J. Bullot et al., PHOTOCONDUCTIVITY IN HYDROGENATED AMORPHOUS-SILICON .1. THERMAL EMISSION AND HOPPING OF TRAPPED CHARGES, Philosophical magazine. B. Physics of condensed matter. Structural, electronic, optical and magnetic properties, 67(6), 1993, pp. 751-762
The photoconductivity of hydrogenated amorphous silicon is found to be
the result of three different processes that have been identified usi
ng the dual-beam-modulated photoconductivity technique: the release of
electrons trapped either in D- states or in the conduction-band tail,
and a quenching process related to recombination. Each process follow
s its own generation rate dependence. A model has been established in
which the above three processes add up to give the observed steady-sta
te photoconductivity. The relative contributions of these processes ha
ve been determined over large temperature and light flux ranges. This
allowed us to show firstly that the thermal release of electrons from
gap states, either from D- or from the band tail, is still an importan
t process at 90 K, secondly that the relative contributions of D- and
tail states change drastically at about 165 K, that is when the electr
on quasi-Fermi level is located at about 0.3 eV below the mobility edg
e and thirdly that at low temperatures the above three processes canno
t account for the whole photoconductivity and hopping transport of ele
ctrons has to be taken into account, the contribution from which is ab
out 50% at 90 K.