bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Design and Synthesis of New Organic Molecules with Large Two-Photon Absorption Cross-Sections for Optical Limiting Applications
Authors: Reinhardt B. A. , Brott L. L. , Clarson S. J. , Kannan R. , Dillard A. G. 
Published: September 2012

AbstractThe molecular structure / nonlinear optical (NLO) property relationship is explored with seven recently synthesized chromophores. Two symmetrical compounds were made using electron withdrawing groups separated by an electron rich core while five asymmetrical molecules were developed using electron donating and withdrawing groups coupled by a it electron bridging group. Pendant chains were added to some of the chromophores to improve processibility. Their syntheses are described and their optical limiting properties discussed.
Structure-Optical Property Relationships of Porphyrins
Authors: Su Weijie , Cooper Thomas M. , Tang Nansheng , Krein Doug , Jiang Hao , Brandelik Donna , Fleitz Paul , Brant Mark C. , Mclean Daniel G. 
Published: September 2012

AbstractPorphyrins are attractive compounds for optical applications. We have been investigating the relationship between molecular structure and optical properties of a number of porphyrin compounds. Structural variations explored include insertion of metal ions, extension of conjugation, halogenation and formation of multimers. The characterization of these chromophores includes measurement of UV/Vis, fluorescence and fluorescence lifetimes. Furthermore, we have investigated their nonlinear absorption, refraction, excitation dynamics as well as oxidation/reduction behavior. Based on our observations, clear recommendations can be made for the design of optical limiting chromophores.
Semi-Empirical Calculations and Measurements of Modified Carbocyanines Optical Properties
Authors: McLean Daniel G. , Day Paul , Wang Zhiqiang , Tang Nansheng , Su Weijie , Brandelik Donna M. , Si Jiaoshi , Pachter Ruth 
Published: September 2012

AbstractWe report semi-empirical calculations of modified 3,3'-diethylthiacyanine iodide (DTC1), 3,3'-diethylthiacarbocyanine iodide (DTC3) and 3,3'-diethylthiadicarbocyanine iodide (DTC5), particularly with halogen substitution at the meso position in the polymethine bridge. Primarily we investigate geometrical changes and infer spectral trends from the molecular orbital levels. The semi-empirical calculations for the unsubstituted DTC3 agree well with an ab initio Hartree-Fock result. In all molecules multiple conformations are examined. Significant changes in dipole moments are noted between the cis and trans forms. Calculated electronic spectra at the CI singles level are compared to the measured spectra with reasonable agreement. Halogenation effects show a rotation of the benzthiazole groups out of planarity. We draw conclusions about consequent effects on solubility and excited state spectral changes.
Prediction of Nonlinear Optical Effects for Phenyl-Substituted Nickel Dithiolenes
Authors: Trohalaki Steven , Zellmer Robert J. , Pachter Ruth 
Published: September 2012

AbstractNickel dithiolene complexes, known for their strong NIR absorptions, high thermal stability, and lightfastness, are being considered for nonlinear-optical applications. In particular, phenylsubstituted nickel dithiolenes may be of interest because it has recently been shown that the hyperpolarizability of a phenyl-substituted 1,3-dithiole can be strongly dependent on the torsional angle of the phenyl group relative to the dithiole ring [1]. We report results from Hartree-Fock ab initio calculations of the second hyperpolarizability as a function of phenyl torsion.
Optical Limiting in Modified meso-alkynyl Porphyrins
Authors: Tang Nansheng , Su Weijie , Krein Douglas M. , McLean Daniel G. , Brant Mark C. , Fleitz Paul A. , Brandelik Donna M. , Sutherland Richard L. , Cooper Thomas M. 
Published: September 2012

AbstractWe characterize the optical properties of modified meso-alkynyl porphyrins by a number of techniques. Our data show that extending the conjugation and incorporating metal in the center of the ring in these molecules serve the objective of tuning the absorption maximum wavelength. We evaluate the optical limiting performance by determining the effective reversesaturable- absorption (RSA) lifetime and the change in the absorption cross-sections. The ultimate optical limiting performance of these molecules is tested in both picosecond and nanosecond regimes.
Limiting Mechanisms in a Photoconducting Liquid Phthalocyanine
Authors: Pong Richard G. S. , Shirk James S. , Flom Steven R. , Snow Arthur W. 
Published: September 2012

