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- ItemSilver electrodeposits in ion-exchanged oxide glasses(American Physical Society, 1993-02) Roy, Saibal; Chakravorty, D.; Department of Science and Technology, Ministry of Science and TechnologySilver electrodeposits have been grown within a wide range of oxide glass sytems. The latter are first of all subjected to an alkali-metal–silver-ion exchange reaction. The experimental configuration ensures two-dimensional growth. The fractal dimension of the electrodeposits has been estimated for each sample. Glasses having lithium ions (in the original composition) show a fractal dimension around 1.85 whereas those containing sodium ions give a fractal dimension of 1.68 for their respective silver deposits. Dendritic growth has been observed in glasses containing nanometer-sized metal particles of either bismuth or aluminum. The fractal dimension of all the electrodeposits appears to increase as the length scale is reduced.
- ItemThe operation of multiquantum-well barriers(University College Cork, 1996-12) Morrison, Alan P.; Lambkin, John D.; van der Poel, Carel J.The multiquantum barrier (MQB), proposed by Iga et al in 1986, has been shown by several researchers to be an effective structure for improving the operating characteristics of laser diodes. These improvements include a reduction in the laser threshold current and increased characteristic temperatures. The operation of the MQB has been described as providing an increased barrier to electron overflow by reflecting high energy electrons trying to escape from the active region of the laser.This is achieved in a manner analogous to a Bragg reflector in optics. This thesis presents an investigation of the effectiveness of the MQB as an electron reflector. Numerical models have been developed for calculating the electron reflection due to MQB. Novel optical and electrical characterisation techniques have been used to try to measure an increase in barrier height due to the MQB in AlGaInP.It has been shown that the inclusion of MQB structures in bulk double heterostructure visible laser diodes can halve the threshold current above room temperature and the characteristic temperature of these lasers can be increased by up to 20K.These improvements are shown to occur in visible laser diodes even with the inclusion of theoretically ineffective MQB structures, hence the observed improvement in the characteristics of the laser diodes described above cannot be uniquely attributed to an increased barrier height due to enhance electron reflection. It is proposed here that the MQB improves the performance of laser diodes by proventing the diffusion of zinc into the active region of the laser. It is also proposed that the trapped zinc in the MQB region of the laser diode locally increases the p-type doping bringing the quasi-Fermi level for holes closer to the valence band edge thus increasing the barrier to electron overflow in the conduction band.
- ItemBroadband ac conductivity of conductor-polymer composites(American Physical Society, 1998-01-15) Connor, Marco T.; Roy, Saibal; Ezquerra, Tiberio A.; Baltá Calleja, Francisco J.; Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung; Ministerio de Educación, Cultura y Deporte; European Commission; Dirección General de Investigación Científica y TécnicaThe electrical conductivity of a composite model system formed by highly structured carbon black (CB) filled, within an amorphous polymer, poly(ethylene terephtalate) composite is studied. The dc conductivity as a function of CB content follows a scaling law of the type σ∝(p−pc)t yielding for the percolation concentration, pc=0.011 and for the exponent, t=2.17. The analysis of the temperature dependence of the conductivity suggests that for temperatures larger than 45 K, conduction can be ascribed to thermal fluctuation induced tunneling of the charge carriers through the insulating layer of polymer separating two CB aggregates. At lower temperatures, conductivity becomes temperature independent, which is typical of conventional tunneling. The frequency dependence of the conductivity is also studied between dc and 109 Hz. By the introduction of a shift factor ap, a procedure for the construction of a master curve based on a “time-length equivalence principle” is proposed. Finally, a model is introduced to describe the frequency dependence of the conductivity of CB-filled composites based on the behavior of charge carriers placed in a fractal object.
