Browsing by Subject "Germanium nanocrystals"
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Item Open Access Crystallization of Ge in SiO2 matrix by femtosecond laser processing(American Vacuum Society, 2012-01-19) Salihoglu, O.; Kürüm, U.; Yaglioglu, H. G.; Elmali, A.; Aydınlı, AtillaGermanium nanocrystals embedded in a siliconoxide matrix has been fabricated by single femtosecond laser pulse irradiation of germanium doped SiO2 thin films deposited with plasma enhanced chemical vapor deposition. SEM and AFM are used to analyze surface modification induced by laser irradiation. Crystallization of Ge in the oxide matrix is monitored with the optic phonon at 300 cm(-1) as a function of laser fluence. Both the position the linewidth of the phonon provides clear signature for crystallization of Ge. In PL experiments, strong luminescence around 600 nm has been observed.Item Open Access Dc-switchable and single-nanocrystal-addressable coherent population transfer(2010) Gunceler, D.; Bulutay, C.Achieving coherent population transfer in the solid-state is challenging compared to atomic systems due to closely spaced electronic states and fast decoherence. Here, within an atomistic pseudopotential theory, we offer recipes for the stimulated Raman adiabatic passage in embedded silicon and germanium nanocrystals. The transfer efficiency spectra display characteristic Fano resonances. By exploiting the Stark effect, we predict that transfer can be switched off with a dc voltage. As the population transfer is highly sensitive to structural variations, with a choice of a sufficiently small two-photon detuning bandwidth, it can be harnessed for addressing individual nanocrystals within an ensemble. © 2010 American Institute of Physics.Item Open Access A figure of merit for optimization of nanocrystal flash memory design(2008) Dâna, A.; Akca, I.; Aydınlı, Atilla; Turan, R.; Finstad, T. G.Nanocrystals can be used as storage media for carriers in flash memories. The performance of a nanocrystal flash memory depends critically on the choice of nanocrystal size and density as well as on the choice of tunnel dielectric properties. The performance of a nanocrystal memory device can be expressed in terms of write/erase speed, carrier retention time and cycling durability. We present a model that describes the charge/discharge dynamics of nanocrystal flash memories and calculate the effect of nanocrystal, gate, tunnel dielectric and substrate properties on device performance. The model assumes charge storage in quantized energy levels of nanocrystals. Effect of temperature is included implicitly in the model through perturbation of the substrate minority carrier concentration and Fermi level. Because a large number of variables affect these performance measures, in order to compare various designs, a figure of merit that measures the device performance in terms of design parameters is defined as a function of write/erase/discharge times which are calculated using the theoretical model. The effects of nanocrystal size and density, gate work function, substrate doping, control and tunnel dielectric properties and device geometry on the device performance are evaluated through the figure of merit. Experimental data showing agreement of the theoretical model with the measurement results are presented for devices that has PECVD grown germanium nanocrystals as the storage media. CopyrightItem Open Access Low dimensional structures for optical and electrical applications(Bilkent University, 2008) Akça, İmranLow dimensional structures such as quantum dots have been particularly attractive because of their fundamental physical properties and their potential applications in various devices in integrated optics and microelectronics. This thesis presents optical and electrical applications of low dimensional structures. For this purpose we have studied silicon and germanium nanocrystals for flash memory applications and InAs quantum dots for optical modulators. As a quantum dot, nanocrystals can be used as storage media for carriers in flash memories. Performance of a nanocrystal memory device can be expressed in terms of write/erase speed, carrier retention time and cycling durability. Charge and discharge dynamics of PECVD grown nanocrystals were studied. Electron and hole charge and discharge currents were observed to differ significantly and strongly depend on annealing conditions chosen for the formation of nanocrystals. Our experimental results revealed that, discharge currents were dominated by the interface layer acting as a quantum well for holes and route for direct tunneling for electrons. On the other hand, possibility of obtaining quantum dots with enhanced electro-optic and/or electro-absorption coefficients makes them attractive for use in light modulation. Therefore, waveguides of multilayer InAs quantum dots were studied. Electro-optic measurements were conducted at 1.5 µm and clear Fabry-Perot resonances were obtained. The voltage dependent Fabry-Perot measurements revealed that 6 V was sufficient