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We present a theoretical estimation of optical, vibrational, and structural properties of II–VI semiconducting quaternary alloy Zn0.5Cd0.5SySe1−y for 0 < y < 1 giving total 10 compositions. The estimation of refractive index, elastic constants, bulk modulus, and vibrational frequencies are performed using the important input parameters provided by the empirical pseudopotential method. In this method, the bandgaps are computed, and the alloying effects are modeled through the modified virtual crystal approximation. We have computed the static refractive index, static and high-frequency dielectric constants, longitudinal and transverse optical phonon frequencies, elastic constants, bulk modulus, and cohesive energy for 10 compositions of the alloy. The results are compared to other experimental and theoretical values wherever available., Зроблено теоретичну оцiнку оптичних, вiбрацiйних та структурних властивостей чотирикомпонентного напiвпровiдникового сплаву з елементiв II та VI груп Zn0.5Cd0.5SySe1−y для 0 < y < 1, всього для 10 варiантiв композицiй. Розраховано показник заломлення, коефiцiєнти пружностi, об’ємний модуль пружностi та частоти коливань з використанням вихiдних параметрiв методу емпiричного псевдопотенцiалу. Знайдено щiлини енергетичного спектра i змодельовано ефекти сплавлення у модифiкованому наближеннi вiртуального кристала. Розраховано статичний показник заломлення, дiелектричнi константи у статичному випадку та для високих частот, обчислено частоти оптичних фононiв, коефiцiєнти пружностi, об’ємний модуль пружностi та енергiю когезiї для 10 композицiй сплаву. Результати порiвнюються з iншими теоретичними i експериментальними даними.

The purpose of this work is to incorporate numerically, in a discontinuous Galerkin (DG) solver of a Boltzmann-Poisson model for hot electron transport, an electronic conduction band whose values are obtained by the spherical averaging of the full band structure given by a local empirical pseudopotential method (EPM) around a local minimum of the conduction band for silicon, as a midpoint between a radial band model and an anisotropic full band, in order to provide a more accurate physical description of the electron group velocity and conduction energy band structure in a semiconductor. This gives a better quantitative description of the transport and collision phenomena that fundamentally define the behaviour of the Boltzmann - Poisson model for electron transport used in this work. The numerical values of the derivatives of this conduction energy band, needed for the description of the electron group velocity, are obtained by means of a cubic spline interpolation. The EPM-Boltzmann-Poisson transport with this spherically averaged EPM calculated energy surface is numerically simulated and compared to the output of traditional analytic band models such as the parabolic and Kane bands, numerically implemented too, for the case of 1D $n^+-n-n^+$ silicon diodes with 400nm and 50nm channels. Quantitative differences are observed in the kinetic moments related to the conduction energy band used, such as mean velocity, average energy, and electric current (momentum)., Comment: submission to CMAME (Computer Methods in Applied Mechanics and Engineering) Journal as a reply to the reviewers on February 2017

WOS: 000369480800013, The properties of a semiconductor are drastically modified when the crystal point group symmetry is broken under an arbitrary strain. We investigate the family of semiconductors consisting of GaAs, GaSb, InAs and InSb, considering their electronic band structure and deformation potentials subject to various strains based on hybrid density functional theory. Guided by these first-principles results, we develop strain-compliant local pseudopotentials for use in the empirical pseudopotential method (EPM). We demonstrate that the newly proposed empirical pseudopotentials perform well close to band edges and under anisotropic crystal deformations. Using the EPM, we explore the heavy hole-light hole mixing characteristics under different stress directions, which may be useful in manipulating their transport properties and optical selection rules. The very low 5 Ry cutoff targeted in the generated pseudopotentials paves the way for large-scale EPM-based electronic structure computations involving these lattice mismatched constituents., TUBITAK; Scientific and Technological Research Council of Turkey [112T178], We would like to thank TUBITAK, The Scientific and Technological Research Council of Turkey for their financial support through project No. 112T178. The numerical calculations reported in this paper were partially performed at TUBITAK ULAKBIM, High Performance and Grid Computing Center (TRUBA resources).

