JP4672753B2 - GaN-based nitride semiconductor free-standing substrate manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing a GaN-based nitride semiconductor free-standing substrate.

  In order to realize blue, ultraviolet high-power laser diodes, high-brightness LEDs, and high-power electronic devices, the performance of elements based on nitride semiconductors is increasing. Currently, sapphire substrates are mainly used as crystal growth substrates in the production of GaN-based optical or electronic devices.

However, in the case of a sapphire substrate, the dislocation density is about 10 ¹⁰ / cm ^{2 due} to a large lattice mismatch and a difference in thermal expansion coefficient between the substrate and the GaN-based nitride semiconductor. This large defect density causes problems such as deterioration of device characteristics.
Various buffer layers were used to reduce the dislocation density, and low-defect thin film growth became possible using selective growth such as LEO (lateral epitaxial overgrowth) and PENDEO epitaxy and lateral growth techniques.

  However, such a growth technique causes a production unit cost to increase because a substrate manufacturing process by a large number of processes is necessary before the growth, and there is a problem in reproducibility and yield. Research on a GaN substrate is underway in search of measures to solve the problem of crystal defects from the substrate. In order to ensure high performance and reliability of the element, it is essential to realize a high-quality single crystal film.

  In reality, large-diameter GaN bulk growth is impossible. Instead, a GaN-based nitride semiconductor free-standing substrate using the HVPE (Hydride Vapor Phase Epitaxy) method with a high growth rate of several hundred microns per hour has been put into practical use. Yes. In this method, after a thick GaN-based nitride semiconductor having a thickness of 300 microns or more is grown on a sapphire substrate, the thick GaN-based nitride semiconductor is separated from the sapphire substrate.

  In the case of a blue-violet laser diode that is very sensitive to the defect density of the thin film, devices are practically produced on such a GaN-based nitride semiconductor free-standing substrate. In the future, even in the case of high-intensity LEDs for illumination having a huge market, in order to operate the same element as the laser diode, a high current density and element reliability are preferentially required.

  Various methods have been proposed to realize a high-quality GaN-based nitride semiconductor free-standing substrate. There are two technologies that are important for realizing a free-standing substrate. These include a thick-film GaN-based nitride semiconductor free-standing substrate growth technology and a post-growth thick-film GaN-based nitride semiconductor on a sapphire substrate. Lift-off process technology that separates from Even if the thick film growth without cracks and warpage is successful on the sapphire substrate, a huge stress exists at the interface between the sapphire substrate and the thick film GaN-based nitride semiconductor.

  When the sapphire substrate is removed using a general mechanical polishing method, cracks are likely to occur inside the crystal of the thick GaN-based nitride semiconductor due to the relaxation of the stress present at the interface. In addition, after removing the sapphire substrate, there is also a problem in processes such as surface polishing.

  For this reason, various lift-off techniques for separating the sapphire substrate have been proposed. As typical separation techniques, laser lift-off technique, VAS (Void Assisted Separation), and etching technique are known.

  Laser lift-off technology separates a substrate by absorption from an interface caused by high-power laser irradiation, but it is difficult to produce a large-diameter self-standing substrate. The VAS technique is based on the natural peeling phenomenon, but requires complicated processes such as preparation of a GaN template and vapor deposition of a metal layer.

Although there is a method of growing thick GaN on a GaAs substrate and removing the GaAs substrate by chemical etching, there are problems such as decomposition of the GaAs substrate and interdiffusion at the interface during high temperature growth, and special growth techniques are required. It is. Due to the complexity of the manufacturing process and the problem of yield, it is not used for general purposes.
In summary, the conventional GaN-based free-standing substrate has a large problem of low yield and high cost due to a complicated process.
Japanese Patent Laid-Open No. 2005-1928 Japanese Patent Laid-Open No. 2005-119921 JP 2006-173147 A JP-T-2006-527480 JP 2008-74671 A

  An object of the present invention is to provide an inexpensive and stress-free method for manufacturing a GaN-based nitride semiconductor free-standing substrate by a simple process.

