High-efficient ultraviolet emission in phonon-reduced ZnO films: The role of germanium

Photoluminescence (PL) properties have been studied for Ge-doped ZnO films grown on Si wafers by RF-magnetron sputtering. A PL line, named as G line, appears at 3.324 eV by Ge doping and is attributed to Ge suboxide states including GeO color centers. As Ge concentration (n(Ge)) increases, the intensities of free-exciton-, and neutral-donor-bound-exciton-, two-electron-satellite-, and G-PL lines increase, while those of their phonon replicas decrease. By Ge doping, no-phonon line deconvoluted from the near-band-edge (NBE) PL at 300 K is enhanced, but its LO phonon replicas are reduced, resulting in the enhancement of the NBE PL with its reduced bandwidth. It is suggested that these results are due to the increase of the Ge suboxide states with increasing n(Ge), which is also confirmed by the analysis of the Ge 3d core-level spectra by x-ray photoelectron spectroscopy. (C) 2008 American Institute of Physics.