Theses - Department of Physics

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Open Access
Electronic structure of Si/Ge semiconductor superlattices
(Bilkent University, 1988) Gülseren, Oğuz
A brief review about the two dimensional electron systems and especially band offsets is given. The electronic properties of the Si„/Ge„(001) strained superlattices as a function of the superlattice periodicity and the band misfit is investigated by using the empirical tight-binding method. The difference between the direct and indirect band gaps is reduced from 2.01 eV for bulk Si to 0.01 eV for n = 6. Consequently, the superlattice with n = 6 can be considered as quasi-direct, while it is at least 0.15 eV for n = 4 case. For the cases n=5,6, and 8, the band gap might become direct for large values of band misfit.
Open Access
Atomic theory of the scanning tunneling microscope
(Bilkent University, 1988) Tekman, Ahmet Erkan
The Scanning Tunneling Microscope is proven to be one of the most powerful tools for surface structure determination. Present theories are able to explain the operation of the microscope when the tip is far from the surface. For the small tip height case the atomic-scale interaction of the tip and the surface has to be included in the theory. The electronic structure of the combined system of the tip and the surface is calculated with an Empirical Tight Binding approach for graphite. It is found that in the vicinity of the tip some Tip Induced Localized States are formed. These states play an important role in the tunneling phenomenon. The contribution of these states to the tunneling current is calculated.
Open Access
High Tc superconductivity
(Bilkent University, 1989) Gedik, M. Zafer
critical summary of the recent developments in high temperature superconductivity is given. Physical properties of the new materials are summarized. The new theories proposed to explain high Tc are reviewed. In the case of a special type anisotropic gap, the currents and conductivity for single particle tunneling in high Tc superconductor junctions are calculated. Tunneling is assumed to be specular. The position of the peak in the conductivity curve, is found to be determined by the shape of the Fermi surface. Two geometries are considered: tunneling paxallel and perpendicular to CuO^ planes. The order of the peaks for these two configurations turns out to be opposite to experimental results. It is concluded that, conventional one band theory of superconductivity is not enough to explain observed tunneling spectra of high Tc superconductors.
Open Access
Construction of a scanning tunneling microscope and first results
(Bilkent University, 1990) Oral, Ahmet
In this thesis, construction of a Scanning Tunneling Microscope in air is explained. A step motor sample approach mechanism and a tripod scanner are used in the construction. Atomic resolution images of graphite samples are obtained in both constant current and constant height modes. Loss of trigonal symmetry in some Graphite images are also observed. This anomaly is attributed to the multiple atom tip or slipped top layer of Graphite.
Open Access
Ballistic transport and tunneling in small systems
(Bilkent University, 1990) Tekman, A Erkan
Ballistic transport and tunneling of electrons in mesoscopic systems have become one of the most important subjects of condensed matter physics. The quantum point contacts and scanning tunneling microscope form the basic experimental tools in this area and have been used for understanding many features of small systems. In this work ballistic transport and tunneling in small systems are investigated theoretically. Ballistic transport through narrow constrictions is investigated for a variety of configurations. It is found that for a uniform constriction the conductance is quantized in units of the quantum of conductance (2e^/A) for long channels. The interference of waves in the constriction gives rise to the resonance structure superimposed on the quantized steps. The lack of the resonance structure in the experimental results are attributed to temperature effects and/or adiabatic transport due to tapering of the constriction. It is shown that elastic scattering by an impurity distorts the quantization of conductance. Novel resonant tunneling effects due to formation of bound states are predicted for an attractive impurity or a local widening at the center of the constriction. It is shown that the probing in scanning tunneling microscopy have very much in common with narrow constrictions. The transition from tunneling to point contact regime is explained by the vanishing effective potential barrier as a result of tip-sample interaction. For noble and simple metals it is conjectured that lateral position dependent interaction between the tip and sample leads to corrugation of the potential barrier and in turn to atomic corrugation observed by scanning tunneling microscopy. The focused field emission of electrons from point sources is analyzed in a systematical way. The effective barrier due to the lateral confinement and nonadiabatic transport through the horn-like opening are found to be responsible for focusing. The nonequilibrium nature of transport is investigated by use of Keldysh Green’s function technique. The effects of elastic and inelastic scattering are analyzed in a strictly one-dimensional geometry. The features of voltage and current probes are studied and the Landauer formulae are examined for multiprobe measurements.
