bluuebunny/crossref_metadata_2025 huggingface dataset:
Fretting Tribology of Al2O3/Cu Composite Prepared by Powder Metallurgy
Authors: Ding Hong Yan , Zhou Guang Hong , Zhang Yue , Liu Yi Fa
Published: March 2009

The nano-Al2O3 reinforced copper matrix composite was prepared by powder metallurgy technique and the fretting test against 440C stainless steel was performed at room temperature by using the ball-on–flat configuration with 300μm amplitude at various normal loads in the range of 0.1N to 1N. The influences of load and proportion of Al2O3 on the friction coefficient and volume wear -loss were investigated. Results showed that the volume wear-loss decreased firstly and then increased as the proportion of Al2O3 increased from 0.5% to 4%, and the minimum proportion was about 2%, in which the electrical conductivity can reach up to 81% according to International Annealed Copper Standard (IACS). The friction coefficient of the copper will slightly increases as the load increased. However, in the case of composites, it increased firstly then decreased. The volume wear-loss of the composite is always lower than that of copper, giving the highest relative wear-ability of 1.55, which indicated a higher wear resistance. The wear mechanism of copper is adhesive wear, whereas it becomes oxidation wear in composites.
Microstructure Characterization of the Cu-Ti-C Composites Prepared by Mechanical Alloying and Spark Plasma Sintering
Authors: Zhuang Jian , Liu Yong Bing , Cao Zhan Yi , Li Yue Ying , Liu X.R.
Published: March 2009

The evenly dispersed Cu-Ti-C composites have been synthesized successfully from the element powders by mechanical alloying (MA) and spark plasma sintering（SPS）techniques. The evolution of microstructure with different processing parameters has been investigated by XRD and SEM. The Cu-TiC and the Cu-Ti2Cu3-C composites with different hardness and density were obtained from different content of Cu, milling time and sintering temperature. The effect of Cu content in the compound powders, milling times and sintering temperatures on the solid solution and reaction process of Cu-Ti-C system was also discussed and analyzed.
Comparison of As-Cast Microstructures and Mechanical Properties for Mg-Ce-Mn-Sc and Mg-Ce-Mn-Zn Magnesium Alloys
Authors: Shen Jia , Yang Ming Bo , Pan Fu Sheng , Cheng Ren Ju
Published: March 2009

The as-cast microstructures and mechanical properties of Mg-3Ce-1.2Mn-0.9Sc and Mg-3Ce-1.2Mn-1Zn magnesium alloys were investigated and compared. The results indicate that the as-cast microstructure of Mg-3Ce-1.2Mn-0.9Sc alloy was mainly composed of -Mg, Mg12Ce and Mn2Sc phases, and that the as-cast microstructure of Mg-3Ce-1.2Mn-1Zn alloy was mainly composed of -Mg, Mg12Ce and MgZn phases. In addition, the as-cast tensile and creep properties of Mg-3Ce-1.2Mn-0.9Sc alloy were higher than that of the Mg-3Ce-1.2Mn-1Zn alloy. The difference of the two alloys in as-cast tensile and creep properties may be related to the initial microstructures of the two alloys.
Study on the Filtering Purification of AZ91 Magnesium Alloy
Authors: Le Qi Chi , Zhang Zhi Qiang , Cui Jian Zhong , Chang Shou Wei
Published: March 2009

High quality magnesium casting depends significantly on the purification of its alloy melt. However, it is very difficult to obtain high-quality magnesium alloy melt due to its high reactivity and its tendency of combustion. The fluxing processing, a traditional purification method for magnesium melt, not only bears the risk of flux inclusions but also is facing more and more environmental pressure today. Therefore, the effective substitutes for fluxing processing, such as physical filtering method, are paid more attention recently. In this paper, the effects of technological conditions including the pore size of stainless steel mesh and the filtering temperature on the purification degree of AZ91 alloy melt were investigated in the mesh filtering processing, in which 2 wt.% calcium was added into the melt in order to obtaining ignition-proof property. The results indicated that mesh filtering processing could improve the purification degree markedly, and the purification degree increased with the increase of filtering temperature properly or the decreased of pore size of mesh. However, too high filtering temperature would lead to the increasing of the tendency of oxidation and combustion, and too small pore size would decrease the filtering ability of mesh which would lead to the interrupting of purification operation.
Effects of Strontium Addition on Microstructure and Properties of AE42 Magnesium Alloy
Authors: Jiang Yong , Xie Wei Dong , Peng Xiao Dong , Wei Qun Yi , Feng Yan Bing , Wang Shou Cheng
Published: March 2009

The samples with different strontium contents of AE42 magnesium alloy were prepared. The metalloscope and electronic microscope and XRD were used to characterize the microstructure and composition of the samples. The elevated temperature mechanical properties of the samples were tested and the strontium phases in the AE42 and their influence to the mechanical properties analyzed. The results showed that the main strontium phase in the AE42 was Al4Sr. Al4Sr pinned the crystal boundary of magnesium matrix and hindered dislocation climbing and sliding through the crystal boundary effectively. As a result, the crystal axial range was reduced and the crystal grain refined, and hence the creep resistance increased.
Study on the Moisture Absorption Properties of Poly[acrylate-co-methacrylate]
Authors: Zhang Chun Xiao , Bai Fu Chen , Zhang Xi Yao , Liu Jian , Zhang Wan Xi
Published: March 2009