- ItemSilicon-based resonant-cavity-enchanced photodiode with a buried SiO2 reflector(AIP Publishing, 1999) Sinnis, Vasileios S.; Seto, M.; 't Hooft, G. W.; Watabe, Y.; Morrison, Alan P.; Hoekstra, W.; de Boer, W. B.We report on a silicon-based resonant cavity photodiode with a buried silicon dioxide layer as the bottom reflector. The buried oxide is created by using a separation by implantation of oxygen technique. The device shows large Fabry-Perot oscillations. Resonant peaks and antiresonant troughs are observed as a function of the wavelength, with a peak responsivity of about 50 mA/W at 650 and 709 nm. The leakage current density is 85 pA/mm(2) at -5 V, and the average zero-bias capacitance is 12 pF/mm(2). We also demonstrate that the buried oxide prevents carriers generated deep within the substrate from reaching the top contacts, thus removing any slow carrier diffusion tail from the impulse response. (C) 1999 American Institute of Physics. (DOI: 10.1063/1.123499).
- ItemCollisional broadening and shift of spectral lines in quantum dot lasers(AIP Publishing, 1999) Uskov, Alexander V.; Nishi, K.; Lang, R.We estimate homogeneous broadening and shift of optical transition lines in self-assembled quantum dots (SAQD) caused by elastic Coulomb collisions of carriers in wetting layer with carriers in the SAQD. In particular, we demonstrate that the dephasing time for lasing transitions can be similar to 0.1-1 ps at carrier densities in wetting layer similar to 10(15) m(-2). (C) 1999 American Institute of Physics. (DOI: 10.1063/1.124068)
- ItemProperties of latent interface-trap buildup in irradiated metal-oxide-semiconductor transistors determined by switched bias isothermal annealing experiments(AIP Publishing, 2000) Jaksic, Aleksandar B.; Pejovic, M. M.; Ristic, G. S.Isothermal annealing experiments with switched gate bias have been performed to determine the properties of the latent interface-trap buildup during postirradiation annealing of metal-oxide-semiconductor transistors. It has been found that a bias-independent process occurs until the start of the latent interface-trap buildup. During the buildup itself, oxide-trap charge is not permanently neutralized, but is temporarily compensated. (C) 2000 American Institute of Physics. (DOI: 10.1063/1.1336159)
- ItemSi/SiGe electron resonant tunneling diodes with graded spacer wells(AIP Publishing, 2001) Paul, D. J.; See, P.; Bates, R.; Griffin, N.; Coonan, Barry P.; Redmond, Gareth; Crean, Gabriel M.; Zozoulenko, I. V.; Berggren, K. F.; Hollander, B.; Mantl, S.; Fourth Framework ProgrammeResonant tunneling diodes have been fabricated using graded Si(1-x)Ge(x) (x=0.3 -->0.0) spacer wells and strained Si(0.4)Ge(0.6) barriers on a relaxed Si(0.7)Ge(0.3) n-type substrate which demonstrates negative differential resistance at up to 100 K. This design is aimed at reducing the voltage at which the peak current density is achieved. Peak current densities of 0.08 A/cm(2) with peak-to-valley current ratios of 1.67 have been achieved for a low peak voltage of 40 mV at 77 K. This represents an improvement of over an order of magnitude compared to previous work. (C) 2001 American Institute of Physics. (DOI: 10.1063/1.1381042)
- ItemSi(100)-SiO2 interface properties following rapid thermal processing(AIP Publishing, 2001-04) O'Sullivan, B. J.; Hurley, Paul K.; Leveugle, C.; Das, J. H.An experimental examination of the properties of the Si(100)-SiO2 interface measured following rapid thermal processing (RTP) is presented. The interface properties have been examined using high frequency and quasi-static capacitance-voltage (CV) analysis of metal-oxide-silicon (MOS) capacitor structures immediately following either rapid thermal oxidation (RTO) or rapid thermal annealing (RTA). The experimental results reveal a characteristic peak in the CV response measured following dry RTO and RTA (T > 800 degreesC), as the Fermi level at the Si(100)-SiO2 interface approaches the conduction band edge. Analysis of the QSCV responses reveals a high interface state density across the energy gap following dry RTO and RTA processing, with a characteristic peak density in the range 5.5x10(12) to 1.7x10(13) cm(-2) eV(-1) located at approximately 0.85-0.88 eV above the valence band edge. When the background density of states for a hydrogen-passivated interface is subtracted, another peak of lower density (3x10(12) to 7x10(12) cm(-2) eV(-1)) is observed at approximately 0.25-0.33 eV above the valence band edge. The experimental results point to a common interface state defect present after processes involving rapid cooling (10(1)-10(2) degreesC/s) from a temperature of 800 degreesC or above, in a hydrogen free ambient. This work demonstrates that the interface states measured following RTP (T > 800 degreesC) are the net contribution of the P-b0/P-b1 silicon dangling bond defects for the oxidized Si(100) orientation. An important conclusion arising from this work is that the primary effect of an RTA in nitrogen (600-1050 degreesC) is to cause hydrogen desorption from pre-existing P-b0/P-b1 silicon dangling bond defects. The implications of this work to the study of the Si-SiO2 interface, and the technological implications for silicon based MOS processes, are briefly discussed. The significance of these new results to thin oxide growth and optimization by RTO are also considered.
