Electronic Band Offset Determination of Oxides Grown by Atomic Layer Deposition on Silicon

Abstract

Minimizing electrical losses at metal/silicon interfaces in high-efficiency single-junction silicon solar cells requires the use of carrier-selective passivating contacts. The electronic barrier heights at the insulator/silicon interface are necessary for calculating the probability of quantum tunneling of charge carriers at these interfaces. Thus, precise knowledge of these parameters is crucial for the development of contact schemes. Using a photoemission-based method, we experimentally determine the electronic band offsets of Al ₂ O ₃ , HfO ₂ and SiO ₂ layers grown by atomic layer deposition (ALD) on silicon. For Al ₂ O ₃ /Si, we determine a valence band offset (Δ E_V ) and conduction band offset (Δ E_C ) of 3.29 ± 0.07 eV and 2.24 ± 0.13 eV, respectively. For HfO ₂ /Si, Δ E_V and Δ E_C are determined as 2.67 ± 0.07 eV and 1.81 ± 0.21 eV, while for SiO ₂ /Si, Δ E_V and Δ E_C are 4.87 ± 0.07 eV and 2.61 ± 0.12 eV, respectively. Using technology computer-aided design simulations, we incorporate our experimental results to estimate the contact resistivity that would be attained at various dielectric layer thicknesses. We find that for achieving the 100 mΩ·cm ² contact resistivity benchmark, Al ₂ O ₃ layers should be no thicker than 1.65 nm for a p -type polysilicon-based hole-selective contact, assuming hole tunneling masses taken from the literature. Correspondingly, for HfO ₂ and SiO ₂ , an upper limit of 1.4 nm is determined as the thickness threshold in order to utilize these ALD-grown layers for contacts in high-performance silicon photovoltaics.