Effect of Composition and Annealing on Some of the Optical Parameters of Gex Te100-x Thin Films

The dependence of the absorption coefficient α on photon energy hν
was determined in the spectral range from 1.1 to 3.9 eV at room temperature,
for evaporated Gex Te100-x films. The variation of the optical gap Eg
opt ,
dielectric constant ε ,and absorption coefficient,α with composition and
annealing are reported. The optical gap increases with x followed by a sharpe
decrease beyond x =20. This can be explained in terms of increase in the
number of Te-Te bonds and formation of GeTe4 tetrahedral with an increase
in the chalcogen content. The observed behaviour is explained on the basis of
chemical bond approach and on Mott and Davis model for the density of states
in amorphous solids. The absorption coefficient for Ge -Te films exhibits
exponential dependence on photon energy obeying Urbach’s rule in the
absorption edge . The optical absorption measurements on the as-deposited
and annealed films indicate that the absorption mechanism is due to direct
transition. The optical gap Eg as well as the high frequency dielectric constant,
ε ,depends on the film composition and the annealing temperature. The shift in
the absorption edge due to the heat treatment is explained by a change in the
defect structure of the films. The decrease in the optical gap after annealing is
due to the effect of oxidation . The optical gap, Eg
opt , as well as the high
frequency dielectric constant ,ε ,depends on the annealing temperature.