Laminate

Abstract

To provide a novel method for manufacturing a GaN single crystal having a reduced dislocation density.SOLUTION: A GaN single crystal manufacturing method has a seed preparation step for preparing a single crystal GaN (0001) substrate, and a HVPE step for growing a GaN crystal by HVPE on the (0001) surface of the single crystal GaN (0001) substrate. In the HVPE step, after growing a pitted layer, operation for growing a flattened layer is performed by changing a growth condition. A thickness of the pitted layer may be 100 μm or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for producing a GaN single crystal. [Background technology]

[0002] When growing GaN crystals on a sapphire substrate by HVPE (Hydride Vapor Phase Epitaxy), conditions that initially cause pits to form on the growth surface are used, and then conditions that cause the pits to close are used. It has been reported that by changing the conditions, GaN crystals having a uniformly reduced dislocation density were obtained (Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-52102 [Patent Document 2] Special Publication No. 2007-519591 Summary of the Invention [Problem to be solved by the invention]

[0004] The present invention relates to a novel method for producing GaN single crystals with reduced dislocation density. [Means for solving the problem]

[0005] The inventors have 6 cm -2 The inventors discovered that it is possible to grow a pitted GaN crystal layer on the (0001) surface of a single-crystal GaN substrate with a uniform dislocation density reduced to the low half-micron range by sufficiently lowering the growth temperature. Based on this finding, they conducted further research and finally completed the present invention.

[0006] Embodiments of the present invention include the following GaN single crystal production method. [1] A method for producing a GaN single crystal, comprising a seed preparation step of preparing a single crystal GaN (0001) substrate, and an HVPE step of growing a GaN crystal on the (0001) surface of the single crystal GaN (0001) substrate by HVPE, wherein in the HVPE step, after growing a pitted layer, an operation of changing the growth conditions to grow a smoothing layer is performed. [2] The dislocation density on the (0001) surface of the single-crystal GaN (0001) substrate is 10 6 cm -2 The method according to [1] above, wherein the first half of the [3] The oxygen concentration of the single-crystal GaN (0001) substrate is 2 × 10 16 atoms / cm 3 The method according to [2] above, which is as follows: [4] The method according to any one of [1] to [3] above, wherein the pitted layer has a thickness of 100 μm or less. [5] The method according to any one of [1] to [4] above, wherein the growth temperature of the pitted layer is 900° C. or less. [6] The method according to [5] above, wherein the growth temperature of the planarizing layer is 1000°C or higher. [7] The method according to any one of [1] to [6] above, wherein the operation is carried out two or more times in the HVPE step. [8] The method according to any one of [1] to [7] above, wherein the thickness of the planarization layer grown last in the HVPE step is 1 mm or more. [9] The method according to any one of [1] to [8] above, comprising the step of processing the planarizing layer to obtain a c-plane single crystal GaN substrate.

[10] The method according to any one of [1] to [9] above, wherein the single-crystal GaN (0001) substrate prepared in the seed preparation step is manufactured by growing a GaN crystal thick film on a sapphire substrate via a delamination layer by HVPE, and then spontaneously delaminating the GaN crystal thick film from the heterosubstrate.

[11] The method according to

[10] , wherein the single-crystal GaN (0001) substrate is produced in a reactor of an HVPE apparatus and is subjected to the HVPE process without being removed from the reactor.

[12] The method according to

[11] above, wherein the temperature of at least one of the gallium boat and the susceptor of the HVPE apparatus is maintained at 700° C. or higher between the seed preparation step and the HVPE step. [Effects of the Invention]

[0007] A novel method for producing GaN single crystals with reduced dislocation density is provided. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a flow diagram of a method for producing a GaN single crystal. [Figure 2] 2(a) to 2(c) are cross-sectional views of stacks that can be formed by the HVPE process. [Figure 3] FIG. 3 is a fluorescence microscope image of a cross section of a stack formed by growing a GaN crystal layer on a single-crystal GaN (0001) substrate. [Figure 4] FIG. 4 is a fluorescence microscope image of a cross section of a stack formed by growing a GaN crystal layer on a single-crystal GaN (0001) substrate. DETAILED DESCRIPTION OF THE INVENTION

[0009] Unless otherwise specified, the GaN crystal referred to in this specification means a hexagonal GaN crystal having a wurtzite crystal structure. In GaN crystals, the crystal axis parallel to

