Low surface recombination velocities achieved by silicon dioxide grown electrochemically in nitric acid

Abstract

This work investigates the surface passivation achieved by growing silicon dioxide (SiO2) electrochemically in concentrated nitric acid (HNO3) at room temperature, a procedure that has the potential to be significantly less ex pensive than the thermal oxides used in high-efficient solar cells and test structures. The SiO2 layers are formed by two methods: direct-current (DC) electrochemical oxidation and alternating-current (AC) electrochemical oxidation. Prior to annealing, both methods offer poor passivation, however after annealing in oxygen and then forming gas, surface recombination velocities (SRV) of 35 cm/s and 15 cm/s are achieved for the DC and AC methods, respectively. In the case of the DC oxidation, the low SRV is achieved by the presence of a high positive charge density of Qf = 3.1012 cm-2 and a high interface defect density of Dit >1013 cm-2eV-1, whereas the SRV obtained by the AC oxidation results from a lower Qf of <1.1012 cm-2 and Dit of 1011 cm-2eV-1, which is more desirable for solar cell passivation. In quantifying the SRV more precisely, we have used a HF passivation method to monitor the bulk lifetime. In some cases the bulk lifetime has been shown to decrease from ∼ 11 ms to ∼ 500 μs after DC and AC oxidation method followed by a low temperature anneal (400°C). However by cleaning the silicon wafers using the RCA method prior to oxidation, very little contamination is observed.