Rationalizing nucleation and growth in the vapor-liquid-solid (VLS) methods

Recently, there has been an immense technological interest in the bulk growth of III-nitrides, synthesis of one-dimensional nanostructures ("nanowires") and low temperature epitaxy layers. The processes employ vapor-liquid-solid (VLS) methods in which gas phase solutes get dissolved into liquid or solvent before precipitating as solid phases. By comparision, the chemical vapor deposition (CVD) methods employ gas-solid reactions to accomplish synthesis of solid phases. However, the lack of understanding about nucleation under vapor-liquid-solid schemes limits their usefulness.; In this work, a model is proposed based on spinodal decomposition theory to understand nucleation and growth from molten metal phases in the V-L-S schemes. Furthermore, the theory is applied to rationalize both nucleation and growth of solutes from low-melting metal melts. The validation of the theory is performed through systematic experiments aimed at studying the nucleation of different solutes from lowmelting metal melts. Primarily, the experiments were conducted using binary systems involving low-melting metals as solvent and solutes such as non-reacting solutes (Si and Ge) and reacting solutes (nitrides and oxides).; The results based on the proposed model and experimental evidence suggest that nuclei diameter is primarily a function of synthesis temperature and is also dependent upon other process variables to a lesser extent. In the case of Ga-Ge and Ga-Si systems, the nucleation density of the resulting Ge nanowires increases with increasing Ga droplet diameter and decreases with increasing synthesis temperature, in agreement with the proposed model.; In the case of GaN-Ga binary system, it is determined that the interfacial energy plays a key role in the morphology of the resulting crystals. It is found that increases in hydrogen concentration in the inlet gas mixture increase the surface energy of the gallium melt thereby resulting in nanowire growth. However, it is also noted that the introduction of ammonia in the gas phase drastically reduces the surface energy of the gallium droplets owing to increased dissolution of nitrogen into the melt.