bluuebunny/crossref_metadata_2025 huggingface dataset:
p-GaAs Base Regrowth for GaN HBTs and BJTs
Authors: Dang G. , Zhang A.P. , Cao X.A. , Ren F. , Pearton S.J. , Cho H. , Hobson W.S. , Lopata J. , van Hove J.M. , Klaassen J.J. , Polley C.J. , Wowchack A.M. , Chow P.P. , King D.J.
Published: April 2011

Low resistance ohmic contacts are difficult to form to p-type GaN and AlGaN due to the unavailability of growth methods for highly p-doped GaN and AlGaN. A p-type carbon-doped GaAs regrowth on p-GaN prior to ohmic metallization has been shown in previous work to improve contact resistance to p-GaN [13]. Applying the regrowth method to the p-base regions of npn structured bipolar transistors, AlGaN/GaN heterojunction bipolar transistors and GaN bipolar junction transistors have been demonstrated. GaN/AlGaN epilayers were grown with a molecular beam epitaxy system. Highly carbon-doped p-GaAs (1020 cm−3) was regrown on the devices (∼500 Å) in the base contact region by metal organic chemical vapor deposition after emitter mesa etching. Emitter and base mesa structures were formed by Inductively Coupled Plasma etching under low damage conditions with a Cl2/Ar chemistry. SiO2 was used for emitter sidewall formation to reduce leakage current to the emitter, as well as for a mask for GaAs base regrowth. Very high current densities were obtained for common base operation in both device types. The devices were operable at 250 °C.
Stable Ti/TaSi2/Pt Ohmic Contacts on N-Type 6H-SiC Epilayer at 600°C in Air
Authors: Okojie Robert S. , Spry David , Krotine Jeff , Salupo Carl , Wheeler Donald R.
Published: April 2011

We report preliminary electrical and diffusion barrier characteristics of Ti (100nm)/TaSi2 (200nm)/Pt (300nm) thermally stable ohmic contact metallization on n-type 6H-SiC epilayers. These contacts exhibited linear ohmic characteristics with contact resistance in the range of (0.3×10−4−8×10−4) Δcm2 on n-type epilayer doped between 0.6 to 2×1019cm−3. The I-V characteristics and contact resistance remained stable after heat treatments at 500°C and 600°C in air for over 600 hours and 150 hours, respectively. Auger Electron Spectroscopy (AES) was used to analyze the metal and semiconductor interfaces to understand the prevailing reactions. The thermal stability of the ohmic contact between 500°C and 600°C in air is believed to be due to the formation of silicides and carbides of titanium after being annealed at 600°C in H2 (5%)/N2 forming gas for 30 minutes. The oxidation of silicon species that migrated after TaSi2 decomposition is proposed as the diffusion barrier mechanism.
Influence of LaNiO3 as an Electrode on the Properties of Ferroelectric Oxides
Authors: Kalare R. , Vedawyas M. , Kumar A.
Published: April 2011

An electrode plays an important role in realising a ferroelectric thin film as a potential memory device. We have investigated LaNiO3 (LNO) as a potential electrode material and evaluated the ferroelectric properties of oxide materials like strontium bismuth tantalate (SBT) and barium titanate(BT). We have successfully deposited epitaxial films of LNO on Pt coated Si(100) and LaAlO3 (LAO) substrates using the pulsed excimer laser deposition technique. We are able to grow high quality SBT and BT films on top of this LNO layer. The X-ray diffraction revealed the epitaxy of the LNO, SBT and BT films. The ferroelectric properties of SBTand BT were investigated using the RT66A ferroelectric tester.
Growth, Characterization and Electrical Properties of PZT thin Film Heterostructures on Silicon by Pulsed Laser Deposition
Authors: Chattopadhyay Soma , Kvit Alex , Sreenivasank V. , Sharma A.K. , Lee C. B. , Narayan J.
Published: April 2011

Epitaxial thin films of PbZr0.52Ti0.48O3 (PZT) have been synthesized successfully on SrRuO3/SrTiO3/MgO/TiN/Si heterostructures by pulsed laser deposition. The films were single phase and had (001) orientation. The deposition parameters were varied to obtain the best epitaxial layer for each of the compounds. Transmission electron microscopy indicated good epitaxy for the entire heterostructure and sharp interfaces between the epilayers. Dielectric and P-E hysteresis loop measurements were carried out with evaporated Ag electrodes. The dielectric constant for the films was found to be between 400-450. The value of saturation polarization Ps, was between 55-60 νC/cm2 and the coercive field Ec varied from 60-70 kV/cm. Integration of PZT films with silicon will be useful for future memory and micromechanical devices.
Light Scattering from Pulsed Laser Deposited BaBi4Ti4O15 Thin Films
Authors: Soni R.K. , Dixit Anju , Katiyar R. S. , Pignolet A. , Satyalakshmi K.M. , Hesse D.
Published: April 2011

Light scattering investigations are carried out on BaBi4Ti4O15 (BBiT) which is a member of the Bi-layer structure ferroelectric oxide with n = 4. The BBiT thin films, thickness ∼ 300 nm, were grown on epitaxial conducting LaNiO3 electrodes on epitaxial buffer layers on (100) silicon by pulsed laser deposition. Micro-Raman measurements performed on these films reveal a sharp low-frequency mode at 51 cm−1 along with broad highfrequeficy modes corresponding to other lattice vibrations including TiO6 octahedra. No temperature dependence of the low frequency mode is seen while a weak dependence of the broad high frequency vibrations are observed in the mixed oriented regions. Raman polarization carried out at room temperature indicates that the prominent modes have Alg and Eg symmetries in the BaBi4Ti4O15 thin films.
Lattice Defects in Epitaxial Ba2Bi4Ti5O18 Thin Films Grown by Pulsed Laser Deposition Onto LaNiO3 Bottom Electrodes
Authors: Zakharov N.D. , James A.R. , Pignolet A. , Senz S. , Hesse D.
Published: April 2011

