High mobility IGZO/ITO double-layered transparent composite electrode: A thermal stability study

The fabrication of a thin film optoelectronic device involves the exposure of the transparent conductive oxide (TCO) to a high process temperature. Indium gallium zinc oxide (InGaZnCO₄ or IGZO) is a well known TCO with high optical transparency, moderate conductivity and high mobility. However, its electrical properties deteriorate after subsequent high temperature processes in air atmosphere. On the other hand indium tin oxide (ITO) has higher conductivity than IGZO and better thermal stability. Therefore, IGZO/ITO bilayers have been deposited on glass by radio frequency magnetron sputtering at room temperature and subsequently annealed at high temperatures in order to study their thermal stability. In the present work, a-IGZO layers with a thickness ranging from 10 nm to 100 nm were deposited over a 50 nm thick ITO layer. Results are compared with those from a single IGZO layered thin film without the ITO bottom layer. The structural optical and electrical properties of the multilayers are studied with the use of scanning electron microscopy, UV-Vis spectroscopy and Hall measurement. An IGZO optimal thickness of 50 nm is found to improve the bilayer thermal stability at temperatures upto 400°C keeping good opto-electrical properties. The sheet resistance for the optimized IGZO/ITO composite films is about 22 Ohm/sq, and the transmittance in the visible range is about 90%. The composite shows an excellent mobility above 40 cm²/V-s and thus can be potentially applied as channel layer in thin film transistors (TFTs)