bluuebunny/crossref_metadata_2025 huggingface dataset:
The Change in Bioavailability of Organic Matter Associatedwith
Clay-Based Buffer Materials as a Result of Heat Andradiation
Treatment
Authors: Stroes-Gascoyne S. , Haveman S. A. , Vilks P.
Published: September 2012

Compacted clay-based buffer surrounds corrosion-resistant waste containers
in the Canadian concept for nuclear fuel waste disposal. Clays naturally
contain small quantities of organic matter that may be resistant to
bacterial degradation. The containers with highly radioactive material would
subject the surrounding buffer to both heat (max. 95°C) and radiation (max.
52 Gy/h). Both could potentially break down complex organic material to
smaller, more bioavailable compounds. This could stimulate microbial growth
and possibly affect gas production, microbially-influenced corrosion or
radionuclide migration. Experiments were carried out in which buffer (50
wt.% Na-bentonite, 50 wt.% sand) was heated at 60 and 90°C for periods of 2,
4 and 6 weeks, in some cases followed by irradiation to 25 kGy. Unheated
buffer was also irradiated to 25 and 50 kGy at different moisture contents.
The treated materials were subsequently suspended in distilled water, shaken
for 24 h and centrifuged to remove the solids. The 0.22 μm filter-sterilized
leachates were inoculated with equal volumes of fresh groundwater and
incubated at room temperature for 10 d to determine the increase in total
and viable bacteria compared to a groundwater control. Results indicated
that leachates from buffer subjected to heat, radiation or combinations of
these, had a stimulating effect on both total and viable cell counts in
groundwater, compared to unamended groundwater controls. This stimulating
effect was generally most pronounced for viable counts and could be larger
than two orders of magnitude. Leachates from untreated buffer material also
stimulated the growth of groundwater bacteria, but to a lesser extent than
leachates from heat-and radiation-treated buffer material. The effects of
heat and radiation on nutrient availability in clay-based sealing materials
(at relevant clay/moisture ratios) should, therefore, be taken into account
when attempting to quantify the effects of microbial activity on vault
performance.
Imaging of Silicon Naphthalocyanine in Toluene
Authors: Becker R. J. , Goedert R. V. , Clements A. F. , Whittaker T. A.
Published: September 2012

Pulsed-laser shadowgraphs and images of scattered light are combined with limiting and scattering data to build a model of the response of silicon napthalocyanine (SiNc) at concentrations on the order of 10−3 M in toluene to 12–16 ns 532 nm laser pulses over an energy range from 10 nJ to 2 mJ. The inherent spherical aberration induced by the sample has a profound effect on the response. The scattering is extremely intense above pulse energies of 100 μJ. The data indicates the response at pulse energies below 300 mJ is due to reverse saturable absorption. Full absorbance is not observed until the agglomerates vaporize. At fluences approaching 1 J/cm2 a plasma begins to form and the liquid near the window starts to boil. Intense scatter from the boiling liquid combined with plasma absorption produces hard clamping.
Biologically Inspired Design: Color on Wings
Authors: Srinivasarao Mohan , Padilla Luis
Published: September 2012

Brilliant, iridescent colors found on the bodies and wings of many birds, butterflies and moths are produced by structural variations and have been the subject of study for centuries. Such brilliant colors have been described as metallic colors due to thesaturation or purity ofthe color produced and have attracted the attention of great scientists like Newton, Michelson and Lord Rayleigh. It was recognized early on that such colors arise from physical effects such as interference or diffraction as opposed to colors that are normally produced due to the presence of chromophores which absorb or emit light. Common examples of physical colors are some butterfly wings [1], color of Indigo snake skin [2], hummingbird feathers [3,4], arthropod cuticles [which are due to selective reflection of color from the solidified cholesteric phase of chitin crystallites] [5], gemstones like opal [6,7], and some crystals like potassium chlorate [8]. While the origins of such colors are well understood the properties of color and color specification have not received much attention.
Chitosan Gel Systems as Novel Host Materials for Optical Limiters
Authors: Jiang Hao , Su Weijie , Brant Mark , Tomlin David , Bunning Timothy J.
Published: September 2012

