bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemical Beam Epitaxy Of Ganxpi- Using A N Radical Beam Source
Authors: Li N. Y. , Tomich D. H. , Wong W. S. , Solomon J. S. , Tu C. W.
Published: April 2011

In this study we report the growth behavior of GaNxP1−x by chemical beam epitaxy using triethylgallium, tertiarybutylphosphine, and a RF-plasma N radical beam source. We demonstrate that the N radical beam source is an effective N source for the growth of GaNxP1−x, compared to ammonia (NH3) with co-injection of phosphine (PH3) or tertiarybutylphosphine (TBP). At a growth temperature of 640°C, the N composition increases slowly from 2.5 to 2.8% even though the N2 flow rate is doubled. When the N2 flow rate is increased further, the reflection high-energy electron diffraction pattern (RHEED) becomes spotty. The N composition, however, shows a strong dependence on the growth temperature. For a fixed N plasma radical beam flux, the lower the substrate temperature is, the higher the N incorporation. The N composition can be adjusted from 0.7 to 10.2% by lowering the growth temperature from 690 to 400°C.
GaN Film Growth by a Supersonic Arcjet Plasma
Authors: Cappelli M. A. , Kull A. E. , Schwendner K. , Lee H. , Harris S. J. , Mroczkowski J.
Published: April 2011

This paper reports on preliminary studies of GaN growth obtained using a supersonic nitrogen arcjet plasma expanding into low pressure. A hydrogen-free and carbon-free growth environment was achieved by use of a Ga vapor source positioned downstream of the expanding plume. GaN growth rates of around 8 μm/hour were obtained in these preliminary studies. We believe these growth rates are the highest reported for a non-chemical decomposition process. The growth rates are attributed to the very high nitrogen atom fluxes estimated to be on the order of 1019 atoms/cm2/sec. The initial growth rates are believed to be well below those possible with a fully optimized system. The GaN films are characterized by Raman spectroscopy, visible-IR transmission, and x-ray diffraction. The deposited films appear to be single crystal and epitaxial on basal plane sapphire substrates.
Gan Growth By Nitrogen Ecr-Cvd Method
Authors: Chen K. H. , Chao C. H. , Chuang T. J. , Yang Y. J. , Chen L. C. , Chen C. K. , Huang Y. F. , Yang C. H. , Lin H. Y. , Chang I. M. , Chen Y. F.
Published: April 2011

A new approach toward GaN growth using electron cyclotron resonance assisted microwave plasma enhanced chemical vapor deposition (ECR-CVD) method has been implemented. This growth technique allows for low- as well as high-temperature deposition, the use of pure nitrogen source, and a wide operating pressure that is between MOCVD and MOMBE. The unique features of this technique enable the growth of the epitaxial layer of GaN on a variety of substrates including sapphire, silicon, and LiGaO2. SEM, XRD, Raman, photoluminescence (PL), and Hall measurement are employed to characterize the deposited films. Highly oriented, (0001) textured films in the expected wurtzite structure with blue emission have been obtained.
High Temperature Surface Degradation of III-V Nitrides
Authors: Vartuli C. B. , Pearton S. J. , Abernathy C. R. , MacKenzie J. D. , Zolper J. C. , Lambers E. S.
Published: April 2011

The surface stoichiometry, surface morphology and electrical conductivity of AIN, GaN, InN, InGaN and InAIN was examined at rapid thermal annealing temperatures up to 1150 °C. The sheet resistance of the AIN dropped steadily with annealing, but the surface showed signs of roughening only above 1000 °C. Auger Electron Spectroscopy (AES) analysis showed little change in the surface stoichiometry even at 1150 °C. GaN root mean square (RMS) surface roughness showed an overall improvement with annealing, but the surface became pitted at 1000 °C, at which point the sheet resistance also dropped by several orders of magnitude, and AES confirmed a loss of N from the surface. The InN surface had roughened considerably even at 650 °C, and scanning electron microscopy (SEM) showed significant degradation. In contrast to the binary nitrides the sheet resistance of InAIN was found to increase by ˜ 102 from the as grown value after annealing at 800 °C and then remain constant up to 1000 °C, while that of InGaN increased rapidly above 700 °C. The RMS roughness increased above 800 °C and 700 °C respectively for InAIN and InGaN samples. In droplets began to form on the surface at 900 °C for InAIN and at 800 °C for InGaN, and then evaporate at 1000 °C leaving pits. AES analysis showed a decrease in the N concentration in the top 500 Å of the sample for annealing ≥800 °C in both materials.
Low Volume Resistivity Chemical Vapor Deposited Boron Doped Polycrystalline Thin Diamond Film Growth on Sapphire
Authors: Golestanian Hassan , Mirzakuchaki S. , Charlson E. J. , Stacy T. , Charlson E. M.
Published: April 2011

Hot-filament chemical vapor deposited (HFCVD) boron doped polycrystalline diamond thin films having low volume resistivity were grown on sapphire. The films were characterized using scanning electron microscope (SEM), X-ray diffraction, and current-voltage measurements. SEM micrographs show good crystalline structure with preferred (100) orientation normal to the surface of the film. X-ray diffraction pattern revealed diamond characteristics with the four typical diamond peaks present. Finally, the obtained I-V characteristics indicated that the film's volume resistivity is at least two orders of magnitude lower than those of HFCVD polycrystalline diamond thin films grown on silicon under similar growth conditions.
Factors Influencing the Electrical and Optical Properties of Aigan Layers on Sapphire
Authors: Shin M. , Polyakov A. Y. , Skowronski M. , Greve D. W. , Wilson R. G. , Freitas J. A.
Published: April 2011

