bluuebunny/crossref_metadata_2025 huggingface dataset:
Excimer Laser Crystallisation of Poly-Si TFTs for AMLCDs
Authors: Brotherton S D , McCulloch D J , Gowers J P , Ayres J R , Fisher C A , Rohlfing F W
Published: April 2011

There is interest in reducing the shot number in the poly-Si laser crystallisation process in order to improve its throughput. Two distinct shot number dependent effects have been identified, which are both laser intensity dependent. The critical laser energy density is that which causes full film melt-through, and the major issue occurs at energies greater than this, where there is a considerable degradation in device uniformity with reducing shot number. The cause of this is non-uniform recovery of the full-melt-through fine grain poly-Si, and it is demonstrated that by extending the trailing edge of the beam, the material uniformity at reduced shot number can be improved. For energies less than this, the issue is not so much uniformity, as a general degradation in overall device properties with reducing shot number, which has been correlated with reducing grain size.

In more demanding, future applications (such as system-on-panel), it will be necessary to improve circuit performance and approach that of current MOSFET devices. This will require short channel, self-aligned (SA) TFTs, and some of the issues with this architecture, particularly lateral ion implantation damage beneath the gate edge and drain field relief are discussed.
A Novel Approach for True Work Function Determination of Electron-Emissive Materials by Combined Kelvin Probe and Photoelectric Effect Measurements
Authors: Lägel Bert , Baikie Iain D. , Dirscherl Konrad , Petermann Uwe
Published: April 2011

For the development of new electron-emissive materials knowledge of the work function Φ and changes in Φ is of particular interest. Among the various methods, the ultra-high vacuum (UHV) compatible scanning Kelvin Probe has been proven to be a superior technique to measure work function changes due to e.g. UHV cleaning processes, chemical contamination, thermal processing etc. with high accuracy (<1meV).

The Kelvin Probe measures local work function differences between a conducting sample and a reference tip in a non-contact, truly non-invasive way over a wide temperature range. However, it is an inherently relative technique and does not provide an absolute work function if the work function of the tip (Φtip) is unknown.

Here, we present a novel approach to measure Φtip with the Kelvin Probe via the photoelectric effect, where a Gd foil is used as the photoelectron source. This method thus provides the true work function of the sample surface with an accuracy of approx. 50meV. We demonstrate the application of the technique by in situ work function measurements on evaporated layers of the low work function material LaB6 on a Re substrate and follow the changes in Φ of LaB6 due to the surface adsorption of residual gas molecules. Thus, the extended Kelvin Probe method provides an excellent tool to characterise and monitor the stability of low work function surfaces.
Study on Depletion Mode Operation of Protruding-Tip Field Emitter Triodes
Authors: Luo Y-C. , Shibata M. , Okada H. , Onnagawa H.
Published: April 2011

Electrical characteristics of Spindt-type Molybdenum (Mo) field emitter triode devices with varied emitter tip-height have been studied based on device modeling and experiment. Potential and electric field distributions with varied the emitter tip-height has been simulated. It is observed that the electric field of the top of the higher emitter tip was strongly affected with the anode-gate distance and the anode voltage compared to conventional field emitter triode device. Experimental results with varied different tip-height were in good agreement with that of calculated results. We present the possibility of “depletion mode” field emitter triode device.
Capacitance and Transient Photocapacitance Studies of Tetrahedral Amorphous Carbon
Authors: Palinginis Kimon C. , Ilie A. , Milne W.I. , Cohen J. David
Published: April 2011

We have applied junction capacitance and transient photocapacitance measurements to undoped tetrahedral amorphous carbon (ta-C)/silicon carbide (SiC) heterostructures to deduce defect densities and defect distributions in ta-C. The junction capacitance measurements show two thermally activated processes. One can be related to the activation of carriers out of defects at the ta-C/SiC interface while the other one with an activation energy of 0.36eV is an intrinsic property of the ta-C. The defect density at the ta-C/SiC interface is estimated to be roughly 9 ± 2 × 109 cm−2. The transient photocapacitance measurements have allowed us to observe the broader band tail of ta-C, giving a value (Urbach energy) of 230meV.
Formation of Carbon Nanotube by using RF plasma CVD equipment from Acetylene and Hydrogen gases
Authors: Show Y. , Matsukawa T. , Iwase M. , Izumi T.
Published: April 2011

The carbon nanotubes were grown on the round Ni particles at the growth temperature below 60 °C by the radio frequency plasma CVD equipment from the acetylene and the hydrogen gases. For the low growth of the carbon nanotubes, the acetylene concentration in the hydrogen gas was essential parameter. In the case of the high acetylene concentration above 30 %, the cauliflower-like carbon film was grown. On the other hand, the low acetylene concentration of 10 % realized the low temperature growth of the carbon nanotubes.
Fabrication of Freestanding Carbon Nanotube Arrays in Large Scale
Authors: Huang Z.P. , Moser J. , Sennett M. , Gibson H. , Naughton M.J. , Wen J.G. , Ren Z.F.
Published: April 2011

We have successfully fabricated many freestanding carbon nanotube arrays on silicon substrates. Two sizes of nickel dot arrays have been made by E-beam lithography. It has been found that the size of the carbon nanotubes is closely related to the size of the dot. Compared with our previous report on diameters of about 300 –400 nm, much thinner carbon nanotubes of 100 –150 nm have been made. With even smaller dots, it is expected that even thinner nanotubes of a few tens of nanometers could be made. The nanotube height is controlled by the growth time and nanotube uniformity has been greatly improved by introduction of a two-phase process of nucleation and growth.
Reinforcement Mechanisms in Polymer Nanotube Composites: Simulated Non-Bonded and Cross-Linked Systems
Authors: Frankland S. J. V. , Caglar A. , Brenner D. W. , Griebel M.
Published: April 2011

