Paper TF+EM+NS+SS-FrM10
Wurtzite Phase Stable Mg_xZn_1-xO Epilayers with High Mg Compositions for High Performance Ultraviolet Photodetectors

Friday, November 1, 2013, 11:20 am, Room 104 A

Wide band gap semiconductors such as Mg_xZn_1-xO represent an excellent choice for making optical photodetectors and emitters operating in the UV spectral region. High crystal and optical quality Mg_xZn_1-xO thin films were grown epitaxially on c-plane sapphire substrates by plasma-assisted Molecular Beam Epitaxy. ZnO thin films with high crystalline quality, low defect and dislocation densities, and sub-nanometer surface roughness were achieved by applying a low temperature nucleation layer. The critical growth conditions were discussed to obtain a high quality film: the sequence of Zn and O sources for initial growth of nucleation layer, growth temperatures for both ZnO nucleation and growth layers, and Zn/O ratio. Resultant epitaxial ZnO films demonstrated a root-mean-square surface roughness of 0.373nm for 1μm × 1μm atomic force microscope images with clear hexagonal shaped terrace steps. The x-ray diffraction FWHM for (0002) peak was measured to be 13 arc sec for ZnO. By tuning Mg/Zn flux ratio, wurtzite Mg_xZn_1-xO thin films with Mg composition as high as x=0.46 were obtained without phase segregation. The steep optical absorption edges were shown with a cut-off wavelength as short as 278nm, indicating of suitability of such material for solar blind photo detectors. Consequently, Metal-Semiconductor-Metal photoconductive and Schottky barrier devices with interdigital electrode geometry and active surface area of 1 mm² were fabricated and characterized. Photoconductor based on Mg_0.46Zn_0.54O showed ~10² A/W peak responsivity at wavelength of ~260nm. The spectral cutoff of the devices was close to 315nm with more than two orders of magnitude visible rejection ratio (R_260nm/R_400nm) these devices good candidates for solar blind applications.