Session E37a: Complex Oxide Interfaces and Heterostructures- Stannates, Superconductivity

Mobility-Electron Density Relation Probed via controlled Oxygen Vacancy Doping in Epitaxial BaSnO$_{\mathrm{\mathbf{3}}}$

The recently discovered high 300 K mobility in wide band gap semiconducting BaSnO$_{\mathrm{3}}$ is of exceptional interest for perovskite oxide heterostructures. Critical issues in epitaxial films include determination of the optimal dopant, and understanding the mobility-electron density ($\mu $-$n)$ relation. These are addressed here through a transport study of BaSnO$_{\mathrm{3}}$ films with oxygen vacancy doping controlled \textit{via} reduction temperature. Single-phase, close-to-stoichiometric, smooth, epitaxial films were grown \textit{via} high pressure oxygen sputter deposition. $n$ at 300 K can be tuned from 5$\times$ 10$^{\mathrm{19}}$ cm$^{\mathrm{-3}}$ to as low as 2$\times$ 10$^{\mathrm{17}}$ cm$^{\mathrm{-3}}$, which drives a weak- to strong-localization transition accompanied by a 10$^{\mathrm{4}}$-fold increase in resistivity. This reveals $\mu \quad \propto $ $n^{\mathrm{0.65}}$ over the entire $n$ range probed, important for understanding mobility-limiting scattering mechanisms. La-doping has also been explored with this growth method; results at high electron density will be discussed, with a view to mobility optimization. Work supported by the NSF MRSEC under DMR-1420013.   

Carrier Density Modulation over an Exceptional Voltage Window in BaSnO$_{\mathrm{3}}$ Films \textit{via} Ion Gel Gating

BaSnO$_{\mathrm{3}}$ has drawn interest recently due to its outstanding room temperature mobility and potential applications in oxide transistors, transparent conductors, \textit{etc}. Here we report effective control of the electronic transport properties of sputtered oxygen-vacancy-doped~BaSnO$_{\mathrm{3}}$~(BaSnO$_{\mathrm{3-\delta }})$ films \textit{via} ion gel gating in electric double layer transistor structures. The electron densities of the starting films is tuned by thickness, from 4 $\times$ 10$^{\mathrm{19}}$ cm$^{\mathrm{-3}}$ at 13 nm to much lower densities at lower thickness. The response to gate voltage is found to be notably robust, with largely reversible response (even in vacuum) over an exceptionally wide window from -4 to $+$4 V, even at 300 K. The data support predominantly electrostatic response, unlike many other oxides, which we ascribe to Sn redox stability. In this manner the sheet resistance of 13-nm-thick BaSnO$_{\mathrm{3}}$ films can be modulated by a factor of 50 at 300 K, increasing to almost 10$^{\mathrm{3}}$ at low temperatures. Similar measurements at lower thickness/electron density will also be discussed.   

Structural characterization of LaInO$_{\mathrm{3}}$/BaSnO$_{\mathrm{3}}$ interface via synchrotron scattering

The alkaline earth stannate BaSnO$_{\mathrm{3}}$ has one of the highest measured room-temperature mobilities of the conducting perovskite oxides. FETs based on BaSnO$_{\mathrm{3}}$ display a large on/off ratio, I$_{\mathrm{on}}/$I$_{\mathrm{off}}=$10$^{\mathrm{7}}$, and high field effect mobility, $\mu =$90 cm$^{\mathrm{2}}$/Vs. [1] It has been suggested that in these polar devices, which use a LaInO$_{\mathrm{3}}$ dielectric, the polar discontinuity between the polar LaInO$_{\mathrm{3\thinspace }}$dielectric and the nonpolar Ba$_{\mathrm{0.93}}$La$_{\mathrm{0.07}}$SnO$_{\mathrm{3\thinspace }}$channel leads to an electronic reconstruction. LaInO$_{\mathrm{3\thinspace }}$remotely dopes Ba$_{\mathrm{0.93}}$La$_{\mathrm{0.07}}$SnO$_{\mathrm{3\thinspace }}$with electrons, creating the high observed mobility. Using synchrotron radiation, we measure crystal truncation rods (CTRs) of thin film LIO/BSO/STO grown by pulsed-laser deposition. Fitting these CTRs, we determine a layer-resolved atomic structure for the LIO/BSO interface. We observe octahedral rotations and polarization in the LIO layer for films as thin as 6 unit-cells, similar to the rotations observed in bulk LIO. We discuss how these rotations may be coupled to the polarization near the interface. [1] Kim et al. APL Mater. 3, 036101 (2015)   

Structure, Defects and Electronic Transport in High-Mobility BaSnO$_{\mathrm{3}}$ Films and Heterostructures

