Combined Spectroscopy and Electrical Characterization of La: BaSnO3 Thin Films a
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 content. 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. Our results expand the current understanding of the interplay between the doping concentration, electronic band structure, and transport properties of epitaxial La: BaSnO3 films.
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VOLUME 01, ISSUE 01. DOI-
The perovskite La-doped BaSnO3 (La: BaSnO3) is a novel transparent oxide semiconductor that exhibits outstanding room temperature (RT) electron mobility (µe) with high carrier density together with a high optical transmittance [1–3]. Owing to its unique electronic and optical properties, La: BaSnO3 has the potential for applications in transparent electronics [4–7], photovoltaics [8–11], as well as in thermoelectric [12–15], and multifunctional perovskite-based optoelectronic devices [10, 16, 17]. Furthermore, its low-power consumption combined with its ability to be heavily doped and its good stability at high temperatures make La: BaSnO3 a suitable material for integration in thermally stable capacitors, field-effect transistors, and power electronic devices [3, 4, 17–19]. The discovery of an RT µe of 320 cm2 V −1 s −1 (with corresponding carrier density, n = 8 × 1019 cm−3 ) in La: BaSnO3 single crystals [1–3] stimulated intense investigation into this material . Particularly, the potential of La: BaSnO3 for device applications and heterostructures triggered considerable interest in thin films grown from this compound [5–7, 15, 17, 19–38]. However, the reported µe in La: BaSnO3 thin films have only reached a maximum value of 183 cm2 V −1 s −1 (n ' 1.2× 1020 cm−3 ) for epitaxial films grown by molecular beam epitaxy (MBE) . Other growth techniques resulted in the following electron mobilities: 140 cm2 V −1 s −1 (n ' 5.2 × 1020 cm−3 ) for pulsed laser deposition (PLD) , 121 cm2 V −1 s −1 (n ' 4.0×1020 cm−3 ) for high-pressure magnetron sputtering , and 53 cm2 V −1 s −1 (n ' 2.0 × 1020 cm−3 ) for chemical solution deposition . Various strategies to improve mobility in La: BaSnO3 epitaxial films have been explored. Such efforts include, for example, the incorporation of undoped BaSnO3 buffer layers to compensate for the lattice mismatch between the substrate and the active La: BaSnO3 top layers [7, 19, 26, 33], adsorption-controlled MBE for improved stoichiometry control [31–33, 40], very high temperature grown insulating buffer layer to reduce the density of threading dislocations , and post-growth annealing processes [22, 24, 41]. Besides the ongoing efforts for RT µe improvement, to gain a better understanding of the conduction mechanisms in La: BaSnO3 films, it is important to establish a proper correlation between the transport characteristics and the behaviour of the electronic states in the conduction band. This is crucial because the high ambient µe in La: BaSnO3 has been proposed to originate from both the small effective mass of the electrons at the conduction band minimum (CBM) [25, 42], which is associated with the largely dispersive Sn-5s conduction band and the low optical phonon scattering rate [19, 43]. Although several studies used photoemission spectroscopy techniques to investigate the electronic structure of La: BaSnO3 films [32, 43–46], only a few reports have combined electronic transport and spectroscopic studies to explore the evolution of electronic states in La: BaSnO3 films and heterostructures at different Ladoping levels [32, 43]. In particular, recent ex-situ hard x-ray photoemission spectroscopy (HAXPES) experiments on La: BaSnO3 films demonstrated that both the CBM and the valence band maximum (VBM), as well as the core electrons, are effectively modified with increasing carrier density . Thus, this result calls for additional combined spectroscopic and electrical characterizations to facilitate a more quantitative exploration of the evolution of the intrinsic properties of La: BaSnO3 films and heterostructures at different doping levels.
In summary, we have systematically investigated the evolution of electronic states in the band structure of
La: BaSnO3 films at different La doping levels. A close connection between the transport and the spectroscopic characteristics is demonstrated. In particular, increasing the carrier concentration in the conduction band by doping is observed to significantly affect the core and valence band spectra. The Sn 3d core line shape presents a pronounced asymmetry variation with the carrier density and is fitted following the plasmon model applicable to metallic systems. Scans around the valence band spectra allowed the detection of the occupied states in the conduction bands. It is determined that surface contamination could potentially induce surface carrier accumulation, supported by the increase in the intensity of the CBM detected on the surface exposed to contamination. This study presents a detailed characterization of the chemical composition of the near-surface region of La: BaSnO3, and it provides a better picture of the interplay between the doping concentration, electronic band structure, and transport properties of epitaxial La: BaSnO3 films.
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 The La:BaSnO3 films were prepared at the Max Planck Institute for Solid State Research, Stuttgart, Germany.