Cataloged from PDF version of thesis. Includes bibliographical references (leaves 42-49). Thesis (M.S.): Bilkent University, Department of Physics, İhsan Doğramacı Bilkent University, 2015. Self-assembled quantum dots composed of III-V compounds receive considerable attention due to their potential applications on spintronics and quantum informa- tion processing. Here, lattice mismatch between two materials causes a remark- able strain and this subsequently affects not only carriers but also nuclear spins due to electric quadrupole interaction. In this thesis, the behavior of electronic band structure and deformation potentials under various strains are investigated in the family of semiconductors consisting of InAs, GaAs, InSb and GaSb. Com- putations are performed using semi-empirical pseudopotential method (EPM) by generating a new set of strain-compliant pseudopotentials. In order to both lead and validate EPM calculations, density functional theory based on hybrid functionals has been employed. Our results on hydrostatic and shear strain de- formation potentials obtained by either technique are in very good agreement with the experimental data. We demonstrate that the newly proposed empirical pseudopotentials perform well around band edges under anisotropic crystal de- formations. This paves the way for large-scale electronic structure computations involving lattice mismatched constituents. by Aslı Çakan. M.S.

Το οξείδιο του ψευδαργύρου είναι ένας ημιαγωγός της ομάδας II-VI και έχει μεγάλη ποικιλία σε τεχνολογικές εφαρμογές όπως οι αισθητήρες διαφόρων χημικών αερίων, τα lasers, οι δίοδοι εκπομπής φωτός, οι νανο-γεννήτριες, τα ηλιακά κύτταρα και πολλές άλλες. Το ευρύ του ενεργειακό κενό (3.445 eV) το καθιστά ένα πολλά υποσχόμενο υλικό για φωτονικές εφαρμογές στην περιοχή του UV ή του ιώδους, ενώ ταυτόχρονα η υψηλή ενέργεια συνοχής του εξιτονίου που το χαρακτηρίζει (περίπου στα 60 meV) επιτρέπει την αποτελεσματική εξιτονική εκπομπή σε θερμοκρασία δωματίου. Οι πιο πρόσφατες εξελίξεις στον τομέα του νανοδομημένου ZnO είναι οι νανοδρόμοι, οι νανογέφυρες, οι νανοπροπέλες, οι νανοδακτύλιοι, τα νανονήματα κ.α. Στην παρούσα διπλωματική εργασία μελετώνται οι ηλεκτρονιακές και οπτικές ιδιότητες νανονημάτων οξειδίου του ψευδαργύρου (ZnO) για ένα εύρος διαμέτρων από 2 έως 6 nm και με την βοήθεια της εμπειρικής μεθόδου των ψευδοδυναμικών και της Configuration Interaction (CI). Μια ανασκόπηση των ιδιοτήτων και χαρακτηριστικών του bulk ZnO, όπως η κρυσταλλική και η ενεργειακή του δομή, κάποιες τεχνολογικές εφαρμογές και μέθοδοι ανάπτυξης δίνονται στο πρώτο κεφάλαιο. Το δεύτερο κεφάλαιο περιέχει την περιγραφή διαφόρων υπολογιστικών μεθόδων όπως της προσέγγισης ενεργούς μάζας ( Effective Mass Approximation), της θεωρίας του συναρτησιακού της πυκνότητας (Density Functional Theory) και τέλος, της εμπειρικής μεθόδου των ψευδοδυναμικών που χρησιμοποιείται στους υπολογισμούς των ηλεκτρονιακών και οπτικών ιδιοτήτων των νανοδομών που μελετάμε. Στο τρίτο και τελευταίο κεφάλαιο, παρατίθενται τα αριθμητικά αποτελέσματα . Αυτά, αφορούν στο εξαρτώμενο από το μέγεθος, οπτικό ενεργειακό κενό, το Stokes shift, και το φάσμα φωτοφωταύγειας. Στο τέλος του κεφαλαίου περιγράφονται τα συμπεράσματα. Zinc oxide (ZnO), a typical group II-VI compound, has a great variety of device applications, such as chemical sensors, lasers, light-emitting diodes, nanogenerators, solar cells and so forth. The wide band gap (3.445 eV) makes it a promising material for photonic applications in the UV or the blue range, while the high exciton binding energy (around 60 meV at room temperature) allows efficient excitonic emission at room temperature. The most recent developments are towards the nanostructured ZnO, such as nanorods, nanobridges, nanopropellers, nanorings, nanowires, et al. In the present master thesis, the electronic and optical properties of ZnO nanowires within the range of 2-6 nm in diameter are studied by means of atomistic empirical pseudopotential method and configuration interaction. A review of the bulk ZnO, such as the crystal and band structures, technological applications and synthesis methods, is presented in chapter one. The second chapter is devoted to the discussion of various types of methods, e.g., effective-mass approximation, density-functional theory (DFT), and especially the empirical pseudopotential method used herein, for the calculations of the electronic and optical properties of nanostructured ZnO. The numerical results, based on the empirical pseudopotential methods and configuration interaction approach, are present in the following chapter. These results cover the size-dependent optical band gap, Stokes shift and photoluminescence spectrum. A summarization of the results is given in the last chapter.