Epitaxial layers of A1xGa1-xN were grown by MOCVD on sapphire substrates and characterized by temperature dependent Hall effect, optical absorption, SIMS and photoluminescence. The concentration of native donors was found to decrease with increase of growth temperature. Deep levels found in A1GaN layers were interpreted as A1 double donor state and T2 donor state of nitrogen vacancy.
Influence of Forced Diffusion of Boron on Electrical Conductivity of Diamond Films
Authors: Sung T. , Popovici G. , Prelas M. A. , Wilson R. G. , Bigelow Kim , Chacon J. , Loyalka S. K.
Published: April 2011

Four CVD diamond films grown on tungsten carbide were used for diffusion. Diffusion was performed in a hydrogen atmosphere. Three methods of diffusion were used: conventional diffusion due to concentration gradient, forced diffusion under a dc electric bias with thermal ionization and forced diffusion with optical and thermal ionization of boron acceptor level in diamond. The temperature dependence of the electrical conductivity of the diffused samples was measured in the temperature range 300–600 K. The increase of approximately six order of magnitude in conductivity of the films doped by forced diffusion was obtained. The samples diffused with boron by conventional diffusion due to concentration gradient showed only one order of magnitude increase in electrical conductivity.
Measurement of The Activation Energy in Phosphorous Doped Polycrystalline Diamond Thin Films Grown on Silicon Substrates by Hot Filament Chemical Vapor Deposition
Authors: Mirzakuchaki S. , Golestanian H. , Charlson E. J. , Stacy T.
Published: April 2011

Although many researchers have studied boron-doped diamond thin films in the past several years, there have been few reports on the effects of doping CVD-grown diamond films with phosphorous. For this work, polycrystalline diamond thin films were grown by hot filament chemical vapor deposition (HFCVD) on p-type silicon substrates. Phosphorous was introduced into the reaction chamber as an in situ dopant during the growth. The quality and orientation of the diamond thin films were monitored by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Current-voltage (I-V) data as a function of temperature for golddiamond film-silicon-aluminum structures were measured. The activation energy of the phosphorous dopants was calculated to be approximately 0.29 eV.
Electronic Structure and Temperature Dependence of Excitons in GaN
Authors: Buyanova I. A. , Bergman J. P. , Lil W. , Monemar B. , Amano H. , Akasaki I.
Published: April 2011

We present an optical study of the excitonic properties of epitaxial GaN using reflectivity and photoluminescence (PL) measurements. The values for the intrinsic exciton energies are found to be dependent on the built in strain developed due to the difference in thermal expansion coefficients between the GaN epilayer and the foreign substrate. For GaN on sapphire thecompressive biaxial strain causes an upshift of A and B excitons by typically 15 meV relative to the strain free sample, in accordance with previous data. For GaN on SiC, on the other hand, a downshift ˜ 8 meV in the free exciton energies is observed at 2K. Only two excitonic peaks about 18 meV apart, are resolved in reflectivity spectra for GaN on SiC, probably due to the overlapping of A and B excitons. The suggested explanation implies the reduction of the bandgap energy and of the valence band splitting under the action of a biaxial tensional strain in the GaN layer. For all structures the strain-induced shifts of the bandgap energy are much smaller at elevated temperatures, presumably reflecting the temperature dependence of the accumulated strain energy. The exciton-polariton structure of the GaN is also discussed. The enhanced intensity of the no-phonon (NP) A line compared to its longitudinal (LO) phonon replica is suggested to be partially attributed to strong defect scattering.
Optical Properties and Morphology of Gan Grown by MBE on Sapphire Substrates
Authors: Tuncel E. , Oberman D. B. , Lee H. , Ueda T. , Harris J. S.
Published: April 2011

A series of GaN films grown by MBE on sapphire substrates with different nitrogen sources are characterized by optical transmission, spectroscopic ellipsometry (SE), photoluminescence (PL) and cross-sectional atomic force microscopy (AFM). The film thicknesses determined from broad spectral range transmission measurements and the AFM images are used in the analysis of the SE spectra. Interface roughnesses between the constituent layers, such as the substrate, the buffer and GaN layers are included in the modelling of the SE spectra and are also imaged by cross-sectional AFM. An effective medium type model is used for the modelling of interface and surface roughnesses in the SE spectra. The optical constants of the films in the band edge spectral range are determined in such a way as to simultaneously satisfy the transmission and the ellipsometry data.
Characterisation of Diamond-like Carbon by Raman Spectroscopy and Optical Constants
Authors: Mölßner C. , Grant P. , Tran H. , Clarke G. , Lockwood D. J. , Labbé H. J. , Mason B. , Berriche R.
Published: April 2011

Crystalline diamond coatings and, increasingly, diamond like amorphous carbon (DLC) films are used for tribological and protective layers for their hardness and chemical inertness. They are also under investigation for their electron emitting properties, with possible applications in field emission displays. DLC films were deposited by laser ablation using a KrF excimer laser and fluences between 0.5 and 2 J/cm2. FTIR measurements did not show the presence of hydrogen in the films. Raman spectra allowed for the determination of the nature of the graphitic and diamond bonds (sp2 and sp3) as well as information about the disorder and short range order in the films. For a better determination of the sp3-content, which is often hidden in the Raman spectra, a correlation with optical properties in the near IR to near UV region was established. These values depended strongly on the substrate temperature and the laser fluence. DLC formation could be demonstrated even at substrate temperatures close to room temperature. Vickers hardness values and first measurements on the electron emissivity of the films can be correlated to the diamond character and the preparation method of the films.
Electron Field Emission From Diamond-Like Carbon
Authors: Robertson J , Silva S R P , Amaratunga G A J , Milne W I
Published: April 2011

