the thickness of the intermediate layer 300 is not limited. However, the thickness of the intermediate layer 300 may be, for example, 10 μm or less. By setting the thickness of the intermediate layer 300 to 10 μm or less, the total thickness of the stack on the underlying substrate 100 can be adjusted to, for example, 15 μm or less. Thereby, cracks caused by the difference in the linear expansion coefficients between the underlying substrate 100 and each layer such as the intermediate layer 300 can be suppressed.

[0046] (S40: Cover layer formation step) After forming the intermediate layer 300, as shown in FIG. 3A, for example, a cover layer 400 containing group III nitride crystals is formed on the intermediate layer 300 by a vapor phase growth method (a cover layer 400 made of group III nitride crystals is formed).

[0047] In the present embodiment, for example, GaN crystals are grown as the cover layer 400. Thereby, the crystal growth of the cover layer 400 can be easily controlled by a vapor phase growth method. As a result, the crystallinity of the cover layer 400 can be stably improved. Furthermore, the carrier concentration of the cover layer 400 can be easily controlled.

[0048] Specifically, for example, by the HVPE method, a Si-doped GaN layer is grown as the cover layer 400 on the intermediate layer 300 under the same conditions as in the intermediate layer formation step S30 except that the supply amount of SiH2Cl2 gas as an n-type dopant gas is less than that in the intermediate layer formation step S30. Note that the cover layer 400 grows with the +c plane as the growth plane.

[0049] In the present embodiment, the carrier concentration in the cover layer 400 is made lower than, for example, the carrier concentration in the intermediate layer 300. In other words, the n-type impurity concentration in the cover layer 400 is made lower than, for example, the n-type impurity concentration in the intermediate layer 300. Specifically, the carrier concentration (and n-type impurity concentration) in the cover layer 400 is, for example, 1×10 18 cm -3Assume the following. As a result, in the subsequent porous process S50, the intermediate layer 300 can be selectively made porous while suppressing the etching of the cover layer 400. That is, under a predetermined voltage, the size of the voids 360 in the intermediate layer 300 can be increased while preventing the micropores in the cover layer 400 from growing larger.

[0050] The lower limit of the carrier concentration in the cover layer 400 is not limited. However, the carrier concentration in the cover layer 400 may be, for example, 1×10 16 cm -3 or higher, or 1×10 17 cm -3 or higher. In this way, since the cover layer 400 itself also has conductivity, in the electrochemical treatment of the porous process S50 for making the intermediate layer 300 porous, the intermediate layer 300 can be connected to the anode 842 via the cover layer 400, and the entire intermediate layer 300 can be made equipotential with the anode 842.

[0051] Here, in the state where the cover layer formation process S40 is completed, the base layer 200, the intermediate layer 300, and the cover layer 400 have, for example, a plurality of dislocations D penetrating in the thickness direction. The dislocation density on the surface of the cover layer 400 is, for example, 1×10 8 cm -2 or higher and 1×10 9 cm -2 or lower. The dislocations D in the cover layer 400 are utilized in the following porous process S50.

[0052] In this embodiment, the thickness of the cover layer 400 may be, for example, 10 nm or more and 2 μm or less, or 50 nm or more and 1.5 μm or less.

[0053] By setting the thickness of the cover layer 400 to 10 nm or more, or 50 nm or more, even if outgassing such as N2 gas occurs when the intermediate layer 300 is etched in the porous process S50 described later, the cover layer 400 can be maintained, and the outgassing can be released outside the cover layer 400 through the dislocation D of the cover layer 400. Thereby, peeling of the cover layer 400 from the intermediate layer 300 can be suppressed.

[0054] On the other hand, by setting the thickness of the cover layer 400 to 2 μm or less, or 1.5 μm or less, the electrolytic solution can stably reach the intermediate layer 300 through the dislocation D of the cover layer 400 in the porous process S50 described later. Thereby, void formation in the intermediate layer 300 can be stably performed.

[0055] After growing the cover layer 400, the temperature of the lower base substrate 100 is lowered from the growth temperature of the group III nitride crystal to room temperature. Thereby, a laminate including the lower base substrate 100, the lower layer 200, the intermediate layer 300, and the cover layer 400 is formed.

[0056] At this time, as shown in FIG. 3B, due to the difference in the linear expansion coefficient between the lower base substrate 100 and the group III nitride crystal layer including the lower layer 200, the intermediate layer 300, and the cover layer 400, the entire laminate warps. Specifically, since the linear expansion coefficient of sapphire as the lower base substrate 100 is larger than the linear expansion coefficient of the group III nitride, the entire laminate warps so that the surface of the cover layer 400 becomes convex.

[0057] The above-described lower layer formation step S20, intermediate layer formation step S30, and cover layer formation step S40 are continuously performed in the same chamber without exposing the lower base substrate 100 to the atmosphere. Thereby, at the interface between the lower layer 200, the intermediate layer 300, and the cover layer 400, unintended incorporation of at least one of oxygen (O) and silicon (Si) as n-type impurities can be suppressed. As a result, it is possible to suppress the influence on the carrier concentration in these layers.

[0058] (S50: Porous Process) After forming the cover layer 400, as shown in FIG. 4, by electrochemical treatment, while maintaining the surface state of the cover layer 400, the intermediate layer 300 is made porous through the dislocations D of the cover layer 400.

[0059] Specifically, for example, the electrochemical treatment is performed according to the following procedure.

[0060] As shown in FIG. 4, first, a treatment tank 820, a power source 840, and an ammeter 860 are prepared. Note that the power source 840 and the ammeter 860 may be incorporated into one device as a current-voltage power source.

[0061] The treatment tank 820 is filled with an electrolytic solution 810. The electrolytic solution 810 is a solution containing ions capable of electrochemically etching group III nitrides. Examples of the electrolytic solution 810 include an aqueous solution containing oxalic acid, nitric acid, hydrofluoric acid, sulfuric acid, sodium sulfate (Na2SO4), sodium hydroxide (NaOH), etc. Here, the electrolytic solution 810 is an oxalic acid solution.

[0062] Also, each electrode for performing the electrochemical treatment is prepared. Specifically, in the laminate in which the above-described cover layer forming step S40 is completed, an anode 842 is provided on the surface of the cover layer 400, and the anode 842 is connected to the power source 840. On the other hand, a cathode 844 is prepared and the cathode 844 is connected to the power source 840. As the cathode 844, a material having corrosion resistance and allowing current to flow easily is used. Specific examples of the cathode 844 include stainless steel (SUS), platinum (Pt), gold (Au), and boron-doped diamond.

[0063] After the connection of each electrode is completed, the laminate connected to the anode 842 and the cathode 844 are immersed in the electrolytic solution 810 in the treatment tank 820. In this state, a predetermined voltage is applied between the anode 842 and the cathode 844 by the power source 840. Thereby, the electrochemical treatment is performed. At this time, based on the change in current in the ammeter 860, the progress of the electrochemical treatment is confirmed.

[0064] At this time, due to the electrochemical treatment, C2O4 is permeated through the dislocation D of the cover layer 400 with a relatively low carrier concentration toward the intermediate layer 300 with a relatively high carrier concentration. 2- That is, the dislocation D of the cover layer 400 is used as a nano-sized path through which the electrolytic solution permeates. The electrolytic solution that has reached the intermediate layer 300 in this way selectively etches the intermediate layer 300. As a result, a plurality of voids 360 are formed respectively in the vicinity of a plurality of dislocations D in the intermediate layer 300. As a result, the intermediate layer 300 can be made porous.

[0065] On the other hand, according to the following reaction formula, group III element ions (Ga 3+ ) generated when the intermediate layer 300 is etched and nitrogen (N2) gas are released outside the cover layer 400 through the dislocation D of the cover layer 400. 2GaN + 6h + → 2Ga 3+ + N2

[0066] By the above-described electrochemical treatment, each of the plurality of voids 360 in the intermediate layer 300 is formed at a position overlapping, for example, each of the plurality of dislocations D in the cover layer 400 described later. The plurality of voids 360 in the intermediate layer 300 extend, for example, from the lower surface of the cover layer 400 in the thickness direction toward the underlying substrate 100. Note that the voids 360 do not necessarily reach the underlying layer 200.

[0067] On the other hand, the partition portions other than the voids 360 of the intermediate layer 300 connect the upper part of the underlying layer 200 or the lower part of the intermediate layer 300 and the cover layer 400. Thereby, even if the intermediate layer 300 has a plurality of voids 360, a certain thickness is maintained.

[0068] At this time, in the present embodiment, the length of each of the plurality of voids 360 in the intermediate layer 300 in the direction along the main surface 120 of the base substrate 100 may be, for example, 30 nm or more, or 100 nm or more when viewed from an arbitrary cross-section orthogonal to the main surface 120 of the base substrate 100. Thereby, in the regrowth step S60 described later, the voids 360 of the intermediate layer 300 can be maintained. As a result, in the peeling step S70 described later, at least a part of the intermediate layer 300 maintained in a porous state can be easily and stably peeled from the base substrate 100.

[0069] The upper limit value of the length of the voids 360 in the intermediate layer 300 in the direction along the main surface 120 of the base substrate 100 is not limited. However, the length of the voids 360 in the direction along the main surface 120 of the base substrate 100 may be 10 μm or less. Thereby, by appropriately adjusting the electrochemical treatment conditions in the porous step S50, peeling of the cover layer 400 caused by outgas generated when the intermediate layer 300 is etched can be suppressed.

[0070] At this time, in the present embodiment, the depth of each of the plurality of voids 360 in the thickness direction of the intermediate layer 300 may be, for example, more than 100 nm, or 500 nm or more, or 1 μm or more. Also by this, in the regrowth step S60 described later, the voids 360 of the intermediate layer 300 can be maintained. As a result, in the peeling step S70 described later, the regrowth layer 500 can be easily and stably peeled from the base substrate 100.

