Mechanisms of charge carrier transport in polycrystalline silicon passivating contacts

Abstract

We use temperature-dependent contact resistivity (rho c) measurements to systematically assess the dominant electron transport mechanism in a large set of poly-Si passivating contacts, fabricated by varying (i) the annealing temperature (Tann), (ii) the oxide thickness (tox), (iii) the oxidation method, and (iv) the surface morphology of the Si substrate. The results show that for silicon oxide thicknesses of 1.3-1.5 nm, the dominant transport mechanism changes from tunneling to drift-diffusion via pinholes in the SiOx layer for increasing Tann. This transition occurs for Tann in the range of 850 degrees C-950 degrees C for a 1.5 nm thick thermal oxide, and 700 degrees C-750 degrees C for a 1.3 nm thick wet-chemical oxide, which suggests that pinholes appear in wet-chemical oxides after exposure to lower thermal budgets compared to thermal oxides. For SiOx with tox = 2 nm, grown either thermally or by plasma-enhanced atomic layer deposition, carrier transport is pinhole-dominant for Tann = 1050 degrees C, whereas no electric current through the SiOx layer could be detected for lower Tann. Remarkably, the dominant transport mechanism is not affected by the substrate surface morphology, although lower values of rho c were measured on textured wafers compared to planar surfaces. Lifetime measurements suggest that the best carrier selectivity can be achieved by choosing Tann right above the transition range, but not too high, in order to induce pinhole dominant transport while preserving a good passivation quality.