[0001] and [000-1] is called the c-axis, the crystal axis parallel to <10-10> is called the m-axis, and the crystal axis parallel to <11-20> is called the a-axis. The crystal plane perpendicular to the c-axis is called the c-plane, the crystal plane perpendicular to the m-axis is called the m-plane, and the crystal plane perpendicular to the a-axis is called the a-plane. Hereinafter, when a crystal axis, a crystal plane, a crystal orientation, etc. are mentioned, they mean the crystal axis, crystal plane, crystal orientation, etc. of a GaN crystal unless otherwise specified. The Miller indices (hkil) of hexagonal crystals are sometimes written in three digits as (hkl) because of the relationship h+k=-i. For example, (0004) written in three digits is (004). Unless otherwise specified, the term "dislocation" used in this specification refers to a threading dislocation. In this specification, screw dislocations, mixed dislocations, and edge dislocations are not distinguished from each other, and are collectively referred to as threading dislocations. Hereinafter, an embodiment of the present invention will be described with reference to the drawings as appropriate.

[0010] 1. GaN single crystal manufacturing method In a GaN single crystal production method according to an embodiment of the present invention, as shown in the flow chart of FIG. 1, a seed preparation step P1 for preparing a single crystal GaN (0001) substrate and an HVPE step P2 for growing a GaN crystal by HVPE on the (0001) surface of the single crystal GaN (0001) substrate prepared in the seed preparation step P1 are carried out in this order. The HVPE step P2 involves growing a pitted layer followed by growing a planarizing layer under modified growth conditions. Each step is described in detail below.

[0011] 1.1.Seed preparation process The single-crystal GaN (0001) substrate prepared in the seed preparation step P1 serves as a seed when a GaN crystal layer is grown in the subsequent HVPE step P2. A single-crystal GaN (0001) substrate is a free-standing wafer consisting of only single-crystal GaN, with a large-area surface parallel to the (0001) crystal plane or slightly off-cut from the (0001) crystal plane, referred to as the (0001) surface. The off-cut angle is preferably 2.5° or less, more preferably 1.5° or less.

[0012] The single-crystal GaN (0001) substrate to be prepared in the seed preparation step P1 does not have inversion domains (microdomains where the polarity of the crystal is locally inverted). The single-crystal GaN (0001) substrate to be prepared in the seed preparation step P1 has a good uniformity of dislocation density, and in a preferred example, the difference in dislocation density between the lowest and highest points on the (0001) surface is 4 times or less. The dislocation density here refers to the dislocation density measured in an observation area of ​​100 μm × 100 μm. The single crystal GaN (0001) substrate of this preferred embodiment may exceptionally have localized regions with a significantly higher dislocation density than the surrounding regions, but the density of such exceptional regions is less than 1 region / cm 2 and their total area is less than 0.1% of the area of ​​the (0001) surface.

[0013] A single-crystal GaN (0001) substrate with a good uniformity of dislocation density, such as this preferred example, is usually 2 × 10 16 atoms / cm 3 GaN crystals produced by facet growth, which inevitably involves the formation of numerous dislocation-concentrated regions or inversion domains, cannot have such low oxygen concentrations. The dislocation density on the (0001) surface of the single-crystal GaN (0001) substrate to be prepared in the seed preparation step P1 is, for example, 10 6 cm -2 One of the features of the GaN single crystal production method according to the embodiment is that a GaN single crystal having a lower dislocation density can be grown on a GaN substrate having such a dislocation density.

[0014] Dislocations present on the (0001) surface of a single-crystal GaN (0001) substrate are observed as dark spots in a cathodoluminescence image. When the carrier concentration is too low to count the dark spots in the cathodoluminescence image, the dislocation density can be determined by counting etch pits. When etched for one hour with 89% sulfuric acid heated to 270°C, etch pits corresponding to dislocations are formed on the (0001) surface of a single-crystal (0001) GaN substrate.

[0015] In the seed preparation step P1, a single-crystal GaN (0001) substrate may be produced by growing a GaN crystal thick film on a heterosubstrate via a separation layer by HVPE, and then separating the GaN crystal thick film from the heterosubstrate. A hetero substrate is a single crystal substrate made of a compound with a different composition from GaN, such as a sapphire substrate.

