1. Field of the Invention
The present invention relates to a nitride based semiconductor device and a process for preparing the same, and more particularly to a nitride based semiconductor device which has nanorods and an amorphous matrix layer on a substrate and thus is capable of inhibiting occurrence of cracks and a process for preparing the same
2. Description of the Related Art
Generally, light emitting diodes (LEDs) are broadly divided into blue and green visible light, and UV light LEDs, using a nitride based semiconductor, and green and red LEDs using semiconductor materials such as GaAs or GaP. Recently, owing to development of a blue light LED using the nitride based semiconductor such as gallium nitride, the primary colors of light are realized by the LED and thereby realization of a full color display has become feasible. Further, improved luminance of LEDs is rapidly expanding the number of fields in which LEDs may be utilized, including use thereof as a lighting source, catalyzing world-wide efforts to develop new applications for LEDs.
Up to now, in order to realize a great part of nitride based semiconductor devices including nitride based LEDs, a single crystal sapphire (Al.sub.2O.sub.3) substrate or silicon carbide (SiC) substrate is used. However, such substrates are expensive and the size of the substrate is limited to a range of about 2 to 3 inches, resulting in difficulty of large diameter substrate production and thus being unsuitable for mass production.
Members of industries and academic areas relating to semiconductors have already acknowledged that in order to implement true mass production of nitride based semiconductors, use of the silicon (Si) substrate is most preferred, and research into methods for implementation of the nitride based semiconductor devices using the silicon substrate are underway. However, large lattice constant and thermal expansion coefficient differences are present between the Si substrate and group III nitride semiconductor such as GaN. For such reasons, a GaN layer formed on the Si substrate undergoes significant cracking. Such cracks formed on the GaN layer lead to significant deterioration of performance of devices manufactured using the GaN layer and have fatal effects on the service life of such devices. In order to overcome these problems, a great deal of research and study is underway throughout the world.
One method of solving the above-mentioned problems is to grow the GaN layer on a buffer layer of Al.sub.xGa.sub.1-xN after formation of the buffer layer on the Si substrate. U.S. Pat. No. 6,649,287 discloses a method of forming a transition layer of Al.sub.xGa.sub.1-xN with an Al composition varying in the direction of the thickness thereof on the silicon substrate, in order to alleviate lattice constant and thermal expansion coefficient differences between the Si substrate and GaN layer.
FIG. 1 is a cross-sectional view of a conventional nitride based semiconductor device including a GaN layer formed on a Si substrate. Referring to FIG. 1, an Al.sub.xGa.sub.1-xN buffer layer 13 is formed on a Si substrate 11 and an n-type GaN layer 15 is formed on the buffer layer 13. The Al.sub.xGa.sub.1-xN buffer layer 13 serves to reduce stress or cracks occurring when forming the GaN. layer on the silicon substrate. Meanwhile, the Al.sub.xGa.sub.1-xN buffer layer 13 has a different Al composition along the direction of thickness, in order to alleviate differences of lattice constant and thermal expansion coefficient between Si and GaN. That is, stress due to differences of crystal structure between Si and GaN is alleviated by controlling the Al composition of the Al.sub.xGa.sub.1-xN buffer layer 13 to be higher on the Si substrate 11 side than on the GaN layer 15 side. Therefore, the GaN layer 15 grown on the Al.sub.xGa.sub.1-xN buffer layer 13 exhibits decreased cracking.
However, formation of the AlxGa1-xN buffer layer 13 fails to effectively alleviate tensile stress due to thermal expansion coefficient differences between the Si substrate 11 and GaN layer 15, thus causing production of a crack network on the surface of the grown GaN layer 15. Such a crack network not only degrades the performance of optical devices such as LEDs prepared on the basis of the GaN layer 15 but also drastically reduces the service life of the devices.
Another method to solve problems associated with cracks occurring upon formation of the GaN layer on the Si substrate is to form a GaN layer on a ZnO buffer layer formed on a silicon substrate. However, this method also fails to solve fundamental problems such as cracking. Therefore, there remains a need for a method capable of forming a good quality GaN layer on the Si substrate by solving cracking problem due to differences in lattice constant and thermal expansion coefficient between the Si substrate and GaN layer.