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Articles 30601 - 30630 of 36660

Full-Text Articles in Physical Sciences and Mathematics

Magnetic Activity In The Extreme Ultraviolet, Mihalis Mathioudakis Jan 1999

Magnetic Activity In The Extreme Ultraviolet, Mihalis Mathioudakis

Turkish Journal of Physics

The wealth of astronomical observations obtained in the extreme ultraviolet in recent years, have allowed comprehensive studies of the stellar transition regions and coronae to be carried out. For the first time we have been able to resolve individual coronal lines of various elements formed over a large temperature range (10^{5} - 10^{7.2} K). The temperature, densities, abundances and magnetic field strengths of the stellar coronae can now be determined. Here we review some of the observations in the field of cool stars and discuss the implications on atmospheric heating requirements.

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The Lattice Dynamics Of \Gamma-Iron, Kemal Çolakoğlu, G. Uğur, M. Çakmak, H. M. Tütüncü Jan 1999

The Lattice Dynamics Of \Gamma-Iron, Kemal Çolakoğlu, G. Uğur, M. Çakmak, H. M. Tütüncü

Turkish Journal of Physics

Lattice dynamical calculations are performed on \gamma-iron using the Clark-Gazis-Wallis (CGW) model to represent the ion-ion interactions, and a modified form of the Sharma-Joshi model to include ion-electron interactions. The theory is used to compute the phonon dispersion curves, frequency spectra and the lattice specific heat of \gamma-iron. The obtained results are in good agreement with the experimental findings, and are better than those calculated using the other theories.

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Ionizing Radiations And Annealing Influence On Mosfet Charge States, Zainabidinov Ciragiddin, Atamuratov Atabek, Ysupov Akhmed, Adinaev Kuvondik Jan 1999

Ionizing Radiations And Annealing Influence On Mosfet Charge States, Zainabidinov Ciragiddin, Atamuratov Atabek, Ysupov Akhmed, Adinaev Kuvondik

Turkish Journal of Physics

The threshold voltage shift \Delta V_T and its components due to trapped-oxide charges \Delta V_{Not} and Si-SiO_2 interface traps \Delta V_N in MOSFET exposed to Bremsstrahlung, Co_{60} irradiation and annealing were studied. Several effects caused by differences in the photon energies from two types of sources are discussed as well as a mechanism of changing the trapped-oxide and Si-SiO_2 interface traps by annealing. The mechanism is based on previously available models.

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Fundamental Research And Device Technology, B. K. Ridley Jan 1999

Fundamental Research And Device Technology, B. K. Ridley

Turkish Journal of Physics

Highlights of the interaction between fundamental research and semiconductor device technology are summarized.

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Heat Pulse Studies Of The Emission And Absorption Of Acoustic Phonons In Gaas Quantum Wells And Wires, A. J. Kent Jan 1999

Heat Pulse Studies Of The Emission And Absorption Of Acoustic Phonons In Gaas Quantum Wells And Wires, A. J. Kent

Turkish Journal of Physics

This paper reviews some recent experiments in which heat pulse techniques have been used to study the interaction of acoustic phonons with electrons in semiconductor quantum wells and wires. Heat pulse experiments provide temporal and spatial resolution of the phonons that are emitted or absorbed by the electrons and so give more detailed information about the electron-phonon interaction than can be obtained by other methods (e.g. transport measurements). Phonon experiments demonstrate clearly the effect of the electron confinement on the carrier-phonon interaction and the overall energy loss rate due to acoustic phonon emission. A qualitative explanation of the results is …

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Electron Transport In Gaas Quantum Wells: Effect Of Interface Roughness Scatterin, Rita Gupta Jan 1999

Electron Transport In Gaas Quantum Wells: Effect Of Interface Roughness Scatterin, Rita Gupta

