Investigation Of Doped ZnO Films And ZnO-based Heterojunction LEDs

ZnO, a multifunctional semiconductor with a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV, has been extensively studied in recent years for various optoelectronic applications, such as, ultraviolet light-emitting diodes (LEDs) and laser diodes, thin film transistors, ultraviolet detectors, gas sensors and diluted magnetic semiconductors (DMSs), etc. In this dissertation, our works are focused on ZnO:Mn DMS films, transparent conducting ZnO:F films and ZnO/GaN heterojunction LEDs.(â… ) Mn-only doped and (Mn, N)-codoped ZnO films were fabricated by reactive magnetron co-sputtering. All the films have the single-phase wurtzite structure with c-axis preferred orientation. Mn incorporation results in the increase of the lattice constant and band-gap energy of ZnO, and also induces microstructural disorder. At room temperature, Mn-doped ZnO films only show a paramagnetic behavior. In contrast, (Mn, N)-codoped ZnO films exhibit ferromagnetism with a Curie temperature above 300 K. The mechanism of ferromagnetic coupling in codoped films is discussed based on a bound magnetic polaron model.(â…¡) F-doped ZnO polycrystalline films were prepared from thermal oxidation of ZnF₂ film grown by electron beam evaporation. ZnO:F film has a resistivity lower than 10^-3Ωcm and a transmittance higher than 90% in visible light range. F-donor doping not only increases the carrier concentration of the film, but also passivates the surface defects of ZnO nanocrystals. F passivation (1) significantly decreases the grain boundary potential barrier (⁵ meV), thus increasing carrier mobility; (2) quenches the defect-related visible emission, thus increasing the efficiency of ZnO UV luminescence. By studying electron transport mechanisms in ZnO:F film, it is...