JP2008074671A - Manufacturing method of free-standing nitride substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a free-standing nitride substrate having little warpage or residual strain. <P>SOLUTION: A nitride buffer layer 130 comprising GaN etc. is grown on a ZnMgO buffer layer 120 (a). Through a HVPE method, a nitride layer 140 comprising GaN etc. is grown on the nitride buffer layer 130 at a low substrate temperature (700-900°C) (b). An ZnO film is lifted off in situ during the growth of the nitride layer 140 (b)-(c). A nitride thick film (100 μm or larger) 150 is formed through nitride growth employing the HVPE method at a high substrate temperature (900-1,500°C) (d). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a method for manufacturing a self-standing nitride substrate using a ZnO layer.

There are (1) HVPE method, (2) MOCVD method, (3) solution growth method using Na solvent, etc., and (4) solvothermal method. The solution growth method and the solvothermal method are still in the research stage because it is difficult to increase the diameter and the growth rate is slow. Also, the MOCVD method has a slow growth rate. At present, only the HVPE method (Hydride Vapor Phase Epitaxy) has succeeded in commercializing a group III nitride semiconductor substrate.

The manufacture of a nitride substrate by the HVPE method is generally performed by the following process.
(1) As a substrate, a sapphire substrate or a single crystal substrate such as a GaAs substrate, a SiC substrate, or a Si substrate is used.
(2) A buffer layer is formed on the substrate. As the buffer layer, a low-temperature GaN buffer layer, an AlN layer, or a metal buffer layer (Ti or the like) is used. These buffer layers are formed by HVPE method, MOCVD method (Metal Organic Chemical Vapor Deposition), MBE method (Molecular Beam Epitaxy).
(3) A high-temperature nitride thick film (about 400 microns thick) is grown by the HVPE method.
(4) The nitride thick film is separated from the substrate to obtain a free-standing nitride substrate.
Examples of the separation method include a laser lift-off (LLO) method, a method of dissolving a substrate with a chemical solution, and a method of scraping off the substrate by polishing.

  However, residual stress is generated in the nitride layer and the substrate due to lattice irregularities between the substrate and the nitride epitaxy layer grown thereon and differences in thermal expansion coefficients. For this reason, the occurrence of high-density dislocations, the presence of residual strain, the warpage of the substrate, the generation of cracks, etc. occur in the nitride film growth process or the temperature-falling process after growth. In addition, the process of separating the substrate, that is, the polishing and the LLO process also causes cracks due to the presence of residual strain.

In order to achieve the above-described problem, that is, separation that does not induce residual stress reduction or substrate damage, a method using a ZnO layer as a buffer has been proposed.
Advantages of ZnO buffer layer are (1) small lattice mismatch with GaN (2%)
(2) Since it is easily dissolved by a chemical solution, a separation process from a substrate by a chemical lift-off (CLO) method is easy. The following non-patent documents 1 to 4 and patent documents 1 to 6 are described as these techniques.

Non-Patent Document 2 and Non-Patent Document 4 describe that a ZnO buffer is formed on a sapphire substrate and a GaN layer is formed thereon by HVPE, but it is disclosed that the sapphire substrate is lifted off. Not.
Non-Patent Document 3 describes that a ZnO buffer is formed on a sapphire substrate and a GaN layer is grown on the HVPE. Here, after the temperature is lowered, a GaN free-standing substrate is obtained using a chemical solution (aqua regia). Sometimes, the GaN substrate may spontaneously peel off when the temperature drops after growth. However, the sample size is limited to 2 mm × 4 mm due to the occurrence of cracks. The occurrence of cracks suggests the presence of residual stress.

Patent Documents 2 and 3 describe that a ZnO buffer layer and a low-temperature GaInN buffer layer are stacked on a sapphire substrate, and are structurally similar. The techniques described are as follows.
(1) Laminate ZnO by sputtering.
(2) The buffer layer growth step includes a step of etching ZnO using HCl gas.
(3) A GaInN layer is used as the protective layer of the ZnO layer, and the growth temperature is 1000 ^OC (Patent Document 2). Further, baking in a hydrogen atmosphere 1050 ^O C to grow immediately before (Patent Document 3).
However, the ZnO film produced by the sputtering method generally has poor crystallinity, and the polarity is not controlled and domains having different polarities are mixed. Therefore, the diffusion of Zn into the GaInN layer through the ZnO / GaInN interface and the diffusion of Ga and In into ZnO are large, and the yield for obtaining good quality crystals is not good.
Further, ZnO film, there is a possibility that are etched before forming the growth temperature (1000 ^O C) and high temperature in a hydrogen atmosphere (1050 ^O C) in GaN of GaInN layer.

