bluuebunny/crossref_metadata_2025 huggingface dataset:
7 kV 4H-SiC GTO Thyristors
Authors: Campen Stephen Van , Ezis Andris , Zingaro John , Storaska Garrett , Clarke R. Chris , Temple Vic , Thompson Mark , Hansen Todd
Published: April 2011

High Power asymmetric SiC GTOs (Gate Turn-Off Thyristors) were fabricated on n-type 4H-SiC substrates with multiple epi-layers and were tested to investigate breakdown voltage, maximum current density, switching characteristics, and temperature dependences.

Comparison of breakdown voltages achieved by GTOs with Guard Ring edge termination and GTOs with a proprietary Junction Termination Extension (JTE) fabricated on the same wafer are made. Individual GTOs with a nominal area of 4 mm2 (2 mm × 2 mm) utilizing the JTE were tested in forward bias and found to support over 6 kV at leakage currents of less than 5 μA. In addition, GTOs with a nominal area of 0.25 mm2 (0.5 mm × 0.5 mm) utilizing the JTE were found to support over 7 kV. These blocking voltages are the highest reported for GTOs in SiC. [1–5] Wafer maps of the breakdown voltages for older and newer wafers suggest an improvement of the material. The defect density for the newer wafer is estimated to be greater than the micropipe density.

The on-state characteristics are equally impressive, with 4 mm2 GTOs carrying > 1000 A/cm2. The GTO has a forward voltage drop of 3.66 V at a current density of 300 A/cm2 at room temperature and a forward voltage drop of 3.1 V at 300 A/cm2 at 224 °C. This indicates that on-state losses should not be excessive, even at high current densities and high temperatures.
Distinct Magnesium Incorporation Behavior in Laterally Grown AlGaN
Authors: Liu R. , Bell A. , Ponce F. A. , Cherns D. , Amano H. , Akasaki I.
Published: April 2011

Different magnesium incorporation behavior has been observed in heavily Mg-doped AlGaN epitaxial layers. The films were grown by metal-organic vapor phase epitaxy involving a lateral overgrowth technique on patterned sapphire substrates. TEM observations show that direct growth on sapphire exhibits pyramidal defects, while lateral overgrowth is homogeneous and free of structural defects. The orientation of the growth front significantly influences the microstructure, and the {0001} growth facet appears to be essential for the formation of the pyramidal defects. In addition, cylindrical and funnel-shaped nanopipes have been observed at dislocations with an edge component. The relationship between Mg segregation and these defects is discussed, and formation mechanisms are proposed taking into consideration the orientation of the growth front.
Lateral growth of AlxGa1–xN and GaN on SiC substrates patterned by photo-electrochemical etching
Authors: Rossów U. , Riedel N. , Hitzel F. , Riedl T. , Hangleiter A.
Published: April 2011

The large defect densities in heteroepitaxially grown group-III-nitride layers on sapphire or SiC cannot be tolerated in applications such as lasers. We report here on a defect reduction by overgrowth of patterned n-6H-SiC(0001)surfaces.

First, we formed mesa structures in the windows of metal masks and then after removal of the masks layers of AlxGa1–xN and GaN were grown by low-pressure MOVPE under conditions of high lateral growth rates. We demonstrate that layers and layered structures can be grown with smooth surfaces and reduced defect densities.
Blue Luminescence in Undoped and Zn-doped GaN
Authors: Reshchikov M. A. , Morkoç H. , Molnar R. J. , Tsvetkov D. , Dmitriev V.
Published: April 2011

A broad band with a maximum at about 2.9 eV (blue band) is widely observed in the photoluminescence (PL) and cathodoluminescence (CL) spectra of unintentionally doped GaN grown by metalorganic chemical vapor deposition (MOCVD) or by hydride vapor phase epitaxy (HVPE). In some samples this band exhibits fine structure attributed to electron-phonon coupling, in others it appears featureless. Different defect origin and recombination mechanisms responsible for the blue band have been suggested in the past. The situation is complicated by the fact that bands similar in shape and position were observed also in Zn- and Mg-doped GaN, as well as in undoped GaN after dry or wet etching. We investigated PL in HVPE-grown undoped, Si- and Zn-doped GaN layers in wide temperature and excitation intensity ranges. We have found that the shape, temperature and excitation intensity dependencies of the blue band in undoped GaN are almost identical to those in GaN lightly doped with Zn. Moreover, in both undoped and Zn-doped samples we observed a distinctive set of peaks related to an exciton bound to the Zn acceptor. Although the exact structure of the Zn acceptor is still unknown, our experimental results unambiguously demonstrate that Zn impurity is responsible for the blue band in unintentionally doped GaN. The results of transient PL study are also consistent with the above attribution.
Self-Induced Photon Absorption by Screening of the Electric Fields in Nitride-based Quantum Wells
Authors: Kalliakos S. , Lefebvre P. , Taliercio T. , Gil B.
Published: April 2011

We have calculated the change of interband absorption spectra of a quantum well based on hexagonal group-III nitride semiconductors under photo-injection of high densities of electron-hole pairs. The screening of internal electric fields by such optical excitation is known to blue-shift and reinforce the ground-state optical transition. Due to the large values of densities of states and of internal fields, we predict novel properties that rather concern optical absorption via transitions between excited states. The absorption coefficient can be strongly enhanced by the optical excitation itself, in this particular spectral region, yielding the possibility for self-induced absorption properties. In other words, if sufficiently intense, an excitation laser can increase the absorption coefficient of the system at its own wavelength, thus providing a strong nonlinear optical response. Finally, we briefly discuss the potential application of these optical phenomena.
Ohmic and Rectifying Contacts to n and p-type GaN Films
Authors: Hall H. P. , Awaah M. A. , Kumah A. , Das K. , Semendy F.
Published: April 2011

