Production method for nitride crystal substrate and nitride crystal substrate

Abstract

To stably obtain a nitride crystal substrate containing AlGaN.SOLUTION: A production method for a nitride crystal substrate includes steps of: (a) preparing a base substrate; (b) forming an intermediate layer including n-type group III nitride crystal, above the base substrate; (c) forming a cover layer on the intermediate layer, the cover layer including group III nitride crystal having a carrier concentration lower than a carrier concentration of the intermediate layer; (d) making the intermediate layer porous through dislocations in the cover layer by performing an electrochemical process, while maintaining a surface condition of the cover layer; (e) epitaxially growing a regrowth layer comprising group III nitride crystal, on the cover layer; and (f) peeling off the regrowth layer from the base substrate by using at least a portion of the intermediate layer made porous as a boundary, where in the step (e), crystal represented by a composition formula of AlxGa1-xN, where an Al composition ratio x is 0.05 or more and 0.8 or less, is grown as the regrowth layer.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a method for producing a nitride crystal substrate, and to a nitride crystal substrate. [Background technology]

[0002] Various methods have been disclosed as manufacturing methods for obtaining nitride crystal substrates made of Group III nitride crystals (for example, Patent Document 1 and Non-Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2003-178984 A [Non-patent literature]

[0004] [Non-Patent Document 1] ECS Journal of Solid State Science and Technology 10 (3), 035001, 2021 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present disclosure is to stably obtain nitride crystal substrates containing AlGaN. [Means for solving the problem]

[0006] According to an aspect of the present disclosure, (a) preparing a base substrate; (b) forming an intermediate layer containing n-type Group III nitride crystals above the base substrate; (c) forming a cover layer on the intermediate layer, the cover layer including a Group III nitride crystal having a carrier concentration lower than that of the intermediate layer; (d) rendering the intermediate layer porous through dislocations in the cover layer by electrochemical treatment while maintaining the surface state of the cover layer; (e) epitaxially growing a regrowth layer made of a Group III nitride crystal on the cover layer; (f) peeling the regrowth layer from the base substrate along at least a portion of the porous intermediate layer; Equipped with In (e), The regrown layer is made of Al having an Al composition ratio x of 0.05 or more and 0.8 or less. x Ga 1-x Grow a crystal with the formula N A method for producing a nitride crystal substrate is provided.

[0007] According to another aspect of the present disclosure, A nitride crystal substrate, Al x Ga 1-x It consists of crystals represented by the formula N, The Al composition ratio x in the composition formula is 0.05 or more and 0.8 or less, The nitride crystal substrate satisfies the following formula (1): 0.58≦a / c≦0.66 (1) Where: c is the diffraction angle of the (0002) plane diffraction peak of AlGaN by X-ray diffraction measurement of the nitride crystal substrate <0001> is the lattice constant in the axial direction, a is the lattice constant in the <11-20> axis direction of the nitride crystal substrate, which is determined based on c and the diffraction angle of the (10-12) plane diffraction peak of AlGaN by X-ray diffraction measurement. A nitride crystal substrate is provided. Effect of the Invention

[0008] According to the present disclosure, nitride crystal substrates containing AlGaN can be obtained stably. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a flowchart showing a method for manufacturing a nitride crystal substrate according to an embodiment of the present disclosure. [Figure 2A] FIG. 2A is a schematic cross-sectional view illustrating a method for manufacturing a nitride crystal substrate according to an embodiment of the present disclosure. [Figure 2B] FIG. 2B is a schematic cross-sectional view showing a method for manufacturing a nitride crystal substrate according to an embodiment of the present disclosure. [Figure 3A] FIG. 3A is a schematic cross-sectional view showing a method for manufacturing a nitride crystal substrate according to an embodiment of the present disclosure. [Figure 3B] FIG. 3B is a schematic cross-sectional view showing a method for manufacturing a nitride crystal substrate according to an embodiment of the present disclosure. [Figure 4] FIG. 4 is a schematic cross-sectional view illustrating a method for manufacturing a nitride crystal substrate according to an embodiment of the present disclosure. [Diagram 5] FIG. 5 is a diagram showing temperature changes during a regrowth process according to an embodiment of the present disclosure. [Figure 6] FIG. 6 is a schematic cross-sectional view illustrating a method for manufacturing a nitride crystal substrate according to an embodiment of the present disclosure. [Figure 7] FIG. 7 is a schematic cross-sectional view illustrating a method for manufacturing a nitride crystal substrate according to an embodiment of the present disclosure. [Figure 8] FIG. 8 is a schematic cross-sectional view illustrating a method for manufacturing a nitride crystal substrate according to an embodiment of the present disclosure. [Figure 9] FIG. 9 is a schematic cross-sectional view illustrating a method for manufacturing a nitride crystal substrate according to an embodiment of the present disclosure. [Figure 10A] FIG. 10A is a plan view showing a nitride crystal substrate according to an embodiment of the present disclosure. [Figure 10B] FIG. 10B is a cross-sectional view showing a nitride crystal substrate according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] <Knowledge gained by the inventors> Light emitting diodes (LEDs) and laser diodes (LDs) with wavelengths ranging from about 365 nm to 530 nm (green) have been realized by using substrates made of gallium nitride (GaN) crystals (hereinafter also referred to as GaN crystal substrates).

[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, it is difficult to realize a high-quality device due to lattice mismatch between the AlGaN layer and the GaN crystal substrate. In another case, an AlGaN layer may be grown on an AlN crystal substrate manufactured by sublimation as described in Non-Patent Document 1. However, in this case, too, it is difficult to realize a high-quality device due to lattice mismatch between the AlGaN layer and the AlN crystal substrate. For this reason, it is desirable to realize a high-quality AlGaN crystal substrate.

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

[0013] Alternatively, in order to obtain a GaN crystal substrate, a GaN main growth layer may be grown on a template in which the surface of the GaN layer is partially covered with a mask having openings.

[0014] In either case, GaN crystals grow in island shapes at the beginning of the growth of the GaN growth layer. Then, when the thick GaN crystal grows, the island-shaped GaN crystals coalesce to obtain a GaN growth layer as a continuous film. In the process of coalescing these island-shaped GaN crystals, the crystal strain introduced during the growth of the thick GaN crystal is significantly alleviated. As a result, it is possible to grow a crack-free thick GaN growth layer even on a heterogeneous substrate.

[0015] An aluminum nitride (AlN) crystal substrate can be obtained by sublimation deposition, for example, 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 used as a seed crystal to increase its thickness by sublimation deposition. Thereafter, 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 layer is grown again by sublimation deposition using the AlN crystal substrate as a seed crystal to obtain an AlN ingot. 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, it is practical to grow an AlGaN main growth layer on a template such as the above-mentioned VAS method. However, in this case, it is very difficult for Al atoms to move, and it is difficult to realize the growth process including island-like growth, the association of island-like crystals, and the flattening of the crystal as in the case of GaN.

[0017] In particular, when island-shaped AlGaN crystals grow three-dimensionally, the movement of Al atoms is reduced as described above, resulting in the formation of areas with high Al composition ratios and areas with low Al composition ratios within the plane. This makes it easier for crystal distortion to occur in the AlGaN main growth layer. As a result, cracks may occur in the AlGaN main growth layer.

[0018] Therefore, in order to obtain an AlGaN crystal substrate from an AlGaN main growth layer, a method is considered in which a thick GaN layer is first grown, the surface of the GaN layer is planarized, and then a thick AlGaN main growth layer is grown. However, even with this method, it is difficult to obtain an AlGaN main growth layer without cracks due to the lattice mismatch between the GaN layer and the AlGaN main growth layer.

