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

This work investigates the surface passivation achieved by growing silicon dioxide (SiO ₂) electrochemically in concentrated nitric acid (HNO ₃) 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 SiO ₂ 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 Q _f = 3.10 ¹² cm ^-2 and a high interface defect density of D _it >10 ¹³ cm ^-2eV ^-1, whereas the SRV obtained by the AC oxidation results from a lower Q _f of <1.10 ¹² cm ^-2 and D _it of 10 ¹¹ 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.