bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase Formation in Zr/Fe Multilayers During Kr Ion Irradiation
Authors: Motta A. T. , Paesano A. , Birtcher R. C. , Ryan E. A. , Bruckmann M. E. , Teixeira S. R. , Amaral L.
Published: April 2011

A detailed study has been conducted of the effect of Kr ion irradiation on phase formation in Zr-Fe metallic multilayers, using the Intermediate Voltage Electron Microscopy (IVEM) at Argonne National Laboratory. Metallic multilayers were prepared with different overall compositions (near 50–50 and Fe-rich), and with different wavelengths (repetition thicknesses). These samples were irradiated with 300 keV Kr ions at various temperatures to investigate the final products, as well as the kinetics of phase formation. For the shorter wavelength samples, the final product was in all cases an amorphous Zr-Fe phase, in combination with Fe, while specially for the larger wavelength samples, in the Fe-rich samples the intermetallic compounds ZrFe2 and Zr3Fe were formed in addition to the amorphous phase. The dose to full reaction decreases with temperature, and with wavelength in a manner consistent with a diffusion-controlled reaction.
Role of Interstitial and Interstitial-Impurity Interaction on Irradiation-Induced Segregation in Austenitic Steels
Authors: Nastar M. , Bellon P. , Martin G. , Ruste J.
Published: April 2011

Segregation under irradiation in austenitic steels is due to a permanent flux of vacancies and interstitials produced by irradiation towards sinks like surfaces and interfaces. A model based on a mean field lattice rate theory is proposed where kinetics and thermodynamics are treated in a mutually consistent way. For a Fe-Ni-Cr ternary alloy, the 15 parameters defining the jump frequencies of vacancies were fitted on equilibrium properties including ordering energies and tracer diffusion experiments with no use of segregation data. Measurements of RIS by Auger Electron Spectroscopy (AES) were used in the last step of the fitting procedure in order to choose the best set of the 27 interstitial jump frequencies. This fitting procedure strongly supports the idea that the interstitials are contributing to RIS in Fe-Cr-Ni alloys. We also simulate the trapping of interstitials by an impurity model and reproduce the total inhibition of RIS by this impurity as observed experimentally [1].
Response of Zircon to Electron and Ne+ Irradiation
Authors: Devanathan R. , Weber W. J. , Boatner L. A.
Published: April 2011

Zircon (ZrSiO4) is an actinide host phase in vitreous ceramic nuclear waste forms and a potential host phase for the disposition of excess weapons plutonium. In the present work, the effects of 800 and 900 keV electron, and 1 MeV Ne+ irradiations on the structure of single crystals of ZrSiO4 have been investigated. The microstructural evolution during the irradiations was studied in situ using a high-voltage electron microscope interfaced to an ion accelerator at Argonne National Laboratory. The results indicate that electron irradiation at 15 K cannot amorphize ZrSiO4 even at fluences an order of magnitude higher than that required for amorphization by 1.5 MeV Kr + ions. However, the material is readily amorphized by I MeV Ne+ irradiation at 15 K. The temperature dependence of this amorphization is discussed in light of previous studies of radiation Zdamage in ZrSiO4.
Non-Equilibrium Formation Of Silicon Nitride During Both Ball Milling And Ion Bombardment
Authors: Li Z. L. , Williams J. S. , Llewellyn D. J. , Wong-Leung J. , Giersig M. , Chivers D. J.
Published: April 2011

Phase evolution during ball milling of Si in NH3 gas and during subsequent annealing has been studied and compared with nitride formation during ion bombardment of Si. X-ray diffraction, differential thermal analysis, Rutherford backscattering and channeling, combustion analysis and transmission electron microscopy have been used as analytical techniques. Results have shown that an amorphous SixNy(Fe) phase forms during milling which transforms into α-Si3N4 and FeSi2 on annealing. During ion bombardment, slightly N-rich Si3N4 is formed but it is mostly crystalline at temperatures between 150 and 450°C.
Densification and Crystallization Kinetics of Cordierite Glass-Ceramic Coatings on Rigid Substrates
Authors: Calata Jesus Noel , Wen Sihua , Lu Guo-Quan
Published: April 2011

We report our findings on the sintering kinetics of slip-cast glass-ceramic coatings on rigid substrates at temperatures between 900°C and 1000°C; the glass-ceramic crystallizes into cordierite at sintering temperatures above 900°C. Changes in coating thickness during sintering were monitored in situ using an optical setup, and the crystallization of the coatings was determined by x-ray diffraction. The coatings went through a rapid shrinkage due to initial densification, followed by an expansion stage caused by the transformation of the glass into a crystalline phase with a lower density. The rate and extent of the expansion were more noticeable at sintering temperatures above 900°C. We also found that wetting between the glass-ceramic and the substrate was important to the sintering kinetics and sintered microstructure of the coatings. For coatings formed on silicon substrate, growth of large pores was observed at the glass-ceramic/silicon interface. Using wafer-curvature measurement technique, we found a significant tensile stress build-up in the coatings during the initial stage of sintering. We believe that the combination of poor wetting and the tensile stress build-up led to the observed desintering phenomenon at the glass-ceramic/silicon interface.
Influence of Mo on the Epitaxial Crystallization of Silicon
Authors: Thoma S. , Lindner J. K. N. , Stritzker B.
Published: April 2011