Moisture-absorbent poly- (acrylate-co-methacrylate) was synthesized by solution polymerization, using N,N’-methylene bisacrylamide as the crossinglinking agent, sodium bisulfate and ammonium persulfate as the initiator. The moisture absorption properties of the copolymer in different temperatures and humidities have been systematically studied. Experimental results indicated that the moisture absorption behavior has been enhanced with the increase amount of methacrylate. Under different relative humidity, the moisture adsorption ability was distinct. When the relative moisture reached 40%, the humidity behavior became less evident. As the increase of moisture, the absorbency for moisture improves correspondingly. When the relative moisture get to 90%, the properties of this copolymer was obviously improved compared with the traditional materials, such as silica gel and molecular sieve.
First-Principles Study of the Mechanical Properties of Mg-Based Alloys with FCC and Hexagonal Structure
Authors: Wang Na , Yu Wei Yang , Zhang Wei Bing , Tang Bi Yu , Zeng Xiao Qin , Ding Wen Jiang
Published: March 2009

More and more research has been focused on the improvement of the mechanical properties and the optimal design of the new excellent Mg-based alloys. In spite of many experimental investigations, the theoretical studies of the mechanical properties are very scarce. First-principle calculations of the elastic constants and mechanical properties of typical Mg-based alloys become necessary to understand the fundamental mechanism governing the observed mechanical properties. In this paper, the single-crystal elastic constants Cijs of the typical fcc and hexagonal structured Mg-based alloys (Mg3Zn3Y2 and CaMg2) were calculated, using density functional theory within the generalized gradient approximation. Then the bulk modulus B, shear modulus G, Young’s modulus E, Poisson’s ratio ν and anisotropy value A were derived from single-crystal elastic constants. The mechanical properties such as the ductility and stiffiness of the alloys are analyzed and discussed in comparison with experimental observations.
Effect of Ga on the Microstructure and Mechanic Al Properties of AZ81 Alloy
Authors: Sun Jing , Mao Ping Li , Liu Zheng , Guo Quan Ying , Yi Yang , Zhao Zhong Gang
Published: March 2009

The effect of Ga on the microstructure and mechanical properties of Mg-8wt.%Al alloy at room temperature were investigated by SEM and X-ray analysis. The microstructure of as-cast Mg-8wt.%Al alloy was consist of α-Mg matrix, β- Mg17Al12 and α+β eutectic phases. The primary β- Mg17Al12 phase precipitate along the α-Mg matrix grain boundary and in the discontinuous reticular shape. Ga addition in the as-cast Mg-8wt.%Al alloy change the microstructure significantly. When the addition is less then 2% Gd could be detected and distributed mainly within β- Mg17Al12 phase and in the interface of α-Mg matrix and β- Mg17Al12. With increasing Ga content α-Mg matrix grains and β- Mg17Al12 phase are refined. The amount of α+β eutectic structure increase as well. The holes within β phase are diminished. When Ga content was more than 2% with increasing of Ga content β phase was broken along the concentration of Ga. With the increase of Ga content to 4%, a new phase of Ga2Mg form in the microstructure and β phase precipitate in the shape of smooth baboon along the grain boundary. The mechanical properties test showed that as-cast Mg-8wt.%Al-2wt.%Ga alloy has the optimum combination of ultimate tensile strength of 248 MPa and elongation of 10%.
Influence of KH-550 Silane Treatment on the Corrosion Resistance of AZ31 Magnesium Alloy
Authors: Zhang Wei , Pan Fu Sheng , Yang Ming Bo , Zhang Jin , Wen Tao
Published: March 2009

Silane treatment is an effective method of chemical conversion as a replacement for chromium treatments. In this paper, KH-550 silane treatment was adopted to improve the corrosion resistance AZ31 magnesium alloy. Hydrolysis process of KH-550 in the silane solutions was detected by the conductivity detection method. It was found that KH-550 silane could be hydrolyzed easily and its hydrolysis time was about 10 minutes. Polarization curve and salt immersion were carried out to investigate the corrosion behavior of the coated magnesium alloy treated by five kinds of silane solutions with different silane concentrations. When the concentration of KH-550 in the silane solution was 8%, the coated specimens by heating at 100°C for half an hour after the surfaces dry had more excellent corrosion resistance than others. It was shown that the KH-550 silane treatment could form a -Si-O-M covalent bonded film/metal interface and -Si-O-Si siloxane chains to improve the corrosion resistance of coated magnesium alloy.
Synergistic Effects of Al and Cr Addition on Microstructure of Mg2Ni Hydrogen Storage Alloy
Authors: Xie Zhao Ming , Pan Fu Sheng , Xiang Ya Fang , Chen Yuan
Published: March 2009

The Mg-based hydrogen storage alloys of Mg2Ni (A2B), Mg1.7Al0.3Ni, Mg2Ni0.8Cr0.2 and Mg1.8Al0.2Ni0.8Cr0.2 were successfully prepared by mechanical alloying and sintering. The effect of the substitution side A by Al and side B by Cr on microstructure of Mg2Ni alloy has been studied by XRD and SEM. The results indicated that the synergistic effect occurred when the elements Al and Cr coexist in Mg2Ni alloy, element Cr could inhibit the formation of non-hydrogen-adsorption phase of Al3Ni2, in addition, it will promote the formation of hydrogen-adsorption phase of Mg3AlNi2; the existence of Al could inhibit the formation of non-hydrogen-adsorption phase of -[ Cr, Ni].
Thermal Mechanical Behaviors and Microstructure Characteristics in Twin-Roll-Casting Process of Magnesium Alloy AZ31
Authors: Hu Xiao Dong , Zhao Hong Yang , Ju Dong Ying
Published: March 2009

Twin-Roll-Casting process is a rapid solidification process combining with hot rolling deformation. The liquid metal will be squeezed out but not sucked in from the mush zone caused by rolling deformation. This is different from conventional continuous casting process and will greatly affect the microstructure morphology and surface cracks of as-cast strip. In this paper, a 2D FEM model was employed to simulate the thermal mechanical behavior during twin-roll casting process of Mg alloy AZ31. The FEM results were compared with experimental results and show the effect of rolling deformation on the microstructure morphology and surface cracks of as-cast sheet.
The Experimental Research of a Heavyweight High-Strength Concrete
Authors: Luo Ying She , Wang Hong , Liu Wen Bin , Deng Shou Chang , Ma Yuan Rong
Published: March 2009

