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

Abstract

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.

Stable chemical enhancement of passivating nanolayer structures grown by atomic layer deposition on silicon

“…Earlier studies into HfO 2 passivation stated that for HfO 2 -based layers to be incorporated into photovoltaic processing schemes, the temperatures they were exposed to could not exceed 400 °C. 16 We have since shown that HfO 2 -based films perform well up to ∼500 °C, 30 but, nevertheless 400–500 °C temperature range provides a useful benchmark for testing thermal stability.…”

“…The passivation quality of both HfO 2 and Al 2 O 3 are known to be dependent on annealing temperature, with optimal passivation achieved in both cases on annealing at 450-500 °C. 11,30 Photoconductance decay lifetime curves for the three structures activated at 450 °C are shown in Fig. 1(a).…”

“…The average passivation quality of HF-dipped stacks exceeds that of untreated Si/HfO 2 / Al 2 O 3 at all temperatures studied. Higher temperature ranges could not be studied, as above ∼500 °C, HfO 2 -based passivation quality is known to degrade, 30 and hence it would not be possible to separate contributions from the dielectric layers and the chemical treatment at these annealing temperatures.…”

“…Thus, the observed improvement seems related to properties of the complete stack, not the individual layers. Changes in CPD could also be due to changes in the dielectric constant, 30 but this does not seem likely for a simple room temperature chemical treatment.…”

“…The difference in behaviour for ≥30 s of HF immersion for 1 nm and 10 nm interlayers is likely because of differences in crystallisation level between the HfO 2 films of different thicknesses. 10 nm thick films are known to crystallise upon annealing at 450 °C, 30 becoming highly resistant to etching in HF. 65 ∼1 nm HfO 2 films are unlikely to crystallise fully without being annealed at 700-800 °C, 66 and thus have not developed resistance to etching in HF.…”