A summary of three simulations related to the issue of load transfer in carbon nanotube/polymer composites is given. Each simulation considers a (10,10) nanotube in a polyethylene matrix modeled with either a united-atom or fully-atomic polyethylene representation. The first simulation uses molecular dynamics to model a 100 nm nanotube in polyethylene in which the nanotube and matrix interact through weak non-bonded interactions. No permanent stress transfer is observed for this system. The second simulation estimates the shear yield strength for the nanotube in each of the model polyethylene matrices. The third simulation addresses a chemically functionalized nanotube in which there are cross-links between the nanotube and matrix. The results indicate that the cross-linked system has an enhanced shear yield strength of one to two orders of magnitude larger than the non-bonded composites.
The Crystallography of Metal Halides formed within Single Walled Carbon Nanotubes
Authors: Sloan J. , Brown G. , Bailey S.R. , Coleman K.S. , Flahaut E. , Friedrichs S. , Xu C. , Green M.L.H. , Dunin-Borkowski R.E. , Hutchison J.L. , Kirkland A.I. , Meyer R.R.
Published: April 2011

The crystal growth behaviour and crystallography of a variety of metal halides incorporated within single walled carbon nanotubes (SWNTs) as determined by high resolution electron microscopy (HRTEM) is described. Simple packed structures, such as the alkali halides, form related structures within SWNTs that are found to be integral atomic layers in terms of their thickness as a function of the encapsulating SWNT diameter. An enhanced HRTEM image restoration technique reveals precise data concerning lattice distortions present in these crystals. More complex structures, such as those derived from 3D complex, layered and chain halides form related crystal structures within SWNTs. In narrow SWNTs (i.e. with diameters less than ca. 1.6 nm), structures consisting of individual 1D polyhedral chains (1D-PHCs) were obtained that were derived from the corresponding bulk halides structures. In the case of infinite 3D network and layered halides, the 1D polyhedral chains form with lower co-ordinations than in the bulk. Molecular halides also intercalate into SWNTs but these do not readily form organised structures within SWNTs.
Low-Voltage Electron Transport in Self-Assembled Nanocrystal Arrays
Authors: Black C. T. , Murray C. B. , Sandstrom R. L. , Sun Shouheng
Published: April 2011

Electrons traverse two-dimensional nanocrystal arrays by sequential tunneling between neighboring nanocrystals. Analysis of array conductance at zero bias-voltage gives information about underlying nanocrystal uniformity, as well as the relevant single-electron charging energy. We discuss low-temperature measurements of two-dimensional self-assembled superlattices composed of 10 nanometer diameter cobalt nanocrystals, with ∼2 nm inter-nanocrystal spacing.
Status of Ion Projection Lithography
Authors: Bruenger Wilhelm H. , Kaesmaier Rainer , Loeschner Hans , Springer Reinhard
Published: April 2011

As part of the European MEDEA project on Ion Projection Lithography (IPL), headed by Infineon Techologies, a process development tool (PDT) has been assembled at IMS, Vienna, with the final target of 50 nm resolution in a 12.5 mm exposure field at 4× demagnification. The ion-optical system (PDT-IOS) has been integrated, including the LEICA mask changer and a sophisticated metrology stage with in-situ diagnostics. In parallel, the LEICA wafer stage and the vacuum compatible off-axis ASML wafer alignment system have been realized. At the moment (Nov00) the He+ ion beam is aligned until the mask level. Ion beam proximity wafer exposures directly behind the mask show a performance of the illumination optics as predicted. 150 mm stencil masks with 125mm diameter, 3μm Si membranes, 50mm × 50mm design field, have been produced by IMS-Chips, Stuttgart. There is expectation to start the PDT-IOS test phase in Q1/01. Using the experimental ion projector at the Fraunhofer-Institute ISiT in Berlin recent resolution tests have demonstrated 50 nm lines and spaces without proximity effect in standard Shipley DUV resist UVIIHS at an exposure dose of 0.5 μC/cm2 for 75 keV He+ions. This was accomplished by 8.5 × demagnification of a new generation of stencil test masks from IMSChips. One further promising application of IPL is the resistless structuring of thin magnetic films to produce magnetic nano dots for future ≥ 100 G bit/in2 storage devices. A consortium of IBM Germany - Speichersysteme Mainz, Fraunhofer-ISiT, LEICA Jena and IMS-Chips in cooperation with IMS-Vienna has been formed to evaluate this technology.
Spatially controlled CdSe nanocrystal distribution in phase separated polymer blend films
Authors: Asami Harumi , Saita Soichiro , Kamiya Itaru , Yoshie Kenichi
Published: April 2011

We report the fabrication and characterization of the self-assembled structure of tri-n-octylphosphine oxide (TOPO)-capped CdSe nanocrystals in binary polymer phase separated films of polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(2-vinylpyridine) (P2VP). These structures are formed via demixing of CdSe nanocrystals and binary polymer during spin coating. The nanocrystals preferentially segregate to the PS-rich phase in phase separated PS/PMMA and to the P2VP-rich phase in phase separated PS/P2VP, as shown by optical microscopy and photoluminescence images. For CdSe/PS/PMMA, we attribute the driving force for CdSe segregating to the PS-rich domain to the stronger attractive interaction of TOPO on PS with respect to PMMA due to polarity, and in the case of CdSe/PS/P2VP, segregating to the P2VP-rich domain to the interaction such as coordinate bonds being formed between pyridine groups and surface Cd atoms.
Dynamic Light Scattering at CdSe Nanocrystals and CdSe Cluster-Molecules
Authors: Eichhöfer A. , Hänisch C.V. , Jacobsohn M. , Banin U.
Published: April 2011