Two chitosan gel systems, chitosan/acetic anhydride and chitosan/glutaraldehyde, were studied as host materials for optical limiters. Both gels are transparent and have a very high laser damage threshold. The chitosan/acetic anhydride gel has a damage threshold > 540 J/cm2 while the chitosan/glutaraldehyde gel, which is slightly yellow in color, has a damage threshold > 600 J/cm2 (measurements made with 6.8 ns laser pulses at 532 nm). Different chromophore dopants, including porphyrin and CuPc, were tested. The optical limiting behavior of the guest/host gel systems was similar to their corresponding solution systems. The morphological structure of the gel systems was studied and the gelation process is discussed. Our current research explores the effect of gel morphology on the optical limiting properties of the chromophores and studies the relation between chromophores, cross-linking agents and the host materials. We have also investigated the relationship between optical properties and chemical structure of the gel/chromophore systems in order to optimize the optical behavior.
Optical Switching in Thin Film NdNiO3
Authors: DeNatale J. F. , Kobrin P. H.
Published: September 2012

Crystalline thin films of NdNiO3 are known to exhibit characteristic metal-insulator electrical transitions. Infrared transmission measurements of these films have demonstrated the corresponding reversible optical switching response, transforming between IR-transparent and IR-opaque states. Their low-temperature transmission spectra exhibit a sharp increase in transmittance at an onset wavelength of approximately 3 mm (˜0.4eV), with an appearance suggesting formation of a distinct band gap structure. The transmission and reflection characteristics of these films differ significantly from those reported for bulk polycrystalline material.
Spatial and Temporal Characteristics of the Semiconductor to Metallic Phase Transition in VO2 Thin Films
Authors: Corbett Mark , Lewis Keith L , Pitt Andrew M
Published: September 2012

Vanadium dioxide exhibits a semiconductor to metallic phase transition at a temperature of about 68 °C. This can be exploited in the form of optical thin film structures which will exhibit non-linear behaviour when exposed to pulsed infra-red radiation. Since the phase transition is structural in nature, it is of interest to explore the temporal and spatial properties when irradiated with pulsed laser sources. Fast CMT detectors have been used to resolve nanosecond temporal detail on an experimental basis and the spatial charateristics have been explored using a simple adiabatic heating model. The dynamic transmission values measured for VO2 devices are complex combinations of the temporally and spatially varying characteristics of the film.
Semiconductor Infrared Nonlinear Optical Materials
Authors: Guha Shekhar , Jessen Gregg , Hood Patrick J.
Published: September 2012

Large values of the infrared nonlinearity make semiconductors important in many optical applications. Along with the two-photon absorption coefficient and free-carrier absorption cross-section, the values of the decay rates of the generated charge carriers control the nonlinear optical performance of semiconductors. In this paper, calculations of the limits of the charge carrier decay rates for five infrared materials are provided and some experimental results, performed in the long-wave and the mid-wave infrared wavelengths on three of these materials are presented. To the authors' best knowledge, this is the first report of measurement of optical nonlinearity of Hg0.65Cd0.35Te in the 4 -5 jtm wavelength range. Comparisons of room temperature and low temperature nonlinear optical behavior of all the materials studied are presented.
Nonlinear Optical Properties of Modified Carbocyanines
Authors: Tang Nansheng , Su Weijie , Cooper Thomas M. , McLean Daniel G. , Brandelik Donna M. , Sutherland Richard L.
Published: September 2012

We studied the optical nonlinearities of modified 3,3′-diethylthiadicarbocyanine iodides (X-DTDCI) in dimethyl sulfoxide (DMSO) solutions (X = H, Cl, Br at meso position) by a pump-probe technique with crossed polarized chirped laser pulses at room temperature. Reverse-saturable-absorption (RSA) with monoexponential lifetimes is observed at 532 nm in all the samples studied. We determined the effective excited-state absorption cross-sections and their lifetimes for all the samples investigated and found that they clearly correlate to the substituent X at the meso position. Empowered by the chirped pulses, a much faster nonlinearity with a decay time of ˜3.1 ps that is much shorter than the laser pulses used, is unveiled in Cl-DTDCI. We expressed both the absorptive and the refractive part of this fast nonlinearity as the equivalent molecular hyperpolarizability γ1212 and found γ1212 = (8.1 + i9.8) × 10−32erg−1.
Broad Band Optical Power Limiting in Vanadium Doped Cd0.55Mn0 45Te Crystals
Authors: Trivedi S. B. , Chen R. J. , Grasza K. , Kutcher S. W. , Nathan G. V. J. , Steier W. H. , Bacher David , Guha S. , Hood Patrick
Published: September 2012