- ItemStress modelling of multi level interconnect schemes for future deep submicron device generations(Springer, 2001-09) Gonzales Montes DeOca, Carlos; Foley, Sean; Mathewson, Alan; Rohan, James F.; Tsoukalas, D.; Tsamis, C.Copper and low dielctric constantant (k) materials are poised to become the dominant interconnect scheme for integrated circuits for the future because of the low resistance and capacitance that they offer which can improve circuit performance by more than 30% over conventional interconnect schemes. This paper addresses the thermomechanical stresses in the Cu/Low k interconnect scheme through numerical simulation and identifies the locations of maximum stress in the structure with view to providing information on the impact that different dielectric materials have on the stress distribution in the interfaces between metals and dielectric layers.
- ItemSelective electroless nickel deposition on copper as a final barrier/bonding layer material for microelectronics applications(Elsevier, 2001-12-11) Rohan, James F.; O'Riordan, Gerald; Boardman, Jane; Enterprise IrelandA low cost, selective electroless metallisation of integrated circuit (IC) copper bond pads with nickel and gold is demonstrated. This metallurgy can function as a barrier layer/bondable material when deposited as a thin layer or as the chip bump for flip chip applications when deposited to greater heights. Four alternative activation steps for selective electroless nickel deposition on bond pad copper are demonstrated. Selective low cost deposition has been achieved with a proprietary electroless plating bath developed at NMRC and three commercial baths on both sputtered copper substrates and electrolessly deposited copper on titanium nitride barrier layer material.
- ItemLasing from semiconductor microring on the end of an optical fiber(AIP Publishing, 2002) Corbett, Brian M.; Lambkin, Paul M.; Wu, Guanghong; Houlihan, John; Huyet, Guillaume; Enterprise IrelandIsolated InGaAsP microrings with an outer diameter of 5.8 mum, a width of 1 mum, and a thickness of 0.41 mum were fabricated by epitaxial separation. Individual devices were bonded to multimode optical fiber using Van der Waals forces and optically pumped through the fiber. Lasing around 1505 nm was measured under pulsed and cw pumping at room temperature. The threshold pump power for pulsed operation was estimated to be 38 and 80 muW for cw operation. Multiple radial and azimuthal modes were present due to strong, three-dimensional confinement. Under strong pulsed pumping thermal effects caused the emission wavelength to chirp. (C) 2002 American Institute of Physics. (DOI: 10.1063/1.1496496)
- ItemComparing leakage currents and dark count rates in Geiger-mode avalanche photodiodes(AIP Publishing, 2002) Jackson, Jennifer C.; Hurley, Paul K.; Lane, Bill; Mathewson, Alan; Morrison, Alan P.This letter presents an experimental study of dark count rates and leakage current in Geiger-mode avalanche photodiodes (GM APD). Experimental results from circular diodes over a range of areas (20-500 mum diam), exhibit leakage current levels orders of magnitude higher than anticipated from dark count rates. Measurements of the area and peripheral components of the leakage current indicate that the majority of the current in reverse bias does not enter the high-field region of the diode, and therefore, does not contribute to the dark count rate. Extraction of the area leakage current term from large-area devices (500 mum) corresponds well with the measured dark count rates on smaller devices (20 mum). Finally, the work indicates how dark count measurements represent 10(-18) A levels of leakage current detection in GM APDs. (C) 2002 American Institute of Physics. (DOI: 10.1063/1.1483119)
