MOCVD Growth and Structural Properties of ZnS Nanowires: A Case Study of Polytypism

Controlling the morphology, orientation, and crystal phase of semiconductor nanowires is crucial for their future applications in nanodevices. In this work, zinc sulfide (ZnS) nanowires have been grown by metalorganic chemical vapor deposition (MOCVD), using gold or gold-gallium alloys as catalyst. At first, basic studies on MOCVD growth regimes (mass-transport, zinc- or sulfur- rich conditions) have been carried out for ZnS thin films. Subsequently, the growth of ZnS nanowires was investigated, as a function of key parameters such as substrate temperature, S/Zn ratio, physical state and composition of the catalyst droplet, and supersaturation. A detailed analysis of the structural properties by transmission electron microscopy (TEM) is given. Depending on the growth conditions, a variety of polytypes is observed: zinc-blende (3C), wurtzite (2H) as well as an uncommon 15R crystal phase. It is demonstrated that twinning superlattices, i.e., 3C structures with periodic twin defects, can be achieved by increasing the Ga concentration of the catalyst. These experimental results are discussed in the light of growth mechanisms reported for semiconductor nanowires. Hence, in this work, the control of ZnS nanowire structural properties appears as a case study for the better understanding of polytypism in semiconductor 1D nanostructures.