bluuebunny/crossref_metadata_2025 huggingface dataset:
Reliable Method for Eliminating Stacking Fault on 3C-SiC(001)
Authors: Hatta Naoki , Kawahara Takamitsu , Yagi Kuniaki , Nagasawa Hiroyuki , Reshanov Sergey A. , Schöner Adolf
Published: May 2012

A reliable method for reducing the stacking faults (SFs) is demonstrated on the 3C-SiC (001) surface. It is a practical method based on Monte Carlo (MC) simulations of SF propagation during 3C-SiC epitaxial growth, which showed that introducing some discontinuity on the (001) surface enhanced SF reduction. The method is implemented by patterning on the 3C-SiC (001) surface and subsequent homo-epitaxial growth, and this sufficiently reduced the SF density to less than 400 cm-1. A yield of 97.4 % was estimated for a device-ready area of 10 mm2 by statistical measurements of SF density on the entire epitaxial layer surface.
Transition Metal Defects in Cubic and Hexagonal Polytypes of SiC: Site Selection, Magnetic and Optical Properties from Ab Initio Calculations
Authors: Ivády Viktor , Somogyi Bálint , Zólyomi Viktor , Gällström Andreas , Son Nguyen Tien , Janzén Erik , Gali Adam
Published: May 2012

Relatively little is known about the transition metal defects in silicon carbide (SiC). In this study we applied highly convergent and sophisticated density functional theory (DFT) based methods to investigate important transition metal impurities including titanium (Ti), vanadium (V), niobium (Nb), chromium (Cr), molybdenum (Mo) and tungsten (W) in cubic 3C and hexagonal 4H and 6H polytypes of SiC. We found two classes among the considered transition metal impurities: Ti, V and Cr clearly prefer the Si-substituting configuration while W, Nb, and Mo may fractionally form a complex with carbon vacancy in hexagonal SiC even under thermal equilibrium. If the metal impurity is implanted into SiC or when many carbon impurities exist during the growth of SiC then complex formation between Si-substituting metal impurity and the carbon vacancy should be considered. This complex pair configuration exclusively prefers the hexagonal-hexagonal sites in hexagonal polytypes and may be absent in cubic polytype. We also studied transition metal doped nano 3C-SiC crystals in order to check the effect of the crystal field on the d-orbitals of the metal impurity.
Fundamental Study of Sublimation-Recrystallization Phenomena in PVT-Growth of SiC Single Crystals
Authors: Fujimoto Tatsuo , Ohtani Noboru , Sato Shinya , Katsuno Masakazu , Tsuge Hiroshi , Ohashi Wataru
Published: May 2012

Sublimation-recrystallization processes occurring during PVT are investigated from the viewpoint of quasi-equilibrium phase transitions of SiC. In addition to the elemental reaction processes of PVT, other phenomena such as silicon droplet formation and in-situ etching are also discussed based upon the Si-C binary phase diagram, and possible mechanisms are proposed.
Investigation of Intrinsic Carbon-Related Defects in 4H-SiC by Selective-Excitation Photoluminescence Spectroscopy
Authors: Gällström Andreas , Ivanov Ivan G. , Coble R. , Devaty Robert P. , Choyke W.J. , Janzén Erik
Published: May 2012

Emission of carbon-related defects is investigated by means of selectively-excited photoluminescence in high purity 4H-SiC electron-irradiated with very low dose. Two new centers with clearly associated phonon replicas are observed, one of which is tentatively assigned to the carbon split interstitial at hexagonal site. The temperature dependence of the spectrum is also studied and indicates that at least some of the observed luminescence lines arise from recombination of excitons bound to isoelectronic centers.
Status of Large Diameter SiC Single Crystals
Authors: Gupta Avinash K. , Wu Ping , Rengarajan Varatharajan , Xu Xue Ping , Yoganathan Murugesu , Martin Christ , Emorhokpor Ejiro , Souzis Andy , Zwieback Ilya , Anderson Tom
Published: May 2012

Large-diameter SiC single crystals are grown at II-VI by the sublimation technique. 100mm substrates of semi-insulating 6H SiC and n-type 4H SiC are produced as commercial products; in development, diameter expansion to 125mm has been achieved. Over the last two years, significant improvements have been made in crystal quality. The values of FWHM of x-ray rocking curves are typically 20-40 arc-seconds for 6H SI wafers and 12-30 arc-seconds for 4H n+ wafers. Micropipe density is less than 3 cm-2, and less than 0.1 cm-2 in best substrates. Electrical resistivity of SI substrates is, typically, of 1011 Ω•cm or above. For 4H n+ substrates, the typical dislocation density is about 9×103 cm-2 and the typical BPD density is less than 1×103 cm-2.
Fourier Transform Analysis of Basal Plane Dislocation Structure in Repeated A-Face Grown Crystals
Authors: Gunjishima Itaru , Urakami Yasushi , Hirose Fusao , Adachi Ayumu , Onda Shoichi , Nishikawa Koichi
Published: May 2012

Morphological features, such as the orientation and linearity of basal plane dislocations (BPDs) in SiC crystals, were analyzed by applying a two-dimensional fast Fourier transform (2D-FFT) to X-ray topographic images of the BPDs. An SiC crystal fabricated by an improved repeated a-face (RAF) method and an SiC crystal fabricated by an conventional RAF method discussed in a previous study were evaluated. In the 2D-FFT images of the improved crystal, streaks along the directions were observed, indicating that the BPDs were highly oriented along the directions. The degree of orientation of the BPDs, which may reflect their linearity, was calculated, and the improved RAF crystal had a much higher degree of orientation than the conventional RAF crystal.
Basal Plane Dislocation Multiplication via the Hopping Frank-Read Source Mechanism and Observations of Prismatic Glide in 4H-SiC
Authors: Wang Huan Huan , Byrapa Sha Yan , Wu F. , Raghothamachar Balaji , Dudley Michael , Sanchez Edward , Hansen Darren M. , Drachev Roman , Mueller Stephan G. , Loboda Mark J.
Published: May 2012