[0016] A preferred exfoliation layer is formed by growing a GaN layer several hundred nanometers thick on a sapphire substrate via a low-temperature buffer layer by MOVPE, then forming a Ti (titanium) layer several tens of nanometers thick on top of that by vacuum deposition, and then annealing it in a mixed gas of 80% H2 (hydrogen gas) and 20% NH3 (ammonia) at 1060°C for 30 minutes, for example. A method for producing a single-crystal GaN substrate, in which a thick GaN crystal film is grown on such a separation layer by HVPE and then the thick GaN crystal film is spontaneously separated from the sapphire substrate, is known as the VAS (Void-Assisted Separation) method [Y. Oshima, et al., Japanese Journal of Applied Physics 42 (2003) pp. L1-L3]. In a preferred example, a single-crystal GaN (0001) substrate is produced by the above-mentioned VAS method in the reactor of an HVPE apparatus used in the subsequent HVPE step P2, and then subjected to the HVPE step P2 without being removed from the reactor.

[0017] In the VAS method, when the GaN crystal growth temperature is 1050°C, the temperature at which thermal stress causes delamination during the cooling process after growth is estimated to be above 950°C [T. Yoshida, et al., Journal of Crystal Growth 310 (2008) pp. 5-7]. Therefore, in one example, after growth of a thick GaN crystal film, the GaN crystal thick film is delaminated from the sapphire substrate in the reactor of the HVPE equipment while maintaining the temperature of the gallium boat and susceptor of the HVPE equipment at above 700°C, preferably above 750°C, more preferably above 800°C, and particularly preferably above 850°C, to form a free-standing single-crystal GaN (0001) substrate, which can then be used for the next HVPE process. This eliminates the need to significantly change the temperature of the HVPE equipment between processes, improving production efficiency. In another example, in order to ensure that the thick GaN crystal film is delaminated from the sapphire substrate in the reactor, the temperature of the susceptor may be reduced by 400°C or more, 500°C or more, or 600°C or more from the growth temperature of the thick GaN crystal film before proceeding to the HVPE process.

[0018] 1.2.HVPE process In the HVPE step P2, a GaN crystal is grown by the HVPE method on the (0001) surface of the single-crystal GaN (0001) substrate prepared in the seed preparation step P1. In the HVPE step P2, a pitted layer is grown, followed by growth of a planarizing layer under different growth conditions, which may be repeated two or more times. 2(a) to 2(c) are cross-sectional views showing stacks that can be formed in the HVPE step P2. Each stack consists of a single-crystal GaN (0001) substrate 1 and a GaN crystal 2 grown on its (0001) surface.

[0019] In the example of FIG. 2( a ), the GaN crystal 2 is made up of a pitted layer 22 grown directly on a single-crystal GaN (0001) substrate 1 and a flattening layer 24 laminated on the pitted layer 22 . In the example of FIG. 2(b), the GaN crystal 2 is composed of an underlayer 21 grown flat directly on the single-crystal GaN (0001) substrate 1, a pitted layer 22 stacked on the underlayer 21, and a flattening layer 24 stacked on the pitted layer 22. In the example of FIG. 2(c), the GaN crystal 2 is formed by repeating twice the operation of growing a pitted layer 22 and then growing a flattened layer 24 thereon.

[0020] During the growth of the pitted layer, dislocations gather toward the pits, and when the pits coalesce, the propagation direction of the dislocations is bent, increasing the probability of dislocation annihilation. Therefore, the dislocation density of the GaN crystal is lower in the smoothing layer stacked on the pitted layer than below the pitted layer. The thickness of the pitted layer is usually 10 μm or more, preferably 30 μm or more, more preferably 50 μm or more. Surprisingly, the formation of a pitted layer having a thickness of only 100 μm or less results in a sufficiently effective reduction in dislocation density.

[0021] The growth of a pitted GaN layer is possible by sufficiently lowering the growth temperature within the range where single crystal growth is not difficult, and does not require the SAG (Selective Area Growth) technique, i.e., there is no need to provide a selective growth mask on the single crystal GaN (0001) substrate prepared in the seed preparation step P1, or to perform pattern etching to create irregularities.

[0022] According to experiments by the inventors, when HVPE conditions were used, such as a V / III ratio of about 5, an F value of the carrier gas of about 0.4, and a growth temperature of about 900°C, 10 6 cm -2 The surface of a GaN crystal layer grown on a CMP-finished flat (0001) surface of a single-crystal GaN (0001) substrate with a uniform dislocation density in the low-order order was pitted. On the other hand, when the same seed substrate, V / III ratio, and F value of the carrier gas were used, the surface of the GaN crystal layer grown at a temperature of 950°C or higher was flat and not pitted. Here, the V / III ratio refers to the molar ratio of NH3 (ammonia) and GaCl (gallium chloride) supplied as raw materials, and the F value of the carrier gas refers to the molar ratio of H2 (hydrogen gas) in the carrier gas introduced into the reactor.