[0071] The upper limit value of the depth of the voids 360 is not limited. However, the depth of the voids 360 may be equal to or less than the thickness of the intermediate layer 300. Thereby, in the peeling step S70 described later, excessive spread of peeling from the intermediate layer 300 to other layers can be suppressed.

[0072] On the one hand, in the electrochemical treatment, almost no etching occurs on the surface of the cover layer 400 where the carrier concentration is relatively low. That is, even if the cover layer 400 has a plurality of dislocations D as described above, excessive etching does not occur on the surface near the dislocations D of the cover layer 400. Thereby, the surface state of the cover layer 400 can be maintained flat.

[0073] At this time, in the present embodiment, after the porous process S50, the arithmetic mean roughness (Ra) of the surface of the cover layer 400 is, for example, 1.0 nm or less, and the root mean square roughness (RMS) of the surface of the cover layer 400 is, for example, 2.0 nm or less. Alternatively, the Ra of the surface of the cover layer 400 may be, for example, 0.5 nm or less, and the RMS of the surface of the cover layer 400 may be, for example, 1.0 nm or less. Here, Ra and RMS are values when the surface of the cover layer 400 is observed in a 5 μm square field of view by an atomic force microscope (AFM).

[0074] As described above, by maintaining the surface roughness of the cover layer 400 small, a stable growth of a recrystallized layer 500 of a thick film with good crystallinity can be achieved on the cover layer 400.

[0075] The lower limit values of Ra and RMS of the surface of the cover layer 400 are not limited and may be close to the Ra and RMS of the main surface 120 of the lower base plate 100. Specifically, the lower limit values of Ra and RMS of the surface of the cover layer 400 may be 0.1 nm and 0.2 nm, respectively.

[0076] Specific conditions of the electrochemical treatment capable of realizing the selective etching of the intermediate layer 300 described above include, for example, the following. The treatment voltage is adjusted based on the carrier concentration such as that of the intermediate layer 300. The treatment current is adjusted based on the treatment area (the area of the lower base plate 100). The treatment time is adjusted based on the thickness of the intermediate layer 300.

[0077] Temperature of the electrolytic solution: room temperature (10 °C or more and 30 °C or less) Processing voltage: 1 V or more and 200 V or less, or 10 V or more and 20 V or less Processing current: 0.01 mA or more and 60 A or less, or 0.1 mA or more and 10 A or less Processing time: 0.1 min or more and 180 min or less, or 1 min or more and 30 min or less

[0078] As the intermediate layer 300 becomes porous by the above-described electrochemical treatment, as shown in FIG. 4, with the porous intermediate layer 300 interposed therebetween, the cover layer 400 and the laminate including the underlying substrate 100 and the underlayer 200 are separated. Since the voids 360 of the intermediate layer 300 have the above-described size, such separation can occur significantly.

[0079] Thereby, the warp of the cover layer 400 in the porous step S50 can be reduced compared to the warp of the cover layer 400 before the porous step S50. That is, the cover layer 400 can be made to be in a state closer to flat.

[0080] On the other hand, the seed substrate 10 is in a state close to a configuration in which the group III nitride crystal layer on the underlying substrate 100 is thin. Thereby, the warp of the underlying substrate 100 in the porous step S50 can be reduced compared to the warp of the underlying substrate 100 before the porous step S50.

[0081] After the electrochemical treatment, the nitride crystal growth seed substrate 10 is taken out from the electrolytic solution in the treatment tank 820. Thereafter, the nitride crystal growth seed substrate 10 taken out from the treatment tank 820 is washed with pure water or the like and dried. Thereby, the electrolytic solution remaining in the voids 360 of the intermediate layer 300 is removed. Thus, the porous step S50 is completed.

[0082] In this way, the seed substrate 10 is obtained. The seed substrate 10 is used in the regrowth step S60 and the peeling step S70 described later.

[0083] (S60: Regrowth step) When the porous process S50 is completed, a regrown layer 500 made of a group III nitride crystal is epitaxially grown on the cover layer 400. As a method for growing the regrown layer 500, for example, a vapor growth method is used.

[0084] In the present embodiment, as the regrown layer 500, a crystal represented by the composition formula of Al x Ga 1-x N with an Al composition ratio x greater than 0 and less than or equal to 1 is grown. At this time, the Al composition ratio x is periodically changed in the thickness direction of the regrown layer 500.

[0085] In the present embodiment, by growing the regrown layer 500 made of the above-described AlGaN crystal on the flat cover layer 400 on the porous intermediate layer 300, the crystal strain generated in the along-surface direction of the regrown layer 500 can be relaxed more than when the porous process S50 is not performed.

[0086] Furthermore, in the present embodiment, by periodically changing the Al composition ratio x in the thickness direction of the regrown layer 500, the crystal strain generated in the along-surface direction of the regrown layer 500 can be stably relaxed.

[0087] In the regrowth process S60 of the present embodiment, as shown in FIG. 5, the growth temperature may be adjusted in two steps. Specifically, the regrowth process S60 may include, for example, a first regrowth process S62 and a second regrowth process S64.

[0088] In this case, in the regrowth process S60, first, as shown in FIG. 5, the temperature of the seed substrate 10 is raised from room temperature to the first growth temperature T1.

[0089] (S62: First regrowth process) When the temperature of the seed substrate 10 reaches the first growth temperature T1, as shown in FIG. 6, a first regrown layer 520 is grown on the cover layer 400 at the first growth temperature T1.

[0090] Specifically, for example, by the HVPE method, an AlGaN layer is grown by supplying AlCl3 gas, GaCl gas, and NH3 gas to a seed substrate 10 heated to a first growth temperature T1. Thereby, an AlGaN layer is epitaxially grown as a first regrowth layer 520 on the surface of the cover layer 400. Note that various dopants may be added to the AlGaN layer as the first regrowth layer 520.

[0091] At this time, in the present embodiment, the first growth temperature T1 in the first regrowth step S62 is made lower than the second growth temperature T2 in the second regrowth step S64 as the main growth described later. That is, the first growth temperature T1 in the first regrowth step S62 is made slightly lower than the growth temperature of typical group III nitride crystals.

[0092] At this time, as the temperature rises from room temperature to the first growth temperature T1, the base substrate 100 expands. However, since the first growth temperature T1 is lower (than the growth temperature of the underlying layer 200 etc.), the base substrate 100 does not become completely flat and is maintained in a state where the main surface 120 of the base substrate 100 is slightly convex.

[0093] On the other hand, since the linear expansion coefficient of the cover layer 400 is smaller than that of the base substrate 100, the elongation of the cover layer 400 is smaller than that of the base substrate 100. For this reason, the cover layer warps so that the surface of the cover layer 400 becomes concave. However, since the first growth temperature T1 is low, excessive warping of the cover layer 400 can be suppressed. Thereby, the generation of cracks in the cover layer 400 can be suppressed. As a result, the generation of cracks in the regrowth layer 500 on the cover layer 400 can be suppressed.

[0094] At this time, in the present embodiment, the first growth temperature T1 is set to, for example, 970 °C or lower. Thereby, in the first regrowth step S62, the generation of cracks in the cover layer 400 can be stably suppressed. As a result, the generation of cracks in the regrowth layer 500 on the cover layer 400 can be stably suppressed.

[0095] On the one hand, the first growth temperature T1 may be, for example, 800°C or higher. Thereby, the group-III nitride crystal as the first regrowth layer 520 can be stably grown.

[0096] As described above, by adjusting the first growth temperature T1, the first regrowth layer 520 with a predetermined thickness can be grown on the cover layer 400 while suppressing the generation of cracks in the cover layer 400. Thereby, the total thickness of the group-III nitride crystal layers (the cover layer 400 and the first regrowth layer 520) above the porous intermediate layer 300 can be stably increased. As a result, in the second regrowth step S64 as the main growth described later, when the temperature is raised to the second growth temperature T2, the group-III nitride crystal layer above the intermediate layer 300 can be made less likely to crack.

[0097] At this time, in the present embodiment, by starting the growth of the first regrowth layer 520 on the cover layer 400 having a flat surface, at least in the initial growth stage (the stage where the thickness is thin) of the first regrowth layer 520 as the regrowth layer 500, the regrowth layer 500 can be grown by step-flow growth. That is, in the initial growth stage, the island growth of the regrowth layer 500 made of AlGaN crystal can be suppressed. Thereby, the regrowth layer 500, which is a continuous film made of AlGaN crystal, can be grown by step-flow growth with the c-plane 522 (+c-plane) as the growth plane over the entire surface of the cover layer 400. As a result, the Al composition ratio x can be made uniform in the plane of the first regrowth layer 520. Furthermore, the crystallinity of the first regrowth layer 520 can be improved.

[0098] Thereafter, when gradually growing the first regrowth layer 520 after step-flow growth in the initial growth stage of the first regrowth layer 520, an inclined interface other than the c-plane may be formed in at least a part of the first regrowth layer 520. That is, after step-flow growing the first regrowth layer 520 with a predetermined thickness, the first regrowth layer 520 may be three-dimensionally grown. As a result, at least a part of the plurality of dislocations D propagating in the direction along the c-axis of the first regrowth layer 520 from the cover layer 400 can be bent and propagated in a direction substantially perpendicular to the inclined interface at the position where the inclined interface is exposed. Thereby, at least a part of the plurality of dislocations D can be locally collected. By causing the locally collected dislocations D to disappear, the dislocation density can be reduced. As a result, the number of pits on the surface of the regrowth layer 500 can be decreased.

[0099] At this time, in the present embodiment, the Al composition ratio x in the first regrowth layer 520 may be made uniform in the thickness direction of the first regrowth layer 520. In this case, the Al composition ratio x in the first regrowth layer 520 may be, for example, more than 0 and 1 or less, or may be 0.05 or more and 0.8 or less. By setting the Al composition ratio x in the first regrowth layer 520 to be 0.05 or more and 0.8 or less, appropriate crystal strain can be applied to the porous intermediate layer 300.

