bluuebunny/crossref_metadata_2025 huggingface dataset:
Neuroactive steroids have multiple actions to potentiate GABAA receptors
Authors: Akk Gustav , Bracamontes John R. , Covey Douglas F. , Evers Alex , Dao Tim , Steinbach Joe Henry
Published: May 2004

The effects of neuroactive steroids on the function of GABAA receptors were studied using cell‐attached records of single channel activity recorded from HEK293 cells transfected with α1 β2 γ2L subunits. Activity was elicited with a half‐maximal (50 μm) concentration of GABA. Two steroids were studied in detail: ACN ((3α,5α,17β)‐3‐hydroxyandrostane‐17‐carbonitrile) and B285 ((3α,5β,17β)‐3‐hydroxy‐18‐norandrostane‐17‐carbonitrile). Four effects on channel activity were seen, two on open time distributions and two on closed times. When clusters of openings were elicited in the absence of steroid, the open time distribution contained three components. ACN produced concentration‐dependent alterations in the open time distribution. The prevalence of the longest duration class of open times was increased from about 15% to about 40% (EC50 about 180 nm ACN), while the duration of the longest class increased from 7.4 ms to 27 ms (EC50 about 35 nm ACN). B285 also increased the prevalence of the longest duration open times (EC50 about 18 nm B285) but increased the duration only at concentrations close to 10 μm. The differences in the actions of these two steroids suggest that the effects on proportion and duration of the long duration open time component are produced by independent mechanisms and that there are separate recognition sites for the steroids which are associated with the two functional actions. The closed time distributions also showed three components in the absence of steroid. The rate of occurrence of the two brief duration closed time components decreased with increasing ACN, with an EC50 of about 50 nm ACN. In contrast, B285 did not reduce the rate of occurrence of the brief closings until high concentrations were applied. However, both B285 and ACN reduced the rate of occurrence of the activation‐related closed state selectively, with comparable IC50 concentrations (about 40 nm ACN, 20 nm B285). As in the case for action on open times these data suggest that there are two recognition sites and two independent mechanisms, perhaps the sites and mechanisms associated with actions on open times. The presence of  1 μm ACN had no effect on the estimated channel opening rate or on the apparent affinity of the receptor for GABA. Mutation of the carboxy terminus of the γ2 subunit, but not the α1 or β2 subunits, abolished the ability of ACN to increase the duration of OT3 but had no effect on the reduction of the rate of occurrence of the activation‐related closed state. These observations are also consistent with the idea that there is more than one distinguishable steroid recognition site on the GABAA receptor.
A Consistent Analytical Model to Predict the Structural Behavior of Flexible Risers Subjected to Combined Loads
Authors: Ramos, Roberto , Pesce Celso Pupo
Published: May 2004

This work presents an analytical model for the structural analysis of flexible risers subjected to combined loads of bending, twisting and tension, as well as internal and external pressures applied to the riser. The proposed formulation, which comprises equilibrium conditions, constitutive equations and geometrical relations, leads to the solution of the problem in terms of the loads supported by the layers and deformation components of a flexible riser subjected to the above mentioned combined loads. All the modeling hypotheses are highlighted in the work. Comparisons between analytical results using the full-slip model and experimental results obtained in literature are shown and discussed. Some proposals leading to improvement of the presented model are drawn in the conclusions.
Modeling and Identification of a Nonlinear SDOF Moored Structure, Part 1—Hydrodynamic Models and Algorithms
Authors: Narayanan S. , Yim S. C. S.
Published: May 2004

The highly nonlinear responses of compliant ocean structures characterized by a large-geometry restoring force and coupled fluid-structure interaction excitation are of great interest to ocean and coastal engineers. Practical modeling, parameter identification, and incorporation of the inherent nonlinear dynamics in the design of these systems are essential and challenging. The general approach of a nonlinear system technique using very simple models has been presented in the literature by Bendat. In Part 1 of this two-part study, two specific nonlinear small-body hydrodynamic Morison type formulations: (A) with a relative-velocity (RV) model, and (B) with an independent flow-field (IFF) model, are formulated. Their associated nonlinear system-identification algorithms based on the reverse multiple-input/single-output (R-MI/SO) system-identification technique: (A.1) nonlinear-structure linearly damped, and (A.2) nonlinear-structure coupled hydrodynamically damped for the RV model, and (B.1) nonlinear-structure nonlinearly damped for the IFF model, are developed for a specific experimental submerged-sphere mooring system under ocean waves exhibiting such highly nonlinear response behaviors. In Part 2, using the measured input wave and output system response data, the algorithms derived based on the MI/SO linear analysis of the reverse dynamic systems are applied to identify the properties of the highly nonlinear system. Practical issues on the application of the R-MI/SO technique based on limited available experimental data are addressed.
Facet coatings for nonhermetically packaged lasers
Authors: Caballero J. A. , He C. , Djavani-Tabrizi S. , Yin T. , Mallard R. , Das S. R.
Published: May 2004