[0019] Therefore, the inventors have investigated the following method for manufacturing an AlGaN crystal substrate. First, an intermediate layer and a cover layer containing a group III nitride crystal are grown in this order on a base substrate, and the laminate is subjected to an electrochemical treatment. In this way, a seed substrate is produced in which the intermediate layer is porous while maintaining the flatness of the cover layer. Next, using the seed substrate, a regrowth layer made of AlGaN crystal as a continuous film is regrowthed on the flat cover layer. This method has made it possible to solve the above-mentioned problems, and has succeeded in obtaining an AlGaN crystal substrate without cracks in the regrowth layer.

[0020] The present disclosure below is based on the above-mentioned new findings discovered by the inventors.

[0021] <Details of the embodiment 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, but is defined by the claims, and is intended to include all modifications within the meaning and scope of 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 nitride crystal substrate The method for manufacturing a nitride crystal substrate according to this embodiment will now be described with reference to Figures 1 to 9. Note that in Figures 2A to 4 and 6 to 9, hatching of cross sections other than that of intermediate layer 300 has been omitted.

[0024] In the following, in a group III nitride crystal having a wurtzite structure, <0001> The axis (e.g., the <0001> axis) is referred to as the "c-axis" and the (0001) plane is referred to as the "c-plane". The (0001) plane is sometimes referred to as the "+c-plane (III group element polar plane)" and the (000-1) plane 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". In this disclosure, "carrier concentration" refers to 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 this embodiment includes, for example, a base substrate preparation step S10, a base 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: Board preparation process) First, as shown in FIG. 2A, a base substrate 100 is prepared.

[0027] In this embodiment, for example, a base substrate 100 made of a material other than a group III nitride is prepared. Specifically, the base substrate 100 may be, for example, a sapphire substrate, a SiC substrate, a silicon (Si) substrate, or a gallium arsenide (GaAs) substrate. The base substrate 100 may be insulating or conductive. Here, the base substrate 100 is, for example, a sapphire substrate.

[0028] The diameter of the base 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. This makes it possible to grow a large-area regrown layer 500, which will be described later.

[0029] The thickness of the base substrate 100 is, for example, not less than 150 μm and not more than 3 mm.

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

[0031] In this embodiment, the c-plane of the base substrate 100 may be inclined with respect to the major surface 120. In other words, the c-axis of the base substrate 100 may be inclined at a predetermined off-angle with respect to the normal to the major surface 120. The off-angle of the base substrate 100 is, for example, not less than 0° and not more than 5°.

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

[0033] (S20: Base layer formation process) After the base substrate 100 is prepared, as shown in FIG. 2A, for example, a vapor phase growth method is used to form a base layer 200 containing Group III nitride crystals on the base substrate 100 (the base layer 200 made of Group III nitride crystals is formed).

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

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

[0036] In this embodiment, the carrier concentration in the GaN layer of the underlayer 200 is set to be lower than the carrier concentration in the intermediate layer 300, for example. In other words, the n-type impurity concentration in the GaN layer of the underlayer 200 is set to be lower than the n-type impurity concentration in the intermediate layer 300, for example. Specifically, the carrier concentration (and n-type impurity concentration) in the underlayer 200 is set to be lower than, for example, 2×10 18 cm -3 or less, or 1×10 18 cm -3 This allows the base layer 200 to function as an etching stopper below the intermediate layer 300 in the porous step S50, which will be described later, of making the intermediate layer 300 porous by electrochemical treatment.

[0037] The lower limit of the carrier concentration in the GaN layer of the underlayer 200 is not limited. However, the carrier concentration in the GaN layer of the underlayer 200 is not limited to, for example, 1×10 16 cm -3 In this way, since the underlayer 200 itself has some electrical conductivity, etching of the intermediate layer 300 can proceed toward the lower part of the intermediate layer 300 by electrochemical processing in the porous step S50 described later.

[0038] The thickness of the underlayer 200 is not particularly limited. However, the thickness of the underlayer 200 may be, for example, more than 0 nm and 5 μm or less. By setting the thickness of the underlayer 200 to 5 μm or less, the total thickness of the layers on the undersubstrate 100 can be adjusted to, for example, 15 μm or less. This makes it possible to suppress cracks caused by differences in the linear expansion coefficient between the undersubstrate 100 and each layer such as the underlayer 200.

[0039] In this embodiment, the c-plane of the GaN layer is the low-index crystal plane closest to the surface of the underlayer 200. By forming a flat surface of the underlayer 200 having such a crystal plane in advance, the intermediate layer 300 and the cover layer 400 with good crystallinity can be grown on the surface of the underlayer 200.

[0040] (S30: Intermediate layer formation process) After the underlayer 200 is formed, as shown in Fig. 2B, an intermediate layer 300 containing n-type Group III nitride crystals is formed on the underlayer 200 located above the undersubstrate 100 by, for example, vapor phase growth. (In this step, an intermediate layer 300 made of Group III nitride crystals that does not contain voids is formed.)

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

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

[0043] The upper limit of the carrier concentration in the intermediate layer 300 is not limited. However, the carrier concentration in the intermediate layer 300 is not limited to, for example, 1×10 20 cm -3or less, or 5×10 19 cm -3 This can suppress the deterioration of the crystallinity of the intermediate layer 300.

[0044] In this embodiment, the thickness of the intermediate layer 300 may be, for example, more than 100 nm, or may be 500 nm or more, or may be 1 μm or more. This allows large voids to be formed in the intermediate layer 300 in the porous step S50 described below. This allows the voids 360 in the intermediate layer 300 to be maintained in the regrowth step S60 described below. As a result, in the peeling step S70 described below, the regrowth layer 500 can be easily and stably peeled off from the base substrate 100 at least a portion of the intermediate layer 300 maintained in a porous state as a boundary.

[0045] The upper limit 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 layers on the base substrate 100 can be adjusted to, for example, 15 μm or less. This makes it possible to suppress cracks caused by differences in the linear expansion coefficient between the base substrate 100 and each layer such as the intermediate layer 300.

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

[0047] In this embodiment, for example, GaN crystal is grown as the cover layer 400. This allows the crystal growth of the cover layer 400 to be easily controlled by vapor phase epitaxy. 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, a Si-doped GaN layer is grown as the cover layer 400 on the intermediate layer 300 by the HVPE method 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. The cover layer 400 is grown with the +c plane as the growth surface.

[0049] In this embodiment, the carrier concentration in the cover layer 400 is set to be lower than the carrier concentration in the intermediate layer 300, for example. In other words, the n-type impurity concentration in the cover layer 400 is set to be lower than the n-type impurity concentration in the intermediate layer 300, for example. Specifically, the carrier concentration (and n-type impurity concentration) in the cover layer 400 is set to be lower than, for example, 1×10 18 cm -3 As a result, in the porous step S50 described later, the intermediate layer 300 can be selectively made porous while suppressing etching of the cover layer 400. In other words, under a predetermined voltage, it is possible to increase the size of the voids 360 in the intermediate layer 300 while preventing the microvoids in the cover layer 400 from becoming large.

[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 is not limited to, for example, 1×10 16 cm -3 or more, or 1×10 17 cm -3 In this way, since the cover layer 400 itself has electrical conductivity, in the electrochemical treatment of the porous step 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 a state where the cover layer forming step S40 is completed, the underlayer 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 More than 1×10 9cm -2 The dislocations D in the cover layer 400 are utilized in the following porous step 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 making the thickness of the cover layer 400 10 nm or more, or 50 nm or more, even if outgassing such as N2 gas is generated when the intermediate layer 300 is etched in the porous process S50 described later, the cover layer 400 is maintained, and the outgas can be released to the outside of the cover layer 400 through the dislocations D of the cover layer 400. This makes it possible to suppress peeling of the cover layer 400 from the intermediate layer 300.

[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 electrolyte can stably reach the intermediate layer 300 through the dislocations D of the cover layer 400 in the porous step S50 described later. This allows void formation in the intermediate layer 300 to be stably performed.

