Effects of NH3 flow on structural and optical properties of ZnO films grown by atmospheric-pressure chemical vapor deposition

Abstract

Structural and optical properties of zinc oxide (ZnO) films doped with nitrogen (N) grown on r-plane sapphire substrates by the atmospheric-pressure chemical vapor deposition using zinc, water, and ammonia (NH3) as source materials have been studied. The small amount of NH3 flow was found to be effective for improving the film orientation and surface morphology. Photoluminescence (PL) spectra of the as-grown undoped and N-doped ZnO films were composed of a near-band-edge emission with two peaks at ~376 and ~386 nm and a green band (GB) emission with a peak at ~540 nm. PL and PL excitation (PLE) measurements for the as-grown N-doped films grown at the different NH3 flow rates indicated the possibility of the contribution of the N and/or hydrogen (H) related defect to both the appearance of the peak at ~386 nm and the excitation process of the GB emission. The comparison between the PL and photoacoustic (PA) spectra showed that the N atom occupying oxygen site (NO) or its complex defect is the most conceivable candidate for the non-radiative center responsible for the intra-band-gap PA absorption. After post annealing under the different conditions, a red band (RB) emission appeared at ~720 nm besides the GB emission. Both the RB and GB emissions observed for the annealed films were found to be effectively excited by the band-to-band (inter-band-gap) and free exciton transitions. Regardless of the difference in annealing condition, all the PLE spectra for the RB emissions showed a weak excitation band extending from 460 nm to 680 nm, suggesting the formation of the deep impurity level probably due to the NO acceptor. It was also confirmed by X-ray photoelectron spectroscopy that the NO deep acceptors were formed in the films, but passivated by the H atoms and/or compensated by the H related donors and/or by the nitrogen molecule substituting oxygen site [(N2)O] double donors.