BaSnO3 Thin Films Research Articles

Electronic properties of p-type BaSnO3 thin films

Cubic perovskite oxides are attracting wide attention for applications in electronics and optoelectronics due to their high electronic mobility, however, realization of p-type conductivity in these oxides is still a matter. Barium stannate (BaSnO3) is one of such materials, wherein low-amount of lanthanum-doping can induce an extremely high room-temperature electron mobility. In this work, anion-doping strategy was explored to realize the p-type conductivity in BaSnO3, and the underlying physics of transport was investigated. It is found that nitrogen substitution for oxygen can induce the p-type conductivity in BaSnO3 thin films. By analyzing the temperature-dependent resistance and negative magnetoresistance data, Efros-Shklovskii variable-range hopping conduction is identified as the dominant transport mechanism in p-type BaSnO3 thin films. Additionally, nitrogen-doped BaSnO3 homojunctions exhibit good rectifying and photo-response properties. This study provides an alternative strategy for realizing the p-type conductivity in stannate-based perovskite oxides, and is helpful for designing the properties of stannate-based devices.

Scattering mechanisms and mobility enhancement in epitaxial BaSnO3 thin films probed via electrolyte gating

The wide-gap semiconducting perovskite BaSnO3 has attracted attention since the discovery of outstanding mobility at high electron densities, spurred on by potential applications in oxide, transparent, and power electronics. Despite progress, much remains to be understood in terms of mobility-limiting scattering in BaSnO3 thin films and thus mobility optimization. Here, we apply solid-state ion-gel-based electrolyte gating to electrostatically control electron density over a wide range (1018 cm−3 to >1020 cm−3) in BaSnO3 films. Temperature- and gate-voltage-dependent transport data then probe scattering mechanisms and mobility vs electron density alone, independently of sample-to-sample defect density variations. This is done on molecular-beam-epitaxy- and sputter-deposited films as a function of thickness, initial chemical doping, and initial mobility. Remarkably universal behavior occurs, the mobility first increasing with electron density to ∼1020 cm−3 before decreasing slightly. This trend is quantitatively analyzed at cryogenic and room temperatures using analytical models for phonon, ionized impurity, charged dislocation, surface/interface roughness, and electrolyte-induced scattering. The mobility maximum is thus understood to arise from competition between charged impurity/dislocation scattering and electrolyte scattering. The gate-voltage-induced mobility enhancement is found as large as 2000%, realizing 300 K mobility up to 140 cm2 V−1 s−1. This work thus significantly advances the understanding of mobility-limiting scattering processes in BaSnO3, resulting in outstanding room temperature mobilities.

Tunable permittivity of La-doped BaSnO3 thin films for near- and mid-infrared plasmonics

We report tunable permittivity of La-doped BaSnO3 (LBSO) epitaxial films in the near- and mid-infrared range. The structural, electrical transport, and optical properties of the LBSO thin films can be effectively modified by controlling oxygen concentration during film growth. The permittivity of the LBSO films can be modified as a function of the oxygen pressure and temperature during deposition, which allows the tuning of their epsilon-near-zero wavelength from 2 µm to 5.6 µm. The ability to tune the permittivity in this infrared range enables the use of LBSO films as active components in infrared plasmonic devices.

Carbon impurity concentrations in BaSnO3 films grown by molecular beam epitaxy using a tin oxide source

Achieving high carrier mobilities and low carrier densities has been one of the major challenges in the growth of BaSnO3 thin films. In this study, unintentional carbon impurities are investigated as one possible cause for poor electrical transport. Carbon concentrations in La-doped BaSnO3 films grown by molecular beam epitaxy using solid SnO2 sources are quantified using secondary ion mass spectroscopy. High carbon concentrations of mid-to-low 1018 atoms/cm3 are detected in samples grown from SnO2 powder and pellet sources, respectively. The carbon contamination is attributed to adsorbents present on the surface of the air exposed source material. Use of an oxygen plasma during growth does not reduce the amount of carbon detected in the films. Possible steps to reduce carbon contamination are discussed.

