bluuebunny/crossref_metadata_2025 huggingface dataset:
Grazing Incidence Small Angle X-Ray Scattering Study on Low Dielectric Thin Films
Authors: Hsu C.-H. , Lee Hsin-Yi , Liang K.S. , Jeng U-Ser , Windover D. , Lu T.-M. , Jin C.
Published: April 2011

Highly porous silica films with pore size in the nanometer scale are being extensively studied as potential candidates for interlevel dielectrics. Because these dielectric materials appear in the form of thin films with a thickness of only several thousand Angstroms, conventional techniques are difficult to be readily applied to study their structure and porosity. We employed small angle scattering in the grazing incidence geometry in this study. Using high resolution xray beamline with synchrotron radiation source, we demonstrate that the small angle x-ray scatteirng (SAXS) data of the porous films can be obtained. The structure of sol-gel derived silica - xerogel films on silicon substrate studied by specular reflectivity and grazing incidence small angle x-ray scattering (GISAXS) will be presented.
Processing, Characterization and Reliability of Silica Xerogel Films for Interlayer Dielectric Applications
Authors: Jain Anurag , Rogojevic Svetlana , Wang Feng , Gill William N. , Wayner Peter C. , Plawsky Joel L. , Haberl Arthur , Lanford William
Published: April 2011

Surface modified silica xerogel films of high porosity (25-90 %) and uniform thickness (0.4-2 µm) were fabricated at ambient pressure on silicon and other substrates. Mechanical reliability of the films was determined by measuring fracture toughness (adhesive) as a function of aging time and temperature using the modified edge-lift-off technique. There is an optimum aging time at 60 °C aging to obtain maximum fracture toughness for the procedure used here.

Cu/xerogel/Si and Ta/xerogel/Si structures were annealed at different temperatures and in different ambient environments were analyzed using RBS and optical microscopy to assess the extent of interaction with the xerogel film. When annealed in N2 with trace amounts of O2 (equivalent to 10-7-10-6 Torr vacuum), RBS analysis does not show diffusion of Cu or Ta through the xerogel up to 450 °C. At higher temperatures, or in the presence of larger concentrations of O2, Cu and Ta oxidize. Cu oxidation leads to significant diffusion through the xerogel. Ta oxidation also results in diffusion-like RBS spectra. Using the micron-size ion beam to probe the Ta surface, this was found to be solely due to buckling of Ta films on xerogel. A thin SiNx layer on top of Cu and Ta prevents metal oxidation up to 640 °C, Cu diffusion, and Ta buckling.
Strain Measurements from Single Grains in Passivated Aluminum Conductor Lines by X-Ray Microdiffraction During Electromigration
Authors: Hwang K. J. , Cargill G. S. , Marieb T.
Published: April 2011

We describe a method for determining the local strain state of passivated aluminum metal lines from single grains within 2.6 µm × 7.0 µm × 0.75 µm sized regions along the line. X-ray microbeam diffraction is used to obtain localized measurements of thermal and electromigration-induced strain during 37 hours of electromigration in a passivated 2.6 µm-wide, 300 µm-long pure Al conductor line at a current density of 4.2×105 A/cm2 and temperature of 270°C. Diffraction from single grains is used to measure both the in-plane and normal components of strain and their evolution during electromigration at several positions along the line.
Experimental Studies of the Reliability of Interconnect Trees
Authors: Hau-Riege S.P. , Thompson C.V.
Published: April 2011

The electromigration resistance of simple straight-line interconnects is usually used to estimate the reliability of complex integrated circuits. This is generally inaccurate, and overly conservative at best. The shapes and connectedness of interconnects is not accounted for in standard reliability assessments. We have identified the interconnect tree as the fundamental reliability unit. An interconnect tree consists of connected conducting line segments lying within a single layer of metallization, and terminating at two or more nodes at which there is a diffusion barrier such as a W-filled via. We performed electromigration experiments on the simplest tree structures, such as ‘L’- and ‘T’-shaped interconnects, as well as straight lines with an additional via in the middle of the line, passing currents of different magnitudes and directions through the limbs of the trees. We found that metal limbs ending in other limbs can act as reservoirs for electromigrating metal atoms. Passive reservoirs, which are limbs that do not carry electrical current, are generally beneficial for reliability, whereas limbs that do carry electrical current, called active reservoirs, can be beneficial or detrimental, depending on the direction and magnitude of the current in the reservoir. However, our experiments show that bends in interconnects do not affect their reliability significantly. We also found that the reliability of an interconnect tree can be conservatively estimated by considering void-growth and void-nucleation-limited failures at the most heavily stressed junction in the tree, which can be found by analyzing the geometry and current configuration. Our experimentally verified model for tree reliability can be used with layout tools for reliability-driven computer-aided design (RCAD), through ranking of the reliabilities of trees in order to identify areas at risk from electromigration damage.
The Influence of Stress-Induced Voiding on the Electromigration Behavior of AlCu Interconnects
Authors: Zitzelsberger A.E. , Fischer A.H.
Published: April 2011

The influence of stress-induced voiding on the electromigration (EM) behavior has been investigated on narrow via-line structures. For this purpose the EM performance of metal lines containing stress-induced voids already before the electrical operation has been compared with samples without stress-induced voids. We found, that pre-existing stress voids in the metal lines do not affect the activation energy Ea and lead only to a small decrease of the EM median time to failure, but cause a relevant reduction of the current density exponent down to n = 1. As a consequence, a tremendous decrease of the electromigration limited life time is obtained.
Evolution of Surface Morphology During Cu(TMVS)(hfac) Sourced Copper CVD
Authors: Yang Daewon , Hong Jongwon , Cale Timothy S.
Published: April 2011