[0055] After the cover layer 400 has been grown, the temperature of the base substrate 100 is reduced from the growth temperature of the Group III nitride crystal to room temperature, thereby forming a stack including the base substrate 100, the base layer 200, the intermediate layer 300 and the cover layer 400.

[0056] 3B, the entire stack is warped due to the difference in linear expansion coefficient between the base substrate 100 and the Group III nitride crystal layer including the base layer 200, intermediate layer 300, and cover layer 400. Specifically, since the linear expansion coefficient of sapphire used as the base substrate 100 is greater than that of the Group III nitrides, the entire stack is warped such that the surface of the cover layer 400 becomes convex.

[0057] The above-mentioned underlayer forming step S20, intermediate layer forming step S30, and cover layer forming step S40 are performed consecutively in the same chamber without exposing the undersubstrate 100 to the atmosphere. This makes it possible to suppress unintended incorporation of at least one of oxygen (O) and silicon (Si) as n-type impurities at the interfaces between the underlayer 200, intermediate layer 300, and cover layer 400. As a result, it becomes possible to suppress the influence on the carrier concentration in these layers.

[0058] (S50: Porous process) After the cover layer 400 is formed, as shown in FIG. 4, an electrochemical treatment is performed to make the intermediate layer 300 porous through dislocations D in the cover layer 400 while maintaining the surface state of the cover layer 400.

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

[0060] 4, first, there are prepared a treatment tank 820, a power source 840, and an ammeter 860. The power source 840 and the ammeter 860 may be incorporated into one device as a current and 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 a group III nitride. Examples of the electrolytic solution 810 include aqueous solutions containing oxalic acid, nitric acid, hydrofluoric acid, sulfuric acid, sodium sulfate (Na2SO4), sodium hydroxide (NaOH), and the like. 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-mentioned 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 a 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 that has corrosion resistance and allows a current to easily flow is used. Specific examples of the cathode 844 include stainless steel (SUS), platinum (Pt), gold (Au), and boron-doped diamond.

[0063] After completing the connection of each electrode, the laminate with the anode 842 connected and the cathode 844 are immersed in the electrolyte 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. This causes the electrochemical treatment to be performed. At this time, the progress of the electrochemical treatment is confirmed based on the change in current in the ammeter 860.

[0064] At this time, the electrochemical treatment causes C2O4 to flow through dislocations D in the cover layer 400, which has a relatively low carrier concentration, toward the intermediate layer 300, which has a relatively high carrier concentration. 2- In other words, the dislocations D in the cover layer 400 are used as nano-sized paths for the electrolyte to penetrate. The electrolyte that reaches the intermediate layer 300 in this manner selectively etches the intermediate layer 300. As a result, a plurality of voids 360 are formed near each of the 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, the Group III element ions (Ga) generated when the intermediate layer 300 is etched are reacted with the following reaction formula: 3+ ) and nitrogen (N 2 ) gas are released to the outside of the cover layer 400 through dislocations D in the cover layer 400. 2GaN + 6h + → 2Ga 3+ + N2

[0066] By the above-mentioned electrochemical treatment, each of the multiple voids 360 in the intermediate layer 300 is formed, for example, at a position overlapping with each of the multiple dislocations D in the cover layer 400 described below. The multiple 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 undersubstrate 100. Note that the voids 360 do not necessarily need to reach the underlayer 200.

[0067] On the other hand, the partition wall portions of the intermediate layer 300 other than the voids 360 connect the upper part of the foundation layer 200 or the lower part of the intermediate layer 300 to the cover layer 400. As a result, even if the intermediate layer 300 has a plurality of voids 360, it maintains a constant thickness.

[0068] At this time, in this embodiment, the length of each of the multiple 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 in any cross section perpendicular to the main surface 120 of the base substrate 100. This makes it possible to maintain the voids 360 in the intermediate layer 300 in the regrowth step S60 described below. As a result, in the peeling step S70 described below, the regrowth layer 500 can be easily and stably peeled off from the base substrate 100 at at least a portion of the intermediate layer 300 maintained in a porous state as a boundary.

[0069] There is no upper limit to the length of voids 360 in intermediate layer 300 in the direction along main surface 120 of base substrate 100. However, the length of voids 360 in the direction along main surface 120 of base substrate 100 may be 10 μm or less. This makes it possible to suppress peeling of cover layer 400 caused by outgassing generated when intermediate layer 300 is etched by appropriately adjusting the electrochemical treatment conditions in porous step S50.

[0070] At this time, in this embodiment, the depth of each of the multiple 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. This also makes it possible to maintain the voids 360 in the intermediate layer 300 in the regrowth step S60 described below. As a result, the regrowth layer 500 can be easily and stably peeled off from the base substrate 100 in the peeling step S70 described below.

[0071] There is no upper limit to the depth of void 360. However, the depth of void 360 may be equal to or less than the thickness of intermediate layer 300. This makes it possible to suppress excessive spread of peeling from intermediate layer 300 to other layers in the peeling step S70 described below.

[0072] On the other hand, in the electrochemical treatment, the surface of the cover layer 400, which has a relatively low carrier concentration, is hardly etched. In other words, even if the cover layer 400 has a plurality of dislocations D as described above, the surface of the cover layer 400 near the dislocations D is not excessively etched. This makes it possible to maintain the surface state of the cover layer 400 flat.

[0073] In this embodiment, after the porous step 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 ​​obtained when the surface of the cover layer 400 is observed with an atomic force microscope (AFM) in a field of view of 5 μm square.

[0074] As described above, by maintaining the surface roughness of the cover layer 400 small, the regrown layer 500 having good crystallinity and a thick film can be stably grown on the cover layer 400 .

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

[0076] Specific conditions for the electrochemical process that can realize the selective etching of the intermediate layer 300 described above include, for example, the following: The process voltage is adjusted based on the carrier concentration of the intermediate layer 300, etc. The process current is adjusted based on the process area (area of ​​the base substrate 100). The process time is adjusted based on the thickness of the intermediate layer 300.

[0077] Electrolyte temperature: Room temperature (10℃ to 30℃) Processing voltage: 1V to 200V or 10V to 20V Processing current: 0.01mA to 60A or 0.1mA to 10A Processing time: 0.1 min to 180 min, or 1 min to 30 min

[0078] As the intermediate layer 300 becomes porous due to the electrochemical treatment described above, the cover layer 400 and the laminate including the base substrate 100 and the base layer 200 are separated from each other by the porous intermediate layer 300, as shown in Fig. 4. When the voids 360 in the intermediate layer 300 have the above-mentioned size, this separation can occur significantly.

[0079] This makes it possible to reduce the warping of the cover layer 400 in the porous step S50 compared to the warping of the cover layer 400 before the porous step S50. That is, the cover layer 400 can be made nearly 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 base substrate 100 is thin. This makes it possible to reduce warping of the base substrate 100 in the porous step S50 compared to the warping of the base substrate 100 before the porous step S50.

[0081] After the electrochemical treatment, the nitride crystal growth seed substrate 10 is taken out of the electrolytic solution in the treatment tank 820. The nitride crystal growth seed substrate 10 taken out of the treatment tank 820 is then washed with pure water or the like and dried. This removes the electrolytic solution remaining in the voids 360 of the intermediate layer 300. This completes the porous step S50.

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

[0083] (S60: Regrowth process) After the porous step S50 is completed, a regrown layer 500 made of a group III nitride crystal is epitaxially grown on the cover layer 400. The regrown layer 500 is grown by, for example, vapor phase epitaxy.

[0084] In this embodiment, the regrown layer 500 is made of Al. x Ga 1-x A crystal represented by the formula N is grown.

[0085] In this embodiment, by growing the regrowth layer 500 made of the above-mentioned AlGaN crystals on the flat cover layer 400 on the porous intermediate layer 300, the crystal distortion occurring in the regrowth layer 500 can be alleviated more than in the case where the porous step S50 is not performed.

