Investigation of the growth process of silicon from the gas phase in the Dynatec Centrifuge and Free Space Reactor

Abstract

Silicon is by far the most important semiconductor for use in electronicand photovoltaic applications. As a result from policy changes in countries such as Germany, Japan and others the installation of photovoltaic modules increased rapidly in the beginning of the last decade. This led to a short sale on the photovoltaic market. However after China entered the market, flooding it with cheap modules, many producers were pushed out of the marked as they could no longer compete with the low prices. This price drop encouraged producers to investigate new, less energy consuming process methods for silicon production. In this masters thesis the results of two such methods are discussed with respect to production processes on the microscopic scale. Samples from a Dynatec Centrifuge Reactor (DCR) and a Free Space Reactor (FSR) were analysed using visible light and electron microscopes as well as electron and X-Ray diffraction. The results suggest a strong dependence of the formation of crystallised structures on temperature. Phosphorous doping, however, can reduce this temperature. The major part of particles produced in the FSR at 575 °C and below grew to a size between 50 and 200 nm and were predominantly amorphous. At 600 °C the size and the fraction of crystalline particles grew significantly. The location within the DCR has a major effect on the density, crystallinity and hardness on the end product. The wall area shower crystalline silicon, the ceiling amorphous and crystalline structures and the exhaust almost only amorphous particles. The change of silane content within the reaction gas inside the reactor is one of the reasons for this change. The effect of carbon contamination on the growth process and the end product has been investigated. It has been found that it influenced the homogeneous growth and led to a diverse end product. Areas with no Carbon whatsoever, as well as those with crystalline Carbon, graphite and SiC structures were found.

Silicon is by far the most important semiconductor for use in electronicand photovoltaic applications. As a result from policy changes in countries such as Germany, Japan and others the installation of photovoltaic modules increased rapidly in the beginning of the last decade. This led to a short sale on the photovoltaic market. However after China entered the market, flooding it with cheap modules, many producers were pushed out of the marked as they could no longer compete with the low prices. This price drop encouraged producers to investigate new, less energy consuming process methods for silicon production. In this masters thesis the results of two such methods are discussed with respect to production processes on the microscopic scale. Samples from a Dynatec Centrifuge Reactor (DCR) and a Free Space Reactor (FSR) were analysed using visible light and electron microscopes as well as electron and X-Ray diffraction. The results suggest a strong dependence of the formation of crystallised structures on temperature. Phosphorous doping, however, can reduce this temperature. The major part of particles produced in the FSR at 575 °C and below grew to a size between 50 and 200 nm and were predominantly amorphous. At 600 °C the size and the fraction of crystalline particles grew significantly. The location within the DCR has a major effect on the density, crystallinity and hardness on the end product. The wall area shower crystalline silicon, the ceiling amorphous and crystalline structures and the exhaust almost only amorphous particles. The change of silane content within the reaction gas inside the reactor is one of the reasons for this change. The effect of carbon contamination on the growth process and the end product has been investigated. It has been found that it influenced the homogeneous growth and led to a diverse end product. Areas with no Carbon whatsoever, as well as those with crystalline Carbon, graphite and SiC structures were found.