| Summary: | 碩士 === 長庚大學 === 光電工程研究所 === 93 === GaN epitaxial substrates are mainly on sapphire and SiC. Since sapphire or SiC has many disadvantages such as cost a lot of money, difficult to grow high quality large size bulk substrate, some scientists try to grow the high-quality GaN film on silicon. Actually, silicon has many advantages of its high crystal quality, low cost, large area size, and the most important is silicon substrate has the potential application in integration of well-established Si electronics industry. However, due to the large mismatch of lattice constant (16.9%) and the thermal expansion coefficient (53%) between GaN and Si, it is really difficult to grow high-quality GaN on Si substrate. Recently, many groups have reported on metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) system growth of GaN on Si substrate. To overcome these difficulties of GaN on Si, they used various buffer layers such as AlN internal layer , AlxGa1-xN/GaN supper lattice, and the AlN seed layer with AlGaN intermediate layer as a composite buffer layer. Among these technologies, composite buffer layer provides the highest quality without crack, mirrorlike, large area GaN film.
In our study, we report GaN epitaxy on Si(111) substrate using a composite buffer layer structure by metalorganic chemical vapor deposition (MOCVD).In order to use thinnest buffer layer get highest quality GaN epitaxy layer, we try to fix the total thickness of GaN with composite buffer layer, and change the GaN/AlGaN thickness ratio. From the result we find that the small GaN/AlGaN thickness ratio the decreasing of dislocation density and without crack on GaN film. However, in order to consider the cost of epitaxy and the number of cracks on GaN film, we choose the optimize GaN/AlGaN thickness ratio between 2.8 to 6.6 near 2.8. In our transmission electron microscopy (TEM) observation process, we find the phenomenon of dislocation close loop near the AlGaN/GaN interface; we also report the reason why it possible occurs. Besides, We compare the influence of (CBL+1 LT AlN)、2(CBL+LT AlN)、(CBL+SL(AlGaN/GaN)+LT AlN) buffer layer structure in LED on threading dislocation density.
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