bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical Extraction of The in-Plane Dielectric Constant of Fluorinated Polyimide
Authors: Loke Alvin L.S. , Wetzel Jeffrey T. , Stankus John J. , Simon Wong S.
Published: April 2011

Fluorinated polyimide can potentially replace TEOS as an interlevel dielectric in future ULSI interconnect technologies because its lower dielectric constant offers reduced crosstalk, signal propagation delay, and dynamic power dissipation. One issue associated with polyimides is the anisotropy in dielectric constant, where the smaller out-of-plane dielectric constant, typically measured using parallel-plate capacitors, can misleadingly exaggerate the advantage in reducing crosstalk. In this paper, we present a novel electrical technique to estimate the in-plane dielectric constant of DuPont FPI-136M fluorinated polyimide without requiring dielectric gapfill.

A blanket FPI-136M film is deposited over interdigitated inlaid Al(0.5%Cu) structures and the crosstalk capacitance is measured. Identical inlaid structures with air and TEOS passivations are also measured for capacitance calibration. Differences in measured capacitances reflect electric fields fringing in the various passivation dielectrics above the inlaid metal. With the known dielectric constants of air and TEOS, the effective dielectric constant of FPI-136M is interpolated to be 2.8. Interconnect simulations confirm that the effective dielectric constant extraction technique is valid and accurate provided that the passivation layer is sufficiently thick to contain the fringing fields.

To estimate the in-plane dielectric constant, we use simulations to determine the combination of in-plane and out-of-plane dielectric constants that is equivalent to the extracted effective dielectric constant. With an out-of-plane dielectric constant of 2.6, the in-plane dielectric constant of FPI-136M is estimated to be 3.0. This technique is applicable to other dieletrics.
New Methodologies for Characterization of Low Dielectric Constant Thin Films
Authors: Todd Ryan E. , Cho Taiheui , Malik Irfan , Zhao Jie-Hua , Lee J.K. , Ho Paul S.
Published: April 2011

New methodologies are being developed in our laboratory that allow measurements of the Young's modulus (E) and thermal expansion coefficient (TEC) for materials that cannot be prepared as free standing films. The first employs a bending beam measurement of stress versus temperature. We measure the thermal stress of a given film on two substrates, Si and GaAs. The use of two substrates allows the determination of both E and TEC. A TEOS film is used to demonstrate the technique. Another technique is a variation of the stress-strain measurement of E. The brittle material of interest is coated onto a polymer substrate, and the composite film is elongated to obtain the stress-strain curve of the composite film. Subtraction of the polymer substrate contribution to this curve yields the stress-strain behavior of the brittle film and allows calculation of E.

The in-plane dielectric constants of potential interlayer dielectrics (ILD) are determined from the interline capacitance between metal lines passivated with the material. We have developed a finite difference simulation to account for the fringe capacitance, and it allows the extraction of the in-plane dielectric constant from the measured capacitance. The method was used to measure the anisotropies in two polymer ILDs. A flexible chain polymer showed the same anisotropy in both a blanket film and on patterned lines, but a rigid rod-like polymer was found to be much less anisotropic on the patterned lines relative to the blanket film. This result indicates that the molecular orientation of the film is different on the patterned lines.
Diagnostics, Mechanisms of Deposition, and Process Control of Thin Fluoropolymeric Films in RF Discharges
Authors: d'Agostino R.
Published: April 2011

Plasma deposited fluoropolymer films are shown to have composition, chemical structures and properties variable continuously in a very broad range. Films obtained under high fragmentation and structure retention conditions are discussed and compared. The mechanism of deposition is shown to explain etching-deposition competition. Finally, actinometric spectroscopy is discussed briefly as a semi-quantitative diagnostics tool enabling the control of the chemistry of the process under high fragmentation conditions‥
Novel Approaches to Linear Polyimides
Authors: Marasco J. P. , Garapon J. , Sillion B.
Published: April 2011

Polyamic amides or polyamic esters are alternative polyimides precursors exhibiting a better storage-stability than classical polyamic acids. Two original synthetic routes to these polymers were investigated through either diisoimides and diimidazolides in order to enhance stability and to obtain pure para catenation.
Thermal Stability of a-C:F,H Films Deposited by Electron Cyclotron Resonance Plasma Enhanced Chemical Vapor Deposition
Authors: Theil Jeremy A. , Mertz Francoise , Yairi Micah , Seaward Karen , Ray Gary , Kooi Gerrit
Published: April 2011

Amorphous carbon films grown with fluorohydrocarbons can be grown to have dielectric constant values around 2.0. The behavior of these films when subjected to thermal excursion is studied. We have investigated material deposited in an ECR plasma, and find that the F:H ratio of the gas mixture is a good guide to material properties. Films deposited at 5°C were placed in a vacuum chamber at 400°C as long as 60 minutes. The film thickness, dielectric constant, and infrared absorption spectrum change with the F:H ratio of the incoming gas and thermal cycling. It was found that the dielectric constant and loss tangent decrease upon heating and that there is an apparent increase in C=C groups. As expected, as the F:H ratio increases, the dielectric constant and thermal stability decrease. Good thermal stability is shown for F:H ratios of 1.5, which result in films with a dielectric constant of ∼2.4 after heating.
Low K Sog Etchback and Teos Oxide Capping
Authors: Huang K.G. , Cheung D. , Nguyen D. , Forester L.
Published: April 2011

