Disilane Adsorption on Ge(111): A Multiple Internal Reflection Infrared Spectroscopy Study
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DISILANE ADSORPTION ON Ge(111): A MULTIPLE INTERNAL REFLECTION INFRARED SPECTROSCOPY STUDY JOHN E. CROWELL AND GUANGQUAN LU Department of Chemistry, University of California at San Diego, La Jolla, CA 92093-0314
ABSTRACT The adsorption and decomposition of disilane on the Ge(1 11) surface has been investigated using multiple internal reflection infrared spectroscopy (MIRIRS). At temperatures < 110 K, Si 2H6 weakly bonds to the Ge surface in a molecular fashion. Upon mild heating to temperatures between 110 - 150 K, both desorption and dissociation occur. Dissociative adsorption also occurs upon exposure at temperatures > 110 K. Molecular adsorption is no longer observed for exposure at temperatures above 150 K. Detection of the low frequency deformation modes provides direct evidence for dissociation of the Si-Si bond between 120 150 K. The primary species present at 150 K is silyl, SiH 3. Si-H bond scission takes place from 150 - 500 K, resulting in GeH formation. Production of SiH 2 and SiH is spectroscopically identified in this temperature range.
INTRODUCTION Determination of the surface chemical processes occurring during the growth of semiconductor superlattices is important for controlling device fabrication. This is especially true for low temperature epitaxial growth of heterostructures using ultra high vacuum - chemical vapor deposition (UHV-CVD) [1-3]. Heterostructures made from ultrathin films of Si / Ge and SiGe alloys are promising materials for optoelectronic devices [3,4]. The importance of these materials is exemplified by the recent announcement [3] of a 45 GHz NPN heterojunction bipolar transistor fabricated using SiGe (= 11% Ge) as the base. The speed of this device, grown using UHV-CVD low temperature epitaxy, is due to the graded SiGe base structure. The SiGe system is of special interest due to its unique electrical properties, and the fact that it is readily incorporated into existing Si technology. We have studied the adsorption of disilane on Ge( 111) as a function of temperature and exposure using multiple internal reflection infrared spectroscopy (MIRIRS). The MIRIRS technique enables high resolution IR spectroscopy to be used to study adsorption and reaction processes on semiconductor surfaces. MIRIRS can provide a direct probe of the chemical nature of a semiconductor surface, and hence used to determine the identity and geometry of adsorbates present on the surface. Furthermore, the high resolution of the method, together with the good surface sensitivity, allows distinction between species with similar vibrational frequencies. In this study, the choice of reactant is made based on the significantly higher adsorption probability expected for disilane vs. that of silane [5]. Little is known about the reaction of disilane on Ge surfaces. Furthermore, in performing MIRIRS studies, the choice of Ge per1 mits vibrational modes to be detected as low as = 700 cm- , whereas free carrier absorption in 1 of the low frequency bending detection . The = 1450 cnbelow frequencies all Si obscures v
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