Impurity Gettering by Atomic-Layer-Deposited Aluminium Oxide Films on Silicon at Contact Firing Temperatures

Abstract

Aluminium oxide (Al2O3) thin films deposited on silicon surfaces, synthesised by plasma-assisted atomic layer deposition, are recently reported to possess impurity gettering effects for the silicon wafer bulk during annealing at 425 °C, a typical temperature used for activating the surface passivation quality of the Al2O3 films. This paper investigates the gettering effects of Al2O3 films at higher temperatures of 700–900 °C, which are commonly used for contact firing in silicon solar cell fabrication. Iron is used as a marker impurity in silicon to study the gettering effectiveness. Results show that Al2O3 films also generate strong impurity gettering effects at 700–900 °C, through a segregation gettering mechanism. The as-deposited Al2O3 films are found to be more effective at gettering than the 425 °C-activated Al2O3 films, demonstrating gettering processes that are largely limited by the impurity diffusivity in silicon. For both as-deposited and activated Al2O3 films, gettering during high temperature annealing occurs by impurity accumulation at the Al2O3/Si interfaces, similar to the gettering action at 425 °C. However, some iron is found to redistribute into the bulk of the Al2O3 films after long annealing at a high temperature.