High performance dynamic light scattering (DLS) has been used to determine the hydrodynamic diameters of CdSe-nanocrystals as well as CdSe cluster molecules in a size range of 1 to 10 nm. The method enables the determination of the particle sizes including the ligand shells directly in solution. Consistent with the blue shift of the absorption onset by reducing the particle sizes the values for the hydrodynamic radii decrease. For the CdSe nanoparticles the measured sizes were compared with the diameters determined by TEM experiments; the sizes of the cluster molecules were on the other hand estimated from the space filling models of the single crystal X-ray structure determination. Regarding both types of compounds, DLS yield comparable values for the radii, indicating that it provides a potentially important tool in addition to other size determining techniques like TEM, which operates at more drastic conditions, and powder X-ray diffraction being difficult to interpret below 5 nm.
Interband Transitions in Si Quantum Wires grown in {100} plane
Authors: Zianni X. , Nassiopoulou A.G.
Published: April 2011

We present calculations on the spontaneous emission rates of Si rectangular quantum wires grown in the {100} plane. The electron and holes states are calculated within an effective mass approximation model. For holes, the minimum of the one-dimensional sub-bands is at the Γ- point. For electrons, the six anisotropic valleys at the conduction band minimum of bulk Si are not equivalent in the quantum wires and so, the minima of the energy sub-bands depend on the growth direction of the wire. For some wire directions in the {100} plane, the minimum of the ground electron sub-band is at the Γ-point. The directional dependence of the electron states is reflected on the spontaneous emission of the quantum wires. Light emission from Si quantum wires of diameters of a few nanometers can have direct transition character in the visible range. Phonon assisted transitions are then also present, but their intensity is three orders of magnitude smaller than the intensity of direct transitions. In Si quantum wires with indirect band gap the intensity and the peak position of the phonon assisted transitions exhibit dependence on the wires direction.
Control of the Arrangement of Nano Holes on Silicon Surface
Authors: Ohno Y. , Ichihashi T. , Takeda S.
Published: April 2011

We have found that nanoholes are formed on silicon surfaces by the irradiation of electrons whose energy is above 40 keV. We have systematically investigated the size and distribution of nanoholes. In the temperature range from 300 K to 600 K, both the radii of nanohole and the distance of nearest-neighbor nanoholes increase with increasing temperature; the planar density of nanoholes decreased with increasing temperature. The data follow a simple Arrhenius law, suggesting that nanoholes are formed through the diffusion of surface vacancies. The diffusion energy is estimated to be 50 meV. Surface nanoholes are formed even at 4 K at which surface vacancies cannot diffuse thermally, presumably due to athermal diffusion of surface vacancies.
Erbium Emission from Silicon Based Photonic Bandgap Materials
Authors: Lopez Herman A. , Lugo J. Eduardo , Chan Selena , Weiss Sharon M. , Striemer Christopher C. , Fauchet Philippe M.
Published: April 2011

Control over the 1.5 µm emission from erbium is desirable for communication and computational technologies because the erbium emission falls in the window of maximum transmission for silica based fiber optics. Tunable, narrow, directional, and enhanced erbium emission from silicon based 1-D photonic bandgap structures will be demonstrated. The structures are prepared by anodic etching of crystalline silicon and consist of two highly reflecting Bragg reflectors sandwiching an active layer. The cavities are doped by electro-migrating the erbium ions into the porous silicon matrix, followed by high temperature oxidation. By controlling the oxidation temperature, porosity, and thickness of the structure, the position of the erbium emission is tuned to emit in regions where the normal erbium emission is very weak. The erbium emission from the cavity is narrowed to a full width at half maximum (FWHM) of 12 nm with a cavity quality factor Q of 130, highly directional with a 20 degree emission cone around the normal axis, and enhanced by more than one order of magnitude when compared to its lateral emission. Erbium photoluminescence (PL) from porous silicon 2-D photonic bandgap structures is also demonstrated.
Er3+ Photoluminescence Properties of Erbium-doped Si/SiO2 Superlattices with sub-nm Thin Si Layers
Authors: Ha Yong Ho , Kim Sehun , Moon Dae Won , Jhe Ji-Hong , Shin Jung H.
Published: April 2011

The effect of varying the Si layer thickness on the Er3+ photoluminescence properties of Er-doped Si/SiO2 superlattice is investigated. We find that as the Si layer thickness is reduced from 3.6 nm down to a monolayer of Si, the Er3+ luminescence intensity increases by over an order of magnitude. Temperature dependence of the Er3+ luminescence intensity and time-resolved measurement of Er3+ luminescence intensity identify the increase in the excitation rate as the likely cause for such an increase, and underscore the importance of the Si/SiO2 interface in determining the Er3+ luminescence properties.
Charge Storage Mechanism in Nano-Crystalline Si Based Single-Electron Memories
Authors: Hinds Bruce J. , Yamanaka Takayuki , Oda Shunri
Published: April 2011

