Abhinav Prakash

Band alignment at epitaxial BaSnO3/SrTiO3(001) and BaSnO3/LaAlO3(001) heterojunctions

We have spectroscopically determined the optical bandgaps and band offsets at epitaxial interfaces of BaSnO3 with SrTiO3(001) and LaAlO3(001). 28 u.c. BaSnO3 epitaxial films exhibit direct and indirect bandgaps of 3.56 ± 0.05 eV and 2.93 ± 0.05 eV, respectively. The lack of a significant Burstein-Moss shift corroborates the highly insulating, defect-free nature of the BaSnO3 films. The conduction band minimum is lower in electron energy in 5 u.c. films of BaSnO3 than in SrTiO3 and LaAlO3 by 0.4 ± 0.2 eV and 3.7 ± 0.2 eV, respectively. This result bodes well for the realization of oxide-based, high-mobility, two-dimensional electron systems that can operate at ambient temperature, since electrons generated in the SrTiO3 by modulation doping, or at the BaSnO3/LaAlO3 interface by polarization doping, can be transferred to and at least partially confined in the BaSnO3 film.

Wide bandgap BaSnO3 films with room temperature conductivity exceeding 104 S cm−1

Wide bandgap perovskite oxides with high room temperature conductivities and structural compatibility with a diverse family of organic/inorganic perovskite materials are of significant interest as transparent conductors and as active components in power electronics. Such materials must also possess high room temperature mobility to minimize power consumption and to enable high-frequency applications. Here, we report n-type BaSnO3 films grown using hybrid molecular beam epitaxy with room temperature conductivity exceeding 104 S cm−1. Significantly, these films show room temperature mobilities up to 120 cm2 V−1 s−1 even at carrier concentrations above 3 × 1020 cm−3 together with a wide bandgap (3 eV). We examine the mobility-limiting scattering mechanisms by calculating temperature-dependent mobility, and Seebeck coefficient using the Boltzmann transport framework and ab-initio calculations. These results place perovskite oxide semiconductors for the first time on par with the highly successful III–N system, thereby bringing all-transparent, high-power oxide electronics operating at room temperature a step closer to reality.

Hybrid molecular beam epitaxy for the growth of stoichiometric BaSnO3

Owing to its high room-temperature electron mobility and wide bandgap, BaSnO3 has recently become of significant interest for potential room-temperature oxide electronics. A hybrid molecular beam epitaxy (MBE) approach for the growth of high-quality BaSnO3 films is developed in this work. This approach employs hexamethylditin as a chemical precursor for tin, an effusion cell for barium, and a radio frequency plasma source for oxygen. BaSnO3 films were thus grown on SrTiO3 (001) and LaAlO3 (001) substrates. Growth conditions for stoichiometric BaSnO3 were identified. Reflection high-energy electron diffraction (RHEED) intensity oscillations, characteristic of a layer-by-layer growth mode were observed. A critical thickness of ∼1 nm for strain relaxation was determined for films grown on SrTiO3 using in situ RHEED. Scanning transmission electron microscopy combined with electron energy-loss spectroscopy and energy dispersive x-ray spectroscopy confirmed the cube-on-cube epitaxy and composition. The importan...

Adsorption-controlled growth and the influence of stoichiometry on electronic transport in hybrid molecular beam epitaxy-grown BaSnO3 films

High room-temperature electron mobility and optical transparency in the visible spectrum distinguishes BaSnO3 from other perovskite oxides. The origin of low mobility in thin films as compared to their bulk counterpart is attributed to the presence of dislocations in films with nearly no discussion on the role of point defects such as cation non-stoichiometry. Using high-resolution X-ray diffraction, Rutherford backscattering spectrometry, thermal, and electronic transport measurements, we show that a growth window, in which cation stoichiometry is self-regulating, can be achieved for BaSnO3 films on SrTiO3(001) and (La0.3Sr0.7)(Al0.65Ta0.35)O3(001) (LSAT) substrates using a hybrid molecular beam epitaxy approach. BaSnO3 films on SrTiO3 grown within the growth window yielded a mobility value of 105 cm2 V−1 s−1 at a density, 2.5 × 1020 cm−3. Bulk-like thermal conductivity of 13.3 ± 1.46 W m−1 K−1 was achieved for stoichiometric films. Both Ba- and Sn-deficient films resulted into charge compensation and low mobility, with a stronger dependence for Sn-deficient films.

Mobility-electron density relation probed via controlled oxygen vacancy doping in epitaxial BaSnO3

The recently discovered high room temperature mobility in wide band gap semiconducting BaSnO3 is of exceptional interest for perovskite oxide heterostructures. Critical open issues with epitaxial films include determination of the optimal dopant and understanding the mobility-electron density (μ-n) relation. These are addressed here through a transport study of BaSnO3(001) films with oxygen vacancy doping controlled via variable temperature vacuum annealing. Room temperature n can be tuned from 5 × 1019 cm−3 to as low as 2 × 1017 cm−3, which is shown to drive a weak- to strong-localization transition, a 104-fold increase in resistivity, and a factor of 28 change in μ. The data reveal μ ∝ n0.65 scaling over the entire n range probed, important information for understanding mobility-limiting scattering mechanisms.

