Role of ZnO and MgO interfaces on the growth and optoelectronic properties of atomic layer deposited Zn1−xMgxO films

Zn1−xMgxO films with very precise Mg content are of strong interest for the development of buffer layers on copper-indium-gallium-sulfide solar cells. Atomic layer deposition (ALD) has been successfully used for buffer layers with appropriate electronic properties; however, a good understanding of the growth properties of the ternary oxide is still lacking. Here, we investigate the role of the ZnO/MgO interface on the growth and resulting optoelectronic properties by varying the supercycle parameters (pulse ratio and bilayer period) of the ALD process. We demonstrate that the growth of the MgO layer is enhanced by the ZnO surface, describing the interplay between ZnO and MgO interfaces on the growth of Zn1−xMgxO films. The optical properties of the film not only depend on the Mg content but also on the bilayer period at a given Mg content. More specifically, the bandgap for a given Mg composition is high for the smallest bilayer period 5, starts decreasing slightly for bilayer periods between 10 and 20 due to the increase in thickness of the ZnO layer and confinement effects, and falls to a bandgap of ZnO as the bilayer period increases further >40. With the change in Zn1−xMgxO films from well-mixed to multilayer material as the bilayer period is varied, we illustrate that the optical properties of Zn1−xMgxO can be tuned effectively without largely altering the composition. Probing the effect of the bilayer period on the ternary oxides by ALD is a useful tool in understanding the mixing and interplay of binary materials that can be applied for many other materials.