Zinc oxide thin film deposition, characterization, and its applications
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The objective of this research focuses on modifying optical and electrical properties of ZnO thin films by different deposition processes and various post-annealing procedures in order to improve the performances of metal-semiconductor-metal photodetectors (MSM-PDs) and provide an approach to produce p-type ZnO thin films. The discussion in this dissertation has been divided into ZnO thin films and MSM-PDs. ZnO thin films were deposited, annealed, doped, analyzed for chemical composition and surface morphology, and characterized in optical and electrical properties. MSM-PDs were fabricated with those ZnO thin films and characterized. Preliminary results in solar cell applications are also addressed in the final chapter. ZnO thin films were deposited by laser assisted molecular beam deposition (LAMBD) and radio frequency magnetron sputtering (RF) sputtering. In RF sputtering, nitrogen was introduced into the chamber during sputtering for doping purposes in some batches. After deposition, tube furnace, rapid thermal annealing (RTA), and laser annealing (LA) were applied to modify optical and electrical properties of films. Some samples have been doped with nitrogen ion implantation and RTA annealing. Different annealing methods provide a specific improvement in different applications. The 1:1 stoichiometry of Zn to O was observed by electron spectroscopy for chemical analysis (ESCA). LAMBD gave better quality of ZnO thin films, but easily picked up carbon at the surface. Laser post annealing efficiently improved the bandgap emission and healed the defects leading to less defect emission. Tube furnace annealing provided long duration annealing in nitrogen ambient, which may change the properties of ZnO and produce larger grain size, thus leading to better MSM I-V characteristics. In terms of ZnO:N, the films deposited with nitrogen doping, better quality was obtained with (002) orientation of ZnO, crystal size of 10 nm, and the full width at half maximum (FWHM) = 46 meV. Photoluminescence was obtained with a bandgap emission at 3.2 eV. Higher conductivity was obtained due to higher amount of nitrogen introduced causing larger grain size. Rapid thermal annealing (RTA) post annealing not only reconstructed lattice structure but also activated the N + in the films. P-type ZnO has been achieved with the results of mobility = 9.6 cm 2 /Vs, conductivity = 0.72 S/cm, and carrier density = 1.2 x10 18 cm -3 . MSM-PDs using ZnO:N thin films with ion implantation and RTA annealing exhibited high photo to dark ratio of 2030, and high responsivity of 2.1 A/W with Yb/Au MSM structure Schottky contacts. The fall time of pulse response was improved from micro-second to nano-second by post-annealing. Dark current was suppressed by nitrogen doped, tube furnace annealing, and a thin SiO 2 interlayer formed between ZnO and Si. I-V-T measurement showed the current transport mechanisms were dominated by space charge limited current (SCLC).