Browsing by Subject "Photoluminescence measurements"

Now showing 1 - 6 of 6

Open Access
Analysis of defect related optical transitions in biased AlGaN/GaN heterostructures
(2010) Bengi, A.; Lisesivdin, S.B.; Kasap, M.; Mammadov, T.; Ozcelik, S.; Özbay, Ekmel
The optical transitions in AlGaN/GaN heterostructures that are grown by metalorganic chemical vapor deposition (MOCVD) have been investigated in detail by using Hall and room temperature (RT) photoluminescence (PL) measurements. The Hall measurements show that there is two-dimensional electron gas (2DEG) conduction at the AlGaN/GaN heterointerface. PL measurements show that in addition to the characteristic near-band edge (BE) transition, there are blue (BL) and yellow luminescence (YL) bands, free-exciton transition (FE), and a neighboring emission band (NEB). To analyze these transitions in detail, the PL measurements were taken under bias where the applied electric field changed from 0 to 50 V/cm. Due to the applied electric field, band bending occurs and NEB separates into two different peaks as an ultraviolet luminescence (UVL) and Y4 band. Among these bands, only the yellow band is unaffected with the applied electric field. The luminescence intensity change of these bands with an electric field is investigated in detail. As a result, the most probable candidate of the intensity decrease with an increasing electric field is the reduction in the radiative lifetime. © 2010 Elsevier Ltd. All rights reserved.
Open Access
Characterization of an AlN buffer layer and a thick-GaN layer grown on sapphire substrate by MOCVD
(Springer, 2010-10-23) Çörekçi, S.; Öztürk, M. K.; Bengi, A.; Çakmak, M.; Özçelik, S.; Özbay, Ekmel
An AlN buffer layer and a thick-GaN layer for high-electron-mobility transistors (HEMTs) were grown on sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The structural and morphological properties of the layers were investigated by high resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) techniques. The optical quality of the thick-GaN layer was also evaluated in detail by a photoluminescence (PL) measurement. It was found that the AlN buffer layer possesses high crystal quality and an atomically flat surface with a root-mean-square (rms) roughness of 0.16 nm. The screw-and edge-type dislocation densities of the thick-GaN layer were determined as 5.4 9 107 and 5.0 9 109 cm-2 by means of the mosaic crystal model, respectively. It was observed that the GaN layer has a smooth surface with an rms of 0.84 nm. Furthermore, the dark spot density of the GaN surface was estimated as 6.5 9 108 cm-2 over a scan area of 4 μm2. © Springer Science+Business Media, LLC 2010.
Open Access
Fabrication of flexible polymer–GaN core–shell nanofibers by the combination of electrospinning and hollow cathode plasma-assisted atomic layer deposition
(Royal Society of Chemistry, 2015) Ozgit Akgun, C.; Kayaci, F.; Vempati S.; Haider A.; Celebioglu A.; Goldenberg, E.; Kizir S.; Uyar, Tamer; Bıyıklı, Necmi
Here we demonstrate the combination of electrospinning and hollow cathode plasma-assisted atomic layer deposition (HCPA-ALD) processes by fabricating flexible polymer-GaN organic-inorganic core-shell nanofibers at a processing temperature much lower than that needed for the preparation of conventional GaN ceramic nanofibers. Polymer-GaN organic-inorganic core-shell nanofibers fabricated by the HCPA-ALD of GaN on electrospun polymeric (nylon 6,6) nanofibers at 200 °C were characterized in detail using electron microscopy, energy dispersive X-ray analysis, selected area electron diffraction, X-ray diffraction, X-ray photoelectron spectroscopy, photoluminescence measurements, and dynamic mechanical analysis. Although transmission electron microscopy studies indicated that the process parameters should be further optimized for obtaining ultimate uniformity and conformality on these high surface area 3D substrates, the HCPA-ALD process resulted in a ∼28 nm thick polycrystalline wurtzite GaN layer on polymeric nanofibers of an average fiber diameter of ∼70 nm. Having a flexible polymeric core and low processing temperature, these core-shell semiconducting nanofibers might have the potential to substitute brittle ceramic GaN nanofibers, which have already been shown to be high performance materials for various electronic and optoelectronic applications.
Open Access
Low-temperature grown wurtzite InxGa1−xN thin films via hollow cathode plasma-assisted atomic layer deposition
(Royal Society of Chemistry, 2015-08) Haider A.; Kizir S.; Ozgit Akgun, C.; Goldenberg, E.; Leghari, S. A.; Okyay, Ali Kemal; Bıyıklı, Necmi
Herein, we report on atomic layer deposition of ternary InxGa1−xN alloys with different indium contents using a remotely integrated hollow cathode plasma source. Depositions were carried out at 200 °C using organometallic Ga and In precursors along with N2/H2 and N2 plasma, respectively. The effect of In content on structural, optical, and morphological properties of InxGa1−xN thin films was investigated. Grazing incidence X-ray diffraction showed that all InxGa1−xN thin films were polycrystalline with a hexagonal wurtzite structure. X-ray photoelectron spectroscopy depicted the peaks of In, Ga, and N in bulk of the film and revealed the presence of relatively low impurity contents. In contents of different InxGa1−xN thin films were determined by energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. Transmission electron microscopy also confirmed the polycrystalline structure of InxGa1−xN thin films, and elemental mapping further revealed the uniform distribution of In and Ga within the bulk of InxGa1−xN films. Higher In concentrations resulted in an increase of refractive indices of ternary alloys from 2.28 to 2.42 at a wavelength of 650 nm. The optical band edge of InxGa1−xN films red-shifted with increasing In content, confirming the tunability of the band edge with alloy composition. Photoluminescence measurements exhibited broad spectral features with an In concentration dependent wavelength shift and atomic force microscopy revealed low surface roughness of InxGa1−xN films with a slight increase proportional to In content.
Open Access
Synthesis of ultra-small Si / Ge semiconductor nano-particles using electrochemistry
(Elsevier, 2012) Alkis, S.; Ghaffari, M.; Okyay, Ali Kemal
In this paper, we describe the formation of colloidal Si/Ge semiconductor nano-particles by electrochemical etching of Ge quantum dots (GEDOT), Silicon-Germanium graded layers (GRADE) and Silicon-Germanium multi-quantum well (MQW) structures which are prepared on Silicon wafers using low pressure chemical vapor deposition (LPCVD) technique. The formation of Si/Ge nano-particles is verified by transmission electron microscope (TEM) images and photoluminescence (PL) measurements. The Si/Ge nano-particles obtained from GEDOT and GRADE structures, gave blue emissions, upon 250 nm, and 300 nm UV excitations. However, the nano-particles obtained from the MQW structure did exhibit various color emissions (orange, blue, green and red) upon excitation with 250 nm, 360 nm, 380 nm and 400 nm wavelength light.
Open Access
Unconventional pairing in excitonic condensates under Spin-orbit coupling
(The American Physical Society, 2009) Can, M. A.; Hakioǧlu T.
It is shown that Rashba and Dresselhaus spin-orbit couplings enhance the conclusive power in the experiments on the excitonic condensate by at least three low temperature effects. First, spin-orbit coupling facilitates the photoluminescence measurements via enhancing the bright contribution in the otherwise dominantly dark ground state. The second is the presence of a low temperature power law dependence of the specific heat and weakening of the second order transition at the critical temperature. The third is the appearance of the nondiagonal elements in the static spin susceptibility.