BaSnO3 Films Research Articles

Control of Threading Dislocation Formation in La-doped BaSnO3 Films Grown by Hybrid Molecular Beam Epitaxy

Control of Threading Dislocation Formation in La-doped BaSnO3 Films Grown by Hybrid Molecular Beam Epitaxy

Electric field control of the energy gap in ZnO and BaSnO3 films grown on PMN-PT

ZnO and BaSnO3 (BSO) thin films grown on Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) substrates have been studied using electrical resistance and photoconductivity (PC) spectra measurements under different applied electric fields on the substrate. The behavior of the resistance and the energy gap (EG) extracted from the PC spectra are modified by the polarization state of the substrate in the case of the ZnO film, while for BSO, these physical parameters depend on the strain imposed by the substrate when a voltage is applied on the PMN-PT. In the latter case, an in-plane tensile (compressive) strain leads to a reduction (increase) in the resistance and the energy gap when an external electric field is applied on the substrate. The behavior of ZnO and BSO can be explained by the different crystalline structure in both films and by the fact that ZnO is also a piezoelectric material. In ZnO, a change in the polarization state of the substrate is associated with an imposed strain and an induced polarization on the film that leads to a modification of the band bending and hence of the energy gap. In the case of BSO, a shift of the impurity and conduction band generates a modification of the energy gap for the different types of strain.

Effect of substrate on structural and transport properties of La0.05Sr0.95SnO3 and La0.05Ba0.95SnO3 epitaxial films

Perovskite-structured stannate La0.05Sr0.95SnO3 and La0.05Ba0.95SnO3 films were deposited on SrTiO3, LaAlO3, LSAT, PMN-PT, and MgO substrates using pulsed laser deposition, respectively. The effects of substrates on the structure, transport properties, and optical properties of the films were investigated in detail. High-resolution X-ray diffraction results revealed the epitaxial growth of all films on each substrate without any impurity phase, and a high lattice quality indicated by the rocking curves. Atomic force microscopy images confirmed the low root-mean-square roughness observed in all films. Hall effect measurements showed that the maximum mobility was obtained for both La0.05Sr0.95SnO3 films and La0.05Ba0.95SnO3 films grown on SrTiO3 substrates, with values of 26.46 cm2/Vs and 43.09 cm2/Vs, respectively. The temperature dependence of resistivity at low temperatures was attributed to e−-e− interactions. Optical transmittance spectra showed high transparency of all films in both visible and near-infrared regions. Collectively, these results indicate that La-doped SrSnO3 and BaSnO3 films exhibit excellent performance across various substrates, providing a wide range of options for optoelectronic device applications.

Ni doping induced property enhancement in laser ablated BaSnO3 films suitable for optoelectronic applications

Pulsed laser deposition is a straightforward approach for preparing films with superconducting to dielectric properties with atomic layer precision. The deep-seated mechanisms involved in the particle transport from target to substrate and subsequent film formation still need to be fully comprehended. This manuscript reports the property enhancement observed in laser ablated perovskite BaSnO3 films with Ni doping. Films' crystallinity improvement is observed, and an intensity enhancement of 1150% is observed on 3 mol% Ni-doping. The optimum Ni-doping concentration in BaSnO3 is found to be 3 mol%. Herein, Ni-doped BaSnO3 films deposited by PLD showed an unusual increase in film thickness (i.e., from 615 nm in the pure film to 1317 nm in the film with 7 mol% Ni-doping as revealed by lateral SEM analysis and spectroscopic ellipsometry). We propose an “Induced Magnetic field-assisted Particle Convergence (IMPC)” effect for this superficial growth enhancement. The film's optical properties are modified with an increased nickel doping level, and the bandgap energy shows renormalization. All the films show excellent transmittance (80–90%) in the Vis.-NIR region. Hall-effect measurement reveals the increased carrier concentration by three orders (2.98 × 1011 to 3.50 × 1014 cm−3). In addition, the enhancement in mobility from 3.13 to 20.93 cm2V−1s−1 and a decrease in electrical resistivity by six orders (i.e., from 4.05 × 109 to 1.13 × 103 Ω cm) are observed on 7 mol% Ni doping. XPS measurements reveals that the Ba, Sn and Ni ions are at 2+, 4+ and 2+ oxidation states. Using spectroscopic ellipsometric method, we estimated the optical constants of the films, the refractive index, dielectric constant, and extinction coefficient show a normal dispersion behavior. The high crystallinity, high transmittance, suitable surface topography, and improved electrical performances of the Ni-doped BaSnO3 films make them excellent candidates for optoelectronic devices and solar cells.

