Photoluminescence phenomena prevailing in c-axis oriented intrinsic ZnO thin films prepared by RF magnetron sputtering

Abstract

Substantial c-axis orientation of the hexagonal ZnO crystals with wurtzite structure demonstrates only those two preferred peaks in the first-order spectra which are permitted by the Raman scattering selection rule viz., the E^high₂ and A₁ (LO) modes, that identify the improved structural quality of the undoped ZnO film grown by magnetron sputtering in Ar ambient at an RF power of P = 200 W. The presence of a substantial amount of hydroxyl groups attached to the Zn lattice has been correlated to the dominant c-axis orientation of the ZnO crystals which exhibited a distinct UV luminescence band that arises due to the typical exciton emission or near-band-edge emission. At higher applied powers, disorder-activated Raman scattering introduces a well resolved B^high₁ mode and gradually growing second order Raman peaks, (E^high₂ − E^low₂) and (B^high₁ − B^low₁), which are caused by the breakdown of translational symmetry of the lattice by defects or impurities and lead to deviation from preferred c-axis orientation with I₀₀₂/I₁₀₃ < 1. Out diffusion of oxygen from the network creates increasing oxygen vacancy states and in addition, various other defects e.g., Zn interstitial , doubly ionized Zn vacancy and oxygen antisite (O_Zn) as the dynamic acceptor defects, act as the origins of different visible photoluminescence components classified in the UV-violet, violet, violet-blue, blue and green regions.