bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural and Optical Properties of Al2O3 with Si and Ge Nanocrystals
Authors: Yerci Selcuk , Yildiz Ilker , Seyhan Ayse , Kulakci Mustafa , Serincan Ugur , Shandalov Michael , Golan Yuval , Turan Rasit
Published: April 2011

Si and Ge nanocrystals were formed in alumina matrix by ion implantation and subsequent annealing. The phase separation of the Si nanocrystals was observed using X-ray photoelectron spectroscopy by monitoring Si 2p electrons. During nanocrystal formation with a high temperature annealing Si0 signals corresponding to Si nanoclusters increases while Si4+ signals related to a-SiO2 disappears from the spectrum. The transition from amorphous to nanocrystalline phase for both Si and Ge nanoclusters and the compressive stress exerted on the formed nanocrystals were also studied by Raman spectroscopy. Photoluminescence spectra of the Al2O3 containing nanocrystals were discussed by means of Ti and Cr impurities, as well as F centers. The existence of the amorphous Ge nanoclusters in alumina matrix significantly enhances the light emission of Ti3+ impurities.
DMC Study of the Optoelectronic Properties of Diamondoids
Authors: Drummond Neil , Williamson Andrew , Needs Richard , Galli Giulia
Published: April 2011

We have performed density-functional theory (DFT) and quantum Monte Carlo (QMC) calculations of the optoelectronic properties of hydrogen-terminated carbon nanoparticles (diamondoids). Both the QMC and DFT results show that quantum confinement effects disappear in diamondoids larger than 1 nm in diameter, which have optical gaps below that of bulk diamond. Furthermore, our QMC calculations predict a negative electron affinity for diamondoids up to 1 nm in diameter. Both these properties result from the delocalized nature of the lowest unoccupied molecular orbital.
Ni-Fe Competition in the Catalytic Growth of Carbon Nanotubes
Authors: Mordkovich Vladimir. Z. , Kharitonov D. N. , Maslov I. A. , Mitberg E. B.
Published: April 2011

The paper presents the results of competitive catalysis investigation of the carbon nanotube growth in situ of the partial oxidation process of methane. The competition between Ni and Fe results in suppression of Ni catalytic activity and the growth of Fe-capped carbon nanotubes. The discrimination is so strong that iron is segregated from Ni-Fe based stainless steel alloy leaving characteristic Ni-enriched corrosion caverns. The process strongly depends on temperature. Depending on particular catalyst bed composition, the nanotubes of various morphology may occur. In particular, the use of perovskite-type catalyst leads to formation of “olive-branch”-like peculiar carbon nanostructures.
Theoretical Analysis of Non-Catalytic Growth of Nanorods on a Substrate
Authors: Kwon S. Joon , Park Jae-Gwan
Published: April 2011

A theoretical analysis explaining non-catalytic growth of one-dimensional (1D) nanorods on a substrate is presented. The nuclei undergo cluster migration which continues until the mean free time of the adatoms is larger than surface diffusion time during several consecutive nuclei growth steps. The most probable mechanism is the migration of six adatoms into one fixed adatom. After the cluster migration, the nuclei grow in an isotropic manner, until the nucleus reaches the size limit. The 1D growth of nanorods on the nuclei begins when the reactant dose is smaller than a certain value. The growth rate of the height is greater than that of the radius. This difference in the growth rate causes the aspect ratio to increase with growth time. The presented analysis explains well the experimental results of the non-catalytic growth of nanorods.
A Facile Solution Method to Synthesis of C60 Nanorods
Authors: Liu Bing bing , Wang Lin , Zou Guang tian , Iwasiewicz A. , Sundqvist Bertil
Published: April 2011

We report here a facile solution method to synthesis of C60 nanorods. A very simple and effective solvent, neat m-xylene, from which C60 can rapidly grow into individual novel nanorods with widths and thicknesses of the order of nanometers was found as a shape controller. These unusual nanorods can easily grow on various substrates such as glass and silicon, and nanorods with different diameters and length-to-diameter ratios can be synthesized under different growth conditions. The smallest nanorods we obtained have widths smaller than 30 nm. The nanorods obtained are highly crystalline and single phase.
Thermal Transport in MWNT Sheet: Extremely High Radiation From The Carbon Nanotube Surface
Authors: Aliev Ali E. , Guthy C. , Zhang M. , Zakhidov A. A. , Fischer J. E. , Baughman R. H.
Published: April 2011

Laser flash and self-heating 3ω techniques were employed to determine the anisotropic thermal conductivity and thermal diffusivity of highly oriented free standing multiwalled carbon nanotube (MWNT) sheet drawn from a sidewall of a MWNT forest that was grown by chemical-vapor deposition. The thermal conductivity and the thermal diffusivity along the alignment are 50±5 W/m·K and 45±5 mm2/s, respectively, and are mostly limited by intrinsic defects of individual nanotubes and phonon-phonon interaction within bundles which form the supporting matrix of the MWNT sheet. The long tube-tube overlapping substantially decreases the electrical and thermal interconnection resistances which are usually dominate in randomly deposited mat-like nanotube assemblies. The extremely large surface area of the MWNT sheet leads to excessive heat radiation that dose not allow to transfer the heat energy by means of phonons to distances > 2 mm.
Preparation and Characterization of NPC/SWNT Nanocomposite
Authors: Yi Bo , Rajagopalan Ramakrishnan , Foley Henry C
Published: April 2011

