Influence of Ge4+ doping on photo- and electroluminescence properties of ZnGa2O4

Nanocrystalline ZnGa2O4 samples doped with varying concentrations of Ge4+ ions were prepared and their photo- and electroluminescence properties were investigated in detail before and after annealing at 900 °C. X-ray diffraction (XRD) studies confirmed that Ge4+ doping even to a level of 0.5 at.% at the expense of Ga3+ in ZnGa2O4, leads to incorporation of significant extent Ga3+ ions (∼29%) at Zn2+ site (tetrahedral site) in ZnGa2O4 lattice thereby increasing relative extent of distorted gallium-oxygen (GaOx) structural units. XRD, UV-Visible optical reflectance and lifetime measurements confirmed that a maximum of 0.65 at.% Ge4+ is doped in nanocrystalline ZnGa2O4. Observed blue shift in photoluminescence maximum and decrease in line width with Ge4+ doping is explained based on increased electro-negativity of Ge4+ compared to Ga3+ and lack of formation of oxygen vacancies particularly for 900 °C annealed samples. Lack of oxygen vacancies in Ge4+ doped annealed samples facilitates selective excitation of regular GaO6 and GaOx structural units, upon application of AC voltages, leading to around 50% reduction in line width of electroluminescence peak from doped sample compared to undoped one. Observed variation in electroluminescence properties between doped and undoped samples have been understood based on difference in nature of defects generated in the lattice.