A memory device sensitive to a single electron stored in a nanocrystalline Si dot has been synthesized allowing for the study of charge retention lifetime. A 50 nm by 20 nm transistor channel is synthesized by E-beam lithography followed by reactive ion etching of thin (20nm) Silicon-on-Insulator (SOI) channel. This small area of the narrow channel allows for the isolation of a single nano-crystalline Si dot and elimination of channel percolation paths around the screening charge. Remote Plasma Enhanced CVD is used to form 6±2nm diameter nc-Si dots in the gas phase from a pulsed SiH4 source. Electrons stored in a dot results in an observed discrete threshold shift of 90 mV. Analysis of lifetime as a function of applied potential and temperature show the dot to be an acceptor site with nearly Poisson lifetime distributions. An observed 1/T2 dependence of lifetime is consistent with a direct tunneling process, thus interface states are not the dominant mechanism for electron storage in this device structure. Median lifetimes are modeled by a direct tunneling process, which is influenced by gate bias and dot size.
Ballistic Electron Surface-Emitting Cold Cathode by Porous Polycrystalline Silicon Film Formed on Glass Substrate
Authors: Komoda Takuya , Ichihara Tsutomu , Honda Yoshiaki , Aizawa Koichi , Koshida Nobuyoshi
Published: April 2011

It is demonstrated that a porous polycrystalline silicon (PPS) film is useful as a ballistic electron emitter for excitation source of a flat panel display. A 1.5 µ m polysilicon layer is deposited on a silicon substrate by Low Pressure Chemical Vapour Deposition (LPCVD) technique and subsequently anodised in an ethanoic HF solution and oxidised in a Rapid Thermal Oxidation (RTO) furnace. A thin Au film is deposited onto the RTO-treated PPS layer and used as a top electrode. The electron emission current Ie and the diode current Ips are measured as a function of the bias voltage Vps. Electron emission of which onset voltage is about 8 V rapidly increases with increasing Vps. The Ie value reaches about 2 mA/cm2 for Vps= 20 V at which the emission efficiency defined as Ie/Ips is about 1 %. The emission mechanism has also been investigated in terms of the correlation between the emitted electron energy and the structure of PPS layer. The observed energy distribution curve of output electrons suggests that the PPS layer acts as a ballistic transport medium and the emission occurs based on multiple tunnelling through silicon nanocrystallites. The PPS layer is also formed on the polysilicon layer deposited on a glass substrate by Plasma Enhanced Chemical Vapour Deposition (PCVD) technique. In this case, the film is treated by an electrochemical oxidation (ECO) in an H2SO4 solution. Similar emission characteristics are observed, although the emission current is lower than that formed on silicon substrate. We also demonstrate the 2.6 inches diagonal 53×40 pixels multicolour flat panel display. We name it ballistic electron surface-emitting display device (BSD). BSD shows the possible application to the future flat panel display.
CVD growth of Si Nanocrystals on Dielectric Surfaces for Nanocrystal Floating Gate Memory Application
Authors: Madhukar Sucharita , Smith K. , Muralidhar R. , O'Meara D. , Sadd M. , Nguyen B-Y. , White B. , Jones B.
Published: April 2011

Initial stages of growth of silicon on dielectric surface proceeds through Volmer-Webber growth mode (through island formation) and coalescence of islands to form a continuous thin film layer. Growth of silicon nanocrystals on dielectric surface can be achieved by controlling the nucleation and growth kinetics of these nanocrystals on the dielectric surface. In this study, we present CVD growth of Si nanocrystals on oxide, oxynitride and nitride surfaces using silane based chemistry. The effect of process conditions such as deposition temperature, precursor flow rate, carrier gas on nanocrystal incubation time, size and density of nanocrystals was studied. The nucleation and growth rate of the silicon nanocrystals is also a strong function of the surface chemistry, surface structure and strain in the dielectric film. We have studied the effect of oxide deposition temperature and surface pretreatments on the nucleation and growth characteristics of the silicon nanocrystals.
Oxidation of Silicon Nanocrystals
Authors: Scheer Kristen C. , Madhukar Sucharita , Muralidar Ramachandran , Lozano Jose , O'Meara David , Bagchi Sandeep , Conner James , Perez Carlos , Sadd Michael , Jones R. E. , White B. E.
Published: April 2011

Silicon nanocrystals have made a recent appearance in the literature because of their potential importance in microelectronic[1] and optoelectronic [2] devices. For example, replacing the traditional Si floating gate in nonvolatile memory field effect transistors with nanocrystals has been shown to produce more reliable and lower power memory devices than traditional transistors [1]. Controlled oxidation of Si nanocrystals is desirable in the processing of Si quantum dots for these devices. If controlled oxidation is attainable, then it could be used to control nanocrystal size, achieve high nanocrystal density, and achieve a high quality interface between Si dot and SiO2 gate oxide layer in nanocrystal based memory. In this work, we report the oxidation properties of various sized silicon nanocrystals, deposited by low-pressure chemical vapor deposition, in different oxidizing environments. In the literature, when a Si column or dot is oxidized below the viscoelastic temperature of SiO2 (950 oC), the oxidation will self-limit to a Si core size that is dependent on the oxidizing conditions and the initial particle size. This self-limiting phenomenon is said to occur because of the presence of a compressive stress in the oxide layer that limits the diffusion of the oxidizing agent through the SiO2 to the Si- SiO2 interface. This compressive stress is present at the interface because of the difference in the density of the oxide and the Si and is enhanced by the radius of curvature of the dot. This self- limiting oxidation phenomenon will be studied experimentally using microscopy techniques and the effect of the constrained structure will be characterized.
Theory of electronic transport in semiconductor nanostructures.
Authors: Niquet Y.M. , Delerue C. , Allan G. , Lannoo M.
Published: April 2011

