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Register a new userVisible light carrier generation in co-doped epitaxial titanate films
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Perovskite titanates such as SrTiO$_{3}$ (STO) exhibit a wide range of important functional properties, including high electron mobility, ferroelectricity, and excellent photocatalytic performance. The wide optical band gap of titanates limits their use in these applications, however, making them ill-suited for integration into solar energy harvesting technologies. Our recent work has shown that by doping STO with equal concentrations of La and Cr we can enhance visible light absorption in epitaxial thin films while avoiding any compensating defects. In this work, we explore the optical properties of photoexcited carriers in these films. Using spectroscopic ellipsometry, we show that the Cr$^{3+}$ dopants, which produce electronic states immediately above the top of the O 2p valence band in STO reduce the direct band gap of the material from 3.75 eV to between 2.4 and 2.7 eV depending on doping levels. Transient reflectance spectroscopy measurements are in agreement with the observations from ellipsometry and confirm that optically generated carriers are present for longer than 2 ns. Finally, through photoelectrochemical methylene blue degradation measurements, we show that these co-doped films exhibit enhanced visible light photocatalysis when compared to pure STO.
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Doping ferroelectric Hf0.5Zr0.5O2 with La is a promising route to improve endurance. However, the beneficial effect of La on the endurance of polycrystalline films may be accompanied by degradation of the retention. We have investigated the endurance - retention dilemma in La-doped epitaxial films. Compared to undoped epitaxial films, large values of polarization are obtained in a wider thickness range, whereas the coercive fields are similar, and the leakage current is substantially reduced. Compared to polycrystalline La-doped films, epitaxial La-doped films show more fatigue but there is not significant wake-up effect and endurance-retention dilemma. The persistent wake-up effect common to polycrystalline La-doped Hf0.5Zr0.5O2 films, is limited to a few cycles in epitaxial films. Despite fatigue, endurance in epitaxial La-doped films is more than 1010 cycles, and this good property is accompanied by excellent retention of more than 10 years. These results demonstrate that wake-up effect and endurance-retention dilemma are not intrinsic in La-doped Hf0.5Zr0.5O2.
Transparent and conductive ZnO:Ga thin films are prepared by laser molecular-beam epitaxy. Their electron properties were investigated by the temperature-dependent Hall-effect technique. The 300-K carrier concentration and mobility were about $n_s sim 10^{16}$ cm$^{-3}$ and 440 cm$^{2}$/Vs, respectively. In the experimental `mobility vs concentration curve, unusual phenomenon was observed, i.e., mobilities at $n_s sim 5times$ 10$^{18}$ cm$^{-3}$ are significantly smaller than those at higher densities above $sim 10^{20}$ cm$^{-3}$. Several types of scattering centers including ionized donors and oxygen traps are considered to account for the observed dependence of the Hall mobility on carrier concentration. The scattering mechanism is explained in terms of inter-grain potential barriers and charged impurities. A comparison between theoretical results and experimental data is made.
Room-temperature metallicity of lightly doped SrTiO$_3$ is puzzling, because the combination of mobility and the effective mass would imply a mean-free-path (mfp) below the Mott Ioffe Regel (MIR) limit and a scattering time shorter than the Planckian time ($tau_P=hbar/k_BT$). We present a study of electric resistivity, Seebeck coefficient and inelastic neutron scattering extended to very high temperatures, which deepens the puzzle. Metallic resistivity persists up to 900 K and is accompanied by a large Seebeck coefficient whose magnitude (as well as its temperature and doping dependence) indicates that carriers are becoming heavier with rising temperature. Combining this with neutron scattering data, we find that between 500 K and 900 K, the Bohr radius and the electron wave-length become comparable to each other and twice the lattice parameter. According to our results, between 100 K and 500 K, metallicity is partially driven by temperature-induced amplification of the carrier mass. We contrast this mass amplification of non-degenerate electrons with the better-known case of heavy degenerate electrons. Above 500 K, the mean-free-path continues to shrink with warming in spite of becoming shorter than both the interatomic distance and the thermal wavelength of the electrons. The latter saturates to twice the lattice parameter. Available theories of polaronic quasi-particles do not provide satisfactory explanation for our observations.
SrTiO$_3$ is a promising $n$-type oxide semiconductor for thermoelectric energy conversion. Epitaxial thin films of SrTiO$_3$ doped with both La and oxygen vacancies have been synthesized by pulsed laser deposition (PLD). The thermoelectric and galvanomagnetic properties of these films have been characterized at temperatures ranging from 300 K to 900 K and are typical of a doped semiconductor. Thermopower values of double-doped films are comparable to previous studies of La doped single crystals at similar carrier concentrations. The highest thermoelectric figure of merit ($ZT$) was measured to be 0.28 at 873 K at a carrier concentration of $2.5times10^{21}$ cm$^{-3}$.
The combination of ferromagnetism and semiconducting behavior offers an avenue for realizing novel spintronics and spin-enhanced thermoelectrics. Here we demonstrate the synthesis of doped and nanocomposite half Heusler Fe$_{1+x}$VSb films by molecular beam epitaxy. For dilute excess Fe ($x < 0.1$), we observe a decrease in the Hall electron concentration and no secondary phases in X-ray diffraction, consistent with Fe doping into FeVSb. Magnetotransport measurements suggest weak ferromagnetism that onsets at a temperature of $T_{c} approx$ 5K. For higher Fe content ($x > 0.1$), ferromagnetic Fe nanostructures precipitate from the semiconducting FeVSb matrix. The Fe/FeVSb interfaces are epitaxial, as observed by transmission electron microscopy and X-ray diffraction. Magnetotransport measurements suggest proximity-induced magnetism in the FeVSb, from the Fe/FeVSb interfaces, at an onset temperature of $T_{c} approx$ 20K.
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