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Register a new userThermopower modulation clarification of the intrinsic effective mass in a transparent oxide semiconductor, BaSnO3
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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.
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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.
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.
To clarify the electronic density of states (DOS) around the conduction band bottom for state of the art transparent amorphous oxide semiconductors (TAOSs), InGaZnO4 and In2MgO4, we fabricated TAOS-based transparent thin film transistors (TTFTs) and measured their gate voltage dependence of thermopower (S). TAOS-based TTFTs exhibit an unusual S behavior. The |S|-value abruptly increases, but then gradually decreases as Vg increases, clearly suggesting the anti-parabolic shaped DOS is hybridized with the original parabolic shaped DOS around the conduction band bottom.
We demonstrate that the interfacial dipole associated with bonding across the SrTiO3/Si heterojunction can be tuned through space charge, thereby enabling the band alignment to be altered via doping. Oxygen impurities in Si act as donors that create space charge by transferring electrons across the interface into SrTiO3. The space charge induces an electric field that modifies the interfacial dipole, thereby tuning the band alignment from type-II to type-III. The transferred charge, resulting in built-in electric fields, and change in band alignment are manifested in electrical transport and hard x-ray photoelectron spectroscopy measurements. Ab initio models reveal the interplay between polarization and band offsets. We find that band offsets can be tuned by modulating the density of space charge across the interface. Functionalizing the interface dipole to enable electrostatic altering of band alignment opens new pathways to realize novel behavior in semiconducting heterojunctions.
A multi-band effective-mass Hamiltonian is derived for lattice-matched semiconductor nanostructures in a slowly varying external magnetic field. The theory is derived from the first-principles magnetic-field coupling Hamiltonian of Pickard and Mauri, which is applicable to nonlocal norm-conserving pseudopotentials in the local density approximation to density functional theory. The pseudopotential of the nanostructure is treated as a perturbation of a bulk reference crystal, with linear and quadratic response terms included in k.p perturbation theory. The resulting Hamiltonian contains several interface terms that have not been included in previous work on nanostructures in a magnetic field. The derivation provides the first direct analytical expressions showing how the coupling of the nonlocal potential to the magnetic field influences the effective magnetic dipole moment of the electron.
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