Epitaxial, ferroelectric Ba2Bi4Ti5O18 films grown on LaNiO3/CeO2/ZrO2:Y2O3 epitaxial layers on Si(100) are investigated by cross-section high-resolution transmission electron microscopy (HRTEM). The films are perfectly oriented and consist of well-developed grains of rectangular shape. The grain boundaries are strained and contain many defects, especially a new type of defect, which can be described as a staircase formed by repeated lattice shifts of Δ ∼ c/12 ∼ 4.2 Å in the [001] direction. This repeated shift results in seemingly bent ribbons of stacked Bi2O2 planes, involving, however, individual Bi2O2 planes which remain strongly parallel to the (001) plane. These defects contain an excess of bismuth. Other defects found in the grain interior include mistakes in the stacking sequence originating from the presence of single, well-oriented, non-stoichionietric layers intergrown with the stoichiometric Ba2Bi4Ti5O18 film matrix.
Optimizing Fabrication of Buried Oxide Channel Field Effect Transistors
Authors: Schrott A.G. , Misewich J.A. , Copel M. , Abraham D.W. , Neumayer D.A.
Published: April 2011

In this paper we describe improved methods of fabrication for an oxide channel field effect transistor (OxFET) similar in architecture to a conventional FET. We demonstrate that a substrate treatment consisting of a low power oxygen ashing followed by annealing yields a strontium (A-site) terminated surface in single-crystal strontium titanate (STO). This surface termination of the substrate results in pulsed laser deposited cuprate-channel films of improved quality.
Properties of a Novel Amorphous Transparent Conductive Oxide, InGaO3(ZnO)m
Authors: Orita M. , Ohta H. , Hirano M. , Hosono H. , Morita K. , Tanji H. , Kawazoe H.
Published: April 2011

Novel amorphous transparent conductive oxides, InGaO3(ZnO)m, where m is an integer less than four, was developed. Optical transmittance in the visible region exceeded over 80 % and the electric conductivity at 300 K was as large as 400 S/cm. Both Seebeck and Hall coefficients exhibited negative values, indicating the conduction was n-type. It was suggested that 4s orbital of Zn2+ played a significant role for the formation of the extended state responsible for the conduction, while In3+ acted as a modifier for the stabilization of amorphous state.
Preparation of PZT-YBCO Heterostructure on YSZ Coated Si by KrF Laser Ablation
Authors: Ebihara Kenji , Mitsugi Fumiaki , Ikegam Tomoaki , Narayan Jagdish
Published: April 2011

KrF excimer laser ablation technique is used to fabricate the ferroelectric Pb(ZrxTi1−xO3)(PZT) capacitor on Si(100) substrate. The superconducting YBa2Cu3O71−x(YBCO) and the colossal magnetoresistive La0.8sr0.2MnO3(LSMO) thin films are used as bottom electrodes for the ferroelectric capacitive structure. The YBCO and LSMO films were studied to understand the interface problems with perovskite oxide films. The fabricated PZT/YBCO/YSZ/Si capacitor shows the ferrolectric properties of the remanent polarization of 20pC/cm2 and the coercive field of 40 kV/cm. Post-thermal annealing improves the ferroelctric properties and the results are comparable with that of the PZT/YBCO/MgO(100) structure. The leakage current of the PZT capacitor is discussed.
Single Quantum Well Heterostructures of MgZnO/ZnO/MgZnO on C-Plane Sapphire
Authors: Choopun S. , Chalk D. M. , Yang W. , Vispute R. D. , Ogale S. B. , Sharma R. P. , Venkatesan T.
Published: April 2011

The single quantum well heterostructures of MgZnO/ZnO/MgZnO were grown on c-plane sapphire substrate by pulsed laser deposition. The well width was varied from 10 nm to 40 nm by controlling the deposition rate via number of laser pulsed on ZnO target. Using photoluminescence spectroscopy, we have observed a blue shift with respect to a thick ZnO reference sample when the well width was decreased. These results were fitted with calculations based on the simple square well model using the appropriate electron and holes effective masses. The quantized-energy and band offset as a function of well width, growth conditions, interface roughness, and possible quantum size effects on the quantum wells are discussed.
Properties of Amorphous and Crystalline Ta2O5 Thin Films Prepared by Metalorganic Solution Deposition Technique for Integrated Electronic Devices
Authors: Joshi P.C. , Cole M.W.
Published: April 2011

We report on the properties of Ta2O5 thin films prepared by the metalorganic solution deposition (MOSD) technique on Pt-coated Si, n+-Si, and poly-Si substrates. The effects of postdeposition annealing temperature on the structural, electrical, and optical properties were analyzed. The electrical measurements were conducted on MIM and MIS capacitors. The dielectric constant of amorphous Ta2O5 thin films was in the range 29.2-29.5 up to 600°C, while crystalline thin films, annealed in the temperature range 650–750°C, exhibited enhanced dielectric constant in the range 45.6–51.7. The dielectric loss factor did not show any appreciable dependence on the annealing temperature and was in the range 0.006–0.009. The films exhibited high resistivities of the order of 1012–1015 Δ-cm at an applied electric field of 1 MV/cm in the annealing temperature range of 500-750 °C. The temperature coefficient of capacitance was in the range 52-114 ppm/°C for films annealed in the temperature range 500-750°C. The bias stability of capacitance, measured at an applied electric field of 1 MV/cm, was better than 1.41 % for Ta2O5 films annealed up to 750°C.
Laser Guided Direct Writing
Authors: Renn Michael J.
Published: April 2011

