The modification of ferroelectric LiNbO3(0001) surfaces using chromium oxide thin films

Abstract

The impact of ferroelectric polarization on the chemical and electronic properties of atomically thin layers of non-polar chromium oxide deposited on positively and negatively poled LiNbO₃(0001) was studied. Chromium(III) oxide readily forms on LiNbO₃; however, annealing at high temperatures was required to maintain well-ordered films as the thickness increased. Prolonged heating at these temperatures caused Cr diffusion into the LiNbO₃ substrate. Comparing Cr 2p X-ray photoelectron spectroscopy (XPS) peak positions as a function of temperature and substrate polarization revealed no evidence of shifts from the peak positions expected for Cr₂O₃. The lack of any band offset between Cr₂O₃ on the oppositely poled surface suggests that charge compensation of the ferroelectric substrate occurs at least predominantly at the surface of the film, as opposed to the film-substrate interface. No evidence of shifts due to oxidation or reduction of the Cr was observed indicating that charge compensation did not involve a change in the ionic state of the Cr. Exposing the films to reactive oxygen species emitted from an oxygen plasma, however, caused a distinct high binding energy shoulder on the Cr 2p_3/2 XPS peaks that could be associated with oxygen adsorption on surface Cr and concomitant oxidation to Cr⁵⁺. This feature was used to gauge the concentration of O adatoms on the surfaces as a function of temperature for oppositely poled substrates; these measurements did not reveal any significant polarization dependence for oxygen desorption. Further, temperature programmed desorption measurements for a Cr₂O₃ film on α-Al₂O₃ showed a similar trend in O₂ desorption. Therefore, it is concluded that the reactivity of Cr₂O₃ toward O is at least largely independent of substrate polarization despite data suggestive of charge compensation at the film surfaces.