SOG etchback was studied using Applied Materials' 200mm MxP+ etch chamber on Allied Signal's SOG (spin-on-glass) T11 and low κ SOP (spin-on-polymer) 418 together with liner films of PE-oxide and oxynitride for IMD (inter-metal dielectric) planarization. Excellent chamber performance and wide process window were demonstrated. Etch selectivities between various SOG and liner films are tunable in a wide range allowing optimization of IMD planarization.
In-Situ Transmission Electron Microscopy of the Solid-Phase Epitaxial Growth of GaAs: Sample Preparation and Artifact Characterization
Authors: Belay K. B. , Ridgway M. C. , Llewellyn D. J.
Published: April 2011

In-situ transmission electron microscopy (TEM) has been used to characterize the solidphase epitaxial growth of amorphized GaAs at a temperature of 260°C. To maximize heat transfer from the heated holder to the sample and minimize electron-irradiation induced artifacts, non-conventional methodologies were utilized for the preparation of cross-sectional samples. GaAs (3xI) mm rectangular slabs were cut then glued face-to-face to a size of (6x3) mm stack by maintaining the TEM region at the center. This stack was subsequently polished to a thickness of ~ 200 ýtm. A 3 mm disc was then cut from it using a Gatan ultrasonic cutter. The disc was polished and dimpled on both sides to a thickness of ~15 mimT.h is was ion-beam milled at liquid nitrogen temperature to an electron-transparent layer. From a comparison of in-situ and ex-situ measurements of the recrystallization rate, the actual sample temperature during in-situ characterization was estimated to deviate by ≤ 20°C from that of the heated holder. The influence of electron-irradiated was found to be negligible by comparing the recrystallization rate and microstructure of irradiated and unirradiated regions of comparable thickness. Similarly, the influence of “thin-foil effect” was found to be negligible by comparing the recrystallization rate and microstructure of thick and thin regions, the former determined after the removal of the sample from the microscope and further ion-beam milling of tens of microns of material. In conclusion, the potential influence of artifacts during in-situ TEM can be eliminated by the appropriate choice of sample preparation procedures.
Phase Separation: From the Initial Nucleation Stage to the Final Ostwald Ripening Regime
Authors: Sagui Celeste , O'Gorman Dean Stinson , Grant Martin
Published: April 2011

In this work we have re-examined the classical problem of nucleation and growth. A new model considers the correlations among droplets and naturally incorporates the crossover from the early-stage, nucleation dominated regime to the scaling, late-stage, coarsening regime within a single framework.
Time Dependent Nucleation in a Bulk Metallic Glass Forming Alloy
Authors: Croat T. K , Kelton K. F.
Published: April 2011

The effect of composition on the time-dependent nucleation rates in Zr65Al7.5Ni10CU17.5 glasses is investigated to better understand nucleation processes in partitioning systems. Asquenched glasses were annealed to produce a homogeneous dispersion of nanocrystals within the amorphous matrix. The nucleation rates were estimated from the number of crystallites produced as function of annealing time, using scanning and transmission electron microscopy. Experimental results for single and multiple-step annealing treatments are presented. The nucleation results are discussed briefly within the time-dependent model of the classical theory of nucleation.
Computer Simulations of the Chessboard-Like Microstructure Formation in Decomposing Alloys
Authors: Bouar Y. Le , Khachaturyan A. G.
Published: April 2011

The mechanism of formation of a chessboard-like structure observed during coherent decomposition in alloys with several orientation variants of the precipitate phase is considered. It is based on the accommodation of coherency strain achieved by spatial rearrangement of orientation variants of the precipitate phase in an optimal pattern. The computational model of precipitation of the ordered tetragonal phase, based on the continuum stochastic field kinetic equations for the composition and lro parameters profiles, is formulated. Its numerical solution describes the spatial and temporal evolution of the microstructure, from nucleation to coarsening. It is shown that the chessboard-like microstructure is produced by strain-driven, self-assembling of orientation variants of the precipitate phase, faceted by the planes normal to the elastically soft directions. Coarsening of such microstructures occurs by consequent disappearance of entire rows of “tiles” in the pattern.
Approximations for Vibrational Thermodynamics of Disordered Alloys: Effective Supercells and the Quasiharmonic Method
Authors: De Fontaine Didier , Althoff Jeffrey , Morgan Dane , Asta Mark , Foiles Stephen , Quong Andrew , Johnson Duane
Published: April 2011

Recent work has suggested that vibrational effects can play a significant role in determining alloy phase equilibria. In order to better understand these effects and the methods used in their calculation, we investigate the vibrational properties of disordered Ni3A1 using the Embedded-Atom Method. We examine the effectiveness of the Special Quasirandom Structure (SQS) approximation, and find that an SQS-8 can accurately represent the vibrational thermodynamics of the disordered state. By the use of Monte Carlo (MC) techniques, we also find that the quasiharmonic approximation becomes less accurate as we approach the melting temperature, but that the accuracy may be extended to higher temperatures by resorting to the MC equation of state giving the specific volume as a function of temperature.
Competition Between SRO and LRO in CuAu3
Authors: Kompatscher M. , Lang H. , Püschl W. , Pfeiler W.
Published: April 2011

Ordering in CuAu3 was investigated by residual resistivity and X-ray scattering. L12 longrange ordering (LRO) in this alloy is quite sluggish, therefore for low quenching temperatures and sufficiently short annealing times only effects of short-range order (SRO) are observed. With increasing quenching temperature and/or increasing annealing time evolution of LRO was observed by both residual resistometry and X-ray scattering. From the resistivity measurements information can be gained on the boundaries of the order-disorder two-phase field. An analysis of SRO-relaxation yields an ordering activation energy of 2.1±0.1 eV.
Reentrance and Multistriped Phases in the Blume-Emery-Griffiths Model with Biased Diffusion
Authors: Seshaaiyar Geetha , McKay Susan R.
Published: April 2011