[0086] On the other hand, in this embodiment, by setting the Al composition ratio x of the regrowth layer 500 to be 0.05 or more and 0.8 or less, it is possible to apply an appropriate crystal strain to the porous intermediate layer 300. As a result, it is possible to improve the peelability of the regrowth layer 500 across the intermediate layer 300.

[0087] In the regrowth step S60 of this embodiment, the growth temperature may be adjusted in two stages as shown in Fig. 5. Specifically, the regrowth step S60 may include, for example, a first regrowth step S62 and a second regrowth step S64.

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

[0089] (S62: 1st regrowth step) When the temperature of the seed substrate 10 reaches the first growth temperature T1, a first regrowth layer 520 is grown on the cover layer 400 at the first growth temperature T1, as shown in FIG.

[0090] Specifically, for example, 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 by the HVPE method. As a result, an AlGaN layer is epitaxially grown as a first regrown layer 520 on the surface of the cover layer 400. Various dopants may be added to the AlGaN layer as the first regrown layer 520.

[0091] At this time, in this embodiment, the first growth temperature T1 in the first regrowth step S62 is set lower than the second growth temperature T2 in the second regrowth step S64 as the main growth, which will be described later. In other words, the first growth temperature T1 in the first regrowth step S62 is set slightly lower than a typical growth temperature of a Group III nitride crystal.

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

[0093] In contrast, since the linear expansion coefficient of the cover layer 400 is smaller than that of the base substrate 100, the expansion of the cover layer 400 is smaller than that of the base substrate 100. As a result, the cover layer 400 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. This makes it possible to suppress the occurrence of cracks in the cover layer 400. As a result, it becomes possible to suppress the occurrence of cracks in the regrown layer 500 on the cover layer 400.

[0094] At this time, in this embodiment, the first growth temperature T1 is set to, for example, 970° C. or less. This makes it possible to stably suppress the occurrence of cracks in the cover layer 400 in the first regrowth step S62. As a result, it becomes possible to stably suppress the occurrence of cracks in the regrowth layer 500 on the cover layer 400.

[0095] On the other hand, the first growth temperature T1 may be set to, for example, not less than 800° C. This allows the Group III nitride crystal as the first regrowth layer 520 to grow stably.

[0096] As described above, by adjusting the first growth temperature T1, the first regrowth layer 520 can be grown to a predetermined thickness on the cover layer 400 while suppressing the occurrence of cracks in the cover layer 400. This makes it possible to stably increase the total thickness of the Group III nitride crystal layer (cover layer 400 and first regrowth layer 520) above the porous intermediate layer 300. 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 this embodiment, by starting the growth of the first regrowth layer 520 on the cover layer 400 having a flat surface, the regrowth layer 500 can be grown by step-flow growth at least in the initial growth stage (thin thickness stage) of the first regrowth layer 520 as the regrowth layer 500. That is, in the initial growth stage, island-like growth of the regrowth layer 500 made of AlGaN crystals can be suppressed. This allows the regrowth layer 500, which is a continuous film made of AlGaN crystals, to be grown by step-flow growth over the entire surface of the cover layer 400 with the c-plane 522 (+c-plane) as the growth plane. As a result, the Al composition ratio x can be made uniform within the surface of the first regrowth layer 520. Furthermore, the crystallinity of the first regrowth layer 520 can be improved.

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

[0099] At this time, in this embodiment, the thickness of the first regrowth layer 520 may be, for example, 1 μm or more, or 10 μm or more, or 20 μm or more. This makes it possible to stably prevent the Group III nitride crystal layer above the intermediate layer 300 from cracking when the temperature is raised to the second growth temperature in the second regrowth step S64 as the main growth described later.

[0100] On the other hand, there is no particular upper limit to the thickness of first regrown layer 520. However, the thickness of first regrown layer 520 may be, for example, 300 μm or less, or 100 μm or less.

[0101] When the thickness of the first regrown layer 520 reaches a predetermined thickness, the supply of the AlCl3 gas and the GaCl gas is stopped to terminate the growth of the first regrown layer 520. Note that the supply of the NH3 gas is continued.

[0102] Thereafter, as shown in FIG. 5, the temperature of the seed substrate 10 is increased from the first growth temperature T1 to a second growth temperature T2.

[0103] (S64: 2nd regrowth process) After the temperature of the seed substrate 10 is increased from the first growth temperature T1 to the second growth temperature T2, growth of the regrowth layer 500 as the second regrowth layer 540 is resumed as shown in Fig. 7. That is, at the second growth temperature T2, the second regrowth layer 540 as a thick film is grown on the first regrowth layer 520 as main growth.

[0104] Specifically, for example, the second regrowth step S64 is performed in the same chamber as the first regrowth step S62 without exposure to the atmosphere. In the second regrowth step S64, an AlGaN layer is epitaxially grown as the second regrowth layer 540 on the first regrowth layer 520 under the same conditions as the first regrowth step S62 except that the growth temperature is set to a second growth temperature T2. Various dopants may be added to the AlGaN layer as the second regrowth layer 540.

[0105] At this time, in this embodiment, as described above, even if the second growth temperature T2 in the second regrowth step S64 as the main growth is made higher than the first growth temperature T1 in the first regrowth step S62, the 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 is large, so that the Group III nitride crystal layer above the intermediate layer 300 is less likely to crack. This makes it possible to stably grow the regrowth layer 500.

[0106] At this time, in this embodiment, as described above, by setting the second growth temperature T2 in 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) using the c-plane 542 as the growth surface. This allows the second regrowth layer 540 to be grown as a thick film with good crystallinity while improving the flatness of the surface. Note that the c-axis, which is the normal to the c-plane 542 of the second regrowth layer 540, may be inclined at an off-angle inherited from the c-axis of the base substrate 100, which is inclined at a predetermined off-angle.

[0107] At this time, in this embodiment, the second growth temperature T2 may be, for example, 980° C. or higher, or 10,000° C. or higher. This allows the second regrown layer 540 to be stably grown by step-flow growth. As a result, the second regrown layer 540 can be stably grown as a thick film with good crystallinity while improving the flatness of the second regrown layer 540.

[0108] On the other hand, the second growth temperature T2 may be, for example, equal to or lower than 1200° C. This can prevent the surface of the second regrown layer 540 from becoming rough due to an excessively high growth temperature.

[0109] At this time, in the first regrowth step S62 of this embodiment, when an inclined interface is generated in at least a part of the first regrowth layer 520 (when the first regrowth layer 520 is grown three-dimensionally), the second regrowth layer 540 is grown laterally as described above, so that the inclined interface in the second regrowth layer 540 can be gradually reduced, that is, the c-plane 542 can be gradually enlarged. This makes it possible to grow the second regrowth layer 540 having a mirror-finished surface.

[0110] At this time, in this embodiment, the total thickness of the regrowth layer 500 (first regrowth layer 520 and second regrowth layer 540) may be, for example, 600 μm or more, or 1 mm or more. The upper limit 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.

[0111] Here, dislocations D move like a random walk during the growth process of the thick regrowth layer 500. As a result, dislocations D meet with each other or form loops during the growth of the regrowth layer 500. Due to this phenomenon, it is possible to reduce the number of dislocations D that reach the surface of the second regrowth layer 540 of the thick film.

[0112] Alternatively, even if an inclined interface is generated in at least a part of the first regrowth layer 520 (the first regrowth layer 520 is grown three-dimensionally) in the first regrowth step S62 of this embodiment, dislocations can be locally collected at the meeting point of adjacent inclined interfaces in the process of laterally growing the second regrowth layer 540. Even in this case, dislocations D reaching the surface of the thick second regrowth layer 540 can be reduced.