Nonhermetic packaging is one avenue of reducing the module cost of optical devices; however, this requires the coatings applied to the facets of lasers and modulators to be highly stable and impervious to the effects of high humidity under high temperature/high power operating conditions. We have developed two types of facet coatings for nonhermetic applications. One involves electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) of a SiNx cap layer and the other utilizes ion beam deposition (IBD) of a Ta2O5 cap layer, both on top of a two-layer SiO2/a-Si high-reflectance (HR) coating deposited on the facets of Fabry–Perot (F–P) ridge waveguide (RWG) lasers operating at 1310 nm. The lasers were aged under 85 °C/85% relative humidity (RH) conditions and ex situ tested using drive currents of 10–20 mA (subthreshold) and 80–120 mA operating currents (Iop). The output power–current–voltage (L–I–V) tests were conducted at room temperature and at various time intervals up to 5000 h. For comparison, tests were also done on devices aged in a dry oven.
Optical modulator in silicon-on-insulator with a low thermal signature
Authors: Waldron P. D. , Jessop P. E. , Yuen A. P. , Winchiu L. , Chyurlia P. , Tarr N. G. , Smy T. J. , Golub I. , Xu D.-X. , Janz S.
Published: May 2004

We describe the design, fabrication, and characterization of an optical modulator in silicon-on-insulator (SOI). The modulator is based upon a rib waveguide, with a transverse p–i–n diode being used to produce a refractive index change through the plasma dispersion effect. Measurements of optical modulation show submicrosecond transition times and confirm that electronic switching dominates over thermal effects.
Charge collection and absorption-limited x-ray sensitivity of pixellated x-ray detectors
Authors: Kabir M. Zahangir , Kasap S. O.
Published: May 2004

The charge collection and absorption-limited x-ray sensitivity of a direct conversion pixellated x-ray detector operating in the presence of deep trapping of charge carriers is calculated using the Shockley–Ramo theorem and the weighting potential of the individual pixel. The sensitivity of a pixellated x-ray detector is analyzed in terms of normalized parameters; (a) the normalized x-ray absorption depth (absorption depth/photoconductor thickness), (b) normalized pixel width (pixel size/thickness), and (c) normalized carrier schubwegs (schubweg/thickness). The charge collection and absorption-limited sensitivity of pixellated x-ray detectors mainly depends on the transport properties (mobility and lifetime) of the charges that move towards the pixel electrodes and the extent of dependence increases with decreasing normalized pixel width. The x-ray sensitivity of smaller pixels may be higher or lower than that of larger pixels depending on the rate of electron and hole trapping and the bias polarity. The sensitivity of pixellated detectors can be improved by ensuring that the carrier with the higher mobility-lifetime product is drifted towards the pixel electrodes.
Highly conductive n+ hydrogenated microcrystalline silicon and its application in thin film transistors
Authors: Lee Czang-Ho , Striakhilev Denis , Nathan Arokia
Published: May 2004

n +  μc- Si:H films were deposited using 13.56 MHz plasma enhanced chemical vapor deposition from a gas mixture of silane, phosphine, and hydrogen. To study the effect of deposition conditions and thickness dependence on film properties, and to optimize deposition regimes, we varied the rf power and doping ratio for two different hydrogen dilutions. The film properties were investigated using Raman, x-ray diffraction, secondary-ion-mass spectroscopy, and optical transmittance measurements, as well as dark conductivity. The growth mechanism for a formation of n+ μc-Si:H is explained in terms of a hydrogen dilution effect using the combination of surface diffusion and selective etching models. The optimal conductivity [25 (Ωcm)−1] 50 nm n+ μc-Si:H film was obtained with 99.6% hydrogen dilution of silane. Thin film transistors with this n+ μc-Si:H ohmic contact layer demonstrate a device mobility of 0.9 cm2/Vs, a threshold voltage of 3 V, an ON/OFF current ratio of above 107, a subthreshold slope of 0.5 V/dec, and a leakage current of the order of 10−13 A.
Variable current transport in polymer thin film transistors
Authors: Marinov Ognian , Deen M. Jamal , Yu Jianfei , Vamvounis George , Holdcroft Steven , Woods William
Published: May 2004

The performance of polymer thin film transistors, made of different semiconducting polymers, depends mostly on the type of polymer and its deposition conditions. For these polymer field-effect transistors (PFETs), the current transport is limited by the carrier injection from the source electrode into the polymer. The disordered polymer molecules near the injection interface randomize and decrease the injection barrier, resulting in a large variation of the PFET characteristics, such as threshold voltage, leakage current, and mobility. The PFET current–voltage characteristics degrade at temperatures higher than 40 °C and the low frequency noise increases by 0.3 dB/°C.
Optical and compositional characterization of SiOxNy and SiOx thin films deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition
Authors: Roschuk T. , Wojcik J. , Tan X. , Davies J. A. , Mascher P.
Published: May 2004

Thin silicon oxynitride (SiOxNy) and silicon-rich silicon-oxide (SiOx,x⩽2) films of varying composition have been deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition. Films were deposited using various source gas flow rates while maintaining a constant chamber pressure. Thicknesses and refractive indices for these films were determined using ellipsometry. Bonding of the constituent atoms was analyzed using Fourier transform infrared (FTIR) spectroscopy. FTIR spectroscopy also allowed for the detection of bonded species such as hydrogen. Compositional characteristics were determined using various forms of ion beam analysis such as Rutherford backscattering and elastic recoil detection. These analysis techniques were used to determine the values of x and y, the molar fractions of oxygen and nitrogen, respectively, and the total amount of hydrogen present in the films. Using the results obtained from these methods the film characteristics were determined as a function of the deposition conditions.
Single crystal growth of Ge1−xSix alloys using the traveling solvent method
Authors: Labrie D. , George A. E. , Jamieson M. , Obruchkov S. , Healey J. P. , Paton B. E. , Saghir M. Z.
Published: May 2004

