arXiv subjects

A. Ohtomo

Publications and source records attributed to A. Ohtomo.

13 recordsLinked to original sources

arXiv2023

Optical Properties of LiNbO$_2$ thin films

The complex dielectric functions of LiNbO$_2$ were determined using optical transmittance and reflectance spectroscopies at room temperature. The measured dielectric function spectra reveal distinct structures at several bandgap energies. The bandgaps (exciton resonances) in the spectrum were observed at ca. 2.3, 3.2, 3.9, and 5.1 eV, respectively. These experimental data have been fit using a model dielectric function based on the electronic energy-band structure near critical points plus excitonic effects. The features of measured dielectric functions are, to some extent, reproduced quantitatively by an ab-initio calculation including the interaction effects between electrons and holes.

cond-mat.mtrl-sci↗

arXiv2022

Modulation of magnetic property of double-perovskite La2MnCoO6 films by controlling B-antisite disorder

The predicted physical properties of double perovskites have usually been compromised by their intrinsic B-antisite (AS) disorder effect, but the relationship between them is still unclear for the La2MnCoO6 (LMCO) system. This study focuses on controlling the AS disorder and quantitatively reveals correlations with magnetic and electronic states in epitaxial LMCO films grown on (111) SrTiO3 substrates. The AS fraction was precisely controlled by tuning the growth conditions as evaluated from saturation magnetization and x-ray reflection profiles. The saturation magnetization at 5 K decreased linearly from 6 uB per formula unit by 50% as the AS fraction increased from 0 (perfectly ordered) to 0.5 (fully disordered). The x-ray absorption spectroscopy revealed that valence states of Mn and Co were 4+ (3d^3) and 2+ (3d^7), respectively, regardless of AS fraction. A local-spin-moment model is proposed to explain the net magnetizations with and without AS disorder and Mn/Co valence states, suggesting that only the rearrangement of local spin moment of Co at the antisite plays a key role in tuning the magnetism in this LMCO system.

cond-mat.mtrl-sci↗

arXiv2016

Superconductivity in higher titanium oxides

Recent renewal of the highest transition temperature in a conventional superconductor of the sulfer hydride attracts much attention to exploring simple compounds with the lighter elements, situated in unconventional conditions. We report the discovery of superconductivity in simple oxides of Ti4O7 and g-Ti3O5 in a thin-film form having deliberately tuned epitaxial structures and off-stoichiometry. These higher titanium oxides join in a class of simple-oxide superconductors, and g-Ti3O5 now holds the highest superconducting transition temperature of 7.1 kelvin among them. The mechanism behind the superconductivity is discussed on the basis of electrical measurements and theoretical predictions. We conclude that superconductivity arises from unstabilized bipolaronic insulating states.

cond-mat.mtrl-sci↗

arXiv2011

Ferrimagnetism and spontaneous ordering of transition-metals in La2CrFeO6 double-perovskite films

We report on atomic ordering of B-site transition-metals and magnetic properties of epitaxial La2CrFeO6 double-perovskite films grown by pulsed-laser deposition under various conditions. The highest ordered sample exhibited a fraction of antisite-disorder of only 0.05 and a saturation magnetization of ~2μ_{B} per formula unit at 5 K. The result is consistent with the antiferromagnetic ordering of local spin moment (3d^{3}_{\downarrow}3d^{5}_{\uparrow}; S = -3/2+5/2 = 1). Therefore, the magnetic ground state of La2CrFeO6 double-perovskite that has been long debate is unambiguously revealed to be ferrimagnetic. Our results present a wide opportunity to explore novel magnetic properties of binary transition-metal perovskites upon epitaxial stabilization of the ordered phase.

cond-mat.mtrl-sci↗

arXiv2008

Spin susceptibility and effective mass of two-dimensional electrons in MgxZn1-xO/ZnO heterostructures

We report measurements of the spin susceptibility and the electron effective mass for two-dimensional electrons confined at the interfaces of MgxZn1-xO/ZnO single heterostructures (x = 0.05, 0.08, and 0.11), grown by molecular-beam epitaxy on (0001) ZnO substrates. By tuning the built-in polarization through control of the barrier composition, the electron density was systematically varied in the range of 5.6 x 10^11 to 1.6 x 10^12 cm^-2, corresponding to a range of 3.1 < rs < 5.2, where rs is the average electron spacing measured in units of the effective Bohr radius. We used the coincidence technique, where crossings of the spin-split Landau levels occur at critical tilt angles of magnetic field, to evaluate the spin susceptibility. In addition, we determined the effective mass from the temperature dependence of the Shubnikov-de Haas oscillations measured at the coincidence conditions. The susceptibility and the effective mass both gradually increase with decreasing electron density, reflecting the role of electron-electron interaction.

cond-mat.mtrl-sci↗

arXiv2006

Shifting donor-acceptor photoluminescence in N-doped ZnO

We have grown nitrogen-doped ZnO films grown by two kinds of epitaxial methods on lattice-matched ScAlMgO$_4$ substrates. We measured the photoluminescence (PL) of the two kinds of ZnO:N layers in the donor-acceptor-pair transition region. The analysis of excitation-intensity dependence of the PL peak shift with a fluctuation model has proven that our observed growth-technique dependence was explained in terms of the inhomogeneity of charged impurity distribution. It was found that the inhomogeneity in the sample prepared with the process showing better electrical property was significantly smaller in spite of the similar nitrogen concentration. The activation energy of acceptor has been evaluated to be $\approx 170$ meV, which is independent of the nitrogen concentration.