Open Access
Electronic structure of low dimensional semiconductor systems
(Bilkent University, 1992) Gülseren, Oğuz
Recent progress made in the growth techniques has led to the fabrication of the artificial semiconductor systems of lower dimension. Electrons and holes in these materials have quantization different from those of the three dimensional systems presenting unusual electronic properties and novel device applications. In this work, the important features of the free carriers in semiconductor superlattices are examined, and the electronic structure of some novel 2D semiconductor systems are investigated theoretically. This thesis studies various systems of lower dimensionality such as: the strained Si/Ge superlattices, i-doping. Si (100) surface and the tip-sample interaction in scanning tunneling microscopy (STM) study of this surface, and Wannier-Stark localization in finite length superlattices. The electronic energy structure of pseudomorphic Ge„i/Si„ superlattices is investigated by using the empirical tight binding method. Effects of the band offset, sublattice periodicity and the lateral lattice constant on the transition energies have been investigated. It is found that Ge„i/Si„ superlattices grown on Ge (001) can have a direct band gap, if m + n = 10 and m = 6. However, optical matrix elements for in-plane and perpendicular polarized light are negligible for the transition from the highest valence band to the lowest conduction band state at the center of the superlattice Brillouin zone. The electronic structure of the Si i-layer in germanium is explored by using the Green’s function formalism with layer orbitals. We found two dimensional parabolic subbands near the band edges. This approach is extended to treat the electronic structure of a single quantum well without invoking the periodically repeating models. Quantum well formation in Ge,„Si„ superlattices is also studied by using different number of ^-layers. Subband structure is observed by changing the height of the Si quantum well. The confinement of acoustical modes within 2DEG due to only the electronphonon interaction is proposed. The confined modes split out from the bulk phonons, if the 2DEG is created by means of modulation doping. This occurs even if the lattice has uniform parameters. The effect is more pronounced when the wave vector q of the modes increases and is maximum a,t q = 2kp {kp is the Fermi wave vector). In the case of several electron sheets the additional features of the confinement effect appear. Green’s function method is also applied to treat the modifications of electronic state density in STM. The tip-sample interaction in STM study of Si (100) surface is explored by calculating the Gieen’s function within the empirical tight binding method. Both of the proposed reconstruction models, buckled and symmetrical dimer model, is investigated. A dip occurs in the change of density of states of surface atoms at the energy of surface states for small tip-sample distances, and it decreases with increasing tip-sample separation. Although, in-plane tip position (above the up- or down-surface atom) affects the surface atoms differently in buckled dimer model, it influences the surface atoms symmetrically in symmetric dimer model. Recent experimental studies revealed the significant information on the Wannier-Stark localization. Following these experimental results, the WannierStark ladder is investigated by carrying out numerical calculations on a multiple quantum well structure under an applied electric field. The variation of the Wannier-Stark ladder energies and localization of the corresponding wave II function are examined for a wide range of applied electric field. Our results show that Wannier-Stark ladder do exist for finite but periodic system which consists of a large number of quantum well having multi-miniband structure. It is found that the miniband states are localized in the well regions with the applied electric field, while the continuum states preserve their extended character. Energies of the well states show a linear shift with the electric field except the small field values in which a nonlinear shift is resulted. Multiband calculations show that there is a mixing between the different band states although they are localized in different well regions.
Open Access
Superconducting systems of low dimensionality
(Bilkent University, 1992) Gedik, M. Zafer
It is possible to call the last five years as the golden age of superconductivity. The two most important developments in the field are the discovery of copper oxide and fullerene superconductors. In this work, some possible pairing mechanisms for these materials ai’e examined by giving emphasis on the reduced dimensionality. First, an older problem, spatially separated electron-hole system, is investigated to identify the possible phases in coupled double quantum well structures in electric field. Secondly, the superconducting transition temperature and response to external magnetic fields of layered systems with varying number of layers are studied by means of a microscopic model and its GinzburgLandau version. It is also shown that an interlayer pairing mechanism, phonon assisted tunneling, can induce superconductivity. Finally, effects of the spherical structure of fullerenes are examined by solving a two fermion problem on an isolated molecule where the particles interact via a short range attractive potential. As a possible mechanism of superconductivity in alkali metal doped fullerenes, coupling between electrons and the radial vibrations of the molecule is investigated.
