Density of localized electronic states in a-Se from electron time-of-flight photocurrent measurements

Electron time-of-flight transient photocurrents have been investigated in stabilized a-Se as a function of electric field, annealing, aging (relaxation), and alloying with As and doping with Cl. The distribution of localized states (DOS) in stabilized a-Se has been investigated by comparing the measured and calculated transient photocurrents. The samples were prepared by conventional vacuum deposition techniques. The theoretical analysis of multiple-trapping transport has been done by the discretization of a continuous DOS and the use of Laplace transform formalism. The resulting DOS has distinct features: A first peak at ∼0.30eV below Ec with an amplitude ∼1017eV−1cm−3, a second small peak (or shoulder) at 0.45–0.50 eV below Ec with an amplitude 1014–1015eV−1cm−3, and deep states with an integral concentration 1011–1014cm−3 lying below 0.65 eV, whose exact distribution could not be resolved over the time scale of present experiments. The influence of doping, aging, annealing, and substrate temperature on the DOS distribution has been investigated. The doping with Cl does not affect the amplitudes of the first and second peaks while the concentration of deep states increases dramatically. The alloying with As reduces the density of deep states and seems to increase the amplitude of first and second peaks. The aging substantially reduces the deep states density and the amplitude of the second peak while the amplitude of the first one remains practically unchanged. The results have been interpreted primarily in terms of thermodynamic and intrinsic structural defects in the chalcogenide glass structure.