We report new multistriped phases at high densities in a three-state model (Si = 0, ± 1) driven by an external field E and influenced by repulsive nearest neighbor interactions. The dynamics are dictated by local Metropolis rates with the ratio of particle-particle to particle-hole exchange given by γ. In the noninteracting limit, this model shows an ordered, low-current phase above a density-dependent critical field. In this phase the two types of charges are segregated from each other and from the vacancies, forming stripes transverse to the field direction. Addition of a repulsive bilinear interaction J favors mixing of the charges. This interaction competes directly with the charge-segregated ordering and the system undergoes a nonequilibrium transition to one of four other types of striped phases, some with checkerboard ordering. We have calculated the staggered charge density and used the amplitude of its longest wavelength Fourier component along the field direction as an order parameter to characterize this new type of striped ordering. The phase diagram in the parameter space of E, J, γ, and density shows both first and second order transitions, with second order transitions most prevalent in the high-E/J, high density regions. Our study also shows a reentrant phase diagram with only second order transitions for high γ and a tricritical topology at low γ. For field strengths close to zero, the system orders in a checkerboard fashion above a J-dependent density. Results were obtained using Monte Carlo simulations on a 30×30 square lattice with periodic boundary conditions and charge conservation.
Transitory Metastable Phases in Refractory Metals-Based Systems Solidified by Drop-Tube Processing: A First-Principles Approach
Authors: Pasturel A. , Vinet B.
Published: April 2011

Refractory metals and alloys are deeply undercooled in an ultrahigh vacuum drop-tube facility. During rapid solidification, some brightness traces give evidences of two successive phase transformations. First-principles calculations of the structural stability in these systems are developed to appreciate the possibilities of obtaining metastable phases.
Diffusion via native defects, and the appropriate choice of independent thermodynamic variables in both quasi-equilibrium and nonequilibrium experimental designs
Authors: Cohen R. M.
Published: April 2011

It is often incorrectly assumed that temperature alone defines the equilibrium state of a crystal. However, the Gibbs phase rule shows that the number of independent thermodynamic variables required to define the equilibrium state depends upon the experimental design. In most practical cases, this means that temperature and components of the external phase or phases in proximate contact with the sample will determine the equilibrium state, e.g., the equilibrium native defect concentrations, impurity solubility, etc. If native defect concentrations approach their equilibrium values in a time which is short compared to the time of an experiment, then impurity diffusion can be described well by a thermodynamic model and impurity diffusion is analogous to classic tracer diffusion. If native defect concentrations require a long time to approach their equilibrium values, then diffusivities will exhibit significant time dependence and simple models do not apply. However, multi-phase, multi-component systems generally have several possible equilibrium regions within their phase diagram. Relating the phase diagram to a given experimental design allows one to qualitatively understand how the native defect concentrations change as a crystal in a nonequilibrium state relaxes toward one (of several possible) well-defined equilibrium states. Examples will focus largely on the diffusion of impurities in single crystal GaAs. Evidence will be presented that native defect concentrations can rapidly approach equilibrium in a limited group of experimental designs. We shall show why a description of diffusivity in terms of carrier concentration, a dependent thermodynamic variable, sometimes succeeds but often fails. Examples of commonly used experimental designs, in which there is inadequate control over the independent variables, will demonstrate some large variations in measured diffusivity. An enormous range of reported diffusivities, in GaAs covered by SiO2 or Si3N4 encapsulant layers, will demonstrate the difficulty and the futility of measuring diffusion when the solid is cut off from the external phases which define the equilibrium state.
Electric-Field Effects on Reactions Between Oxides
Authors: Johnson Matthew T. , Gilliss Shelley R. , Carter C. Barry
Published: April 2011

Thin films of In2O3 and Fe2O3 have been deposited on (001) MgO using pulsed-laser deposition (PLD). These thin-film diffusion couples were then reacted in an applied electric field at elevated temperatures. In this type of solid-state reaction, both the reaction rate and the interfacial stability are affected by the transport properties of the reacting ions. The electric field provides a very large external driving force that influences the diffusion of the cations in the constitutive layers. This induced ionic current causes changes in the reaction rates, interfacial stability and distribution of the phases. Through the use of electron microscopy techniques the reaction kinetics and interface morphology have been investigated in these spinel-forming systems, to gain a better understanding of the influence of an electric field on solid-state reactions.
Crystallography of Hydride Formation in Zr-Nb Alloys
Authors: Srivastava D. , Dey G. K. , Banerjee S. , Ranganathan S.
Published: April 2011

The crystallographic and morphological aspect associated with the formation of γ hydride phase (fct) from the β phase in β abilized Zr-20%Nb alloy has been reported. In this paper the βto γ transformation has been considered in the terms of the phenomenological theory of martensitic crystallography in order to predict the crystallographic features of the γ hydride in the β to γ transformation. The prediction made in the present analysis has been found to match very closely to the experimentally observed habit plane. The possibility of the α to γ transition through the formation of a transient β configuration has been examined.
Effects of Aging and Thermal Cycling Treatments on the Damping Capacity of Cu-Mn Alloys
Authors: Chung T. S. , Jun J. H. , Lee Y K.
Published: April 2011