The low electron affinity of a-C:H is related to that of diamond surfaces and is studied using a chemical bonding model. The electron field emission from hydrogenated amorphous carbon (a-C:H) and nitrogen modified a-C:H showing low turn on fields are described. Nitrogen improves the field emission, apparently by raising the Fermi level.
Characteristics of XeCI Excimer-Laser Annealed Insulator
Authors: Jang Keun-Ho , Choi Hong-Seok , Jun Jae-Hong , Yoo Jhun-Suk , Han Minkoo
Published: April 2011

The laser annealing effects on the TEOS (Tetra-Ethyl-Ortho-Silicate) oxide of MOS (Al/TEOS/n+ Silicon ) structures was investigated with different initial oxide conditions, such as breakdown field. The breakdown field increased upto the 170 mJ/cm2 with increasing laser energy density and decreased at 220 mJ/cm2. It is considered that the increase of breakdown field is originated from the restore of strains which exist mainly at the metal/oxide interface.
Helical Resonator Plasma Oxidation of Amorphous and Polycrystalline Silicon for Flat Panel Displays
Authors: Kaluri Sita R. , Howell Robert S. , Hess Dennis W. , Hatalis Miltiadis K.
Published: April 2011

A low temperature process for the oxidation of amorphous and polycrystalline silicon was studied using a helical resonator plasma source. Constant current anodization of amorphous silicon was performed at 350°C and 30 mtorr in an oxygen plasma. Oxidation rate data were analyzed by both parabolic and power law expressions: parabolic rate coefficient B = 23.45 nm2/min and power law rate coefficients α = 0.3807 and C* = 7.3 nm/minα. Oxidation at substrate floating conditions yielded thin oxides; the growth rate saturated after 20 mins, at a thickness of about 10 nm. For TFr fabrication, a two layer oxide was used. The first layer was grown by plasma oxidation to a thickness of about 10 nm to ensure a good interface, and the second layer deposited by PECVD to the final thickness of 35 nm. Electrical properties of control oxides grown on c-Si at the same conditions were promising, with a fixed oxide charge of 1.98 × 1011 charges/cm2 and a mean breakdown field of 5.3 MV/cm after post metallization anneal. The TF's had an average effective electron mobility of 31.1 V/cm2 and an average threshold voltage of 4.6 V.
The Poly-Si Tfts Fabricated by Novel Oxidation Method with Intermediate Oxide
Authors: Park C-M , Yoo J-S , Min B-H , Han M-K
Published: April 2011

We have fabricated a poly-Si TFT using a novel oxidation method, which improves the surface roughness at the interface between the poly-Si layer and the gate oxide layer. Compared with the poly-Si TFTs fabricated by the conventional oxidation method, the proposed poly-Si TFT exhibits the remarkable enhancement of the electrical parameters, such as the subthreshold swing and the threshold voltage. It is observed that the proposed poly-Si TFT has a higher dielectric strength and the device characteristics are not degraded significantly after an electrical stress. The improvement of the surface roughness at oxide/poly-Si interface is found to be critical to enhance the device performance.
Improvement of Poly-Silicon Thin Films and Thin Film Transistors Using Ultrasound Treatment
Authors: Ostapenko S. , Jastrzebski L. , Lagowski J. , Smeltzer R. K.
Published: April 2011

Ultrasound treatment (UST) was applied to improve electronic properties of polycrystalline silicon films on silica-based substrates. A strong decrease of sheet resistance by a factor of two orders of magnitude was observed in hydrogenated films at UST temperatures lower than 100°C. This is accompanied by improvement of a film homogeneity as confirmed by spatially resolved photoluminescence study. The UST effect on sheet resistance demonstrates both stable and metastable behavior. A stable UST effect can be accomplished using consecutive cycles of UST and relaxation. An enhanced passivation of grain boundary defects after UST was directly measured by nano-scale contact potential difference with atomic force microscope. Two specific UST processes based on interaction between ultrasound and atomic hydrogen are suggested: enhanced passivation of grain boundary states and UST induced metastability of hydrogen related defects. We also demonstrate a utility of UST for improvement of leakage current and threshold voltage in hydrogenated poly-Si thin film transistors.
Solid-Phase Crystallization of Amorphous Si0.7Ge0.3/Si and Si/Si0.7Ge0.3 Bilayer Films on SiO2
Authors: Kim Tae-Hoon , Ryu Myung-Kwan , Kim Jin-Won , Kim Chang-Soo , Kim Ki-Bum
Published: April 2011

We have investigated the solid phase crystallization of a-(Si/Si0.7Ge0.3) and a-(Si0.7Ge0.3/Si) bilayer films deposited on SiO2 for an annealing temperature of 550 °C. It was found that, in case of a-(Si0.7Ge0 3/Si), nucleation of crystalline phases occurred at the free surface, while in a-(Si/Si0.7Ge0.3) crystalline phase nucleated at Si0.7Ge0.3/SiO2 interface. The crystallization rate of an a-(Si0.7Ge0.3/Si) is much slower than that of an a-(Si/Si0.7Ge0.3) films. After full crystallization, poly-(Si0.7Ge0.3/Si) has many equiaxed grains and the defect density of the upper Si0.7Ge0.3 was much lower than that of lower Si0.7 Ge0.3 in a poly-(Si/Si0.7Ge0.3) film whose grain morphology was elliptical. The average grain size of poly-(Si0.7Ge0.3/Si) was ˜7 μm and this film had strong (111) preferential orientation, while poly-(Si/Si0.7Ge0.3) had weak (311) or random oriented grains with the average size of˜0.3 μm.
Characterization of Polysilicon films using atomic force microscopy
Authors: Fehlner Francis P. , Moore Chad B. , Couillard J. Greg
Published: April 2011