The definition of heavyweight high-strength concrete（HHC for short）is firstly introduced. According to the characteristic targets of a HHC, suitable experimental material and project are chosen. Via analyzing and commenting on various properties of different minerals such as silica powder and fly ash, a feasibility study for mixed them into concrete to manufacture HHC is conducted, and its mechanics properties and Alkali-resistant property are tested. On the basis of them, HHC whose strength is C60 and apparent density is 2800 kg/m3 is successfully manufactured. The optimum proportion is given out.
Nonlinear Stability Analysis on the Concrete Casting Step of Long-Span Concrete-Filled Steel Tube Arch Bridge
Authors: Wang Ji , Zhang Ming Zhong , Guo Xiao Li
Published: March 2009

In recent years, concrete-filled steel tube (CFST) was developed and used extensively in civil engineering in China. In this paper, the method for stability analysis of long-span CFST arch bridge was introduced. Based on Jingyang River Bridge in Hubei province of China and finite element method, the spatial model was set up. Both linear and nonlinear stability of long-span CFST arch bridge in construction process were analyzed. The result indicated the influence of geometrical nonlinearity was small and the influence of material nonlinearity was evident. So, for analyzing the stability of long-span CFST arch bridge, the influence of geometrical nonlinearity and material nonlinearity must be considered at the same time. The results of paper were used to provide the basis for the construction control of the bridge.
Dislocations and Triangular Defect in Low-Temperature Halo-Carbon Epitaxial Growth and Selective Epitaxial Growth
Authors: Das Hrishikesh , Melnychuk Galyna , Koshka Yaroslav
Published: March 2009

Dislocations were investigated in the halo-carbon low-temperature epitaxial growth and low-temperature selective epitaxial growth (LTSEG) conducted at 13000C. The origin of triangular defects was investigated in low-temperature epilayers grown at higher growth rates with HCl addition. Due to the conversion of substrates’ basal plane dislocations (BPD) into threading dislocations, the concentration of BPDs was about an order of magnitude lower than the concentration of threading dislocations at moderate growth rates. An additional order of magnitude conversion of BPDs into threading dislocations was observed at higher grow rates achieved with HCl addition. In LTSEG epilayers, dislocation concentration away from the mesa walls was comparable to the blanket (i.e., regular non-selective) growth. High concentrations of BPDs were found only at mesa edges located on the “upstream” side with respect to the step-flow direction. No substrate defects could be traced to the triangular defects. Instead, the disturbances causing the triangular defect generation are introduced during the epitaxial process.
Growth of Graphene Layers on Silicon Carbide
Authors: Strupiński Wlodek , Bożek Rafał , Borysiuk Jolanta , Kościewicz Kinga , Wysmolek Andrzej , Stepniewski Roman , Baranowski Jacek M.
Published: March 2009

The so-called “growth” of graphene was performed using a horizontal chemical vapor deposition (CVD) hot-wall reactor. In-situ etching in the mixture (H2-C3H8) was performed prior to growth at 1600oC temperature under 100 mbar. Systematic studies of the influence of the decomposition temperature and time, substrates roughness, etching of the substrates, heating rate, SiC dezorientation and other process parameters on the graphene thickness and quality have been conducted. Morphology and atomic scale structure of graphene was examined by Scanning Tunnelling Microscopy (STM), Transmission Electron Microscopy (TEM) and Raman scattering methods.
Growth of Few Graphene Layers on 6H, 4H and 3C-SiC Substrates
Authors: Camara Nicolas , Huntzinger Jean Roch , Tiberj Antoine , Rius G. , Jouault B. , Pérez-Murano Francesco , Mestres Narcis , Godignon Philippe , Camassel Jean
Published: March 2009

We report a comparative investigation of few layers graphene grown on 6H, 4H and 3C-SiC substrates. We show that the size of the graphitic domains depends more on the <0001> SiC surface orientation than the polytypism.
Temperature Dependence and Selective Excitation of the Phosphorus Related Photoluminescence in 4H-SiC
Authors: Ivanov Ivan G. , ul Hassan Jawad , Henry Anne , Janzén Erik
Published: March 2009

The paper presents experimental data on the temperature dependence and the excitation properties of the phosphorus-related photoluminescence in 4H SiC. Two main sets of phonon replicas can be observed with selective excitation, which are attributed to two of the no-phonon lines observed in the spectrum. Some of the excited states are also attributed to one of the no-phonon lines on the ground of the selectively excited spectra. A tentative explanation of the observed features in terms of multiple bound excitons is proposed.
Long Carrier Lifetime in 4H-SiC Epilayers Using Chlorinated Precursors
Authors: Shrivastava Amitesh , Klein Paul B. , Glaser E.R. , Caldwell Joshua D. , Bolotnikov A.V. , Sudarshan Tangali S.
Published: March 2009

In this work we report the measurement of minority carrier lifetimes using the time resolved photoluminescence technique. It was found that 4H-SiC homo-epilayers grown using chlorine-based precursors have longer carrier lifetimes if used in conjunction with a tantalum carbide coated (TaC-coated) graphite susceptor rather than a SiC-coated graphite susceptor. Longer carrier lifetimes were obtained by optimal combinations of precursor gases and susceptor type. The controllable variation in lifetime from 250 ns to 9.9 s was demonstrated.
Optical Investigation of Electronic Properties in Bulk and Surface Region of Sublimation-Grown 3C-SiC Crystals
Authors: Manolis Georgios , Jarašiūnas Kęstutis , Galben-Sandulache Irina G. , Chaussende Didier
Published: March 2009

