bluuebunny/crossref_metadata_2025 huggingface dataset:
Thin Film Diamond Field Effect Transistors For High Power Applications
Authors: Looi Hui Jin , Pang Lisa Ys , Jackman Richard B.
Published: April 2011

Early predictions that diamond would be a suitable material for high performance, high power devices were not supported by the characteristics of diodes and field effect transistors (FETs) fabricated on boron doped (p-type) thin film material. In this paper commercially accessible polycrystalline thin film diamond has been turned p-type by the incorporation of near surface hydrogen. Schottky diodes and metal-semiconductor FETs (MESFETs) have been fabricated using this approach which display unprecedented performance levels; diodes with a rectification ratio > 106, leakage currents < l nA, no indication of reverse bias breakdown at 100V and an ideality factor of 1.1 have been made. Simple MESFET structures that are capable of withstanding VDS values of 100V with low leakage and current saturation (pinch-off) characteristics have also been fabricated. Predictions based upon experiments performed on these devices suggest that optimised device structures will be capable of operation at power levels up to 20 W/mm, implying that thin film diamond may after all be an interesting material for power applications.
Junction Field Effect Transistors For High-Temperature Or High-Power Electronics
Authors: Zolperw J. C.
Published: April 2011

Junction field effect transistors (JFETs) are attractive for high-temperature or highpower operation since they rely on a buried semiconductor junction, and not a metal semiconductor interface as in a metal semiconductor (MESFET) or heterojunction field effect transistor (HFET), for modulating the transistor channel. This is important since a metal/semiconductor interface often degrades at elevated temperatures, either due to the ambient temperature or to Joule heating at high current levels, while a buried semiconductor junction can withstand higher temperatures. In fact, for proper design, the JFET becomes limited by thermal carrier generation in the semiconductor and not metallurgical degradation of the gate electrode.

In this talk an overview is given of JFET technology based on GaAs, SiC, and GaN. While impressive room temperature, high-frequency, results have been reported for GaAs JFET's with unit current gain cut-off frequencies up to 50 GHz, more work is needed to limit substrate conduction for optimum operation at 300 °C and above. For SiC JFETs, well behaved transistor operation has been maintained up to 600 °C, however, increased frequency performance is needed. More recently, a GaN JFET has also been demonstrated that is promising for similarly high temperature operation but is presently limited by buffer conduction. Future directions for each of these technologies, and potential extension to high power switching devices such as thyristors, will be presented at the conference.
A Thin-Film Silicon Solar Cell: Design and Processing Approach
Authors: Sopori Bhushan L. , Chen Wei , Madjdpour Jamal , Symko Marta
Published: April 2011

We present a new device structure for a high efficiency, thin-film, silicon solar cell. A preliminary design and an approach for fabrication of such a cell are discussed. The cell structure uses interface texturing and a back surface reflector for effective light trapping. A theoretical analysis is applied to determine the major parameters of the cell. These analyses indicate that a cell efficiency of about 18% is attainable with a Si film thickness of 10–15 μm, and grain size of about 50 μm. A method for making a large-grain thin cell is proposed.
In-Situ Surface Composition Measurements of CuGaSe2 Thin Films
Authors: Fons P. , Yamada A. , Niki S. , Oyanagi H.
Published: April 2011

Two CuGaSe2 films were grown by molecular beam epitaxy onto GaAs (001) substrates with varying Cu/Ga flux ratios under Se overpressure conditions. Growth was interrupted at predetermined times and the surface composition was measured using Auger electron spectroscopy after which growth was continued. After growth, the film composition was analyzed using voltage dependent electron microprobe spectroscopy. Film structure and morphology were also analyzed using x-ray diffraction and atomic force microscopy. The film with a Cu/Ga ratio larger than unity showed evidence of surface segregation of a second Cu-rich phase with a Cu/Se composition ratio slightly greater than unity. A second CuGaSe2 film with a Cu/Ga ratio of less than unity showed no change in surface composition with time and was also consistent with bulk composition measurements. Diffraction measurements indicated a high concentration of twins as well as the presence of domains with mixed c and a axes in the Ga-rich film. The Cu-rich films by contrast were single domain and had a narrower mosaics. High sensitivity scans along the [001] reciprocal axis did not exhibit any new peaks not attributable to either the substrate or the CuGaSe2 thin film.
Piezoelectric Photoacoustic Spectra In CuGaSe2 Thin Films Grown by Molecular Beam Epitaxy
Authors: Yoshino Kenji , Maruoka Daisuke , Kawahara Masakazu , Fukuyama Atsuhiko , Maeda Kouji , Fons Paul J. , Niki Shigeru , Yamada Akimasa , Ikari Tetsuo
Published: April 2011

The piezoelectric photoacoustic (PPA) spectra for Cu-rich CuGaSe2 (CGS)/GaAs epitaxial layers were successfully observed between liquid nitrogen and room temperatures for the first time. Bandgap energy of CGS (A band) is estimated to be 1.72 eV at liquid nitrogen temperature. The activation energies of three possible intrinsic defect levels are estimated to be about 80, 130 and 190 meV.
Sulfur Diffusion In Polycrystalline Thin-Film CdTe Solar Cells
Authors: Aslan M. H. , Song W. , Tang J. , Mao D. , Collins R. T. , Levi D. H. , Ahrenkiel R. K. , Lindstrom S. C. , Johnson M. B.
Published: April 2011