Laser-induced optical forces are used to guide and deposit 100 nm - 10 µm diameter particles onto solid surfaces in a process called laser-guided direct-writing. Nearly any particulate material, including both biological and electronic materials, can be manipulated and deposited with micrometer accuracy. Potential applications include three-dimensional cell patterning for tissue engineering, hybrid biological and electronic device construction, and biochip array fabrication
Photo-Induced Large Area Growth of Dielectrics With Excimer Lamps
Authors: Boyd Ian W. , Zhang Jun-Ying
Published: April 2011

In this paper, UV-induced large area growth of high dielectric constant (Ta2O5, TiO2and PZT) and low dielectric constant (polyimide and porous silica) thin films by photo-CVD and sol-gel processing using excimer lamps, as well as the effect of low temperature UV annealing, are discussed. Ellipsometry, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), UV spectrophotometry, atomic force microscope (AFM), capacitance-voltage (C-V) and current-voltage (I-V) measurements have been employed to characterize oxide films grown and indicate them to be high quality layers. Leakage current densities as low as 9.0×10−8 A·cm−2and 1.95×10−7 A·cm−2at 0.5 MV/cm have been obtained for the as-grown Ta2O5 films formed by photo-induced sol-gel processing and photo-CVD, respectively – several orders of magnitude lower than for any other as-grown films prepared by any other technique. A subsequent low temperature (400°C) UV annealing step improves these to 2.0×10−9A·cm−2and 6.4×10−9A·cm−2, respectively. These values are essentially identical to those only previously formed for films annealed at temperatures between 600 and 1000°C. PZT thin films have also been deposited at low temperatures by photo-assisted decomposition of a PZT metal-organic sol-gel polymer using the 172 nm excimer lamp. Very low leakage current densities (10−7A·cm2) can be achieved, which compared with layers grown by conventional thermal processing. Photo-induced deposition of low dielectric constant organic polymers for interlayer dielectrics has highlighted a significant role of photo effects on the curing of polyamic acid films. I-V measurements showed the leakage current density of the irradiated polymer films was over an order of magnitude smaller than has been obtained in the films prepared by thermal processing. Compared with conventional furnace processing, the photo-induced curing of the polyimide provided both reduced processing time and temperature. A new technique of low temperature photo-induced sol-gel process for the growth of low dielectric constant porous silicon dioxide thin films from TEOS sol-gel solutions with a 172 nm excimer lamp has also been successfully demonstrated. The dielectric constant values as low as 1.7 can be achieved at room temperature. The applications investigated so far clearly demonstrate that low cost high power excimer lamp systems can provide an interesting alternative to conventional UV lamps and excimer lasers for industrial large-scale low temperature materials processing
Charged Molten Metal Droplet Deposition as a Direct Write Technology
Authors: Orme M. , Courter J. , Liu Q. , Zhu J. , Smith R.
Published: April 2011

The formation of highly uniform charged molten metal droplets from capillary stream breakup has recently attracted significant industrial and academic interest for applications requiring high-speed and high-precision deposition of molten metal droplets such as direct write technologies. Exploitation of the high droplet production rates intrinsic to the phenomenon of capillary stream break-up and the unparalleled uniformity of droplet sizes and speeds attained with proper applied forcing to the capillary stream make many new applications related to the manufacture of electronic packages, circuit board printing and rapid prototyping of structural components feasible. Recent research results have increased the stream stability with novel acoustic excitation methods and enable ultra-precise charged droplet deflection. Unlike other modes of droplet generation such as Drop-on-Demand, droplets can be generated at rates typically on the order of 10,000 to 20,000 droplets per second (depending on droplet diameter and stream speed) and can be electrostatically charged and deflected onto a substrate with a measured accuracy of ±12.5 µm. Droplets are charged on a drop-to-drop basis, enabling the direct writing of fine details at high speed. New results are presented in which fine detailed patterns are “printed” with individual molten metal solder balls, and issues relevant to the attainment of high quality printed artifacts are investigated.
Pattern Writing by Implantation in a Large-Scale PSII System With Planar Inductively Coupled Plasma Source
Authors: Wu Lingling , Gao Hongjun , Manos Dennis M.
Published: April 2011

A large-scale plasma source immersion ion implantation (PSII) system with planar coil RFI plasma source has been used to study an inkless, deposition-free, mask-based surface conversion patterning as an alternative to direct writing techniques on large-area substrates by implantation. The apparatus has a 0.61 m ID and 0.51 m tall chamber, with a base pressure in the 10−8 Torr range, making it one of the largest PSII presently available. The system uses a 0.43 m ID planar rf antenna to produce dense plasma capable of large-area, uniform materials treatment. Metallic and semiconductor samples have been implanted through masks to produce small geometric patterns of interest for device manufacturing. Si gratings were also implanted to study application to smaller features. Samples are characterized by AES, TEM and variable-angle spectroscopic ellipsometry. Composition depth profiles obtained by AES and VASE are compared. Measured lateral and depth profiles are compared to the mask features to assess lateral diffusion, pattern transfer fidelity, and wall-effects. The paper also presents the results of MAGIC calculations of the flux and angle of ion trajectories through the boundary layer predicting the magnitude of flux as a function of 3-D location on objects in the expanding sheath
Lateral Dye Distribution With Ink-Jet Dye Doping of Polymer Organic Light Emitting Diodes
Authors: Madigan Conor F. , Hebner Thomas R. , Sturm J. C. , Register Richard A. , Troian Sandra
Published: April 2011