(1) forming a step of preparing a substrate, forming a GaN dot and NH 4 _Cl layer on the substrate, a low temperature buffer layer made of a group III-V nitride semiconductor on the GaN dots and NH 4 _Cl layer A step of forming a GaN-based nitride semiconductor layer on the low-temperature buffer layer, and a step of naturally peeling the GaN-based nitride semiconductor layer from the substrate by returning the substrate temperature to room temperature. A method for manufacturing a semiconductor free-standing substrate.
(2) The method for producing a GaN-based nitride semiconductor free-standing substrate according to (1), wherein the substrate is a sapphire substrate.
(3) including a step of nitriding the substrate surface and locally forming AlN _X O _1-X (0 <X ≦ 1) before the step of forming the GaN dots and the NH ₄ Cl layer on the substrate. (2) The manufacturing method of the GaN-based nitride semiconductor free-standing substrate according to (2).
(4) the step of forming the GaN dots and _NH 4 Cl layer is characterized by a step of forming a GaN dot and _NH 4 Cl layer by flow of HCl and NH ₃ in Ga gas atmosphere ( 1) A method for manufacturing a GaN-based nitride semiconductor free-standing substrate according to any one of (3).
(5) The method for producing a GaN-based nitride semiconductor free-standing substrate according to any one of (1) to (4), wherein the GaN dots are aligned in the c-axis direction.
(6) The low-temperature buffer layer is a layer containing GaN, AlN, InN, BN, or any of these mixed crystal semiconductors as a constituent material. A method for producing a GaN-based nitride semiconductor free-standing substrate according to claim 1.
(7) The method for manufacturing a GaN-based nitride semiconductor free-standing substrate according to any one of (1) to (6), wherein the low-temperature buffer layer is formed at a temperature of 400 ° C. or higher and 800 ° C. or lower.
(8) The method for manufacturing a GaN-based nitride semiconductor free-standing substrate according to any one of (1) to (7), wherein each of the above steps is continuously performed in a single HVPE apparatus.

According to the present invention, an inexpensive and stress-free GaN free-standing substrate can be manufactured by using the NH ₄ Cl layer.

Regarding the present invention, a method for producing a GaN free-standing substrate using the HVPE method is illustrated as an example.
In this embodiment, the following steps 1) to 5) are basically used.
1) High-temperature nitridation process of sapphire substrate 2) NH ₄ Cl layer and GaN dot formation process 3) Low-temperature GaN buffer layer growth process 4) High-temperature thick film GaN layer formation process 5) GaN free-standing substrate self-lift-off process and this implementation In the example, a c-plane-grown GaN free-standing substrate is obtained through the steps 1) to 5).
Hereinafter, the steps 1) to 5) will be described in detail.

1) High-temperature nitriding step of sapphire substrate This step is a step of forming AlN _X O _1-X (0 <X ≦ 1) locally on the surface of the sapphire substrate by high-temperature nitriding treatment of the sapphire substrate in the HVPE reactor. is there.
FIG. 1 shows a schematic view of AlN _X O _1-X (0 <X ≦ 1) in which an sapphire substrate is locally formed by high-temperature nitriding in an HVPE reactor in an NH ₃ gas atmosphere. AlN _X O _1-X serves as a seed layer for GaN dot formation performed in the next step.

The NH ₃ flow rate was 1 L / min, the substrate temperature was 1080 ° C., the nitriding time was 30 minutes, and the carrier gas was 2.51 L / min hydrogen. In order to form a uniform AlN _X O _1-X (0 <X ≦ 1) layer on the sapphire substrate, it is necessary to optimize nitriding conditions. The surface density of the GaN dots is determined by the nitriding conditions in this step, and finally has a close relationship with the natural self-lift-off conditions.

2) NH ₄ Cl layer and GaN dot formation step This step is a step of forming an NH ₄ Cl layer and a GaN dot on the sapphire substrate nitrided in step 1). FIG. 2 shows the formed GaN dots and the NH ₄ Cl layer. This step is performed continuously to the nitriding step of step 1). The temperature of the substrate was lowered from 1080 ° C. to 500 ° C., and the temperature of the Ga boat was set to 450 ° C.

