Intrinsic photoluminescence from low temperature deposited zinc oxide thin films as a function of laser and thermal annealing

An investigation into the modification of low temperature deposited ZnO thin films by different annealing processes has been undertaken using laser, thermal and rapid thermal annealing of 60 nm ZnO films deposited by high-target-utilization sputtering. Single-pulse laser annealing using a KrF excimer laser (λ = 248 nm) over a range of fluences up to 318 mJ cm−2 demonstrates controlled in-depth modification of internal film microstructure and luminescence properties without the film degradation produced by high temperature thermal and RTA processes. Photoluminescence (PL) properties show that the ratio of defect related deep level emission (DLE, 450–750 nm, 2.76–1.65 eV) to excitonic near band-edge emission (NBE at 381 nm, 3.26 eV) is directly correlated to processing parameters. Thermal and rapid thermal processing results in the evolution of a strong visible orange/red DLE PL (with peaks at 590 nm, 2.10 eV and 670 nm, 1.85 eV) dominated by defects related to excess oxygen. At higher temperatures, the appearance of a green/yellow emission (530 nm, 2.34 eV) indicates a transition of the dominant radiative transfer mechanism. In contrast, laser processing removes defect related DLE and produces films with intense NBE luminescence, correlated to the observed formation of large grains (25–40 nm).