X-ray diffraction and photoluminescence measurements have been used to characterize the diffusion of S into CdTe during post growth annealing of CdTe solar cells. For anneals at 410°C in the presence of CdCl2, evidence that both a CdTe1 xSx phase and nearly-pure CdTe are present near the back contact is observed. The ternary phase becomes more prominent and the S concentration increases with depth reaching roughly 4–5% near the CdS interface. Much less diffusion is observed at 350°C while for a 460°C anneal, CdTe1-xSx with a S concentration near 5% is found throughout the layer. The presence of CdC12 during the anneal enhances the interdiffusion.
Interface Reactions in CdTe Solar Cell Processing
Authors: Albin D. , Dhere R. , Swartzlander-Guest A. , Rose D. , Li X. , Levi D. , Niles D. , Moutinho H. , Matson R. , Sheldon P.
Published: April 2011

Currently, the best performing CdS/CdTe solar cells use a superstrate structure in which CdTe is deposited on a heated CdS/SnO2/Glass substrate. In the close-spaced-sublimation (CSS) process, substrate temperatures in the range 550°C to 620°C are common. Understanding how these high processing temperatures impact reactions at the CdS/CdTe interface in addition to reactions between previously deposited layers is critical. At the SnO2/CdS interface we have determined that SnO2 can be susceptible to reduction, particularly in H2 ambients. Room-temperature sputtered SnO2 shows the most susceptibility. In contrast, higher growth temperature chemical vapor deposited (CVD) SnO2 appears to be much more stable. Elimination of unstable SnO2 layers, and the substitution of thermal treatments for H2 anneals has produced total-area solar conversion efficiencies of 13.6% using non-optimized SnO2 substrates and chemical-bath deposited (CBD) CdS. Alloying and interdiffusion at the CdS/CdTe interface was studied using a new lift-off approach which allows enhanced compositional and structural analysis at the interface. Small-grained CdS, grown by a low-temperature CBD process, results in more CdTe1-x.Sx alloying (x=12–13%) relative to larger-grained CdS grown by high-temperature CSS (x7sim;2–3%). Interdiffusion of S and Te at the interface, measured with lift-off samples, appears to be inversely proportional to the amount of oxygen used during the CSS CdTe deposition. Our highest efficiency to date using CSS-grown CdS is 10.7% and was accomplished by eliminating oxygen during the CdTe deposition.
Multijunction InGaAs Thermophotovoltaic Devices
Authors: Fatemi Navid S. , Wilt David M. , Jenkins Phillip P. , Weizer Victor G. , Murray Christopher S.
Published: April 2011

A monolithic interconnected module (MIM) structure has been developed for thermophotovoltaic (TPV) applications. The MIM consists of many individual InGaAs cells series-connected on a single semi-insulating (S.I.) InP substrate. An infrared (IR) back surface reflector (BSR), placed on the rear surface of the substrate, returns the unused portion of the TPV radiator output spectrum back to the radiator for recuperation, thereby providing for high system efficiencies. MIMs were fabricated with an active area of 0.9×l cm, and with 15 cells monolithically connected in series. Both lattice-matched and lattice-mismatched InGaAs/InP devices were fabricated, with bandgaps of 0.74 and 0.55 eV, respectively. The 0.74 eV MIMs demonstrated an open-circuit voltage (Voc) of 6.16 V and a fill factor of 74.2% at a short-circuit current (Jsc) of 0.84 A/cm2, under flashlamp testing. The 0.55 eV MIMs demonstrated a Voc of 4.85 V and a fill factor of 57.87percnt; at a Jsc of 3.87 A/cm2 Electrical performance results for these MIMs are presented.
The Effect of The Zn Interstitial Defect on the Performance of p/n InP Solar Cells
Authors: Hoffman R. W. , Fatemi N. S. , Weizer V. G. , Jenkins P. , Stan M. A. , Ringel S. A. , Sieg R. M. , Scheiman D. A. , Wilt D. M. , Brinker D. J.
Published: April 2011

The bandgap of InP is near ideal for the high theoretical conversion efficiency of the air mass zero (AM0) light spectrum experienced in space. This combined with the well known radiation resistance makes InP an ideal candidate for space solar cell use. The p/n configuration has several advantages over the n/p configuration, however, the conversion efficiency of p/n InP cells has been low compared to the n/p configuration, mostly do to poor performance of the Zn doped emitter. We have recently achieved a 17.6% AMO conversion efficiency for a p/n InP cell on an InP substrate and have achieved this record efficiency through understanding of the role of Zn interstitial defects and the hydrogen interactions with these defects. The effect of the Zn defect on device performance will be described.
Spectral Utilization in Thermophotovoltaic Devices
Authors: Clevenger Marvin B. , Murray Christopher S. , Riley David R.
Published: April 2011

Multilayer assemblies of epitaxially-grown, Ill-V semiconductor materials are being investigated for use in thermophotovoltaic (TPV) energy conversion applications. It has been observed that thick, highly-doped semiconductor layers within cell architectures dominate the parasitic free-carrier absorption (FCA) of devices at wavelengths above the bandgap of the semiconductor material. In this work, the wavelength-dependent, free-carrier absorption of p- and n-type InGaAs layers grown epitaxially onto semi-insulating InP substrates has been measured and related to the total absorption of long-wavelength photons in thermophotovoltaic devices. The optical responses of the TPV cells are then used in the calculation of spectral utilization factors and device efficiencies.
SiGe Thin-Film Structures for Solar Cells
Authors: Bremond G. , Daami A. , Laugier A. , Seifert W. , Kittler M. , Poortmans J. , Caymax M. , Said K. , Konuma M. , Gutjahr A. , Silier I.
Published: April 2011