We applied a picosecond dynamic grating technique for studies of nonequilibrium carrier dynamics in a 0.8 mm thick bulk 3C-SiC crystal grown by the continuous feed physical vapor transport (CF-PVT) on 6H-SiC (0001) substrate. Investigation of carrier dynamics at surface or bulk excitation conditions was performed for excess carrier density in range from ~ 1017 cm-3 to ~ 1020 cm3 using for excitation weakly or strongly absorbed illumination. In DPBs free domains, the bipolar diffusion coefficient and carrier lifetime value at 300K were found gradually increasing with carrier density. The bipolar mobility vs. temperature dependence, μ. ~ T -k, provided a value k = 1.2 - 2 in range T < 100 K, thus indicating a negligible scattering by point and extended defects. These data indicated strong contribution of the carrier-density dependent but not defect-density governed scattering mechanisms, thus indicating high quality of the CF-PVT grown bulk cubic SiC. These studies were found in good correlation with the structural and photoluminescence characterization of the given crystal.
Spin-Coupling in Heavily Nitrogen-Doped 4H-SiC
Authors: Savchenko D.V. , Pöppl Andreas , Kalabukhova Ekaterina N. , Greulich-Weber Siegmund , Rauls Eva , Schmidt Wolf Gero , Gerstmann Uwe
Published: March 2009

EPR and ESE in nitrogen doped 4H- and 6H-SiC show besides the well known triplet lines of 14N on quasi-cubic (Nc,k) and hexagonal (Nc,h) sites additional lines (Nx) of comparatively low intensity providing half the hf splitting of Nc,k. Frequently re-interpreted as spin-forbid¬den lines, arising from Nc,k pairs and triads or resulting from hopping conductivity, only re¬cent¬ly the theoretical calculation of the corresponding g-tensors lead to a tentative model of distant NC donor pairs on inequivalent lattice sites which are coupled to S = 1 assuming a fine-struc¬ture splitting too small to be observed in the EPR and ESE experiments. In this work, we pre¬sent ESE nutation measurements confirming S = 1 for the Nx center. Analysing the nutation frequencies in comparison with that of the Nc,k (S = 1/2) spectrum as well as the line width of ESE and EPR spectra we obtain a rough estimate between 5104 cm-1 and 50104 cm-1 for the fine-structure splitting demonstrating efficient spin-coupling between nitrogen donors in 4H-SiC.
SiCCSiAntisite Pairs as Dominant Irradiation Induced Defects in p-Type 4H-SiC
Authors: Gerstmann Uwe , Seitsonen A.P. , Mauri Francesco , von Bardeleben Hans Jürgen
Published: March 2009

In this work we elucidate the microscopic origin of the dominant radiation induced I-II
spectra in p-type doped 4H-SiC. By calculating the electronic g-tensor from first principles in the
framework of density functional theory, basal antisite pairs SiCCSi
+ are shown to give rise to the
characteristic anisotropic g-tensors found in the electron paramagnetic resonance (EPR)
measurements. Additional central hyperfine (hf) splittings of about 100 MHz due to the SiC antisite
nuclei are predicted theoretically and also resolved experimentally. We have, thus, identified
antisite pairs as a dominant defect in electron and proton irradiated p-type doped 4H-SiC.
Defects Introduced by Electron-Irradiation at Low Temperatures in SiC
Authors: Son Nguyen Tien , Isoya Junichi , Morishita Norio , Ohshima Takeshi , Itoh Hisayoshi , Gali Adam , Janzén Erik
Published: March 2009

Defects introduced by electron irradiation at ~80-100 K in 3C-, 4H- and 6H-SiC were studied by electron paramagnetic resonance (EPR). A number of EPR spectra, labelled LE1-10, were detected. Combining EPR and supercell calculations, we will show that the LE1 center in 3C-SiC with C2v symmetry and an electron spin S=3/2 is related to the (VSi-Sii)3+ Frenkel pair between the silicon vacancy and a second neighbour Sii interstitial along the <100> direction. Results on other centers, possibly also related to interstitials, are discussed.
Role of Spontaneous Polarization Effect in the Formation of the Energy Diagram of the NH/3C/NH-SiC Heterostructure: An Analytical Approach
Authors: Davydov Sergey Y. , Lebedev Alexander A.
Published: March 2009

The three-layer heterostructure formed by the two domains of the cubical 3C-SiC polytype and hexagonal NH-SiC (N = 4, 6, 8) layer is considered. The Poisson equation with the account of spontaneous polarization for the hexagonal component has been solved with the assumption that the resulting electric field can be treated as a sum of contact and polarization field components. The analytical expression for the value of the dimensionless band-bending potentials on the interfaces is found. It is demonstrated that the account of the spontaneous polarization leads to the asymmetry of the energy diagram, which results in an inequality of the quantum wells located at the interfaces. The possibility of the indirect electronic transition between the states of these quantum wells is considered.
Two-Branch Boron Diffusion from Gas Phase in n-Type 4H-SiC
Authors: Bolotnikov A.V. , Muzykov Peter G. , Agarwal Anant K. , Zhang Qing Chun Jon , Sudarshan Tangali S.
Published: March 2009

In this work the analysis of thermal diffusion of boron carried out from vapor phase was performed. Two-branch diffusion associated with kick-out and substitution mechanisms was observed. The activation energy and prefactor were calculated from Arrhenius plot for each diffusion branch. It has been established that the surface layer of diffused boron mostly consists of shallow boron acceptors, while the tail of the diffusion profile has mostly deep level D centers.
Diffusion of Point Defects from Ion Implanted 4H-SiC: Cathodoluminescence Observation
Authors: Mitani Takeshi , Hattori Ryo , Yoshikawa Masanobu
Published: March 2009