A variety of approaches have been used for optical limiting in the past [1–6].A relatively new approach, an electro-optic power limiter (EOPL) has a simple and compact design, wavelength agile sensitivity, the ability to respond to coherent as well as incoherent radiation, low threshold and a large field of view (up to 15°). EOPL devices allow the transmission of low intensity background radiation while simultaneously blocking the damaging high intensity beam. We have demonstrated EOPL in the past using the II-VI semiconductors CdTe:In [7,8], CdTe:V [9] and ZnTe [10]. In this paper, we are reporting on EOPL using a new ternary II-VI semiconductor Cdl-xMnxTe:V. Cd0.55Mn0.45Te is capable of limiting optical radiation from visible (0.630.tm) to near infrared (1.5pm) wavelengths, which covers the wavelength ranges of both CdTe and ZnTe combined. Additionally, crystals of Cd0.55Mn0.45Te are relatively easier to grow than ZnTe. We report our preliminary results of optical limiting in Cd0.55Mn0.45Te:V at 0.780μm, 0.895μm, and 1.06μm, and we discuss the materials issues involved in improving and optimizing Cd1-xMnxTe for electro-optic power limiting.
Hydrated Transition Metal Oxide Films for Reversible Switching in Near IR Region
Authors: Shiyanovskaya I. , Hepel M.
Published: September 2012

Electro-optical properties of thin films of hydrated transition metal oxides, WO3 2(H2O) and WO3 1/3(H2O), have been investigated in situ using photoelectrochemical methods. These films can reversibly change their light absorption in the near IR region due to the double charge injection upon the applied electric field in electrolyte media. Taking into account the high stability and fast switching time, these hydrated tungsten oxide films can be utilized for efficient light modulation in the near infrared region of the spectrum.
Photorefractive Beam Fanning Optical Limiter
Authors: Miller M. J. , Wood G. L. , Salamo G. J.
Published: September 2012

We have investigated the potential for using photorefractive phenomena in optical limiting devices. The major obstacle preventing the photorefractive limiter from reaching this potential has been speed of response. In this paper we report on several techniques which are successful in enhancing the photorefractive beam fanning response time. In particular, we demonstrate and characterize the use of applied electric fields, gratings to seed the beam fan, and cylindrical focusing. We also report the first demonstration of a prototype beam fanning optical limiter encompassing many of the proposed improvements.
Optical Limiting in a Photorefractive Polymer
Authors: Grunnet-Jepsen A. , Thompson C. L. , Moerner W. E.
Published: September 2012

Optical limiting has been suggested as a possible device application of high gain photorefractive (PR) polymers. The principle of operation relies on the existence of amplified scattering (beam fanning) to deplete the intensity of the incident coherent optical beam. The newest photorefractive polymers show a combination of high gain and reasonable speed, and we show how these materials behave as a beam fanning optical limiter and as a limiter based on self-pumped phase conjugation. A prototype cascaded beam fanning device is demonstrated and its salient features are discussed.
Effects of Varying Surfactants on the Electrooptical Switching Characteristics of Volume holograms recorded in PDLC'S
Authors: Tondiglia V. P. , Natarajan L. V. , Neal R. M. , Sutherland R. L. , Bunning T. J.
Published: September 2012

The effects of adding various chemical surfactants to the prepolymer syrup on the electro-optical switching properties of Bragg gratings recorded in polymer dispersed liquid crystals (PDLC's) have been studied. The gratings were holographically recorded in prepolymer recipes substituting hexanoic, heptanoic, propylpentanoic and octanoic acids as surfactants in the recipe. A small percentage of monomer containing an attached long chain alkyl group (vinyl neononanoate) was also used instead of a surfactant. The addition of surfactants resulted in lowering the required switching field from 17 V/μm with no surfactant to 1.5 to 8 V/μm at a concentration of about 6.7% by weight, depending on the surfactant. Field on response times for electrical switching decreased with the addition of surfactant, and off times increased.
Saturation Effects in Optical Limiters Utilizing Thick Nonlinear Media
Authors: Chapple P. B. , Hermann J. A.
Published: September 2012