[0113] As a result, it is possible to reduce the dislocation density in the second regrown layer 540. (Note that because FIG. 7 is simplified, the number of dislocations in FIG. 7 is not significantly reduced.)

[0114] (S70: Peeling process) Once the regrowth step S60 is completed, the regrowth layer 500 is peeled off from the base substrate 100 along at least a portion of the porous intermediate layer 300 as a boundary, as shown in FIG.

[0115] In this embodiment, the regrown layer 500 is spontaneously peeled off from the base substrate 100 while the temperature is lowered after the regrowth step S60. This makes it possible to eliminate the need for a separate step for performing the peeling, thus simplifying the manufacturing method.

[0116] 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 base substrate 100). This is due to, for example, as described above, the dislocation density decreasing as the thickness of the regrowth layer 500 increases.

[0117] The tensile stress thus generated in the regrowth layer 500 warps the c-plane 510 of the regrowth layer 500 into a spherical shape with a concave upper side. This causes the regrowth layer 500 to spontaneously and gradually peel off from the outer periphery toward the center of the base substrate 100. That is, by utilizing the warping of the c-plane 510 of the regrowth layer 500, the regrowth layer 500 can be gradually peeled off from the outer periphery toward the center of the base substrate 100. In other words, the regrowth layer 500 can be peeled off evenly in a concentric manner with respect to the center of the base substrate 100. As a result, the peeling off of the large-area regrowth layer 500 can be easily and stably performed.

[0118] By the above peeling step S70, a peeled intermediate body 20 is formed that includes at least the cover layer 400 and the regrown layer 500. Residual pieces of the intermediate layer 300 may remain on the lower surface of the cover layer 400 of the peeled intermediate body 20.

[0119] (S80: Post-processing process) 9, the regrowth layer 500 is sliced, for example, by a wire saw along a cutting plane perpendicular to the normal direction at the center of the surface of the regrowth layer 500. This forms a nitride crystal substrate 50 (hereinafter sometimes abbreviated as "substrate 50") as an as-sliced ​​substrate.

[0120] Next, both sides of the substrate 50 are polished by a polishing device, whereby the main surface of the substrate 50 is made mirror-finished.

[0121] Through the above steps, the substrate 50 made of AlGaN crystal of this embodiment is obtained.

[0122] (Fabrication process of laminate or semiconductor device) Thereafter, a laminate or a semiconductor device may be fabricated using the above-mentioned substrate 50. The laminate or the semiconductor device includes, for example, the above-mentioned substrate 50 and a semiconductor layer including a group III nitride crystal provided on the substrate 50. This allows the manufacture of a high-quality semiconductor device such as an ultraviolet LED or an ultraviolet LD.

[0123] (2) Nitride crystal substrate A nitride crystal substrate 50 according to this embodiment will be described with reference to FIGS. 10A and 10B.

[0124] In this embodiment, the substrate 50 is, for example, Al x Ga 1-x The substrate 50 is made of a crystal represented by a composition formula of N. The Al composition ratio x in the composition formula of the substrate 50 is, for example, not less than 0.05 and not more than 0.8.

[0125] By setting the Al composition ratio x in the composition formula of substrate 50 to be 0.05 or more and 0.8 or less, the peelability of regrowth layer 500 at intermediate layer 300 is improved as described above, and substrate 50 can be stably obtained from regrowth layer 500.

[0126] From the viewpoint of application to semiconductor devices, by setting the Al composition ratio x in the composition formula of the substrate 50 to 0.05 or more, when the substrate 50 is applied to an ultraviolet LED or ultraviolet LD, the transmittance of ultraviolet light through the substrate 50 can be improved. On the other hand, if the Al composition ratio x in the composition formula of the substrate 50 exceeds 0.8, the generation of Al vacancies becomes significant. Therefore, the conductivity of the substrate 50 is easily lost. In contrast, by setting the Al composition ratio x in the composition formula of the substrate 50 to 0.8 or less, the generation of Al vacancies can be suppressed. As a result, it is possible to impart conductivity to the substrate 50 made of AlGaN crystal.

[0127] The diameter of the substrate 50 is, for example, 1 inch (25 mm) or more, or 2 inches (50 mm) or more, or 4 inches (100 mm) or more, and the thickness of the substrate 50 is, for example, 300 μm or more and 1 mm or less.

[0128] The substrate 50 has a primary surface 50s whose nearest low-index crystal plane is, for example, a c-plane 50c. The primary surface 50s of the substrate 50 is, for example, mirror-finished, and the root-mean-square roughness RMS of the primary surface 50s of the substrate 50 is, for example, less than 1 nm.

[0129] In this embodiment, the substrate 50 does not include inversion domains, for example, by growing the regrown layer 500 using the c-plane as a continuous growth plane.

[0130] In this embodiment, the above-described manufacturing method can relax the crystal distortion occurring in the regrown layer 500 made of AlGaN crystals. As a result, the substrate 50 obtained from the regrown layer 500 does not have cracks.

[0131] In this embodiment, the crystal strain occurring in the regrown layer 500 is relaxed, thereby relaxing the crystal strain in the substrate 50 obtained from the regrown layer 500. As a result, the ratio a / c of ​​the lattice constant in the a-axis direction to the lattice constant in the c-axis direction of the substrate 50 becomes close to the ratio a / c (=0.62) of an unstrained AlGaN crystal.

[0132] Specifically, the substrate 50 satisfies the following formula (1). 0.58≦a / c≦0.66 (1)

[0133] Here, c is the diffraction angle of the substrate 50 determined based on the diffraction peak of the (0002) plane diffraction of AlGaN by X-ray diffraction measurement (2θ-ω scan). <0001> is the lattice constant in the axial direction. a is the lattice constant in the <11-20> axis direction of the substrate 50, which is determined based on the above c and the diffraction angle of the (10-12) plane diffraction peak of AlGaN by X-ray diffraction measurement (2θ-ω scan).

[0134] In this embodiment, each of the (0002) plane diffraction patterns and the (10-12) plane diffraction patterns of AlGaN obtained in the above-mentioned X-ray diffraction measurement of the substrate 50 has a single peak.

[0135] In this embodiment, by alleviating the crystal distortion occurring in the regrown layer 500, the crystallinity of the substrate 50 obtained from the regrown layer 500 can be improved.

[0136] Specifically, the full width at half maximum (FWHM) of the (0002) plane diffraction of AlGaN obtained by X-ray rocking curve measurement of the substrate 50 may be, for example, 300 arcsec or less, or 100 arcsec or less. The full width at half maximum (FWHM) of the (10-12) plane diffraction of AlGaN obtained by X-ray rocking curve measurement of the substrate 50 is, for example, 500 arcsec or less, or may be 300 arcsec.

[0137] The X-ray rocking curve measurement is carried out under the following conditions. X-ray: Monochromatic Cu Kα1 light obtained from an X-ray source via an X-ray mirror and two Ge(220) crystals. Goniometer radius: 420mm Entrance slit width: 0.1 mm Receiving slit side: analyzer crystal

[0138] In this embodiment, the regrown layer 500 is not grown in islands, but is grown as a continuous film made of AlGaN crystals, thereby making the Al composition ratio x uniform in the thickness direction of the regrown layer 500. As a result, in the substrate 50 obtained from the regrown layer 500, the Al composition ratio xr of the back surface of the substrate 50 and the Al composition ratio xm of the main surface of the substrate 50 are equal to or close to each other.

[0139] Specifically, the ratio xm / xr of the Al composition ratio xm in a region from the main surface 50s of the substrate 50 to a position 35 μm in the thickness direction to the Al composition ratio xr in a region from the back surface of the substrate 50 to a position 35 μm in the thickness direction is, for example, 0.9 or more and 1.1 or less.

