Thermal stability of silicon surface passivation by APCVD Al ₂O₃

We investigate the thermal stability of silicon surface passivation provided by aluminium oxide (Al₂O₃) 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 Al₂O₃ 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 Al₂O₃ 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/cm² on a diffusion with a sheet resistance of 100 Ω/□ and surface dopant concentration of ~1.3×10¹⁹ 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.