Nanoscale etching of perovskite oxides for field effect transistor applications

The etching of epitaxially grown perovskite oxide BaSnO3 (BSO) and BaTiO3 (BTO) thin films is studied using Cl-based (BCl3/Ar) and F-based (CF4/Ar) plasma chemistries in an inductively coupled plasma reactive ion etching (ICP-RIE) system for the development of field effect transistors (FETs). It is found that the BCl3/Ar process has a time-independent and a higher etch rate and creates a smooth etched surface, while the etch rate of BSO and BTO in CF4/Ar plasma decreases with the etching time duration. For the BCl3/Ar etching process, the etch rate increases with both ion density and ion energy, suggesting the combination of chemical plasma etching and physical ion sputtering mechanisms. Using the Cl-based etching process, BaSnO3 and BaTiO3 heterojunction FETs are developed. The devices with a gate length of 1.5 μm have a saturation current density of 287.6 mA/mm, a maximum transconductance of gm = 91.3 mS/mm, an FET mobility of 45.3 cm2/V s, and a threshold voltage of −1.75 V. The etching processes developed in this work will enable further development of perovskite oxide heterostructure electronic devices.

Fabrication of BaSnO3 thin films on SiO2 glass substrates using excimer laser-assisted metal organic decomposition

The perovskite oxide BaSnO3 has received much attention because of its excellent electronic properties for use in electronic devices such as solar cells. In this study, BaSnO3 thin films were successfully deposited onto SiO2 glass substrates using excimer laser-assisted metal organic decomposition (ELAMOD). The films could not be obtained using the thermal decomposition process because Ba reacts readily with a SiO2 substrate. Results of a transmission electron microscopy and numerical simulation of heat generation and transfer under laser irradiation revealed local and rapid generation of heat in the ELAMOD process resulting in a suppression of the reaction between Ba and SiO2 and hence enabling BaSnO3 thin film formation on SiO2 substrates.

Metalorganic chemical vapor deposition and investigation of nonstoichiometry of undoped BaSnO3 and La-doped BaSnO3 thin films

Wide bandgap (~3 eV) perovskite transparent conductive oxide material exhibiting superior electric properties is highly desired in today's optoelectronics. Just recently La-doped barium stannate (LBSO) bulk n-type semiconductor with outstanding carrier mobility (~320 cm2 V−1 s−1) at high carrier density (1020 cm−3) has been discovered. Highly conductive thin LBSO film synthesis has been challenging due to large lattice mismatch and nonstoichiometry as being the least investigated. Therefore, in this work, the Pulsed injection metalorganic chemical vapor deposition method was used for the growth of undoped BaSnO3 (BSO) and La-doped BaSnO3 thin films. Film depositions were carried out on monocrystalline LaAlO3, SrTiO3, Al2O3 substrates in wide stoichiometric range. Deviation from stoichiometric composition in BSO (Sn/Ba) and LBSO (Sn/(Ba + La)) had a significant impact on microstructure, optical, and electric properties while maintaining cubic symmetry. Near-stoichiometric films even grown at a high rate of ~10 nm/min showed sufficient electric properties.

Improved optoelectronic properties in solution-processed epitaxial rare-earth-doped BaSnO3 thin films via grain size engineering

Epitaxial BaSnO3 (BSO) thin films doped with different rare-earth elements (La, Pr, and Nd) were prepared by a simple solution deposition method. Grain size engineering through processing was achieved to tune both lateral and vertical grain sizes, resulting in obvious increments in carrier mobility. Improved performance with the room-temperature mobility exceeding 35 cm2 V−1 s−1 and resistivity as low as 0.55 mΩ cm was realized. The results will open a low-cost route for preparation of large-area BSO thin films with improved optoelectronic properties through the solution method.

