Relaxation Effects due to Tunnelling of Hydrogen in Metals and Semiconductors

Abstract
The mobility of hydrogen and its isotopes in metals has been the object of investigation for several years, whereas the diasion studies of H in doped semiconductors started more recently. Although the H diffusion coefficient in metals may be several orders of magnitudes higher than in semiconductors, the dynamics of H in metals and semiconductors presents many common features, like precipitation, trapping by heavier impurities and, as indicated by recent results, quantum tunnelling at low temperature.
In metals two regimes of the H mobility are observed : hopping with deviations from a classical Arrhenius motion, and a much faster tunnelling within few close sites. In the latter regime the H dynamics does not consist of jumps but of transitions between the quantized energy levels of the tunnel systems. The types of interactions assisting the H transitions and the geometry of the tunnel systems are an open problem : although the two-level tunnel system (TLS) has been widely used to explain neutron diffision, specific heat, and acoustic spectroscopy results in interstitial solutions (NbO_xH_y), recently this model has appeared not to be valid in substitutional solutions (NbZr_xH_y, NbTi_xH_y) where the tunnel systems have a higher symmetry. The four-level systems seem to be more appropriate, although the corresponding model has not been developed as much as the TLS yet.
In boron doped silicon, the relaxation rates τ^-1(T) of H around B obtained from anelastic relaxation were joined with those from infrared absorption : the remarkably wide range obtained (11 decades) clearly shows a deviation of τ^-1(T) from the classical dependence at low temperature. However, no conclusions can be drawn at present on the mechanism governing the H(D) dynamics. Most recently, the investigation of the dynamics of H(D) in GaAs doped with Zn revealed a dissipation peak at 20 K in the kHz range. This relaxation has the highest rate found so far for H in a semiconductor : more than 15 orders of magnitude higher than in all the other semiconductors measured so far. The analysis of the dissipation curves indicates that the nature of the H reorientation is strongly quantistic.