Mobility - lifetime estimates in amorphous hydrogenated silicon ( a -

Estimates of the majority photocarrier mobility-lifetime (pt) product obtained from time-of-flight transient photoconductivity and from steady-state photo-conductivity are compared. It is shown that in principle both experiments measure the integral of the transient drift mobility; the demonstration is based solely on linear response theory and is independent of the specific microscopic transport and recombination model. Estimates of pz for undoped amorphous hydrogenated silicon (a-Si : H) based on the two techniques disagree by a factor of approximately 100 in comparable specimens. Possible origins of the discrepancy are discussed in relation to experimental procedures, optical bias effects and surface inhomogeneity. It is concluded that in undoped a-Si:H the dangling-bond defect observed in electron-spin resonance acts predominantly as a photocarrier trap and not as a simple recombination centre. It is widely known that steady-state photoconductivity measurements yield an estimate of a majority photocarrier mobility-lifetime (yz) product. Recently several authors have employed time-integrated transient photoconductivity in the time-of-flight geometry (the 'charge collection' technique) to obtain an alternative y~ estimate for both electrons and holes in amorphous hydrogenated silicon (a-Si : In this paper I demonstrate that the two yz techniques-charge collection and steady-state photoconductivity-in principle yield identical estimates of the majority-carrier pr product if surface inhomogeneity can be neglected. The demonstration is a simple consequence of linear response theory and is independent of any detailed model for transport or recombin-ation. Both measurements are estimates of the time-integral of the transient majority-carrier drift mobility p,(t). For a-Si:H there have of course been many estimates of the steady-state photoconductivity mobility-lifetime product, p.rss. Particularly useful measurements are those of Evangelisti, Fiorini, Fortunato and Giovanella (1983) and Stutzmann, Jackson and Tsai (1985). Stutzmann et al. (1985) demonstrated that the steady-state photoconductivity was inversely proportional to the neutral dangling-bond defect (T!) density determined from electron-spin resonance when the specimen was modified by