  Patent Documents 1, 4 and 5 describe a technique for removing a ZnO buffer layer by etching after forming a GaN film, and do not include lift-off in the growth process. It is difficult to obtain a large area size because cracks occur during chemical lift-off.

A method using a ZnO substrate instead of using a ZnO buffer layer between the substrate and the nitride epitaxy layer has also been proposed (see Patent Documents 1, 8, and 9). However, it has not been put into practical use because (1) ZnO substrates are expensive and (2) large-diameter ZnO substrates are not commercial (currently 10 mm × 10 mm).
In particular, Patent Document 1 relates to a technique for growing a GaN thick film on a ZnO substrate, and removes the ZnO substrate when forming a GaN high-temperature layer after forming the GaN buffer layer. However, using a ZnO substrate as described above is costly. Further, when ZnO is removed after the formation of the GaN buffer layer, the residual stress is not sufficiently reduced and finally residual strain exists, which causes problems such as warpage and cracks.

  Patent Documents 6 and 7 are technically related in that a single-crystal ZnO buffer layer with controlled polarity is formed on a sapphire substrate.

Shulin Gu, Rong Zhang, Jingxi Sun, Ling Zhang, and TF Kuech: “Role of interfacial compound formation associated with the use of ZnO buffers layers in the hydride vapor phase epitaxy of GaN” Appl. Phys. Lett. 76, 3454 (2000 ) X. W. Sun, R. F. Xiao, H. S. Kwoka: “Epitaxial growth of GaN thin film on sapphire with a thin ZnO buffer layer by liquid target pulsed laser deposition” J. Appl. Phys. 84, 5776 (1998). T. Detchprohm, K. Hiramatsu, H. Amano and 1. Akasaki: “Hydride vapor phase epitaxial growth of a high quality GaM film using a ZnO buffer layer” Appl. Phys. Lett. 2688 Appl. Phys. Lett. 61 (22 ), 30 November 1992 T. Ueda, TF Huang, S. Spruytte, H. Lee, M. Yuri, K. Itoh, T. Baba, JS Harris Jr., “Vapor phase epitaxy growth of GaN on pulsed laser deposited ZnO buffer layer” J. Cryst Growth 187, 340 (1998).

JP 2005-127713 A (Applicant: Samsung Electric Co., Ltd.) JP 2003-37069 A (Applicant: Toyoda Gosei Co., Ltd.) Japanese Patent No. 3744155 (Applicant: Toyoda Gosei Co., Ltd.) JP 2003-31501 A (Applicant: Fuji Photo Film Co., Ltd.) JP 2003-277196 A (Applicant: Hitachi Cable) Japanese Patent No. 3700786 (Applicant: Tohoku University) Japanese Patent No. 3595829 (Applicant: Tohoku University) Japanese Patent Laying-Open No. 2005-67988 (Applicant: Material Organization + Tokyo Radio Co., Ltd.) Japanese Patent Laying-Open No. 2005-174951 (Applicant: Tokyo Radio Co., Ltd.)

The problem of group III nitride compound semiconductor growth is
(1) The substrate on which the semiconductor is grown, the presence of residual strain due to the difference in thermal expansion coefficient of the semiconductor, and the generation of warpage and cracks induced thereby (2) The process of separating the semiconductor from the substrate (lift-off ·process)
It is. The former is an obstacle to a microfabrication process such as lithography when a semiconductor device is manufactured using the semiconductor. As for the latter, the current technology has two methods: (A) laser lift-off and (B) polishing and extinguishing the substrate, both of which are high in process cost, take time, and have a poor yield.
An object of the present invention is to provide a method for solving the above two problems at once.