for full on/off modulation. Electroabsorption measurements were conducted at both 1.3 and 1.5 µm. Since the structure lases at 1285 nm, high absorption values at 1309 nm were obtained. The absorption spectrum of the samples was also studied under applied electric field. Absorption spectra of all samples shift to lower photon energies with increasing electric field.Item Open Access Matrix density effect on morphology of germanium nanocrystals embedded in silicon dioxide thin films(Materials Research Society, 2011) Alagoz, A. S.; Genisel, M. F.; Foss, Steinar; Finstad, T. G.; Turan, R.Flash type electronic memories are the preferred format in code storage at complex programs running on fast processors and larger media files in portable electronics due to fast write/read operations, long rewrite life, high density and low cost of fabrication. Scaling limitations of top-down fabrication approaches can be overcome in next generation flash memories by replacing continuous floating gate with array of nanocrystals. Germanium (Ge) is a good candidate for nanocrystal based flash memories due its small band gap. In this work, we present effect of silicon dioxide (SiO 2) host matrix density on Ge nanocrystals morphology. Low density Ge+SiO 2 layers are deposited between high density SiO 2 layers by using off-angle magnetron sputter deposition. After high temperature post-annealing, faceted and elongated Ge nanocrystals formation is observed in low density layers. Effects of Ge concentration and annealing temperature on nanocrystal morphology and mean size were investigated by using transmission electron microscopy. Positive correlation between stress development and nanocrystal size is observed at Raman spectroscopy measurements. We concluded that non-uniform stress distribution on nanocrystals during growth is responsible from faceted and elongated nanocrystal morphology.Item Open Access Nanofibrous nanocomposites via electrospinning(Bilkent University, 2011) Deniz, Ali EkremIn recent years, numerous studies have been reported for fabrication of composite nanofibers from polymeric and inorganic materials by using electrospinning method. In the first part of this study, TiO2 and ZnO inorganic nanofibers were produced by electrospinning from their precursors by using polymeric carrier matrix and their photocatalytic activity of these metal oxide nanofibers were studied. Moreover, electrospun TiO2 nanofibers were crushed into short nanofibers (TiO2-SNF) and embedded in electrospun polymeric nanofiber matrixes such as poly(methyl methacrylate) (PMMA), polyacrylonitrile (PAN), polyethylene terephthalate (PET), polycarbonate (PC) and polyvinylidene fluoride (PVDF). Different weight loading of TiO2-SNF ranging from 2% to 8% (w/w, respect to polymer) incorporated into PVDF nanofibrous matrix was applied and the structural and morphological changes along with their photocatalytic activities were also examined. In the second part, metallic nanoparticles produced by laser ablation method were incorporated into nanofibrous polymeric matrix by using electrospinning technique. For example, gold (Au) and silver (Ag) nanoparticles (NPs) were produced from their metallic sources by laser ablation method directly in the polymer solutions. The NPs/polymer mixtures were electrospun and surface plasmon resonance effect of Au-NPs and Ag-NPs on optical properties of the nanofibers was studied. In addition, germanium nanocrystals produced by means of laser ablation were mixed with PVDF polymer solution and consequently electrospun into composite polymeric nanofiber matrix.Item Open Access Photoluminescent electrospun polymeric nanofibers incorporating germanium nanocrystals(Elsevier, 2013) Ortaç, B.; Kayaci, F.; Vural, H. A.; Deniz, A. E.; Uyar, TamerThe photoluminescent germanium nanocrystals (Ge-NCs) were successfully incorporated into electrospun polymeric nanofiber matrix in order to develop photoluminescent nanofibrous composite web. In the first step, the synthesis of Ge-NCs was achieved by nanosecond pulsed laser ablation of bulk germanium wafer immersed in organic liquid. The size, the structural and the chemical characteristics of Ge-NCs investigated by TEM, XPS, XRD and Raman spectroscopy revealed that the Ge-NCs were highly pure and highly crystalline having spherical shape within 3-20 nm particle size distribution. In the second step, Ge-NCs were mixed with polyvinyl alcohol (PVA) polymer solution, and then, Ge-NC/PVA nanofibers were obtained via electrospinning technique. The electrospinning of Ge-NCs/PVA nanoweb composite structure was successful and bead-free Ge-NCs/PVA nanofibers having average fiber diameter of 185 ± 40 nm were obtained. The STEM analysis of the electrospun Ge-NCs/PVA nanofibers elucidated that the Ge-NCs were distributed homogeneously in the polymeric nanofiber matrix. The UV-Vis absorption and photoluminescence spectroscopy studies indicated the quantum confinement effect of Ge-NCs on the optical properties of the electrospun Ge-NCs/PVA nanoweb. © 2013 Elsevier Ltd. All rights reserved.