Single crystal growth of Ge1−xSix alloys with Si composition ranging from 2 to 15 at. %. using the traveling solvent method (TSM) is presented. The growths were carried out using Ge 〈111〉 seeds or a self-seeding method. Electron microprobe, Laue x-ray, and Hall effect techniques were used to characterize the quality of the materials. The standard deviations associated with the composition profiles along and transverse to the growth direction were less than 0.4 and 0.3 at. %, respectively. A change in conductivity from n to p type in the TSM samples was observed at a Si composition <5 at. %.
Nano patterning on optical fiber and laser diode facet with dry resist
Authors: Kelkar P. S. , Beauvais J. , Lavallée E. , Drouin D. , Cloutier M. , Turcotte D. , Yang Pan , Mun Lau Kien , Legario R. , Awad Y. , Aimez V.
Published: May 2004

Semiconductor micro and nanofabrication lithography techniques for application in microelectronics as well as in micromechanics and optoelectronics can gain significantly from using a dry resist process, since it enables the deposition of a very uniform lithographically sensitive layer on a potentially very small area. This would otherwise be extremely difficult to achieve by using a traditional spin coated resist, such as poly(methylmethacrylate) (PMMA). We demonstrate the use of an electron sensitive sterol based evaporated electron beam resist to fabricate high-resolution features (down to 100 nm) on a small surface area. This electron beam resist has a sensitivity comparable to PMMA and is deposited using a simple thermal evaporation. Two practical applications are explored: first, this resist makes it possible to fabricate a Fresnel zone plate lens on the tip of an optical fiber in order to demonstrate the principle and the potential of highly efficient coupling of diode laser emission into the fiber; second, we use this evaporated electron beam resist in order to pattern an optical diffractive element on the facet of a semiconductor laser.
Implant isolation in an indium phosphide optoelectronic device: A scanning spreading resistance microscopy study
Authors: Dixon-Warren St. J. , Haysom J. E. , Betty I. , Lu J. , Hewitt K.
Published: May 2004

Scanning spreading resistance microscopy has been used to delineate the implant isolated regions of an InP optoelectronic device. This scanning probe microscopy-based technique provides direct insight into the effect of helium implant isolation on the electrical conductivity of the material. The nanometer-scale resolution of the technique allows us to directly image the implanted region of the device structure. The results of the scanning probe measurements are compared with theoretical simulations of the helium implant damage. The optimal implant isolation conditions were determined by direct measurements of the series resistance across implanted and reference device material. The scanning spreading resistance microscopy results were able to provide fundamental insight explaining the implant conditions required for optimal isolation.
Deep-ultraviolet-induced damage of charge coupled device sensors
Authors: Li Flora , Nathan Arokia , O Nixon
Published: May 2004

In order to facilitate the inspection of deep-submicron features, a generation of semiconductor inspection systems are being pushed to image using deep-ultraviolet (UV) sensitive cameras at increasingly shorter wavelengths. In this article, we present a deep-UV sensitive thinned front-illuminated linear charge coupled device (CCD) image sensor structure and investigate its behavior after exposure to F2 (λ=157 nm) excimer laser. Two key manifestations of radiation damage are observed: (1) Extrinsic quantum efficiency drifts with increasing 157 nm exposure, and (2) dark current increases almost exponentially with 157 nm exposure. These fluctuations in CCD parameters can be caused by several factors including UV-induced color center formation in the oxide, charge generation in the oxide, interface modification, and structural rearrangement. These UV-induced effects alter the optical and electrical properties of the oxide and Si–SiO2 interface, resulting in both temporal and permanent shifts in device performance. The experimental results suggest that careful control of the oxide thickness and the Si–SiO2 interface quality are critical for realizing CCD sensors with high responsivity and stability for deep-UV imaging.
Cold wall chemical vapor deposition of single walled carbon nanotubes
Authors: Finnie P. , Bardwell J. , Tsandev I. , Tomlinson M. , Beaulieu M. , Fraser J. , Lefebvre J.
Published: May 2004

Single walled carbon nanotubes (SWNTs) are emerging as a material system for electronics and optoelectronics. Hot wall thermal chemical vapor deposition (CVD) has become a standard method of preparing SWNTs. We have been using a closely related synthesis process, cold wall CVD, to produce SWNTs. Grown materials are characterized by scanning electron microscopy and photoluminescence (PL) spectroscopy. Nanotubes exceeding 150 μm in length are observed, with growth rates of at least 0.5 μm/s. PL was used to confirm the growth of SWNTs, and underscores the suitability of the material for optical and optoelectronic applications. Advantages and disadvantages of the cold wall CVD method will be described. This work shows that the cold wall CVD technique specifically is capable of producing SWNTs, and also demonstrates the use of PL as a characterization tool.
Extraction of negative ions from pulsed electronegative inductively coupled plasmas having a radio-frequency substrate bias
Authors: Subramonium Pramod , Kushner Mark J.
Published: May 2004

