Author/Authors :
Alwan, Tariq Jaffar AI-Mustansiriyah University - College of Education - Department of Physics, Iraq
Abstract :
A systematic investigation on the effect of substrate temperature on the structure, optical absorption and density of states of vacuum evaporated Ge 0.2 Te 0.8 thin films is reported. The X-ray diffraction analysis shows a transformation occurrence from amorphous to polycrystalline structure in the films deposited at higher temperature substrates (473K). The thickness of the film (3500+8 A°) is measured by an optical interference fringes method. Optical characteristics of the Ge 0.2Te 0.8 sample have been analyzed using reflection and transmission spectrum result. The absorption mechanism has been recognized as the allowed indirect transition for amorphous and polycrysfalline films. The absorptioncoefficient (alpha) and the effect of substrate temperature (Ts) on it were investigated. The tailwidth for each prepared films was calculated. The density of state (DOS) was calculated by depending on DC conductivity measurements; the density of extended state N(Eext)increases with increasing the Ts, while the density of localized state N(Eloc) decreases with increasing the Ts. Low field conduction enables one to determine the density of states near Fermi level N (Ef) in amorphous and polycrystalline films, that needs measurements of the DC conductivity under low temperature by using liquid nitrogen. The experimental results are interpreted in terms of variations in the density of localized states due to progressive decrease of the unsaturated bonds during deposition.