In this paper, we describe an experimental study of the nucleation and growth stages during Cu(TMVS)(hfac) sourced Cu CVD on TaN substrates. In particular, we have investigated the effects of water vapor as a co-reactant on evolving surface morphology. The results of short (less than 10 s) depositions without/with water vapor indicate that water vapor helps to reduce the incubation time and to enhance the nuclei formation, uniformity, and adhesion (based on AFM analysis). Introducing water vapor during only the initial stage of deposition results in lower roughnesses, larger grain sizes, and lower short-range roughnesses as compared to the films deposited without water vapor. From this study, we conclude that water vapor enhances Cu nucleation and that a relatively small amount of water vapor before or during the initial stage of deposition improves surface morphology in terms of roughness and grain size.
Modeling of Ionized Magnetron Sputtering of Copper
Authors: Bloomfield M.O. , Cale T.S.
Published: April 2011

Computer simulations are used to study ionized physical vapor deposition, with ionized magnetron sputtering of copper as the primary system of interest. The effects of sputtering-ion energy and sputtering-ion angular flux distributions on the evolution of sub-micron scale features during IPVD are explored using the EVOLVE simulator. Our goal is to develop semi- quantitative engineering relationships that accurately predict the trends in experimental responses to changes in operating conditions. A sticking- factor model is used to describe deposition by neutrals. The sticking-factor of incident Cu atoms can depend on arrival angle and energy. Copper ions can also become part of the growing film. Energy and angular dependent sputter yields for both copper and argon ions are taken alternately from MD simulation results and a semi-empirical model. Sputtered material is ejected from the surface, tracked through the gas phase, and allowed to redeposit. Redeposition is also modeled via a sticking-factor based approach. The redistribution of film material results in non-intuitive profiles and complex relationships between final profiles and process parameters such as sample bias and neutral-to-ion flux ratios.
Computational Experiment on CVD of SiC: Growth Rate, C/Si Ratio, Parasitic Phase Formation
Authors: Vorob'ev Andrei N. , Komissarov Alexandre E. , Bogdanov Maxim V. , Karpov Sergey Yu. , Bord Olga V. , Zhmakin Alexandre I. , Lovtsus Andrei A. , Makarov Yuri N.
Published: April 2011

The heat and mass transport model extended to describe silicon cluster formation in the gas phase is employed for a numerical analysis of SiC CVD in a commercial vertical rotating disc reactor. The growth rate is studied as a function of precursor flow rates varied in a wide range of values. It is found that the growth rate is limited by the gas mixture depletion in silicon atoms due to homogeneous nucleation. The secondary phase formation on the growing surface is analyzed. The SiC growth window depending on the precursor flow rates is calculated, and a significant effect of the homogeneous nucleation on the window width is found. The model predicts that the Si/C ratio on the wafer can considerably differ from that at the reactor inlet.
Low Temperature Growth Of Barium Strontium Titanate Films By Ultraviolet-Assisted Pulsed Laser Deposition
Authors: Craciun V. , Howard J. M. , Lambers E. S. , Singh R. K.
Published: April 2011

Barium strontium titanate (BST) thin films were grown directly on Si substrates by an in situ ultraviolet (UV)-assisted pulsed laser deposition (UVPLD) technique. With respect to films grown by conventional (i.e. without UV illumination) pulsed laser deposition (PLD), the UVPLD grown films exhibited improved structural and electrical properties. The dielectric constant of a 40-nm thick film deposited at 650 °C was determined to be 281, the leakage current density was approximately 4×10−8 A/cm2at 100 kV/cm, and the density of interface states at the flat-band voltage was found to be approximately 5.6×1011 eV−1 cm−2 X-ray photoelectron spectroscopy investigations found that the surface of the grown films exhibited an additional Ba-containing phase, besides the usual BST perovskite phase, which was likely caused by stress and/or oxygen vacancies. The amount of this new phase was always smaller and very superficial for UVPLD grown films, which can explain their better overall properties.
Photoluminescence Charateristics of SrTiO3:Pr3+,Ga3+ Single Crystal
Authors: Byun Jaedong , Lee Yongjei , Jang Boyun , Yu Youngmoon , Ryou Sunyoun , Suh Kyungsoo
Published: April 2011

SrTiO3 crystals containing varying concentrations of Pr and Ga were grown by a standard floating zone method and their photoluminescence characteristics were investigated. It was found that only a small fraction of Pr ions added are incorporated in the lattice. When the Ga ions are co-doped, the solubility of Pr ions is increased considerably. Comparing the ionic sizes of the host and dopants, it seems that the increase of solubility of Pr by Ga co-doping is due to charge compensation achieved by the additional Ga3+. The addition of Ga in Pr - activated SrTiO3 resulted also in a considerable enhancement of Pr3+ emission band at 615 nm. This result is attributed to the hole trapping effect of Ga3+ ions.
Plastic Deformation of Thin Foil Substrates with Amorphous Silicon Islands into Spherical Shapes
Authors: Hsu Pai-hui I. , Huang Min , Wagner Sigurd , Suo Zhigang , Sturm J. C.
Published: April 2011

There is a growing interest in the application of large area electronics on curved surfaces. One approach towards realizing this goal is to fabricate circuits on planar substrates of thin plastic or metal foil, which are subsequently deformed into arbitrary shapes. The problem that we consider here is the deformation of substrates into a spherical shape, where the strain is determined by geometry and cannot be reduced by simply using a thinner substrate. The goal is to achieve permanent, plastic deformation in the substrates, without exceeding fracture or buckling limits in the device materials.

