Browsing by Subject "Semiconductors"

Now showing 1 - 20 of 23

Open Access
Anharmonicity in GaTe layered crystals
(Wiley-VCH Verlag GmbH & Co. KGaA, 2002) Aydınlı, Atilla; Gasanly, N. M.; Uka, A.; Efeoglu, H.
The temperature dependencies (10-300 K) of seven Raman-active mode frequencies in layered semiconductor gallium telluride have been measured in the frequency range from 25 to 300 cm -1. Softening and broadening of the optical phonon lines are observed with increasing temperature. Comparison between the experimental data and theories of the shift of the phonon lines during heating of the crystal showed that the experimental dependencies can be explained by contributions from thermal expansion and lattice anharmonicity. Lattice anharmonicity is determined to be due to three-phonon processes.
Open Access
Atomic force microscopy: Methods and applications
(Elsevier, 2017) Baykara, Mehmet Z.; Schwarz, U. D.; Lindon, J.; Tranter, G. E.; Koppenaal, D.
This chapter provides an overview of atomic force microscopy, covering the fundamental aspects of the associated instrumentation and methodology as well as representative results from the literature highlighting a variety of application areas. In particular, atomic-resolution imaging and spectroscopy capabilities are emphasized, in addition to applications in biology, nanotribology and catalysis research. Finally, an outlook on emerging aspects and future prospects of atomic force microscopy is provided.
Open Access
Deformed octagon-hexagon-square structure of group-IV and group-V elements and III-V compounds
(American Physical Society, 2019) Görkan, T.; Aktürk, E.; Çıracı, Salim
We report the prediction of a two-dimensional (2D) allotrope common to group-IV and group-V elements and III-V compounds, which consist of two nonplanar atomic layers connected by vertical bonds and form deformed octagon, hexagon, and squares (dohs) with threefold and fourfold coordinated atoms. Specifically for silicon, it is a semiconductor with cohesion stronger than silicene and can be chemically doped to have localized donor and acceptor states in the band gap. This allotrope can be functionalized to construct quasi-2D clathrates with transition metal atoms and attain spin polarized metallic, half-metallic, or semiconducting states. It is demonstrated that these properties can be maintained, when it is grown on a specific substrate. Stringent tests show that the atomic structure is dynamically stable and can sustain thermal excitation at high temperatures. Additionally, stable bilayer, as well as 3D layeredlike structures, can be constructed by the vertical stacking of single-layer dohs. Surprisingly, C, Ge, AlP, and GaAs can form also similar 2D semiconducting structures. In contrast to semiconducting black and blue phosphorene, P-dohs is a semimetal with band inversion. While the premise of using well-developed silicon technology in 2D electronics has been hampered by the semimetallic silicene, the realization of this 2D, semiconducting allotrope of silicon and compounds can constitute a productive direction in 2D nanoelectronics/spintronics.
Open Access
Degrading effect on electrical properties of printed gallium sulfide based photodetector
(Institute of Physics Publishing Ltd., 2024-05-24) Odacı, Cem; Khan, Muhammad Shaukat; Jose, Manoj; Kisielewska, Marta; Roshanghias, Ali; Aydemir, Umut
Layered GaS structures have been attracting increasing research interest due to their highly anisotropic structural, electrical, optical, and mechanical properties. However, the investigation of the performance based on the responsivity, external quantum efficiency, and detectivity of printed GaS based photodetector on a flexible PET substrate with respect to a period of time under the environmental conditions have not been reported so far. This experimental study shows that the printed GaS based photodetector stored in ambient conditions undergoes a change in terms of performance in a few weeks after the fabrication. This work also holds an importance being premier study experimentally investigating the printed III–VI group monochalcogenide based photodetector stored under the environmental conditions and contributing the literature to improve the printed device performance in further applications.
Open Access
Donor - acceptor pair recombination in Tl2InGaS4 layered crystals
(2005) Goksen, K.; Gasanly, N.M.; Ozkan H.; Aydınlı, Atilla
Photoluminescence (PL) spectra of Tl2InGaS4 layered single crystals were studied in the temperature range 15-150 K and wide laser excitation intensity range 0.01-110.34 Wcm-2. We observed a total of three PL bands, one centered at 542 nm (2.286 eV, A-band), one at 607 nm (2.041 eV, B-band), and one at 707 nm (1.754 eV, C-band), at various excitation intensities. The A- and the B-bands were determined to be due to radiative transitions from moderately deep donor levels located at 0.189 and 0.443 eV below the bottom of the conduction band to the shallow acceptor levels at 0.025 and 0.016 eV above the top of the valence band, respectively. The blue shift of the C-band peak energy and the quenching of the PL with increasing temperature are explained within the configuration coordinate model. The observation in the PL spectra of different emission bands in the sequence of B-, C- and A-bands at low, moderate, and high excitation laser intensities, respectively, are attributed to the shift of the quasi-Fermi level with increasing excitation intensity.