Perovskite stannate La-doped BaSnO3 films for near- and mid-infrared plasmonic applications

La-doped BaSnO3 (LBSO) epitaxial thin films were grown on (001) MgO substrates by pulsed laser deposition using a La0.04Ba0.96SnO3 target. The structural, electrical, and optical properties of the LBSO films were investigated as a function of oxygen pressure during deposition. The carrier mobility can be tuned from 13 cm2V−1s−1 to 44 cm2V−1s−1 by varying oxygen partial pressure from 2 Pa to 12 Pa, respectively. This improved mobility is attributed to the reduced film strain via reducing oxygen defect concentration or reducing off-stoichiometry in the film. The optical permittivity of LBSO films can also be modified as a function of the oxygen pressure during deposition, allowing tuning of their epsilon-near-zero wavelength from 1.9 µm to 5.6 µm. These results demonstrate that LBSO thin films grown on MgO can be used for plasmonic devices at mid-wave infrared wavelengths.

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.

Modulation of vortex pinning by matrix defects in YBa2Cu3O7 nanocomposite film

Nanorods are a very effective pinning center in YBa2Cu3O7 films, and the vortex pinning mechanism should be understood to further improve the critical current density (J c) in the nanocomposite films. Matrix defects are naturally formed during nanocomposite growth, and in this study the effect of matrix defects on the J c in the YBa2Cu3O7 films containing nanorods is discussed. YBa2Cu3O7 + BaHfO3 and YBa2Cu3O7 + BaSnO3 films were prepared by varying the laser repetition frequency to control the matrix defects. All the films contained the nanorods. The YBa2Cu3O7 + BaHfO3 film of 2 Hz exhibits high J c for magnetic field parallel to the c-axis (B//c) due to the nanorods and the isolated random point defects. In the YBa2Cu3O7 + BaHfO3 film of 10 Hz, while the short stacking faults and networked random point defects decreased the J c for B//c, the short stacking faults improved the J c for tilted magnetic field. In the YBa2Cu3O7 + BaSnO3 film of 10 Hz, the isolated random point defects improved J c regardless of the magnetic field direction. While the ab-plane correlated defects are analyzed by the structural observation, the network or isolated nature of the random point defects can be discussed by the normal state resistivity. The matrix defects of YBa2Cu3O7 nanocomposite films should be designed considering the correlated/network/isolated nature which is determined by the morphology and defect concentration.

Hexagonal Boron Nitride Seed Layer-Assisted van der Waals Growth of BaSnO3 Perovskite Films.

We have succeeded in obtaining BaSnO3 perovskite thin films with remarkable near-infrared luminescence by van der Waals growth. The films were grown on quartz glass substrates by pulsed laser deposition using hexagonal boron nitride as the seed layer, and their crystallinity was confirmed by X-ray diffraction and cross-sectional transmission electron microscopy. The near-infrared emission of the grown film exhibited a broad emission peak centered at 920 nm. The transparency of the BaSnO3 film (thickness = 1000 nm)/ hexagonal boron nitride /double-sided optically polished quartz glass substrate was approximately 90% at approximately 500 nm with or without the BaSnO3 film. Films showing remarkable near-infrared emission and high transparency obtained by van der Waals-type growth could be used in practical wavelength conversion devices that improve the efficiency of Si single-crystal solar cells. The hexagonal boron nitride seed layer supporting the van der Waals growth is an effective method for high-quality crystal growth of films. It can be used for perovskite-type oxides with many functionalities.

Employing high-temperature-grown SrZrO3 buffer to enhance the electron mobility in La:BaSnO3-based heterostructures

We report a synthetic route to achieve high electron mobility at room temperature in epitaxial La:BaSnO3/SrZrO3 heterostructures prepared on several oxide substrates. Room-temperature mobilities of 157, 145, and 143 cm2 V−1 s−1 are achieved for heterostructures grown on DyScO3 (110), MgO (001), and TbScO3 (110) crystalline substrates, respectively. This is realized by first employing pulsed laser deposition to grow at very high temperature the SrZrO3 buffer layer to reduce dislocation density in the active layer, then followed by the epitaxial growth of an overlaying La:BaSnO3 active layer by molecular-beam epitaxy. Structural properties of these heterostructures are investigated, and the extracted upper limit of threading dislocations is well below 1.0×1010 cm−2 for buffered films on DyScO3, MgO, and TbScO3 substrates. The present results provide a promising route toward achieving high mobility in buffered La:BaSnO3 films prepared on most, if not all, oxide substrates with large compressive or tensile lattice mismatches to the film.

