Hafnium Oxide Surface Passivation for Silicon Solar Cells

Hafnium oxide (HfO₂) is a wide bandgap electrical insulator, frequently used in the electronics industry due to its dielectric properties. With its high dielectric constant, high refractive index, inherent mechanical robustness, and potential for charge storage, HfO₂ makes an excellent candidate for silicon surface passivation. Importantly, HfO₂ can easily be deposited using established industrial processes, such as atomic layer deposition (ALD), which is now widely used in the silicon PV industry to deposit Al₂O₃ films. Importantly, in contrast to Al₂O₃, HfO₂ generally provides a positively charged film¹, giving an additional degree of freedom for field effect passivation in certain solar cell designs.<br/><br/>We have investigated the effects of various post-deposition annealing conditions on the carrier lifetime produced by ALD-grown HfO₂ films, focusing on optimising the temperature, time and ambient. Prior research on this topic used float-zone wafers^1–3, whose lifetime is has been found to degrade at the temperatures at which the passivation is activated^4,5. In contrast, we use Czochralski silicon (the material of choice for high efficiency solar cells), in conjunction with room temperature superacid passivation to isolate surface and bulk effects which may have influenced previous work.<br/><br/>We deposit our hafnium oxide using plasma-enhanced ALD at 200 °C, and subsequently anneal the sample to activate the surface passivation. We find that effective carrier lifetime initially increases with increasing temperature, up to around 450-500^oC, and at higher temperatures is begins to degrade. This temperature is significantly higher than past reports which all used float zone silicon, which we believe could have been influenced by bulk carrier lifetime degradation effects not related to surface passivation⁴^,⁵. Through this optimisation process we have been able to achieve surface recombination velocities &lt;5 cm/s.<br/><br/>To support our lifetime results we conduct a series of superacid re-passivation experiments, whereby the HfO₂ films are etched away and the sample is subsequently re-passivated at room temperature using a superacid-based method^6,7. This approach, which has not been applied in this context previously, enables us to separate competing surface and bulk effects. Using this method ensures that the measured effects are solely attributed to the HfO₂ films themselves, and not influenced by bulk degradation due to the thermally induced recombination centres. Initial experiments show no obvious degradation to the bulk of the Czochralski silicon at these temperatures, indicating the beneficial effects of higher temperature activation in the context of photovoltaic cells.<br/><br/>With the aim of understanding the origin of the effects observed, we use high resolution grazing incidence X-ray diffraction to characterise the HfO₂ films as a function of activation temperature. Results suggest the crystallinity of the film is important in achieving good passivation but there are other factors involved, including changes to field effect and chemical passivation. We study these competing effects by analysing the injection dependence of the effective lifetime and also through Kelvin probe measurements. This work clarifies the role both HfO₂ crystallinity and silicon bulk effects play within the passivation process, and furthers the development of HfO₂ as the main candidate for a positively charged ALD-grown passivation layer for solar cells.<br/><br/>1 J. Cui, et al., <i>Appl. Phys. Lett.</i>, 2017, <b>110</b>, 021602.<br/>2 A. B. Gougam, et al. <i>Mater. Sci. Semicond. Process.</i>, 2019, <b>95</b>, 42–47.<br/>3 S. Tomer, et al., <i>IEEE J. Photovoltaics</i>, 2020, <b>10</b>, 1614–1623.<br/>4 N. E. Grant, et al., <i>Phys. Status Solidi - Rapid Res. Lett.</i>, 2016, <b>10</b>, 443–447.<br/>5 N. E. Grant, et al., <i>Phys. Status Solidi Appl. Mater. Sci.</i>, 2016, <b>213</b>, 2844–2849.<br/>6 N. E. Grant, et al., <i>IEEE J. Photovoltaics</i>, 2017, <b>7</b>, 1574–1583.<br/>7 A. I. Pointon, et al., <i>Sol. Energy Mater. Sol. Cells</i>, 2018, <b>183</b>, 164–172.