A simplified technique for characterizing crystallites in polysilicon films has been demonstrated based on use of the atomic force microscope (AFM). The crystallization of films deposited by two different techniques was examined.
The effect of phenyl benzoate self-assembled monolayers on liquid crystal alignment
Authors: Ondris-Crawford R. J. , Lander L. M. , Frank C. W. , Twjeg R. J.
Published: April 2011

We have synthesized compounds of the form M-R-SiCl3where M is a mesogen based on octyloxy phenyl benzoate, and R is an alkyl chain (tether) that connects M to the silane head group. Tethering lengths from the mesogenic unit to the silane moiety involved 3, 6, or 10 methylene units. These compounds were used to form self-assembled monolayers (SAMs) on silicon wafers, fused silica, and SiO2coated indium tin oxide (ITO) coated glass. Characterization of these systems includes contact angle measurements, ellipsometry, and atomic force microscopy. Results suggest that the monolayer packing is governed by the lateral interactions between the mesogenic units and not by the tethering length to the solid surface. We have investigated the utility of these chemically-tuned surfaces for liquid crystal alignment. The octyloxy phenyl benzoate self-assembled monolayers induce homeotropic alignment of a nematic liquid crystal.
The Effect of Technology and Materials Choice on Field Emission Display Performance
Authors: Slusarczuk Marko M. G.
Published: April 2011

The Field Emission Display (FED) is often portrayed as the emerging alternative to the Active Matrix Liquid Crystal Display (AMLCD). FED displays are based on cold cathodes that emit electrons which are accelerated across a potential in a vacuum to strike a phosphor and emit light. There are numerous material and technology choices at each point of the display system design. Each alternative is characterized by advantages and consequences that affect subsequent choices and system performance. This paper discusses the alternative approaches to cathode material and structure, electron beam focusing, support structure, phosphor and system integration. These choices are related to ultimate display performance and cost.
Investigation Of The Nucleation And Growth Of Thin-Film Phosphors
Authors: Moss T. S. , Springer R. W. , Peachey N. M. , Dye R. C.
Published: April 2011

The deposition of CaGa2S4:Ce has been accomplished using a commercial liquid delivery system on two substrate surfaces: ZnS and SrS. However, the film deposited on ZnS was not of satisfactory quality because of the formation of an amorphous layer and a high amount of residual porosity within the deposition. The use of a SrS layer on top of the ZnS improved the nucleation by reducing the interfacial energy between the substrate and deposition. It greatly reduced the porosity in the coating and reduced the formation of the amorphous layer. The crystallinity of the CaGa2S4 400 peak was also increased by a factor of ten when a layer of SrS was used. Further, the FWHM of the 400 peaks from the two depositions was not significantly different, indicating that the crystallite size and strain was approximately the same. The B40 was increased by a factor of two, from 1.84 cd/m2 for ZnS to 3.67 cd/m2 for SrS. This increase is an improvement in the performance of the films and is attributable to the increase in the crystallinity.
Influence Of The Density of States and Series Resistance on the Field-Effect Activation Energy in a-Si:H TFT
Authors: Chen Chun-Ying , Kanicki Jerzy
Published: April 2011

We have proposed a new two-dimensional simulation model, which takes into account the density of states of hydrogenated amorphous silicon (a-Si:H) and temperature-dependence of the source/drain series resistances (Rs), to explain the dependence of the activation energy (Eact) of drain-source current (IDs) on gate-source bias (VGs) in a-Si:H thin-film transistors (TFTs). We found that the influence of series resistance cannot be ignored, else an overestimated Eact will result. The results of our simulation are in agreement with experimentally observed saturation of the Eact at higher VGs.
Investigation of the off-current in amorphous silicon thin film transistors for SiO2 and SiNx. gate insulators
Authors: Kim Jeong Hyun , Choi Woong Sik , Hong Chan Hee , Soh Hoe Sup
Published: April 2011

The off current behavior of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs) with an atmospheric pressure chemical vapor deposition (APCVD) silicon dioxide (SiO2) gate insulator were investigated at negative gate voltages. The a-Si:H TFT with SiO2 gate insulator has small off currents and large activation energy (Ea) of the off current compared to the a-Si:H TFT with SiNx gate insulator. The holes induced in the channel by negative gate voltage seem to be trapped in the defect states near the a-Si:H/SiO2 interface. The interface state density in the lower half of the band gap of a-Si:H/SiO2 appears to be much higher than that for a-Si:H/SiNx.
Physics of Below Threshold Current Distribution in a-Si:H TFTs
Authors: Slade H. C. , Shur M. S. , Deane S. C. , Hack M.
Published: April 2011

We have examined the material properties and operation of bottom-gate amorphous silicon thin film transistors (TFTs) using temperature measurements of the subthreshold current. From the derivative of current activation energy with respect to gate bias, we have deduced information about the density of states for several different transistor types. We have demonstrated that, in TFTs with thin active layers and top nitride passivation, the current conduction channel moves from the gate insulator interface to the passivation insulator interface as the transistor switches off. Our 2D simulations clarify these experimental results. We have examined the effect of bias stress on the transistors and analyzed the resulting reduction in the subthreshold slope. Based on these results, we have extended our analytic amorphous silicon TFI SPICE model to include the effect of bias stress.
Statistical Theory on Polymer Liquid Crystals
Authors: Kimura Hatsuo
Published: April 2011