The influence of Mo atoms on the solid phase epitaxial crystallization of amorphous silicon layers on (100) and (111) Si substrates has been studied by RBS/channeling and cross-sectional TEM. For this purpose, Mo doped amorphous surface layers were produced by low temperature 180 keV Mo+ ion implantation with different doses. Mo is observed to cause enhanced crystallization rates for both (100) and (111) substrates, compared to literature data on pure amorphous silicon. Similar to pure Si, recrystallization in <100> directions is much faster than in <111> directions, where two different velocities are found. For (111) substrates, the formation of thick, uniformly twinned layers is observed. Annealing for several hours at 550° C does not lead to detectable changes of the Mo depth distribution, but for high doses the formation of hexagonal MoSi2 precipitates is observed.
Metastable Phase Formation and Microstructural Evolution During Self-Propagating Reactions in Ai/Ni and Ai/Monel Multilayers
Authors: Van Heerden D. , Gavens A. J. , Jayaraman S. , Weihs T. P.
Published: April 2011

The quenched microstructures of self-propagating reactions in Al/Ni and Al/Monel foils are examined. The quenched reaction fronts in both films are shown to be approximately 30μm long and exhibit a continuous evolution from an as-deposited to a fully reacted microstructure. In both multilayers the first phase to form is isostructural with Al9Co2, and nucleates on the Al side of the layer interface. Growth of the intermetallic occurs in a two step process. First the grains grow laterally along the interface until they impinge on each other, followed by growth normal to the interface into the Al layer. It is shown that planarity of this interface during the latter growth stage is determined by the width of the intermetallic grains immediately prior to their growth normal to the interface into the Al layer. On the basis of these observations it is argued that in modeling these reactions diffusion through the intermetallic during the reaction must be considered, and that atomic diffusion may be considerably more three- dimensional in practice than the current models suggest.
Modeling Self-Propagating Exothermic Reactions in Multilayer Systems
Authors: Jayaraman S. , Mann A. B. , Knio O. M. , Heerden D. Van , Bao G. , Weihs T. P.
Published: April 2011

Self-propagating reactions in free-standing multilayer foils provide a unique opportunity to study very rapid, diffusion-based transformations in non-equilibrium material systems. To fully understand the coupling between mass and thermal diffusion controlling these reactions and to optimize the commercial use of reactive foils, we have undertaken analytical and numerical modeling. Our analytical model predicts an increase in the reaction velocities with decreasing bilayer thickness down to a critical bilayer thickness and a reversal in this trend below the critical thickness. Predicting reaction characteristics such as the flame thermal width, the reaction zone width and the effect of variations in material properties with temperature has proven analytically intractable. To overcome these limitations, we have also used numerical methods to determine the composition and temperature profiles ahead of the reaction front for different multilayer periods and premixing. The results are compared with experimental values where possible.
Effects of Finite Rate Phase Transformation Kinetics on The Steady-State Solidification Front Propagation Speed in Undercooled Pure Liquids
Authors: Norris J. A. , Kassoy D. R.
Published: April 2011

This novel approach to modeling the steady-state solidification of undercooled pure liquids is based upon first principles. Continuum equations are used to describe a volumetrically averaged, coexisting mixture of solid and liquid in the thin phase transformation zone between regions of pure liquid and pure solid. These equations are coupled with a dynamic equilibrium based rate law that describes temperature dependent phase transformation kinetics. The time scale associated with finite rate phase transformation is found to depend on a solidification activation energy, thermal energy, and the enthalpy of fusion. The model leads naturally to an eigenvalue problem for the solidification front propagation speed. In addition, the variation of the volumetrically averaged liquid fraction across the solidification zone is predicted.
Phase Transformation as a Function of Particle Size in Nanocrystalline Zirconia
Authors: Chraska Tomas , King Alexander H. , Berndt Christopher C. , Karthikeyan J.
Published: April 2011

Bulk zirconia undergoes a pressure-induced transformation from a (low pressure) monoclinic phase to a high pressure tetragonal phase, at around 3GPa (above 900K). We have studied the structures of zirconia nanoparticles formed by plasma-spraying an organo-metallic precursor. Inspection of the particles in the TEM reveals that they adopt one of two distinct crystal structures, depending upon their size. The smallest particles have the tetragonal structure, while larger ones are monoclinic. Interpolation of the data reveals a critical size above which the monoclinic structure is stable. Upon annealing, the zirconia particles coarsen and the small tetragonal particles transform to the monoclinic structure at about the critical size. Coarsening under these conditions produces irregular particle size distributions. We estimate that the surface-stress induced internal pressure in the tetragonal nanoparticles can be as high as 5 GPa and the corresponding surface stress is about 6N/m.
Oxidation Dynamics of Nanophase Aluminum Clusters: A Molecular Dynamics Study
Authors: Ogata Shuji , Campbell Timothy J. , Tsuruta Kenji , Nakano Aiichiro , Kalia Rajiv K. , Vashishta Priya , Loong Chun-K.
Published: April 2011

Oxidation of an aluminum nanocluster (252,158 atoms) of radius 100Å placed in gaseous oxygen (530,727 atoms) is investigated by performing molecular-dynamics simulations on parallel computers. The simulation takes into account the effect of charge transfer between Al and 0 based on the electronegativity equalization principles. We find that the oxidation starts at the surface of the cluster and the oxide layer grows to a thickness of ∼28Å. Evolutions of local temperature and densities of Al and 0 are investigated. The surface oxide melts because of the high temperature resulting from the release of energy associated with Al-O bondings. Amorphous surface-oxides are obtained by quenching the cluster. Vibrational density-of-states for the surface oxide is analyzed through comparisons with those for crystalline Al, Al nanocluster, and α-Al2O3
Phase Transitions in Lead(II) Fluoride Upon Milling
Authors: DÉnÉ Georges , Madamba M. Cecilia
Published: April 2011