AbstractPhotoexcitation of a new pure liquid phthalocyanine gives both a strong nonlinear absorption and photoalectrical charge generation. The novel photoelectrical properties of the material lead to plasma formation, an additional limiting mechanism, at moderate fluences.
Elastic Polymer Guest-Host Optical Limiters
Authors: Rosa M. E. De , Su W. , Brant M. C. , Mclean D. G. 
Published: September 2012

AbstractThis paper presents the results of the bulk laser damage performance of undoped epoxy thermosets with glassy and rubbery mechanical properties and two commercial polymethylmethacrylate (PMMA) samples. We demonstrate how thermomechanical properties such as glass transition temperature (Tg) and equilibrium shear modulus (Ge) affect laser damage threshold. The rubbery epoxy elastomer shows a damage threshold fluence one order of magnitude higher than PMMA and 2.5 orders of magnitude higher than a glassy epoxy thermoset. A solid-state guest-host limiter is made by doping the epoxy elastomer with zinc octabromotetraphenylporphyrin (ZnOBP). The limiting performance of the elastomer limiter is compared to a solution of ZnOBP and C60 in toluene. Photochemical hysteresis effects in the solid
limiter were also investigated.
Room Temperature CW Operation of GaN-Based Blue Laser Diodes by GaInN/GaN Optical Guiding Layers
Authors: Koike Masayoshi , Yamasaki Shiro , Tezen Yuta , Nagai Seiji , Iwayama Sho , Kojima Akira 
Published: September 2012

AbstractGaN-based short wavelength laser diodes are the most promising key device for a digital versatile disk. We have been improving the important points of the laser diodes in terms of optical guiding layers, mirror facets. The continuous wave laser irradiation at room temperature could be achieved successfully by reducing the threshold current to 60 mA (4 kA/cm2). We have tried to apply the multi low temperature buffer layers to the laser diodes for the first time to reduce the crystal defects.
High-Sensitivity Visible-Blind AlGaN Photodiodes and Photodiode Arrays
Authors: Brown J.D. , Matthews J. , Harney S. , Boney J. , Schetzina J.F. , Benson J.D. , Dang K.V. , Nohava Thomas , Yang Wei , Krishnankutty Subash 
Published: September 2012

AbstractVisible-blind UV cameras based on a 32 × 32 array of backside-illuminated GaN/AlGaN p-i-n photodiodes have been successfully demonstrated. The photodiode arrays were hybridized to silicon readout integrated circuits (ROICs) using In bump bonds. Output from the UV cameras were recorded at room temperature at frame rates of 30-240 Hz. These new visible-blind digital cameras are sensitive to radiation from 285-365 nm in the UV spectral region.
Characteristics of Ti/Pt/Au Ohmic Contacts on p-type GaN/AlxGa1-xN Superlattices
Authors: Zhou L. , Khan F. , Ping A.T. , Osinski A. , Adesida I. 
Published: September 2012

AbstractTi/Pt/Au metallization on p-type GaN/AlxGa1-xN (x=0.10 and 0.20) superlattices (SL) were investigated as ohmic contacts. Current-voltage and specific contact resistance measurements indicate enhanced p-type doping in the superlattice structures compared to that in GaN. Ti/Pt/Au is shown to be an effective ohmic metallization scheme on p-type GaN/AlxGa1-xN superlattices. A specific contact resistance of Rc = 4.6×10-4 Ω-cm2 is achieved for unalloyed Ti/Pt/Au on GaN/Al0.2Ga0.8N SL. This is reduced to 1.3×10-4 Ω-cm2 after annealing for 5 minutes at 300 °C.
High Quality Non-Alloyed Pt Ohmic Contacts to P-Type GaN Using Two-Step Surface Treatment
Authors: Jang Ja-Soon , Park Seong-Ju , Seong Tae-Yeon 
Published: September 2012