Electrical contacts to both n and p-type GaN films have been investigated using electron-beam evaporated and sputtered films of metals such as Al, Au, Cr, Cu, Ni, Pt, and Ti. Films deposited by electron-beam evaporation for the n-type films with doping levels of 1 × 1018/cm3 and lower showed rectifying characteristics with all the metals studied with the exception of Al. Aluminum contact diodes were ohmic in the as-deposited state. The Pt rectifying contact was near-ideal with an ideality factor close to 1.0. Ideality factors for the other metals were much greater than 1. This deviation from thermionic behavior was interpreted as space charge limited current conduction in the presence of deep-level states. Sputtered films showed very similar characteristics to electron-beam deposited films, with the exception of Ti. The Ti contact was ohmic in the as-deposited state. Non-linear Cu contacts to n-type films became ohmic on annealing. However, for p-type films, Ar ion sputter-cleaning prior to metal deposition by sputtering created ohmic contacts with Cu and Pt. Low resistance ohmic contacts were achieved by ion implantation and anneal of Si in n-type and Mg in p-type films, prior to metallization. The implant parameters and anneal temperatures are currently being optimized.
Epitaxial Growth of AlN on 6H-SiC (1120) by Molecular-Beam Epitaxy and Effect of Low-Temperature Buffer Layer
Authors: Onojima N. , Suda J. , Matsunami H.
Published: April 2011

Aluminum nitride (AlN) has been grown on 6H-silicon carbide (SiC) substrates with the non-polar (1120) face using rf plasma-assisted molecular-beam epitaxy (rf-MBE). Reflection high-energy electron diffraction (RHEED) revealed that AlN and 6H-SiC (1120) had an exact epitaxial relationship, i.e., [1120]AlN|[1120]SiC and [0001]AlN∥[0001]SiC. From the result of microscopic Raman scattering spectroscopy, the stacking structure of the AlN epitaxial layer was suggested to be a 2H structure, not a 6H structure. A directly grown AlN layer and layer with AlN low-temperature (LT) buffer layer were investigated based on atomic force microscopy (AFM) and X-ray diffraction (XRD).
Study on Chemical Treatment and High Temperature Nitridation of Sapphire for III-Nitride Heteroepitaxial Growth
Authors: Dwikusuma F. , Saulys D. , Kuech T. F.
Published: April 2011

We have systematically studied the effects of wet chemical etching and high temperature nitridation on the resulting sapphire surface morphology and chemical transformation. The etching of c-plane sapphire substrates using H2SO4, H3PO4, and a 3:1 H2SO4 :H3PO4 mixture as a function of temperature and etching time was studied and compared with H2 etching at 1100°C and air-annealing at 1400°C. The surface nitridation using NH3 and N2 at 1100°C was studied as a function of NH3 concentration, nitridation time, and surface pretreatment. Atomic force microscopy and x-ray photoelectron spectroscopy were used to study the surface morphology and chemical composition. The detailed surface morphology after chemical etching was a function of the chemical composition and the specific time and temperatures. The smoothest, pit-free sapphire surface was obtained by etching in pure H2SO4 at 300°C for 30 min. Sulfuric acid etching at higher temperatures or for longer periods generated an insoluble mixture of Al2(SO4)3 and Al2(SO4)3·17H2O crystalline deposits on the surface. Phosphoric acid and the 3:1 H2SO4:H3PO4 mixture etched the sapphire preferentially at defect sites and resulted in pits formation on the surface. The high temperature sapphire nitridation resulted in nitrogen incorporation into the surface. The nitrogen content of nitridation layer depends on NH3 concentration, nitridation time, and surface pretreatment. The nitrogen contents of sapphire treated with H2SO4 and 3:1 H2SO4:H3PO4 are about the same as the as-received sapphire. While the nitrogen content of the air-annealed sapphire is ∼1.6 times higher then the nitrogen content of the as-received sapphire.
Screw Dislocations in MBE GaN Layers Grown on Top of HVPE Layers: Are They Different?
Authors: Liliental-Weber Z. , Zakharov D. , Jasinski J. , Washburn J. , O'Keefe M. A. , Morkoc H.
Published: April 2011

Transmission Electron Microscopy was applied to study HVPE template and MBE over-layers in plan-view and cross-section. It was observed that screw dislocations in the HVPE layers are decorated by small voids arranged along the screw axis. However, no voids were observed along screw dislocations in MBE overlayers grown with excess Ga, despite the fact that Ga droplets were observed on the layer surface as well as imbedded in the layer. By applying a direct reconstruction of the phase and amplitude of the scattered electron wave from a focal series of high-resolution images, the core structures of screw dislocations in both materials have been studied and show that all screw dislocations have filled cores. Dislocation cores in MBE samples grown Ga-rich and N-rich show no substantial differences and no stoichiometric change compared to the matrix. However, in HVPE materials, single atomic columns show substantial differences in intensities and indicate the possibility of Ga presence. These Ga-rich cores might be responsible for the attraction impurities forming voids in their close vicinity.
Low Temperature Laser-Assisted Gas Phase Reactivity of TMGa with NH3 and Oxygen-Containing Compounds (H2O, CH3OH, O(CH3)2) in Constrained Pulsed Expansions
Authors: Demchuk Alexander , Lynch Michael , Simpson Steven , Koplitz Brent
Published: April 2011