[0100] At this time, in the present embodiment, the thickness of the first regrowth layer 520 may be, for example, 1 μm or more, or may be 10 μm or more, or may be 20 μm or more. Thereby, in the second regrowth step S64 as the main growth described later, when the temperature is raised to the second growth temperature, the group III nitride crystal layer above the intermediate layer 300 can be made difficult to crack stably.

[0101] On the other hand, the upper limit value of the thickness of the first regrowth layer 520 is not particularly limited. However, the thickness of the first regrowth layer 520 may be, for example, 300 μm or less, or may be 100 μm or less.

[0102] When the thickness of the first regrowth layer 520 reaches a predetermined thickness, the supply of AlCl3 gas and GaCl gas is stopped, and the growth of the first regrowth layer 520 is terminated. Note that the supply of NH3 gas is continued.

[0103] Thereafter, as shown in FIG. 5, the temperature of the seed substrate 10 is raised from the first growth temperature T1 to the second growth temperature T2.

[0104] (S64: Second regrowth process) After the temperature of the seed substrate 10 is raised from the first growth temperature T1 to the second growth temperature T2, as shown in FIG. 7, the growth of the regrowth layer 500 as the second regrowth layer 540 is resumed. That is, at the second growth temperature T2, a thick film second regrowth layer 540 is grown as a main growth on the first regrowth layer 520.

[0105] Specifically, for example, the second regrowth process S64 is carried out in the same chamber as the first regrowth process S62 without exposing to the atmosphere. In the second regrowth process S64, an AlGaN layer is epitaxially grown as the second regrowth layer 540 on the first regrowth layer 520 by supplying AlCl3 gas, GaCl gas, and NH3 gas to the seed substrate 10 heated to the second growth temperature T2. Note that various dopants may be added to the AlGaN layer as the second regrowth layer 540.

[0106] At this time, as shown in FIG. 7, in the present embodiment, within the range where the Al composition ratio x is greater than 0 and less than or equal to 1, the Al composition ratio x is periodically changed in the thickness direction of the second regrowth layer 540. In FIG. 7, the gray portions in the second regrowth layer 540 indicate regions where the Al composition ratio x is relatively large.

[0107] Specifically, for example, as shown in FIG. 8, as the second regrowth layer 540 is grown, the Al flow ratio R is changed. The "Al flow ratio R" referred to here is obtained by the following formula (A) as the ratio of the Al source gas flow rate to the total flow rate of the Al source gas flow rate and the Ga source gas flow rate. R = [AlCl3] / ([AlCl3] + [GaCl]) ···(A) Here, [AlCl3] is the flow rate of AlCl3 gas as the Al source gas, and [GaCl] is the flow rate of GaCl gas as the Ga source gas.

[0108] That is, in the second regrowth step S64, a step of adjusting the Al flow rate ratio R to the minimum value Rmin, a step of gradually increasing the Al flow rate ratio R from the minimum value Rmin as the second regrowth layer 540 grows, a step of adjusting the Al flow rate ratio R to the maximum value Rmax, a step of gradually decreasing the Al flow rate ratio R from the maximum value Rmax as the second regrowth layer 540 grows, and repeating a cycle including these steps.

[0109] Thus, by adjusting the Al flow rate ratio R when growing the second regrowth layer 540, the Al composition ratio x can be periodically changed in the thickness direction of the second regrowth layer 540. As a result, the local strain amount in the thickness direction of the second regrowth layer 540 can be made unbalanced, and slip of atomic planes can be caused. Consequently, the crystal strain generated in the along-surface direction of the second regrowth layer 540 can be stably relaxed.

[0110] At this time, in this embodiment, the average value xavg of the Al composition ratio x of the second regrowth layer 540 is set to be 0.05 or more and 0.8 or less. Specifically, the average value Ravg of the Al flow rate ratio R is adjusted to be 0.05 or more and 0.8 or less so that the average value xavg of the Al composition ratio x is within the above range. Thereby, appropriate crystal strain can be applied to the porous intermediate layer 300 based on the average value xavg of the Al composition ratio x of the second regrowth layer 540.

[0111] At this time, in the present embodiment, the period of the Al composition ratio x in the thickness direction of the second regrowth layer 540 may be 5 nm or more and 2000 nm or less, or 5 nm or more and 500 nm or less. Specifically, the period tp of varying the Al flow rate ratio R is adjusted so that the period of the Al composition ratio x falls within the above range. Thereby, within the portion where the Al composition ratio x changes, the above-described slip of the atomic plane can be stably generated.

[0112] At this time, in the present embodiment, the difference between the maximum value and the minimum value of the Al composition ratio x of the second regrowth layer 540 is set to be 0.2xavg or more and xavg or less when the average value of the Al composition ratio x is xavg. Specifically, the difference (fluctuation range) Rmax - Rmin between the maximum value and the minimum value of the Al flow rate ratio R is adjusted to be 0.2Ravg or more and Ravg or less so that the difference between the maximum value and the minimum value of the Al composition ratio x falls within the above range. Thereby, the above-described slip of the atomic plane can be stably generated.

[0113] At this time, in the present embodiment, as described above, even if the second growth temperature T2 of the second regrowth step S64 as the main growth is made higher than the first growth temperature T1 of the first regrowth step S62, due to the large total thickness of the cover layer 400 and the first regrowth layer 520 as the group III nitride crystal layer above the intermediate layer 300, the group III nitride crystal layer above the intermediate layer 300 is less likely to crack. Thereby, it becomes possible to stably grow the regrowth layer 500.

[0114] At this time, in the present embodiment, as described above, by making the second growth temperature T2 of the second regrowth step S64 higher than the first growth temperature T1, the second regrowth layer 540 can be stably grown by step-flow growth (two-dimensional growth, lateral growth) with the c-plane 542 as the growth surface. Thereby, while improving the flatness of the surface, a second regrowth layer 540 having a good crystalline thick film can be grown. Note that the c-axis, which is the normal line of the c-plane 542 of the second regrowth layer 540, may be inclined at an off-angle that follows the c-axis of the underlying substrate 100 inclined at a predetermined off-angle.

[0115] At this time, in the present embodiment, the second growth temperature T2 may be, for example, 980°C or higher, or may be 10000°C or higher. Thereby, the second regrowth layer 540 can be stably grown in step flow. As a result, while improving the flatness of the second regrowth layer 540, a second regrowth layer 540 with a thick film having good crystallinity can be stably grown.

[0116] On the other hand, the second growth temperature T2 may be, for example, 1200°C or lower. Thereby, roughening of the surface of the second regrowth layer 540 due to an excessively high growth temperature can be suppressed.

[0117] At this time, in the first regrowth step S62 of the present embodiment, when an inclined interface is formed in at least a part of the first regrowth layer 520 (when the first regrowth layer 520 is three-dimensionally grown), by laterally growing the second regrowth layer 540 as described above, the inclined interface in the second regrowth layer 540 can be gradually reduced, that is, the c-plane 542 can be gradually expanded. Thereby, a second regrowth layer 540 having a mirror-finished surface can be grown.

[0118] At this time, in the present embodiment, the total thickness of the regrowth layer 500 (the first regrowth layer 520 and the second regrowth layer 540) may be, for example, 600 μm or more, or may be 1 mm or more. The upper limit value of the total thickness of the regrowth layer 500 is not particularly limited. However, from the viewpoint of improving productivity, the total thickness of the regrowth layer 500 may be, for example, 100 mm or less.

[0119] Here, in the growth process of the thick film regrowth layer 500, the dislocations D move like a random walk. Thereby, during the growth of the regrowth layer 500, the dislocations D meet each other or the dislocations D form loops. Due to such a phenomenon, the dislocations D reaching the surface of the second regrowth layer 540 of the thick film can be reduced.

[0120] Alternatively, even when an inclined interface is formed in at least a part of the first regrowth layer 520 (when the first regrowth layer 520 is three-dimensionally grown) in the first regrowth step S62 of the present embodiment, dislocations can be locally concentrated at the junction of adjacent inclined interfaces during the process of laterally growing the second regrowth layer 540. Even in this case, the dislocations D reaching the surface of the thick second regrowth layer 540 can be reduced.

[0121] As a result, it becomes possible to reduce the dislocation density of the second regrowth layer 540. (Note that since FIG. 7 is simplified, the number of dislocations in FIG. 7 has not decreased much.)

[0122] (S70: Peeling step) When the regrowth step S60 is completed, as shown in FIG. 9, the regrowth layer 500 is peeled from the lower base substrate 100 with at least a part of the porous intermediate layer 300 as a boundary.

[0123] In the present embodiment, the regrowth layer 500 is spontaneously peeled from the lower base substrate 100 while the temperature is lowered after the regrowth step S60. Thereby, a special separate step for peeling can be made unnecessary. That is, the manufacturing method can be simplified.

[0124] Here, in the regrowth step S60, tensile stress is generated in the regrowth layer 500 (in the direction along the main surface 120 of the lower base substrate 100). This is caused by, for example, the fact that the dislocation density decreases as the thickness of the regrowth layer 500 increases as described above.

[0125] The tensile stress generated in the regrowth layer 500 in this way causes the c-plane 510 of the regrowth layer 500 to warp into a spherical shape with the upper side being concave. As a result, the regrowth layer 500 is spontaneously and gradually peeled off from the outer periphery of the lower base substrate 100 toward the center. That is, by utilizing the warp of the c-plane 510 of the regrowth layer 500, the regrowth layer 500 can be gradually peeled off from the outer periphery of the lower base substrate 100 toward the center. In other words, the regrowth layer 500 can be peeled off concentrically and evenly with respect to the center of the lower base substrate 100. As a result, the peeling of the large-area regrowth layer 500 can be performed easily and stably.