PbF2 is known to exist under two different polymorphic structures. Orthorhombic α-PbF 2 is stable at ambient temperature. It has the PbC12 structure. Cubic β-PbF2 is obtained by heating α-PbF2. It does not transform back to α-PbF2 on cooling, and it seems to be infinitely stable in the metastable state under ambient conditions. β-PbF2 crystallizes in the fluorite (CaF2) type. Owing to the large number of potential interstitial sites, many F- Frenkel defects can be formed, which make β-PbF2 the highest performance fluoride ion conductor among binary fluorides. In this work,
both phases of PbF2 have been ball milled. Milling α-PbF2 results in a partial transformation to microcrystalline β-PbF2. The energy required for obtaining the high temperature phase is probably provided in the mechanical form. Milling β-PbF2 leads to partial amorphization and formation of α-PbF2. In this case, milling transforms the high temperature polymorph to the low temperature form, by providing the energy required to overcome the activation energy that keeps PbF2 trapped in the high temperature β-form after cooling.
Growth of Single Crystal Yig Fibers by the Laser Heated Pedestal Growth Method
Authors: Lim HanJin , DeMattei R. C. , Feigelson R. S.
Published: April 2011

Yttrium iron garnet (YIG) single crystal fibers of nominal composition Y3Fe5O12were grown by the laser heated pedestal growth (LHPG) technique, a miniaturized floated-zone process. YIG which melts incongruently, was grown at a temperature below the peritectic decomposition temperature under self-adjusting conditions even though it has very narrow solidification region according to the Y2O3-Fe2O3phase diagram. YIG fibers in diameter ranges from 100 to 750 μm were grown at various growth rates and conditions, and analyzed by x-ray diffraction, electron microprobe, and IR-VIS spectroscopy. Infrared transparent YIG fibers were grown at rates below 12 mm/h in air. At these growth rates, yttrium orthoferrite and iron-oxide inclusions within the YIG fiber, which act as IR scattering centers, were significantly reduced. The transparency of the fibers was more dependent on the growth rate than the stability of the molten zone. Surface ridges containing an Fe-rich composition were observed at all growth rates. These were associated with molten zone instability.
Formation of a Metastable BCC Solid Solution and Decomposition to aC15 Laves Phase in Melt-Spun CrNb10Ti10
Authors: Chen Katherine C. , Kotula Paul G. , Chu Fuming , Thoma Dan J.
Published: April 2011

A metastable, disordered bcc phase has been formed from the melt in a Cr-rich alloy of the Nb-Cr-Ti system, where large volume fractions of the Laves phase would develop under equilibrium solidification conditions. X-ray diffraction (XRD) studies and lattice constant determinations confirm that the melt-spun ribbons contain a bcc phase beyond its terminal solid solution limits. Solidification pathways are proposed based upon metastable and equilibrium phase diagrams. Microstructures have been studied using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Cross-sectional TEM reveals tiny, dispersed Laves phase precipitates within the metastable bcc grains near the chill (wheel) side. Away from the wheel, the microstructure changes to a dendritic structure of the bcc andC15 Laves phase. Annealing of the melt-spun ribbons produces a fine, two-phase microstructure of the equilibrium phases. Microstructures from the melt-spun and arc-melted processing conditions are compared. The microstructural control afforded through this metastable processing route enables a methodology to tailor phase distributions for optimized toughness in Laves phase alloys.
Current Controlled Photoelectrochemical Etching of Gan Leaving Smooth Surfaces
Authors: Rotter T. , Uffmann D. , Ackermann J. , Aderhold J. , Stemmer J. , Graul J.
Published: April 2011

We have etched GaN grown by plasma source MBE in aqueous solutions of KOH in an electrochemical cell under HeCd laser illumination and additional current control.The etch rate was dramatically enhanced up to 8 μm/h by an applied current density of 6.4 mAcm-2. Photocurrent control leads to etched GaN surfaces exhibiting mirror-like appearance with uniform interference color. According to mechanical profilometry, they have a roughness of less than 3.5 nm after etching of several hundred nanometers, which is comparable to the roughness prior to etching. This etching process allows in situ control via photocurrent and induced yellow luminescence.
Pulsed Laser Etching of GaN and AIN Films
Authors: Chen H. , Vispute R. D. , Talyansky V. , Enck R. , Ogale S. B. , Dahmas T. , Choopun S. , Sharma R. P. , Venkatesan T. , Iliadis A. A. , Salamanca-Riba L. G. , Jones K. A.
Published: April 2011

Due to limited success in wet etching of GaN and AIN, dry etching techniques have become more relevant for the processing of the GaN films. Here we demonstrate the results of an alternative dry etching process, namely, pulsed laser etching, for GaN and AIN. In this method, a KrF pulsed excimer laser (λ=248 nm, τ=30 ns) was used to etch epitaxial GaN and AIN films. The dependence of the etching characteristics on the laser energy density and the number of pulses has been studied. The etch depth showed a linear dependence on the number of pulses over a wide range of laser energy densities. The threshold intensity for GaN etching was determined to be 0.33 J/cm2. The etching rate was found to be a strong function of laser energy density. Above the threshold, the etch rate was found to be 300–1400 Å per pulse leading to etching rates of 0.1–1μm/sec depending upon the laser energy density and the pulse repetition rate. It is shown that the etching mechanism is based on laser induced absorption, decomposition and layer by layer removal of the GaN.
High Indium Content Ingan Films and Quantum Wells.
Authors: Van Der Stricht W. , Jacobs K. , Moerman I. , Demeester P. , Considine L. , Thrush E. J. , Crawley J. A , Ruterana P.
Published: April 2011