Based on common molecular model, polymer liquid crystals of hard-rod and flexible main-chain are studied statistical mechanically. The hard-core repulsion and also the attractive potentials are assumed between molecules. The nematic-isotropic phase transitions, and molecular orientations at interfaces in polymers and polymer-liquid crystal composite systems are discussed.
Molecular Dynamics of Side Chain Liquid Crystalline Polysiloxanes
Authors: Podojil G. M. , Farmer B. L. , Bunning T. J. , Pachter R. , Adams W. W.
Published: April 2011

Molecular dynamics simulations have been used to characterize the development and longevity of associations between cholesterol and biphenyl mesogens when attached to linear siloxane oligomers by flexible spacer groups. Single substituents, alternating substituents, and diblock and triblock arrangements of the substituents were considered. The backbone and spacer groups allow sufficient flexibility that long-lived associations between cholesterol mesogens form quite rapidly, as do more fluid associations between biphenyl and cholesterol mesogens. The study of the individual mesogen interactions and how these lead to larger scale aligned structures has provided insight into the nature of the liquid crystalline state in these materials.
A Novel Rheological Microscope for Flow Studies of Thermotropic Polymers and Polymer Blends: Droplet Deformation
Authors: Mather P. T. , Stüber H. R. , Chaffee K. P. , Haddad T. S. , Romo-Uribe A. , Lichtenhan J. D.
Published: April 2011

We have recently begun investigating the rheological behavior of blends containing thermotropic liquid crystalline polymers (LCPs) and isotropic polymers. To aid us in our studies, we have designed a unique instrument for simultaneous visualization of morphology evolution and rheological measurements. Rectilinear shear is used, with a top glass bar being translated relative to a fixed bottom glass bar. The sample is heated by conduction from mica heaters embedded in the substrate materials supporting the bounding glass bars. Shear forces are obtained by measuring the (small) deflection of the bottom glass bar and heating assembly which is designed to be slightly compliant in the flow direction while rigid in direction normal to the sample.
Highly Uniform Oriented Liquid Crystalline Thermosets
Authors: Amendola E. , Carfagna C. , Giamberini M. , Komitov L.
Published: April 2011

Curing of mesogenic epoxy monomer p-Bis(2, 3-epoxypropoxy)-cz-methylstilbene (DOMS) with aromatic amines is discussed. The crosslinking density is controlled using a mixture of biand tetra-functional hardener. The unidirectional alignment of the molecules was achieved during the curing process by external factors such as surface-liquid crystal interactions.
Liquid Crystals in Ternary Polyoxyethylene Trisiloxane Surfactant - Silicone Oil - H2O System
Authors: Li X. , Hill R. M. , Scriven L. E. , Davis H. T.
Published: April 2011

Polyethylene trisiloxane surfactants M(D′En)M (n=6, 8, 10, 12) self assemble into sheet-like structure and form various lyotropic cubic, hexagonal and lamellar liquid crystal phases in ternary trisiloxane surfactant - silicone oil - water systems. Small angle X-ray scattering (SAXS) was used to identify and characterize the liquid crystal phases. Cyclic octamethylcyclotetrasiloxane (D4), decamethyl-cyclopentasiloxane(D5) and short linear decamethyltetrasiloxane (MD2M) silicone oils facilitate the formation of liquid crystals. The structures in liquid crystals (LC) progress from zero curvature lamellar liquid crystals to higher curvature cubic liquid crystals with increasing the surfactant hydrophilic head group size. The effects of surfactant and oil concentration on the lamellar bilayer thickness and molecular packing were studied with small angle X-ray scattering.
Polarized Fluorescence from Vitrified Liquid Crystalline Films
Authors: Conger Brooke M. , Shi H. , Chen S. H. , Tsutsui T.
Published: April 2011

Linearly and circularly polarized photoluminescence (PL) were observed from neat vitrified liquid crystal (LC) films. Electronic properties of the compounds were investigated through measurement of absorption and emission spectra in solution and in film. The nematic film exhibited a polarization factor (linear polarization) of 2.8 at an order parameter of 0.70, whereas the chiral nematic film gave rise to an absolute dissymmetry factor (circular polarization) of 0.27 at an order parameter of 0.54.
Dynamics of Defect Annihilation in Vitrified Liquid Crystalline (VLC) Thin Films Upon Thermal Annealing
Authors: Shi H. , Katsis D. , Chen S. H. , De Rosa M. E. , Adams W. W. , Bunning T. J.
Published: April 2011

Based on a series of model nematic and chiral-nematic compounds as well as a polymer analog, it was found that the annihilation of defects in a VLC system occurs at a much faster rate than its polymer analog. In chiral-nematic blends, the degree of structural similarity between the nematic host and the chiral dopant plays an important role; a wider biphasic region contributes to a slower annihilation process.
Cyclic Liquid Crystalline Siloxanes - Chemistry and Applications
Authors: Kreuzer F.-H. , Maurer R. , Stohrer J.
Published: April 2011

Cyclic Siloxanes, bearing mesogenic groups, connected with the backbone via aliphatic spacers exhibit phases analogue to the classic calamitics. Bearing two different mesogenic groups, these materials can be considered as cooligomers, whose degree of polymerization is given by the H-siloxanes used. These cooligomers consist of a mixture of molecules with substitution patterns determined by the statistics of the addition reaction of the mesogenic groups to the H-siloxane. Two types of materials are available, crosslinkable and non crosslinkable. As most important applications may be mentioned: Cholesteric reflectors and filters, storage materials for optical informations, retardation plates and pigments for iridescent or polarizing coatings.
Morphology and Mechanical Properties of Pc/Lcp in Situ Composites With Compatibilizer
Authors: Leng Y. , Zheng W. G.
Published: April 2011