Optical Limiters which operate by exploiting nonlinear absorption and nonlinear refraction are most effective towards an incoming laser beam when the active material is somewhat thicker than one Rayleigh length. A variety of thick optical limiter configurations are described and modeled by appropriately integrating the formulas for thin nonlinear media with gaussian-shaped beams. The attenuating effects of linear losses are allowed for, and the approach is exact to the first order in optical power or irradiance. Irradiance-dependent and fluence-dependent absorbers are considered. For a gaussian-shaped input beam, we find that the combined effects of diffraction and nonlinear absorption produce, in the thick-medium limit, a transmitted beam which retains a gaussian-shaped profile at the far field, but which is now uniformly attenuated according to the magnitudes of the nonlinear and linear absorption parameters. This implies that the normalized transmittance is independent of the size of the receiving aperture. As the optical beam power increases the limiting behaviour saturates. We have discovered a simple saturation formula which agrees well with numerically calculated transmittances for thick irradiance-dependent absorbers within the strongly nonlinear regime. Under certain conditions this formula also describes the behaviour of refractive limiters, providing a simple and accurate technique for modeling limiters over a broad parameter range.
Broadband Optical Limiting and CCD Sensor Protection From Nanosecond-Pulsed Laser Threat With Reverse Saturable Absorbers
Authors: Dentan V. , Feneyrou P. , Soyer F. , Vergnolle M. , Barny P. Le , Robin Ph.
Published: September 2012

We study in detail the linear and nonlinear optical properties of a heavy metal substituted naphthalocyanine reverse saturable absorber (RSA). The excited states involved in the RSA process under nanosecond pulsed laser excitation in the visible are fully characterized. We then demonstrate that this material can be used as an efficient optical limiter for protecting silicium CCD arrays from destruction due to nanosecond-pulsed laser irradiation in the visible.
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

The 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

Porphyrins 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

We 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

Nickel 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

We 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

Photoexcitation 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

This 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

GaN-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

Visible-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

Ti/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

Two-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

Defects 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

We 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

The 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

We 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

An 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

High 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

The 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

Lateral 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

Changes 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

Phonon 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

Electronic 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

Single 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

An 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

GaN-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

Photoluminescence (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.
Spectroscopic Ellipsometry Analysis of InGaN/GaN and AlGaN/GaN Heterostructures Using a Parametric Dielectric Function Model
Authors: Wagner J. , Ramakrishnan A. , Obloh H. , Kunzer M. , Köhler K. , Johs B.
Published: September 2012

Spectroscopic ellipsometry (SE) has been used for the characterization of AlGaN/GaN and InGaN/GaN heterostructures. The resulting pseudodielectric function spectra were analyzed using a multilayer approach, describing the dielectric functions of the individual layers by a parametric oscillator model. From this analysis, the dielectric function spectra of GaN, AlxGa1−xN (x≤0.16), and In0.13Ga0.87N were deduced. Further, the dependence of the AlxGa1−xN band gap energy on the Al mole fraction was derived and compared with photoluminescence data recorded on the same material. The SE band gap data are compatible with a bowing parameter close to 1 eV for the composition dependence of the AlxGa1−xN gap energy. Finally, the parametric dielectric functions have been used to model the pseudodielectric function spectrum of a complete GaN/AlGaN/InGaN LED structure.
Dynamics of Anomalous Temperature-Induced Emission Shift in MOCVD-grown (Al, In)GaN Thin Films
Authors: Cho Yong-Hoon , Gainer G. H. , Lam J. B. , Song J. J. , Yang W. , Jhe W.
Published: September 2012

We present a comprehensive study of the optical characteristics of (Al, In)GaN epilayers measured by photoluminescence (PL), integrated PL intensity, and time-resolved PL spectroscopy. For not only InGaN, but also AlGaN epilayers with large Al content, we observed an anomalous PL temperature dependence: (i) an “S-shaped” PL peak energy shift (decrease-increase-decrease) and (ii) an “inverted S-shaped” full width at half maximum (FWHM) change (increase-decrease-increase) with increasing temperature. Based on time-resolved PL, the S shape (inverted S shape) of the PL peak position (FWHM) as a function of temperature, and the much smaller PL intensity decrease in the temperature range showing the anomalous emission behavior, we conclude that strong localization of carriers occurs in InGaN and even in AlGaN with rather high Al content. We observed that the following increase with increasing Al content in AlGaN epilayers: (i) a Stokes shift between the PL peak energy and the absorption edge, (ii) a redshift of the emission with decay time, (iii) the deviations of the PL peak energy, FWHM, and PL intensity from their typical temperature dependence, and (iv) the corresponding temperature range of the anomalous emission behavior. This indicates that the band-gap fluctuation responsible for these characteristics is due to energy tail states caused by non-random inhomogeneous alloy potential variations enhanced with increasing Al content.
Time-Resolved Spectroscopy of InGaN
Authors: Pophristic Milan , Long Frederick H. , Tran Chuong , Ferguson Ian T.
Published: September 2012