- ItemDefect study of GaInP/GaAs based heterojunction bipolar transistor emitter layer(AIP Publishing, 2002-01) Cherkaoui, Karim; Murtagh, M. E.; Kelly, P. V.; Crean, Gabriel M.; Cassette, S.; Delage, S. L.; Bland, S. W.; European CommissionDefects in the emitter region of Ga0.51In0.49P/GaAs heterojunction bipolar transistors (HBTs) were investigated by means of deep-level transient spectroscopy. Both annealed (635 degreesC, 5 min) and as grown metalorganic chemical vapor deposition epitaxial wafers were investigated in this study, with an electron trap observed in the HBT emitter space-charge region from both wafers. The deep-level activation energy was determined to be 0.87+/-0.05 eV below the conduction band, the capture cross section 3x10(-14) cm(2) and the defect density of the order of 10(14) cm(-3). This defect was also found to be localized at the emitter-base interface.
- ItemInterface controlled electrical and magnetic properties in Fe-Fe3O4-silica gel nanocomposites(American Institute of Physics, 2002-04) Roy, Saibal; Das, D.; Chen, J.; Chakravorty, D.; Department of Science and Technology, Ministry of Science and Technology; Ministry of Science and Technology of the People's Republic of ChinaIron nanoparticles with a shell of Fe3O4Fe3O4 phase with a total diameter of 5.3 nm have been grown within a silica gel matrix in the percolative configuration by suitable reduction followed by oxidation treatments. dc electrical resistivity measurements were carried out in the temperature range 80–300 K. The resistivity of the nanocomposites was found to be about 7 orders of magnitude lower than that of the reference gel. The electrical conduction has been explained on the basis of a small polaron hopping mechanism. The activation energy in the case of the composites was calculated from experimental data to be about one-fifth that for the reference sample. An interfacial amorphous phase is believed to cause such reduction in resistivity. The effective dielectric constant of this phase was estimated to be about four times that of the reference glass. Magnetization measurements on these specimens were carried out in the temperature range 5–300 K both in zero field cooled and field cooled states. A peak in the magnetization at ∼120 K was ascribed to an order–disorder (Verwey) transition. Another peak at ∼55 K was explained as arising due to a spin glass like disorder at the interface between the ferromagnetic iron ores and the ferrimagnetic Fe3O4Fe3O4 shell. A loop shift was observed as a result of the spin freezing below this temperature.
- ItemThin silicon in a novel 3-D format for implementation in distributed autonomous micro modules(Viktoria Institute, Göteborg, Sweden, 2002-09) Cussen, Teresa; Majeed, Bivragh; Delaney, Kieran; Ó Mathúna, S. CianSilicon thinning, interconnection and packaging are key innovative hardware technologies that can be used to realise distributed autonomous micro-modules (DAMM) for future ad-hoc networks in ambient systems and intelligent environments. These would interact, respond and learn from their surroundings making integration of engineering, computer science and human intelligence a reality. This paper investigates thinning silicon sensors and packaging these sensors in a tetrahedral format. This form was chosen because it is vastly expandable and when miniaturised can be used as a building block for DAMM’s, which can be designed to physically integrate into materials from which artefacts are fabricated.
