Metal oxide films as functional materials for optoelectronic applications

Abstract

Zinc oxide (ZnO) and gallium oxide (Ga2O3), as novel wide band gap semiconductor materials have unique advantages such as wide band gap, low preparation cost, environmentally friendly and high exciton binding energy (180-270 meV for Ga2O3). These characteristics make them suitable for deep blue/UV light-emitting diodes devices, laser and photodetectors. The investigations on wide band gap semiconductor materials become a research hotspot and they have received great attention from the scientific and industrial communities in recent years. This thesis mainly focuses on the wide band gap semiconductor materials of ZnO and Ga2O3 films. Their electrical properties, defect distribution, electronic band structure and their applications in organic light emitting diodes (OLEDs) and solar blind UV detectors are systematically studied. Firstly, by optimizing the doping ratio of Er element and film growth conditions, the highly conductive Er doped ZnO films (ErZO) with the low resistivity of ~10-4 Ω.cm and high optical transmittance of ~90% were prepared. PL spectra and UPS measurements were used to observe the changes of film defects and work function. It is found that the highest current efficiency of OLEDs was increased from 76.0 cd/A (ITO) to 86.5 cd/A (ErZO). Secondly, combined PLD and ion implantation techniques, the dielectric performance of ZnO films were optimized by (Ga, Cu) co-doping method. By optimize the doping concentration and post-annealing temperature, the GCZO films with high permittivity, high stability in wide frequency range and low dielectric loss were achieved (δ=87 and tan δ=0.17 at the frequency of 1 kHz). Through ac conductivity, impedance and modulus analysis, the enhancement of permittivity was attributed to the correlated potential barrier hopping of electrons between the defect complex states induced by the acceptor-donor co-doping. Thirdly, the deposition, conduction mechanism, defect distribution and relaxation process of the amorphous state and β-phase Ga2O3 are comprehensively studied. Through the variable temperature dc conductivity and variable temperature ac modulus spectrum, the position of the defect energy level in the Ga2O3 film is determined, which are ~0.02 eV, ~0.2 eV and ~0.4 eV. It was also found that an activation energy at ~0.180 eV that only exist in the amorphous Ga2O3. In addition, we studied the energy band shift between β-Ga2O3 and electrode materials (such as ErZO, GZO), and realized the construction of β-Ga2O3 ohmic electrodes using ErZO film which also exhibited sensitive response to the UV light.