Our experiments consist of the planar fabrication of amorphous silicon device structures onto stainless steel or Kapton® polyimide substrates, followed by permanent deformation into a spherical shape. We will present empirical experiments showing the dependence of the results on the island/line size of the device materials and the deformation temperature. We have successfully deformed Kapton® polyimide substrates with 100 [.proportional]m wide amorphous silicon islands into a one steradian spherical cap, which subtends 66 degrees, without degradation of the silicon. This work demonstrates the feasibility of building semiconductor devices on plastically deformed substrates despite a 5% average biaxial strain in the substrate after deformation.
Sequential Lateral Solidification of Ultra-Thin a-Si Films
Authors: Cho Hans S. , Kim Dongbyum , Limanov Alexander B. , Crowder Mark A. , Im James S.
Published: April 2011

This paper demonstrates that Sequential Lateral Solidification (SLS) of Si can be carried out on films as thin as – and potentially much thinner than – 250 Å. When compared to thicker Si films, however, the SLS-processed ultra-thin films contain more twins, and successful processing requires irradiation within a narrower laser energy density range and a smaller per-pulse translation distance. The physical interpretation of these findings is formulated by analyzing the details of the microstructures observed in single-pulse-irradiation-induced Controlled Super-Lateral Growth (C-SLG) experiments. SEM and TEM analyses reveal complicated microstructural details that we interpret as originating from breakdown of epitaxial growth during lateral solidification, an effect that is detrimental to the SLS process. Based on considerations of far-from- equilibrium solidification behavior of Si, it is argued that undercooling of the solidification interface below a threshold value at which solidification no longer proceeds epitaxially – arising from reduction in interfacial recalescence during lateral solidification of ultra-thin Si films, relative to that of thicker films – is responsible for the breakdown. Based on this model, we discuss how external parameters may be adjusted so as to permit optimal crystallization of ultra-thin Si films using SLS.
Field Emission from Carbon Systems
Authors: Robertson John
Published: April 2011

Electron field emission from diamond, diamond-like carbon, carbon nanotubes and nanostructured carbon is compared. It is found that in all practical cases that emission occurs from regions of positive electron affinity with a barrier of ∼5 eV and with considerable field enhancement. The field enhancement in nanotubes arises from their geometry. In diamond, the field enhancement occurs by depletion of grain boundary states. In diamond-like carbon we propose that it occurs by the presence of sp2-rich channels formed by the soft conditioning process.
Mo and Co Silicide FEAs
Authors: Lee Jong Duk , Shim Byung Chang , Jin Sung Hun , Park Byung-Gook
Published: April 2011

For enhancement and stabilization of electron emission, Mo and Co silicides were formed from Mo mono-layer and Ti/Co bi-layers on single crystal silicon FEAs, respectively. Using the slope of Fowler-Nordheim curve and tip radius measured from SEM, the effective work function of Mo and Co silicide FEAs are calculated to be 3.13 eV and 2.56 eV, respectively. Compared with silicon field emitters, Mo and Co silicide exhibited 10 and 34 times higher maximum emission current, 10 V and 46 V higher failure voltage, and 6.1 and 4.8 times lower current fluctuation, respectively. Moreover, the emission currents of the silicide FEAs depending on vacuum level are almost same in the range of 10−9 ∼ 10−6 torr. This result shows that silicide is robust in terms of anode current degradation due to the absorption of air molecules.
Under-Gate Triode Type Field Emission Displays with Carbon Nanotube Emitters
Authors: Kang J. H. , Choi Y. S. , Choi W. B. , Lee N. S. , Park Y. J. , Choi J. H. , Kim H. Y. , Lee Y. J. , Chung D. S. , Jin Y. W. , You J. H. , Jo S. H. , Jung J. E. , Kim J. M.
Published: April 2011

A new structure of triode type field emission displays based on single-walled carbon nanotube emitters is demonstrated. In this structure, gate electrodes are situated under cathode electrodes with an in-between insulating layer, so called under-gate type triode. Electron emission from the carbon nanotube emitters is modulated by changing gate voltages. A threshold voltage is approximately 70 V at the anode bias of 275 V.
Breit-Wigner-Fano Lineshape Analysis of the angential G-band of Metallic Nanotubes
Authors: Brown Sandra D. , Jorio Ado , Corio Paola , Dresselhaus Mildred , Dresselhaus Gene , Kneipp Katrin
Published: April 2011

We present a resonant Raman study of the tangential G-band in metallic SWNTs. By measuring the Raman spectra for isolated SWNTs, we show that the two different lineshapes observed for semiconducting and metallic SWNTs in bundles also occur for isolated SWNTs. A lineshape analysis of the tangential G-band feature for metallic SWNT bundles is presented, showing that only two components are present, the higher frequency component having a Lorentzian lineshape, and the lower one having a Breit–Wigner–Fano (BWF) line-shape. Through comparisons of the Raman tangential G-band spectra from three different diameter distributions of carbon nanotubes, we find that both the frequency and linewidth of the BWF component are diameter dependent.
Structure and Application of Flourinated Carbon Nanofibers
Authors: Hayashi T. , Terrones M. , Kim Y.A. , Nakajima T. , Endo M.
Published: April 2011

