Koustav Ganguly

Structure and transport in high pressure oxygen sputter-deposited BaSnO3−δ

BaSnO3 has recently been identified as a high mobility wide gap semiconductor with significant potential for room temperature oxide electronics. Here, a detailed study of the high pressure oxygen sputter-deposition, microstructure, morphology, and stoichiometry of epitaxial BaSnO3 on SrTiO3(001) and MgO(001) is reported, optimized conditions resulting in single-phase, relaxed, close to stoichiometric films. Most significantly, vacuum annealing is established as a facile route to n-doped BaSnO3−δ, leading to electron densities above 1019 cm−3, 5 mΩ cm resistivities, and room temperature mobility of 20 cm2 V−1 s−1 in 300-A-thick films on MgO(001). Mobility limiting factors, and the substantial scope for their improvement, are discussed.

Critical thickness and strain relaxation in molecular beam epitaxy-grown SrTiO3 films

We report on the study of the critical thickness and the strain relaxation in epitaxial SrTiO3 film grown on (La0.3Sr0.7)(Al0.65Ta0.35)O3 (001) (LSAT) substrate using the hybrid molecular beam epitaxy approach. No change in the films lattice parameter (both the in-plane and the out-of-plane) was observed up to a film thickness of 180 nm, which is in sharp contrast to the theoretical critical thickness of ∼12 nm calculated using the equilibrium theory of strain relaxation. For film thicknesses greater than 180 nm, the out-of-plane lattice parameter was found to decrease hyperbolically in an excellent agreement with the relaxation via forming misfit dislocations. Possible mechanisms are discussed by which the elastic strain energy can be accommodated prior to forming misfit dislocations leading to such anomalously large critical thickness.

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.

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.

Study of Strain and Intermixing at the BaSnO 3 /SrTiO 3 and BaSnO 3 /LaAlO 3 Interfaces Using STEM and EELS

The interfaces between two different perovskite oxides have been widely studied since the discovery of metallic behavior at the interface of SrTiO3/LaAlO3 [1]. While various novel electronic properties, such as two-dimensional electron gas [2], superconductivity [3], and quantum hall effect [4], have been observed in some perovskite materials, the origins of these emergent properties are still under debate. More studies of these properties in perovskite materials are desired, and transparent and insulating cubic perovskite BaSnO3 is a promising candidate for such studies. Recently, bulk BaSnO3 reportedly showed unusually high carrier mobility at room temperature that was comparable to Si [5]. The heterostructures of BaSnO3 with other perovskite oxides are also expected to exhibit unusual electronic properties. Motivated by this, we conducted scanning transmission electron microscopy (STEM) and electron energyloss spectroscopy (EELS) studies of two BaSnO3 interfaces: BaSnO3/LaAlO3 and BaSnO3/SrTiO3.