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2004

Electrical and Computer Engineering Publications

Articles 1 - 16 of 16

Full-Text Articles in Engineering

Transient Photovoltage In Gan As Measured By Atomic Force Microscope Tip, M. A. Reshchikov, S. Sabuktagin, D. K. Johnstone, H. Morkoç Jan 2004

Transient Photovoltage In Gan As Measured By Atomic Force Microscope Tip, M. A. Reshchikov, S. Sabuktagin, D. K. Johnstone, H. Morkoç

Electrical and Computer Engineering Publications

We studied restoration of the band bending at the surface of undoped GaN layers after illumination with above-bandgap light. The photovoltage saturated with illumination at about 0.2–0.3 eV at room temperature, although the upward band bending for GaN in the dark is of the order of 1 eV. We attribute the photovoltage effect to charging of the surface states, the density of which is estimated at about 10^12 cm^−2. Restoration of the barrier after a light pulse is simulated by a phenomenological model whereby the acceptorlike surface states are emptied of electrons under illumination and filled back in dark due …

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Determination Of The Carrier Concentration In Ingaasn∕Gaas Single Quantum Wells Using Raman Scattering, Patrick A. Grandt, Aureus E. Griffith, M. O. Manasreh, D. J. Friedman, S. Doğan, D. Johnstone Jan 2004

Determination Of The Carrier Concentration In Ingaasn∕Gaas Single Quantum Wells Using Raman Scattering, Patrick A. Grandt, Aureus E. Griffith, M. O. Manasreh, D. J. Friedman, S. Doğan, D. Johnstone

Electrical and Computer Engineering Publications

Raman scattering from longitudinal optical phonon-plasmon coupled mode was observed in a series of InGaAsN∕GaAs single quantum well samples grown by metalorganic vapor phase epitaxy. The phonon-plasmon mode spectra were fitted with the dielectric constant function based on Drude model that contains contributions from both lattice vibrations and conduction electrons. The carrier concentration is calculated directly from the plasmon frequency, which is obtained from the fitting procedure. An empirical expression for the electron concentration, [n], in InGaAsN∕GaAs samples is determined as [n]≈{2.35×1016(ωm−502)}cm−3, where ωm is the peak of the upper frequency branch, L+, of the phonon-plasmon mode measured in unit …

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Alternate Spintronic Analog Of The Electro-Optic Modulator, S. Bandyopadhyay, M. Cahay Jan 2004

Alternate Spintronic Analog Of The Electro-Optic Modulator, S. Bandyopadhyay, M. Cahay

Electrical and Computer Engineering Publications

There is significant current interest in spintronic devices fashioned after a spin analog of the electro-opticmodulator proposed by Datta and Das [Appl. Phys. Lett.56, 665 (1990)]. In their modulator, the “modulation” of the spin-polarized current is carried out by tuning the Rashba spin-orbit interaction with a gate voltage. Here, we propose an analogous modulator where the modulation is carried out by tuning the Dresselhaus spin-orbit interaction instead, using a split gate. Additionally, the magnetization of the source and drain contacts in our device is transverseto the channel, whereas in the Datta-Dasdevice, it is along the channel. Therefore, in the …

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Thermal Stability Of Electron Traps In Gan Grown By Metalorganic Chemical Vapor Deposition, D. Johnstone, S. Doğan, J. Leach, Y.-T. Moon, Y. Fu, Y. Hu, Hadis Morkoç Jan 2004

Thermal Stability Of Electron Traps In Gan Grown By Metalorganic Chemical Vapor Deposition, D. Johnstone, S. Doğan, J. Leach, Y.-T. Moon, Y. Fu, Y. Hu, Hadis Morkoç

Electrical and Computer Engineering Publications

Deep level transient spectroscopy was used to investigate the thermal stability of electron traps in n-type GaN grown by metalorganic chemical vapor deposition. The concentration of traps at 160 and at 500K increased more than fivefold over the course of several 700Kanneal cycles, while a peak at 320K increased by a factor of only 1.19. The increase in the trap concentration with repeated annealing might be due to a mobile trap or loss of passivant. Hydrogen is very likely present in high concentration in the epilayer, and its passivating effects may be lost with annealing.

