Positron annihilation studies of the AlOₓ/SiO₂/Si interface in solar cell structures

Abstract

Film and film/substrate interfacecharacteristics of 30 and 60 nm-thick AlOₓfilmsgrown on Si substrates by thermal atomic layer deposition(ALD), and 30 nm-thick AlOₓfilms by sputtering, have been probed using variable-energy positron annihilation spectroscopy (VEPAS) and Doppler-broadened spectra ratio curves. All samples were found to have an interface which traps positrons, with annealing increasing this trapping response, regardless of growth method. Thermal ALD creates an AlOₓ/SiOₓ/Si interface with positron trapping and annihilation occurring in the Si side of the SiOₓ/Si boundary. An induced positive charge in the Si next to the interface reduces diffusion into the oxides and increases annihilation in the Si. In this region there is a divacancy-type response (20 ± 2%) before annealing which is increased to 47 ± 2% after annealing.Sputtering seems to not produce samples with this same electrostatic shielding; instead, positron trapping occurs directly in the SiOₓinterface in the as-deposited sample, and the positron response to it increases after annealing as an SiO₂ layer is formed. Annealing the film has the effect of lowering the film oxygen response in all film types. Compared to other structural characterization techniques, VEPAS shows larger sensitivity to differences in film preparation method and between as-deposited and annealed samples. Show more