Structural and optical properties of ZnMgO thin films grown by pulsed laser deposition using ZnO-MgO multiple targets

We report on structural and optical properties for Zn1-xMgxO (ZMO) thin films produced by pulsed laser ablation. ZMO thin films were grown on a-plane Al2O3 substrates at 400 degrees C. In order to efficiently incorporate Mg into ZnO thin films, we used multiple ZnO-MgO ablation targets. Pulses from a Nd:YAG laser (4th harmonic generation: 266 nm) were directed on the ZnO-MgO ablation targets, which consisted of MgO single crystals mounted on ZnO ceramic targets. The ZMO films were characterized by x-ray diffraction, optical transmittance and cathodeluminescence (CL) measurements. Highly c-axis oriented ZMO(0002) reflections corresponding to the wurtzite-phase were observed. The c-axis lattice constants of the films were determined from the ZnMgO(0002) peak. The c-axis length of the ZMO films decreased linearly with Mg content. From the optical transmittance spectra of ZMO films, we observed a blue shift in the absorption edge with increasing Mg content. Band gap energies of ZMO thin films were determined from the optical transmittance and CL spectra. We found that the band gap energy changed from 3.27 eV to 3.95 eV. The Mg content of ZMO films increased monotonically with the number of laser pulses which struck the MgO target. These results show that laser ablation using multiple targets of ZnO and MgO is effective for band engineering of ZMO.