The present work reports on the study of III-V gas phase reactivity in constrained gas pulse expansions of trimethylgallium (TMGa) and oxygen derivative compounds (H2O, CH3OH, O(CH3)2) with and without ammonia. The precursors are introduced separately into a high vacuum chamber via a multipulsed gas nozzle assembly. The gas mixtures are then exposed to a UV pulse from an ArF excimer laser (λ=193 nm) and the products are mass analyzed with a quadrupole mass spectrometer. The efficient laser-assisted growth of Ga-O-containing clusters in the form of [(CH3)2GaOR] x, where R is H or CH3, has been revealed. Different behavior can be seen in the reaction of TMG and the oxygen species depending on the presence of H atoms bonded to the oxygen. Significant influence of NH3 on cluster formation and oxygen incorporation is demonstrated.
Morphology and surface reconstructions of m-plane GaN
Authors: Lee C. D. , Feenstra R. M. , Northrup J. E. , Lymperakis L. , Neugebauer J.
Published: April 2011

M-plane GaN(1100) is grown by plasma assisted molecular beam epitaxy on ZnO(1100) substrates. A low-temperature GaN buffer layer is found to be necessary to obtain good structural quality of the films. Well oriented (1100) GaN films are obtained, with a slate like surface morphology. On the GaN(1100) surfaces, reconstructions with symmetry of c(2×2) and approximate “4×5” are found under N- and Ga-rich conditions, respectively. We propose a model for Ga-rich conditions with the “4×5” structure consisting of ≥ 2 monolayers of Ga terminating the GaN surface.
Lattice Relaxation of AlN Buffer on Surface-Treated SiC in Molecular-Beam Epitaxy for Growth of High-Quality GaN
Authors: Suda Jun , Miura Kouhei , Honaga Misako , Onojima Norio , Nishi Yusuke , Matsunami Hiroyuki
Published: April 2011

The effects of SiC surface treatment on the lattice relaxation of AlN buffer layers and the crystalline quality of GaN layers grown on the buffer layers were studied. AlN buffer layers and GaN main layers were grown by plasma-assisted molecular-beam epitaxy on on-axis 6H-SiC (0001)Si substrates. High-temperature HCl-gas etching resulted in an atomically flat SiC surface with (√3×√3)R30° surface reconstruction, while HCl-gas etching followed by HF chemical treatment resulted in an atomically flat surface with (1×1) structure. The AlN layer grown on the (1×1) surface showed slower lattice relaxation. GaN grown on the AlN buffer layer exhibited a (0002) X-ray rocking curve of 70 arcsec and 107 cm−2 of screw-type dislocation density, which was superior than that of GaN grown on (√3×√3)R30° surface.
In as a Surfactant for the Growth of AlGaN/GaN Heterostructures by Plasma Assisted MBE
Authors: Monroy E. , Gogneau N. , Bellet-Amalnc E. , Enjalbert F. , Barjon J. , Jalabert D. , Brault J. , Dang Le Si , Daudin B.
Published: April 2011

In this paper, we study the surfactant capability of In for the growth of AlGaN/GaN heterostructures by plasma-assisted molecular beam epitaxy. Growth conditions were determined to have a self-regulated 1×1 In adlayer on AlxGa1-xN (0001). The presence of this In film favors two dimensional growth of AlGaN under stoichiometric conditions, and inhibits the formation of metal droplets on the surface. The quality of these layers was assessed by high resolution X-ray diffraction, atomic force microscopy and photoluminescence.
Room and Cryogenic Temperature Operation of 280 nm Deep Ultraviolet Light Emitting Diodes
Authors: Shatalov Maxim , Adivarahan Vinod , Zhang Jian Ping , Chitnis Ashay , Wu Shuai , Pachipulusu Radhika , Mandavilli Vasavi , Asif Khan M.
Published: April 2011

We present a study of the electrical and optical characteristics of 280 nm emission deep ultraviolet light emitting diodes (LED) at room and cryogenic temperatures. At low bias the defect assisted carrier tunneling primarily determines the current conduction. The room-temperature spectral performance and optical power are limited mostly by pronounced deep level defect assisted radiative and non-radiative recombination as well as poor electron confinement in the active region. At temperatures below 100 K the electroluminescence peak intensity increases by more than one order of magnitude due to suppression of non-radiative recombination channels indicating that with a proper device design and improved material quality, milliwatt power 280 nm LED are viable.
Material and Device Issues of AlGaN/GaN HEMTs on Silicon Substrates
Authors: Javorka P. , Alam A. , Marso M. , Wolter M. , Fox A. , Heuken M. , Kordoš P.
Published: April 2011

Results on the preparation and properties of AlGaN/GaN HEMTs on silicon substrates are presented and selected issues related to the material structure and device performance devices are discussed. Virtually crack-free AlGaN/GaN heterostructures (xAlN ≅ 0.25), with low surface roughness (rms of 0.64 nm), ns ≅ 1×1013 cm−2 and μ ≅ 1100 cm2/V s at 300 K, were grown by LP-MOVPE on 2-inch (111)Si substrates. HEMT devices with Lg = 0.3–0.7 μm were prepared by conventional device processing steps. Photoionization spectroscopy measurements have shown that a trap level of 1.85 eV, additional to two levels of 2.9 and 3.2 eV found before on GaN-based HEMTs on sapphire, is present in the structures investigated. Self-heating effects were studied by means of temperature dependent dc measurements. The channel temperature of a HEMT on Si increases with dissipated power much slower than for similar devices on sapphire substrate (e.g. reaches 95 and 320 °C on Si and sapphire, respectively, for 6 W/mm power). Prepared AlGaN/GaN/Si HEMTs exhibit saturation currents up to 0.91 A/mm, a good pinch-off, peak extrinsic transconductances up to 150 mS/mm and static heat dissipation capability up to ∼16 W/mm. Unity current gain frequencies fT up to 21 and 32 GHz were obtained on devices with gate length of 0.7 and 0.5 μm, respectively. The saturation current and fT values are comparable to those known for similar devices using sapphire and SiC substrates. Properties of AlGaN/GaN/Si HEMTs investigated show that this technology brings a prospect for commercial application of high power rf devices.
Dynamic growth mechanism and interface structure of crystalline zirconia on silicon
Authors: Wang S J , Huan A C H , Ong C K
Published: April 2011