We review the orthodox theory of single charge tunneling in a semiconductor quantum dot and we extend it to treat both single electron and hole charging effects. We analyze recent tunneling spectroscopy experiments. We show that for sufficiently large bias voltages V, both electrons and holes can tunnel into the quantum dot, leading to specific features in the I(V) curve. We present detailed simulations of the I(V) curves based a tight binding method for the electronic structure. A very good agreement is obtained with available experiments on InAs nanocrystals, allowing a complete interpretation of the spectra. Finally, we make some predictions concerning Si nanocrystals.
Drastic Reduction of Threading Dislocation Density of AlGaN on SiC Wafer by Using Highly-Si-Incorporated AlGaN Superlattice
Authors: Hirayama Hideki , Ainoya Makoto , Kinoshita Atsuhiro , Hirata Akira , Aoyagi Yoshinobu
Published: April 2011

A new in-situ technique to reduce threading dislocation density (TDD) within sub- micron growth is demonstrated by using metal-organic vapor-phase epitaxy (MOVPE). We achieved drastic reduction of TDD of AlGaN buffer on SiC substrate by inserting highly-Si- incorporated AlGaN/undoped AlGaN superlattice (SL). TDD of AlGaN was decreased from 2×1010 to 7×107 cm−2 by inserting the SL with the total growth thickness of 0.8νm. Si incorporation in AlGaN SL was estimated to be 1.2×1020 cm−3. This technique is exactly in- situ process without complicated fabrication processes, and the surface is kept flat throughout the total growth. This method is especially useful on SiC wafer in order to prevent cracks with thin growth layer. We confirmed the similar effects for GaN and AlGaN buffer on sapphire substrates.
Emission Characteristics of GaN-Based EL Device with AC Operation
Authors: Honda Tohru , Kawanishi Hideo
Published: April 2011

GaN-based electroluminescence devices (ELDs) were fabricated using a GaN powder as an emission layer. The electroluminescence spectra of the GaN ELDs under AC operation were observed at room temperature. The emission characteristics of GaN-based ELDs were studied to compare the EL spectra and the cathodoluminescence (CL) spectra. It was clarified that the EL spectra were similar to the CL spectra of a GaN emission layer. The emission peaks in the EL spectra were shifted toward the high-energy side with increasing operation frequency.
High Resistivity GaN Formed by Ion Implantation
Authors: Kudo Jun , Hishida Yuji , Watanabe Masanori , Hatayama Tomoaki , Fuyuki Takashi
Published: April 2011

Advanced high frequency devices using GaN-based semiconductor require the technology to form high resistivity regions with high thermal stability to electrically isolate the active elements from surroundings, as well as from the underlying substrate. The present paper describes the preparation of thermally stable, high resistivity GaN layers by ion implantation. It was confirmed that C or Zn implantation yielded high resistivity layers. In particular, Zn implantation yielded the layers with resistivity on the order of 1010 cm, which could be sustained at temperatures as high as 1000.
Hot Electron Transport in AlN
Authors: Collazo Ramón , Schlesser Raoul , Roskowski Amy , Davis Robert F. , Sitar Z.
Published: April 2011

The energy distribution of electrons that were transported through a thin intrinsic AlN film was directly measured as a function of the applied field. The measurements were realized by extracting the electrons into vacuum through a semitransparent Au contact and measuring their energy using an electron spectrometer. At moderate applied fields (350 kV/cm), the energy distribution followed a Maxwellian model corresponding to an electron temperature of 2700 K and a drift component below the spectrometer resolution. At higher fields, intervalley scattering was evidenced by the presence of a second peak at 0.7 eV. This coincides well with the energy position of the L-M valley in AlN. To the best of our knowledge, these are the first measurements that offer direct evidence of intervalley scattering in any solid system.
Synthesis, Structure, and Luminescence of A2B4C5 Nitrides
Authors: Bondar Vyacheslav , Axelrud Lev , Davydov Vladimir , Felter Tom
Published: April 2011

In order to extend the possibilities of doping of nitrides with activators of different valent and coordination states the complex A2B4C5 nitrides were investigated. The crystallochemical computer simulation of the new phosphor materials based on Mn, Ti and Eu-doped nitrides A2B4C5 has been made. Methods of synthesis of CaSiN2, and MgSiN2 have been developed, and X-ray diffraction analysis of these materials has been carried out. ESR and cathodoluminescent properties of MgSiN2, and CaSiN2 doped with manganese and europium have been studied.
Deep Level Formation in Undoped and Oxygen-Doped GaN
Authors: Gregie J. M. , Korotkov R. Y. , Wessels B. W.
Published: April 2011

Deep level defects in oxygen doped GaN grown by metal-organic vapor phase epitaxy were investigated. Using steady-state photocapacitance (SSPC) spectroscopy, three deep levels with optical ionization energies of 1.0, 1.4, and 3.25 eV were observed in both nominally undoped and oxygen-doped samples. The total deep level defect concentrations ranged from 6 × 1015 cm-3 in undoped films to 3 × 1016 cm-3 in oxygen-doped films. The concentration of the 3.25 eV level defect increased upon oxygen doping, while the concentrationof the 1.0 and 1.4 eV levels were essentially dopant independent. From the measured concentrations the formation energies of the defects were calculated and compared to energies calculated using density functional theory.
TEM Analysis of Threading Dislocations in ELO-GaN Grown with Controlled Facet Planes
Authors: Kuwano Noriyuki , Horibuchi Kayo , Miyake Hideto , Hiramatsu Kazumasa
Published: April 2011

