bluuebunny/crossref_metadata_2025 huggingface dataset:
Nucleation Modes in Sharp Concentration Gradients
Authors: Hodaj F. , Gusak A. M. , Kovalchuk A. O. , Desre P. J.
Published: April 2011

Reaction kinetics in bimetallic multilayers have demonstrated that sharp unidirectional concentration gradient, which develop as interdiffusion proceeds at the interface are able to delay or to suppress nucleation of intermetallics. It has been found that the existence of a critical gradient beyond which nucleation is inhibited is strongly dependent on the mechanism of formation of the embryo [1–7].

A mechanism of nucleation under concentration gradient (∇c) is proposed and treated on the basis of the Fokker–Planck equation for the distribution in the size space.

The influence of the aspect ratio of the embryo on the critical concentration gradient is also studied. Due to the fluctuations of the embryo shape, it is shown that the minimization of the thermodynamic potential leading to the aspect ratio of the embryo is only significant beyond a certain value of the concentration gradient. Application is presented to the nucleation of the compound Ni10Zr7 in an amorphous layer Ni-Zr.
Particle Nucleation in Dissipative Systems
Authors: Russell K. C. , Rangel J. C. F.
Published: April 2011

Dissipative regimes may induce athermal processes in addition to the thermally activated single atom addition or subtraction events considered by classical nucleation theory. Displacement cascades or impinging energetic ions could destroy sub-critical precipitates. Similarly, either displacement cascades or energetic knock-on atoms could remove single atoms or groups of atoms from subcritical clusters. Cluster injection is also possible, as in vapor deposition of clusters or in the cores of displacement cascades providing void nuclei.
Domain-Spatial Correlation Function of Spherulitic Domain Evolution in Polymer Films
Authors: Huang Tao , Tsuji Tomohiro , Kamal M. R. , Rey A. D.
Published: April 2011

We present a new theoretical model of nucleation and growth in term of a novel domainspatial correlation function. This model probes the patterns and spatio-temporal evolution of nucleation and growth process and agrees very well with experimental data. The dynamic domain-spatial correlation function directly and simultaneously explores the transformed volume fraction, the time-dependent domain size distribution function, and the spatial correlation function of domain core centers for the entire process, including the post-nucleation, domain growth and grain formation stages.
Effects of Hafnium, Heat Treatment and Cycling under an- Applied Stress on The Transformations of Cold Worked Niti-Based Shape Memory Alloys
Authors: Zhan Chen , Zee Ralph H. , Thoma Paul E. , Boehm John J.
Published: April 2011

The effect of thermal cycling under a constant tensile load on the transformation temperatures (TTs) of NiTi-based shape memory alloys (SMAs) is investigated. Three SMAs are examined in this study: a near equiatomic binary Ni49Ti51 alloy and two ternary Ni49Ti51-xHfx alloys with 1 at% and 3 at% Hf. The SMAs are in the form of wires with 40% cold work (reduction in area) and heat treated between 300°C and 600°C. These SMA wires are thermally cycled between their martensite (M) and austenite (A) phases for 100 cycles under an axial tensile stress of 206.8MPa (30Ksi) in air. Results show that the effect of thermal cycling on the M and A TTs depends on heat treatment (HT) temperature and composition in a complex manner. For example, the M TT, of the binary NiTi SMA heat treated between 300°C and 450°C, increases during thermal cycling. However, with HT temperatures between 500°C and 600°C, the M TIT decreases slightly during thermal cycling. For the NiTi-based SMAs that contain 1 at% and 3 at% Hf, the M TT increases during thermal cycling for HT temperatures up to 500°C, and the M TT decreases during thermal cycling when heat treated at 600°C. These results are due to changes in internal stress and structure, such as dislocation density and arrangement, which are affected by HT temperature and thermal cycling. The influence of Hf content on the changes in the M and A TTs during thermal cycling is also shown.
A Calorimetric Evaluation of the Role of Impurities in the Nucleation of Secondary Phases in 1xxx Al Alloys
Authors: Allen C. M. , O'Reilly K. A. Q. , Evans P. V. , Cantor B.
Published: April 2011

Superpurity (> 99.995wt% pure) lxxx series Al-0.3wt%Fe-0.lwt% Si alloys with different impurity additions have been melt spun to produce ribbons with microstructures of fine scale ∼0.1–0.2μm diameter secondary phase particles entrained in a higher melting point Al matrix. The melting and resolidification behaviour at 2Kmin−1of the secondary phase particles while entrained in the solid matrix was monitored using differential scanning calorimetry, to simulate the nucleation of secondary phases in liquid interdendritic regions during the final stages of solidification in casting. The melting and resolidification behaviour of 500ppm V and Zr doped superpurity ribbons is the same as that of a commercial purity ribbon. This indicates that V is responsible for the promotion of the metastable FeAlmphase in the commercial purity ribbon. The presence of FeAlmin direct chill (DC) cast commercial purity alloys indicates that V also influences phase selection in the faster cooled (∼5-10Ks−1) regions of these alloys.
Melting vs. Solidification of a Pure Metal Analysed by DSC
Authors: Clavaguera N. , Clavaguera-Mora M. T. , Fontán J. , Touron J. L. , Comas C.
Published: April 2011

The aim of the present paper is to analyse the melting/solidification of pure metals resulting from non-equilibrium conditions. Attention is focused on the direct measurement of the calorimetric signal obtained under isothermal hold of the sample at a temperature close to the equilibrium melting temperature Tm, which results in both melting of an overheated solid and solidification of an undercooled melt. The non-equilibrium transformations are monitored by DSC under isothermal regime, with previous continuous heating/continuous cooling of the sample. The dependence of the calorimetric signal on thermodynamic factors, ΔH and ΔG, is explored. Here ΔH and ΔG are, respectively, the melting enthalpy and Gibbs free energy difference between the crystal and the liquid. In particular, the results of the investigation performed on the melting/solidification behaviour of pure In and Pb are presented.
Iron Precipitation and Dissolution in Float-Zone Silicon
Authors: Ramappa Deepak A. , Henley Worth B.
Published: April 2011

