Optimization of optical and electrical properties of hydrogenated amorphous silicon thin films prepared by reactive sputtering

This work reports on DC reactive sputtered amorphous silicon thin films. The films were coated using BALZERS BAE 250 coating system. Ellipsometry and spectrophotometry techniques were used for optical characterization, while a four point probe and the Van der Pauw method were employed for electrical characterization of the films. The influence of various deposition parameters such as DC power, hydrogen and argon partial pressure, base pressure, substrate temperature, hydrogen-argon ratio and film thickness on optical and electrical properties of the deposited films was studied. The absorption coefficients for the films were determined from transmittance and reflectance measurements. The maximum absorption coefficient reported is about 3.5 x 105 cm-1. Optical band gaps (Eopt) of the films were derived from the Tauc plot. For a-Si thin films the optical band gap varied from 1.62 to 1.68 eV. a-Si:H thin films were found to have energy gap ranging from 1.84 to 2.07 eV. An inverse variation of the absorption coefficient with optical band gap and deposition rate was observed. Ellipsometry was specifically used for determination of refractive indices. The refractive indices varied from 2.1 - 3.8. An inverse variation of the refractive index with the deposition rate was observed. The influence of DC power on optical properties of the thin films was studied. The absorption coefficient increased with increase in DC power. The optical band gap decreased with increase in DC power. An inverse variation of the refractive index with DC power is reported. The base pressure was found to play a significant role on the absorption coefficient and deposition rate of a-Si. The deposition rate and absorption coefficient of a-Si thin films were decreasing with increase in the base pressure. The optical band gap increased with the increase in base pressure. However the deposition rate of a-Si:H thin films was found to be insensitive to the base pressure. The effect of hydrogen partial pressure on the properties of the films was studied. The optical energy gap increased with the increase in hydrogen partial pressure. The deposition rate and refractive index were found to vary inversely with the hydrogen partial pressure. There was an inverse variation of the optical band gap with substrate temperature. The highly absorbing film was found for a sample deposited at low temperature (less than 200 °C}. For substrate temperatures higher than or equal to 150 °C the refractive index increased with the increase in substrate temperature, but for temperatures lower than 150 °C the opposite was true. A-Si films prepared at low argon partial pressure had high absorption coefficient and low optical band gap. The refractive index decreased with the increase in argon partial pressure. For a-Si:H thin films deposited at high hydrogen-argon ratio, low optical band gap and high absorption coefficient were observed. The refractive index varied inversely with hydrogen-argon ratio. The variation of film properties with film thickness was also studied. The absorption coefficient increased with increasing thickness. The optical bandgap and refractive index decreased with the increase in film thickness. For the case of phosphorus-doped a-Si:H, the resistivity of films decreased with the increase in hydrogen partial pressure and DC power. Furthermore, the influence of substrate temperature on the film resistivity showed that lowest resistivity could be achieved at substrate temperature of 200 °C. The carrier concentration NeH obtained for the film with lowest resistivity was 1016cm-3.