Silicon Surface Passivation by Al2O3: Recombination Parameters and Inversion Layer Solar Cells

Abstract

The interface between p- and n-type FZ-Si and an amorphous aluminum oxide (Al2O3) surface passivation layer deposited by plasma-assisted atomic layer deposition (ALD) was investigated by frequency-dependent conductance measurements. The hole capture cross section in the lower half of the bandgap, σp = (4±3)×10-16cm2, was found to be independent of energy. The electron capture cross section σn in the upper half of the bandgap decreases from σn = (7±4)×10-15cm2 at midgap over two orders of magnitude towards the conduction band edge. Numerical simulations of the effective surface recombination velocity based on these recombination parameters show a good agreement with experimental surface recombination velocities for a wide range of excess carrier and surface charge densities. Carrier transport in the inversion layer formed at the n-Si/Al2O3 interface was investigated yielding a sheet resistance of 15 kΩ/□, which was reduced to 6 kΩ/□ for a surface charge density of –2×1013cm2 obtained by corona charging. The applicability of Al2O3 inversion layers as emitters in n-type inversion layer solar cells was demonstrated by short circuit current densities of up to 25mA/cm2, which show a pronounced dependence on surface charge density.