A quantitative analysis of the phase changes of iron in silicon from interstitial to precipitate phase and vice-versa is presented. Temperature dependent iron precipitation and dissolution in float-zone grown silicon wafers is experimentally investigated. A quantitative analysis of iron silicide precipitate stability and dissolution with respect to time and temperature is also presented. Precipitation of iron in silicon was analyzed by a quantitative assessment of change in interstitial iron using a surface photo voltage minority carrier lifetime analysis technique. Contamination levels of iron in the range 1011 to 1013 atoms/cm3 are investigated. It is concluded that maximum iron precipitation occurs in the temperature range of 500°C to 600°C. Iron precipitation is rapid in this region where more than 90% of the interstitial iron precipitates in a period of 30 minutes. Iron silicide precipitates were found to dissolve above a temperature of 760°C restoring iron back to an interstitial phase in the silicon matrix.
From Semiconductor to Metal in a Flash: Observing Ultrafast Laser-Induced Phase Transformations
Authors: Callan J. P. , Kim A. M.-T. , Huangt L. , Glezer E. N. , Mazur E.
Published: April 2011

We use a new broadband spectroscopic technique to measure ultrafast changes in the dielectric function of a material over the spectral range 1.5–3.5 eV following intense 70-fs laser excitation. The results reveal the nature of the phase transformations which occur in the material following excitation. We studied the response of GaAs and Si. For GaAs, there are three distinct regimes of behavior as the pump fluence is increased — lattice heating, lattice disordering, and a semiconductor-to-metal transition.
Phase Transformation and Ion Beam Induced Crystallization in SiC Layers Formed by Mevva Implantation of Carbon into Silicon
Authors: Chen Dihu , Wong S. P. , Ho L. C. , Yan H. , Kwok R.W.M.
Published: April 2011

Buried SiC layers were synthesized by carbon implantation into silicon with a metal vapor vacuum arc ion source under various implantation and annealing conditions. The infrared absorption spectra of these samples were deconvoluted into two or three gaussian components depending on the preparation conditions. One component peaked at around 700 cm-1was assigned to amorphous SiC (a-SiC). The other two components, both peaked at 795 cm-1 but with different values of full width at half maximum (FWHM), were attributed to β-SiC. The one with a larger (smaller) FWHM corresponds to β-SiC of smaller (larger) grains. With this deconvolution scheme, the fraction of various SiC phases in these samples were determined. It was found that for the as-implanted samples there are critical energies and doses at which the crystalline SiC fraction increases abruptly. This was attributed to the ion beam induced crystallization (IBIC) effect. It was also shown that the IBIC effect leads to strong dependence of the β-SiC fraction on the order of implantation for samples synthesized by double-energy implantation. Analysis of the evolution of the β-SiC fraction with annealing time indicated that the crystallization process in these SiC layers could well be described by the classical random nucleation and growth theory.
Investigation Of Phase Formation During Cold Rolling Of Elemental Zr-Al-Ni-Cu Foils With Bulk Glass Forming Composition
Authors: Sagel A. , Sieber H. , Fecht H. J. , Perepezko J. H.
Published: April 2011

Elemental Zr-Al-Ni-Cu foils of bulk glass forming composition were cold rolled at ambient temperature. The phase transformations during the cold deformation process were monitored with electron and X-ray diffraction and thermal analysis. After 120 deformation cycles a fully amorphous sample was obtained, as indicated by a distinct endothermic glass transition and a large exothermic crystallization reaction. The characteristics of the amorphous phase are compared with a liquid quenched metallic glass and mechanically alloyed elemental powder mixtures of similar composition. The amorphization reaction during cold rolling is similar to mechanical alloying and proceeds by the formation of a fine layered structure of the constituents, a rapid reduction of the layer thickness and of the average grain size to a nanometer scale and dissolution reactions in the hcp Zr lattice. While in the beginning of the cold rolling process the elemental foils were ductile, after several rolling passes the multilayered sample became brittle and, finally, displayed large elastic flexure in the amorphous state. Several thermal treatments of the multilayered and amorphous samples were performed in order to study thermally activated solid-state amorphization reaction as well as phase formation during crystallization.
Crystallization of Amorphous Ribbon in NiTi-Cu Shape Memory Alloy
Authors: Yang F. , Ma J. L. , Pu Z. J. , Wu K. H.
Published: April 2011

The objective of this work is to study the crystallization process of the amorphous ribbon of NiTi-Cu based shape memory alloys. An amorphous material with a composition of Ti50Ni25Cu25 was used to conduct this study. First, a study was conducted to understand the crystallization kinetics of this amorphous material at the isothermal model and continuous heating mode, respectively. The characteristic parameters associated with the crystallization process, such as the start and finish time for isothermal crystallization, the peak temperature for continuous heating crystallization, and activation energy, are obtained. Based on the study of experimental data of crystallization kinetics, a series of isothermal annealing experiments was conducted to study the crystallization process and microstructure of fully crystallized materials. The TEM micrograph shows the crystal phase growths in the amorphous matrix as a perfect geometric sphere. A system analysis was conducted to explain the crystallization micro-mechanism.
Amorphous to Crystalline Phase Transformation in Metglas® Studied by Mössbauer Spectroscopy
Authors: Marquina V. , Gómez R. , Aburto S. , Jiménez M. , Ridaura R. , Marquina M. L. , Valenzuela R.
Published: April 2011