Effect of substrate temperature, laser energy and post-deposition annealing on the structural, morphological and optical properties of laser-ablated perovskite BaSnO3 films

Perovskite BaSnO3 thin films suitable for excitonic solar cell applications were deposited on quartz substrate by pulsed-laser ablation method. The effect of various deposition parameters on the structural, morphological and optical properties is systematically studied by various characterization techniques. Phase-pure films with cubic crystalline nature were evolved at a substrate temperature of 600 °C with a laser energy of 60 mJ and post-deposition annealing further improved the structural purity and optical quality of the films. An exponential growth in film thickness was observed with increasing laser energy in which Stranski–Krastnov growth mechanism dominated. Chemical states of BaSnO3 were confirmed from XPS analysis. The deposited films present luminescence in the visible region at 452 nm, 480 nm and 580 nm wavelength.

Growth of La-Doped BaSnO₃ Film via Two-Step Spin Coating.

We successfully grew a uniform and high-transmittance La-doped BaSnO₃ (LBSO) thin film by reducing the LBSO particle size and improving film uniformity. We report an alternating magnetic stirring and ultrasonication process that can effectively reduce the size of particles in the colloid. A two-step spin coating process can improve thin film uniformity. A two-step spin coating process consists of two main steps: aggregation and gradual evolution. Aggregation is achieved by a single coating, which causes the colloidal to particles agglomerate into uneven clusters on the substrate. The gradual evolution step improves the quality of the films by filling the pinholes left by the initial coating.

Velocity saturation in La-doped BaSnO3 thin films

BaSnO3, a high mobility perovskite oxide, is an attractive material for oxide-based electronic devices. However, in addition to low-field mobility, high-field transport properties such as the saturation velocity of carriers play a major role in determining the device performance. We report on the experimental measurement of the electron saturation velocity in La-doped BaSnO3 thin films for a range of doping densities. The predicted saturation velocities based on a simple LO-phonon emission mode, using an effective LO phonon energy of 120 meV show good agreement with the measurements of velocity saturation in La-doped BaSnO3 films. Density-dependent saturation velocity in the range of 1.8 × 107 cm/s reducing to 2 × 106 cm/s is predicted for δ-doped BaSnO3 channels with carrier densities ranging from 1013 cm−2 to 2 × 1014 cm−2, respectively. These results are expected to aid the informed design of BaSnO3 as an active material for high-charge density electronic transistors.

Epitaxial BaSnO3 and SrSnO3 perovskite growth on SrTiO3(001) via atomic layer deposition

The authors report epitaxial BaSnO3(001) and SrSnO3(001) growth on SrTiO3(001) (STO) substrates via atomic layer deposition (ALD) at 180 °C, where the BaSnO3 growth rate is 0.46 ± 0.03 Å and the SrSnO3 growth rate is 0.69 ± 0.04 Å per ALD unit cycle. Reflection high-energy electron diffraction, x-ray diffraction (XRD), and rocking curve analyses verify the single crystalline nature of BaSnO3(001) and SrSnO3(001) thin films on STO(001), while the atomic force microscopy analyses reveal the smooth surface of the stannate films with rms roughness values of ∼0.4 nm, which is only slightly higher than the STO substrate roughness of 0.32 nm. Out-of-plane XRD and reciprocal space mapping show that both BaSnO3(001) and SrSnO3(001) (∼10 nm) are fully relaxed on STO(001), owing to the large lattice mismatches (5.1% for BaSnO3/STO and 3.2% for SrSnO3/STO). The visible light transmittance spectra indicate that ALD-grown BaSnO3 and SrSnO3 thin films have high transparency at 400–800 nm that matches the transparency of STO(001) substrates.