Depth profiles of ion-implantation induced defect centers have been investigated by cross-sectional CL measurements in the energy range from visible to near infrared. CL observation has shown that point defects diffused out from implanted region to ~10 µm depth during activation annealing. Annealing temperature dependence of the depth distribution of CL intensity of these defects has suggested that structural transformation of point defects proceeds as “silicon vacancy (VSi) → carbon vacancy-antisite pair (VC-CSi ; UD2) → antisite pair (CSi-SiC ; DI)”.
Atomistic Scale Modeling and Analysis of Sodium Enhanced Oxidation of Silicon Carbide
Authors: Chatterjee Aveek , Matocha Kevin
Published: March 2009

We explain the phenomenon of sodium enhanced oxidation (SEO) using computational techniques, and analyze a set of probable hypotheses, which can elucidate the observation of high inversion channel mobility achieved with SEO. The ability of the Na to screen the interface traps, reduce carbon cluster type of defect formation, and enhance the oxidation rate can be explained using these calculations. We observe an increased availability of electrons near the interface in the presence of Na. The sodium atom also helps in breaking the intramolecular oxygen bond. The electronic and atomic structure of the interface cluster, and electric field computations showed that the carbon cluster formed at the oxide side of the interface could be screened by the Na ion.
Interface States in 4H- and 6H-SiC MOS Capacitors: A Comparative Study between Conductance Spectroscopy and Thermal Dielectric Relaxation Current Technique
Authors: Løvlie Lars S. , Pintilie Ioana , Kumar C.P. S. , Grossner Ulrike , Svensson Bengt Gunnar , Beljakowa Svetlana , Reshanov Sergey A. , Krieger M. , Pensl Gerhard
Published: March 2009

The purpose of this work is to compare the density of shallow interface states (Dit) at the interface of SiO2/SiC MOS capacitors as deducted by the conductance spectroscopy (CS) and thermally dielectric relaxation current (TDRC) techniques. Both capacitors of 4H- and 6H-SiC (n-type) are investigated, and both ordinary dry oxidation and an improved industrial procedure have been employed. The two techniques are found to give rather good agreement for interface states located ≥0.3 eV below the conduction band edge (Ec) while for more shallow states vastly different distributions of Dit are obtained. Different reasons for these contradictory results are discussed, such as strong temperature and energy dependence of the capture cross section of the shallow interface states.
Characterization of 4H-SiC–SiO2 Interfaces by a Deep Ultraviolet Spectroscopic Ellipsometer
Authors: Seki H. , Wakabayashi T. , Hijikata Yasuto , Yaguchi Hiroyuki , Yoshida Sadafumi
Published: March 2009

We have characterized 4H-SiC–oxide interfaces fabricated by thermal oxidation of SiC using spectroscopic ellipsometry in the wide spectral range from visible to deep UV region. It was found that there exists an interface layer, around 1 nm in thickness, regardless of the oxide thickness from 15 nm to 40 nm. The optical constants of the interface layer have similar spectral dependence to those of SiC, though the absolute value of the refractive indices is 0.5–1 larger than that of SiC. We have discussed the structure of the interface layer based on the oxidation mechanism of SiC, like the Si-emission model.
Electrical Characterization of MOS Structures with Deposited Oxides Annealed in N2O or NO
Authors: Grieb Michael , Noborio Masato , Peters Dethard , Bauer Anton J. , Friedrichs Peter , Kimoto Tsunenobu , Ryssel Heiner
Published: March 2009

In this work, the electrical characteristics and the reliability of 80nm thick deposited oxides annealed in NO and N2O on the 4H-SiC Si-face for gate oxide application in MOS devices is analyzed by C-V, I-V measurements and by constant current stress. Compared to thermally grown oxides, the deposited oxides annealed in N2O or NO showed improved electrical properties. Dit-values lower than 1011cm-2eV-1 have been achieved for the NO sample. The intrinsic QBD-values of deposited and annealed oxides are one order of magnitudes higher than the highest values reported for thermally grown oxides. Also MOSFETS were fabricated with a channel mobility of 20.05 cm2/Vs for the NO annealed deposited oxide. Furthermore annealing in NO is preferred to annealing in N2O regarding µFE- and QBD-values.
Effect of High Temperature Oxidation of 4H-SiC on the Near-Interface Traps Measured by TDRC
Authors: Allerstam Fredrik , Sveinbjörnsson E.Ö.
Published: March 2009

In this work the effect of oxidation temperature of 4H-SiC on the density of near-interface traps is studied. It is seen that the portion of traps with slower emission times decreases with increasing oxidation temperature. Despite this reduction, high temperature oxidation alone is not useful to achieve low density of interface traps at the SiO2/4H-SiC interface.
AlON/SiO2 Stacked Gate Dielectrics for 4H-SiC MIS Devices
Authors: Hosoi Takuji , Harada Makoto , Kagei Yusuke , Watanabe Yuu , Shimura Takayoshi , Mitani Shuhei , Nakano Yuki , Nakamura Takashi , Watanabe Heiji
Published: March 2009

We propose the use of an aluminum oxynitride (AlON) gate insulator for 4H-SiC MIS devices. Since direct deposition of AlON on 4H-SiC substrate generates a large amount of interface charge due to an interfacial reaction, a thick AlON layer was deposited on underlying thin SiO2 thermally grown in N2O ambient. To reduce the negative fixed charge density in the aluminum oxide (Al2O3) film, we used reactive sputtering of Al in an N2/O2 gas mixture. The fabricated MIS capacitor with AlON/SiO2 stacked gate dielectric shows no flat band voltage shift and negligible capacitance-voltage hysteresis (30 mV), indicating the dielectric is almost free from both fixed charges and electrical defects. Owing to the high dielectric constant of AlON (k=6.9), as compared to single N2O-SiO2 gate insulator, significant gate leakage reduction was achieved by AlON/SiO2 stacked gate dielectrics even at high-temperature, especially in a high electric field condition (>5 MV/cm).
Reliability of 4H-SiC(000-1) MOS Gate Oxide Using N2O Nitridation
Authors: Suzuki Takuma , Senzaki Junji , Hatakeyama Tetsuo , Fukuda Kenji , Shinohe Takashi , Arai Kazuo
Published: March 2009

