Interfacial oxides for charge control of hafnium oxide surface passivation of silicon

Surface passivating stacks are fabricated on n-type silicon using plasma-enhanced atomic layer deposition (ALD) with the structure comprising a positively charged silicon oxide interlayer capped with negatively charged hafnium oxide. Without the inclusion of an ALD-grown silicon oxide interlayer, high-resolution transmission electron microscopy reveals the existence of a ∼2 nm thick silicon oxide layer between the hafnium oxide and the silicon before and after a 450 °C activation anneal. The thickness of the silicon oxide interlayer is increased with the intentional deposition of silicon oxide by plasma-enhanced ALD (up to 33 nm). By increasing the thickness of silicon oxide, we demonstrate control of the stack's passivating properties, finding an inverse relationship between interlayer thickness and passivation level. Corona charging experiments demonstrate a reduction in the net negative charge density (from −1012 q cm−2 to −1011 q cm−2) with increasing interlayer thickness. For the interlayer thicknesses considered, Kelvin probe measurements confirm that the charge polarity was not flipped as has previously been observed for an analogous stack with an aluminium oxide capping layer. ALD-SiOx/HfOx stacks provide well-defined effective fixed charge densities which can be used to provide flexibility in field-effect passivation for silicon photovoltaic solar cells.