Thermal stability of silicon surface passivation by APCVD Al2O3

Abstract

Abstract We investigate the thermal stability of silicon surface passivation provided by aluminium oxide (Al 2 O 3 ) films deposited using atmospheric pressure chemical vapour deposition (APCVD) and fired in a belt furnace at a peak temperature of ~810 °C. Firing stability is investigated for p - and n -type substrates as a function of Al 2 O 3 film thickness both with and without a plasma-enhanced chemical vapour deposition (PECVD) SiN x capping layer, and for boron-diffused surfaces with a ~10 nm Al 2 O 3 film only. Excellent thermal stability of the passivation is demonstrated, with effective carrier lifetimes in n -type silicon wafers remaining stable or even improving after firing, and lifetimes in p -type wafers initially degrading slightly but recovering to above their initial values following ~10 min illumination by a halogen lamp at ~20 mW/cm 2 . Film thickness appears to be unimportant to stability, as does the presence of the capping layer. Surface recombination velocities of less than 3 cm/s for 1.35 Ω cm p -type and less than 2 cm/s for 1.2 Ω cm n -type substrates are measured after firing and illumination. The passivation of boron-diffused surfaces is also shown to improve slightly following firing, with a post-firing saturation current density of 42 fA/cm 2 on a diffusion with a sheet resistance of 100 Ω/□ and surface dopant concentration of ~1.3×10 19 cm −3 . Capacitance–voltage ( C – V ) measurements show that short firing times result in an initial reduction of the interface defect density D it and a slight increase of the negative insulator fixed charge density Q f , while longer firing results in a substantial increase in both Q f and D it .