We present differential scanning calorimetry (DSC) and in situ Mössbauer spectroscopy results for Metglas ribbons, to which different heat treatments were made. The Curie temperature of the amorphous phase is determined and the evolution of the magnetic field of this phase is studied as a function of temperature.
Phases Driven Far From Equilibrium by Applying Mechanical Energy: Phase Transformations to γ-PbSnF4 Upon Ball Milling
Authors: DÉnÉs Georges , Roux Delphine Le , Madamba M. Cecilia
Published: April 2011

The technique of ball milling has been applied to various phases of superionic PbSnF4, namely on (i): highly stressed tetragonal α-PbSnF 4(aq1) obtained by precipitation from aqueous solutions, (ii): highly stressed tetragonal α-PbSnF4(aq2) obtained by reaction of a solid with an aqueous solution, (iii): stressed orthorhombic o-PbSnF4 obtained by precipitation from aqueous solutions, (iv): non-stressed tetragonal α-PbSnF4(ssr) obtained by direct reaction between SnF2 and
PbF2 at high temperature, and on (v): non-stressed tetragonal β-PbSnF4 obtained by direct reaction between SnF2 and PbF2 at high temperature. In all cases, transformation to microcrystalline cubic γ-PbSnF4 is observed very rapidly. This is a unique method for stabilizing high temperature γ-PbSnF4 at ambient temperature, which cannot be done by conventional methods, such as quenching. The phases obtained are totally disordered, microcrystalline, and have the memory of their origin.
Fabrication and Optical Pumping of Laser Cavities Made by Cleaving and Wet Chemical Etching
Authors: Stocker D. , Schubert E. F. , Grieshaber W. , Redwing J. M. , Boutros K. S. , Flynn J. S. , Vaudo R. P.
Published: April 2011

We report on fabrication and characterization of cleaved laser facets and photoelectrochemically wet etched laser facets in III-Nitrides grown by MOVPE on c-plane sapphire. The roughness of the cleaved facets in the InGaN/GaN double heterostructure (DH) laser cavities with a 1000-Å-thick active region is ≈25 rim, while that of the wet etched GaN facets is ≈100 nm. A theoretical model is developed for the maximum allowable laser facet roughness, which yields a value of 18 nrm for uncoated GaN and 22 rm for the uncoated DH. Optically pumped laser action at room temperature is demonstrated in the cleaved DH laser cavities. 2 Above the incident threshold pumping power of 1.3 MW/cm2, the differential quantum efficiency increases by a factor of 34, the emission linewidth decreases to 13.5 meV, and the output becomes highly TE polarized. Wet chemical etching a 1-mm-long laser cavity into the GaN homostructure is found to increase the differential quantum efficiency by a factor of 2.
Luminescence Properties of As, P, and Bi as Isoelectronic Traps in GaN
Authors: Jadwisienczak W. M. , Lozykowski H. J.
Published: April 2011

Photoluminescence spectra of high quality GaN epilayers, grown by MOCVD on sapphire substrates and implanted by isoelectronic ions: As, P, and Bi, were investigated. Post implant annealing was done at temperatures of up to 1150 °C, in a tube furnace under flowing NH3 or N2, and in a rapid thermal annealing system in ambient of N2. The PL of GaN: P annealed at 1150 °C in NH 3 exhibited strong pair-type modulated structures on the short wavelength shoulder of an emission band.The band at 2.914 eV is due to the recombination of bound exciton to P-hole isoelectronic traps (P-BE), and the modulated structure results from electron-hole recombination at pairs of neutral donors and a hole on P isoelectronic traps. The PL of GaN: As, GaN: Bi showed an emission with peaks at 2.597 eV and 3.241 eV, due to the recombination of an exciton bond to As and Bi isoelectronic-hole traps. We also studied thermal quenching excitation spectra, and PL kinetics. The experimental results are discussed using a simple spherical potential-well model for isoelectronic traps: As, P, and Bi replacing nitrogen in GaN.
High Temperature GaN and AlGaN Photovoltaic Detectors for UV Sensing Applications
Authors: Hove J. M. Van , Chow P. P. , Hickman R. , Klaassen J. J. , Wowchak A. M. , Polley C. J.
Published: April 2011

AlGaN photodiode detectors are grown on (0001) sapphire by RF atomic nitrogen plasma molecular beam epitaxy. Both individual detectors and 1 × 10 element arrays are fabricated. The individual detectors have active areas of 0.5 mm2, 1.0 mm2, and 2.0 mm2. Individual elements in the l × 10 detector arrays range in size from 250×250 μm to 450×450 μm. The detectors are fabricated using a chlorine-based reactive ion etch (RIE) and refractory metal ohmic contacts. At room temperature, GaN p-i-n photovoltaic detectors show peak responsivity at 360 nm as high as 0.198 A/W, corresponding to an internal quantum efficiency of 85%. These detectors also exhibit five orders of magnitude of rejection for radiation longer than 500 nm. The electrical and spectral characteristics of these detectors are examined at elevated temperatures. The short wavelength UV responsivity remains fairly constant at elevated temperatures, while the peak responsivity actually increases with increasing temperature. The smooth surface morphology of heavily doped p-type material grown by MBE makes possible diode structures with a p-type bottom layer. The effect of the spectrally broader p-type material in the photodiode responsivity will be discussed.
Ohmic contact to GaN grown by MOCVD
Authors: Kim Dae-Woo , Baik Hong Koo , Kim Cha Yeon , Kim Sung Woo , Hong Chang Hee
Published: April 2011

