Electronic charges and electric potential at LaAlO3/SrTiO3interfaces studied by core-level photoemission spectroscopy

Abstract

We studied LaAlO3/SrTiO3 interfaces for varying LaAlO3 thickness by core-level photoemission spectroscopy. In Ti 2p spectra for conducting "n-type" interfaces, Ti 3+ signals appeared, which were absent for insulating "p-type" interfaces. The Ti 3+ signals increased with LaAlO3 thickness, but started well below the critical thickness of 4 unit cells for metallic transport. Core-level shifts with LaAlO3 thickness were much smaller than predicted by the polar catastrophe model. We attribute these observations to surface defects/adsorbates providing charges to the interface even below the critical thickness.

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

“…Its observation requires low photon energies, rarely used for core level studies. 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?…”

“…34 This hypothesis, if confirmed, could be consistent with the observation that BE shifts of electrostatic nature are hardly found in LaAlO 3 /SrTiO 3 . 30,[36][37][38] At the end we address the general importance of the observed Sr-segregation for the physics of the oxide heterostructures. Any attempt to directly link this phenomenon to interface conductivity would be highly speculative at this stage.…”

Control of electronic conduction at an oxide heterointerface using surface polar adsorbates

“…These data also suggest that many of the conflicting (and often contradictory) studies of this popular system can be explained by this observation, that is, whether measurements are made in situ 31 , or after various surface exposures (for example, atmospheric water, or solvent cleaning) [32][33][34] , the sensitive electronic structure of the interface may be dramatically altered. For example, the predicted electric field across the LaAlO 3 layer [18][19][20] was not observed in X-ray photoemission studies 32 that may be explained consistently because the surface adsorbates (in particular atmospheric water) induced by the ex-situ processing reduce V Uncom , or equally, the electrical field across the LaAlO 3 layer.…”

A polarity-induced defect mechanism for conductivity and magnetism at polar–nonpolar oxide interfaces

“…1) 3,4 . The other three mechanisms involve various defects, including the oxygen vacancies at the interface (denoted as V O (I), where 'I' means 'Interface') [14][15][16] , oxygen vacancies at LaAlO 3 overlayer surface (denoted as V O (S), where S means 'Surface') [17][18][19][20][21][22] , and the La-onSr (La Sr ) antisite donor defects induced by interfacial cation intermixing [23][24][25][26][27][28][29] . As Table 1 shows, each of these proposed mechanisms represents one aspect of the interface physics, explains some experimental findings, but conflicts with a few others 2 .…”