Cross sectional transmission electron microscope (TEM) observation has been performed for specimens of ELO-GaN (ELO: epitaxial-lateral-overgrowth) in order to analyze the behavior of dislocations, with special reference to the effect of facet plane orientation and the size of mask. An ELO-GaN layer was grown overlying on a thick GaN layer with a patterned mask by MOVPE with a carrier gas of hydrogen (H2) under a low-ambient pressure. The growth temperature and the reactor-pressure were controlled in a two-step way during the growth of ELO-GaN layer in order to change the dominant facet-planes and the aspect ratio in growth rate. The experimental results revealed that (a+c)-type threading dislocations (TDs) show a 90-degree-bending in the specimen with slanting facets (2 1 1 2), but not in those with vertical ones (2110). a-type TDs run upward without bending irrespective of the orientation of the facet planes. Dislocations lying on (0001) planes, or horizontal dislocations (HDs), have been generated in the specimens with wide mask-terraces. It is thought that the formation of HDs relieved stresses in the ELO-GaN and then suppressed the bending of a-type TDs. In the specimens with narrow terraces, the both type TDs penetrate upward without bending and few HDs are generated. The behavior of dislocations is attributable to the fact that the small size of terrace generates small stresses and promotes a fast meeting of wings of ELO-GaN.
Integration of InxGa1−xN Laser Diodes with Dissimilar Substrates by Laser Lift-off
Authors: Wong William S. , Kneissl Michael , Mei Ping , Treat David W. , Teepe Mark , Johnson Noble M.
Published: April 2011

Continuous-wave (cw) indium-gallium nitride (InGaN) multiple-quantum-well (MQW) laser diodes (LDs) were successfully transferred from sapphire onto copper and diamond substrates using a two-step laser lift-off (LLO) process. Reduced threshold currents and increased differential quantum efficiencies were measured for LDs on Cu due to a 50% reduction of the thermal impedance. Light output for LDs on Cu was three times greater than comparable LDs on sapphire with a maximum output of 30 mW. Increased light output for LDs on diamond were also measure with a maximum output of 80 mW.
The Investigation of InGaN MQW Electroabsorption Modulator using the LED/Modulator/Detector Monolithically Integrated Structure
Authors: Chung S.W. , Zhao Y.S. , Lin C.H. , Lee H.P.
Published: April 2011

The strong piezoelectric effect and quantum confined stark effect (QCSE) in the InGaN/GaN quantum well structures allow one to modify the free exciton absorption by the extrinsic field. The QCSE is investigated using a monolithically integrated three-section device comprising an LED, electroabsorption modulator and a detector section. The experimental results show that the LED output can be modulated as indicated by the detector signal. The strength of modulation decreases monotonically with increasing In composition in the InGaN/GaN MQW. The result can be explained on the basis of the Stokes' shift between the emission and absorption spectra in the InGaN/GaN QW structure, and a blue shift of the absorption spectrum due to the QCSE as a result of the piezoelectric effect.
Investigation of Buffer Layers for GaN Grown by MBE
Authors: Yun Feng , Reshchikov Michael A. , Visconti Paolo , Jones Keith M. , Wang Dongfeng , Redmond Marc , Cui Jie , Litton Cole W. , Morkoç Hadis
Published: April 2011

The structural quality of the buffer layer juxtaposed to the substrate is pivotal in attaining high quality GaN layers. In MBE deposition, low temperature, medium temperature and high temperature AlN buffer layers are at the disposal of the grower. There are quite a few reports, some discussing the benefits of high temperature buffer layers and others doing the same for low temperature buffer layers. The reports emanate from different laboratories; and due to stringent parameter control required, it is difficult to compare one type of buffer with another. To gain some insight, we undertook an investigation wherein these varieties of buffer layers were grown on nitridated sapphire substrate under similar conditions for a comparative analysis. In addition to the single buffer layers of both GaN and AlN varieties, some combinations of stacked buffer layers, including cases where these buffer layers were separated by GaN layers, were employed. Structural analysis by high resolution X-ray diffractometry and topological analysis by AFM were carried out to assess the quality of the epilayers grown on these buffers. Hall measurements at room temperature were carried out to characterize the electrical transport properties.
Surface Segregation and Composition Fluctuations in ammonia MBE and MOVPE of InGaN
Authors: Karpov Sergey Yu. , Talalaev Roman A. , Yakovlev Eugene V. , Makarov Yuri N.
Published: April 2011

Surface segregation and phase separation are investigated as processes limiting the indium incorporation in InGaN grown by ammonia Molecular Beam Epitaxy (MBE) and Metal- Organic Vapor Phase Epitaxy (MOVPE). It is shown that a significant concentration of indium on the growing surface may prevent the adsorption of ammonia via site blocking mechanism and result in appearance of In droplets instead of InGaN growth. Another conclusion is that the composition fluctuations in InGaN are related to coexistence of strained and relaxed InGaN islands rather than to the phase separation as commonly assumed.
Strain and Compositional Analysis of InGaN/GaN Layers
Authors: Pereira Sérgio , Correia Maria. R. , Pereira Estela , Trager-Cowan C. , Sweeney F. , Edwards P.R. , O'Donnell K.P. , Alves E. , Sequeira A.D. , Franco N.
Published: April 2011

