1. Field of the Invention
The present invention relates to a method for manufacturing a gallium nitride (GaN) based single crystalline substrate, and more particularly to a method for manufacturing a gallium nitride (GaN) based single crystalline substrate using a sapphire substrate.
2. Description of the Related Art
Recently, an optical disk field has been developed to record data at high density and to reproduce data with high resolution. In order to meet these requirements, semiconductor devices, which emit light in a short wavelength band, have been developed. A gallium nitride (GaN) based single crystalline substrate is mainly used as a material for producing these semiconductor devices which emit light in a short wavelength band. A GaN single crystal has an energy band gap of 3.39 eV, thus being suitable for emitting short wavelength blue light.
Till now, GaN based single crystals have been grown on a substrate made of a different material using vapor growth, such as MOCVD (Metal Organic Chemical Vapor Deposition) or HVPE (Hydride Vapor Phase Epitaxy), or MBE (Molecular Beam Epitaxy). Generally, a sapphire (α-Al2O3) substrate or a SiC substrate is used as the substrate. Particularly, the sapphire substrate has a hexagonal structure the same as that of GaN and is cheaper and more stable at a high temperature than the SiC substrate, thus being widely used as the substrate for producing the above semiconductor devices.
However, since a difference between lattice constants of sapphire and GaN is approximately 13% and a difference between thermal coefficients of expansion (TCEs) of sapphire and GaN is approximately −34%, strain is exerted on an interface between the sapphire substrate and the GaN single crystals, thus causing several problems such as lattice defects and cracks in the GaN single crystals. These problems cause difficulty in growing a GaN film of a high quality on the sapphire substrate, and shorten the expected lifetime of a semiconductor device produced on the GaN film.
Accordingly, there is required a method for forming a GaN film on a GaN single crystalline layer using homo-epitaxy. However, this GaN single crystalline layer has a high vapor pressure of nitrogen, thus not being able to be used in a conventional method for producing a Si substrate or a GaAs substrate.
Therefore, the GaN based single crystalline layer is obtained by growing a GaN bulk on a growth substrate made of sapphire or SiC using vapor growth such as MOCVD (Metal Organic Chemical Vapor Deposition) or HVPE (Hydride Vapor Phase Epitaxy), or MBE (Molecular Beam Epitaxy). For example, in case that HVPE is used, the GaN bulk can be grown on the growth substrate so that the growth thickness of the GaN bulk is several μm˜ several hundreds μm per hour. That is, the GaN bulk with a desired thickness can be grown on the growth substrate in a short period of time.
However, in this case, stress is still exerted on both the sapphire substrate and the GaN layer due to the difference between TCEs of sapphire and GaN, thus causing the same problems such as lattice defects and cracks in the GaN based single crystals. In case that strain acting on the sapphire substrate by the GaN layer is less than a yielding point, the GaN layer is not cracked, but is warped toward the sapphire substrate. The warpage of the GaN layer depends on the thickness of the GaN layer. When the thickness of the GaN layer increases, the radius of curvature of the warped GaN layer is reduced and the surface of the warped GaN layer is roughened, thus causing difficulty in polishing the surface of the GaN layer.
In order to solve the above-described problems, there is required a freestanding GaN based substrate. The freestanding GaN based substrate is obtained by growing a GaN based single crystalline bulk on a sapphire substrate and then removing the sapphire substrate from the GaN based single crystalline bulk. Here, the sapphire substrate is removed from the GaN single crystalline bulk by mechanical polishing using diamond powder, chemical etching, or etc.
In case that the sapphire substrate is removed from the GaN single crystalline bulk by the mechanical polishing, stress exerted on the sapphire substrate provided with GaN single crystals grown thereon is in the range of the limit of elasticity of the sapphire substrate. Accordingly, the sapphire substrate is not cracked, but is warped. However, during the progress of the mechanical polishing, the sapphire substrate is reduced in thickness so that equilibrium of strength in the sapphire substrate is lost, thus being cracked. The cracks of the sapphire substrate may be transmitted to the GaN layer, and then the GaN layer may be cracked also. On the other hand, in case that the sapphire substrate is removed from the GaN single crystalline bulk by the chemical etching, it is difficult to obtain an etchant which has a high etching rate and selectively etches the sapphire substrate.
Recently, there is proposed a method for growing a GaN single crystalline bulk on a sapphire substrate using HVPE and then separating the sapphire substrate from the GaN single crystalline bulk using an ultraviolet laser beam. The ultraviolet laser beam passes through the sapphire substrate with a high-energy band gap, and is absorbed by the GaN single crystalline bulk. Thus, this laser beam irradiated onto the lower surface of the sapphire substrate dissolves GaN into nitrogen gas and gallium, thereby separating the sapphire substrate from the GaN single crystalline bulk.
This method for separating the sapphire substrate from the GaN single crystalline bulk by means of the irradiation of the ultraviolet laser beam can be used in a small-sized substrate without causing cracks. However, in case that this method is used in a large-sized substrate with a diameter of more than 2 inches, which is generally required in manufacturing a semiconductor device, cracks form on the substrate.
More specifically, in case that a laser beam is irradiated on the lower surface of a growth substrate 11 made of sapphire or SiC as shown in FIG. 1, since the area of the irradiation of the laser beam is narrow (maximally 10 mm×10 mm until now), the laser beam is sequentially irradiated on limited local areas of the sapphire substrate 11 so that the laser beam can be irradiated on the entire surface of the sapphire substrate 11. Thereby, the level of stress generated by lattice mismatching and a difference between TCEs of the sapphire substrate 11 and a GaN single crystalline bulk 15 grown on the sapphire substrate 11 becomes more serious, thus causing cracks on the GaN single crystalline layer 15. Further, the cracks are propagated into the inside of the GaN single crystalline bulk 15 along cleavages, and then may cleave the GaN single crystalline bulk 15. The GaN single crystalline bulk 15 obtained by the conventional separation method is not suitably used as a substrate for manufacturing a semiconductor device thereon.
Accordingly, there is required a method for manufacturing a GaN single crystalline substrate, in which a GaN single crystalline bulk is grown on a growth substrate and the growth substrate is separated from the GaN single crystalline bulk without damaging the GaN single crystalline bulk.