Doktora Tezleri

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Cosmological implications of affine gravity
(Izmir Institute of Technology, 2018-04) Azrı, Hemza; Demir, Durmuş Ali
The main aim of this thesis is to reveal some interesting aspects of the purely affine theory of gravity and its cosmological implication. A particular attention will be devoted to its consequences when applied to cosmological inflation. Primarily, affine spacetime, composed of geodesics with no notion of length and angle, accommodates gravity but not matter. The thesis study is expected to reveal salient properties of matter dynamics in affine spacetime and may reveal an intimate connection between vacuum state and metrical gravity. An interesting application of the framework is the inflationary regime, where it is shown that affine gravity prefers only a unique metric tensor such that the transition from nonminimal to minimal coupling of the inflaton is performed only via redefinition of latter. This allows us to avoid the use of the so called conformal frames. In fact, unlike metric gravity, the metric tensor in affine gravity is generated and not postulated a priori, thus this tensor is absent in the actions and conformal transformation does not make sense. Last but not least, we try to show how metric gravity can be induced through a simple structure that contains only affine connection and scalar fields. General relativity arises classically only at the vacuum, and this view of gravity may be considered as a new way to inducing metric elasticity of space, not through quantum corrections as in standard induced gravity, but only classically. The thesis is concluded by analyzing affine gravity in a particular higher-dimensional manifold (product of two spaces) in an attempt to understand both, the cosmological constant and matter dynamically.
Electronic struture of organic molecules containing transition-metal atoms
(Izmir Institute of Technology, 2019-07) Kandemir, Zafer; Bulut, Nejat
Hemoglobin including iron atom, vitamin B12 containing cobalt atom and ruthenium- based dye molecules are examples of organic molecules. We explore whether electron correlations arising from transition-metal atoms have any special role in the functioning of organic molecules using the effective multi-orbital Anderson model. We choose deoxy and oxy-heme molecules which are examples of hemoglobin derivatives because they have many experimental and theoretical studies. The experimental magnetic susceptibility measurements find that deoxy and oxy-heme molecules exhibit a high-spin to low-spin transition. We use four different computational methods: density functional theory (DFT), DFT+U, DFT+mean-field approximation (DFT+MFA) and DFT+quantum Monte Carlo (DFT+QMC) to study this transition. In this thesis, we compare the results of these methods with each other and the experimental results. DFT and DFT+U methods do not yield the high-spin state for deoxy-heme. DFT method correctly does not find the location of impurity bound state (IBS) known as correlated new electronic states. These methods obtain low-spin for oxy-heme, but they find that magnetic correlations are very small. DFT+MFA works well for high-spin, but this technique does not obtain low-spin because it does not find the location of IBS correctly. DFT+QMC gives the high(low)- spin state for deoxy-heme (oxy-heme) and finds IBS and magnetic correlations. We obtain that DFT+QMC works better among these methods for deoxy and oxy-heme molecules. Moreover, we investigate whether we can observe the IBS and magnetic correlations for vitamin B12, dye molecules and single-atom catalysts by using these computational approaches.
Electronic, magnetic and optical properties of disordered graphene quantum dots
(Izmir Institute of Technology, 2018-07) Altıntaş, Abdulmenaf; Güçlü, Alev Devrim
In this thesis, we theoretically investigate electronic, magnetic and optical properties of disordered graphene quantum dots. The numerical calculations are performed using a combination of tight-binding, mean-field Hubbard and configuration interaction methods. We focus on the effects of long-range disorder and electron-electron interactions on the optical properties and the effects of atomic defect related short-range disorders and electron-electron interactions on Anderson type localization and the magnetic properties of hexagonal armchair graphene quantum dots. For the case of long-range disorder, we show that, when the electron-hole puddles are present, tight-binding method gives a poor description of the low-energy absorption spectra compared to meanfield and configuration interaction calculation results. As the size of the graphene quantum dot is increased, the universal optical conductivity limit can be observed in the absorption spectrum. When disorder is present, calculated absorption spectrum approaches the experimental results for isolated monolayer of graphene sheet. On the other hand, for the case of short-range related disorder, we observe that randomly distributed defects with concentrations between 1-5% of the total number of atoms leads to electronic localization alongside magnetic puddle-like structures. We show that localization length is not affected by magnetization if there is an even distribution of defects between the two sublattices of the honeycomb lattice. However, for an uneven distributions, localization is found to be significantly enhanced.