Molecular beam epitaxy growth of SnO2 using a tin chemical precursor

The authors report on the development of a molecular beam epitaxy approach for atomic layer controlled growth of phase-pure, single-crystalline epitaxial SnO2 films with scalable growth rates using a highly volatile precursor (tetraethyltin) for tin and rf-oxygen plasma for oxygen. Smooth, epitaxial SnO2 (101) films on r-sapphire (101¯2) substrates were grown as a function of tin precursor flux and substrate temperatures between 300 and 900 °C. Three distinct growth regimes were identified where SnO2 films grew in a reaction-, flux-, and desorption-limited mode, respectively, with increasing substrate temperature. In particular, with increasing tin flux, the growth rates were found to increase and then saturate indicating any excess tin precursor desorbs above a critical beam equivalent pressure of tin precursor. Important implications of growth kinetic behaviors on the self-regulating stoichiometric growth of perovskite stannates are discussed.

Electronic structure of BaSnO3 investigated by high-energy-resolution electron energy-loss spectroscopy and ab initio calculations

There has been growing interest in perovskite BaSnO3 due to its desirable properties for oxide electronic devices, including high electron mobility at room temperature and optical transparency. As these electronic and optical properties originate largely from the electronic structure of the material, here the basic electronic structure of epitaxially grown BaSnO3 films is studied using high-energy-resolution electron energy-loss spectroscopy in a transmission electron microscope and ab initio calculations. This study provides a detailed description of the dielectric function of BaSnO3, including the energies of bulk plasmon excitations and critical interband electronic transitions, the band structure and partial densities of states, the measured band gap, and more.

THz characterization and demonstration of visible-transparent/terahertz-functional electromagnetic structures in ultra-conductive La-doped BaSnO3 Films

We report on terahertz characterization of La-doped BaSnO3 (BSO) thin-films. BSO is a transparent complex oxide material, which has attracted substantial interest due to its large electrical conductivity and wide bandgap. The complex refractive index of these films is extracted in the 0.3 to 1.5 THz frequency range, which shows a metal-like response across this broad frequency window. The large optical conductivity found in these films at terahertz wavelengths makes this material an interesting platform for developing electromagnetic structures having a strong response at terahertz wavelengths, i.e. terahertz-functional, while being transparent at visible and near-IR wavelengths. As an example of such application, we demonstrate a visible-transparent terahertz polarizer.

Terahertz spectroscopy and demonstration of visible-transparent/terahertz-functional electromagnetic structures in La-doped BaSnO3 films (Conference Presentation)

BaSnO_3 (BSO) is a transparent conductive oxide. This category of materials is interesting for applications such as optically transparent electrodes in solar cells and displays. This perovskite-material possesses many interesting properties including a wide bandgap, 3 eV, and a high electrical conductivity (exceeding 10^4 S/cm at room-temperature), which make it very interesting for visible-transparent applications. The DC conductivity in BSO can be superior to that in ITO, which is a commonly used transparent conductive oxide. Thin films used in our study were grown by molecular beam epitaxy (MBE) on LSAT substrates. The epitaxial structure of the samples consist of 45 nm of La-doped BSO on top of a 45 nm thick undoped BSO film grown on LSAT. The BSO films were characterized by means of terahertz spectroscopy. The terahertz-extracted optical conductivity was ~0.8x10^3 S/cm in the 0.1 to 2 THz frequency range. Using these films, upon patterning into stripes, we demonstrate a terahertz polarizer. The polarizer is transparent at visible wavelengths, and functional at terahertz wavelengths; it achieves 96% transmission for terahertz polarization parallel to the stripes and 16% transmission for the perpendicular polarization. Furthermore, we also show that resonant structures, such as cross resonators, are also realizable in this material. The large optical conductivity in BSO films at terahertz frequencies, together with being transparent at visible wavelengths, makes it a very good candidate for developing visible-transparent electromagnetic structures responding in the terahertz frequency range.

Frequency- and temperature-dependent dielectric response in hybrid molecular beam epitaxy-grown BaSnO3 films

We report on the dielectric response of epitaxial BaSnO3 films grown on Nb-doped SrTiO3 (001) substrates using a hybrid molecular beam epitaxy approach. Metal-insulator-metal capacitors were fabricated to obtain frequency- and temperature-dependent dielectric constant and loss. Irrespective of film thickness and cation stoichiometry, the dielectric constant obtained from Ba1−xSn1−yO3 films remained largely unchanged at 15-17 and was independent of frequency and temperature. A loss tangent of ∼1 × 10−3 at 1 kHz < f < 100 kHz was obtained for stoichiometric films, which increased significantly with non-stoichiometry. Using density functional theory calculations, these results are discussed in the context of point defect complexes that can form during film synthesis.We report on the dielectric response of epitaxial BaSnO3 films grown on Nb-doped SrTiO3 (001) substrates using a hybrid molecular beam epitaxy approach. Metal-insulator-metal capacitors were fabricated to obtain frequency- and temperature-dependent dielectric constant and loss. Irrespective of film thickness and cation stoichiometry, the dielectric constant obtained from Ba1−xSn1−yO3 films remained largely unchanged at 15-17 and was independent of frequency and temperature. A loss tangent of ∼1 × 10−3 at 1 kHz < f < 100 kHz was obtained for stoichiometric films, which increased significantly with non-stoichiometry. Using density functional theory calculations, these results are discussed in the context of point defect complexes that can form during film synthesis.