We investigate strain and composition of epitaxial single layers of wurtzite InxGa1−xN (0<x<0.25) grown by MOCVD on top of GaN/Al203 substrates. It is shown that significant inaccuracies may arise in composition assessments if strain in InxGa1−xN/GaN heterostructures is not properly taken into account. Rutherford backscattering spectrometry (RBS) measures composition, free from the effects of strain and with depth resolution. Using X-ray diffraction (XRD) we measure both a- and c- parameters of the strained wurtzite films. By measuring both lattice parameters and solving Hooke's equation, a good estimation for composition can be obtained from XRD data. The agreement between RBS and XRD data for composition allows reliable values for perpendicular (εzz) and parallel strain components ( (εxx) to be determined. RBS and depth resolved cathodoluminescence (CL) measurements further indicate that the indium content is not uniform over depth in some samples. This effect occurs for the most strained layers, suggesting that strain is the driving force for compositional pulling.
Direct Observation of Bulk and Interface States in GaN on Sapphire grown by Hydride Vapor Phase Epitaxy
Authors: Goss S. H. , Young A. P. , Brillson L. J. , Look D. C. , Molnar R. J.
Published: April 2011

We have used an ultrahigh vacuum scanning electron microscope to carry out cross sectional secondary electron imaging, cathodoluminescence spectroscopy, and cathodoluminescence imaging on GaN grown on sapphire by hydride vapor phase epitaxy. These measurements provide evidence for deep level defects highly localized at the GaN, sapphire interface as well as defects extending into both the semiconductor film and the substrate. The different spatial distributions of these radiative defects provide information on the physical origin of these electrically active features.
Relationship between Microscopic Structure and Optical Property of Polycrystalline GaN on Silica Glass
Authors: Kagatsume Hidetaka , Aono Hiroaki , Araki Tsutomu , Nanishi Yasushi
Published: April 2011

We have investigated relationships between the microscopic structure and optical property of polycrystalline GaN grown by (electron-cyclotron-resonance plasma-excited molecular beam epitaxy) ECR-MBE on silica glass substrates, using scanning electron microscope (SEM) and cathodoluminescence (CL). It was found that CL intensity was stronger for the samples with a large columnar domain size. These individual columnar domains showed clear luminescence. It was found that the origin of strong luminescence from polycrystalline GaN is due to such a columnar domain. That luminescence was closely related to the morphology of the columnar domains. It was revealed that the columnar domain with a homogeneous and hexagonal shape showed clear luminescence.
The Influence of Defects and Piezoelectric Fields on the Luminescence from InGaN/GaN Single Quantum Wells
Authors: Henley S. J. , Cherns D.
Published: April 2011

High spatial resolution cathodoluminescence (CL) studies have been carried out on GaN/InGaN/(0001)GaN single quantum well (SQW) structures in a field emission scanning electron microscope at 5kV and temperatures down to 8K. Direct comparison of QW CL maps with transmission electron microscope studies of plan-view samples showed that edge type threading dislocations act as non-radiative recombination centers. Spectra taken from extended areas showed a progressive blue shift in the QW emission from around 460nm at low beam intensities to about 445nm as the beam intensity was increased. This effect which correlated with a decrease in the spatial resolution is interpreted as due to an increase in the diffusion length of carriers in the SQW due to a combination of screening of the piezoelectric field and band filling effects.
UV Raman Study of A1(LO) and E2 Phonons in InGaN Alloys Grown by Metal-Organic Chemical Vapor Deposition on (0001) Sapphire Substrates
Authors: Alexson Dimitri , Bergman Leah , Nemanich Robert J. , Dutta Mitra , Stroscio Michael A. , Parker C.A. , Bedair S.M. , El-Masry N.A. , Adar Fran
Published: April 2011

We report on UV Raman spectroscopy of InxGa1−xN thin films grown on (0001) sapphire substrates using a specially designed metal-organic chemical vapor deposition (MOCVD) reactor. Eight films were examined in the compositional range 0<×<0.50. Mid and deep-UV Raman spectroscopy was done with the 325.2 nm line of the HeCd laser and the 244 nm line of a double frequency Ar ion laser. The mode behavior of the A1(LO) and E2 phonons was also investigated. We have found compelling evidence for one-mode behavior for the A1(LO) phonon mode, while our data for the E2 mode deviates from the predictions for one-mode behavior. The results for the A1(LO) mode are consistent with the previously reported phonon mode behavior in AlGaN alloys. Also, evidence regarding the presence of compositional inhomogeneities and spinodal decomposition in InGaN thin films is presented.
Morphological Instabilities during Explosive Crystallization of Germanium Films
Authors: Chojnacka Aleksandra , Thompson Michael O.
Published: April 2011

Explosive crystallization, a self-sustaining transformation of an amorphous phase to a crystalline phase mediated by a thin liquid layer, exhibits three distinct kinetic and morphological regimes in germanium. Dynamics of these growth processes and the resulting morphologies have been examined in detail. Steady-state crystallization velocities were measured as a function of heat loss into the substrate. Dark field optical microscopy, tapping mode atomic force microscopy, transmission electron microscopy, and x-ray diffraction were used to examine the crystallized films. Analyses of the experimental results provide evidence for two distinct processes governing explosive crystallization in limits of high substrate temperature (low heat loss) and low substrate temperature (high heat loss). The low temperature growth mode produces a “scalloped” structure with propagation velocities that monotonically increase with temperature. At high substrate temperatures, the velocity is independent of temperature and a “columnar” pattern with preferred texture is formed.
Microstructure and Optical Properties of Co Sputter Deposited Si-Al Nanocomposite Thin Films
Authors: Niu F. , Dobson P.J. , Cantor B.
Published: April 2011

