The B-H complex in Si can be aligned by stress and reorients with an activation energy of roughly 0.2 eV. We combine new measurements of the reorientation kinetics of the B-H complex made by the stress-induced dichroism technique with previous internal friction results to show that the reorientation kinetics are non-Arrhenius.These results support Stoneham's suggestion [Phys. Rev. Lett. 63, 1027] that reorientation occurs by thermally assisted tunneling.We have also discovered the remarkable fact that the reorientation rate for B-D is greater than that of B-H for T & 57 K. PACS numbers: 61.72.Ji, 78.30.Hv, 78.50.6e The thermally activated reorientation of acceptorhydrogen complexes in semiconductors is well known [1 -8]. The activation energies for reorientation are typically a few tenths of an eV. The best studied example is the B-H complex in Si for which the reorientation kinetics have been studied by stress-induced dichroism[1] at low temperature (=65 K) and internal friction [7,8] at high temperature (=130 K). Adiabatic total energy surfaces have been calculated theoretically for the B-H complex [9],and the agreement of the measured activation energy for reorientation with the calculated barrier height is excellent.In spite of what appears to be good agreement between experiment and theory, Stoneham [10,11] has suggested that the reorientation of the acceptor-H complexes is by a thermally assisted tunneling mechanism and that a model in which the hydrogen jumps over a barrier is inappropriate. This is well known to be the case for hydrogen diffusion in metals [12] and has been studied for many years. One of the classic signatures of quantum diffusive motion of light interstitials in metals is non-Arrhenius hopping kinetics. Here, by combining new stress-induced dichroism data with previous results obtained by internal friction [7,8], we show for the first time that the reorientation kinetics of the B-H complex are non-Arrhenius and thereby provide experimental support for a tunneling mechanism for reorientation. Further, the reorientation kinetics of the B-H and B-D complexes have been measured together in the same samples to convincingly show that the activation energies for reorientation are different and that the reorientation rate of the B-D complex is faster than that of B-H above 57 K.The samples for most of our experiments were prepared from floating zone silicon that had been ion implanted with B at energies of 30, 100, and 180 keV, each to a dose of 7 X 10' cm . The implants were activated by a rapid thermal anneal at 1200 C for 60 s. Oriented bar-shaped samples with dimensions 2 x 2 x 9 mrn were prepared for stress studies. Both hydrogen and deuterium were introduced into each sample at 120 C in a Technics Planar Etch II plasma reactor. In order to check that our results were independent of the sample fabrication procedure and doping level, stress samples were also prepared with a lower implantation dose, i.e. , 2 X 10' cm at 30, 100, and 180 keV, and from bulkdoped material with [B] = 2 X 1...

Perturbation of the 2201-cm Ϫ1 absorption line of the Al-H complex in Si by uniaxial stress is shown to be consistent with a stretching mode of a center with an effectively trigonal symmetry. Quantitative analyses of the effects of the stresses and of varying the temperature reveal the presence of a very-low-frequency transverse mode which transforms as the E irreducible representation in the trigonal symmetry. Its frequency is 64 cm Ϫ1 in the ground state of the stretching mode ͑representation A 1 ͒. An A 1 ϩE combination mode, observed only under stress, lies 24 cm Ϫ1 above the excited state of the A 1 mode. Upon substituting D for H, the frequencies of the stretching and transverse modes are reduced by approximately a factor of 1/&, as expected for vibrations which predominantly involve the H ͑D͒ atom. The symmetries of the center and of its modes are unchanged by the isotopic substitution. The Al-H axis may be preferentially aligned along one of the ͗111͘ axes by an applied stress. The barrier to reorientation is ϳ370 meV. ͓S0163-1829͑96͒03139-6͔