Pulsed electronegative plasmas are promising candidates for reducing charge buildup during microelectronics fabrication by extracting negative ions into features. By modulating power in inductively coupled plasmas (ICPs), the plasma potential collapses during the power-off period, thereby allowing negative ions to be extracted. In principle, application of a radio-frequency (rf) substrate bias should accelerate these ions into features. In practice, this goal is not always achieved due to the unfavorable dynamics of the plasma potential. We computationally investigated the extraction of negative ions in the afterglow of pulsed ICPs having rf substrate biases sustained in Ar/Cl2 gas mixtures. We found that the extraction of negative ions is optimized by delaying the transition to a capacitive heating mode in the afterglow, which can be achieved by the addition of Ar to Cl2 plasmas. Increasing the bias voltage causes a capacitive heating mode to begin earlier, which prevents negative ions from being extracted. To circumvent this effect, schemes were investigated in which the rf bias is applied for only a portion of the pulse period. At high rf frequencies (≈10 MHz), ions striking the substrate have only thermal energies due to the majority of the applied bias being dropped across the bulk plasma. At lower frequencies (≈2 MHz), negative ions with 2–25 eV energy were extracted with an anisotropic angular distribution due to more favorable sheath formation.
Cathode glow polymerization of trimethylsilane
Authors: Yasuda Hirotsugu , Yu Qingsong
Published: May 2004

In glow discharges of Ar, negative glow develops away from the cathode and constitutes the most luminous gas phase, and the cathode remains in the dark, i.e., there is no cathode glow. The location of negative glow is where the maximum value of electron temperature is observed, indicating that the energy of photon-emitting species is in the vicinity of ionization of the atoms involved. In glow discharges of trimethylsilane, cathode glow, the primary glow of glow discharge, appears on the cathode surface that is caused by dissociation reactions of organic molecules upon low-energy electron impact, which can be more properly termed molecular dissociation glow. Dissociation glow develops at the very onset in various glow discharges and, at that stage, negative glow is barely visible, but becomes visible as the gas phase composition changes due to plasma polymerization, which deposits polymerizable species into solid thin films and leaves nonpolymerizable species such as hydrogen atoms and molecules in the gas phase. Dissociation glow is not observed with simple molecular gases such as O2, N2, and CO2, which do not deposit material.
Electron-cyclotron-resonance plasma etching of the ZnO layers grown by molecular-beam epitaxy
Authors: Ogata K. , Honden T. , Tanite T. , Komuro T. , Koike K. , Sasa S. , Inoue M. , Yano M.
Published: May 2004

Etching of high-quality ZnO layers grown by molecular-beam epitaxy was examined by using electron-cyclotron-resonance plasma etching. Etching rates of ZnO layers were larger using CH4 than CF4, and four times more enhanced by using a mixture of the two gases. For a ZnO surface covered with photoresist layers by the plasma-etching procedure, degradation was mostly recovered by thermal annealing in an O2 atmosphere at the proper temperatures. This was found to be effective for the recovery of the layers.
Process for uniformly electroplating a patterned wafer with electrically isolated devices
Authors: Zhang S. , Meng X. , Zheng X. , Das S. R. , Shepherd F.
Published: May 2004

This article reports a technique to electroplate a patterned wafer with electrically isolated devices. A seed layer of Ti/Au was deposited on a SiO2/InP substrate. After lithography, 1.2-μm-thick Au was electroplated on the patterned wafer. Then Ti (10 nm) was deposited on the wafer by vacuum evaporation. Ti on the photoresist was lifted off in PRS 2000, while Ti on the plated Au was retained and was used as an etch-protection layer. The wafer was etched in saturated KI solution to remove the unprotected Au of the seed layer. Finally, exposed Ti in the seed layer, as well as on the plated gold surface, was etched with a HF:HNO3:H2O solution. Auger electron spectroscopy analysis of the wafer did not detect any residual Ti on the gold surface or on the SiO2. Electrical probing between two isolated devices exhibited infinite resistance, which indicated that the unwanted seed layer was completely removed. Scanning electron microscopy confirmed that the metal (seed layer plus electroplated Au) on the devices had the desired cross section. This technique has been used to successfully fabricate T electrodes on traveling wave polarization converter devices and bond pads for p-side down bonding of uncooled directly modulated lasers.
Optical phonons in InSb1−xAsx/InSb
Authors: Rowell N. L. , Lockwood D. J. , Yu G. , Gao Y. Z. , Gong X. Y. , Aoyama M. , Yamaguchi T.
Published: May 2004

We have examined the optical phonons of InSb1−xAsx using oblique-incidence infrared spectroscopy. Reflectance spectra of thick (∼2 μm) relaxed layers grown on InSb (100) by liquid phase epitaxy exhibit for x<0.65 a weak reflectivity peak near 235 cm−1 together with a strong substrate peak. No other prominent features were seen. Fitting the spectra with a factorized model yielded the transverse and longitudinal mode frequencies and damping constants. The InSb-like mode frequencies were only weakly dependent on Sb concentration for x<0.65. Although it had been expected that InAs1−xSbx would exhibit one-mode behavior, this proved to be true only for x<0.49 above which concentration two modes were seen.
Direct UV patterning of waveguide devices in As2Se3 thin films
Authors: Bryce R. M. , Nguyen H. T. , Nakeeran P. , DeCorby R. G. , Dwivedi P. K. , Haugen C. J. , McMullin J. N. , Kasap S. O.
Published: May 2004

