Deciphering the Vibrational Spectrum of Interstitial H 2 in Si
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Deciphering the Vibrational Spectrum of Interstitial H2 in Si Michael Stavola, E Elinor Chen, and W. Beall Fowler Department of Physics and Sherman Fairchild Laboratory, Lehigh University, Bethlehem, Pennsylvania 18015, USA ABSTRACT H2 is a fascinating molecule whose properties revealed the influence of nuclear spin on the molecular wave function in the 1920s. As an interstitial defect in Si, the H2 molecule has given rise to a number of perplexing puzzles since the discovery of its vibrational spectrum. The absence of an ortho-para splitting for the H2 vibrational line and an apparent low symmetry found in stress experiments misled several researchers into thinking that interstitial H2 in Si must have a barrier to rotation. Our discovery of a new vibrational line for HD in Si and its interpretation, along with the recognition that certain transitions are possible for HD, but not for H2 or D2, establish that H2 in Si is a nearly free rotator after all. Additional puzzles such as the anomalous intensity of the HD line, the absence of an isotope dependence for the uniaxial stress splitting of the H2 and D2 vibrational lines, and the properties of an O-H2 complex are also explained naturally. Recent Raman studies confirm that interstitial H2 in Si is a free rotator but raise interesting new questions about the diffusivities of the ortho and para species.
INTRODUCTION The importance of interstitial H2 molecules in semiconductors was suggested by theoretical work performed in the early 1980s when interest in the properties of H in semiconductors was beginning to grow [1-3]. The formation of H2 molecules was also suggested to explain the diffusion of H into Si and Ge [4]. Nonetheless, the H2 molecule in a semiconductor was not observed directly until recently when vibrational lines for the H2, HD, and D2 molecules in GaAs [5] and Si [6-8] were discovered. In addition to the isolated H2 defect, an O-H2 complex is formed when the H2 molecule becomes trapped near an oxygen impurity in Czochralski-grown Si [7]. Interstitial H2 and O-H2 in Si are shown schematically in Fig. 1. These experimental results motivated a number of theoretical studies of the microscopic properties of interstitial H2 in semiconductors [9-14]. For several years, it was not possible to reconcile a growing body of experimental results for interstitial H2 in Si with the properties observed for H2 in GaAs or with the predictions of theory. The Raman band of the H2 molecule in GaAs is split into two components, 8 cm-1 apart, with an intensity ratio of ~ 3:1 (ref. 5). These lines were assigned to ortho- and para-H2, whose frequencies differ because of ro-vibrational coupling. This interpretation leads naturally to the conclusion that H2, sitting at a Td interstitial site in GaAs, is freely rotating. For the Si host, several theoretical calculations for H2 at a tetrahedral interstitial site found that , , and orientations have similar energies, suggesting that interstitial H2 in Si should also rotate nearly freely [9-12]. Furthermore, molecular dynam
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