Open Access
Phase transitions in tetrahedral Ising lattices
(Bilkent University, 1993) Kabakçıoğlu, Alkan
After a review of the Renormalization CJroup theory, the phase diagram of unisotro])ic tetrahedral Ising lattice is explored l)y the motivation gained through the recent experimental findings about SiGe alloys. Renormalization Group approcich and the mean-field R(J approximation previously pro])osed by Kinzel ¿ire used. Four different ordered pluises are olxserved. The critical expoiKMit // is Ciih’uhited using the liiUNirized t.ra.nsform
Open Access
Hard spin mean field theory of 3D stacked-triangular-lattice system
(Bilkent University, 1994) Akgüç, Gürsoy Bozkurt
Closed form solution of ‘Hard spin mean field theory’ is constructed and applied to ‘Three dimensional stacked-triangular-system’. The phase diagram of this system is examined. ‘Free energy’ is calculated to reveal the thermodynamically stable state in the phase diagram. A new second order phase transition line is found near the zero external magnetic field. Strong evidence is found for the tricriticality point behaviour in the multicritical region.
Open Access
Fabrication, characterization, and extraction of GaAs mesfets
(Bilkent University, 1994) Ata, Erhan Polatkan
Metal Semiconductor Field Effect Transistor (MESFET) is the most widely used active element of today’s microwave industry. After development of the MESFET technology, the microwave industry gained a high acceleration, especially in the telecommunication field. In this study, GaAs MESFETs with various dimesions and geometries were fabricated. Characterization and parameter extraction of these devices were performed, by means of low and high frequency measurements. The low cutoff frequency of the MESFETs produced were attributed to the non-optimized gate recess etch.
Open Access
Investigation of Si1-xGex alloy formation by using STM
(Bilkent University, 1994) Oral, Ahmet
Open Access
Controlled lateral and perpendicular motion of atoms on metal surfaces
(Bilkent University, 1994) Buldum, Alper
Nanoscale modification of matter has been the subject of interest. Recently, several experimental studies have demonstrated that by using a scanning tunneling microscope one can translate atoms on metal surfaces to a desired position. Furthermore, it has been shown that an atom between surface and tip can be transferred reversibly which results in bistable conductance. The controlled dynamics of adsorbed species has opened a new field of research. This thesis work provides a theoretical investigation of the controlled lateral and perpendicular motion of an inert gas atom (Xe) on metal surfaces. The lateral motion of Xe on the Ni(llO) and P t(lll) surfaces is manipulated by a W tip. The interaction energy of the physisorbed atom with the tip and metal surface is described by an empirical potential. Using molecular statics the energy surfaces are calculated and the adsorbtion sites are determined. By using the molecular dynamics calculations, the variation in the coordinates of the adsorbate Xe with the tip moving at a given height are obtained. Three different modes of Xe translation are distinguished depending on the height of the tip. These are i) carriage on the tip, ii) pushing and, iii) pulling modes. The range of the tip height where one of these modes occur is strongly depended on the relaxation of electrodes and the geometry of the tip. Controlled and reversible transfer of atoms between the metal surface and the tip is studied by the transfer of Xe between two flat P t(lll) surfaces. Physisorption of Xe on the P t(lll) surface is studied by an empirical potential including short and long-range interactions and yielding correct account of several experimental data. Effective charge on Xe and the dipole moment constructed therefrom are calculated as a function of the Xe-surface separation. The potential energy curve of Xe between two P t(lll) surfaces and quantum states of Xe therein are calculated as a function of the applied voltage and separation between two P t (lll) surfaces. Within this model, various mechanisms, such as tunneling of Xe, dipole excitation and resonant tunneling, electromigration contributing to the transfer of Xe are examined. The transfer rate of Xe is then calculated for different mechanisms. Its dependence on the bias voltage is explored. The overall behavior of the total transfer rate is not a power law. While at low bias voltages thermal assisted atom tunneling is effective, the dipole excitation and resonant tunneling becomes dominant at high bias voltages.
Open Access
Al(x)Ga(1-x)As/GaAs graded index separate confinement heterostructure single quantum well lasers
(Bilkent University, 1994) Bozkurt, Mümtaz Koray
"Stimulated emission of photons could be produced in semiconductors by recombination of carriers injected across a p-n junction This idea was first suggested by Basov et al} in 1961. Soon afterwards diode lasers were first demonstrated at cryogenic temperatures in pulsed operation in 1962 by separate groups in US.^"® Until the first use of heterostructures in diode lasers^ in 1969, advances in the diode laser area were not as good as was expected. New era of the diode Icisers begin with use of the heterostructures in laser diode technology which allowed them to run at room temperatures in continuous wave operations. Also, introduction of MBE and LPE techniques in crystal growth area supplied the forecoming materials and enabled growing of nanocrystal layers for semiconductor laser diode applications. Reaching to reliable, compact and an efficient components for applications is the major factor which forces the laser diode designs to maturity. In this work, ridge type Single Quantum Well Graded Index Separately Confined Heterostructure lгıser diodes which were made by reactive ion etching in CCI2F2 and lift-off of low temperature PECVD SİO2, is taken from its crystal growth aspects through design and fabrication steps to its characterization.
