Characterization of ultrathin II-VI semiconductor nanowires by transmission electron microscopy

Using the molecular-beam epitaxy technique, high-quality and ultra thin ZnSe and ZnS nanowires have been fabricated on different substrates based on the Au-catalytic vapor-liquid-solid (VLS) reaction. The nucleation, initial growth, growth rates, defects, interface structures and growth direction of the nanowires were systematically investigated by high-resolution transmission electron microscopy (HRTEM). The possible mechanisms of nanowire formation were proposed by combining the classical VLS process with the surface diffusion theory. The electrical and optical properties were also characterized. Particularly, this study has focused on how the growth temperature and the size of the catalysts influence the morphology, structure and growth direction of these nanowires. The nanowires were grown on different substrates, e.g., GaAs (001), (110) and (111). The Au catalysts were prepared by depositing a thin layer of Au on the substrate at 150°C and annealing the substrate subsequently at 530°C for 10 min. Uniform Au-alloy droplets formed on the substrate surface by the annealing process. The sizes of Au droplets were mainly determined by the thickness of the Au thin layer. The Au droplets initially reacted with the substrate of GaAs to form AuGa2 phase and As evaporated. For the ZnSe substrates, Au-Zn alloy formed and Se evaporated. The nanowires were then grown at temperatures ranging from 390°C to 530°C, using a ZnSe or ZnS compound source. The morphology and structure of the ZnSe nanowires grown by MBE are in some way dependent on the growth conditions such as substrate temperature. The nanowires grown at high temperature (TS=520°C) exhibit smooth surfaces and uniform diameters. In contrast, the nanowires grown at a low temperature (TS=400°C) have a tapered shape and slightly sawtooth facets. In addition, the decrease of growth temperature greatly increases the density of defects. But the interfaces between the catalysts and nanowires are generally the lowest energy {111} planes, which is independent of the growth conditions and nanowire diameters. This study has focused on how growth temperatures and the sizes of the catalysts influence the growth direction of the ultrathin nanowires. TEM studies revealed that the structure and growth direction of ultra thin nanowires are highly sensitive to growth temperatures and nanowire diameters. At a low growth temperature (400°C) <111> direction was resulted in most nanowires. Planar defects (mainly stacking faults and twins) were found to extend throughout the nanowires. At a high growth temperature (520°C), structurally uniform <110> and <112> nanowires were typically found in nanowires with diameters around 10nm, while <111> nanowires were mainly found in nanowires with diameter larger than 20nm. The size-dependent and temperature-dependent growth directions of the nanowires were analyzed based on the estimation of the nanowire surface and interface energies. The relationship among the growth direction, temperature and the diameter were quantitatively elucidated. According to the classical VLS growth model, an important parameter, the critical thickness LC, was introduced for the first time which determined the transition of the growth directions of nanowires. In addition, the electrical transport through an individual nanowire was characterized via in-situ measurement by the STM-TEM equipment. The resistivity of the ultrathin ZnSe nanowires was measured to be about 1x102Ωcm. When applying a large current through a single crystalline ZnSe nanowire, we observed melting phenomena and structural changes