Formation of Aluminum Oxide Films on Silicon Surface by Aluminum Evaporation in Oxygen Gas Atmosphere
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Formation of Aluminum Oxide Films on Silicon Surface by Aluminum Evaporation in Oxygen Gas Atmosphere Y. Nagatomi1, S.Yoshidomi1, M. Hasumi1, T. Sameshima1, and A. Kohno2 1 Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan 2 Fukuoka University, 8-19-1 Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan ABSTRACT We report formation of thin aluminum oxide AlOx films on the silicon surface by a simple method of Al metal evaporation in oxygen gas atmosphere. 520 Pm thick 30-:cm p-type-silicon substrates with a top bare surface and a rear surface coated with 100 nm thick thermally grown SiO2 layers were prepared. AlOx films were formed on the top surfaces by Al metal evaporation up to 20 s in oxygen gas atmosphere at 0.8 Pa with a flow rate of 3 sccm. Samples were subsequently annealed with 9.0x105 Pa H2O vapor at 260oC for 3 h. Measurement of capacitance response to a modulation voltage at 500 kHz as a function of bias gate voltages C-V revealed that AlOx films had the effective oxide thickness ranging from 2.0 and 2.6 nm were formed. C-V measurements also revealed that negative fixed charges were accumulated with a density of 5x1012 cm-2 in AlOx films. Photo-induced carrier microwave absorption measurement resulted in a high minority carrier effective lifetime Weff of 3.6x10-4 s comparable to that of 4.1x10-4 s for thermally grown SiO2 passivation. Field effect passivation was probably caused by negative charges in AlOx so that the surface recombination velocity decreased to 70 cm/s. X-ray reflectivity analysis indicated that the interfacial layer like SiOx was formed between AlOx and Si substrate. High pressure H2O vapor heat annealing caused increase in the density and decrease in the thickness of AlOx layers, although it increased the density and thickness of the interfacial SiOx layer thickness. H2O vapor treatment is effective to improve the quality of nanometer thick AlOx layer. INTRODUCTION Crystalline silicon solar cells are attractive devices, which produce electrical power from sunlight with no damage to the global environment [1]. Surface passivation with a low minority carrier recombination velocity is very important for generation of substantial photo-induced carriers in silicon. Field effect passivation has widely investigated using insulation layer with positive charges, for example, silicon nitride, formed on silicon surface to decrease the recombination velocity. Field effect passivation with positive charges is naturally effective for surface passivation of n-type silicon [2]. On the other hand, field effect passivation with negative charges is necessary for passivation of p-type silicon surface. Aluminum oxide AlOx is one of attractive insulator with accumulating negative charges [3]. Development in simple and low cost fabrication process steps is also important for device production. In this paper, we propose simple fabrication of AlOx films on the silicon surface by combination of Al metal evaporation in oxygen gas atmosphere with high-pressure H2O vapor heat treatment. We repo
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