First-principles investigation of structural, electronic, and energetic properties of BaSnO3 (001) surfaces

Cubic perovskite oxide BaSnO3 has attracted increasing attention because of its exceptional optoelectronic properties and promising device applications. However, its composition of the most stable surface and relative thermodynamic stability of the different surface terminations remain controversial. Here we report a comprehensive computational and theoretical study of various (001) surface structures of BaSnO3 using first-principles electronic structure calculations. Our calculations show that the stoichiometric BaO surface termination has a slightly lower surface energy than SnO2 surface termination, although their values are comparable to each other and about one-half of the cleavage energy of BaSnO3. The calculated phase stability diagrams of the BaSnO3 (001) surface show that BaO surface termination is thermodynamically preferred to other surface terminations in the allowed range of BaSnO3 without forming secondary phases. Our calculations of cleavage energy reveal that eliminating the surface layer of BaO⋅H2O requires less energy compared to the surface layer of SnO2⋅H2O. These results explain the experimental findings why BaSnO3 films have a Ba-excess surface after oxygen annealing while SnO2 surface termination can only be achieved upon water leaching. This work may offer some guidelines for achieving precise control over the surface terminations of BaSnO3.

The role of defects in the persistent photoconductivity of BaSnO3 thin films

Time-dependent photoconductivity (PC) and PC spectra have been studied in oxygen deficient BaSnO3 thin films grown on different substrates. X-ray spectroscopy measurements show that the films have epitaxially grown on MgO and SrTiO3 substrates. While on MgO the films are nearly unstrained, on SrTiO3 the resulting film is compressive strained in the plane. Electrical conductivity in dark is increased in one order of magnitude for the films on SrTiO3 in comparison to the one on MgO. This leads to an increase of PC in the latter film in at least one order of magnitude. PC spectra show a direct gap with a value of eV for the film grown on MgO while on SrTiO3 eV. For both type of films, time-dependent PC curves show a persistent behavior after illumination is removed. These curves have been fitted employing an analytical procedure based on the frame of PC as a transmission phenomenon showing the relevant role of donor and acceptor defects as carrier traps and as a source of carriers. This model also suggests that in the BaSnO3 film on SrTiO3 more defects are created probably due to strain. This latter effect can also explain the different transition values obtained for both type of films.

Combined spectroscopy and electrical characterization of La:BaSnO3 thin films and heterostructures

For La-doped BaSnO3 thin films grown by pulsed laser deposition, we combine chemical surface characterization and electronic transport studies to probe the evolution of electronic states in the band structure for different La-doping contents. Systematic analyses of spectroscopic data based on fitting the core electron line shapes help to unravel the composition of the surface as well as the dynamics associated with increasing doping. These dynamics are observed with a more pronounced signature in the Sn 3d core level, which exhibits an increasing asymmetry to the high binding energy side of the peak with increasing electron density. The present results expand the current understanding of the interplay between the doping concentration, electronic band structure, and transport properties of epitaxial La:BaSnO3 films.

Optical and electrical properties of BaSnO3 and In2O3 mixed transparent conductive films deposited by filtered cathodic vacuum arc technique at room temperature