In the present invention, a free-standing nitride substrate is manufactured by the following process.
(1) A Zn _1-x Mg _x O layer (0 ≦ x ≦ 0.3) whose polarity is controlled is formed on the substrate.
(2) A nitride buffer layer is formed by a growth method such as the MBE method. The nitride buffer layer acts as _{Zn 1-x} Mg x O protective layer and _{Zn 1-x} Mg x O layer / nitride layer mutual diffusion suppressing film at the interface. The nitride buffer _{layer, GaN, AlN, Al x Ga} 1-x N (0 ≦ x ≦ 1), Al x Ga 1-x-y In y N (0 ≦ x ≦ 1,0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1) or a combination thereof. The nitride buffer layer is preferably cation polarity (Ga polarity in the case of GaN).
(3) A nitride layer (for example, a GaN layer) is grown by HVPE (Hydride Vapor Phase Epitaxy).
Before starting the growth, when the temperature of the substrate is raised to a predetermined temperature for growth by the HVPE method, the temperature may be raised to a predetermined nitride layer growth temperature in an inert gas such as nitrogen gas or argon gas.
By introducing HCl and NH ₃ into the growth furnace, a nitride layer is grown by the HVPE method. During the growth of the nitride layer, the ZnO buffer layer is etched by a reaction with a gas such as HCl or NH ₃ to lift off the nitride layer. The temperature of the nitride layer is preferably 700 to 900 ° C. in the case of GaN. The thickness of the nitride layer is preferably 80 microns or more.
(4) A thick nitride film is grown by HVPE at a higher temperature than in the previous step. In the step (3), since the nitride layer has already been separated from the substrate, the nitride thick film grown by the relatively high temperature HVPE method is homoepitaxially grown on the nitride layer, and the residual A self-supporting nitride substrate with little distortion and therefore very low warpage can be produced.
The free-standing nitride substrate that can be produced is GaN, Al _x Ga _1-x N (0 ≦ x ≦ 1), AlN, Al _x Ga _1-xy In _y N (0 ≦ x ≦ 1, 0 ≦ y ≦ 0). .3, 1 ≦ x + y ≦ 1) is possible.

By the manufacturing method of the present invention described above,
(1) Almost no residual strain,
(2) Very small warpage (for example, a curvature radius of 10 m or more),
(3) No cracks
(4) Substrate manufacturing process (lift-off process and polishing) has been greatly reduced.
A nitride semiconductor substrate can be grown.

In the present invention, a nitride film such as GaN is formed on a Zn _1-x Mg _x O layer (0 ≦ x ≦ 0.3) formed on a substrate by the HVPE method (Hydride Vapor Phase Epitaxy). In this process, a technique for obtaining a nitride thick film free from residual strain is provided by a lift-off technique in which a ZnO film is controlled by chemical etching using in-situ (growth place).
With this technique, a nitride (eg, GaN) substrate free from warpage and cracks due to thermal expansion strain can be produced. A nitride substrate that is currently about 3 inches long can be made into nitrides with no warp at 4 inches or more. Provide substrate fabrication technology.

The method of an embodiment of the present invention is schematically illustrated in FIG.
(1) A ZnO layer 120 that is easily removed chemically is formed on the substrate 110 (see FIG. 1A). The polarity of the ZnO layer 120 may be either Zn polarity (cation polarity) or O polarity (anion polarity). In order to control the polarity, for example, an MgO buffer layer is first grown on the substrate. As a result, it is known that when the thickness of the MgO layer is 3 nm or less, the ZnO film grown on the MgO layer becomes O-polarity when the thickness is 3 nm or more. The film thickness of the ZnO layer may be 100 nm to 100 μm.
For the formation of these layers, a growth method capable of growing a high-quality ZnO layer with controlled polarity, for example, MBE (Molecular Beam Epitaxy) can be used.
The substrate 110 may be either a sapphire substrate for which polarity control technology has been established, or an SiC substrate that already has polarity (ie, Si polarity plane, C polarity plane) and has the same Wurtz structure as ZnO.
A Zn _1-x Mg _x O layer (0 ≦ x ≦ 0.3) may be used instead of the ZnO layer.
ZnO has a wurtzite structure, and MgO has a rock salt structure. Because the crystal structure is different, the epitaxial film has a wurtzite structure by growing a ZnO buffer layer on the sapphire substrate until the composition of the Zn _1-x Mg _x O layer on it is about x = 0.5. Can be formed. However, in order to produce a Zn _1-x Mg _x O layer having good crystallinity with polarity control, x is limited to 0.3.

