Browsing by Author "Prakash, Abhinav"
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Item Data for "Scattering mechanisms and mobility enhancement in epitaxial BaSnO3thin films probed via electrolyte gating"(2020-08-05) Wang, Helin; Prakash, Abhinav; Reich, Konstantin; Ganguly, Koustav; Leighton, Chris; leighton@umn.edu; Leighton, Chris; Leighton Electronic and Magnetism LabData includes temperature-dependent electronic transport (sheet resistance, electron density, and mobility) of ion-gel-gated BaSnO3 thin films of various thicknesses and growth methods. The mobility vs electron density experimental data and the fitting results (fits) are also provided.Item Defect Engineering in Perovskite Oxide Thin Films and Heterostructures(2019-03) Prakash, AbhinavPerovskite oxide (ABO3 type compounds) is an important class of materials exhibiting a wide range of functionalities. However, in comparison to conventional semiconductors such as silicon, they possess orders of magnitude lower room-temperature electron mobilities. For example, in doped SrTiO3, the best reported room-temperature value of electron mobility has remained below 10 cm2 V−1 s−1 for over five decades. The realization of a perovskite oxide semiconductor with high room-temperature mobility would constitute a significant advancement, enabling novel physical and perhaps even a plethora of new and more realistic device concepts. Very recently a key step in this direction was taken via the growth of bulk doped BaSnO3, where room-temperature mobilities as high as 320 cm2 V−1 s−1 was reported. Thin films of BaSnO3 show much lower room-temperature mobility values ranging between 1-180 cm2 V−1 s−1 and highly dependent on the growth method, choice of substrate, and dopants. Although these findings have been encouraging for fundamental studies and potential applications in room-temperature oxide electronics, there still remains many open fundamental questions and challenges including the role of defects on the properties of BaSnO3 and the scattering mechanisms that limit the mobility in thin films from reaching values close to bulk mobility. These questions will be addressed in this thesis by studying thin films of BaSnO3 grown by molecular beam epitaxy. One of the challenges with the growth of BaSnO3 is the high electronegativity (low oxidation potential) of tin suggesting that stronger oxidizing conditions such as ozone or high-pressure oxygen plasma are required to achieve full oxidation of Sn. Such extreme oxidation conditions in an ultra-high vacuum molecular beam epitaxy system may lead to undesirable consequences such as oxidation of elemental sources leading to flux-instabilities, filament oxidation, and potential damage to vacuum pumps. As the first step in this direction, a new radical-based hybrid MBE approach for tin-based compounds is developed. For BaSnO3 growth, Ba is supplied through effusion from a cell, Sn using a chemical precursor (hexamethylditin – (CH3)6Sn2), and oxygen using a radio frequency plasma source. The unique aspect of our approach is that hexamethylditin forms highly reactive Sn• radicals, which facilitate the growth of phase-pure, stoichiometric films even in weak oxidizing environment such as molecular oxygen. Using this approach, synthesis of phase-pure and epitaxial BaSnO3 with scalable growth rates and layer-by- layer control over thicknesses is reported. Reflection high-energy electron diffraction is used to describe the strain relaxation behavior of BaSnO3. Various characterization techniques are employed for establishing the stoichiometric growth condition such as X-ray diffraction for lattice parameter measurements, Rutherford backscattering spec- trometry for quantification of cation (Sn:Ba) ratio, atomic force microscopy for imaging the surface morphology, electronic transport for measuring the carrier concentrations, resistivity, and electron mobility in lanthanum-doped BaSnO3 films, and time-domain thermoreflectance for determining the thermal conductivity. With the combination of these techniques, existence of a self-regulating “growth-window” is demonstrated. Through controlled La-doping in BaSnO3 films, a highest room-temperature electron mobility of 120 cm2 V−1 s−1 is achieved on a -5.12 % lattice-mismatched SrTiO3 substrate. The optimal doping range for the highest mobility is found to be 5.0 × 1019 cm−3 to 5.0 × 1020 cm−3. Mobility decreases at higher or lower doping concentrations. Temperature-dependent measurements of mobility provide insights into the scattering mechanisms limiting the mobility at different doping concentrations and temperatures. While dislocation scattering is found to be dominant at low doping regime, ionized impurity scattering plays a major role at high doping levels. At intermediate doping concentrations, both scattering mechanisms control the transport behavior. Phonon scattering accounts for the decreasing trend in mobility with increasing temperature. Building upon these findings which revealed mobility-limiting mechanisms in uniformly doped BaSnO3, the final step involves the development of modulation doping approach n BaSnO3-based heterostructures. The basic idea behind modulation doping is to sep- arate electrons from their ionized donors. Favorable band offsets in BaSnO3–SrTiO3 and BaSnO3–SrSnO3 systems are established. Taking BaSnO3–SrSnO3 as the model heterostructure, electron transfer from La-doped SrSnO3 to BaSnO3 is demonstrated, resulting in dramatic changes in the transport behavior. Results are encouraging and clearly suggest that electrons in BaSnO3 can be separated from ionized dopants. The transport, however, is still limited by dislocations and defects at the interface which should be the focus of future studies.Item Dopant segregation inside and outside dislocation cores in perovskite BaSnO 3 and reconstruction of the local atomic and electronic structures(2021-04-26) Yun, Hwanhui; Prakash, Abhinav; Birol, Turan; Jalan, Bharat; Mkhoyan, K. Andre; yunxx133@umn.edu; Yun, HwanhuiDistinct dopant behaviors inside and outside dislocation cores are identified by atomic-resolution electron microscopy in perovskite BaSnO3 with considerable consequences on local atomic and electronic structures. Driven by elastic strain, when A-site designated La dopants segregate near a dislocation core, the dopant atoms accumulate at the Ba sites in compressively strained regions. This triggers formation of Ba-vacancies adjacent to the core atomic sites resulting in reconstruction of the core. Notwithstanding the presence of extremely large tensile strain fields, when La atoms segregate inside the dislocation core, they become B-site dopants, replacing Sn atoms and compensating the positive charge of the core oxygen vacancies. Electron energy-loss spectroscopy shows that the local electronic structure of these dislocations changes dramatically due to segregation of the dopants inside and around the core ranging from formation of strong La-O hybridized electronic states near the conduction band minimum to insulator-to-metal transition.Item Supporting data for Metallic line defect in wide-bandgap transparent perovskite BaSnO₃(2021-01-22) Yun, Hwanhui; Topsakal, Mehmet; Prakash, Abhinav; Jalan, Bharat; Jong Seok, Jeong; Birol, Turan; Mkhoyan, K Andre; yunxx133@umn.edu; Yun, HwanhuiA 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.