[0023] To grow a flattening layer on the pitted layer, the growth conditions can be changed. The growth temperature of the flattening layer is preferably 1000°C or higher. If necessary, in order to accelerate the flattening of the surface, either or both of the V / III ratio and the F value of the carrier gas when growing the flattening layer may be lower than those when growing the pitted layer.

[0024] The planarizing layer is grown so that pits on the surface are substantially eliminated. The state in which pits are substantially eliminated is when the pit density on the surface of the planarizing layer, excluding the periphery, is 1 pit / cm. 2 The reason for excluding the vicinity of the periphery is that pits tend to occur easily in this area regardless of the growth conditions. The pit density on the surface of the planarization layer is preferably 0.5 pits / cm 2 Less than or equal to 0.1 pits / cm, more preferably 0.1 pits / cm 2 Below 0 pits / cm, most preferably 0 pits / cm 2 is. During the growth of the planarizing layer, the pits may substantially disappear from the surface of the planarizing layer.

[0025] Even after the growth of the smoothing layer, the thickness of the underlying pitted layer can be investigated by observing a cross section of the GaN crystal perpendicular to the c-plane using a fluorescence microscope. The pitted layer and the smoothing layer are easily distinguishable by their distinct color tones in the fluorescence microscope image, likely due to different concentrations and / or types of impurities incorporated during growth. In the fluorescence microscope image, the pitted layer appears darker than the smoothing layer. The impurity whose concentration differs particularly between the pitted layer and the flattened layer is oxygen, and the pitted layer has a higher oxygen concentration than the flattened layer.

[0026] In the HVPE step P2, the thickness of the planarization layer grown last (hereinafter also referred to as the "final planarization layer") is usually 1 mm or more, preferably 2 mm or more, more preferably 4 mm or more, and even more preferably 8 mm or more. 2(a) and 2(b), the final planarization layer is the single planarization layer 24 included in the GaN crystal 2. In the example of FIG. 2(c), of the two planarization layers 24 included in the GaN layer 2, the one that is grown later is the final planarization layer. The growth conditions for the final planarization layer may be the same throughout or may be changed during growth.

[0027] In the final planarization layer, which grows after the surface has been substantially cleared of pits, the oxygen concentration is typically 2×10 16 atoms / cm 3 The following is the result. The final planarization layer can be doped with donor impurities such as Si (silicon) or Ge (germanium), or with impurities that act to make the GaN semi-insulating through donor impurity compensation, such as Fe (iron), Mn (manganese), Cr (chromium), and C (carbon). Doping of the final planarization layer may begin after the surface pits have substantially disappeared. When the planarizing layer is grown multiple times, only the final planarizing layer may be doped with impurities, and all the other planarizing layers may be grown undoped.

[0028] By slicing the final planarized layer, a c-plane single crystal GaN substrate with desirable quality can be produced. A c-plane single crystal GaN substrate is a free-standing wafer made of single crystal GaN only, and has a large-area c-plane surface that is parallel to the c-plane or slightly off-cut from the c-plane. The thickness of the c-plane single crystal GaN substrate can be set appropriately between 200 μm and 1 mm depending on the area of ​​the c-plane surface. Of the final planarization layers, a particularly preferred material for a c-plane single crystal GaN substrate is the portion grown after the pits have substantially disappeared from the surface.

[0029] 2. Uses of GaN substrates The c-plane single crystal GaN substrate produced by the GaN single crystal production method according to the embodiment can be used in the production of various nitride semiconductor devices, including light-emitting devices such as light-emitting diodes (LEDs) and laser diodes (LDs), and electronic devices such as rectifiers, bipolar transistors, field-effect transistors, and HEMTs (High Electron Mobility Transistors). Nitride semiconductors are also called nitride III-V compound semiconductors, group III nitride compound semiconductors, GaN-based semiconductors, etc., and include GaN as well as compounds in which part or all of the gallium in GaN is replaced with other Group 13 elements of the periodic table (B, Al, In, etc.).

