New Perspectives on Surface Passivation: Understanding the Si‒Al₂O₃ Interface

Abstract

High-efficiency crystalline silicon solar cells must suppress recombination at their p-type surfaces. Thin-film, amorphous aluminium oxide (Al₂O₃) has been shown to provide very effective passivation of such surfaces, assisted by its negative fixed charge. However, many details of Al₂O₃ passivation remain poorly understood. Furthermore, conventional means of depositing passivating Al₂O₃ are too slow or too expensive to be suitable for high-volume commercial production. This thesis addresses these issues in three ways: 1) by contributing to a deeper understanding of semiconductor-dielectric interfaces and semiconductor surface recombination mechanisms in general, 2) by investigating the properties of Al₂O₃ as a passivating dielectric for silicon surfaces, and 3) by demonstrating the viability of APCVD as a high-throughput, industrially compatible deposition method for Al₂O₃, enabling its application to commercial solar cells. Using Al₂O₃ as a test case, it is shown how a novel analysis of the extended conductance method can be used to i) distinguish the separate contributions to the interface state distribution at a semiconductor-dielectric interface, and ii) determine their capture cross-sections for both minority and majority carriers. Furthermore, the direct link between these measured interface state properties and the recombination rate at the semiconductor surface is experimentally demonstrated by showing that the former can be used to accurately predict the latter. Investigations of the surface passivation properties of Al₂O₃ reveal a remarkably consistent picture. It is shown that the properties of the Si‒Al₂O₃ interface states are essentially independent of the Al₂O₃ deposition conditions and technique. The interface properties are found to be independent of the surface dopant concentration at boron- and phosphorus-doped surfaces, while recombination is shown to be only weakly dependent on surface orientation and morphology as a result of the remarkable orientation-independence of the Si‒Al₂O₃ interface state properties. Meanwhile, the chemical origin of the charge at the Si‒Al₂O₃ interface is elucidated by correlating FTIR and electrical measurements. APCVD is clearly shown - for the first time - to be capable of depositing Al₂O₃ films with exceptional surface passivation properties, comparable to the best results achieved using other deposition techniques. In the best case, interface state densities as low as 5 x 10¹⁰ eV¯¹ cm¯² at midgap, and negative fixed charge concentrations of 3.3 x 10¹²cm¯² are measured, resulting in a saturation current density of 7fAcm¯² on undiffused p-type surfaces. The APCVD films are shown to be thermally stable under standard solar cell processing conditions and are demonstrated in large-area solar cells with peak efficiencies of 21.3 %. These results demonstrate the viability of APCVD Al₂O₃ as a surface passivation layer for industrial silicon solar cells.