Interface polarization model for a 2-dimensional electron gas at the BaSnO3/LaInO3 interface

In order to explain the experimental sheet carrier density n2D at the interface of BaSnO3/LaInO3, we consider a model that is based on the presence of interface polarization in LaInO3 which extends over 2 pseudocubic unit cells from the interface and eventually disappears in the next 2 unit cells. Considering such interface polarization in calculations based on 1D Poisson-Schrödinger equations, we consistently explain the dependence of the sheet carrier density of BaSnO3/LaInO3 heterinterfaces on the thickness of the LaInO3 layer and the La doping of the BaSnO3 layer. Our model is supported by a quantitative analysis of atomic position obtained from high resolution transmission electron microscopy which evidences suppression of the octahedral tilt and a vertical lattice expansion in LaInO3 over 2–3 pseudocubic unit cells at the coherently strained interface

2\sqrt{2}×\times2R45∘\sqrt{2}R45^\circ surface reconstruction and electronic structure of BaSnO3_3 film

We studied surface and electronic structures of barium stannate (BaSnO$_3$) thin-film by low energy electron diffraction (LEED), and angle-resolved photoemission spectroscopy (ARPES) techniques. BaSnO$_3$/Ba$_{0.96}$La$_{0.04}$SnO$_3$/SrTiO$_3$ (10 nm/100 nm/0.5 mm) samples were grown using pulsed-laser deposition (PLD) method and were \emph{ex-situ} transferred from PLD chamber to ultra-high vacuum (UHV) chambers for annealing, LEED and ARPES studies. UHV annealing starting from 300$^{\circ}$C up to 550$^{\circ}$C, followed by LEED and ARPES measurements show 1$\times$1 surfaces with non-dispersive energy-momentum bands. The 1$\times$1 surface reconstructs into a $\sqrt{2}$$\times$$\sqrt{2}R45^\circ$ one at the annealing temperature of 700$^{\circ}$C where the ARPES data shows clear dispersive bands with valence band maximum located around 3.3 eV below Fermi level. While the $\sqrt{2}$$\times$$\sqrt{2}R45^\circ$ surface reconstruction is stable under further UHV annealing, it is reversed to 1$\times$1 surface by annealing the sample in 400 mTorr oxygen at 600$^{\circ}$C. Another UHV annealing at 600$^{\circ}$C followed by LEED and ARPES measurements, suggests that LEED $\sqrt{2}$$\times$$\sqrt{2}R45^\circ$ surface reconstruction and ARPES dispersive bands are reproduced. Our results provide a better picture of electronic structure of BaSnO$_3$ surface and are suggestive of role of oxygen vacancies in the reversible $\sqrt{2}$$\times$$\sqrt{2}R45^\circ$ surface reconstruction.Comment: 7 pages, 4 figures, Journa

Physical properties of transparent perovskite oxides (Ba,La)SnO3 with high electrical mobility at room temperature

Transparent electronic materials are increasingly in demand for a variety of optoelectronic applications. BaSnO3 is a semiconducting oxide with a large band gap of more than 3.1 eV. Recently, we discovered that La doped BaSnO3 exhibits unusually high electrical mobility of 320 cm^2(Vs)^-1 at room temperature and superior thermal stability at high temperatures [H. J. Kim et al. Appl. Phys. Express. 5, 061102 (2012)]. Following that work, we report various physical properties of (Ba,La)SnO3 single crystals and films including temperature-dependent transport and phonon properties, optical properties and first-principles calculations. We find that almost doping-independent mobility of 200-300 cm^2(Vs)^-1 is realized in the single crystals in a broad doping range from 1.0x10^19 to 4.0x10^20 cm^-3. Moreover, the conductivity of ~10^4 ohm^-1cm^-1 reached at the latter carrier density is comparable to the highest value. We attribute the high mobility to several physical properties of (Ba,La)SnO3: a small effective mass coming from the ideal Sn-O-Sn bonding, small disorder effects due to the doping away from the SnO2 conduction channel, and reduced carrier scattering due to the high dielectric constant. The observation of a reduced mobility of ~70 cm^2(Vs)^-1 in the film is mainly attributed to additional carrier-scatterings which are presumably created by the lattice mismatch between the substrate SrTiO3 and (Ba,La)SnO3. The main optical gap of (Ba,La)SnO3 single crystals remained at about 3.33 eV and the in-gap states only slightly increased, thus maintaining optical transparency in the visible region. Based on these, we suggest that the doped BaSnO3 system holds great potential for realizing all perovskite-based, transparent high-frequency high-power functional devices as well as highly mobile two-dimensional electron gas via interface control of heterostructured films.Comment: 31 pages, 7 figure

