Mechanisms of Silicon Surface Passivation by Negatively Charged Hafnium Oxide Thin Films

We have studied the mechanisms underpinning effective surface passivation of silicon with hafnium oxide (HfO ₂ ) thin films grown via atomic layer deposition (ALD). Plasma-enhanced ALD with O ₂ plasma and a tetrakis(dimethylamido)hafnium precursor was used to deposit 12 nm thick HfO ₂ films at 200 °C on high-lifetime 5 Ωcm n -type Czochralski silicon wafers. The passivation was activated by postdeposition annealing, with 30 min in air at 475 °C found to be the most effective. High-resolution grazing incidence X-ray diffraction measurements revealed the film crystallized between 325 and 375 °C, and this coincided with the onset of good passivation. Once crystallized, the level of passivation continued to increase with higher annealing temperatures, exhibiting a peak at 475 °C and yielding surface recombination velocities of <5 cm s ⁻¹ at 5 × 10 ¹⁴ cm ⁻³ injection. A steady decrease in effective lifetime was then observed for activation temperatures >475 °C. By superacid repassivation, we demonstrated this reduction in lifetime was not because of a decrease in the bulk lifetime, but rather because of changes in the passivating films themselves. Kelvin probe measurements showed the films are negatively charged. Corona charging experiments showed the charge magnitude is of order 10 ¹² qcm ⁻² and that the reduced passivation above 475 °C was mainly because of a loss of chemical passivation. Our study, therefore, demonstrates the development of highly charged HfO ₂ films and quantifies their benefits as a standalone passivating film for silicon-based solar cells.