Browsing by Subject "Carrier mobility"

Now showing 1 - 5 of 5

Open Access
Effect of reactor pressure on optical and electrical properties of InN films grown by high-pressure chemical vapor deposition
(Wiley - V C H Verlag GmbH & Co. KGaA, 2015) Alevli, M.; Gungor, N.; Alkis, S.; Ozgit Akgun, C.; Donmez, I.; Okyay, Ali Kemal; Gamage, S.; Senevirathna, I.; Dietz, N.; Bıyıklı, Necmi
The influences of reactor pressure on the stoichiometry, free carrier concentration, IR and Hall determined mobility, effective optical band edge, and optical phonon modes of HPCVD grown InN films have been analysed and are reported. The In 3d, and N 1s XPS spectra results revealed In-N and N-In bonding states as well as small concentrations of In-O and N-O bonds, respectively in all samples. InN layers grown at 1 bar were found to contain metallic indium, suggesting that the incorporation of nitrogen into the InN crystal structure was not efficient. The free carrier concentrations, as determined by Hall measurements, were found to decrease with increasing reactor pressure from 1.61×1021 to 8.87×1019 cm-3 and the room-temperature Hall mobility increased with reactor pressure from 21.01 to 155.18 cm2/Vs at 1 and 15 bar reactor pressures, respectively. IR reflectance spectra of all three (1, 8, and 15 bar) InN samples were modelled assuming two distinct layers of InN, having different free carrier concentration, IR mobility, and effective dielectric function values, related to a nucleation/interfacial region at the InN/sapphire, followed by a bulk InN layer. The effective optical band gap has been found to decrease from 1.19 to 0.95 eV with increasing reactor pressure. Improvement of the local structural quality with increasing reactor pressure has been further confirmed by Raman spectroscopy measurements. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Exchange-correlation effects in the impurity-limited mobility of GaAs quantum wires
(Sci Tech Res Counc Turkey, Ankara, Turkey, 1999) Tanatar, Bilal
We study the many-body effects described by the local-field corrections on the mobility of quasi-one dimensional electron systems. The low temperature mobility due to remote-impurity doping and interface-roughness scattering is calculated within the relaxation time approximation. We find that correlation effects significantly reduce the mobility at low density.
Open Access
Investigation of the effect of donor-acceptor substitution on band gap, band width, and conductivity
(Elsevier, 2001) Salzner, U.
Polymers of two donor-acceptor systems, 3-cyano,3′-hydroxybithiophene 1 and 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b′],3,4-ethylenedioxythiophene (CDM-EDOT) 2, were analyzed with density functional theory. As predicted by perturbation theory, interactions between donors and acceptors with very different energy levels are greatly reduced compared to those between fragments with similar energy levels. This leads to localized states and bands with little dispersion. For poly-1 these localized states lie below the valence band and above the conduction band. For 2 localized unoccupied levels lie within the band gap. These acceptor levels account for the high electron affinity of poly-2 and allow for self-doping. Self doping explains the increased intrinsic conductivity of poly-2, the localized nature of the low lying MOs rationalizes the low mobility of n-type carriers in poly-2.
Open Access
An online adaptive cooperation scheme for spectrum sensing based on a second-order statistical method
(Institute of Electrical and Electronics Engineers, 2012) Yarkan S.; Töreyin, B. U.; Qaraqe, K. A.; Çetin, A. Enis
Spectrum sensing is one of the most important features of cognitive radio (CR) systems. Although spectrum sensing can be performed by a single CR, it is shown in the literature that cooperative techniques, including multiple CRs/sensors, improve the performance and reliability of spectrum sensing. Existing cooperation techniques usually assume a static communication scenario between the unknown source and sensors along with a fixed propagation environment class. In this paper, an online adaptive cooperation scheme is proposed for spectrum sensing to maintain the level of sensing reliability and performance under changing channel and environmental conditions. Each cooperating sensor analyzes second-order statistics of the received signal, which undergoes both correlated fast and slow fading. Autocorrelation estimation data from sensors are fused together by an adaptive weighted linear combination at the fusion center. Weight update operation is performed online through the use of orthogonal projection onto convex sets. Numerical results show that the performance of the proposed scheme is maintained for dynamically changing characteristics of the channel between an unknown source and sensors, even under different physical propagation environments. In addition, it is shown that the proposed cooperative scheme, which is based on second-order detectors, yields better results compared with the same fusion mechanism that is based on conventional energy detectors.
Open Access
Surface evolution of 4H-SiC(0001) during in-situ surface preparation and its influence on graphene properties
(Trans Tech Publications, Switzerland, 2013) Ul Hassan J.; Meyer, A.; Çakmakyapan, Semih; Kazar, Özgür; Flege J.I.; Falta J.; Özbay, Ekmel; Janzén, E.
The evolution of SiC surface morphology during graphene growth process has been studied through the comparison of substrate surface step structure after in-situ etching and graphene growth in vacuum. Influence of in-situ substrate surface preparation on the properties of graphene was studied through the comparison of graphene layers on etched and un-etched substrates grown under same conditions. © (2013) Trans Tech Publications, Switzerland.