[0140] The Al composition ratios xr and xm referred to here are determined based on the diffraction angle of the (0002) plane diffraction peak by X-ray diffraction measurement of the back surface and main surface of the substrate 50. The position of "35 μm" in the thickness direction from each surface is the penetration depth from each surface in X-ray diffraction measurement using Cu Kα1 radiation.

[0141] Furthermore, in the substrate 50 obtained from the regrown layer 500, the Al composition ratio xo at the outer periphery of the principal surface of the substrate 50 and the Al composition ratio xc at the center of the principal surface of the substrate 50 are equal to or close to each other.

[0142] Specifically, the ratio xc / xo of the Al composition ratio xc at the center of the main surface 50s of the substrate 50 to the Al composition ratio xo at a position 5 mm from the outer periphery toward the center of the main surface 50s of the substrate 50 is, for example, 0.9 or more and 1.1 or less.

[0143] The Al composition ratios xo and xc herein are determined based on the diffraction angle of the (0002) plane diffraction peak by X-ray diffraction measurement of the outer periphery and center of the main surface of the substrate 50, respectively.

[0144] In this embodiment, dislocations can be reduced during the growth of the thick regrowth layer 500. This makes it possible to reduce the dislocation density in the substrate 50 made of an AlGaN crystal obtained from the regrowth layer 500.

[0145] Specifically, the dislocation density on the main surface 50s of the substrate 50 is, for example, 3×10 8 cm -2 or less than or equal to 1×10 7 cm -2 or less, or 5×10 6 cm -2 or less, or 3×10 6 cm -2 It may be the following.

[0146] The conductivity type of the substrate 50 is not particularly limited. However, the substrate 50 of this embodiment may be, for example, an n-type. In this case, examples of n-type impurities include Si, germanium (Ge), oxygen (O), and tin (Sn). The concentration of n-type impurities in the substrate 50 is, for example, 1×10 17 cm -3 More than 1×10 19 cm -3 It may be the following.

[0147] Alternatively, the substrate 50 of the present embodiment may be, for example, a p-type. In this case, the substrate 50 may contain, for example, magnesium (Mg). The Mg concentration in the substrate 50 is, for example, 1×10 16 cm -3 It may be more than that.

[0148] Alternatively, the substrate 50 of this 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 It may be more than that.

[0149] (3) Summary of this embodiment According to this embodiment, one or more of the following advantages can be obtained.

[0150] (a) In this embodiment, a regrowth layer 500 made of AlGaN crystals is grown on a cover layer 400 provided on a porous intermediate layer 300, and the easily deformable porous intermediate layer 300 can reduce the stress applied to the regrowth layer 500.

[0151] (b) Furthermore, by growing the regrowth layer 500 made of AlGaN crystals on the flat cover layer 400, it is possible to grow the regrowth layer 500 as a continuous film while suppressing island-like growth of the regrowth layer 500. This makes it possible to make the Al composition ratio x uniform throughout the regrowth layer 500.

[0152] By growing the regrowth layer 500 as shown in (a) and (b), it is possible to alleviate crystal distortion occurring in the regrowth layer 500 made of AlGaN. This makes it possible to suppress the occurrence of cracks in the regrowth layer 500. Furthermore, it is possible to improve the crystallinity of the regrowth layer 500. As a result, it is possible to stably obtain a substrate 50 made of high-quality AlGaN crystal from the regrowth layer 500.

[0153] By using the above-described substrate 50 obtained in this embodiment, it is possible to manufacture high-quality semiconductor devices such as ultraviolet LEDs or ultraviolet LDs.

[0154] (c) In this embodiment, the crystal strain applied to the regrown layer 500 can be relaxed, while an appropriate crystal strain can be applied to the porous intermediate layer 300 .

[0155] By setting the Al composition ratio x of the regrowth layer 500 to 0.05 or more, appropriate crystal distortion can be applied to the porous intermediate layer 300 due to lattice mismatch between the regrowth layer 500 and the layer located below the regrowth layer 500 as the regrowth layer 500 grows. This can increase the fragility of the porous intermediate layer 300. As a result, the peelability of the regrowth layer 500 across the intermediate layer 300 can be improved.

[0156] On the other hand, by setting the Al composition ratio x of the regrowth layer 500 to 0.8 or less, it is possible to suppress excessive crystal distortion occurring in the porous intermediate layer 300 due to lattice mismatch between the regrowth layer 500 and the layer located below the regrowth layer 500. This makes it possible to suppress disappearance of the voids 360 in the intermediate layer 300 and suppress adhesion between the regrowth layer 500 and the layer located below the regrowth layer 500. As a result, it is possible to suppress a decrease in the peelability of the regrowth layer 500 across the intermediate layer 300.

[0157] As described above, also from the viewpoint of the peelability of the regrown layer 500, the substrate 50 made of an AlGaN crystal can be obtained stably.

[0158] (4) Modifications of the embodiment The above-described embodiment can be modified as necessary as in the following modified examples. 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.

[0159] (Variation 1) In the cover layer forming step S40 of the first modification, the cover layer 400 is, for example, Al x Ga 1-x It is also possible to grow a crystal represented by a composition formula of N. In this case, the Al composition ratio x in the cover layer 400 may be, for example, greater than 0 and equal to or less than 1, or equal to or greater than 0.05 and equal to or less than 0.8.

[0160] According to the first modification, by growing an AlGaN crystal as the cover layer 400, the regrown layer 500 can be homoepitaxially grown on the cover layer 400. This makes it possible to stably grow the regrown layer 500 with good crystallinity.

[0161] (Variation 2) In the first regrowth step S62 of the second modification, the growth temperature may be gradually increased from the first growth temperature T1 to the second growth temperature T2 while the first regrowth layer 520 is being grown.

[0162] According to the second modification, the rate of temperature rise from the first growth temperature T1 to the second growth temperature T2 can be made gentler, and as a result, cracks in the group III nitride crystal layer above the intermediate layer 300 caused by the temperature rise process from the first growth temperature T1 to the second growth temperature T2 can be stably suppressed.

[0163] (Variation 3) In the regrowth step S60 of Modification 3, the growth temperature of the regrowth 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 above-mentioned second growth temperature T2).

[0164] According to the third modification, the first regrowth step S62 can be omitted, which makes it possible to shorten the manufacturing process.

[0165] <Other embodiments of the present disclosure> Although the embodiments of the present disclosure have been specifically described above, the present disclosure is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit and scope of the present disclosure.

[0166] In the above embodiment, the case where the base layer forming step S20 is performed has been described, but the base layer forming step S20 does not have to be performed. In other words, the base layer 200 does not have to be provided. In the intermediate layer forming step S30, the intermediate layer 300 may be formed directly on the base substrate 100.

[0167] In the above embodiment, the upper layer of the underlayer 200 and the intermediate layer 300 each contain GaN crystals, but the present disclosure is not limited to this case. Each layer is not limited to GaN crystals, and may be made of, for example, Group III nitride crystals such as aluminum nitride (AlN), aluminum gallium nitride (AlGaN), indium nitride (InN), indium gallium nitride (InGaN), and aluminum indium gallium nitride (AlInGaN), i.e., In x Al y Ga 1-x-y The crystal may be represented by the composition formula N (where 0≦x≦1, 0≦y≦1, 0≦x+y≦1).

[0168] In the above embodiment, the case where the base substrate 100 is made of a material other than Group III nitrides has been described, but the present disclosure is not limited to this case. The base substrate 100 may be, for example, a free-standing substrate made of Group III nitride crystals.

[0169] In the above-described embodiment, 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. However, 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.

[0170] In the above embodiment, the above-mentioned vapor phase epitaxy method is used as the growth method of the underlayer 200, the intermediate layer 300, the cover layer 400, and the regrown layer 500, but the present disclosure is not limited to this case. Metal-organic vapor phase epitaxy (MOVPE) may be used as the growth method of at least one of the underlayer 200, the intermediate layer 300, the cover layer 400, and the regrown layer 500. Alternatively, a growth method other than the vapor phase epitaxy method may be used as the growth method of at least one of the underlayer 200, the intermediate layer 300, the cover layer 400, and the regrown layer 500.