The oxide reliability of metal-oxide-semiconductor (MOS) capacitors on 4H-SiC(000-1) carbon face was investigated. The gate oxide was fabricated by using N2O nitridation. The effective conduction band offset (Ec) of MOS structure fabricated by N2O nitridation was increased to 2.2 eV compared with Ec = 1.7 eV for pyrogenic oxidation sample of. Furthermore, significant improvements in the oxide reliability were observed by time-dependent dielectric breakdown (TDDB) measurement. It is suggested that the N2O nitridation as a method of gate oxide fabrication satisfies oxide reliability on 4H-SiC(000-1) carbon face MOSFETs.
Phase Inhomogeneity and Electrical Characteristics of Nickel Silicide Schottky Contacts Formed on 4H-SiC
Authors: Nikitina Irina P. , Vassilevski Konstantin , Horsfall Alton B. , Wright Nicolas G. , O'Neill Anthony G. , Ray S.K. , Johnson C. Mark
Published: March 2009

Nickel silicide Schottky contacts were formed on 4H-SiC by consecutive deposition of a titanium adhesion layer, 4 nm thick, and nickel, 100 nm thick, followed by annealing at temperatures from 600 to 750 °C. It was found that contacts with barrier heights of 1.45 eV, consisting mainly of NiSi phase, formed in the 600-660 °C temperature range, while annealing at around 750 °C led to the formation of Ni2Si phase with barrier heights of 1.1 eV. Annealing at intermediate temperatures resulted in the nucleation of Ni2Si grains embedded in the NiSi film which were directly observed by micro-Raman mapping. It was concluded that the thermodynamically unfavourable NiSi phase appeared in the 600-660 °C temperature range due to the fact that the solid state chemical reaction between Ni and SiC at these temperatures is controlled by nickel diffusion through the titanium barrier.
Basal Plane Dislocation Mitigation in 8º Off-Cut 4H-SiC through In Situ Growth Interrupts during Chemical Vapor Deposition
Authors: VanMil Brenda L. , Stahlbush Robert E. , Myers-Ward Rachael L. , Picard Yoosuf N. , Kitt S.A. , McCrate J.M. , Katz S.L. , Gaskill D. Kurt , Eddy Charles R.
Published: March 2009

Conversion of basal plane dislocations (BPD) to threading edge dislocations (TED) in 8° off-cut 4H-SiC within an n+ buffer layer would eliminate the nucleation site for Shockley-type stacking faults in active device regions grown on such buffer layers. To that end, the propagation and conversion of BPDs through in situ growth interrupts is monitored using ultraviolet photoluminescence (UVPL) wafer mapping. The optimized growth interrupt scheme lasts for 45 minutes with a propane mass flow of 10 sccm at growth temperature. This scheme has shown a conversion efficiency of up to 99% for samples with electron (hole) concentrations < 5x1014 cm-3 (8x1015 cm-3). Samples subjected to a 45 or 90 minute interrupt under 10 sccm of propane, regardless of conversion efficiency, exhibit a “slit” in the surface morphology associated with each BPD and oriented perpendicular to the off-cut and BPD propagation direction. Repetition of the optimal interrupt sequence with a 5 μm epilayer spacer grown between the two interrupts resulted in the same conversion efficiency as a single optimal growth interrupt. Incorporation of the optimal interrupt into an n+ layer is more complicated as attempts to do so in layers doped with nitrogen to 2x1018, 2x1017 and 2x1016 cm-3 resulted in conversion efficiencies of ~6%.
Bottom-Up Routes to Porous Silicon Carbide
Authors: Greulich-Weber Siegmund , Friedel B.
Published: March 2009

We report on constructive methods providing a large range of high purity porous SiC products. All methods are based on modified sol-gel processes combined with carbothermal re¬duction. We obtain monodisperse regular pores of well defined diameters by using carbon sphere templates which are removed after SiC infiltration. A different way is a sol-gel based conversion of graphite bodies into SiC, which transfers the porosity from the graphite matrix into the final SiC product. Thus a large variety of porosity features are available, originating either from natural poro¬sity of graphite or from priorly created nano-/ microstructures in the carbonaceous base material. Whereas all our pristine porous sol-gel derived silicon carbide products are semi-insulating, doping is possible, during the growth to modifiy the electrical and optical properties.
Fabrication and Characterization of Cr-Based Schottky Diode on n-Type 4H-SiC
Authors: Koliakoudakis C. , Dontas J. , Karakalos S. , Kayambaki M. , Ladas S. , Konstantinidis G. , Kennou S. , Zekentes Konstantinos
Published: March 2009

The behavior of 200nm Cr Schottky contacts on n-type 4H-SiC has been investigated with photoelectron spectroscopy (XPS) and standard (I-V and C-V) electrical measurements at different measurement temperatures. A barrier height close to 1.2 eV was calculated from XPS data under no-current and no-bias conditions on ultra-thin Cr films grown in-situ under UHV conditions. The I-V measurements on as-deposited contacts resulted in a barrier height of 1.06 eV while a value of 1.2 eV has been extracted from the C-V measurements.
High-Temperature Reliability Assessment of 4H-SiC Vertical-Channel JFET Including Forward Bias Stress
Authors: Cheng Lin , Mazzola Michael S. , Sheridan David C.
Published: March 2009

