High throughput optimizations of alloy and doped films based on ZnO and parallel synthesis of ZnO/Mg_xZn_1-xO quantum wells using combinatorial laser MBE towards ultraviolet laser

We report high-throughput optimizations for various material parameters of Mg_xZn_1-xO and Zn_1-xCd_xO alloy films, Al-doped ZnO films, Mg_xZn_1-xO/ZnO single quantum wells and superlattice structures with using combinatorial laser MBE (CL-MBE). Combinatorial chips including nine thin film pixels were grown on lattice-matched ScAlMgO₄ (0001) substrates by switching the mask patterns and targets during pulsed laser deposition. For alloy experiments, two chips (two runs) provided us with complete and systematic understanding of the band gap in a range from 3.0 to 3.6 eV. An Al-doped chip revealed systematic variation of the conductivity from 3 to 2 × 10³ Scm^-1. A single quantum well chip showed clear systematic variation of photoluminescence peak energy as a function of well layer thickness. Two superlattice chips showed clear systematics on the quantum sub-band levels in absorption spectra even at room temperature, giving all desired information being useful for designing an efficient active layer in possible ultraviolet light emitting devices. The excitonic stimulated emissions could be observed even at very low optical pumping at room temperature. The threshold excitation intensity changed in a range of 11-40 kW/cm² and the emission energy can be tuned between 3.2 and 3.4 eV, depending on the well layer thickness and/or the Mg content in the barrier layers. The excitonic stimulated emission could be observed up to 100°C and a characteristic temperature was as high as 87 K for the best sample optimized by the combinatorial approach using only 8 deposition runs carried out in a week.