(2) A nitride buffer layer 130 such as GaN is grown at a low temperature (for example, 1000 ° C. or less) on the ZnO buffer layer 120 with controlled polarity so as to have a cation polarity (see FIG. 1A). . The nitride buffer layer 130 of GaN or the like is formed to serve as a protective film for the ZnO layer 120 and an interdiffusion suppression film at the ZnO / nitride interface.
Incidentally, for example, in the case of GaN is ^{600 O C~900} O C, which is the case of AlN 1000 ^O C.
In order to obtain a positive polarity (Ga polarity in the case of GaN), for example, the Zn-polar ZnO buffer layer 120 is irradiated with Zn immediately before the growth, and then simultaneously irradiated with Ga molecular beam and N plasma. Thus, a cation-polar nitride (Ga-polar GaN) grows. In the case of an O-polar ZnO buffer, for example, a Zn ₃ N ₂ crystal layer having an inversion symmetry center is formed by irradiating N plasma, and the GaN buffer layer 130 grown thereon becomes Ga-polar. Of course, Mg ₃ N ₂ having a reversal symmetry center similarly becomes Ga polarity even if a crystal layer is formed. That is, by growing a crystal layer having an inversion symmetry center on the O-polar ZnO layer 120, the GaN buffer layer 130 thereon becomes Ga-polar. The thickness of the nitride buffer layer 130 is preferably 100 nm to 10 μm.
In the above description, the nitride buffer layer 130 has been described as an example of a GaN buffer layer. However, Al _x Ga _1-x N (0 ≦ x ≦), which is a nitride semiconductor having a lattice constant close to the crystal structure similar to that of GaN. 1) A buffer layer is formed of AlN, Al _x Ga _1-xy In _y N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0.7 ≦ x + y ≦ 1) single crystal thin film or a combination thereof. May be.
For the formation of these nitride buffer layers, for example, the MBE method (Molecular Beam Epitaxy) may be used similarly to the ZnO layer 120 described above.
The reason why the MBE method is used so far is that low temperature growth and polarity control are indispensable to finally obtain a nitride buffer layer with controlled polarity while minimizing ZnO / GaN interface diffusion. is there. In addition, as described later, since the nitride layer is first formed at a relatively low temperature by using the HVPE method after the nitride buffer layer is formed, the temperature control is easily performed. .

(3) In the initial state of the HVPE method (Hydride Vapor Phase Epitaxy), the temperature is raised to a predetermined temperature in an inert gas atmosphere such as N ₂ or argon gas, and then NH ₃ and HCl are introduced. Then, a nitride layer 140 such as GaN and a substrate temperature relatively lower than the normal crystal growth temperature (eg, 700 ° C. to 900 ° C.) is grown on the nitride-attached buffer layer 130 (FIG. 1B). Here, when the nitride layer is GaN, when the temperature is 900 ° C. or higher, the nitride is thermally decomposed when NH ₃ is low, and when the temperature is 700 ° C. or lower, the growth rate of the nitride is extremely reduced. It is.
In the case of AlN, since the reaction of AlCl ₃ and NH ₃ is used as the elementary reaction process on the substrate surface, the nitride layer 140 can be formed at 1000 ° C. to 1100 ° C. Note that the reaction of GaCl and NH ₃ is used for the elementary reaction process in the substrate during the growth of GaN.
The nitride layer 140 is preferably 80 to 200 microns thick. The ZnO film 120 is chemically etched with NH ₃ and HCl during the growth of the nitride layer 140 and lifts off in situ (FIGS. 1B to 1C).
Here, as the nitride layer 140, other than GaN, Al _x Ga _1-x N (0 ≦ x ≦ 1), AlN, which can be grown by HVPE using HCl and NH ₃ like GaN. And Al _x Ga _1-xy In _y N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1).
If the nitride layer has a thickness of 80 μm or more, a thick nitride film without cracks can be grown by the process (4) described below.
(4) Next, by performing nitride growth by the HVPE method at a higher substrate temperature (for example, 900 ° C. to 1500 ° C.) than in the previous stage, a free-standing nitride thick film (100 μm or more) 150 can be manufactured. A nitride thick film is grown at a higher temperature than the previous stage by the HVPE method. The growth temperature is 1000 ° to 1100 ° C. for GaN and 1100 ° to 1500 ° C. for AlN. This thick nitride film 150 is of high quality because it grows at a relatively high temperature, and has almost no residual strain due to in-situ lift-off. Further, warpage is extremely small and cracks are small (curvature radius of 10 m or more) (see FIG. 1D).