Such a temperature profile is for maintaining a stable gas flow in the HVPE reactor. Most of the HCl gas passing through the Ga boat reacts with NH ₃ gas on the surface of the sapphire substrate in a pure HCl gas state without reacting with Ga to form an NH ₄ Cl layer. However, some HCl gas reacts with a small amount of Ga to form GaCl, and further reacts with NH ₃ gas on the surface of the sapphire substrate to form GaN. GaN dots are selectively formed on the AlN _X O _1-X (0 <X ≦ 1) layer locally formed on the sapphire substrate by the high temperature nitriding process. The surface density of GaN dots is determined by the nitriding conditions in step 1).

In the present invention, the surface density of the GaN dots is 2 to 4 × 10 ⁹ cm ⁻² . The ratio of the NH ₄ Cl layer to the GaN dots is determined by various growth conditions such as HCl flow rate, NH ₃ flow rate, Ga boat temperature, and substrate temperature.

An SEM photograph of the sample produced in this step is shown in FIG. The thickness of the NH ₄ Cl layer is 800 nm, and the diameter of the GaN dot is 200 nm. In part of the photograph, the NH ₄ Cl layer was removed and GaN dots were observed reliably. A flat surface was obtained where there was an NH ₄ Cl layer. As preferable growth conditions, HCl flow rate: 40 sccm, NH ₃ flow rate: 1 L / min, and carrier nitrogen gas flow rate: 2.5 L / min.

The thickness of the NH ₄ Cl layer formed in this step is an important factor that determines the density of voids finally formed at the interface together with the high-temperature nitriding conditions.
FIG. 4 shows the ω-2θXRD results for the NH ₄ Cl layer. As can be seen from FIG. 4, the (100), (110), (200), and (220) peaks in the polycrystalline state and the (0002) peak from the GaN dots were observed.

In order to confirm the presence or absence of GaN dots and the surface density, an H ₂ O etching experiment was performed on the sample manufactured in this step. NH ₄ Cl is a substance that can be easily chemically etched with H ₂ O.
FIG. 5 is an SEM photograph of the surface of the sapphire substrate after etching. It was found that 200 nm GaN dots were uniformly distributed. The XRD result of the sapphire substrate surface after this etching is shown in FIG. The NH ₄ Cl peak completely disappeared by etching, and only a (0002) peak due to GaN aligned with the c-axis in a small amount of single crystal was observed.

3) Growth Step of Low Temperature GaN Buffer Layer This step is a step of growing a low temperature buffer layer on the NH ₄ Cl layer including the GaN dots formed in step 2). This by growing a low-temperature buffer layer at a temperature higher than the formation temperature of the NH 4 _Cl layer, suppressing the synthesis of NH ₄ Cl layer due to an increase in the substrate temperature, and forms a seed layer of high-temperature GaN layer.
7 shows a low-temperature GaN buffer layer formed at a temperature higher than the formation temperature of the _NH 4 Cl layer on the _NH 4 Cl layer schematically.

The temperature of the Ga boat was set to 450 ° C. as in step 2), and the substrate temperature was increased from 500 ° C. to 600 ° C. The low temperature buffer layer grows continuously while the substrate temperature rises.
The low temperature buffer layer can be formed at a temperature of 400 ° C. or higher and 800 ° C. or lower.
However, regarding the formation temperature of the GaN low temperature buffer layer, a temperature range of 500 ° C. to 600 ° C. is more desirable.
The boiling temperature of the NH ₄ Cl layer is 520 ° C. Above this temperature, the amount of NH ₄ Cl synthesized is drastically reduced and the growth rate of GaN formed on the sapphire substrate is increased.

The GaN dots aligned on the c-axis became seeds for the low-temperature buffer layer, and the low-temperature buffer layer also showed the c-axis crystal direction in the same way.
FIG. 8 shows an end face SEM photograph. The thickness of the low temperature buffer layer is 2.3 microns, and the growth conditions are HCl flow rate: 40 sccm, NH ₃ flow rate: 1 L / min, and carrier nitrogen gas flow rate: 2.5 L / min. GaN dots were observed in a part of the photograph.
FIG. 9 shows the ω-2θXRD result of the low temperature GaN buffer layer. The (0002) peak of the low-temperature GaN buffer layer including the GaN dots aligned with the c-axis is shown together with the NH ₄ Cl peak in the polycrystalline state.

