Physical Properties and Local Structures of Amorphous Zinc-Tin-Oxygen and Amorphous Indium-Gallium-Oxygen Films

The electrical properties and local structures of amorphous Zn-Sn-O ( a-ZTO) and amorphous In-Ga-O (a-IGO) transparent conducting oxide (TCO) films were investigated in this work. With appropriate control of deposition oxygen pressure, good electrical conductivities (~ 500 S/cm for a-ZTO films, ~ 550-2000 S/cm for a-IGO films) could be obtained. Local structures, which play a crucial role in determining the electrical properties, were examined using X-ray absorption spectroscopy. It is observed that the cations (Zn, In, Ga, and Sn) prefer to retain a nearest neighbor structure close to that of their native oxides. Consistent under-coordination was observed around the host cations (In for a-IGO films and Sn for a-ZTO films), which is consistent with the proposed oxygen vacancy model, with vacancies present as both neutral and charged (donor) species. The carrier concentration is governed by the balance between neutral and charged oxygen vacancies. The doping cations, with different bonding preferences from their host cations,introduce local disorder and "strain" in the films, which changes the potential barrier distribution around cations and plays a significant role in the carrier mobility change with doping. In the case of the a-ZTO90 film, the carrier mobility was improved with the addition of Zn, which could be related to the improved structural homogeneity on the local scale. In most cases, however, the doping cations, being "substitutional" on the host cation sites, tend to form neutral defect species or structural elements that nevertheless act as scattering centers, and thereby decrease the carrier mobilities. Changes in the local structures of these films with thermal annealing were investigated in parallel with the changes in physical properties. Low-temperature annealing retained or even improved the good electrical properties of the as-prepared films, which was correlated with thermal/strain relaxation of the local structure. Upon annealing at higher temperature, the development of longer scale order ultimately leading to differences in the crystalline quality of crystallized films, can further affect the electrical properties of the resulting films. Higher carrier mobilities were obtained in the films with better crystallinity. Although electrical conductivities decreased with the increase in the doping level, an increase in the crystallization temperature was consistently obtained, which can be attributed to the local structure differences around the constituent cations (Zn, In, Ga, and Sn). This provides an explanation for the improved thermal stabilities of doped vs. undoped a-TCO films.