AbstractComprehensive measurements of secondary photoconductivity with steady optical excitation (SSPC), transient excitation (TPC), and steady state optical bias plus transient excitation (OB) were carried out on samples of glow-discharge produced n-type a-Si:H from two sources, lightly doped with phosphorus and arsenic respectively.
The data from the short time TPC, measured as a function of excitation density are consistent with multiple trapping and saturation of the trap states above the electron quasi-Fermi level,
which allows determination of the distribution of conduction band tail (CBT) states, and hence comparison of the quality of the two materials. A broad exponential tail of high state density was found for the As-doped material while the CBT in the P-doped material was found to be characterised by a shallow shoulder (-0.17eV below the conduction mobility edge) below which the density of states falls sharply.
Both the SSPC and the long time TPC results from the P-doped sample were interpreted in terms of a recombination model in which the recombination centres are exclusively dangling bond defects mainly in the D state. The band tail states trap most of the charge, act as carrier reservoirs and manifest themselves only in the charge neutrality relation. The recombination rate limiting step is hole release and subsequent monomolecular recombination via D states.
In addition, the model explains the SSPC maximum observed in annealed n-type a-Si:H at relatively high temperatures and predicts the thermal quenching caused by light soaking of such material. It is of interest to note that the independence of the SSPC with temperature at very low temperatures, coupled with a linear response to excitation can also be explained by the model in terms of a changeover to hole conduction.
|Date of Award||Dec 1990|