Optical properties of ZnO and MgZnO nanocrystals below and at the phase separation range

Abstract

This paper presents studies of the optical properties of MgxZn1−xO nanocrystals below and at the phase transition range where the solubility is limited. For the nanocrystals of Mg composition 0%–30%, the samples were found to consist of the wurtzite structure and exhibited band edge absorption as well as photoluminescence (PL) blueshift of ∼0.25 eV. The cold temperature PL of the ZnO nanocrystals exhibits the A-exciton and the bound exciton with values similar to that of the bulk ZnO reference sample. In addition, the PL of the ZnO nanocrystals was found to be dominated by an emission, referred to as the ε-PL, at ∼3.32 eV that is ∼56 meV below the A-exciton. The characteristics of the ε-PL agree well with model calculations of an emission that involves a transition of conduction-band electrons to acceptorlike states having an acceptor binding energy of ∼116 meV. The room temperature PL exhibited a strong exciton component, which is convoluted with the ε-PL. The PL line shapes of the MgxZn1−xO samples of Mg composition ranging from 3% up to 30% were found to exhibit the inherent alloy broadening, which masks the excitonic emissions. For samples at the Mg composition range of 40%–50%, the PL exhibited multiple emission peaks at a wide range of the UV spectrum starting from ∼3.22 eV, which is that of ZnO, up to 4 eV. Additionally, the x-ray diffraction of these samples was found to have diffractions corresponding to the wurtzite as well as to the cubic structures. The results for higher Mg composition imply that the nanoalloys have the onset of phase separation at Mg composition of ∼40%.