Step Flow Surface Morphology in Plasma Assisted Molecular Beam Epitaxy Grown GaN
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Step Flow Surface Morphology in Plasma Assisted Molecular Beam Epitaxy Grown GaN
Kazuhide Kusakabe, Akihiko Kikuchi and Katsumi Kishino Department of Electrical and Electronics Engineering, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo, Japan 102-8554 ABSTRACT The surface morphologies of homoepitaxial GaN films grown by molecular beam epitaxy (MBE) on metalorganic chemical vapor deposition (MOCVD) grown GaN template layers were investigated, using atomic force microscopy (AFM). Typical surface morphology of MBE-grown films on MOCVD-templates was dominated by spiral hillocks due to the high density of dislocations having a screw character and large driving force of MBE growth. Introduction of the AlN multiple interlayer (AlN -MIL) into MBE-GaN layers suppressed the formation of spiral hillocks. It was attributed to obstructing the dislocation propagation by AlN-MIL. Migration enhanced epitaxy (MEE) growth of GaN also reduced the density and tightness of spiral hillocks. This observation was attributed to that MEE growth technique decreased the driving force of growth. INTRODUCTION The recent demonstrations of both gallium nitride based blue laser diodes and high power field effect transistors contain quantum heterostructures [1]. In the growth of these heterostructures, the flatness of a growth surface is considerably demanded to obtain abrupt heterointerfaces, since an interface roughness can result in degradation of device performances. To date, the atomically stepped hexagonal (0001) GaN surfaces grown by molecular beam epitaxy (MBE) [2] and by metalorganic chemical vapor deposition (MOCVD) [3] have been reported. However, the observed morphologies of step terraces routinely had some inhomogeneity explained as the dislocation mediated structures, which are dominated by the high density of threading dislocations intersecting a free surface [4]. In fact, on the surfaces of MOCVD-grown GaN, pit- like surface depletions at both mixed and pure edge dislocations and nearly straight steps pinned at mixed dis locations were observed. A fine step flow surface extended to 10 × 10 µm2 area was achieved in MOCVD-grown GaN layers on the Si side surface SiC (0001) substrates [5]. In contrast, the surfaces of MBE-GaN are governed by hexagonal-shaped spiral hillocks, each of which includes a mixed dislocation at the center. Such spiral hillocks are induced due to non-equilibrium growth kinetics of MBE by pinning of steps at mixed dislocations, and thus can be suppressed with reduction of mixed dislocations. G3.33.1
So developme nt and understanding of the fundamental mechanism governing MOCVD-GaN growth are fairly ongoing, but improvement of the surface structures in MBE-GaN layers is still an open subject. Under such situations, a deep consideration concerning with the MBE growth mode of GaN films is necessary for further developments in the nitride devices. Especially, it is important to elucidate how the formation of spiral hillocks on the surfaces of MBE-GaN can be suppressed, since the spiral hillocks induce fluctuat
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