Photodarkening of thermally evaporated amorphous As2Se3 chalcogenide thin films was generated by a UV mercury light source in a standard mask aligner. The refractive index modification of the chalcogenide glass was determined by applying Swanepoel’s method. Index changes of +0.04 were obtained for 500 s exposure. Using these photoinduced index changes, waveguides with losses of approximately 1 dB/cm at 980 nm were fabricated. Another set of waveguides was fabricated by UV exposure and subsequent selective etching to form rib structures. Those waveguides exhibited loss of approximately 2 dB/cm at 980 nm. Silver photodoping of As2Se3 was also performed on a mask aligner with index increases on the order of 0.3 obtained. Due to the rapid and large photomodification obtainable with standard photolithographic equipment, these processes are promising for integrated optic device fabrication.
Monitoring chamber walls coating deposited during plasma processes: Application to silicon gate etch processes
Authors: Joubert O. , Cunge G. , Pelissier B. , Vallier L. , Kogelschatz M. , Pargon E.
Published: May 2004

During plasma etching processes, organic or mineral layers are deposited on the chamber walls. In general, these layers cause large and uncontrolled shifts in the etch process, which is becoming a major issue in some of the plasma processes used in integrated circuit fabrication. The chemical nature of these layers and their deposition mechanisms remain poorly understood due to the lack of in situ surface diagnostics available to monitor the reactor walls. In this article, we present a simple technique using x-ray photoelectron spectroscopy (XPS) analyses to monitor the chemical composition of the layer deposited on a sample floating on top of a 200-mm-diam wafer where the layers deposited are identical to those deposited on the chamber walls. The principle of the technique is to stick a small Al2O3 sample onto the 200-mm-diam silicon wafer, with an air gap between the sample and the wafer. Providing that the air gap is thick enough, the Al2O3 surface will be electrically floating even when the silicon wafer is rf biased. During the etching process, the Al2O3 sample thus experiences exactly the same plasma conditions as the reactor walls. As the sample is physically clamped on the wafer, it can then be transferred under vacuum to an XPS analyzer, allowing quasi-in situ analyses of the deposited layers. The validity of the technique has been tested during silicon gate etching in HBr/Cl2/O2 plasmas, which are known to deposit silicon oxychloride layers on the chamber walls. The influence of CF4 addition in the plasma which has been recently introduced in gate etching manufacturing is also analyzed using the same technique. In a second step, we show that the presence of photoresist on the etched wafer profoundly affects the chemical nature of the layers formed on the chamber walls, mainly by significantly increasing the carbon concentration in the deposited layer.
Composition, structural, and electrical properties of fluorinated silicon–nitride thin films grown by remote plasma-enhanced chemical-vapor deposition from SiF4/NH3 mixtures
Authors: Fandiño J. , Ortiz A. , Rodrı́guez-Fernandez L. , Alonso J. C.
Published: May 2004

Fluorinated silicon–nitride films (SiNx:F) have been prepared at 250 °C by remote plasma-enhanced chemical-vapor deposition using mixtures of SiF4/NH3 in different proportions. The structure, relative composition, and fluorine content of the films were evaluated by Fourier-transform infrared spectroscopy, Rutherford backscattering, ellipsometry, and resonant nuclear-reaction analysis. The electrical properties of the films were also assessed from the current–voltage characteristics of Al–SiNx:F–Si metal–insulating–semiconductor structures. It was found that the SiF4/NH3 ratio produces little influence on the refractive index and density of the films, but this ratio has important effects on the fluorine content, deposition rate, and electrical properties. In general, these SiNx:F films are free of Si–H bonds, chemically stable, and show breakdown fields above 8 MV/cm.
Effects of atomic hydrogen on the selective area growth of Si and Si1−xGex thin films on Si and SiO2 surfaces: Inhibition, nucleation, and growth
Authors: Schroeder T. W. , Lam A. M. , Ma P. F. , Engstrom J. R.
Published: May 2004

Supersonic molecular beam techniques have been used to study the nucleation of Si and Si1−xGex thin films on Si and SiO2 surfaces, where Si2H6 and GeH4 have been used as sources. A particular emphasis of this study has been an examination of the effects of a coincident flux of atomic hydrogen. The time associated with formation of stable islands of Si or Si1−xGex on SiO2 surfaces—the incubation time—has been found to depend strongly on the kinetic energy of the incident molecular precursors (Si2H6 and GeH4) and the substrate temperature. After coalescence, thin film morphology has been found to depend primarily on substrate temperature, with smoother films being grown at substrate temperatures below 600 °C. Introduction of a coincident flux of atomic hydrogen has a large effect on the nucleation and growth process. First, the incubation time in the presence of atomic hydrogen has been found to increase, especially at substrate temperatures below 630 °C, suggesting that hydrogen atoms adsorbed on Si-like sites on SiO2 can effectively block nucleation of Si. Unfortunately, in terms of promoting selective area growth, coincident atomic hydrogen also decreases the rate of epitaxial growth rate, essentially offsetting any increase in the incubation time for growth on SiO2. Concerning Si1−xGex growth, the introduction of GeH4 produces substantial changes in both thin film morphology and the rate nucleation of poly-Si1−xGex on SiO2. Briefly, the addition of Ge increases the incubation time, while it lessens the effect of coincident hydrogen on the incubation time. Finally, a comparison of the maximum island density, the time to reach this density, and the steady-state polycrystalline growth rate strongly suggests that all thin films [Si, Si1−xGex, both with and without H(g)] nucleate at special sites on the SiO2 surface, and grow primarily via direct deposition of adatoms on pre-existing islands.
Etching mechanism of a GaN/InGaN/GaN heterostructure in Cl2- and CH4-based inductively coupled plasmas
Authors: Kim H. K. , Lin H. , Ra Y.
Published: May 2004

