Roughness Analysis of Episurfaces Grown on Ion-Beam Processed GaSb Substrates
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Roughness Analysis of Episurfaces Grown on Ion-Beam Processed GaSb Substrates K. Krishnaswami,1 D.B. Fenner,2 S.R. Vangala,1 C. Santeufemio,1 M. Grzesik,1 L.P. Allen,3 G. Dallas,3 and W.D. Goodhue1 1. Photonics Center, Dept. of Physics, University of Massachusetts, Lowell, MA 01854 2. Dept. of Physics, University of Connecticut, Storrs, CT 06269 3. Galaxy Compound Semiconductors, 9922 East Montgomery #7, Spokane, WA 99206, USA ABSTRACT High-quality GaSb substrates with minimal surface roughness and thin, uniform oxide layers are critical for developing low-power, epitaxy-based, electronic and optoelectronic devices. Ion-beam processing techniques of gas-cluster ion beam (GCIB) and bromine ion-beam assisted etching (Br-IBAE) were investigated as to their potential for improving the suitability of substrate surfaces for molecular beam epitaxial (MBE) growth. Statistical analysis of the residual surface roughness provides insight into ion-beam processing and its impact on epitaxial growth. Images of episurfaces grown on chemical mechanical polished (CMP), Br-IBAE, and GCIB finished substrates were obtained using atomic force microscopy (AFM) and these were statistically analyzed to characterize their surface roughness properties. Autocorrelation analysis of the first two types of episurfaces showed a quick loss of correlation within ~100 nm. The episurface with Br-IBAE also showed isotropic mound roughness with sharp point-like protrusions. The GCIB prepared episurfaces exhibited the formation of uniform step-terrace patterns with monatomic steps and wide terraces as indicated by the strong, long range (>0.5 µm) correlations. Statistical analysis of the GCIB episurfaces showed self-similar random fractal behavior over eight orders of magnitude in the power spectral density (PSD) with a fractal dimension of ~2.5. I.
INTRODUCTION
Recently, there has been a lot of interest in antimonide based compound semiconductor (ABCS) materials, in particular GaSb, due to their unique material properties such as low effective electron/hole mass, high mobility, and low threshold voltage operations (~1V).1,2 They can be employed as substrates, barrier layers, or active layers, in conjunction with other ternary or quaternary compounds in the GaInAlAsSb material system, to produce novel photonic and optoelectronic devices that can operate over a large range of bandgaps, ranging from 0.2 to 2.2eV. However, the difficulty in producing defect and damage free GaSb substrates surfaces with thermally desorbable surface oxides presently inhibits the wider-scale commercial applications of these materials. Surface topography of a substrate and growth conditions strongly influence the quality of epitaxially grown films. Several wet etch techniques have been developed to prepare GaSb surfaces for epitaxial growth with varying degrees of success.3 Recently, dry-etch techniques using the gas-cluster ion beam (GCIB) 4,5,6 technique, incorporating either O2 or CF4/O2 as the source gas, and the bromine ion-beam assisted etching (Br-IBAE)7 techn
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