In this paper, we report on the synchrotron white beam topographic (SWBXT) observation of “hopping” Frank-Read sources in 4H-SiC. A detailed mechanism for this process is presented which involves threading edge dislocations experiencing a double deflection process involving overgrowth by a macrostep (MP) followed by impingement of that macrostep against a step moving in the opposite direction. These processes enable the single-ended Frank-Read sources created by the pinning of the deflected basal plane dislocation segments at the less mobile threading edge dislocation segments to “hop” from one slip plane to other parallel slip planes. We also report on the nucleation of 1/3< >{ } prismatic dislocation half-loops at the hollow cores of micropipes and their glide under thermal shear stress.
Analysis of Dislocations Nucleated after Nano Indentation Tests at Room Temperature in 4H-SiC
Authors: Demenet Jean Luc , Amer Madyan , Tromas Christophe , Rabier Jacques
Published: May 2012

4H-SiC intrinsic homoepitaxied single crystals have been nano indented at room temperature using a spherical indentor and the related deformation microstructures have been analyzed by Transmission Electron Microscopy. Dislocations are lying in the basal plane but have been found to be perfect, in contrast with observations made at higher temperature. Although such a change in deformation mechanism has been observed in other semiconductors such as Silicon and Indium Antimonide, it was unexpected in a very low stacking fault material such as SiC.
Characterization of Triangular-Defects in 4° off 4H-SiC Epitaxial Wafers by Synchrotron X-Ray Topography and by Transmission Electron Microscopy
Authors: Yamashita Tamotsu , Momose Kenji , Muto Daisuke , Shimodaira Yoshiki , Yamatake Kuniaki , Miyasaka Yoshihiko , Sato Takayuki , Matsuhata Hirofumi , Kitabatake Makoto
Published: May 2012

We report our investigation results on triangular-defects formed on 4deg. off 4H-SiC epi- taxial wafers. Triangular-defects that had neither down-falls nor basal-plane dislocations previously reported as origins of triangular-defects at the tips of triangle were investigated by TEM. Our TEM results revealed that foreign materials contamination that were different from well-known down- -falls in size and in composition caused one of the defect formations and abnormal domain forma- tions were implied to occur and thought to relate to defect formations. We also report that several types of microstructure existed in the isosceles of defect during dislocation analyses around triangular-defects by X-ray topography.
Variation of Etch Pit Size by Screw Dislocation Tilt in 4H-SiC Wafer
Authors: Ishikawa Yukari , Yao Yong Zhao , Sugawara Yoshihiro , Danno Katsunori , Suzuki Hiroshi , Kawai Yoichiro , Shibata Noriyoshi
Published: May 2012

The wide size distribution of the hexagonal etch pit of screw dislocations (SD) in 4H-SiC wafer was found in spite of the narrow size distribution of the SD pit in epitaxial film. Calculation on the basis of the strain energy equation indicated that etch pit size depends on the Burgers vector and dislocation tilt. Size variation of SD etch pits in 4H-SiC wafer fabricated by sublimation method is explained to be caused by the dislocation tilt by observing the sizes and the positions of etch pits from the surface of the epitaxial film to the inside of 4H-SiC wafer. The SDs in 4H-SiC wafer fabricated by sublimation method propagate to c-axis direction in macroscopic but changing tilt in microscopic.
Effect of Stacking Faults Originating from Half Loop Arrays on Electrical Behavior of 10 kV 4H-SiC PiN Diodes
Authors: Stahlbush Robert E. , Zhang Qing Chun Jon , Agarwal Anant K. , Mahadik Nadeemullah A.
Published: May 2012

The effects of Shockley stacking faults (SSFs) that originate from half loop arrays (HLAs) on the forward voltage and reverse leakage were measured in 10 kV 4H-SiC PiN diodes. The presence of HLAs and basal plane dislocations in each diode in a wafer was determined by ultraviolet photoluminescence imaging of the wafer before device fabrication. The SSFs were expanded by electrical stressing under forward bias of 30 A/cm2, and contracted by annealing at 550 °C. The electrical stress increased both the forward voltage and reverse leakage. Annealing returned the forward voltage and reverse leakage to nearly their original behavior. The details of SSF expansion and contraction from a HLA and the effects on the electrical behavior of the PiN diodes are discussed.
Low Temperature Photoluminescence Signature of Stacking Faults in 6H-SiC Epilayers Grown on Low Angle Off-Axis Substrates
Authors: Sun J.W. , Robert T. , Jokubavicius Valdas , Juillaguet Sandrine , Yakimova Rositza , Syväjärvi Mikael , Camassel Jean
Published: May 2012

The radiative recombination spectra of 6H-SiC epilayers grown on low angle (1.4° off-axis) substrates have been investigated by low temperature photoluminescence spectroscopy. Four different types of stacking faults have been identified, together with the presence of 3C-SiC inclusions. From the energy of the momentum-conserving phonons, four excitonic band gap energies have been found with Egx equal to 2.837, 2.698, 2.600 and 2.525 eV. These photoluminescence features, which give a rapid and non-destructive approach to identify stacking faults in 6H-SiC, provide a direct feedback to improve the material growth.
Electrically Detected Magnetic Resonance (EDMR) Studies of SiC-SiO2 Interfaces
Authors: Umeda Takahide , Kosugi Ryouji , Fukuda Kenji , Morishita Norio , Ohshima Takeshi , Esaki K. , Isoya J.
Published: May 2012