InGaN films and InGaN/GaN quantum wells with high indium content have been grown by MOVPE and characterised to evaluate the growth process and the indium incorporation efficiency. The characterisation techniques include photoluminescence, DC X-ray and TEM. The closed spaced vertical rotating disk reactor configuration results in a very high Indium incorporation for InGaN material, compared to other configurations. InGaN layers with an indium composition up to 56 % have been deposited which still exhibit very good optical properties (intense PL emission). The influence of various growth conditions on the InGaN composition and quality have been investigated to optimize the layer quality. TEM diffraction patterns have shown that the ternary InGaN layer can be chemically ordered. The In and Ga atoms occupy respectively the two simple hexagonal sublattice sites related by the glide mirrors and helicoidal axes of the P6 3mc symmetry group of the wurtzite GaN.
Ni/Si-Based Ohmic Contacts to p- and n-Type GaN
Authors: Kamińska E. , Piotrowska A. , Barcz A. , Guziewicz M. , Kasjaniuk S. , Bremser M. D. , Davis R. F. , Dynowska E. , Kwiatkowski S.
Published: April 2011

Ni/Si-based ohmic contact scheme for GaN, based on the solid-phase regrowth (SPR) mechanism have been developed. Using Mg and Si as dopant species, ohmic contacts with a resistivity of ∼1*10-3Ωcm2 to p-GaN (p≈3*1017 cm-3) and n-GaN (n≈2*1017cm-3), respectively, have been obtained. SIMS, XRD, and RBS analysis show in as-deposited contacts, an initial reaction at GaN/Ni interface, leading to the formation of an Ni-Ga-N layer. The ohmic behavior of contacts, observed after annealing at 400°C, is accompanied by structural transformations in the contact region: i) the decomposition of Ni-Ga-N layer and ii) the growth of NiSi compound.
Metal Contacts to n- AlXGa1-xN
Authors: Sampath A. , Ng H. M. , Korakakis D. , Moustakas T. D.
Published: April 2011

In this paper we report on the formation of ohmic contacts to n- AlxGa1-xN alloys. The films were produced by plasma-assisted MBE and doped n- type with silicon at doping levels between 1018 to 1019 cm-3. Contacts were formed by sequential deposition of 200 Å of Ti and 2000 Å of Al and the contact resistivities were determined from TLM measurements. For low Al- content (x<. 10) the I-V characteristics are linear with contact resistivities of between 10-4 to 10-5 cm2. The contacts become progressively non-ohmic at Al concentrations greater than 10%. There results are consistent with the Schottky limit being applicable to these alloys and thus the Ti/Al contact forms Schottky barriers with higher barrier height as the conduction band of the alloy moves towards the vacuum level.
Status of Nitride Based Light Emitting and Laser Diodes on SiC
Authors: Doverspike K. , Bulman G. E. , Sheppard S. T. , Kong H. S. , Leonard M. , Dieringer H. , Weeks T. W. , Edmond J. , Brown J. D. , Swindle J. T. , Schetzina J. F. , Song Y- K , Kuball M. , Nurmikko A.
Published: April 2011

Single crystal thin films with compositions from the A1N-InN-GaN system were grown via metal-organic chemical vapor deposition (MOCVD) on single crystal 6H-SiC substrates. Blue light emitting (LED) and laser diode (LD) structures were fabricated. The conducting buffer layer LEDs employed an AlGaN buffer layer which provides a conduction path between SiC and the active device region. The external quantum efficiency of the LEDs was 3% at 20 mA- 3.6V and peak emission wavelength of 430 nm. Violet and blue LDs were fabricated and consisted of an 8-well InGaN/GaN multiple quantum well (MQW) active region in a separate confinement heterostructure (SCH) design. Lasing was obtained both on structures using an insulating buffer layer, and also on structures using a conducting buffer layer. The resulting lasers operated at room temperature using pulsed and continuous wave operation with an emission wavelength of 404-435 rim. The lowest threshold current density obtained for lasing was 11 kA/cm2.
The First Nitride Laser Diode on Silicon Carbide
Authors: Brown J. D. , Swindell J. T. , Johnson M. A. L. , Yu Zhonghai , Schetzina J. F. , Bulman G. E. , Doverspike K. , Sheppard S. T. , Weeks T. W. , Leonard M. , Kong H. S. , Dieringer H. , Carter C. , Edmond J. A.
Published: April 2011

This paper reports the events at NCSU leading up to and including those of June 5, 1997 which produced the first demonstration of a nitride laser diode on silicon carbide – and the very first nitride laser demonstration outside of Japan. All of the laser diode samples tested at NCSU were designed, grown, and fabricated into cleaved cavity test structures at Cree Research. Laser testing at NCSU consisted of spectral emission versus current measurements, light output power versus current (L-I) measurement, and light output polarization measurements versus current. The first successful laser on silicon carbide emitted at 402.6 nm. Subsequently, lasers displaying outputs ranging from 402.6 to 430.2 have been successfully tested at NCSU.
InGaN Laser Diodes Grown on SiC Substrate Using Low-Pressure Metal-Organic Vapor Phase Epitaxy
Authors: Kuramata A. , Domen K. , Soejima R. , Horino K. , Kubota S. , Tanahashi T.
Published: April 2011

We report the crystal growth and the characteristics of InGaN multiple quantum well (MQW) laser diodes grown on a 6H-SiC substrate using a low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). We discuss the buffer layer, the control of InGaN and AlGaN alloy composition, the magnesium doping of GaN and AlGaN, and the characteristics of the MQW structure. We also demonstrate the room-temperature pulsed operation of the laser diode. The threshold voltage was reduced to 15 V by improving the p-contact resistance. The threshold current was reduced to 500 mA by changing the MQW structure and employing high reflection coating.
A Kinetic Model for GaN Growth
Authors: Koleske D. D. , Wickenden A. E. , Henry R. L. , Desisto W. J. , Gorman R. J.
Published: April 2011

