MBE-VLS grown ZnSe and ZnS nanowires : growth mechanisms and photoluminescence properties

A lot of effort was recently devoted in realizing semiconducting nanowires (NWs) that are considered as a channel for electrons and photons no wider than a few thousand atoms. Such a one-dimensional structure will find potential applications in nano-optoelectronics and nano-photonics. The control of the growth orientation of NWs is particularly important because it will eventually affect their optical and transport properties. The ordering of NWs has also been a recent focus due to the fact that some of the above mentioned applications require the NWs fabricated in regular and periodic arrays. In this study, the growth of ZnSe and ZnS nanowires using the molecular beam epitaxy technique via the vapor-liquid-solid reaction with Au alloy droplets as the catalyst is successfully demonstrated. The nanowires resulting from this approach were found to orient along some specific crystallographic directions. Through detailed structural characterization, we have revealed that the preferred growth orientation of NWs depends on the chosen size of the catalyst and growth temperature. A phenomenological model based on the minimization of the total system energy of a nanowire was proposed to explain these observations. Based on these findings, we have successfully grown vertical ZnSe nanowires with a diameter around 10 nm on a GaAs(110) substrate. The interactions between the catalyst and the direct-contact materials (either the GaAs substrate or the ZnSe buffer) were analyzed in details. Through this study, it was found that thermal annealing of Au alloy catalyst droplets on a ZnSe buffer surface could result in nano-trenches along the <110> directions. Based on the results obtained from a number of surface profiling and chemical analysis techniques, a model is proposed to describe the possible formation mechanisms of the observed nanotrenches An ordered ZnSe NW array fabricated on a GaAs (111) substrate with a novel pre-patterning method associated with plasma etching is also demonstrated in this study. This array shows a high degree of ordering and a good size uniformity of the as-grown NWs. The diameter of the NWs in the array is around 80nm and most of them were found to orient vertically but some tilt to one of the six possible directions of the <111> direction family. ZnS NWs fabricated on a sapphire substrate at growth temperature of 4300C were found to mainly consist of the cubic phase but a little portion is in the hexagonal phase. The analysis of the temperature-dependent band-edge emission of these NWs and that of a ZnS thin film reveals that the energy shift of the interband transition on temperature in ZnS is mainly attributed to the electron-phonon interactions. The observed blue-shift of the band-edge emission of ZnS NWs could be quantitatively explained by the confinement of the excited excitons in the NW geometry