bluuebunny/crossref_metadata_2025 huggingface dataset:
In-Situ Mass Spectroscopy of ECR Silane Plasmas for Amorphous and Microcrystalline Silicon Growth
Authors: Song Young J. , Guliants Elena , Lee Hak-Gyu , Anderson Wayne A.
Published: April 2011

ECR silane plasmas for the deposition of a-Si:H and μc-Si films were investigated by in-situ mass spectroscopy (MS) using a quadrupole residual gas analyzer. The results showed that the intensities of ionic and neutral species (H, H2, He, Ar, Si and SiHx) in the 2 % SiH4/He plasma are strongly dependent on the deposition conditions such as chamber pressure, input power and hydrogen dilution. In all cases, the prevalence of Si ions was observed over SiH, SiH2 and SiH3 ions, suggesting a high decomposition rate of the silane in the plasma. In particular, the population of atomic hydrogen in the plasma seems to play a key role in the properties of both a-Si:H and μc-Si films. For example, the increased intensity of atomic hydrogen, compared to that of molecular hydrogen, resulted in the better quality a-Si:H film, showing a higher photo and dark conductivity ratio of ~105. The intensity of the hydrogen species was especially sensitive to the chamber pressure. The correlation between MS spectra and film properties is presented.
N-Type Silicon Films Produced by Hot Wire Technique
Authors: Ferreira Isabel M. M. , Cabrita Ana M. F. , Fortunato Elvira M. C. , Martins Rodrigo F. P.
Published: April 2011

The role of the deposition pressure (p) and the type of filaments (tungsten, W or tantalum, Ta) used to produce large area (10cm×10cm) n-type Si:H films by hot wire chemical vapour (HW-CVD) deposition technique was investigated. The data show that the electro-optical properties of the films produced are dependent on the gas pressure used. In the pressure range of 1×10-3 Torr to 1.0 Torr, the room dark conductivity (σd) varies from 1×10-8 to 2 S/cm for films produced at the same hydrogen dilution and filament temperature (Tfil.). On the other hand, the hydrogen concentration (CH) decreases from 10% to 2%, while the growth rate (R) shows an exponential increase, from 1 to 9 Å/s. The SIMS analysis, within the detection limits, does not reveal the existence of any significant W or Ta contamination in the films produced.
The Influence of W Filament Alloying on the Electronic Properties of Hwcvd Deposited a-Si:H Films
Authors: Mahan A. H. , Mason A. , Nelson B. P. , Gallagher A. C.
Published: April 2011

In depositing a-Si:H by HWCVD using W filaments, one issue common to this technique is that of filament lifetime. When using undiluted silane as the source gas, a buildup of silicon at the colder ends of the filament is routinely observed (thickening), and it is here that filament breakage usually occurs. Less well understood is the effect of filament alloy formation on a-Si:H electronic properties. In this work we combine ambipolar diffusion length (SSPG) measurements on consecutively deposited a-Si:H films with SEM surface topography and sputter (SP) Auger depth profiling of the filament Si/(W+Si) composition to track film electronic properties as a function of the Si buildup on short filaments entirely exposed to the growing film surface. We find that with increasing exposure time of the filament to silane, appreciable Si exists everywhere along the filament length, even in the non-thickened central regions. We discuss the effect of this alloying on the film deposition rate and electronic properties, and suggest that the nature of the filament surface must be carefully considered when optimizing a- Si:H film electronic properties. Finally, we discuss possible ways to minimize this alloying by post deposition treatments, which include different filament ‘run’ temperatures.
Thin Single Crystal Silicon on Oxide by Lateral Solid Phase Epitaxy of Amorphous Silicon and Silicon Germanium
Authors: Greene Brian J. , Valentino Joseph , Hoyt Judy L. , Gibbons James F.
Published: April 2011

The fabrication of 250 Å thick, undoped, single crystal silicon on insulator by lateral solid phase epitaxial growth from amorphous silicon on oxide patterned (001) silicon substrates is reported. Amorphous silicon was grown by low pressure chemical vapor deposition at 525°C using disilane. Annealing at temperatures between 540 and 570°C is used to accomplish the lateral epitaxial growth. The process makes use of a Si/Si1-xGex/Si stacked structure and selective etching. The thin Si1-xGex etch stop layer (x=0.2) is deposited in the amorphous phase and crystallized simultaneously with the Si layers. The lateral growth distance of the epitaxial region was 2.5 μm from the substrate seed window. This represents a final lateral to vertical aspect ratio of 100:1 for the single crystal silicon over oxide regions after selective etching of the top sacrificial Si layer. The effects of Ge incorporation on the lateral epitaxial growth process are also discussed. The lateral epitaxial growth rate of 20% Ge alloys is enhanced by roughly a factor of three compared to the rate of Si films at an anneal temperature of 555°C. Increased random nucleation rates associated with Ge alloy films are shown to be an important consideration when employing Si1-xGex to enhance lateral growth or as an etch stop layer.
Via Electromigration Lifetime Improvement of Aluminum Dual-Damascene Interconnects by Using Soft Low-k Organic SOG Interlayer Dielectrics
Authors: Kaneko Hisashi , Usui Takamasa , Ito Sachiyo , Hasunuma Masahiko
Published: April 2011