4) Formation process of high-temperature thick GaN layer In this process, in order to form a thick high-temperature GaN layer on the buffer layer formed in steps 2) and 3), the growth temperature is increased to 1040 ° C. to increase the thickness. This is a step of forming a film GaN layer. At this time, there is a heat treatment effect at a growth temperature of 1040 ° C., and with the complete decomposition of the NH ₄ Cl layer, many voids are formed at the interface.
FIG. 10 schematically shows that a high-temperature thick GaN layer is formed on the low-temperature buffer layer, and voids are formed at the interface during the temperature rise.
FIG. 11 shows a photograph of a preferable void. This is a case where the low temperature buffer layer is formed at 180 ° C. and 180 nm before the high temperature thick GaN layer is formed.

The purpose of this step is to cause natural lift-off by relaxation of stress from the GaN dot layer locally formed on the sapphire substrate when the growth temperature is lowered to room temperature after forming the high-temperature thick film GaN by void formation.
Also, due to the heat treatment effect, the thickness of the buffer layer formed through steps 2) and 3) was reduced to 1.2 microns.

FIG. 12 is an end face SEM photograph after the heat treatment. After the heat treatment, many voids were formed at the interface and the thickness decreased.
FIG. 13 shows the ω-2θXRD result of this sample. The NH ₄ Cl layer formed in step 2) was completely decomposed, and the peak of the NH ₄ Cl layer was not observed, and only the (0002) peak of the GaN buffer layer was observed. That is, the decomposition of the NH ₄ Cl layer is indirectly known by the heat treatment. The heat treatment was performed at 1040 ° C. in an NH ₃ atmosphere for 10 minutes.

5) Self-lift-off process of GaN free-standing substrate FIG. 14 schematically shows self-lift-off due to separation from the interface while the substrate temperature is being cooled to room temperature.
This step is a step of producing a GaN free-standing substrate by natural exfoliation accompanying relaxation of stress at the interface when growing a high-temperature thick film GaN and lowering the substrate temperature to room temperature.

FIG. 15 shows an end-surface SEM photograph of a 200-micron GaN free-standing substrate obtained by this process. The cooling rate of the substrate was adjusted to 100 ° C. per hour in an NH ₃ atmosphere. FIG. 16 shows an SEM photograph of the surface side of the sapphire substrate observed after lift-off.

  It was found that a part of the GaN residue remained on the sapphire substrate by the interface separation. The XRD result of this sample is shown in FIG. A (0002) peak from the GaN residue was observed on the sapphire substrate. As a result of the lift-off, it was possible to obtain a GaN free-standing substrate having good quality crystals without cracks and warping.

The a-axis and c-axis lattice constants of the GaN free-standing substrate by XRD evaluation are 3.1893.1 and 5.185Å, respectively, which are numerical values that coincide with the lattice constants of the strain-free bulk.
FIG. 18 shows a PL result of the GaN free-standing substrate. The donor-acceptor pair emission peak (3.4718 eV) was found to be the same as the emission position of the strain-free bulk crystal.

After the growth of the high-temperature thick GaN layer, the voids cause spontaneous separation by relaxation of stress at the interface during natural cooling. The sapphire substrate and the thick GaN are separated in situ by natural peeling.
High-quality GaN growth becomes difficult only by forming the NH ₄ Cl layer on the sapphire substrate. In order to solve such a problem, a high-quality GaN crystal can be grown by using a GaN dot and a low-temperature GaN buffer layer that are rarely used in HVPE.

The purpose of using the NH ₄ Cl layer formed at a low temperature is to decompose the NH ₄ Cl layer as the substrate temperature rises and to form a large number of voids between the sapphire substrate and GaN during the growth of the high-temperature GaN layer. Thick GaN grown on the interface where such voids are present is free from cracks and warpage, and a large-diameter self-standing substrate can be produced.
In addition, the selectively formed GaN dots cause the sapphire substrate and the thick film GaN to be naturally lifted off during the natural cooling by void formation, and the sapphire substrate can be easily separated without additional processes.

FIG. 19 shows a comparative example in which the formation of the low temperature buffer layer is omitted. As can be seen from the right side of FIG. 19, even if the substrate is prepared in the same manner as the present invention and the step of forming the GaN dots and the NH ₄ Cl layer on the substrate is omitted, the formation of the low temperature buffer layer is omitted. Since no void is formed at the interface after the growth of the high-temperature thick GaN layer, natural peeling does not occur at the interface during natural cooling.