A bisphenol A epoxy resin is used to modify compatibility between PC and LCP (Vectra B950, a wholly aromatic liquid crystalline polymer) during injection molding. LCP can effectively reinforce PC, since it forms in situ micro-fibrils in the thermoplastic PC matrix during melt flow. However, the immiscibility of PC and LCP affects the mechanical performance of the LCP in situ composite. Preliminary results of this work indicate that epoxy might improve the interfacial bonding between PC and LCP fibrils, and also modify the tensile properties of the PC/LCP system.
Peculiarities of the Structure and Properties of Liquid-Crystal Composites Based on Polymer Complexes
Authors: Lazareva T. G. , Vashuk E. V. , Yakovleva O. V. , Prodan S. A.
Published: April 2011

Some features of the: formation of liquid-crystal structures based on polymer complexes are discussed for composites of polyvinyl alcohol with cellulose esters, polyacids, inorganic acids and variable-valence metals. Rheological and optical methods of research as well as a dielectric relaxation method have revealed the formation of several types of interpolymer complexes in liquid flowable media. This leads to formation of associates and results in an increase of the molecular mobility of macromolecules and their fragments. The structure and properties of the complexes depend on the media compositions and on the method of additive introduction. The composites obtained -can be used for producing light-, electric- and heat-sensitive film materials and sensors of various types.
Switchable Bragg Gratings Formed in Situ Within a Polymer-Dispersed Liquid Crystal Composite Medium
Authors: Sutherland R. L. , Natarajan L. V. , Bunning T. J. , Tondiglia V. P. , Adams W. W.
Published: April 2011

Holographic photo-curing of a penta-acrylate monomer-liquid crystal mixture forms a unique system of liquid crystal microdomains confined to Bragg planes. We examine the physics of this structure as it relates to the formation of electrically switchable holograms, for which many potential applications exist. The results of scanning electron microscopy and laser characterization studies lead to the development of concepts and models for explaining the microscopic morphology and electro-optical properties of these holograms. We find that a model incorporating a shaped-droplet analysis of the electro-mechanical properties of LC domains combined with standard coupled-wave theory of holography offers good numerical agreement with diffraction efficiency data for ppolarized probe light.
Investigations of Silicon Nitride Films for Silicon Solar Cells
Authors: Elmiger J. R. , Kunst M.
Published: April 2011

Silicon nitride films on crystalline silicon were deposited in a low-temperature (< 400 °C ) Plasma Enhanced Chemical Vapour Deposition process. The deposition process is monitored with in situ Time Resolved Microwave Conductivity measurements leading to an on-line quality control of the deposited films. It is shown that at the start of the deposition there is a strong decrease of the lifetime of the measured transient signal due to plasma induced damage at the silicon surface. Afterwards an increase of the lifetime is observed due to passivation of the interface. For thin films (< 30 nm), the lifetime and the film composition depend on the film thickness. Furthermore, the film composition has a strong impact on the passivation of thick (100 nm ) silicon nitride films. The best passivation is obtained for almost stoichiometric films characterized by a refractive index of 1.95.
Effects of the Concentration of Ga on Junction Formation in Thin-film ZnO/CdS/CuInXGal-XSe2 /Mo Photovoltaic Devices
Authors: Matson R. J. , Contreras M. A. , Tuttle J. R. , Swartzlander A. B. , Parilla P. A. , Noufi R.
Published: April 2011

Charge-collection microscopy (CCM)—commonly known as electron-beam-induced current [EBIC] microscopy—in a scanning electron microscope (SEM) was used to obtain charge-collection efficiency profiles of cleaved thin-film ZnO/CdS/CuInXGal-XSe2/Mo photovoltaic devices on glass substrates, with x varied between 0 and 100%. We observed considerable variation in the EBIC peak position, the uniformity (between and within devices), and overall charge-collection profile of the junction as a function of Ga concentration. Whereas using only CuInSe2 (CIS) absorber material results in a rather wide and buried junction in the CIS, the systematic addition of Ga to the p-CuInSe2 matrix affects the conductivity type of the material, thereby creating an increasingly abrupt, uniform, and shallow junction near the heteroface. Although the substitution of Ga for In improves overall device performance up to ∼ X=25% and the Voc continues to improve, the Jsc, FF, and device efficiency degrade with additional Ga.
Enhanced Mo Adhesion on Glass With Cr Interlayers for Copper Indium Diselenide Thin Film Devices
Authors: Alleman Jeff , Ginley Dave , Hasoon Falah , Asher Sally , Noufi Rommel
Published: April 2011

A key element of current Copper Indium Diselenide (CIS) and Copper Indium Gallium Diselenide (CIGS) thin film solar cells is the use of a Mo back contact on soda lime glass (SGL). Because of surface preparation problems, high process temperatures, and mismatch of thermal expansion coefficients, adhesion of the Mo to the soda lime glass can be variable. Also beneficial is the Na facile diffusion of the glass into the absorber layer. We report on the use of thin Cr interlayers to improve the adhesion at the Mo/glass interface. The films were subsequently annealed in vacuum under normal process conditions. Adhesion was excellent and quite uniform for Mo layers with a Cr interlayer of 50 to 800 Å compared to control samples without Cr. X-ray Photoelectron Spectroscopy (XPS) data suggests CrO bonding at the glass interface and Cr metallic bonding at the Cr Mo interface. Secondary Ion Mass Spectrometry (SIMS) data for Mo/Cr films shows diffusion of Na throughout the Mo layer identical to that for Mo alone samples. Resistivities of the films have been measured to be 11 μhms-cm, twice that for bulk material of 5.7 μohms-cm. CIGS films were then grown for comparison to films grown on Mo only substrates.
Growth and Characterization of CuInSe2Epitaxial Films for Device Applications
Authors: Niki S. , Kurafuji T. , Fons P. J. , Kim I. , Hellman O. , Yamada A.
Published: April 2011

