Kinetic and thermodynamic modelling of ternary nanowire growth

Sammanfattning: Among nanoscale structures of different shapes and dimensions nanowires are one of the most promising because of its truly unique properties different from their bulk counterparts. The energy quantization, perfect defect-free structure, and the possibility to grow them within the miscibility gap in combination with the benefits of bottom-up design and the possibility of integration on silicon substrates make nanowires ideal candidates for applications in optoelectronics, biotechnology and energy harvesting. Today the research focus shifts toward the investigation of more complex materials, namely ternary and quaternary nanowires. For the majority of applications, a critical step in the nanowire-based device design is the ability to control the composition and crystal structure of ternary III-V nanowires. Such tuning is impossible without understanding of the underlying growth mechanism. Theoretical modelling may provide insight into the growth processes and help to assess optimal growth conditions. Moreover, simulation of nanowire growth allows one to reduce the number of experiments, which is essential in view of its high cost. In this perspective, a set of models that encompass a variety of aspects of ternary nanowire formation including their composition and crystal structure has been developed. Within the modelling both thermodynamic and kinetic approaches have been used. The first model is based on two-component nucleation theory and describes the formation of the critical nucleus from a quaternary liquid. An analytic expression that links the compositions of the ternary nucleus and liquid particle is derived. The nucleus composition of four materials systems is discussed in details. Next, we explain how the surface energy influences the miscibility gap and the liquid-solid composition dependence during nucleation from a liquid melt. The second model is based on the consideration of the incorporation rates of binary species into the monolayer and describes the evolution of the solid composition from the nucleated-limited composition to the kinetic one. The kinetic steady state regime is used to fit an experimental data set, namely the liquid-solid composition dependence obtained during growth of InxGa1-xAs nanowires in an environmental transmission electron microscope. Finally, a model which describes the composition dependence of the zinc blende – wurtzite polytypism in ternary nanowires growing by the vapor-liquid-solid mechanism is developed.