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

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 Q_f = 3·10¹² cm^-2 and a high interface defect density of D_it >;10¹³ cm^-2eV^-1, whereas the SRV obtained by the AC oxidation results from a lower Q_f of <;1·10¹² cm^-2 and D_it 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.