High-resolution transmission electron microscopy (HRTEM) study of heavily fluorinated carbon nanofibers was performed under an ultra high voltage transmission electron microscope (UHVTEM). TEM observation showed rapid transformation of the disordered structure of fluorinated nanofiber to the cylindrical layer structure that is same as the non-fluorinated nanofiber. This observation suggests that electron beam irradiation helps fluorine atoms to dissociate from the carbon atoms. Li-ion secondary battery test cell using fluorinated nanofibers was prepared and the battery performance was tested as a function of cycling.
Single Walled Carbon Nanotubes (SWNTs) as a Gas Sensor
Authors: Pradhan Bhabendra K. , Sumanasekera Gamini U. , Adu Clement K. W. , Romero Hugo , Eklund Peter C.
Published: April 2011

A thermoelectric “nano-nose” has been built from tangled bundles of single- walled carbon nanotubes (SWNT). The detector's thermoelectric response [.Delta]S is sensitive to the logarithmic energy derivative of the additional bundle resistivity ([.rho]a) which is identified with the adsorbed molecules. The response is therefore specific to the details of the interaction of the adsorbed molecule with the nanotube wall; even gases such as He, N2 and H2 can be easily detected. Plots of [Δ]S vs. [ρ]a are sensitive to whether oxidation or reduction of the tube wall is taking place, and to whether the gas molecule is physisorbed or chemisorbed. The utility of the sensor stems from the amphoteric nature of the SWNT, the quasi-one-dimensional character of the charge conduction and the high specific surface area of SWNTs.
Electronic and Structural Properties of Carbon Nanotubes Molecular Junction
Authors: Machado M. , Piquini P. , Mota R. , Fazzio A.
Published: April 2011

The electronic and structural properties of finite junctions produced by connecting two C nanotubes, saturated with hydrogens in both edges, are investigated using first-principles calculations, through self-consistent field Hartree-Fock-Roothaan method. The main target of our study is a molecular junction, which connects (10,0) and (6,6) tubes by the introduction of pentagon-heptagon pair defects diametrically opposed in the structure. The charge distributions, the character of the highest occupied molecular orbitals and the densities of states are determined for the finite (10,0) and (6,6) nanotubes and for the formed junction. An energetic analysis is also performed using ab-initio approach and empirical Tersoff potential.
Imaging Oriented Aggregates of Lyotropic Chromonic Mesogenic Dyes by Atomic Force Microscopy
Authors: Schneider Tod , Smith Ashley , Lavrentovich Oleg D.
Published: April 2011

We use a layer-by layer electrostatic assembly technique to obtain aggregates of a chromonic mesogenic dye on mica substrates. The aqueous solutions of dye used for deposition are in the nematic phase. Atomic force microscopy of the dried films reveals that the chromonic molecules adsorb at the charged substrate in the form of ordered aggregates of elongated shape. These elongated aggregates form orientationally ordered arrays. The overall orientation of the array can be changed by shear.
Demonstration of Arrays of Sub-Micrometer Solid-State Fresnel Lenses for Electrons
Authors: Ito Y. , Bleloch A. L. , Brown L. M.
Published: April 2011

We demonstrate the focusing action of a compact solid state pixelated Fresnel phase (PFP) lens for electrons (700 nm in diameter), consisting of an array of holes (“pixelated”) directly drilled by a finely focused electron beam in a thin AlF3 thin film on carbon supporting film. The depths of holes, hence the phase of the exit electron wave is varied as a function of radius from the center of the pattern so that the wavelet from each hole can be in phase on axis at a designated focal point thus producing a lens. An array of two types of lenses, convergent and divergent, with nominal focal length of 1 mm for 200 keV electrons was produced. The estimated full-width-half-maximum of the focus is 8 nm. With improvement of the efficiency, these lenses may find applications in parallel electron-beam lithography, in X-ray optics and in light optics.
FTIR Spectrometry as a Quality Control Method for Surface Engineering of Nanomaterials
Authors: Baraton Marie-Isabelle
Published: April 2011

Controlled surface engineering of nanopowders cannot be achieved without a specific characterization of the surface chemical groups and the monitoring of the reactions or interactions responsible for the surface modifications. As an example of controlled engineering of nanopowder surfaces, the surface of tin oxide nanoparticles (n-SnO2) is modified by grafting hexamethyldisilazane (HMDS) while the process leading to chemical and electrical modifications of n-SnO2 is monitored in situ by Fourier transform infrared spectrometry. It appears that the effects of the grafting reaction on the surface chemical composition are complex and that a desorption at increasing temperatures leads to strong perturbations of the chemical groups originally grafted on n-SnO2. The present work clarifies the issues related to surface engineering of nanomaterials and elucidates the case of HMDS-grafted n-SnO2.
Chromium Oxide Sub-Micron Particles Fabricated by a Unique Technique: Laser-Induced Solution Deposition
Authors: Zhong Zhenchen
Published: April 2011

We have demonstrated that well-dispersed sub-micron chromium oxide particles can be fabricated by laser-induced solution deposition (LISD) from the solution of CrCl2 solute dissolved in organic solvents containing THF and cyclohexane. The particles are uniform in composition and contain little contamination. The distribution of the particles is narrow in three sizes: 230 nm, 350 nm and 400 nm. We have discussed the kinetics of forming these three kinds of sub-micron particles and the difficulty in obtaining single-phase chromium oxide. We have successfully shown that LISD is a unique technique for fabricating submicron chromium oxide particles with uniform composition and controllable size with narrow particle size distribution. The kinetics (nucleation and growth mechanism) of forming three sizes of particles is discussed.
Characterization of Microcrystalline Si Films by MeV Ion Scattering Techniques
Authors: Bohne W. , Röhrich J. , Selle B. , Birkholz M. , Fenske F. , Fuhs W. , Platen-Schwarzkopf J. , Reinig P.
Published: April 2011