A kinetic model is presented to explain GaN growth. The model is based on established values for the N and Ga desorption kinetics and well founded assumptions on the adsorption and decomposition of the N and Ga containing precursors. When grown on similar nucleation layers, it is shown that high quality GaN films are achieved when the V/III ratio is chosen to be slightly larger than the Ga and N desorption rates. The model is verified by comparing the structural, optical, and electrical properties of the GaN to the growth temperature and V/III ratio. The model explains several features of GaN growth including, growth conditions for smooth surface morphology, growth conditions for highly resistive GaN, and a possible explanation for the origin of Ga and N vacancies in GaN. Based on the growth model, ordering of the GaN during growth is achieved via an adsorption/desorption cycle where Ga and N containing species are exchanged between the gas phase boundary layer and the solid surface. Consequences of the model on establishing growth conditions and run-to-run reproducability are also discussed.
The Effect of Growth Temperature On The Microstructure of Movpe AlN/Si (111)
Authors: Zhou Mei , Perkins N. R. , Rehder E. , Kuech T. F. , Babcock S. E.
Published: April 2011

The epitaxy, defect structure, and heterointerface microstructure of MOVPE-grown AlN on <111> oriented Si were studied by transmission electron microscopy (TEM) in an investigation of the affects of the growth temperature (925°C–1 175°C) on the microstructure of low thickness (∼0.2 μm) films. A film grown at 1175°C was single crystalline with the following orientation relationship: (0001)AlN||(111)Si, <1120> and <1010>AlN||<211>Si. All AlN films grown at temperatures below 1100°C were wire-textured polycrystals with (0001)AlNlargely parallel to (111)siand some in-plane preferred orientation similar to that of 1175°C AlN epifilms. Columnar grains/domains with faceted top surfaces were observed for the entire growth temperature range. The mean column diameters decreased from 50 nm to 35 nm as the growth temperature decreased. The columnar grain/domain structure and the faceted tops resulted in peakto-valley surface roughness on the order of 20 nm. Sharp interfaces of AlN/Si were generally observed in all AIN films studied by HREM except for the 1075°C film where a transitional layer of approximately 15Å thick was observed at some interface regions. Defects typically encountered in large lattice mismatched heteroepitaxial systems were observed.
Bulk Gan Crystal With Low Defect Density Grown By Hydride Vapor Phase Epitaxy
Authors: Usui A.
Published: April 2011

A new approach to grow thick GaN layers by hydride vapor phase epitaxy (HVPE) is described. Selective growth is carried out at the beginning of growth. The coalescence of selectively grown facet structures makes it possible to achieve a flat surface over the entire substrate. As a result, crack-free GaN films with mirror-like surfaces are successfully grown even to a thickness of about 100 μm on a 2-inch-diameter sapphire substrate. The extended defect density is as low as 6×107 cm−2. The reduction mechanism for dislocation is discussed based on TEM observation. The high optical properties of FIELO GaN are confirmed by 5 K photoluminescence and reflectance measurements.
MOCVD Growth of GaN on bulk AlN Substrates
Authors: Lu Hong-Qiang , Bhat Ishwara B. , Lee Byung-Chan , Slack Glen A. , Schowalter Leo J.
Published: April 2011

In this paper, the growth of epitaxial GaN layers on c-plane and a-plane bulk AIN substrates by metalorganic vapor phase epitaxy is reported. The AlN boules were grown by the sublimationrecondensation technique. Single crystal GaN films grown on the c-plane orientation replicate the substrate orientation. However the surface of the epilayer had a high density of cross-hatch defect lines, presumably caused by mechanical polishing damage. The low temperature PL spectra of these films were dominated by exciton emission at 3.470 eV with a FWHM of 14 meV at 7 K. On the other hand, GaN grown on the a-plane orientation AlN was polycrystalline and the surface was rough with ridge-like facets. The PL from this film showed a dominate peak at 3.406 eV which may originate from defect-bound excitons. The quality of the GaN layers grown on these AIN bulk substrates appeared to be limited by the surface preparation method, which has not been optimized.
Atomic Scale Aluminum and Strain Distribution in a Gan/AlxGa1−XN Heterostructure
Authors: Kisielowski Christian , Schmidt Olaf , Yang Jinwei
Published: April 2011

A GaN/AlxGalxN multi-quantum well test structure with Al concentrations 0 ≤ xAl ≤ 1 was utilized to investigate the growth of AlxGal–xN barrier layers deposited by metal organic chemical vapor deposition (MOCVD). A transition from a two dimensional (2D) to a three dimensional (3D) growth mode was observed in AlxGa1–xN barriers with XAl ≥ 0.75. It is argued that the transition occurs because of growth at temperatures that are low compared with the materials melting points Tmelt. The resulting rough AlxGa1–xN surfaces can be planarized by overgrowth with GaN. Quantitative high resolution electron microscopy (HREM) was applied to measure composition and strain profiles across the GaN/AlxGa1−xN stacks at an atomic level. The measurements reveal a substantial variation of lattice constants at the AlxGa1−xN/GaN interfaces that is attributed to an Al accumulation.
A Combined Tem/Rheed, Sem/Cl Study Of Epitaxial Gan
Authors: Brown P. D. , Tricker D. M. , Xin Y. , Chengt T. S. , Foxont C. T. , Evanst D. , Galloway S. A. , Brock J. , Humphreys C. J.
Published: April 2011

