Optical Properties of ZnO and Bandgap Engineered MgZnO

Dr. Leah Bergman
Department of Physics
University of Idaho

ZnO and MgxZn1-xO are emerging materials capable of high-efficiency luminescence in the ultraviolet (UV) spectral range with promising applications in optoelectronic devices. ZnO has the hexagonal wurtzite structure of bandgap ~ 3.4 eV, while MgO has the NaCl cubic structure of bandgap ~ 7.7 eV; thus, alloying in principle can provide a bandgap engineered alloy system. Due to their different crystal structures a very low solubility limit of the ZnO-MgO alloy system has been previously predicted. However, the solubility limit may be improved by non-thermal equilibrium conditions of a growth process.

We present studies of the optical properties of MgxZn1-xO nanocrystals, and ceramics up to ~ 50% Mg composition. For the samples of Mg composition 0-30%, the materials consist of the wurtzite structure and were found to exhibit band edge absorption as well as PL blueshift of ~ 0.30 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. The principal finding is that at the cold temperature regime, the PL was found to be dominated by an emission, referred to as the e-PL, at ~ 3.32 eV that is ~ 56 meV below the A-exciton. The e-PL is discussed in terms of surface defects of the crystallites. In contrast to the resolved PL spectra of the ZnO nanocrystals, the PL lineshapes of the MgxZn1-xO samples of Mg composition up to 30% were very broad, indicating the inhomogeneous nature of the nanoalloys.

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 (XRD) of these samples was found to have diffractions corresponding to the wurtzite as well as to the cubic structure. These results for the higher Mg composition imply that the MgxZn1-xO nanoalloys have onset of phase separation, due to the structural solubility limit.