HYDROGEN MOTION AND STRETCHED-EXPONENTIAL RELAXATION IN A-SI-H

Relaxation phenomena involving the motion of hydrogen in hydrogenated amorphous silicon, that have traditionally been described with a distr ibution of relaxation times and a stretched-exponential functional, we re recently analysed by Van de Walle (Van de Walle C G 1996 Phys. Rev. B 53 11292) in terms of retrapping in a three-level energy diagram. W e show that the expression derived by Van de Walle can also be obtaine d on the basis of hydrogen transport via interstitial positions which are in thermal equilibrium with a set of hydrogen traps located about 1 eV below the hydrogen chemical potential. However, the models based on hydrogen retrapping only give a good account of the relaxations for temperatures above room temperature. Dispersive behaviour is assumed to be responsible for the discrepancy at lower temperatures.