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Articles 301 - 330 of 364

Full-Text Articles in Electrical and Computer Engineering

Increased Carrier Lifetimes In Gan Epitaxial Films Grown Using Sin And Tin Porous Network Layers, Ü. Özgür, Y. Fu, Y. T. Moon, F. Yun, H. Morkoç, H. O. Everitt Jan 2005

Increased Carrier Lifetimes In Gan Epitaxial Films Grown Using Sin And Tin Porous Network Layers, Ü. Özgür, Y. Fu, Y. T. Moon, F. Yun, H. Morkoç, H. O. Everitt

Electrical and Computer Engineering Publications

Improved structural quality and radiative efficiency were observed in GaN thin films grown by metalorganic chemical vapor deposition on SiN and TiN porous network templates. The room-temperature decay times obtained from biexponential fits to time-resolved photoluminescence data are increased with the inclusion of SiN and TiN layers. The carrier lifetime of 1.86 ns measured for a TiN network sample is slightly longer than that for a 200μm-thick high-quality freestanding GaN. The linewidth of the asymmetric x-ray diffraction (XRD) (101¯2) peak decreases considerably with the use of SiN and TiN layers, indicating the reduction in threading dislocation density. However, no direct …

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A Study On The Nature Of Anomalous Current Conduction In Gallium Nitride, Joshua K. Spradlin Jan 2005

A Study On The Nature Of Anomalous Current Conduction In Gallium Nitride, Joshua K. Spradlin

Theses and Dissertations

Current leakage in GaN thin films limits reliable device fabrication. A variety of Ga and N rich MBE GaN thin films grown by Rf, NH3, and Rf+ NH3, are examined with electrical measurements on NiIAu Schottky diodes and CAFM. Current-voltage (IV) mechanisms will identify conduction mechanisms on diodes, and CAFM measurements will investigate the microstructure of conduction in GaN thin films. With CAFM, enhanced conduction has been shown to decorate some extended defects and surface features, while CAFM spectroscopy on a MODFET structure indicates a correlation between extended defects and field conduction behavior at room temperature. A remedy for poor …

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Investigation Of Surface States And Device Surface Charging In Nitride Materials Using Scanning Kelvin Probe Microscopy, Mohammed Shahriar Sabuktagin Jan 2005

Investigation Of Surface States And Device Surface Charging In Nitride Materials Using Scanning Kelvin Probe Microscopy, Mohammed Shahriar Sabuktagin

Theses and Dissertations

In this work Scanning Kelvin Probe Microscopy (SKPM) was used to characterize surface states and device surface charging in nitride materials. Samples grown by Molecular Beam Epitaxy (MBE), Metal Organic Chemical Vapor Deposition (MOCVD) and Hydride Vapor Phase Epitaxy (HVPE) typically show a high surface band bending of about 1 eV. In an n-type sample with 3X1017 cm-3 carrier concentration, 1 eV upward band bending corresponds to 1.7X1012 cm-2 trapped charge density in the surface states. Under continuous ultraviolet (UV) illumination up to 0.6 eV surface photo voltage effect could be observed in some samples, which further indicates that surface …

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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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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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High Quality Zno Epitaxial Grown By Plasma Assisted Molecular Beam Epitaxy, Yun Zhang Jan 2004

High Quality Zno Epitaxial Grown By Plasma Assisted Molecular Beam Epitaxy, Yun Zhang

Theses and Dissertations

Described in this thesis are the growth and characterization of high quality ZnO epitaxy layers.Zinc oxide (ZnO) epitaxy layers were grown on sapphire and epi-GaN substrates respectively, using plasma assisted molecular beam epitaxy (MBE) . Various growth conditions, such as growth temperature, II/VI ratio, and buffer layers, were employed to optimize the quality of the ZnO film. The subsequent characterization of the films was carried out to evaluate the surface, optical and crystalline properties of the film, using AFM, SEM, PL and XRD techniques. It was found out that the high quality of the ZnO film was grown on epi-GaN …

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Gan Epitaxy On Melt Grown Thermally Prepared Bulk Zno Substrates, Xing Gu Jan 2004

Gan Epitaxy On Melt Grown Thermally Prepared Bulk Zno Substrates, Xing Gu

Theses and Dissertations

Different methods were developed for the preparation of bulk ZnO substrates. Remarkable improvement on the surface, optical and crystalline quality of the bulk ZnO substrate was achieved. ZnO substrates with an atomically flat surface exhibiting terrace-like features were used as a substrate for GaN grown by MBE. High-resolution x-ray diffraction and low temperature PL results show that similar high quality GaN layers can be achieved on both annealed O-face and Zn-face ZnO substrates. The prospect of the device applications of GaN epitaxy on ZnO, including AlGaN/GaN MODFET structure on ZnO and GAN/ZnO based p-n junction were discussed.