Open Access
Enhanced memory effect via quantum confinement in 16 nm InN nanoparticles embedded in ZnO charge trapping layer
(AIP Publishing, 2014) El-Atab, N.; Cimen, F.; Alkis, S.; Ortac, B.; Alevli, M.; Dietz, N.; Okyay, Ali Kemal; Nayfeh, A.
In this work, the fabrication of charge trapping memory cells with laser-synthesized indium-nitride nanoparticles (InN-NPs) embedded in ZnO charge trapping layer is demonstrated. Atomic layer deposited Al2O3 layers are used as tunnel and blocking oxides. The gate contacts are sputtered using a shadow mask which eliminates the need for any lithography steps. High frequency C-Vgate measurements show that a memory effect is observed, due to the charging of the InN-NPs. With a low operating voltage of 4 V, the memory shows a noticeable threshold voltage (Vt) shift of 2 V, which indicates that InN-NPs act as charge trapping centers. Without InN-NPs, the observed memory hysteresis is negligible. At higher programming voltages of 10 V, a memory window of 5 V is achieved and the Vt shift direction indicates that electrons tunnel from channel to charge storage layer. © 2014 AIP Publishing LLC
Open Access
First-principles study of thin TiOx and bulklike rutile nanowires
(American Physical Society, 2009) Çakır, D.; Gülseren, O.
We have systematically investigated structural, electronic and magnetic properties of very thin TiOx (x=1,2) nanowires as well as bulklike (110) rutile nanowires by using the first-principles plane-wave pseudopotential calculations based on density functional theory. A large number of different possible structures have been searched via total-energy calculations in order to find the ground-state structures of these nanowires. Three-dimensional structures are more energetically stable than planar ones for both of the stoichiometries (i.e., x=1,2). The stability of TiOx nanowires is enhanced with its increasing radius as a result of reaching sufficient coordination number of Ti and O atoms. All stoichiometric TiO2 nanowires studied exhibit semiconducting behavior and have nonmagnetic ground state. There is a correlation between binding energy (Eb) and energy band gap (Eg) of TiO2nanowires. In general, Eb increases with increasing Eg. In TiO nanowires, both metallic and semiconductor nanowires result. In this case, in addition to paramagnetic TiO nanowires, there are also ferromagnetic ones. We have also studied the structural and electronic properties of bulklike rutile (110) nanowires. There is a crossover in terms of energetics, and bulklike nanowires are more stable than the thin nanowires for larger radius wires after a critical diameter. These (110) rutile nanowires are all semiconductors.
Open Access
Improved InGaN/GaN light-emitting diodes with a p-GaN/n-GaN/p-GaN/n-GaN/p-GaN current-spreading layer
(Optical Society of American (OSA), 2013) Zhang, Z.-H.; Tan, S.T.; Liu W.; Ju, Z.; Zheng, K.; Kyaw, Z.; Ji, Y.; Hasanov, N.; Sun X.W.; Demir, Hilmi Volkan
This work reports both experimental and theoretical studies on the InGaN/GaN light-emitting diodes (LEDs) with optical output power and external quantum efficiency (EQE) levels substantially enhanced by incorporating p-GaN/n-GaN/p-GaN/n-GaN/p-GaN (PNPNP-GaN) current spreading layers in p-GaN. Each thin n-GaN layer sandwiched in the PNPNP-GaN structure is completely depleted due to the built-in electric field in the PNPNP-GaN junctions, and the ionized donors in these n-GaN layers serve as the hole spreaders. As a result, the electrical performance of the proposed device is improved and the optical output power and EQE are enhanced. © 2013 Optical Society of America.
Open Access
Influence of channel layer thickness on the electrical performances of inkjet-printed In-Ga-Zn oxide thin-film transistors
(IEEE, 2010-12-10) Wang, Y.; Sun, X. W.; Goh, G. K. L.; Demir, Hilmi Volkan; Yu, H. Y.
