8 research outputs found
Ge nanocrystals embedded in ultrathin Si3N4 multilayers with SiO2 barriers
Multilayers of germanium nanocrystals (NCs) embedded in thin films of silicon nitride matrix separated with SiO2 barriers have been fabricated using plasma enhanced chemical vapor deposition (PECVD). SiGeN/SiO2 alternating bilayers have been grown on quartz and Si substrates followed by post annealing in Ar ambient from 600 to 900 degrees C. High resolution transmission electron microscopy (HRTEM) as well as Raman spectroscopy show good crystallinity of Ge confined to SiGeN layers in samples annealed at 900 degrees C. Strong compressive stress for SiGeN/SiO2 structures were observed through Raman spectroscopy. Size, as well as NC-NC distance were controlled along the growth direction for multilayer samples by varying the thickness of bilayers. Visible photoluminescence (PL) at 2.3 and 3.1 eV with NC size dependent intensity is observed and possible origin of PL is discussed.UNAM-National Nanotechnology Research Center at Bilkent Universit
Dielektrik matrislerde germanium nanokristaller : yapısal ve optik analiz ve zor evrimi
Cataloged from PDF version of article.Thesis (Ph.D.): Bilkent University, Department of Physics, İhsan Doğramacı Bilkent University, 2017.Includes bibliographical references (leaves 88-99).Group IV semiconductor nanocrystals, namely silicon and germanium have attracted
much interest in the past two decades due to their broad applications
in photovoltaic, memory, optoelectronic, medical imaging and photodetection
devices. Generally, there are two major features of semiconducor nanocrystals:
First, spatial confinement of charge carriers which leads to the significant changes
in optical and electronic properties of materials as a function of size. This effect
gives the possibility to use the size and shape of the nanocrystals to tune the energy
of electronic energy states. Second feature of nanocrystals, is the increased
of surface area to volume ratio of the nanocrystal with reducing size. This leads
to an enhanced role of the effects related to surface and interface of the nanocrystal.
Furthermore, stress on the nanocrystals can lead modification of the band
structure as well as in
uencing the crystallization of the nanomaterials. Recent
works show that measurement and control of the stress can open the way for
strain engineering of the electronic band structure, thereby opening the way for
new physics and applications. In this thesis, we first carry out a study on the
synthesis of germanium embedded in silicon nitride and oxide matrices. In
uence
of the annealing method as well as germanium concentration on the formation of
nanocrystals is discussed. It was found that Ge concentration and annealing play
important roles in the formation of the Ge nanocrystals. With crystallographic
data obtained from high resolution transmission electron microscopy, quantitative
analysis of stress state of germanium nanocrystals have been done by analyzing
Raman peak shift of embedded nanocrystals taking into account the phonon
confinement effect. Finally, using stressors as buffer layers, superlattices of Ge
nanosheets were studied to understand the effects of the stressors on the stress
state of Ge nanocrystals. We demonstrate that it is possible to tune the stress
on the Ge nanocrystals from compressive to tensile. Finally we showed a three
dimensional Ge quantum solid that can be used in optoelectronic applications.by Rahim Bahariqushchi.xvi, 99 leaves : illustrations (some color), chart ; 30 c
Optical bandgap of semiconductor nanostructures: Methods for experimental data analysis
Determination of the optical bandgap (E-g) in semiconductor nanostructures is a key issue in understanding the extent of quantum confinement effects (QCE) on electronic properties and it usually involves some analytical approximation in experimental data reduction and modeling of the light absorption processes. Here, we compare some of the analytical procedures frequently used to evaluate the optical bandgap from reflectance (R) and transmittance (T) spectra. Ge quantum wells and quantum dots embedded in SiO2 were produced by plasma enhanced chemical vapor deposition, and light absorption was characterized by UV-Vis/NIR spectrophotometry. R&T elaboration to extract the absorption spectra was conducted by two approximated methods (single or double pass approximation, single pass analysis, and double pass analysis, respectively) followed by Eg evaluation through linear fit of Tauc or Cody plots. Direct fitting of R&T spectra through a Tauc-Lorentz oscillator model is used as comparison. Methods and data are discussed also in terms of the light absorption process in the presence of QCE. The reported data show that, despite the approximation, the DPA approach joined with Tauc plot gives reliable results, with clear advantages in terms of computational efforts and understanding of QCE.ENERGETIC - PON00355_3391233MIUR under project Beyond-Nano - PON a3_0036
