Disentangling specific versus generic doping mechanisms in oxide heterointerfaces

More than a decade after the discovery of the two-dimensional electron system (2DES) at the interface between the band insulators ${\mathrm{LaAlO}}_3$ (LAO) and ${\mathrm{SrTiO}}_3$ (STO) its microscopic origin is still under debate. Several explanations have been proposed, the main contenders being electron doping by oxygen vacancies and electronic reconstruction, i.e., the redistribution of electrons to the interface to minimize the electrostatic energy in the polar LAO film. However, no experiment thus far could provide unambiguous information on the microscopic origin of the interfacial charge carriers. Here we utilize a novel experimental approach combining photoelectron spectroscopy (PES) with highly brilliant synchrotron radiation and apply it to a set of samples with varying key parameters that are thought to be crucial for the emergence of interfacial conductivity. Based on microscopic insight into the electronic structure, we obtain results tipping the scales in favor of polar discontinuity as a generic, robust driving force for the 2DES formation. Likewise, other functionalities such as magnetism or superconductivity might be switched in all-oxide devices by polarity-driven charge transfer.

Figure 1

(a) In resonant photoemission spectra of oxygen-depleted ${\mathrm{SrTiO}}_3$, recorded across the Ti $L$ edge, two distinct features with different resonance behavior appear in the Ti $3d$ spectral region, a metallic quasiparticle (QP) peak at the Fermi energy $(E_F=0$ eV) and a broad structure in the gap (IG) at about 1.3 eV binding energy, reflecting localized states induced by oxygen vacancies, as labeled in the inset. (b) Schematic diagram of the conduction- (CB) and valence-band (VB) edges for epitaxial ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$. The inset shows how the band alignment alters between an oxygen-depleted (red) and the fully oxidized state (blue) of epitaxial ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ as obtained from relative core level shifts (see Appendix pp3). (c) The photoemission setup of beamline I09 at DIAMOND light source allows for directing a soft (SX) and a hard x-ray (HX) beam onto the same sample spot. Due to the high intensity of the synchrotron radiation, oxygen vacancies are created in ${\mathrm{SrTiO}}_3$, which can be counteracted by directing molecular oxygen onto the sample using a metallic capillary. Within the light cones ${\mathrm{O}}_2$ molecules are dissociated into atomic, highly reactive oxygen, refilling the oxygen vacancies.

Figure 2

Valence-band spectra and evolution of QP and IG intensities of the metallic LAO/STO(001) heterointerface with a 4-unit-cell (uc)-thick film (top) and bare Nb-doped STO (lower panels) for varying oxygen vacancy concentrations. (a), (b), and (d) Evolution of spectra under irradiation without oxygen dosing on the QP [(a) and (d)] and IG resonances [bottom graph in (b)]. The top graph in (b) shows spectra under O dosing. The fully oxidized state is shown in blue, the one with saturated O depletion in red. The black arrows indicate the direction of time. (c) and (e) QP (pink curve) and IG (green curve) spectral weights as function of irradiation time with the oxygen dosing switched off and on. With the constant remaining QP spectral weight in the fully oxidized state of the heterostructure subtracted (grey curve), the QP spectral weight scales in direct proportion to that of the IG state (c) as is the case for the bare surface (e).

Figure 3

(a) Evolution of valence-band spectra for a 1.7-nm-thick amorphous LAO film grown on undoped STO under irradiation without oxygen dosing on the QP resonance. (b) Evolution of valence-band spectra for a 2-uc-thick epitaxial LAO film grown on Nb-doped STO under irradiation without oxygen dosing on the QP resonance. In each case the state with saturated O depletion is shown in red, while the fully oxidized state is shown in blue. Under full oxygen dosing the IG and QP weights are fully quenched for both heterostructures.

Figure 4

Transport characterization of the epitaxial, (001)-oriented LAO/STO heterointerface. Temperature-dependent two-dimensional charge carrier concentration as inferred from Hall measurements. (Inset) Charge carrier mobility as a function of temperature.

Figure 5

Valence-band analysis of the LAO/STO(001) heterointerface. QP and IG weights as a function of irradiation time with oxygen dosing switched off and on for different measurement series with/without intermittent hard x-ray irradiation.

Figure 6

Hard x-ray photoemission core level spectra of the epitaxial LAO/STO(001) heterointerface. (a) Ti $2p$ HAXPES spectra of the LAO/STO(001) heterointerface for various oxygen vacancy concentrations. (b) Sr $3d$ and Al $2s$ HAXPES spectra of the LAO/STO(001) heterointerface in the fully oxidized and the maximally O-depleted state. The inset shows a schematic diagram of conduction- (CB) and valence-band (VB) edges as well as the Al $2s$ and Sr $3d$ core levels for an oxygen-depleted (red lines) and the fully oxidized state (blue lines) of epitaxial LAO/STO.

Figure 7

Valence-band spectra of the a-LAO/STO heterointerface for various oxygen vacancy concentrations. Evolution of spectra under irradiation without oxygen dosing on the QP (a) and IG resonance (b). The valence-band spectrum of the fully oxidized state is shown in blue. (c) QP and IG weights as a function of irradiation time with oxygen dosing switched off and on.

Figure 8

(a)–(c) Valence-band spectra of a 4-uc-thick LAO film grown on Nb:STO(001) for various oxygen vacancy concentrations. Evolution of spectra under irradiation without oxygen dosing on the QP (a) and IG resonances (b). The fully oxidized state is shown in blue. (c) QP and IG weights as a function of irradiation time with oxygen dosing switched off and on. (d)–(f) Valence-band spectra of a 2-uc-thick LAO film grown on Nb:STO(001) for various oxygen vacancy concentrations. Evolution of spectra under irradiation without oxygen dosing on the QP (d) and IG resonances (e). The fully oxidized state is shown in blue. (f) QP and IG weights as a function of irradiation time with oxygen dosing switched off and on.

Figure 9

Valence-band spectra of the bare STO(111) surface for various oxygen vacancy concentrations. (a) Evolution of spectra under irradiation without oxygen dosing on the QP resonance. The valence-band spectrum of the fully oxidized state is shown in blue. (b) QP and IG weights as a function of irradiation time with oxygen dosing switched off and on.

Figure 10

Valence-band spectra of an epitaxial eight bilayer (BL) LAO film grown on STO(111) for various oxygen vacancy concentrations. Evolution of spectra under irradiation without oxygen dosing on the QP (a) and IG resonances (b). The fully oxidized state is shown in blue, the maximally oxygen-depleted state in red. The inset in (a) shows valence-band spectra on the QP resonance recorded during oxygen dosing of a previously irradiated and hence oxygen-depleted spot. (c) QP and IG weights as a function of irradiation time with oxygen dosing switched off and on. The solid (dashed) lines depict measurement series in which the sample was only exposed to soft x rays (intermittently exposed to hard x rays).