Epitaxial GaN grown by MBE has been characterised using the combined techniques of scanning electron microscopy / cathodoluminescence, reflection high energy electron diffraction (RHEED), and conventional transmission electron microscopy. Variations in spatial and spectral distributions of luminescence can arise due to embedded cubic inclusions within the hexagonal GaN matrix. The strong effect of doping on the crystallinity of the GaN deposit is illustrated, as determined by RHEED in a TEM.
Comparative Study of Typical Defects in III-Nitride Thin Films and Their Alloys
Authors: Dovidenko K. , Oktyabrsky S. , Narayan J. , Joshkin V. , Razeghi M.
Published: April 2011

The microstructure and typical defects in GaN and GaN/GaAlN thin film heterostructures grown on (0001)α-A12O3 were investigated using different transmission electron microscopy (TEM) techniques including diffraction contrast analysis, multiple dark field imaging, and highresolution TEM. The films were grown by metal-organic chemical vapor deposition (MOCVD) technique. All of the films exhibited good electrical/optical properties. Yet, films were found to be of two distinctive types in terms of the microstructure. Films of the first type (A) were found to contain high, up to the 109 cm2, density of inversion domains (IDs) as well as pure edge (b=l/3[11 20]), screw and mixed type dislocations with the average density of 109 - 1010 cm−2. Smoother surface of the film, absence of IDs, and low (down to 107 cm−2 in the device quality layers) density of screw and mixed type dislocations were found to be characteristic for the second type (B) microstructure. The majority of defects present in these B-type GaN and GaN/GaA1N thin films were found to be threading pure edge dislocations associated with low angle tilt sub-grain boundaries. Despite the 1010 cm−2 density of the edge dislocations, the films displayed the devicequality electrical characteristics. Type A and type B microstructure can be obtained by the variation of growth conditions. The correlation between the optical and structural properties are discussed.
Dislocation Distribution and Subgrain Structure of GaN Films Deposited on Sapphire by HVPE and MOVPE
Authors: Dunn K. A. , Babcock S. E. , Vaudo R. , Phanse V. , Redwing J.
Published: April 2011

Transmission electron microscopy (TEM) was used to characterize the microstructure in GaN films deposited by two different methods. An 11 μm thick film was deposited directly on a sapphire substrate by HVPE; an 8 μm thick film was deposited on a 15 nm buffer layer of AIN on sapphire by MOVPE. The dislocation densities in the top layer of the HVPE and MOVPE ilms were ∼109 cm-2 and ∼5 x 109 cm-2 respectively. In the HVPE film this was almost exclusively threading dislocations (TDs), ∼70% of which had edge character. In addition to the TDs, the MOVPE sample also contained an appreciable number of dislocations lying in the basal plane. The microstructure of each film was dominated by a subgrain structure of slightly misoriented cells. In the MOVPE specimen, approximately 90% of the TDs were associated with subgrain walls, whereas only approximately 75% of the dislocations in the HVPE specimen were associated with walls. Both the HVPE and MOVPE samples experienced 40% coarsening of the cells through the thickness of the film. The subgrains of the MOVPE sample were 75% smaller than those in the HVPE sample (350 and 1300 nm, respectively). The average dislocation spacing in the walls was 50% smaller in the MOVPE sample than in the HVPE sample (82 and 180 nm, respectively).
Comparative Analysis of Strain and Stress in MBE and MOCVD grown GaN thin films on sapphire
Authors: Krüger Joachim , G.S. Sudhir , Corlatan Dorina , Cho Yonah , Kim Viihwan , Klockenbrink Ralf , Rouvimov Sergei , Liliental-Weber Zuzanna , Kisielowski Christian , Rubin Michael , Weber Eicke R. , McDermott Brian , Pittman R. , Gertner Edward R.
Published: April 2011

In this study, the causes of biaxial and hydrostatic stress components in epitaxially grown thin GaN films on sapphire are analyzed. It is observed that growth by Molecular Beam Epitaxy (MBE) and by Metal Organic Chemical Vapor Deposition (MOCVD) are governed by very similar physical principles. Differences in the absolute stress values are mainly due to the difference in growth temperature. It is argued that in the case of MOCVD growth the onset of plasticity for higher growth temperatures is responsible for a larger stress relaxation in the buffer layer. It is further found that either process can result in highly off-stoichiometric GaN layers, as manifested by the large variations in the a and c lattice parameters caused by intrinsic point defects.
The core Structure Of Pure Edge Threading Dislocations In Gan Layers Grown On [0001] SiC Or Sapphire By Mbe
Authors: Ruterana P. , Potin V. , Nouet G.
Published: April 2011

The best layers of GaN grown on SiC or sapphire contain high densities of threading dislocations, which do not seem to exhibit a critically important electrical activity. It is possible that the electrical activity of these dislocations may change with time. The large majority of the threading dislocations are a type, with 1/3 <1120> Burgers vector and their line parallel to the [0001] growth direction. The atomic structure of the a threading dislocations was found to correspond to 5/7 or 8 atoms rings core with rather equal frequency. The 8 atoms ring cores contain dangling bonds and will present favourable sites for atomic impurities which eventuallly can modify the good performances of the devices.
Structural Properties of Nitrides Grown by Omvpe on Sapphire Substrate
Authors: Amano H. , Takeuchi T. , Yamaguchi S. , Nitta S. , Kariya M. , Iwaya M. , Wetzel C. , Akasaki I.
Published: April 2011