In present report, we have studied the initial stage of the growth of crystalline yttria-stabilized zirconia (YSZ) films on the natively oxidized Si (100) wafer by pulsed-laser deposition. X-ray photoelectron spectroscopy (XPS) and high-resoluti on transmission electron microscopy (HRTEM) show that, for the first few monolayers of crystalline YSZ deposited on Si (100), the dynamic processes appear to be the decomposition of SiO2 to SiO, the formation of ZrO2, and the desorption of SiO. The native amorphous silicon oxide layer is removed completely with the continued deposition of YSZ and the oxygen in this layer is used as oxygen source for forming stable crystalline oxide film. XPS depth profile and HRTEM investigation showed that the interface of crystalline YSZ film in contact with silicon was found to be atomically sharp and commensurately crystallized without an amorphous layer. The interface structure is suggested to have a sequence of-Si-O-Zr-O-. For the film with electrical equivalent oxide thickness 1.46 nm, the leakage current is about 1.1×10-3 A/cm2 at 1 V bias voltage. The hysteresis and interface state density in this film are measured to be less than 10 mV and 2.0×1011eV-1cm-2.
High Temperature Plasma Etching of PZT Capacitor Stacks for High Density FERAMs
Authors: Egger Ulrich , Tomioka Kazuhiro , Stojakovic George , Taniguchi Yasuyuku , Bruchhaus Rainer , Zhuang Haoren , Kanaya Hiroyuki , Beitel Gerhard , Sugimoto Shigeki
Published: April 2011

A 32 Mbit chain FeRAM™ stack with 0.20μm minimum feature size was etched with two subsequent lithography/RIE steps: in mask step 1 the platinum/SRO (strontium ruthenium oxide) top electrode and the PZT (lead zirconate titanate) layer, in mask step 2 the bottom electrode together with the Ir/IrO2 diffusion barrier were etched. The stack was etched with various chlorine based chemistries. High temperature etching processes were applied to suppress residues by the formation of volatile etching byproducts resulting in a highly anisotropic etching profile and low redeposition.

Profile angles of 75° for step 1 and 80° for step 2 could be achieved. For the thin SRO-layer a separate etching recipe was developed to avoid surface roughening caused by micromasking. The influence of etching temperature and different gas chemistries on the etching behavior was evaluated. Reliable end point detection and good uniformity of the individual etching processes were obtained, both being crucial for the application of a multi-step recipe. The ferroelectric properties of the capacitor were confirmed by hysteresis measurements. This demonstrates that the ferroelectric properties were conserved during RIE etch processes at high temperature.
Chemical Solution Deposition of PLZT Films on Base Metal Foils
Authors: Kim D.-J. , Kaufman D. Y. , Streiffer S. K. , Lee T. H. , Erck R. , Auciello O.
Published: April 2011

In an effort to develop cost effective, volumetrically efficient, high charge density and high energy density capacitors, Pb(Zr,Ti)O3 and La-doped Pb(Zr,Ti)O3 films were deposited by chemical solution deposition on nickel and alloy foils. PZT films deposited on bare foils exhibited lower permittivity and more electric field hysteresis compared to films deposited on platinized silicon substrates, due to the formation of low capacitance interfacial layers and/or diffusion of foil elements into the PZT. However, an ultimate dielectric breakdown strength of approximately 1.35 MV/cm was obtained for a film thickness of 1.8 μm, corresponding to a withstand voltage of 245 V. A reduced temperature dependence of capacitance was observed with decreasing film thickness. In order to improve the dielectric response, barrier layers of LaNiO3, Ru, or Ir were deposited on top of the metal foils used as substrates. The barrier improved relative permittivity and reduced hysteresis in relative permittivity as a function of dc bias.
Basic Study on High-density Ferroelectric Data Storage Using Scanning Nonlinear Dielectric Microscopy
Authors: Hiranaga Yoshiomi , Fujimoto Kenjiro , Wagatsuma Yasuo , Cho Yasuo , Onoe Atsushi , Terabe Kazuya , Kitamura Kenji
Published: April 2011

Scanning Nonlinear Dielectric Microscopy (SNDM) is the method for observing ferroelectric polarization distribution, and now, its resolution has become to the sub-nanometer order, which is much higher than other scanning probe microscopy (SPM) methods for the same purpose. Up to now, we have studied high-density ferroelectric data storage using this microscopy. In this study, we have conducted fundamental experiments of nano-sized inverted domain formation in LiTaO3 single, and successfully formed inverted dot array with the density of 1.5 Tbit/inch2.
Dielectric Properties of Pulsed Excimer Laser Ablated BaBi2Nb2O9 Thin Films
Authors: Laha Apurba , Krupanidhi S. B. , Saha S.
Published: April 2011

The dielectric response of BaBi2Nb2O9 (BBN) thin films has been studied as a function of frequency over a wide range of temperatures. Both dielectric constant and loss tangent of BBN thin films showed a ‘power law’ dependence with frequency, which was analyzed using the Jonscher's universal dielectric response model. Theoretical fits were utilized to compare the experimental results and also to estimate the value of temperature dependence parameters such as n(T) and a(T) used in the Jonscher's model. The room temperature dielectric constant (ε') of the BBN thin films was 214 with a loss tangent (tanδ) of 0.04 at a frequency of 100 kHz. The films exhibited the second order dielectric phase transition from ferroelectric to paraelectric state at a temperature of 220 °C. The nature of phase transition was confirmed from the temperature dependence of dielectric constant and sponteneous polarization,respectively. The calculated Currie constant for BBN thin films was 4 × 105°C.
Raman Spectroscopy of Ferroelectric Thin Films
Authors: Katiyar R. S. , Dixit A. , Jain M. , Savvinov A. A. , Dobal P. S.
Published: April 2011

