Temperature Dependent Photoluminescence Spectra of Nanocrystalline Zinc Oxide: Effect of processing condition on the excitonic and defect mediated emissions

Nanocrystalline zinc oxide (ZnO) with crystallite size in the range 85 -89 nm are prepared by the thermal decomposition of carbonate precursor under three different atmospheres, viz., in air ambiance, under nitrogen flow, and in vacuum. O 1s X-ray photoelectron spectra indicate that the atomic percentage of oxygen vacancies is more in the sample prepared under vacuum while adsorbed oxygen is more in the sample prepared in air ambience. Optical band gaps for the samples are in the range 3.17-3.20 eV. Room temperature photoluminescence spectra of the samples reveal band edge emission in the UV range (-3.18 eV) and broad defect mediated visible emission (-1.60-2.70 eV). Emission profile of excitonic emission at 80 K can be resolved into two free excitonic peaks at 3.42 eV (FX-A) and 3.46 eV (FX-B), donor bound excitonic peaks (D0X), peak due to transition of free electrons to acceptor levels (FA) donor-acceptor pair (DAP) emission and phonon replicas. D0X peak at -3.35 eV has contribution from oxygen vacancies (Vo) and Zn interstitials (Zni). The most intense peak in the case of all the three samples is the FA peak at - 3.31 eV which has contribution from zinc vacancies (VZn). Temperature dependent variation of the dominant D0X and FA peaks in the range 80-400 K are analyzed using modified Varshni model which considers contributions from electron-phonon scattering and lattice thermal expansion. Defect dependent emission intensity is maximum at 80 K due to freeze-out of phonons and decreases with increase in temperature. The evolution of the colour of luminescence with temperature is studied using CIE plot method.