A Monte-Carlo simulation of the time-of-flight experiment in non-uniformly defected thincrystalline layers

In the present paper we report on the results of the Monte Carlo simulation of the time of flight (TOF) experiment for r and r- epsilon hopping transport in highly defected very thin crystalline layers. The defects are considered as localized states, between which a hopping motion of the injected carriers is possible. The total (integrated over energy) density of defects is assumed to be spatially non-uniform on the macroscopic scale, i.e. the scale comparable with the layer thickness. In particular, we consider an exponential dependence of the total density of hopping centres on the distance from the layer contacts. The results of simulations performed for various defect concentrations, various defect distributions in energy, and various degrees of the layer spatial non-uniformity are discussed. It is shown that both r and r- epsilon hopping transient currents measured in the classical TOF experiment are highly sensitive to the spatial macroscopic scale variations of the total centre concentration. The detailed shape of the transients depends in a complicated way on the system dilution, the energetic centre distribution, and the character of the spatial variations of the total centre density. The existence of the spatial non-uniformity of the layer could be recognized experimentally by observation of the qualitative changes of the current shape with increasing temperature, which leads to lower dispersion, and thus more pronounced characteristic features of the x-dependent total centre density, such as a higher polarity dependence, or the appearance of the current maxima or plateaux just before the effective TOF.