Al-H and Al-D complexes in Si: A uniaxial-stress study of the hydrogen vibrational modes

Perturbation of the 2201-cm(-1) absorption line of the Al-II 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(-1) in the ground state of the stretching mode (representation A(1)). An A(1) + E combination mode, observed only under stress, lies 24 cm(-1) above the excited state of the Al mode. Upon substituting D for H, the frequencies of the stretching and transverse modes are reduced by approximately a factor of 1/root 2, as expected for vibrations which predominantly involve the H (D) atom. The symmetries of the center and of its modes are unchanged by the isotopic substitution. The Al-Il axis may be preferentially aligned along one of the [111] axes by an applied stress. The barrier to reorientation is similar to 370 meV.