Interface and material characterization of thin ALD-Al2O3 layers on crystalline silicon

Aluminum oxide (Al2O3) films renownedly supply excellent surface passivation properties on crystalline Si surfaces. This is of major importance for PV applications due to the use of thin Si wafers with good material quality for current state-of-the-art solar cells. For Al2O3 passivation layers the chemical passivation by reduction of the electronic surface state density is further improved by a field effect passivation mechanism at the Al2O3/Si interface. This is assigned to fixed negative charges. However, the atomic origin of the fixed charges is currently under discussion. Within this work thin films of Al2O3 with thicknesses between the submonolayer region and a few nanometers have been grown on Si by thermal atomic layer deposition (ALD). The elemental composition of the samples has been quantified by x-ray photoelectron spectroscopy (XPS) as a function of the film thickness. Modifications at the interface upon thermal annealing have been studied in detail. After the first few ALD cycles an imperfect Al2O3 layer is found. Continued deposition leads to the growth of stoichiometric Al2O3. Within the first similar to 1 nm from the Si interface additional O atoms (excess O), surpassing the Al2O3 and SiO2 stoichiometry, are observed. During thermal annealing the excess O does not completely react with the Si surface to SiO2. Therefore, it is suggested that interstitial O in near-interface Al2O3 constitutes the fixed negatively charged states. (C) 2012 Published by Elsevier Ltd. Selection and peer-review under responsibility of the scientific committee of the SiliconPV 2012 conference.