Turkish Journal of Physics

The importance of interface roughness (IFR) scattering of electrons and LO-phonons for electron transport in semiconductor quantum wells is discussed. Modulation doping of quantum wells minimizes the effect of impurity scattering on the low-field electron mobility so that IFR scattering of electrons in the well becomes the major limiting factor. A model calculation of IFR scattering of electrons in quantum wells is presented and it is shown that (both)\Lambda and \Delta, the parameters defining IFR, can be estimated by comparing the theoretical and experimental electron mobilities. The application of high electric field leads to a distribution of hot electrons which …

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Well-Width Dependence Of Warm Electron Relaxation And Interface Roughness Scattering In Gaas/Ga_{1-X}Al_Xas Multiple Quantum Wells, M. Cankurtaran, H. Çeli̇k, E. Tiraş, A. Bayrakli, N. Balkan Jan 1999

Well-Width Dependence Of Warm Electron Relaxation And Interface Roughness Scattering In Gaas/Ga_{1-X}Al_Xas Multiple Quantum Wells, M. Cankurtaran, H. Çeli̇k, E. Tiraş, A. Bayrakli, N. Balkan

Turkish Journal of Physics

We review our recent results concerning the well-width dependence of the acoustic-phonon-assisted energy relaxation of two-dimensional (2D) warm electrons in modulation-doped GaAs/Ga_{1-x}Al_xAs multiple quantum wells. Electron energy-loss rates via the emission of acoustic phonons are determined from the amplitude of Shubnikov-de Haas (SdH) oscillations, measured as a function of lattice temperature and applied electric field. Experimental results are compared with the existing theoretical models that involve deformation-potential and screened and unscreened piezoelectric scattering. Well-width dependence of the quantum and transport mobilities of 2D electrons in the same samples have also been determined by measuring the quantum oscillations in both the …

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Large-Bandgap Semiconductors, B. K. Ridley Jan 1999

Large-Bandgap Semiconductors, B. K. Ridley

Turkish Journal of Physics

The general features of large-bandgap semiconductors that make them interesting for electronic and optoelectronic applications are briefly reviewed. The nitrides AIN, GaN and InN are singled out for a more focused treatment.

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Angular Dependence And Mode Distribution Of Acoustic Phonon Emission By Hot 2d Electrons In Gaas/Algaas Heterojunctions And Quantum Wells, Dietmar Lehmann, Czeslaw Jasiukiewicz Jan 1999

Angular Dependence And Mode Distribution Of Acoustic Phonon Emission By Hot 2d Electrons In Gaas/Algaas Heterojunctions And Quantum Wells, Dietmar Lehmann, Czeslaw Jasiukiewicz

Turkish Journal of Physics

We report a detailed theoretical study of the angular dependence and mode distribution of the acoustic phonon emission by hot two-dimensional electron gases in GaAs/AlGaAs heterojunctions and quantum wells and compare the results with some recent heat pulse measurements for carrier temperatures below 50 K. Common to all the experimental results was the strong dependence of the ratio of emitted longitudinal acoustic (LA) phonons to transverse acoustic (TA) phonons from the width of the quantum well and the absence of LA phonons propagating in a direction close to the 2DEG normal for GaAs/AlGaAs heterojunctions. To explain these phenomena and to …

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Electrons In Large-Bandgap Bulk Semiconductors And Quantum Wells, N. A. Zakhleniuk, M. Babiker, C. R. Bennett, B. K. Ridley Jan 1999

Electrons In Large-Bandgap Bulk Semiconductors And Quantum Wells, N. A. Zakhleniuk, M. Babiker, C. R. Bennett, B. K. Ridley

Turkish Journal of Physics

The analytical theory of hot electrons interacting with lattice vibrations, impurities, and interface roughness in quantum wells is developed. We have obtained new distribution functions which describe all the kinetic properties of the non-equilibrium electron gas. As a specific example we present the electric field dependence of the electron mobility in GaN-based quantum wells and in bulk GaN. The relative importance of the different scattering mechanisms is analysed in detail.

