Chemical and structural study of electrically passivating Al2O3/Si interfaces prepared by atomic layer deposition

Aluminum oxide (Al2O3) layers, prepared by atomic layer deposition (ALD), provide excellent surface passivation properties on crystalline Si surfaces, which are of major importance for photovoltaic applications. Beyond the chemical passivation by reduction of the electronic surface state density, a supportive field effect passivation mechanism emerges at the Al2O3/Si interface. The atomic origin of the fixed negative charges that are responsible for the field effect is currently under discussion. In this contribution, thin layers of Al2O3 with thicknesses ranging from the submonolayer region to several nanometers have been grown on Si substrates by means of thermal ALD. The principle elements of the samples have been quantified by x-ray photoelectron spectroscopy as a function of the film thickness. Changes at the interface upon thermal annealing have been investigated in detail. After the first few ALD cycles an imperfect Al2O3 layer is found together with the formation of an ultrathin SiOx interlayer. Continued deposition leads to stoichiometric Al2O3 growth. Within the first ∼1 nm from the Si interface, additional O (“excess O”), surpassing the Al2O3 and SiO2 stoichiometry, is observed. The excess O does not completely react with the Si surface to SiO2 during thermal annealing. Therefore, interstitial O in near-interface Al2O3 is suggested to provide the fixed negatively charged states.