We discuss the results of electrically detected magnetic resonance (EDMR) spectroscopy on SiC-SiO2 interfaces interacting with hydrogen and nitrogen. Using EDMR, three types of 4H-SiC MOSFETs, which were prepared by dry oxidation (“Dry” sample), post hydrogen anneal (“Hydrogen” sample), and post nitridation anneal (“Nitrogen” sample), were examined in the temperature range of 4–300 K. These samples revealed several different results from the earlier ESR (electron spin resonance) and EDMR studies on SiC-SiO2 interfaces. The most significant finding was the high-density doping of nitrogen into the channel region after the post nitridation anneal. The incorporated nitrogen donors were observed as the “Nh” EDMR signal at 4–20 K. Roles of these nitrogen donors are discussed in correlation with the electrical properties of SiC MOSFETs.
Sodium, Rubidium and Cesium in the Gate Oxides of SiC MOSFETs
Authors: Tuttle B.R. , Dhar S. , Ryu S.H. , Zhu X. , Williams J.R. , Feldman L.C. , Pantelides S.T.
Published: May 2012

Large group-I elements such as sodium, rubidium and cesium have recently been incorporated in the gate oxide of SiC power MOSFETs. In the case of sodium incorporation, enhanced field effect mobilities have been definitively observed. Based on density functional calculations, we find large group-I elements serve as a shallow impurities near the interface. The enhanced mobility, observed in the case of sodium, can be explained in terms of an impurity band model.
Detection of Mobile Ions in the Presence of Charge Trapping in SiC MOS Devices
Authors: Habersat Daniel B. , Lelis Aivars J. , Green Ronald
Published: May 2012

Since power devices such as DMOSFETs will operate at high temperature where mobile ion effects are enhanced, identifying their presence is a key reliability issue for power electronics applications. We have detected the presence of mobile ion contamination in some SiC MOS device sample sets and correlated those results with observed high temperature bias instability. The differing behaviors of these devices to bias stressing as a function of temperature suggests that in some cases mobile ion drift may be counteracting the typical charge trapping effect. Triangular voltage sweep (TVS) data indicates the presence of roughly 1-4x1012 cm-2 mobile ions in samples where the bias instability significantly decreased with higher temperature, while samples with a nearly flat or positive-trending response showed overall lower ion contaminations of roughly 6-9x1011 cm-2. These results, although preliminary, support the theory that mobile ion contamination is the cause of negative bias instability at elevated temperatures in SiC MOS.
Doping-Induced Lattice Mismatch and Misorientation in 4H-SiC Crystals
Authors: Sasaki Sho , Suda Jun , Kimoto Tsunenobu
Published: May 2012

The c- and a-lattice constants of nitrogen-doped 4H-SiC were measured in the wide temperature range (RT - 1100°C). The samples used in this study were heavily doped substrates and lightly-doped free-standing epilayers. The lattice constants at room temperature are almost identical for all the samples. However, the lattice contraction by heavy nitrogen doping was clearly observed at high temperatures, which indicates that the thermal expansion coefficients are dependent on the nitrogen concentration. The lattice mismatch (Δd/d) between a lightly-doped free-standing epilayer (Nd = 6x1014 cm-3) and a heavily-doped substrate (Nd = 2x1019 cm-3) was calculated as 1.7x10-4 at 1100°C. The authors also investigated lattice constants of high-dose N+, P+, and Al+-implanted 4H-SiC. Reciprocal space mapping (RSM) was utilized to investigate the lattice mismatch and misorientation. The RSM images show the c-lattice expansion and c-axis tilt of the ion-implanted layers, irrespective of ion species. The authors conclude that the lattice expansion is not caused by heavy doping itself, but by secondary defects formed after the ion-implantation and activation-annealing process.
Stress Evaluation on Hetero-Epitaxial 3C-SiC Film on (100) Si Substrates
Authors: Anzalone Ruggero , Camarda M. , Locke C. , Carballo J. , Piluso N. , D'Arrigo G. , Severino A. , Volinsky A.A. , Saddow S.E. , La Via F.
Published: May 2012

SiC is a candidate material for micro- and nano-electromechanical systems (MEMS and NEMS). In order to understand the impact that the growth rate has on the residual stress of CVD-grown 3C-SiC hetero-epitaxial films on Si substrates, growth experiments were performed and the resulting stress was evaluated. Film growth was performed using a two-step growth process with propane and silane as the C and Si precursors in hydrogen carrier gas. The film thickness was held constant at ~2.5 µm independent of the growth rate so as to allow for direct films comparison as a function of the growth rate. Supported by profilometry, Raman and micro-machined free-standing structures, this study shows that the growth rate is a fundamental parameter for low-defect and low-stress hetero-epitaxial growth process of 3C-SiC on Si substrates. Stress analysis performed by modify Stoney’s equation trough optical curvature measurement, Raman shift analysis and micro-machining of free-standing structures that shows apparent disagreement about the nature of the stress. These odds between the experimental data can be explained assuming a strong stress field located in the substrate and related to defects generated in the silicon during the growth process.
The Atomic Step Induced by off Angle CMP Influences the Electrical Properties of the SiC Surface
Authors: Tanaka Yayoi , Kanda Takao , Nagatoshi Kazuyuki , Yoshimura Masamichi , Eryu Osamu
Published: May 2012

