Growth of Germanium on Porous Silicon (001)
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M Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois USA Department of Materials Science, Coordinated Science Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 C.C. Cameron Applied Research Center and Department of Electrical Engineering, University of North Carolina, Charlotte, NC 28223
ABSTRACT The surface morphology of Ge grown on Si (001) and porous Si(0O1) by molecular beam epitaxy at 380 'C is examined using atomic force microscopy (AFM). For layer thicknesses of 30 nm, the surface shows islanding while still maintaining some of the underlying roughness of the surface of porous Si. For thicknesses in the 100 nm range, the surface roughness is not visible, but the islanding persists. Unlike the case of silicon where islands tend to merge and nearly disappear as the thickness of the deposited layer rises, we observe on the porous layer the persistence of the islands with no merging even for macroscopic thicknesses as large as 0.73 microns. INTRODUCTION The discovery of strong, visible photoluminescence in porous silicon was a surprise due to its indirect band gap [1, 2]. This raises the possibility of incorporating an opto-electronic device on silicon which would eliminate many of the problems associated with the compound semiconductors that have been developed to create opto-electronic devices. They are expensive and not easily integrated with silicon due to the difference in their lattice spacing. Heteroepitaxial growth of metals, insulators and semiconductors and their compounds on silicon substrates is a rapidly expanding field of research in Si molecular beam epitaxy (MBE). In particular, the growth of device-quality material on Si is of great interest for several reasons, including strong incentive to utilize large diameter, structurally robust, and inexpensive Si wafers as substrates, advantages in integrating the high-speed and optical properties of the III - V devices with Si very large scale integration (VLSI) technology. The interest is also driven by the anticipated band gap engineering, quantum confinement of carriers, and the incorporation of direct band gap materials such as GaAs in well established Si microdevice technology. Recently, growth of a high quality epitaxial Ge film as an intermediate layer prior to subsequent device quality growth of GaAs and the other III-V compounds has been suggested as a means for alleviating many of the problems of growing on Si that are encountered due to lattice mismatch among other things [3]. In the hope of combining optically active porous silicon with GaAs, we have studied the growth of Ge on porous silicon We compare the surface topography of epitaxial growth of Ge on porous Si(001) to growth on Si(001) using atomic force microscopy (AFM), and examine the dependence of the surface topography on the thickness of the germanium layer. For nanometer scale thicknesses, the surface topography shows islanding while reflecting some of the underlying roughness of the surface of porous Si. For thickness
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