Open Access
Ridge waveguide GaAS
(Bilkent University, 1994) Türkoğlu, Abdullah Kamuran
The study of solid-state laser structures in low dimensions has gained great deal of attention in recent years. The theory originated in early 1960s developed really fast along with new fabrication methods bringing geometries from macroscopic to sub-micron scale. This, in turn, made it possible to realize more complex semiconductor laser structures having multiple quantum wells in their active region with sub-milliampere threshold currents and tens of mW att/facet optical light outputs. Today, after a long way of effort in the interest for MQW laser structures, quite challenging performances have been achieved.* However, due to complexity encountered during manufacturing and testing processes of these new lasing structures, it seems that overall technique still needs to be improved. In this research conducted at BU Advanced Research Laboratories, design, fabrication and characterization of CaAs/ALGaı-^As Multiple Quantum Well Icisers is aimed. In the subsequent chapters, first the basic theoretical background for QW lasers is summarized, then the method followed during fabrication is reported, and finaly, typical characteristics obtained after test studies are presented.
Open Access
Phonon renormalization effects in low dimensional electron-hole systems
(Bilkent University, 1995) Güven, Kaan
Ia)vv (linuMisioiial sciniromluclor st i iid iii4's lia\4‘ 1и‘(М1 an (‘Xt(Misiv(‘ i4\soa,rrh iwcix ill (4)iul(4iscd iiiatlcr physics. In j^aii iculai', iiindi (‘llorl has 1)(Ч'п dcvolnd to lhc‘ study ol ciuasi-one-diiiK'iisional siMniconductor sli-iiclures in гесчмП, yc'ars. The IK'W physic'al pluMioiiK'iia. in\'ol\4*d in tlu'sc* sysitMUs arisin,^ diu‘ to th(‘ Г(‘(1п(чм1 dimensionality point to various |)ot(Milial applicalions for iuture teclinologi(.‘s. y\ltliough the theory is lainiliar with tin* "oiK'-dimensionar’ pi’oblem for a long time, the realization of such structures (also known as (juanlum wires) extends onl}^ to ¿1 decade belonx Ilowciver, the по\ч'1 рго(1 исГкя1 t(‘chni([n('s led to a rapid increase in the experimental studies which, in turn, required a mor(' realistic and comi)rc*hensive theory to anal^'/c' and int('rpr('t tln^ obtaiiKnl data. This tlu'sis work intends to make a contribution in i1k‘ diiH’ctioii of th(‘S(' imj>rovem(‘nts. Our study is based on a (juasi-one-diiiKMisional eh'ctron-hole syst em as realized in pliotoexcit(‘d quantum wires, interacting with tlu' bulk bO-])honons. VVe investigate the polaronic corrections to the band gap and the c:arri(n* eilective mass, cirid the dependence of this correction itself to carrier density, temperature, and quantum finite size effects. VVe apply two different formal approaches; tho' p(n·Lurl.)гıtion theory and the vaiiational iiK'thod. 'I'he latter enables the investigation of dynamical screening elfects, thereby clarifying tin? (luestion of validity of the static screi'iiing approximation in one dimension. Our results have shown that dynamical screening is r<'levant in low dimensions. The dielectric function, which is a key quantity in describing tin' many-particle properties, is analyzf'd under diihn(*nt tip'proximations such as the 1 lartree-Fock ap[)roximation, random-|)hase api)roximation. and the more advaiuaxl local-field correction. Several confinement |)otentials (inlinit(‘ well, paralrolic, cj lindrical) are presented. I'kxplicit results iirc.' ol)tained hn' a (¡a.As (|uan(iim wiiv. VVe compare the results of tin' |)olar(niic corn'cticnis with tluit of tin? (‘xchangecorndalion induced corrections. W'e loiiinl that they are comi)arable in orrler of magnitudi', indicating that cari'i('r-|)lu)non int('iactions are more enhanced as the dimensionality reduces, and Ik.micc' should be treated on tin etpial footing along with the carrier-carri(‘r interact i(jiis. WV* make comparison with the pohironic corrections in two diiiK'iisiinial sysUnus. kdiially, we brielly discuss the renormalization due to confined phuiions as well.
Open Access
Time dependent study of quantum bistabiliity
(Bilkent University, 1995) Ecemiş, Mustafa Ihsan
The analysis of quantum transport phenomena in small systems is a prominent topic of condensed matter physics due to its numerous technological applications. The current analytical theories are not adequate for studying realistic problems. Computational methods provide the most convenient approaches. Numerical integration of the time-dependent Schrödinger equation is one of the most powerful tools albeit the implementation of the blackbody boundary conditions is problematic. In this work, a novel method which render possible this implementation is described. A number of sample calculations are presented. The method is applied to several one- and two-dimensional systems. A description of the time-dependent behavior of quantum bistable switching is given.
