Browsing by Subject "Semiconductors."
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Item Open Access Carrier dynamics in silicon and Germanium nanocrystals(Bilkent University, 2008) Sevik, CemThis is a computational work on the Si and Ge nanocrystals (NCs) embedded in wide band gap host matrices. As the initial task, extensive ab initio work on the structural and electronic properties of various NC host matrices, namely, SiO2, GeO2, Si3N4, and Al2O3 are preformed. The structural parameters, elastic constants, static and optical dielectric constants are obtained in close agreement with the available results. Furthermore, recently reported high density cubic phase of SiO2 together with GeO2 and SnO2 are studied and their stable highdielectric constant alloys are identified. Based on the ab initio study of host matrices, two related high field phenomena, vital especially for the electroluminescence in Si and Ge NCs, are examined. These are the hot carrier transport through the SiO2 matrix and the subsequent quantum-confined impact ionization (QCII) process which is responsible for the creation of electron-hole pairs within the NCs. First, the utility and the validity of the ab initio density of states results are demonstrated by studying the high field carrier transport in bulk SiO2 up to fields of 12 MV/cm using the ensemble Monte Carlo technique. Next, a theoretical modeling of the impact ionization of NCs due to hot carriers of the bulk SiO2 matrix is undertaken. An original expression governing the QCII probability as a function of the energy of the hot carriers is derived. Next, using an atomistic pseudopotential approach the electronic structures for embedded Si and Ge NCs in wide band-gap matrices containing several thousand atoms are employed. Effective band-gap values as a function of NC diameter reproduce very well the available experimental and theoretical data. To further check the validity of the electronic structure on radiative processes, direct photon emission rates are computed. The results for Si and Ge NCs as a function of diameter are in excellent agreement with the available ab initio calculations for small NCs. In the final part, non-radiative channels, the Auger recombination (AR) and carrier multiplication (CM) in Si and Ge NCs are investigated again based on the atomistic pseudopotential Hamiltonian. The excited electron and excited hole type AR and CM and biexciton type AR lifetimes are calculated for different sized and shaped NCs embedded in SiO2 and Al2O3. Asphericity is also observed to increase the AR and CM rates. An almost monotonous size-scaling and satisfactory agreement with experiment for AR lifetime is obtained considering a realistic interface region between the NC core and the host matrix. It is further shown that the size-scaling of AR can simply be described by slightly decreasing the established bulk Auger constant for Si to 1.0×10−30cm6 s −1 . The same value for germanium is extracted as 1.5×10−30cm6 s −1 which is very close to the established bulk value. It is further shown that both Si and Ge NCs are ideal for photovoltaic efficiency improvement via CM due to the fact that under an optical excitation exceeding twice the band gap energy, the electrons gain lion’s share from the total excess energy and can cause a CM. Finally, the electron-initiated CM is predicted to be enhanced by couple orders of magnitude with a 1 eV of excess energy beyond the CM threshold leading to subpicosecond CM lifetimes.Item Open Access Cascading and modifying nonradiative energy transfer mechanisms in strong coupling region of plasmons and excitons in semiconductor quantum dots(Bilkent University, 2010) Akın, OnurNonradiative energy transfer finds important applications in nanophotonics and nanobiotechnology including nanoscale optical waveguiding and biological nanosensors. Various fluorophores can take part in such energy transfer interactions in close proximity of each other. Their emission kinetics can be strongly modified and controlled as a result. For example, colloidal semiconductor quantum dots, also known as nanocrystals, have widely been shown to serve as donors and acceptors among themselves or with other fluorescent species to transfer excitation energy nonradiatively. In their close proximity, emission characteristics of such fluorophores can also be altered when coupled with plasmonic structures, e.g., metal nanoparticles. One favored result of these plasmon-exciton interactions is the emission enhancement. In principle it is possible to plasmon-couple acceptor-donor pairs of nonradiative energy transfer to modify their transfer rate. Such plasmon-mediated energy transfer has been demonstrated, where both acceptor-donor pairs are plasmoncoupled. In these cases, however, the resulting plasmon-exciton interactions are not controlled to take place either at the donor site or the acceptor site but at both of the sites. Therefore, it has previously