Work Function Mapping Across a-In2Se3 to α-In2Se3 to γ-InSe in RF-Sputtered Thin Films

Abstract

Indium selenide is a phase-change chalcogenide whose polymorphism enables a variety of physical properties to be tuned. Here we directly quantify the evolution of the surface work function across the amorphous-to-crystalline transition in RF-sputtered In2Se3 thin films grown on c-plane Al2O3 (001). By varying deposition temperature (100-500 °C) and film thickness, we establish processing windows for a- In2Se3, α-In2Se3, and γ-InSe, and correlate structure with electronic and morphological properties. X-ray diffraction shows films deposited at 100 to 200 °C are amorphous, 300 to 400 °C yields α-In2Se3, whereas 500 °C yields γ-InSe. Kelvin probe force microscopy (KPFM) maps the surface potential and yields spatially averaged work functions spanning 5.26 to 6.64 eV across the amorphous-crystalline transformation; pronounced intra-film heterogeneity is observed, with select α-phase and γ-InSe grains exhibiting work functions exceeding the local mean. Topographical distinctions are found between phases with hexagonally faceted grains in the crystalline state, whereas homogeneous nano-mounds are found in amorphous films. Analysis of Tauc plots revealed optical bandgaps in the range of 2.50 to 1.55 eV across the observed phases. X-ray fluorescence (XRF) measurements further indicated that the indium to selenium concentration ratio varied between 0.70 {\pm} 0.1 to 1.01 {\pm} 0.1 as the deposition temperature increased from 100 to 500 °C. These measurements provide direct, spatially resolved quantification of work-function evolution through the phase change, supplying parameters essential for contact engineering and device integration of In2Se3 and InSe.