During a ferroelectric phase transition or domain rearrangement, ions or molecules in a ferroelectric material move in a highly cooperative manner from their initial lattice positions into the final positions they occupy and the collective response results into a “soft” lattice vibrational mode. Moreover, the structural changes are always accompanied by at least a few other changes in the normal mode behavior of the material. In the present work, Raman spectroscopy is conveniently employed to study such vibrational modes and other related phenomena in ferroelectric materials at the sub-microscale levels. We have investigated ferroelectric thin films of various lead and barium based perovskites prepared by sol-gel technique. The effect of processing conditions, A- and B- site substitutions, and size dependence on their Raman spectra were analyzed in terms of the structure-property correlations.
Integration Processes and Properties of One Transistor Memory Devices
Authors: Li Tingkai , Hsu Sheng Teng , Ulrich Bruce , Zhang Fengyan , Evans Dave
Published: April 2011

MFMPOS (Metal, Ferroelectrics, Metal, Polysilicon, Oxide, and Silicon) one-transistor (1T) ferroelectric memory devices have been fabricated. However, the yield of 1T-memory devices is lower. We find that the main problems of 1T MFMPOS memory devices are shorts, opens, no memory window, smaller memory windows and blank. In order to solve these problems, we studied the reasons resulted in the problems. Then, the integration processes for one transistor memory device were optimized. Fabrication of nMOSFET 1T memory devices starts with shallow trench isolation (STI) on p-type Si. A gate oxide is thermally grown after p-well implantation. Phosphorus ions were implanted after polysilicon gate definition for the formation of self-aligned source, drain, and n-type floating gate. A damascene process using MOCVD PGO deposition and chemical mechanical polishing (CMP) were used to avoid etching damage. Electrodes for the ferroelectric capacitor, i.e., the floating Ir bottom electrode and Pt top electrode, are deposited by E-Beam evaporation. The area ratio of the top and floating gate electrodes is 1:1. After inter-level dielectric (ILD) deposition, contact etching stops on Pt at gate and on Si at source/drain (S/D) without difficulty because of high etch rate selectively to the Pt. Finally, the high quality 1T memory devices have been made. The one-transistor memory devices showed memory windows around 2 – 3V. The memory windows are almost saturated from operation voltage of 3V. The ratios of “on” state current to the “off” state current are closed to 8 – 9 orders. The one-transistor memory devices also show a very good memory characteristics and retention properties.
X-ray Photoelectron and UV Photoyield Spectroscopic Studies on Structural and Electronic Properties of SrxBiyTa2O9 Films
Authors: Takahashi Mitsue , Noda Minoru , Okuyama Masanori
Published: April 2011

Photoelectron spectra by X-ray photoelectron spectroscopy (XPS) and UV-photoyield spectroscopy (UV-PYS) have been analyzed to study O2-annealing effects on band diagrams of ferroelectric SrxBiyTa 2O9 (SBT) thin films deposited by pulsed laser deposition (PLD) and metalorganic dcecomposition (MOD) methods. XPS studies on the annealed PLD-SBT film has shown a rapid shift in its Bi 4f core peaks from the oxidized to the metallic ones, as the film suffer Ar+ bombardment. Surface of the annealed film has exhibited lower Fermi level than the as-deposited one in UV-PYS. The result suggests O2-annealing can suppress leakage current through PLD-SBT films. The UV-PYS studies on MOD-SBT have shown almost the intrinsic Fermi levels before and after the additional annealing. The XPS studies have shown that the additional annealing is not effective to improve stabilities of (Bi2O2)2+ layers, once the film is baked and crystallized.
Acoustic Emission Monitoring of Nanoindentation-Induced Slip and Twinning in Sapphire
Authors: Tymiak Natalia I. , Daugela Antanas , Wyrobek Thomas J. , Warren Oden L.
Published: April 2011

Monitoring with an Acoustic Emission (AE) sensor integrated into an indenter tip was utilized for the evaluation of the earliest stages of indentation-induced plasticity in sapphire single crystal. The evaluated surfaces included basal (C), rhombohedral (R) and two different prismatic orientations (A and M). The differences between the mechanisms of the initial stages of plasticity for the various crystallographic orientations were reflected in the following aspects of AE activity: detection of a specific type of AE waveform that correlated to the presence of linear surface features near the indentation impressions; AE signal associated with the yield point, consisting either of one or two distinct waveforms; and presence or absence of AE signals after the yield point. Moreover, analysis of AE activity revealed loading rate effects on the yield point mechanism for the M plane. The possibility of plasticity onset mechanisms involving both slip and twinning is discussed.
Characterization and Tribological Properties of Nanostructured Copper/Carbon Composite Films Prepared by Microwave Plasma-Assisted Dual Deposition Processes
Authors: Thiery François , Pauleau Yves , Pelletier Jacques
Published: April 2011

