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Passivation is a technique that is used in many industries to prevent or minimize corrosion of metals. It involves the creation of an outer layer of shield material created through chemical reaction or allowed to build naturally from spontaneous oxidation in air.
In the context of photovoltaics, aluminium oxide (AI2O3) is a versatile surface passivation material with field-effect and chemical passivation capability. It is characterized by a high negative fixed charge density that repels electrons from crystalline silicon surfaces, reducing the number of defects and making it an excellent material for passivation.
Aluminium oxidation at elevated temperatures and oxygen pressures can be monitored using optical transmission spectroscopy with Al thin films deposited via ALD. The oxidation rate is governed by the Drude behavior of free Al electrons and increases with temperature and varies with the concentration of O2 below 100 mbar, but is nearly pressure-independent above.
Optical transmission spectra of different Al metal film thicknesses prior to oxidation reveal a steep decrease in the transmissivity at short wavelengths, due to the Drude behavior and the rising transparency of the sample, which is accompanied by the onset of the first oxidation regime. This regime accounts for a large percentage of the total metal/oxide conversion and lasts for about 100 s.
The second oxidation regime, which is triggered by the initiation of the electric-field-driven process and reaches saturation coverage of oxygen, exhibits lower rates, but with considerably longer lifetimes. The conversion of Al to oxide occurs in two distinct reaction regimes, which become progressively faster and more pressure-independent as the saturation coverage of oxygen is achieved.