In this work, we made an off-axis surface of SiC with Mechano-chemical polishing method to examine the influence that an atomic step gave for electronic properties of devices. Low energy ion scattering spectroscopy was shown Si-face and C-face with SiC structure were polished with monocrystal. We consider that there results above the atomic step on the surface influence to a device properties, especially a leakage current of Schottky diode. C atom at the step and terrace cause leakage event at metal-semiconductor interface of Schottky diode have been clarified by conductive atomic force microscopy.
Graphene on Carbon-Face SiС{0001} Surfaces Formed in a Disilane Environment
Authors: Srivastava Nishtha , He Guo Wei , Feenstra R.M.
Published: May 2012

The formation of epitaxial graphene on SiC(000-1) in a disilane environment is studied. The higher graphitization temperature required, compared to formation in vacuum, results in more homogeneous thin films of graphene. Some areas of the surface display unique electron reflectivity curves not seen in vacuum-prepared samples. Using selected area diffraction, these areas are found to have a graphene/SiC interface structure with a graphene-like buffer layer [analogous to what occurs on SiC(0001) surfaces].
The Registry of Graphene Layers Grown on SiC(000-1).
Authors: Johansson Leif I. , Watcharinyanon Somsakul , Zakharov Alexei A. , Yakimova Rositza , Virojanadara Chariya
Published: May 2012

Graphene samples were grown on the C-face of SiC, at high temperature in a furnace and an Ar ambient, and were investigated using LEEM, XPEEM, LEED, XPS and ARPES. Formation of fairly large grains (crystallographic domains) of graphene exhibiting sharp 1x1 patterns in m-LEED was revealed and that different grains showed different azimuthal orientations. Selective area constant initial energy photoelectron angular distribution patterns recorded showed the same results, ordered grains and no rotational disorder between adjacent layers. A grain size of up to a few mm was obtained on some samples.
Micro- and Nano-Scale Electrical Characterization of Epitaxial Graphene on Off-Axis 4H-SiC (0001)
Authors: Giannazzo Filippo , Bongiorno Corrado , Di Franco Salvatore , Rimini Emanuele , Raineri Vito
Published: May 2012

The electrical properties of epitaxial graphene (EG) grown on 8° off-axis 4H-SiC (0001) by annealing at 1600 °C in inert gas ambient (Ar) were studied. The sheet resistance of the EG layers (Rsh=740±50Ω/sq) and the specific contact resistance of Ni contacts to EG (ρc≈6x10-5 Ωcm2) were evaluated on micrometer scale by measurements on transmission line model (TLM) structures. Si3N4 was evaluated as a gate dielectric, showing excellent coverage to EG and a limited effect on its conductivity. The high n-type doping (~1013 cm-2) of EG, as well as the field effect mobility (μ) dependence on n were determined using top gated field effect transistors (FETs) with Si3N4 gate dielectric. Electron mean free path (lloc) and mobility (μloc) were also locally determined, on submicrometer scale, by scanning probe microscopy, showing a broad distribution of μloc values, with the most probable value very similar to macroscopic carrier mobility μ.
Decoupling the Graphene Buffer Layer from SiC(0001) via Interface Oxidation
Authors: Ostler Markus , Koch Roland J. , Speck Florian , Fromm Felix , Vita Hendrik , Hundhausen Martin , Horn Karsten , Seyller Thomas
Published: May 2012

Epitaxial graphene (EG) grown on SiC(0001) resides on the so-called buffer layer. This carbon rich (6√3×6√3)R30° reconstruction is covalently bound to the topmost silicon atoms of the SiC. Decoupling the graphene buffer layer from the SiC interface is a well studied topic since successful intercalation has been shown for hydrogen [1-3]. Recently, intercalation was also shown for oxygen [4, 5]. We present ARPES, XPS and Raman spectroscopy studies to determine the quality of oxygen intercalated buffer layer samples in terms of decoupling and integrity of the transformed graphene layer. The decoupling effect is demonstrated by ARPES measurements showing a graphene-like π band. XPS shows whether the oxidation takes place in the buffer layer or at the interface. Raman spectroscopy is well suited to investigate oxygen induced defects in graphene-like material.
Suppression of Hole Current in Graphene Transistors with N-Type Doped SiC Source/Drain Regions
Authors: Nagahisa Yuichi , Tokumitsu Eisuke
Published: May 2012

To achieve graphene channel transistors which have high on/off drain current ratio and unipolar behavior of drain current – gate voltage (ID-VG) characteristics, we fabricated and characterized the top gated graphene channel transistors with n-type doped SiC source/drain regions. Graphene layer was formed on SiC by high temperature annealing in vacuum, and Al2O3 was used as a gate insulator. For the graphene channel transistor with heavily doped n-SiC source/drain regions (doping concentration ND=4.5x1019cm-3) and a 4~6ML graphene channel, ambipolar behavior was observed. On the other hand, when ND was reduced to 4.5x1018cm-3 and a thin graphene layer was used, the suppression of hole current in ID-VG curve was observed.
SiO2/SiC Interfacial Region: Presence of Silicon Oxycarbides and Effects of Hydrogen Peroxide and Water Vapor Thermal Treatments
Authors: Stedile Fernanda Chiarello , Radtke Cláudio , Soares Gabriel Vieira , Pitthan Eduardo , Palmieri Rodrigo , Corrêa Silma A.
Published: May 2012

This work provides data corroborating the presence of silicon oxycarbides (SiOxCy) in the SiO2/SiC interface region. Besides, it presents results on the efficiency of hydrogen peroxide annealings for reducing the SiO2/SiC interfacial region thickness. Finally, influences of water vapor thermal treatments on dielectric films thermally grown are presented. In most of the samples, isotopes rare in nature (18O and 2H) were used in thermal treatments associated with ion beam analyses.
Identification of Slow States at the SiO2/SiC Interface through Sub-Bandgap Illumination
Authors: Spargo Christopher M. , Furnival Benjamin J.D. , Mahapatra R.M. , Goss J.P. , Wright Nicolas G. , Horsfall Alton B.
Published: May 2012

