[Paper Review] Reversible and Persistent Photoconductivity at the NdGaO3/SrTiO3 Conducting Interface

U. Scotti di Uccio, C. Aruta+11 more|arXiv (Cornell University)|Jun 22, 2012

Electronic and Structural Properties of Oxides19 references3 citations

TL;DR

This study demonstrates reversible and persistent photoconductivity at the NdGaO3/SrTiO3 oxide interface, where a two-dimensional electron gas forms due to polar discontinuity. Using atomic-resolution EELS and electrical measurements, it is shown that low-energy photon irradiation induces a giant persistent photoconductivity effect in highly resistive samples, while enhancing low-temperature mobility without increasing carrier density, highlighting unique charge dynamics in all-perovskite oxide heterostructures.

ABSTRACT

The interface between the band gap insulators LaAlO3 and SrTiO3 is known to host a highly mobile two-dimensional electron gas. Here we report on the fabrication and characterization of the NdGaO3/SrTiO3 interface, that shares with LaAlO3/SrTiO3 an all-perovskite structure, the insulating nature of the single building block and the polar-non polar character. Our work demonstrates that in NdGaO3/SrTiO3 a metallic layer of mobile electrons is formed, with properties comparable to LaAlO3/SrTiO3. The localization of the injected electrons at the Ti sites, within a few unit cells from the interface, was proved by Atomic-scale-resolved EELS analyses. The electric transport and photoconduction of samples were also investigated. We found that irradiation by photons below the SrTiO3 gap does not increase the carrier density, but slightly enhances low temperature mobility. A giant persistent photoconductivity effect was instead observed, even under irradiation by low energy photons, in highly resistive samples fabricated at non-optimal conditions. We discuss the results in the light of different mechanisms proposed for the two-dimensional electron gas formation. Both the ordinary and the persistent photoconductivity in these systems are addressed and analyzed.

Motivation & Objective

• To investigate the formation and transport properties of a two-dimensional electron gas at the NdGaO3/SrTiO3 interface.
• To explore the origin of photoconductivity in this all-perovskite heterostructure, particularly under low-energy illumination.
• To compare the behavior of NdGaO3/SrTiO3 with the well-known LaAlO3/SrTiO3 system in terms of electron mobility and photoresponse.
• To determine whether persistent photoconductivity arises from defect states or intrinsic interfacial mechanisms.

Proposed method

• Epitaxial NdGaO3 thin films were grown on SrTiO3 substrates via pulsed laser deposition.
• Atomic-scale electron energy-loss spectroscopy (EELS) was used to probe the spatial localization of electrons at Ti 3d orbitals near the interface.
• Electrical transport measurements were performed to analyze carrier density and mobility under varying conditions.
• Photoconductivity was measured under irradiation with photons below the SrTiO3 bandgap (Eg ≈ 3.2 eV).
• Samples were fabricated under non-optimal conditions to enhance resistivity and study persistent photoconductivity effects.
• Comparative analysis with LaAlO3/SrTiO3 system was conducted to assess similarities and differences in interfacial electron behavior.

Experimental results

Research questions

• RQ1 Does the NdGaO3/SrTiO3 interface support a two-dimensional electron gas similar to LaAlO3/SrTiO3?
• RQ2 What is the role of low-energy photon irradiation in modifying the electrical properties of the NdGaO3/SrTiO3 interface?
• RQ3 Why does persistent photoconductivity occur in highly resistive NdGaO3/SrTiO3 samples despite no increase in carrier density?
• RQ4 How are the interfacial electrons localized in space, and what is their electronic configuration?
• RQ5 What mechanisms underlie the observed reversible and persistent photoconductivity effects?

Key findings

• A metallic layer of mobile electrons forms at the NdGaO3/SrTiO3 interface, with electronic properties comparable to those in the LaAlO3/SrTiO3 system.
• Electrons are localized within a few unit cells from the interface, primarily occupying Ti 3d orbitals, as confirmed by atomic-resolution EELS.
• Irradiation with photons below the SrTiO3 bandgap does not increase carrier density but enhances low-temperature electron mobility.
• A giant persistent photoconductivity effect is observed in highly resistive samples under low-energy illumination, indicating long-lived charge carriers.
• The persistent photoconductivity effect is attributed to defect-mediated trapping or intrinsic interfacial mechanisms rather than free carrier excitation.
• The results suggest that the NdGaO3/SrTiO3 system is a promising platform for studying interfacial conductivity and optoelectronic phenomena in oxide heterostructures.