The simulation of transport and optical properties of semiconductors is based on the availability of an accurate description of the band structures for the materials under investigation. Ab initio techniques are invaluable for predicting the electronic band structure particularly in the absence of reliable experimental information. Nevertheless, the Empirical Pseudopotential Method (EPM) remains the method of choice for transport studies because of its optimum compromise between complexity, accuracy, and computational efficiency. We have developed a set of integrated, interactive software tools which may greatly help in the delicate process of finding the best EPM band structure of a semiconductor compound. At the core of our computational method is an efficient yet general f90 implementation of nonlocal EPM for zinc-blende and wurtzite crystals. A robust simplex algorithm allows the optimization of the EPM adjustable parameters in order to match a set of reference values, i.e. energy gaps in several points of the Brillouin zone, and effective masses along all the principal directions, both for direct and indirect-gap materials. This computational procedure has been integrated with an array of Matlab functions, providing interactive functionalities for defining the local part of the atomic pseudopotentials, checking the convergence of the optimization process, plotting the resulting band structure, and computing detailed information about. e.g. the position and value of minima not located at symmetry points, the nonparabolicity coefficients of secondary valleys, etc.

For the design of InAs/GaSb superlattice (SL) heterojunction infrared photodetectors with very low dark current we have extended the standard two-component superlattice empirical pseudopotential method (SEPM) and implemented a four-component model including interface layers. For both models, the calculated bandgap values for a set of SL samples are compared to bandgaps determined by photoluminescence measurements. While the bandgap resulting from the two-component model agrees well with experimental data for SL structures with individual layer thicknesses of 7 monolayers and more, we show that for SLs with thinner GaSb layers the four-component SEPM model is accurate, when the As-content in the interface and barrier layers is included in the model.