Novel Si-Al nanocomposite thin films were made by radio frequency co-sputtering of Si and Al with Al content from 0 at.% to 69 at.%. Microstructure and optical properties of the films were characterised by conventional and high resolution transmission electron microscopyand spectrometry in the wavelength range from 200 to 3000 nm. The film microstructure consisted of Al nanoparticles (2-9 nm) embedded in an amorphous Si-Al matrix. Optical absorption spectra of the films up to 50 at.% Al exhibited a sharp absorption peak below500 nm and relatively low absorption above 500 nm. In addition, the absorption peak shifted towards longer wavelengths and the general absorption above 500 nm increased remarkably as Al content increased. For the Si-69at.%Al films, however, an absorption plateau appeared between 300 nm to 700 nm and a second weak and broad absorption peak appeared at around 900 nm. The results are analysed and compared with the optical absorption predicted by various effective medium theories.
Self-Assembly of Ruthenium Porphyrins into Monolayer and Multilayer Architectures via Heterogeneous Coordination Chemistry
Authors: Sarno David M. , Matienzo Luis J. , Jones Wayne E.
Published: April 2011

Self-assembly chemistry is finding use as a means of controlling the nanoscale structures at materials interfaces. Building on the success of a previous methodology, Ruthenium porphyrins have been coordinated to the pyridine-terminated surface of a self-assembled alkylsilane coupling layer on silica glass. These arrays are built in a stepwise fashion in which the surface of each layer allows further reaction to yield the next layer. Characterization by UV-vis and XPS confirms the deposition of the alkyl chains and pyridine moieties of the coupling layer, as well as coordination of the subsequent Ru-porphyrin layer. AFM analysis indicates very smooth surfaces, with roughness measured at 0.43 nm and 1.43 nm for the pyridine and porphyrin layers, respectively. The occasional appearance of large surface features (15-30 nm diameter) suggests aggregation among Φ-systems, with further characterization underway. Strategies toward the construction of a multilayer porphyrin architecture are also reported.
Investigation of the Layered Structure of Polycrystalline Diamond Thin Films Grown by ECR-Assisted CVD by Spectroscopic Phase Modulated Ellipsometry
Authors: Gupta S. , Weiner B. R. , Morell G.
Published: April 2011

Polycrystalline diamond thin films deposited by the electron cyclotron resonance-assisted chemical vapor deposition (ECR-CVD) were investigated using spectroscopic ellipsometry (SE) from the near IR to UV range (830 nm-270 nm). Employing the conventional Bruggeman effective medium approximation (EMA) and linear regression analysis (LRA) to the raw ellipsometry data (ψλi), δ(λi)) provided the details about the film microstructure: (i) the multilayer structure and the ovearall thickness of the films; (ii) the volume fraction of the constituents (sp3 - and sp2- bonded carbon) and of voids in the bulk layer; (iii) the inhomogeneity of the structure along the growth axis and its variation with the seeding density; and (iv) the surface roughness layer. A simplified three-layer structural model consisting of an interfacial layer, an intermediate (or bulk) layer and the top surface roughness layer has been proposed to simulate the ellipsometry data. The results obtained through ellipsometry modeling such as surface roughness layer and overall film thickness were compared with rms surface roughness from atomic force microscopy (AFM) and profilometry respectively, in order to validate the model employed. The results such as fv and fsp2C for the bulk layer and its behavior with respect to process parameters are discussed.
Hydrothermal Growth Kinetics of BaTiO3 on TiO2 Single Crystal Surfaces
Authors: Fuenzalida Víctor M. , Lisoni Judit G.
Published: April 2011

TiO2 (rutile) single crystal plates with (001) and (110) orientation were immersed in an aqueous solution of Ba(OH)2 0.5 M at temperatures of 100 and 150 °C for 1 and 4 hours in order to grow BaTiO3on them. SEM micrographs of the samples fabricated on the (001) surface of rutile displayed isolated grains with an average height ranging from 200 nm at 100 °C to 700 nm at 150 °C. On the other hand, samples with the (001) orientation exhibited no growth at 100 °C and only a few grains along lines attributed to the polishing process of the substrate at 150 °C. The image of backscattered electrons indicated that barium is concentrated on the grains in all cases. Only the (001) samples exhibited reflections of cubic BaTiO3, as indicated by x-ray dif- fraction, as well as distinct Ba signals under x-ray photoelectron spectrometry. These results agree with the hypothesis of a dissolution-precipitation growth mechanism, in which dissolution is possible for the (001) face, but not for the (110) one, which is the most stable of the low-index faces of this material. Similar treatments were applied to ZrO2:Y2O3 crystals, leading to no film growth.
Sliding Friction between Polymer-brush-bearing Surfaces: Crossover from Brush-brush Interfacial Shear to Polymer-substrate Slip.
Authors: Klein Jacob
Published: April 2011

A model is presented for the shear (or frictional) forces Fs between two surfaces a distance D apart as they slide past each other while bearing mutually compressed polymer brushes, on the assumption that sliding takes place at the brush-brush interface. The predictions of the model for the rapid increase in Fs at increasing compressions are in reasonable agreement with experiments on polystyrene brushes immersed in toluene over two decades in Fs. At higher compressions (smaller D) the experimental shear forces increase only slowly, and diverge from the calculated ones which continue to increase rapidly; at the same time the form of the shear force response at these higher compressions reverts from a viscous-like one to a stick-slip behaviour. These observations strongly indicate that at sufficiently high compressions the plane of slip crosses over from the brush-brush interface to the polymer-solid surface.