First-principles Study Of Nucleation And Growth Of Semiconductor Nanowires

Semiconductor nanowires have some excellent and unique physical and chemical properties, together with compatibility with silicon technology. So semiconductor nanowires have promising potential applications in micro/nano-electronics, micro/nano-optoelectronics, and micro/nano-eletro-mechanical system. Synthesizing uniform and high quality semiconducting nanowires is a great challenge for utilizing nanowires’ full potential in future applications. Deeply understandings of nucleation and growth have important directive significance to controlled nanowire growth. In this thesis, the first-principles calculations based on the density functional theory have been performed on Si and Ga As systems, to investigate several universal problems in nucleation and growth of nanowires. The main results are as follows:1. The phase separation between a catalyst particle and an embedded NW nucleus was investigated. Here, we develop a theoretical model to explore the mechanism of NW’s nucleation from a catalyst particle. Depending on competition between surface energies of the nanowire, catalyst and their interface, two symmetric and one asymmetric configurations, are predicated to exist in a diagram of the surface energies. Among the three potential configurations, only the asymmetric one is suitable for the catalyzed NW growth. Therefore, a precondition of catalyzed CVD NW growth is drawn. Furthermore, we carefully explore the gold catalyzed Si NW growth and found that all the successful experiments fall into the right zone of the diagram as predicated by the model. Such a model of nanowire nucleation and the drawn precondition allow us to design proper catalyst for desired NW growth and predicting the growth behavior.2. The formation and propagation of multi-steps at the interface between catalyst and nanowires observed frequently during vapor-solid-solid growth of nanowires were investigated. While the mechanism at atomic scale has never been revealed. By considering the formation energies of the steps at the interface, we propose that the lattice mismatching is responsible for the formation of multi-steps. As an example of Si nanowires grown from a solid Au catalyst, our density functional calculation showed that the formation energy of step increased drastically due to the local structural distortion induced by lattice mismatch and thus the multi-steps with minimum lattice mismatching is energetically preferred. This study properly explained the experimental puzzle of the formation of multi-steps in nanowire VSS growth and can be applied for the controllable growth of nanowires.3. The Wurtzite-Zinc Blende polytypism in III-V nanowire growth was investigated. The nucleation in Ga As nanowire growth is numerically discussed by a theoretical model based on the nucleation of experimental fabrication. By the ab initio calculations of the Ga As surfaces and steps, we have revealed the competition between the Wurtzite and Zinc Blende phase nucleations under different chemical potential. The model predicts that the driving force of Wurtzite phase nucleation is the low formation energy of the(1 0-1 0) surface of Wurtzite structure. Further analysis indicates that Wurtzite phase nucleation is easy happen, and can be suppressed by high As chemical potential in environment. Such an analysis may be helful to design the growth experiments for other III-V nanowires.