  The effects of annealing and variation of temperature on the electrical and
thermoelectric properties of e-beam evaporated InSe thin films has been
investigated in details. The XRD study demonstrates that the as-deposited InSe
thin films are amorphous while they become polycrystalline with the presence of
In3Se4 phase after annealing. The SEM micrographs reveal that the surfaces of
as-deposited films are smooth whereas they become non-uniform due to annealing.
The heating and cooling cycles of the as-deposited films exhibit that the
resistivity of the films shows an irreversible phase-transition and become
stable after 3-4 successive heat-treatment operations in air. The electrical
conductivity of annealed InSe thin films shows a highly degenerate
semiconducting (metallic) behavior. The thermopower of the annealed films
indicates that InSe thin film is a highly degenerate n-type semiconductor i.e.
metallic. Thickness dependence thermopower obeys the Fuchs-Sondheimer theory.
The optical band gap of the annealed films increases as compared to the
as-deposited films. These results indicate that InSe thin films encounter a
phase-transformation from In2Se3 to a new In3Se4 metallic phase with an optical
band gap of ~1.8 eV due to heat-treatment.
