Kinetics of metastability in doped hydrogenated amorphous silicon

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Abstract
Thermodynamic equilibrium of doped hydrogenated amorphous Si (a-Si: H) is explored using dark-conductivity measurements of small thermal perturbations. Raising the temperature increases the electron and hole densities owing to the activation of additional dopants, while decreasing the temperature causes the opposite effect of dopant passivation. The equilibration kinetics of P- and B-doped material are different and depend strongly on the deposition temperature. The activation kinetics of P-doped a-Si:H follow a stretched exponential with a temperature-independent β value of 0.85 whereas dopant passivation in the same temperature range is also stretched-exponential decay, but with values of β<0.8. In contrast, the kinetics of B activation and passivation are of a stretched-exponential type with equal β values. The time constant τ to achieve equilibrium for both activation and passivation is thermally activated with energies of about 1.1 eV for P- and B-doped a-Si: H. τ has a weak dependence on the magnitude of the thermal perturbation, while an increase in deposition temperature leads to a larger activation energy. The discussion and interpretation of the data are based on interactions between the electronic states and the H-bonding structure.