We investigated the electrical properties and interfacial reactions of the Si/Ti based ohmic contacts to n-GaN grown by metal organic chemical vapor deposition and the electrical properties were related to the material reactions. The perfect ohmic characteristics were obtained after annealing at 700 °C for 3 min under N2 ambient, and Ti silicide was formed in Ti-Si based contact systems. The lowest value for the specific contact resistance of 3.86×10-6 Ωcm2 was obtained for Au(1000 Å)/Ni(400 Å)/Ti(400 Å)/Si(1460 Å)/Ti(150 Å) after annealing at 900 °C for 3min. It could be concluded from the material analyses that the ohmic characteristics of Ti-Si based contact systems were due to the low barrier height by the formation of Ti silicides with a low work function
InGaN/GaN/AiGaN-Based Laser Diodes with an Estimated Lifetime of Longer than 10,000 Hours
Authors: Nakamura Shuji
Published: April 2011

InGaN multi-quantum-well (MQW) structure laser diodes with A10.14Ga0.86N/GaN modulation doped strained-layer superlattice cladding layers grown on an epitaxially laterally overgrown GaN substrate were demonstrated to have an estimated lifetime of more than 10,000 hours under continuous-wave operation at 20°C. Under operation at a high temperature of 50°C, the lifetime was longer than 1,000 hours. The activation energy of the lifetime was estimated to be 0.5 eV. With the operating current increasing above the threshold, a self-pulsation with a high frequency of 3.5 GHz was observed. The relative intensity noise (RIN) less than -145 dB/Hz was obtained even at the 6% optical feedback using the high-frequency modulation of 600 MHz. The threshold carrier density of the InGaN MQW-structure LDs was estimated to be 3 × 1019/cm3 using a carrier lifetime of 1.8 ns.
-Pulsed- Operation of -Cleaved-Facet- InGaN Laser -Diodes-
Authors: Sink R. K. , Abare A. C. , Kozodoy P. , Mack M. P. , Keller S. , Coldren L. A. , DenBaars S. P. , Bowers J. E.
Published: April 2011

Blue-emitting nitride laser diodes have been fabricated on a-plane sapphire (1120). The active region is composed of 10 In.18Ga.82N quantum wells, which were grown by MOCVD at atmospheric and low pressure in a modified two-flow Thomas-Swan Ltd. horizontal reactor. The chemical precursors used were trimethylgallium (TMGa), trimethylindium (TMIn), trimethylaluminum (TMAI), ammonia, and disilane. The n- and p-contacts were formed by depositing Ti/Al/Ni/Au and Ni/Au/Ni/Au, respectively. Diode wafers were thinned to less than 50 μm before they were cleaved along the sapphire r-plane (1120).Lasers show TE polarization, spectral line narrowing, and far field interference patterns above the lasing threshold. The laser emission spectra peak at 410–420 nm. Under pulsed operation at room temperature, the lowest observed threshold current density was 15 kA/cm2 with threshold voltages ranging from 50–90 V. Differential efficiencies are as high as 7% with maximum output powers greater than 50 mW. Near and far field mode patterns are presented. Structures are gain-guided devices with each device occupying a mesa with a width of 125 μm. Device widths range from 3 to 20 μm, with lengths of 500 to 1200 μm.
Detection and Analysis of Phase Separation in Metalorganic Chemical Vapor Deposition InGaN
Authors: Piner E. L. , El-Masry N. A. , Liu S. X. , Bedair S. M.
Published: April 2011

InGaN films in the 0–50% InN composition range have been analyzed for the occurrence of phase separation. The ñ0.5 jum thick InGaN films were grown by metalorganic chemical vapor deposition (MOCVD) in the 690 to 780°C temperature range and analyzed by θ−20 x-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area diffraction (SAD). As-grown films with up to 21% InN were single phase. However, for films with 28% InN and higher, the samples showed a spinodally decomposed microstructure as confirmed by TEM and extra spots in SAD patterns that corresponded to multiphase InGaN. An explanation of the data based on the GaN-InN pseudo-binary phase diagram is discussed.
Vacuum Pressure MOVCD Growth and Characterization of AlN Films On MgO(100), Sapphire, and Si
Authors: Serra A. D. , Magtoto N. P. , Ingram D. C. , Richardson H. H.
Published: April 2011

abstract

Films of AlN were grown on MgO(100), Al2O3, and Si under vacuum pressure (10-3 to 10-4 Torr) at different substrate temperatures. They were examined ex situ with infrared reflectance spectroscopy, scanning electron microscopy, x-ray diffraction and rutherford backscattering spectroscopy. Highly oriented smooth films were grown at film thicknesses below 1 μm. Thicker films showed significantly more roughness but remained oriented with respect to the substrate. AIN growth was faster on Si than MgO(100) or Al2O3 and Si was the only substrate that growth was observed at 500°C.
Phase Separation and Atomic Ordering in AlGaInN Alloys
Authors: Moustakas T. D. , Singh R. , KOrakakis D. , Doppalapudi D. , Ng H.m. , Sampath A. , Iliopoulos E. , Misra M.
Published: April 2011

This paper reviews experimental results of phase separation and atomic long-range ordering in A1GaInN alloys, grown by plasma-assisted MBE. Phase separation has been observed in InxGal-x alloys and atomic ordering has been observed in AlxGa1-xN alloys, using XRD studies. Specifically, we find that alloys with In content in excess of 30% show an extra diffraction peak corresponding to an alloy with high In concentration (close to pure InN). The effect of phase separation on the optical properties of the films was also investigated by transmission and photoluminescence measurements. From these studies, we find that the degree of phase separation depends on the thickness of the alloy layer. Atomic long-range ordering has been observed in AlxGal-xN alloys over the entire alloy composition. The phenomenon was investigated by studying the superlattice peaks (0001), (0003) and (0005). The relative intensity of these peaks was found to be largest for the Al content in the 40–50 % range in qualitative agreement with expectations for an ordered structure of ideal Al0.5Ga0.5N stoichiometry. We found that the ratio of III/V fluxes has a significant effect on the degree of ordering.
Controlling 2d/3d Growth of Gan by Molecular Beam Epitaxy: From Superlattices to Quantum Dots
Authors: Daudin B. , Feuillet G. , Widmann F. , Samson Y. , Rouviere J. L. , Pelekanos N , Fishman G.
Published: April 2011