A nanocomposite carbon composed with single wall carbon nanotube (SWNT) and nanoporous carbon (NPC) was prepared by grafting a carbonizable polymer, poly(furfuryl alcohol) (PFA) to a SWNT. The SWNT was first functionalized with arylsulfonic acid groups on sidewall (SA-SWNT) and then converted to PFA-functionalized SWNT (PFA-SWNT) by in situ polymerization of furfuryl alcohol (FA). SWNT/NPC nanocomposite carbon was generated by heating PFA-SWNT in argon at 600°C. A continuous phase was formed between SWNT and NPC. The deformation of the nanocomposite carbon at high temperature was studied by heating it at temperatures from 1200 to 2000 °C and characterized with HRTEM and Raman spectra. It was found that NPC tended to graphitize along the axis of neighboring nanotubes at temperature higher than 1400°C. Complete graphitization of NPC and SWNTs was obtained at 2000 °C, when the NPC transformed to graphitic nanoribbon (GNR) and SWNT or DWNT collapsed within the confines of the GNR. Nanocomposite polymer and carbon fibers were prepared by dispersing small amount of SA-SWNT in FA, followed with polymerization, thermosetting and pyrolysis. The composite polymer fibers' Young's modulus was lower than the pure PFA fibers prepared at the same conditions. However, after heated in argon at 300 °C, 400 °C, 500 °C and 600 °C respectively, the Young's modulus of the nanocomposite carbon fibers turned to be higher than the fibers derived from pure PFA and the enhancement by SWNT increase with increasing temperature. For the nanocomposite carbon fibers treated at 600 °C, the fibers had ∼20% increase of Young's modulus over the fibers from pure PFA with only 0.1wt% of SA-SWNT in FA.
Effects of Hydrogen Post Treatment and Duty Factor on Carbon Nanotube Field Emission Stability
Authors: Wang Sigen , Chang Sha
Published: April 2011

Carbon nanotubes (CNTs) have been investigated as field emission electron sources for a number of applications such as x-ray source and microwave power amplifier. These applications often require that nanotubes provide a stable field emission at a high emission current. In this paper, we investigated the emission stability of CNTs as functions of hydrogen post-treatment and duty factor. The experimental results show that the hydrogen plasma treatment and a reduced duty cycle from 100% to 35% can noticeably improve the emission stability of CNTs at a high current value of 4 mA. Our study also shows that emission induced anode heating at high duty factor and high emission current level degrades vacuum level and thus the emission stability of CNTs.
Investigation of DNA Decorated Carbon Nanotube Chemical Sensors
Authors: Chen Michelle , Khamis S. M. , Johnson R. R. , Staii C. , Klein M. L. , Fischer J. E. , Johnson A. T.
Published: April 2011

We demonstrate a versatile class of nanoscale sensors based on single-stranded DNA (ss-DNA) as the chemical recognition site and single-walled carbon nanotube field effect transistors (swCN-FETs) as the electronic readout component. Coating swCN-FETs with ss-DNA causes a current change when exposed to gaseous analytes, whereas bare swCN-FETs show no detectable change. The responses differ in sign and magnitude depending both on the type of gaseous analyte and the sequence of DNA. Our results suggest that the conformation of ss-DNA on swCN-FET plays a role in determining the sensor response to gaseous analytes. The conformation depends not only on the base content of the oligomer, but also on the specific arrangement of the bases in the ss-DNA. We compare our results with the molecular dynamic simulations for understanding of the sensing mechanisms. SsDNA/swCN-FETs possess rapid recovery and self-regenerating ability, which could lead to realization of large arrays for sensitive electronic olfaction and disease diagnosis.
Reliability Study of a Fluorescent Blue Organic Light-Emitting Device
Authors: Lee Jiun-Haw , Ho Yu-Hsuan , Lin Tien-Chun , Wu Chia-Fang
Published: April 2011

In this paper, we measured and analyzed the operation lifetime of a high efficiency blue OLED which consists of N,N' –Vdiphenyl -N,N'-bis(1-napthyl) -1,1'-biphenyl-4,4'- diamine (NPB) as the hole-transport layer (HTL), 4,4'-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl (DPAVBi) doped in 9,10-bis(2';-naphthyl) anthracene (ADN) as the emitting layer (EML), and bis(10-hydroxyben-zo[h]quinolinato)beryllium (Bebq2) as the electron-transport layer (ETL). Due to the high electron mobility of the ETL (one order of magnitude higher than Alq3), the carrier balance is achieved and a blue OLED with a high external quantum efficiency of 8.32% is obtained. The device structure of our blue OLED device is ITO /HTL (40nm)/EML (45nm, 4% dopant)/ETL (15nm)/ LiF(1.2nm)/Al (100nm). In our operation lifetime measurement, we fixed the initial luminescence of the blue OLEDs at 12500, 10000, 7000, 5000 cd/m2 with a constant current driving. The resulting half-lifetime are 5.58, 16.56, 27, 109.819 hours, respectively. To estimate the half-lifetime of this device, we use a well-known relation in our fitting: L*t1/2n= constant where n is the acceleration coefficient, and t1/2 is the half-lifetime. In our blue OLED, the n value is 3.088. By using the equation, we can calculate that the estimated half lifetime at an initial luminance of 1000 cd/m2 achieves 15611 hours in our device. For further investigating the lifetime mechanism in our blue OLED, we fit all the luminance versus time curves obtained under different driving condition. We found that luminance is inversely proportional to the square of the time, rather than a typically stretched exponential decay which means the luminance decay is a second-order reaction in our blue OLED.
Fabrication of Organic Thin Film Transistors Using Low Temperature, Soluble Silicon Oxide as the Gate Dielectrics
Authors: Wei Jeng-Hua , Lin HorngJiunn , Chen Ying-Ren
Published: April 2011

