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.
Highlights
Interface layer partially screening the polar electric field[11]
Similar concepts are expected to hold for LaGaO3/SrTiO3 and NdGaO3/SrTiO3, sharing the same perovskite-related structure and the same nominal cationic valences of LaAlO3/SrTiO3 both at the A and at the B site
Photoemission spectroscopy (PES) is sensitive to electrostatic fields in the LaAlO3 overlayer predicted by the electronic reconstruction scenario by detecting line broadening and shifting[17,18,19,20,26]
Summary
Interface layer partially screening the polar electric field[11]. The full screening of the field and the complete flattening of the LaAlO3 band is only expected to take place for an infinitely thick LaAlO3 film. PES is sensitive to electrostatic fields in the LaAlO3 overlayer predicted by the electronic reconstruction scenario by detecting line broadening and shifting[17,18,19,20,26]. Such effects are reported to be essentially absent or an order of magnitude smaller than expected. The Ti 3dxy orbital could be thereby identified as the one that is lowest in energy and hosts the 2DEG In spite of such broad activity on LaAlO3/SrTiO3, the similar structures LaGaO3/SrTiO3 and NdGaO3/ SrTiO3 are almost unexplored[33]. From a methodological point of view it is mandatory to expand the field of investigations to these systems, in order to separate general from material-specific properties and try to identify the electronic features that should be considered as signatures of oxide heterostructures hosting a 2DEG
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