The via electromigration(EM) reliability of aluminum(Al) dual-damascene interconnects by using Niobium(Nb) new reflow liner is described. It has been found that the via EM lifetime was improved by introducing low-k organic spin on glass(SOG)-passivated structure than the conventional TEOS-SiO2/SiN-passivated structure. Higher EM activation energy of 1.08 eV was obtained for the SOG-passivated structure than the conventional TEOS-passivated structure of 0.9 eV, even though no significant Al micro-crystal structure difference was found for both structures. It has been turned out that the low-k SOG material has the 1/7 Young's modulus (8 GPa) of TEOS-SiO2 (57 GPa) or thermal SiO2(70 GPa). The small Young's modulus means that SOG is more elastically deformable and/or softer than TEOS or thermal SiO2. This elastic deformation of the low-k SOG could retard the tensile stress evolution due to the Al atom migration near the cathode via, and elongated the time until the Al interconnect tensile stress exceeds the critical stress value for void nucleation. It has been concluded that the small-RC and reliable multi-level Al interconnect can be realized by the Nb-liner reflow-sputtered process with soft and low-k SOG dielectric materials.
Synthetic Control and Properties of Processible Poly(Methylsilsesquioxane)S
Authors: Lee Jin-Kyu , Char Kookheon , Kim Hie-Joon , Rhee Hee-Woo , Ro Hyun-Wook , Yoo Dae Young , Yoon Do Y.
Published: April 2011

Processible poly(methylsilsesquioxane)s (PMSSQs) were prepared in THF solution under nitrogen atmosphere in the presence of HCl catalyst. It was found that various reaction parameters such as concentration, temperature, reaction time, the amount of water, and the amount of acid catalyst could affect the molecular weight and the amount of functional end groups of PMSSQ samples. Thin films prepared from our PMSSQ samples by spin-coating followed by curing to 420°C exhibited a much better crack resistance than those presented in the literature, while the dielectric constant remained practically the same, i.e., ca. 2.7.
Characterisation of Low-K Dielectric Films by Ellipsometric Porosimetry
Authors: Baklanov M.R. , Mogilnikov K.P.
Published: April 2011

Ellipsometric porosimetry (EP) is a simple and effective method for the characterization of the porosity (volume of both open and close pores), average pore size, specific surface area and pore size distribution (PSD) in thin porous films deposited on top of any smooth solid substrat e. Because a laser probe is used, small surface area can be analyzed. Therefore, EP can be used on patterned wafers and it is compatible with microelectronic technology. This method is a new version of adsorption (BET) porosimetry. In situ ellipsometry is used to determine the amount of adsorptive which adsorbed/condensed in the film. Change in refractive index is used to calculate of the quantity of adsorptive present in the film. EP also allows the study of thermal stability, adsorption and swelling properties of low-K dielectric films. Room temperature EP based on the adsorption of vapor of some suitable organic solvents and method of calculation of porosity and PSD is discussed. Examination of the validity of Gurvitsch rule for various organic adsorptives (toluene, heptane, carbon tetrachloride and isopropyl alcohol) has been carried out to assess the reliability of measurements of pore size distribution by the ellipsometric porosimetry.
Structural Analyses of Fluorine-Doped Silicon Dioxide Dielectric Thin Films by Micro-Raman Spectroscopy
Authors: Coffer Jeffery L. , Zerda T. Waldek , Taylor Kelly J. , Martin Scott
Published: April 2011

Fluorine-doped silicon dioxide, a dielectric material compatible with copper integration, has received considerable attention for applications requiring a k value in the 3.5 to 4.0 range. Given the influence of structure on desired properties, convenient experimental structural probes of this type of material are of widespread interest. This work focuses on Raman spectroscopic analyses of ring defects in fluorine-doped silicon dioxide films prepared by plasma enhanced chemical vapor deposition (PECVD) as well as high density plasma methods (HDP). These measurements are complemented by ab initio computational simulations of the ring defects in these films and the impact of nearby fluorine on their stability. The impact of aging on these structures and correlations of observed trends with experimental techniques such as X-ray fluorescence are also described.
Seed-layer Deposition for Sub 0.25 µm Cu Metallization Using a Line Cusp Magnetron Plasma Source
Authors: Wickramanayaka Sunil , Nagahama Hanako , Watanabe Eisaku , Hayashi Toshihiko , Sato Makoto , Nakagawa Yukito , Hasegawa Shinya , Mizuno Shigeru , Numasawa Yoichiro
Published: April 2011

A magnetically enhanced capacitively coupled plasma source was developed for sputter deposition of Cu seed layers on sub 0.25µm via or contact holes. The plasma source is of planer parallel plate configuration where the Cu target plate is one of the electrodes. For the generation of plasma, 60 MHz rf power is selected in order to increase the plasma density. Additionally, a line cusp magnetic field is used to further increase the plasma density. The film deposition rate and uniformity obtained with this plasma source is∼200 nm/min and ∼±5%, respectively. The Cu film resistivity lies around 2 µωcm. This sputtering system yields good film coverage on bottom and sidewalls of via holes with an aspect ratio > 5; therefore, a perfect Cu filling could be realized by electroplating process.
Organic Solution Deposition of Copper Seed Layers onto Barrier Metals
Authors: Gu H. , Fang R. , O'Keefe T. J. , O'Keefe M. J. , Shih W.-S. , Snook J. A. M. , Leedy K. D. , Cortez R.
Published: April 2011