[0126] Through the above peeling step S70, a peeling intermediate 20 including at least the cover layer 400 and the regrowth layer 500 is formed. Residual pieces of the intermediate layer 300 may remain on the lower surface of the cover layer 400 of the peeling intermediate 20.

[0127] (S80: Post-treatment step) When the peeling step S70 is completed, as shown in FIG. 10, for example, the regrowth layer 500 is sliced by a wire saw along a cutting plane perpendicular to the normal direction of the center of the surface of the regrowth layer 500. Thereby, a nitride crystal substrate 50 as a sliced substrate (hereinafter, may be abbreviated as "substrate 50") is formed.

[0128] Next, both surfaces of the substrate 50 are polished by a polishing device. Thereby, the main surfaces of the substrate 50 are mirror-finished.

[0129] Through the above steps, a substrate 50 made of AlGaN crystal according to the present embodiment is obtained.

[0130] (Manufacturing process of laminate or semiconductor device) Thereafter, a laminate or a semiconductor device may be manufactured using the above-described substrate 50. The laminate or the semiconductor device includes, for example, the above-described substrate 50 and a semiconductor layer containing a group III nitride crystal provided on the substrate 50. Thereby, for example, a high-quality semiconductor device such as an ultraviolet LED or an ultraviolet LD can be manufactured.

[0131] (2) Nitride crystal substrate Referring to FIGS. 11A to 13, the nitride crystal substrate 50 according to this embodiment will be described.

[0132] (Basic features) In this embodiment, the substrate 50 is made of, for example, a crystal represented by the composition formula of Al x Ga 1-x N. The Al composition ratio x in the composition formula of the substrate 50 is, for example, greater than 0 and less than or equal to 1.

[0133] The diameter of the substrate 50 is, for example, 1 inch (25 mm) or more, or may be 2 inches (50 mm) or more, or may be 4 inches (100 mm) or more. Also, the thickness of the substrate 50 is, for example, 300 μm or more and 2 mm or less.

[0134] The substrate 50 has, for example, a main surface 50s whose closest low-index crystal plane is the c-plane 50c. Note that the main surface 50s of the substrate 50 is, for example, mirror-finished, and the root mean square (RMS) of the main surface 50s of the substrate 50 is, for example, less than 1 nm.

[0135] In this embodiment, the substrate 50 does not include a polarity inversion region (inversion domain), for example, by growing a regrowth layer 500 with the c-plane as a continuous growth surface. In other words, the closest low-index crystal plane to the main surface 50s of the substrate 50 is the c-plane 50c over the entire main surface 50s. In this embodiment, the c-plane 50c as the closest low-index crystal plane to the main surface 50s of the substrate 50 may be curved in a concave spherical shape with respect to the main surface 50s, for example, due to the warping in the peeling step S70 described above.

[0136] (Periodic structure of Al composition ratio) As shown in FIGS. 11B and 12, in this embodiment, the Al composition ratio x in the substrate 50 changes periodically in the thickness direction of the substrate 50. In FIG. 11B, the gray portion in the substrate 50 indicates a region where the Al composition ratio x is relatively large.

[0137] Specifically, the Al composition ratio x in the substrate 50 has, for example, a so-called sine curve distribution in the thickness direction (depth direction) of the substrate 50. That is, the substrate 50 alternately has, in the thickness direction, for example, a minimum point P1 where the Al composition ratio x reaches the minimum value xmin and a maximum point P2 where the Al composition ratio x reaches the maximum value xmax. The Al composition ratio x in the substrate 50 gradually increases from the minimum point P1 to the maximum point P2 in the thickness direction of the substrate 50. On the other hand, the Al composition ratio x in the substrate 50 gradually decreases from the maximum point P2 to the minimum point P1 in the thickness direction of the substrate 50.

[0138] The above-described slip of the atomic plane is considered to be caused by an imbalance in the amount of local strain in the thickness direction of the substrate 50 created by the composition change in the substrate 50. By making the distribution of the Al composition ratio x in the thickness direction of the substrate 50 a smooth sine curve, the imbalance in strain that causes the slip of the atomic plane can be made to occur continuously in the thickness direction of the substrate 50. As a result, it becomes possible to efficiently cause the slip of the atomic plane.

[0139] In this embodiment, the average value xavg of the Al composition ratio x in the substrate 50 is, for example, 0.05 or more and 0.8 or less.

[0140] By setting the average value xavg of the Al composition ratio x in the substrate 50 to 0.05 or more and 0.8 or less, the peelability of the regrown layer 500 with the intermediate layer 300 as a boundary can be improved as described above, and the substrate 50 can be stably obtained from the regrown layer 500.

[0141] From the perspective of application to a semiconductor device, by setting the average value xavg of the Al composition ratio x in the substrate 50 to 0.05 or more, when the substrate 50 is applied to an ultraviolet LED or an ultraviolet LD, the transmittance of ultraviolet light of the substrate 50 can be improved. On the other hand, when the average value xavg of the Al composition ratio x in the substrate 50 exceeds 0.8, the generation of Al vacancies becomes remarkable. For this reason, the conductivity of the substrate 50 is likely to be lost. In contrast, by setting the average value xavg of the Al composition ratio x in the substrate 50 to 0.8 or less, the generation of Al vacancies can be suppressed. As a result, it becomes possible to impart conductivity to the substrate 50 made of an AlGaN crystal.

[0142] In the present embodiment, the period Tp of the Al composition ratio x in the thickness direction of the substrate 50 is, for example, 5 nm or more and 2000 nm or less, or may be 5 nm or more and 500 nm or less. The "period Tp of the Al composition ratio x" as used herein means the thickness from the first maximum point P2 of the Al composition ratio x that is first reached when measuring the Al composition ratio x in the thickness direction of the substrate 50 to the second maximum point P2 that is adjacent to (next reaches) the first maximum point P2 with the minimum point P1 in between.

[0143] In the present embodiment, since the substrate 50 has the period Tp of the Al composition ratio x as described above, the number of cycles in which the Al composition ratio x in the substrate 50 fluctuates is larger than the number of cycles such as multiple quantum wells of a general laser diode. Specifically, the number of cycles of the Al composition ratio x in the thickness direction of the substrate 50 is, for example, 150 cycles or more and 4×10 5 cycles or less, or may be 600 cycles or more and 4×10 5 cycles or less.

[0144] In this embodiment, the difference xmax - xmin between the maximum value and the minimum value of the Al composition ratio x in the substrate 50 is, for example, not less than 0.2xavg and not more than xavg, where xavg is the average value of the Al composition ratio x. The "difference xmax - xmin between the maximum value and the minimum value of the Al composition ratio x" as used herein means the fluctuation range in which the Al composition ratio x fluctuates. In other words, xmax - xmin is a value corresponding to the amplitude when the fluctuation of the Al composition ratio x is assumed to be "vibration" or "wave".

[0145] The substrate 50 may, for example, satisfy the following formulas (B) and (C). 0.1xavg ≦ xmax - xavg ≦ 0.5xavg ···(B) 0.1xavg ≦ xavg - xmin ≦ 0.5xavg ···(C)

[0146] In this embodiment, in the periodic structure of the Al composition ratio x in the substrate 50, each layer (isocompositional layer) having the same Al composition ratio x may not be parallel to the main surface 50s of the substrate 50, for example. Specifically, according to the warp of the regrown layer 500 in the above-described peeling step S70, each layer having the same Al composition ratio x may also have a concave spherical warp with respect to the main surface 50s of the substrate 50. In this case, the main surface 50s of the substrate 50 may have a periodic structure of the Al composition ratio x distributed concentrically with respect to the center of the main surface 50s, for example. Alternatively, by tilting the crystal during polishing to obtain the main surface 50s, the main surface 50s of the substrate 50 may have a periodic structure of the Al composition ratio x distributed linearly (strip-like), arcuately, or elliptically, for example.

[0147] The periodic structure of the Al composition ratio x in the above substrate 50 can be confirmed, for example, by secondary ion mass spectrometry (SIMS) and X-ray diffraction measurement described later.

[0148] (Structure without cracks) In this embodiment, by applying the above manufacturing method and forming the periodic structure of the above Al composition ratio x in the regrowth layer 500, the crystal strain generated in the surface direction of the regrowth layer 500 made of AlGaN crystal can be relaxed. As a result, the substrate 50 obtained from the regrowth layer 500 does not have cracks.

[0149] (Feature 1 regarding X-ray diffraction measurement) In this embodiment, since the substrate 50 has the periodic structure of the above Al composition ratio x, the diffraction pattern obtained by X-ray diffraction measurement of the substrate 50 has fringes.

[0150] Specifically, as shown in FIG. 13, when the 2θ-ω scan measurement of X-ray diffraction of the substrate 50 is performed under the condition that the (0002) plane diffraction of AlGaN is measured by symmetric reflection using the Kα1 line of Cu (wavelength 0.15405 nm), the diffraction pattern obtained by the 2θ-ω scan measurement has, for example, a 0th-order peak and a 1st-order peak as a satellite peak. The 0th-order peak is the peak with the highest intensity of the (0002) plane diffraction of AlGaN. The satellite peak is a peak that occurs at an interval from the 0th-order peak and has an intensity lower than that of the 0th-order peak, depending on the periodic structure of the Al composition ratio x.

[0151] In this embodiment, the 0th-order peak occurs, for example, in the range where the diffraction angle 2θ is 34.62° or more and 35.73° or less. Based on the diffraction angle 2θ of the 0th-order peak, the average value of the lattice constant in the <0001> axis direction of the substrate 50 is obtained. The fact that the diffraction angle 2θ of the 0th-order peak is within the above range corresponds to the fact that the average value xavg of the Al composition ratio x in the substrate 50 is 0.05 or more and 0.8 or less based on the above average value of the lattice constant.

