Mass Transport, Faceting and Behavior of Dislocations in GaN
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F99W2.8
such structures as p-n junctions, quantum dots, buried heterostructure lasers, and gratings for distributed feedback lasers and wafer fusion. Therefore, it is one of the most important key technologies for the fabrication of novel devices. However, there has been no report concerning the mass transport of HT-nitrides. We found that mass transport at the surface of HT-GaN occurs at around the growth temperature [13]. We also found that this phenomenon can be applied to grow dislocation-free areas in HT-GaN films on sapphire. In this study, a detailed TEM analysis was performed to clarify the behavior of threading dislocations in the mass-transported region, thereby allowing us to understand the mass transport process on an atomic scale. EXPERIMENT GaN 7 µm in thickness was grown by metal organic vapor phase epitaxy (MOVPE) on a sapphire (0001) substrate using a LT-AlN buffer layer. Trench stripes were patterned on the surface along the direction by reactive ion etching. Several samples of trenches with different depths and widths were prepared. Then, in order to perform mass transport, these samples were annealed at 1100 in a MOVPE reactor. The temperature was monitored using a thermocouple inserted into the graphite susceptor. During annealing, 0.20 mol/min of ammonia with nitrogen gas was supplied. No group-III alkyl source gas was supplied. After annealing for 12 min, the grooves were almost buried by mass transport processes. To analyze the mass transport process and behavior of dislocations, cross-sectional TEM study was carried out. RESULTS Fundamental mechanism of mass transport in shallow trench The process of the mass transport of GaN is schematically shown in Fig. 1. Mass transport is generally thought to proceed toward the minimization of total surface energy [14]. Assuming a single mesa step, asF99W2.8 shown in Fig. 1(a), at the convex part of the surface, the binding of crystal (Ga-N) is weakened by the higher surface energy. Therefore, GaN is decomposed and free Ga atoms diffuse to the surrounding. On the contrary, at concave part of the surface, Ga atoms are easily reincorporated into the solid and thus growth of GaN occurs. However, the high crystallographic anisotropy, which is characteristic of GaN, modulates the surface shape during mass transport. Therefore, it is hard to form round surface corners composed of higher index facets as observed in the case of InP [14]. {1-101} facets of GaN are energetically much more stable than other facets, except for {0001}, as has already been reported by other groups [15,16]. Thus, once the facets are formed at the upper trench edges, decomposition does not continue there. Therefore, the Ga species are supplied from the outer area of the surface, as shown in Fig. 1(b). Other stable facets such as {1-100} are formed before the {1-101} facet plane is completely formed, as shown in Fig. 1(c). Then, once {1-101} facets from both upper edges are connected to each other or reach the bottom of the groove, higher index facets are formed and the trench is gra
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