not been possible to identify the coupled interactions. In this thesis, we propose and demonstrate cascaded plasmonic - nonradiative energy transfer interactions that are controlled by selectively plasmon-coupling either only the donor quantum dots or only the acceptor quantum dots. For that, we designed a novel self-assembly architecture of our hybrid layered systems of semiconductor nanocrystals and metal nanoparticles in a bottom-up fashion through precise spatial and spectral control. This scheme uniquely allowed for the ability to spatially control plasmonexciton interactions to take place either at the “start” site (donors) or “finish” site (acceptors) of the energy transfer. This control was achieved by placing the plasmonic layer in the right proximity of the donors (for strong donor-exciton plasmon-coupling) while sufficiently being far away from the acceptors (for weak acceptor-exciton plasmon-coupling), or vice versa. Here we comparatively studied and analyzed consequent modifications of quantum dot emission kinetics in response to both cases of plasmon-coupling to only the donors and to only the acceptors through steady-state and time-resolved photoluminescence measurements, along with their lifetime and rate calculations. Such cascaded energy transfer interactions in the strong exciton-plasmon coupling region hold great promise for innovative near-field photonic devices and biological tags. system.Item Open Access Electric field dependent optoelectronic nature of InGaN/GaN quantum structures and devices(Bilkent University, 2012) Sarı, EmreIn the past two decades we have been witnessing the emergence and rapid development of III-Nitride based optoelectronic devices including InGaN/GaN light-emitting diodes (LEDs) and laser diodes with operation wavelengths ranging from green-blue to near-UV. These InGaN/GaN devices are now being widely used in applications important for lighting, displays, and data storage, collectively exceeding a total market size of 10 billion USD. Although InGaN/GaN has been studied and exploited very extensively to date, its field dependent nature is mostly unknown and is surprisingly prone to quite unexpected behavior due to its intrinsic polarization property. In this thesis, we report our systematic study on the electric field dependent characteristics of InGaN/GaN quantum structures and devices including modulators and LEDs. Here we present our comparative study of electroabsorption in polar c-plane InGaN/GaN multiple quantum wells (MQWs) with different built-in polarization induced electrostatic fields. Analyzing modulator structures with varying structural MQW parameters, we find that electroabsorption grows stronger with decreasing built-in electrostatic field strength inside the well layer, as predicted by our theoretical model and verified by our experimental results. To further explore the field dependent optoelectronic nature of c-plane grown InGaN/GaN quantum structures, we investigate radiative carrier dynamics, which is of critical importance for LEDs. Our time and spectrum resolved photoluminescence measurements and numerical analyses indicate that the carrier lifetimes, the radiative recombination lifetimes, and the quantum efficiencies all decrease with increasing field. We also study the physics of electroabsorption and carrier dynamics in InGaN/GaN quantum heterostructures grown intentionally on nonpolar a-plane of the wurtzite crystal structure, which are free of the polarization-induced electrostatic fields. We compare these results with the conventional c-plane grown polar structures. In the polar case, we observe blue-shifting absorption profile and decreasing carrier lifetimes with increasing electric field. In the nonpolar case, however, we observe completely the opposite: a red-shifting absorption profile and increasing carrier lifetimes. We explain these observations in the context of basic physical principles including Fermi‟s golden rule and quantum-confined Stark effect. Also, we present electroabsorption behavior of InGaN/GaN quantum structures grown using epitaxial lateral overgrowth (ELOG) in correlation with their dislocation density levels and in comparison to steady state and time-resolved photoluminescence measurements. The results reveal that ELOG structures with decreasing mask stripe widths exhibit stronger electroabsorption performance. While keeping the ELOG window widths constant, compared to photoluminescence behavior, however, electroabsorption surprisingly exhibits the largest performance variation, making the electroabsorption the most sensitive to the mask stripe widths. This thesis work provides significant insight and important information for the optoelectronics of InGaN/GaN quantum structures and devices to better understand their field dependent nature.Item Open Access Electronic structure of Si/Ge semiconductor superlattices(Bilkent University, 1988) Gülseren, OğuzA 