[0152] In this embodiment, the primary peak as a satellite peak occurs, for example, at a position where the diffraction angle 2θ is separated from the primary peak by 0.005° or more and 1.85° or less. This corresponds to the period Tp of the Al composition ratio x in the thickness direction of the substrate 50 being 5 nm or more and 2000 nm or less based on the following formula (D). Alternatively, the primary peak may occur, for example, at a position where the diffraction angle 2θ is separated from the primary peak by 0.018° or more and 1.85° or less. This corresponds to the period Tp of the Al composition ratio x in the thickness direction of the substrate 50 being 5 nm or more and 500 nm or less based on the following formula (D). Tp = λ / {2(sinθ1 - sinθ0)} ···(D) Here, λ is the wavelength of the Kα1 line of Cu. θ0 is the angle of the primary peak. θ1 is the angle of the primary peak.

[0153] (Feature 2 regarding X-ray diffraction measurement) In this embodiment, by relaxing the crystal strain generated in the regrown layer 500, the crystal strain in the substrate 50 obtained from the regrown layer 500 is relaxed. As a result, the ratio a / c of the average value of the lattice constant in the a-axis direction to the average value of the lattice constant in the c-axis direction of the substrate 50 is close to the ratio a / c (=0.62) of the strain-free AlGaN crystal.

[0154] Specifically, the substrate 50 satisfies the following formula (1). 0.58 ≦ a / c ≦ 0.66 ···(1)

[0155] Here, c is the average value of the lattice constant in the <0001> axis direction of the substrate 50 obtained based on the diffraction angle of the primary peak with the highest intensity of the (0002) plane diffraction of AlGaN by X-ray diffraction measurement (2θ-ω scan). a is the average value of the lattice constant in the <11-20> axis direction of the substrate 50 obtained based on the diffraction angle of the primary peak with the highest intensity of the (10-12) plane diffraction of AlGaN by the above-mentioned c and X-ray diffraction measurement (2θ-ω scan).

[0156] (Feature 3 regarding X-ray diffraction measurement) In this embodiment, by relaxing the crystal strain generated in the regrowth layer 500, the crystallinity of the substrate 50 obtained from the regrowth layer 500 can be improved.

[0157] Specifically, the full width at half maximum (FWHM) of the 0th order peak with the highest intensity of the (0002) plane diffraction of AlGaN obtained by the X-ray rocking curve measurement of the substrate 50 is, for example, 300 arcsec or less, or may be 100 arcsec or less. The full width at half maximum (FWHM) of the 0th order peak with the highest intensity of the (10-12) plane diffraction of AlGaN obtained by the X-ray rocking curve measurement of the substrate 50 is, for example, 500 arcsec or less, or may be 300 arcsec.

[0158] The X-ray rocking curve measurement is performed under the following conditions. X-ray: Monochromatic light of Cu Kα1 obtained from an X-ray source via an X-ray mirror and two crystals of Ge(220) plane Gonio radius: 420 mm Incident slit width: 0.1 mm Light receiving slit side: Analyzer crystal

[0159] (Feature 4 regarding X-ray diffraction measurement) In this embodiment, instead of growing the regrowth layer 500 in an island-like manner, by growing the regrowth layer 500 as a continuous film made of AlGaN crystals, within the main surface 50s of the substrate 50 obtained from the regrowth layer 500, the average value xavg of the Al composition ratio x averaged within a predetermined thickness range from the main surface 50s is uniform.

[0160] Specifically, the ratio xc / xo of the average value xc of the Al composition ratio at the center of the main surface 50s of the substrate 50 to the average value xo of the Al composition ratio at a position 5 mm from the outer periphery to the center of the main surface 50s of the substrate 50 is, for example, 0.9 or more and 1.1 or less.

[0161] The average values xo and xc of the Al composition ratio are determined based on the diffraction angle of the 0th peak with the highest intensity of the (0002) plane diffraction of AlGaN when X-ray diffraction measurement is performed within a range of 35 μm of the penetration length of the Kα1 line of Cu from the main surface at the outer peripheral side and the center of the main surface of the substrate 50, respectively.

[0162] (Dislocation density) In this embodiment, during the growth process of the thick film regrowth layer 500, dislocations can be reduced. Thereby, in the substrate 50 made of an AlGaN crystal obtained from the regrowth layer 500, the dislocation density can be reduced.

[0163] Specifically, the dislocation density on the main surface 50s of the substrate 50 is, for example, 3×10 8 cm -2 or less, or may be 1×10 7 cm -2 or less, or may be 5×10 6 cm -2 or less, or may be 3×10 6 cm -2 or less.

[0164] (Conductivity type) The conductivity type of the substrate 50 is not particularly limited. However, the substrate 50 of this embodiment may be, for example, n-type. In this case, examples of the n-type impurity include Si, germanium (Ge), oxygen (O), and tin (Sn). The n-type impurity concentration in the substrate 50 is, for example, 1×10 17 cm -3 or more and 1×10 19 cm -3 or less.

[0165] Alternatively, the substrate 50 of this embodiment may be, for example, p-type. In this case, the substrate 50 may contain, for example, magnesium (Mg). The Mg concentration in the substrate 50 may be, for example, 1×10 16 cm -3 or more.

[0166] Alternatively, the substrate 50 of the present embodiment may be, for example, semi-insulating. In this case, the substrate 50 may contain, for example, iron (Fe), manganese (Mn), or carbon (C). The concentration of Fe, Mn, or C in the substrate 50 may be, for example, 1×10 16 cm -3 or more.

[0167] (3) Summary of the present embodiment According to the present embodiment, one or more of the following effects can be obtained.

[0168] (a) In the present embodiment, by growing the regrown layer 500 made of AlGaN crystals on the cover layer 400 provided on the porous intermediate layer 300, the stress applied to the regrown layer 500 can be relaxed by the easily deformable porous intermediate layer 300.

[0169] (b) Further, by growing the regrown layer 500 made of AlGaN crystals on the flat cover layer 400, the regrown layer 500 can be grown as a continuous film while suppressing the island growth of the regrown layer 500. Thereby, the Al composition ratio x in the in-plane direction of the regrown layer 500 can be made uniform over the entire regrown layer 500.

[0170] (a) and (b), by growing the regrown layer 500 in this way, the crystal strain generated in the in-plane direction of the regrown layer 500 made of AlGaN can be relaxed. Thereby, the occurrence of cracks in the regrown layer 500 can be suppressed. Further, the crystallinity of the regrown layer 500 can be improved.

[0171] (c) In this embodiment, the Al composition ratio x in the regrowth layer 500 changes periodically in the thickness direction of the regrowth layer 500. Thereby, the local strain amount in the thickness direction of the regrowth layer 500 can be made unbalanced. Due to such an imbalance in the local strain amount in the thickness direction, dislocations can be bent in the surface direction, causing slip of atomic planes. By causing slip of atomic planes, the crystal strain in the surface direction of the regrowth layer 500 can be released, and the crystal strain in the surface direction can be further relaxed. As a result, the generation of cracks in the regrowth layer 500 can be stably suppressed.

[0172] (a) By growing the regrowth layer 500 as in (a) to (c), it becomes possible to stably obtain the substrate 50 made of high-quality AlGaN crystal without cracks from the regrowth layer 500.

[0173] By using the above-described substrate 50 obtained in this embodiment, for example, it becomes possible to manufacture high-quality semiconductor devices such as ultraviolet LEDs or ultraviolet LDs.

[0174] (d) In this embodiment, the average value xavg of the Al composition ratio x in the regrowth layer 500 (i.e., the substrate 50) is 0.05 or more and 0.8 or less. Thereby, while relaxing the crystal strain applied to the regrowth layer 500, appropriate crystal strain can be applied to the porous intermediate layer 300.

[0175] Specifically, by setting the average value xavg of the Al composition ratio x of the regrowth layer 500 to 0.05 or more, due to the lattice mismatch between the regrowth layer 500 and the layer located below the regrowth layer 500 as the regrowth layer 500 grows, appropriate crystal strain can be applied to the porous intermediate layer 300. Thereby, the vulnerability of the porous intermediate layer 300 can be increased. As a result, the peelability of the regrowth layer 500 with the intermediate layer 300 as the boundary can be improved.

[0176] On the other hand, by setting the average value xavg of the Al composition ratio x of the regrowth layer 500 to 0.8 or less, it is possible to suppress an excessive crystal strain generated in the porous intermediate layer 300 due to lattice mismatch between the regrowth layer 500 and the layer located below the regrowth layer 500. As a result, disappearance of the voids 360 in the intermediate layer 300 can be suppressed, and adhesion between the regrowth layer 500 and the layer located below the regrowth layer 500 can be suppressed. Consequently, a decrease in the peelability of the regrowth layer 500 with the intermediate layer 300 as a boundary can be suppressed.

[0177] As described above, from the viewpoint of the peelability of the regrowth layer 500, it becomes possible to stably obtain the substrate 50 made of an AlGaN crystal.

[0178] (e) In the present embodiment, the period of the Al composition ratio x in the thickness direction in the regrowth layer 500 (that is, the substrate 50) is 5 nm or more and 2000 nm or less, or may be 500 nm or less.

[0179] By setting the period of the Al composition ratio x in the thickness direction in the regrowth layer 500 to 5 nm or more, a portion where the Al composition ratio x changes can be stably formed. As a result, stable slip of atomic planes can be caused within the portion where the Al composition ratio x changes. Consequently, the crystal strain in the in-plane direction of the regrowth layer 500 can be stably relaxed.

