Understanding Surface Treatment and ALD AlOx Thickness Induced Surface Passivation Quality of c-Si Cz Wafers

In this paper, the passivation quality of crystalline silicon (c-Si) wafers, when passivated by atomic layer deposited aluminum oxide (ALD AlO _x ), is investigated. Specifically, we investigated the effect of surface modification of the c-Si interface prior to the ALD AlO _x deposition (via -H and -OH termination of the c-Si wafer) over a large range of AlO _x thicknesses (0.4-80 nm). Fourier transform infrared (FTIR) studies confirmed the presence of -H and -OH termination on c-Si surface. In general, surface passivation on -OH terminated wafers was found to be better than their -H terminated counterparts, which is consistent with correspondingly measured density of interface defects, and density of fixed interface charge. In all cases, the formation of an additional interfacial SiO _x layer between the c-Si/AlO _x interface was revealed. The corresponding Si-oxide peaks, as measured by X-ray photoelectron spectroscopy (XPS), were found to be stronger for -OH terminated c-Si surfaces. The thickness of the interfacial SiO _x layer was measured by high-resolution transmission electron microscopy. Thus, despite the superior surface passivation of -OH terminated c-Si wafers, for the ALD AlO _x tunnel layers of 0.4-2 nm (i.e., on the order necessary to form passivated contacts for solar cells), there is a trade-off between the better passivation achieved by the thicker SiO _x interfacial layers (-OH termination), and the correspondingly higher tunneling resistance due to the overall larger effective tunnel layer thickness.