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Modelling Of Devices For Optoelectronic Applications: The Quantum Confined Stark Effect And Self-Electrooptic Effect Devices, Eckehard Scholl Jan 1999

Modelling Of Devices For Optoelectronic Applications: The Quantum Confined Stark Effect And Self-Electrooptic Effect Devices, Eckehard Scholl

Turkish Journal of Physics

Electro-optical effects, such as the Franz-Keldysh effect in bulk materials or the quantum confined Stark effect in quantum well structures, lead to strong optoelectronic nonlinearities which form the basis for optical modulators and optically bistable devices. They result from a modification of the optical absorption properties by an applied electric field and are particularly pronounced in the case of low dimensional semiconductors. We review theoretical modelling and computer simulations of such optoelectronic devices in particular for ZnSe based quantum well structures, where excitonic features dominate even at room temperature. The field dependent absorption spectra are calculated by a many-body theory …

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Butt-Coupling Loss Of 0.1 Db/Interface In Inp/Ingaas Multi-Quantum-Well Waveguide-Waveguide Structures Grown By Selective Area Chemical Beam Epitaxy, C. A. Verschuren, M. R. Leys, H. Vonk, J. H. Wolter, P. J. Harmsma, Y. S. Oei Jan 1999

Butt-Coupling Loss Of 0.1 Db/Interface In Inp/Ingaas Multi-Quantum-Well Waveguide-Waveguide Structures Grown By Selective Area Chemical Beam Epitaxy, C. A. Verschuren, M. R. Leys, H. Vonk, J. H. Wolter, P. J. Harmsma, Y. S. Oei

Turkish Journal of Physics

The lateral coupling of waveguiding structures in both [011] and [0\bar{1}1] directions is studied using embedded selective area epitaxy by Chemical Beam Epitaxy. All growth steps are carried out under the same growth conditions on (100) lnP substrates misoriented by 0.5° towards (111)B. Both planar and selectively grown material exhibits bright luminescence and narrow PL line widths (8 meV FWHM at 4K), up to the lateral junction. Moreover, no degradation of the original material properties is observed after regrowth. SEM images show very flat layers and excellent lateral coupling for all four types of junctions. After reactive ion etching of …

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Silicon-Based Optoelectronics: Progress And Challenges, Tamim P. Sidiki, Clivia M. Sotomayor Torres Jan 1999

Silicon-Based Optoelectronics: Progress And Challenges, Tamim P. Sidiki, Clivia M. Sotomayor Torres

Turkish Journal of Physics

We review the status of silicon-based optoelectronics with emphasis on light emitting diodes. Erbium-doped Si, por-Si and silicon-based superlattices and nanostructures are discussed. The origin behind light emission in silicon with feature sizes below about 60 nm still remains poorly understood.

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3 \Mu M Intersubband Quantum Well Photodetector (Qwip), Rita Gupta, A. L. Y. Wong, S. K. Haywood Jan 1999

3 \Mu M Intersubband Quantum Well Photodetector (Qwip), Rita Gupta, A. L. Y. Wong, S. K. Haywood

Turkish Journal of Physics

In recent years photodetectors operating in the mid- to far infrared region of 3-15 \mu m have been designed based on electron and hole intersubband transitions in multiple quantum wells and superlattices. In general, QWIPs based on electron transitions show greater detectivity compared to the hole-based photodetectors. However, selection rules for electron intersubband transitions usually forbid the TE mode operation, associated with normal light incidence; Therefore, special coupling structures/geometries have been employed to couple light into the device. The spectral region 3-5 \mu m is of interest for a variety of applications such as environmental gas sensing, thermal imaging etc. …

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Single Si \Delta-Doped Gaas Investigations By New Photothermal Wavelength Modulated Photocurrent Technique, Fi̇kret Hajiev, Yener Özkan, M. Çeti̇n Arikan Jan 1999