Inkjet-printed In-Ga-Zn oxide (IGZO) thin-film transistors (TFTs) with bottom-gate bottom-contact device architecture are studied in this paper. The impact of the IGZO film thickness on the performance of TFTs is investigated. The threshold voltage, field-effect mobility, on and off drain current, and subthreshold swing are strongly affected by the thickness of the IGZO film. With the increase in film thickness, the threshold voltage shifted from positive to negative, which is related to the depletion layer formed by the oxygen absorbed on the surface. The field-effect mobility is affected by the film surface roughness, which is thickness dependent. Our results show that there is an optimum IGZO thickness, which ensures the best TFT electrical performance. The best result is from a 55-nm-thick IGZO TFT, which showed a field-effect mobility in the saturation region of 1.41 cm(2)/V . s, a threshold voltage of 1 V, a drain current on/off ratio of approximately 4.3 x 10(7), a subthreshold swing of 384 mV/dec, and an off-current level lower than 1 pA.
Open Access
Infrared photoluminescence from TlGaS2 layered single crystals
(Wiley - V C H Verlag GmbH & Co., 2004) Yuksek, N. S.; Gasanly, N. M.; Aydınlı, Atilla; Ozkan, H.; Acikgoz, M.
Photolimuniscence (PL) spectra of TlGaS2 layered crystals were studied in the wavelength region 500-1400 nm and in the temperature range 15-115 K. We observed three broad bands centered at 568 nm (A-band), 718 nm (B-band) and 1102 nm (C-band) in the PL spectrum. The observed bands have half-widths of 0.221, 0.258 and 0.067 eV for A-, B-, and C-bands, respectively. The increase of the emission band half-width, the blue shift of the emission band peak energy and the quenching of the PL with increasing temperature are explained using the configuration coordinate model. We have also studied the variations of emission band intensity versus excitation laser intensity in the range from 0.4 to 19.5 W cm-2. The proposed energy-level diagram allows us to interpret the recombination processes in TlGaS2 crystals.
Open Access
Liquid-phase synthesis of nanoparticles and nanostructured materials
(Elsevier, 2018) Karatutlu, A.; Barhoum, A.; Sapelkin, A.
Nanoparticles less than 100nm in size have attracted significant interest over the past 20 years due to their unique properties led by quantum size effect. This chapter evaluates the synthesis methods in liquid phase conducted under operation in high/room temperature and at vacuum/atmospheric environment for nanoparticles and nanostructured materials. We draw attention to the fact that various synthesis methods for formation of colloidally stable matrix-free nanoparticles are available. These methods including chemical stain etching, electrodeposition methods, direct-precipitation methods, sol-gel methods, colloidal synthesis methods, hot-injection synthesis methods, hydrothermal and solvothermal methods, microwave-assisted synthesis methods, ultrasonic synthesis methods, and laser ablation in liquid-phase.
Unknown
Low thermal-mass LEDs: Size effect and limits
(Optical Society of American (OSA), 2014) Lu, S.; Liu W.; Zhang, Z.-H.; Tan, S.T.; Ju, Z.; Ji, Y.; Zhang X.; Zhang, Y.; Zhu, B.; Kyaw, Z.; Hasanov, N.; Sun X.W.; Demir, Hilmi Volkan
In this work, low thermal-mass LEDs (LTM-LEDs) were developed and demonstrated in flip-chip configuration, studying both experimentally and theoretically the enhanced electrical and optical characteristics and the limits. LTM-LED chips in 25 × 25 μm2, 50 × 50 μm2, 100 × 100 μm2 and 200 × 200 μm2 mesa sizes were fabricated and comparatively investigated. Here it was revealed that both the electrical and optical properties are improved by the decreasing chip size due to the reduced thermal mass. With a smaller chip size (from 200 μm to 50 μm), the device generally presents higher current density against the bias and higher power density against the current density. However, the 25 × 25 μm2 device behaves differently, limited by the fabrication margin limit of 10 μm. The underneath mechanisms of these observations are uncovered, and furthermore, based on the device model, it is proven that for a specific flip-chip fabrication process, the ideal size for LTM-LEDs with optimal power density performance can be identified. ©2014 Optical Society of America
Unknown
Nonradiative resonance energy transfer directed from colloidal CdSe/ZnS quantum dots to epitaxial InGaN/GaN quantum wells for solar cells
(Wiley, 2010-06-04) Nizamoglu, S.; Sari, E.; Baek, J. H.; Lee, I. H.; Demir, Hilmi Volkan
We report on Förster-type nonradiative resonance energy transfer (NRET) directed from colloidal quantum dots (QDs) to epitaxial quantum wells (QWs) with an efficiency of 69.6% at a rate of 1.527 ns-1 for potential application in III-nitride based photovoltaics. This hybrid exciton generation-collection system consists of chemically-synthesized cyan CdSe/ZnS core/shell QDs (λPL = 490 nm) intimately integrated on epitaxially-grown green InGaN/GaN QWs (λPL = 512 nm). To demonstrate directional NRET from donor QDs to acceptor QWs, we simultaneously show the decreased photoluminescence decay lifetime of dots and increased lifetime of wells in the hybrid dipole-dipole coupled system.