Nanocrystalline copper/hydrogenated amorphous carbon films have been deposited on Si substrates at the floating potential using a distributed electron cyclotron resonance microwave plasma reactor. In this deposition technique, the microwave plasma-enhanced chemical vapor deposition process of carbon from argon-methane or argon-acetylene mixtures of various compositions was associated with the sputter deposition of copper from a copper target. The total pressure was fixed at 0.13 Pa. For deposition, the substrates mounted on a water-cooled substrate holder were maintained at ambient temperature. The composition of films determined by Rutherford backscattering spectroscopy, energy recoil detection analyses and nuclear reaction analyses was investigated as a function of the gas phase composition. The structure of films was identified by X-ray diffraction (XRD) techniques and the size of copper crystallites incorporated in the amorphous carbon matrix was deduced from XRD data. The magnitude of residual stresses developed in these films was calculated from the radius of curvature of film/substrate samples determined by profilometry. The residual stress values were found to be nearly independent on the composition of films and deposition parameters.
In situ Fracture and Adhesion Failure of Al-Cu-Fe Quasicrystalline Films
Authors: Daniels M. J. , French B. L. , King David , Bilello J. C.
Published: April 2011

Quasicrystalline precursor coatings were deposited on single crystal silicon and sapphire wafers by RF sputtering from an AlCuFe powder composite target. Synchrotron white beam radiography/topography and stress analysis were performed in situ on the wafers during heating to 495 or 585°C, and subsequent cooling. A plateau region of constant stress was present throughout most of the 1 hour anneals before a large tensile stress developed in the film during cooling due to coefficient of thermal expansion mismatch. Cracking was observed for films on both substrates at an average film stress of approximately 930 MPa. Distinct differences in the fracture behavior were observed for the two different substrates. X-ray diffraction performed on films after annealing suggested that texturing took place during the transition to a fully developed quasicrystalline structure.
A Novel Technique for Mixed-mode Thin Film Adhesion Measurement
Authors: Wang Junlan , Sottos Nancy R. , Weaver Richard L.
Published: April 2011

Guided by recent parametric studies of laser-induced tensile spallation, we successfully developed a new test method for studying mixed-mode interfacial failure of thin films. By allowing an initial longitudinal wave to mode convert at an oblique surface, a high amplitude shear wave is generated in a fused silica substrate and propagated toward the thin film surface. A shear wave was generated with amplitude large enough to fail an Al film/fused silica interface and the corresponding shear stress was calculated from high-speed interferometric displacement measurements. Examination of the interfaces failed under mixed-mode conditions revealed significant wrinkling and tearing of the film, in great contrast to blister patterns observed in similar Al films failed under tensile loading. Slightly higher interfacial strength was measured for the same Al film/fused silica substrate under mixed-mode loading than under tensile loading. The mixed-mode loading experiment developed in the current work provides a new tool to explore a range of thin film interfacial failure modes as well as the role of extrinsic effects such as residual stress, surface roughness, and imperfections.
Ion Energy/Momentum Effects During Ion Assisted Growth of NbXNY Films
Authors: Klingenberg M. L. , Demaree J. D. , Hirvonen J. K. , Messier R.
Published: April 2011

In a previous paper, it was shown that the tribological properties of NbxNy thin films produced by ion beam assisted deposition (IBAD) depend strongly on the beam energy and the ion-to-atom (R) ratio. This study was designed to separate ion energy vs. ion momentum effects on film stress, crystalline phase, grain size, morphology, and composition, all of which influence the tribological properties of the films. Inert ion beams (Kr, Ar, and Ne) were used in conjunction with a nitrogen gas backfill to independently control ion energy and ion momentum transfer to NbxNy films. The ion species, energies, and R ratios were chosen to create a matrix of coatings that exhibited the same total energy deposition with different momentum transfer or the same momentum transfer but different total energy deposition. The resultant films were characterized using Rutherford Backscattering Spectroscopy (RBS), x-ray diffraction (XRD), atomic force microscopy (AFM), and residual stress analysis. Crystalline phases and texture, as well as residual stress, were more closely correlated with ion momentum transfer to the coating atoms than with overall ion energy input.
Investigations of Multifunctional Graded Zirconium Carbide PVD-Coatings for the Application on Machine Parts
Authors: Lugscheider E. , Knotek O. , Bobzin K. , Maes M. , Arntz K.
Published: April 2011

An efficient way to reduce energy consumption and costs of machines is to reduce the friction in tribological systems such as gear wheels, spindle bearings and regular ball bearings. Tribological PVD (Physical Vapour Deposition)-coatings have thus been developed at the Materials Science Institute in the context of the special research project SFB 442 named “Environmentally Compatible Tribosystems”. The main challenge facing PVD applications is the complex geometry of machine parts as well as the low deposition temperature. Therefore, the use of Magnetron Sputtering Ion Plating (MSIP-PVD) offers the advantage of producing coatings with a much wider range of properties. A superstoichiometric coating with an over saturated amount of carbon is adjusted by PVD parameters. This creates a shear-weak top coating with a low friction coefficient and a self smoothing surface. The coatings were analysed in standard tests such as the calo-test, scratch-test and HRC-test. Furthermore, the nanoindentation and SEM observations proved a clear correlation between chemical composition and physical properties.
Photochemical Lamination of Low Refractive Index Transparent SiO2 Film at Room Temperature for Antireflection Coating
Authors: Ogawa Y. , Murahara M.
Published: April 2011

A transparent low refractive index SiO2 film laminated on a glass substrate at room temperature by photochemical reactions with the Xe2* excimer lamp (172nm). This SiO2 film grown on the fused silica glass was proved to avoid reflection of light.

