Hydrogen diffusion in crystalline semiconductors

The diffusion of hydrogen in semiconductors is complicated by the existence of several charge states (notably H⁺ in p-type material and H^- or H⁰ in n-type material, at least for Si) and also that hydrogen is present in a number of different forms, namely atomic, molecular or bound to a defect or impurity. Since the probability of formation of these different states is dependent on the defect or impurity type and concentration in the material and on the hydrogen concentration itself, then the apparent hydrogen diffusivity is a function of the sample conductivity and type and of the method of hydrogen insertion. Under conditions of low H⁺ concentration in p-type Si, for example, the diffusivity is of the order of 10^-10 cm² · s^-1 at 300 K and is consistent with the value expected from an extrapolation of the Van Wieringen and Warmoltz expression D_H = 9.4 × 10^-3 exp-0.48 eV/kT] cm² · s^-1. The characteristics of hydrogen diffusion in n- and p-type Si and GaAs are reviewed in this paper, and the retardation of hydrogen permeation by molecular formation and impurity trapping is discussed. The measurement of several key parameters, including the energy levels for the hydrogen donor and acceptor in Si and the diffusivity of the H⁰ and H^- species, would allow a more quantitative treatment of hydrogen diffusion in semiconductors.