Universal electronic structure of polar oxide hetero-interfaces

Abstract

The electronic properties of NdGaO3/SrTiO3, LaGaO3/SrTiO3, and LaAlO3/SrTiO3 interfaces, all showing an insulator-to-metal transition as a function of the overlayer-thickness, are addressed in a comparative study based on x-ray absorption, x-ray photoemission and resonant photoemission spectroscopy. The nature of the charge carriers, their concentration and spatial distribution as well as the interface band alignments and the overall interface band diagrams are studied and quantitatively evaluated. The behavior of the three analyzed heterostructures is found to be remarkably similar. The valence band edge of all the three overlayers aligns to that of bulk SrTiO3. The near-interface SrTiO3 layer is affected, at increasing overlayer thickness, by the building-up of a confining potential. This potential bends both the valence and the conduction band downwards. The latter one crossing the Fermi energy in the proximity of the interface and determines the formation of an interfacial band offset growing as a function of thickness. Quite remarkably, but in agreement with previous reports for LaAlO3/SrTiO3, no electric field is detected inside any of the polar overlayers. The essential phenomenology emerging from our findings is discussed on the base of different alternative scenarios regarding the origin of interface carriers and their interaction with an intense photon beam.

Surface functionalization of WSe2 by F16CoPc

“…Charge transfer to or from semiconducting or insulating materials has also been investigated at various, different heterojunctions, and in some cases, it was demonstrated that even a two‐dimensional metal can be realized at some of these heterointerfaces as a consequence of the charge transfer. Famous examples comprise polar oxide interfaces (LaAlO 3 /SrTiO 3 or LaGaO 3 /SrTiO 3 ) or purely organic interfaces such as TTF/TCNQ (tetrathiofulvalene/tetracyanoquinodimethane) or rubrene/F 16 CoPc (rubrene/fluorinated cobalt phthalocyanine) . Such two‐dimensional electron systems confined to an interface can exhibit fascinating physical properties and, therefore, are of particular interest to fundamental as well as applied research.…”

Identifying Ionic and Electronic Charge Transfer at Oxide Heterointerfaces

“…This uncompensated field could be due to the partial filling of LAO surface oxygen vacancies (which have been suggested as the driver behind electronic reconstruction) [71] when exposed to oxygen atmosphere. [66] The BE of the low-BE-Sr component is at a highly unusual value, as throughout the current literature secondary components in STO based systems are almost exclusively reported at the higher BE side with respect to bulk STO. [44,60,61] Exotic defect states, such as complex Sr/Ti anti-site defects were reported to yield low BEs, [72] but these are unlikely to form in a reversible manner under the light thermodynamic treatments in our study.…”

“…This uncompensated field could be due to the partial filling of LAO surface oxygen vacancies (which have been suggested as the driver behind electronic reconstruction) [ 71 ] when exposed to oxygen atmosphere. [ 66 ]…”

“…In accordance with the literature, we assume a close to flat band scenario within the LAO layer under UHV conditions, which is motivated by the absence of significant core-level broadening. [66][67][68][69] In Figure 3a, we show Al 2p and Ti 2p spectra as a function of oxygen partial pressure for 6 uc LAO/STO. In the absence of interfacial chemistry, BE shifts reflect changes in the electrostatic potential of the LAO and STO, respectively.…”

Manifold field effects at a complex oxide interface

“…By using solid solutions like La x Sr 2y Al x+y Ta y O 3 or (LaAlO 3 ) 0.5 (SrTiO 3 ) 0.5 , the polar discontinuity can be weakened, leading to a higher critical thickness and an alteration of interface properties (111)(112)(113). All these systems may have different properties emerging from the interaction of the SrTiO 3 surface with the (polar) overlayer, although their electronic structure is remarkably similar (114). Therefore, studying the SrTiO 3 surface states that form at the interface with LaAlO 3 should yield results applicable to all such interfaces.…”