Direct Observation and Control of Surface Termination in Perovskite Oxide Heterostructures.

Interfacial behavior of quantum materials leads to emergent phenomena such as quantum phase transitions and metastable functional phases. Probes for in situ and real time surface-sensitive characterization are critical for control during epitaxial synthesis of heterostructures. Termination switching in complex oxides has been studied using a variety of probes, often ex situ; however, direct in situ observation of this phenomena during growth is rare. To address this, we establish in situ and real time Auger electron spectroscopy for pulsed laser deposition with reflection high energy electron diffraction, providing structural and compositional surface information during film deposition. Using this capability, we show the direct observation and control of surface termination in heterostructures of SrTiO3 and SrRuO3. Density-functional-theory calculations capture the energetics and stability of the observed structures, elucidating their electronic behavior. This work demonstrates an exciting approach to monitor and control the composition of materials at the atomic scale for control over emergent phenomena and potential applications.

Electron Doping BaZrO3 via Topochemical Reduction.

We report the topochemical reduction of epitaxial thin films of the cubic perovskite BaZrO3. Reduction with calcium hydride yields n-type conductivity in the films, despite the wide band gap and low electron affinity of the parent material. X-ray diffraction studies show concurrent loss of out-of-plane texture with stronger reducing conditions. Temperature-dependent transport studies on reduced films show insulating behavior (decreasing resistivity with increasing temperature) with a combination of thermally activated and variable-range hopping transport mechanisms. Time-dependent conductivity studies show that the films are stable over short periods, with chemical changes over the course of weeks leading to an increase in electrical resistance. Neutron reflectivity and secondary ion mass spectrometry indicate that the source of the carriers is most likely hydrogen incorporated from the reducing agent occupying oxygen vacancies and/or interstitial sites. Our studies introduce topochemical reduction as a viable pathway to electron-dope and meta-stabilize low electron affinity and work function materials.

Bandgap Control via Structural and Chemical Tuning of Transition Metal Perovskite Chalcogenides.

Transition metal perovskite chalcogenides are a new class of versatile semiconductors with high absorption coefficient and luminescence efficiency. Polycrystalline materials synthesized by an iodine-catalyzed solid-state reaction show distinctive optical colors and tunable bandgaps across the visible range in photoluminescence, with one of the materials' external efficiency approaching the level of single-crystal InP and CdSe.