In this paper, a unique water-based, liquid phase deposited silicon oxide (LPD SiO2) is adapted to the fabrication process of the organic thin film transistor (OTFT). Through the use of this process, an OTFT with a silicon oxide gate insulator is successfully fabricated at 100°C or less. At this low process temperature, the SiO2 functions efficiently as a gate dielectric with the breakdown field being larger than 5 MV/cm, the leakage current being near 1 pA/um2 with a gate bias of 20 V and the surface roughness being less than 1nm. Due to the high quality silicon oxide, the oxide-gated OTFT shows the low threshold voltage (-1 ∼ -2V) and medium on/off current ratio (∼1000). Because this oxide is a water-based process, it is highly resistant to the following soluble semiconductor material and its solvent.
Gate Dielectric Dependent on Pentacene Growth and Electrical Stability in OTFTs
Authors: Chang Su Kim , Sung Jin Jo , Sung Won Lee , Hong Koo Baik
Published: April 2011

We report on the effects of modifying the gate dielectrics by spin coating of HMDS, PVP and PVA which cause different surface energy and surface roughness owing to the different functional groups. In changing the surface state with applying various surface treatments, I-V and C-V measurements of the OTFTs were performed. The PVP-coated OTFTs, which has smoother and lesser amount of OH-groups on the gate dielectric surface, showed enhanced pentacene growth and nearly free hysteresis behavior than that of the HMDS and PVA-coated OTFTs.
Ab Initio Study of Switching Properties of Photochromic Dithienylethene Molecules
Authors: Odell Anders , Delin Anna
Published: April 2011

We report an ab initio study of a photochromic dithienylethene derivative. The mechanisms for switching between open and closed conformations are investigated based on total energy calculations for the ground and excited state. An explanation for the central ring closure based on relaxation of the excited state is presented.
Controlling the pKa for Protonic Doping of Polyaniline by Non-covalent Complexation
Authors: Wan Hui , Yang Sze C
Published: April 2011

We report a synthetic method for shifting the pKa for the deprotonaton of polyaniline by the formation of non-covalent complexes between polyaniline and a polyanion. Depending on the choice of the polyanion and the condition of the synthesis, we can control the pKa value for the dedoping reaction to spread the pH range from 5.5 to 9.0. From a systematic study of the structural influence on the pKa values, we have verified that the electrostatic stabilization of the polaron is the origin for this unusually large pKa shift.
Molecular-Scale Structure of Pentacene Interfaces with Si (111)
Authors: Seo Soonjoo , Evans Paul G.
Published: April 2011

The morphology and crystal structure of the first few molecular layers of organic semiconductor thin films at organic-inorganic interfaces are important from both electronic and structural perspectives. The first upright layer of pentacene on Si (111) forms on top of a disordered layer of strongly bonded pentacene molecules in a structure similar to the pentacene monolayers formed on insulators. We describe a high-resolution structural study of this crystalline phase of pentacene using low-temperature scanning tunneling microscopy (STM). The arrangement of molecules in these layers observed with STM agrees the results of with structural studies using scattering techniques. The imaging conditions and sample preparation techniques necessary to achieve molecular resolution can be adapted to subsequent STM and scanning tunneling spectroscopy experiments probing individual structural defects including vacancies, dislocations and grain boundaries within and between islands.
In-plane Structure and Polymorphism of Pentacene Thin Films
Authors: Kakudate Toshiyuki , Yoshimoto Noriyuki , Saito Yoshio
Published: April 2011

The structure and polymorphism of pentacene thin films on SiO2 substrate were investigated by grazing incidence x-ray diffractometry (GIXD). The in-plane GIXD patterns were successfully obtained from vacuum deposited ultra-thin films of a few monolayers thick. The two-dimensional lattice constants were determined for both thin-film and bulk phases from the observed GIXD patterns. Considering the obtained unit cell parameters, the mechanism of the transformation between polymorphs was discussed using the classical nucleation theory.
Electronic Structure of Ionic Liquids Studied by UV Photoemission and Inverse Photoemission Spectroscopy
Authors: Nishi Toshio , Kamizuru Yasunori , Kanai Kaname , Ouchi Yukio , Seki Kazuhiko
Published: April 2011

Electronic structures of the ionic liquids were studied by ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). The sample materials contain 1-buthyl-3-alkylimidazolium ion [Cnmim]+ (n=4, 8, 10) as the cation in combination with fluorine-containing anions (tetrafluoroboronate BF4−, hexafluorophosphate PF6−). Comparing the calculated density of states with the observed spectra, we found that the ionization thresholds of these ionic liquids are determined by the highest occupied molecular orbitals (HOMO) of the cation, although the calculated HOMOs of the isolated anions are higher than that of isolated cation. The combination of the UPS and IPES results reveals that the band gaps of these ionic liquids are determined by only cation.
The Effect of Ultraviolet Light Curing on the Material and Fracture Properties of a k∼2.5 Low-k Dielectric
Authors: Smith Ryan S. , Tsui Ting , Ho Paul S.
Published: April 2011

In this paper, we investigated the role of ultra-violet light (UV) curing on the molecular structure and fracture properties of a porous organosilicate glass (OSG) dielectric with k∼2.5. A correlation between material properties, including fracture toughness, and molecular structure was found. Multiple analytical techniques including Fourier Transform IR (FTIR) Spectroscopy, x-ray photo-emission spectroscopy (XPS) and x-ray reflectivity were used to investigate changes in the molecular bonding and material properties with UV curing. Overall, UV seems to be highly penetrating leading to uniform composition and density changes in the low k film to improve its properties. The decrease of methyl and hydrogen content relative to the Si-O-Si bonding structures led to increasing density and dielectric constant with UV exposure. UV curing increases the critical fracture toughness, while the sub-critical fracture toughness is insensitive to UV cure. The correlation between critical fracture toughness and material properties is discussed.
The Impact of Dielectric Films and Post-Metal Etch Wet Treatment on Charge-Induced Corrosion of Tungsten Vias
Authors: Lee Szetsen , Ni Chi-Jung
Published: April 2011

