La-doped BaSnO3 Thin Films Research Articles

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.

Low-dimensional electronic state at the surface of a transparent conductive oxide

Materials that blend physical properties that are usually mutually exclusive could facilitate devices with novel functionalities. For example, the doped perovskite alkaline earth stannates BaSnO3 and SrSnO3 show the intriguing combination of high light transparency and high electrical conductivity. Understanding such emergent physics requires deep insight into the materials’ electronic structures. Moreover, the band structure at the surfaces of those materials can deviate significantly from their bulk counterparts, thereby unlocking novel physical phenomena. Employing angle-resolved photoemission spectroscopy and ab initio calculations, we reveal the existence of a 2-dimensional metallic state at the SnO2-terminated surface of 1% La-doped BaSnO3 thin films. The observed surface state is characterized by a distinct carrier density and a lower effective mass compared to the bulk conduction band, of about 0.12me. These particular surface state properties place BaSnO3 among the materials suitable for engineering highly conductive transition metal oxide heterostructures.

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.

Effect of Threading Dislocations on the Electronic Structure of La-Doped BaSnO3 Thin Films.

In spite of great application potential as transparent n-type oxides with high electrical mobility at room temperature, threading dislocations (TDs) often found in the (Ba,La)SnO3 (BLSO) films can limit their intrinsic properties so that their role in the physical properties of BLSO films need to be properly understood. The electrical properties and electronic structure of BLSO films grown on SrTiO3 (001) (STO) and BaSnO3 (001) (BSO) substrates are comparatively studied to investigate the effect of the TDs. In the BLSO/STO films with TD density of ~1.32 × 1011 cm−2, n-type carrier density ne and electron mobility are significantly reduced, as compared with the BLSO/BSO films with nearly no TDs. This indicates that TDs play the role of scattering-centers as well as acceptor-centers to reduce n-type carriers. Moreover, in the BLSO/STO films, both binding energies of an Sn 3d core level and a valence band maximum are reduced, being qualitatively consistent with the Fermi level shift with the reduced n-type carriers. However, the reduced binding energies of the Sn 3d core level and the valence band maximum are clearly different as 0.39 and 0.19 eV, respectively, suggesting that the band gap renormalization preexisting in proportion to ne is further suppressed to restore the band gap in the BLSO/STO films with the TDs.

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.

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.

Role of oxygen partial pressure on structure and properties of sputtered transparent conducting films of La-doped BaSnO3

Single crystal film of La doped BaSnO3 (LBSO) has been reported with excellent conductivity and transparency but in order to improve its applicability, it is important to grow polycrystalline LBSO films on inexpensive substrate. A series of polycrystalline LBSO thin films of ~570 nm in thickness are grown on quartz substrate by radio frequency magnetron sputtering at different oxygen pressures varying from 0 to 0.13 Pa. Both carrier concentration and mobility of the films seem to be decreasing with increasing oxygen pressure. The estimated values of mobility, carrier concentration and conductivity are 5.21 cm2/V s, 9.1 × 1019 cm−3 and 2.8 × 102 Ω−1 cm−1, respectively, with optical transmittance of 68% for the film deposited at lowest oxygen partial pressure (0.01 Pa). The optical transmittance of the films improves significantly upto 82% with increasing oxygen pressure to 0.13 Pa. The suitable condition for deposited films is observed at 0.05 Pa in terms of transmittance (~80%) and conductivity (0.67 × 102 Ω−1 cm−1) which is almost 8 times superior to previously reported conductivity (9 Ω−1 cm−1) for polycrystalline LBSO films and comparable to the conductivity (102 Ω−1 cm−1) of the hetero-structured LBSO thin films. Overall, understanding of changes in microstructure, electrical and optical properties of thin film by varying oxygen pressure have been reported effectively. The LBSO films prepared in the current work could be further utilized in TCO applications.

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.

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.

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.

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.

Enhancing electron mobility in La-doped BaSnO3 thin films by thermal strain to annihilate extended defects

We report the enhancement of room-temperature electron mobility in La-doped BaSnO3 (LBSO) thin films with thermal strain induced by high temperature nitrogen (N2) annealing. Simple annealing under an N2 environment consistently doubled the electron mobility of the LBSO films on the SrTiO3 (STO) substrates to as high as 78 cm2 V−1 s−1 at a carrier concentration of 4.0 × 1020 cm−3. This enhancement is mainly attributed to annihilation of extended defects as a consequence of compressive strain induced by the difference in the thermal expansion coefficients of LBSO and STO. Our study suggests that thermal strain can be exploited to reduce extended defects and to facilitate electron transport in transparent oxide semiconductors.