Stress evolution of Ge nanocrystals in dielectric matrices
Germanium nanocrystals (Ge NCs) embedded in single and multilayer silicon oxide and silicon nitride matrices have been synthesized using plasma enhanced chemical vapor deposition followed by conventional furnace annealing or rapid thermal processing in N-2 ambient. Compositions of the films were determined by Rutherford backscattering spectrometry and x-ray photoelectron spectroscopy. The formation of NCs under suitable process conditions was observed with high resolution transmission electron microscope micrographs and Raman spectroscopy. Stress measurements were done using Raman shifts of the Ge optical phonon line at 300.7 cm(-1). The effect of the embedding matrix and annealing methods on Ge NC formation were investigated. In addition to Ge NCs in single layer samples, the stress on Ge NCs in multilayer samples was also analyzed. Multilayers of Ge NCs in a silicon nitride matrix separated by dielectric buffer layers to control the size and density of NCs were fabricated. Multilayers consisted of SiNy:Ge ultrathin films sandwiched between either SiO2 or Si3N4 by the proper choice of buffer material. We demonstrated that it is possible to tune the stress state of Ge NCs from compressive to tensile, a desirable property for optoelectronic applications. We also observed that there is a correlation between the stress and the crystallization threshold in which the compressive stress enhances the crystallization, while the tensile stress suppresses the process
Stress evolution of Ge nanocrystals in dielectric matrices
Germanium nanocrystals (Ge NCs) embedded in single and multilayer silicon oxide and silicon nitride matrices have been synthesized using plasma enhanced chemical vapor deposition followed by conventional furnace annealing or rapid thermal processing in N-2 ambient. Compositions of the films were determined by Rutherford backscattering spectrometry and x-ray photoelectron spectroscopy. The formation of NCs under suitable process conditions was observed with high resolution transmission electron microscope micrographs and Raman spectroscopy. Stress measurements were done using Raman shifts of the Ge optical phonon line at 300.7 cm(-1). The effect of the embedding matrix and annealing methods on Ge NC formation were investigated. In addition to Ge NCs in single layer samples, the stress on Ge NCs in multilayer samples was also analyzed. Multilayers of Ge NCs in a silicon nitride matrix separated by dielectric buffer layers to control the size and density of NCs were fabricated. Multilayers consisted of SiNy:Ge ultrathin films sandwiched between either SiO2 or Si3N4 by the proper choice of buffer material. We demonstrated that it is possible to tune the stress state of Ge NCs from compressive to tensile, a desirable property for optoelectronic applications. We also observed that there is a correlation between the stress and the crystallization threshold in which the compressive stress enhances the crystallization, while the tensile stress suppresses the process
Tailoring morphology to control defect structures in ZnO electrodes for high-performance supercapacitor devices
Zinc oxide (ZnO) nanostructures were synthesized in the form of nanoparticles, nanoflowers and nanourchins. Structural, electronic and optical characterization of the samples was performed via standard techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence, Raman and ultraviolet-visible (UV-Vis) spectroscopy. Point defect structures which are specific to each morphology have been investigated in terms of their concentration and location via state-of-the-art electron paramagnetic resonance (EPR) spectroscopy. According to the core-shell model, all the samples revealed core defects; however, the defects on the surface are smeared out. Finally, all three morphologies have been tested as electrode materials in a real supercapacitor device and the performance of the device, in particular, the specific capacitance and the storage mechanism, has been mediated by the point defects. Morphology-dependent defective ZnO electrodes enable the monitoring of the working principle of the supercapacitor device ranging from electric double-layer capacitors (EDLC) to pseudo-supercapacitors. © 2020 The Royal Society of Chemistry