[0180] On the other hand, by setting the period of the Al composition ratio x in the thickness direction in the regrowth layer 500 to 2000 nm or less, the number of cycles in which the Al composition ratio x fluctuates can be sufficiently ensured within the regrowth layer 500. That is, in the regrowth layer 500, a sufficient number of portions where slip of atomic planes can occur can be formed. As a result, the crystal strain in the in-plane direction of the regrowth layer 500 can be stably relaxed.

[0181] Furthermore, by setting the period of the Al composition ratio x in the thickness direction in the regrowth layer 500 to 500 nm or less, in the diffraction pattern obtained by the 2θ-ω scan measurement of X-ray diffraction of the substrate 50 obtained from the regrowth layer 500, a sufficient interval can be ensured between the 0th order peak and the satellite peak. Thereby, after manufacturing the substrate 50, the periodic structure of the Al composition ratio x in the substrate 50 can be stably confirmed.

[0182] (f) In the present embodiment, the difference xmax - xmin between the maximum value and the minimum value of the Al composition ratio x in the regrowth layer 500 (that is, the substrate 50) is 0.2xavg or more and xavg or less when the average value of the Al composition ratio x is xavg.

[0183] By setting xmax - xmin in the regrowth layer 500 to 0.2xavg or more, local strain can be caused to change by an appropriate amount in the thickness direction of the regrowth layer 500. Thereby, slip of atomic planes can be stably caused. As a result, the crystal strain in the plane direction of the regrowth layer 500 can be stably relaxed.

[0184] On the other hand, by setting xmax - xmin in the regrowth layer 500 to xavg or less, it is possible to suppress the local strain amount in the thickness direction of the regrowth layer 500 from becoming excessively large. Thereby, the occurrence of cracks can be more stably suppressed.

[0185] (4) Modification example of an embodiment The above-described embodiment can be changed as shown in the following modification examples as needed. Hereinafter, only elements different from the above-described embodiment will be described, and elements substantially the same as those described in the above-described embodiment will be denoted by the same reference numerals and their description will be omitted.

[0186] (Modification example 1) In the cover layer forming step S40 of modification example 1, as the cover layer 400, for example, Al x Ga 1-xCrystals represented by the composition formula of N may be grown. At this time, the Al composition ratio x in the cover layer 400 may be, for example, more than 0 and 1 or less, or may be 0.05 or more and 0.8 or less.

[0187] According to Modification 1, by growing an AlGaN crystal as the cover layer 400, a regrown layer 500 can be homoepitaxially grown on the cover layer 400. As a result, it becomes possible to stably grow a regrown layer 500 with good crystallinity.

[0188] (Modification 2) In the first regrowth step S62 of Modification 2, while growing the first regrown layer 520, the growth temperature may be gradually increased from the first growth temperature T1 toward the second growth temperature T2.

[0189] According to Modification 2, the rate of temperature increase from the first growth temperature T1 to the second growth temperature T2 can be made gentle. As a result, cracking of the group III nitride crystal layer above the intermediate layer 300 due to the temperature increase process from the first growth temperature T1 to the second growth temperature T2 can be stably suppressed.

[0190] (Modification 3) In the regrowth step S60 of Modification 3, the growth temperature of the regrown layer 500 may be adjusted in one step. That is, the temperature of the seed substrate 10 may be directly increased from room temperature to the growth temperature of the regrowth step S60 (a temperature corresponding to the second growth temperature T2 described above).

[0191] According to Modification 3, the first regrowth step S62 can be omitted. As a result, the manufacturing process can be shortened.

[0192] (Modification 4) In the first regrowth step S62 of Modification 4, similar to the second regrowth step S64, the Al composition ratio x may be periodically changed in the thickness direction of the first regrown layer 520.

[0193] In the first regrowth layer 520, the average value of the Al composition ratio x, the period of the Al composition ratio x in the thickness direction, and the difference between the maximum value and the minimum value of the Al composition ratio x may be the same as those in the second regrowth layer 540.

[0194] According to Modification 4, by forming a periodic structure of the Al composition ratio x also in the first regrowth layer 520, the crystal strain generated in the plane direction of the first regrowth layer 520 can be stably relaxed.

[0195] (Modification 5) In Modification 5, the regrowth layer 500 may alternately have two layers with different Al composition ratios x in the thickness direction as follows.

[0196] In the second regrowth step S64 of Modification 5, as shown in FIG. 14A, By adjusting the Al flow rate ratio R to the minimum value Rmin, the step S642 of forming the first composition layer, and By adjusting the Al flow rate ratio R to the maximum value Rmax, the step S644 of forming the second composition layer on the first composition layer, and The cycle including is repeated.

[0197] In the step S642 of forming the first composition layer, the Al flow rate ratio R is kept constant at the minimum value Rmin over the time tp1. In the step S644 of forming the second composition layer, the Al flow rate ratio R is kept constant at the maximum value Rmax over the time tp2.

[0198] The substrate 50 obtained from the regrowth layer 500 of Modification 5 has, as shown in FIG. 14B, a first composition layer 52 with the Al composition ratio x being the minimum value xmin and a second composition layer 54 with the Al composition ratio x being the maximum value xmax. The Al composition ratio x in the first composition layer 52 is constant in the thickness direction of the first composition layer 52. The Al composition ratio x in the second composition layer 54 is constant in the thickness direction of the second composition layer 54.

[0199] In Modification 5, the average value xavg of the Al composition ratio x in the substrate 50, the period Tp of the Al composition ratio x in the thickness direction of the substrate 50, and the difference xmax - xmin between the maximum value and the minimum value of the Al composition ratio x of the substrate 50 may be the same as those in the above-described embodiment.

[0200] In Modification 5, the period Tp of the Al composition ratio x in the thickness direction of the substrate 50 is the total thickness of the thickness Tp1 of the first composition layer 52 and the thickness Tp2 of the second composition layer 54. The thickness Tp1 of the first composition layer 52 and the thickness Tp2 of the second composition layer 54 may be equal to each other, or may be different from each other.

[0201] According to Modification 5, an interface where the Al composition ratio x changes can be clearly formed between the first composition layer 52 and the second composition layer 54. Thereby, within the interface where the Al composition ratio x changes, slippage of atomic planes can be stably generated. As a result, crystal strain in the regrowth layer 500 can be stably relaxed.

[0202] <Other Embodiments of the Present Disclosure> As described above, the embodiments of the present disclosure have been specifically described. However, the present disclosure is not limited to the above-described embodiments, and various modifications can be made without departing from the gist thereof.

[0203] In the above-described embodiment, the case where the underlayer formation step S20 is performed has been described, but the underlayer formation step S20 may not be performed. That is, the underlayer 200 may be absent. In the intermediate layer formation step S30, the intermediate layer 300 may be directly formed on the underlying substrate 100.

[0204] In the above-described embodiment, the case where each of the upper layer of the underlayer 200 and the intermediate layer 300 contains GaN crystals has been described, but the present disclosure is not limited to this case. Each layer is not limited to GaN crystals, and for example, group III nitride crystals such as aluminum nitride (AlN), aluminum gallium nitride (AlGaN), indium nitride (InN), indium gallium nitride (InGaN), aluminum indium gallium nitride (AlInGaN), that is, In xAl y Ga 1-x-y It may be composed of a crystal represented by the composition formula of N (where 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ x + y ≤ 1).

[0205] In the above-described embodiment, the case where the base substrate 100 is made of a material different from the group III nitride has been described, but the present disclosure is not limited to this case. The base substrate 100 may be, for example, a self-supporting substrate made of a group III nitride crystal.

[0206] In the above-described embodiment, the case where the upper layer of the base layer 200, the intermediate layer 300, the cover layer 400, and the regrown layer 500 contain Si as an n-type impurity has been described, but at least one of the upper layer of the base layer 200, the intermediate layer 300, the cover layer 400, and the regrown layer 500 may contain, for example, O or Ge as an n-type impurity.

[0207] In the above-described embodiment, the case where each of the above-described vapor phase growth methods is used as the growth method of the base layer 200, the intermediate layer 300, the cover layer 400, and the regrown layer 500 has been described, but the present disclosure is not limited to this case. As the growth method of at least any one of the base layer 200, the intermediate layer 300, the cover layer 400, and the regrown layer 500, a metalorganic vapor phase epitaxy (MOVPE) method may be used. Alternatively, as the growth method of at least any one of the base layer 200, the intermediate layer 300, the cover layer 400, and the regrown layer 500, a growth method other than the vapor phase growth method may be used.

[0208] In the above-described embodiment, the case where the first regrown layer 520 is three-dimensionally grown after step-flow growth in the initial growth stage of the first regrown layer 520 has been described, but the present disclosure is not limited to this case. The step-flow growth of the first regrown layer 520 may be maintained.

Example

[0209] Hereinafter, experimental results verifying the effects of the above-described embodiment will be described.

[0210] (1) Preparation of Samples The following three samples were fabricated.

[0211] [Sample A] Using the manufacturing method of the above-described embodiment, a nitride crystal substrate of Sample A was fabricated by the following procedure.

