Main properties of Al2O3 thin films deposited by magnetron sputtering of an Al2O3 ceramic target at different radio-frequency power and argon pressure and their passivation effect on p-type c-Si wafers

Abstract

In this work, 50-nm thick Al2O3 thin films were deposited at room temperature by magnetron sputtering from an Al2O3 ceramic target at different RF power and argon pressure values. The sputtering technique could be preferred to conventional atomic layer deposition for an industrial application, owing to its simplicity, availability, and higher deposition rate. The resulting thin films were characterized by UV/Vis/NIR spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The deposited Al2O3 material was always highly transparent and amorphous in nature. It was found that the O/Al ratio is higher when the Al2O3 layer is deposited at lower RF power or higher argon pressure. Also, some argon incorporation into the films was observed at low deposition pressure. On the other hand, the performance of the previously characterized Al2O3 thin films in the passivation of 2.25-Ωcm p-type float zone c-Si wafer surfaces was evaluated by the quasi-steady-state photoconductance technique. The best effective carrier lifetime value at one-sun illumination, 0.34 ms (corresponding to a surface recombination velocity of 41 cm/s), was obtained with the 50-nm Al2O3 deposited at the higher argon pressure studied, 0.67 Pa (5.0 mTorr), with the lowest RF power studied, 150 W (corresponding to a power density of 3.3 W/cm2), and after an annealing process, in this case at 350ºC for 20 min with forming gas. It was assumed that the reduction of the surface passivation quality at higher RF power or lower argon pressure is a consequence of an increased surface damage, and, probably, to a decrease of the O/Al ratio of the Al2O3 passivation material. These assumptions were confirmed with the obtainment of a lifetime of 0.73 ms (a surface recombination velocity equal to 19 cm/s) with a simple experiment with Al2O3 deposited with progressively varied sputtering conditions started from minimal silicon surface damage conditions: 50 W (corresponding to a power density of 1.1 W/cm2) and 6.67 Pa (50 mTorr). Finally, comments about further improvement of the effective lifetime (up to 1.25 ms, corresponding to a surface recombination velocity of 11 cm/s) with preliminary experiments about the incorporation of an intrinsic hydrogenated amorphous silicon interlayer are included.