Single Si \Delta-Doped Gaas Investigations By New Photothermal Wavelength Modulated Photocurrent Technique, Fi̇kret Hajiev, Yener Özkan, M. Çeti̇n Arikan

Turkish Journal of Physics

New Photothermal Wavelength Modulated Photocurrent (PWMPC) technique is reported. This technique is used for investigation of the MBE grown p-GaAs sample in which a single Si-layer was embedded with a \delta-type profile. Two spectral features were observed at 1.5137 eV and 1.5115 eV at 20 K. These peaks were attributed to the (D°,X) and (A°,X)-excitons bound to neutral donors and acceptors, respectively. We studied the temperature dependence of these excitonic peaks positions at temperatures between 20-90 K. Additionally, we demonstrate a blue shift of photocurrent spectra under a low level illumination intensity. PWMPC technique and the nature of the registered …

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Semiconductor And Dielectric Microcavity Spectroscopy, Ali̇ Serpengüzel Jan 1999

Semiconductor And Dielectric Microcavity Spectroscopy, Ali̇ Serpengüzel

Turkish Journal of Physics

Semiconductor and dielectric microcavities are used for the localization of photons as well as the enhancement of photon density of states. The enhancement of photoluminescence, electroluminescence, and lasing by the use of microcavities leads to novel active and passive optoelectronic and photonic devices such as channel droppping filters, semiconductor lasers, and resonant cavity enhanced devices. Experimental results showing photoluminescence enhancement in active planar, lasing in active ellipsoidal microcavities as well as light scattering in passive and spherical microcavities are presented.

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Experimental Separation Of Phonon And Extrinsic Scattering In 2d Carrier Gases In Gaas, F. F. Ouali, H. R. Francis, H. C. Rhodes Jan 1999

Experimental Separation Of Phonon And Extrinsic Scattering In 2d Carrier Gases In Gaas, F. F. Ouali, H. R. Francis, H. C. Rhodes

Turkish Journal of Physics

A new method which separates phonon scattering from extrinsic scattering in 2D gases in GaAs is presented. In contrast to previous ones, the technique makes no assumptions about the temperature dependence of the extrinsic scattering. The preliminary measurements of the phonon limited mobility \mu_{ph} on two electron gas samples show a Bloch-Grüneisen regime in the temperature range 1.2-4.5K and the results agree reasonably well with other experimental and theoretical work.

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Stability Of Quasi-Two-Dimensional Bipolarons, R. Tuğrul Senger, Ati̇lla Erçelebi̇ Jan 1999

Stability Of Quasi-Two-Dimensional Bipolarons, R. Tuğrul Senger, Ati̇lla Erçelebi̇

Turkish Journal of Physics

The stability criteria of quasi-two-dimensional dimensional bipolarons have been studied within the framework of strong coupling and path-integral theories. It is shown that the critical values of the electron-phonon coupling constant (\alpha), and the ratio of dielectric constants (\eta = \epsilon_{\infty}/\epsilon_{0}) exhibit some non-trivial features as the effective dimensionality is tuned from three to two.

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The Effect Of Non-Equilibrium Acoustic Phonons On The Tunnel Current In Gaas/Alas Superlattices, S. A. Cavill, F. F. Ouali, L. J. Challis, A. J. Kent, M. Henini, A. V. Akimov Jan 1999

The Effect Of Non-Equilibrium Acoustic Phonons On The Tunnel Current In Gaas/Alas Superlattices, S. A. Cavill, F. F. Ouali, L. J. Challis, A. J. Kent, M. Henini, A. V. Akimov

Turkish Journal of Physics

We present the first study of the effect of ballistic acoustic phonons generated by a heated metal film on the tunnel current in GaAs/AlAs superlattices. The phonon-induced increase of the tunnel current as a function of applied voltage has a maximum at a voltage that varies linearly with heater temperature both in zero and applied magnetic field. The behaviour is consistent with phonon-assisted tunnelling by stimulated phonon emission. The system acts as a phonon spectrometer in the < 1THz frequency region with a resolution of approximately 200 GHz.