In order to study their applicability as the active base material in Si thin crystalline film solar cell technology, SiGe relaxed layers grown by Liquid Phase Epitaxy (LPE) and Chemical Vapor Deposition (CVD) on Si substrates are investigated by optical and electrical measurements (TEM, EXD, PL, EBIC). The main results of this work is to point out the improvement of the SiGe active base layer by using smooth Ge graded SiGe buffer layer and remote plasma hydrogenation. TEM, EXD, PL experiments show the effect of the Ge graded buffer layer grown using LPE, by confining the threading dislocations in the SiGe buffer layer close to the Si/SiGe interface. EBIC measurements reveal low recombination activity of dislocations at 300 K providing the diffusion length exceeds the 15 μm layer thickness The enhanced luminescence of SiGe near bandgap indicates that remote plasma hydrogenation induces a decrease of the non-radiative recombination pathways due to dislocations on CVD layers where defect recombinations dominate as indicated by EBIC measurements.

This study points out the importance of controlling relaxed SiGe layers with good minority carrier recombination quality as a key issue for the optimization of new SiGe/Si based solar cells.
Amorphous Silicon Based Waveguides And Light Modulators For Silicon Low-Cost Photonic Integrated Circuits
Authors: Cocorullo G. , Corte F. G. Della , Rosa R. De , Rendina I. , Rubino A. , Terzini E.
Published: April 2011

This paper reports about the fabrication and experimental test of an interferometric light intensity modulator integrated in a low loss (0.7 dB/cm), amorphous silicon based waveguide. It measures approximately 1 mm in length, while its cross section is 30-μm-wide and 3-μm-high. The device, which exploits the strong thermo-optic effect in thin film a-Si for its operation, is designed for application at the infrared wavelengths of 1.3 and 1.55 μm. The measured maximum operating on-off switching frequency of the device is 600 kHz. The very simple fabrication technology involves maximum process temperatures of 230 °C, and is therefore compatible with the standard microelectronic technology. This offers a new opportunity for the integration of optical and electronic functions on the same substrate.
Er-Doped Porous Silicon Led For Integrated Optoelectronics
Authors: Tsybeskov L. , Grom G. F. , Hirschman K. D. , Lopez H. A. , Chan S. , Fauchet P. M. , Bondarenko V. P.
Published: April 2011

Porous silicon (PSi) was doped by Er using electromigration from a solution and converted to Er-doped silicon-rich silicon oxide (SRSO:Er) by partial thermal oxidation at 600–950°C following densification at 1100°C in an inert atmosphere. Room-temperature photoluminescence (PL) at ∼1.5 μm is intense and decreases by less than 20% from 12 K to 300 K. The PL spectrum of SRSO:Er reveals no luminescence bands related to Si-bandedgerecombination, point defects or dislocations, and shows that the Er3+ centers are the most efficient radiative recombination centers. A light-emitting diode (LED) with an active layer made of SRSO:Er was manufactured using a pre-oxidation cleaning step to increase the quality of the interface between SRSO:Er and the top electrode. Room temperature electroluminescence at ∼1.5 μm was demonstrated.
Photo- and Electro-Luminescence from Deuterium Terminated Porous Silicon
Authors: Matsumoto Takahiro , Masumoto Yasuaki , Koshida Nobuyoshi
Published: April 2011

We have studied the optical and the electrical properties of deuterium-terminated porous silicon. The photoluminescence spectrum was different from that of usual hydrogen-terminated porous Si despite both porous Si showing the same structure and the same absorption spectrum. In addition to these basic results, it is found that the replacement of hydrogen with deuterium reduces the degradation not only of photoluminescence but also of electroluminescence. The luminescence degradation mechanism will be discussed from the point of a photoinduced oxidation model.
Temperature-Dependent Photoluminescence of Silicon Nanocrystallites Prepared by Inert-Gas-Ambient Pulsed Laser Ablation
Authors: Umezu I. , Yamaguchi S. , Shibata K. , Sugimura A. , Yamada Y. , Yoshida T.
Published: April 2011

The inert-gas-ambient pulsed laser ablation technique is a promising method for preparing Si nanocrystallites. We measured the temperature dependence of photoluminescence (PL) spectra to investigate radiative and nonradiative recombination processes in the nanocrystallites prepared using this method. The Si nanocrystallites showed visible PL bands in the red (1.6 eV) and green (2.1 eV) spectral regions. The intensities of the red and green PL increased with decreasing temperature and then saturated below 80 K. This temperature dependence was compared with that of other photoluminescent Si materials. It was shown that the PL quantum efficiency of the Si nanocrystallites was larger than that of a-Si:H at high temperatures. One of the reasons for the difference in the temperature dependence between the Si nanocrystallite and a-Si:H is the change in the role of defects in the nonradiative recombination process.
Formation of Silicon Nanocrystallites by Electron Cyclotron Resonance Chemical Vapor Deposition and Ion Beam Assisted Electron Beam Deposition
Authors: Kim Eun Kyu , Choi Won Chel , Min Suk-Ki , Park Chong-Yun
Published: April 2011