Spontaneous deposition of copper seed layers from metal bearing organic based solutions onto sputter deposited titanium, titanium nitride, and tantalum diffusion barrier thin films has been demonstrated. Based on electrochemically driven cementation exchange reactions, the process was used to produce adherent, selectively deposited copper metal particulate films on blanket and patterned barrier metal thin films on silicon substrates. The organic solution deposited copper films were capable of acting as seed layers for subsequent electrolytic and electroless copper deposition processes using standard plating baths. Electroless and electrolytic copper films from 0.1µm to 1.0µm thick were produced on a variety of samples on which the organic solution copper acted as the initial catalytic seed layer. The feasibility of using organic solution deposited palladium as a seed layer followed by electroless copper deposition has also been demonstrated. In addition, experiments conducted on patterned barrier metal samples with exposed areas of dielectric such as polyimide indicated that no organic solution copper or palladium deposition occurred on the insulating materials.
Structural Characterization Of Laser Lift-Off GaN
Authors: Stach Eric A. , Kelsch M. , Wong W.S. , Nelson E.C. , Sands T. , Cheung N.W.
Published: April 2011

Laser lift-off and bonding has been demonstrated as a viable route for the integration of III-nitride opto-electronics with mainstream device technology. A critical remaining question is the structural and chemical quality of the layers following lift-off. In this paper, we present detailed structural and chemical characterization of both the epitaxial layer and the substrate using standard transmission electron microscopy techniques. Conventional diffraction contrast and high resolution electron microscopy indicate that the structural alteration of the material is limited to approximately the first 50 nm. Energy dispersive electron spectroscopy line profiles show that intermixing is also confined to similar thicknesses. These results indicate that laser lift-off of even thin layers is likely to result in materials suitable for device integration. Additionally, because the damage to the sapphire substrate is minimal, it should be possible to polish and re-use these substrates for subsequent heteroepitaxial growths, resulting in significant economic benefits.
Magnetic Field Generation at Early-Stage KrF Excimer Laser Ablation Of Solid Substrates
Authors: Hong M. H. , Lu Y. F. , Foong A. , Chong T.C.
Published: April 2011

Magnetic field generation at early-stage of KrF excimer laser ablation of solid substrates (for delay time less than 200 ns) is investigated. Based on classical electrodynamics, fast and dynamic emission of electrons and positive ions at the beginning of laser ablation induces an electromagnetic field nearby. A tiny iron probe wrapped with 50-turn coil is applied to detect this dynamic magnetic field. It is found that signal waveform is closely related to probe distance from the laser spot. For probe distance less than a threshold, the signal has a double-peak profile with a negative peak appearing first and peak durations in tens of nanoseconds. As probe distance increases, the amplitude of positive peak reduces much faster than the negative one. It disappears for probe distance up to the threshold and signal waveform becomes a negative peak with a wider duration. Mechanism on magnetic field generation at early-stage of laser ablation is analyzed to obtain more information on charged particle dynamics. Dependence of the magnetic signal on laser fluence, substrate bias and pulse number is also studied during laser ablation of solid substrates and removal of metallic oxide layer on the surface.
Optimization of The Processing Parameters for Pulsed Laser Deposition of Nickel Silicide Ohmic Contacts On SiC
Authors: Richardson C. J. K. , Wisnioski M. H. , Spicer J. B. , Demaree J. D. , Cole M. W. , Hubbard C. W. , Joshi P. C. , Hirvonen J. K. , Kim H. , Pique A. , Chrisey D. B.
Published: April 2011

This research investigates the potential of pulsed laser deposition to create reliable high current ohmic contacts of Ni2Si on single crystal 4H-SiC. Since this stoichiometry is the stable interphase in the nickel-silicon carbide diffusion couple, direct deposition eliminates the detrimental excess carbon normally formed by direct sintering Ni on SiC, the surface roughening that results from this sintering as well as the need for post-deposition high-temperature (900°C) anneals that are required in complex multi-component contacts. This study examines the processing parameters that must be used during deposition to obtain the desired microstructural characteristics for the contact. Pulsed laser deposition of nickel silicide produces smooth films with an amorphous or nanocrystalline structure interspersed with macroparticles. Macroparticle formation on the resulting films appear in the form of solidified droplets of the eutectic composition nickel silicide (3:1) that form during the long term target processing. The dependence of the number and size distributions of these droplets on laser fluence sample temperature is examined.
Atomistic Simulations of the (1014) Surface of Carbonate Minerals
Authors: Wright Kate , Cygan Randall T. , Slater Ben
Published: April 2011

Atomistic simulation methods have been used to model the structure of the (1014) surfaces of calcite, dolomite, and magnesite under dry and wet conditions. The potential parameters for the carbonate and water species contain shell terms to model the polarizability of the oxygen atoms. These static calculations show that the surfaces undergo relaxation leading to the rotation and distortion of the carbonate groups with associated movement of cations. The dry surface energies are 0.322, 0.247, and 0.256 Jm−2 for calcite, dolomite, and magnesite respectively. The influence of water on the surface structure and energies has been investigated for monolayer coverage. When fully hydrated with a monolayer of water, the surface energy for calcite is reduced indicating a stabilization of the surface with hydration. The extent of carbonate group distortion is greater for the dry surfaces compared to the hydrated surfaces, and for the dry calcite relative to that for dry magnesite.
Organic Light-Emitting Diodes Using Triphenylamine Based Hole Transporting Materials
Authors: Fujikawa Hisayoshi , Ishii Masahiko , Tokito Shizuo , Taga Yasunori
Published: April 2011

The durability of the tris(8-quinolinolato) aluminum based light-emitting diode (LED) is related to the thermal stability of the hole transport layer. Several linear linkage triphenylamine oligomers were used for the hole transport layer. The thermal stability was clearly seen to depend on a glass transition temperature (Tg) of the hole transporting material, and a linear relationship between the Tg and the thermal stability was found. A lowering of “turn-on voltage” for light emission and an increase of luminous efficiency with increasing temperature was also observed. Excellent durability of the organic LED with a tetramer of triphenylamine was achieved at a high temperature of 120°C. Our results indicate that the linear linkage of triphenylamine leads to a high Tg and high device performance at high temperatures.
Simulation of the Influence of Interface Charge on Electron Emission
Authors: Jensen Kevin L. , Shaw Jonathan L.
Published: April 2011

