Structural Properties of GaN films grown by Molecular Beam Epitaxy on vicinal SiC(0001)
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Structural Properties of GaN films grown by Molecular Beam Epitaxy on vicinal SiC(0001) C. D. Lee,1 R. M. Feenstra,1 O. Shigiltchoff,2 R. P. Devaty2 and W. J. Choyke2 1 Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213 2 Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260 ABSTRACT Gallium nitride films are grown by plasma-assisted molecular beam epitaxy (MBE) on vicinal 6H-SiC(0001) substrates with [1 1 00] and [11 2 0] miscut directions. The hydrogen-etched substrates display straight, or chevron shaped steps respectively, and the same morphology is observed on the GaN films. X-ray rocking curves display substantially reduced width for films on the vicinal substrates compared to singular substrates, for the same Ga/N flux ratio used during growth.
INTRODUCTION
A number of research groups have in recent years studied the growth using molecular beam epitaxy (MBE) of GaN on SiC [1-7]. Hydrogen etching of the substrates is becoming a standard preparation step for removing polishing damage [8]. Most growth has been performed on singular (no intentional miscut, or on-axis) (0001) Si-face substrates, although a few studies have employed vicinal (miscut, or off-axis) substrates. One report in particular, by Xie et al. [6], has reported significantly improved film quality on the vicinal substrates, as discussed in more detail below. In this work we report on results for GaN film growth by plasma-assisted MBE on vicinal 6H-SiC(0001) substrates with [1 1 00] and [11 2 0] miscut directions. After H-etching, 1 1 00 oriented steps are observed on both types of miscut substrates. Subsequent GaN film growth is found to preserve this general morphology. For high Ga/N flux ratios (where smooth films are achieved), the structural properties of the films grown on the vicinal substrates are found to be substantially better than for those films grown on the singular substrates. EXPERIMENTAL
The 6H polytype of SiC was used for all the studies reported here. The on-axis SiC and the substrates miscut 3.5° towards [11 2 0] were n-type material, purchased commercially from Cree, Inc. The substrates misoriented towards [1 1 00] were cut from a boule of n-type material. Samples were cut at an angle of 3.5±0.5° from the [0001] direction towards [1 1 00] using a goniometer accurate to ±0.5°. X-ray Laue patterns were taken to verify the angle. These samples were polished with 90, 30, 15, 6, 3, 1, 0.5 and finally 0.25 µm diamond paste. The last four stages took about 20-25 hours each. No visible scratches on the resulting surfaces were detected using the naked eye. Hydrogen etching of the SiC was performed at 1600–1700°C and 1 atm pressure to remove polishing damage [8]. Following transfer of the substrates to the MBE system, Si deposition followed by annealing was done to remove surface oxide and obtain a surface with well-defined structure and stoichiometry, namely, the √3×√3-R30° reconstruction I3.40.1
containing 1/3 monolayer (ML) of Si adatoms on the Si-face SiC(0001) surface. This su
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