The prevention of charge-induced corrosion of tungsten vias after metal etch has been stud-ied with several types of commonly used wet chemical solutions and two kinds of dielectric film materials, silicon dioxide and silicon oxynitride. It was found that one of the solutions, leaving essentially no polymer residue on metal lines, could effectively prevent corrosion of tungsten vias. Other solutions either produced minor residues or severe sidewall erosion on metal lines. This study has shown that the combination of wet treatment with oxynitride as the dielectric charge shielding film was as effective as other conventional methods for preventing tungsten vias corrosion. However, for metal lines capped with silicon dioxide, significant sidewall ero-sion, surface roughness, and polymer residue were observed. Chemical reaction mechanisms are proposed for the preservation of tungsten vias after metal etch.
A Low Temperature Photonic Crystal Technology for Integration with Modern CMOS Technologies
Authors: Bayat Khadijeh , Farrokh Baroughi Mahdi , Chaudhuri Sujeet K. , Safavi-Naeini Safieddin
Published: April 2011

In this paper, low temperature amorphous silicon oxynitride (a-SixOyNz:H) thin film technology is proposed for implementation of CMOS compatible photonic crystal (PC) based optical integrated circuits (OICs). The a-SixOyNz films of different refractive indices were developed by plasma enhanced chemical vapor deposition (PECVD) technique using silane, nitrous oxide, and ammonia as gas phase precursors at 300°C. The films with refractive index between 1.43 − 1.75 were obtained by changing gas flow ratios. Such thin films can be used as cladding and core layers in photonic crystal structure.

The bandgap and guiding properties of the a-SixOyNz based PCs were simulated and was shown that the a-SixOyNz:H based PC technology offers larger feature sizes than a conventional silicon based photonic crystals.
Engineered Solder-Directed Self-Assembly Across Length Scales
Authors: Knuesel Robert , Bose Shameek , Zheng Wei , Jacobs Heiko O
Published: April 2011

We report on recent progress in the directed self-assembly of discrete inorganic semiconductor device components. Different from prior research, the goal is to enable the integration of increasingly small dies while supporting unique-angle orientation and contact pad registration. The process is based on the reduction of surface free energy between liquid solder coated areas on the substrate and metal-coated binding sites on the semiconductor dies. Recent advances include flip-chip assembly with unique angular orientation accomplished using “two-element” docking sites that contain pedestals that act as chaperones for the solder directed assembly to take place. The scale reduction to 20 μm sized components involves the use of a liquid-liquid interface to concentrate component delivery and speed up the self-assembly process to prevent oxidative dissolution of the solder sites prior to completion.
Improved Electromigration Lifetime for Copper Interconnects using Tantalum Implant
Authors: Gambino Jeff , Sullivan Timothy D. , Gill Jason , Chen Fen , Mongeon Steve , Adams Edward D. , Burnham Jay , Pokrinchak Phil , Rodbell Kenneth P.
Published: April 2011

In this study, a novel method is explored for improving the electromigration lifetime of Cu wires, using Ta implantation into Cu. For high implant doses (2E15 cm−2), the electromigration lifetime is improved by over 5X using this method. An increase in lifetime is achieved, even for an average surface concentration of Ta on the order of 0.1 atm%. We propose that the improvement in electromigration lifetime is due to the reduction of defects at the SiN/Cu interface due to the presence of Ta. The line-to-line leakage at high voltages (> 5V) increases with the Ta implant, with higher leakage at higher Ta concentrations, so the Ta dose must be limited to avoid excessive leakage.
Impact of Fabrication Process, Layout Variation and Packaging Process on Cu/Low-k Interconnect Reliability
Authors: Karmarkar Aditya , Xu Xiaopeng , Pramanik Dipu , Lin Xi-Wei , Rollins Greg , Lin Xiao
Published: April 2011

The industry trend towards smaller feature size and higher integration density leads to multi-level Cu/low-k interconnect schemes with reduced line width and spacing. Mechanical stress is generated during interconnect fabrication. The spatial distribution of the stress is strongly affected by the layout variation. The packaging process generates a global chip level stress that permeates to the local interconnect level. Stress related failures and yield loss are major areas of concern for Cu/low-k interconnects. The effects of fabrication process, layout variation, and packaging process on the final stress distributions in Cu/low-k interconnect structures are examined and the reliability impact of mechanical stress is assessed.
Metal-Organic Chemical Vapor Deposition (MOCVD) of GeSbTe-based Chalcogenide Thin Films
Authors: Tompa Gary S. , Sun Shangzhu , Rice Catherine E , Cuchiaro Joe , Dons Edwin
Published: April 2011

