Microstructural differences in thin film ZnGa2O4:Mn phosphor produced by differences in sputtering gas pressure

Abstract

The authors report on the microstructural characteristics of sputter-deposited thin film ZnGa2O4:Mn phosphors, with an emphasis on the role of energetic particle bombardment. The thin film ZnGa2O4:Mn phosphors were deposited by radio frequency planar magnetron sputtering of a 2mol% Mn-doped ZnGa2O4 target in an Ar–O2 gas mixture at gas pressures ranging from 2to20mTorr. The growth rate of the ZnGa2O4:Mn films was decreased from 40to23Å∕min as the gas pressure was raised due to both increased gas-phase scattering as well as reduced target self-bias voltage. Owing to the thermalization of impinging energetic particles and the randomization in their incidence directions when arriving at the substrate, the ZnGa2O4:Mn films produced at an elevated gas pressure exhibited a porous composite microstructure in which larger columns consisted of bundles of smaller columns separated by voided boundaries. Energetic particle bombardment of the growing film surface at a low gas pressure yielded a densely packed zone-T-type microstructure due to porosity annihilation by knock-on processes and bombardment-enhanced adatom mobility. Atomic force microscopy and grazing incidence x-ray reflectivity data revealed that as the gas pressure was decreased from 20to2mTorr, the rms surface roughness of the deposited ZnGa2O4:Mn films was reduced from 4.95to1.23nm and the film density increased from 5.314to5.681g∕cm3, consistent with the postulated effects of energetic particle bombardment upon film microstructure.