By controlling the Stranski-Krastanov growth mode of strained GaN deposited by molecular beam epitaxy on AlN, it is shown that islands with nanometric dimensions can be elaborated. The luminescence energy of the wetting layer is 4.6 eV, determined by cathodoluminescence experiments. The luminescence of dots with a mean size of 14 nm in diameter and 2nm in height is 3.8 eV and remains constant in intensity with increasing temperature, as an evidence of 0-dimension confinement effect. Theoretical analysis of the wetting layer luminescence energy is carried out, taking into account the non parabolicity in the conduction band.
Stress Controlled MBE-growth of GaN:Mg and GaN:Si
Authors: Kim Y. , Klockenbrink R. , Kisielowski C. , Krueger J. , Corlatan D. , Sudhir G. S , Peyrot Y. , Cho Y. , Rubin M. , Weber E. R.
Published: April 2011

The stress in GaN thin films grown on sapphire is shown to be determined by lattice mismatch, by differences in thermal-expansion-coefficients and by the incorporation of point defects. It can be controlled by the buffer layer thickness, the buffer layer growth temperature, the V/III flux ratio, and by doping. It is argued that a Fermi-level dependence of defect formation energies affects the material stoichiometry and thereby lattice constants and stresses. We observed that stress relaxation occurred if the stresses exceeded a critical compressive or tensile stress value. The stress changes materials properties. As an example, it is demonstrated that the electron Hall mobility in GaN:Si can be increased with constant electron carrier concentration if large compressive stress is present.
Selective Area Growth of GaN by MOVPE and HVPE
Authors: Hiramatsu K. , Matsushima H. , Shibata T. , Sawaki N. , Tadatomo K. , Okagawa H. , Ohuchi Y. , Honda Y. , Matsue T.
Published: April 2011

Recent successful results on the selective area growth (SAG) of GaN that has been done by MOVPE and HVPE are shown. The SAG were carried out on MOVPE-grown GaN (0001) / sapphire substrates with lined or dotted SiO2 masks. Sub-micron GaN dot and line structures are fabricated by the SAG in MOVPE, so that smoothly overgrown GaN layers are successfully realized using the epitaxially lateral overgrowth (ELO) technique. The ELO structures are confirmed to be good quality GaN single crystal with a smooth surface, no grain boundaries, and low-dislocation densities. In addition, thick GaN bulk single crystals without any cracks are grown by the SAG in HVPE. Crystalline and optical properties of the GaN bulk are much improved. The reduction in the thermal strain due to the growth on the limited area as well as the ELO are found to be effective to reduce crystalline defects of the GaN bulk single crystals.
The Evolution of Nitride Semiconductors
Authors: Akasaki I.
Published: April 2011

The great scientific and commercial success of the group-III nitrides in recent years is the result of persistent fundamental research over a time span of three decades. In the late 60's and in the early 70's the very heart of gallium nitride research was located in J.I. Pankove's laboratory at RCA. There the first single crystalline GaN was grown by Maruska and Tietjen and the very first GaN light emitting diodes were produced by Pankove in September 1971, 26 years ago. Since then the community of nitride research has come a long and troublesome way, but it has succeeded. This 1997 Fall Meeting Symposium on Nitride Semiconductors of the Materials Research Society is dedicated to Professor J.I. Pankove for his outstanding and groundbreaking contributions in the early development of group-III nitride research. This paper reports a historical summary of the evolution of the field summarizing the landmark contributions that have led to the current status of success.
Scanning tunneling microscopy observation of surface reconstruction of GaN on sapphire and 6H-SiC
Authors: Smith A. R. , Ramachandran V. , Feenstra R. M. , Greve D. W. , Neugebauer J. , Northrup J. E. , Shin M. , Skowronski M.
Published: April 2011

We report studies of the surface structure of MBE-grown GaN layers on sapphire (0001) and 6H-(0001) SiC substrates. A different set of reconstructions is observed for nitrogen-face and gallium-face layers. The gallium-face has so far only been grown on MOCVD GaN/ sapphire substrates, while the nitrogen-face has been obtained on SiC and bare sapphire substrates.
Chemical and Structural Analysis of Nitridated Sapphire
Authors: Cho Y. , Rouvimov S. , Kim Y. , Liliental-Weber Z. , Weber E. R.
Published: April 2011

The incorporation of nitrogen into sapphire substrates during nitridation was studied by xray photoelectron spectroscopy (XPS). An increase in the intensity of nitrogen 1s peak in XPS was observed upon longer nitridation. The surface morphology of the substrates was characterized by atomic force microscopy (AFM). High resolution electron microscopy (HREM) was employed for structural analysis. The cross sectional TEM showed a thin layer of AlN buried between amorphous AlNxO1−x and sapphire. This is the first direct observation of AlN on sapphire. The TEM images show a deeper penetration depth of nitrogen into a longer nitridated sapphire.
Effect Of Mg, Zn, Si, And O On The Lattice Constant of Gallium Nitride Thin Films
Authors: Sudhir G. S. , Peyrot Y. , Krüger J. , Kim Y. , Klockenbrink R. , Kisielowski C. , Rubin M. D. , Weber E. R. , Kriegseis W. , Meyer B. K.
Published: April 2011

