Tatsuo Schimizu

The effect of strain on the permittivity of SrTiO3 from first-principles study

Abstract The frequencies of Γ transverse-optical phonons in the cubic perovskite phase of SrTiO3 were investigated within the frozen-phonon scheme using the first-principles method with ultrasoft pseudopotentials. The calculated frequencies in the cubic phase showed good agreement with the experimental data. Strain was found to have a strong influence on the frequency of the lowest frequency mode. The atomic motion of the lowest frequency mode mainly consisted of the Slater mode, but the Last mode, in which the strontium atom shows an out-of-phase vibration relative to the rigid TiO3 cluster, was found to be very important. At T = 0, the reciprocal of the permittivity was approximately in proportion to strain x (the difference of the lattice constant from the theoretically determined lattice constant). The calculated permittivity ϵ strongly depends on strain x at finite temperatures T; ϵ = C(x) [T − T 0 (x)] , where C(x) ≈ 9.0 × 10 4 (1 – 23.7x) and T0(x)≈38.0 + 1.53x.

Lattice deformation and magnetic properties in epitaxial thin films of Sr1−xBaxRuO3

Crystal structure and magnetic properties in epitaxially grown Sr1−xBaxRuO3 on SrTiO3 substrates were determined. Epitaxial Sr1−xBaxRuO3 exhibits a simple perovskite structure in the whole region of the Ba/Sr ratio, in contrast to the complex hexagonal layered perovskite of Ba-rich bulk Sr1−xBaxRuO3, which has plane-sharing oxygen octahedra. Tetragonal deformation was enhanced from pseudocubic in SrRuO3 to a highly distorted tetragonal lattice in BaRuO3. Electronic properties such as conductivity and magnetization were examined. A metal–insulator transition was not observed in this system, and metallic conductivity was maintained in the whole region of Ba concentration. Ferromagnetic ordering at 160 K seen in bulk SrRuO3 was observed to be suppressed in the Sr1−xBaxRuO3 films with increasing tetragonal deformation and Curie temperatures decreased to 50 K in BaRuO3.

Investigation of stability of the effective work function on LaAlO3 and La2Hf2O7

The stability of the effective work function (ϕeff) on La-based high-k materials was studied in detail by changing the annealing ambient and the gate dielectric stack. ϕeff for a LaAlO3/SiO2/Si stack with Pt gate electrode was not affected by the annealing ambient, whereas that for a Pt gate electrode on an La2Hf2O7/SiO2/Si stack increased sharply when O2 annealing was performed after forming gas annealing (FGA). Comparison with the results for a stack without SiO2 indicates that this anomalous phenomenon in the La2Hf2O7/SiO2/Si stack is caused by oxygen-vacancy-related dipoles at La2Hf2O7/SiO2 interface produced by FGA.

Band offset design with quantum-well gate insulating structures

The authors propose a concept, a nanoscaled quantum-well gate insulating structure. The effective conduction band offset (ΔEc) can be controlled with an appropriate combination of high-K and high-ΔEc materials. The electronic structures of SrTiO3 and Sr2TiO4 were studied by means of first-principles calculations to investigate the change in the band structures induced by SrO-layer intercalation. The ΔEc of Sr2TiO4 is raised by about +0.8eV. A quantum-well gate insulating structure with off-resonance condition is also proposed. The ΔEc becomes as high as the barrier height of the barrier material.