Several materials are promising candidates for electron sources. For diamond, a tunneling interface at the back contact limits injecting charge into the conduction band, but a purely geometric model of internal field emission is inadequate to explain experimental data. The presence of a defect, modeled by a coulomb charge, within the tunneling barrier region significantly enhances transmission and, in concert with a geometrical model, may better account for observed current levels. Charge has been suggested to play a similar role in the SiO2 covering on a single tip silicon field emitter to explain experimental data. The tunneling theory in both cases is similar. In the present work, a general method for estimating electron transport and energy distributions through potential profiles, which describe both semiconductor interfaces and field emission potential barriers when a charged particle modifies the tunneling barrier, is developed. While the model is intended for treating a metal-semiconductor interface, it is cast here in terms of in a thin SiO2 coating over a silicon field emitter tip to enable qualitative comparisons with experimental data. Tunneling probabilities are found by numerically solving Schrödinger's Equation for a piece-wise linear potential using an Airy Function approach. A qualitative comparison to experimental energy distribution findings is possible by utilizing an analytical model of the field emitter tip from which current-voltage relations may be found.
Observation of Hot Electron Field Emission
Authors: Shaw Jonathan
Published: April 2011

We report energy distributions of silicon Field Emitter Arrays coated with 50A of ZnO. The distributions reflect changes in the ZnO conductivity induced by annealing in vacuum, temperature, and annealing in hydrogen. An additional coating of titanium performed in-situ produced large additional changes. Emission from the ZnO at energies near the Fermi level increased with gate voltage only after hydrogen annealing, when hot, and after Ti coating. In those same cases the emission distribution contained a tail at energies above EF. The highenergy emission tail is due to a many-body or Auger process whereby holes injected below EF create hot electrons. Although emission from ZnO occurred at energies up to 8eV below EF, no high-energy tail was observed in the normal case. Thus emission appears to occur from isolated electrons in ZnO gap states in cases where the distribution lacks a high-energy tail. Conversely, emission above EF suggests that emission occurred from a metallic state such as an accumulated conduction band.
Effect of Temperature Gradient near the Target and Gas Flow Rate on the Diameter Distribution of Single-Walled Carbon Nanotubes Grown by the Laser Ablation Technique
Authors: Sen Rahul , Kataura Hiromichi , Ohtsuka Yohsuke , Ishigaki Toshinobu , Suzuki Shinzo , Achiba Yohji
Published: April 2011

Gas dynamic and time resolved imaging studies have been performed on the growth of single-walled carbon nanotubes (SWNTs) in the laser ablation process. SWNTs were synthesized by laser ablation of Ni-Co catalyzed graphite targets at 1200°C under argon flow. The effects of the temperature gradient near the target and the gas flow rate were studied in order to understand the effect of gas dynamics over the diameter distribution of SWNTs. The gas flow rate affects the diameter distribution of SWNTs especially when the growth species flow through a large temperature gradient. Scattering images from the growth species at different flow rates was recorded by high-speed video imaging. The results indicate that the velocities of these species are dependent on the gas flow rate but this dependence is evident 30 ms after the laser ablation. These findings are used to estimate the time period for the nucleation and the growth of SWNTs.
Multiwall Carbon Nanotubes by Hydrothermal Treatment
Authors: Calderon-Moreno Jose M. , Yoshimura Masahiro
Published: April 2011

Multiwall carbon nanotubes have been obtained from carbon soot after hydrothermal treatment at 800°C and 100 MPa. High-resolution electron microscopy (HRTEM) study reveals multiwall carbon nanotubes and carbon nanoparticles made of a hollow core enclosed in well-ordered concentric graphitic layers after hydrothermal treatment. Condensed solid products are free of the amorphous phase. Micro-Raman spectroscopy reveals that the hydrothermal multiwall nanotubes have a characteristic perfectly closed graphitic lattice in the basal plane, without edges or plane terminations.
Carbon Nanotube/Polyelectrolyte Composites as Novel Actuator Materials
Authors: Chattopadhyay Debjit , Galeska Izabela , Baughman Ray , Zakhidov Anwar , Papadimitrakopoulos Fotios
Published: April 2011

Mechanical actuators that simultaneously provide high power densities and large force generation capacities are of great scientific and technological interest. Recently single wall carbon nanotube (SWNT) papers (“bucky” papers) were shown to possess significant promise as electrochemical actuators. Embedding polyelectrolytes, like Nafion™, within the nanotube matrix has the potential to address the limitations of SWNT bundling and tube slippage thus increasing force generation. In this paper two types of Nafion/SWNT composite actuators have been investigated depending on the method of fabrication. In the first case, infiltration of Nafion within SWNT paper matrix was followed by annealing at 150°C to invert Nafion's micellar structure and render it insoluble. This has resulted in a substantially exfoliated layer morphology that causes a reduction in both conductivity and actuation strain (c.a. 0.03%). In the second case, slow casting of a methanolic suspension of Nafion and SWNT soot, followed by annealing at 150°C, resulted in a more homogeneous structure. This composite, upon electrochemical cycling between -1 and +1 V in aqueous electrolytes, exhibited actuation strains (as high as 0.43%). However, these higher strains are accompanied by an order of magnitude reduction in modulus largely due to Nafion swelling.
NMR chemical shift of single-wall carbon nanotubes
Authors: Latil Sylvain , Goze Christopher , Bac Goze , Bernier Patrick , Henrard Luc , Rubio Angel
Published: April 2011