Electronic, magnetic and transport properties of graphene quantum dots with charged impurities
(Izmir Institute of Technology, 2020-12) Polat, Mustafa; Güçlü, Alev Devrim; Izmir Institute of Technology
In this thesis, electronic, magnetic, and transport properties of armchair edged hexagonal and zigzag edged triangular graphene quantum dots (GQDs) are investigated in the presence of charged impurities. In this manner, a special attention has been paid to the Coulomb impurity problem in these structures. The collapse of the wave functions starting from the 1S$_{1/2}$ state is studied in the presence of not only the Coulomb impurity but also in the presence of a Coulomb charged vacancy with the help of tight-binding and extended mean-field Hubbard (MFH) models. Here, we report an interaction induced renormalization of the critical coupling constant ($\beta_{c}$). In addition, our results suggest that the induced charge for the interacting fermions is smaller than that of the non-interacting fermions. Furthermore, the transport coefficients reveal two different characteristics of the subcritical ($\beta$ $<$ $\beta_{c}$) and supercritical ($\beta$ $>$ $\beta_{c}$) regimes. As for the charged vacancy, the bare carbon vacancy induces a local magnetic moment in the hexagonal GQDs, but it is suppressed when the vacancy is charged with the subcritical Coulomb potential. Except the pristine cases of the GQDs, we numerically study a Coulomb impurity problem for the interacting fermions restricted in disordered hexagonal GQDs. In the presence of randomly distributed lattice defects and spatial potential fluctuations induced by Gaussian impurities, the response of $\beta_{c}$ for atomic collapse is mainly investigated by local density of states (LDOS) calculations within the MFH model. We find that both types of disorder cause an amplification of the critical threshold. As for the zigzag edged triangular GQDs, in the presence of the bare vacancy, we exactly obtain the spin splitting with the help of LDOS calculations in the energy spectrums, which are dominated by the edge states around the Fermi level. Similar to the hexagonal GQDs, if the vacancy is charged, the local magnetic moment disappears in these GQDs.
Electronic, magnetic, and mechanical properties of novel two dimensional monolayer materials
(Izmir Institute of Technology, 2017-07) Yağmurcukardeş, Mehmet; Senger, Ramazan Tuğrul; Şahin, Hasan
Layered materials exhibit different properties when they are thinned down to a few monolayers. Following the successful isolation of graphene in 2004, there has been a rapid increase in the number of studies focusing on other novel two dimensional (2D) materials such as hexagonal Boron Nitride (BN), transition metal dichalcogenides (TMDs), post transition metal chalcogenides (PTMCs), and in-plane anisotropic monolayers (Redichalcogenides and blackphosphorus). In addition to their electronic, optical, and magnetic properties, mechanical properties of 2D materials are of fundamental importance. Measurements of elastic constants of 2D materials are still challenging. Therefore, theoretical investigation of the mechanical properties is particularly important. Moreover, investigation of Raman spectra of these materials requires a through understanding of their vibrational properties. In these regards, we investigate the electronic, magnetic, and mechanical properties of some novel monolayer 2D materials (such as, auxetic pentagonal monolayers, flexible monolayers of holey graphene crystals, ultra-flexible monolayers of PTMCs, and in-plane anisotropic monolayers of ReS2 and blackphosphorus) by means of first-principles calculations based on density functional theory (DFT). In addition, tuning electronic properties of a van der Waals heterobilayer structure composed of monolayers of Mg(OH)2 and WS2 upon an external out-of-plane electric field is studied. The effect of biaxial strain on the vibrational properties of novel 2D materials is also studied through their off-resonant Raman activities. Our findings will be useful to clarify several issues related to the experiments of novel 2D materials.