The effects of aging and thermal cycling treatments on the damping capacity and microstructure of Cu-(47, 55, 65)%Mn alloys and Cu-65%Mn-4.5%Ni alloy have been studied using an optical microscope, a transmission electron microscope (TEM), and a cantilever type damping measuring apparatus. The maximum damping capacity appeared after aging at 400°C for 18 hours in Cu-47%Mn, 8 hours in Cu-55%Mn, 4 hours in Cu-65%Mn, and 36 hours in Cu-65%Mn-4.5%Ni alloy, respectively. Storage at 100°C slightly reduced damping capacity of these alloys, and the higher the Mn content, the smaller the amount of the degradation. This result might be ascribed to α-Mn precipitation inside microtwins in Cu-65%Mn alloy, and to the microstructural change from tweed structure to mottled structure in the other alloys. The thermal cycling treatment between room temperature (20°C) and 250°C led to an increase in damping capacity, which is probably due to the refinement of microstructure(tweed or twin).
Amorphization Of Zr-Al Under Mechanical Alloying At Different Temperatures: A Re-Entrant Melting Phenomenon
Authors: Sheng H. W. , Lu K. , Ma E.
Published: April 2011

Zr100−xAlx powder blends have been subjected to ball milling at different temperatures to investigate the amorphization process. At low temperatures the Zr-Al solid solutions amorphized under the polymorphous constraints, whereas at higher temperatures there was an obvious twophase coexistence region. The Al concentration for the complete amorphization of Zr-Al increased with increasing temperature, suggesting a re-entrant melting behavior. Both of the temperature- and composition-dependent amorphization mechanisms are analyzed in terms of the thermodynamic properties of the phases involved, as well as the dynamic effects brought in by the non-equilibrium milling process.
Microstructure of Ni2B Laser-Induced Surface-Alloyed α-Fe
Authors: Dehm G. , Scheu C. , Bamberger M.
Published: April 2011

Substrate surfaces of α-Fe were laser alloyed with a CW-CO2laser using Ni2B powder. Single laser scans and laser scans with a 50% overlap between successive passes were carried out to obtain hard surface coatings on the α-Fe substrate. High hardness values in the range of 520 - 600 Hv1 were achieved by laser boriding of α-Fe, compared to a substrate hardness of ∼ 110 Hv1. The improvement in hardness is caused by an off-eutectic microstructure, which consists of Fe dendrites and Fe3B and Fe lamellae. Both phases, Fe3B and Fe, contain Ni in solid solution. For samples treated with a single laser scan the Fe phase solidifies in the bcc structure. In contrast, a 50 % overlap between successive laser passes causes the iron to crystallize in the fcc γ-phase. The difference in crystallographic structure is caused by the amount of Ni dissolved in Fe during laser alloying: only ∼ 9 at.% Ni is contained in the α-Fe phase after a single laser scan, while up to ∼ 40 at.% of Ni is dissolved in the γ-Fe phase after a 50 % overlap between each laser scan due to the nearly complete melting of all injected Ni2B particles. However, the Fe3B phase solidifies during laser alloying independent of the amount of Ni in the orthorhombic Fe3C structure, which is metastable for Fe contents exceeding 45 at.%.
Atomic Scale Study Of Precipitate / Matrix Interfaces in a Metallic Alloy
Authors: Rozdilsky I. , Cerezo A. , Smith G. D. W. , Watson A.
Published: April 2011

The study of phase transformations involves making assumptions about interfaces within a material in order to apply mathematical models. An example of this is the Gibbs classical theory of nucleation and growth which is based upon the assumption that the precipitate / matrix interface is sharp. Recent developments in atom probe microscopy have made it possible for the first time to characterize complex three-dimensional internal interfaces within materials to sub-nanometre accuracy. We have used the OPoSAP (Optical Position Sensitive Atom Probe) to characterize the precipitate / matrix interface in the dilute Cu-Co system which is a model alloy for the study of homogeneous nucleation and growth. Interface widths derived from radial composition profiles were measured to be 0.9 ± 0.2 nm in size. The effects of thermal energy, positioning inaccuracies, and statistical limitations on the measurement of interface widths are examined through the use of mathematical models and computer simulations. Similar widths are measured for precipitates in the coarsening regime which shows the interface velocity does not affect the interface width.
Interfacial Reaction in Aluminum/Carbon Multilayer Thin Films
Authors: Landry K. , Sieber H. , Sui M. , Perepezko J. H.
Published: April 2011

The reaction at the interface between Al and amorphous C in Al/C multilayer thin films with modulation wavelengths of about 25nm and 125nm has been investigated by differential scanning calorimetry, X-ray diffraction, transmission electron microscopy/selected area electron diffraction and high resolution transmission electron microscopy. The reaction was found to take place in two steps. The first step onsets at about 300°C, and was identified as the diffusion of C into Al. The second step starts above 400°C, at a temperature strongly dependent on the modulation wavelength of the film, and is the formation of A14C3. The carbide has been observed to nucleate and grow inside the Al layers. The multilayer structure is preserved in the samples up to at least 550°C, and Al grains start to grow at or below 300°C.
In-Situ UHV Tem Investigations of the Initial Oxidation Stage of Copper Thin Films
Authors: Yang J. C. , Yeadon M. , Kolasa B. , Gibson J. M.
Published: April 2011