Wide bandgap perovskite oxides with high room-temperature conductivities that are structurally compatible with other perovskite materials are of significant current interest as transparent conductors and as active components in high performance power electronics. Such materials must also possess high room-temperature mobility to minimize power consumption and to enable high frequency applications. In this talk, I will focus on the detailed electronic transport study of La-doped BaSnO$_{\mathrm{3}}$ films and heterostructures grown using the hybrid molecular beam epitaxy technique. We will discuss the mobility-limiting scattering mechanisms by calculating temperature-dependent mobility, and Seebeck coefficient using the Boltzmann transport framework and \textit{ab initio }calculations. Finally, we will provide pathways to realize high mobility two-dimensional electron gases in BaSnO$_{\mathrm{3}}$ via bandgap engineering approaches.   

High Mobility and Scattering Mechanisms in La-doped BaSnO$_{\mathrm{3}}$ Films Grown by a Radical-based Oxide MBE Approach

Using experiment and transport modeling, we will present on the detailed electronic transport study of La-doped BaSnO$_{\mathrm{3}}$ films grown \textit{via} a radical-based oxide MBE approach. Using a chemical precursor of tin, effusion cell for Ba and rf plasma for oxygen, we will first present the discovery of an ``MBE growth window'', in which cation stoichiometry of BaSnO$_{\mathrm{3}}$ films was maintained as 1:1 for a range of Ba/Sn flux ratios. Temperature dependent electronic transport measurements were then performed to investigate the effect of La doping on mobility ($\mu )$ and carrier concentrations (n) in stoichiometric BaSnO$_{\mathrm{3}}$ films grown on SrTiO$_{\mathrm{3}}$ (001) substrates. We will discuss the role of charged dislocations, non-stoichiometry and dopant density on the electronic transport properties. Using \textit{ab initio} calculation and Boltzmann transport equations, temperature-dependent mobility and Seebeck coefficient are calculated and will be presented to elucidate different mobility-limiting scattering mechanisms as a function of n and temperature.   

Non-stoichiometry, Compensation and Disorder in Hybrid MBE-grown Alkaline Earth Stannate

Alkaline earth stannate has recently been of significant interest for transparent conducting oxide and power electronic applications owing to its wide band gap and high conductivity. In this talk, we will present a novel hybrid molecular beam epitaxy approach utilizing elemental solid source for Ba and Sr, a chemical precursor source for Sn and a rf plasma source for oxygen, for the growth of BaSnO$_{\mathrm{3}}$ and SrSnO$_{\mathrm{3}}$ films on (001) SrTiO$_{\mathrm{3}}$ and (110) GdScO$_{\mathrm{3}}$ substrates. High-resolution x-ray diffraction and reflection high-energy electron diffraction revealed single phase, epitaxial films and a layer-by-layer growth mode, respectively. Films' cation stoichiometry was determined using high energy Rutherford backscattering spectrometry. Using temperature dependent electronic transport measurements of films with varying cation stoichiometry, we will discuss how cation stoichiometry, charge compensation and structural disorder influence the metal-to-insulator transition, electron density, and mobility in La-doped BaSnO$_{\mathrm{3}}$ and SrSnO$_{\mathrm{3}}$ films.   

Electrical transport of La-doped SrTiO$_{\mathrm{3}}$ films on silicon

The integration of complex oxides such as SrTiO$_{\mathrm{3}}$ (STO) onto silicon using molecular beam epitaxy (MBE) allows one to combine the multifunctional properties of oxide heterostructures with traditional semiconductor platforms. It has been shown that STO can support high 2D carrier densities from either cation doping (i.e. La$_{\mathrm{x}}$Sr$_{\mathrm{1-x}}$TiO$_{\mathrm{3}}$ -- LSTO) or as a 2D electron gas at an interface. One current challenge is that the room temperature mobility is low and limited by phonon scattering. One approach to improve the mobility of the STO/Si interface is to increase the conduction band offset between the oxide and the silicon by changing the amount of oxygen at the oxide-semiconductor interface. We measure the electrical transport of films grown on Si by MBE and show that as-grown films exhibit multiple channels of conduction from Hall measurements. We show that by increasing the oxygen content in the film by growth in a plasma, we can increase the conduction band offset of STO/Si as measured by x-ray photoemission, and consequently observe a limited increase in the density of electrons in the Si. While the achievable density is small, this approach demonstrates the tunability of band offsets in oxide-semiconductor systems in order to control electron distribution and mobility for device applications.   

