Characterization of High Ge Content SiGe Heterostructures and Graded Alloy Layers Using Spectroscopic Ellipsometry
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A. R. Heyd*'t, S. A. Alterovitz*, and E. T. Crokel *NASA Lewis Research Center, 21000 Brookpark Road, MS 54-5, Cleveland, OH 44135 tHughes Research Laboratories, 3011 Malibu Canyon Rd, MS RL63, Malibu, CA 90265 tThis work was performed while the author held a National Research Council-NASA Research Associateship ABSTRACT SiGel-, heterostructures on Si substrates have been widely studied due to the maturity of Si technology. However, work on SixGel-x heterostructures on Ge substrates has not received much attention. A SiXGel-x layer on a Si substrate is under compressive strain while SiTGel-. on Ge is under tensile strain; thus the critical points will behave differently. In order to accurately characterize high Ge content Si.Gel-. layers the energy shift algorithm, which is used to calculate alloy compositions, has been modified. These results have been used along with variable angle spectroscopic ellipsometry (VASE) measurements to characterize SixGel_.T/Ge superlattices grown on Ge substrates. The results are found to agree closely with high resolution x-ray diffraction measurements made on the same samples. The modified energy shift algorithm also allows the VASE analysis to be upgraded in order to characterize linearly graded layers. In this work VASE has been used to characterize graded SiGel_, layers in terms of the total thickness, and the start and end alloy composition. Results are presented for a 1 pm SixGel-,,, layer linearly graded in the range 0.5 < x < 1.0. INTRODUCTION SiTGeiT/Si epilayers have recently started to be used for device fabrication. The ability to vary the Si composition, x, in the SiGel-x layer offers the device designer a wide range of options. To date, most work has been focused on SixGel_,/Si heterostructures grown on Si substrates. To our knowledge little work has been done on high Ge content SixGel-x structures grown on Ge substrates. The ability to deposit quality SiTGel_,/Ge heterostructures will further expand the material properties and band offsets available to device engineers. As interest in high Ge content SiGel-, materials increases, accurate characterization techniques of such materials and structures will be needed. Recently, variable angle spectroscopic ellipsometry (VASE) has been shown to be a powerful, non-destructive technique for the post-deposition characterization of SixGell/Si superlattices (SLs) and device structures [1, 2]. In these studies VASE was used to determine the layer thicknesses, alloy composition, oxide thickness, sample homogeneity, and number of SL periods. The alloy composition was determined by interpolating between the Si.Gel-. dielectric functions of Jellison et al. 13]. However, only the Si rich materials, x > 0.5, were considered. In this current work, we have turned our attention to high Ge content SiGel_•/Ge SLs grown on Ge substrates. To accomplish this, the SiGel-x interpolation scheme has been modified so that it can be used for SiýGe_.T at all compositions, 0.0 < x < 1.0. In order to relieve stress in SiGel-x layers, a graded layer is
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