We have used time-resolved photoluminescence (PL), with 400 nm (3.1 eV) excitation, to examine InxGa1−xN/GaN light-emitting diodes (LEDs) before the final stages of processing at room temperature. We have found dramatic differences in the time-resolved kinetics between dim, bright and super bright LED devices. The lifetime of the emission for dim LEDs is quite short, 110 ± 20 ps at photoluminescence (PL) maximum, and the kinetics are not dependent upon wavelength. This lifetime is short compared to bright and super bright LEDs, which we have examined under similar conditions. The kinetics of bright and super bright LEDs are clearly wavelength dependent, highly non-exponential, and are on the nanosecond time scale (lifetimes are in order of 1 ns for bright and 10 ns for super bright LED at the PL max). The non-exponential PL kinetics can be described by a stretched exponential function, indicating significant disorder in the material. Typical values for β, the stretching coefficient, are 0.45 – 0.6 for bright LEDs, at the PL maxima at room temperature. We attribute this disorder to indium alloy fluctuations.

From analysis of the stretched exponential kinetics we estimate the potential fluctuations to be approximately 75 meV in the super bright LED. Assuming a hopping mechanism, the average distance between indium quantum dots in the super bright LED is estimated to be 20 Å.
High Density Plasma Damage Induced in n-GaN Schottky Diodes Using Cl2/Ar Discharges
Authors: Zhang A.P. , Dang G. , Ren F. , Cao X.A. , Cho H. , Lambers E.S. , Pearton S.J. , Shul R.J. , Zhang L. , Baca A.G. , Hickman R. , Hove J.M. Van
Published: September 2012

The effects of dc chuck self-bias and high density source power (which predominantly control ion energy and ion flux, respectively) on the electrical properties of n-GaN Schottky diodes exposed to Inductively Coupled Plasma of Cl2/Ar were examined. Both parameters were found to influence the diode performance, by reducing the reverse breakdown voltage and Schottky barrier height. All plasma conditions were found to produce a nitrogen-deficient surface, with a typical depth of the non-stoichiometry being ∼500 Å. Post-etch annealing was found to partially restore the diode characteristics.
Wet Etching of Ion-implanted GaN Crystals by AZ-400K Photoresist
Authors: Carosella C.A. , Molnar B. , Schiestel S. , Sprague J.A.
Published: September 2012

The photoresist developer AZ-400K, commonly used to remove AlN encapsulant layers on GaN crystalline films, is found to also etch certain as-grown GaN films. Even as-grown GaN films, which can not be etched in AZ-400K, however can be etched if amorphized by ion implantation. Etch rates of as high as 450 Å/min. were observed. The etching proceeds linearly in GaN in the first few minutes to a depth corresponding to the depth of the amorphous region. Subsequently, the etching rate saturates. Annealing of the highly amorphized samples up to 1000°C for one minute in a N2/H2 gas mixture does not reduce the etch rate, but for lower doses we observed a reduction of the etch rate. Observations of etching depth under various ion-implanted conditions could be correlated with the number of displacements per atoms (dpa) required for amorphization.
Oxidation of Gallium Nitride Epilayers in Dry Oxygen
Authors: Chen P. , Zhang R. , Xu X.F. , Chen Z.Z. , Zhou Y.G. , Xie S.Y. , Shi Y. , Shen B. , Gu S.L. , Huang Z.C. , Hu J. , Zheng Y.D.
Published: September 2012

The oxidation of GaN epilayers in dry oxygen has been studied. The 1-μm-thick GaN epilayers grown on (0001) sapphire substrates by Rapid-Thermal-Processing/Low Pressure Metalorganic Chemical Vapor Deposition were used in this work. The oxidation of GaN in dry oxygen was performed at various temperatures for different time. The oxide was identified as the monoclinic β-Ga2O3 by a θ-2θ scan X-ray diffraction (XRD). The scanning electron microscope observation shows a rough oxide surface and an expansion of the volume. XRD data also showed that the oxidation of GaN began to occur at 800°C. The GaN diffraction peaks disappeared at 1050°C for 4 h or at 1100°C for 1 h, which indicates that the GaN epilayers has been completely oxidized. From these results, it was found that the oxidation of GaN in dry oxygen was not layer-by-layer and limited by the interfacial reaction and diffusion mechanism at different temperatures.