Nano-crystalline silicon (nc-Si) thin films were directly deposited by electron cyclotron resonance chemical vapor deposition (ECR-CVD) and ion beam assisted electron beam deposition (IBAED) method. In the sample deposited by ECR-CVD, the room temperature photoluminescence originated from the nc-Si and the silicon-hydrogen bond were appeared. It was confirmed that the size of the nc-Si could be controlled up to about 3 nm with the low substrate temperature during the deposition process and then the hydrogen atoms play a very important role in the formation of the nc-Si. The IBAED method was also found to an useful technique for nc-Si formation by the control of ion beam power.
Photoexcitation of Si-Si Radiative Surface States in Nanocrystallites
Authors: Nayfeh M. H. , Rigakis N. , Yamani Z.
Published: April 2011

Recently, intrinsic localized excitonic radiative surface states belonging to interacting dangling bonds of expanded, unstrained Si-Si dimers on the surface of nanocrystallites have been predicted. Those are connected, via a potential barrier, to the nonradiative delocalized excitonic states on the unexpanded, strained dimer. We present a theoretical analysis of the photoexcitation pathways involved in populating the radiative states. We include both direct above barrier, and indirect excitation from the photoexcited delocalized excitonic states via quantum tunneling and thermal activation. Our calculation gives an enhancement in the efficiency for sizes below a critical size of 1.4 nm, the size for which the outer trapped states become stable against tunneling and thermal activation. It is as if the material were changed from an indirect gap to a somewhat direct gap material. The results show that the photoluminescence exhibits a large Stokes shift resulting from the expansion of the radiative dimers. Moreover, the emission bandwidth is found to be quite wide especially for the ultra small crystallites (0.8 nm) where it encompasses nearly all of the visible spectrum, and the near infrared. Spectra simulated over size distributions are presented.
Characterization of Erbium Doped SiO2 Layers Formed On Silicon By Spark Processing
Authors: John John V. St. , Coffer Jeffery L. , Rho Young Gyu , Diehl Patrick , Pinizzotto Russell F. , Culp Thomas D. , Bray Kevin L.
Published: April 2011

Deposition of a rare earth salt layer on a silicon substrate with subsequent spark processing yields a porous Si layer and SiO 2 cap doped with the rare earth ion. We have characterized luminescent Er-doped porous SiO2 on Si by scanning electron microscopy, energy dispersive Xray spectroscopy, as well as visible and near IR photoluminescence (PL) spectroscopies. Energydispersive x-ray maps indicate that the erbium concentration in the porous layer can be controlled by varying the molarity of the erbium solution deposited on the substrate prior to spark processing. Visible PL measurements reveal that the concentration of Er3+ is proportional to the resultant intensity of the visible fluorescence transitions; however, for the near IR fluorescence peak at 1.54 gim, self-quenching due to erbium clustering occurs at higher concentrations. Erbium-doped porous silicon layers can also be obtained by diffusion of an erbium salt into porous silicon formed by anodic etching of Si in hydrofluoric acid. Densification of the porous Si layers through high temperature oxidation after erbium diffusion forms erbium-doped SiO2 layers.
Time Decay Characteristics of the Yb3+- Related 0.98 μm Emissions in Porous Silicon
Authors: Kimura Tadamasa , Nishida Yasuhiro , Dejima Tohru , Saito Riichiro , Isshiki Hideo
Published: April 2011

Enhanced emissions at 0.98 μm of Yb3+ ions (2F5/2 → 2F7/2) incorporated in porous silicon are obtained when the host porous silicon is pre-annealed in an O2 containing atmosphere. Time decay characteristics are measured for the Yb 3+-related 0.98 μm emissions as well as for the emissions of these host porous silicon between 20 K and 300 K. The decay of the Yb3+ peak at 20 K is characterized with a fast (˜ 30 μs) and a slow decay time (≤ ˜ 400 μs), whereas it is fitted with one slow decay time (˜ 390 μs) at 300 K. The results are explained in terms of two optical centers: one has a fast fluorescent lifetime with a large temperature quenching, and the other has a slow fluorescent lifetime which is nearly constant between 20 K and 300 K.
The Origin Of Light Emission From Porous Silicon
Authors: Ben-Chorin M. , Heckler H. , Kovalev D. , Averboukh B. , Polisski G. , Schwartzkopff M. , Koch F.
Published: April 2011

We report on luminescence hole burning experiments, which prove that radiative recombination between quantum confined states is the only viable model for the mechanism of the light emission from porous silicon. We find that more than 90% of the luminescence originates from quantum confined states inside the Si nanocrystals.
Dispersive Fourier Transform Spectroscopy Of Free-Standing Porous Silicon Films
Authors: Salonen J. , Saarinen K. , Peura J. , Vilnikanoja J. , Salomaa I. , Laine E. , Kauppinen J
Published: April 2011