Conduction band edge effective mass of La-doped BaSnO3

BaSnO3 has attracted attention as a promising material for applications requiring wide band gap, high electron mobility semiconductors, and moreover possesses the same perovskite crystal structure as many functional oxides. A key parameter for these applications and for the interpretation of its properties is the conduction band effective mass. We measure the plasma frequency of La-doped BaSnO3 thin films by glancing incidence, parallel-polarized resonant reflectivity. Using the known optical dielectric constant and measured electron density, the resonant frequency determines the band edge electron mass to be 0.19 ± 0.01. The results allow for testing band structure calculations and transport models.

Determination of the effective mass and nanoscale electrical transport in La-doped BaSnO3 thin films

Epitaxial La-doped BaSnO3 thin films were grown by radio-frequency (RF) magnetron sputtering technique on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (001) substrates. The n-type degenerate semiconductor was demonstrated in the La-doped BaSnO3 thin film from Hall-effect measurement, and its electron effective mass ∼0.396m0 (m0, the free electron mass) was determined from combined Seebeck coefficient and carrier density. Additionally, the local current–voltage curve measured using conductive atomic force microscopy exhibits non-linear characteristic and the transport mechanism at high bias is found to be the Fowler-Nordheim tunneling.

Electrical and optical behaviors of La-doped BaSnO3 thin film

La-doped BaSnO3 is regarded as a very essential material to construct transparent perovskite devices due to its super high electrical mobility in perovskite transparent conducting oxides. For understanding the high electrical mobility, the effective mass of the carrier in La-doped BaSnO3 is a critical factor and should be determined. In this work, the performances of epitaxial La-doped BaSnO3 thin films grown on (LaAlO3)0.3 (SrAl0.5Ta0.5O3)0.7 (001) substrate by radio-frequency (RF) magnetron sputtering technique are investigated. The electrical properties (resistivity, carrier density, mobility and Seebeck coefficient) and the optical transmittance are analyzed. In addition, it is proved from both the Hall effect and thermoelectric power measurements that the La-doped BaSnO3 thin films are n-type degenerate semiconductor. At 300 K, the resistivity, carrier density, mobility and Seebeck coefficient are 0.987 mΩ·cm, 2.584×1020 cm-3, 24.49 cm2·V-1·s-1 and 45.71 μV/K, respectively. The electron effective mass ～ 0.31m0 (m0, the free electron mass) is extracted by combining the Seebeck coefficient and carrier density. Ba0.99La0.01SnO3 (BLSO) thin film exhibits a high optical transmittance of 73% in the visible spectral region. In order to derive the band-gap energy, the complex dielectric constant, and the film thickness, the transmittance spectrum is simulated based on the dielectric model comprising the band-gap transition (O'Leary-Johnson-Lim model) and free electron excitation (Drude-Lorentz model). The band-gap energy, exponential band tail and thickness of the BLSO thin film are 3.43 eV, 0.27 eV and 781.2 nm, respectively. Wavelength-dependence of complex dielectric function of the BLSO thin film is also obtained from the fitted line. Additionally, the parameters (optical carrier density and mobility) resulting from the optical measurement are in agreement with the results from the electrical measurement, which supports the calculated electron effective mass aforementioned.

Structural and optical properties of La-doped BaSnO3 thin films grown by PLD

In this study the structural and optical properties of lanthanum-doped BaSnO3 powder samples and thin films deposited on fused silica were investigaed using laser ablation. Under an oxygen pressure of 5×10−4mbar, phase pure BaSnO3 films with a lattice constant of 0.417nm and grain size of 21nm were prepared at 630°C. The band gap of BaSnO3 powder sample and thin films was calculated to be 3.36eV and 3.67eV, respectively. There was a progressive increase in conductivity for thin films of BaSnO3 doped with 0~7 at% of La. The highest conductivity, 9Scm−1, was obtained for 7 at% La-doped BaSnO3. Carrier concentration, obtained from Burstein-Moss (B-M) shift, nearly matches the measured values except for 3 at% and 10 at% La-doped BaSnO3 thin films.

Infrared conductivity of PED-grown (La,Ba)SnO3 thin films

We have synthesized La-doped BaSnO3 thin films on SrTiO3 substrates by using the pulsed electron deposition technique. By performing reflection/transmission measurements on the films and a Drude model analysis of the data, we determined the carrier density n = 6.7 × 1019 cm−3 and the room-temperature mobility µ = 17 cm2V−1s−1 of the conducting electrons. The standard Drude optical conductivity of the films demonstrates that the electron correlation effect is minimal in the present transparent conducting oxide material.