Gauged and geometric vector fields at the MeV scale
(Izmir Institute of Technology, 2020-07) Puliçe, Beyhan; Demir, Durmuş Ali
In this thesis, we have studied gauged and geometric vector fields at the MeV scale in two main parts. The basic framework of these two parts are given briefly as follows. In the first part (Chapter \ref{chapter-U(1)}), we have built a family-nonuniversal $U(1)^\prime$ model populated by an MeV-scale sector with a minimal new field content which explains the recent anomalous beryllium decays. Excited beryllium has been observed to decay into electron-positron pairs with a $6.8~\sigma$ anomaly. The process is properly explained by a $17$ MeV proto-phobic vector boson. In this thesis, we consider a family-nonuniversal $U(1)^{\prime}$ that is populated by the $U(1)^{\prime}$ gauge boson $Z^\prime$ and a scalar field $S$. The kinetic mixing of $Z^\prime$ with the hypercharge gauge boson, as we show by a detailed analysis, generates the observed beryllium anomaly. We show that beryllium anomaly can be explained by an MeV-scale sector with a minimal new field content. In the second part (Chapter \ref{chapter-GDM}), we have shown how a light vector particle can arise from metric-affine gravity and how this particle fits the current data and constraints on the dark matter. We show that, metric-affine gravity , which involves metric tensor and affine connection as two independent fields, dynamically reduces, in its minimal form, to the usual gravity plus a massive vector field. The vector $Y_\mu$ is neutral and long-living when its mass range lies in the range $9.4\ {\rm MeV} < M_Y < 28.4\ {\rm MeV}$. Its scattering cross section from nucleons, which is some 60 orders of magnitude below the current bounds, is too small to facilitate direct detection of the dark matter. This property provides an explanation for whys and hows of dark matter searches. We show that due to its geometrical origin the $Y_\mu$ couples only to fermions. This very feature of the $Y_\mu$ makes it fundamentally different than all the other vector dark matter candidates in the literature. The geometrical dark matter we present is minimal and self-consistent not only theoretically but also astrophysically in that its feebly interacting nature is all that is needed for its longevity.
Modelling electronic and structural properties of graphene and transition metal chacogenide nanostructures
(Izmir Institute of Technology, 2016-07) Özaydın, Hediye Duygu; Senger, Ramazan Tuğrul
The purpose of this thesis is to investigate the electronic and structural properties of one- and two-dimensional materials such as graphene, graphene-like transition metal chalcogenides by using density functional theory. The single-atom thickness of graphene sheet is a novel material and attracts great interest due to its unique features. In recent years, theoretical and experimental studies on graphene provide quick knowledge and have opened up possibilities for many other two-dimensional new materials. Among these materials, especially transition metal chalcogenides have recently been the focus of studies of condensed matter physics. Unlike many superior properties of graphene, lack of band gap in electronic structure have highlighted the necessity of such transition metal chalcogenides materials for electronic applications. As compared to graphene, transition metal chalcogenides have various physical properties and possess sizable band gaps, for this reason they are promising candidate for many applications. Many experiments have revealed that the surfaces of graphene and graphene-like structures can play an active role as a host surface for clusterization of metal atoms. Motivated by these observations, we investigate characteristic properties of Pt atoms on graphene, MoS2 and TaS2. Similarly, TiSe2 is very recently synthesized two-dimensional transition metal dichalcogenide material and stable in 1T phase. Two-dimensional TiSe2 has a metallic electronic property and widely studied material. We analyze how to change the structural and electronic properties of TiSe2 by functionalization with hydrogen atom. Again to the effects of hydrogenation on two-dimensional TiSe2 monolayer we also study the structural and electronic properties of this material in nanoribbon form. At the same time, PtSe2 which is also very recently synthesized two-dimensional transition metal dichalcogenide and stable in 1T phase like TiSe2, its nanoribbon structural and electronic properties have also been investigated and compared with TiSe2 nanoribbons. Finally, TiS3 which is also transition metal chalcogenide but entirely different crystal structure, is recently widely studied materials. The structural and electronic properties as well as carrier mobility and strain response of TiS3 nanoribbons have been investigated. Besides many comprehensive theoretical studies, a lot of experimental studies are avaibale about the synthesis of these materials. In brief, these materials which tackles a contemporary and rapidly developing field, the nanoribbon form and functionalization of them that hold promise for many other applications.
