Characterization of a-SiC x :H Films for c-Si Surface Passivation
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Characterization of a-SiCx:H Films for c-Si Surface Passivation M. Vetter, I. Martín, A. Orpella, C. Voz, J. Puigdollers and R. Alcubilla Departament d’Enginyeria Electrònica, Universitat Politècnica de Catalunya Gran Capità S/N, Mòdul C4, 08034 Barcelona, Spain. ABSTRACT Amorphous intrinsic silicon carbide (a-SiCx:H(i)) films and amorphous phophorous doped silicon carbide (a-SiCx:H(n)) films deposited by plasma enhanced chemical vapor deposition (PECVD) from silane/methane mixtures provide excellent electronic passivation of p-type c-Si. Effective surface recombination velocities (Seff) lower than 23 cm s-1 have been reported for aSiCx:H(i) films and Seff < 11 cm s-1 for a-SiCx:H(n) films. The analysis of the dependence of Seff on the injection level indicates that the good electronic passivation is due to field-effect passivation resulting from a high fixed charge (Qf) created in the a-SiCx:H film. In this work the absorption of SiH bonds in infrared transmission spectra of a-SiCx:H films is quantitatively analysed resulting in about 30% smaller amount of SiH bonds in phosphorous doped films compared to intrinsic films. Furthermore, a strong reciprocal correlation of the hydrogen content in the films and the Qf created at the a-SiCx:H/c-Si interface is observed. INTRODUCTION The electronic passivation of the surface in crystalline silicon (c-Si) solar cells is of paramount importance to obtain high conversion efficiencies. In high quality c-Si silicon solar cells recombination occurs mainly at the surfaces especially if the thickness of the solar cell is reduced to values lower than the diffusion length of minority charge carriers. Recently, amorphous intrinsic (a-SiCx:H(i)) and phosphorous doped silicon carbide (aSiCx:H(n)) deposited by PECVD have been shown to provide excellent electronic passivation of p-type c-Si [1, 2, 3]. From the analysis of the dependence of the effective surface recombination velocity (Seff) on the injection level (∆n) measured with the quasi-steady-state photoconductance method [4] results that the good electronic passivation is due to a strong band bending at the c-Si surface. The origin of the band bending at the c-Si surface is the creation of a high amount of positive charges (Qf) in the a-SiCx:H film together with a low interface state density. The creation of fixed charges in insulators is known in the silicon oxide (SiO2)/c-Si interfaces or amorphous silicon nitride(a-SiNx)/c-Si interfaces and is due to the formation of defect centers (E-center in SiO2 [5], K-center in SiNx [6]) in the band gap of these insulators because of Si dangling bond formation. The creation of these centers depends strongly on preparation conditions e.g. on SiO2 oxidation temperature and post oxidation ambient [7] and in case of a-SiNx:H on hydrogen content in the plasma [8]. Recent investigations of a-SiCx:H films have shown that also phosphorous doping of these films affects the charge creation at the aSiCx:H/c-Si interface [3]. This paper reports on infrared (IR) spectroscopic transmission measurements
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