High Mobility in a Stable Transparent Perovskite Oxide

We discovered that La-doped BaSnO3 with the perovskite structure has an unprecedentedly high mobility at room temperature while retaining its optical transparency. In single crystals, the mobility reached 320 cm^2(Vs)^-1 at a doping level of 8x10^19 cm^-3, constituting the highest value among wide-band-gap semiconductors. In epitaxial films, the maximum mobility was 70 cm^2(Vs)^-1 at a doping level of 4.4x10^20 cm^-3. We also show that resistance of (Ba,La)SnO3 changes little even after a thermal cycle to 530 Deg. C in air, pointing to an unusual stability of oxygen atoms and great potential for realizing transparent high-frequency, high-power functional devices.Comment: 15 pages, 3 figure

Transparent thin film transistors of polycrystalline SnO2-x and epitaxial SnO2-x

We report on transparent thin film field effect transistors (TFTs) based on polycrystalline SnO2-x and epitaxial SnO2-x. Polycrystalline SnO2-x TFTs of the top and the bottom gate geometries exhibited high mobility values of 145.7 cm(2)/V s and 160.0 cm(2)/V s, respectively. However, our polycrystalline SnO2-x devices showed non-ideal behaviors in their output and transfer characteristics; a large hysteresis was observed along with large voltage dependence. The probable origin of these non-ideal behaviors is the barrier formation across grain boundaries of polycrystalline SnO2. To confirm this, we used SnO2-x epitaxially grown on r-plane sapphire substrates as a channel layer and compared their performance with those of polycrystalline SnO2-x based TFTs. Although the mobility of epitaxial SnO2-x TFTs was not as high as that of the polycrystalline SnO2-x TFTs, the non-ideal voltage dependence in output and transfer characteristics disappeared. We believe our direct experimental comparison clearly demonstrates the grain boundary issue in polycrystalline SnO2-x.Y

Selective chemical etching for termination layer control of BaSnO3 and 2DEG formation at the LaInO3/BaSnO3 interface

An ex situ chemical etching method was developed to achieve a SnO2-terminated surface in BaSnO3 films. An SnO2-terminated surface is crucial for the formation of a (LaO)+/(SnO2)0 interface structure to form the two-dimensional electron gas (2DEG) state at the LaInO3 (LIO)/BaSnO3 (BSO) interface. By employing a 9:1 mixture of acetone and water, the etching rate of the surface barium oxide (BaO) layer could be effectively controlled, taking advantage of the solubility of BaO in water. To determine the optimal etching conditions, we investigated the relationship between the etching time and the resulting 2DEG conductance. The optimum times for maximizing the conductance of the 2DEG state were found to be 90 s on SrTiO3 substrates and 40 s on MgO substrates, generating a higher conductance than the in situ SnO2 dusting method reported earlier. The surface properties before and after the chemical etching were analyzed by angle reserved x-ray photoelectron spectroscopy

The role of coherent epitaxy in forming a two-dimensional electron gas at LaIn1-xGaxO3/BaSnO3 interfaces

A 2D electron gas is known to form at the interface of some oxides. Here, 2D electron density is studied in the LaIn1-xGaxO3/Ba0.997La0.003SnO3 interface, revealing that increased alloying causes the migration of dislocations to the interface, destroying coherency and preventing 2D electron gas formation. Some oxide interfaces are known to exhibit unique properties such as a 2D electron gas, controlled by epitaxial strain and coherency between the two layers. Here, we study variation in the 2D electron density in the polar LaIn1-xGaxO3/Ba0.997La0.003SnO3 interface with changing x and LaIn1-xGaxO3 layer thickness. We find that the 2D electron density decreases as the gallium alloying ratio increases and the interface conductance eventually disappears, which shows that an interface with polar discontinuity is not a sufficient condition for 2D electron gas formation. The interface conductance reaches its maximum value when the LaIn1-xGaxO3 layer thickness is approximately 20 angstrom, beyond which conductance decreased to a constant value. Atomistic imaging reveals that dislocations start to form as the gallium ratio increases, forming away from the interface and then moving closer with increasing gallium alloying. The dislocations eventually destroy coherency in the case of LaGaO3 and suppress the formation of a 2D electron gas.Y

Remote Doping of the Two-Dimensional-Electron-Gas State at the LaInO3/BaSnO3 Polar Interface

We investigate the transport properties of a modified interface by intentionally inserting a nanometer-scale undoped BaSnO3 spacer layer at the LaInO3/Ba1-xLaxSnO3 interface, thereby creating remotely doped heterostructures. Both the carrier density (n(s)) and the Hall mobility (mu(H)) continuously decrease as the thickness of the BaSnO3 spacer layer at the interface increases, indicating a changing electron-density profile as a function of the spacer thickness. We find the behavior is consistent with the recently proposed "interface-polarization" model by self-consistent one-dimensional Poisson-Schrodinger calculations. The decrease of n s makes it difficult to see the effect of the spacer layer on the mobility in the remotely doped structures due to the simultaneous decrease of mu caused by the ineffective screening of the remote Coulomb scattering from ionized donors in addition to the threading dislocation scattering. Hence, we control the band bending continuously via the field effect with a fixed spacer-layer thickness, leading to observation of enhanced mobility (mu(FE)) in the remotely doped 2DEG heterostructures in spite of high-density threading dislocations acting as the background charged impurities.N