Chalcogenide Random Access Memory (C-RAM) has shown significant promise in combining the desired attributes of an ideal memory, including: nonvolatility, fast read/write/erase speed, low read/write/erase voltage/power, high endurance, and radiation hardness. Current C-RAM production technology relies on sputtering to deposit the active chalcogenide layer. The sputtering process leads to difficulties in meeting requirements for device conformality (in particular – filling vias), film adherence, compositional control, wafer yield, and surface damage. Ultimately, a viable CVD manufacturing process is needed for high-density products to realize the full potential of C-RAM. In this work, we discuss the Metal-Organic Chemical Vapor Deposition (MOCVD) tool technology used to produce the films and report the materials properties of GeSbTe-based chalcogenide thin films grown in small research scale and in large production scale MOCVD reactors. Films were grown at low pressures at temperatures ranging from 350 C to 600 C. X-Ray Fluorescence (XRF) and Auger Electron Spectroscopy (AES) were performed and determined that the film composition is controllable and uniform.
Energetics of Both Minority and Majority Carrier Transitions through Deep Defects in Wide Bandgap Pentenary Cu(In,Ga)(Se,S)2 Thin Film Solar Cells
Authors: Halverson Adam , Nishiwaki Shiro , Shafarman William , Cohen J. David
Published: April 2011

We report and electronic and optical characterization of three wide-bandgap alloys of the Cu(InxGa1-x)(SeyS1-y)2 pentenary material system. Devices were characterized using admittance spectroscopy as well as drive-level capacitance profiling. The devices showed activated defect behavior typical of thin-film solar cell devices. Optical characterizations were carried out with Transient Photocapacitance and Transient Photocurrent spectroscopies. These data showed broad exponential bandtails with large Urbach energies, indicative of a moderately high degree of compositional and/or structural disorder. The temperature dependence of the TPC spectra was examined in detail and we were able to observe the thermal emission of electrons from defects into the conduction band. The emission energy of these features corresponds well with the measured optical threshold and the known bandgap of the cells. Thus we infer an upper bound of about 50meV for the lattice relaxation energy following the optical transition into the defect.
Exploring Back Contact Technology to Increase CdS/CdTe Solar Cell Efficiency
Authors: Fahrenbruch Alan L.
Published: April 2011

The primary routes for increasing CdS/CdTe solar cell efficiency involve increasing free carrier density, reducing bulk and interface recombination, and/or reducing back contact barrier height. This paper focuses on the role of the back contact barrier in increasing cell efficiency. Measurement of barrier height and back surface recombination are outlined and three CdTe/MX/M back contact prototypes, each with particular strengths, are discussed to bring out important issues.
A Neutral Barrier at CGS Grain Boundaries - Compositional and Structural Dependencies
Authors: Hafemeister Michael , Siebentritt Susanne , Sadewasser Sascha , Frank-Rotsch Christiane , Lux-Steiner Martha Ch.
Published: April 2011

Unlike other photoactive layers (e.g. Si) polycrystalline chalcopyrite layers achieve higher efficiency than monocrystalline ones. Although grain boundaries may play an important role for this phenomenon, there is currently no commonly accepted model for the electronic structure of grain boundaries. First experimental results on CuGaSe2 absorbers indicated a small neutral barrier for majority carriers (20-40meV) at sigma 3 grain boundaries, which is the predominant grain boundary in polycrystalline chalcopyrite absorbers [1]. These results are in discrepancy with theory, which predicts a 10 times higher barrier; we suspect that the copper excess might reduce the barrier height.

Here we present a study using Hall-effect and Kelvin Probe Force Microscopy (KPFM) measurements to investigate the composition dependence of the barrier height of epitaxially grown CuGaSe2 layers containing a sigma 3 grain boundary as a function of the Cu/Ga ratio. First results show that the barrier height is independent of the copper content. In addition, we present initial results on sigma 9 grain boundaries which are observed to occur in high efficiency absorbers.
Electronic Defects and Device Performance in CuGaSe2 Solar Cells
Authors: Rembold J. Jedediah , Curtis Todd W. , Heath Jennifer T. , Young David L. , Johnston Steve W. , Shafarman William N.
Published: April 2011

The electronic and materials properties of two series of wide-bandgap solar cells with Cu-poor CuGaSe2 (CGS) absorbers have been studied, to better understand limitations on the device performance. One series of samples displayed distinct lateral non-uniformities in Cu/Ga ratio, Na content, and thickness, likely due to a limited supply of Se during CGS growth. The second series of samples appeared uniform. The most prominent electronic difference was the presence of a distinct band of near-interface defect states in the more non-uniform set of samples. The device performance did not appear to be limited by defects in the bulk CGS film until the defect density was larger than 2×1016 cm-3. Instead, interface recombination appears to be a significant factor limiting Voc in both sets of samples.
The Mechanism of J-V “Roll-Over” in CdS/CdTe Devices
Authors: Zhou Jie , Wu Xuanzhi , Yan Yanfa , Asher Sally , Da Silva Juarez , Wei Suhuai , Weinhardt Lothar , Bar Marcus , Heske Clemens
Published: April 2011

The “roll-over” phenomenon in current-voltage (J-V) curves of CdS/CdTe devices is recognized as a result of the formation of a higher back barrier. When Cu has not been intentionally added to the back contact, roll-over is understandable. However, the mechanism was unclear for forming J-V roll-over in a CdTe cell with a back contact containing Cu. We did extensive characterizations, including XRD, XPS, SIMS, TEM, and EDS, and “recontact” experiments to understand this phenomenon. The results show that the roll-over comes from the formation of Cu-related oxides at the back side of the device during processing, rather than the diffusion of Cu to the front side of the device. Discussions related to the J-V roll-over mechanisms will also be presented.
Pressure Dependent Rapid Thermal Processing of CuInS2 Thin Films Investigated by In-Situ Energy Dispersive X-ray Diffraction
Authors: Kötschau Immo Michael , Rodriguez-Alvarez Humberto , Streeck Cornelia , Weber Alfons , Klaus Manuela , Denks Ingwer Asmus , Gibmeier Jens , Genzel Christoph , Schock Hans-Werner
Published: April 2011

