Phase controlled epitaxy of wurtzite ZnS thin films by metal organic chemical vapor deposition

Abstract

In many semiconductors, metastable polytype phases are attractive to tune physical properties. Owing to a non-centrosymmetric structure, ZnS with a wurtzite (WZ) phase would exhibit additional piezoelectric properties, optical non-linearity as well as Rashba effect. Because bulk single crystalline ZnS-WZ is not commercially available in useful sizes and attractive prices, large area thin films with controlled crystalline phase are required. We report the synthesis of ZnS films deposited on m-plane CdS substrates as a proof of concept of the replication of the WZ stacking on non-polar surfaces. First, a chemical mechanical polishing is required for the substrate preparation. ZnS layers are subsequently grown using metal organic chemical vapor deposition. The results show a strong impact of the growth temperature on the CdS substrate surface that is highly detrimental on the crystalline quality of the layer. However, the m-plane ZnS layers grown at 360 °C have WZ structure with a perfect epitaxial orientation relationship with the CdS-WZ substrate. The hexagonality in ZnS-WZ is about 90 %. Strain relaxation occurs through the formation of misfit dislocations at the interface forming basal and prismatic stacking faults on {11–20} planes. Additional pyramidal planar defects are evidenced. With the optimisation of a buffer layer or a more suitable substrate, this ZnS-WZ layers may find various applications not limited to optics such as piezoelectricity or spintronics. Additionally, they may be used as a platform for the growth of other materials with the envisioned WZ structure.