The exact intrinsic carrier effective mass m * of a well-studied transparent oxide semiconductor BaSnO 3 is unknown because the reported m * values are scattered from 0.06m 0 to 3.7m 0 . This paper identifies the intrinsic m * of BaSnO 3 , m * = 0.40 ± 0.01m 0 , by the thermopower modulation clarification method and determines the threshold of the degenerate/nondegenerate semiconductor. At the threshold, the thermopower values of both the La-doped BaSnO 3 and BaSnO 3 thin-film transistor structures are 240 μV K −1 , the bulk carrier concentration is 1.4 × 10 19 cm −3 , and the two-dimensional sheet carrier concentration is 1.8 × 10 12 cm −2 . When the Fermi energy E F is located above the parabolic shaped conduction band bottom, the mobility is rather high. In contrast, E F below the threshold exhibits a very low carrier mobility, most likely because the tail states suppress the carrier mobility. The present results are useful to further develop BaSnO 3 -based oxide electronics. DOI: 10.1103/PhysRevMaterials.1.034603Transparent oxide semiconductors (TOSs) with a relatively high electrical conductivity and a large band gap (E g > 3.1 eV) are commonly used as transparent electrodes and channel semiconductors for thin-film transistor (TFT) driven flat panel displays (FPDs) such as liquid crystal displays (LCDs) and organic light emitting diodes (OLEDs) [1]. TOS materials include Sn-doped In 2 O 3 (ITO) and InGaZnO 4 -based oxides. Novel TOSs exhibiting higher carrier mobilities have been intensively explored since the TFT performance strongly depends on the carrier mobility of the channel semiconductor. In 2012, Kim et al. reported that a La-doped BaSnO 3 (space group P m3m, cubic perovskite structure, a = 4.115Å, E g ∼ 3.1 eV) single crystal grown by the flux method exhibits a very high mobility (μ Hall ∼ 320 cm 2 V −1 s −1 ) at room temperature [2,3]. This report inspired the current interest in BaSnO 3 films and BaSnO 3 -based TFTs [4][5][6][7][8][9].Since the mobility is expressed as μ = eτ m * −1 , where e, τ , and m * are the electron charge, carrier relaxation time, and carrier effective mass, respectively, the high mobility of the Ladoped BaSnO 3 single crystal should be due to both a small m * and a large τ . Generally, the τ value of epitaxial films is smaller than that of the bulk single crystal due to the fact that the carrier electrons are scattered at dislocations, which originated from the lattice mismatch (δ) and at other structural defects, in addition to optical phonon scattering. The estimated misfit dislocation spacing d is 7.4 nm because δ between BaSnO 3 and SrTiO 3 (a = 3.905Å) is +5.3%. We hypothesized that the experimental values include errors since the substrate contributes to the optical spectra of the BaSnO 3 films. To overcome this difficulty, we use the thermopower S modulation clarification method to determine the intrinsic m * of BaSnO 3 as S clearly reflects the energy derivative of the density of states (DOS) at the Fermi energy E F . This study measures the S of La-doped BaSnO 3 ...

La-doped SrSnO 3 (LSSO) is known as one of deep-ultraviolet (DUV)-transparent conducting oxides with an energy bandgap of ~4.6 eV. Since LSSO can be grown heteroepitaxially on more wide bandgap substrates such as MgO (E g ~7.8 eV), LSSO is considered to be a good candidate as a DUV-transparent electrode. However, the electrical conductivity of LSSO films are below 1000 S cm −1 , most likely due to the low solubility of La ion in the LSSO lattice. Here we report that high electrically conducting (>3000 S cm −1 ) LSSO thin films with an energy bandgap of ~4.6 eV can be fabricated by pulsed laser deposition on MgO substrate followed by a simple annealing in vacuum.From the X-ray diffraction and the scanning transmission electron microscopy analyses, we found that lateral grain growth occurred during the annealing, which improved the

In this study, we report that the carrier mobility of 2%-La-doped BaSnO 3 (LBSO) films on (001) SrTiO 3 and (001) MgO substrates strongly depends on the thickness whereas it is unrelated to the film/substrate lattice mismatch (+5.4 % for SrTiO 3 , −2.3 % for MgO). The films exhibited large differences in the lattice parameters, the lateral grain sizes (~85 nm for SrTiO 3 , ~20 nm for MgO), the surface morphologies, the threading dislocation densities, and the misfit dislocation densities. However, the mobility dependences on the film thickness in both cases were almost the same, saturating at ~100 cm 2 V −1 s −1 while the charge carrier densities approached the nominal carrier concentration (=[2 % La 3+ ]). Our study clearly indicates that the carrier mobility in LBSO films strongly depends on the thickness. These results would be beneficial for understanding the carrier transport properties and fruitful to further enhance the mobility of LBSO films.program funded from following programs of each country: International

shows rather high carrier mobility at room temperature (≈320 cm 2 V −1 s −1 ). [3,4] The crystal structure of BaSnO 3 is cubic perovskite type, ABO 3 with space group of Pm-3m (a = 0.4116 nm), and the A-site can be fully substituted with Sr (SrSnO 3 ), though the crystal structure is orthorhombic perovskite with space group of Pbnm (a = 0.57082 nm, b = 0.57035 nm, c = 0.80659 nm). [5] The bandgap of SrSnO 3 is ≈4.1 eV [2] and the La-doped SrSnO 3 films also show high carrier mobility (40-55 cm 2 V −1 s −1 [6,7] ) at room temperature. Thus, BaSnO 3 -SrSnO 3 solid-solutions (E g = 3.1-4.1 eV) [8] are good candidates to realize large bandgap (≈4 eV) TOS-based TFTs. Very recently, a metal-semiconductor TFT operating as depletion-mode has been reported by Chaganti et al. [9] They used the La-doped SrSnO 3 film as the channel layer. However, the TFT performance of undoped BaSnO 3 -SrSnO 3 solid-solutions, which would be better to fabricate an accumulation-mode TFT, has not been reported thus far probably due to the lack of fundamental knowledge especially the effective thickness (t eff ) and the carrier effective mass (m*), which are essential information to design the TFTs.Although several researchers have reported on the m* of SrSnO 3 , the reported m* values are scattered ranging from 0.14 to 4 m 0 . In 2007, Hadjarab et al. [10] measured the magnetic susceptibility of Sr 0.98 La 0.02 SnO 3−δ ceramic and extracted m* of 4 m 0 . Moreira et al. [11,12] and Liu et al. [13] performed the band structure calculation of SrSnO 3 and BaSnO 3 , and reported much lighter m* values; SrSnO 3 : m* = 0.14-0.23 m 0 , BaSnO 3 : 0.03-0.20 m 0 . Recently, Ong et al. [14] reported that the m* of SrSnO 3 is ≈0.4 m 0 , whereas the calculated effective mass of BaSnO 3 is m* = 0.26 m 0 with the same method. Generally, the m* of n-type TOS, in which the conduction band is composed of ns orbitals, strongly depends on the overlap integral of the neighboring ns orbitals. [15] Further, the overlap integral of ns orbitals would be insensitive to the bond angle. [16] If this assumption is correct, since the interatomic distances of neighboring Sn ions in SrSnO 3 and BaSnO 3 crystals are 0.4035 and 0.4116 nm, respectively, the m* of SrSnO 3 should be lighter than that of BaSnO 3 , which is in opposite relationship with the band calculation results. [11][12][13]

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.