Rutherford Backscattering Spectroscopy (RBS) and channeling studies with 1.4 MeV−4He ions as well as Heavy-Ion Elastic Recoil Detection Analysis (HI-ERDA) with 230 MeV 129Xe ions have been applied to characterize structural properties and the impurity content of thin Si films. The analytical potential of these ion-beam techniques is demonstrated for two types of samples: (1) μc-Si films prepared by dc magnetron sputtering in a pure Ar plasma and (2) homoepitaxial Si films deposited by low-temperature electron-cyclotron resonance PECVD at the transition from oriented to disordered growth. For μc-Si the atomic area density N.d obtained by RBS was compared with the optical thickness n.d (n=refractive index) derived from the interference structure of IR reflection spectra. It is shown that the ratio R=n.d/N.d of these quantities can serve as a figure of merit for the degree of crystalline order. An apparent similarity was found in the case of the epitaxially grown films between the Si disorder profiles evaluated from the RBS channeling spectra and the hydrogen and oxygen profiles determined by HI-ERDA. This suggests that hydrogen and oxygen are preferentially incorporated in the disordered parts of the films.
Impact of Erbium-Doped Silicon Nanocrystals on the Properties of Polyphenylene Vinylene Films
Authors: Coffer Jeffery L. , Ji Junmin
Published: April 2011

This work describes our initial studies of Er-doped Si nanocrystal/poly(phenylene vinylene) (PPV) composites. Preliminary results have focused on the characterization of these materials via atomic force microscopy (AFM), profilometry, absorption/fluorescence spectroscopies, and current-voltage measurements. A particular point of emphasis here concerns the influence of the Er-doped Si nanocrystals on PPV photoluminescence and carrier transport.
Pulsed Laser Induced Recrystallization of Amorphous Si
Authors: Tosto Sebastiano
Published: April 2011

The paper introduces a 3D model to simulate the recrystallization microstructure of amorphous Si induced by pulsed laser irradiation. The computer simulation shows how the microstructure is modified when introducing into the model the assumption of rapid solidification quenching.
Identification of tetrahedrally coordinated atoms in supercooled liquid silicon
Authors: Brambilla Luigi , Celino Massimo , Cleri Fabrizio , Colombo Luciano , Conversano Roberto , Rosati Mario , Rosato Vittorio
Published: April 2011

An atomic-scale model of liquid silicon has been cooled from high temperatures down in the temperature range between the amorphous and the crystalline melting temperatures by nanosecond scale molecular dynamics simulations with the Stillinger-Weber potential. Tetrahedrally coordinated sites have been identified, in the supercooled liquids, by using a few structural order parameters. The local structure and the stability of these crystalline-like regions (c-type sites and clusters) have been characterized. These have been regarded as candidates for crystalline embryos.
Three-dimensional silicon-based nanostructures in opal matrix: Preparation and properties
Authors: Pevtsov A.B. , Golubev V.G. , Kosobukin V.A. , Kurdyukov D.A. , Medvedev A.V.
Published: April 2011

Three-dimensional opal-silicon composites with both direct (a variable extent of filling of opal voids with silicon) and inverted structures have been synthesized. A structural analysis of these fabricated systems is performed. Reflectance spectra from the (111) surface of the composites are measured within the spectral range 400-900 nm. Observed spectral features are interpreted as a manifestation of the [111] direction photonic band gap that is tunable in position and width in the visible and near-infrared spectral ranges.
Electrical transport in porous silicon from improved complex impedance analysis
Authors: Remaki B. , Perichon S. , Lysenko V. , Barbier D.
Published: April 2011

An improved analysis of the electrical transport parameters in meso-porous silicon is presented. Our approach is based on a separate contribution of the crystallites and their interconnections to the total impedance of meso-porous silicon layers. Meso-porous silicon morphology exhibits a columnar structure without quantum confinement. The electrical conduction is thus, partially due to the bulk conductivity within continuous paths of crystallites. The samples were realized on 0.02ω-cm p-type Si substrates. Porous silicon layers of 100µm of thickness and 50% of porosity were inserted in Al/SiO2/porous-Si/Si structures. Their electronic transport parameters were determined using complex impedance measurements. A frequency range of 102 - 107 Hz was used allowing an accurate determination of the impedance components. Combined with thermal stimulation, theses measurements provide a powerful tool for the interpretation of basic properties such as the carriers density in the crystallites and the trapping mechanisms. Our results were interpreted in terms of free carriers conduction in partially compensated crystallites prevailing at low frequencies. At high frequencies (above 10 kHz), the electrical conductivity is mainly controlled by hopping transport on localized states in the chaotic porous structure. Finally, the free carriers mobility, evaluated from SCLC measurement is discussed.
Lateral Superlattices Fabricated with Interferometric Lithography for Nanoscale Device Applications
Authors: Striemer Christopher C. , Fauchet Philippe M. , Tsybeskov Leonid
Published: April 2011

