Optical Properties of ZnO Deposited by Atomic Layer Deposition on Sapphire: A Comparison of Thin and Thick Films

Abstract

Optical properties of thin (≈100 nm) ZnO films in comparison with thick (≈1 μm) layers deposited on sapphire substrates by atomic layer deposition (ALD) at temperatures from 100 to 300 °C are investigated by optical transmittance and luminescence. It is found that growth temperature, the thickness of the layers, and post‐grown annealing influence the measured optical bandgap values and low‐temperature photoluminescence. However, the values of the bandgaps corrected according to the Burstein–Moss effect do not depend on layer thickness, whereas an increase in growth temperature leads to a decrease in the energy gap. On the other hand, post‐growth annealing of the layers results in increasing the value of the energy gap and decreasing Urbach energy. It is observed that thicker and annealed samples show more intensive and narrower PL bands, which are assigned to donor and acceptor bound excitons, free electrons to acceptors, and donor–acceptor pair transitions. Their energy position and relative intensity depend on the growth temperature applied.