CuInSe2(CIS) epitaxial layers have been grown on both GaAs (001) and In0.29Ga0.71As pseudo lattice-matched substrates by molecular beam epitaxy, and characterized for device applications. Despite a large lattice mismatch of Δa/a˜2.2%, epitaxial growth of CuInSe2has been demonstrated on GaAs (001) showing their film properties strongly dependent on the Cu/In ratio. In-rich films had a large number of twins on {112} planes, and were found to be heavily compensated. On the other hand, Cu-rich films showed distinct photoluminescence emissions indicating significantly higher film quality in comparison with In-rich films. Two dimensional reciprocal x-ray intensity area mapping and cross-sectional transmission electron microscopy showed the formation of an interfacial layer in the vicinity of the CuInSe2/GaAs interface resulting from the strain-induced interdiffusion between CuInSe2and GaAs. Reduction in lattice mismatch to Δa/a˜0.2% by using In0.29Ga0.71Aspseudolattice-matched substrates made possible the growth of high quality CuInSe2with predominant free exciton emissions in their photoluminescence spectra and with residual defect densities of as low as p˜l×1017cm-3implying the growth of device quality CuInSe2epitaxial films.
Growth of CuInS2 Films on Crystalline Substrates
Authors: Hunger R. , Scheer R. , Alt M. , Lewerenz H. J.
Published: April 2011

CuInS2 films were grown by molecular beam epitaxy (MBE) on hydrogen terminated Si(111) substrates with 4° miscut. X-ray diffraction (XRD) texture analysis reveals that CuInS2 was grown heteroepitaxially with the epitaxial relationships CuInS2(112) II Si(111) and [111] II [111]. Moreover, a substantial amount of rotational twins is observed. The crystalline order is maintained across the interface as observed by cross-sectional transmission electron microscopy (XTEM). XRD and scanning electron microscopy (SEM) investigations show that nonstoichiometric preparation greatly influences the growth morphology and leads to the formation of secondary phases.
Thin-Film Cadmium Sulfide/Cadmium Telluride Alloys
Authors: Jensen D. G. , McCandless B. E. , Birkmire R. W.
Published: April 2011

Thin films of CdTel-xSx with bulk atomic compositions, x≡[S]/([S]+[Te]), ranging from 0 to 0.45 were deposited by vacuum co-evaporation of CdTe and CdS with substrate temperatures of 200 and 250°C. X-ray diffraction analysis revealed that films with x < 0.3 were predominately single phase having the zincblende structure. Films with 0.35 < x < 0.45 contained the wurtzite modification. Lattice parameter determination indicated that each phase exists with compositions well within the miscibility gap shown on published equilibrium phase diagrams. The variation of the optical band gap with x was determined by measuring transmission and reflection of the films. Heat treatment at 415°C in the presence of CdC12 caused the films to segregate into two phases consistent with the phase diagram. If the CdCl2 is assumed to only promote the phase segregation process, then the compositions of the two phases after heat treatment may be taken as measurements of the solubility limits of S in CdTe and Te in CdS respectively. The solubility limit of S in CdTe was thus determined to be 5.8% at 415°C which is the temperature used for the common CdC12 treatment of CdTe-based solar cells. An analysis of CdTe/CdS solar cell device structures shows that the atomic composition of alloys created by interdiffusion are consistent with these solubility limits.
Effects of Magnetic Field Configuration on Rf Sputtering for CdS/CdTe Solar Cells
Authors: Compaan A. , Shao M. , Fischer A. , Grecu D. , Jayamaha U. , Contreraspuente G. , Bohn R. G.
Published: April 2011

We report studies of solar cells prepared by rf planar magnetron sputtering in which the films were deposited using magnetic field structures ranging from approximately balanced to strongly unbalanced in the “type II” configuration. For films grown with the unbalanced configurations, we find much stronger photoluminescence and much better cell performance than for the balanced configuration. The CdTe films show differences in electrical performance depending on magnetic field as well. These effects are interpreted as arising from the enhanced electron and ion bombardment of the film growth interface for the unbalanced magnetrons. Using two unbalanced magnetrons we have fabricated an all-rf-sputtered cell with NREL-verified efficiency of 11.66% at air mass 1.5 illumination.
Deposition Schemes for Low Cost Transparent Conductors for Photovoltaics
Authors: Delahoy A. E. , Cherny M.
Published: April 2011

Production requirements for transparent conductors for thin film photovoltaics are reviewed, with particular reference to magnetron sputtered zinc oxide. On-going optimization processes for ZnO:AI prepared on 0.43 m2substrates by bipolar, reactive sputtering from ZnO:A12O3targets and by reactive sputtering from a Zn:Al target are described. Optical emission from the plasma is used to study the state of the sputtering target. Attention is drawn to the spatial dependence of deposition conditions under the cathode.
High Performance Transparent Conducting Films of Cadmium Indate Prepared by Rf sputtering.
Authors: Coutts T. J. , Wu X. , Mulligan W. P.
Published: April 2011