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Binding Energies Of Excitons In Symmetric And Asymmetric Coupled Double-Quantum Well Structures In A Magnetic Field, Esi̇n Kasapoğlu, Hüseyi̇n Sari, Yüksel Ergün, Sezai̇ Elagöz, Naci̇ Balkan, İsmai̇l Sökmen Jan 1999

Binding Energies Of Excitons In Symmetric And Asymmetric Coupled Double-Quantum Well Structures In A Magnetic Field, Esi̇n Kasapoğlu, Hüseyi̇n Sari, Yüksel Ergün, Sezai̇ Elagöz, Naci̇ Balkan, İsmai̇l Sökmen

Turkish Journal of Physics

The binding energy of excitons in the symmetric and asymmetric coupled double GaAs/Ga_{1-x}Al_xAs quantum wells is calculated by using variational approach. Results have been obtained as a function of the potential symmetry, the size of the quantum well, and the coupling parameter of the wells in the presence of a magnetic field applied parallel to the growth direction. The role of the asymmetric barriers, magnetic field, barrier and well width in determining the tunability of the excitonic binding parameters of the GaAs/Ga_{1-x}Al_xAs system is discussed.

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A Gainasp/Inp Vertical Cavity Surface Emitting Laser For 1.5 \Mu M Operation, R. Sceats, N. Balkan, M. J. Adams, J. Masum, A. J. Dann, S. D. Perrin, I. Reid, J. Reed, P. Cannard, M. A. Fisher, D. J. Elton, M. J. Harlow Jan 1999

A Gainasp/Inp Vertical Cavity Surface Emitting Laser For 1.5 \Mu M Operation, R. Sceats, N. Balkan, M. J. Adams, J. Masum, A. J. Dann, S. D. Perrin, I. Reid, J. Reed, P. Cannard, M. A. Fisher, D. J. Elton, M. J. Harlow

Turkish Journal of Physics

We present the results of our studies concerning the pulsed operation of a bulk GaInAsP/InP vertical cavity surface emitting laser (VCSEL). The device is tailored to emit at around 1.5 \mu m at room temperature. The structure has a 45 period n-doped GaInAsP/InP bottom distributed Bragg reflector (DBR), and a 4 period Si/Al_2O_3 dielectric top reflector defining a 3-\lambda cavity. Electroluminescence from a 16 \mu m diameter top window was measured in the pulsed injection mode. Spectral measurements were recorded in the temperature range between 125K and 240K. Polarisation, lasing threshold current and linewidth measurements were also carried out at …

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An Investigation Of Electrical Properties Of Porous Silicon, G. Algün, M. Ç. Arikan Jan 1999

An Investigation Of Electrical Properties Of Porous Silicon, G. Algün, M. Ç. Arikan

Turkish Journal of Physics

In this work, electrical properties of porous silicon structures, formed with electrochemical anodization in HF acid solution under two different current densities, were investigated. In these experiments, Sb doped (111)-oriented n-type silicon samples with 0.006-0.015 \Omega cm resistivity was used. Samples were anodized in a solution of 38% HF and 99% C_2H_2OH at 1:1 ratio for 15 minutes. After anodization, the structures that formed at low current density (J = 5 mA/cm^2) was compared with structures that formed at high current density (J = 30 mA/cm^2). Both structures and electrical properties were investigated.

