Unveiling the electrical and thermoelectric properties of highly degenerate indium selenide thin films: Indication of In3Se4 phase

TL;DR

This study investigates how annealing affects the electrical and thermoelectric properties of InSe thin films, revealing a phase transition to a metallic In3Se4 phase with significant implications for optoelectronic applications.

Contribution

It provides detailed analysis of phase transformation and property changes in InSe thin films induced by annealing, highlighting the emergence of a metallic In3Se4 phase.

Findings

InSe thin films become polycrystalline with In3Se4 phase after annealing.

Resistivity shows irreversible phase transition stabilizing after several heat cycles.

Annealed films exhibit highly degenerate n-type metallic behavior with increased optical band gap.

Abstract

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.