We are examining various spinel-structured thin films (e.g., Cd2SnO4, Zn2SnO4) to develop higher-quality transparent conducting oxides (TCO) than more conventional materials such as indium tin oxide. Here, we report on cadmium indate (CdIn2O4, CIO), which is another member of this family. Thin films of CIO were deposited by radio-frequency (RF) magnetron sputtering, from an oxide target, onto borosilicate glass substrates. The variables included the substrate temperature, sputtering gas composition, and pressure. Film properties were measured before and after heat treatment. Characterization involved Hall effect measurements, optical and infrared spectrophotometry, X-ray diffraction, and atomic-force microscopy. Film resistivities as low as 2.3x10-4Ω cm were achieved for a film thickness of 0.55 μm. The transmittance was 90% in the visible region of the spectrum, without correction for substrate losses and without an anti-reflection coating. The plasma resonance occurred at longer wavelengths than for other materials and this, with a bandgap of approximately 3.1 eV, presents a wide window for optical transmittance. The highest mobility was 54 cm2 V-s-1 and the highest carrier concentration was 7.5x1020 cm-3.
Full-Optical Characterization of Thin Films in Photovoltaic Cells
Authors: Romanov D. A. , Victoria N. , Kalameitsev A. V.
Published: April 2011

We present a handy new method of optical investigation of thin semiconductor layers regardless of the substrate's composition and structure. The method was especially developed for the particular case of thin (˜ 1μm ) CdTe films obtained by means of electrochemical deposition on conducting glass for further solar cell applications. The procedure consists in transmission and reflection (or the transmission only) measurements under different thicknesses of the semiconductor layer and following semi-analytical treatment of the data. Finally we obtain the frequency dependencies of the real and imaginary parts of the refractive index for the semiconductor material of the layer.
Ftir and UV Study of Amorphous Silicon-Boron Alloys Deposited by LPCVD
Authors: Yang G.-R. , Zhao Y.-P. , Tong B. Y.
Published: April 2011

As-deposited and wet oxidized a-Si:B alloy deposited by low pressure chemical vapor deposition (LPCVD) with various boron content was studied by UV Spectroscopy and Fourier Transform Infrared Spectroscopy. It is found that the optical band gap of a-Si:B varies with respect to the boron content, which provides potential application probability in narrow-gap photovoltaic devices. This effect is associated with the structure change induced by B content. The oxidation behaviors of a-Si:B films with boron content 3 at.% – 25 at.% are different from those with boron content exceeded 30 at.%. Possible oxidation mechanism for different boron content a-Si:B film are proposed.
Single-Crystalline and Bamboo Al Lines Fabricated by Graphoepitaxy: Microstructure and 1/f Noise Measurements
Authors: Van Den Homberg Marc J.C. , Verbruggen A. H. , Alkemade P. F. A. , Radelaar S.
Published: April 2011

The continuing scaling-down of integrated circuits leads to increased metallization reliability problems, especially electromigration. We used 1/f noise measurements to study the relation between electromigration and microstructure. These measurements are very sensitive to the microstructural attributes, such as grain boundaries and dislocations. Al lines were grown by graphoepitaxy: First, a pure Al film was grown by dc magnetron sputtering on a groove pattern etched into a SiO2 substrate. The growth was then followed by an in situ rapid thermal anneal that resulted in a complete filling of the grooves with Al. These Al lines were carefully characterized with SEM and Backscatter Kikuchi Diffraction. Depending on the presence of a temperature gradient during the anneal, the lines were either nearly single-crystalline or bamboo with one grain per ∼ 3 μm. The resistivity was ∼ 2.8 μΩcm, only slightly higher than for bulk Al. We measured the 1/f noise with the two-channel ac technique at RT. We found in both bamboo as well as the single-crystalline lines a very low noise intensity; a factor two lower than in conventionally sputter deposited and annealed Al lines. No clear difference between the noise spectra of the bamboo and the single-crystalline lines was observed. We concluded that grain boundaries are not the only contributor to 1/f noise; other types of defects must play a role as well.
Characterization of the early stages of EM in Al-based metal lines by means of a HRRMT based on an extremely stable ambient temperature
Authors: D'Haeger V. , De Ceuninck W. , De Schepper L. , Stals L. M.
Published: April 2011

A high resolution resistometric measurement technique (HRRMT) is used to study electromigration (EM) in different Al-based metal lines. Instead of using a bridge-method, the high resolution is now obtained by performing experiments in a very stable temperature environment. The main advantage of this technique is that resistance variations are measured, due to processes that occur in the tested metal line only. During EM, both resistivity and geometrical variations can occur, both resulting in a resistance change. A new method is proposed in order to separate these two contributions on-line during the whole EM-experiment. It is shown that even at low current stress, geometrical changes are the main cause for the EM-induced resistance increase.
Microstructure of Passivated Al-Cu and Al-Si-Cu Conductor Lines: Interaction of Precipitates, Defects and Mechanical Stresses
Authors: Dirks A. G.
Published: April 2011

In this paper the results of our investigations of passivated, narrow lines of both Al-Cu and Al-Si-Cu (0.4 at.% Cu, 1.0 at.% Si) by transmission electron microscopy will be presented. Thin alloy films have been deposited on either oxide or refractory-metal films. During the standard anneal treatment of 0.5 h at 400 °C the atomic mobility is relatively high. At this temperature the binary Al-Cu films are one-phase alloys. Depending on the cooling rate, transformation occurs into a two-phase mixture of (initially) finely distributed, Cu-enriched particles and Al. The precipitates eventually form the equilibrium θ-Al2Cu phase, which tends to form large conglomerates upon further annealing. In contrast, the ternary Al-Si-Cu films are two-phase alloys already at 400 °C: at this temperature Si precipitates are pre:sent in the Al matrix, whereas the onset of Cu precipitation is not far below this temperature.