We have investigated optical constants of free-standing porous silicon films by dispersive Fourier transform spectroscopy (DFTS) in the NIR-VIS range. This allows the spectral variation of both the absorption coefficient and the refractive index of a material to be determined from the measurements of the attenuation and phase shift imposed on an electromagnetic wave by its interaction with a specimen. Using these optical constants, we have studied the complex dielectric function and the complex conductivity. To avoid the additive error in the absorption spectra arising from the pseudocoherence, we measured the transmission spectra by conventional Fourier transform spectroscopy (FTS). Using the refraction spectrum derived from the DFTS measurements, we have corrected for reflection losses in calculation of the absorption spectrum from the FTS transmission spectrum. The changes in the absorption coefficient and the refractive index due to oxidation, which is the most common aging phenomenon in porous silicon, have been studied using samples with different types of oxidization.
Guided Wave Fluorescence In Thin Film Er- Doped Barium Titanate
Authors: Gill D. M. , Ford G. M. , Block B. A. , Wessels B. W. , Ho S. T.
Published: April 2011

Guided-wave absorption and fluorescence in epitaxial Er:BaTiO3 thin film channel waveguides on MgO are reported. Guided- wave absorption is strongly dependent on post- growth annealing. Stimulated emission over a 40 nm bandwidth (λcente - 1.54 μm) has been achieved in oxygen-annealed waveguides. At 1.54 μm the absorption is reduced from ∼ -2 dB/cm to -∼-1.3 dB/cm in a 2.2 mm long 5 μm wide channel waveguide due the presence of the pump light.
Photoluminescence Stability Of Silicon Suboxide Thin Films
Authors: Wang F. , Hinds B. J. , Wolfe D. W. , Lucovsky G.
Published: April 2011

Photoluminescence (PL) stability of silicon suboxide (SiOx, 0<x<l.5) under light soaking and thermal processing has been investigated. As the oxygen content increases, the PL peak position shifts to the visible, with improved stability against measurement temperature and light soaking. With increasing annealing temperature, the PL spectra shifted to infrared, characteristic of a- Si:H. With annealing above 550 °C, visible PL is lost while infrared PL can be recovered by rehydrogenation of Si-dangling bond defects. Infrared absorption, transmission electron microscopy, visible absorption, and photocurrent indicate a disproportionation reaction resulting in a-Si phase segregation.
Materials For Monolithic Silicon Microphotonics
Authors: Giovane L. M. , Lim D. R. , Ahn S. H. , Chen T. D. , Foresi J. S. , Liao L. , Oulette E. J. , Agarwal A. M. , Duan X. , Michel J. , Thilderkvist A. , Kimerling L. C.
Published: April 2011

The path for silicon materials development has been charted. By the year 2010 we will have fabricated integrated circuit chips containing 109 transistors with 40 Å thick gate oxides and 1000 Å minimum feature sizes running at 4 GHz clock speeds. It is conceivable that incremental advances on the current chip architecture will satisfy the required materials and process improvements. The interconnection problem is the only challenge without a proposed solution. The signal propagation delay between devices is now longer than the individual device gate delay. The resistance and capacitance associated with fine line Al interconnects limit speed and increase power consumption and crosstalk. High power line drivers are limited by the reliability constraint of electromigration. There is no current paradigm for 4 GHz, electronic clock distribution. Optical interconnection can remove the electronic transmission bandwidth limit. The main challenge is development of a silicon-compatible, microphotonic technology.

Rare earth doping has provided a means of sharp-line electroluminescence from silicon at λ = 1.54 μm. Silicon's high index of refraction and low absorption in the near infrared yield an ideal optical waveguide. As with microelectronics, the silicon / silicon-dioxide materials system allows high levels of integration and functionality. The applications of silicon materials to light emission (Si:Er), optical waveguides (Si/SiO2), photonic switching (Si/SiO2) and photon detection (SiGe) are reviewed. These developments are discussed in the context of systems applications to communications and computation.
Visible Photonic Band Gap Waveguide Devices
Authors: Charlton M. D. B. , Parker G. J.
Published: April 2011

Photonic band gap (PBG) structures are multidimensional lattices, formed by a strong modulation of dielectric constant which prevent the propagation of photons over a forbidden energy range1. Since the first demonstration of a PBG at microwave frequencies2-7, it has proved technologically difficult to create a PBG in the optical region of the spectrum8-11 since the band gap centre wavelength is of the order of twice the lattice pitch. Recently, we have succeeded in fabricating novel waveguide designs which incorporate a TE polarised visible photonic band gap12,13 By reducing the band gap to visible wavelengths, we greatly simplify the experimental investigation of the behaviour of these complicated structures.

In this paper, we present the fabrication and initial optical testing of devices with band gaps in the visible wavelength range 545nm-632.8nm. These devices demonstrate unusual optical effects dependent upon the number of lattice periods such as: broadband wavelength filtering, wavelength dependent beam splitting, and broadband polarisation selective routing. In contrast to the well known active applications of PBGs, we suggest a host of new passive applications for PBGs in optical computing and wavelength division multiplexed (WDM) communications.
Mineralization of Multilayer Hydrogels as a Model for Mineralization of Bone
Authors: Calvert Paul , Frechette Joelle , Souvignier Chad
Published: April 2011

Extrusion freeform fabrication is a 3-D layerwise writing technique for forming objects directly under the control of a CAD program. This method is one of a family of rapid prototyping methods which include stereolithography, selective laser sintering and fused deposition modeling. This system can be used to build shapes, layer by layer, from hydrogels of agarose, polyacrylamide or other cross-linked water-soluble acrylic polymers.