The nucleation and growth of Cu2O due to oxidation of Cu(001) films were monitored at various temperatures and oxygen partial pressures. For all examined temperatures and pressures, Cu2O islands were observed to form epitaxially with respect to the copper film. The nucleation of these oxide islands was homogeneous –no clear evidence was observed for either steps or dislocations being preferential nucleation sites. Based on this data, we have developed a semiquantitative model of the initial oxidation stage where the dominant mechanism for transport, nucleation and growth of oxide islands is oxygen diffusion on the Cu surface. We are presently comparing our experimental data with nucleation rate theory, where the predictions qualitatively describe our observations, but not quantitatively.
The Formation of Cu3Si from Cu/a-Si Multilayers
Authors: Chromik R. R. , Neils W. K. , Cotts E. J.
Published: April 2011

The kinetics of the formation of Cu3Si in Cu/a-Si diffusion couples have been investigated by means of differential scanning calorimetry and x-ray diffraction. Multilayered composites of average stoichiometry Cu3Si were prepared by sputter deposition with individual layer thicknesses varying in different samples between 2 and 100 nm. We observed diffusion limited growth of Cu3 Si upon annealing these diffusion couples below 500 K. Reaction constants were measured for a temperature range of 455 to 495 K for thicknesses of growing Cu3Si between 2.6 and 80 nm. The temperature dependence of the reaction constant, k2, was characterized as k2 = k0 exp(− Ea/kbT) with activation energy, Ea = 1.0 eV/atom and pre-factor, k0 = 1.9×10−3 cm2/s.
Metastable Phase Formation in Mechanically Alloyed Sm-Co-Fe
Authors: Smith P. A. I. , Ding J. , McCormick P. G. , Street R.
Published: April 2011

A detailed phase analysis of mechanically alloyed (Sm0.18Co0.82)100-xFex powders has been performed using X-ray diffraction and Mössbauer spectroscopy. A two-phase structure develops as the Fe content is increased, with an increasing proportion of bcc Fe-Co in addition to amorphous Sm-Co-Fe. Both phases become richer in Fe, but Fe is concentrated in the bcc phase, due to a limited ability of Fe to substitute in amorphous Sm-Co. Changes in phase formation with increasing Fe content can be correlated with changes in the calculated free energy of mixing of amorphous Sm-Co-Fe.
The Solidification Microstructure of Ai-Cu-Si Alloys Metal Matrix Composites
Authors: Garbellini O. , Palacio H. , Biloni H.
Published: April 2011

The relationship between solidification microstructure and fluidity in MMC was studied. The composites were fabricated by infiltration of liquid metal into a alumina SAFFIL fibers preform under a gas pressure, using alloys of the AlCuSi system as matrices. The fluidity was measured in terms of classic foundry practice (i.e., the distance of flow liquid metal into the preform, while solidifying). The characterization of solidification microstructure in the cast composite was analysed and correlated with the results of fluidity. The attention was particularly focused on such effects as the presence or absence of selective nucleation, the refinement of certain solidifying phases in the presence of fibers and their influence on microstructure formation and segregation of certain elements present in the liquid at the fiber matrix interface. By comparing reinforced and non reinforced zones, it was shown that the presence of fibers resulted in a refinement of the dendritic arm spacing of the α-Al phase, with nucleation of Si on the fibers and without nucleation of primary Al dendrites. The results were discussed and compared with the microstructures and fluidity test of the unreinforced Al-Cu-Si alloys.
Formation of Ni/Pt/Au Ohmic Contacts to p-GaN
Authors: Jang Ja-Soon , Kim Hyo-Gun , Park Kyung-Hyun , Um Chang-Sub , Han Il-Ki , Kim Sun-Ho , Jang Heong-Kyu , Park Seong-Ju
Published: April 2011

We report a new Ni/Pt/Au (20/30/80 nm) metallization scheme to achieve a low ohmic contacts to p-type GaN with a carrier concentration of 9.4 × 1016 cm-3. A Mg-doped GaN layer (0.5 μm) was grown on (0001) sapphire substrate by metalorganic chemical vapor deposition (MOCVD). All metal thin films were deposited on the p-GaN layer in an electron-beam evaporation system. Samples were annealed by a rapid thermal annealing (RTA) process at a range of temperatures from 300 °C to 850 °C under a flowing Ar atmosphere. A circulartransmission line model (c-TLM) was employed to calculate the specific contact resistance, and current-voltage (I-V) data were measured with HP4155A. The Ni/Pt/Au contacts without the annealing process showed nearly rectifying characteristics. The ohmic contacts were formed on the samples annealed at 500 °C for 30 sec and the I-V data showed a linear behavior. The specific contact resistance was 2.1 × 10-2 Ωcm2. However with increasing the annealing temperature above 600 °C, ohmic contacts were again degraded. Auger electron spectroscopy (AES) depth profiles were used to investigate the interfacial reactions between the trilayer and GaN. AES results suggested that Pt plays a significant role in forming ohmic contact as an acceptor at the interface. Atomic force microscope (AFM) also showed that the samples with good ohmic contact have very smooth surface.
Electrical and Optical Characterisation of Homojjunction Gallium Nitride Light Emitting Diodes
Authors: Laws G M , Morgan J , Ren G B , Harrison I , Larkins E C , Orton J W , Hoopert S E , Chengt T , Foxont C T
Published: April 2011

We report on the fabrication and characterisation gallium nitride light emitting diodes (LEDs) grown by molecular beam epitaxy on (0001) oriented sapphire and (111)B GaAs substrates. The current voltage characteristics of the devices grown on sapphire show turn on voltages of between 4 and 5V with large on-series resistance of 600Ω; for corresponding devices grown on GaAs these parameters are between 6 and 7V and 150 Ω, respectively.