We show that it is possible to obtain information relating to deep level interface traps, or so called ‘slow states’, by using the photo-CV characterisation method. Sub-bandgap illumination has been chosen in order to avoid band-to-band excitation for the creation of minority carriers. This enables information to be extracted from trapping states at the SiO2/SiC interface that are energetically deep within the band gap. Empirical observations of deep level trapping states with life times in the order of tens of hours are reported and the interface trap density as a function of energy has been extracted using the Terman method. Characterisation of these interface states will aid the development of new fabrication processes, with the aim of reducing the interface trap density to the same level as that of the SiO2/Si interface and facilitating the production of higher quality SiC based devices.
The Thickness-Ratio Effects of Ni/Nb Electrode on Wire Bonding Strength with N-Type 4H-SiC
Authors: Jung Kunhwa , Ando Daisuke , Sutou Yuji , Oyamada Tetsuya , Tanaka Masamoto , Koike Junichi
Published: May 2012

The thickness-ratio effects of Ni/Nb bi-layer electrodes were studied for power device applications. The reaction microstructure and electrical contact property were investigated after annealing at 1000 °C and compared with the results of an Ni electrode. Microstructure-related problems of the Ni electrode could be successfully resolved without sacrificing ohmic contact behavior by the addition of Nb to a Ni based electrode. Carbon precipitation was reduced with increasing Nb thickness by the formation of carbides, which led to good adhesion between the electrode and a wiring pad. High shear strength of the bonded wire was also obtained by the elimination of the carbon precipitates on the electrode surface.
Curious Relationship between Orientation of SiC Substrates and Chemical Reactivity
Authors: Hatayama Tomoaki , Koketsu Hidenori , Yano Hiroshi , Fuyuki Takashi
Published: May 2012

Relationship between the chemical reactivity and the orientation of SiC substrates was investigated. Thermal etching of 4H-SiC in the mixed gas of oxygen and chlorine was carried out as the chemical reaction. The etching rate did not change monotonously with the increase of the off angle in 4H-SiC (000-1) C substrate. By the use of such tendency in the thermal etching, the three dimensional structure with the specific pyramidal plane was able to be obtained.
High-Resolution TEM Observation of 4H-SiC (0001) Surface Planarized by Catalyst-Referred Etching
Authors: Pho Bui Van , Sadakuni Shun , Okamoto Takeshi , Sagawa Ryusuke , Arima Kenta , Sano Yasuhisa , Yamauchi Kazuto
Published: May 2012

A novel abrasive-free planarization method “called catalyst-referred etching (CARE)” has been invented. After the CARE process, a flat and well-ordered surface is obtained as observed by atomic force microscopy (AFM). To determine the atomic structure at the topmost surface, in this study, CARE-processed surfaces of a standard commercial 2-inch n-type 4H-SiC (0001) wafer cut 8o off-axis toward the [1-100] direction were observed by high-resolution transmission electron microscopy (HRTEM). The HRTEM images showed alternating wide and narrow terraces and a single-bilayer step height. The relationship between the width of the terraces and the 4H-SiC crystal structure has been clarified.
Novel Cleaning Method of SiC Wafer with Transition Metal Complex
Authors: Kubo Maiko , Hidaka Makoto , Kageyama Motohiro , Okano Tomomichi , Kobayashi Hisayoshi
Published: May 2012

In this article, we report a new cleaning method for silicon carbide (SiC) wafers. We found that the dipping treatment in hydrogen fluoride (HF) solution damages the SiC in the “RCA cleaning process”, so we have designed a new cleaning method that does not use HF and reduced the cleaning process to three steps. The characteristic factor of this new method is using a transition metal complex. Generally, no metals have been used for wafer cleaning, but we deliberately used metal and found it could clean the wafer surface very well. After cleaning, the atomic force microscope (AFM) and “Candela” images showed that the particles on most parts of the SiC surface had been removed and the contact angle for ultra-pure water became very low.
SIMS Analyses Applied to Open an Optical Window in 4H-SiC Devices for Electro-Optical Measurements
Authors: Lazar Mihai , Jomard François , Nguyen Duy Minh , Raynaud Christophe , Pâques Gontran , Scharnholz Sigo , Tournier Dominique , Planson Dominique
Published: May 2012

4H-SiC vertical bipolar power diodes have been fabricated with bilayer metallic anode contact based on an Al-Ti-Ni ohmic contact and a thick Al over-metallization. An optical window of 100 × 100 μm2 has been created through the anode contact with a SIMS Cameca IMS 4F equipment using Cs+ primary ions at 10 kV and with a beam spot size of 100 nm. The current/voltage characteristics of the diodes show that the SIMS process does not induce an increase of the leakage currents in forward nor in reverse bias. OBIC UV photogeneration occurs under the optical window and not under the contact metal.
Hexagonal Faceted SiC Nanopillars Fabricated by Inductively Coupled SF6/O2 Plasma Method
Authors: Choi J.H. , Latu-Romain Laurence , Baron Thierry , Chevolleau Thierry , Bano Edwige
Published: May 2012

We demonstrate a top-down fabrication technique for nanometer scale silicon carbide (SiC) pillars by using inductively coupled SF6/O2 plasma etching. The obtained SiC nanopillars exhibit high anisotropy features (aspect ratio ~ 6.5) with high etch depth (>7 μm). The etch characteristics of SiC nanopillars obtained under these conditions show a high etch rate (550 nm/min) and a high selectivity (over 60 for Ni mask). We obtained hexagonal symmetry of SiC nanopillar, which might be attributed to the crystallographic structure of the SiC phase.
4H-SiC Trench Structure Schottky Diodes
Authors: Aketa Masatoshi , Yokotsuji Yuta , Miura Mineo , Nakamura Takashi
Published: May 2012