In this work we investigate the lateral dye distribution resulting from the dye doping of a thin polymer film by ink-jet printing (IJP) for the integration of color organic light emitting diodes (OLED's). The dye is found to segregate into distinct outer rings following rapid droplet evaporation, while slower evaporation rates are found to significantly reduce (or eliminate) this effect. The dye segregation phenomena are found to depend critically on the mechanisms of droplet evaporation. Good dye uniformity was obtained using a low vapor pressure solvent, and integrated, 250 micron red, green, and blue polymer organic light emitting diodes (OLED's) were fabricated with this technique. These devices had good color uniformity over most of the device area and similar electrical properties to comparable spin-coated devices without IJP.
All-Printed Inorganic Logic Elements Fabricated by Liquid Embossing
Authors: Bulthaup Colin , Wilhelm Eric , Hubert Brian , Ridley Brent , Jacobson Joe
Published: April 2011

The liquid embossing process, devices made by this process, and their characteristics are presented. Structures fabricated and discussed here include: conductive lines with cross line resistance greater than 100 GΩ and resistivities 4 times that of the bulk material, multilayer structures with etched sacrificial materials, vias that conduct through an embossed insulating layer, photodetectors made with nanocrystal solutions of CdSe, and all printed inorganic field effect transistors.
Using Layered Manufacturing to Create Textured Microstructures in Si3N4 Ceramics
Authors: Rangarajan S. , Harper B. , McCuiston R. , Safari A. , Kalman Z. , Mayo W. , Danforth S. C. , Gasdaska C.
Published: April 2011

In recent years, seeding has been shown to be an effective method to create textured microstructures in Si 3N4, Al2O3 and PZT ceramics. The objective of this research is to create anisotropic and textured Si3N4 parts using the Fused Deposition of Ceramics (FDC) process. This technique is currently being developed to fabricate high performance structural Si 3N4 based components. In order to create the textured microstructures, rod-like β-Si3N4 seed particles are introduced into the FDC feedstock filaments. In this study, anisometric β-Si3N4 seeds were introduced into the starting α-Si3N4 powder (Honeywell's AS800 grade) at a 10 vol.% loading. The effects of the seeds (aspect ratio ∼4) on the viscosity and resultant microstructure were evaluated using capillary rheology, scanning electron microscopy, coefficient of thermal expansion (CTE) measurements, and x-ray diffraction. It is observed that the seeds do align during filament extrusion and a significant texture has been detected by x-ray diffraction and CTE measurements in FDC feedstock filaments and sintered FDC parts.
Investigation into Freeform Fabrication of Multi-Material Parts by 3D Welding and Milling Process
Authors: Song Yong-Ak , Park Sehyung
Published: April 2011

Solid Freeform Fabrication (SFF) gives engineers a new freedom to build parts which have proved impossible to manufacture using conventional techniques. However, the surface finish nd accuracy of SFF parts are still lower than those of parts which have been machined in conventional operations. A process combination of additive and subtractive techniques is currently being developed in order to overcome this problem. The hybrid approach called “3D Welding and Milling” uses welding as an additive and conventional milling as a subtractive technique, thereby exploiting the advantages of both processes. In this paper, the attempt was made to build multi-material parts, especially molds, by using two different deposition strategies. The results show that it is a feasible approach to build multi-material metal parts directly.
Dimensionally Accurate Mold Inserts and Metal Components by Direct Metal Laser Sintering
Authors: Lind Jan-Erik , Kotila Juha , Syvänen Tatu , Nyrhilä Olli
Published: April 2011

One of the main deficiencies in today‘s rapid tooling techniques is the capability of producing only near net-shape parts. Direct Metal Laser Sintering (DMLS) is a technique that enables the fabrication of true net-shape parts in just a few hours with only minimum postprocessing. The DMLS is a laser-based rapid tooling process developed in Europe by Rapid Product Innovations (formerly Electrolux Rapid Development Finland) and EOS GmbH Germany. Rapid Product Innovations was also the first to implement this technology to functional prototyping by using two proprietary bronze-based powders and a new steel-based powder. The technique enables the fabrication of tailored microstructures from porous matrix to near full density. Thus, functionally gradient structures can be fabricated, i.e. material is sintered to full density only where it is needed. This paper describes the philosophy of fabricating true net-shape mold inserts and metal components, but also concentrates on how to maintain the dimensional accuracy even after the post-processing. The study shows that even +/− 0.05 mm accuracy can be obtained. The results of the study also illustrate the beneficial effects of reduced layer thickness and post-processing on the surface roughness and mechanical properties as well as the suitability of various conventional and non-conventional coatings. With reduced layer thickness, the step-shaped effect of the layers was no longer visible. The surface was even further improved by shot peening and coating. In addition, case studies from injection molding, pressure die-casting and direct metal component fabrication are presented.
Selective Laser Sintering of Zirconia
Authors: Harlan Nicole R. , Bourell David , Park Seok-Min , Beaman Joseph J.
Published: April 2011

A combination of Selective Laser Sintering and colloidal infiltration has been used to create partially stabilized zirconia molds for titanium casting. The mold material system was chosen for its low reactivity with molten titanium and thermal shock resistance. The base material, stabilized zirconia mixed with a copolymer binder, was pre-processed before laser sintering into the desired green shape. The average density of the fired parts could be increased to twice that of the green density. Hole sizes as small as 180 m are possible in thin ceramic components.
Matrix Assisted Pulsed Laser Evaporation Direct Write (MAPLE DW): A New Method to Rapidly Prototype Active and Passive Electronic Circuit Elements
Authors: Fitz-Gerald J.M. , Chrisey D.B. , Piqu A. , Auyeung R.C.Y. , Mohdi R. , Young H.D. , Wu H.D. , Lakeou S. , Chung R.
Published: April 2011