For the crystalline temperature of BaSnO3 (BTO) was above 650 °C, the transparent conductive BTO-based films were always deposited above this temperature on epitaxy substrates by pulsed laser deposition or molecular beam epitaxy till now which limited there application in low temperature device process. In the article, the microstructure, optical and electrical of BTO and In2O3 mixed transparent conductive BaInSnO x (BITO) film deposited by filtered cathodic vacuum arc technique (FCVA) on glass substrate at room temperature were firstly reported. The BITO film with thickness of 300 nm had mainly In2O3 polycrystalline phase, and minor polycrystalline BTO phase with (001), (011), (111), (002), (222) crystal faces which were first deposited at room temperature on amorphous glass. The transmittance was 70%–80% in the visible light region with linear refractive index of 1.94 and extinction coefficient of 0.004 at 550-nm wavelength. The basic optical properties included the real and imaginary parts, high frequency dielectric constants, the absorption coefficient, the Urbach energy, the indirect and direct band gaps, the oscillator and dispersion energies, the static refractive index and dielectric constant, the average oscillator wavelength, oscillator length strength, the linear and the third-order nonlinear optical susceptibilities, and the nonlinear refractive index were all calculated. The film was the n-type conductor with sheet resistance of 704.7 Ω/□, resistivity of 0.02 Ω ⋅cm, mobility of 18.9 cm2/V⋅s, and carrier electron concentration of 1.6 × 1019 cm−3 at room temperature. The results suggested that the BITO film deposited by FCVA had potential application in transparent conductive films-based low temperature device process.

High-Mobility Field-Effect Transistor Using 2-Dimensional Electron Gas at the LaScO3/BaSnO3 Interface

A 2-dimensional electron gas (2DEG) system with high mobility was discovered at the interface of two perovskite oxides: a polar orthorhombic perovskite LaScO3 and a nonpolar cubic perovskite BaSnO3. Upon depositing the LaScO3 film on the BaSnO3 film, we measured the conductance enhancement and the resulting 2DEG density (n2D). Comparing the results with the previously reported LaInO3/BaSnO3 polar interface, we applied the “interface polarization” model to the LaScO3/BaSnO3 system, in which the polarization exists only over four pseudocubic unit cells in LaScO3 from the interface and vanishes afterward like the LaInO3/BaSnO3 interface. Based on the calculations of the self-consistent Poisson–Schrödinger equations, the LaScO3 thickness dependence of n2D of the LaScO3/BaSnO3 heterointerface is consistent with this model. Furthermore, a single subband in the quantum well is predicted. By use of the conductive interface and the LaScO3 as a gate dielectric, a 2DEG transistor composed of only perovskite oxides with high field-effect mobility (μFE) close to 100 cm2 V–1 s–1 is demonstrated.

Precise composition control and cation nonstoichiometry in La-doped BaSnO3 thin films grown by MOCVD

La:BaSnO3 has recently emerged as an outstanding perovskite oxide material due to its high electron mobility. To-date, lower electron mobility in thin films compared to the single-crystal are discussed in terms of the presence of dislocations, nearly neglecting the role of cation nonstoichiometry. However, by controlling Sn/Ba compositional ratio, we demonstrate the significance of stoichiometry and resulting point defects on microstructure, morphology, electrical properties, and Raman defect modes in epitaxial La:BaSnO3 films. A narrow compositional window is demonstrated to yield high mobility and smooth morphology in La:BaSnO3 thin films using the metalorganic chemical vapor deposition. The room temperature electron mobility of 121 cm2V−1s−1 and 85 cm2V−1s−1 was achieved on SrTiO3 and MgO substrates, respectively. Correlation between the lowering of the A1 g soft mode near 140 cm−1 frequency and increase in the carrier mobility is demonstrated by Raman spectroscopy analysis. It was suggested that the position of this soft mode might be employed as a marker band for probing the electrical properties of La-doped BaSnO3 perovskites.

Epitaxial BaSnO3 thin films with low dislocation density grown on lattice matched LaInO3 substrates

The use of LaInO3 with (110) surface orientation was investigated as a novel orthorhombic substrate for the epitaxial growth of semiconducting BaSnO3 thin films. On the basis of reflection high-energy electron diffraction, energy dispersive x-ray analysis and inductively coupled plasma-optical emission spectrometry measurements, we revealed that slight Ba doping of LaInO3 crystals is beneficial to stabilize the substrate surface, which facilitates the epitaxial growth of well-ordered BaSnO3 thin films by pulsed laser deposition. Fully strained BaSnO3 films without misfit dislocations found by means of transmission electron microscopy were achieved due to the negligible lattice mismatch between BaSnO3 film and Ba-doped LaInO3 substrate. Electric properties of La-doped BaSnO3 films exhibit a Hall-mobility of 69 cm2 V−1 s−1 at room temperature and 99 cm2 V−1 s−1 at 20 K at a constant charge carrier density of 3.8·1019 cm−3.