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.Item Open Access Ground-state properties of double-wire semiconducting systems(Bilkent University, 1997) Mutluay Müstecaplıoğlu, NihalWith the recent advances in nanometer-scale semiconductor device fabrication technology, it became experimentally possible to produce strongly confined electron systems. Quantum wires are among these systems, and are attracting increasing interest due to their potential applications in solid-state device technology such as high-speed transistors, efficient photodetectors and lasers. Quantum wires are quasi-one-dimensional systems where electrons are free to move in one dimension, but their motion is restricted in the remaining two dimensions. Various models for qucisi-one-dimensional structures have been proposed in the literature, such as cylindrical, square-well and parabolic confinements. in this thesis, we examine ground-state correlations in double-quantum-wire systems within the self-consistent scheme of Singwi et ai, namely the STLS approximation. The model we adopt consists of two parallel cylindrically-confined quantum wires. The cases when both wires have electrons as charge carriers and when one wire has electrons while the other has holes are considered. Under the assumption that only one subband is occupied in each quantum wire and there is no tunneling between them, we calculate the local-field factors and static correlation functions. Ground-state energy and collective modes are discussed within the RPA, Hubbard and STLS approximations in order to compare the results. Charge-density-wave instabilities in these structures are examined at small and finite q values. Our numerical results are given for systems where the carrier densities and the radii of both wires are equal. As the charge carrier density is lowered, we observe that the importance of local field corrections increases so that the RPA or Hubbard approximations do not give reliable results in this region. We find that the interwire correlations become quite important for electron-hole systems. Taking into account the exchange-correlation hole around electrons, STLS provides a much better description to this many-body problem compared to the previous models.Item Open Access Hot electron interactions in nanostructures(Bilkent University, 1997) Kaya, Ismet InonuModern semiconductor growth and processing techiques have provided the capability of fabricating a huge variety of devices which have atomically precise layered structures and lateral patterns with nanometer sizes. This not only provided novel device possibilités but also opened a new field in condensed matter physics, so called mesoscopics. It does not seem likely that the mesoscopic electronic devices will be available in the near future. Two main obstacles for mesoscopic electronics are the low temperature requirements and the breakdown of the phase coherence of the carriers as their energies exceed the Fermi level. This strongly suggests the investigation of the excited carriers with energies well in excess of their thermal equilibrium energy as the dimensions shrink. In this thesis, the interactions of hot electrons in semiconductor and metal structures with deep submicron characteristic dimensions have been studied. Tunneling Hot Electron Transfer Amplifier (THETA) constructed by abrupt semiconductur heterojunctions is a perfect system to analyze the interaction of hot electrons with cold electrons and the other possible excitations in solids. Recently, it has been discovered that an electron multiplication effect took place in such devices under certain conditions and resulted in a transfer ratio of greater than unity. In this work a novel fabrication technique has been developed. It would make it possible to utilize this effect for fabrication of a high frequency oscillator in the THz regime, in a future work. In addition, a kind of lateral THETA device has been constructed using a Two Dimensional Electron Gas structure. Electron multiplication effect for the first time has been observed in 2DEG structures. Moreover, the dependence of the effect on parameters such as injection energy, emitter and collector barrier heights and electron transit length has been investigated. The other direction of the work has been the investigation of metal wires under extremely high current densities. A strong nonlinearity in conductivity is introduced when a free standing submicrometer wire is biased to heat upto very high temperatures. The geometry of two crossing wires has been investigated under this condition.Item Open Access InGaN/GaN quantum electroabsorption modulators with record breaking electroabsorption in blue(Bilkent University, 2007) Sarı, EmreItem Open Access Light scattering from core-shell nano-structures : structural coloration(Bilkent University, 2013) Dolaş, Muhammet HalitIn this work, we produced kilometer-long semiconducting cylindrical nanostructures by using a top-to-bottom