Effect of thickness on metal to semiconductor transition in La doped BaSnO3 films deposited on high mismatch LSAT substrates

1% La doped BaSnO3 thin films of different thicknesses, ranging from 15 to 300 nm, were obtained on single crystal Lanthanum Aluminate-Strontium Aluminate Tantalate [LSAT(001)] substrates via Pulsed Laser Deposition. The films grow epitaxially on these substrates (cube-on-cube epitaxy) and are almost relaxed with a strain of ≈0.51% for 300 nm films. All films show n-type conducting behavior with their conductivity varying from 65.36 S cm−1 to 465.11 S cm−1 as the thickness of the film is increased. Low temperature carrier concentration measurements indicate that the films are degenerate semiconductors. Films with a thickness ≥30 nm exhibit metal to semiconductor transition (MST) at low temperatures. Temperature dependent resistivity analysis of the films shows evidence of electron-electron interaction rather than weak localization as the governing transport mechanism below MST. The transition temperature shifts toward lower values at higher thicknesses, strengthening the metallic transport in such films.

Cr-doped BaSnO3 nanoporous thin films with tunable band gap via a facile colloidal solution route

Cr-doped BaSnO3 (BaSn1-xCrxO3, BSCO-x, x = 0–0.15) thin films with nanoporous structure were deposited on fluorine-doped tin oxide substrates via a colloidal solution route. The BSCO-x films exhibited single cubic perovskite phase and nanoporous feature, with the typical specific surface area of 18.98 m2/g and pore size of 10.75 nm (BSCO-0.15). The lowest band gap of 2.68 eV and a high transparency (above 70% in 500–800 nm) were realized in BSCO-0.15 film. This work paves a way toward perovskites with tunable band gap and nanoporous structures for varied photo and photoelectric devices.

Effect of oxygen pressure on the structural and optical properties of BaSnO3 films prepared by pulsed laser deposition method

BaSnO3 thin films were deposited on quartz substrate by pulsed laser deposition technique under different background oxygen pressures and the effects of oxygen pressure on the structural, morphological and optical properties of BaSnO3 thin films are systematically investigated using different characterization techniques. The BaSnO3 films deposited without and with oxygen pressures are polycrystalline in nature with cubic crystalline phase. Moderate oxygen ambience favors enhanced crystallinity of the BaSnO3 films and 0.02 mbar is found to be optimum oxygen pressure for highest crystallinity. The surface morphology of the deposited films was strongly affected by the oxygen pressure in the deposition chamber. A systematic increase of film thickness and decrease of RMS surface roughness is observed with increase in oxygen pressure. XPS analysis reveals that barium is in the + 2 oxidation state and Sn is in the + 4 oxidation state in the film deposited at optimum oxygen pressure of 0.02 mbar. The deposited BaSnO3 films have photoluminescence emissions in the visible region and have high transmittance in the visible and infrared regions. The BaSnO3 films deposited at oxygen ambience shows a blue shift in the optical band gap. The optimized film shows high crystallinity, high value of transmittance and wide band gap energy which indicates its suitability for optoelectronic devices.

Epitaxial Co doped BaSnO3 thin films with tunable optical bandgap on MgO substrate

Co doped BaSnO3 films [BaSn1 − xCoxO3(0 ≤ x ≤ 0.50), i.e., BSCO] have been grown on MgO single-crystal substrates via pulsed laser deposition. Effects of the Co-doping level on the crystallinity, the microstructure, and optical properties of the BSCO films are investigated. All the BSCO films are of high crystallinity and grown epitaxially on MgO substrates. The lattice parameter of the (200) plane drops linearly with the increase of the ‘x’ value. The Co doping is found to increase the film roughness and the grain size, and a RMS roughness of 8.33 nm and grain size up to 100 nm are observed in the film with x = 0.50. The low Co-doping level (x = 0.05, 0.1) has slight anti-reflection effects on the incident visible light, and the Co-doping level higher than 0.2 reduces the optical transmittance significantly. The optical bandgap shows a first-rising then falling trend with the increase of the Co content, and the lowest bandgap of 1.94 eV is realized in the BSCO film with x = 0.50.