In this work, we report the most recent reliability results of the 1200-V SiC vertical-channel JFETs (VJFETs) under reverse and forward bias of the gate-source diode at temperatures up to 200 °C. The preliminary results indicate that continuous forward bias stress of the gate-source diode at 200 °C for 112 hours produced no observable change in the forward conduction or transient or reverse blocking characteristics of the vertical-channel JFET. This preliminary result suggests that devices based on this structure, such as the enhancement-mode (normally off) SiC VJFET, may not be effected by the recombination enhanced defect creation process and the associated increase in on-resistance, related to body-diode conduction in the SiC DMOSFET and the SiC lateral-channel depletion-mode JFET. Since the vertical-channel JFET has no body diode, no degradation is possible from the reverse conduction mode of operation.
Effects of N Implantation before Gate Oxidation on the Performance of 4H-SiC MOSFET
Authors: Poggi Antonella , Moscatelli Francesco , Solmi Sandro , Nipoti Roberta , Tamarri Fabrizio , Pizzochero G.
Published: March 2009

The electrical characteristics of MOSFETs fabricated on 4H-SiC with a process based on N implantation in the channel region before the growth of the gate oxide are reported as a function of the N concentration at the SiO2/SiC interface up to 6  1019 cm-3. The field effect mobility improves with increasing N concentration. At room temperature values change from 4 cm2/Vs for the not implanted sample up to 42 cm2/Vs for the sample with the highest N concentration. Furthermore, the field effect mobility increases with temperature and presents values above 60 cm2/Vs at 200 °C. The MOSFETs with the better electrical characteristics (higher mobility, lower threshold voltage, lower subthreshold swing) were fabricated by a low thermal budget oxidation process, thank to the use of a high N implantation dose able to produce an amorphous SiC surface layer. A strong correlation among the increasing of the N concentration at the SiO2/SiC interface, the reduction of the interface state density located near the conduction band and the improvement of the MOSFETs performance was obtained.
High Channel Mobility of 4H-SiC MOSFETs Fabricated by Over-Oxidation of the N-/Al-Coimplanted Surface Layer
Authors: Reshanov Sergey A. , Beljakowa Svetlana , Frank Thomas , Zippelius Bernd , Krieger M. , Pensl Gerhard , Noborio Masato , Kimoto Tsunenobu
Published: March 2009

Conventional MOSFETs and Hall-bar MOSFETs are fabricated side by side by over-oxidation of N-implanted or N-/Al-coimplanted 4H-SiC layers. It is demonstrated that the N-/Al-coimplanted MOSFETs possess a positive threshold voltage at room temperature and reach high values of the channel mobility. The effective electron mobility and Hall mobility in Hall-bar MOSFETs are 31 cm2/Vs and 150 cm2/Vs, respectively, indicating a high density of interface traps in spite of the excellent high mobility values.
Effect of Oxidant in MOCVD-Growth of Al2O3 Gate Insulator on 4H-SiC MOSFET Properties
Authors: Moriya Hitoshi , Hino Shiro , Miura Naruhisa , Oomori Tatsuo , Tokumitsu Eisuke
Published: March 2009

We have examined the effect of oxidant in metalorganic chemical vapor deposition (MOCVD) of Al2O3 gate insulator on MOSFET electrical properties. High channel mobility of 311 cm2/Vs for Al2O3/SiC MOSFET is demonstrated when the Al2O3 gate insulator is deposited on HF-treated substrate at 190oC using triethyl-aluminum (TEA) and O2 as Al source and oxidant gas, respectively. This is much higher than that of Al2O3/SiC MOSFET when Al2O3 gate insulator was deposited with TEA and H2O at the same temperature. In addition, channel mobility at high gate electric field can be improved by using O2 as oxidant gas and effective mobility of 207 cm2/Vs is obtained at SiO2 equivalent gate electric field of 1.5 MV/cm.
Ab Initio Calculations of SiO2/SiC Interfaces and High Channel Mobility MOSFET with (11-20) Face
Authors: Okuno Eiichi , Endo Takeshi , Sakakibara Toshio , Onda Shoichi , Itoh Makoto , Uda Tsuyoshi
Published: March 2009

Ab initio calculations were carried out to study the origin of the trap at the SiO2/SiC (MOS: Metal-Oxide-Semiconductor) interface with the three different faces of the substrate, (0001), (000-1), and (11-20). In a previous report we experimentally discovered that the (11-20) face is suitable for high channel mobility. The calculation in this report showed that the MOS interface achieved the intermediate states due to distortion and thus acted like an interface trap. The interface trap density of the MOS interface on the (11-20) face substrate was smaller than those on the other faces. The interface trap densities were 2.14, 3.36, and 1.40 in units of 1015 cm-2 for the above listed substrate orientations, respectively. For clarity, the channel mobility was compared experimentally to reveal that it realized a larger value for the (11-20) substrate than the other two faces. From our results, we concluded that (11-20) face substrate was more suitable for high power device applications than the (0001) face or (000-1) face substrates.
Interface Trap Density and Mobility Characterization of Silicon Carbide MOSFET Inversion Layers
Authors: Tilak Vinayak , Matocha Kevin , Dunne Greg
Published: March 2009

Silicon Carbide (SiC) based metal oxide semiconductor field effect transistors (MOSFETs) were fabricated and characterized using gated hall measurements with different p-type substrate doping concentration (7.2X1016cm-3 and 2X1017 cm-3). An interface trap state density of 5X1013 cm-2eV-1 was observed nearly 0.1 eV above the conduction band edge leading to the conclusion that these states are present in the silicon dioxide rather than the interface. The Hall mobility of the MOSFETs decreased from 26.5 to 20 cm2/Vs as the doping was increased from 7.2X1016 to 2X1017cm-3. The decrease in mobility is primarily due to an increase in the surface electric field that causes an increase in surface roughness scattering. The inversion layer mobility when plotted as a function of average surface electric field is not independent of doping concentration as is the case in silicon MOSFETs because the dominant scattering mechanism is not phonon scattering.
Implications of Threshold-Voltage Instability on SiC DMOSFET Operation
Authors: Lelis Aivars J. , Habersat Daniel B. , Green Ronald , Goldsman Neil
Published: March 2009

