Near room temperature atomic layer deposition of ZnO thin films on poly (methyl methacrylate) (PMMA) templates: A study of structure, morphology and photoluminescence of ZnO as an effect of template confinement

Abstract

ZnO thin films were grown on confined Poly (methyl methacrylate) (PMMA) templates, with high coverage and sharp interface, by atomic layer deposition (ALD) at near room temperature (at 35 °C). Crystalline ZnO films, with c-axis orientation along surface normal, were formed on PMMA templates. With decreasing PMMA template thickness, from 80 nm (5Rg, Rg ∼ 16 nm, the radius of gyration of PMMA) to 32 nm (2Rg) and below (5 nm, 0.3Rg), the photoluminescence (PL) of ALD grown ZnO films changes significantly. Pronounced deep level emission (DLE), corresponding to the oxygen vacancy related defect level (VO++) of ZnO grown on 5Rg PMMA, reduced noticeably for ZnO on 2Rg PMMA and below. Furthermore, in the near-band-edge emission (NBE) of ZnO, the contribution of free-exciton (FX) with respect to its phonon replica (FX-2LO) increased with reduction of PMMA template thickness from 5Rg to 2Rg or below. The observed trend of the photoluminescence spectra indicates significant reduction of surface induced defects in the ZnO nanostructures, formed on the thinner PMMA templates (2Rg and 0.3Rg) and can be ascribed to the formation of smaller ZnO nanoparticles on 5Rg PMMA, which is consistent with the results obtained from structure and surface morphology study.