One-step epitaxy of high-mobility La-doped BaSnO3 films by high-pressure magnetron sputtering

As unique perovskite transparent oxide semiconductors, high-mobility La-doped BaSnO3 films have been successfully synthesized by molecular beam epitaxy and pulsed laser deposition. However, it remains a big challenge for magnetron sputtering, a widely applied technique suitable for large-scale fabrication, to grow high-mobility La-doped BaSnO3 films. In this work, we developed a method to synthesize high-mobility epitaxial La-doped BaSnO3 films (with mobility up to 121 cm2 V−1 s−1 at the carrier density of ∼4.0 × 1020 cm−3 at room temperature) directly on SrTiO3 single crystal substrates using high-pressure magnetron sputtering. The structural and electrical properties of La-doped BaSnO3 films were characterized by combined high-resolution x-ray diffraction, x-ray photoemission spectroscopy, and temperature-dependent electrical transport measurements. The room-temperature electron mobility of La-doped BaSnO3 films achieved in this work is two to four times higher than the reported values of the films grown by magnetron sputtering. Moreover, in the high carrier density range (n > 3 × 1020 cm−3), the electron mobility value of 121 cm2 V−1 s−1 achieved in our work is among the highest values for all reported doped BaSnO3 films. It is revealed that high argon pressure during sputtering plays a vital role in stabilizing the fully relaxed films and inducing oxygen vacancies, which facilitates high mobility at room temperature. Our work provides an easy and economical way to massively synthesize high-mobility transparent conducting films for transparent electronics.

The influence of thermal annealing on the photoconducting properties of BaSnO3 films

Starting from high-quality oxygen-deficient BaSnO3 films, we have monitored the evolution of their electrical conducting and photoconducting properties after subsequent post-thermal annealing in oxygen. In this way, we have been able to modify the electrical conductivity of the film by at least three orders of magnitude (from 18.2 to 0.013 Ω−1 m−1) by simply reducing the oxygen vacancies concentration after each thermal annealing. Even though the film holds its semiconducting-like behavior, we have observed a modification of the hopping parameters concomitant with a decrease in the Fermi energy level as the electrical conductivity is reduced. Similarly, the effective energy gap extracted from photoconductance spectroscopy measurements decreases as the Fermi energy level decreases suggesting the presence of in-gap states generated by oxygen vacancies. A direct energy bulk gap value of (3.8±0.1) eV was obtained. While the photoconductivity increases from ≃4.6 to 73%, its slow time constants become less dominant as the electrical conductivity is decreased in accordance with a reduction of the oxygen vacancies density, which play a key role as electron-traps.

Opto-electronic properties of poly-crystalline La doped BaSnO3 films deposited on quartz substrates

Highly conducting and transparent thin films of La x Ba1 − x SnO3 (x = 0, 0.01 and 0.05) were grown on quartz substrates via pulsed laser deposition. Conductivity increases in orders of magnitude upon vacuum annealing of the as–deposited films. This is attributed to the significant enhancement in carrier concentration due to the increased oxygen vacancy defects (V). Enhanced carrier concentration improves the metallicity in the films at room temperature. Metal to semiconductor transition (MST) is observed in the films with n e 1019 cm−3 at low temperatures. The MST shifts towards absolute zero temperature as the carrier concentration in the film increases. Low–temperature resistivity analysis suggests that the charge transport is mainly governed by strong electron–electron interaction. UV–Vis spectroscopy confirms a 60%–80% transmittance in the visible range (400–800 nm) for all the films. Due to the large change in carrier concentration, Burstein Moss (BM) shift is observed, and the bandgap ranges between 3.65–3.98 eV. The effective mass of electron and refractive index estimated from the BM shift and transmittance are found to be 0.6 m e and 2, respectively.

Metallic line defect in wide-bandgap transparent perovskite BaSnO3.

A line defect with metallic characteristics has been found in optically transparent BaSnO3 perovskite thin films. The distinct atomic structure of the defect core, composed of Sn and O atoms, was visualized by atomic-resolution scanning transmission electron microscopy (STEM). When doped with La, dopants that replace Ba atoms preferentially segregate to specific crystallographic sites adjacent to the line defect. The electronic structure of the line defect probed in STEM with electron energy-loss spectroscopy was supported by ab initio theory, which indicates the presence of Fermi level-crossing electronic bands that originate from defect core atoms. These metallic line defects also act as electron sinks attracting additional negative charges in these wide-bandgap BaSnO3 films.