3, Fraunhofer Institute of Mechanics of Materials, Freiburg, , Germany
2, University of Basel, Basel, , Switzerland
The optoelectronic properties of transparent conducting oxides (TCOs) are linked to the presence of defects in the films. These defects may range from the nanometer to the atomic scale and can lead to reduction of the carrier mobility and optical transparency. Here we present an experimental and computational study of both the nature and influence on the optoelectronic properties of subgap defects in state-of-the-art amorphous zinc tin oxide (ZTO) and tin oxide (SnO2) films deposited by sputtering. Based on this, passivation strategies are proposed to improve the optoelectronic properties of the TCOs.
The optoelectronic properties and microstructure of amorphous ZTO thin films were investigated as a function annealing temperature and atmosphere (oxygen-rich, or reducing) up to 500 °C. We demonstrate that the detrimental visible absorption centers in ZTO after deposition are suppressed by thermal treatments at temperatures > 400 °C in O-rich atmospheres. Defect suppression by oxygen intake results in an improvement of the mobility by 75%, up to 35 cm2/Vs as the number of ionized scattering centers is decreased. Conversely, annealing in a H2 atmosphere > 400 °C increases both subgap absorption and free carrier concentration but decreases the mobility. Overall, the films retain their amorphous dense microstructure throughout these experiments, indicating that the changes in optoelectronic properties are linked to a change in atomic defect population rather than microstructure.
To investigate the influence of point defects on the optoelectronic properties, amorphous atomic structures having the same composition as the sputtered thin films were generated using molecular dynamics. The subsequent analysis of these structures using density functional theory , revealed that the presence of oxygen deficiencies (Vo) and atomic hydrogen within the amorphous network plays a crucial role in the resulting properties of ZTO. Undercoordinated Sn atoms that result from the presence of Vo may provide free carriers but generate absorptance centers in the visible spectra and limit the electron mobility. In accordance with experimental data, defect passivation by adding oxygen atoms in the simulated structure removes these absorption centers. Furthermore, the addition of hydrogen in the vicinity of Vo shifts the associated states deeper into the bandgap.
To reduce the temperature at which defects are passivated and match the thermal requirement of several devices, we demonstrate that co-sputtering either ZTO or SnO2 together with SiO2 also decreases subgap absorption. Interestingly, for films that contain 2.0 wt% SiO2, the decrease in absorptance does not affect the electrical properties of the TCO. Co-sputtering Sn-based TCOs with SiO2 is hence an effective strategy to passivate subgap defects even at temperatures < 200°C, an effect that could not be reached by tuning the oxygen flow during deposition or by annealing in an O-rich atmosphere at these temperatures.