[0171] In the above embodiment, the first regrown layer 520 is three-dimensionally grown after step-flow growth in the initial growth stage of the first regrown layer 520, but the present disclosure is not limited to this case. The step-flow growth of the first regrown layer 520 may be maintained. EXAMPLES

[0172] Hereinafter, experimental results that support the effects of the above-described embodiment will be described.

[0173] (1) Preparation of samples The following three samples were prepared.

[0174] [Sample A] Using the manufacturing method of the above-described embodiment, a nitride crystal substrate of sample A was fabricated in the following procedure.

[0175] (Base board preparation process) A base substrate for obtaining a seed substrate was prepared. Substrate: Sapphire substrate Orientation of the main surface of the base substrate: +c plane Substrate diameter: 2 inches (50.8 mm) Thickness of the substrate: 430μm

[0176] (Underlying layer formation process) An AlN buffer layer and a GaN layer were formed in this order as an underlying layer on a base substrate by HVPE under the following conditions. Growth temperature of the underlayer: 1055°C. The thickness of the AlN buffer layer and the GaN layer are 100 nm and 4 μm, respectively. Carrier concentration in the GaN layer as the underlayer: about 1×10 18 cm -3

[0177] (Intermediate layer formation process) Next, a Si-doped GaN layer was grown as an intermediate layer on the underlayer by HVPE under the following conditions: Intermediate layer growth temperature: 1055°C. Interlayer thickness: 3000nm Carrier concentration in the intermediate layer: 6×10 18 cm -3

[0178] (Cover layer forming process) Next, a GaN layer was grown as a cover layer on the underlayer by HVPE under the following conditions: Cover layer growth temperature: 1055°C. Cover layer thickness: 150nm Carrier concentration in the cover layer: approx. 5×10 17 cm -3

[0179] (Porous process) The intermediate layer was then rendered porous by electrochemical treatment through rearrangements in the cover layer under the following conditions: Electrolyte temperature: Room temperature (23℃) Processing voltage: 20V Processing current: Maximum 100mA Processing time: 10 minutes

[0180] (1st regrowth process) Next, Al was grown as the first regrowth layer on the cover layer by HVPE under the following conditions: 0.6 Ga 0.4 N layers were grown. First growth temperature: 835℃. Thickness of the first regrowth layer: 50 μm

[0181] (Second regrowth process) Next, in the same chamber as the first regrowth step, a second regrowth layer was formed on the first regrowth layer. 0.6 Ga 0.4 N layers were grown. Second growth temperature: 1055℃. Total thickness of regrowth layer: 1mm

[0182] (Peeling process) During the temperature drop after the regrowth step, the regrowth layer was peeled off from the base substrate.

[0183] (Post-processing) The peeled regrown layer was sliced ​​to form a nitride crystal substrate. After slicing, both sides of the nitride crystal substrate were polished to obtain a 430 μm-thick Al substrate as sample A1. 0.6 Ga 0.4 A nitride crystal substrate made of N crystal was obtained.

[0184] [Sample B1 (direct growth on a GaN free-standing substrate)] In the preparation of sample B1, a GaN free-standing substrate was used as the base substrate. A 1 mm thick Al layer was grown as the main growth layer directly on the GaN free-standing substrate by the HVPE method under the same conditions as those for the second regrowth layer of sample A. 0.6 Ga 0.4 An N layer was then grown, thereby obtaining a laminate of sample B1.

[0185] [Sample B2 (VAS method)] The VAS method was used to fabricate sample B2. First, a template was prepared on a sapphire substrate with a diameter of 2 inches, which had a GaN layer with a thickness of 300 nm and containing voids, and a mesh-like TiN layer with a thickness of 20 nm, in that order. Next, a 1 mm thick AlN layer was grown as a main growth layer on the above-mentioned GaN layer and TiN layer by the HVPE method under the same conditions as those for the second regrowth layer of sample A. 0.6 Ga 0.4 Then, the grown layer was peeled off from the sapphire substrate at the boundary of the GaN layer having voids. 0.6 Ga0.4 N layers were obtained.

[0186] (2) Evaluation [Optical microscope] Each sample was observed under an optical microscope.

[0187] [X-ray diffraction measurement] X-ray diffraction measurements (2θ-ω scan) were performed on the (0002) and (10-12) plane diffraction of AlGaN on the nitride crystal substrate of sample A. The measurement positions included the center of the main surface of the nitride crystal substrate of sample A, points set at 1 cm intervals on a line passing through the center of the main surface and along the m-axis direction, and points set at 1 cm intervals on a line passing through the center of the main surface and along the a-axis direction perpendicular to the m-axis direction. The a / c ratio of the nitride crystal substrate of sample A was calculated based on the results obtained by the X-ray diffraction measurements.

[0188] In addition, by performing X-ray diffraction measurements on the nitride crystal substrate of sample A, the ratio xc / xo was obtained based on the Al composition ratio xc at the center of the main surface of the substrate relative to the Al composition ratio xo at a position 5 mm from the outer periphery toward the center of the main surface of the substrate.

[0189] Furthermore, by performing X-ray diffraction measurements at each of the above-mentioned overlapping measurement positions on the main surface and back surface of the nitride crystal substrate of sample A, the ratio xm / xr was obtained based on the Al composition ratio xm in a region from the main surface of the substrate to a position 35 μm in the thickness direction, relative to the Al composition ratio xr in a region from the back surface of the substrate to a position 35 μm in the thickness direction.

[0190] [X-ray rocking curve measurement] By performing X-ray rocking curve measurements at each of the measurement positions described above on the nitride crystal substrate of sample A, the full width at half maximum (FWHM) of the (0002) plane diffraction and the (10-12) plane diffraction of AlGaN were obtained.

[0191] [Multiphoton excitation microscope] At each of the measurement positions on the nitride crystal substrate of sample A, the dislocation density was determined by observing the main surface with a multiphoton excitation microscope in a field of view of 250 μm square.

[0192] (3) Results The results for samples A, B1 and B2 will now be described.

[0193] [Sample B1] In sample B1, cracks were generated in the growth layer after it was grown on the GaN free-standing substrate. It is believed that the cracks were generated in the growth layer in sample B1 due to lattice mismatch between the GaN free-standing substrate and the growth layer made of AlGaN crystals.

[0194] [Sample B2] In sample B2, cracks occurred in parts of the main growth layer that had peeled off from the sapphire substrate. In sample B2, there was little movement of Al atoms when island-shaped AlGaN crystals grew three-dimensionally on the GaN layer containing voids and the mesh-like TiN layer. This caused variation in the Al composition ratio within the surface. This variation in the Al composition ratio caused crystal distortion in the main growth layer, which is made of thick AlGaN crystals. It is believed that this caused cracks in the main growth layer in sample B2.

[0195] [Sample A] In sample A, no cracks were generated in the peeled regrown layer or in the nitride crystal substrate obtained from the regrown layer.

[0196] In sample A, the crystal distortion occurring in the regrowth layer could be alleviated by growing a regrowth layer made of AlGaN crystal on a flat cover layer provided on a porous intermediate layer. As a result, it was confirmed that a nitride crystal substrate made of high-quality AlGaN crystal without cracks could be obtained in sample A.

