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In this publication, the activation and degradation of the passivation quality of plasma-enhanced chemical vapor deposited aluminum oxide (Al2O3) layers with different thicknesses (10 nm, 20 nm, and 110 nm) on crystalline silicon (c-Si) during long and high temperature treatments are investigated. As indicated by Fourier Transform Infrared Spectroscopy, the concentration of tetrahedral and octahedral sites within the Al2O3 layer changes during temperature treatments and correlates with the amount of negative fixed charges at the Si/Al2O3 interface, which was detected by Corona Oxide Characterization of Semiconductors. Furthermore, during a temperature treatment at 820 °C for 30 min, the initial amorphous Al2O3 layer crystallize into the γ-Al2O3 structure and was enhanced by additional oxygen as was proven by x-ray diffraction measurements and underlined by Density Functional Theory simulations. The crystallization correlates with the increase of the optical density up to 20% while the final Al2O3 layer thickness decreases at the same time up to 26%. All observations described above were detected to be Al2O3 layer thickness dependent. These observations reveal novel aspects to explain the temperature induced passivation and degradation mechanisms of Al2O3 layers at a molecular level like the origin of the negative fixe charges at the Si/SiOx/Al2O3 interface or the phenomena of blistering. Moreover, the crystal phase of Al2O3 does not deliver good surface passivation due to a high concentration of octahedral sites leading to a lower concentration of negative fixed charges at the interface. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we present a systematic study of the surface recombination velocity of boron-diffused Si wafer passivated with plasma-enhanced chemical-vapor-deposited (PECVD) AlOx layers. Saturation current densities in the range of 5.2–38fAcm−2 (at 300K) were achieved on planar surfaces. In particular, we present an industrially relevant boron emitter, allowing for about 700mV open circuit voltage on textured surfaces, after a firing process. This high passivation quality could be achieved using AlOx passivation layers deposited by PECVD. The passivation quality is found to be equivalent to AlOx layers deposited by plasma-assisted atomic-layer-deposition. A wide range of surface doping concentration (2.5–70×1018 cm−3) was investigated. The emitters used here allow a high resolution in the determination of the surface recombination velocity upper limit. Our simulations, based on Fermi-Dirac statistics, indicate that only very shallow emitters can be used to further increase the resolution on the determination of the surface recombination velocity. [Copyright &y& Elsevier]

Abstract: We are studying the thermal stability of thick hydrogenated amorphous aluminum oxide (Al2O3) layers (20–50nm) prepared by a high-throughput plasma-enhanced chemical-vapor-deposition (PECVD) technique for the electrical passivation of crystalline silicon surfaces. These passivation layers can be applied alone or covered by a capping layer like amorphous hydrogenated silicon nitride (SiNx) or amorphous hydrogenated silicon oxide (SiOx), also prepared by PECVD. After firing at 870°C for approximately 3s, the layers show blistering for Al2O3 of 30nm or higher, independently from the capping layer. For thinner Al2O3, no blistering can be observed even using scanning electron microscope (SEM). Very long carrier lifetimes up to 900μs was obtained in passivated p-Si (1Ωcm) wafer after annealing and firing, without observing a strong influence of the layer thickness and the capping layer. All the layer stacks, including the stacks with SiNx capping layer, show high negative charge densities in the layer (1–4×1012 cm−2). Additionally, low interface defect densities (∼1011 cm−2 eV−1), which could be achieved with and without a hydrogenated capping layer, were measured even after firing. To explain these phenomena, hydrogen concentration depth profiles were measured by nuclear reaction analysis. These measurements have shown that, at the Al2O3–Si interface, hydrogen atomic concentration ranging 5–7% after annealing and 4% after firing are obtained independently from the capping hydrogen concentration. We conclude that PECVD Al2O3 layers of 20nm or thicker can provide enough hydrogen to passivate the interface defects, even after a high temperature step. However, the layer thickness should be limited to 30nm in order to avoid the blistering. [Copyright &y& Elsevier]

Ultrathin (7 nm) atomic layer deposited Al2O3 layers and high-deposition-rate plasma-enhanced chemical vapor deposited AlOx layers have been applied and characterized as rear-surface passivation for high-efficiency silicon solar cells. The excellent efficiency values (up to 21.3%-21.5%) demonstrate that both aluminum oxide deposition processes have a very high potential comparable to the reference cells with SiO2 passivation. The high voltages (~680 mV), the excellent long-wavelength quantum efficiency, and the high short-circuit currents of these cells (~40 mA/cm²) are a proof for the low rear-surface recombination velocity and excellent internal rear-surface reflection. [ABSTRACT FROM AUTHOR]