[0212] (Substrate preparation step) A substrate for obtaining a seed substrate was prepared. Substrate: Sapphire substrate Plane orientation of the main surface of the substrate: +c plane Diameter of the substrate: 3 inches (76.2 mm) Thickness of the substrate: 430 μm

[0213] (Underlayer formation step) By the HVPE method, an AlN buffer layer and a GaN layer were formed in this order as an underlayer on the substrate under the following conditions. Growth temperature of the underlayer: 1055 °C Thickness of the AlN buffer layer and the GaN layer: 100 nm and 4 μm, respectively Carrier concentration in the GaN layer as the underlayer: about 1×10 18 cm -3

[0214] (Intermediate layer formation step) Next, by the HVPE method, an Si-doped GaN layer was grown as an intermediate layer on the underlayer under the following conditions. Growth temperature of the intermediate layer: 1055 °C Thickness of the intermediate layer: 3000 nm Carrier concentration in the intermediate layer: 6×10 18 cm -3

[0215] (Cover layer formation step) Next, by the HVPE method, a GaN layer was grown as a cover layer on the underlayer under the following conditions. Growth temperature of the cover layer: 1055 °C Thickness of the cover layer: 150 nm Carrier concentration in the cover layer: about 5×10 17 cm-3

[0216] (Porous process) Next, by electrochemical treatment, the intermediate layer was made porous through the dislocations of the cover layer under the following conditions. Temperature of the electrolytic solution: room temperature (23 °C) Treatment voltage: 20 V Treatment current: maximum 100 mA Treatment time: 10 min

[0217] (First regrowth process) Next, by the HVPE method, under the following conditions, an Al 0.6 Ga 0.4 N layer was grown as the first regrowth layer on the cover layer. First growth temperature: 835 °C Thickness of the first regrowth layer: 50 μm

[0218] (Second regrowth process) Next, in the same chamber as the first regrowth process, an AlGaN layer was grown as the second regrowth layer on the first regrowth layer while periodically changing the Al composition ratio x in the thickness direction. Second growth temperature: 1055 °C Adjustment sequence of the Al flow ratio R: Figure 8 Average value Ravg of the Al flow ratio R: 0.6 Minimum value Rmin of the Al flow ratio R: 0.45 Maximum value Rmax of the Al flow ratio R: 0.75 Period tp for varying the Al flow ratio R: Adjusted so that the period Tp of the Al composition ratio x in the thickness direction of the second regrowth layer is about 30 nm. Total thickness of the regrowth layer: 1 mm

[0219] (Peeling process) During the temperature drop after the regrowth process, the regrowth layer was peeled off from the underlying substrate.

[0220] (Post-treatment process) A nitride crystal substrate was formed by slicing the peeled regrown layer. After slicing, both surfaces of the nitride crystal substrate were polished. Further, the outer periphery of the nitride crystal substrate was bevelled. As a result, a nitride crystal substrate made of an AlGaN crystal with a diameter of 2 inches (50.8 mm) and a thickness of 430 μm was obtained as Sample A1.

[0221] [Sample B1 (Direct growth on GaN free-standing substrate)] In the preparation of Sample B1, a GaN free-standing substrate was used as the base substrate. By the HVPE method, a 1-mm-thick Al 0.6 Ga 0.4 N layer was grown directly on the GaN free-standing substrate under the same conditions as those of the second regrown layer of Sample A. Thus, a laminate of Sample B1 was obtained.

[0222] [Sample B2 (VAS method)] In the preparation of Sample B2, the VAS method was adopted. First, a template having a GaN layer with a thickness of 300 nm and having voids and a mesh-shaped TiN layer with a thickness of 20 nm in this order was prepared on a 3-inch-diameter sapphire substrate. Next, by the HVPE method, a 1-mm-thick Al 0.6 Ga 0.4 N layer was grown as the main growth layer on the above-mentioned GaN layer and TiN layer under the same conditions as those of the second regrown layer of Sample A. Then, the main growth layer was peeled off from the sapphire substrate with the GaN layer having voids as the boundary. Thus, an Al 0.6 Ga 0.4 N layer of Sample B2 was obtained.

[0223] (2) Evaluation [Optical microscope] Each sample was observed with an optical microscope.

[0224] [2θ-ω scan measurement of X-ray diffraction 1] Using the Kα1 line of Cu (wavelength 0.15405 nm), 2θ-ω scan measurement of X-ray diffraction of the nitride crystal substrate of Sample A was performed under the condition that diffraction of the (0002) plane of AlGaN was measured by symmetric reflection.

[0225] The 2θ-ω scan measurement 1 of X-ray diffraction was performed at each of the following measurement positions on the nitride crystal substrate of sample A. · The center of the main surface of the substrate · Points set at 1 cm intervals on a straight line passing through the center of the main surface of the substrate and along the m-axis direction · Points set at 1 cm intervals on a straight line passing through the center of the main surface of the substrate and along the a-axis direction perpendicular to the m-axis direction · Points set at a position 5 mm from the outer periphery of the main surface of the substrate toward the center

[0226] As a result of the measurement, based on the diffraction angle 2θ of the 0th-order peak with the highest intensity of the (0002) plane diffraction of AlGaN, the average value of the lattice constant c in the <0001> axis direction of the substrate was determined. Based on the average value of the lattice constant c, the average value xavg of the Al composition ratio x (averaged within the range of the X-ray penetration length of 35 μm) in the nitride crystal substrate of sample A was determined.

[0227] Based on the diffraction angle of the 0th-order peak and the diffraction angle of the 1st-order peak, the period Tp of the Al composition ratio x in the thickness direction of the nitride crystal substrate of sample A was determined by the above formula (D).

[0228] Furthermore, based on the average value xc of the Al composition ratio at the center of the main surface of the substrate with respect to the average value xo of the Al composition ratio at a position 5 mm from the outer periphery of the main surface of the substrate toward the center, the ratio xc / xo was determined.

[0229] [2θ-ω scan measurement 2 of X-ray diffraction] Using the Kα1 line of Cu, the 2θ-ω scan measurement of X-ray diffraction of the nitride crystal substrate of sample A was performed under the condition that the (10-12) plane diffraction of AlGaN was measured by asymmetric reflection.

[0230] The 2θ-ω scan measurement 2 of X-ray diffraction was performed at each of the above measurement positions in the same manner as the 2θ-ω scan measurement 1 of X-ray diffraction.

[0231] As a result of the measurement, based on the diffraction angle 2θ of the 0th order peak with the highest intensity of the (10-12) plane diffraction of AlGaN, the average value of the lattice constant a in the <11-20> axis direction of the substrate was determined.

[0232] [X-ray rocking curve measurement 1] At each of the above measurement positions of the nitride crystal substrate of Sample A, by performing X-ray rocking curve measurement, the full width at half maximum (FWHM) of the 0th order peak with the highest intensity of the (0002) diffraction of AlGaN was determined.

[0233] [X-ray rocking curve measurement 2] At each of the above measurement positions of the nitride crystal substrate of Sample A, by performing X-ray rocking curve measurement, the full width at half maximum (FWHM) of the 0th order peak with the highest intensity of the (10-12) diffraction of AlGaN was determined.

[0234] [Multiphoton excitation microscope] At each of the above measurement positions of the nitride crystal substrate of Sample A, by observing the main surface at a field of view of 250 μm angle with a multiphoton excitation microscope, the dislocation density was determined.

[0235] (3) Results The results of Sample A, B1, and B2 will be described.

[0236] [Sample B1] In Sample B1, after the growth of the main growth layer on the GaN free-standing substrate, cracks occurred in the main growth layer. In Sample B1, it is considered that cracks occurred in the main growth layer due to the lattice mismatch between the GaN free-standing substrate and the main growth layer composed of AlGaN crystal.

[0237] [Sample B2] In Sample B2, cracks occurred in a part of the grown layer peeled off from the sapphire substrate. In Sample B2, when island-shaped AlGaN crystals grew three-dimensionally on the GaN layer and the mesh-like TiN layer containing voids, the movement of Al atoms was small. Therefore, the Al composition ratio varied in the plane. Due to the variation in the Al composition ratio, crystal strain occurred in the grown layer composed of thick-film AlGaN crystals. As a result, it is considered that cracks occurred in the grown layer in Sample B2.

[0238] [Sample A] In Sample A, cracks did not occur in the peeled regrown layer and the nitride crystal substrate obtained from the regrown layer.

[0239] FIG. 13 shows a diffraction pattern obtained by 2θ-ω scan measurement 1 of X-ray diffraction performed at the center of the main surface of the nitride crystal substrate of Sample A. As shown in FIG. 13, the diffraction pattern of Sample A had fringes. Thus, it was confirmed that in Sample A, the Al composition ratio x in the nitride crystal substrate changed periodically in the thickness direction of the substrate.

[0240] As a result of the measurement, the average value of the lattice constant c of the nitride crystal substrate of Sample A determined based on the diffraction angle 2θ of the 0th peak with the highest intensity of the (0002) plane diffraction of AlGaN was 0.5065 nm. The average value xavg of the Al composition ratio x in the nitride crystal substrate of Sample A determined based on the average value of the lattice constant c was 0.593.

[0241] Based on the diffraction angle of the 0th peak and the diffraction angle of the 1st peak, the period Tp of the Al composition ratio x in the thickness direction of the nitride crystal substrate of Sample A determined by the above formula (D) was 30.9 nm.

[0242] Furthermore, it was confirmed that the following characteristics were obtained from the measurement results at each of the above measurement positions for the nitride crystal substrate of Sample A.

[0243] xavg: 0.56 to 0.60 Ratio xc / xo: approximately 1.07 Period Tp: 28 - 33 nm 0.58 ≤ a / c ≤ 0.66 ···(1) FWHM of diffraction from (0002) plane < 250 arcsec FWHM of diffraction from (10 - 12) plane < 450 arcsec Dislocation density < 3×10 6 cm -2

[0244] As shown by the above results, in Sample A, by growing a regrown layer made of AlGaN crystals on a flat cover layer provided on a porous intermediate layer, the crystal strain generated in the in-plane direction of the regrown layer could be relaxed. Further, in Sample A, by periodically changing the Al composition ratio x in the regrown layer in the thickness direction, the crystal strain generated in the in-plane direction of the regrown layer could be further relaxed. As a result, it was confirmed that in Sample A, a nitride crystal substrate made of high-quality AlGaN crystals without cracks could be stably obtained.

[0245] <Supplementary Note> Hereinafter, aspects of the present disclosure will be supplemented.

[0246] (Supplementary Note 1) A nitride crystal substrate, composed of crystals represented by the composition formula of AlxGa1 - xN, wherein the Al composition ratio x in the composition formula is greater than 0 and less than or equal to 1, and the Al composition ratio x changes periodically in the thickness direction of the nitride crystal substrate Nitride crystal substrate.

