Hydrogen Motion in the B—H Complex

The BC configuration and the H motion of interest are shown in Fig. 23. The stress-induced-alignment technique described in Sec. II.3 was used by Stavola et al. (1988b) to measure the kinetics of such an H jump. The longitudinal H stretching mode that is used as a probe of the center s orientation lies at 1903 cm-1 at low temperature (Sec. III. 1). 

The orientation dependence of the stress alignment effect is consistent with the trigonal symmetry of the B—H complex. Stress along the [110] direction lifts the orientational degeneracy of the four BC sites about the boron while stress along the [100] direction does not. (A [111] stress also leads to a dichroism of the expected magnitude.) The sites perpendicular to 

To determine the kinetics of the hydrogen motion from BC to BC site adjacent to the B, the decay of the stress-induced dichroism was measured at several temperatures. A [110] stress was applied at an elevated temperature (77 K) to align the B—H centers. The samples were cooled to the measurement temperature with the stress maintained. The stress was removed and a and a// were measured as a function of time at fixed temperature. 

It was found that In D decays linearly with time [where D is the dichroic ratio defined in Eq. (2)]. The time constant for the decay of the dichroism, r, is plotted vs temperature-1 in Fig. 25. It was shown (Stavola et al., 1988b) that the time constant, r, for a single H jump from one BC site adjacent to the B to another given by r=4r. From the fit to the data shown in Fig. 25 the following expression for the time constant for a single H jump was obtained  

Denteneer et al. (1988, 1989a,b,c) have predicted that there is a low energy pathway for H motion about the B in which the H can move from 

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In Chapter 8, Stavola and Pearton discuss the local vibrational modes of complexes in Si that contain hydrogen or deuterium. They also show how one can use applied stress and polarized light to determine the symmetry of the defects. In the case of the B-H complex, the bond-center location of H is confirmed by vibrational and other measurements, although there are some remaining questions on the stress dependence of the Raman spectrum. The motion of H in different acceptor-H complexes is discussed for the Be-H complex, the H can tunnel between bond-center sites, while for B-H the H must overcome a 0.2 eV barrier to move between equivalent sites about the B. In the case of the H-donor complexes, instead of bonding directly to the donor, H is in the antibonding site beyond the Si atom nearest to the donor. The main experimental evidence for this is that nearly the same vibrational frequency is obtained for the different donor atoms. There is also a discussion of the vibrational modes of H tied to crystal defects such as those introduced by implantation. The relationship of the experimental results to recent theoretical studies is discussed throughout. 

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