Two-dimensional periodic arrays of inverted pyramid holes with nanometer scale have been patterned on the surface of a (100) silicon wafer and studied for possible application in nanoscale silicon based devices. The surface patterning employed a simple microelectronic processing scheme in which the standing wave intensity pattern from two interfering 458nm laser beams was used to expose holes in a photoresist layer. Subsequent dry etching through an underlying oxide mask layer, followed by a KOH etching step yielded a highly periodic, large area array of inverted pyramids. The pyramid geometry is formed during the anisotropic KOH etch, which stops at the (111) pyramid walls. Therefore, the tips of all inverted pyramids are formed by the intersection of (111) silicon crystal planes and have identical geometry. This study focuses on the use of these features as templates for the controlled crystallization of amorphous silicon layers and also as electric field concentrating “funnels” in MOS-type structures. We will discuss a proposed device in which silicon nanocrystals will be incorporated into the concentrated electric field region at the tip of each inverted pyramid. With this structure, the charging of identical addressable nanocrystals may be possible, leading to the development of practical nanoscale silicon devices.
The Novel Synthesis of Silicon and Germanium Nanocrystallites
Authors: Kauzlarich Susan M. , Liu Qi , Yin Shih-Chieh , Lee Howard W. H. , Taylor Boyd
Published: April 2011

Interest in the synthesis of semiconductor nanoparticles has been generated by their unusual optical and electronic properties arising from quantum confinement effects. We have synthesized silicon and germanium nanoclusters by reacting Zintl phase precursors with either silicon or germanium tetrachloride in various solvents. Strategies have been investigated to stabilize the surface, including reactions with RLi and MgBrR (R = alkyl). This synthetic method produces group IV nanocrystals with passivated surfaces. These nanoparticle emit over a very large range in the visible region. These particles have been characterized using HRTEM, FTIR, UV-Vis, solid state NMR, and fluorescence. The synthesis and characterization of these nanoclusters will be presented.
Optical Applications of Macroporous Silicon
Authors: Lehmann Volker
Published: April 2011

Two promising optical applications of macroporous silicon are presented. Due to the high contrast in dielectric constant between the air filled pores and the silicon walls the porous structure exhibits a photonic band gap for infrared radiation perpendicular to the pore axis. By photolithograpic patterning waveguides and optical cavities can be realized in this two-dimensional photonic crystal. Along the pore axis a short-pass filter characteristic is observed for ultraviolet and visible light. Such macropore filters are of high optical quality and may replace conventional filters in imaging systems.
A New Way to Form Three-Dimensional Microstructures by Electrochemical Etching of Silicon
Authors: Kleimann P. , Linnros J. , Juhasz R.
Published: April 2011

A new technique of bulk micromachining using anodic etching of (100)-oriented n-type silicon is presented. For particular conditions the transition regime between porous silicon formation and electropolishing enables the formation of high aspect ratio microtips which correspond to inverted macropore structures. This unusual property can be explained by the distortion of current lines near the basis of formed structures. The distortion, which prevents the tip dissolution, is due to the electrical field in the space charge region at the silicon-electrolyte interface. The same property can be used to form three-dimensional microstructures. The position and shape of the structures can be defined by forming steps of a few microns depth, prior tothe electrochemical etching. Then the etching parameters (HF concentration, light intensity, etching current density) are adjusted in order to electropolish the sample except where vertical walls are needed. This enables to form microstructures without a periodic pattern. The feasibility of this technique is demonstrated by forming 100μm wide pores, free-standing beams as well as high aspect ratio micro-needles and micro-tubes.
AlN Acoustic Wave Sensors Using Excimer Laser Micromachining Techniques
Authors: Zhong Feng , Huang Changhe , Auner Gregory W.
Published: April 2011

Aluminum Nitride (AlN) is a promising piezoelectric material for Acoustic Wave (AW) sensor application due to its high acoustic velocity, linear thermal coefficient and high electromechanical coupling coefficient. Epitaxial AlN thin films were successfully grown on the Sapphire C plane substrate by Plasma Source Molecular Beam Epitaxy (PSMBE). Standard two-port resonator structures were fabricated on the AlN/Sapphire by standard photolithography. Excimer laser micromachining techniques were utilized to fabricate microgroove gratings on the surface of thin film. Beside Surface Acoustic Wave (SAW), Surface Transverse Wave (STW) propagation was also found on the devices with those microgroove and classical metal strip gratings. Laser micromachinined microgroove gratings were found to enhance the propagation of STW. Further, sensors based on STW showed little attenuation in the liquid environment, while sensors based on SAW suffered excess loss when exposed to liquid.
Characterization of GaN MOS Structures Using Photoanodically Grown Oxides with Respect to FET Devices
Authors: Mistele D. , Rotter T. , Ferretti R. , Fedler F. , Klausing H. , Semchinova O.K. , Stemmer J. , Aderhold J. , Graul J.
Published: April 2011

Photoanodically grown Ga2O3 layers were characterized with respect to their suitability as gate dielectrics for GaN based MOSFET Device applications. The Ga2O3 layers were produced in a photoelectrochemical cell using aqueous solutions of KOH. IV characterization of MOS structures show insulating behavior of the oxide layers and CV measurements indicate a small density of states at the oxide/GaN interface. Integrating the wet chemical oxide growth in a MOSFET device fabricating process includes tungsten as gate metal together with H2O2 as etching solution for the gate metal. Source/drain areas were made free of oxide by the alkaline developer of a conventional lithographic step and metallization was done by using the liftoff technique. MOS structures show no inversion mode but strong depletion in reverse biasing mode.
Defect and Stress Control of Algan and Fabrication of High-Efficiency Uv-Led
Authors: Amano H. , Iwaya M. , Nitta S. , Terao S. , Nakamura R. , Ukai T. , Saitoh S. , Kamiyama S. , Wetzel C. , Akasaki I.
Published: April 2011

