ABSTRACT:The discovery of two-dimensional electron gases (2DEGs) in SrTiO 3 -based heterostructures provides new opportunities for nanoelectronics. Herein, we create a new type of oxide 2DEG by the epitaxial-strain-induced polarization at an otherwise nonpolar perovskite-type interface of CaZrO 3 /SrTiO 3 . Remarkably, this heterointerface is atomically sharp, and exhibits a high electron mobility exceeding 60,000 cm 2 V -1 s -1 at low temperatures. The 2DEG carrier density exhibits a critical dependence on the film thickness, in good agreement with the polarization induced 2DEG scheme.* Corresponding Author. Email:yunc@dtu.dk. Phone: +45 4677 5614.2 KEYWORDS: Complex oxide interfaces, oxide electronics, two-dimensional electron gases, strain induced polarization.Atomically engineered complex oxide heterostructures exhibit a variety of exotic interfacial properties because of strong interactions among the spin, charge, and orbital freedoms as well as lattice vibrations.One particular example is the emergence of high mobility two-dimensional electron gases (2DEGs) at the interface between two oxide insulators, 1,2 one of which is SrTiO 3 (STO), the basis material of oxide electronics. These complex oxide 2DEGs consist of strongly coupled electrons and give rise to a rich set of physical phenomena 3-5 , for example, superconductivity 6,7 , magnetism 8,9 , and tunable metal-insulator transitions on nanoscale, 10,11 providing new opportunities for nanoelectronics and mesoscopic physics. Under optimized conditions, the CZO films deposited by pulsed laser deposition (PLD) can be epitaxially grown on the (001) TiO 2 -terminated STO substrates within a layer-by-layer two-dimensional growth mode, as confirmed by the presence of periodic intensity oscillations of the reflection highenergy electron diffraction (RHEED) pattern monitored in-situ during film growth (Supporting information, Fig.S1). Both RHEED intensity oscillations and sharp RHEED patterns can persist up to a film thickness over 50 unit cells (uc), suggesting high quality film growth. A terrace surface of the grown heterostructure is detected by atomic force microscopy (AFM), which shows a regular step height of 0.4 nm (Fig. 1b). High-resolution X-ray diffraction (XRD) further confirms the epitaxial growth of the (001) We further investigated the atomic structure and interface chemistry of our CZO/STO heterostructures by an aberration corrected scanning transmission electron microscopy (STEM) in combination with electron energy-loss spectroscopy (EELS). Figure 2a shows a high-angle annular dark field (HAADF) STEM image of a CZO/STO sample with the CZO layer of approximately 50 uc (~20 nm). The CZO film is found to be coherent with the STO substrate with no obvious defects or dislocations at the interface. The averaged line profiles (Fig. 2b) Fig.1. In a similar zirconate system of SrZrO 3 /SrTiO 3 , the compressive strain has been reported to result in ferroelectricity in its superlattices. 27 Theoretical calculations by first principle density functional ...
The single layer of Zirconium pentatelluride (ZrTe 5 ) has been predicted to be a large-gap two-dimensional (2D) topological insulator, which has attracted particular attention in the topological phase transitions and potential device application. Here we investigated the transport properties in ZrTe 5 films with the dependence of thickness from a few nm to several hundred nm. We find that the temperature of the resistivity anomaly's peak (T p ) is inclining to increase as the thickness decreases, and around a critical thickness of ~40 nm, the dominating carriers in the films change from n-type to p-type. With comprehensive studying of the Shubnikov-de Hass (SdH) oscillations and Hall resistance at variable temperatures, we demonstrate the multi-carrier transport instinct in the thin films. We extract the carrier densities and mobilities of two majority carriers using the simplified two-carrier model. The electron carriers can be attributed to the Dirac band with a non-trivial Berry's phase π, while the hole carriers may originate from the surface chemical reaction or unintentional doping during the microfabrication process. It is necessary to encapsulate ZrTe 5 film in the inert or vacuum environment to make a substantial improvement in the device quality.
Deep traps and enhanced photoluminescence efficiency in nonpolar a-GaN/InGaN quantum well structures J. Appl. Phys. 111, 033103 (2012) Temperature dependence of the intraexcitonic AC Stark effect in semiconductor quantum wells Appl. Phys. Lett. 100, 051109 (2012) Photoreflectance study of direct-gap interband transitions in Ge/SiGe multiple quantum wells with Ge-rich barriers Appl. Phys. Lett. 100, 041905 (2012) Time-resolved photocurrents in quantum well/dot infrared photodetectors with different optical coupling structures Appl. Phys. Lett. 100, 043502 (2012) Structural and optical studies of nitrogen incorporation into GaSb-based GaInSb quantum wells Deep-UV optical gain has been demonstrated in Al 0.7 Ga 0.3 N/AlN multiple quantum wells under femtosecond optical pumping. Samples were grown by molecular beam epitaxy under a growth mode that introduces band structure potential fluctuations and high-density nanocluster-like features within the AlGaN wells. A maximum net modal gain value of 118 6 9 cm À1 has been measured and the transparency threshold of 5 6 1 lJ/cm 2 was experimentally determined, corresponding to 1.4 Â 10 17 cm À3 excited carriers. These findings pave the way for the demonstration of solid-state lasers with sub-250 nm emission at room temperature.
The exploration of topological Dirac semimetals with intrinsic superconductivity can be a most plausible way to discover topological superconductors. We propose that type-II Dirac semimetal states exist in the band structure of TaC, a well-known s-wave superconductor, by using the first-principles calculations and the k ⋅ p effective model. The tilted gapless Dirac cones, which are composed of Ta d and C p orbitals and are protected by C 4v symmetry, are found to be below the Fermi level. The bands from Ta d orbitals are greatly coupled with the acoustic modes around the zone boundary, indicating their significant contribution to the superconductivity. The relatively high transition temperature ∼10.5 K is estimated to be consistent with the experimental data. To bring the type-II Dirac points close to chemical potential, hole doping is needed. This seems to decrease the transition temperature a lot, making the realization of topological superconductivity impossible.
Vertical electronic transport in periodic GaN/AlGaN multiple-quantum-well structures grown on free-standing GaN substrates is investigated. Highly nonlinear current-voltage characteristics are measured, displaying a clear transition from a high-resistance state near zero applied bias to a low-resistance state as the voltage is increased. The measurement results, including their temperature dependence and the variations in turn-on voltage with subband structure and bias polarity are in full agreement with a picture of sequential tunneling through the ground-state subbands of adjacent coupled quantum wells. Scattering-assisted tunneling due to interface roughness or structural defects appears to be the dominant transport mechanism. The potential role of photon-assisted tunneling is also investigated.
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