Crystalline quality of nitrides on sapphire by OMVPE has been investigated. First, in-situ observations of the crystallization process of the low temperature deposited AIN buffer layer or GaN buffer layer on sapphire substrate have been performed. Small hexagonal mesas were formed from the sapphire to the surface and finally they formed a stacked structure. Secondly, a low temperature deposited buffer layer located between the high temperature grown GaN was found to reduce the etch pit density of GaN films. Thirdly, structural properties of Ga1−xInxN (0 ≤x≤0.21, and x= l) on GaN and GaInN/GaN MQWs on GaN have been characterized by Xray diffraction. Coherently grown GaInN showed almost the same twisting as the underlying GaN layer, while free standing InN showed large twisting. Thickness of the well layers in MQWs has been controlled within one monolayer preciseness, and the fluctuation of alloy composition has been controlled to within 2%
Raman Analysis Of AlxGa1-xN Films
Authors: Bergman Leah , Dutta Mitra , Bremser Michael D. , Nam Ok-Hyun , Perry William G. , Alexon Dimitri , Davis Robert F. , Balkas Cengiz M. , Nemanich Robert J.
Published: April 2011

Raman analysis of the E2 mode of AlxGal-xN in the composition range 0 ≤ x ≤ 1 is presented. The lineshape was observed to exhibit a significant asymmetry and broadening toward the high energy range. The spatial correlation model is discussed, and is shown to account for the lineshape. The model calculations also indicate the lack of a long-range order in the CVD (chemical vapor deposition) alloys. These results were confirmed by X-ray scattering: the relative intensity of the superlattice line was found to be negligible. The line broadening of the E2 mode was found to exhibit a maximum at a composition x∼0.5 indicative of a random disordered alloy system. The stress state of the alloys was found to be tensile and was attributed to the difference in the thermal expansion coefficients of the SiC substrate and the film.
Nondestructive, Room Temperature Determination Of The Nature Of The Band-Bending (Carrier Type) In Group III Nitrides Using Contactless Electroreflectance And Surface Photovoltage Spectroscopy
Authors: Krystek Wojciech , Pollak Fred H. , Feng Z. C. , Schurman M. , Stall R. A.
Published: April 2011

Using contactless electroreflectance and surface photovoltage spectroscopy at room temperature we have nondestructively evaluated the band bending (carrier type) at the surface of epitaxial n- and p-type GaN/sapphire samples as well as at both the InGaN surface and the GaN at the InGaN/GaN interface of samples of epitaxial InGaN grown on top of thick GaN epilayers/sapphire, having average n- and p-type character.
Localized Excitons in InGaN
Authors: ChichiBu S. , Deguchi T. , Sota T. , Wada K. , Nakamura S.
Published: April 2011

Emission mechanisms of the device-quality quantum well (QW) structure and bulk three dimensional (3D) InGaN materials grown on sapphire substrates without any epitaxial lateral overgrown GaN (ELOG) base layers were investigated. The InxGx1−xN layers showed various degree of spatial potential (bandgap) fluctuation, which is probably due to a compositional inhomogeneity or monolayer thickness fluctuation produced by some kinetic driving forces initiated by the threading dislocations (TDs) or growth steps during the growth. The degree of fluctuation changed remarkably around nominal InN molar fraction x=0.2, which changes to nearly 8–10 % for the strained InxGa1−xN. This potential fluctuation induces energy tail states both in QW and 3D InGaN, showing a large Stokes-like shift combined with the red shift due to quantum confined Stark effect (QCSE) induced by the piezoelectric field. The spontaneous emission from undoped InGaN single quantum well (SQW) light-emitting diodes (LED's), undoped 3D double heterostructure (DH) LED's, and multiple quantum well (MQW) laser diode (LD) wafers was assigned as being due to the recombination of excitons localized at the potential minima, whose area was determined by cathodoluminescence (CL) mapping to vary from less than 60 nm to 300 nm in lateral size in the case of QW's. The lasing mechanisms of the cw In0.15Gao.85N MQW LD's having small potential fluctuation, whose bandgap broadenings are less than about 50 meV, can be described by the well-known electron-hole-plasma (EIHP) picture with Coulomb enhancement. The inhomogenous MQW LD's are considered to lase by EHP in segmented QW's or Q-disks. It is desirable to use entire QW planes with small potential inhomogeneity as gain media for higher performance LD operation.
Cathodoluminescence Studies of InGaN Quantum Wells
Authors: Ponce F. A. , Galloway S. A. , Goetz W. , Kern R. S.
Published: April 2011

Low temperature cathodoluminescence has been used to investigate the spatial characteristics of light emission in InxGa1−xN single quantum wells. High spatial resolution, narrow band pass imaging shows the luminescence to be strongly inhomogeneous in wavelength as well as in intensity on a sub-micron scale. Cathodoluminescence spectra correlate favorably with photoluminescence spectra. However, when spectra are recorded from different areas in spot mode, the quantum emission varies significantly in wavelength. The observed variations are consistent with composition inhomogeneities in the quantum well.
Surface Characterization of GaN Formation on GaAs(100) Using Ammonia
Authors: Huh Chul , Ahn Sook , Han Jeong Yeul , Cho Keum Jae , Seo Jae Myung , Park Seong-Ju
Published: April 2011

The GaN formation on GaAs(100) using NH3 has been investigated using synchrotron radiation photoemission spectroscopy and an atomic force microscope (AFM). This study showed the effect of nitridation temperature on the compositional change, the chemical states, and the surface morphological changes in the nitridated surface layer. It was observed that ammonia is decomposed to an activated nitrogen atom above 700°C forming GaN on the surface. A thermally nitridated layer was composed of metallic Ga and GaN islands elongated along the [011] direction to relax the tensile strain in the [011] direction. As the nitridation temperature increased, the composition of GaN increased in the nitridated layer due to the efficient thermal decomposition of NH3 and the subsequent incorporation of the N atom to the metallic Ga. The surface morphology of the nitridated layer was also sensitive to the nitridation temperature and a smooth surface morphology could be obtained on the surface nitridated at 700°C.
Optical Properties of Ingan/GaN Multi Quantum Well Structures
Authors: Bergman J. P. , Saksulv N. , Dalfors J. , Holtz P. O. , Monemar B. , Akasaki I , Amano H.
Published: April 2011