Molecular beam epitaxy growth and characterization of CdTe heterostructures on GaAs-effect of interface, growth, and annealing conditions to crystal quality
(Izmir Institute of Technology, 2017-07) Arı, Ozan; Öztürk, Orhan
Highly crystalline CdTe structures are desired for solar cells, x-ray detectors, electro-optical modulators, and especially in Hg1-xCdxTe infra-red detectors. Epitaxial growth of Hg1-xCdxTe infra-red layers are usually performed on lattice matched bulk CdZnTe substrates. But, limited size and fragile nature of the CdZnTe have led to a push for alternative substrates such as GaAs. The large lattice mismatch between Hg1-xCdxTe and GaAs requires an implantation of a buffer layer such as CdTe. In addition (211)B orientation is preferred due to high sticking coefficient of Hg on this orientation and suppression of twin formation. In the first part of this study, the effect of the thermal deoxidation of GaAs(211)B surface on which CdTe layers grown was investigated by various in situ and ex situ experimental techniques. The changes in the surface chemical structure and morphology of GaAs(211)B substrates with As4 and In assisted deoxidation under various conditions were presented. Secondly, the effect of the growth conditions on CdTe epilayers by using molecular beam epitaxy were investigated in two parts; (1) the initiation of the CdTe growth and (2) the equilibrium growth conditions. The correlations between the structural defects, twins, point defects, and dislocations with the growth conditions are determined. Thirdly, the effect of the cyclic annealing to the crystal and surface quality of the CdTe epilayers were investigated by using different temperatures during the annealing. Finally, the effect of the temperature uniformity during the production of the CdTe layers was investigated by the two substrate heater geometries consisting of rotational symmetric and tilted at the edge. A new approach to study the dislocations with different types of cores proposed by Ayers is applied to the zinc blende (211) crystal orientation. It has been shown that the dislocations having two different cores responded differently to both growth and annealing conditions. The results of the experimental techniques probing the dislocation density in CdTe layers are not well correlated with each other due to dual origin of these dislocations. The compressive and biaxial stresses building in the CdTe layers due to growth and annealing conditions were resolved with the investigation of the optical properties of the layers.
Optical and electronic properties of atomically thin layered materials: First principles calculations
(Izmir Institute of Technology, 2019-07) İyikanat, Fadıl; Senger, Ramazan Tuğrul; Şahin, Hasan
The extraordinary interest in two-dimensional (2D) materials is increasing day by day. Thanks to advances in the experimental techniques, monolayer form of another material is synthesized every day with features not seen in the bulk form. Ab initio methods provide useful tools for characterizing and functionalizing the various properties of these materials. The results obtained through first principles quantum-mechanical calculations can help to predict and understand the experimental data, such as the position and source of the spectroscopic peaks in the Raman or optical absorption spectra. The aim of this thesis is to predict and functionalize the optical and electronic properties of atomically thin layered materials using density functional theory and approaches beyond. Within the scope of this thesis, possible technological applications of various 2D materials ranging from perovskite crystals to transition metal dichalcogenites are investigated by using several functionalization methods. In order to accurately predict the optical properties of these materials, it is very important to use approaches that take into account the many-body effects. Recent studies have shown that many-body perturbation theory in the form of GW approximation is highly reliable to calculate the quasiparticle properties of materials. By solving the Bethe Salpeter equation on top of GW calculation, the quasiparticle energies and excitonic properties, which have dominant effect in the optical properties of ultra-thin materials are examined in detail.