This paper presents three different structures of Schottky diodes that were fabricated with low Schottky barrier heights. To reduce the forward voltage drop, the introduction of a lower Schttoky barrier is necessary. One of key issues associated with diodes having a low Schottky barrier height and a planar structure is an excessively high leakage current. By introducing the novel trench structure, the leakage current was reduced to a reasonable level. Furthermore it was confirmed that they have minimal switching time during turn-off and high avalanche capability. Thus trench structure Schottky diodes are able to reduce not only switching losses but also conductive losses and demonstrate sufficient robustness.
Single Photolithography/Implantation 120-Zone Junction Termination Extension for High-Voltage SiC Devices
Authors: Snook Megan , McNutt Ty , Kirby Chris , Hearne Harold , Veliadis Victor , Nechay Bettina , Woodruff Sharon , Howell R.S. , White Joseph , Davis Stuart
Published: May 2012

The multi-zone junction termination extension (MJTE) is a widely used edge termination technique for achieving high voltage SiC devices. It is commonly implemented with multiple lithography and implantation events. In order to reduce process complexity, cycle time, and cost, a single photolithography and single implant MJTE technique has been successfully developed. The method utilizes a pattern of finely graduated oxide windows that filter the implant dose and create a graded MJTE in a single implant and single photolithography step. Based on this technique, 6 kV / 0.09 cm2 PiN diodes were fabricated utilizing a 120-zone single-implant JTE design. This novel single-implant MJTE design captures 93% of the ideal breakdown voltage and has comparable performance and yield to a baseline three implant process.
Novel Fork Architectures of Ag2S Nanoparticles Synthesized through In Situ Self-Assembly inside Chitosan Matrix
Authors: Hashmi Lubna , Sana Prabha , Malik M.M. , Siddiqui A.H. , Qureshi M.S.
Published: May 2012

In a simple reaction between Silver nitrate and Thiourea under the presence of Chitosan, novel fork architectures of silver sulfide (Ag2S) were obtained throughIn-situself-assembly of Ag2S nanoparticles inside Chitosan matrix in aqueous basic environment. Samples were characterized by SEM, TEM, XRD, FTIR, Raman Spectroscopy and UV-vis Spectroscopy. Average particle size was found to be 8 nm from TEM analysis. Formation of crystalline monoclinic α-Ag2S was confirmed by SAED and X-ray diffraction analysis. The experimental results show that the fork architectures of Ag2S can be obtained from Tape like assemblies of Ag2S nanoparticles at the later stage of their growth with temperature. Raman spectra confirm the incorporation of Ag2S nanoparticles inside Chitosan matrix. Multiple resonant Raman peaks indicates that the yielded Ag2S nanoparticles possess good optical quality and crystalinity. FTIR spectra indicates that the Ag+ion is coupled with the C=O bond of the Chitosan molecules through the electrostatic interaction. UV-vis absorption spectrum of the Ag2S Fork architectures has a broad absorption band in the UV to visible region, and the blue-shifting of the band is observed along with the presence of both direct and indirect band gap.
Synthesis of CdS Nanocrystals Using Polymer Metal Complex as a Single Source Precursor and their Physicochemical Properties
Authors: Ahamad Tansir , Alshehri Saad M.
Published: May 2012

Polymeric ligand (TSCF) have been synthesized via polycondansation of thiosemicarbazide with formaldehyde and its polymer metal complex [TSCF-Cd (II)] has been prepared with Cd (II) ion. Two different batches of CdS chalcogenide, semiconductor nanocrystals have been synthesized by hydrothermal treatment of TSCF-Cd (II)] using a glass Q tube reactor at 200 °C and 250 °C, and marked as (CdS)1and (CdS)2respectively. The resultant nanocrystals were characterized by a variety of methods for thier physochemcial properties. The TEM micrographs showed well-defined, close to hexagonal particles, and the lattice fringes in the HRTEM images confirmed their crystalline nature. The sizes of (CdS)1and (CdS)2were 40 and 50 nm respectively but their morphologies were similar. The optical band gap energies (2.52 eV/ 2.48 eV) and photoluminescence peaks (532/534 nm) of the synthesized CdS nanocrystals suggest that they can be promising photocatalysts. The conductivities and the dielectric constants of the CdS nanocrystals were also studied and the activation energy value of (CdS)1and (CdS)2nanocrystals were found to be 0.64 eV and 0.70 eV.
Characterization of Electron Beam Irradiated Polyvinylpyrrolidone-Dextran (PVP/DEX) Blends
Authors: Dumitraşcu Maria , Albu Mădălina Georgiana , Vîrgolici Marian , Vancea Cătălin , Meltzer Viorica
Published: May 2012

In the past years an increased interest to create new polymeric blends with application in the medical area for development of new types of biomaterials has appeared. Electron beam irradiation is well known as a method of producing important changes in polymer structure, being an alternative to chemical synthesis of biomaterials based on polymeric materials. The aim of the present study was to investigate the behaviour of some polyvinylpyrrolidone-dextran (PVP/DEX) blends under electron beam irradiation. Aqueous solutions of PVP with molecular weights of 360 000 Da (PVP 360), 40 000 Da (PVP 40), and DEX with molecular weight of 500 000 Da (DEX), were mixed as to obtain 50:50 blends of PVP40/DEX and PVP360/DEX. The obtained blends were irradiated with electron beam at different radiation doses and after irradiation treatment were processed by freeze-drying. PVP/DEX blends were characterized by infrared spectroscopy (FT-IR) and thermal analysis. The analyses were conducted in order to establish the relation between radiation dose and changes of structural and thermal properties.
Analysis of Deformation Characteristics of Cortical Bone Tissue
Authors: Li Si Min , Demirci Emrah , Silberschmidt Vadim V.
Published: May 2012