Open Access
A photometric investigation of ultra-efficient LEDs with high color rendering index and high luminous efficacy employing nanocrystal quantum dot luminophores
(Optical Society of America, 2009-12-24) Erdem, T.; Nizamoglu, S.; Sun, X. W.; Demir, Hilmi Volkan
We report a photometric study of ultra-efficient light emitting diodes (LEDs) that exhibit superior color rendering index (CRI) and luminous efficacy of optical radiation (LER) using semiconductor quantum dot nanocrystal (NC) luminophores. Over 200 million systematically varied NC-LED designs have been simulated to understand feasible performance in terms of CRI vs. LER. We evaluated the effects of design parameters including peak emission wavelength, full-width-at-half-maximum, and relative amplitudes of each NC color component on LED performance. Warm-white LEDs with CRI >90 and LER >380 lm/W at a correlated color temperature of 3000 K are shown to be achieved using nanocrystal luminophores. (C) 2009 Optical Society of America
Open Access
Photonic band gaps with layer-by-layer double-etched structures
(A I P Publishing LLC, 1996-09-03) Biswas, R.; Özbay, Ekmel; Ho, K. M.
Periodic layer‐by‐layer dielectric structures with full three‐dimensional photonic band gaps have been designed and fabricated. In contrast to previous layer‐by‐layer structures the rods in each successive layer are at an angle of 70.5° to each other, achieved by etching both sides of a silicon wafer. Photonic band‐structure calculations are utilized to optimize the photonic band gap by varying the structural geometry. The structure has been fabricated by double etching Si wafers producing millimeter wavephotonic band gaps between 300 and 500 GHz, in excellent agreement with band calculations. Overetching this structure produces a multiply connected geometry and increases both the size and frequency of the photonic band gap, in very good agreement with experimental measurements. This new robust double‐etched structure doubles the frequency possible from a single Si wafer, and can be scaled to produced band gaps at higher frequencies. © 1996 American Institute of Physics
Open Access
Physics of nonradiative energy transfer in the complex media of 0D, 2D and 3D materials
(2016-07) Yeltik, Aydan
Quantum-confined colloidal nanostructures with strong excitonic properties have emerged as promising light harvesting components in photonics and optoelectronics over the past 20 years. With their favorable photophysical characteristics, three-dimensional-confined colloidal quantum dots and 2D-confined colloidal quantum wells have garnered great attention in the fields ranging from biology and chemistry to physics and engineering. It is technologically significant to utilize the key characteristics of these brightly luminescent nanomaterials through hybridizing and/or interfacing with various technological materials including 3D bulk silicon, graphene based 2D structures such as graphene oxide and reduced graphene oxide, and 2D layered transition metal dichalcogenides such as molybdenum disulphide. Compelling partnership of these appealing materials can be achieved through the nonradiative energy transfer (NRET), which is a phenomenon involving both the exciton and charge transfer mechanisms. Along with the hybrids of low dimensional particles with the conventional bulk materials, the closely interacting structures of these colloidal and layered nanomaterials have widespread interest at both the fundamental science and application levels. From these physical and technological points of view, in this thesis, we addressed important scientific problems and proposed innovative solutions including both the experimental and theoretical approaches in interfacing complex media of 0D, 2D and 3D materials and showing strong NRET interactions. Our key achievements include high excitonic enhancement in silicon and graphene based materials with the integration of nanoparticles, comprehensive photophysical investigation of the newly emerging nanomaterials and successful tailoring of the colloidal nanostructures to the next-generation optoelectronic applications.
Open Access
Quadrupolar spectra of nuclear spins in strained InxGa1−xAs quantum dots
(American Physical Society, 2012) Bulutay, C.