The rapid thermal processing (RTP) of Cu-rich Cu/In precursors for the synthesis of CuInS2 thin films is possible within a broad processing window regarding leading parameters like top temperature, heating rate, and Cu excess. The key reaction pathway for the CuInS2 phase formation has already been investigated by in-situ energy dispersive X-ray diffraction (EDXRD) for various precursor stoichiometries, heating rates and top temperatures at sulphur partial pressure conditions which are typical for physical vapour deposition processes. According to the phase diagrams of the binary sulphide phases, the sulfur partial pressure strongly determines the occuring crystalline phases. However, a controlled variation of the maximum sulphur partial in a typical RTP experiment has not been carried out yet. In order to study the influence of this parameter a special RTP reaction chamber was designed suitable for in-situ EDXRD experiments at the EDDI beamline at BESSY, Berlin. In a typical in-situ RTP/EDXRD experiment sulphur and a Cu/In/Mo/glass precursor are placed in an evacuated graphite reactor. The amount of sulphur determines the maximum pressure available at the top temperature of the experiment. As the RTP process proceeds a complete EDXRD spectrum is acquired every 10 seconds and thus the various stages of the reaction path and the crystalline phases can be monitored. The first experiments show already a significant change in the reaction pathway and the secondary Cu-S phases which segregate on top of the CuInS2 thin film during the reaction.
Synthesis and Sintering of Aluminium Nitride Nano-particles
Authors: HAN Zhao , YANG Mei , ZHU Hongmin
Published: April 2011

Aluminum nitride (AlN) nano-particles were synthesized from aluminum chloride by sodium reduction in liquid ammonia. A liquid solution of sodium dissolved in ammonia was employed as a reduction-nitridation agent, which enabled direct nitridation of aluminum chloride at −45°C. The synthesized particles were heat-treated at 1000°C in vacuum, and were characterized by X-ray diffraction (XRD), scan electron microscopy (SEM), transmission electron microscopy (TEM) and BET surface area measurement. The results indicated that the product were hexagonal wurtzite AlN particles, with a specific surface area of 262 m2g−1. Spark plasma sintering (SPS) process was used to consolidate the as-prepared AlN nanoparticles, and a dense AlN ceramic (>98.5%) with average grain size of 200-400nm was obtained at 1600°C without the use of sintering additives.
Preparation of AlN thin films by means of CVD using iodide source under atmospheric pressure
Authors: Iwane Hiroki , Wakiya Naoki , Sakamoto Naonori , Nakamura Takato , Suzuki Hisao
Published: April 2011

Epitaxial aluminum nitride (AlN) thin films were successfully prepared on the (0001) sapphire substrate by chemical vapor deposition (CVD) using aluminum iodide (AlI3) and ammonia (NH3) under atmospheric pressure at 750 ºC. The crystallographic relationship between AlN thin films and Al2O3 substrate is in the following; AlN(0001)//Al2O3(0001) and AlN[1010]//Al2O3[1120]. Lattice parameters of AlN thin film measured by X-ray diffraction revealed that c=0.498 and a=0.311 nm, respectively. Residual stress estimated by modified sin2ψ method was 0.38 GPa in compressive stress. Cross-sectional TEM observation revealed that an interlayer lies between the AlN films and the sapphire substrate. It was suggested that relaxation of residual stress caused by the mismatching of lattice parameter and thermal expansion coefficient was brought about by the interlayer.
Preparation of InN by Means of AP-HCVD Using In Buffer Layers
Authors: Yokoo Hiroaki , Wakiya Naoki , Sakamoto Naonori , Nakamura Takato , Suzuki Hisao
Published: April 2011

We have grown indium nitride (InN) films using In buffer layer on an a-plane sapphire substrate under atmospheric pressure by halide CVD (AP-HCVD). Growth was carried out by two steps: deposition In buffer layer at 900 °C and subsequent growth of InN layer at 650 °C. In order to compare, we also grown InN films on an a-plane sapphire. The InN films are investigated on crystal quality, surface morphology and electrical property using high-resolution X-ray diffraction (HR-XRD), X-ray pole figure, scanning electron microscope (SEM), Hall measurement. The results show that the crystal quality, surface morphology and electrical property of InN films are improved by using In buffer layer.
Predicting Efflorescence and Subflorescences of Salts
Authors: Espinosa Rosa , Franke Lutz , Deckelmann Gernod
Published: April 2011

Crystallization of salts is a common cause of damage in porous building materials. Understanding of the crystallization mechanism of salts is important in order to prevent or avoid the problem. Subflorescence of salts (i.e., crystallization within the pores of the body) can induce scaling and cracking, while efflorescence (i.e., crystallization in a film of solution on the exterior surface of the body) does not generally affect the coherence and endurance of the building materials.

In this paper, we deal with the crystallization behavior of two salts, sodium sulfate and sodium chloride, in two bricks with different capillary porosity. The results reveal quite different crystallization behavior depending on salt and substrate.