AbstractTwo-step surface-treatment is introduced to obtain low resistance Pt contacts to ptype GaN. The first step is performed after the mesa etching process using buffered oxide etch (BOE) and ammonium sulfide [(NH4)2Sx]. This is followed by the second step using BOE. The Pt contact, which was treated sequentially using ultrasonically boiled BOE (10 min) and boiled (NH4)2Sx (10 min), produces a specific contact resistance of 3.0 (±3.8)×10-5 Ωcm2. However, the contact, that was simply BOE-treated, yields 3.1 (±1.1)×10-2 Ωcm2. This indicates that the two-step surface treatment is promising technique for obtaining high quality ohmic contacts to p-GaN. Investigation of the electronic transport mechanisms using current-voltage-temperature (I-V-T) data indicates that thermionic field emission is dominant in the surface-treated Pt contacts.
Electrical Measurements in GaN: Point Defects and Dislocations
Authors: Look David C. , Fang Zhaoqiang , Polenta Laura 
Published: September 2012

AbstractDefects can be conveniently categorized into three types: point, line, and areal. In GaN, the important point defects are vacancies and interstitials; the line defects are threading dislocations; and the areal defects are stacking faults. We have used electron irradiation to produce point defects, and temperature-dependent Hall-effect (TDH) and deep level transient spectroscopy (DLTS) measurements to study them. The TDH investigation has identified two point defects, an 0.06-eV donor and a deep acceptor, thought to be the N vacancy and interstitial, respectively. The DLTS study has found two point-defect electron traps, at 0.06 eV and 0.9 eV, respectively; the 0.06-eV trap actually has two components, with different capture kinetics. With respect to line defects, the DLTS spectrum in as-grown GaN includes an 0.45-eV electron trap, which has the characteristics of a dislocation, and the TDH measurements show that threading-edge dislocations are acceptor-like in n-type GaN. Finally, in samples grown by the hydride vapor phase technique, TDH measurements indicate a strongly n-type region at the GaN/Al2O3 interface, which may be associated with stacking faults. All of the defects discussed above can have an influence on the dc and/or ac conductivity of GaN.
Lattice Location of Deuterium in Plasma and Gas Charged Mg Doped GaN
Authors: Wampler W. R. , Barbour J. C. , Seager C. H. , Myers S. M. , Wright A. F. , Han J. 
Published: September 2012

AbstractWe have used ion channeling to examine the lattice configuration of deuterium in Mg doped GaN grown by MOCVD. The deuterium is introduced by exposure to gas phase or ECR plasmas. A density functional approach including lattice relaxation, was used to calculate total energies for various locations and charge states of hydrogen in the wurtzite Mg doped GaN lattice. Results of channeling measurements are compared with channeling simulations for hydrogen at lattice locations predicted by density functional theory.
Surface Conversion Effects in Plasma-Damaged p-GaN
Authors: Cao X.A. , Pearton S.J. , Dang G.T. , Zhang A.P. , Ren F. , Shul R.J. , Zhang L. , Hickman R. , Van Hove J.M. 
Published: September 2012

AbstractThe near-surface (400-500Å) of p-GaN exposed to high density plasmas is found to become more compensated through the introduction of shallow donors. At high ion fluxes or ion energies there can be type-conversion of this surface region. Two different methods for removal of the damaged surface were investigated; wet etching in KOH, which produced self-limiting etch depths or thermal annealing under N2 which largely restored the initial electrical properties.
Zirconium Mediated Hydrogen Outdiffusion from p-GaN
Authors: Kaminska E. , Piotrowska A. , Barcz A. , Jasinski J. , Zielinski M. , Golaszewska K. , Davis R.F. , Goldys E. , Tomsia K. 
Published: September 2012