As described above, the method for producing a GaN free-standing substrate using the HVPE method has been exemplified as an example, but the present invention is not limited to this.
For example, GaN is exemplified as a free-standing substrate, but the present invention can also be applied to GaN-based nitride semiconductors such as AlGaN and InGaN.
Moreover, although the GaN buffer layer was illustrated as a low temperature buffer layer, any of AlN, InN, BN, and those mixed crystal semiconductors may be sufficient.

Next, in an example, a sapphire substrate is prepared to form a GaN dot and NH ₄ Cl layer on the substrate, and AlN _X O _1-X (0 <X ≦) locally on the sapphire substrate by a high-temperature nitridation process in advance. 1) Although a layer is formed, the substrate is not necessarily a sapphire substrate if the GaN dots and the NH ₄ Cl layer are reliably formed on the substrate. Further, the high temperature nitriding step of the substrate can be omitted.

Schematic diagram of AlN _X O _1-X (0 <X ≦ 1) locally formed by high temperature nitriding treatment of sapphire substrate Schematic diagram of formation of GaN dots and NH ₄ Cl layer SEM photo of NH ₄ Cl layer and GaN dots Ω-2θXRD results showing NH ₄ Cl layer and GaN dots SEM photograph of sapphire substrate surface after H ₂ O etching XRD result of sapphire substrate surface after H ₂ O etching Schematic diagram of low-temperature GaN buffer layer formation SEM photo of end face of low-temperature GaN buffer layer Ω-2θXRD result of low-temperature GaN buffer layer Schematic diagram of high-temperature thick GaN layer formation Boyd Pictures Cross-sectional SEM photograph after heat treatment Ω-2θXRD result after heat treatment Schematic diagram of self-lift-off SEM photo of end face of GaN free-standing substrate by natural peeling SEM photograph of the surface side of the sapphire substrate after lift-off XRD result of sapphire substrate surface after lift-off GaN free-standing substrate PL results Comparative example of the present invention

Claims (8)

A step of preparing a substrate, forming a step of forming a GaN dot and NH 4 _Cl layer on the substrate, a low temperature buffer layer made of a group III-V nitride semiconductor on the GaN dots and NH 4 _Cl layer A GaN-based nitride semiconductor free-standing substrate comprising: a step of forming a GaN-based nitride semiconductor layer on a low-temperature buffer layer; and a step of naturally peeling the GaN-based nitride semiconductor layer from the substrate by returning the substrate temperature to room temperature Manufacturing method.   The method for producing a GaN-based nitride semiconductor free-standing substrate according to claim 1, wherein the substrate is a sapphire substrate. The method includes a step of nitriding the substrate surface and locally forming AlN _X O _1-X (0 <X ≦ 1) before the step of forming the GaN dots and the NH ₄ Cl layer on the substrate. A method for producing a GaN-based nitride semiconductor free-standing substrate according to claim 2. The step of forming the GaN dots and _NH 4 Cl layer is 1 to claim, characterized in that a step of forming a GaN dot and _NH 4 Cl layer by flow of HCl and NH ₃ in Ga gas atmosphere 4. A method for manufacturing a GaN-based nitride semiconductor free-standing substrate according to any one of items 3 to 3.   5. The method for producing a GaN-based nitride semiconductor free-standing substrate according to claim 1, wherein the GaN dots are aligned in a c-axis direction.   The said low-temperature buffer layer is a layer which uses either GaN, AlN, InN, BN, and these mixed crystal semiconductors as a constituent material, The any one of Claim 1 thru | or 5 characterized by the above-mentioned. GaN-based nitride semiconductor free-standing substrate manufacturing method.   The method for manufacturing a GaN-based nitride semiconductor free-standing substrate according to any one of claims 1 to 6, wherein the low-temperature buffer layer is formed at a temperature of 400 ° C or higher and 800 ° C or lower.   The method for manufacturing a GaN-based nitride semiconductor free-standing substrate according to any one of claims 1 to 7, wherein each of the steps is continuously performed in a single HVPE apparatus.