We report calculation of the NMR chemical shift anisotropy (CSA) tensor →μ of single-wall carbon nanotubes, within the London approximation (ring currents contribution). Our results indicate that the isotropic line as measured by high resoultion experiments is splited about 11ppm between metallic and semiconductor nanotubes. We carefully check that this result remains vaild and observable when the bundle packing is taken into account. The resulting broadening is aroune 20ppm, but reduces onto a sharp lorentzian (<1ppm) when averaging by high resoultion NMR measurements.
Adsorption of Hydrogen on Carbon Single-Walled Nanotubes as Measured by the Volumetric Technique
Authors: Parilla Philip A. , Dillon Anne C. , Gennett Thomas , Alleman Jeff L. , Jones Kim M. , Heben Michael J.
Published: April 2011

We report on hydrogen adsorption onto carbon single-walled nanotube (SWNT) material as measured by the volumetric technique. This method determines the amount adsorbed by monitoring the pressure in a known volume at a known temperature with a given number of moles of hydrogen introduced. The raw material for the samples is produced by pulsed laser vaporization of a graphite target containing Ni (0.6 at%) and Co (0.6 at%) dopants. This is followed with a purification procedure and then by a cutting procedure. Finally, the sample is degassed before the hydrogen adsorption is measured. The cutting procedure introduces a titanium alloy which participates in the overall hydrogen adsorption of the SWNT material. Importantly though, the amount of hydrogen adsorbed in the SWNT material cannot be explained by absorption from the alloy alone. A description of the apparatus that has allowed us to measure these samples is given and we discuss crucial experimental procedures needed to activate the SWNT samples for hydrogen adsorption.
Defect energetics, thermal stability and localized electronic states in carbon nanotubes
Authors: Conversano Roberto , Cleri Fabrizio , D'Agostino Gregorio , Rosato Vittorio , Volpe Manuela
Published: April 2011

Tight Binding molecular dynamics simulations have been performed on single wall carbon nanotubes, in order to evaluate thermal stability and the effect of the most relevant defects (the single vacancy and a Stone-Wales -SW- defect). The nanotubes are stable up to the graphite instability temperature. Both the considered defects have a large formation energy (EF(vac)=6.10 eV, EF(SW)= 5.55 eV).
A Soft Lithographic Approach to the Fabrication of Single Crystalline Silicon Nanostructures with Well-Defined Dimensions and Shapes
Authors: Yin Yadong , Gates Byron , Xia Younan
Published: April 2011

A procedure was developed for large-scale fabrication of nanometer-sized structures of single crystalline silicon with well-defined dimensions and shapes. Near-field optical lithography was used to define the nanostructures in a thin film of positive-tone photoresist with an elastomeric phase mask. The nanostructures were then transferred into the underlying silicon-on-insulator (SOI) substrate through a reactive ion etching (RIE) process. With this method, we can routinely generate silicon nanostructures ∼130 nm in lateral dimension. They can be supported on the surface of a solid substrate as a patterned array, or released into a freestanding form. The lateral dimension of these silicon structures could be further reduced to as small as ∼40 nm using stress-limited oxidation at elevated temperatures. The flexibility of this approach was demonstrated by fabricating nanoscale wires, rods, rings, and interconnected triangles of silicon. Using a two-step exposure method, the silicon nanowires can be precisely “cut” into silicon nanorods with specific lengths.
Electrochemical Synthesis of Multi-Material Nanowires as Building Blocks for Functional Nanostructures
Authors: Pena David J. , Razavi Baharak , Smith Peter A. , Mbindyo Jeremiah K. , Natan Michael J. , Mayer Theresa S. , Mallouk Thomas E. , Keating Christine D.
Published: April 2011

Nanostructures are electrochemically deposited into alumina or polycarbonate templates resulting in monodisperse, anisotropic particles with a range of tunable sizes. These particles have been synthesized with diameters of 20–250 nm and with lengths of 1–10 μm. Currently, structures have been made with stripes of Au, Ag, CdSe, Co, Cu, Ni, Pd, and Pt. These materials offer a variety of different properties. In particular, many of the metals in this group are excellent conductors, meaning these particles can actually be used as nanowires. Co and Ni are ferromagnetic and may be used for separation or assembly. CdSe is a semiconductor, possibly allowing for the synthesis of electronic devices such as transistors. Furthermore, many of these materials have different surface chemistries, making the orthogonal functionalization and assembly of these nanowires more accessible. This research focuses on increasing the number of materials available, especially semiconductors, incorporating these potentially useful materials into multilayered nanowires and evaluating their electrical properties, either individually or in small bundles. In addition, the surface chemistry of the various materials in the nanowires is being compared to aid in orthogonal self-assembly of functional nanostructures such as memory devices. The work presented will demonstrate the effects of rod composition on electrical properties. In particular, the effects of changing the work function of the materials on either side of a semiconductor to form Schottky junctions or ohmic contacts will be shown.
Surface Potentials of Conjugated Molecules on Metal Surfaces: Measurements Using Electrostatic Force Microscopy and Calculations using a Preliminary Physically-Based Model.
Authors: Howell S. , Kuila D. , Ghosh A. W. , Rakshit T. , McNally H. , Janes D. , Datta S. , Kashibhatla B. , Kubiak C. , Reifenberger R.
Published: April 2011