What makes high-\textit{T}$_{\mathbf{c}}$\textbf{ cuprate superconductors so special?}

We have measured the precise dependence of key normal- and superconducting-state parameters on doping, temperature, and external fields in La$_{\mathrm{2-x}}$Sr$_{\mathrm{x}}$CuO$_{\mathrm{4}}$ (LSCO) films. This called for significant advances in both synthesis and measurement techniques. Single-crystal and mutilayer films are synthesized and engineered using atomic-layer-by-layer molecular beam epitaxy. The doping level is tuned in steps of 0.01{\%}. The absolute value of the magnetic penetration depth $\lambda $?is measured to the accuracy better than 1{\%}. The study took over a decade and encompassed thousands of cuprate samples. The large statistics reveals clear trends and intrinsic properties; this is essential when dealing with complex materials. The findings bring in some great surprises. Even in extremely overdoped LSCO, both the superconducting and the normal state are quite unusual, departing qualitatively from the standard BCS / Fermi Liquid paradigm.\\ \\ References: Bozovic et al., Nature 536, 309 (2016); Wu et al., PNAS 113 (2016); Wu et al., Nature Mater. 12, 877 (2013); Bollinger et al., Nature 472, 458 (2011); Bilbro et al., Nature Phys. 7, 298 (2011); Morenzoni et al., Nature Comm. 2, 272 (2011); Gozar et al., Nature 455, 782 (2008).   

Superconductor to Non-Superconductor Transitions at Oxide Interface Tuned by Electrostatic Dual Gates

The quantum phase transitions from superconductor to non-superconductor in two dimensions (2D) are determined by various factors including carrier density, disorder, and dissipation coupling. By applying a dual gate technique on the LaAlO$_{3}$/SrTiO$_{3}$ interface superconductor, we obtained the degrees of freedom to tune these factors and induce different phase transitions electrostatically. Our device is formed by simultaneous gating from the top of the epitaxially grown LaAlO$_{3}$ and the back of the SrTiO$_{3}$ (100) substrate. The structural asymmetry of the dual gate device and the large and nonlinear dielectric constant of SrTiO$_{3}$ enable independent and wide-range parameter tuning. We found, by top gating, a superconductor-metal transition can be achieved within the highly conducting limit ($k_{F}l $\textgreater \textgreater 1, where $k_{F}$ is the Fermi wave vector and $l$ is the electron mean free path). By back gating, a superconductor-insulator transition can be observed with the collapse of $k_{F}l$. Our results provide a comprehensive perspective for the quantum phase transitions for the oxide interface systems.   

Tunable Superconducting Order at the (111) LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ Interface

Interfacial superconductivity in (001) LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ (LAO/STO) heterostructures is well studied and is known to be tunable, coexists with ferromagnetism, and exhibits a superconductor-to-insulator transition. On the other hand, the (111) orientation of LAO/STO has only been shown relatively recently to play host to a 2DEG, and is particularly interesting due to its complex Fermi surface, and potential topological characteristics. We have previously reported detailed transport studies on (111) LAO/STO devices, conducted at 4.4K, which are strikingly different from results in the (001) heterostructures. In particular, these heterostructures exhibit strong anisotropy along two orthogonal, in plane crystal directions as a function of electrostatic back-gate voltage.$^{\mathrm{1}}$ Here we report on the superconducting properties of PLD grown (111) LAO/STO heterostructures at mK temperatures. Specifically, we measure the low frequency transport properties along both two orthogonal, in-plane, crystal directions in response to an applied electrostatic back gate potential as well as post growth surface treatments. 1) S. Davis, V. Chandrasekhar, Z. Huang, K. Han, Ariando, T. Venkatesan, \textit{Arxiv, }\textbf{2015 }1603.04538.   

Phase Domain Structure of a Manganite

X-ray Bragg Coherent Diffraction Imaging of nanocrystals of La0.5Ca0.5MnO4, carried out at the 34-ID-C beamline of APS, at one of the bulk crystal Bragg peaks has revealed an interesting new structure. 3D series of diffraction patterns, obtained while ``rocking'' the sample through a fraction of a degree, showed a strong speckle structure attributed to a domain structure rather than the simple fringed flares expected from a facetted crystal. The 3D diffraction pattern was inverted to a complex image showing massive strain effects, containing a mosaic of domains of regions with different phase of the complex density function, in the range --$\pi $\textless $\varphi $\textless $\pi $, conveniently represented on a color wheel. These correspond to rigid shifts of the crystal lattice of zero to one unit cell in magnitude This is a little like traditional picture of mosaic spread, except that the domains are only shifted (translated) and not rotated. The behavior has not been noted before. We identify this preliminary result as a new form of crystal mosaic disorder. Further investigation is planned to discover whether these phase domains are fundamental in nature or coupled to the crystal growth and subsequent treatment to prepare nanomaterials.