In the prior art, residual strain due to the difference in thermal expansion coefficient, and further, warpage of the substrate could not be avoided. On the other hand, in the present invention, (A) the step of forming the nitride layer 140 while lifting off the ZnO buffer layer 120, and (B) the thickness of the nitride thick film layer 150 on the nitride layer 140 after the lift-off is performed. A step of growing the film. For this reason, the nitride thick film layer 150 grown at a high temperature essentially undergoes homoepi growth, and therefore, distortion and warping due to differences in thermal expansion coefficients that cause residual strain do not occur. There are no cracks. That is, residual strain occurs during the growth of the high temperature nitride buffer layer 140, but the ZnO buffer layer is etched and the growth of the nitride thick film 150 thereon is homoepitaxy, which is caused by a difference in thermal expansion coefficient. Residual strain is small and a nitride layer with low warpage can be formed by high-temperature growth of HVPE.
Since there are no warping and cracks as described above, almost no steps are required after the nitride thick film 150 is manufactured, and the yield of the substrate manufacturing is greatly increased and the manufacturing cost is also increased to simplify the substrate manufacturing process. Therefore, an inexpensive nitride semiconductor substrate can be supplied.

  The manufacturing method of the free-standing GaN substrate of the example and the produced free-standing GaN substrate will be described in detail with reference to FIGS. FIG. 2 is a diagram of the process of the embodiment, and FIG. 3 is a graph showing the relationship between temperature and time in HVPE growth. FIG. 4 is an SEM photograph of the surface and cross section of the GaN film in each step of FIG. FIG. 5 is a photograph showing the surface morphology of a free-standing GaN substrate by an atomic force microscope (AFM). FIG. 6 shows the result of X-ray diffraction. FIG. 7 is a graph of a photoluminescence (PL) spectrum. FIG. 8 shows the result of evaluating the side surface of the self-standing GaN substrate with the microscopic PL. FIG. 9 is a diagram showing (002) reciprocal lattice space mapping of an in-situ lift-off free-standing GaN substrate.

FIG. 2 shows a process for making a free-standing GaN substrate. Components corresponding to those in FIG. 1 are denoted by the same reference numerals.
First, a 200 nm thick ZnO thin film 120 with controlled polarity was grown on a sapphire substrate 110 by an MBE method using oxygen plasma. Growth temperature was 400~1100 ^O C. Here, it grew using MgO buffer. When the thickness of the MgO buffer layer is 3 nm or less, O-polar ZnO grows, and when it is 3 nm or more, Zn-polar ZnO grows.
Next, a Ga-polar GaN buffer layer 130 having a thickness of 1 μm was stacked on the ZnO film 120 by a plasma-assisted MBE method (see FIG. 2A). Growth temperature was 600~900 ^O C.

Then, by HVPE GaN layers 140 to the GaN / ZnO / sapphire templates on site grown at 700 to 900 ^O C. The relationship between temperature and time at this time is shown in FIG. In the example of FIG. 3, it grows at 900 ^OC . In HVPE growth, HCl gas is flowed to react with metal gallium to generate GaCl. This GaCl gas is transported to the substrate and reacts with NH ₃ to grow GaN on the substrate. The ZnO layer is etched by NH ₃ or unreacted HCl gas (see FIG. 2B).
Regarding the flow rate during growth, hydrogen was 1-10 liter / min, NH ₃ was 0.3-5 liter / min, and HCl was 10-200 cc / min. The growth rate was 10-200 μm / hr.
Desirably, hydrogen growth temperature is 850 to 900 ^O C is 2.5-4 liters / min, NH ₃ is 0.5-2 liters / min, HCl was 20-100cc / min.