Self-assembled quantum dots (QDs) are born out of lattice mismatched ingredients where strain plays an indispensable role. Through the electric quadrupolar coupling, strain affects the magnetic environment as seen by the nuclear spins. To guide prospective single-QD nuclear magnetic resonance (NMR), as well as dynamic nuclear spin polarization experiments, an atomistic insight to the strain and quadrupolar field distributions is presented. A number of implications of the structural and compositional profile of the QD have been identified. A high aspect ratio of the QD geometry enhances the quadrupolar interaction. The inclined interfaces introduce biaxiality and the tilting of the major quadrupolar principal axis away from the growth axis; the alloy mixing of gallium into the QD enhances both of these features while reducing the quadrupolar energy. Regarding the NMR spectra, both Faraday and Voigt geometries are investigated, unraveling in the first place the extend of inhomogeneous broadening and the appearance of the normally forbidden transitions. Moreover, it is shown that from the main extend of the NMR spectra the alloy mole fraction of a single QD can be inferred. By means of the element-resolved NMR intensities it is found that In nuclei has a factor of 5 dominance over those of As. In the presence of an external magnetic field, the borderlines between the quadrupolar and Zeeman regimes are extracted as 1.5 T for In and 1.1 T for As nuclei. At these values the nuclear spin depolarization rates of the respective nuclei get maximized due to the noncollinear secular hyperfine interaction with a resident electron in the QD.
Restricted
Resources-a theoretical soup stone ?
(1984) Navon, David
Open Access
Strain engineering of electronic and optical properties of monolayer diboron dinitride
(American Physical Society, 2021-11-29) Demirci, Salih; Rad, Soheil Ershad; Jahangirov, Seymur
We studied the effect of strain engineering on the electronic, structural, mechanical, and optical properties of orthorhombic diboron dinitride (o-B2N2) through first-principles calculations. The 1.7-eV direct band gap observed in the unstrained o-B2N2 can be tuned up to 3 eV or down to 1 eV by applying 12% tensile strain in armchair and zigzag directions, respectively. Ultimate strain values of o-B2N2 were found to be comparable with that of graphene. Our calculations revealed that the partial alignment of the band edges with the redox potentials of water in pristine o-B2N2 can be tuned into a full alignment under the armchair and biaxial tensile strains. The anisotropic charge carrier mobility found in o-B2N2 prolongs the average lifetime of the carrier drift, creating a suitable condition for photoinduced catalytic reactions on its surface. Finally, we found that even in extreme straining regimes, the highly anisotropic optical absorption of o-B2N2 with strong absorption in the visible range is preserved. Having strong visible light absorption and prolonged carrier migration time, we propose that strain engineering is an effective route to tune the band gap energy and band alignment of o-B2N2 and turn this two-dimensional material into a promising photocatalyst for efficient hydrogen production from water splitting.
Embargo
The enhanced lifetime of printed GaS-based photodetectors with polymer encapsulation
(Elsevier BV, 2024-12-02) Odacı, Cem; Khan, Muhammad Shaukat; Beduk, Tutku; Jose, Manoj; Kisielewska, Marta; Aydemir, Umut; Roshanghias, Ali
Exhibiting excellent absorption in the UV-visible wavelength range makes layered gallium sulfide (GaS) semiconductor material a promising candidate for use in electronics and optoelectronics applications. Recently, a fully printed GaS-based photodetector has been proposed and fabricated, rendering a low-cost fabrication process in flexible electronics. However, the degradation of the semiconductor layer due to environmental conditions causes reliability issues and shortens their lifetime. Thus, in this study, an attempt has been made to encapsulate printed GaS-based photodetector using different polymers to hinder the degradation. It is demonstrated that encapsulating the printed GaS-based photodetector by utilizing the polymer-capping method with styrene copolymers, Polystyrene-block-polyisoprene-block-polystyrene, highly hydrogenated poly(styrene)-block-poly (butadiene), partially hydrogenated poly(styrene)-block-poly(butadiene), increases the performance of the photodetector. The efficiency of the GaS-based photodetector printed on flexible polyethylene terephthalate (PET) substrate has reached up to 123 % in responsivity in 6 weeks after the polymer coating. Also, the device figure of merit, the detectivity value of the printed photodetector, has increased more than three times after the polymer coating compared to its as-deposited state. Meanwhile, it is observed that the fall and rise times of the printed GaS photodetector have remained constant. Based on these results attained in this study, it can be claimed that the polymer coating provides high performance and long stability in the printed GaS-based photodetectors on flexible substrates, which will pave the way for the further implementations of III-VI group layered semiconductor materials in electronics and optoelectronics applications.