Plasma-assisted Molecular Beam Epitaxy Growth and Properties of Tin Oxide

Currently, transparent conducting oxides (TCOs) ZnO, SnO2, In2O3, and Ga2O3 are widely used in electrical and optoelectronic devices. With a wide-bandgap of 3.6 eV transparent to the visible and near-UV spectrum, SnO2, has generated many interests not only due to the curiosity of fundamental research on material science but also potential technological uses in the industry. This dissertation focuses on the development of SnO2 epitaxy by plasma-assisted molecular beam epitaxy (PAMBE). The detailed growth mechanism, physical properties and electrical doping studies will be addressed along with the application as a transparent contact for GaN-based LEDs.;High-quality single-crystalline SnO2 films were grown on TiO 2 in the Volmer--Weber growth mode. A growth rate diagram was constructed by the dependence of growth rate on tin flux under a constant oxygen BEP. This growth rate diagram consisted of two growth regimes: an increased growth rate under an oxygen-rich regime and a decreased growth rate under a tin-rich regime. The decreased growth rate was due to the formation of volatile SnO. No SnO2 growth was observed when the impinging tin flux was larger than twice the stoichiometric flux.;A detailed study on transport properties was discussed to evaluate the origin of n-type conductivity in SnO2. Lowest electron concentration was measured on the oxygen-rich grown sample. Based on the evolution of the electron concentrations before and after annealing, oxygen vacancies were considered as the dominant donors in our SnO2 films.;Controllable electrical conductivity by Ga-doping was performed. The decrease of electron concentrations with increasing Ga concentrations indicated Ga acted as a compensation center in SnO2. A semi-insulating SnO 2 film was achieved with the Ga concentration of 1.5 x10 18 cm-3. A deterioration of crystalline quality was observed beyond this concentration due to the formation of Ga2O 3 secondary phases.;Finally, highly conductive Sb-doped SnO2 (ATO) was shown as potential transparent p-contacts for InGaN/GaN LEDs. A systematic growth study on various substrate temperatures was performed for achieving a better device performance. The LEDs showed a forward-bias voltage of 3.8 V at an injection current of 20 mA, which was comparable to the typical LEDs with ITO contacts.