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Studies Of Ingan∕Gan Multiquantum-Well Green-Light-Emitting Diodes Grown By Metalorganic Chemical Vapor Deposition, K. S. Ramaiah, Y. K. Su, S. J. Chang, C. H. Chen, F. S. Juang, H. P. Liu, I. G. Chen Jan 2004

Studies Of Ingan∕Gan Multiquantum-Well Green-Light-Emitting Diodes Grown By Metalorganic Chemical Vapor Deposition, K. S. Ramaiah, Y. K. Su, S. J. Chang, C. H. Chen, F. S. Juang, H. P. Liu, I. G. Chen

Electrical and Computer Engineering Publications

InGaN(3nm)∕GaN(5nm) three period multiquantum green-light-emitting diodes(LEDs) grown by the metalorganic chemical vapor deposition technique have been studied using high-resolution transmission electron microscopy (HRTEM), double crystal high resolution x-ray diffraction (HRXRD) and low temperature photoluminescence. HRTEM analysis showed that the defect density gradually decreased in the growth direction with increasing thickness. Self-assembled quantum dot-likestructures in the wells and black lumps between the well and barrier due to In segregation and strain contrast were observed, respectively. The HRXRD spectrum of the green LEDstructure was simulated using the kinematical theory method to obtain the composition and thickness of the well and barrier. The …

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Reexamination Of Some Spintronic Field-Effect Device Concepts, S. Bandyopadhyay, M. Cahay Jan 2004

Reexamination Of Some Spintronic Field-Effect Device Concepts, S. Bandyopadhyay, M. Cahay

Electrical and Computer Engineering Publications

Current interest in spintronics is largely motivated by a belief that spin-based devices (e.g., spin field-effect transistors) will be faster and consume less power than their electronic counterparts. Here we show that this is generally untrue. Unless materials with extremely strong spin-orbit interaction can be developed, the spintronic devices will not measure up to their electronic cousins. We also show that some recently proposed modifications of the original spin field-effect transistor concept of Datta and Das [Appl. Phys. Lett.56, 665 (1990)] actually lead to worse performance than the original construct.

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The Effect Of Hydrogen Etching On 6h-Sic Studied By Temperature-Dependent Current-Voltage And Atomic Force Microscopy, S. Doğan, D. Johnstone, F. Yun, S. Sabuktagin, J. Leach, A. A. Baski, Hadis Morkoç, G. Li, B. Ganguly Jan 2004

The Effect Of Hydrogen Etching On 6h-Sic Studied By Temperature-Dependent Current-Voltage And Atomic Force Microscopy, S. Doğan, D. Johnstone, F. Yun, S. Sabuktagin, J. Leach, A. A. Baski, Hadis Morkoç, G. Li, B. Ganguly

Electrical and Computer Engineering Publications

6H–SiC was etched with hydrogen at temperatures between 1000 and 1450°C. The etchedSi-terminated face for the 6H‐SiC wafer was investigated by atomic force microscopy and temperature-dependent current–voltage (I–V–T)measurements. Mechanical polishing damage was effectively removed by hydrogen etching at temperatures above 1250°C. Atomic force microscopy images revealed that very good surface morphology, atomic layer flatness, and large and large step width were achieved. Schottky diode characteristics were investigated in detail by current–voltage and temperature-dependent current–voltage measurements, and the results showed a transition from defect assisted tunneling to thermionic emission as the annealingtemperature was increased from 1250 to 1450°C.

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Stimulated Emission And Time-Resolved Photoluminescence In Rf-Sputtered Zno Thin Films, Ü. Özgür, A. Teke, C. Liu, S.-J. Cho, Hadis Morkoç, H. O. Everitt Jan 2004

Stimulated Emission And Time-Resolved Photoluminescence In Rf-Sputtered Zno Thin Films, Ü. Özgür, A. Teke, C. Liu, S.-J. Cho, Hadis Morkoç, H. O. Everitt

Electrical and Computer Engineering Publications

Stimulated emission (SE) was measured from ZnOthin filmsgrown on c-plane sapphire by rf sputtering. Free exciton transitions were clearly observed at 10 K in the photoluminescence(PL), transmission, and reflection spectra of the sample annealed at 950 °C. SE resulting from both exciton-exciton scattering and electron hole plasma formation was observed in the annealed samples at moderate excitation energy densities. The SE threshold energy density decreased with increasing annealing temperature up to ∼950 °C. The observation of low threshold exciton-exciton scattering-induced SE showed that excitonic laser action could be obtained in rf-sputtered ZnOthin films. At excitation densities below the SE threshold, …

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Investigation Of Forward And Reverse Current Conduction In Gan Films By Conductive Atomic Force Microscopy, J. Spradlin, S. Doǧan, J. Xie, R. Molnar, A. A. Baski, Hadis Morkoç Jan 2004