The supersaturation of the solution is induced in our experiments by evaporation. Indeed, the main reason for the different behavior of these salts is the different ability for supersaturation. Thus, the sodium sulfate solution is prone to be much more supersaturated than sodium chloride. Furthermore, the solution transport, which depends on salt properties, material porosity, pore-clogging and climatic conditions, affects the position of the drying front and, with it, the crystallization front leading to the formation either of efflorescence or of subflorescence. A simulation of the experiments helps us to understand the effect of the influencing factors on the crystallization pattern. Therefore, considering both factors, supersaturation ratio and solution transport, it is possible to predict the different crystallization behavior observed in the experiments.
Defect Healing of Carbon Nanotubes by Rapid Vacuum Arc Annealing
Authors: Tsai Jeff Tsung-Hui , Li Jason , Tseng Andy
Published: April 2011

A rapid thermal annealing process is demonstrated for healing the defects in carbon nanotubes using a DC vacuum arc discharge system. Multi-walled carbon nanotubes (MWCNTs) grown by chemical vapor deposition at a relatively low temperature (∼650 °C) showed structural imperfections inside the tubes which are known as "bamboo-like" defects. These defects can be thermally annealed to reconstruct the graphitic structure. A vacuum arc discharge system was used to generate high temperatures (∼1800 °C) followed by rapid cooling. The MWCNTs can be rapidly annealed in such a system by several heating and cooling cycles. The annealed samples were characterized by Raman spectroscopy and transmission electron microscopy. The defects were found to be healed when the environment contained water vapor, indicating that oxygen may play an important role in breaking the imperfect graphitic structure and removing the weakly bonded defects during the rapid heating cycles. After breaking the “bamboo” segment, the graphene shell was then reconstructed during the cooling process to produce multi-shell perfection. This method produces effective defect healing and bamboo structure removal from MWCNTs.
Growth of Single Crystalline ZnO Nanotubes and Nanosquids
Authors: Prasad Abhishek , Mensah Samuel , Wang Jiesheng , Pandey Archana , Yap Yoke Khin
Published: April 2011

The growth of ZnO nanotubes and nanosquids is obtained by conventional thermal chemical vapor deposition (CVD) without the use of catalysts or templates. Characterization of these ZnO nanostructures was conducted by X-ray powder diffraction (XRD), Field-emission scanning electron microscopy (FESEM), Raman spectroscopy, and photoluminescence (PL). Results indicate that these ZnO nanostructures maintain the crystalline structures of the bulk wurtzite ZnO crystals. Our results show that rapid cooling can be used to induce the formation of ZnO nanotubes and ZnO nanosquids. The self-assembly of these novel ZnO nanostructures are guided by the theory of nucleation and the vapor-solid crystal growth mechanism.
Dielectrophoretic Deposition of Carbon Nanotubes with Controllable Density and Alignment
Authors: Moscatello Jason , Kayastha Vijaya , Pandey Archana , Ulmen Benjamin , Yap Yoke Khin
Published: April 2011

Controlled deposition of carbon nanotubes (CNTs) across desired electrodes is important for the fabrication of nanoelectronic devices. Dieletrophoresis (DEP) has been recognized as a convenient and affordable technique for the deposition of nanotubes and nanowires on electrodes. Although DEP has been quite well studied for dielectric particles, the application for depositing nanotubes is still at the early stage of development. Here, we show that multi-walled CNTs can be deposited by DEP with controllable density and degree of alignment.
Real-Time Electrical Characterization of Dielectrophoretic Assembly of Multi-Walled Carbon Nanotubes
Authors: An Libao , Friedrich Craig
Published: April 2011

This paper reports on a real-time monitoring method for the assembly of a small number of metallic carbon nanotubes (CNTs) by dielectrophoresis (DEP). A time-varying impedance model of the electrode gap was developed to evaluate the number of CNTs which span the gap by measuring the simultaneous variation of gap impedance during the DEP process. Sudden decreases of gap impedance signals were detected during the DEP assembly of multi-walled carbon nanotubes (MWNTs) corresponding to assembly of single or multiple tubes across the gap. The method reduces the requirement of scanning electron microscopy (SEM) inspection and could help automate DEP assembly of CNTs.
Dependence of GaN Defect Structure on the Growth Temperature of the AlN Buffer Layer
Authors: Wong Yuen-Yee , Chang Edward Yi , Yang Tsung-Hsi , Chang Jet-Rung , Chen Yi-Cheng , Ku Jui-Tai
Published: April 2011

The defect structure of the GaN film grown on sapphire by plasma-assisted molecular beam epitaxy (PAMBE) technique was found to be dependent on the AlN buffer layer growth temperature. This buffer growth temperature controlled the defect density in GaN film but had shown contrary effects on the density of screw threading dislocation (TD) and edge TD. The density of screw TD was high on lower temperature buffer but low on the higher temperature buffer. Meanwhile the density of edge TD had shown the opposite. Further examinations have suggested that the defect structure was closely related to the stress in the GaN film, which can be controlled by the growth temperature of the AlN buffer. Using the 525°C AlN buffer, optimum quality GaN film with relatively low screw and edge TDs were achieved.
An Analytical Compact Direct-Current and Capacitance Model for AlGaN/GaN High Electron Mobility Transistors
Authors: Li Miao , Cheng Xiaoxu , Wang Yan
Published: April 2011

We develop an analytical model for the Direct-Current (DC) and capacitance-voltage (CV) characteristics of AlGaN/GaN High Electron Mobility Transistors (HEMTs), providing accurate predictions of the transconductance and the gate capacitance in both linear and velocity saturation regions. The models provide an accurate and smooth connection in the area near the knee point for the DC characteristics, which is attributed to precise descriptions of the channel charge. An accurate model for the low-field mobility of the two dimensional electron gases (2DEG) has been developed, considering the nonmonotonic dependence of the carrier velocity on the electric field perpendicular to the channel for the first time. The calculated transconductance and output conductance are proved accurate and to have high order continuity, especially in large voltage biases, which is hard for some other analytical or numeral models. The gate capacitance has been obtained analytically and verified by experimental data. The slight decrease in the measured gate-to-source capacitance over the velocity saturation region which indicates the results of neutralization of donors and the contribution of the free electron in the AlGaN layer has been modeled accurately and smoothly for the first time. The predicted cut-off frequency is in excellent agreements with measured data over the full range of applied biases. The models are implemented into the HSPICE simulator for the DC, AC and transient simulations, with good speed and convergence characteristics.
Simulation of Capacitance-Voltage characteristics of Ultra-thin Metal-Oxide-Semiconductor Structures with Embedded Nanocrystals
Authors: Rahman Mosur , Lojek Bo , Kalkur Thottam
Published: April 2011