Numerical modeling of bones is necessary for design of efficient surgical cutting tools that can provide low cutting forces, reduce damage and prevent thermal necrosis of bone tissue. Development of realistic numerical models of cortical bone tissue requires deep knowledge of its deformation behaviour. Deformation mechanisms of bones differ from those of metals, polymers and composites since bones consist of a living tissue with hierarchical microstructure. The aim of this study is to analyse deformation characteristics of the cortical bone tissue from both experimental and numerical perspectives. Initially, Vickers hardness tests were conducted at various anatomical positions on a cross-section of a bovine femur bone to observe location-based variation of its mechanical response. Various load magnitudes ranging between 1 kgf and 100 kgf were applied in the Vickers hardness tests to analyse the effect of anisotropy on damage evolution. These tests were simulated using a finite element scheme to reproduce the mechanical behaviour of bones in indentation. Finally, results of the hardness tests were compared with those obtained from finite element simulations.
Drilling-Induced Damage in CFRP Laminates: Experimental and Numerical Analysis
Authors: Phadnis Vaibhav A. , Makhdum Farrukh , Roy Anish , Silberschmidt Vadim V.
Published: May 2012

The use of composite materials such as carbon fiber-reinforced plastic (CFRP) has grown considerably in recent years, especially in aerospace, automotive, sports and construction industries. The properties such as high strength and stiffness, low weight, excellent fatigue and corrosion resistance have made them a useful material for light-weight applications. Though parts made from CFRP are often manufactured to a near-net shape, various machining processes such as drilling, can be used to facilitate assembly of structures. Drilling CFRPs involve penetrating through several plies of laminate, which causes high stresses and strains in the vicinity of the drilled hole. Thus, the machining process not only affects the overall hole quality but also initiates discrete damage phenomena such as micro-cracking, matrix burning; delamination and fiber pull out in the specimen. Moreover, the cutting edges of a drill wear dramatically out due to presence of highly abrasive fibers in the matrix, resulting in increased thrust forces that can cause interply delamination.
Ultrasonically Assisted Drilling of Carbon Fibre Reinforced Plastics
Authors: Makhdum Farrukh , Norddin Dk Nurdiyana Pg. , Roy Anish , Silberschmidt Vadim V.
Published: May 2012

Carbon fibre-reinforced plastics (CFRPs) gained substantial acclaim in recent decades and are used in aerospace, automotive and structural applications due to their high strength-to-weight ratio, high stiffness, high fatigue and corrosion resistance. CFRPs are manufactured near to net shape but some machining processes such as drilling cannot be avoided. Drilling induces damage (delamination, matrix cracking, matrix burning, lamina cracking and fibre pull out) in CFRP because of high axial thrust forces and a temperature rise. In this research an attempt is made to use ultrasonically assisted drilling (UAD) to reduce the axial thrust forces. In UAD high frequency (~ 20 kHz) vibrations are superimposed on a drill bit, preferably in axial direction, to reduce the thrust forces. In this study, experiments are conducted in two stages. At the first stage an initial setup with an existing UAD transducer is used to compare UAD with conventional drilling (CD) of CFRP. A reduced thrust force is experienced in case of UAD when compared to CD. At the second stage, drilling dynamics, i.e. feed speed, is changed along with the improvement of the transducer, and an enormous amount of force reduction (>80%) is observed in case of UAD (as compared to CD).
Estimation of the Constitutive Law by Dual Small Punch Test and Instrumented Indentation
Authors: Guillemot Gildas , van Gorp Adrien , Coorevits Thierry , Iost Alain
Published: May 2012

In this paper an inverse finite element procedure was developed to assess the true- stress strain relationship of a DP dual phase. Experiments were performed both by means of instrumented indentation and small punch tests on small samples cut in the same steel sheet. Tests are developed on a Zwick macro-hardness tester. Single test are first proposed in order to estimate the parameters of the constitutive law (K,n). We propose to conduct SPT tests using PTFE lubricant and hard (WC / Co) ball. An inverse method based on the Nelder and Mead algorithm has been developed in order to found experimental curves. Nevertheless we show that this method lead to poor estimation of the parameter of the constitutive law. We propose to improve these estimations by cross-mechanical tests. Indeed we show that the estimation of the low-error (K,n) domains deduced from each single test can be crossed. This method leads to a better estimation of the constitutive law. Moreover both load/displacement curves of SPT and macrohardness can be found again.
Experimental Study of Bimaterial Shear Strength and Strain Concentrations by Iosipescu Based Test Using Digital Image Correlation System
Authors: Sadowski Tomasz , Kneć Marcin
Published: May 2012

Adhesive bonding of two different materials appears in many modern engineering applications, e.g.: airplanes, boats, cars etc. In many practical problems the adhesive bonding is subjected to shear loading. Therefore this is important to investigate the whole deformation process of the considered type of joints under monotonic loading, to get information about the shear strength and strain concentrations. Such concentrations lead to microdefects initiation and their further coalescence to create a main crack. The unstable crack propagation leads to final failure of the adhesive joint. The Digital Image Correlation (DIC) System - ARAMIS allows for constant monitoring of the deformation state up to the final failure. The tests were performed for bi-material specimens made of adhesively bonded PMMA and aluminum strips (Fig.1) and for pure PMMA and pure aluminum specimens. Additionally, two strain gauges on each homogeneous specimen and four on the bimaterial ones are used for strains estimations. The four point bending Iosipescu tests were performed using MTS machine with constant speed. In the first method (DIC) the ARAMIS system recorded a displacement distribution in samples with frequency 1Hz. In the second method the strains were recorded by the strain gauges - using analog output channels of the HOTTINGER data Acquisition System - MGCPlus, the current value of the load using analog output channel of the MTS machine was recorded too. The load-displacement curves were obtained for the whole deformation process and the shear strength of the joints was estimated. The energy absorption of the joints was calculated.
Modeling and Prediction of Mechanical Behavior of Plastic Gears in Simulated Wear Situation
Authors: Koffi Demagna , Kassegne K.A. , Wotodzo K.F. , Bedja K.
Published: May 2012