Here, the initial temperature of HVPE growth is 700 ° C to 900 ° C. This is because at 900 ° C. or higher, GaN is thermally decomposed in a situation where NH ₃ is low. Moreover, it is because the growth rate of GaN extremely decreases below 700 ° C. The temperature was raised to a predetermined temperature (900 ° C.) under an N ₂ atmosphere.
The flow rate of nitrogen when raising the temperature from room temperature to 900 ° C. was 1-15 liters / min. 2-10 liter / min is desirable.
Next, NH ₃ and HCl are introduced to grow a GaN layer.
The in-situ etching rate shown in FIG. 2B is determined by the substrate temperature, NH ₃ flow rate, and HCl gas flow in the first stage.
Under the desirable conditions described above, the etching rate of ZnO is approximately 1-10 μm / sec.
The GaN high temperature buffer layer 140 grown in the first stage was grown at 900 ° C. and had a thickness of 100 microns or more. During this growth process, the ZnO layer between the sapphire substrate and the GaN layer was completely etched as schematically shown in FIG.
In In-situ lift-off, GaN layer growth and ZnO etching occur simultaneously, so the growth temperature in the first stage and the NH ₃ and HCl gas flow rates are extremely important. Since the etching of the ZnO buffer starts from the end face of the sample, it is important that the GaN layer is sufficiently thick. Therefore, the growth film thickness of the GaN high-temperature buffer layer in the first stage needs to be 80 μm or more, preferably 100 μm or more.

The second stage GaN thick film growth was carried out at 1040 ° C. on the above-mentioned in-situ lift-off GaN layer (grown at 900 ° C.) (FIG. 2D). In the process of increasing the temperature from 900 ° C. to 1040 ° C., the supply of HCl gas is stopped. In this growth at 1040 ° C., a high quality GaN thick film can be grown.
Regarding gas flow rates during growth, hydrogen was 1-10 liter / min, NH ₃ was 0.3-5 liter / min, and HCl was 10-200 cc / min. The growth rate was 10-200 μm / hr. Desirably, hydrogen was 2.5-4 liter / min, NH3 was 0.5-2 liter / min, and HCl was 20-100 cc / min.

  The GaN / ZnO heterostructure (GaN layer 130 / ZnO120 in FIG. 2) was grown on the c-plane sapphire substrate 110 by the plasma-assisted MBE method. GaN layers 140 and 150 are grown on the ZnO / GaN template by the HVPE method. GaN having a thickness of 250 microns or more is grown on this template by a two-step growth method.

  FIG. 4 is a SEM photograph of GaN at each growth step. FIGS. 4A, 4B, and 4C are a surface and cross-sectional SEM photographs of the GaN layer 140 grown at 900 ° C., respectively. 4D, 4E, and 4F are SEM photographs of the surface, cross section, and back surface of the free-standing GaN thick film grown at 1040 ° C. on the GaN layer grown at 900 ° C. FIG.

  In FIG. 4 (a), grains of several microns appear, but as shown in FIG. 4 (c), atomic layer steps and terraces are observed in each grain. A free-standing GaN substrate can be grown without causing cracks. This is because the residual strain was greatly reduced because the ZnO buffer layer 120 was etched during the growth of the GaN layer 140 at 900 ° C.

  The surface of the 250-micron-thick freestanding GaN substrate formed by the two-step growth is a mirror surface (FIG. 4D). In FIG. 4E, the interface between the 900 ° C. grown GaN layer 140 and the 1040 ° C. grown GaN thick film 150 is observed. As shown in FIG. 4F, cracks are scattered on the back surface, but no cracks are observed on the surface. This is because cracks are generated at the ZnO / GaN interface with the progress of etching of the ZnO layer 120 performed simultaneously with the growth of the 900 ° C. GaN layer 140, but the cracks are filled by the growth and lateral growth of the GaN layer 140. Since cracks generated on the back surface disappear during the formation of the GaN layer 140, no cracks are observed on the front surface side of the GaN layer 150 due to high temperature growth.

  Etching of the ZnO buffer layer 120 starts from the end face of the sample. As a result, the crack density in the vicinity of the end face on the back surface is higher than that in the central portion. Although cracks occur in the first stage growth at 900 ° C., it is confirmed by the cross-sectional SEM observation that the cracks disappear as the growth proceeds. That is, as shown in FIG. 4E, no cracks penetrating the self-standing GaN substrate are observed.

  FIGS. 5A and 5B are surface morphologies of a free-standing GaN substrate observed with an atomic force microscope (AFM) (a: 5 μm × 5 μm; b: 0.5 μm × 0.5 μm size). The surface shows a flat morphology at the atomic level, and the rms roughness is 0.46 nm in FIG. 5A and 0.11 nm in FIG. 5B. In FIG. 5B, steps and terraces at the atomic level are observed. Although the 900 ° C. growth GaN layer 140 has a rough surface, it can be seen that the surface flatness is restored by the GaN thick film 150 grown at 1040 ° C.