Optical properties of ultra-thin materials
(Izmir Institute of Technology, 2017-07) Bacaksız, Cihan; Senger, Ramazan Tuğrul; Şahin, Hasan
Many years of research effort, after the synthesis of graphene, have revealed that atomically thin two-dimensional materials have mechanical, electronic, and optical properties which are different from their bulk counterparts. Thus, the interest in twodimensional materials is growing which is also fueled by fast advances in synthesis and measurement techniques. In this regard, the theoretical and computational simulations provide physical insight to the experiments in this new and demanded field; a tool for characterizing these materials; and also a reliable prediction approach to possible stable structures. The density functional theory (DFT) is one of the most powerful and commonly used methods for such theoretical investigations. The DFT-based computational determination of optical properties, as compared to other usual DFT-based calculations, is in its early stage due to high computational resource requirements and lack of established documentation. Therefore, the present thesis aims at giving the methodology and computing the optical properties of ultra-thin materials by using DFT and beyond-DFT approaches. More precisely, the thesis provides an overview of light matter interaction; basics of DFT, GW approximation for many-body effects, Bethe-Salpeter equation for excitonic effects; and several applications of these on atomically-thin systems.
Performance enhancement of graphene/silicon based near-infrared Schottky photodiodes
(Izmir Institute of Technology, 2022-07) Fidan, Mehmet; Çelebi, Cem
This thesis presents an experimental investigation on the performance enhancement of graphene/silicon based near-infrared Schottky photodiodes. The photodiode devices were fabricated by transferring CVD graphene layers onto n-type silicon (n-Si) substrates. The samples exhibited strong Schottky diode character and had high spectral sensitivity at 905 nm peak wavelength. The Schottky contact characteristics of the samples (e.g., barrier height, ideality factor and sheet resistance) were determined by analyzing the current-voltage measurement data. All the samples demonstrated a clear photovoltaic activity under light illumination. The Schottky barrier height (SBH) in Gr/n-Si photodiodes was tuned as a function of light power density. Light power density driven modification of the SBH was correlated with the variation in the measured open-circuit voltage. The impact of junction area and number of graphene layers on the spectral responsivity and response speed of Gr/n-Si based Schottky photodiodes were also investigated. Firstly, three batches of Gr/n-Si photodiode samples with junction area of 4 mm2, 12 mm2 and 20 mm2 were produced by transferring monolayer CVD graphene on individual n-Si substrates. The sample with 20 mm2 junction area reached a spectral response of 0.76 AW-1, which is the highest value reported in the literature for self-powered Gr/n-Si Schottky photodiodes without the modification of graphene electrode. In contrast to their spectral responsivities, the response speed of the samples was found to be lowered as a function of the junction area. After that, we increased the number of graphene layers on n-Si. Wavelength-resolved and time-dependent photocurrent measurements demonstrated that both spectral responsivity and response speed are enhanced as the number of graphene layers is increased from 1 to 3 on n-Si substrates. This thesis showed that the device performance of Gr/n-Si Schottky photodiodes can be modified simply by changing the size of graphene electrode and/or as well as the number of graphene layers on n-Si without need of external doping of graphene layer or engineering Gr/n-Si interface.
Photonic crystal textiles
(Izmir Institute of Technology, 2022-07) Çetin, Zebih; Sözüer, Hüseyin Sami
Photonic crystals are man-made structures that can be used to manipulate the flow of light. They are classified as one-, two- and three-dimensional photonic crystals according to the periodic variation of the dielectric profile in space. Apart from artificial photonic crystals there are numerous examples of naturally occurring photonic crystals which have evolved mostly for structural coloration, such as wings of butterflies, natural opal gem stone, peacock feathers to name a few. Using photonic crystal structures the propagation of electromagnetic waves can entirely be prohibited by means of photonic band gap. Considering the fact that approximately two thirds of the heat loss of the human body occurs through electromagnetic radiation with a wavelength around 10 microns, it becomes important to consider photonic crystals for the purpose of reducing heat loss in textiles. We observe that the textile, by virtue of the fact that it has been produced by weaving, already has a periodic structure, and thus is a potential candidate for a photonic crystal. With the right fiber that the textile is woven and the right weave pattern, the textile itself would be a photonic crystal. The most common weave patterns used in the textile industry are plain weave, basket weave, dutch weave and twill weave. In this thesis, we used the finite-difference time-domain method to search for the optimum weave pattern to minimize heat loss by the human body.
