Hydrogen and Muonium in the Lattice

The energy surfaces for H in various semiconductors exhibit a number of common features. In this part of the chapter, I will mainly address the neutral impurity results for other charge states will be presented in Section V. In the first part of this section, I will present a general discussion, which will be illustrated with results for silicon. Subsequent sections will contain results specific to other semiconductors. 

Since motion of the host atoms is so essential at the bond-center site, only calculations that allow for relaxation will produce it as a stable site. Among those are recent cluster calculations for Si by Estreicher (1987), who used the PRDDO method as well as ab initio minimal-basis-set 

One study (DeLeo et al., 1988 Fowler et al., 1989) has found that neutral H at the B site in Si has a tendency to preferentially bind to one of the two Si neighbors, leading to an asymmetric configuration, with Si—H distances of 1.48 A and 1.77 A respectively. This tendency was interpreted in terms of a pseudo-Jahn-Teller distortion. However, the potential barrier that leads to the asymmetric position is so low ( 0.2 eV) that it can readily be surmounted by zero-point motion of the proton. Experimental observation of such an asymmetry is therefore unlikely, except maybe through an isotope shift measurement in an infrared experiment (DeLeo et al., 1988). None of the other theoretical approaches has produced this type of asymmetry. 

Finally, the hexagonal interstitial (H) site, which lies in the (111) direction halfway between two T sites, was found to be a local minimum along the (111) direction but only a saddle point when considered in three dimensions (Van de Walle et al., 1989). 

The location of H in diamond has been investigated in several cluster studies. When no relaxation of the host is allowed, most calculations find a minimum of the energy surface at T, with a potential well deep enough to 

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From the discussion of location of hydrogen and muonium in the lattice, it is clear that the bond-center position is the lowest-energy site in many semiconductors. In GaAs and Si, this site has only recently been experimentally associated with anomalous muonium, which is relatively immobile (Kief et al., 1987, 1988). The agreement between theoretical results for hyperfine parameters (Van de Walle, 1990) and experiment allows an unambiguous identification of Mu with the bond center. Other models, such as the vacancy model proposed by Sahoo etal. (1985, 1989), are no longer considered acceptable candidates for Mu. ... 

The simulation of hydrogen by muons has proved to be extremely valuable in the identification of potential sites for hydrogen in semiconductors and insulators. Although the muon has a mass one-ninth that of the proton, its interaction with the host lattice, both electronically and chemically, is virtually identical to that of a proton. During its 2.2 microsecond lifetime (experiments are frequently undertaken over a timescale of up to ten lifetimes), the muon can diffuse, interact with, and adopt positions in the lattice that protons themselves would occupy. If the temperature is sufficiently low, muons can capture electrons to form muonium atoms. The reduced mass of muonium is within 0.5% of that of... 

Anomalous muonium in silicon, and apparently also the AA9 hydrogen center, is stable because there is a large lattice relaxation of the two closest silicon neighbors to the muon or proton. This is the conclusion of theory,... 

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