Electronic Structure and Hyperfine Parameters for Hydrogen and Muonium in Silicon
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ELECTRONIC STRUCTURE AND HYPERFINE
PARAMETERS FOR HYDROGEN AND MUONIUM IN SILICON CHRIS G. VAN DE WALLE Philips Laboratories, North American Philips Corporation, Briarcliff Manor, NY 10510
ABSTRACT First-principles spin-density-functional calculations are used to evaluate hyperfine and superhyperfine parameters for hydrogen and muonium at various sites in the Si lattice. The results can be directly compared with values from muon-spin-rotation experiments, leading to an unambiguous identification of "anomalous muonium" with the bond-center site. The agreement found in this case instills confidence in the general use of spin-density-functional calculations for predicting hyperfine parameters of defects. INTRODUCTION The behavior of hydrogen in semiconductors has been the focus of intense experimental and theoretical interest in recent years. The observation that hydrogen can passivate shallow impurities' illustrated the technological importance of the subject; it also emphasized the need for a more profound understanding of hydrogen as an impurity in a semiconductor lattice. Theory'-s has been able to significantly enhance insight in the interactions between hydrogen (H) and the semiconductor host atoms, and with other impurities. In the case of isolated interstitial hydrogen, an unambiguous identification of the proposed structures with the experimentally observed defects was still missing, however. Experimental information about defect configurations can be obtained from electron 9 paramagnetic resonance (EPR) or muon spin rotation (,iSR), which probe the interaction between electronic wavefunctions and nuclear spins. A conclusive structural identification of the defect is possible if reliable theoretical calculations of the hyperfine parameters for various configurations are available. Comparison of theory and experiment then determines whether a proposed structure is consistent with experimental observations. In this paper I present theoretical results for hyperfine parameters calculated for H at various sites in the Si lattice. Apart from the parameters for the impurity itself, I also give values for the so-called superhyperfine parameters which describe the interaction of the defect wavefunctions with neighboring host atoms. The electronic-structure calculations, based on pseudopotential-spin-density-functional theory, are shown to yield values in good agreement with experiment, allowing an unambiguous identification of "normal muonium" with the tetrahedral interstitial site, and "anomalous muonium" with the bond-center site. Experimentally, it has proved very difficult to obtain results for an isolated hydrogen 9 atom. Fortunately, a vast amount of information is available about muonium. Muonium is a pseudo-isotope of hydrogen; it consists of an electron bound to a positive muon (t) The mass of p+ is 1/9 of that of the proton, and its lifetime is 2.2 ps. Muon spin rotation and muon level-crossing resonance are the experimental techniques that have provided a 9 0 wealth of information about muonium in solid-s
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