AbstractWe have shown that Zr-based metallization can effectively remove hydrogen from the p-type GaN subsurface, which eventually leads to the formation of an ohmic contact. As the release of hydrogen starts at ∼900°C, the thermal stability of the contact system is of particular importance. The remarkable thermal behavior of the ZrN/ZrB2 metallization is associated to the microstructure of each individual Zr-based compound, as well as to the interfacial crystalline accommodation.
Electrical Transport of an AlGaN/GaN Two-Dimensional Electron Gas
Authors: Saxler A. , Debray P. , Perrin R. , Elhamri S. , Mitchel W. C. , Elsass C.R. , Smorchkova I.P. , Heying B. , Haus E. , Fini P. , Ibbetson J.P. , Keller S. , Petroff P.M. , DenBaars S.P. , Mishra U.K. , Speck J.S. 
Published: September 2012

AbstractAn AlxGa1-xN/GaN two-dimensional electron gas structure with x = 0.13 deposited by molecular beam epitaxy on a GaN layer grown by organometallic vapor phase epitaxy on a sapphire substrate was characterized. Hall effect measurements gave a sheet electron concentration of 5.1×1012 cm-2 and a mobility of 1.9 × 104 cm2/Vs at 10 K. Mobility spectrum analysis showed single-carrier transport and negligible parallel conduction at low temperatures. The sheet carrier concentrations determined from Shubnikov-de Haas magnetoresistance oscillations were in good agreement with the Hall data. The electron effective mass was determined to be 0.215±0.006 m0 based on the temperature dependence of the amplitude of Shubnikov-de Haas oscillations. The quantum lifetime was about one-fifth of the transport lifetime of 2.3 × 10-12 s.
Correlation between Sheet Carrier Density-Mobility Product and Persistent Photoconductivity in ALGAN/GAN Modulation Doped Heterostructures
Authors: Li J. Z. , Li J. , Lin J. Y. , Jiang H. X. 
Published: September 2012

AbstractHigh quality Al0.25Ga0.75N/GaN modulation-doped heterojunction field-effect transistor (MOD-HFET) structures grown on sapphire substrates with high sheet carrier density and mobility products (nsμ > 1016/Vs at room temperature) have been grown by metal organic chemical vapor deposition (MOCVD). The optimized structures were achieved by varying structural parameters, including the AlGaN spacer layer thickness, the Si-doped AlGaN barrier layer thickness, the Si-doping concentration, and the growth pressure. In these structures, the persistent photoconductivity (PPC) effect associated with the two-dimensional electron gas (2DEG) system was invariantly observed. As a consequence, the characteristic parameters of the 2DEG were sensitive to light and the sensitivity was associated with permanent photoinduced increases in the 2DEG carrier mobility (μ) and sheet carrier density (ns). However, we observed that the magnitude of the PPC and hence the photoinduced instability associated with these heterostructures were a strong function of only one parameter, the product of ns and μ, which is the most important parameter for the HFET device design. For a fixed excitation photon dose, the ratio of the low temperature PPC to the dark conductivity level was observed to decrease from 200% to 3% as the nsμ (300 K) product was increased from 0.048 × 1016/Vs to 1.4 ‘times; 1016/Vs. Based on our studies, we suggest that the magnitude of the low temperature PPC can be used as a sensitive probe for monitoring the electronic quality of the AlGaN/GaN HFET structures.
Temperature Distribution in InGaN-MQW LEDs Under Operation
Authors: Schwegler Veit , Seyboth Matthias , Schad Sven , Scherer Marcus , Kirchner Cristoph , Kamp Markus , Stempfle Ulrich , Limmer Wolfgang , Sauer Rolf 
Published: September 2012

AbstractThe temperature distribution in InGaN-MQW light emitting diodes was examined during operation with spatially resolved micro-Raman and micro-Electroluminescence measurements. The experimental results were compared to finite element simulations. A good agreement between the different experimental and calculated data is found. Maximum operation temperatures up to 140 °C at a moderate forward currents of 30 mA are revealed by all three independent methods. Influences of substrate thickness, different substrates, and even bond-wires are shown.
Vertical Transport Properties of GaN Schottky Diodes Grown by Molecular Beam Epitaxy
Authors: Misra M. , Sampath A.V. , Moustakas T.D. 
Published: September 2012

