Wide bandgap magnesium zinc oxide semiconducting thin films and applications to solar/visible blind ultraviolet photodetectors

Research on Mg_xZn_1−xO (χ = 0∼1) represents long-lasting efforts of searching for suitable wide-bandgap semiconductors for ultraviolet optoelectronic applications. With a tunable direct bandgap of 3.27∼7.8 eV and excellent material properties, especially the large exciton binding energy (60 meV) that enables low-threshold stimulated emission, Mg_xZn_1−xO is believed to be one of the most promising optoelectronic materials for semiconductor ultraviolet lasers and photodetectors. The objective of this dissertation is to provide an in-depth picture of this oxide semiconductor, and to explore its potential applications in the scope of ultraviolet optoelectronics.; Single crystal Mg_xZn_1−xO thin films were epitaxially grown on sapphire and silicon using pulsed laser deposition techniques. Large-scale deposition of Mg_xZn_1−xO polycrystalline films on glass and polymer was also realized. The overall Mg_xZn _1−xO film quality improved significantly with the new target-making procedure and the optimized laser ablation conditions. By using combinatorial techniques, Mg_xZn_1−xO thin films with continuously varying composition from ZnO to MgO were epitaxially grown on a single substrate. Characterizations of Mg_xZn_1-xO composition spreads reveal a panoramic picture of the structural, electrical, and optical properties of Mg_xZn_1−xO alloys and their evolution with film composition. Phase separation, bandgap discontinuity, solid solubility, and other application-related issues are discussed.; In view of the p-type doping challenge that impedes p-n junction based MgxZn_1−xO devices, the optoelectronic applications of Mg_xZn_1−xO thin films are focused on ultraviolet photodetectors with a metal-semiconductor-metal structure. Solar or visible-blind Mg_xZn_1−xO photodetectors with high degree of visible rejection were fabricated on sapphire, glass, and silicon substrates. Large responsivity and moderate detection speed, which enable these Mg_xZn_1−xO photodetectors to have a variety of practical applications, were realized. The mechanisms of photodetector internal gain and the relationship of carrier lifetime and photodetector response time are interpreted. Based on Mg_xZn_1−xO composition spread thin films, wavelength-distinguishable ultraviolet photodetector arrays were also demonstrated. Each photodetector in the array has a unique composition-dependent photoresponse spectrum, which is consistent with the Mg_xZn _1−xO bandgap-composition relationship. The potential applications of using Mg_xZn_1−xO photodetector arrays in ultraviolet spectral analyzing are also illustrated.