Room temperature electroluminescence (EL) spectra from the GaN LEDs ,grown on sapphire substrates, show a dominant emission at 3.2 eV (397nm) with a full width half maximum (FWHM) of 335 meV which is attributed to free electron to acceptor transitions (e, A−Mg). A broad low intensity deep level emission is also observed centred at 2.4 eV (506nm). The peak of the EL from the devices grown on GaAs is at 3.1eV rather than 3.2eV. The differences between the two sets of devices are probably caused by the different device geometry.

Preliminary results show that an “annealing” effect caused by electrical stressing resulted in an improvement of the EL spectra. The stressed samples show an increase in the near band edge emission intensity, a 20meV reduction in the FWHM and a significant reduction in the intensity of the deep level emission. The devices have a large 1/f noise contribution which does not appear to change after electrical stressing.
Microstructural Evaluation of Zno Thin Films Deposited by Mocvd
Authors: Gorla C. R. , Liang S. , Emanetoglu N. , Mayo W. E. , Lu Y.
Published: April 2011

ZnO films are being used in many emerging technologies such as high frequency RF filters, substrate/buffer layer for GaN deposition, and as active layers in blue-UV LEDs and lasers. These applications require epitaxial films with high structural quality. In the present paper, results on the structure of ZnO films deposited by MOCVD on c-plane and r-plane sapphire substrates are presented. These films have been investigated by XRD, SEM and TEM. X-ray phi-scans show that the films have a single epitaxial relationship with each substrate orientation. (11.0) ZnO grows on (01.2) sapphire (r-plane), while (00.1) oriented films grow on (00.1) sapphire (c-plane). Smooth films are obtained on r-sapphire, while films with a columnar morphology are obtained on c-sapphire. Based on the morphology of the columnar grains grown on c-sapphire, it is expected that the films are oxygen terminated. The interface between ZnO and sapphire is atomically sharp as observed by HRTEM. Misfit dislocations at the interface between ZnO and r-sapphire have been observed. Grain boundaries between coalescing islands during film growth are the main type of defects. Under certain conditions, very long whiskers were observed to grow by the VLS mechanism. By modifying our system, we have been able to prevent the growth of these whiskers.
Growth Mechanism and Structure of Ain Films Grown on Sapphire by MOCVD
Authors: Li Yun-Xin , Salamanca-Riba L. , Wongchotigul K. , Zhou P. , Spencer M. G. , Jones V. K.
Published: April 2011

AIN films grown on sapphire by MOCVD with different V/IAR ratios were investigated by XRD, TEM and AFM. The AIN films show single crystalline character as well as columnar structure. The growth of AIN has three zones: (1) high-density nucleation layer (2) fine columnar growth and (3) grain merging and lateral growth. The films grown at intermediate V/III ratio have the maximum value of [0002] FWHM. When the V/HI ratio increases, the thickness of the nucleation zone and the film misorientation increase, but when the ratio is increased further, the nucleation zone decreases and the AIN film has a more highly oriented growth. These results suggest that the AIN films with optimum crystalline quality can be obtained by varying the V/III ratio during growth.
GaN Crystals: Growth and Doping Under Pressure
Authors: Grzegory I. , Bockowski M. , Lucznik B. , Wroblewski M. , Krukowski S. , Weyher J. , Nowak G. , Suski T. , Leszczynski M. , Teisseyre H. , Suski T. , Litwin-Staszewska E. , Porowski S.
Published: April 2011

The recent progress in high pressure crystallization of GaN is reported. The results of the growth from the solutions of atomic nitrogen in pure Ga and in its alloys with Mg, Ca and Zn are discussed. It is shown that the growth mechanisms and the physical properties of the crystals depend on the type of dopant added into the solution. In particular, high resistivity (104-106Ωcm) GaN crystals of improved structural quality can be grown from solutions containing Mg. It was also observed that the addition of Mg, Ca and Zn suppress yellow photoluminescence commonly observed in GaN crystals grown without an intentional doping. The preparation of surfaces of GaN substrates for homoepitaxy by mechanical and mechanochemical polishing is discussed. It is shown that atomically flat, thermally stable surfaces are possible to obtain by the applied procedures.

Some most interesting results concerning homoepitaxial growth by MOCVD and MBE is shortly reviewed. In particular, it is shown that perfectly matched (strain free) GaN layers can be deposited on the highly resistive GaN:Mg substrates.
Drastic Change in the Gan Film Quality by In-Situ Controlling Surface Reconstructions In Gsmbe
Authors: Shen X. Q. , Tanaka S. , Iwai S. , Aoyagi Y.
Published: April 2011

GaN growth was performed on 6H-SiC (0001) substrates by gas-source molecular beam epitaxy (GSMBE), using ammonia (NH3) as a nitrogen source. Two kinds of reflection high-energy electron diffraction (RHEED) patterns, named (1×1) and (2×2), were observed during the GaN growth depending on the growth conditions. By careful RHEED study, it was verified that the (1×1) pattern was corresponded to a H2-related nitrogen-rich surface, while (2×2) pattern was resulted from a Ga-rich surface. By x-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM) characterizations, it was found that the GaN quality changed drastically grown under different RHEED patterns. GaN film grown under the (1×1) RHEED pattern showed much better qualities than that grown under the (2×2) one.
Growth of GaN on Lithium Gallate Substrates for Development of a GaN Thin Compliant Substrate
Authors: Doolittle W. A. , Kropewnicki T. , Carter-Coman C. , Stock S. , Kohl P. , Jokerst N. M. , Metzger R. A. , Kang S. , Lee K. , May G. , Brown A. S.
Published: April 2011