This paper presents an approach to model quantum mechanical effects in solid-state devices such as Metal Oxide Semiconductor (MOS) capacitor with and without nanocrystal in the oxide at the device simulation level. This quantum-mechanical model is developed to understand finite inversion layer width and threshold voltage shift. It allows a consistent determination of the physical oxide thickness based on an agreement between the measured and modeled C-V curves. However, as for thinner oxides finite inversion layer width effects become more severe, quantum-mechanical model predicts higher threshold voltage than the classical model. The inversion-layer charge density and MOS capacitance in multidimensional MOS structures are simulated with various substrate doping profiles and gate bias voltages. The effectiveness of the QM correct method for modeling quantum effects in ultrathin oxide MOS structures is also investigated. The CV characteristic is used as a tool to compare results of the QM correction with that of the Schrödinger–Poisson (SP) solution and Classical solution The variation of (different parameters) for various doping profiles at different gate voltages is investigated.
Stability of Larger Ferromagnetic Chain-of-sphere Nanostructure Comprising Magnetic Vortices
Authors: Barpanda Prabeer
Published: April 2011

Chain-of-sphere (CoS) nanostructure containing Permalloy (Fe20Ni80) nanospheres of uniform size (d=50 nm) has been studied using micromagnetic simulation. These large-size Permalloy nanospheres support magnetic vortex structure upon relaxation. The presence of magnetic vortices in CoS architecture affects its magnetic properties significantly. Micromagnetic behaviour of Permalloy CoS system was studied focusing on the magnetization reversal process. The presence of magnetic vortices triggers a vortex creation and annihilation mechanism (VCA) involving the formation and breaking of an inversion symmetry (IS) feature. This VCA mechanism has been studied using 3D micromagnetic simulation and results of coercivity and vortex parameters are presented.
New Physical and Chemical Treatments to Improve the Quantum Efficiency in Polymer Solar Cells
Authors: Yoshikawa Osamu , Sonobe Taro , Sagawa Takashi , Yoshikawa Susumu
Published: April 2011

The performance of the devices of bulk heterojunction polymer-based solar cells were investigated by using poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM) as light absorption (viz. active) layer, with TiOx as interlayer as follows: ITO/PEDOT:PSS/P3HT-PCBM/TiOx/Al [1] through the treatment of microwave irradiation (single mode of 2.45 GHz, 800 W for 1, 2.5, or 5 min). Such treatments enabled to increase the short-circuit current density Jsc (from 4.53 mA cm−2 to 7.27 mA cm−2) and fill factor FF (from 0.41 to 0.66) of the cell, though the open circuit voltage Voc was decreased (from 0.61 V to 0.57 V) along the irradiation. Absorption spectra of P3HT-PCBM blended film before and after the microwave treatment were observed. Shoulders at 550 nm and 600 nm appeared after the irradiation. This result implies that the microcrystallization of P3HT was slightly promoted through the microwave treatment.
Octadecyltriethoxysilane Surface Modification of Zinc Oxide
Authors: Allen Cary , Baker Darick J. , Furtak Thomas E. , Collins Reuben T. , White Matthew S. , Gillaspie Dane T. , Olson Dana C. , Ginley David S.
Published: April 2011

Zinc Oxide (ZnO) is actively investigated for hybrid organic inorganic device applications. The interface greatly influences the electronic properties of these devices. Molecular surface modification of ZnO is being investigated for its potential to control the alignment of energy levels, charge transfer, as well as, interfacial chemical characteristics that influence device fabrication. In this study, octadecyltriethoxysilane (OTES) treatments of thin film ZnO produced by sol-gel decomposition were explored. The ZnO films were hydroxylated and then modified using OTES in solution. The condensation reaction of the OTES at the surface was promoted by the addition of a protoamine catalyst. Contact angle and infrared spectroscopy studies confirmed the surface modification and indicated that the coverage of the OTES was submonolayer. The modified ZnO films were reproducible and stable for long periods. The effects of the modification on subsequently spin-cast poly[3-hexylthiophene](P3HT) and on hybrid ZnO/P3HT organic solar cell performance are discussed.
Diethynyl Aryl Derivatives for P-Channel and N-Channel Organic Field-Effect Transistors
Authors: Yasuda Takeshi , Kashiwagi Kimiaki , Morizawa Yoshitomi , Tsutsui Tetsuo
Published: April 2011

Organic field-effect transistors (OFETs) consisted of vacuum-evaporated diethynyl aryl derivatives were prepared and the device characteristics were evaluated. The fabricated OFETs showed typical p-type characteristics for diethynyl naphthalene derivative with two end naphthyl groups. By optimizing the fabrication process, the device exhibited a high field-effect hole mobility up to 0.12 cm2V−1s−1 and a high on/off current ratio of 3.3×105. On the other hand, OFETs showed typical n-type characteristics for diethynyl aryl derivative with two end heptafluoronaphthyl groups. We have observed clear changes from p-channel to n-channel conductions in OFETs by chemically modifying diethynyl aryl derivatives.