This study analyzes the impact of most common impurities and dopants on the c lattice parameter for thin films of Gallium Nitride (GaN) deposited on basal plane sapphire. Both Mg (∼1017 cm-3) and Zn (∼3 × 1020 cm-3) doping were found to expand the c lattice parameter as much as +0.38% and +0.62%, respectively. On the contrary, Oxygen up to concentrations 9 1021 cm-3 is shown to replace N in GaN thin films reducing the c parameter only by a small amount. Incorporation of Si leads to a large decrease of the c parameter which can not be attributed to the different size of Ga and Si atom. It is suggested that doping alters the film stoichiometry by a predicted Fermi level dependence of defect formation energies. The impact of stoichiometry on c lattice parameter and the effect of hydrostatic strain on resistivity in undoped and doped GaN is discussed.
Time-Resolved Photoluminescence of GaN / Ga0.93Al0 .07N Quantum Wells
Authors: Lefebvre P. , Allègre J. , Gil B. , Kavokine A. , Mathieu H. , Kim W. , Salvador A. , Botchkarev A. , Morkoç H.
Published: April 2011

The recombination dynamics of excitons in GaN / Ga0.93Al0.07N multiple quantum wells is studied versus lattice temperature. The average decay time of photoluminescence measured at 8K is of ∼330 ps, with a substantial variation of times within the emission line. This is interpreted in terms of carrier localization due to alloy disorder and to well width and depth variations. The radiative lifetime τr of excitons in the wells is found to increase linearly with temperature, with ∂τr / ∂T = 20.5 ± 0.7 ps.K−1. The radiative lifetime of free excitons in the low-temperature limit is deduced to be 2.4 ps, consistent with a longitudinal-transverse splitting ћωLT in GaN of 0.6 meV, in excellent agreement with recent estimations. The ratio of the lifetimes of localized and free excitons is found coherent with the picture of electrons and holes independently localized on short-range defects, instead of excitons localized as a whole on long-range potential fluctuations.
Well Thickness And Doping Effects, And Room Temperature emission mechanisms in InGaN/GaN And GaN/AlGaN Multiple Quantum Wells
Authors: Zeng K. C. , Smith M. , Lin J. Y. , Jiang H. X. , Tang H. , Salvador A. , Kim W. , Morkoc H. , Khan M. Asif
Published: April 2011

Effects of well thickness and Si-doping on the photoluminescence (PL) emission properties of GaN/AlxGa1-xN (x ∼ 0.07) and InxGa1-xN/GaN (x ∼ 0.15–0.2) multiple quantum wells (MQWs) grown both by MOCVD and reactive MBE have been studied. The room temperature emission mechanisms have also been addressed.
Depth-Resolved and Excitation Power Dependent Cathodo-Luminescence of MBE Grown Cubic GaN Epilayers
Authors: As D. J. , Wang C. , Schöttker B. , Schikora D. , Lischka K.
Published: April 2011

We present a detailed investigation of the cathodoluminescence (CL) of MBEgrown cubic GaN (c-GaN) epitaxial layers. Under high excitation conditions the room temperature CL-spectrum is dominated by a single excitonic emission band at about 3.21 eV with a full width at half maximum of 55 meV. Under low electron beam excitation a second, deep emission band at 2.4 eV is observable. A strong variation of the ratio between the deep luminescence intensity and the near band edge luminescence intensity ratio is observed with increasing excitation intensity. Using a simple model based on bimolecular rate equations the concentration of defects involved in this transition is estimated to be about 1015 cm−3. CL-measurements with varying excitation intensity reveal that these recombination levels have only minor influence on the performance of high injection optoelectronic devices like laser diodes. Depth resolved measurements and CL-images from a cleaved c-GaN/GaAs reveal that the deep recombination centers are homogeneously distributed within the epilayer, however the concentration of nonradiative recombination centers is increasing towards the GaN/GaAs interface.
Cross-sectional cathodoluminescence of GaN epitaxial films
Authors: Zaldivar M. Herrera , Fernández P. , Piqueras J.
Published: April 2011

Cathodoluminescence (CL) in the scanning electron microscope has been used to investigate the variation along the growth axis direction of luminescence emission from epitaxial GaN films. CL spectra recorded at different positions of the sample cross-section as well as monochromatic CL images show strong spatial variations of the different luminescence emissions along the growth axis. At the buffer layer-substrate interface and at the top part of the sample, which corresponds to a Si doped epilayer, enhanced CL emission is observed as compared with the relatively low emission in the central region of the cross-section. The nature of the defects responsible for the observed CL distribution is discussed.
Luminescence Of A New Material: GaN Grown On NdGaO3
Authors: Korona K. P. , Pakula K. , Wysmolek A. , Baranowski J. M. , Bergman J. P. , Monemar B. , Łukasiewicz T. , Łuczyński Z.
Published: April 2011

It is shown that heteroepitaxial GaN layers grown on NdGaO3, in spite of a very high conductivity (˜ 10 Ω−1cm−1) have very efficient luminescence properties. It is shown that a high electrical conductivity is caused by contamination of GaN layers with oxygen. Efficient emission due to donor bound excitons (at hv = 3.475 eV), free excitons and free electron – hole recombination have been identified. The total PL emission in the exciton region exceeds the intensity from the homoepitaxial GaN layers. It is argued that a high oxygen concentration eliminates nonradiative channels connected with point defects, leading to efficient radiative recombination.
Excitonic Enhanced Optical Gain Of GaN/AlGaN Quantum Wells With Localized States
Authors: Uenoyama Takeshi
Published: April 2011

