DC reactive sputtering of ZnON thin films: band gap engineering and associated evolution of microstructures

Abstract

Zinc oxynitride (ZnON) has recently emerged as a highly promising band gap-tunable semiconductor material for optoelectronic applications. In this study, a novel DC reactive sputtering protocol was developed to fabricate ZnON films with varying elemental concentrations, by precisely controlling the working pressure. The working pressure was varied from 0.004 mbar to 0.026 mbar.For working pressure greater than 1.6 x 10$^{-3}$mbar, the mean free path of ions decrease, the sputtering rate decreases and the concentration of nitrogen in the films decreases. The band gap of the film obtained from UV Vis Spectroscopy initially decreases and reaches a minimum of 1.6 eV at a flow rate of 20 sccm of nitrogen, after which it drastically increases. The correlation between the micro structure and band gap was investigated. The initial alloy structure of the film was found to exist when the band gap was between 1.66 eV and 2.15 eV, beyond which, a distorted wurtzite structure began to emerge. At a band gap of 2.7 eV, the spectrum peaks indicated the coexistence of both alloy and wurtzite structures. With an increasing band gap, the wurtzite structure became dominant, completely replacing the alloy structure at 3.25 eV. This study revealed the existence of intermediate structures formed during the tuning of the band gap, which can have important implications for future research aimed at developing heterostructures and 2D superlattices for photonics applications.