Local dynamics of H and D trapped by substitutional dopants in semiconductors

The motion regimes of hydrogen and deuterium in p-type Si:B and GaAs:Zn have been studied by anelastic spectroscopy (elastic energy loss and dynamic modulus) between 1 and 600 K in the frequency range (1-40) kHz. Interstitial H or D in Si:B are bound to the acceptor acting as a trapping centre, and display rather high jumping rates between the four equivalent bond centre sites around B; the respective relaxation peaks are nearly single Debye processes. The H(D) reorientation rate ?-1(T) is satisfactorily interpreted by the classical Arrhenius law in terms of overbarrier hopping, within the temperature range of measurements. However, by joining the data from anelastic relaxation with those from infrared absorption, the range of the measured H(D) relaxation rates is widened to 11 orders of magnitude, and a slight deviation from the Arrhenius law to higher rates is observed below liquid nitrogen temperature. The analysis of the relaxation curves indicates that the models of incoherent tunnelling used to interpret the behaviour of hydrogen (deuterium) in metals hardly apply to the presently investigated systems. The influence of quantum tunnelling is even more evident in GaAs:Zn charged with H or D, which display relaxation processes at liquid helium temperatures characterized by transition rates of more than 10 orders of magnitude faster than all those previously reported in hydrogenated semiconductors. The mobility parameters obtained and the features of the relaxation spectra may suggest that the mobile species causing the effect performs coherent tunnelling within close sites, and the relaxation rates are not related to jumps, but to transitions among the quantized energy levels of the tunnel systems. Although the species causing the effect has not yet been univocally individualized, the data shows that the presence of both H(D) and Zn in the relaxing complex is required.