Band Alignment and Surface Photo-Voltage Analysis of C-Si/Sio_x/Poly-Silicon Passivating-Contact Stacks Through X-Ray...

Ultrathin SiO_x layers and c-Si/SiO_x interfaces find application in tunnel-oxide passivated contacts (TOPcon) for high-efficiency silicon solar cells. Here, we investigate their detailed microscopic properties, with specific attention for the case of c-Si(100) substrates, capped either by p-type or n-type poly-silicon layers [c-Si/SiO_x/poly-Si (p⁺) or c-Si/SiO_x/poly-Si (n⁺)]. Our focus is on the effects of the substrate preparation conditions (either by a dry-plasma or wet SiO_x process) and the high-temperature annealing step (as required for the poly-Si crystallization) on the SiO_x stoichiometry and its microscopic structure. Through advanced photoemission techniques, we find a clearly decreased valence band offset between the c-Si and SiO_x (from 4.5 eV to 4.15 eV) when comparing the dry SiO_x with the wet SiO_x process, independent of the SiO_x film thickness, but correlating with the relative fraction of sub-stochiometric Si states. We lastly examine the magnitude of band-bending of the contact structure through controlled in-situ exposure to light of the surfaces and subsequent tracking of core and valence band levels via a surface photovoltage and a junction photo-voltage (JPV) effect. By analyzing this JPV effect qualitatively, we find it to be proportional to the expected quasi fermi level splitting within the c-Si wafer.