We demonstrate a novel laser-based approach to perform rapid prototyping of active and passive circuit elements called MAPLE DW. This technique is similar in its implementation to laser induced forward transfer (LIFT), but different in terms of the fundamental transfer mechanism and materials used. In MAPLE DW, a focused pulsed laser beam interacts with a composite material on a laser transparent support transferring the composite material to the acceptor substrate. This process enables the formation of adherent and uniform coatings at room temperature and atmospheric pressure with minimal post-deposition modification required, i.e., ≤ 400°C thermal processing. The firing of the laser and the work piece (substrate) motion is computer automated and synchronized using software designs from an electromagnetic modeling program validating that this technique is fully CAD/CAM compatible. The final properties of the deposited materials depend on the deposition conditions and the materials used, but when optimized, the properties are competitive with other thick film techniques such as screenprinting. Specific electrical results for conductors are < 5X the resistivity of bulk Ag, for BaTiO3/TiO2 composite capacitors the k can be tuned between 4 and 100 and losses are < 1-4%, and for polymer thick film resistors the compositions cover 4 orders of magnitude in sheet resistivity. The surface profiles and fracture cross-section micrographs of the materials and devices deposited show that they are very uniform, densely packed and have minimum resolutions of ∼10 µm. A discussion of how these results were obtained, the materials used, and methods to improve them will be given.
The Inverse Hall-Petch Effect—Fact or Artifact?
Authors: Koch Carl C. , Narayan J.
Published: April 2011

This paper critically reviews the data in the literature which gives softening—the inverse Hall-Petch effect—at the finest nanoscale grain sizes. The difficulties with obtaining artifactfree samples of nanocrystalline materials will be discussed along with the problems of measurement of the average grain size distribution. Computer simulations which predict the inverse Hall-Petch effect are also noted as well as the models which have been proposed for the effect. It is concluded that while only a few of the experiments which have reported the inverse Hall-Petch effect are free from obvious or possible artifacts, these few along with the predictions of computer simulations suggest it is real. However, it seems that it should only be observed for grain sizes less than about 10 nm.
Vibrational Properties of Silver Nanoparticles and Nanocrystalline Materials
Authors: Meyer Ralf
Published: April 2011

The vibrational density of states of silver in the form of a free cluster, a single crystal and a nanocrystalline material has been calculated with the help of molecular-dynamics simulations. The model for the nanocrystalline material was derived by the simulation of pressureless sintering of nanometer sized silver particles. The results show a broadening of the vibrational density of states in the case of the cluster and the nanocrystalline material.
Optical Properties of Nanoparticle Pair Structures
Authors: Sandrock Marie L. , El-Kouedi Mahnaz , Gluodenis Maryann , Foss Colby A.
Published: April 2011

The synthesis of nanoparticle pair structures via porous host electrochemical template synthesis reviewed. Electrochemical template synthesis offers two advantages over solution methods, namely: 1) control over particle pair structure and orientation; and 2) control over geometry, size and composition of each member of the particle pair. These features of electrochemical template synthesis allow for straightforward comparison of experimental and theoretical spectra. Orientation control allows for the evaluation of second order nonlinear optical properties of centrosymmetric and non-centrosymmetric nanoparticle pair systems. The dependence of Second Harmonic Generation intensity on particle pair shape size and orientation is discussed. The synthesis and linear spectra of metal-semiconductor nanoparticle pair structures are also discussed, with emphasis on interparticle physical and electromagnetic interactions.
Strain Anisotropies in Core/Shell Magnetic Nanostructures
Authors: Papaefthymiou Georgia C.
Published: April 2011

The magnetic properties of nanosized iron-oxo molecular clusters have been investigated via Mössbauer spectroscopy and compared to those of silica coated iron-oxide nanoparticles. The clusters, prepared by controlled hydrolytic iron polymerization reactions, contain a ∼ 1.2 nm diameter magnetic core of spin-coupled iron ions surrounded by a shell of benzoate ligands. The nanoparticles, prepared via sol-gel synthesis, contain a ∼ 4.0 nm average diameter γ-Fe2O3 core coated by a shell of SiO2. Both systems exhibit magnetic bistability at low temperatures with estimated magnetic anisotropy constants of Keff = 0.63x105 J/m3 for the clusters and Keff = 0.55 ×105 J/m3 for the particles. The similar values of Keff indicate that these two systems experience similar degrees of strain at the core/shell interface. This is further supported by the values of the quadrupole splitting, ΔEQ=0.77 mm/s for the clusters and ΔEQ=0.75 mm/s for the particles, pointing to same degree of distortion from pure octahedral or tetrahedral symmetry at the iron coordination sites for either system. Implications of these observations for the surface atomic structure of γ-Fe2O3 nanoparticles are discussed.
Polymeric Surfactants Based on Oleic Acid— Lamellar Liquid Crystal Polymerization of Sodium Oleate/Water/Hexadecane/4-Allyl-1, 6-Heptadiene-4-ol System And Sodium Oleate/Water/Hexadecane/Pentaerythritol Triarylate System
Authors: Fu Qinghong
Published: April 2011