A refractive index of the SiO2 film was 1.36. After annealing the film for one hour at 200 degrees centigrade, the refractive index increased to 1.42. The refractive index increased as the F atom density in the SiO 2 film decreased.
Material Mechanical Properties Influence on the Ratio of Drilling Thrust to Hardness
Authors: Mauvoisin G. , Bartier O. , El Abdi R. , Nayebi A.
Published: April 2011

A new approach using an experimental hole drilling technique has been developed to determine the hardness profile of superficially heat-treated steels. This method was based on the proportionality between the hardness of the material tested and the cutting thrust measured during the drilling test. Although it was observed that, overall, the increase in the cutting thrust was linear with hardness, locally the variation of thrust was not necessarily regular and linear in particular for iron-carbon alloys. In fact, other parameters related to the material, other than hardness, can modify the cutting thrust level. We report here on the influence of parameters, such as the type of microstructure and the ductility or the work hardening of material, on drilling thrust. The cutting thrust obtained for various brittle or ductile materials will be discussed using results obtained from hardness tests, micrographic observations and microhardness of the chips obtained by drilling test.
Nanoindentation of Vacuum Ultraviolet Light-Irradiated Poly(methylmethacrylate) Substrates
Authors: Hozumi Atsushi , Yokogawa Yoshiyuki , Kameyama Tetsuya , Wu Yunying , Sugimura Hiroyuki , Takai Osamu
Published: April 2011

Surface modification of poly(methylmethacrylate) (PMMA) substrates has been demonstrated using an excimer lamp radiating vacuum ultraviolet (VUV) light of 172 nm in wavelength. In this study, we have particularly focused on the effects of atmospheric pressure during VUV irradiation. Each of the substrates was photoirradiated with VUV light under a pressure of 10, 103 or 105 Pa. Changes in nanomechanical properties of the VUV-irradiated sample surfaces were studied based on a scratching test using a nanoindenter. The wear-depth of the PMMA sample treated at 105 Pa was about 137.0 nm, which was much larger than the wear-depth of an untreated PMMA substrate (63.3 nm). On the contrary, when samples were prepared with VUV irradiation conducted at 10 and 103 Pa, their wear-depths markedly decreased down to about 2.2 and 12.5 nm, respectively. The sample treated at 10 Pa was particularly wear-resistant. This high wear-resistance was attributable to the formation of new carbon-carbon bonds such as C=C bonds on the PMMA surfaces.
Nanoscale Control of Friction and Chemistry on Silicon Surfaces
Authors: Baluch Andrew S. , Basu Rajiv , Guisinger Nathan P. , Reagan Kinser C. , Kramer Donald E. , Such Matthew W. , Hersam Mark C.
Published: April 2011

Many materials properties are tailored by controlling the chemical reactivity of surfaces. Consequently, through chemically active nanopatterning, surfaces can be engineered with nanoscale spatial resolution. This paper describes three techniques that enable the chemical reactivity of silicon surfaces to be customized down to the single molecule limit using scanned probe microscopy (SPM). All three techniques use chemically inert hydrogen passivated silicon as a starting substrate. Injected electrons and/or electric field from a conductive SPM tip locally disrupt the passivation, thus allowing the surface chemistry to be modified. Using ultra-high vacuum scanning tunneling microscopy, feedback controlled lithography leads to electron stimulated desorption of hydrogen from Si(100)-2×1:H with atomic resolution. The resulting dangling bonds preferentially react with gas phase molecules, enabling pre-determined self-directed growth of styrene molecular wires. In ambient conditions, conductive atomic force microscopy (cAFM) is used to grow nanoscale hydrophilic domains on Si(111):H via anodic field induced oxidation (FIO). Due to the chemical contrast between the hydroxyl terminated oxide and the hydrophobic Si(111):H surface, octadecyltrichlorosilane selectively binds to FIO nanopatterns. The final nanolithographic scheme, called liquid phase nanolithography (LPN), employs cAFM on surfaces submerged in organic solvents. By encapsulating the tip-sample junction in a solvent with low water and oxygen solubility, anodic oxidation is suppressed, thus enabling direct molecular attachment to the silicon surface from the liquid phase. In this work, LPN has been used to nanopattern undecylenic acid methyl ester. Nanoscale variations in the frictional properties induced by LPN and FIO are directly measured with lateral force microscopy. Through readily available and well-established chemical protocols, this nanolithography toolkit enables arbitrary materials, including polymers and biological molecules, to be covalently bound to silicon with nanometer spatial resolution.
Ag functionalized Carbon Molecular Sieves membranes for separating O2 and N2
Authors: Barsema J. N. , Balster J. , Van der Vegt N. F. A. , Koops G. H. , Jordan V. , Wessling M.
Published: April 2011

In the last two decades substantial progress has been made in the preparation of Carbon Molecular Sieve (CMS) membranes for gas separation. Today, researchers actively study precursor materials and pyrolysis routes to fully explore the merits of CMS membranes. Successful separation of permanent gas mixtures in which the gaseous components posses only little to no affinity to adsorb onto the internal CMS surface relies highly on the exact tailoring of microsieving regions. Preparation of CMS structures, which are highly permselective towards one of such mixture components, is especially cumbersome if the molecular sizes differ only slightly (e.g. O2/N2).