AbstractLateral and vertical electron transport parameters were investigated in lightly doped n-GaN films, grown by MBE. Diodes were fabricated by forming Schottky barriers on n--GaN films using a mesa-etched vertical geometry. Doping concentrations and barrier heights were determined, from C-V measurements, to be 8-9×1016 cm-3 and 0.95-1.0 eV respectively. Reverse saturation current densities were measured to be in the 1-10times;10-9 A/cm2 range. Using the diffusion theory of Schottky barriers, vertical mobility values were determined to be 950 cm2/V-s. Lateral mobility in films grown under similar conditions was determined by Hall effect measurements to be 150-200 cm2/V-s. The significant increase in mobility for vertical transport is attributed to reduction in electron scattering by charged dislocations.
Dependence of Aging on Inhomogeneities in InGaN/AlGaN/GaN Light-Emitting Diodes
Authors: Kudryashov V.E. , Mamakin S.S. , Turkin A.N. , Yunovich A.E. , Kovalev A.N. , Manyakhin F.I. 
Published: September 2012

AbstractChanges of properties of green LEDs based on InxGa1-xN/AlyGa1-y/GaN heterostructures were studied during 150÷200 hours at currents J = 30÷80 mA. The radiation intensity at low currents (0.1÷1 mA) is quite sensitive to such an aging, it falls down 10÷100 times. Quantum efficiency and spectral parameters at normal currents (J ≈ 10 mA) change non-monotonically during aging, some degradation is observed after 168 hours. The degradation is observed also after a short (< 1 min) period of reverse current. These phenomena are discussed in terms of under threshold defect's formation and their migration in the space charge region of p-n-heterojunction. Potential fluctuations in the space charge region are quite sensitive to this process.
Phonons and Free Carriers in a Strained Hexagonal GaN-AlN Superlattice Measured by Infrared Ellipsometry and Raman Spectroscopy
Authors: Schubert M. , Kasic A. , Tiwald T.E. , Woollam J.A. , Harle V. , Scholz F. 
Published: September 2012

AbstractPhonon and free-carrier effects in a strained hexagonal (α) {GaN}l-{AlN}m superlattice (SL) heterostructure (l = 8 nm, m = 3 nm) are studied by infrared spectroscopic ellipsometry (IRSE) and micro (µ)-Raman scattering. Growth of the heterostructures was performed by metal-organic vapor phase epitaxy (MOVPE) on (0001) sapphire. An unstrained 1 µm-thick α-GaN layer was deposited prior to the SL. SL phonon modes are identified combining results from both IRSE and µ-Raman techniques. The shift of the GaN-sublayer phonon modes is used to estimate an average compressive SL stress of σxx ∼ - 4.3 GPa. The IRSE data reveal a free-carrier concentration of ne ∼ 5×1018 cm−3 within the undoped SL GaN-sublayers. According to the vertical carrier confinement, the free-carrier mobility is anisotropic, and the lateral mobility ( µ⊥ ∼ 400 cm2/Vs, polarization E⊥c-axis) exceeds the vertical mobility (µ∥ ∼ 24 cm2/Vs, E∥c) by one order of magnitude.
Electronic Raman Scattering from Mg-Doped Wurtzite GaN
Authors: Tsen K.T. , Koch C. , Chen Y. , Morkoc H. , Li J. , Lin J.Y. , Jiang H.X. 
Published: September 2012

AbstractElectronic Raman scattering experiments have been carried out on both MBE and MOCVD-grown Mg-doped wurtzite GaN samples. Aside from the expected Raman lines, a broad structure (FWHM ≅ 15cm−1) observed for the first time at around 841cm−1 isattributed to the electronic Raman scattering from neutral Mg impurities in Mg-dopedGaN. From the analysis of the temperature-dependence of this electronic Ramanscattering signal binding energy of the Mg impurities in wurtzite GaN has been found to be Eb ≅ 172 ± 20meV. These experimental results demonstrate that the energy between the ground and first excited states of Mg impurities in wurtzite GaN is about 3/5 of its binding energy.
Photoluminescence characterization of Mg implanted GaN
Authors: Ronning C. , Hofsäss H. , Stötzler A. , Deicher M. , Carlson E.P. , Hartlieb P.J. , Gehrke T. , Rajagopal P. , Davis R.F. 
Published: September 2012

