Formation of Aluminum Oxide Films on Silicon Surface by Aluminum Evaporation in Oxygen Gas Atmosphere

摘要

We report formation of thin aluminum oxide AlO_x films on the silicon surface by a simple method of Al metal evaporation in oxygen gas atmosphere. 520 μm thick 30-Ωcm p-type-silicon substrates with a top bare surface and a rear surface coated with 100 nm thick thermally grown SiO_2 layers were prepared. AlO_x 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 scem. Samples were subsequently annealed with 9.0×10~5 Pa H_2O vapor at 260℃ 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 AlO_x 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 5×10~(12) cm~(-2) in AlO_x films. Photo-induced carrier microwave absorption measurement resulted in a high minority carrier effective lifetime τ_(eff) of 3.6×10~(-4) s comparable to that of 4. 1×10~(-4) s for thermally grown SiO_2 passivation. Field effect passivation was probably caused by negative charges in AlO_x so that the surface recombination velocity decreased to 70 cm/s. X-ray reflectivity analysis indicated that the interfacial layer like SiO_x was formed between AlO_x and Si substrate. High pressure H_2O vapor heat annealing caused increase in the density and decrease in the thickness of AlO_x layers, although it increased the density and thickness of the interfacial SiO_x layer thickness. H_2O vapor treatment is effective to improve the quality of nanometer thick AlO_x layer.