cond-mat.mtrl-sci↗

arXiv2006

Monte-Carlo simulation of localization dynamics of excitons in ZnO and CdZnO quantum well structures

Localization dynamics of excitons was studied for ZnO/MgZnO and CdZnO/MgZnO quantum wells (QW). The experimental photoluminescence (PL) and absorption data were compared with the results of Monte Carlo simulation in which the excitonic hopping was modeled. The temperature-dependent PL linewidth and Stokes shift were found to be in a qualitatively reasonable agreement with the hopping model, with accounting for an additional inhomogeneous broadening for the case of linewidth. The density of localized states used in the simulation for the CdZnO QW was consistent with the absorption spectrum taken at 5 K.

cond-mat.mtrl-sci↗

arXiv2005

Majority-Carrier Mobilities in Undoped and \textit{n}-type Doped ZnO Epitaxial Layers

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 $n_s \sim 5\times$ 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.

cond-mat.mtrl-sci↗

arXiv2005

Analysis on reflection spectra in strained ZnO thin films

Thin films of laser molecular-beam epitaxy grown ZnO films were studied with respect to their optical properties. 4-K reflectivity was used to analyze various samples grown at different biaxial in-plane strain. The spectra show two structures at 3.37 eV corresponding to the A-free exciton transition and at 3.38 eV corresponding to the B-free exciton transition. Theoretical reflectivity spectra were calculated using the spatial dispersion model. Thus, the transverse energies, the longitudinal transversal splitting (ELT,), the oscillator strengths, and the damping parameters were determined for both the A- and B-free excitons of ZnO. As a rough trend, the strain dependence of the energy E_LT for the A-excitons is characterized by a negatively-peaking behavior with a minimum around the zero strain, while ELT for the B-excitons is an increasing function of the strain field values.

cond-mat.mtrl-sci↗

arXiv2005

Spectral shape analysis of ultraviolet luminescence in \textit{n}-type ZnO:Ga

Thin films of laser molecular-beam epitaxy grown \textit{n}-type Ga-doped ZnO were investigated with respect to their optical properties. Intense room-temperature photoluminescence (PL) in the near-band edge (NBE) region was observed. Moreover, its broadening of PL band was significantly larger than predicted by theoretical results modeled in terms of potential fluctuations caused by the random distribution of donor impurities. In addition, the lineshape was rather asymmetrical. To explain these features of the NBE bands, a vibronic model was developed accounting for contributions from a series of phonon replicas.

cond-mat.mtrl-sci↗

arXiv2005

Electron transport in ZnO thin films

Epitaxial, n-type ZnO films grown by a laser molecular-beam epitaxy method 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. Transport characteristics are calculated by solving the Boltzmann transport equation using a variational method. Mobility limit of 430 cm$^{2}$/Vs was calculated at 300 K. The temperature dependence of the mobility for an undoped film is calculated and agrees favorably well with experimental data if physical parameters are chosen so as to approach to those. In the experimental `mobility vs concentration' curve, unusual phenomenon was observed, i.e., mobilities at $n_s \sim 5\times$ 10$^{18}$ cm$^{-3}$ are significantly smaller than those at higher densities above $\sim 10^{20}$ cm$^{-3}$. It is qualitatively explained in terms of electron-plasmon interaction.

cond-mat.mtrl-sci↗

arXiv2004

Gallium concentration dependence of room-temperature near-bandedge luminescence in n-type ZnO:Ga

We investigated the optical properties of epitaxial \textit{n}-type ZnO films grown on lattice-matched ScAlMgO$_4$ substrates. As the Ga doping concentration increased up to $6 \times 10^{20}$ cm$^{-3}$, the absorption edge showed a systematic blueshift, consistent with the Burstein-Moss effect. A bright near-bandedge photoluminescence (PL) could be observed even at room temperature, the intensity of which increased monotonically as the doping concentration was increased except for the highest doping level. It indicates that nonradiative transitions dominate at a low doping density. Both a Stokes shift and broadening in the PL band are monotonically increasing functions of donor concentration, which was explained in terms of potential fluctuations caused by the random distribution of donor impurities.

cond-mat.mtrl-sci↗

arXiv2003

Modeling and simulation of polycrystalline ZnO thin-film transistors

Thin film transistors (TFTs) made of transparent channel semiconductors such as ZnO are of great technological importance, because their insensitivity to visible light makes device structures simple. In fact, several demonstrations are made on ZnO TFT achieving reasonably good field effect mobilities of 1-10 cm2/Vs, but reveal insufficient device performances probably due to the presence of dense grain boundaries. We have modeled grain boundaries in ZnO thin film transistors (TFTs) and performed device simulation using a two-dimensional device simulator for understanding the grain boundary effects on the device performance. Actual polycrystalline ZnO TFT modeling is commenced with considering a single grain boundary in the middle of the TFT channel formulating with a Gaussian defect distribution localized in the grain boundary. A double Shottky barrier is formed in the grain boundary and its barrier height are analyzed as functions of defect density and gate bias. The simulation is extended to the TFTs with many grain boundaries to quantitatively analyze the potential profiles developed along the channel. One of the big contrasts of polycrystalline ZnO TFT compared with a polycrystalline Si TFT is that much smaller nanoscaled grain size induces heavy overlap of double Shottky barriers. Through the simulation, we can estimate the total trap state density localized in the grain boundaries for a polycrystalline ZnO by knowing apparent mobility and grain size in the device.

cond-mat↗