Wichtige Hersteller von Mikroprozessoren verbessern seit der Einführung der 90 nm -Technologien die Transporteigenschaften von Silizium durch den Einsatz von Techniken, die mechanische Spannungen im Transistorkanal induzieren. Die gängigsten dieser neuen Techniken, die entweder globale oder lokale Verspannungen am Silizium Wafer einprägen, werden in dieser Arbeit behandelt.Während sich bisher theoretische Arbeiten fast ausschließlich mit biaxial verspanntem Siliziumsubstrat mit {001} Orientierung beschäftigten, sind die physikalischen Ursachen der Verbesserung der Elektronenbeweglichkeit durch uniaxiale Verspannung noch nicht zur Gänze geklärt. In dieser Arbeit wird der Einfluss allgemeiner homogener Verspannung - beschrieben durch den Spannungstensor [epsilon] - auf den Bulk-Transport und die Oberflächenbeweglichkeit von Silizium untersucht. Als Grundlage der Analyse dient die Bandstruktur von verspanntem Silizium, welche einerseits numerisch mittels der nicht-lokalen empirischen Pseudopotentialmethode mit Spin-Bahn Kopplung und andererseits analytisch mit Hilfe der störungstheoretischen kp-Methode berechnet wird. Zusätzlich zur Verschiebung der Leitungsbandtäler relativ zueinander läßt sich aus den Berechnungen der Bandstruktur eine änderung der effektiven Elektronenmassen ableiten. Diese kann auf die Aufhebung der Entartung der zwei untersten Leitungsbänder im X-Punkt zurückgeführt werden, die von einer Scherspannung hervorgerufen wird.Mittels der kp-Methode werden auch analytische Gleichungen zur Beschreibung der Massenänderung hergeleitet.Die Beweglichkeit in verspanntem Silizium wird durch das Lösen der Boltzmanngleichung mit Hilfe einer Monte Carlo Methode berechnet, der sowohl eine analytische Bandbeschreibung als auch die numerisch berechnete Bandstruktur zugrunde gelegt werden kann. Es zeigt sich, dass nur im Fall von kleinen Scherverspannungen (< 0.5 %) die Beweglichkeit bei niedrigen Feldstärken aus Monte Carlo Simulationen mit einer analytischen Beschreibung der Bandstruktur mit den Simulationsergebnissen übereinstimmen, die die numerische Bandstruktur verwenden. Dies unterstreicht die Wichtigkeit die gesamte Bandstruktur einer genauen Modellierung des Ladungsträgertransports in verspannten Silizium zugrunde zu legen.Durch das Aufkommen der Hybrid-Orientation Technologie haben die Substratorierungen {110} und {111} an Bedeutung gewonnen. Da eine mechanische Verspannung zu einer Anisotropie der Ladungsträgerbeweglichkeit führt, muss die Verspannung auf die Richtung des Kanals und die Substratoberfläche abgestimmt werden, um den größten Beweglichkeitszuwachs zu erreichen. Um die Auswirkungen der Verspannung auf die Oberflächenbeweglichkeit zu bestimmen, wird die Subbandstruktur durch selbstkonsistentes Lösen der Schrödinger-Gleichung und der Poisson-Gleichung berechnet. Im zweidimensionalen Elektronengas bewirkt die Scherspannung eine energetische Verschiebung der Subbandleitern und eine änderung der effektiven Massen. Beweglichkeitssimulationen zeigen, dass durch die inhärente Aufspaltung der Subbänder bei starker geometrischer Quantisierung, wie etwa bei Dünnschichttransistoren, die änderung der effektiven Masse gegenüber der Verschiebung der Subbandleitern an Bedeutung gewinnt.Des Weiteren wird in dieser Arbeit der Einfluss des Pauli-Verbots auf den Elektronenladungsträgertransport untersucht. Dies ist vor allem im zweidimensionalen Elektronengas von Bedeutung, wo das Fermi-Niveau bei starker Inversion deutlich über dem Minimum des untersten Leitungsbandes liegt., Starting with the introduction of the 90 nm CMOS technology node many important microprocessor manufacturers improve the transport characteristics of silicon using techniques that induce strain in the transistor channel. The most common techniques that produce strain globally or locally on the wafer are being reviewed in this work.While most theoretical work has been performed for biaxially strained Si with {001} substrate orientation, a thorough theoretical analysis of electron mobility enhancement in arbitrarily strained Si is missing. In this work the effect of a general homogeneous strain - described by the strain tensor [epsilon] - on the transport in bulk Si and Si inversion layers is analyzed. The band structure of strained Si is calculated numerically using the empirical pseudopotential method with nonlocal and spin-orbit corrections. The results of band structure calculations reveal that shear strain changes the effective electron masses in addition to the splitting of the six [Delta]-valleys. The effective mass change can be attributed to the lifting of the degeneracy of the two lowest conduction bands at the X-point due to shear strain. Using the kp-method analytical expressions for the effective mass change are derived.The transport properties of strained Si are investigated by solving the semiclassical Boltzmann equation using the Monte Carlo (MC) method employing fullband and analytical band models. The low-field electron mobility resulting from MC simulations using an analytical description of the electron bands and the fullband description coincide only for not too high shear strain (< 0.5%). At larger shear strain the band deformation is so pronounced, that fullband modeling is required.The hybrid-orientation technology combines different silicon substrate orientations and channel directions on the same wafer and can be used in conjunction with strain techniques. Since strain yields an anisotropic mobility, the proper channel direction and substrate orientation have to be chosen to obtain the maximum mobility enhancement. The effect of strain on the inversion layer mobility of electrons is investigated by calculating the subband structure using a self-consistent Schrödinger-Poisson solver. In the two-dimensional electron gas shear strain results in a shift of the subband ladders and a change of the effective electron masses. From simulations of the effective electron mobility in ultra-thin-body MOSFETs it can be concluded that the change of the effective masses is the dominant effect leading to mobility enhancement, since the strong geometrical confinement yields a large intrinsic splitting of the subband ladders, such that an additional splitting induced by strain has only a little effect.The effect of degeneracy both on the phonon-limited mobility and the effective mobility including surface-roughness scattering is studied using a new MC algorithm developed in this work. By comparison with results from MC simulations where the Pauli exclusion principle is neglected, it is shown that a correct treatment of degenerate carrier statistics of the 2DEG of Si inversion layers is important.