Investigation Of Forward And Reverse Current Conduction In Gan Films By Conductive Atomic Force Microscopy, J. Spradlin, S. Doǧan, J. Xie, R. Molnar, A. A. Baski, Hadis Morkoç

Electrical and Computer Engineering Publications

We have used conductive atomic force microscopy (C–AFM) to investigate the forward and reverse bias current conduction of homo- and heteroepitaxial GaN-based films grown by molecular beam epitaxy. In the case of homoepitaxy, C–AFM shows enhanced current conduction at the centers of ∼30% of spiral hillocks, which are associated with screw dislocations. Local current–voltage spectra taken by C–AFM on and off such hillocks indicate Frenkel–Poole and field emission mechanisms, respectively, for low current levels in forward conduction. In the case of heteroepitaxialGaN films grown on sapphire, the correlation between conduction pathways and topography is more complex. We do observe, however, …

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Effects Of Hydrostatic And Uniaxial Stress On The Schottky Barrier Heights Of Ga-Polarity And N-Polarity N-Gan, Y. Liu, M. Z. Kauser, M. I. Nathan, P. P. Ruden, S. Dogan, Hadis Morkoç, S. S. Park, K. Y. Lee Jan 2004

Effects Of Hydrostatic And Uniaxial Stress On The Schottky Barrier Heights Of Ga-Polarity And N-Polarity N-Gan, Y. Liu, M. Z. Kauser, M. I. Nathan, P. P. Ruden, S. Dogan, Hadis Morkoç, S. S. Park, K. Y. Lee

Electrical and Computer Engineering Publications

We report measurements of the Schottky barrier heights of Ni/Au contacts on Ga-polarity and N-polarity n-GaN under hydrostatic pressure and applied in-plane uniaxial stress. Under hydrostatic pressure the two different polarities of GaN yield significantly different rates of Schottky barrier height increase with increasing pressure. Uniaxial stress parallel to the surface affects the Schottky barrier height only minimally. The observed changes in barrier height under stress are attributed to a combination of band structure and piezoelectric effects.

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Surface Band Bending In As-Grown And Plasma-Treated N-Type Gan Films Using Surface Potential Electric Force Microscopy, Sang-Jun Cho, Seydi Doğan, Shahriar Sabuktagin, Michael A. Reshchikov, D. Johnstone, Hadis Morkoç Jan 2004

Surface Band Bending In As-Grown And Plasma-Treated N-Type Gan Films Using Surface Potential Electric Force Microscopy, Sang-Jun Cho, Seydi Doğan, Shahriar Sabuktagin, Michael A. Reshchikov, D. Johnstone, Hadis Morkoç

Electrical and Computer Engineering Publications

The surface band bending, as well as the effect of plasma-induced damage on band bending, on GaN surfaces, was investigated. The upward band bending, measured by surface potentialelectric force microscopy (a variant of atomic force microscopy), for the as-grown n -type GaNwas about 1.0 eV which increased to ∼1.4 eV after reactive ion etching (RIE). UV illuminationdecreased the band bending by 0.3 eV with time constants on the order of seconds and hundreds of seconds for the as-grown and RIE treated GaN, respectively. This implies that there is a higher density of the surface states in the samples subjected to …

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Characterization Of Ingan/Gan Multi-Quantum-Well Blue-Light-Emitting Diodes Grown By Metal Organic Chemical Vapor Deposition, K. S. Ramaiah, Y. K. Su, S. J. Cheng, B. Kerr, H. P. Liu, I. G. Chen Jan 2004

Characterization Of Ingan/Gan Multi-Quantum-Well Blue-Light-Emitting Diodes Grown By Metal Organic Chemical Vapor Deposition, K. S. Ramaiah, Y. K. Su, S. J. Cheng, B. Kerr, H. P. Liu, I. G. Chen

Electrical and Computer Engineering Publications

The structural,surface morphology, and the temperature dependence photoluminescence of InGaN(3 nm)/GaN(7 nm) 5 period multi-quantum-well blue-light-emitting diode (LED)structures grown by metal organic chemical vapor deposition(MOCVD) have been studied. Quantum dot-likestructures and strain contrast evident by black lumps were observed in the quantum wells using high-resolution transmission electron microscopy(HRTEM) analysis. Double-crystal high-resolution x-ray diffraction (HRXRD) spectra of blue LED were simulated using kinematical theory method, to obtain composition, and period thickness of well and barrier. The “S” shape character shift as red–blue–redshift of the quantum-well emission line, i.e., blue emission peak 2.667 eV at 10 K, was observed with variation of …