4H-SiC Surface Structures and Oxidation Mechanism Revealed by Using First-Principles and Classical Molecular Dynamics Simulations

The 4H-SiC(000-1)C and (0001)Si surface reconstructed structures and the oxidation processes of these surfaces are investigated using a first-principles calculation method. The most stable reconstructed 4H-SiC(000-1)C and (0001)Si surfaces have p-bonded chains. In the topmost SiC bilayer, half of Si and C atoms exchange their positions and C-C or Si-Si bonds formed densely below the surfaces. When we place a SiO2 layer on the p-bonded chain (000-1)C surface, C-C bonds are formed more densely below the interface. We simulate a sequence of O2 molecules arrivals at an interface of tridymite-phase-SiO2 and 4H-SiC(000-1)C. Dissociated O atoms at the interface tended to make bonds with Si atoms. The C-C bonds in the SiC substrate break easily and a local C surface occasionally appears. We have examined how the surface structure changes through an O2 molecule exposure by using a classical molecular dynamics simulation program and confirmed the formation of C-C bonds below the surface and the interface.

Novel Gate Insulator Process by Nitrogen Annealing for Si-Face SiC MOSFET with High-Mobility and High-Reliability

The gate insulator process for SiC-MOSFET was examined and high-quality interface was realized by employing the pre-annealing process before high-temperature N2 annealing. The pre-annealing evidently activated the interface to introduce nitrogen, and then field-effect mobility exceeded 50 cm2/Vs. The fabricated sample also demonstrated superior bias temperature instability (BTI) and excellent breakdown electric field of 11.7 MV/cm.

Hybrid density functional analysis of distribution of carbon-related defect levels at 4H-SiC(0001)/SiO2 interface

Silicon carbide (SiC)-based metal–oxide–semiconductor devices suffer from carrier mobility degradation due to defects at the SiC/SiO2 interface. The carbon-related defects Si2>C=O and Si2>C=C<Si2 are thought to be possible origins of the defect levels. We studied the electronic structures of 4H-SiC(0001)/SiO2 interfaces with C=O and C=C defects by first-principles calculations based on hybrid density functional theory. We found that their defect levels are strongly affected by the local structures around the C=O and C=C groups, and the defect levels form a broad state distribution.

Control of Electronic Properties of High‐k Gate Oxides with Doping from First‐principles Calculation

We investigate the microscopic mechanisms not only of crystallization inhibition but also of band gap narrowing (BGN), focusing on an experimental range of nitrogen (N) concentrations, in order to estimate the real potentiality of N doping, and to select much more efficient dopants than N. Oxygen vacancy, Vo, appearance mechanism with N doping is the exothermic reaction. The binding energy of NsVoNs is too small to fix the Vo near the doped Ns site. Because Vo can move easily, total energy is lowered by crystallization around Vo. A large amount of N is needed to be in amorphous phase. However, a large amount of N induces the conduction band (CB) lowering owing to Vo‐Vo interaction. The most important point is that Vo should be strongly fixed around the dopants to be in amorphous phase without the CB lowering. Binding energy of BaVo is estimated to be more than six times as large as that of NsVoNs. Moreover, Ba‐doping exhibts no BGN. We propose that Ba should be one of the most efficient dopants.

Lattice deformation and magnetic properties in epitaxial thin films of Sr 1-x Ba x RuO 3

The crystal structure and magnetic properties in epitaxially grown Sr 1-x Ba x RuO 3 on SrTiO 3 substrates were determined. Epitaxial Sr 1-x Ba x RuO 3 exhibits a simple perovskite structure in the whole region of Ba/Sr ratio, in contrast to a complex hexagonal layered perovskite in the case of Ba-rich bulk Sr 1-x Ba x RuO 3 which has plane-sharing oxygen octahedra. Tetragonal deformation was enhanced from the pseudo cubic structure of SrRuO 3 to a highly distorted tetragonal lattice in BaRu0 3 . Electronic properties such as conductivity and magnetization were examined and compared to the results of band calculation in which a tetragonal distortion was taken into account. A metal-insulator transition was not observed in this system either in the experiment or in the simulation, and metallic conductivity was maintained in the whole region of Ba content. The ferromagnetic ordering which occurs at 160K in pseudo-cubic bulk SrRuO 3 was suppressed in Sr 1-x Ba x RuO 3 films with increasing tetragonal deformation and Curie temperature decreased to 50K in BaRuO 3 .