Giant photovoltaic effects driven by residual polar field within unit-cell-scale LaAlO₃ films on SrTiO₃

Abstract

For polar/nonpolar heterostructures, Maxwell's theory dictates that the electric potential in the polar components will increase divergently with the film thickness. For LaAlO₃/SrTiO₃, a conceptually intriguing route, termed charge reconstruction, has been proposed to avert such "polar catastrophe". The existence of a polar potential in LaAlO₃ is a prerequisite for the validity of the charge reconstruction picture, yet to date, its direct measurement remains a major challenge. Here we establish unambiguously the existence of the residual polar potential in ultrathin LaAlO₃ films on SrTiO₃, using a novel photovoltaic device design as an effective probe. The measured lower bound of the residual polar potential is 1.0 V. Such a direct observation of the giant residual polar potential within the unit-cell-scale LaAlO₃ films amounts to a definitive experimental evidence for the charge reconstruction picture, and also points to new technological significance of oxide heterostructures in photovoltaic and sensing devices with atomic-scale control.

Figure 1. Band diagram and PV effect.

(a) Schematic band diagrams for an LAO/STO heterostructure, an Au/LAO/STO heterostructure, and an Au/LAO/STO heterostructure under UV illumination with photon energy larger than the LAO gap, respectively. The LAO thickness is assumed to be equal to or larger than 4 uc. The dashed lines refer to the Fermi level in the LAO. (b) Schematic of the PV effect across the LAO overlayer in an Au/LAO/STO heterostructure. (c) I–V curves between the surface Au electrode and the LAO/STO interface with an LAO thickness of 5 uc without and with 6.7-eV light illumination.

Figure 2. Detailed I–V characteristics for LAO film thickness of 5 uc.

(a) Measured V_oc and I_sc as functions of the lateral size of the Au electrode. (b) Dependence of V_oc on the work function of the surface metal contact. (c) I–V curves of the q2-DEG at the LAO/STO interface in without and with 6.7-eV light illumination. (d) V_oc and J_sc under UV illumination of 6.7 eV and 3.4 eV with the same incident photon flux (2.7 × 10¹⁶ cm⁻²s⁻¹, which corresponds to 29 mWcm⁻² for the 6.7-eV light and 15 mWcm⁻² for the 3.4-eV light). The V_oc and J_sc measured with a solar simulator (AM 1.5 G filter at 150 mWcm⁻²) are also shown. The dotted line in (d) refers to the LAO bandgap.

Figure 3. PV effect dependent on light intensity and LAO thickness.

(a) V_oc as a function of the light intensity under 6.7-eV light illumination for the atmospheric-oxygen-pressure annealed LAO samples with thickness of 5 uc. Pt electrodes were adopted for the measurements. (b) Measured V_oc as a function of the LAO thickness for both the samples with in-situ annealing (yellow) and with atmospheric-oxygen-pressure annealing (green), respectively. Au electrodes were adopted for the former, while Pt for the latter.

Figure 4. Power supply to an external resistor.

Response of a commercial CdS photoresistor to an on/off-switchable green light source, driven by a prototypical PV device based on a 5-uc-LAO/STO heterostructure.

Figure 5. Sample growth.

(a) RHEED oscillations for the LAO overlayer grown on a STO substrate. The inset shows the RHEED pattern after growth of 5-uc LAO layers. (b) AFM image of a 5-uc LAO overlayer on a STO substrate. The scale bar is 1 μm. (c) XRD pattern near the (001) peak of a LAO/STO heterostructure with the LAO thickness of 75 uc.

Figure 6. Theoretical modeling.

The optimized atomic structures and corresponding layer-resolved density of states (DOS) of the (LAO)₅(STO)_5.5 and Au₄(LAO)₅(STO)_5.5 slabs, respectively. Zero energy refers to the Fermi level.