Epitaxial integration of high-mobility La-doped BaSnO3 thin films with silicon

La-doped BaSnO3 has been epitaxially integrated with (001) Si using an SrTiO3 buffer layer via molecular-beam epitaxy (MBE). A 254 nm thick undoped BaSnO3 buffer layer was grown to enhance the mobility of the overlying La-doped BaSnO3 layer. The x-ray diffraction rocking curve of the BaSnO3 002 peak has a full width at half maximum of 0.02°. At room temperature, the resistivity of the La-doped BaSnO3 film is 3.6 × 10−4 Ω cm and the mobility is 128 cm2 V−1 s−1 at a carrier concentration of 1.4 × 1020 cm−3. These values compare favorably to those of La-doped BaSnO3 films grown by all techniques other than MBE on single-crystal oxide substrates. Our work opens an exciting arena for integrating hyper-functional oxide electronics that make use of high-mobility oxide films with the workhorse of the semiconductor industry, silicon.

Excellent structural, optical, and electrical properties of Nd-doped BaSnO3 transparent thin films

We epitaxially grew 7 mol. % Nd-doped BaSnO3 (NBSO) thin films on double-side polished SrTiO3 (001) single-crystal substrates and optimized the oxygen pressure (PO2), substrate temperature (TS), and film thickness (t) to achieve excellent structural, optical, and electrical performance. By keeping TS (=800 °C) constant, NBSO films prepared at PO2 = 10 Pa show the best crystallization, yielding a full-width at half-maximum (FWHM) of the x-ray diffraction rocking curve of 0.079° and exhibiting a room-temperature resistivity (ρ) of ∼1.85 mΩ cm and a volume carrier density (n) of ∼8.5 × 1020/cm3. By keeping PO2 (=10 Pa) constant, the room-temperature ρ of NBSO films could be reduced to as low as 0.5 mΩ cm by increasing TS from 700 to 825°; meanwhile, the volume carrier density and mobility show the maximum of 5.04 × 1020/cm3 and 24.9 cm2/Vs, respectively, for TS = 825 °C. For all as-grown NBSO thin films, the optical transmittance in the visible wavelength region is larger than 80%. The optimized comprehensive properties of the NBSO films with FWHM = 0.11°, ρ = 0.5 mΩ cm, μ = 24.9 cm2/Vs, and T > 80% are superior to those of other rare-earth and 4d- and 5d-transition metal-doped BaSnO3 thin films.

Unique Line Defect Discovered in BaSnO3 Thin Film

An abstract is not available for this content so a preview has been provided. As you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

Microstructure characterization of BaSnO3 thin films on LaAlO3 and PrScO3 substrates from transmission electron microscopy

Detailed microstructure analysis of epitaxial thin films is a vital step towards understanding essential structure-property relationships. Here, a combination of transmission electron microscopy (TEM) techniques is utilized to determine in detail the microstructure of epitaxial La-doped BaSnO3 films grown on two different perovskite substrates: LaAlO3 and PrScO3. These BaSnO3 films are of high current interest due to outstanding electron mobility at ambient. The rotational disorder of low-angle grain boundaries, namely the in-plane twist and out-of-plane tilt, is visualized by conventional TEM under a two-beam condition, and the degree of twists in grains of such films is quantified by selected-area electron diffraction. The investigation of the atomic arrangement near the film-substrate interfaces, using high-resolution annular dark-field scanning TEM imaging, reveals that edge dislocations with a Burgers vector along [001] result in the out-of-plane tilt. It is shown that such TEM-based analyses provide detailed information about the microstructure of the films, which, when combined with complimentary high-resolution X-ray diffraction, yields a complete structural characterization of the films. In particular, stark differences in out-of-plane tilt on the two substrates are shown to result from differences in misfit dislocation densities at the interface, explaining a puzzling observation from X-ray diffraction.