Physics of higher spin fields
(Izmir Institute of Technology, 2020-12) Sargın, Ozan; Güçlü, Alev Devrim; Izmir Institute of Technology
Spin-3/2 fields are the next spin multiplet we look for in the general particle search. Although these fields can be either fundamental vector-spinors or just excited leptons and quarks we assume that they are fundamental throughout this thesis. These higher-spin fields, described by the Rarita-Schwinger equations have to obey certain constraints to have correct degrees of freedom when they are on the physical shell. \par In the first chapter after the introduction, we introduce these spinor-vector fields to the reader by first going through the different representations that can be employed to describe them. We then recapitulate some facts on the most general free lagrangian and the propagator for these fields. \par In the next chapters we investigate different phenomenological implications. We start out in chapter \ref{chap:1} with a massive spin-3/2 field hidden in the standard model (SM) spectrum thanks to the form of the special interaction that vanishes when the field falls into the mass shell. Different collider signatures are investigated through analytical computations and numerical predictions. \par In chapter \ref{chap:2}, we assume that the Higgs boson stays stable via a finely tuned hidden sector which involves a spin-3/2 field that is split from the SM and whose sole contact with it at the renormalizable level is through the neutrino portal. Then, the total mass correction to the Higgs mass is used as a constraint to calculate the mass scale of the spin-3/2 field. \par Lastly, we investigate the possible role that a spin-3/2 field could play in leptogenesis. Our model incorporates a spin-3/2 field in addition to the type-I see-saw fields in inducing the CP asymmetry and mitigating the naturalness problem of the Higgs boson. We investigate the plausibility in regard to successful leptogenesis with no side effects, specifically the naturalness of the Higgs boson and correct prediction of the active neutrino masses.
Studies on modified Newtonian dynamics and dark matter
(Izmir Institute of Technology, 2016-12) Karahan, Canan Nurhan; Demir, Durmuş Ali
The flat rotation curves of the galaxies are considered to be anomalous observations according to Newtonian dynamics. There are two dierent approaches to explain this challenge - Dark Matter (DM) and Modified Newtonian Dynamics (MOND). Both of them possess some failures as well as many successes. Beyond these failures, they have much more fundamental diculties such as the lack of any direct or indirect detection of proposed dark matter candidates and the lack of a full-fledged relativistic version of MOND theory. In this thesis, focus will be on these fundamental problems. First, the relativistic MOND theory will be studied. The first successful relativistic version is Tensor-Vector-Scalar (TeVeS) theory based on bimetric gravity. However, the addition of vector and scalar fields into the theory by hand is not much dierent than the addition of dark matter. In this study, TeVeS theory will be constructed in a more natural way. To do this, at first standard Einstein-Hilbert action will be extended by using non-Riemannian structures (torsion, non-metricity, etc.) from metric-ane formalism. It will then be shown that obtained extended theory of gravity could turn into a Tensor- Vector-Scalar theory via decomposition of ane connection as Levi-Civita connection and rank(1,2) tensorial structure composed of lower rank fundamental and composite fields such as vector and scalar fields. Subsequently, it will be continued with a study on the relativistic MOND theory, without requiring an action principle. In this study, energy momentum tensor part will be modified rather than the geometrical part of the Einstein field equations. This could be considered as the first dynamical approach to relativistic MOND in the literature. It will be shown that the modified field equations obtained via this dynamical approach can be reduced to true MONDian force in the non-relativistic limit in some astrophysical domains. This study can be also considered as an extension of Milgrom’s modified inertia approach to relativistic domain. Finally, a new phenomenological model- Higgsed Stueckelberg scenario - involving a hidden vector field with an accompanying scalar field ensuring the gauge invariance will be proposed. It will be shown that the contributions from the hidden fields could stabilize the Higgs boson mass at one-loop, where the set up can accommodate naturally a viable DM candidate.
