A comparative analysis of structural, optical, and electrical characteristics of c-plane and a-plane ZnO:Al thin films fabricated by a pulsed laser ablation technique

Abstract

The effective growth of the non-polar a-plane ZnO:Al thin films with a large grain size using the pulsed laser ablation technique under controlled conditions (in an oxygen-specific environment) over the easily grown polar c-plane films (under ambient conditions) function well as heterojunction diodes possessing excellent rectification behaviour. The absence of a built-in electric field has resulted in enhanced electrical transport properties making the a-plane films better than the c-plane films. Both the thin films, homogeneous and highly oriented along the respective crystal planes as observed in the structural studies and thickness maps, further exhibit defect mediated variations in the transmittance spectra. The values of Drude damping and the imaginary part of the dielectric constant suggest a diminished optical loss in the a-plane ZnO:Al thin film, whereas the c-plane film showed a greater optical loss. An enhanced near-band edge emission in the non-polar a-plane film is inferred from photoluminescence. The open-aperture z-scan measurements indicate an exceptional transition from saturable absorption to reverse saturable absorption in both the a-plane and c-plane oriented films. The closed-aperture measurements exhibit a rare phenomenon of self-focusing mechanism in both the thin films. The detailed structural, optical and electrical studies reveal that the a-plane ZnO:Al thin films are an alternative to the c-plane ZnO:Al thin films for nonlinear and optoelectronic applications.