Although recent fast I-V measurements and subthreshold analysis reveal that the threshold-voltage instability due to low-field bias stressing at room temperature is greater than previously reported when calculated using slower, standard measurements by a parameter analyzer—a result that is consistent with electrons directly tunneling into and out of near interfacial oxide traps, this effect will not prevent the use of power SiC DMOSFET switches in power converter applications if certain precautions are followed. Namely, if the threshold voltage is set high enough so that a negative shift in threshold voltage will not increase the leakage current in the off-state, then the primary effect will be to increase the on-state resistance by decreasing the effective gate voltage. The instability due to ON-state stressing is greater than that for bias stressing alone, but not significantly. For a well behaved device, a 1-hour ON-state stress will result in about a 7 percent increase in conduction losses, which is manageable for power converter applications.
Chloride-Based SiC Epitaxial Growth
Authors: Pedersen Henrik , Leone Stefano , Henry Anne , Beyer Franziska Christine , Lundskog A. , Janzén Erik
Published: March 2009

Some aspects of the chloride-based CVD growth process have been investigated by using both the approach to add HCl to the standard precursors or/and by using the single molecule precursor methyltrichlorosilane (MTS). The efficiency of the process for different precursors, the growth rate stability and the effect that the Cl/Si-ratio has on the growth have been studied. MTS is showed to be the most efficient precursor; the growth can be hindered by to much chlorine in the gas mixture. The Cl/Si-ratio is also found to be a process parameter that affects the amount of incorporated nitrogen in the epilayers.
High Power Density SiC 450A AccuMOSFET for Current Limiting Applications
Authors: Tournier Dominique , Bevilacqua Pascal , Planson Dominique , Morel Hervé , Brosselard Pierre , Montserrat Josep , Lhorte André , Carcouet S. , Leonard D.
Published: March 2009

The expansion of the electrical communications and distribution networks strongly contribute to the increase in the risks of appearance of defaults, such as over-voltage and/or over-current. These developments promote the emergence of safety devices for serial protection commonly named Current Limiting Devices (CLD's). This work presents the design, manufacture and characterization of silicon carbide accuMOSFET of high power density ratings. Components able to limit the current up to 450A @ 350V were manufactured and characterized. Specific characterization test benches were developed, able to provide high energy pulses required for characterization. CLDs behavior subjected to short overloads has been measured experimentally and analyzed be means of simulations for long-time overloads. A maximum sustainable high energy of 35 Joules has been estimated. This achievement give opportunities to build new architectures of serial protection systems.
Growth of Thick 4H-SiC Epitaxial Layers on On-Axis Si-Face Substrates with HCl Addition
Authors: Leone Stefano , Pedersen Henrik , Henry Anne , Rao Shailaja P. , Kordina Olof , Janzén Erik
Published: March 2009

Homoepitaxial growth of 4H-SiC on on-axis Si-face substrates is reported using hydrogen chloride together with silane and ethylene. In this study, the main process parameters, such as temperature, Cl/Si ratio, C/Si ratio, Si/H2 ratio and ramp up conditions, were studied in detail to understand their effects on the growth mechanisms. Two different optimal epitaxial growth conditions were found. Silicon rich conditions and a high Cl/Si ratio were the key parameters to grow thick homoepitaxial layers with a very low background doping concentration and a growth rate higher than 20 μm/h.
GaN Epitaxial Growth on Sapphire Substrate with Al Buffer Layer Prepared by E-Beam Evaporation
Authors: Hwang Hyun Hee , Kim Jung Kyu , Choi Jong Mun , Lee Won Jae , Kim Il Soo , Shin Byoung Chul , Lee Hae Yong
Published: March 2009

GaN epitaxial layers were grown on sapphire substrate deposited an aluminum (Al) buffer layer using a hydride vapor phase epitaxy (HVPE) system with a two-zone resistance furnace. A 10nm-thick Al buffer layer was prepared by an e-beam evaporation in order to reduce the stress resulted from thermal mismatch between the GaN layer and the substrate. The growth temperature and growth rate for GaN epitaxial layer were 1050oC and 40 m/hr, respectively. GaN epitaxial layer grown on substrate with Al buffer layer exhibited uniform and smooth morphology on 2-inch whole substrate and a bow value of 33.5 m. The addition of Al-buffer layer apparently reduced the full width at half maximum (FWHM) value of GaN layer, which indicated the improvement of crystal quality.
Seeded Growth of AlN on (0001)-Plane 6H-SiC Substrates
Authors: Filip Octavian , Epelbaum Boris M. , Bickermann Matthias , Heimann Paul , Nagata S. , Winnacker Albrecht
Published: March 2009

Aluminum nitride (AlN) is a promising substrate material for epitaxy of Al-rich III-nitrides to be employed, e.g., in deep-UV optoelectronic and high-power microwave devices. In this context, preparation of bulk AlN crystals by physical vapor transport (PVT) appears to be of most importance. In this work, seeded growth of AlN on (0001)-plane 6H-SiC substrates was investigated. SiC substrates with a diameter of 15 mm were used. AlN layers with thicknesses up to 3 mm were deposited at growth rates in the range of 10 to 40 μm/hour. Such templates provide large-area seeds, but they are often cracked, especially at thicknesses below 1mm. Besides cracks, other defects from the SiC seed propagate into the AlN layer and subsequently into the bulk AlN crystal. That is why, the aim of this work is to assess structural quality and defect content in thick AlN templates grown on (0001) plane SiC substrates. An optimum thickness-quality, the most appropriate growth stage for further use of the AlN template as a seed for subsequent PVT growth of bulk AlN growth, will be provided. We found that low growth rates mitigate crack propagation; slow cooling as well as optimization of the thermal field inside the crucible can prevent formation of new cracks after growth.