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.

on PERC technologies and get even closer to the theoretical single-junction efficiency limit, electrical losses in the contacted regions must be reduced. [3][4][5] Passivating contacts can help alleviate such losses by simultaneously suppressing the current of non-collected carriers to the contact, and by reducing recombination sites at the interface. Introducing a passivating interlayer between the metal/silicon interface provides a route to reducing the recombination current density, J 0 , [6,7] thereby increasing device voltage. [3] Passivating contacts have achieved some success to date, with the strongest candidates being polysilicon on top of thin silicon oxide layers (e.g., tunnel oxide passivating contacts (TOPCon) or poly silicon on oxide (POLO)) and amorphous silicon (a-Si) heterojunctions. [3,7,8] TOPCon is an efficient electron-selective contact but has a high thermal budget with temperatures around 900 °C needed to reduce the contact resistivity to acceptable levels. [9] An efficient hole-selective layer that can match or exceed the performance of the current electron-selective materials would be of considerable interest. The use of SiO 2 -based hole-selective contacts has so far failed to reach equivalent levels. [10,11] The most promising hole-selective contacting materials are p-type a-Si and siliconrich SiC, but conventional high-temperature Ag screen printing methods are not necessarily compatible with such contacts. [10] Surface passivating thin films are crucial for limiting the electrical losses during charge carrier collection in silicon photovoltaic devices. Certain dielectric coatings of more than 10 nm provide excellent surface passivation, and ultra-thin (<2 nm) dielectric layers can serve as interlayers in passivating contacts. Here, ultra-thin passivating films of SiO 2 , Al 2 O 3 , and HfO 2 are created via plasma-enhanced atomic layer deposition and annealing. It is found that thin negatively charged HfO 2 layers exhibit excellent passivation properties-exceeding those of SiO 2 and Al 2 O 3 -with 0.9 nm HfO 2 annealed at 450 °C providing a surface recombination velocity of 18.6 cm s −1 . The passivation quality is dependent on annealing temperature and layer thickness, and optimum passivation is achieved with HfO 2 layers annealed at 450 °C measured to be 2.2-3.3 nm thick which give surface recombination velocities ≤2.5 cm s −1 and J 0 values of ≈14 fA cm −2 . The superior passivation quality of HfO 2 nanolayers makes them a promising candidate for future passivating contacts in high-efficiency silicon solar cells.

Thin film dielectrics are ubiquitous in the manufacture of electronic devices and are frequently deposited and etched away at various stages of device fabrication. We demonstrate that hafnium oxide (HfO2) thin films grown via atomic layer deposition on silicon and silicon pre-coated with aluminum oxide (Al2O3) have etch resistance properties, which can be tuned simply by changing the post-deposition annealing temperature. The etching rates of films in hydrofluoric acid (HF) solutions were found to be dependent on annealing temperature, with the etch rate decreasing with increasing temperature. A transition region in the etch rate was identified between 300 and 350 °C, corresponding to the crystallization of the HfO2 films, as identified via x-ray diffraction. HfO2 films deposited directly onto silicon annealed above 350 °C were resistant to 10% HF solutions over the course of several hours. In the case of Si/Al2O3/HfO2 stacks, closer inspection reveals the existence of channels, which reduces the etch resistance of HF acid, as evidenced by tetramethylammonium hydroxide etching of the silicon substrate. Crystallized HfO2 can be used to protect other dielectrics in device processing, and we demonstrate its use in single-sided fabrication of patterned structures of Al2O3, which can control the effective charge-carrier lifetime in silicon wafers for use in modulating THz and mm-wave radiation.