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Light Emission From Travelling Space Charge Domains, M. Hostut, N. Balkan Jan 1999

Light Emission From Travelling Space Charge Domains, M. Hostut, N. Balkan

Turkish Journal of Physics

We report light emission from n-doped GaAs epilayers associated with impact ionization when the device is biased in the negative difference resistance (NDR) regime (F > 3.5 kV cm^{-1}). Spectral distribution of the emitted light (electroluminescence) has been measured to identify the energy distribution of the recombining electron-hole pairs. Electron temperatures, calculated from the high energy tail of the electroluminescence spectra, show that the light emission is due to the recombination of the impact ionized holes with the background, channel, and the travelling space charge electrons. The range of the electron temperatures obtained indicate that the contribution to the light emission …

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Electronic Energy Spectrum In A Dqws Within A Tilted Magnetic Field, S. Elagöz, H. Sari, Y. Ergün, P. Karasu, İ. Sökmen Jan 1999

Electronic Energy Spectrum In A Dqws Within A Tilted Magnetic Field, S. Elagöz, H. Sari, Y. Ergün, P. Karasu, İ. Sökmen

Turkish Journal of Physics

The analytical solution of the Schrödinger equation for a DQWS subjected to an externally applied tilted magnetic field are obtained and the results are discussed. The dependency of energy spectrum of the system on the applied magnetic field direction is also given.

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The Concept Of Free Electromagnetic Field In Quantum Domain, Alexander Shumovsky, Özgür Müstecaplioğlu Jan 1999

The Concept Of Free Electromagnetic Field In Quantum Domain, Alexander Shumovsky, Özgür Müstecaplioğlu

Turkish Journal of Physics

By virtue of the consideration of polarization and phase properties of dipole radiation in the quantum domain, it is shown that the concept of free electromagnetic field should be considered as a quite risky approximation in the description of quantum fluctuations of some physical observables.

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Quantum Mechanics Of The Electric Charge, Andrzej Staruszkiewicz Jan 1999

Quantum Mechanics Of The Electric Charge, Andrzej Staruszkiewicz

Turkish Journal of Physics

The Author summarizes the evidence that his quantum theory of the electric charge depends in a nontrivial way on the numerical value of the fine structure constant.

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Elementary Charges In Classical Electrodynamics, Edward Kapuscik, Maciej Widomski Jan 1999

Elementary Charges In Classical Electrodynamics, Edward Kapuscik, Maciej Widomski

Turkish Journal of Physics

In the framework of classical electrodynamics elementary particles are treated as capacitors. The electrostatic potentials satisfy equations of the Schrödinger type. An interesting "quantization condition" for elementary charges is derived.

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Two-Level Atom In A Squeezed Vacuum, Ryszard Tanas Jan 1999

Two-Level Atom In A Squeezed Vacuum, Ryszard Tanas

Turkish Journal of Physics

The master equation for a two-level atom interacting with a strong coherent field and damped into a reservoir formed by a finite bandwidth squeezed vacuum is derived. The master equation extends the Yeoman and Barnett approach to a non-zero detuning of the driving field from the atomic resonance and allows to discuss the role of squeezing bandwidth and the detuning in the level shifts, widths and intensities of spectral lines. The approach is valid for arbitrary values of the Rabi frequency and detuning but for the squeezing bandwidths larger than the natural linewidth in order to satisfy the Markov approximation.

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Medium-Dependent Metric In Electrostatics, Bernard Jancewicz Jan 1999

Medium-Dependent Metric In Electrostatics, Bernard Jancewicz

Turkish Journal of Physics

The use of differential forms allows the formulation of the principal equations of electrodynamics in a metric-independent way. The metric is needed only for finding the solutions. Various metrics can be introduced, depending on the medium. A special metric, connected with the electric permittivity tensor, allows us to reduce all electrostatic problems in anisotropic media to those in in isotropic one.

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Algebraic Implementation Of Gauss' Law, Andrzej Herdegen Jan 1999

Algebraic Implementation Of Gauss' Law, Andrzej Herdegen

Turkish Journal of Physics

A C^*-algebraic model may be constructed for the description of asymptotic fields in quantum electrodynamics. In this model charged fields carry the asymptotic Coulomb field of associated charged particles.

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