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Register a new userLarge thickness dependence of the carrier mobility in a transparent oxide semiconductor, La-doped BaSnO3
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We report herein that the carrier mobility of the 2%-La-doped BaSnO3 (LBSO) films on (001) SrTiO3 and (001) MgO substrates strongly depends on the thickness whereas it is unrelated to the lattice mismatch (+5.4% for SrTiO3, -2.3% for MgO). Although we observed large differences in the lattice parameters, the lateral grain size (~85 nm for SrTiO3, ~20 nm for MgO), the surface morphology and the density of misfit dislocations, the mobility increased almost simultaneously with the thickness in both cases and saturated at ~100 cm2 V-1 s-1, together with the approaching to the nominal carrier concentration (=[2% La3+]), clearly indicating that the behavior of mobility depends on the film thickness. The present results would be beneficial to understand the behavior of mobility and fruitful to further enhance the mobility of LBSO films.
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Transparent oxide semiconductors (TOSs) showing both high visible transparency and high electron mobility have attracted great attention towards the realization of advanced optoelectronic devices. La-doped BaSnO3 (LBSO) is one of the most promising TOSs because its single crystal exhibits a high electron mobility. However, in the LBSO films, it is very hard to obtain high mobility due to the threading dislocations, which are originated from the lattice mismatch between the film and the substrate. Therefore, many researchers have tried to improve the mobility by inserting a buffer layer. While the buffer layers increased the electron mobilities, this approach leaves much to be desired since it involves a two-step film fabrication process and the enhanced mobility values are still significantly lower than single crystal values. We show herein that the electron mobility of LBSO films can be improved without inserting any buffer layers if the films are grown under highly oxidative ozone (O3) atmospheres. The O3 environments relaxed the LBSO lattice and reduced the formation of Sn2+ states, which are known to suppress the electron mobility in LBSO. The resultant O3-LBSO films showed improved mobility values up to 115 cm2 V-1 s-1, which is among the highest in LBSO films on SrTiO3 substrates and comparable to LBSO films with buffer layers.
Although there are so many reports on the carrier effective mass (m*) of a transparent oxide semiconductor BaSnO3, it is almost impossible to know the intrinsic m* value because the reported m* values are scattered from 0.06 to 3.7 m0. Here we successfully clarified the intrinsic m* of BaSnO3, m*=0.40 0.01 m0, by the thermopower modulation clarification method. We also found the threshold of degenerate/non-degenerate semiconductor of BaSnO3; At the threshold, the thermopower value of both La-doped BaSnO3 and BaSnO3 TFT structure was 240 microvolt k-1, bulk carrier concentration was 1.4E19 cm-3, and two-dimensional sheet carrier concentration was 1.8E12 cm-2. When the EF locates above the parabolic shaped conduction band bottom, rather high mobility was observed. On the contrary, very low carrier mobility was observed when the EF lays below the threshold, most likely due to that the tail states suppress the carrier mobility. The present results are useful for further development of BaSnO3 based oxide electronics.
La-doped SrSnO3 (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 (Eg ~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 activation rate of La ion, leading to a significant improvement of carrier concentration (3.26 x 10^20 cm^-3) and Hall mobility (55.8 cm^2 V^-1 s^-1). The present DUV-transparent oxide semiconductor would be useful as a transparent electrode for developing optoelectronic devices, which transmit and/or emit DUV-light.
BaSnO_{3}, a high mobility perovskite oxide, is an attractive material for oxide-based electronic devices. However, in addition to low-field mobility, high-field transport properties such as the saturation velocity of carriers play a major role in determining device performance. We report on the experimental measurement of electron saturation velocity in La-doped BaSnO_{3} thin films for a range of doping densities. Predicted saturation velocities based on a simple LO-phonon emission model using an effective LO phonon energy of 120 meV show good agreement with measurements of velocity saturation in La-doped BaSnO_{3} films.. Density-dependent saturation velocity in the range of 1.6x10^{7} cm/s reducing to 2x10^{6} cm/s is predicted for {delta}-doped BaSnO3 channels with carrier densities ranging from 10^{13} cm^{-2} to 2x10^{14} cm^{-2} respectively. These results are expected to aid the informed design of BaSnO3 as the active material for high-charge density electronic transistors.
As a unique perovskite transparent oxide semiconductor, high-mobility La-doped BaSnO3 films have been successfully synthesized by molecular beam epitaxy and pulsed laser deposition. However, it remains a big challenge for magnetron sputtering, a widely applied technique suitable for large-scale fabrication, to grow high-mobility La-doped BaSnO3 films. Here, we developed a method to synthesize high-mobility epitaxial La-doped BaSnO3 films (mobility up to 121 cm2V-1s-1 at the carrier density ~ 4.0 x 10^20 cm-3 at room temperature) directly on SrTiO3 single crystal substrates using high-pressure magnetron sputtering. The structural and electrical properties of the La-doped BaSnO3 films were characterized by combined high-resolution X-ray diffraction, X-ray photoemission spectroscopy, and temperature-dependent electrical transport measurements. The room temperature electron mobility of La-doped BaSnO3 films in this work is 2 to 4 times higher than the reported values of the films grown by magnetron sputtering. Moreover, in the high carrier density range (n > 3 x 10^20 cm-3), the electron mobility value of 121 cm2V-1s-1 in our work is among the highest values for all reported doped BaSnO3 films. It is revealed that high argon pressure during sputtering plays a vital role in stabilizing the fully relaxed films and inducing oxygen vacancies, which benefit the high mobility at room temperature. Our work provides an easy and economical way to massively synthesize high-mobility transparent conducting films for transparent electronics.
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