The lamellar liquid crystalline (LLC) regions of sodium oleate (NaOL)/ water/hexadecane/4-allyl-1,6-heptadiene-4-ol (AHD-ol) system and sodium oleate (NaOL) /water/hexadecane/pentaerythritol triacrylate (PETA) system were determined when the weight ratio of NaOL to hexadecane was kept at 90:10. Copolymerization was accomplished in the LLC phases of the above systems. AHD-ol and PETA acted as the cross-linking agents between the double bonds on NaOL backbones in their systems respectively. Interlayer spacing determinations showed that the cross-linking agents, AHD-ol and PETA, were solubilized in the middle of the NaOL hydrocarbon chains in each LLC system. As a result, the copolymerization reactions were confined within each monolayer of the amphiphilic bilayer. The systems after the copolymerization were a mixture of water and LLC phase for the former system and a mixture of LLC phase and solid particles for the latter one. The aqueous solutions of copolymers exhibited surface activity, but with both higher CMC and surface tension values than NaOL aqueous solutions.
Fabrication and Characterization of Chromium Oxide Nanoparticles/Thin Films
Authors: Zhong Zhenchen , Cheng Ruihua
Published: April 2011

Well-dispersed nanoscale textured chromium oxide particles/thin films can be fabricated under certain conditions by laser-induced solution deposition (LISD) from organic solutions and by using selective organometallic chemical vapor deposition (OMCVD). The fabricated nanoparticles/thin films are characterized by scanning electron microscope (SEM), EDX, X-ray diffraction, and magnetic measurements. We have successfully demonstrated that the LISD and OMCVD are unique techniques for fabricating uniformly-distributed thin films but anistropic chromium oxide particles, which can be used in electro-magnetic devices. The magnetization measurements show that both types of chromium oxides are presented and that the Curie temperature Tc and the saturation magnetization field may be adjustable by controlling the stoichiometry.
Accurate Dry Etching with Fluorinated Gas for Two-dimensional Si Photonic Crystal
Authors: Takahashi Chiharu , Takahashi Jun-Ichi , Notomi Masaya , Yokohama Itaru
Published: April 2011

Anisortopic Si dry etching is usually carried out with chlorinated gases for electronic devices such as Si-LSIs. We had another look at Si dry etching with fluorinated gases in order to obtain an ideal air hole for two-dimensional Si photonic crystal. We simulated vertical Si etching, and showed the possibility that single crystal Si can be etched vertically with high selectivity to the etching mask using fluorinated gases. We investigated ECR etching with an SF6-CF4 mixture, and vertical Si etching was achieved at room temperature. High Si/Ni selectivity above 100 was also obtained. Two-dimensional Si photonic crystal with a photonic band gap between 1.25 and 1.51 μm was produced using SF6-CF4 ECR plasma and a thin Ni mask.
Stress Effects in p-Type AlGaN/GaN Heterostructures
Authors: Sutandi Agustinus , Ruden P. Paul , Brennan Kevin F.
Published: April 2011

The physics of bulk wurtzite-structure III-nitride materials and of III-nitride heterostructures includes many phenomena that can be modulated by the application of stress. In particular, p-type material is expected to display a rich variety of piezo-resistive and piezo-optic effects that originate from the stress-induced modulation of lattice polarization charges, of valence band energies, and of bulk, surface, and interface defect states in the band gap. Here we focus on the expected effects of in-plane uniaxial on p-channel AlGaN/GaN heterostructures grown along the hexagonal axis on sapphire substrates. The valence band structure in the channel region is calculated self-consistently in the framework of a six-band Rashba-Sheka-Pikus (RSP) Hamiltonian. Stress-effects are included (in linear elastic theory) through deformation potentials and through the modulation of interfacial polarization charges associated with the piezoelectric nature of the constituent materials.
HTCVD growth of semi-insulating 4H-SiC crystals with low defect density
Authors: Ellison A. , Magnusson B. , Hemmingsson C. , Magnusson W. , Iakimov T. , Storasta L. , Henry A. , Henelius N. , Janzén E.
Published: April 2011

The development of a novel SiC crystal growth technique, generically described as High Temperature Chemical Vapor Deposition (HTCVD) is reviewed. The structural, optical and electrical properties of 4H-SiC semi-insulating substrates are investigated with the aim of providing optimal microwave device performances. In particular, alternative compensation mechanisms to vanadium doping in S.I substrates are investigated to eliminate substrate induced trapping effects. Carried out at temperatures above 2100°C, the HTCVD technique uses, as in CVD, gas precursors (silane and a hydrocarbon) as source materials. The growth process can be described as “Gas Fed Sublimation” and proceeds by the gas phase nucleation of Six-Cy clusters, followed by their sublimation into active species that are condensed on a seed. Crystals with diameters up to 45 mm have been obtained with growth rates of 0.6 mm/h. The use of specific process steps, such as in-situ seed surface preparation and micropipe closing are presented and high resistivity wafers with micropipe densities down to 10 cm−2 are demonstrated. 4H-SiC substrates prepared from undoped crystals (with vanadium concentration lower than 5×1014 cm−3) exhibit semi-insulating behavior with a room temperature resistivity of the order of 1010Ω?cm. Infrared absorption measurements show that two types of semi-insulating crystals can be grown, with a spectrum either dominated by the Si-vacancy, or by a previously unreported defect labeled UD-1. These two types of semi-insulating wafers are also differentiated by the temperature dependence of their resistivity, with activation energies of 0.85 and 1.4±0.1 eV, respectively, and by the stability of their resistivity upon an annealing at 1600°C. Initial MESFET devices processed on HTCVD grown substrates show better DC characteristics than devices processed on vanadium doped substrates.
Growth and Characterisation of 4H-SiC MESFET structures grown by Hot-Wall CVD
Authors: Forsberg U. , Henry A. , Danielsson Ö. , Rorsman N. , Eriksson J. , Wahab Q. , Storasta L. , Linnarsson M. K. , Janzén E.
Published: April 2011