The electrostatic surface potential of self-assembled monolayers (SAMs) of aliphatic and aromatic thiols has been measured using electrostatic force microscopy. The variation of the surface potential of chemisorbed alkanethiols, with respect to bare Au(111), is observed to increase with increasing chain length. The trend is similar to that observed in the literature. A preliminary theoretical model, based on treating the monolayer as a sheet of dipoles, has been used to calculate the surface potential of alkanethiols. Similar measurements on several aromatic thiols, with a symmetric and non-symmetric molecular structure, reveal that non-symmetric systems have significantly higher potential (≥ 170mV) than the symmetric molecules.
A Potential Interconnection Method in Molecular Electronics
Authors: Tao Meng
Published: April 2011

A method to electrically connect molecular devices is proposed, which has the potential to develop into an interconnection technology for 3-dimensional molecular electronic circuits. The method is based on electric-bias-induced polarization and electric-pulse-induced chemical reactions. Two molecules to be connected are oppositely biased to induce opposite charges in them. The opposite charges will create electrostatic attraction that pulls together or aligns the two molecules. An electric pulse is then applied across the two molecules to trigger a chemical linking reaction between them. The electric pulse overcomes the activation energy for such a reaction. Chemical linking reactions to produce conjugated molecular chains are proposed for several conjugated molecules, such as phenylene, ethylene, and acetylene based molecules, with different end groups, such as phenyl and acetyl groups. Applications of this method in assembling 3-terminal molecular devices and 3-dimesional molecular electronic circuits are speculated. Major challenges in realizing this interconnection method are also outlined.
Properties of Silicon Nanoparticles Produced by Picosecond Pulsed Laser Ablation
Authors: Wu M. H. , Mu R. , Ueda A. , Henderson D. O.
Published: April 2011

Silicon nanocrystals have been fabricated by picosecond pulsed laser ablation. Size control over the nanocrystals can be achieved by careful selection of experimental geometry, laser fluence, laser wavelength, backing gas pressure and distance from plume center. Measurements of optical absorption and photoluminescence confirm that particle size variation does significantly affect bandgap and emission efficiency. Differences between results published previously for nanosecond pulsed laser ablation will also be discussed.
Resonant excitation of Er3+ by the energy transfer from Si nanocrystals
Authors: Watanabe Kei , Fujii Minoru , Hayashi Shinji
Published: April 2011

Photoluminescence (PL) of SiO2 films co-doped with Si nanocrystals (nc-Si) and Er was studied. The average size of nc-Si was changed in a wide range in order to tune the exciton energy of nc-Si to the energy separations between the discrete electronic states of Er3+. PL from exciton recombination in nc-Si and the intra-4f shell transition of Er3+ were observed simultaneously. At low temperatures, periodic features were observed in the PL spectrum of nc-Si. The period agreed well with the optical phonon energy of Si. The appearance of the phonon structures implies that nc-Si which satisfy the energy conservation rule during the energy transfer process can resonantly excite Er3+. The effects of the quantum confinement of excitons in nc-Si on the energy transfer process are discussed.
Silicon Nanocrystallite Light Emitting Devices Fabricated by Full Pulsed-Laser-Ablation Process
Authors: Yamada Yuka , Makino Toshiharu , Suzuki Nobuyasu , Yoshida Takehito
Published: April 2011

We have developed silicon (Si) nanocrystallite light-emitting devices synthesized by a novel integrated process in which a size-controlling unit of differential mobility analyzer (DMA) is combined to a nanocrystallite formation unit of pulsed laser ablation (PLA). The size-controlled Si nanocrystallites as active layers have been deposited on Si substrates, and have been covered with stoichiometric indium oxide (In2O3) thin films synthesized also by the PLA process. The electroluminescence (EL) spectra had a narrow bandwidth of 0.15 eV peaked at slightly higher energy region (1.17 eV) than the bulk Si energy gap (1.10 eV), at room temperature.
Stabilization of Porous Silicon Electroluminescence by Surface Capping With Silicon Dioxide Films
Authors: Koshida N. , Kadokura J. , Takahashi M. , Imai K.
Published: April 2011

To prolong the operation life of electroluminescence (EL) from porous silicon (PS) diodes without affect on the efficiency, the surface passivation technique is applied to the active PS layer. By combining a postanodization electrochemical oxidation (ECO) technique for the PS layer with a surface capping by SiO2 films sputtered using electron cyclotron resonance (ECR) method, the degradation of the EL efficiency is effectively suppressed for a long time over several hours under a cw operation. It is shown that from thermal desorption spectra analyses, the capability of capped films as a barrier against penetration of water molecules is a key factor for stabilizing the EL operation due to preventing luminescent PS layers from current-induced oxidation.
Charging Effect in Amorphous Silicon Quantum Dots Embedded in Silicon Nitride
Authors: Park Nae-Man , Jeon Sang-Hun , Hwang Hyunsang , Choi Suk-Ho , Park Seong-Ju
Published: April 2011

Capacitance-voltage was investigated for amorphous silicon quantum dots (a-Si QDs) embedded in a silicon nitride as a function of dot size and nitride thickness. a-Sci QDs were grown by plasma enhanced chemical vapor deposition. The electron charging was decreased as the dot size was decreased. These results showed that the conduction band shift is larger than the valence band shift as the dot size decreased and, as a result, electrons are easily discharged in a-Si QDs due to the lower barrier height. For high dot-density-sample, the capacitance-voltage curves were also shifted toward the negative voltage direction when a higher forward bias was applied at forward condition due to the transfer of electrons trapped in the a-Sci QDs from the a-Sci QDs near Si substrate to those near the top metal.
Photoluminescence from nano porous silicon prepared by photoelectrochemical etching of n-type single crystalline silicon
Authors: Lee Chi-Woo , Kim Buem-Suck , Kim Dong-Il , Min Nam-Ki , Hong Suk-In
Published: April 2011