Defects and stress are the most serious issues for growth of AlGaN. Low-temperature deposited (LT -) AlN interlayer between AlGaN and GaN is found to reduce tensile stress during growth, and at the same time suppress the propagation of dislocations having screw components, by which UV-photodetector showing very-low-dark current has been successfully fabricated. However, additional pure-edge dislocations are generated at the LT -interlayer, which resulted in the poor emission property. In addition to the LT -interlayer, lateral growth at the trenched structure was used, thereby achieving crack-free AlGaN and reduction of the density of all types of dislocations in the AlGaN layer. UV light emitting diodes having AlGaN/GaN multi-quantum well active layer was fabricated on the low dislocation density AlGaN. The LED shows strong and sharp UV-emission from GaN-wells.
Fabrication of a High-Power Gan Metal Semiconductor Field-Effect Transistor
Authors: Yoshida Seikoh , Ishii Hirotatsu
Published: April 2011

A high-power metal semiconductor field-effect transistor (MESFET) for operating at a very large-current using GaN is reported for the first time. GaN was grown by metalorganic chemical vapor deposition (MOCVD). Sapphire substrates were used for GaN growth. A GaN MESFET with a large size was fabricated. Multi-finger gates were used for large-current operation. The total gate width was 8 cm and the gate length was 2 νm. The electrode materials of the source and the drain were Al/Ti/Au and the schottky electrodes were Pt/Au. The gate, source, and drain were isolated using SiO2. An FET structure was fabricated using a dry-etching technique. Multi-electrode structures were also fabricated using SiO2 for isolating the source, drain, and gate electrodes, respectively. The FET was operated at a current of over 5 A. The breakdown voltage was over 500 V. The transconductance (gm) was about 12 mS/mm. The pinch-off voltage was about -8 V. We confirmed that this GaN MESFET can also be operated at a current of 10 A.
Comprehensive study of anomalous conduction band structure of InxGa1−xAs1-−yNy
Authors: Skierbiszewski C. , Perlin P. , Wisniewski P. , Presz A. , Suski T. , Geisz J. , Jantsch W. , Mars D.
Published: April 2011

Using free standing layer of InGaAsN we have succeeded in measuring the optical absorption in very broad spectral range (0.8-2.5eV). This gave us an insight into the conduction band density of states for the energies higher than the energy gap of this compound. By combining Hall effect measurements with determination of plasma edge frequency in infrared reflectivity for differently doped samples we were able to deduce the density of states, conduction band electrons effective mass and dispersion relation. In particular it turned out that both i) experimentally measured dispersion relation of the conduction band shows extremely high degree of nonparabolicity and consistently ii) the effective mass of electrons is few times larger than that corresponding to GaAs of the same electron concentration. So far the obtained experimental results are in line with recently proposed band anticrossing model of the electronic structure of III-N-V alloys.
Surface Morphology and Composition Characterization at the Initial Stages of AlN Crystal Growth
Authors: Liu B. , Shi Y. , Liu L. , Edgar J.H. , Braski D.N.
Published: April 2011

The morphology and composition of AlN crystals on 6H-SiC (0001) at the initial stage of crystal growth by sublimation re-condensation technique were investigated by SEM and SAM. Discontinuous AlN coverage occurred after 15 minutes growth. The AlN nuclei size, and growth rate increased as temperature increased or pressure decreased. The SiC substrate decomposed leaving hexagonal hillocks; simultaneously, the AlN nucleated on these SiC hillocks apparently rotated by 15° to 30°. The chemical composition of the substrate and different AlN crystal facets were characterized by SAM. The bare substrate area was stoichiometric SiC with insignificant conversion to silicon nitride, while Si and C preferentially incorporated in the AlN at the initial stages of growth on specific crystal planes.
Heteroepitaxy and Characterization of Low-Dislocation-Density GaN on Periodically Grooved Substrates
Authors: Detchprohm T. , Yano M. , Nakamura R. , Sano S. , Mochiduki S. , Nakamura T. , Amano H. , Akasaki I.
Published: April 2011

We have developed a new method to prepare low-dislocation-density GaN by using periodically grooved substrates in a conventional MOVPE growth technique. This new approach was demonstrated for GaN grown on periodically grooved α-Al2O3(0001), 6H-SiC(0001)Si and Si(111) substrates. Dislocation densities were 2×107 cm−2 in low-dislocation-density area.
Optical Properties of MBE Grown Cubic AlGaN Epilayers and AlGaN/GaN Quantum Well Structures
Authors: As D.J. , Frey T. , Bartels M. , Khartchenko A. , Schikora D. , Lischka K. , Goldhahn R. , Shokhovets S.
Published: April 2011