[0247] (Supplementary Note 2) The average value of the Al composition ratio x is 0.05 or more and 0.8 or less The nitride crystal substrate according to Supplementary Note 1.

[0248] (Supplementary Note 3) The period of the Al composition ratio x in the thickness direction of the nitride crystal substrate is 5 nm or more and 2000 nm or less The nitride crystal substrate described in Supplementary Note 1 or Supplementary Note 2.

[0249] (Supplementary Note 4) When the difference between the maximum value and the minimum value of the Al composition ratio x is 0.2xavg or more and xavg or less, where the average value of the Al composition ratio x is xavg The nitride crystal substrate according to any one of Supplementary Notes 1 to 3.

[0250] (Supplementary Note 5) The thickness of the nitride crystal substrate is 300 μm or more and 2 mm or less, The number of periods of the Al composition ratio x in the thickness direction of the nitride crystal substrate is 150 periods or more and 4×10 5 periods or less The nitride crystal substrate according to any one of Supplementary Notes 1 to 4.

[0251] (Supplementary Note 6) The nitride crystal substrate The minimum point where the Al composition ratio x becomes the minimum value, The maximum point where the Al composition ratio x becomes the maximum value, alternately have in the thickness direction, The Al composition ratio x gradually increases from the minimum point to the maximum point in the thickness direction of the nitride crystal substrate The nitride crystal substrate according to any one of Supplementary Notes 1 to 5.

[0252] (Supplementary Note 7) The nitride crystal substrate The first composition layer where the Al composition ratio x is the minimum value, The second composition layer where the Al composition ratio x is the maximum value, alternately have in the thickness direction The nitride crystal substrate according to any one of Supplementary Notes 1 to 5.

[0253] (Supplementary Note 8) When performing a 2θ-ω scan measurement of X-ray diffraction of the nitride crystal substrate under the condition that the (0002) plane diffraction of AlGaN is measured using the Kα1 line of Cu, The diffraction pattern obtained by the 2θ-ω scan measurement of the nitride crystal substrate is a 0th order peak where the diffraction angle 2θ occurs within the range of 34.62° or more and 35.73° or less, and a satellite peak where the diffraction angle 2θ occurs at a position 0.005° or more and 1.85° or less away from the 0th order peak and has the nitride crystal substrate according to any one of Appendices 1 to 7.

[0254] (Appendix 9) The nitride crystal substrate satisfies the following formula (1), 0.58 ≦ a / c ≦ 0.66 ···(1) where c is the average value of the lattice constant in the <0001> axis direction of the nitride crystal substrate obtained based on the diffraction angle of the 0th order peak with the highest intensity of the (0002) plane diffraction of AlGaN by X-ray diffraction measurement, a is the average value of the lattice constant in the <11-20> axis direction of the nitride crystal substrate obtained based on the diffraction angle of the 0th order peak with the highest intensity of the (10-12) plane diffraction of AlGaN by X-ray diffraction measurement and the c the nitride crystal substrate according to any one of Appendices 1 to 8.

[0255] (Appendix 10) The nitride crystal substrate has a diameter of 25 mm or more and has no cracks the nitride crystal substrate according to any one of Appendices 1 to 9.

[0256] (Appendix 11) The full width at half maximum of the 0th order peak with the highest intensity of the (0002) diffraction of AlGaN by X-ray rocking curve measurement is 300 arcsec or less, and the full width at half maximum of the 0th order peak with the highest intensity of the (10-12) diffraction of AlGaN by X-ray rocking curve measurement is 500 arcsec or less the nitride crystal substrate according to any one of Appendices 1 to 10.

[0257] (Appendix 12) A nitride crystal substrate, A semiconductor layer provided on the nitride crystal substrate and made of a group III nitride crystal, Comprising: The nitride crystal substrate is made of a crystal represented by the composition formula of Al x Ga 1-x N, In the composition formula of the nitride crystal substrate, the Al composition ratio x is greater than 0 and less than or equal to 1, The Al composition ratio x of the nitride crystal substrate changes periodically in the thickness direction of the nitride crystal substrate Stacked product.

[0258] (Appendix 13) (a) A step of preparing a base substrate; (b) A step of forming an intermediate layer containing an n-type group III nitride crystal above the base substrate; (c) A step of forming a cover layer containing a group III nitride crystal having a carrier concentration lower than that of the intermediate layer on the intermediate layer; (d) A step of making the intermediate layer porous through dislocations in the cover layer while maintaining the surface state of the cover layer by electrochemical treatment; (e) A step of epitaxially growing a regrowth layer made of a group III nitride crystal on the cover layer; (f) A step of peeling the regrowth layer from the base substrate with at least a part of the porous intermediate layer as a boundary; Comprising: In (e), As the regrowth layer, a crystal represented by the composition formula of Al with an Al composition ratio x greater than 0 and less than or equal to 1 x Ga 1-x N is grown, The Al composition ratio x is changed periodically in the thickness direction of the regrowth layer Method for manufacturing a nitride crystal substrate.

[0259] (Appendix 14) (e) In, The crystal strain generated in the along-surface direction of the regrowth layer is relaxed more than when (d) is not performed The method for manufacturing a nitride crystal substrate according to Supplementary Note 13.

[0260] (Supplementary Note 15) In (e), at least in the initial growth stage of the regrowth layer, the regrowth layer is grown by step-flow growth The method for manufacturing a nitride crystal substrate according to Supplementary Note 13 or Supplementary Note 14.

[0261] (Supplementary Note 16) In (c), as the cover layer, a GaN crystal is grown The method for manufacturing a nitride crystal substrate according to any one of Supplementary Notes 13 to 15.

[0262] (Supplementary Note 17) In (c), as the cover layer, a crystal represented by the composition formula of Al x Ga 1-x N with an Al composition ratio x greater than 0 and less than or equal to 1 is grown The method for manufacturing a nitride crystal substrate according to any one of Supplementary Notes 13 to 15.

[0263] (Supplementary Note 18) (e) is (e1) a step of growing a first regrowth layer on the cover layer at a first growth temperature; (e2) a step of growing a second regrowth layer on the first regrowth layer at a second growth temperature; and has In (e1), the first growth temperature is made lower than the second growth temperature The method for manufacturing a nitride crystal substrate according to any one of Supplementary Notes 13 to 17.

Explanation of Reference Numerals

[0264] 10 Seed substrate for nitride crystal growth 20 Release intermediate 50 Nitride crystal substrate 100 Substrate 120 Main surface 200 Lower layer 300 Intermediate layer 360 Void 400 Cover layer 500 Regrowth layer 520 First regrowth layer 540 Second regrowth layer 510 c-plane 810 Electrolyte 820 Treatment tank 840 Power supply 842 Anode 844 Cathode 860 Ammeter

Claims

1. A nitride crystal substrate, Al ｘ Ga １－ｘ It consists of crystals represented by the composition formula of N, wherein the Al composition ratio x in the composition formula is greater than 0 and less than or equal to 1, and the Al composition ratio x changes periodically in the thickness direction of the nitride crystal substrate nitride crystal substrate.

2. The average value of the Al composition ratio x is 0.05 or more and 0.8 or less The nitride crystal substrate according to claim 1.

3. The period of the Al composition ratio x in the thickness direction of the nitride crystal substrate is 5 nm or more and 2000 nm or less The nitride crystal substrate according to claim 1 or claim 2.

4. The difference between the maximum value and the minimum value of the Al composition ratio x is 0.2xavg or more and xavg or less when the average value of the Al composition ratio x is xavg The nitride crystal substrate according to claim 1 or claim 2.

5. When the X-ray diffraction 2θ-ω scan measurement of the nitride crystal substrate is performed under the condition that the (0002) plane diffraction of AlGaN is measured using the Kα1 line of Cu, The diffraction pattern obtained by the 2θ-ω scan measurement of the nitride crystal substrate is a 0th order peak that occurs within the range where the diffraction angle 2θ is 34.62° or more and 35.73° or less, and a satellite peak that occurs at a position where the diffraction angle 2θ is 0.005° or more and 1.85° or less away from the 0th order peak having The nitride crystal substrate according to claim 1 or claim 2.

6. The nitride crystal substrate satisfies the following formula (1), 0.58 ≤ a / c ≤ 0.66... (1) where c is the average value of the lattice constant in the <0001> axis direction of the nitride crystal substrate obtained based on the diffraction angle of the 0th order peak with the highest intensity of the (0002) plane diffraction of AlGaN by X-ray diffraction measurement, and a is the average value of the lattice constant in the <11-20> axis direction of the nitride crystal substrate obtained based on the diffraction angle of the 0th order peak with the highest intensity of the (10-12) plane diffraction of AlGaN by X-ray diffraction measurement and the c The nitride crystal substrate according to claim 1 or claim 2.

7. (a) A step of preparing a base substrate, (b) A step of forming an intermediate layer containing an n-type group III nitride crystal above the base substrate, (c) A step of forming a cover layer containing a group III nitride crystal having a carrier concentration lower than that of the intermediate layer on the intermediate layer, (d) A step of making the intermediate layer porous through the dislocations in the cover layer while maintaining the surface state of the cover layer by electrochemical treatment (e) a step of epitaxially growing a regrown layer made of a group-III nitride crystal on the cover layer; (f) a step of peeling the regrown layer from the lower base substrate with at least a part of the porous intermediate layer as a boundary; The method includes: In (e), As the regrowth layer, a crystal represented by a composition formula of Al ｘ Ga １－ｘ N with an Al composition ratio x being more than 0 and 1 or less is grown, ｘ Ga １－ｘ and the Al composition ratio x is periodically changed in the thickness direction of the regrown layer Method for manufacturing a nitride crystal substrate.

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