To facilitate the separation of O2 and N2 we have chosen to functionalize the carbon matrix. By introducing nano-sized (40 nm) metallic Ag-clusters, the affinity of the membrane matrix for O2 significantly increases. We have added a silver containing salt, AgNO3, to a solution of BMTA-TDI/MDI co-polyimide (P84, Lenzing) in NMP in the absence of light to obtain homogeneous flat film polymeric precursors. These precursors were pyrolysed, reducing Ag+ to Ag, at different temperatures (350, 500, 600, 700, and 800 °C) in a N2 atmosphere and characterized using Scanning Electron Microscopy, Atomic Force Microscopy, and gas permeation. Thermo Gravimetrical Analysis was used to follow the pyrolysis in detail. From this we observed an increase of the ideal separation factor of 1.6. Moreover, we observed an increase of the permeability to a maximum of 240 %.
Gas Permeation Characteristics and Stability of Composite Silica-Metal Oxide Membranes
Authors: Asaeda Masashi , Kanezashi Masakoto , Yoshioka Tomohisa , Tsuru Toshinori
Published: April 2011

In order to improve the stability of silica membranes against water (vapor) some metal oxides were added to silica to obtain composite silica-metal oxide membranes by the sol-gel techniques. A Ni-doped silica membrane (Ni/Si=1/2) fired at 500°C showed a relatively large permeance of 1.5×10−5 [m3(STP)/(m2skPa)] with selectivity of 350 (H2/CH4), 4200 (H2/SF6) at 200°C and 100 (CO2/CH4) at 35 °C. After leaving the membrane in humid air (RH: 60%, 40°C) for 70 days, the permeance of H2 decreased by about 50% but the selectivity was improved to 930 for H2/CH4. And little change was observed in the activation energy for H2 permeation, while under the same conditions a silica membrane showed a quite large change in the activation energy from 3.1kJ/mol to 14kJ/mol. There is a possibility that metal oxides added to silica help prevent the densification of silica networks through which hydrogen and helium molecules can permeate.
The Hydrogen Permeability of Sulfur Resistant Palladium-Copper Alloys at Elevated Temperatures and Pressures
Authors: Howard B. H. , Cugini A. V. , Killmeyer R. , Rothenberger K. S. , Ciocco M. V. , Morreale B. D. , Enick R. M. , Bustamante F.
Published: April 2011

Pd-Cu alloys are being considered for hydrogen membrane applications because of their resistance to sulfur poisoning. Therefore the permeance of Pd-Cu alloys containing 53, 60, and 80 wt% Pd has been determined over the 623 – 1173 K temperature range for H2partial pressure drops as great as 2.75 MPa. The results indicate that Pd-Cu alloy composition and thermal history influence membrane permeance. The 60%Pd-40%Cu alloy exhibited very high permeance at 623 K, although both the 53%Pd and 60% Pd alloys exhibited a distinct drop in permeability at higher temperatures due to the transition of the Pd-Cu crystal structure from bcc to fcc. Upon cooling the membrane back to 623 K, the permeability of the 60%Pd alloy was initially an order-of-magnitude less than its initial value, but the permeance increased steadily with time as the Pd – Cu crystal structure slowly reverted to bcc. The fcc 80%Pd alloy was less permeable than the bcc 60% Pd alloy at 623 K, but the 80% Pd alloy was more permeable than the fcc 60%Pd alloy at elevated temperatures.
Filtrate Flux and Sieving Characteristics of Virus Filtration Membranes
Authors: Zydney Andrew L. , Bohonak David M.
Published: April 2011

Virus filtration is increasingly used in the biopharmaceutical industry, but capacity and fouling remain problematic. Experimental studies were conducted in dead-end, stirred filtration cells with Viresolve 180 polyvinylidene fluoride membranes using the protein bovine serum albumin. Data were obtained for membranes in two different flow orientations, with the selective “skin” layer oriented on either the upstream surface or downstream relative to the fluid flow. Compaction of the substructure occurs when the skin layer is downstream, leading to a significant increase in membrane resistance. Concentration polarization in the bulk solution or membrane substructure caused a substantial increase in the protein sieving coefficient, with this effect being greatest when the flow entered through the substructure. Fouling is primarily due to the deposition of large protein aggregates. The effect of this fouling on the flux was reduced when the skin layer was oriented downstream since the substructure acted as a prefilter. These results demonstrate that the membrane morphology and orientation play a critical role in determining the overall performance of virus filtration membranes.
Comparison of Impact of Virgin Polymer Structure on Properties of Irradiated Membranes – Hydrogen Ion Irradiation on Polyimide Isomers
Authors: Xu Xinglong , Hu Ling , Ilconich J.
Published: April 2011

ABSTACT

This study focused on the impact of virgin polymer structure and microstructure on the transport properties of irradiated polyimide. Two fluorine containing polyimide isomers (6FDA-6FpDA and 6FDA-6FmDA) that differ solely in the location of the linkage between the diamine and dianhydride residues were used for this study. While these polymers differ in the location of a single bond, the virgin transport properties are dramatically different. The para connected isomer (6FDA-6FpDA) has much higher permeabilities and lower selectivities than the meta connect isomer (6FDA-6FmDA). The pure gas permeabilities in polyimide-ceramic composite membranes following H+ ion irradiation over a wide range of doses will be compared for these polyimide isomers. In addition, the evolution in chemical structure was monitored using Fourier transform infrared spectroscopy (FTIR). These polymers exhibited different decay rate in chemical structure following ion irradiation. Interestingly, the evolution in gas transport properties of these polymers following H+ ion irradiation was also quite different. We will discuss how the microstructure would affect the gas permeation properties of ion beam modification of polymer.
Molecularly Imprinted Membranes Having Amphiphilic Scaffold Media for Target Molecule Recognition
Authors: Kobayashi Takaomi , Onodera Takashi , Ohta Michiyoshi
Published: April 2011

Polyacrylonitrile copolymer having amphiphilic octyl-dimetyl quarternized ammonium moiety was used as imprint scaffolding media for Theophylline (THO) adsorbent. The THO imprinted membranes were prepared by phase inversion technique; The polymer dimethylsulfoxide (DMSO) solution containing THO was cast and then phase-inversed in water to prepare THO imprinted membranes. After template extraction, adsorbent properties of THO were studied by heterogeneous binding experiments of THO and analogs such as caffeine, uracil, and theobromine. Results showed that imprinting effect was appeared in copolymer containing both octyl-hydrophobic group and positively charged ammonium groups.