Growth of Gan on Porous Sic Substrates by Plasma-Assisted Molecular Beam Epitaxy
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GROWTH OF GaN ON POROUS SiC SUBSTRATES BY PLASMA-ASSISTED MOLECULAR BEAM EPITAXY C. K. Inoki and T. S. Kuan Department of Physics, University at Albany, SUNY, Albany, NY 12222 C. D. Lee, Ashutosh Sagar, and R. M. Feenstra Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213 ABSTRACT We have explored the growth of GaN on porous SiC substrates by plasma-assisted molecular beam epitaxy. The porous 4H- and 6H-SiC(0001) substrates used in this study contain 10 to 100-nm sized pores and a thin skin layer at the surface. This skin layer was partially removed prior to the growth by H-etching. Transmission electron microscopy (TEM) observations indicate that the epitaxial GaN growth initiates from the surface areas between pores, and the exposed surface pores tend to extend into GaN as open tubes and trap Ga droplets. Plan-view TEM observations indicate that the GaN layers grown on porous substrates contain fewer dislocations than layers grown on non-porous substrates by roughly a factor of two. The GaN layers grown on a porous SiC substrate were also found to be mechanically more relaxed than those grown on non-porous substrates; electron diffraction patterns indicate that the former are free of misfit strain or are even in tension after cooling to room temperature. INTRODUCTION Porous SiC and GaN has recently been explored as promising substrates to grow epitaxial SiC or GaN with reduced dislocation density [1-6]. Such porous materials are produced by anodizing n-type SiC in hydrofluoric acid (HF) under ultra-violet illumination [7]. Elongated pores with diameters 10 to 30 nm are typically formed in 4H and 6H SiC depending on the etching conditions (Fig. 1). Preliminary results for GaN growth on porous GaN have shown some promise for improved GaN quality [2-4]. It has been speculated that a porous surface may serve as a template for nano-scale lateral epitaxial overgrowth [4], and that a porous substrate layer may be compliant to any lattice and thermal mismatch strains [2-4]. In this work we grew GaN films on both porous and non-porous SiC substrate under nominally identical conditions and evaluated the effect of using the porous substrate. Transmission electron microscopy (TEM) and electron diffraction were employed to characterize the dislocation density and the amount of strain relaxation. The standard growth conditions optimized for growth on non-porous SiC were used in this study. The porous SiC substrates contain a thin skin layer about 50 nm in thickness with few exposed pores at the surface. As shown in a bright-field, cross-sectional TEM image in Fig. 1, the pores start to form a dense network only below this skin layer. For growth on the porous network, this skin layer must be removed prior to the growth. High temperature H-etching was used for this purpose in the present study, although this process was found to significantly enlarge the pores and modify the pore morphology [8]. Reactive ion beam etching may be a better procedure for skin removal [8]. EXPERIMENTAL The porous 4H and 6H SiC sub
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