Papers by Keyword: Density Functional Theory (DFT)

Abstract: In this work, quenching effect in the photoluminescence (PL) spectrum of divacancy defects in 4H SiC is investigated. Quenching in PL occurs when photoexcitation with an energy below a certain threshold is applied. In order to understand this phenomenon, we carried out Kohn-Sham density functional theory (DFT) calculations. In accordance with recent experimental results, we propose a physical model which explains the quenching phenomenon.

Abstract: Electron energy loss spectroscopy (EELS) and ab initio simulations are combined in this study to produce an atomistic interpretation of the interface morphology in lateral 4H-SiC / SiO₂ MOSFETs with deposited gate oxides. This allows the question of interface abruptness, and the presence the postulated SiO_xC_y interlayer to be explored for a subset of devices with deposited oxides. From comparison between EELS and ab initio calculation the interfaces considered are best described as abrupt, but stepped, transitioning without any of the carbon excess or SiO_xC_y interlayer that have been described for other devices observed.

Abstract: Experimental data indicate that carbon vacancies incorporated in active regions of SiC devices are important electrical defects, responsible for device limiting effects such as carrier lifetime reduction. For field-effect transistors that include a 4H-SiC/SiO2 interface, such as at the gate, the oxidation pro- cess is understood to introduce native defects to the SiC, including injection of carbon self-interstitials and vacancies, that diffuse into the active layer and interact with other defects and impurities. It is therefore important to understand the migration behaviour of primary native defects such as VC in the vicinity of 4H-SiC/SiO2 interfaces. We report here the results of a density-functional theory investi- gation into the diffusion of the carbon vacancy in such a region. We conclude that the migration of VC is significantly hindered in the immediate vicinity of the interface, with the energy of diffusion barrier being approximately 15% greater than the corresponding diffusion in bulk 4H-SiC.

Abstract: Recently, it has become indispensable to use semiconducting nanostructures in the production and development of electronic devices. In this study, the bulk and nanowire heterostructures of the BP / GaN system have been investigated for the structures pure and Te atom doped. In calculations, the plane wave self-consistent field program based on density functional theory was used. The average potentials of the aforementioned systems have been calculated and the interface effect has investigated.

Abstract: Recent studies identified some factors that contribute to the enhancement of T_c in monolayer FeSe/STO superconductor. It has been claimed that electron doping and electron-phonon coupling play a crucial role in high-T_c superconductivity. However, electron doping and electron-phonon mechanism alone cannot fully explain the high-T_c of monolayer FeSe/STO. In this study, we introduce another factor, the Hubbard U correction, and investigate its effect. The electronic structure calculations on single-layer FeSe grown on STO using density functional theory with Hubbard U (DFT+U) is presented. It is found that the Hubbard U suppresses the hole-like band at the Brillouin zone center leading to an electronic structure that resembles the experimental ARPES data. This suggests that electron correlation in monolayer FeSe/STO system plays a crucial role in the origin of high-T_c superconductivity.

Abstract: The structural, electronic, and optical properties of an amorphous SiO₂ (a-SiO₂) model is investigated by using first-principles calculation. Most research works used beta-cristobalite glass structure as a reference to amorphous silica structure. However, only the electronic properties were been presented without any link towards the optical properties. Here, we demonstrate simultaneous electronic and optical properties, which closely matched to a-SiO₂ properties by generating small sample of amorphous quartz glass. Using the Rietveld refinement, amorphous silica structure was generated and optimized using density functional theory in CASTEP computer code. A thorough analysis of the amorphous quartz structure obtained from different thermal treatment was carried out. The structure of amorphous silica was validated with previous theoretical and experimental works. It is shown that small sample of amorphous silica have similar structural, electronic and optical properties with a larger sample. The calculated optical and electronic properties from the a-SiO₂ glass match closely to previous theoretical and experimental data from others. The a-SiO₂ band gap of 5.853 eV is found to be smaller than the experimental value of ~9 eV. This is due to the underestimation and assumption made in DFT. However, the band gap value is in good agreement with the other theoretical works. Apart from the absorption edge at around 6.5 eV, the refractive index is 1.5 at 0eV. Therefore, this atomic structure can served as a reference model for future research works on amorphous structures.

Abstract: A model of undefected and defected amorphous SiO₂ has been constructed from Rietveld Refinement to investigate the effect of defect structure to its properties. Atomic level study for both structure were carried out using plane-wave pseudo potential by density functional theory. A new electrons trapping energy level appears within the 5.853eV band gap of a-SiO₂ for oxygen-excess defected structure. This defect energy level reduces as more number of excess oxygen atoms was added to the structure of a-SiO₂. A spectral emission at 388nm from SiO₂ glass excited with 350nm (200mW) laser demonstrates the existence of the defective states in the structure in trapping electron at 3.273eV energy level.

Abstract: Theoretical molecular dynamic simulations based on plane-wave and pseudopotential density functional theory (DFT) calculations with CASTEP code were employed to explore the pressure influence on the properties of silicon carbide polytypes. The changes in lattice and electronic structures of 2H-, 4H-, and 6H-SiC polytypes at room temperature were investigated when pressures from 10 GPa to 200 GPa were applied. It’s found that the applied pressures didn’t cause a change in the hexagonal structure of the crystals, however the structural and electronic properties clearly affected by the compression. The dependences of volume reduction (V/V_o) and lattice parameters (a and c) on pressure were obtained successfully. The lattice parameters of the polytypes and c/a ratio showed a same trend under the compression with a clear similarity between 4H and 6H. The total energy-volume and enthalpy-pressure relations were estimated. The calculated energy gaps showed a reduction in the band gap width of 4H and 6H with the pressure increase while 2H band gap increased gradually with pressure. The tendency toward decreasing the density of state (DOS) at the conduction band edge was similar among the polytypes.

Abstract: The reversibility of phase transformations in Li₂MnSiO₄ and related materials during charge/discharge of the material is an important factor to enable the practical application of the cathode materials. However, the stability of this material is still unattainable. Here we report the computational identification of a new form of Li₂MnSiO₄ as a stable candidate with acceptable characteristics. The stability could arise due to the presence of the three-dimensional structure of the inorganic framework. The presence of a structure with a compact unit cell forms the basis for high capacity. Surprisingly it was found to have a stable analogue occurring in nature – Na₂CaSiO₄ with the same structure. Using this information the possible routes of obtaining such material are presented. The prediction of such material has been not found in the literature previously. Of course the problems such as phase transformations upon delithiation may exist, and to check the data the experimental and computer studies needed.

Abstract: The structural stability, metallicity and hydrogen adsorption properties of Ti doped superatomic-Al₁₂C cluster have been investigated by first-principles based on density functional theory. The results show that the structural stability of Al₁₂C has been enhanced after replacing the central Al atom of icosahedra Al₁₃ configuration by carbon atom, and the most stable structure and stability of Al₁₂CTi and the lowest energy structures of Al₁₂CTi (H₂)_n (n=1-8) are searched and discussed. Moreover, the Al₁₂CTi (H₂)₆ not only exhibits strong stability according to HOMO-LUMO energy gap analysis, but also absorbs six hydrogen molecules in the absorb energy range of physical and chemical absorption, which could be considered as a candidate for hydrogen absorption materials development.