Optical Properties of Amorphous Selenium Films

by Tan, Wee Chong

Recently there has been a substantial renewed interest in the electrical and optical properties of amorphous selenium (a-Se) films due to its use as an ultra-sensitive photoconductor in the newly developed flat panel x-ray image detector and high definition digital and video camera. This project has examined the optical properties of a range of a-Se films fabricated by conventional vacuum deposition technique. The films were deposited at two substrate temperatures called hot and cold deposition: (i) at 51?C, which is above the glass transition temperature (Tg ? 40?C), and (ii) at 3?C, well below the glass transition temperature. The transmission spectrum T(?) of all the a-Se films were measured over a wide range of wavelengths from 500 nm to 2500 nm, and analyzed using the Swanepoel technique. The thickness, absorption coefficient ?(?), refractive index n(?), and optical bandgap, EgT and EgU, in different absorption regions of a-Se were all extracted using the Swanepoel technique. A systematic way to improve the accuracy of the calculations was also developed and the n vs. ? data was fitted to the Sellmeier dispersion equation. The optical bandgap EgT was obtained from the ?h? vs h? plot of the absorption spectrum and the Urbach tail was obtained from a semi-logarithmic plot of ? vs h?. With all essential optical properties collected, the influence of thermal annealing, the deposition temperature, doping, thickness and aging time on the optical properties of a-Se films was studied.

Amorphous selenium is essentially a glass, and all glasses exhibit some degree of structural relaxation effects during which the physical properties such as the refractive index and the bandgap change with time, called "aging". First of all, thermal annealing an a-Se film at T = 51?C for an hour reduces the thickness by ~15% and increases the refractive index by ~0.5%. The optical bandgap does not change by more than 0.2%. While the decrease in the thickness is thought to be due to the rearrangement of the molecular clusters and the amorphous network, the increase in the refractive index is believed to be linked to the densification and the polarizability of the material. Although the influence of the thickness of the film, the temperature of the glass substrate, and the doping of 67ppm of chlorine on the optical properties of the a-Se film was found to be negligible, there are noticeable changes during aging. It was found that an aged a-Se film that was cold deposited has an increased refractive index that is believed to be caused by densification alone. In a period of 2 months, the thickness of a cold deposited a-Se:67ppm-Cl film shrinks by 3.5% and the refractive index increases by 3.7%. The optical bandgap of the film decreases by 0.6%. Similar aging trends in the thickness, the refractive index, and the optical bandgap were also observed in a cold deposited a-Se film but no significant changes, at least not more than 0.5% deviation, were noted in a period of about 1-3 weeks in any aging a-Se films that were hot deposited or annealed.