1. Field of the Invention
The present invention relates to a method for manufacturing a nitride based single crystal substrate, and more particularly to a method for manufacturing a nitride based single crystal substrate using a ZnO layer and a method for manufacturing a nitride based light emitting diode using the same.
2. Description of the Related Art
Generally, a gallium nitride single crystal is manufactured on a substrate made of a material differing from the gallium nitride single-crystal by vapor growth, such as metal organic chemical vapor deposition (MOCVD) or hydride vapor phase epitaxy (HVPE), or molecular beam epitaxy (MBE).
Here, a sapphire (α-Al2O3) substrate or a SiC substrate is used as the above substrate. The substrate may be cracked by a difference of lattice constants between the substrate and the gallium nitride single crystal and a difference of thermal expansion coefficients between the substrate and the gallium nitride single crystal. For example, since a difference of lattice constants between sapphire and gallium nitride is approximately 13% and a difference of thermal expansion coefficients between sapphire and gallium nitride is −34%, stress is applied on an interface between the sapphire substrate and the gallium nitride single crystal. Since a temperature at which a nitride layer is grown on the substrate is high, the thermal stress caused by a high difference of thermal expansion coefficients therebetween generates the cracks of the substrate and the gallium nitride single crystal.
In order to solve the above problem, two methods have been proposed. One method is that a nitride single crystal is manufactured on a substrate made of a nitride based material using a freestanding nitride based single crystal substrate, and the other method is that a nitride based light emitting structure is separated from a substrate made of a material differing from the nitride single crystal. Here, the freestanding nitride based single crystal substrate is obtained by forming a nitride single crystal on a substrate made of a material differing from the nitride single crystal and then separating the nitride single crystal from the substrate. Accordingly, the above two methods require a technique for separating the nitride single crystal (or the nitride based light emitting structure) from the substrate made of a material differing from nitride.
A laser lift off process is conventionally used to separate the nitride single crystal from the substrate. In the laser lift off process, as shown in FIG. 1, a laser beam is irradiated so that a GaN based single crystal bulk 15 is separated from a sapphire substrate 11 by decomposing the GaN based single crystal bulk 15, located on an interface between the sapphire substrate 11 and the GaN based single crystal bulk 15, into gallium (Ga) and nitride (½N2).
However, when a crystal having a diameter of 2 inches or more or a designated thickness or more is grown on the substrate, the laser lift off process causes severe warpage and cracks (C) of the substrate and the crystal due to a difference of lattice constants therebetween and a difference of thermal expansion coefficients therebetween.
Another conventional technique is that ZnO is used as a buffer layer for growing a nitride crystal. Since a difference of lattice constants between a ZnO single crystal oriented along the c-axis and GaN is only 2% and the ZnO single crystal has a wurzite structure the same as that of GaN, the ZnO single crystal facilitates the growth of a nitride crystal and is eliminated by conventional wet etching, thus being advantageously used in separation of the nitride crystal from the substrate. However, since the ZnO single crystal is thermally and chemically unstable and is easily decomposed even at a relatively low temperature (approximately 500° C.), there is a limit to substantially apply the ZnO single crystal to a step of growing the nitride single crystal performed at a high temperature.
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