Mineralization can be induced in these gels by building a part with alternating layers of gels containing calcium and carbonate or phosphate that can be formed into stacks which then mineralize by cross-diffusion. The write head can be conceived of as a cell which delivers the appropriate minerals to a site within a swollen gel matrix. This diffusive mineralization process can thus be compared with bone mineralization. The gel structure controls the morphology of the mineral. The site of mineralization is controlled by osmotic forces which localize most of the mineral in whichever zone has the higher ionic strength. The mineral content, expressed as a fraction of the polymer content, is similar to that of bone but the water content is much higher than in bone. This raises the question of what process drives the water exclusion during bone mineralization.
Microscale Radiative Effects in Complex Microstructures of Iridescent Butterfly Wing Scales
Authors: Tada Haruna , Mann Seth E. , Miaoulis Ioannis N. , Wong Peter Y.
Published: April 2011

The cellular microstructure of insect scales can be detailed intricately with threedimensional structures and multiple thin-film layers. In butterflies, iridescent scales can reflect bright colors through thin-film interference and other optical phenomena; the balance of radiation is absorbed for thermoregulatory purposes. Results of numerical and experimental investigations into the function, properties, and structure of these scales are presented. Of particular interest are the numerical modeling of the microscale radiative effects in the scales, determining the optical properties of the biological material, and the cellular development of thin-film structures.
Sequence Specific Force Curves Measured by Mechanically Opening the DNA Double Helix
Authors: Bockelmann U. , Essevaz-Roulet B. , Heslot F.
Published: April 2011

Using techniques of molecular biology, we have designed a molecular construction which allows to attach the two complementary strands of one end of a single molecule of bacteriophage λ DNA separately to a glass microscope slide and a microscopic bead. A soft microneedle acting as a force sensor is chemically attached to the bead and its deflection is measured by an optical microscope. Keeping the base of the force lever fixed, the glass slide is displaced slowly, leading to a progressive opening of the double helix. The force measured during the opening process shows a characteristic variation which is related to the sequence of the bases along the DNA molecule. We present a brief summary of the present state of our work.
First Principles Simulations of Glucose in Aqueous Solution
Authors: Molteni C. , Parrinello M.
Published: April 2011

We present an application of first principles molecular dynamics simulations, based on the Car-Parrinello method, to the study of the properties of glucose in aqueous solution. By treating both solute and solvent quantum-mechanically, we provide a reliable description of the hydrogen bonds, which determine the water structure around the glucose. These results show the power of our technique in describing biologically relevant problems, where an accurate microscopic description is essential.
Microscopic Elasticity of DNA from Torsionally-Constrained Stretching
Authors: Moroz J. D. , Nelson P.
Published: April 2011

We investigate the statistical mechanics of a torsionally constrained polymer. The polymer is modelled as an inextensible chain with bend rigidity A, twist rigidity C, and twist-stretch coupling D. In such a model, thermal bend fluctuations couple geometrically to an applied torque through the relation Lk = Tw + Wr. We explore this coupling and find excellent agreement between the predictions of our model and the single λ-DNA molecule stretching experiments of Strick et al. [Science 271 (1996) 1835]. This analysis affords an experimental determination of the microscopic twist rigidity C. Quantitative agreement between theory and experiment is obtained using C = 120 nm and D = 50 nm. The theory further predicts a thermal reduction of the effective twist rigidity induced by bend fluctuations.
Charge Transfer Modeling for Charge-Coupled Devices
Authors: Lavine James P. , Stevens Eric G. , Banghart Edmund K. , Trabka Eugene A. , Burkey Bruce C. , Schneider David J.
Published: April 2011

The three-dimensional Poisson's equation is solved by iterative methods and the resulting electric field is used in Newton's equation to simulate electron transfer in a charge-coupled device (CCD). The time dependence of charge transfer is studied through a random walk simulation of Newton's equation. Potential obstacles of the order of 0.03 V are seen to slow charge transfer. Electron motion is also followed in two spatial dimensions through Newton's equation in order to probe a more varied set of potential obstacles.
Light-Controlled Switching Transients in Mis Silicon Structures with Multichannel Insulator: Physical Processes and New Device Modelling
Authors: Malik A. , Martins R.
Published: April 2011

We present the modelling of a new two-terminal and low-voltage operating optoelectronic device based on MIS silicon structure with multichannel insulator and having as gate a transparent metallic tin-doped indium oxide (ITO) layer deposited by spray pyrolysis technique over the insulator layer. ITO layer has a multiple non-rectifier electrical contact with silicon substrate, in the SiO2 channel's region. Construction details of the process, together with its operating characteristics are given. The devices developed do not require external active electronic components (transistors, microschemes) to execute their functions and to transform analogue input optical signals to digital output form, highly important for a wide range of optoelectronic applications.
The Application of GaAs TCAD in Industry
Authors: Blakey P. A. , Deepak  , Johnson K. , Recker C. , Varadarajan S.
Published: April 2011

The implementation and application of GaAs technology CAD for industrial research and development are described. Existing silicon-oriented software was modified extensively to make it applicable to the simulation of GaAs processes and devices. RF-oriented post-processing capabilities were implemented, and effective applications methodologies were developed. Examples of the practical application of GaAs technology CAD are presented.
Arsenic Deactivation in Silicon
Authors: Berding M. A. , Sher A.
Published: April 2011

