Vacuum deposited amorphous Se-Te photoconductor films: fractionation effects and charge transport in homogeneous films

Abstract

A series of a-Se:Te photoconductor films of thickness 60 μm were prepared by conventional vacuum deposition onto heated Al substrates to study fractionation effects. There is a Te-rich surface region, a plateau of relatively “uniform” bulk region, and a Se-rich zone near the substrate. For a source material of Se:12.5% Te (all compositions in wt %), the substrate surface of the film was about 5.5–6.2% Te and the top surface was Te-rich up to about 25% Te content. The plateau region is between approximately 10 μm to 40 μm from the substrate and has about ∼10% Te. The existence of the plateau region provides a means of preparing relatively homogeneous films by suitably shuttering the substrate during the evaporation to capture this plateau region. A first-order fractionation model of the evaporation process was developed to account for the observed compositional variation across the films. The fractionation model applies concepts from mass transport to the melt by considering a surface region (SR) of a fixed number of atoms in which rapid diffusional changes maintain a relatively uniform Te composition that changes over the course of the evaporation process. The model is able to explain the general features of the experimental results and is in reasonable quantitative agreement with the experiments. A series of nearly homogeneous a-Se_1-xTe_t films (composition homogeneous to within 10%) were prepared by suitably shuttering the vacuum deposition process to capture the plateau region. Charge transport in both homogeneous and fractionated films was studied using the conventional time-of-flight (TOF) and interrupted time-of-flight (IFTOF) transient photoconductivity experiments. As a-Se is alloyed with Te, the hole drift mobility μ_h decreases rapidly with the Te content up to about ∼5% Te beyond which the decrease in the mobility becomes small. Both hole and electron drift mobility has been found to depend on the electric field E as μ ∝ E ⁿ where n is a small index that increases with the Te content in agreement with earlier work on probably fractionated samples. Consequently, the power law dependence is not an artifact of the fractionation effect. Hole lifetime in Cl-doped a-Se:Te films from IFTOF experiments was found to increase with the Te composition, nearly in proportion to the decrease in the drift mobility, in agreement with the shallow-trap-controlled model proposed for hole transport in a-SeTe films.