- ItemMiniaturised modular wireless sensor networks(Viktoria Institute, Göteborg, Sweden, 2002-09) Barton, John; Delaney, Kieran; Ó Mathúna, S. Cian; Paradiso, Joseph A; Higher Education AuthorityThis paper focuses on the development of miniaturised modular wireless sensor networks that can be used to realise distributed autonomous sensors for future ad-hoc networks in the ambient systems and intelligent environments arena. Such modular, mobile networks are key enabling technologies in the field of ubiquitous computing. The modules are fabricated in a 3-D stackable form with a novel modular PCB design which can be mounted on artefacts or on parts of the body, can measure acceleration, rotation, shock, elevation etc. and have a low-power RF channel-shared link to a base station (for sports, exercise, entertainment, health). The modular nature of the design allows for extra panels to be developed and added easily.
- ItemCharacterisation of the electroless nickel deposit as a barrier layer/under bump metallurgy on IC metallisation(Elsevier, 2002-11-07) Rohan, James F.; O'Riordan, Gerard; Enterprise IrelandSelective electroless nickel–phosphorus deposits on integrated circuit (IC) metallisation such as copper and aluminium were characterised using differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) for elemental analysis. Annealing the Ni–P deposits in nitrogen atmospheres at temperatures compatible with organic dielectrics for IC components, such as polyimide, was performed to characterise the deposits. The crystallisation behaviour of the electroless nickel deposits with different concentrations of co-deposited phosphorus was examined.
- ItemIntrinsic limits on electron mobility in dilute nitride semiconductors(AIP Publishing, 2003) Fahy, Stephen B.; O'Reilly, Eoin P.; Science Foundation IrelandA fundamental connection is established between the composition-dependence of the conduction band edge energy and the n-type carrier scattering cross section in the ultradilute limit for semiconductor alloys, imposing general limits on the carrier mobility in such alloys. From the measured nitrogen composition dependence of the bandgap in GaAs1-xNx, the carrier scattering cross section of substitutional nitrogen defects in GaAs is estimated to be 0.3 nm(2). Within an independent scattering approximation, the carrier mobility is then estimated to be similar to1000 cm(2)/V s for a nitrogen atomic concentration of 1%, comparable to the highest measured mobility in high-quality GaInNAs samples at these N concentrations, but substantially higher than that found in many samples. This gives an intrinsic upper bound on the carrier mobility in these materials. (C) 2003 American Institute of Physics. (DOI: 10.1063/1.1622444)
- ItemSuperfluid toroidal currents in atomic condensates(American Physical Society, 2003) Nugent, Eileen; McPeake, Dermot; McCann, J. F.The dynamics of toroidal condensates in the presence of condensate flow and dipole perturbation have been investigated. The Bogoliubov spectrum of a condensate is calculated for an oblate torus using a discrete-variable representation and a spectral method to high accuracy. The transition from spheroidal to toroidal geometry of the trap displaces the energy levels into narrow bands. The lowest-order acoustic modes are quantized with the dispersion relation omegasimilar to\m\omega(s) with m=0,+/-1,+/-2,.... A condensate with toroidal current kappa splits the \m\ co-rotating and counter-rotating pairs by the amount DeltaEapproximate to2\m\h (h) over bar (2)kappa. Radial dipole excitations are the lowest-energy dissipation modes. For highly occupied condensates the nonlinearity creates an asymmetric mix of dipole circulation and nonlinear shifts in the spectrum of excitations so that the center of mass circulates around the axis of symmetry of the trap. We outline an experimental method to study these excitations.
- ItemWave mixing of hybrid Bogoliubov modes in a Bose-Einstein condensate(American Physical Society, 2003) McPeake, Dermot; McCann, J. F.Mode-mixing of coherent excitations of a trapped Bose-Einstein condensate is modeled using the Bogoliubov approximation. Calculations are presented for second-harmonic generation between the two lowest-lying even-parity m=0 modes in an oblate spheroidal trap. Hybridization of the modes of the breather (l=0) and surface (l=4) states leads to the formation of a Bogoliubov dark state near phase-matching resonance so that a single mode is coherently populated. Efficient harmonic generation requires a strong coupling rate, sharply-defined and well-separated frequency spectrum, and good phase matching. We find that in all three respects the quantal results are significantly different from hydrodynamic predictions. Typically the second-harmonic conversion rate is half that given by an equivalent hydrodynamic estimate.