We have evaluated the optical gain of GaN/AlGaN quantum well structures with localized states, taking into account the Coulomb interaction. The localized states axe introduced in the well as quantum dot-like subband states. We have used the temperature Green's function formalism to treat the many-body effects and have found a new excitonic enhancement of the optical gain involved the localized states. This enhancement is stronger than the conventional Coulomb enhancement. It might play an important role to reduce the threshold carrier density.
Dislocations In GaN/Sapphire: Their Distribution And Effect On Stress And Optical Properties
Authors: Jain S. C. , Pinardi K. , Maes H. E. , overstraeten R. Van , Willander M. , Atkinson A.
Published: April 2011

Recent Raman experiments show that strain in GaN layers grown on the c-plane of sapphire is low at the surface, increases with depth, and is maximum at the interface. We show that this stress distribution can be attributed to the presence of a large number of threading dislocations. GaN photonic devices work in spite of large density of dislocations present in the GaN epilayers. We describe a model which shows that the effect of saturation of traps in the dislocations with increasing number of injected carriers can explain the high performance of the GaN devices containing the dislocations.
Effects of Substrate Orientation on the Valence Band Splittings and Valence Band Offsets in GaN and AlN Films
Authors: Majewski J. A. , Städele M.
Published: April 2011

We present a first-principles study of heteroepitaxial interfaces between GaN and both cubic as well as wurtzite AlN substrates oriented along main cubic or hexagonal directions and of stacking fault interfaces between cubic and wurtzite GaN. Our calculations show that all studied heterostructures are of type I. Valence band offsets for GaN/AlN are nearly independent of the substrate orientation and of the order of 0.8 eV. The valence and conduction band offsets for a stacking fault interface are predicted to be 40 meV and 175 meV, respectively.
Phase Separation in InGaN/GaN Multiple Quantum Wells
Authors: Mccluskey M. D. , Romano L. T. , Krusor B. S. , Bour D. P. , Chua C. , Johnson N. M. , Yu Kin Man
Published: April 2011

Evidence is presented for phase separation in In0.27Ga0.73N/GaN multiple quantum wells (MQW's). After annealing for 4 min at a temperature of 1100 °C, the absorption threshold at 2.95 eV is replaced by a broad peak at 2.65 eV. This peak is attributed to the formation of Inrich InGaN phases in the active region. X-ray diffraction measurements show a shift in the diffraction peaks toward GaN, consistent with the formation of an In-poor phase.
Photoluminescence of Wet- and Dry-Etched Gallium Nitride
Authors: Reuter E. E. , Youtsey C. , Adesida I. , Bishop S. G.
Published: April 2011

The photoluminescence (PL) of GaN grown on SiC is studied as a function of etch depth for two types of etches, photoelectrochemical and chemically assisted ion beam. It is found that as the etch proceeds deeper toward the substrate, the PL exhibits an increasing blue-shift and an increase in emission intensity of the donor acceptor pair band, which indicates increasing biaxial compressive stress and increasing impurity concentration near the substrate. The PL spectra of the dry etched GaN tended to have slightly higher intensities than comparably wet etched GaN.
Comparison Of Dry-Etch Techniques For Gan, Inn, And Ain
Authors: Shul R. J. , Vawter G. A. , Willison C. G. , Bridges M. M. , Lee J. W. , Pearton S. J. , Abernathy C. R.
Published: April 2011

Fabrication of group-III nitride devices relies on the ability to pattern features to depths ranging from ∼1000 Å to > 5 μm with anisotropic profiles, smooth morphologies, selective etching of one material over another, and a low degree of plasma-induced damage. In this study, GaN etch rates and etch profiles are compared using reactive ion etch (RIE), reactive ion beam etching (RIBE), electron cyclotron resonance (ECR), and inductively coupled plasma (ICP) etch systems. RIE yielded the slowest etch rates and sloped etch profiles despite dc-biases > −900 V. ECR and ICP etching yielded the highest rates with anisotropic profiles due to their high plasma flux and the ability to control ion energies independently of plasma density. RIBE etch results also showed anisotropic profiles with slower etch rates than either ECR or ICP possibly due to lower ion flux. InN and AIN etch characteristics are also compared using ICP and RIBE.
SiC Epitaxial Growth on Carbon
Authors: Khlebnikov I. , Madangarli V. , Sudarshan T. S.
Published: April 2011

The possibility of single crystal SiC expitaxial growth on freestanding amorphous carbon films (500–1000 Å) as well as thin amorphous carbon layers deposited on mono-crystalline SiC seeds, by conventional physical vapor transport (PVT) technique, is demonstrated. Preliminary experiments indicate that under certain specific growth conditions, 3D SiC single crystals (100 – 600 Å) of different polytypes can be grown on freestanding amorphous carbon layers, with more or less equal probability of formation for each polytype. On the other hand, under low axial temperature gradients (< 30°C/cm), the SiC epitaxial growth on carbon is amorphous in nature. Also, experimental results that demonstrate two-dimensional single crystal SiC epitaxial growth on an amorphous carbon film deposited on mono-crystalline 6H-SiC wafer, is presented. Experiments performed in our laboratory indicate that monocrystalline SiC growth is possible on amorphous carbon layers upto 0.1 μm thickness.
Selective Etching Of Wide Bandgap Nitrides
Authors: Shul R. J. , Willison C. G. , Bridges M. M. , Han J. , Lee J. W. , Pearton S. J. , Abernathy C. R. , MacKenzie J. D. , Donovan S. M.
Published: April 2011

