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Register a new userDiodes with Breakdown Voltages Enhanced by the Metal-Insulator Transition of LaAlO$_3$-SrTiO$_3$ Interfaces
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Using the metal-insulator transition that takes place as a function of carrier density at the LaAlO$_3$-SrTiO$_3$ interface, oxide diodes have been fabricated with room-temperature breakdown voltages of up to 200 V. With applied voltage, the capacitance of the diodes changes by a factor of 150. The diodes are robust and operate at temperatures up to 270 C.
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Multiple experiments have observed a sharp transition in the band structure of LaAlO$_3$/SrTiO$_3$ (001) interfaces as a function of applied gate voltage. This Lifshitz transition, between a single occupied band at low electron density and multiple occupied bands at high density, is remarkable for its abruptness. In this work, we propose a mechanism by which such a transition might happen. We show via numerical modeling that the simultaneous coupling of the dielectric polarization to the interfacial strain (electrostrictive coupling) and strain gradient (flexoelectric coupling) generates a thin polarized layer whose direction reverses at a critical density. The Lifshitz transition occurs concomitantly with the polarization reversal and is first-order at $T=0$. A secondary Lifshitz transition, in which electrons spread out into semiclassical tails, occurs at a higher density.
Surface photovoltage (SPV) spectroscopy, which is a versatile method to analyze the energetic distribution of electronic defect states at surfaces and interfaces of wide-bandgap semiconductor (hetero-)structures, is applied to comparatively investigate heterostructures made of 5-unit-cell-thick LaAlO$_3$ films grown either on TiO$_2$- or on SrO-terminated SrTiO$_3$. As shown in a number of experimental and theoretical investigations in the past, these two interfaces exhibit dramatically different properties with the first being conducting and the second insulating. Our present SPV investigation reveals clearly distinguishable interface defect state distributions for both configurations when interpreted within the framework of a classical semiconductor band scheme. Furthermore, bare SrTiO$_3$ crystals with TiO$_2$ or mixed SrO/TiO$_2$ terminations show similar SPV spectra and transients as do LaAlO$_3$-covered samples with the respective termination of the SrTiO$_3$ substrate. This is in accordance with a number of recent works that stress the decisive role of SrTiO$_3$ and the minor role of LaAlO$_3$ with respect to the electronic interface properties.
Effects of X-ray irradiation on the electronic structure of LaAlO$_3$/SrTiO$_3$ (LAO/STO) samples, grown at low oxygen pressure and post-annealed ex-situ till recovery of their stoichiometry, were investigated by soft-X-ray ARPES. The irradiation at low sample temperature below ~100K creates oxygen vacancies (VOs) injecting Ti t2g-electrons into the interfacial mobile electron system (MES). At this temperature the oxygen out-diffusion is suppressed, and the VOs are expected to appear mostly in the top STO layer. However, we observe a pronounced three-dimensional (3D) character of the X-ray generated MES in our samples, indicating its large extension into the STO depth, which contrasts to the purely two-dimensional (2D) character of the MES in standard stoichiometric LAO/STO samples. Based on self-interaction-corrected DFT calculations of the MES induced by VOs at the interface and in STO bulk, we discuss possible mechanisms of this puzzling three-dimensionality. They may involve VOs remnant in the deeper STO layers, photoconductivity-induced metallic states as well as more exotic mechanisms such as X-ray induced formation of Frenkel pairs.
We have studied the electronic properties of the 2D electron liquid present at the LaAlO$_3$/SrTiO$_3$ interface in series of samples prepared at different growth temperatures. We observe that interfaces fabricated at 650{deg}C exhibit the highest low temperature mobility ($approx 10000 textrm{ cm}^2/textrm{Vs}$) and the lowest sheet carrier density ($approx 5times 10^{12} textrm{ cm}^{-2}$). These samples show metallic behavior and Shubnikov-de Haas oscillations in their magnetoresistance. Samples grown at higher temperatures (800-900{deg}C) display carrier densities in the range of $approx 2-5 times 10^{13} textrm{ cm}^{-2}$ and mobilities of $approx 1000 textrm{ cm}^2/textrm{Vs}$ at 4K. Reducing their carrier density by field effect to $8times 10^{12} textrm{ cm}^{-2}$ lowers their mobilites to $approx 50 textrm{ cm}^2/textrm{Vs}$ bringing the conductance to the weak-localization regime.
Recent experiments have shown that transition metal oxide heterostructures such as SrTiO$_3$-based interfaces, exhibit large, gate tunable, spintronic responses. Our theoretical study showcases key factors controlling the magnitude of the conversion, measured by the inverse Edelstein and Spin Hall effects, and their evolution with respect to an electrostatic doping. The origin of the response can be linked to spin-orbital textures. These stem from the broken inversion symmetry at the interface which produces an unusual form of the interfacial spin-orbit coupling, provided a bulk atomic spin-orbit contribution is present. The amplitudes and variations of these observables are direct consequences of the multi-orbital subband structure of these materials, featuring avoided and topological crossings. Interband contributions to the coefficients lead to enhanced responses and non-monotonic evolution with doping. We highlight these effects using analytical approaches and low energy modeling.
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