Cubic AlyGa1−yN/GaN heterostructures on GaAs(001) substrates were grown by radio-frequency plasma-assisted molecular beam epitaxy. High resolution X-ray diffraction, spectroscopic ellipsometry and cathodoluminescence were used to characterize the structural and optical properties of the alloy epilayers. X-ray diffraction reciprocal space maps demonstrate the good crystal quality of the cubic AlyGa1−yN films. Both SE as well as room temperature CL of the AlyGa1−yN epilayer show a linear increase of the band gap with increasing Al-content. A pseudomorphically strained cubic 10 × (2.4 nm GaN/ 4.8 nm Al0.12Ga0.88N) multi-quantum well (MQW) structure has been realized. Cathodoluminescence clearly demonstrates strong radiative recombination due to quantized states in the GaN well layer at a photon energy of 3.323 eV.
Comparative study of HVPE- and MOCVD-grown nitride structures for UV lasing application
Authors: Lam J. B. , Gainer G. H. , Bidnyk S. , Elgawadi Amal , Park G. H. , Krasinski J. , Song J. J. , Tsvetkov D. V. , Dmitriev V. A.
Published: April 2011

We studied and compared the emission properties of optically excited (Al)GaN structures grown by two different techniques: hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD). We successfully achieved stimulated emission (SE) in an HVPE-grown GaN epilayer and a GaN/AlGaN double heterostructure at 10 K and room temperature. We found that the SE threshold and photoluminescence efficiency of the HVPE-grown samples are similar to those of high-quality MOCVD-grown structures. Photoluminescence measurements from 10 to 300 K show that the HVPE GaN has a high density of non-radiative recombination channels, especially those activated below 100 K. This study represents the first demonstration of SE in HVPE-grown (Al)GaN heterostructures.
Luminescence properties of amorphous AlN thin film phosphors incorporated with mixtures of Tb, Cu or Cu, Cr
Authors: Martin Andrea L. , Caldwell Meghan L. , Kordesch Martin E. , Spalding Chance M. , Patten Paul G. Van , Richardson Hugh H.
Published: April 2011

Amorphous thin films of AlN doped with Cu (blue luminescence), Tb (green luminescence) and Cr (red luminescence) were deposited on p-doped Si (111) substrates using RF magnetron sputtering in a nitrogen atmosphere and made luminescent active by heat treatment at 1000°C. Single layered amorphous AlN films deposited with Tb and Cu showed cathodoluminescence from only the Tb3+ ions. Presumably, energy is transferred from Cu luminescence centers to Tb centers in close proximity. In contrast to this, double layered amorphous AlN films doped with Cu (∼200 nm) on top of Tb (∼50 nm) emits from both ions. This behavior is observed in a double layered amorphous AlN film doped with Cu (∼200 nm) on top of a Cr (∼200 nm) doped film. Secondary-ion mass spectrometry depth profiling revealed that the incorporated metals moved from one layer into the other during the heat treatment step necessary for luminescence activation.
Planar Force-constant Method for Lattice Dynamics of Cubic InN
Authors: Alves H. W. Leite , Alves J. L. A. , Scolfaro L. M. R. , Leite J. R.
Published: April 2011

Using the density-functional theory within the Full Potential Linear Augmented Plane-Wave (FP-LAPW) method, we have calculated, in this work, the equation of state, the effective charges and the phonon dispersion along [100] and [111] directions for the cubic Indium Nitride(c-InN), including hydrostatic strains dependence. A good agreement with the micro-Raman scattering experiment is obtained for the phonon modes at ã. Our results show that the apparent divergence between the known experimental results is a consequence of hydrostatic effects on the sample due to differences of the used Raman methods.
Role of Localized Quantum Well Excitons in InGaN Quantum Well Structure Correlated with Microstructural Analysis
Authors: Chichibu S. F. , Sota T. , Nakamura S.
Published: April 2011

InxGa1−xN multiple-quantum-well laser diode structure, which lased at 405 nm, was shown to have atomically-flat interfaces between each layer. Nanometer-probe compositional analysis showed that InN mole fraction, x, in the wells and barriers are approximately 6 % and 2 %, respectively, which agreed with the result obtained from high-resolution x-ray diffraction measurement. The Stokes-like shift (SS) at 300 K was 49 meV, being approximately 65 % of the luminescence linewidth. The localization depth, E0, of qunatum-well (QW) excitons was estimated to be 35 meV at 300 K though the compositional fluctuation in the well was as small as 1 % or less (detection limit) within adjacent 20-30 nm lateral length scale. Since the well thickness fluctuation is insufficient to reproduce SS or E0, effective bandgap inhomogeneity is attributed to be due to large bandgap bowing in InGaN. The spontaneous emission was thus assigned as being due to the recombination of QW excitons weakly localized in exponential tail-type potential minima in the QW. The size of localization is smaller than the quantum-disk [M. Sugawara, Phys. Rev. B 51, 10743 (1995)]-size. Such small bandgap inhomogeneity can be leveled by injecting high density carriers under lasing conditions, which can explain the general experimental finding that the quantum efficiency decreases with increasing carrier density in InGaN QW devices due to free carrier trapping into threading dislocations.
Pattern Formation via a Two-Step Faceting Transition on Vicinal Si(111) Surfaces
Authors: Men F.K. , Liu Feng , Wang P.J. , Chen C.H. , Cheng D.L. , Lin J.L. , Himpsel F.J.
Published: April 2011

We demonstrate a self-organized pattern formation on vicinal Si(111) surfaces that are miscut toward the [211] direction. All the patterns, consisting of a periodic array of alternating (7×7) reconstructed terraces and step-bunched facets, have the same periodicity and facet structure, independent of the miscut angle; while the width of the facets increases linearly with miscut angle. We attribute such unique pattern formation to a surface faceting transition that involves two transition steps: the first step forms a stress-domain structure defining the universal periodicity; the second step forms the low-energy facets controlling the facet width.