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Decay Of Spin-Polarized Hot Carrier Current In A Quasi-One-Dimensional Spin-Valve Structure, S. Pramanik, Supriyo Bandyopadhyay, M. Cahay Jan 2004

Decay Of Spin-Polarized Hot Carrier Current In A Quasi-One-Dimensional Spin-Valve Structure, S. Pramanik, Supriyo Bandyopadhyay, M. Cahay

Electrical and Computer Engineering Publications

We study the spatial decay of spin-polarized hot carriercurrent in a spin-valve structure consisting of a semiconductorquantum wire flanked by half-metallic ferromagnetic contacts. The current decays because of D’yakonov-Perel’ spin relaxation in the semiconductor caused by Rashba and Dresselhaus spin–orbit interactions in multi-channeled transport. The associated relaxation length is found to decrease with increasing lattice temperature (in the range from 30 to 77 K) and exhibit a nonmonotonic dependence on the electric field driving the current. The relaxation lengths are several tens of microns which are at least an order of magnitude larger than what has been theoretically calculated for …

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Gan Epitaxy On Thermally Treated C-Plane Bulk Zno Substrates With O And Zn Faces, Xing Gu, Michael A. Reshchikov, Ali Teke, D. Johnstone, Hadis Morkoç, Bill Nemeth, Jeff Nause Jan 2004

Gan Epitaxy On Thermally Treated C-Plane Bulk Zno Substrates With O And Zn Faces, Xing Gu, Michael A. Reshchikov, Ali Teke, D. Johnstone, Hadis Morkoç, Bill Nemeth, Jeff Nause

Electrical and Computer Engineering Publications

ZnO is considered as a promising substrate for GaNepitaxy because of stacking match and close lattice match to GaN. Traditionally, however, it suffered from poor surface preparation which hampered epitaxialgrowth in general and GaN in particular. In this work, ZnO substrates with atomically flat and terrace-like features were attained by annealing at high temperature in air. GaNepitaxial layers on such thermally treated basal plane ZnO with Zn and O polarity have been grown by molecular beam epitaxy, and two-dimensional growth mode was achieved as indicated by reflection high-energy electron diffraction. We observed well-resolved ZnO and GaN peaks in the high-resolution …

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Phase-Coherent Quantum Mechanical Spin Transport In A Weakly Disordered Quasi-One-Dimensional Channel, M. Cahay, S. Bandyopadhyay Jan 2004

Phase-Coherent Quantum Mechanical Spin Transport In A Weakly Disordered Quasi-One-Dimensional Channel, M. Cahay, S. Bandyopadhyay

Electrical and Computer Engineering Publications

A transfer matrix technique is used to model phase-coherent spin transport in the weakly disordered quasi-one-dimensional channel of a gate-controlled electron spin interferometer [S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990)]. The model includes the effects of an axial magnetic field in the channel of the interferometer (caused by the ferromagnetic contacts), a Rashba spin-orbit interaction, and elastic (nonmagnetic) impurity scattering. We show that in the presence of an axial magnetic field, nonmagnetic impurities can cause spin relaxation in a manner similar to the Elliott-Yafet mechanism. The amplitudes and phases of the conductance oscillations of the interferometer …

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Excitonic Fine Structure And Recombination Dynamics In Single-Crystalline Zno, A. Teke, Ü. Özgür, S. Doğan, X. Gu, Hadis Morkoç, B. Nemeth, J. Nause, H. O. Everitt Jan 2004

Excitonic Fine Structure And Recombination Dynamics In Single-Crystalline Zno, A. Teke, Ü. Özgür, S. Doğan, X. Gu, Hadis Morkoç, B. Nemeth, J. Nause, H. O. Everitt

Electrical and Computer Engineering Publications

The optical properties of a high quality bulk ZnO, thermally post treated in a forming gas environment are investigated by temperature dependent continuous wave and time-resolved photoluminescence (PL) measurements. Several bound and free exciton transitions along with their first excited states have been observed at low temperatures, with the main neutral-donor-bound exciton peak at 3.3605 eV having a linewidth of 0.7 meV and dominating the PL spectrum at 10 K. This bound exciton transition was visible only below 150 K, whereas the A-free exciton transition at 3.3771 eV persisted up to room temperature. A-free exciton binding energy of 60 meV …

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