nano-fabrication technique which was recently developed in our research group. Comparison of commonly used methods of producing nano-structures such as electrospinning and nano-imprint lithography versus iterative thermal size reduction (ISR) is done in terms of uniformity, geometry control, multi-material compatibility, yield and device integrability. While the others cannot fulfil all requirements, ISR shows impressive results in all aspects. From very beginning to end, all steps of production and characterization of nano-wires produced by ISR, the design, chalcogenide glass production, preform preparation, fiber drawing, iterative size reduction, chemical etching and imaging are explained in details. In addition, production and characterization of nanospheres by in-fiber fluid instability which is based on Plateau-Rayleigh instability is also demonstrated. Theoretical study on scattering from small particles, Mie scattering, which is one of the mechanisms for structural coloration together with thin film interference, multilayer interference, diffraction grating and photonic crystals is done. Structural coloration due to scattering from small particles is simulated using Finite Domain Time Difference (FDTD) method and compared with theoretical results estimated for nano-wire and nano-sphere cases. Results are confirmed with observation of structural coloration by taking dark field optical microscopy images of the final products of ISR and in-fiber fluid instability processesItem Open Access Localized plasmon-coupled semiconductor nanocrystal emitters for innovative device applications(Bilkent University, 2007) Soğancı, İbrahim MuratQuantum confinement allows for the development of novel luminescent materials such as colloidal semiconductor quantum dots for a variety of photonic applications spanning from biomedical labeling to white light generation. However, such device applications require efficient photoluminescence. To this end, in this thesis we investigate the spontaneous emission characteristics of semiconductor nanocrystal emitters under different conditions and their enhancement and controlled modification via plasmonic resonance coupling, placing metallic nanoparticles in their proximity, for innovative device applications. We first present our theoretical and experimental work on the optical characterization of nanocyrstals (e.g., CdSe, CdS, and CdSe/ZnS) including absorption/photoluminescence, time-resolved luminescence, and excitation spectra measurements. Here we demonstrate very strong electromodulation (up to 90%) of photoluminescence and absorption of such nanocrystals (nanodots and nanorods) for optical modulator applications. Second, we present our electromagnetic modeling on the optical response of metal nanoparticles using finite-difference-time-domain method. For the first time, using localized plasmons of metal nanoisland films (nano-silver) carefully spectrally and spatially tuned for optimal coupling conditions, we report very significant controlled modifications of nanocrystal emission including the peak emission wavelength shift (by 14nm), emission linewidth reduction (by 10nm with 22% FWHM reduction), photoluminescence intensity enhancement (15.1- and 21.6-fold compared to the control groups of the same nanocrystals with no plasmonic coupling and those with identical nano-silver but no dielectric spacer in the case of non-radiative energy transfer, respectively), and selectable peaking of surface-state emission at desired wavelengths. Such localized plasmonic engineering of nanocrystal emitters opens new possibilities for our lightemitting and photovoltaic devices.Item Open Access Microfabrication of micro Hall sensors on GaAs and INSB for scanning hall probe microscopy(Bilkent University, 2001) Kaval, Mehmet MuratDifferent techniques have been developed to investigate the surface magnetic structure of materials, including Hall probe, scanning superconductivity quantum interference device (SQUID) and magnetic force microscopies (MFMs), Bitter decoration, Faraday rotation, and electron holography. Recently, the scanning Hall probe microscope (SHPM) has been shown to be a very sensitive, noninvasive instrument with which to obtain quantitative measurements of surface magnetic field profiles with high spatial resolution ( 120 nm)under variable temperature and magnetic field operation. In this thesis we are going to present microfabrication of Hall probes of < 1 micron on GaAs/AlGaAs 2DEG and InSb material to be used for Scanning Hall Probe Microscopy (SHPM) for magnetic imaging at sub-micron scale. First images obtained by InSb Hall probes will be presentedItem Open Access Novel design-based complex nanostructures in hybrid core-shell architectures for high-efficiency light generation(Bilkent University, 2010) Özel, İlkem ÖzgeRecent developments in nanoscience and nanotechnology have given rise to the discovery of hybrid nanostructured multi-component materials