Thermoelectric effect in layered nanostructures
(Izmir Institute of Technology, 2019-07) Özbal Sargın, Gözde; Senger, Ramazan Tuğrul; Sevinçli, Haldun
In this thesis, ballistic transport and thermoelectric (TE) properties of semiconducting and dynamically stable two-dimensional materials are investigated by combining first-principles calculations with Landauer formalism. Motivated by finding novel promising TE materials, transition metal dichalcogenides (TMDs) and oxides (TMOs) (namely MX2 with M = Cr, Mo, W, Ti, Zr, Hf; X = O, S, Se, Te) are studied systematically in their 2H- and 1T-phases in Chapter 3. Having computed structural, as well as ballistic electronic and phononic transport properties for all structures, we analyze the thermoelectric properties of the semiconducting ones. We report for the first time that, 2H-phases of four of the studied structures have very promising thermoelectric properties, unlike their 1T-phases. Next, ballistic transport and thermoelectric (TE) properties of group IIImonochalcogenides (group III-VI) are presented in a wide range temperature from 100 K to 1000 K. This large family composed of 25 compounds which stands out with their unique electronic band structures. In addition to Mexican hat shaped (quartic energy-momentum relation) valence band character, some of the structures exhibit valley degeneracies which can occur either in their conduction and valence bands. Moreover, TE and transport calculations are performed for BO and BS monolayers which consist of lightest species in group III-monochalcogenides. Surprisingly, BO and BS monolayers exhibit high TE efficiency at low temperatures. Low thermal conductance at low temperatures and stepwise electronic transmission at the valence band edge are the physical mechanisms behind achieving large ZT.
X-ray photoelectron spectroscopy analysis of magnetron sputtered Cu2ZnSnS4 based thin film solar cells with CdS buffer layer
(Izmir Institute of Technology, 2017-06) Cantaş Bağdaş, Ayten; Özyüzer, Lütfi
Cu2ZnSnS4 (CZTS) is a novel quaternary compound which contains Cu, Zn, Sn and S elements. It is a p-type semiconductor which has been taken attention in the last years as an absorber layer. Since it consists of abundant, low cost and non-toxic elements, it is one of the most promising candidate as an absorber layer for thin film photovoltaic (PV) application. Having high absorption coefficient, low bandgap value which is theoretically in desired range make this material attractive for solar cell application. In this thesis, CZTS absorber layers were grown using two stages which are the magnetron sputtering of metallic precursors, followed by a heat treatment under sulfur vapor atmosphere. Two types of CZTS were grown such as SLG/Mo/Cu (55nm)/Sn/Zn/Cu (120nm) and SLG/Mo/Cu (120nm)/Sn/Zn/Cu (55nm). For the same stacking order, the effect of Cu thickness sequentially grown with Sn layer on the film quality were investigated. The optical properties, microstructure, surface and bulk composition of CZTS films were investigated in detail. This study revealed a correlation between the CZTS stacking order having different thickness of Cu layer and the improvement of film quality, which was also confirmed by the photo-conversion efficiency of the fabricated devices. In this work, the other investigated layer is CdS which is an n-type semiconductor with bandgap energy of 2.4 eV. Since CdS has well lattice match with the heterojunction between CdS and CZTS, it is one of the most preferred material as a buffer layer for solar cells. In this work CdS buffer layers were deposited by chemical bath deposition technique. The optimization of CdS layers were occurred and optical, structural, bulk and surface compositions were investigated in detail. Finally, SLG/Mo/CZTS/CdS/i-ZnO/AZO devices were fabricated. The effect of structure properties of CZTS films and the thickness of CdS buffer layer on efficiency of fabricated solar cells were investigated.

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