The present work shapes a normal plastic gear and simulates the corresponding worn one in order to predict its mechanical behavior in operation depending on the wear. To predict the mechanical behavior of plastic gears, a modeling of the gears has been done under SOLIDWORKS. Then with ALGOR, which uses the FEM, we studied two types of gear. A normal tooth of each type of gear has been net as well as the corresponding worn tooth. We opted for the study of two cases of charge. The first (case 1) corresponds to the application of strength to the head of the tooth (Fig. 2) and the second (case2) at the pitch point of the tooth (Fig. 3). We noticed the stresses and deformations on the nodes located on the right profile of the tooth, the first node is taken at the head of the tooth. The wear has been assumed uniform on the right profile from the head to the root. The tooth has been assumed embedded at the root. We obtained some results which could allow the prediction of the number of revolutions to breaking off, knowing the wear according to this cycle.
Test Vehicle for Studying Thermal Conductivity of Die Attach Adhesives for High Temperature Electronics
Authors: Slater Conor , Vecchio Fabrizio , Maeder Thomas , Ryser Peter
Published: May 2012

Polymer adhesives offer a viable method for mounting silicon dies for high temperature applications. Here a test vehicle for comparing the thermal conductivity of different die attach materials is presented. The setup can be used to determine the degree of degradation of polymers. It consists of a mock die that has an integrated thick film heater, which is mounted onto a substrate. In operation, the substrate is placed on a heatsink and the die is heated. When the temperature reaches equilibrium the heater is switched off and the temperature of the die is measured as it cools. The time constant of the temperature decay is calculated to give the thermal conductivity. In this paper the thermal conductivity of an epoxy die attach adhesive is compared to its shear strength.
Effects of Thermal Losses on the Heating of a Multifunctional LTCC Module for Atomic Clock Packaging
Authors: Vecchio Fabrizio , Maeder Thomas , Slater Conor , Ryser Peter
Published: May 2012

An innovative multifunctional LTCC module has been designed for miniature atomic clock packaging. Efficient packaging and interconnection of the atomic clock packaging is a critical issue and a precise temperature control is required for some components, such as mini-cell and light source. The great advantage of using LTCC technology for this application is that it allows the integration of different functions, such as heaters and PTCs resistors for temperature measurement and control, and optionally other active elements. In this research, a platform for measuring the thermal conductivity of materials has been developed in order to perform precise thermal studies on the packaging. The relationship between achieved temperature and power dissipated for the heating of the LTCC module has been calculated in different experimental configurations, in order to determine the effects of conduction and convection on the heating and estimate the thermal losses that they introduce into the system.
Finite-Element Analysis of Forces in Drilling of Ti-Alloys at Elevated Temperature
Authors: Muhammad Riaz , Ahmed Naseer , Shariff Yasir Maqbool , Silberschmidt Vadim V.
Published: May 2012

Demand for high-strength alloys in aerospace, marine and off-shore industries has grown significantly over last decades, primarily thanks to their high strength, light weight as well as good fatigue and corrosion-resistance properties. However, these materials are extremely difficult to machine with conventional machining methods. Hot machining is an alternative technique used by many researchers for cutting of hard-to-cut materials in turning and milling operations. In this assisted machining technique, an external heat source is used to reduce shear strength of the machined workpiece, enhancing material removal of such alloys. Drilling is one of the most important and basic operations for producing cylindrical holes in machined components. In this work, a three-dimensional finite-element (FE) model of drilling process is developed in a commercial FE software DEFORM 3D. A nonlinear temperature-dependent material behaviour is incorporated in numerical simulations. The effect of an external heat source on thrust forces and torque on a drill-bit was investigated with the developed FE model. Advantages of hot drilling in reducing thrust force and torque reduction are demonstrated.
Durability Study of a Machine Shaft Based on the Material Behavior of 41Cr4 Steel Subjected to Rotating Bending
Authors: Kun Lorand , Nes Cristian Sorin , Dumitru Ion , Faur Nicolae
Published: May 2012

This paper presents the durability analysis of a machine shaft subjected to bending with torsion, based upon the stress analysis at a notch which joins segments of different cross-sections (circular and square respectively) of the shaft. During the specific operating conditions of the equipment which’s part the shaft is, this notch has proven to be critical, causing premature failure. The durability analysis is carried out based on the real loading conditions, allowing the stress state evaluation at the base of the notch for different radii, using ABAQUS 6.9-3 finite element analysis program. The experimental studies carried out on the shaft’s material, 41Cr4 steel, resulted in the determination of material properties and of torsion fatigue curves in case of two different heat treatments. The cycles composing the equivalent stress spectrum are counted using the rainflow algorithm. The number of loading blocks to failure (number of load spectrum repetitions), considering the real operating conditions, is obtained using Miner’s rule, based on the rotating bending fatigue curve, corrected corresponding to the shaft’s specific characteristics. The proposed calculation method, based on the variation in time of the stress tensor during operating conditions, leads to the determination of the optimal notch radius for which the shaft’s durability falls between prescribed limits.