that serve several tasks all at once. A very important and rapidly growing field of these materials is the development of highly efficient fluorophores to meet the urgent demand of low-energy consuming, high-quality light emitters for future solid-state lighting applications. Such hybrid nanomaterials are entailed to exhibit extraordinary optoelectronic properties compared to the bulk case of their single components such as enhanced quantum efficiency, tunable multi-color emission, and reduction of multiple processing steps. Herein, to address these requirements, we propose and demonstrate novel design-based complex nanomaterials in hybrid multi-shell architectures for high-efficiency light generation. These requirements are made possible by using the concept of hybrid core-shell-… nanostructures comprising at least two units, including hybrid metalcore/dielectric-shell nanoparticles furnished with an outer shell of semiconductor nanocrystals for enhanced emission and different conjugated polymers forming a single multi-polymer nanoparticle and emitting simultaneously at different wavelengths. In the first part of this thesis, we developed and demonstrated Au-silica core/shell nanoparticles that successfully assemble CdTe nanocrystals right on their silica shells for enhanced plasmonexciton interactions, while solving the common problems of lacking control in dielectric spacing and limited film thickness typically encountered in such plasmon-coupled nanocrystals. Here we present the synthesis and characterization results of this new set of multi-shell decorated nanoparticle composites with a tunable dielectric spacing thickness of silica shell precisely controlled by synthesis to optimize plasmon-exciton interactions for enhanced emission. Experimental data obtained from steady-state and time-resolved photoluminescence measurements together with extensive computational analysis clearly verify the strong plasmon-exciton interactions in these designbased multi-shell nanocomposites. In the second part, we construct bi-polymer nanoparticle systems in various architectures of core/shells, for each of which thorough investigations of the non-radiative energy transfer mechanisms are made. Here we present the synthesis and characterization results of these core/shell bi-polymer nanoassemblies. The flexibility of designing such bipolymer nanostructures allows for the optimization of maximum energy transfer efficiency. This concept of complex hybrid nanostructures for high-efficiency light generation opens up new paths for optoelectronic devices and nanophotonics applications including those in solid-state lighting.Item Open Access The phase behavior and synthesis of mesostructured coupled semiconductor thin films : MESO-CdS-TiO2(Bilkent University, 2009) Okur, Halil İbrahimMesostructured [Cd(H2O)4](NO3)2 - titania - P123 ((PEO)20(PPO)70(PEO)20, PEO = -OCH2CH2-, PPO = -OCH(CH3)CH2-) materials have been investigated by changing the [Cd(H2O)4](NO3)2 and titania content of the structures. This has been achieved by making thick samples by casting and thin film samples by spin coating of a butanol solution of [Cd(H2O)4](NO3)2, P123, nitric acid and Ti(OC4H9)4. The film samples are named as meso-xCd(II)-yTiO2, where x is the Cd(II)/P123 and y is TiO2/P123 mole ratios. Increasing the titania amount in the media has transformed the samples from LC-like to soft and then to rigid mesostructured materials. Changing the amount of [Cd(H2O)4](NO3)2 salt in the media only influenced the mesostructure, such that no change on the mechanical properties is observed. However, the synthesis of rigid mesostructured titania materials required controlled humidity. The rigid film samples were prepared first by spin coating and then by aging under a 50% humidity oven. The mesostructure remains stable upon H2S reaction, in the soft and rigid materials region. However, only rigid samples stand to removal of nitrates from the media that is important to keep the CdS nanoparticles stable in or on the pore walls of mesostructured film samples. The phase behavior of the meso-Cd(II)-TiO2, the structural properties of the meso-xCdS-yTiO2 samples, coordination and elimination of the NO3 - ions and the particle size of the CdS nanocrystallites were investigated using diffraction (XRD), spectroscopy (FT-IR, Raman and UV-Vis absorption, EDS) and microscopy (POM, SEM, and TEM) techniques.Item Open Access Silicon Germanium multi quantum wells for high efficiency optoelectronic devices(Bilkent University, 2011) Yeşilyurt, AlperItem Open Access Slow light in Germanium nanocrystals(Bilkent University, 2009) Keleş, ÜmitThe phenomena of quantum coherence has been applied with great success in the atomic systems. For optoelectronic applications the interest is inherently directed towards the semiconductor heterostructures. Large number of works have proposed and analyzed the atomic quantum coherence effects in the semiconductors. In