The etching mechanism of a GaN/InGaN/GaN heterostructure was studied using Cl2- and CH4-based inductively coupled plasma. The Cl2-based plasmas are effective for fast and smooth etching of the homogeneous GaN layer. However, when the layer has a heterogeneous structure of GaN/InGaN/GaN, Cl2-based plasmas cause many pits or pillars on the surface due to a micromasking effect of low volatile In chlorides. The growth of pillars was accompanied by microtrenching at the bottom of each pillar, which transformed into pits after the pillars were detached from their sites. By adding CH4 gas to the Cl2 plasma, the formation of pillars or pits was reduced, and very smooth surface morphology was demonstrated at the gas condition of 35Cl2+20CH4+5Ar (sccm).
Fabrication and characterization of amorphous Si/crystalline Si heterojunction devices for photovoltaic applications
Authors: Baroughi M. Farrokh , Jeyakumar R. , Vygranenko Y. , Khalvati F. , Sivoththaman S.
Published: May 2004

Heterojunction diode and heterojunction photovoltaic cell structures are fabricated with (n+)a-Si/(i)a-Si and rf-sputtered indium-tin-oxide/Al films deposited on p-type crystalline Si using a plasma-enhanced chemical vapor deposition cluster tool system. Dark current–voltage characteristics of the heterojunction diodes are used to determine the carrier transport mechanisms. Experimental results showed the current is recombination-dominated at low forward bias (VA<0.25 V at 27 °C), multitunneling capture emission (MTCE)-dominated at medium bias (0.25<VA<0.45 at 27 °C), a combination of diffusion or thermionic emission and MTCE at high bias, and series resistance limited at very high bias ranges. Measurement results show that the boundary between each region is not fixed and is strongly dependent on temperature. High-frequency capacitance–voltage measurement has been used to extract band diagram parameters of the heterojunction diodes. Measured parameters, built-in potential, and conduction and valence band offsets, have been verified by Analysis of Microelectronic and Photonic Structures numerical device simulator. External quantum efficiency has been measured on photovoltaic cells and used for calculation of short-circuit current under AM1.5 illumination.
Influence of the electrical contact on the reliability of InP-based ridge waveguide distributed feedback semiconductor diode lasers for telecommunications applications
Authors: Zhang S. , Boudreau M. G. , Kuchibhatla R. , Tao Y. , Das S. R. , Griswold E. M. , Sharma U.
Published: May 2004

The design of a ridge waveguide semiconductor diode laser requires the definition of a high-quality metal contact to a p-type semiconductor that has been patterned to form the laser ridge. In the InGaAsP material system, Zn-doped, p-InGaAs is used as the contact layer. An evaporated multilayer metal stack consisting of Ti, Pt, and Au is alloyed to the InGaAs to form a stable ohmic contact with good adhesion. The alloying process is an intermixing of the Ti and As. Pt is not reactive during the alloying process and acts as a diffusion barrier. In this work, the Pt and Ti layer thicknesses were varied to improve the laser reliability. The layer thicknesses were optimized with the goal of producing a minimum stress in the laser active region. A preliminary study of completely processed lasers indicates that the laser reliability is significantly influenced by the changes to the stress induced in the device by the p-contact metal stack. Further work is required to establish the relationship between the stress in the active region of the device, and the stress contribution from the p-contact metal stack.
X-ray-induced recombination effects in a-Se-based x-ray photoconductors used in direct conversion x-ray sensors
Authors: Fogal Bud , Kabir M. Zahangir , O’Leary Stephen K. , Johanson Robert E. , Kasap S. O.
Published: May 2004

Stabilized amorphous selenium (a-Se) is currently used as an x-ray photoconductor in direct conversion flat-panel digital x-ray image detectors. Therefore, there is much interest in x-ray-induced effects in a-Se, especially changes in charge carrier lifetimes that result from x-ray exposure. We have observed that the exposure of an a-Se x-ray detector sample to x rays induces negative capture centers in the bulk and thereby reduces the hole lifetime. By using conventional and interrupted field time-of-flight (IFTOF) transient photoconductivity techniques in a TOF-IFTOF-TOF sequence, we were able to develop a technique that allows the measurement of the capture coefficient Cr between free holes and x-ray-induced negative centers, which we believe to be trapped electrons. We find that the capture process follows the Langevin recombination mechanism, the same recombination mechanism that has been observed in the case of recombination between free holes and free electrons in a-Se. We have shown that the concentration of x-ray-induced negative centers increases almost linearly with the x-ray exposure. As a corollary, in terms of fundamental physics of amorphous semiconductors, we can also conclude that the influence of potential fluctuations in the noncrystalline structure in shielding a charged center in a-Se is relatively small.
Study of lateral-carrier transport in InAs quantum-dot heterostructures by optical spectroscopy
Authors: Lin Ray-Ming , Cheng Chao-Ching , Shen Hui-Tang , Nee Tzer-En
Published: May 2004

