Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 4Citation - Scopus: 5Identifying Threading Dislocations in Cdte Films by Reciprocal Space Mapping and Defect Decoration Etching(American Institute of Physics, 2018) Polat, Mustafa; Bilgilisoy, Elif; Arı, Ozan; Öztürk, Orhan; Selamet, YusufWe study threading dislocation (TD) density of high-quality cadmium telluride (CdTe) layers grown on a (211) oriented GaAs substrate by molecular beam epitaxy. High-resolution X-ray diffraction was performed to calculate the density of screw-type TDs by measuring the broadening of the asymmetrical (511) Bragg reflections of CdTe epilayers. In addition, total TD densities were determined by the Everson-etching method and were compared with screw TDs. Our results show that the total TD densities in CdTe films were dominated by those with screw character. The screw component TDs are estimated to account for more than 90% of the total TD density. CdTe layers grown at a thickness of less than 3.0 μm typically exhibit the screw TD densities in the 106 cm-2 and 107 cm-2 range. It can be noted that as the nucleation temperature increases, i.e., ≥222 °C, both the area density of TDs with the screw component of the CdTe films and the total TD density are roughly four times larger than those of the epilayer grown at the nucleation temperature of 215 °C. Furthermore, we discuss the influence of the II/VI flux ratio on the density of threading dislocations. The contribution of screw TDs to the total TD density showed a significant decrease in roughly 30% in the case of a high II/VI flux ratio. We further examine the reciprocal space maps in the vicinity of the (422) reflections.Article Citation - WoS: 13Citation - Scopus: 16Reciprocal Space Mapping Study of Cdte Epilayer Grown by Molecular Beam Epitaxy on (2 1 1)b Gaas Substrate(IOP Publishing Ltd., 2017) Polat, Mustafa; Arı, Ozan; Öztürk, Orhan; Selamet, YusufWe examine high quality, single crystal CdTe epilayer grown by molecular beam epitaxy (MBE) on (2 1 1)B GaAs substrate using both positions and full width at half maximums (FWHMs) of reciprocal lattice points (RLPs). Our results demonstrate that reciprocal space mapping (RSM) is an effective way to study the structural characteristics of the high-index oriented epitaxial thin films having a large lattice mismatch with the substrate. The measurement method is defined first, and then the influence of shear strain ( xz) on the position of the (5 1 1) node of epilayer is clarified. It is concluded that the lattice tilting is likely to be related with the lattice mismatch. Nondestructive measurement of the dislocation density is achieved by applying the mosaic crystal model. The screw dislocation density, estimated to be 7.56×107 cm2, was calculated utilizing the broadened peakwidths of the asymmetric RLP of the epilayer lattice.Article Citation - WoS: 54Citation - Scopus: 53Nitrogen Doping for Facile and Effective Modification of Graphene Surfaces(Royal Society of Chemistry, 2017) Yanılmaz, Alper; Tomak, Aysel; Akbalı, Barış; Bacaksız, Cihan; Özçeri, Elif; Arı, Ozan; Senger, Ramazan Tuğrul; Selamet, Yusuf; Zareie, Hadi M.We report experimental and theoretical investigations of nitrogen doped graphene. A low-pressure Chemical Vapor Deposition (CVD) system was used to grow large-area graphene on copper foil, using ethylene as the carbon source. Nitrogen-doped graphene (N-graphene) was prepared by exposing the graphene transferred to different substrates to atomic nitrogen plasma. The effect of varying nitrogen flow rates on doping of graphene was investigated while keeping the power and time constant during the process. The N-graphene was characterized via Raman Spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Scanning Tunneling Microscopy and Spectroscopy (STM and STS), and Fourier Transform Infrared spectroscopy (FTIR). Raman mapping of N-graphene was also performed to show homogeneity of nitrogen on the graphitic lattice. XPS results have revealed the presence of different nitrogen configurations in the graphitic lattice with similar doping concentrations. Density functional theory (DFT) based calculations showed that the periodic adsorption of N atoms predominantly occurs on top of the C atoms rather than through substitution of C in our N-graphene samples. Our results indicate a feasible procedure for producing N-graphene with homogenous and effective doping which would be valuable in electronic and optical applications.Article Citation - WoS: 2Citation - Scopus: 2Surface Roughness Estimation of Mbe Grown Cdte/Gaas(211)b by Ex-Situ Spectroscopic Ellipsometry(American Institute of Physics, 2016) Karakaya, Merve; Bilgilisoy, Elif; Arı, Ozan; Selamet, YusufSpectroscopic ellipsometry (SE) ranging from 1.24 eV to 5.05 eV is used to obtain the film thickness and optical properties of high index (211) CdTe films. A three-layer optical model (oxide/CdTe/GaAs) was chosen for the ex-situ ellipsometric data analysis. Surface roughness cannot be determined by the optical