this respect, nanocrystals (NCs) are very promising structures for seeking the quantum coherence phenomena due to their atomic-like electronic structure. Furthermore, their robust structure, integrability and larger excitonic lifetimes with respect to atomic systems makes them more promising candidates for the technological applications. Within an atomistic pseudopotential electronic structure framework, the optical Bloch equations (OBEs) originating from atomic coherence theory are derived and solved numerically for Ge NCs. The results are interpreted in the context of coherent population oscillations (CPO). Narrow dips are observed in the absorption profiles which corresponds to high dispersions within a transparency window and produce slow light. A systematic study of the size-scaling of slow-down factor with respect to NC diameter and controllable slow light by applying external Stark field are provided. The results indicate that Ge NCs can be used to generate optically and electrically controllable slow light. The many-body Coulomb interactions which underlie the quantum coherence and dephasing are of central importance in semiconductor quantum confined systems. The effects of many-body interactions on the optical response of Ge NCs have been analyzed. The semiconductor optical Bloch equations (SBEs) are derived in a semiclassical approach and the Coulomb correlations are included at the level of Hartree-Fock approximation.Item Open Access Synthesis and characterization of highly efficient CdSe/CdS core/shell nanocrystals with silar technique(Bilkent University, 2012) Keleştemur, YusufOwing to their size tunable electronic structure and optical properties, semiconductor nanocrystal quantum dots (NQDs) have become attractive for a wide range of device applications ranging from life sciences to electronics in the last two decades. However, highly efficient and stable NQDs are essential to reaching high performance with these devices utilizing NQDs. In this thesis, to meet these requirements, a new class of CdSe/CdS core/shell NQDs are studied including their colloidal synthesis and nanocharacterization. In this work, CdSe/CdS core/shell NQDs were synthesized with successive ion layer adsorption and reaction (SILAR) technique, which enabled highly precise shell thickness control and uniform coating of the shell material. When compared to the most commonly used CdSe/ZnS core/shell NQDs, CdSe/CdS core/shell NQDs were found to provide important advantages. First, the lattice mismatch within CdSe and CdS (3.9%) is lower than that within CdSe and ZnS (12%), which was very critical for obtaining highly efficient NQDs. Second, as a result of having lower bandgap in CdS, great enhancement in absorption cross section was achieved with more red-shifted emission, which is not possible with CdSe/ZnS core/shell NQDs. Moreover, suppression of Auger recombination was successfully observed with the partial separation of electron and hole wavefunctions in the synthesized CdSe/CdS core/shell NQDs. With all these attractive properties that were experimentally measured, CdSe/CdS core/shell NQDs were found to make better alternatives to CdSe/ZnS core/shell for numerous applications.Item Open Access Thermoelectric efficiency in model nanowires(Bilkent University, 2013) Badalov, SabuhiNowadays, the use of thermoelectric semiconductor devices are limited by their low efficiencies. Therefore, there is a huge amount of research effort to get high thermoelectric efficient materials with a fair production value. To this end, one important possibility for optimizing a material’s thermoelectric properties is reshaping their geometry. The main purpose of this thesis is to present a detailed analysis of thermoelectric efficiency of 2 lead systems with various geometries in terms of linear response theory, as well as 3 lead nanowire system in terms of the linear response and nonlinear response theories. The thermoelectric efficiency both in the linear response and nonlinear response regime of a model nanowire was calculated based on Landauer-B¨uttiker formalism. In this thesis, first of all, the electron transmission probability of the system at the hand, i.e. 2 lead or 3 lead systems are investigated by using R-matrix theory. Next, we make use of these electron transmission probability of model systems to find thermoelectric transport coefficients in 2 lead and 3 lead nanowires. Consequently, the effect of inelastic scattering is incorporated with a fictitious third lead in the 3 lead system. The efficiency at maximum power is especially useful to define the optimum working conditions of nanowire as a heat engine. Contrary to general expectation, increasing the strength of inelastic scattering is shown to be a means of making improved thermoelectric materials. A controlled coupling of the nanowire to a phonon reservoir for instance could be a way to increase the efficiency of nanowires for better heat engines.