  FIG. 6 shows the result of measuring the X-ray diffraction by 2θ-ω scan and evaluating the lattice constant of the self-standing GaN substrate. The a-axis lattice constant is obtained from (10-10) diffraction shown in FIG. 6 (a), and the c-axis lattice constant is obtained from (0002), (0004), (0006) diffraction shown in FIG. 6 (b). It was. From (0002) diffraction, a lattice constant having a thickness of about 10 μm in the vicinity of the surface is obtained. In (0004) diffraction, a lattice constant of 20 μm from the surface is obtained, and in (0006) diffraction, a lattice constant of 30 μm is obtained from the surface. As shown in FIGS. 6A and 6B, the measured lattice constant was 3.189 mm on the a axis and 5.185 mm on the c axis. These values are the same as the values of the free-standing GaN substrate reported so far, and show that the free-standing GaN substrates in the examples are completely strain-relaxed.

A photoluminescence (PL) spectrum at 10K is shown in FIG. The growth surface (surface) of the self-standing GaN substrate was measured. The donor-bound exciton (D ⁰ X) is dominant at 3.472 eV. A donor-acceptor pair (DAP) emission is observed at 3.265 eV. The emission of 3.172 eV is one phonon sideband of DAP emission. Light emission from a deep level is not observed at 2.0 eV to 3.0 eV. FIG. 7B shows details of the emission spectrum in the vicinity of the band edge in FIG. As shown in FIG. 7B, D ⁰ X emission appears at 3.4715 eV, and free exciton emission (FX _A ) is observed at 3.44791 eV. The full width at half maximum is sharp at 4.8 meV and 2.5 meV, respectively, indicating that the residual strain is small. The position of the D ⁰ X emission line is the energy of D ⁰ X emission of bulk GaN (3.4712 eV), the position of the D ⁰ X emission line from the homoepitaxy GaN layer (3.4709 eV and 3.4718 eV), and a self-standing GaN substrate. And the emission line (3.4720 eV, 3.4710 eV ± 2 meV). X-ray diffraction and PL measurement results show that there is almost no residual strain in the GaN thick film layer.

FIG. 8 is a diagram showing the result of evaluating the side surface of the self-standing GaN substrate by microscopic PL at 77K. The definition of the position observed with the microscopic PL is shown in the inset in the graph. As shown in the drawing, the thickness in the growth direction is shown with the interface between the MBE grown GaN layer 130 and the ZnO layer 120 (that is, the back surface of the freestanding GaN substrate) as the origin, and the outermost surface is 250 μm. Micro PL measurement was performed from the 50 μm position to 170 μm in 10 μm increments. The position resolution of the microscopic PL is 1 μm or less. The vicinity of the interface between the 900 ° C. GaN layer 140 and the 1040 ° C. grown GaN thick film layer 150 was measured in steps of several microns. An area of several tens of μm from the front surface or the back surface is not measured because damage when the sample is cut is significant. As shown in FIG. 8, the PL spectra of the upper half layer of the GaN thick film layer 150 grown at 1040 ° C. are A free excitons (FX _A : 3.4863 eV) and B free excitons (FX _B : 3.4815 eV). Donor bound excitons (D ⁰ X: 3.4705 eV) are dominant. The emission energy at 77 K of bulk GaN A excitons is reported to be 3.475 eV. This value almost coincides with the measured value, and it can be seen that the lattice strain is substantially relaxed in the upper half of the GaN layer 150 grown at 1040 ° C. On the other hand, in the lower half, a broad emission around 3.453 eV, which is considered to be a bound exciton (A ⁰ X) of the Zn acceptor, is dominant. Zn is thought to be due to diffusion from ZnO, but this Zn acceptor exciton emission is hardly observed at 120 microns from the interface, and there is no influence of Zn diffusion at 120 microns from the back surface (interface with ZnO). . That is, the influence of Zn diffusion is observed in the 900 ° C. GaN layer 140 with poor crystallinity, but the Zn diffusion is suppressed in the 1040 ° C. grown GaN thick film layer 150 with good crystallinity.