The GaN on LGO system is the near perfect template (due to extremely high etch selectivity) for developing a viable thin film/compliant GaN substrate. Herein, we report on our efforts to grow GaN on LGO, including improvement of the microscopic surface morphology using pre-growthpretreatments. We also report on the first transferred thin film GaN substrate grown on LGO, transferred off of LGO, and mounted on GaAs. With this approach, (InAl)GaN alloys can be grown on thin GaN films, implementing a truly “compliant” substrate for the nitride alloy system. In addition, the flexibility of bonding to low cost Si, metal or standard ceramic IC packages is an attractive alternative to SiC and HVPE GaN substrates for optimizing cost verses thermal conductivity concerns. We have demonstratedhigh quality growth of GaN on LGO. X-Ray rocking curves of 145 arc-seconds are obtained with only a 0.28 μm thick film. We present data on the out of plane crystalline quality of GaN/LGO material. Likewise, we show 2 orders of magnitude improvement in residual doping concentration and factors of 4 improvement in electron mobility as compared to the only previously reported electrical data. We show substantial vendor to vendor and intra-vendor LGO material quality variations. We have also quantified the desorption of Ga and Li from the surface of LGO at typical growth temperatures using in situ desorption mass spectroscopy and XPS.
Atomic Structure Of Grain Boundaries And Interfaces In Iiinitrides Epitaxial Systems
Authors: Ruvimov S. , Liliental-Weber Z. , Washburn J. , Amano H. , Akasaki I. , Koike M.
Published: April 2011

High resolution electron microscopy (HREM) was applied to study atomic structure of stacking faults, grain boundaries and interfaces in III-nitrides epitaxial layers grown by MOVPE on sapphire. Defects formed in GaN epitaxial layers grown by MOVPE were reviewed in comparison with those in MBE grown materials
High-Resolution X-Ray Diffraction Analysis of “Devicequality” Cubic GaN Grown on (001)GaAs Substrate Prepared by Atomic-Hydrogen Treatment At “High Temperatures”
Authors: Yoshikawa A. , Qin Z. X. , Nagano H. , Sugure Y. , Jia A. W. , Kobayashi M. , Shimotomai M. , Kato Y. , Takahashi K.
Published: April 2011

Cubic GaN (c-GaN) layers were grown by if-plasma source MBE on (001) GaAs prepared by atomic-hydrogen treatment at “high temperatures”, and the structural properties of the epilayers were investigated by the high-resolution X-ray rocking curve and the reciprocal space mapping measurements. The growth temperature was varied from 620 to 740°C. It was found that single domain “device-quality” c-GaN layers could be grown for the first time; the FWHM of the X-ray rocking curves for the (002) c-GaN could be as small as 70 - 90 arcsec and the inclusion of h-GaN phase in the c-GaN epilayers grown at temperatures above 680°C could be less than 4×10-3.
Interactions of LO Phonons with Bound Excitons in Homoepitaxial GaN
Authors: Koron K. P. , Wysmolek A. , Baranowskil J. M. , Pakul K. , Bergman J. P. , Monemar B. , Grzegory I. , Porowski S.
Published: April 2011

Photoluminescence connected with excitons and their phonon replicas in undoped homoepitaxial MOCVD grown GaN layers have been studied in the temperature range 2 - 100 K. It is shown that the coupling between LO phonons and neutral acceptor bound excitons (ABE) is much stronger than the coupling between LO phonons and neutral donor bound excitons (DBE). In spite that emission due the DBE no-phonon is one order of magnitude stronger than the ABE one, the predominant feature of the first LO phonon replica of the excitonic structures is related to the ABE. It is argued that this fact is connected with delocalization of the acceptor wavefunction in the k-space which leads to a higher number of interacting LO phonons in the first replica. On the other hand, the second LO phonon replica of the excitonic structures is predominantly connected with the DBE. In the case of two LO phonons interacting with bound excitons the k - conservation has not so direct influence. In addition, the temperature dependence of LO phonon replicas and their kinetics in ps regime are also reported.
Effects of Piezoelectric Fields in GaInN/GaN and GaN/AlGaN Heterostructures and Quantum Wells
Authors: Im Jin Seo , Kollmer H. , Off J. , Sohmer A. , Scholz F. , Hangleiter A.
Published: April 2011

The effects of piezoelectric fields on the static and dynamic optical properties of GaInN/GaN and GaN/AIGaN double heterostructures and single quantum wells are studied by time-resolved photoluminescence. We find a strong increase of the luminescence decay time of the dominating transition with well thickness by several orders of magnitude. For well thicknesses larger than about 5 nm, two emission lines with strongly differing decay times are observed, which are attributed to spatially direct and indirect transitions. Our experimental findings are consistently explained by a quantitative model based on the piezoelectric fields in strained wurtzite quantum wells.
Photocurrent Response In Mg-Doped GaN
Authors: Qiu C. H. , Pankove J. I , Akasaki I. , Amano H.
Published: April 2011

The photoconductivity response of Mg-doped GaN thin films was studied in the time domain of 50 nanoseconds to a few milliseconds in the temperature range of 100K to 390K. The response time, defined as the time when the photocurrent decreased to half its maximum value, is in the sub-microseconds at room temperature, but increased to a few microseconds at low temperatures. The contact capacitance is suspected for this behavior. Slower decay components due to trapping at defect states were also observed.
Characterization Of Si-Doped GaN on (00.1) Sapphire Grown by MOCVD
Authors: Kim Chang Soo , Lee Dong-Kun , Lee Cheul-Ro , Now Sam Kyu , Lee In-Hwan , Bae In-Ho
Published: April 2011

