Generation of charged nanoparticles and their contribution to growth of silicon in the thermal chemical vapor deposition process

Abstract

The growth mechanism of films and nanostructures has been commonly explained based on the terrace-ledge-kink (TLK) model by an atom. In the actually experimental observation, however, many puzzling phenomena, which could not be explained by an atomic growth, occur in the low-pressure synthesis of diamond by chemical vapor deposition (CVD). Hwang et al. suggested the possibility that charged nanoparticles generated in the gas phase during CVD can contribute to the growth of films and nanostructures. The generation of charged nanoparticles in the gas phase has been continually reported in many CVD processes. However, experimental confirmation on the generation of charged nanoparticles in the gas phase is not sufficient to say that the charged nanoparticles should become a building block of films and nanostructures. In this thesis, first, the generation of charged nanoparticles in the gas phase was confirmed experimentally during the CVD of silicon using a differential mobility analyzer (DMA) connected to an atmospheric-pressure chemical vapor deposition (APCVD) reactor at various nitrogen gas flow rates as carrier gas, silane gas flow rates as precursor gas, and furnace temperatures. The processing parameters such as carrier, precursor gas flow rates and furnace temperatures affected not only the growth behavior of nanostructures but also the size distribution and number concentration of both positively and negatively charged nanoparticles. The size distribution and number concentration has a strong correlation with the microstructure evolution of films or nanostructures. Although there are numerous indirect evidences implying that charged nanoparticles should contribute to the growth of films and nanostructures, there has been no direct evidence, making it difficult to prove that charged nanoparticles are the building block of deposited films or nanostructures. Second, we showed the experimental evidences that charged nanoparticles are the building block for films and nanowires. For this, the deposition behavior during silicon CVD was compared between only electrically floated and grounded substrates as fixing the other processing parameters such as temperature and gas flow rates. The microstructure evolution was drastically different between floated and grounded substrates. These results indicate that growth behavior was affected by the electrostatic interaction between charged nanoparticles and the growing surface. Finally, considering that both positively and negatively charged nanoparticles exist abundantly in the gas phase, the charged nanoparticles would be affected by electric field. To exert the electric force on these charged nanoparticles, the alternating current (AC) and direct current (DC) bias was applied to the stainless substrate holder during CVD. The bias frequency and voltage significantly affect the microstructure evolution and the growth rate. These results indicate that the bias such as AC and DC could be applied as a new process parameter in the thermal CVD process where charged nanoparticles are generated.