Metal semiconductor field effect transistor, MESFET, structures have been grown in a hot-wall CVD reactor. Using trimethylaluminium and nitrogen as dopant sources, p- and n-type epitaxial layers were grown on semi insulating substrates. A comprehensive characterization study of thickness and doping of these structures has been performed by using scanning electron microscopy, secondary ion mass spectrometry, capacitance-voltage measurements. Each technique is discussed concerning its advantage and disadvantage. Some transistor properties of MESFETs processed on the grown material are presented.
Optimization of the carbonized buffer layer for the growth of high quality single crystal SiC on Si
Authors: Cloitre T. , Moreaud N. , Vicente P. , Sadowski M. , Moret M. , Aulombard R. L.
Published: April 2011

Carbonized buffer layers were formed on Si (100) nominally oriented substrates with propane diluted in palladium purified hydrogen in a cold wall vertical reactor. Subsequent SiC layers were grown using silane and propane at atmospheric pressure. The growth temperature was ranging from 1150°C to 1350°C. The layers obtained were characterized by LT photoluminescence, IR reflectivity, X-ray diffraction, micro-Raman on cleaved edges, AFM imaging, and optical microscopy. Drastic influence on the layer surface morphology was evidenced depending on the transition step between the carbonization and the SiC epitaxial growth. As a result, we have developed a carbonization process leading to very high quality 3CSiC films grown at 1250°C.
Oxidation study of hydrogenated amorphous silicon carbide films
Authors: Choi W. K. , Lee L. P. , Leoy C. C.
Published: April 2011

We present results of an oxidation study of a-Si1−xCx:H films prepared by the plasma enhanced chemical vapor deposition of silane and acetylene. The composition (i.e. x) of the samples was determined by the flow rates of silane and acetylene. Oxidation was carried out at 400 to 850°C in dry oxygen ambient. The infrared (IR) spectra of the as-prepared films showed the intensity of the Si-C peak decreases and the Si-CH3 peak increases as x increases. The Si-H peak shifts to higher frequency as x increases. Note that the incorporation of CH3 radicals in a-Si1−xCx:H films has shown to introduce voids and increased the porosity of the films. The IR spectra of the oxidized samples showed clear Si-O stretching and rocking/wagging modes for all films. We suggest that the growth of oxide on a-Si1−xCx:H is a result of voids that facilitate the diffusion of oxidants into the film. We shown that the activation energy, obtained from the linear rate region of the oxide growth, was far less than the dissociation energies of the Si-Si, Si-C and Si-H bonds. We suggest that this could be due to the amorphous nature of the samples that caused the various chemical bonds to be weaker during oxidation.
Impurity Activation in N+ Ion-Implanted 6H-SiC with Pulsed Laser Annealing Method
Authors: Eryu O. , Aoyama K. , Abe K. , Nakashima K.
Published: April 2011

We have succeeded in pulsed laser annealing of N+ ion-implanted n-type 6H-SiC for increasing the carrier density near surface in order to decrease contact resistance, which induces little redistribution of implanted impurities after laser irradiation. By repeated laser irradiation at low energy density, the ion–implanted impurities were electrically activated without melting the surface region. SiC substrates with impurity concentration of 2×1018 /cm3 were implanted with 30 keV N+ ions with dose of 4.7×1013/cm2. After pulsed laser annealing, a contact resistance was measured to be 5.7×10−5 Ωcm2 using Al electrode on the N+ -implanted layer.
Fabrication of 3.4kV high voltage n-type 4H-SiC Schottky barrier diodes using thick epitaxial layers
Authors: Tsuji Takashi , Fujisawa Hiroyuki , Ogino Shinji , Tsuchida Hidekazu , Kamata Isaho , Jikimoto Tamotsu , Izumi Kunikazu
Published: April 2011

Fabrication and evaluation of high voltage n-type 4H-SiC Schottky barrier diodes (SBDs) using 27μm thick epitaxial layers were presented. To achieve the ideal value of the breakdown voltage, various parameters of junction termination extension (JTE) were investigated. We concluded that the termination of triple rings with the concentrations of 6×1017, 3×1017, 1.5×1017cm−1 outwardly was best with the simulations. The SBDs with this termination showed the blocking voltage up to 3.4kV, which is almost the ideal value. We also investigated the distribution of leakage currents at -600V in SBDs with various diameters up to 4mm. High yield was obtained in the SBDs with the diameters below 2mm. The SBDs with high leakage currents showed the excess currents in the low forward voltage region and lots of bright spots could be observed by optical beam induced current analysis.
Chemical-Mechanical Polishing and Rapid Thermal Annealing of SiC: Raman Spectroscopy and ESCA (XPS) Studies
Authors: Roughani Bahram , Ramabadran Uma , Phillips Diana , Mitchel W. C. , Neslen C. L.
Published: April 2011