In this paper we examine the properties of arsenic in silicon, using ab initio calculations and a statistical theory. Good agreement is found between theory and experiment for the electronic concentration as a function of temperature and total arsenic concentration. We show that for low arsenic concentrations, full activation is the equilibrium condition. In equilibrium, the neutral complex composed of a lattice vacancy surrounded by four arsenic (VAs4) is the dominant means by which high concentrations of arsenic are rendered inactive. Under constrained equilibrium conditions in which VAs4 cluster formation is prohibited, we show that VAs3Si1 cluster populations increase dramatically and can account for nearly the same degree of compensation as the VAs4 clusters. Even VAs2 clusters alone can account for substantial deactivation in the absence of VAs3 and VAs4 clusters. These smaller complexes are essential not only to the establishment of equilibrium, since SiAs4 clusters are extremely rare, but can also explain some degree of deactivation, even if the formation of VAs4 clusters are kinetically inhibited.
Ab Initio Calculation of Tight-Binding Parameters
Authors: Mcmahan A. K. , Klepeis J. E.
Published: April 2011

We calculate ab initio values of tight-binding parameters for the /-electron metal Ce and various phases of Si, from local-density functional one-electron Hamiltonian and overlap matrix elements. Our approach allows us to unambiguously test the validity of the common minimal basis and two-center approximations as well as to determine the degree of transferability of both nonorthogonal and orthogonal hopping parameters in the cases considered.
Efficient Electronic Energy Functionals for Tight-Binding
Authors: Haydock Roger
Published: April 2011

Generalized functionals are constructed from the exchange-correlation energy by a Legendre transformation which makes the new functionals stationary at the electronic charge density, potential, and wave functions for the ground-state. Using generalized functionals, the density, potential, and wave functions can be independently parameterized and varied to determine the ground-state energy-surface for a system of atoms. This eliminates the computationally awkward steps of constructing densities from wave functions or potentials from densities, and is particularly well suited to parameterizations using tight-binding orbitale together with atomic-like densities and potentials. For each choice of parameters, the only quantities which must be computed are the electron-electron energy for the density, the integral of the potential over the density, and the band structure energy for the wave functions. To second order in the density, potential, and wave functions, the energy for a configuration of atoms is given by the generalized functional evaluated at a superposition of atomic densities, a potential made by stitching together the atomic potentials where they are equal, and atomic wave functions. For more accurate stationary energies the densities, potentials, and wave functions can be improved by one or more conjugate gradient steps.
Electronic Structure, Pressure Dependence and Optical Properties of FeS2
Authors: Nguyen-Manh D. , Pettifor D. G. , Sithole H. M. , Ngoepe P. E. , Arcangeli C. , Tank R. , Jepsen O.
Published: April 2011

A revisited electronic structure study of iron pyrite, FeS2, has been performed using a new Tight-Binding Linear Muffin-Tin Orbital (TB-LMTO) technique in which the radii of overlapping MT spheres are determined from a full potential construction. The interstitial spheres were chosen to provide an efficient packing of space while ensuring that the overlap between the spheres remain small. We have found that this treatment of interstitial spheres results in a dramatic improvement in the description of the electronic structure and the binding energy curves for FeS2 in comparison with a previous LMTO calculation. In particular, the energy band gap, the equilibrium lattice constant and the bulk modulus are all in much better agreement with experimental observations. Moreover, the calculated equation of state is in excellent accord with recent measured P- V data up to pressures of 15GPa with overall deviations of less than 10%. The predicted reflectivity spectrum of FeS2 as a function of pressure gives the observed behaviour of the optical edge. The bonding behaviour the orthorhombic marcasite phase of FeS2 is also discussed within this new TB-LMTO formalism.
Large Scale Quantum Simulations Using Tight-Binding Hamiltonians and Linear Scaling Methods
Authors: Galli Giulia , Kim Jeongnim , Canning Andrew , Raerle Rainer
Published: April 2011

We describe linear scaling methods for electronic structure calculations and quantum molecular dynamics simulations, which are based on an orbital formulation of the electronic problem. In particular, we discuss some open problems which need to be addressed to improve the performances of these methods, and briefly review some applications to carbon and silicon systems, within a Tight-Binding framework.
Tight Binding Simulations of Disordered Systems
Authors: Rosato V. , Celino M.
Published: April 2011

Carbon allotropy allows the formation of a large variety of disordered structures whose properties depend on the density and on the preparation technique. Computer simulations, based on quantum mechanics, can reproduce the essential features of these systems. We first assess the reliability of the amorphous carbon structures generated by molecular dynamics simulations based on a semi-empirical Tight Binding hamiltonian. Then we attempt to understand of the path followed by diamond to reach the amorphous phase via a direct crystalline-to-amorphous transformation. Results concern the large variety of structures which can be obtained and a general adequacy of the semi-empirical Tight Binding hamiltonian to reproduce the essential features of amorphous carbon structure. Moreover, it is shown that the process of direct transformation from the crystalline into the amorphous phase occurs continuously. The formation of three-fold coordinated sites is not followed by an immediate site re-hybridization. When this process takes place, a large strain sites associated with the mis-coordinated sites is released.
A LCAO-OO Approach to the Calculation of Electronic Properties of Materials
Authors: Pou P. , Perez R. , Ortega J. , Flores F.
Published: April 2011