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Iii-Nitride Semiconductors Grown By Plasma Assisted Molecular Beam Epitaxy, Lei He Jan 2004

Iii-Nitride Semiconductors Grown By Plasma Assisted Molecular Beam Epitaxy, Lei He

Theses and Dissertations

III-nitride semiconductors are of great interest owing to their commercial and military applications due to their optoelectronic and mechanical properties. They have been synthesized successfully by many growth methods. Among them, molecular beam epitaxy (MBE) is a promising epitaxial growth method owing to precise control of growth parameters, which significantly affect the film properties, composition, and thickness. However, the understanding of growth mechanism of III-nitride materials grown in this growth regime is far from being complete.In this dissertation, GaN and AIGaN growth mechanism under metal-rich conditions were investigated. The Ga surface desorption behavior during the growth was investigated systematically using …

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Luminescence In Zno, Jin Xu Jan 2004

Luminescence In Zno, Jin Xu

Theses and Dissertations

As a wide band semiconductor with a large binding energy (about 60meV), ZnO is a promising candidate semiconductor material for the next generation of optoelectronic , light emission or high power and high frequency devices. In order to make electrical device from ZnO, it is necessary to investigate optical properties of ZnO.In this thesis, PL setup and the properties of ZnO are introduced briefly and the optical properties of ZnO are investigated in detail. Temperature dependence of PL spectra of ZnO are measured and analyzed. Sharp emission line of PL spectra of rare earth (RE) doped ZnO are also investigated.

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Long-Lasting Photoluminescence In Freestanding Gan Templates, Michael A. Reshchikov, M. Zafar Iqbal, Hadis Morkoç, S. S. Park, K. Y. Lee Jan 2003

Long-Lasting Photoluminescence In Freestanding Gan Templates, Michael A. Reshchikov, M. Zafar Iqbal, Hadis Morkoç, S. S. Park, K. Y. Lee

Electrical and Computer Engineering Publications

We studied time-resolvedphotoluminescence(PL) over a temporal range 10−6–103 s in high-purity freestanding GaN templates. Red, yellow, green, blue, and shallow donor–acceptor emission bands can be resolved in the PLspectrum. Observation of luminescence long after the excitation is switched off is a striking feature of our study. The persistent PL observed for all above bands, except for the green band, is primarily attributed to the donor–acceptor-pair-type recombination. An unusually slow, nonexponential decay of radiative transitions from the conduction band to the shallow acceptor was also observed, pointing to some additional mechanism for the persistent PL. Possible role of the surface states …

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Stimulated Emission And Ultrafast Carrier Relaxation In Algan/Gan Multiple Quantum Wells, Ü. Özgür, Henry O. Everitt, Lei He, Hadis Morkoç Jan 2003

Stimulated Emission And Ultrafast Carrier Relaxation In Algan/Gan Multiple Quantum Wells, Ü. Özgür, Henry O. Everitt, Lei He, Hadis Morkoç

Electrical and Computer Engineering Publications

Stimulated emission (SE) and ultrafast carrier relaxation dynamics were measured in two AlxGa1−xN/GaNmultiple-quantum-well(MQW) structures, grown in a Ga-rich environment with x=0.2 and 0.3, respectively. The threshold density for SE (Ith≃100 μJ/cm2) was found to be independent of x. Room-temperature, time-resolved, differential transmission measurements mapped the carrier relaxation mechanisms for above barrier energy excitation. Photoexcited carriers are observed to relax into the QWs in <1 ps, while carrier recombination times as fast as 30 ps were measured. For excitation above Ith, SE is shown to deplete carriers in the barriers through a cascaded refilling of the QW state undergoing SE. Similar behavior is seen in an Al0.3Ga0.7N/GaNMQW grown with a N-rich atmosphere, but the relaxation phenomena of all AlGaN MQWs are significantly faster than observed in InGaN MQWs of similar structure.