Lateral-carrier-transport characteristics in InAs/GaAs quantum-dot (QD) heterostructures has been studied in depth by photoluminescence spectroscopy. Anomalous temperature-dependent phenomena, including spectral width, emission energy, and quenching behaviors, were attributed to carrier-thermalization processes. Increased temperatures facilitated the photoexcited carriers to redistribute in the heterodot system. Carrier redistrubution led to not only the linewith shrinkages, but also the emission redshifts for the excited- and ground-state transitions in QD ensembles. Plan-view transmission electron microscopy also showed the growth-interruption effect on the dot formation. It was found that the longer interruption time improved the dot-size uniformity.
Electrode fabrication for high-speed polarization mode converter
Authors: Meng X. , Zhang S. , Chen C. , Poirier M. , Shepherd F. R. , Das S. R.
Published: May 2004

As part of the development of a traveling wave polarization converter (TWPC), a technique for producing “T-electrodes” adjacent to the ridge sidewalls and air bridges has been developed. To maximize the conversion efficiency, the rf electrodes on the etched floor are required to be very close to the ridge sidewalls. In the fabrication of the electrode, patterned seed layer stripes were first put on the etched floor on both sides of the waveguide by a metal lift-off process. Onto this seed layer, the T-electrode was electroplated through a patterned thick photoresist. After resist removal, the unwanted seed layer was stripped off by chemical etch back. Through this multistep process, the electrodes were constructed successfully with the desired proximity to the ridge waveguide. For easy chip assembly, the bond pads were designed to be on the same side of the device. Two long and wide metal air bridges over the ridge waveguide were made to connect the electrode on one side to the bond pads on the other side of the ridge. It was found that the air bridges were easily damaged in the bonding process prior to lap and polish because of the particular device structure, i.e., air bridges being the highest point of the wafer and only a small number of air bridges on the wafer. To protect the air bridge a patterned photoresist layer was applied before the wafer was bonded to a carrier.
Low-temperature stress-assisted germanium-induced crystallization of silicon–germanium alloys on flexible polyethylene terephtalate substrates
Authors: Hekmatshoar B. , Shahrjerdi D. , Mohajerzadeh S. , Khakifirooz A. , Robertson M. , Tonita A. , Bennett J. C.
Published: May 2004

The application of mechanical-compressive stress during low-temperature annealing has been investigated for the crystallization of SiGe alloys on plastic substrates. It was observed that crystallization of an amorphous Ge/Cu/Ge “sandwich” can occur at temperatures as low as 130 °C with the application of an equivalent compressive strain of 0.05%. By using this sandwich as a seed for crystallization of an underlying amorphous SiGe film, partial crystallization of the film was observed to occur at a temperature of 180 °C, again under an equivalent compressive strain of 0.05%. Without the application of the compressive strain, crystallization was not observed for either system at the temperatures investigated. The atomic percentage of Si in the SiGe alloy was 35% as confirmed by Rutherford backscattering spectroscopy and the partial crystallization of the SiGe layer was verified by scanning electron microscopy, x-ray diffraction, and transmission-electron microscopy analyses.
Polymethylmethacrylate membrane for fluid encapsulation and release in microfluidic systems
Authors: Srivastava Y. N. , Gadre A. P. , Hylton T. , Monica A. H. , Schneider T. W. , White R. C. , Paranjape M. , Currie J. F.
Published: May 2004

A multipolymeric fabrication process using polymethlymethacrylate (PMMA) as a membrane layer was demonstrated for a robust dermal patch, called the bio-fluidic integrable transdermal (B–FIT) microsystem. B–FIT is a noninvasive microdevice, in the form of an adhesive patch, which samples biomolecules from interstitial fluids using controlled thermal ablation and encapsulated fluid that is released during biomolecule collection. Thus, a membrane material was needed that could encapsulate the fluid and easily liberate it, on demand, with minimal energy and compatibility with the fabrication process. PMMA was selected as the most promising membrane material for the B–FIT as against SU-8 since it had the required properties of low melting temperature (120–150 °C), good adhesion to pre-existing B–FIT materials (gold and SU-8), and could be easily spin-cast into thin uniform films. However, PMMA is not inert towards various solvents like acetone, photoresist developers, hence, a process was developed to protect PMMA from being attacked by the solvents used during the fabrication of the device. In this process, several strategies were employed such as crosslinking and application of thin protection layers on the PMMA. By exploring these fabrication strategies, PMMA was successfully patterned and the desired encapsulation and rupturing of the membrane was obtained.
Analysis and characterization of self-compensating current programmed a-Si:H active matrix organic light-emitting diode pixel circuits
Authors: Sakariya Kapil , Sambandan Sanjiv , Servati Peyman , Nathan Arokia
Published: May 2004

The organic light-emitting diode (OLED) drive current provided by a simple 2 thin film transistor (TFT) hydrogenated amorphous silicon (a-Si:H) active matrix OLED (AMOLED) voltage programmed pixel circuit drops over time due to threshold voltage (VT) increase in the drive TFT. VT shift compensating current programmed 4-TFT circuits have been developed to overcome this problem and to keep the drive current constant, thereby prolonging the life of the display. In this article, we present a thorough analysis of the static and dynamic operation of such circuits and derive theoretical limits for the extent of compensation. Measurement results of circuit characteristics and stability are presented which demonstrate that it is feasible to use a-Si:H for AMOLED back planes. The advantages and drawbacks of current programmed circuits are also outlined from the perspective of active matrix back-plane design.
Thick film resist lithography for a-Si:H devices with high topography and long dry etch processes
Authors: Chan Isaac , Nathan Arokia
Published: May 2004

