ALD tantalum oxide in a passivation stack for silicon solar cells

This is an interesting paper on Ta2O5 ALD for surface passivation in silicon based solar cells. As the authors point out, despite more than four decades of work on Ta2O5, no attempt has yet been made to study this material on c-Si as an electronic passivating layer. Ta2O5 has excellent optical properties:

• a relatively high refractive index 
• a negligible absorption in the visible range

This is why Ta2O5 is often used as antireflection coating (ARC). The work below by researchers at The Australian National University in Canberra was performed in a Picosun R200 Advanced ALD reactor using Tantalum Ethoxide as tantalum precursor and H2O as the oxidant at 250 °C. The reactor is located at the Australian National Fabrication Facility (ANFF).

Established under the National Collaborative Research Infrastructure Strategy, the Australian National Fabrication Facility (ANFF) links 8 university-based nodes to provide researchers and industry with access to state-of-the-art fabrication facilities (http://www.anff.org.au).

Tantalum oxide/silicon nitride: A negatively charged surface passivation stack for silicon solar cells (Open Access)

Yimao Wan, James Bullock and Andres Cuevas
Appl. Phys. Lett. 106, 201601 (2015); http://dx.doi.org/10.1063/1.4921416 

This letter reports effective passivation of crystalline silicon (c-Si) surfaces by thermal atomic layerdeposited tantalum oxide (Ta2O5) underneath plasma enhanced chemical vapour depositedsilicon nitride (SiNx). Cross-sectional transmission electron microscopy imaging shows an approximately 2 nm thick interfacial layer between Ta 2O5 and c-Si. Surface recombination velocities as low as 5.0 cm/s and 3.2 cm/s are attained on p-type 0.8 Ω·cm and n-type 1.0 Ω·cm c-Si wafers, respectively. Recombination current densities of 25 fA/cm2 and 68 fA/cm2 are measured on 150 Ω/sq boron-diffused p + and 120 Ω/sq phosphorus-diffused n + c-Si, respectively. Capacitance–voltage measurements reveal a negative fixed insulator charge densityof −1.8 × 1012 cm−2 for the Ta 2O5 film and −1.0 × 1012 cm−2 for the Ta 2O5/SiNx stack. The Ta2O5/SiNx stack is demonstrated to be an excellent candidate for surface passivation of high efficiency silicon solar cells.