The effects of Chem-Mechanical Polishing (CMP) and Rapid Thermal Annealing (RTA) on n-type 4H:SiC samples doped with nitrogen were investigated using Raman scattering and X-ray Phtoelectron Spectroscopy (XPS a.k.a. ESCA) measurements. A comparison of the Raman spectra from Mechanically Polished (MP) SiC annealed at 600°C and 800°C displays a frequency shift in the coupled plasmon LO-phonon mode. Since the coupled mode frequency is a direct measure of the free carrier concentration, this observation may suggest the removal of polishing induced carrier traps with increasing annealing temperature. The CMP samples did not show this frequency shift, thereby indicating that such polishing traps were not created in that process. The Si-peak observed in the XPS spectra of the unannealed CMP sample indicates primarily a Si-C bonding, while that for the MP sample is more complex, indicating other bonds beside Si-C. Drastic changes in O, C, Si surface content were observed for annealing between 1000°C and 1100°C. The peaks in the XPS spectra associated with the chemical environment for C, O, and N are complex and may be explained as silicon oxycarbide type structures near the surface or possibly around the interface of the SiC substrate with a thin surface oxide layer.
An Analytical Approach for Computing the Energy Structure of InAs Quantum Dots
Authors: Stoleru Valeria G. , Pal Debdas , Towe Elias
Published: April 2011

The experimental and theoretical results of the electronic structure of self-assembled InAs/GaAs quantum dots are presented. We have performed analytical calculations to obtain the spatial strain distribution in truncated pyramidally-shaped InAs quantum dots grown on (001) GaAs substrates by molecular beam epitaxy. The model reproduces our cross sectional high- resolution transmission electron microscopy observations very well. We further take into account the microscopic effects of the spatial strain distribution on carrier confinement potentials. The effects of piezoelectricity have been neglected in our calculations since they minimally affect the energy levels involved in optical transition. The dots considered here are assumed to be in the strong confinement regime; so Coulomb interaction is neglected. Our calculations of the peak luminescence energies are in good agreement with our experimental results and those of others. The calculated energy separation between the ground and the excited states for electrons and for heavy-holes is in very good agreement with the available photoluminescence, capacitance and far- infrared absorption data.
Potential industrial applications of Al-based quasicrystals: plasma sprayed vs. HVOF sprayed coatings
Authors: Lee S.M. , Fleury E. , Kim J.S. , Kim Y.C. , Kim D.H. , Kim W.T. , Ahn H.S.
Published: April 2011

Quasicrystalline coatings with various aluminum based systems were deposited onto 304 stainless steel substrates using two thermal spraying techniques; plasma spraying and high velocity oxy-fuel spraying. The friction and wear performances of the coatings were evaluated using two different testing devices, varying testing conditions and counterpart materials. Values of the friction coefficient were found to be strongly dependent on the testing devices and the counterpart materials, with values ranging from 0.15 to 0.4. In testing conditions corresponding to high sliding velocity, this study showed that the contact problem was posed over a third body system due to the formation of an intermediate layer. Values of the coefficient of friction were found to be approximately the same for all coating layers regardless of the thermal spraying techniques used, however, larger differences were obtained in the wear performance. The tribological properties were also evaluated at high temperature. It is noted that quasicrystal coating layers exhibit better friction and wear performances at 450oC than at room temperature. In comparison to potential coating candidate such as Cr2O3 for piston rings in automotive engines, tribological property of quasicrystalline coating layers seems to be promising one, however, wear performance need to be improved.
Quasicrystalline Catalyst for Steam-Reforming of Methanol
Authors: Tsai A.P. , Yoshimura M.
Published: April 2011

Steam reforming of methanol (CH3OH + H2O -> 3H2 + CO2) has been performed on a number of Al-Cu-based stable quasicrystals and related alloys. Alloys leached in NaOH aqueous solution exhibit the highest activity for steam-reforming of methanol. Among the alloys, AlCuFe quasicrystal, after leaching treatment, reveals excellent activity. The production rate of H2 reaches 235 l/kgmin at 573K for steam reforming of methanol. The activity is due to Cu nanoparticles at the surfaces of quasicrystalline grains which are generated by leaching treatment. The quasicrystals have two advantages: one is their brittle nature, which allows them to be crushed efficiently; the other is the involvement of Fe, which suppresses the sintering of Cu particles.
Transformation In Hydrogenated ZR-CU-NI-AL Quasicrystals
Authors: Zander D. , Köster U. , Khare V.
Published: April 2011

The high number of potential interstitial sites suitable for hydrogen and the favorable hydrogen-metal chemistry make quasicrystalline alloys candidates for hydrogen storage applications. Hydrogen charging was performed electrochemically in a 2:1 glycerin-phosphoric acid electrolyte. The microstructure of quasicrystals in Zr69.5Cu12Ni11Al7.5 was investigated by x-ray diffraction and TEM, the kinetics of transformations by DSC and TDA.

Hydrogen was observed to exhibit a significant effect not only on the formation of Zr-based quasicrystalsfrom the glassy precursor material, but also on their stability. Ribbons with an icosahedral microstructure were found to absorb hydrogen up to H/M = 2.0. With increasing hydrogen content an expansion of the quasilattice constant was observed, followed by the formation of approximants and finally amorphization.

Only partial desorption of hydrogen can occur prior to the decomposition of the quasicrystalline phase. Desorption at lower temperatures is hindered by the formation of a thin ZrO2 barrier. Pd plating, often used to overcome such a barrier, improves the absorption kinetics as well as the desorption behavior significantly. DSC of uncoated Zr-based quasicrystals reveals that at low hydrogen contents the decomposition ofthe icosahedral quasicrystals is shifted to lower temperatures; at high hydrogen contents the formation of ZrH2-x was revealed. In the Pd-coated ribbons fully desorption is observed even below 300°C, butonly after hydrogenation up to H/M = 0.5; higher hydrogen contents lead to irreversible changes.

The micromechanism of transformations during hydrogenation as well as the decomposition during annealing of hydrogenated uncoated as well as Pd coated Zr-based quasicrystals are discussed in some detail.