Wide bandgap (Eg ∼ 3.1 eV) La-doped BaSnO3 (LBSO) has attracted increasing attention as one of the transparent oxide semiconductors since its bulk single crystal shows a high carrier mobility (∼320 cm2 V−1 s−1) with a high carrier concentration (∼1020 cm−3). For this reason, many researchers have fabricated LBSO epitaxial films thus far, but the obtainable carrier mobility is substantially low compared to that of single crystals due to the formation of the lattice/structural defects. Here we report that the mobility suppression in LBSO films can be lifted by a simple vacuum annealing process. The oxygen vacancies generated from vacuum annealing reduced the thermal stability of LBSO films on MgO substrates, which increased their carrier concentrations and lateral grain sizes at elevated temperatures. As a result, the carrier mobilities were greatly improved, which does not occur after heat treatment in air. We report a factorial design experiment for the vacuum annealing of LBSO films on MgO substrates and discuss the implications of the results. Our findings expand our current knowledge on the point defect formation in epitaxial LBSO films and show that vacuum annealing is a powerful tool for enhancing the mobility values of LBSO films.

https://eprints.lib.hokudai.ac.jp/dspace/bitstream/2115/75115/1/1.5054154.pdf

Effects of vacuum annealing on the electron mobility of epitaxial La-doped BaSnO3 films

Wide bandgap (~31 eV) La-doped BaSnO3 (LBSO) has attracted increasing attention as one of the transparent oxide semiconductors since its bulk single crystal shows a high carrier mobility (~320 cm2 V-1 s-1) with a high carrier concentration (~10^20 cm-3) For this reason, many researchers have fabricated LBSO epitaxial films thus far, but the obtainable carrier mobility is substantially low compared to that of single crystals due to the formation of the lattice/structural defects Here we report that the mobility suppression in LBSO films can be lifted by a simple vacuum annealing process The vacuum annealing of the LBSO films on MgO substrate increased the carrier concentrations due to the oxygen vacancy formation, which leads to simultaneous lateral grain growth As a result, the carrier mobility was greatly improved by the vacuum annealing, which does not occur after heat treatment in air These results expand our current knowledge on the point defect formation in epitaxial LBSO films and show that vacuum annealing is a powerful tool for enhancing the mobility values of LBSO films

Oxygen vacancy driven mobility enhancement in epitaxial La-doped BaSnO3 from vacuum annealing

The perovskite oxide BaSnO3 is a highly topical material for next-generation transparent conducting semiconductors. The foreign chemical doping strategy is largely used to realize the high conducti...

Unraveling the Oxygen Effect on the Properties of Sputtered BaSnO3 Thin Films

Highly conductive Nb doped BaSnO3 thin films on MgO substrates by pulsed laser deposition

The wide band gap semiconducting perovskite $\mathrm{BaSn}{\mathrm{O}}_{3}$ is of high current interest due to outstanding room temperature mobility at high electron density, fueled by potential applications in oxide, transparent, and power electronics. Due in part to a lack of lattice-matched substrates, $\mathrm{BaSn}{\mathrm{O}}_{3}$ thin films suffer from high defect densities, however, limiting electron mobility. Additionally, the vast majority of $\mathrm{BaSn}{\mathrm{O}}_{3}$ thin film research has focused on pulsed laser deposition or molecular beam epitaxy. Here, we present an exhaustive optimization of the mobility of ${\mathrm{Ba}}_{0.98}{\mathrm{La}}_{0.02}\mathrm{Sn}{\mathrm{O}}_{3}$ films grown by a scalable, high-throughput method: high-pressure-oxygen sputter deposition. Considering target synthesis conditions, substrate selection, buffer layer structure, deposition temperature, deposition rate, thickness, and postdeposition annealing conditions, and by combining high-resolution x-ray diffraction, reciprocal space mapping, rocking curve analysis, scanning transmission electron microscopy, atomic force microscopy, and temperature-dependent electronic transport measurements, detailed understanding of synthesis-structure-property relationships is attained. Optimized room temperature mobility of $96\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{--}1}$ is achieved in vacuum-annealed $\mathrm{GdSc}{\mathrm{O}}_{3}$(110)/$\mathrm{BaSn}{\mathrm{O}}_{3}$(120 nm)/${\mathrm{Ba}}_{0.98}{\mathrm{La}}_{0.02}\mathrm{Sn}{\mathrm{O}}_{3}$(200 nm) heterostructures, as well as $92\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{--}1}$ on unbuffered substrates and $87\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{--}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{--}1}$ without postdeposition annealing. These results, including important trends in defect densities and a surprising dependence of mobility on lattice mismatch, substantially expand the understanding of the interplay between deposition conditions, microstructure, and transport in doped $\mathrm{BaSn}{\mathrm{O}}_{3}$ films, establishing competitive mobilities in films fabricated via a scalable, high-throughput, industry-standard technique.

Structure-property relationships and mobility optimization in sputtered La-doped BaSn O 3 films: Toward 100 c m 2 V − 1 s − 1 mobility

The correlation between the structural, electrical, and luminescence behaviors of La-doped BaSnO3 (LBSO) epitaxial films was intensively studied. We found that Sn2+ defects and oxygen vacancies control the electrical properties of epitaxial LBSO films that are grown on (001)-oriented SrTiO3 substrates using pulsed laser deposition. Under optimized deposition condition, the films exhibit room temperature resistivity of 16 mΩ·cm, with a mobility of 1.62 cm2V−1s−1. To further reduce the resistivity, the films were vacuum-annealed at various temperatures in the range from 600℃ to 900℃ and the film annealed at 600℃ exhibited the lowest room temperature resistivity of 5 mΩ·cm with the highest mobility of 3.09 cm2V−1s−1. The decrease of resistivity in the film vacuum-annealed at 600℃ originates from the higher concentration of Sn2+ ions and oxygen vacancies, which was also confirmed from photoluminescence studies, in which emission peaks associated with Sn2+ defects were observed at 710 and 910 nm. Raman analysis revealed the presence of defect states related to octahedral tilting in vacuum-annealed LBSO films. Our studies show that the electrical properties of epitaxial films could be controlled by the Sn2+ defects generated with oxygen vacancies during the vacuum-annealing of the films. 
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Sungsu Lee

Papers

Structural, electrical, and luminescence characteristics of vacuum-annealed epitaxial (Ba,La)SnO 3 thin films