AbstractSingle crystalline (0001) gallium nitride layers, capped with a thin epitaxial aluminum nitride layer, were implanted with magnesium and subsequently annealed in vacuum to 1150-1300 oC for 10-60 minutes. Photoluminescence (PL) measurements showed the typical donor acceptor pair (DAP) transition at 3.25 eV after annealing at high temperatures, which is related to optically active Mg acceptors in GaN. After annealing at 1300 °C a high degree of optical activation of the implanted Mg atoms was reached in the case of low implantation doses. Electrical measurements, performed after removing the AlN-cap and the deposition of Pd/Au contacts, showed no p-type behavior of the GaN samples due to the compensation of the Mg acceptors with native n-type defects.
Prism Coupling as a Non Destructive Tool for Optical Characterization of (Al,Ga) Nitride Compounds
Authors: Dogheche E. , Belgacem B. , Remiens D. , Ruterana P. , Omnes F. 
Published: September 2012

AbstractAn optical characterization technique is proposed for GaN based compounds deposited on sapphire. In AlGaN films grown by MOCVD, the film optical behavior and the substrate to layer interface are qualified from the measured optical data. The experimental and theoretical approach used for this purpose is described in detail. The results clearly show bending effects at the interface which may be related to structural defects; a good agreement with transmission electronic microscopy analysis is obtained.
Fabrication and Characterization of Metal-Ferroelectric-Gan Structures
Authors: Li W.P. , Zhang R. , Yin J. , Liu X.H. , Zhou Y.G. , Shen B. , Chen P. , Chen Z.Z. , Shi Y. , Jiang R.L. , Liu Z.G. , Zheng Y.D. , Huang Z.C. 
Published: September 2012

AbstractGaN-based metal-ferroelectric-semiconductor (MFS) structure has been fabricated by using ferroelectric Pb(Zr0.53Ti0.47)O3 (PZT) instead of conventional oxides as gate insulators. The GaN and PZT films in the MFS structures have been characterized by various methods such as photoluminescence (PL), wide-angle X-ray diffraction (XRD) and high-resolution X-ray diffraction (HRXRD). The Electric properties of GaN MFS structure with different oxide thickness have been characterized by high-frequency C-V measurement. When the PZT films are as thick as 1 µm, the GaN active layers can approach inversion under the bias of 15V, which can not be observed in the traditional GaN MOS structures. When the PZT films are about 100 nm, the MFS structures can approach inversion just under 5V. All the marked improvements of C-V behaviors in GaN MFS structures are mainly attributed to the high dielectric constant and large polarization of the ferroelectric gate oxide.
Radiative Recombination between Two Dimensional Electron Gas and Photoexcited Holes in Modulation-doped AlxGa1−xN/GaN Heterostructures
Authors: Shen B. , Someya T. , Moriwaki O. , Arakawa Y. 
Published: September 2012

AbstractPhotoluminescence (PL) of modulation-doped Al0.22Ga0.78N/GaN heterostructures was investigated. The PL peak related to recombination between the two-dimensional electron gases (2DEG) and photoexcited holes is located at 3.448 eV at 40 K, which is 45 meV below the free excitons (FE) emission in GaN. The peak can be observed at temperatures as high as 80 K. The intensity of the 2DEG PL peak is enhanced significantly by incorporating a thin Al0.12Ga0.88N layer into the GaN layer near the heterointerface to suppress the diffusion of photoexcited holes. The energy separation of the 2DEG peak and the GaN FE emission decreases with increasing temperature. Meanwhile, the 2DEG peak energy increases with increasing excitation intensity. These results are attributed to the screening effect of electrons on the bending of the conduction band at the heterointerface, which becomes stronger when temperature or excitation intensity is increased.