The influence of operating parameters in producing light-emitting porous silicon materials was investigated in ethanolic solutions of hydrofluoric acid. Photoluminescence spectra depended on applied potential, the intensity and wavelength of illumination, and electrolyte concentration. When the applied potential and the illumination wavelength increased, the photoluminescence shifted to longer wavelength. Change in HF concentration resulted in different intensity in photoluminescence.
Atomic force microscopy and Raman spectroscopy of nanoscale Si/SiO2superlattices.
Authors: Krishnan R. , Grom G.F. , Fauchet P.M. , Tsybeskov L. , Papernov S. , Sproule G.I , Lockwood D.J.
Published: April 2011

An atomic force microscope (AFM) has been used to optimize the radio frequency (rf)/ radio frequency magnetron sputtering deposition conditions for obtaining atomically smooth, hydrogen free amorphous silicon (a-Si) and silicon dioxide (a-SiO2) thin films. Superlattices composed of periodically repeating units of nanoscale (a-Si/a-SiO2) units were fabricated at these optimized conditions and subsequently crystallized. The amorphous and crystallized superlattices were characterized by AFM and Raman spectroscopy. Raman spectroscopy of the superlattices was performed by enhancing the weak scattered signal and eliminating the silicon substrate signal by using either waveguiding, cross polarization or interferometric enhancement techniques. The enhanced Raman spectrum clearly indicates formation of nanocrystals after crystallization.
Growth and Investigation of GaN/AlN Quantum Dots
Authors: Morkoç Hadis , Reshchikov Michael A. , Jones Keith M. , Yun Feng , Visconti Paolo , Nathan Marshall I. , Molnar Richard J.
Published: April 2011

We have fabricated GaN quantum dots (QDs) in AlN confined layer structures by molecular beam epitaxy. The size distribution and density of the QDs have been estimated from an atomic force microscopy study. Very high quantum efficiency of photoluminescence (PL) has been obtained in some samples with QDs. Compared to the GaN bulk samples, it increased by orders of magnitude. In some samples the quantum size effect dominated, resulting in the blue-shift of the QD related PL peak, whereas in the samples with larger dots a red-shift up to 0.8 eV has been observed, which is related to strong polarization effects. We have observed a blue-shift of the PL peak with excitation intensity in the samples with large dots due to screening effect. The temperature-induced quenching of PL occurs at higher temperatures compared to bulk GaN due to the confinement of nonequilibrium carriers in the QDs. An excited state has been observed in some samples.
Investigation of Sidewall Recombination in GaN Using a Quantum Well Probe
Authors: Haberer E. D. , Woods M. , Stonas A. , Chen C-H. , Keller S. , Hansen M. , Mishra U. , Denbaars S. , Bowers J. , Hu E. L.
Published: April 2011

In this study, we use a quantum well (QW) probe structure to explore the size dependent effects of sidewall recombination in GaN. Mesas 0.8-7 μm in width with pitches of 4 μm, 8 μm, and 12 μm were etched into the QW probe structure, exposing the QW at the sidewalls. Several etch conditions were investigated. Room temperature photoluminescence (PL) measurements, using a He-Cd laser as an excitation source and laser spot size of approximately 230 μm, were taken before and after the mesas were etched. The effects of sidewall formation were quantified by comparing the maximum PL intensity of the QW before and after etch. Higher remaining PL intensity was observed for etch conditions which used both Ar ions and Cl2 gas instead of only Ar ions. The fraction of remaining PL decreased with decreasing mesa width, however the remaining PL intensity was relatively large even for small features. The preliminary data suggested that GaN is relatively insensitive to sidewall damage.
Self-Aligned Process For Emitter- And Base- Regrowth GaN HBTs And BJTs
Authors: Lee K. P. , Zhang A. P. , Dang G. , Ren F. , Han J. , Chu S.N.G. , Hobson W. S. , Lopata J. , Abernathy C. R. , Pearton S. J. , Lee J. W.
Published: April 2011

The development of a self-aligned fabrication process for small emitter contact area (2×4 um2) GaN/AlGaN heterojunction bipolar transistors and GaN bipolar junction transistors is described. The process features dielectric-spacer sidewalls, low damage dry etching and selected-area regrowth of p-GaAs(C) on the base contact or n-GaN/AlGaN on the emitter contact. Series resistance effects are still found to influence the device performance.
High Efficiency Uv Emitter Using High Quality GaN/AlxGa1−xN Multi-Quantum Well Active Layer
Authors: Iwaya M. , Terao S. , Ukai T. , Nakamura R. , Kamiyama S. , Amano H. , Akasaki I.
Published: April 2011

We investigated temperature dependence of the photoluminescence (PL) efficiency of GaN/Al0.08Ga0.92N multi-quantum wells (MQWs) with variations of Si-doping condition and threading dislocation density. Si-doping in the GaN/Al0.08Ga0.92N MQWs, especially in the barrier layer improves the PL efficiency. In addition, reduction of threading dislocation density also improves the PL intensity. The PL intensity of the GaN/Al0.08Ga0.92N MQW is drastically increased at least by a factor of 40, by a combination of the Si-doping and reduction of threading dislocation density. We fabricated a light emitting diode (LED) emitting at 357 nm using such a GaN/Al0.08Ga0.92N MQWs. Electro-luminescence intensity from the region with threading dislocation density of less than 108 cm−2 was much larger than that from the region with threading dislocation density of 6×109 cm−2.
Optical Study of GaN Doped with Mn Grown by Metal Organic Vapor Phase Epitaxy
Authors: Korotkov R. Y. , Gregie J. M. , Wessels B. W.
Published: April 2011