model if oxide is included. We show that roughness can be accurately estimated, without any optical model, by utilizing the correlation between SE data (namely the imaginary part of the dielectric function, <ϵ2 > or phase angle, ψ) and atomic force microscopy (AFM) roughness. <ϵ2 > and ψ values at 3.31 eV, which corresponds to E1 critical transition energy of CdTe band structure, are chosen for the correlation since E1 gives higher resolution than the other critical transition energies. On the other hand, due to the anisotropic characteristic of (211) oriented CdTe surfaces, SE data (<ϵ2 > and ψ) shows varieties for different azimuthal angle measurements. For this reason, in order to estimate the surface roughness by considering these correlations, it is shown that SE measurements need to be taken at the same surface azimuthal angle. Estimating surface roughness in this manner is an accurate way to eliminate cumbersome surface roughness measurement by AFM.Other Erratum To: Mbe-Grown Cdte Layers on Gaas With In-Assisted Thermal Deoxidation(Springer Verlag, 2016) Arı, Ozan; Bilgilisoy, Elif; Özçeri, Elif; Selamet, YusufArticle Citation - WoS: 6Citation - Scopus: 5Mbe-Grown Cdte Layers on Gaas With In-Assisted Thermal Deoxidation(Springer Verlag, 2016) Arı, Ozan; Bilgilisoy, Elif; Özçeri, Elif; Selamet, YusufMolecular beam epitaxy (MBE) growth of thin (∼2 μm) CdTe layers characterized by high crystal quality and low defect density on lattice mismatched substrates, such as GaAs and Si, has thus far been difficult to achieve. In this work, we report the effects of in situ thermal deoxidation under In and As4 overpressure prior to the CdTe growth on epiready GaAs(211)B wafers, aiming to enhance CdTe crystal quality. Thermally deoxidized GaAs samples were analyzed using in situ reflection high energy electron diffraction, along with ex situ x-ray photo-electron spectroscopy (XPS) and atomic force microscopy. MBE-grown CdTe layers were characterized using x-ray diffraction (XRD) and Everson-type wet chemical defect decoration etching. We found that In-assisted desorption allowed for easier surface preparation and resulted in a smoother surface compared to As-assisted surface preparation. By applying In-assisted thermal deoxidation to GaAs substrates prior to the CdTe growth, we have obtained single crystal CdTe films with a CdTe(422) XRD rocking curve with a full-width half-maximum value of 130.8 arc-s and etch pit density of 4 × 106 cm−2 for 2.54 μm thickness. We confirmed, by XPS analysis, no In contamination on the thermally deoxidized surface.Article Citation - WoS: 3Citation - Scopus: 3Growth and Characterization of Cdte Absorbers on Gaas by Mbe for High Concentration Pv Solar Cells(John Wiley and Sons Inc., 2015) Arı, Ozan; Polat, Mustafa; Karakaya, Merve; Selamet, YusufCdTe based II-VI absorbers are promising candidates for high concentration PV solar cells with an ideal band gap for AM1.5 solar radiation. In this study, we propose single crystal CdTe absorbers grown on GaAs substrates with a molecular beam epitaxy (MBE) which is a clean deposition technology. We show that high quality CdTe absorber layers can be grown with full width half maximum of X-ray diffraction rocking curves (XRD RC) as low as 227 arc-seconds with 0.5% thickness uniformity that a 2 μm layer is capable of absorbing 99% of AM1.5 solar radiation. Bandgap of the CdTe absorber is found as 1.483 eV from spetroscopic ellipsometry (SE) measurements. Also, high absorption coefficient is calculated from the results, which is ∼5 x 105cm-1 in solar radiation spectrum.Article Citation - WoS: 6Citation - Scopus: 6Cleavage Induced Rows of Missing Atoms on Znte (110) Surface(American Physical Society, 2013) Çelebi, Cem; Arı, Ozan; Senger, Ramazan TuğrulCleavage induced rows of linear vacancy structures on p-doped ZnTe (110) surface are studied at room temperature by using cross-sectional scanning tunneling microscopy (X-STM). The oscillating contrast superimposed on the Te-driven occupied states neighboring to the vacancy cores are characterized at the atomic scale in order to determine the type of the missing component on the ZnTe surface matrix. We identify three major intensity distributions associated with different vacancy states. The X-STM images of three possible configurations comprising Zn only, Te only, and ZnTe binary vacancy structures on the ZnTe surface are modeled by using ab initio density functional theory calculations. The comparison of the X-STM measurements of each individual vacancy state to the corresponding theoretical simulation showed that unlike the Te vacancy, which leads to a local depression, the absence of Zn only or ZnTe binary gives rise to hillock features on the neighboring Te states of the ZnTe (110) cleaved surface. The theoretical STM images calculated for an undoped ZnTe crystal imply that possible doping-related effects on vacancy-induced features can be disregarded for interpreting the experimentally observed vacancy structures in our samples.