Correlating the growth characteristics with the optical and structural properties of atomic layer deposited zinc tin oxide

Zinc tin oxide (ZTO) films with precise Sn content is of strong interest for the development of channel layers for amorphous oxide based thin film transistors and buffer layers on copper–indium–gallium–sulfide solar cells. Atomic layer deposition (ALD) has been effectively used for buffer layers with appropriate electronic properties, still an in-depth understanding correlating the structural and functional properties with the growth of the ternary oxide remains to be addressed. Here we investigate the mixing of ZnO and ${\mathrm{SnO}}_x$ on the growth and resulting opto-electronic properties by varying the supercycle parameters (cycle ratio and bilayer period) of the ALD process. We show that the growth of ZnO on ${\mathrm{SnO}}_x$ in ZTO thinfilms is more complex than a simple substrate-inhibited growth model. Moreover, we demonstrate a strong dependence of the optical properties with the mixed nature of the films as the optical properties of the film not only depend on the Sn content, but also on the bilayer period at a given Sn content. More specifically, the band-gap for a given Sn composition is high for the well-mixed material at the smallest bilayer period, starts decreasing slightly for bilayer periods between 10–20 and falls to the band-gap of ZnO as bilayer period increases further $>$40. With the change in ZTO films from well-mixed to multilayer material as the bilayer period is varied, we illustrate that the optical properties of ZTO can be tuned effectively without largely altering the composition.