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4h–Sic Photoconductive Switching Devices For Use In High-Power Applications, S. Doǧan, A. Teke, D. Huang, Hadis Morkoç, C. B. Roberts, J. Parish, B. Ganguly, M. Smith, R. E. Meyers, S. E. Saddow Jan 2003

4h–Sic Photoconductive Switching Devices For Use In High-Power Applications, S. Doǧan, A. Teke, D. Huang, Hadis Morkoç, C. B. Roberts, J. Parish, B. Ganguly, M. Smith, R. E. Meyers, S. E. Saddow

Electrical and Computer Engineering Publications

Siliconcarbide is a wide-band-gapsemiconductor suitable for high-power high-voltage devices and it has excellent properties for use in photoconductive semiconductor switches (PCSSs). PCSS were fabricated as planar structures on high-resistivity 4H–SiC and tested at dc bias voltages up to 1000 V. The typical maximum photocurrent of the device at 1000 V was about 49.4 A. The average on-state resistance and the ratio of on-state to off-state currents were about 20 Ω and 3×1011, respectively. Photoconductivity pulse widths for all applied voltages were 8–10 ns. These excellent results are due in part to the removal of the surface damage by high-temperature H2 …

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Improvement Of N-Gan Schottky Diode Rectifying Characteristics Using Koh Etching, J. Spradlin, S. Doğan, M. Mikkelson, D. Huang, L. He, D. Johnstone, Hadis Morkoç, R. J. Molnar Jan 2003

Improvement Of N-Gan Schottky Diode Rectifying Characteristics Using Koh Etching, J. Spradlin, S. Doğan, M. Mikkelson, D. Huang, L. He, D. Johnstone, Hadis Morkoç, R. J. Molnar

Electrical and Computer Engineering Publications

KOH etch was investigated as a means to improve the I–Vcharacteristics of Schottky diodes onn-type GaNgrown by molecular-beam epitaxy on sapphire, or on hydride vapor phase epitaxy templates. Atomic force microscopy images and I–Vcharacteristics are presented. After etching as-grown films in molten KOH, Schottky diodes on c-plane GaN had orders of magnitude reduction in reverse leakage current. The best devices had leakage currents less than 10−12 A (10−8 A/cm2) at −5 V, and ideality factors of 1.04. Measurements on several different sample structures indicate a correlation between surface roughness and saturation current, and an improvement in ideality …

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Electric-Field-Induced Heating And Energy Relaxation In Gan, T. A. Eckhause, Ö. Süzer, Ç. Kurdak, F. Yun, Hadis Morkoç Jan 2003

Electric-Field-Induced Heating And Energy Relaxation In Gan, T. A. Eckhause, Ö. Süzer, Ç. Kurdak, F. Yun, Hadis Morkoç

Electrical and Computer Engineering Publications

Electric-field-induced heating is studied using noise measurements in n-type GaN grown on sapphire substrates. The measured electron temperature is found to be an order of magnitude higher than what is expected based on calculations of electron–phonon coupling via acoustic deformation potential scattering processes in GaN. The discrepancy may be explained by a large thermal boundary resistance between the GaN film and the sapphire substrate.

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Current Mapping Of Gan Films By Conductive Atomic Force Microscopy, A. A. Pomarico, D. Huang, J. Dickinson, A. A. Baski, R. Cingolani, Hadis Morkoç, R. Molnar Jan 2003

Current Mapping Of Gan Films By Conductive Atomic Force Microscopy, A. A. Pomarico, D. Huang, J. Dickinson, A. A. Baski, R. Cingolani, Hadis Morkoç, R. Molnar

Electrical and Computer Engineering Publications

Conductive atomic force microscopy has been used to investigate the local conductivity in hydride vapor-phase epitaxy and molecular-beam epitaxyGaN films, focusing on the effect of off-axis facet planes. We investigated two different types of samples, in which the facet planes were either present on the perimeters of as-grown islands, or on the edges of etch pits created by post-growth chemical etching. The results show that crystallographic planes tilted with respect to the c-plane growth direction show a significantly higher conductivity than surrounding areas. The n-type (or p-type) samples required a negative (or positive) sample bias for current conduction, …

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Observation Of Optical Phonon Instability Induced By Drifting Electrons In Semiconductor Nanostructures, W. Liang, K. T. Tsen, Otto F. Sankey, S. M. Komirenko, K. W. Kim, V. A. Kochelap, Meng-Chyi Wu, Chong-Long Ho, Wen-Jeng Ho, Hadis Morkoç Jan 2003

Observation Of Optical Phonon Instability Induced By Drifting Electrons In Semiconductor Nanostructures, W. Liang, K. T. Tsen, Otto F. Sankey, S. M. Komirenko, K. W. Kim, V. A. Kochelap, Meng-Chyi Wu, Chong-Long Ho, Wen-Jeng Ho, Hadis Morkoç

Electrical and Computer Engineering Publications

We have experimentally proven the Cerenkov generation of optical phonons by drifting electrons in a semiconductor. We observe an instability of the polar optical phonons in nanoscale semiconductors that occurs when electrons are accelerated to very high velocities by intense electric fields. The instability is observed when the electron drift velocity is larger than the phase velocity of optical phonons and rather resembles a “sonic boom” for optical phonons. The effect is demonstrated in p–i–nsemiconductor nanostructures by using subpicosecond Raman spectroscopy.

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