Surface passivation and optical characterization of Al2O3/a-SiCx stacks on c-Si substrates

Abstract

The aim of this work is to study the surface passivation of aluminum oxide/amorphous silicon carbide (Al2O3/a-SiCx) stacks on both p-type and n-type crystalline silicon (c-Si) substrates as well as the optical characterization of these stacks. Al2O3 films of different thicknesses were deposited by thermal atomic layer deposition (ALD) at 200 °C and were complemented with a layer of a-SiCx deposited by plasma-enhanced chemical vapor deposition (PECVD) to form anti-reflection coating (ARC) stacks with a total thickness of 75 nm. A comparative study has been carried out on polished and randomly textured wafers. We have experimentally determined the optimum thickness of the stack for photovoltaic applications by minimizing the reflection losses over a wide wavelength range (300-1200 nm) without compromising the outstanding passivation properties of the Al2O3 films. The upper limit of the surface recombination velocity (S eff,max) was evaluated at a carrier injection level corresponding to 1-sun illumination, which led to values below 10 cm/s. Reflectance values below 2% were measured on textured samples over the wavelength range of 450-1000 nm.

Keywords: aluminum oxide (Al2O3); antireflection coating; atomic layer deposition; silicon carbide (SiCx); surface passivation.

Figure 1

Surface recombination velocity, S_eff,max [cm/s]. (a) and (b) S_eff,max for randomly textured and polished n-type wafers respectively. (c) and (d) S_eff,max for randomly textured and polished p-type wafers respectively. S_eff,max was determined at 1-sun injection level as a function of the Al₂O₃ thickness. The aluminum oxide layers were complemented up to 75 nm with an a-SiC_x film.

Figure 2

Absorbance as a function of the wavelength in the range from 300 to 600 nm. Dashed lines belong to stacks with different Al₂O₃ thicknesses while the continuous red line is the absorbance of 90 nm Al₂O₃.

Figure 3

Reflectance curves of Al₂O₃-coated randomly textured c-Si (a) and polished c-Si (b) for different film thicknesses. Reflectance curves of Al₂O₃/a-SiC_x coated randomly textured c-Si (c) and polished c-Si (d). As a reference, bare polished and textured c-Si reflectances are also included (black line).