We present a selfconsistent LCAO approach for describing the electronic properties of materials. This method introduces many-body effects by means of a new approach, whereby a local orbital potential is defined by calculating the exchange-correlation energy as a function of the different orbital occupancies. A LCAO-pseudopotential is also introduced, keeping all the calculations within the context of the local orbital basis. We have applied the method to the calculation of simple molecules and crystals, in both cases we find results that confirm the validity of our approach.
Covalent Liquids: Tight Binding Simulation Versus Experimental Results
Authors: Gaspard J.-P. , Bichara C. , Raty J. Y.
Published: April 2011

We show that a simple tight binding model with a repulsive potential describes the Peierls distortions in covalent systems and the well-known octet rule. The existence and the intensity of the Peierls distortion is mainly related to the hardness of the repulsive potential as demonstrated both by theoretical calculations and by the experimental systematic analysis of liquid structures. In particular, As is threefold coordinated and Sb is sixfold coordinated in the liquid, the qualitative difference is explained by the ratio of the distortion energy ΔE to the thermal energy kBT. The Asx Sb1-x alloys show continuously varying average coordination numbers showing: the semiconductor-metal transition is continuous with concentration.

In addition, we illustrate in the case of liquid Se that, tight binding Monte Carlo simulations are able to describe quantitatively the structure of liquid elements provided the Van der Waals potential is added.
Structural and Electronic Properties of a-Gaas: A Tight-Binding–Molecular-Dynamics–Art Simulation
Authors: Lewis Laurent J. , Mousseau Normand
Published: April 2011

By combining tight-binding (TB) molecular dynamics (MD) with the recently-proposed activation-relaxation technique (ART), we have constructed structural models of a-GaAs and a-Si of an unprecedented level of quality: the models are almost perfectly four-fold coordinated and, in the case of a-GaAs, exhibit a remarkably low density of homopolar bonds. In particular, the models are superior to structures obtained using melt-and-quench TB-MD or quantum MD. We find that a-Si is best described by a Polk-type model, while a-GaAs resembles closely the mechanical model proposed by Connell and Temkin, which is free of wrong bonds. In this paper, the structural, electronic, and dynamical properties of a-GaAs based on this approach will be reviewed, and compared to experiment and other structural models. Our study provides much-needed information on the intermediate-range topology of amorphous tetrahedral semiconductors; in particular, we will see that the differences between the Polk and Connell-Temkin models, while real, are difficult to extract from experiment, thus emphasising the need for realistic computer models.
Carbon Schwarzites: Properties and Growth Simulation from Fullerene Fragments
Authors: Benedek G. , Colombo L. , Spadoni S. , Gaito S. , Milani P.
Published: April 2011

We investigated by tight-binding molecular dynamics the structure, the bulk modulus and the electronic properties of the smallest four-branched carbon schwarzites fcc-(C28)2, fcc-(C36)2 and fcc-(C40)2 having the shape of a D minimal periodic surface. They are found to have a stability comparable to that of fullerene C60 and to exhibit alternative metallic and insulating characters, with an apparent relationship to their local geometry. We also studied the coalescence of fullerenic fragments and carbon clusters by following the evolution of the topological connectivity. Though different temperature variation protocols lead to irregular structures similar to random schwarzites, their connectivity is found to stabilize at values corresponding either to tubulene or three-branched schwarzites, indicating that long time evolution at constant connectivity is potentially able to yield regular shapes. Experiments on laser-induced transformations of fullerite occasionally yield branched tubular structures with a schwarzite shape.
An ab initio two-center tight-binding approach to simulations of complex materials properties
Authors: Frauenheim Th. , Porezag D. , Elstner M. , Jungnickel G. , Elsner J. , Haugk M. , Sieck A. , Seifert G.
Published: April 2011

We describe theab initioconstruction of two-center tight-binding (TB) hamiltonians, which at a properly chosen input density upon non-selfconsistent solution of the related Kohn-Sham equations transform the energy within density-functional theory (DFT) into a tight-bindinglike expression. In cases, where the electron density of the interacting many-atom structure in good approximation may be represented as a sum of atomic-like densities, the method has been shown to operate highly transferable, being particularly successful in determining the properties of low-energy silicon clusters, in predicting the structure and vibrational signatures of fullerene oligomers, amorphous carbons and carbon nitrides and in simulating elementary growth reactions on diamond surfaces. The uncertainties within the standard non-SCF DF-TB-variant, however, increase if the chemical bonding is controlled by a delicate charge balance between different atomic constituents, as e.g. in organic molecules and in polar semiconductors. Therefore, we extend the standard TB-approach to the operation in a selfconsistent-charge mode(SCC-DFTB)in order to improve total energies, forces, and transferability in the presence of considerable long-range Coulomb interactions. By using a variational technique, we derive a transparent and readily calculable expression for the iterative modification of Hamiltonian matrix elements and show, that the final energy is a second order approximation to the total energy in density-functional theory,see M. Elstner et al., this Symposium.First successful applications to surface studies of GaAs and dislocation modeling in GaN will be presented.
Structure, Energy, and Electronic Properties of the Σ = 13 {510} Tilt Grain Boundary Structure In Si
Authors: Morris J. R. , Lu Z.-Y. , Ring D. M. , Xiang J.-B. , Ho K.-M. , Wang C. Z. , Fu C.-L.
Published: April 2011

We have examined a variety of structures for the {510} symmetric tilt boundary in Si, using first-principles calculations. These calculations show that the observed structure in Si is the lowest energy structure. This structure is more complicated than what is necessary to preserve four-fold coordination. We compare the results to classical and tight-binding models, in order to test these empirical approaches.