[0030] 3. Experimental Results The results of the experiments conducted by the present inventors are described below. Experiment 1 (1) Preparation of seed substrate A single-crystal GaN (0001) substrate was prepared, consisting of a GaN crystal grown by HVPE. The (0001) surface of the substrate was flattened by grinding and then subjected to CMP. The dislocation density on the (0001) surface of the substrate was highly uniform, and observation of the cathodoluminescence image showed a density of 2 × 10 6 cm -2 From 4×10 6 cm -2 was estimated to be between

[0031] (2) Growth of GaN crystal layer by HVPE Using an HVPE apparatus equipped with a quartz hot-wall reactor, a quartz gallium boat, and a pyrolytic graphite susceptor installed inside the reactor, GaN crystals were grown on the (0001) surface of the single-crystal GaN (0001) substrate prepared in (1) above. The total growth time was 8 hours, with the first hour being under condition 1 shown in Table 1 below, and the remaining 7 hours being under condition 2 shown in Table 1 below.

[0032] [Table 1]

[0033] The surface of the grown GaN crystal is essentially pit-free except within a region 5 mm from the periphery, with a pit density of 0.1 pits / cm 2 It was. Figure 3 shows a fluorescence microscope image of a cross section perpendicular to the c-plane of the layered structure formed by GaN crystal growth. The three-layer structure clearly observed in the fluorescence microscope image is the pitted layer in the center, the single-crystal GaN (0001) substrate below, and the planarization layer above. The total thickness of the grown GaN crystal was 440 μm, and the thickness of the pitted layer was 78 μm. Cathodoluminescence imaging revealed that the dislocation density on the surface of the planarization layer was 7.0 × 10 5 ~8.5×10 5 cm -2 The dislocation density was less than half of that on the (0001) surface of the seed GaN substrate.

[0034] Experiment 2 GaN crystals were grown on single-crystal GaN (0001) substrates in the same manner as in Experiment 1 above, except that when growing the GaN crystal layer, growth for 1 hour under Condition 1 above and growth for 4 hours under Condition 2 above were alternately performed twice. The surface of the grown GaN crystal is essentially pit-free except within a region 5 mm from the periphery, with a pit density of 0.6 pits / cm 2 It was. Figure 4 shows a fluorescence microscope image of a cross section perpendicular to the c-plane of the stack formed by the growth of the GaN crystal layer. The fluorescence microscope image clearly shows that the stack has a five-layer structure. The bottom layer of the five-layer structure is the seed substrate, followed by the first pitted layer, then the first planarization layer, then the second pitted layer, and finally the second planarization layer. Cathodoluminescence imaging revealed that the dislocation density on the surface of the second planarization layer was 5.1 × 10 5 ~7.6×10 5 cm -2, which was lower than the dislocation density on the surface of the GaN crystal grown in Experiment 1.

[0035] While the present invention has been described above with reference to specific embodiments, these embodiments are presented as examples and do not limit the scope of the present invention. Each embodiment described in this specification can be modified in various ways without departing from the spirit of the invention, and can be combined with features described in other embodiments to the extent possible. [Explanation of symbols]

[0036] 1. Single-crystal GaN (0001) substrate 2. GaN crystal 21 Base layer 22 Pitted layer 24 Planarization layer P1 Seed preparation process P2 HVPE process

Claims

1. A laminate comprising a single-crystal GaN (0001) substrate and a GaN crystal grown on the (0001) surface of the single-crystal GaN (0001) substrate, the GaN crystal includes a pitted layer and a planarization layer stacked on the pitted layer; the thickness of the pitted layer is 50 μm or more; the dislocation density at the surface of the planarization layer is less than half the dislocation density at the surface of the single-crystal GaN (0001) substrate; the dislocation density on the (0001) surface of the single-crystal GaN (0001) substrate is in the low 10 6 cm −2 range or less; When an observation area of ​​the surface of the single-crystal GaN (0001) substrate is measured at 100 μm×100 μm, the difference in dislocation density between the lowest and highest dislocation density locations is 4 times or less; The dislocation density on the surface of the planarizing layer is 8.5×10 ５ cm －２ The laminate is as follows:

2. The dislocation density on the (0001) surface of the single-crystal GaN (0001) substrate is 10 ６ cm －２ 10. The laminate of claim 1, which is a half-plate.

3. The pit density on the surface of the planarizing layer is 1 pit / cm ２ 3. The laminate according to claim 1 or 2, wherein:

4. 3. The stack according to claim 1 or 2, wherein the GaN crystal further includes a second pitted layer stacked on the planarization layer, and a second planarization layer stacked on the second pitted layer.

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