Open Access
Supercurrents and persistent currents in strongly correlated electron systems
(Bilkent University, 1995) Boyacı, Hüseyin
The lull understanding· ol’the solution Гог the 1-tl Hubbard model is of interest in its own right, and may provide clues to the understanding of higher dimensioned systiMiis. We have found tin? exact solution of tlie model for tw
Open Access
Rabi oscillations in an exciton-polariton system
(Bilkent University, 1995) Müstecaplıoğlu, Özgür Esat
The pure qiiaiitum model d(\scril)ing lîahi oscillations of exciton-polaritons in a ınicro-cavity is consid(‘r(‘(l. IMiolon-c'Xcilon intiT’action Hamiltonian is diagonalized with the aid of Bogoliihov canonical transformations and polariton picture is obtained. We (ind that this picture is ecpiicalent to the two level atomsingle mode held interaction up to the lact that bosonic nature^ of this picture allows many i)articles in both level whose decays give radiation. We demonstrate tluit in contrast to tlie Jaynes-Cummings moded collapse and revivals cannot be seen in our model. Pumping mecluinism and its elfects on the preparation of the initial states are examined. It is found that initial statcîs of such system should form two-modi' coher('nt state'. Markovian da.iiiping is introduc('d in 1 h'isi'iibergLangevin formalism. It is shown that i 1h‘ (oscillations can Ix' obs('rv('d if cavity dtiinping rate does not excc'ed some critical value depending on coupling constant cind detuning (of cavity modex iLxplicit cxxjoii'ssion for la'iiormalized HaJoi frecpiency is found. Strong, weak and critical damioing r('giim‘s are studi('d in detciil.
Open Access
Atomic scale investigation of clean and epi-grown Si(001) surfaces using scanning tunneling microscopy
(Bilkent University, 1996) Özer, H. Özgür
In this thesis, clean and epi-grown Si(001)(2x1)surfaces are analyzed by Scanning Tunneling Microscopy (STM). The STM and Ultra High Vacuum System (UHV) in which the microscope is installed, are described. A brief history of the studies on the reconstruction and fundamental features of the Si(001) surface is also given. First, the sample and tip preparation techniques were optimized. Sample preparation method, which includes both ex situ chemical and in situ heating cleaning procedures, was found not to give routinely the clean and atomically flat surfaces, because of the criticality of the temperature values used during heat treatments. The monoatomic steps, dimer rows, defects such as missing dimer and dimer groups, were observed on clean Si(001) surfaces. Double height step formation due to contamination was also detected on a few samples. Buckling of dimers, which is believed to be due mainly to either the high defect density or tip-surface interaction, was observed on one sample. Si and Ge were grown epitaxially on the silicon substrate, with 0.11 ML and 3.2 ML coverages, respectively. The Si growth on Si(001) was found to occur as island formation because of the low substrate temperature (ca. 300 degrees C). Strong shape anisotropy and diffusional anistropy in the growth have been observed. On the other hand, the large coverage of Ge on Si(001) at a relatively high substrate temperature (ca. 500 degrees C) resulted in step flow growth rather than individual island formation on the terraces.
Open Access
Gain measurements via spontaneous emission in quantum well semiconductor lasers
(Bilkent University, 1996) Azfar, Talal
111 this work ail analysis of gain in single c|iianliini well last'rs as a riinction of sonu' of llu'ir o|)(M’alioiial paranu'l('rs is earrii'd oiil. I^'irsl, a 1 li('or('liea.l inodi'l of gain is |)resent('d. 'riuMi two diderent nudliods of gain iiK'asiir('in('nt, wliieli iis(' the spontaneous emission IVom the iaec't and the nnamplified s[)ontan('ons ('mission from the top of the ridg(', arc' discussed. Indirication procc'sses of lasers to racilitate tJie colh'clion of unampliiic'd spontaiK'ous ('mission are ch'tailed. R('S|)onse of the gain sp('ctrum to chang('s in inject('d cnrr('nt (h'lisity and temperature are measur('d and iinderstood in terms of hand Idling, hand gap renormalization and tenpx'rature d('pendenc(' of tlu' handgap. (!ain sainration above threshold is v('ri(ied and s|)atial variations in spontaiu'ons ('mission in lh(' longitiidnal and hiteral directions are observed.