The optical properties of Mn-doped GaN were investigated. The films were grown by metalorganic vapor phase epitaxy using tricarbonyl (methylcyclopentadienyl) manganese as the dopant source. Two characteristic bands were observed in the absorption spectra of Mn-doped epilayers. The low energy band had a threshold at 1.4 ± 0.05 eV with a maximum at 1.5 ± 0.02 eV, with a full width half maximum of 245 ± 10 meV at 296 K. A second higher energy band was observed as a shoulder to the band edge absorption with a threshold energy of 2.06 eV at room temperature. Using photoluminescence spectroscopy, a new broad band was observed in the infrared spectra of GaN:Mn at 1.27 ± 0.02 eV with a full width half maximum of 0.26 ± 0.01 eV at 20K. From analysis of optical absorption and emission spectra Mn forms a deep acceptor level with optical transitions at 1.4 and 2.06 eV. The deep level nature of Mn indicates that it is a potential dopant for semi-insulating GaN.
Cross-sectional Cathodoluminescence Study in Ga-polar and N-polar GaN Epilayers
Authors: Du X. L. , Lim D. H. , Xu K. , Liu B. L. , Jia A. W. , Takahashi K. , Yoshikawa A.
Published: April 2011

Highly spatial resolved cross-sectional cathodoluminescence (CL) has been used to study the difference in the defect-formation and growth-mechanism between Ga-polar and N-polar GaN epitaxial layers. These epilayers were grown on sapphire substrates by low pressure MOCVD. Their polarities were controlled by the sapphire nitridation and the trimethylaluminum (TMAl) pre-flow just before the conventional two-step growth, and were confirmed with the coaxial impact collision ion scattering spectroscopy (CAICISS). For the Ga-polar GaN epilayers, cross-sectional CL images show distinct two layers, the upper layer with dark lines along c-direction and the layer close to the film-substrate interface with pyramidal dark regions. These two layers correspond to quasi 2D and 3D growth mode respectively. Whereas, the crosssectional CL image taken on the N-polar epilayers is predominated with small dark spots which were randomly distributed throughout the whole GaN epilayer, illustrating the predominant island growth mode in N-polar films.
Comparison of Er3+ Photoluminescence and Photoluminescence Excitation Spectroscopy in In-situ-doped GaN:Er and Er-implanted GaN
Authors: Mitofsky Andrea M. , Papen George C. , Bishop Stephen G. , Lee Dong-Seon , Steckl Andrew J.
Published: April 2011

Site-selective photolumiescence (PL) and photoluminescence excitation (PLE) spectroscopies have been carried out at 6 K on the ∼1540nm 41/32 → 4I1/52 Er3+ transition in in-situ-doped GaN:Er which have detected nine differenct Er3+ sites and associated PL spectra. Three distinct Er3+ sites are identified in the in-situ-doped samples. For the in-situ-doped samples, the concentration of the various Er+3 sites are comparable while for the ion-implanted sample, the concertration of one Er3+ site was higher than the concentration of the other sites. In-situ-doped samples grown with different Ex-cell temperatures were considered, and the width of the PLE spectrum appears to be a function of the Er-cell temperature.
In situ Pyrometric Interferometry For Molecular Beam Epitaxy of AlxGa1−xN on Si (111)
Authors: Nikishin S. A. , Francoeur S. , Temkin H.
Published: April 2011

We apply pyrometric interferometery to control the growth of GaN, AlN, and AlxGa1−xN on Si (111) by molecular beam epitaxy. We show that the period of intensity oscillations in the temperature measured by the pyrometer is caused by optical interference in the growing nitride layer. The period of oscillations is inversely proportional to the growth rate. This effect is used for in-situ measurement of the growth rate of GaN, AlN, and AlxGa1−xN (for 0<×<1). We also show that pyrometric interferometery can be used for in-situ determination of the composition of the growing layer.
Two-dimensional electron gas scattering mechanisms in AlGaN/GaN heterostructures
Authors: Borovitskaya E. , Knap W. , Shur M. S. , Gaska R. , Frayssinet E. , Lorenzini P. , Grandjen N. , Baumont B. , Massies J. , Skierbiszewski C. , Prystawko P. , Leszczynski M. , Grzegory I. , Porowski S.
Published: April 2011

We present the results of the experimental and theoretical studies of the low field mobility of two-dimensional electrons in the homoepitaxial AlGaN/GaN heterostructures and in the AlGaN/GaN heterostructures grown on SiC. We show that, at cryogenic temperatures, the temperature dependence of the mobility is primarily determined by the deformation potential scattering and that most of other important scattering mechanisms are temperature independent. We show also that two-dimensional (2D) and three-dimensional (3D) mobility models yield very close results. The analysis of the mobility dependence on the electron sheet density ns shows two possible explanations of the non monotonic mobility versus carrier density dependence: i) the alloy/interface scattering and ii) transfer of the 2D electrons into 3D states in GaN. We present experimental data suggesting that for high 2D gas densities in the investigated structure grown on SiC the 2D-3D transition should take place and might be responsible for the mobility decrease at high electron sheet densities.