Narrowing of band gap at source/drain contact scheme of nanoscale InAs–nMOS

A multi-scale simulation study of Ni/InAs nano-scale contact aimed for the sub-14 nm technology is carried out to understand material and transport properties at a metal-semiconductor interface. The deposited Ni metal contact on an 11 nm thick InAs channel forms an 8.5 nm thick InAs leaving a 2.5 nm thick InAs channel on a p-type doped (1×1016 cm-3) AlAs0.47Sb0.53 buffer. The density functional theory (DFT) calculations reveal a band gap narrowing in the InAs at the metal-semiconductor interface. The one-dimensional (1D) self-consistent Poisson-Schrödinger transport simulations using real-space material parameters extracted from the DFT calculations at the metal-semiconductor interface, exhibiting band gap narrowing, give a specific sheet resistance of Rsh = 90.9 Ω/sq which is in a good agreement with an experimental value of 97 Ω/sq

Instability, Intermixing and Electronic Structure at the Epitaxial LaAlO3/SrTiO3(001) Heterojunction

The question of stability against diffusional mixing at the prototypical LaAlO3/SrTiO3(001) interface is explored using a multi-faceted experimental and theoretical approach. We combine analytical methods with a range of sensitivities to elemental concentrations and spatial separations to investigate interfaces grown using on-axis pulsed laser deposition. We also employ computational modeling based on the density function theory as well as classical force fields to explore the energetic stability of a wide variety of intermixed atomic configurations relative to the idealized, atomically abrupt model. Statistical analysis of the calculated energies for the various configurations is used to elucidate the relative thermodynamic stability of intermixed and abrupt configurations. We find that on both experimental and theoretical fronts, the tendency toward intermixing is very strong. We have also measured and calculated key electronic properties such as the presence of electric fields and the value of the valence band discontinuity at the interface. We find no measurable electric field in either the LaAlO3 or SrTiO3, and that the valence band offset is near zero, partitioning the band discontinuity almost entirely to the conduction band edge. Moreover, we find that it is not possible to account for these electronic properties theoretically without including extensive intermixing in our physical model of the interface. The atomic configurations which give the greatest electrostatic stability are those that eliminate the interface dipole by intermixing, calling into question the conventional explanation for conductivity at this interface - electronic reconstruction. Rather, evidence is presented for La indiffusion and doping of the SrTiO3 below the interface as being the cause of the observed conductivity

Development and application of embedded cluster methodologies for defects in ionic materials

The main objectives of the present Thesis were to develop a method and computer code for calculations of properties of point defects in insulators, and to study defects and the mechanisms of defect processes in the bulk and at surfaces of ionic crystals. We develop an embedded cluster method and a computer code, which allows us to treat a defect in a crystal quantum mechanically in the effective potential of the rest of the polarisable crystal. This method has several advantages with respect to existing methods: i) it can be applied to bulk and surface defects; ii) it allows application of several quantum-mechanical methods, including the Hartree-Fock method and different configuration interaction techniques, and methods based on the Density Functional. Theory; iii) it allows one to calculate the spectroscopic properties of point defects, such as optical absorption, vibrational spectra and hyperfine interactions, taking into account the defect interaction with the rest of the crystal; iv) the atomic structure of the defect can be optimised self-consistently using an effective procedure. The new method and the computer code have been tested on a number of well-established systems. The method has been applied to study the electronic structure and properties of several defects, and to model the mechanisms of various surface processes. i) We have calculated the position of the top of the (001) MgO surface valence band with respect to the vacuum level and the energy levels of neutral and charged oxygen vacancies with respect to the top of the valence band and the vacuum level. ii) The study of the electronic structure of excitons at low coordinated oxygen sites of the MgO surface has demonstrated a significant dependence of the excitation energies on oxygen coordination. iii) A variety of geometric configurations of the [FeCl_n(CN)_6_-_n]"3"- and [FeCl_n(CN)_6_-_n]"4"- impurities in the bulk NaCI were studied and the most stable configurations identified. iv) It was demonstrated that the accurate account for the lattice polarisation is crucial for quantitative agreement of the optical absorption and luminescence energies with the experimentally observed properties of the Ce"3"+ centres in LiBaF_3. v) Modelling of the interaction of a silicon tip with the NaCI (001) surface has demonstrated that the tip-surface interaction can lead to transfer of surface ions to the tip and allowed us to establish the charge state of the transferred Cl ions. vi) The analysis of the mechanism of laser-induced desorption of positive Mg ions from the (001) MgO surface allowed us to elucidate the atomistic stages of the desorption process. The results of this Thesis have demonstrated the applicability of the method developed to a wide range of defect properties in ionic materials. (author)Available from British Library Document Supply Centre- DSC:DXN038917 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Impact of Body-Thickness-Dependent Band Structure on Scaling of Double-Gate MOSFETs: A DFT/NEGF Study

First principles density functional theory has been used to calculate the 2-D band structure of Si slabs with different thicknesses. From the calculated 2-D band structure, electron longitudinal and transverse effective masses have been extracted as a function of the slab thickness. These thickness-dependent electron effective masses have then been used to simulate IID-V-G characteristics of scaled, sub-10 nm double-gate (DG) MOSFETs and to compare them with the results obtained using bulk masses. The channel thickness dependence of the Si band structure starts to affect noticeably DG MOSFET performance at channel lengths below 10-nm, lowering the ON-current by approximately 10, for transistors with a body thickness of 2.6 mn, and by 20, for transistors with a body thickness of 1.3 nm. On the other hand, the subthreshold swing is improved by 10, in the 6-nm-gate length DG MOSFET and by 15, in the 4-nm-gate length device. Finally, the impact of thickness-dependent effective masses has been related to the behavior of the transmission coefficient

Optical absorption and luminescence energies of F centers in CaO from ab initio embedded cluster calculations

We calculated the optical absorption and luminescence energies of electrons trapped at oxygen vacancies in CaO using a consistent embedded cluster method which accounts for the long-range polarization effects and partial covalence of CaO. Optical absorption and luminescence energies of neutral F center and positively charged F+ center vacancies are calculated by means of time dependent density functional theory using the B3LYP exchange-correlation density functional. Our results demonstrate that using large basis sets to describe a diffuse nature of excited states, and properly accounting for long-range polarization induced by charged and excited defect states, is crucial for accurate predictions of optical excitation and luminescence energies of these defects

Oxygen vacancies in amorphous silica: structure and distribution of properties

We used an ab initio embedded cluster method to study and compare three charged states of the Si-Si dimer configurations of oxygen vacancies in alpha-quartz and amorphous silica. The Si-Si bond in the neutral vacancy remains largely the same in both crystalline and amorphous SiO2. In alpha-quartz the positively charged dimer E' centre exists only as a metastable configuration, whereas in amorphous silica stable dimer configuration can be formed at favorable precursor sites. Our results demonstrate that negatively charged dimer oxygen vacancies can be formed in alpha-quartz