In this article, we present an optimized thick film photoresist process to compensate for issues such as step coverage, linewidth variations, and long dry etch resistance for hydrogenated amorphous silicon (a-Si:H) devices, such as vertical thin film transistors (VTFTs) with significant substrate topography (1 μm or higher). This article describes a systematic approach to develop an optical lithographic process along with practical process models to analyze the key parameters that control the process throughput and resist charactersistics. The resist performance in dry etching and linewidth control over the topographic structure of VTFTs accompanied with the electrical characteristics of the fabricated device are demonstrated.
Electrokinetic movement of micro-objects in fluids using microelectromechanical system electrode arrays
Authors: Ellerington Neil , Hubbard Ted , Kujath Marek
Published: May 2004

In this article we demonstrate methods of transporting micro-objects in fluids using microelectromechanical system electrode arrays to generate nonuniform electric fields. We show the ability to control fluid flow through ac electro-osmosis and the ability to move micro-objects through dielectrophoresis where the velocity is a function of the frequency, voltage, and spacing of arrays. Velocities in excess of 600 μm/s have been achieved with ac voltages as low as 7 Vpp. In addition to one-dimensional arrays that provide single axis control, here we also investigate a two-dimensional array capable of planar control.
Deposition of Hf–silicate gate dielectric on SixGe1−x(100): Detection of interfacial layer growth
Authors: Addepalli S. , Sivasubramani P. , El-Bouanani M. , Kim M. J. , Gnade B. E. , Wallace R. M.
Published: May 2004

The deposition of a stable high-κ dielectric material such as hafnium silicate in direct contact with strained epitaxial SixGe1−x(100) layers on Si(100) provides the prospect of eliminating the Si buffer layer that is currently used to form the gate oxide in SiGe-based devices. In this study, ∼3-nm-thick hafnium silicate films were produced by sputter deposition of hafnium silicide films on precleaned SixGe1−x(100), with subsequent UV-O3 oxidation at room temperature. Prolonged UV-O3 exposure at room temperature leads to the growth of an interfacial layer comprised of a mixture of silicon and germanium oxides. We report on the use of x-ray photoelectron spectroscopy, particularly the x-ray excited Ge (L3MM) Auger feature to optimize UV-O3 exposure time, and minimize the interfacial layer growth of silicon and germanium oxides.
Atomic-level robustness of the Si(100)-2×1:H surface following liquid phase chemical treatments in atmospheric pressure environments
Authors: Baluch A. S. , Guisinger N. P. , Basu R. , Foley E. T. , Hersam M. C.
Published: May 2004

The UHV-prepared Si(100)-2×1:H surface is studied at atomic resolution following liquid phase chemical processing under atmospheric pressure conditions. A custom experimental setup, consisting of an UHV scanning tunneling microscope (STM) chamber that is directly interfaced to an inert atmosphere glovebox, facilitates liquid phase chemical processing without exposing the pristine H-passivated surface to ambient air. While in the inert atmosphere, the Si(100)-2×1:H surface is treated with a variety of organic and aqueous solvents. Atomic resolution STM images reveal that the hydrogen passivation remains largely intact after treatments in toluene and dichloromethane. In addition, by minimizing oxygen levels during processing, perturbation to the Si(100)-2×1:H surface can be significantly reduced following exposure to water. These results are potentially useful in the fields of microelectronics and molecular-beam epitaxy, where liquid phase chemical processing is often avoided in an effort to preserve atomically pristine Si(100) surfaces. Furthermore, this study delineates the conditions under which various organic and biological molecules can be delivered to nanopatterned Si(100)-2×1:H surfaces via liquid phase solvents.
Noise performance of a current-mediated amplified pixel for large-area medical imaging
Authors: Karim Karim S. , Nathan Arokia
Published: May 2004

In contrast to switch-based passive pixel sensor readout circuits, pixel architectures incorporating on-pixel amplification have recently surfaced for large-area, noise-vulnerable, medical imaging applications. In this article, the noise performance of a current-mediated amorphous silicon pixel amplifier readout circuit for diagnostic medical imaging is presented. In addition, reset noise, commonly the limiting noise component in complementary metal-oxide-semiconductor pixel amplifiers, is investigated for amorphous silicon pixel circuits. Theoretical analysis and measurements of pixel noise sources are presented in which the amorphous silicon pixel amplifier appears promising for large-area, noise-sensitive, real-time digital fluoroscopy.
Stress-assisted nickel-induced crystallization of silicon on glass
Authors: Hashemi P. , Derakhshandeh J. , Mohajerzadeh S. , Robertson M. , Tonita A.
Published: May 2004

The effect of external mechanical stress on the crystallization of amorphous silicon deposited on thin, flexible glass substrates has been studied. A thin, 5–10 Å, layer of nickel deposited on the surface of the amorphous silicon layer acted as the seed of crystallization and the crystallization was observed to initiate at the top surface and proceed down towards the glass substrate. Application of a tensile stress during the annealing stage led to a uniform, partial crystallization of the amorphous silicon for annealing temperatures as low as 310 °C. In contrast, the application of compressive stress led to buckling of the silicon films during annealing under mechanical stress and crystallization was nonuniform over the surface of the sample. The crystalline quality of the films was investigated using scanning electron microscopy, x-ray diffraction, and transmission electron microscopy analyses. In addition, lateral polycrystalline growth of the silicon was observed for the case in which the nickel seed layer was less than 5 Å thick.