We investigated structural, electrical and optical properties and their relationships for MOCVD grown Si-doped GaN with different doping levels. The changes in structural properties such as strain, crystallinity and strain relaxation as a function of carrier concentrations were measured by high-resolution triple-axis X-ray diffraction and reciprocal space mapping technique. As the carrier concentration of the samples increases from 1.6×1018 to 9.5×1018/cm3 Hall mobility decreases from 222 to 170cm2/Vsec, and peak intensity ratio of band edge to yellow luminescence in photoluminescence spectra decreases too. The variation of mosaic spread along [00.1] direction determined from full width at half maximum(FWHM) of GaN (00.2) peaks shows good agreement with the variation in Hall mobility. In addition, dislocation density determined from FWHM of (10.2) peaks is shown to be related to the variation in the intensity of yellow luminescence in PL spectra.
Substrate Surface Treatments and “Controlled Contamination” in GaN / Sapphire MOCVD
Authors: Golan Yuval , Fini Paul , Denbaars Steven P. , Speck James S.
Published: April 2011

We have used atomic force microscopy (AFM) to study the effect of common substrate surface treatments for the metal-organic chemical vapor deposition (MOCVD) of GaN on sapphire. It appears that contaminants play a major role in the surface chemistry and strongly influence the morphology of the treated surfaces. In order to investigate the role of these contaminants, we have introduced the concept of “controlled contamination” (CC), namely, exposure of the sapphire surfaces to controlled amounts of potential contaminants in-situ and investigation of the resulting sapphire morphology. The results showed that sapphire, considered to be a very stable oxide surface, is clearly reactive in the GaN MOCVD chemical environment at the high temperatures (HT) employed, allowing us to use CC for obtaining sapphire substrates with controlled roughness. Nevertheless, epitaxial growth using the two-step GaN MOCVD process appears to be very robust and practically insensitive to the (submicronscale) substrate morphology.
The Doping and Characterization of Erbium-Implanted Gan
Authors: Torvik J. T. , Feuerstein R. J. , Qiu C. H. , Pankove J. I. , Namavar F.
Published: April 2011

Strong room temperature Er-related photoluminescence (PL) and electroluminescence (EL) at 1539 nm was observed from Er and 0 implanted n-type GaN. Good device performance requires that the Er-related excitation and emission processes be efficient. Single exponential PL and EL time decays with l/e lifetimes of 2.33 ms and 1.74 ms indicates highly efficient radiative process. The Er excitation process in GaN was studied by comparing the efficiency of direct Erabsorption, electron-hole pair recombination, and hot electron (impact) excitation. The strongest Er luminescence and the lowest pump power was found using impact excitation.
Evidence of Potential Fluctuations in Modulation Doped GaN/AlGaN Heterostructures
Authors: Buyanov A. V. , Sandberg J. , Bergman J. P. , Sernelius B. E. , Holtz P-O , Monemar B. , Amano H. , Akasaki I.
Published: April 2011

We report on novel transport and optical data for GaN/AlGaN modulation doped heterostructures grown by Metal Organic Chemical Vapor Deposition (MOCVD). Variable temperature galvanomagnetic, resistivity, photoluminescence and photoconductivity measurements have been performed. Evidence for potential fluctuations is provided by the observation of weakly localized transport at low temperatures, together with a negative magnetoresistance due to disorder in the interface region. Additional evidence for a built-in electric field caused by the fluctuations near the heterointerface region is given by the observation of photoconductivity dips resonant with free excitons, indicating free exciton ionization. It is concluded that interface roughness, dislocations, and similar structural defects, have a strong influence on the transport properties of the two-dimensional electron gas in these structures.
Local Electronic Structure Of Defects In Gan From Spatially Resolved Electron Energy-Loss Spectroscopy
Authors: Natusch M. K. H. , Botton G. A. , Broom R. F. , Brown P. D. , Tricker D. M. , Humphreys C. J.
Published: April 2011

The optical properties and their modification by crystal defects of wurtzite GaN are investigated using spatially resolved electron energy-loss spectroscopy (EELS) in a dedicated ultra-high vacuum field emission gun scanning transmission electron microscope. The calculated density of states of the bulk crystal reproduces well the features of the measured spectra. The profound effect of a prismatic stacking fault on the local electronic structure is shown by the spatial variation of the optical properties derived from low-loss spectra. It is found that a defect state at the fault appears to bind 1.5 electrons per atom.
Theoretical Investigation Of Extended Defects In Group-III Nitrides
Authors: Wright A. F.
Published: April 2011

We have investigated two types of extended defects commonly found in AIN, GaN and InN films using density-functional techniques. First, basal-plane stacking faults have been studied for all three compounds. Stacking-fault energies were found to be largest in AIN and smallest in GaN consistent with density-functional results for their wurtzite/zinc-blende energy differences. In addition, the 4H and 6H structures were found to have lower energies than zinc blende for all three compounds. Second, we have investigated the electronic structure and formation energy for an edge dislocation in AIN. The full-core dislocation structure was found to have a filled electronic level approximately 0.55 eV above the valence-band edge and an empty level 1.4 eV below the conduction-band edge. An open-core structure was found to have filled and empty electronic levels closer to the middle of the energy gap. Formation energies for these two geometries suggest that the full-core structure would be expected to form in p-type material whereas both are expected in n-type material.