FIG. 9 is a (0002) reciprocal space mapping of an in-situ lift-off free-standing GaN substrate. In FIG. 9, a heavy peak is observed for the omega scan, which indicates that the crystal includes a domain whose c-axis is inclined by a minute angle. This is because a micro crack is generated in the initial stage of growth, and a domain tilted by a micro angle is formed when the crack is filled by lateral growth. The X-ray diffraction half-width is 300-450 arcsec in the omega scan, and there is an influence of the finely tilted domain near the surface, but it is 50 arcsec in the ω-2θ scan, and a high-quality crystal with very little lattice distortion fluctuation is obtained. You can see that it is growing.
In the above-described embodiment, the method for manufacturing the self-standing GaN substrate has been described. However, if the expert in this field, this method may be used for other nitrides (Al _x Ga _1-x N (0 ≦ x ≦ 1). ), AlN, Al _x Ga _1-xy In _y N (0 ≦ x ≦ 1, 0 ≦ y ≦ 0.3, 0 ≦ x + y ≦ 1).
In the above example, a ZnO layer with a controlled polarity is used. More broadly, a Zn _1-x Mg _x O layer (0 ≦ x ≦ 0.3) with a controlled polarity is used instead of the ZnO layer. May be.

It is a figure which shows the outline of the manufacturing method of the self-supporting GaN substrate of embodiment of this invention. It is a figure which shows the manufacturing method of the self-supporting GaN substrate in an Example. It is a figure which shows the relationship between the temperature and time of the growth of a GaN layer by HVPE method, and a lift-off process. 3 is a SEM photograph of the surface and cross section of a GaN layer in each step of FIG. 2. It is a photograph which shows the surface morphology of the self-standing GaN substrate by an atomic force microscope (AFM). It is a figure which shows the result of having carried out the X-ray diffraction of the self-supporting GaN substrate. It is a graph of the photoluminescence (PL) spectrum of a self-standing GaN substrate. It is a figure which shows the result of having evaluated the side surface of the self-standing GaN substrate by microscopic PL. It is a figure which shows the (0002) reciprocal lattice space mapping of the self-standing GaN substrate which carried out the in-situ lift-off.

Claims (6)

A Zn _1-x Mg _x O layer (0 ≦ x ≦ 0.3) is formed on the substrate by controlling the polarity,
A nitride buffer layer is formed on the Zn _1-x Mg _x O layer (0 ≦ x ≦ 0.3) so as to have a cation polarity,
Next, a nitride layer is grown by the HVPE method, and the Zn _1-x Mg _x O layer (0 ≦ x ≦ 0.3) layer is etched by a gas chemical reaction to lift off the nitride layer,
A method for producing a self-supporting nitride substrate, wherein a thick nitride film is grown by an HVPE method on the nitride layer grown by HVPE by raising the growth temperature from the time of growth of the HVPE grown nitride layer. In the manufacturing method of the self-supporting nitride substrate according to claim 1,
A method for producing a self-supporting nitride substrate, wherein the substrate temperature is raised in an inert gas when the substrate temperature is raised from room temperature to a predetermined temperature before forming the nitride layer by HVPE. In the manufacturing method of the self-supporting nitride substrate according to claim 1 or 2,
Nitrides of the nitride buffer _{layer, GaN, AlN, Al x Ga} 1-x N (0 ≦ x ≦ 1) or _{Al x Ga 1-x-y} In y N (0 ≦ x ≦ 1,0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1) or a combination thereof, A method for producing a self-standing nitride substrate. In the manufacturing method of the self-supporting nitride board | substrate in any one of Claims 1-3,
The nitride layer and nitride thick _{film, GaN, AlN, Al x Ga} 1-x N (0 ≦ x ≦ 1), or _{Al x Ga 1-x-y} In y N (0 ≦ x ≦ 1,0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1). A method for producing a self-standing nitride substrate, wherein 5. The method for manufacturing a self-standing nitride substrate according to claim 1, wherein the nitride layer has a thickness of 80 microns or more. On the substrate, the polarity is controlled by the MBE method to form a ZnO layer,
On the ZnO layer, a GaN buffer layer is formed so as to be Ga polarity by MBE method,
A GaN layer is grown on the GaN buffer layer at 700 ° C. to 900 ° C. by HVPE, and the ZnO layer is etched by a chemical reaction using a gas to lift off the GaN layer.
A method for producing a self-standing GaN substrate, wherein a GaN thick film is grown on the GaN layer at a growth temperature higher than that during the growth of the GaN layer by HVPE.