Metastable defect creation in tritiated hydrogenated amorphous silicon and the Staebler–Wronski effect

Abstract

Defect creation through tritium decay in tritiated hydrogenated amorphous silicon provides a unique technique for the study of defect dynamics in hydrogenated amorphous silicon (a-Si:H). Isothermal Capacitance Transient Spectroscopy (ICTS) and Constant Photocurrent Method (CPM) were used to measure the positively charged, D+, and negatively charged, D−, states in the gap of a-Si:H:T, respectively. The samples were thermally annealed prior to the measurement of defect state densities. The ICTS experiments showed a decrease in the concentration of positively charged dangling bonds while the CPM measurements showed an increase in the density of D− states with time. This increase was much larger than the concentration of decayed tritium atoms. The CPM results also showed that the Urbach energy decreased with time. The decrease in Urbach energy indicates a decrease in the concentration of weak bonds in the valence band tail and suggests weak bond to dangling bond conversion. CPM experiments carried out under bias confirm the role of excess carriers in the defect creation process. The dynamic defect pool model was developed to quantitatively explain the experimental data. The defect evolution in a-Si:H:T is similar to the evolution of defect states in light exposure studies, supporting the interpretation of the Staebler–Wronski effect in terms of weak bond to dangling bond conversion.