Role of substrate and annealing temperature on the structure of ZnO and AlxZn1-xO thin films for solar cell applications

Abstract

This paper reports on the deposition of pure and 5 at.% Al doped ZnO (AZO) prepared by sol-gel and applied to the substrates by spin-coating, and the role of annealing temperature on the crystallinity of these layers. It is found that both ZnO and AZO are largely amorphous when coated on glass compared to n-Si(111), as substrates. On both substrates, X-ray diffraction (XRD) shows that the crystallinity improves as annealing temperature is raised from 200 to 600 °C with better crystallinity on Si substrates. The thickness of the films on substrates was determined as 120 nm by Rutherford backscattering spectroscopy (RBS). Specular ultra-violet visible (UV-vis) gives the direct transition optical band gaps (Eg) for AZO as-deposited films are 2.60 and 3.35 eV while that of 600 °C annealed films are 3.00 and 3.60 eV. The Eg calculated from diffuse reflectance spectroscopy (DRS) UV-vis are more diverse in ZnO- and AZO-Si than the ZnO- and AZO-glass samples, although in both sets the Eg tend to converge after annealing 600 °C. The Raman spectra of samples show multiphonon processes of higher order from the AZO and substrates. It is found that residual stresses are related to E2 Raman mode.