High-density plasma etching has been an effective patterning technique for the group-III nitrides due to ion fluxes which are 2 to 4 orders of magnitude higher than more conventional reactive ion etch (RIE) systems. GaN etch rates exceeding 0.68 μm/min have been reported in Cl2/H2/Ar inductively coupled plasmas (ICP) at -280 V dc-bias. Under these conditions, the etch mechanism is dominated by ion bombardment energies which can induce damage and minimize etch selectivity. High selectivity etch processes are often necessary for heterostructure devices which are becoming more prominent as growth techniques improve. In this study, we will report high-density ICP etch rates and selectivities for GaN, AIN, and InN as a function of cathode power, ICP-source power, and chamber pressure. GaN:AIN selectivities > 8:1 were observed in a Cl2/Ar plasma at 10 mTorr pressure, 500 W ICP-source power, and 130 W cathode rf-power, while the GaN:InN selectivity was optimized at ∼ 6.5:1 at 5 mTorr, 500 W ICP-source power, and 130 W cathode rf-power.
Achievement Of Coalesced Oriented Diamond Films On Nickel By Optical Process Control And Methane Enrichment
Authors: Yang P. C. , Wolden C. A. , Liu W. , Schlesser R. , Davis R. F. , Prater J. T. , Sitar Z.
Published: April 2011

Real time in-situ laser reflectometry was used to investigate changes in surface morphology observed during the nucleation of oriented diamond on Ni in a hot filament chemical vapor deposition reactor. Characteristic features observed in the intensities of reflected and scattered light were interpreted by comparison with scanning electron micrographs of the diamond seeded substrates quenched at sequential stages of the process. Based on this analysis, a process was developed in which the scattered light signal was used as a steering parameter. Using this process, oriented nucleation and growth of diamond on Ni can be repeatedly achieved. For the purpose of both countering the loss of carbon atoms and to maximize the density of oriented diamond, different concentrations of methane were added during the high temperature annealing stage to find the optimized conditions. It was found that 0.5% methane in the gas phase produced the best degree of orientation and uniformity. Substrates nucleated at these conditions were grown out into coalesced, 30 μm thick films. Both, (100) and (111) oriented films showed a high degree of orientation. Raman spectra obtained from each orientation showed an intense and narrow diamond signature peak with negligible sp2 carbon or background luminescence.
Surface And Interface Study Of Pdcr/Sic Schottky Diode Gas Sensor Annealed At 425°C
Authors: Chen Liang-Yu , Hunter Gary W. , Neudeck Philip G. , Knight Dak
Published: April 2011

The surface and interface properties of a Pd0.9Cr0.1/SiC Schottky diode gas sensor both before and after annealing are investigated using Auger electron spectroscopy (AES), scanning electron spectroscopy (SEM), and energy dispersive spectroscopy (EDS). At room temperature the alloy reacted with SiC and formed PdxSi only in a very narrow interfacial region. After annealing for 250 hours at 425 °C, the surface of Schottky contact area has much less silicon and carbon contamination than that found on the surface of an annealed Pd/SiC structure. PdxSi formed at a broadened interface after annealing, but a significant layer of alloy film is still free of silicon and carbon. The chromium concentration with respect to palladium is quite uniform down to deep interface region. A stable catalytic surface and a clean layer of Pd0.9Cr0.1film are likely responsible for significantly improved device sensitivity.
The Structural Evolution Of Lely Seeds During The Initial Stages Of Sic Sublimation Growth
Authors: Heydemann V. D. , Sanchez E. K. , Rohrer G. S. , Skowronski M.
Published: April 2011

The as-grown surfaces of 6H SiC seed crystals grown by the Lely method have been characterized by visible light microscopy and atomic force microscopy. The surfaces were then used to seed the sublimation growth of SiC at 2300°C, in 650 torr of Ar, with a gradient of 8 °C/cm. Repeated observations of the seed surface structure after predetermined intervals in the growth reactor demonstrated that transport starts at less than 2160 °C and that voids are nucleated by heterogeneous material on the original seed surface. Near these voids, the concentration of unit screw dislocations is increased and these defects migrate during growth. Screw dislocations of opposite sign act as step sources and build mesa structures around the heterogeneous material. After only 10 min at the growth temperature, micropipe/super screw dislocation complexes form in the centers of the mesas, possibly by the migration of unit dislocations to the voids.
High Temperature Characterization Of Ni, W And Al Contacts To 3C-Silicon Carbide Thin Films
Authors: Jacob C , Pirouz P , Kuo H-I , Mehregany M
Published: April 2011

With the current availability of large-area 3C-SiC films, it is imperative that stable high temperature contacts be developed for high power devices. By comparing the existing data in the literature, we demonstrate that the contact behavior on each of the different polytypes of SiC will vary significantly. In particular, we demonstrate this for 6H-SiC and 3C-SiC. The interface slope parameter, S, which is a measure of the Fermi-level pinning in each system varies between 0.4–0.5 on 6H-SiC, while it is 0.6 on 3C-SiC. This implies that the barrier heights of contacts to 3C-SiC will vary more significantly with the choice of metal than for 6H-SiC.

Aluminum, nickel and tungsten were deposited on 3C-SiC films and their specific contact resistance measured using the circular TLM method. High temperature measurements (up to 400°C) were performed to determine the behavior of these contacts at operational temperatures. Aluminum was used primarily as a baseline for comparison since it melts at 660°C and cannot be used for very high temperature contacts. The specific contact resistance (ρc) for nickel at room temperature was 5 × 10−4 Ω-cm2, but increased with temperature to a value of 1.5 × 10−3 Ω-cm2 at 400°C. Tungsten had a higher room temperature × 10−3 Ω-cm2, which remained relatively constant with increasing temperature up to 400°C. This is related to the fact that there is hardly any reaction between tungsten and silicon carbide even up to 900°C, whereas nickel almost completely reacts with SiC by that temperature. Contact resistance measurements were also performed on samples that were annealed at 500°C.