Time evolution of charged defect states in tritiated amorphous silicon

Abstract

Tritiated hydrogenated amorphous silicon (a-Si:H:T) thin films were deposited on crystalline silicon and high resistivity glass substrates. The time evolution of the density of defect states in these films was studied using the constant photocurrent method (CPM) and isothermal capacitance transient spectroscopy (ICTS). The density of defect states was found to change with time and to recover upon thermal annealing. The ICTS results revealed that, following thermal annealing, in a sample with approximately 1at.% tritium, the concentration of positively charged dangling bonds (D+) decreased by more than an order of magnitude over a period of 300h. The CPM results showed that, over the same period of time, the concentration of negatively charged dangling bonds (D−) increased by over two orders of magnitude. The D+ and D− concentrations followed exponential functions of time, but the rate was different than that of tritium decay. At the same time, the Urbach energy was found to decrease with time to about 1∕2 of its postanneal value. The change in the D+ and D− concentrations is primarily the result of capture of the beta particle generated electrons in dangling bonds and weak bonds, with steady state achieved through the development of a balance between carrier generation and carrier capture processes. The role of excess carriers was confirmed by CPM experiments under electrical bias.