A set of GaN/InGaN multiple quantum wells (QWs) with well thickness 30 Å and barrier thickness 60 Å were grown by MOCVD on sapphire substrates. The n-type Si doping of the InGaN QWs was varied, in order to produce a different electron concentration in the QWs for the different samples. Optical spectra were obtained by time resolved photoluminescence spectroscopy. The data show weak excitonic spectra from the QWs as well as a broad deeper emission with a much stronger intensity. The spectral shape becomes narrower and the energy position shifts to higher energies with increasing doping. The two different emissions are not easily separated in CW or time integrated spectra, but are clearly observed in a time resolved spectral measurement due to their different recombination rates. The deeper emission has a long and non-exponential decay, with an average decay time in the order of several hundred nanoseconds. The higher energy exciton emission has a much faster decay of about 1 ns. The lower energy band is tentatively explained as due to separately localized electron-hole (e-h) pairs in the QW.
High Temperature Photoluminescence and Photoluminescence Excitation Spectroscopy of Er Doped Gallium Nitride
Authors: Hömmerich U. , Thaik Myo , Robinson-Brown T. , MacKenzie J. D. , Abemathy C. R. , Pearton S. J. , Wilson R. G. , Schwartz R. N. , Zavada J. M.
Published: April 2011

We report results of high temperature photoluminescence and photoluminescence excitation studies of Er doped GaN. Er was doped into GaN either by ion-implantation or during growth by metalorganic molecular beam epitaxy (MOMBE). Using above gap excitation (λex=325nm), GaN:Er showed strong 1.54 μm Er3+ luminescence up to 550K which indicates the potential of this material for high temperature opto-electronic applications. In addition, we performed timeresolved photoluminescence excitation (PLE) measurements over the wavelength range 420 to 680 nm using a Nd:YAG pumped Optical Parametric Oscillator (OPO). Similar to our previous PLE results of Er doped AIN, we observed that Er3+ ions in GaN can be excited either through resonant pumping of Er3+ energy levels or through energy-transfer processes involving defect states. The relative contribution of direct and indirect Er3+ excitation, however, seems to depend on the material preparation method.
Cubic Inn Inclusions as the Cause for the Unusually Weak Pressure Shift of the Luminescence in InGaN
Authors: Perlin Piotr , Weinstein Bernard A. , Christensen Niels E. , Gorczyca Iza , Iota Valentin , Suski Tadeusz , Wisniewski Przemek , Osinski M. , Eliseev P. G.
Published: April 2011

We propose a new approach to explain the unusually low pressure coefficients of the luminescence peaks observed in single-quantum-well InGaN-based light emitting diodes manufactured by Nichia Chemical Industries. In view of the most recent first principles band structure calculations for InN under hydrostatic pressure, we find that it is possible to reproduce the measured low (12–16meV/GPa) pressure coefficients of the luminescence by assuming the formation of zincblende InN inclusions in the InGaN quantum well layers. These cubic inclusions, surrounded by the usual wurtzite material, should act like quantum dots giving rise to enhanced electron localization. Obtained this way, the pressure shift of the luminescence peaks in blue and green InGaN-based emitters should be close to 14meV/GPa, in good agreement with our experimental results. This explanation of the observed low pressure coefficients in these devices is consistent with recent independent evidence for InN inclusions in InGaN epilayers.
Observation Of Native Ga Vacancies In Gan By Positron Annihilation
Authors: Saarinen K. , Laine T. , Kuisma S. , Nissilä J. , Hautojärvi P. , Dobrzynski L. , Baranowski J. M. , Pakula K. , Stepniewski R. , Wojdak M. , Ysmolek A. , Suski T. , Leszczynski M. , Grzegory I. , Porowski S.
Published: April 2011

Positron annihilation experiments have been performed to identify native point defects in GaN bulk crystals as well as in epitaxial layers. The results show that Ga vacancies are present at concentrations of 1017 – 1018 cm−3 in undoped GaN bulk crystals and layers, whereas the Mgdoped samples are free of Ga vacancies. The Ga vacancies are negatively charged and their concentration correlates with the intensity of the yellow luminescence. We conclude that the Ga vacancies contribute to the electrical compensation of n-type GaN and their acceptor levels are involved in the yellow luminescence transition.
Stress Gradients In Heteroepitaxial Gallium Nitride Films
Authors: Ager J. W. , Conti G. , Romano L. T. , Kisielowski C.
Published: April 2011

Heteroepitaxial GaN films grown on sapphire are usually in a compressive residual biaxial stress state that is typically 100 – 500 MPa but can be as high as 1 GPa. Spatially resolved measurements of GaN film stresses obtained via micro-Raman spectroscopy using shifts in the frequency of the E2 phonon are presented. The spatial resolution of this stress measurement technique is ca. 2 μm laterally and 10 μm in the axial (growth) direction. Two interesting stress gradients are found near cleaved edges. (1) On the surface of the cleaved edge the compressive film stress is a maximum at the GaN/sapphire interface and relaxes to a nearly stress-free state towards the free edge. (2) In the lateral direction, the stress increases away from the edge, reaching a steady-state value at approximately 10x the film thickness. It is shown that the effects are consistent with existing finite element models used to treat strained growth of surface structures in other semiconductor systems. An axial stress gradient that is not associated with edge effects is found in some thick (>20 μm) HVPEgrown films. The interface region is found to be up to 500 MPa in compression, but this stress declines toward the film surface, which is found to be nearly stress free.