[0197] Furthermore, other analyses were carried out on the nitride crystal substrate of sample A, and it was confirmed that the following characteristics were obtained. Al composition ratio xc / xo: 1.07

[0198] It was also confirmed that the following characteristics were obtained at each measurement position set at 1 cm intervals on a line that included the center of the main surface of the nitride crystal substrate of sample A, passed through the center of the main surface and along the m-axis direction, and on a line that passed through the center of the main surface and along the a-axis direction perpendicular to the m-axis direction. 0.58≦a / c≦0.66 (1) (0002) diffraction FWHM<250arcsec (10-12) plane diffraction FWHM<450arcsec Dislocation density <3×10 6 cm -2

[0199] It was also confirmed that the following characteristics were obtained at each measurement position set at 1 cm intervals on a line that included the centers of the main surface and back surface of the nitride crystal substrate of sample A, passed through the centers of the main surface and back surface, and was aligned along the m-axis direction, and on a line that passed through the centers of the main surface and back surface and was aligned along the a-axis direction perpendicular to the m-axis direction. Al composition ratio xm / xr: 0.95~1.05 Al composition ratio x:0.59~0.63

[0200] <Additional Notes> The following additional notes are given regarding aspects of the present disclosure.

[0201] (Appendix 1) (a) preparing a base substrate; (b) forming an intermediate layer containing n-type Group III nitride crystals above the base substrate; (c) forming a cover layer on the intermediate layer, the cover layer including a Group III nitride crystal having a carrier concentration lower than that of the intermediate layer; (d) rendering the intermediate layer porous through dislocations in the cover layer by electrochemical treatment while maintaining the surface state of the cover layer; (e) epitaxially growing a regrowth layer made of a Group III nitride crystal on the cover layer; (f) peeling the regrowth layer from the base substrate along at least a portion of the porous intermediate layer; Equipped with In (e), The regrown layer is made of Al having an Al composition ratio x of 0.05 or more and 0.8 or less. x Ga 1-x Grow a crystal with the formula N A method for manufacturing a nitride crystal substrate.

[0202] (Appendix 2) In (e), (d) is more effective in reducing crystal distortion in the regrown layer than in the case where the step (d) is not performed. 2. A method for producing the nitride crystal substrate according to claim 1.

[0203] (Appendix 3) In (e), The regrown layer is grown in a step-flow manner at least in an initial stage of growth of the regrown layer. 3. A method for producing a nitride crystal substrate according to claim 1 or 2.

[0204] (Appendix 4) In (c), GaN crystal is grown as the cover layer. 4. A method for producing a nitride crystal substrate according to any one of claims 1 to 3.

[0205] (Appendix 5) In (c), The cover layer is made of Al having an Al composition ratio x of more than 0 and not more than 1. x Ga 1-x Grow a crystal with the formula N 4. A method for producing a nitride crystal substrate according to any one of claims 1 to 3.

[0206] (Appendix 6) (e) is (e1) growing a first regrowth layer on the cover layer at a first growth temperature; (e2) growing a second regrowth layer on the first regrowth layer at a second growth temperature; having In (e1), The first growth temperature is lower than the second growth temperature. 6. A method for producing a nitride crystal substrate according to any one of claims 1 to 5.

[0207] (Appendix 7) A nitride crystal substrate, Al x Ga 1-x It consists of crystals represented by the formula N, The Al composition ratio x in the composition formula is 0.05 or more and 0.8 or less, The nitride crystal substrate satisfies the following formula (1): 0.58≦a / c≦0.66 (1) Where: c is the diffraction angle of the (0002) plane diffraction peak of AlGaN by X-ray diffraction measurement of the nitride crystal substrate <0001> is the lattice constant in the axial direction, a is the lattice constant in the <11-20> axis direction of the nitride crystal substrate, which is determined based on c and the diffraction angle of the (10-12) plane diffraction peak of AlGaN by X-ray diffraction measurement. Nitride crystal substrate.

[0208] (Appendix 8) The nitride crystal substrate has a diameter of 25 mm or more and is free of cracks. 8. The nitride crystal substrate according to claim 7.

[0209] (Appendix 9) the half-width of the AlGaN (0002) plane diffraction obtained by X-ray rocking curve measurement of the nitride crystal substrate is 300 arcsec or less; The half-width of the AlGaN (10-12) plane diffraction obtained by measuring the X-ray rocking curve of the nitride crystal substrate is 500 arcsec or less. 9. The nitride crystal substrate according to claim 7 or 8.

[0210] (Appendix 10) the nitride crystal substrate has a primary surface and a back surface opposite to the primary surface, a ratio xm / xr of an Al composition ratio xm in a region from the main surface to a position 35 μm in a thickness direction of the nitride crystal substrate to an Al composition ratio xr in a region from the back surface to a position 35 μm in a thickness direction of the nitride crystal substrate is 0.9 or more and 1.1 or less. 10. The nitride crystal substrate according to any one of claims 7 to 9.

[0211] (Appendix 11) a ratio xc / xo of an Al composition ratio xc at the center of a main surface of the nitride crystal substrate to an Al composition ratio xo at a position 5 mm from the outer periphery toward the center of the main surface of the nitride crystal substrate is 0.9 or more and 1.1 or less. 11. The nitride crystal substrate according to any one of claims 7 to 10.

[0212] (Appendix 12) A nitride crystal substrate according to any one of Supplementary Note 7 to Supplementary Note 11; a semiconductor layer including a Group III nitride crystal provided on the nitride crystal substrate; Equipped Laminate. [Explanation of symbols]

[0213] 10. Seed substrate for nitride crystal growth 20 Peeling intermediate 50 Nitride crystal substrate 100 Base substrate 120 Main surface 200 Base layer 300 Middle Class 360 Void 400 Cover Layer 500 regrowth layer 520 1st 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) providing a base substrate; (b) forming an intermediate layer containing n-type Group III nitride crystals above the base substrate; (c) forming a cover layer on the intermediate layer, the cover layer including a Group III nitride crystal having a carrier concentration lower than a carrier concentration of the intermediate layer; (d) rendering the intermediate layer porous through dislocations in the cover layer by electrochemical treatment while maintaining the surface state of the cover layer; (e) epitaxially growing a regrowth layer made of a Group III nitride crystal on the cover layer; (f) peeling the regrowth layer from the base substrate along at least a portion of the porous intermediate layer; Equipped with In (e), The regrown layer is an Al layer having an Al composition ratio x of 0.05 or more and 0.8 or less. ｘ G １－ｘ Grow a crystal represented by the formula N A method for producing a nitride crystal substrate.

2. In (e), The regrown layer is grown in a step-flow manner at least in an initial stage of growth of the regrown layer. A method for producing the nitride crystal substrate according to claim 1 .

3. In (c), As the cover layer, a GaN crystal is grown. The method for producing the nitride crystal substrate according to claim 1 or 2.

4. In (c), The cover layer is made of Al having an Al composition ratio x of more than 0 and not more than 1. ｘ G １－ｘ Grow a crystal represented by the formula N The method for producing the nitride crystal substrate according to claim 1 or 2.

5. A nitride crystal substrate, A ｘ G １－ｘ The crystal is represented by the formula: The Al composition ratio x in the composition formula is 0.05 or more and 0.8 or less, The nitride crystal substrate satisfies the following formula (1): 0.58≦a / c≦0.66 (1) Where: c is the lattice constant in the <0001> axis direction of the nitride crystal substrate, determined based on the diffraction angle of the (0002) plane diffraction peak of AlGaN by X-ray diffraction measurement; a is the lattice constant in the <11-20> axis direction of the nitride crystal substrate, which is determined based on c and the diffraction angle of the (10-12) plane diffraction peak of AlGaN by X-ray diffraction measurement. Nitride crystal substrate.

6. The nitride crystal substrate has a diameter of 25 mm or more and is free of cracks. The nitride crystal substrate according to claim 5 .

7. the half-width of the (0002) plane diffraction of AlGaN obtained by X-ray rocking curve measurement of the nitride crystal substrate is 300 arcsec or less; The half-width of the (10-12) plane diffraction of AlGaN obtained by measuring the X-ray rocking curve of the nitride crystal substrate is 500 arcsec or less. The nitride crystal substrate according to claim 5 or 6.

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