Achieving phase stability in ZnSe thin films by thickness and annealing recipes for optical window applications

Abstract

The present work keenly demonstrates the phase stability of Zinc selenide (ZnSe) thin films in order to seek an appropriate substitute for traditionally employed hazardous Cadmium sulphide (CdS) window layer in the development of thin-film solar cells. The ZnSe thin films having thicknesses of 100 nm (T₁) and 220 nm (T₂) are deposited onto glass and Indium-doped Tin oxide (ITO)-coated glass substrates using resistive heating-based thermal evaporation technique followed by post-annealing in an air environment at 100 °C, 200 °C, and 300 °C temperature for 1 h. The structural properties explicitly demonstrate that ZnSe thin films of thickness 100 nm and 220 nm are amorphous and crystalline in nature, respectively. The annealing is found not sufficient to provoke considerable crystallization in 100 nm ZnSe thin films due to low thickness. The 220 nm ZnSe thin films show the transformation of the crystal phase from metastable hexagonal (for as deposited) to stable cubic phase (for annealed films). The optical properties delineate that the absorbance and transmittance of ZnSe films are fluctuated with thickness and annealing where T₂ films exhibited wavy transmittance patterns. The current–voltage characteristics of T₁ and T₂ ZnSe thin films revealed to the Ohmic nature where resistivity is modified with films’ thickness and annealing. The 3D AFM images of as-deposited and 100 °C-annealed ZnSe films have hill and spike-like topographies, respectively. Field emission scanning electron microscopy (FESEM) images of films indicate uniform deposition without any voids and pin holes whereas energy-dispersive spectroscopy (EDS) patterns confirm the deposition of ZnSe films. The experimental results suggest that ZnSe thin films of the thickness of 220 nm annealed at 100 °C might be regarded as an appropriate window/buffer layer in solar cell devices.