We have investigated effects of formation of an ultrathin silicon dioxide (SiO2) layer using the nitric acid oxidation of Si (NAOS) method and the subsequent ammonia (NH3) plasma treatment on single crystalline Si solar cell characteristics and the surface chemical structures, respectively. The NAOS SiO2 layer formed between Si and silicon nitride (SIN) improves the conversion efficiencies from 17.2 to 18.9% for n-Si-based solar cells, and from 16.6 to 17.5% for p-Si-based solar cells. The improvement is attributed to elimination of interface states, but not to fixed charges because the flat band-voltage of the < Al/SiN/Si > MIS diodes isn't largely changed by formation of a NAOS SiO2 layer at the SiN/Si interface. The minority carrier lifetime of the NAOS SiO2/p-Si structure is greatly improved from 12 to 45 mu s (or 35 mu s) by heat treatment in oxygen at 800 degrees C (or 600 degrees C). In the absence of a NAOS SiO2 layer, the NH3 plasma treatment forms an SiN layer of 0.3 nm thickness on the Si surface. In the presence of a NAOS SiO2 layer, on the other hand, only 0.1 monolayer of SiN is formed on the surface, and at the Si interface, silicon oxynitride species is produced. It is concluded that the NAOS SiO2 layer prevents direct nitridation of Si substrates, and thus avoids introduction of plasma damage in Si. (C) 2014 Elsevier B.V. All rights reserved.