Band offsets and density of Ti3+states probed by x-ray photoemission on LaAlO3/SrTiO3

Abstract

A set of LaAlO3/SrT iO3 (LAO-STO) interfaces has been probed by x-ray photoemission spectroscopy in order to contrast and compare the effects of LAO overlayer thickness and of the growth conditions on the electronic properties of these heterostructures. These effects are tracked by considering the band-offset and the density of Ti +3 states, respectively. It is shown that the dominant effects on the local electronic properties are determined by the O2 partial pressure during the growth. In particular, a low P(O2) yields Ti +3 states with higher density and lower binding energy as compared to the sample grown at high P(O2) or to the bare STO reference sample. Band offset effects are all below about 0.7 eV, but a careful analysis of Ti 2p and Sr 3d peaks shows that valence band offsets can be at the origin of the observed peak width. In particular, the largest offset is shown by the conducting sample, that displays the largest Ti 2p and Sr 3d peak widths.

Band bending and alignment at the spinel/perovskiteγ−Al2O3/SrTiO3heterointerface

“…Furthermore, due to its large band gap (≈7.9 eV) [20], the CBM of GAO is above that of STO. Accordingly, in contrast to LAO/STO [15,28,30,40], the heterointerface between GAO and STO is found to be of type I.…”

“…Similar observations have been made for other polar oxide heterostructures. In particular, the related heterostructure LAO/STO does not show a potential gradient as inferred from HAXPES [5,23,[27][28][29][30].…”

“…Furthermore, the positive sign of E VB means that the VBM of STO is above that of GAO, corresponding to an interface of type I. This distinguishes GAO/STO from the case of LAO/STO, where the interface is found to be of type II [5,28,30,40].…”

Band alignment in LaAlO3/SrTiO3oxide heterostructures inferred from hard x-ray photoelectron spectroscopy

“…While agreeing in sign, the value obtained here is significantly higher than the offset reported by other photoemission studies of PLD-grown heterostructures. 10,21,35 Note that a possible small downward bending of the STO-VBM towards the interface would imply an even larger offset directly at the interface. In contrast, from photoemission on samples grown by molecular beam epitaxy (MBE), Segal et al have recently concluded on an offset value similar to ours, but of opposite sign, i.e., a type-I interface.…”

Observation of strontium segregation in LaAlO3/SrTiO3 and NdGaO3/SrTiO3 oxide heterostructures by X-ray photoemission spectroscopy

“…This is probably the reason why this interesting phenomenon escaped the attention of previous investigations, 13,30 although some broadening of the Sr 3d has been reported occasionally. 31 We now turn to the two final and most intriguing open questions: which is the origin of the extra Sr-2 component and what determines the shift in BE? The extra Sr might well migrate from the bulk of SrTiO 3 that can be considered for our purposes as an infinite Sr reservoir.…”