bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface Self-Diffusion Instability in Electric Fields
Authors: Ojovan Michael I. , Poluectov Pavel P.
Published: April 2011

The plane form is the equilibrium one for surfaces of condensed matter. Deviations can be caused usually by crystal structure. Herein we will describe an effect of surface instability due to self- diffusion processes of atoms and molecules in the near surface electric field.

Self-diffusion processes (as it was shown by Mullins) cause relaxation of any deviation (protuberance) from the plane form due to the increased concentration of surface atoms and its consequent smoothing. This process we studied for the case when there is an electric field near the surface. The near surface electric field can be due to either the location of material in an external (homogeneous or inhomogeneous) electrical field or self-charges on the surface. There is an increasing of electric field intensity near protuberances both in external and self-formed electrical fields: the higher is the curvature of surface the stronger is the intensity of the near surface electrical field. Consequently two competing processes occur during surface molecules mass transfer: both the self- diffusion smoothing of surface molecule concentration and drawing of molecules in the strong electric field regions. Depending on the initial shape of the protuberance either relaxation or instability occurs. There is a critical wavelength λ0=RskBT/2Uz, which shows that shorter wavelength deviations decrease their amplitudes and longer wavelength deviations grow in amplitude by time. Here Rs is characteristic of the material, T is temperature, and Uz is the interaction energy of surface molecules with the electric field. Since there are random variations of any surface from the plane form, being placed in an electric field these surfaces will be unstable depending on the intensity of electric field and properties of material.
Electro-resistive Memory Effect in Colossal Magnetoresistive Films and Performance Enhancement by Post-annealing
Authors: Liu Shangqing , Wu Naijuan , Ignatiev Alex , Tavizon Gustavo , Papagianni Christina
Published: April 2011

Colossal magnetoresistive thin films have shown a large electric-pulse-induced resistivity change effect in zero magnetic field and at room temperature. The resistance of such films can be both decreased and increased through multiple nonvolatile intermediate levels by short electrical pulses. The effect provides a potential to develop a novel nonvolatile memory with high density, fast speed, and low power-consumption. An example of this effect has been seen for Pr0.7Ca0.3MnO3 films within which the thermal behavior of the film revealed a method for signal enhancement through annealing. An increase of 700% of the resistance ratio has been demonstrated for a film annealed at 170oC for 30 min. The effect is also observed to be active at room temperature but inefficient at low temperatures, which is interestingly contrary to the behavior of the colossal magnetoresistance effect and provides a clue to understanding the effect.
Unusual Magnetic Properties of Fe/Ni Bilayers on Cu(100)
Authors: Schirmer B. , Liu X. , Wuttig M.
Published: April 2011

Ultrathin iron films grown on Cu(100) have been found to exhibit a rich variety of structural and magnetic phases. In the present work, Fe/Ni bilayers have been prepared by molecular beam epitaxy to explore novel magnetic phenomena introduced by the ferromagnetic (FM) Ni underlayer. Unusual properties have been observed by measuring the temperature dependent magnetic properties. For 5.3 ML Fe on 7 ML Ni, a temperature dependent exchange coupling in the Fe film has been observed between the FM surface layer and FM interface layer.
β-FeSi2 thin-films grown by a pulsed laser deposition
Authors: Uekusa Shin-ichiro , Watanabe Yasuharu , Aida Yasuhiro , Miura Noboru
Published: April 2011

Orthorhombic β-FeSi2 thin-films were prepared on Si(100) and Si(111) substrates by a pulsed laser deposition method. When the substrate temperature was 500°C, β-FeSi2 thin-films were grown on Si(100) and Si(111) substrates. The thin-films grown on Si(100) and Si(111) substrates were polycrystalline and monocrystalline structures, respectively. The values of band-gap energy calculated from transmittance measurements were 0.71-0.72 eV. From Raman scattering measurements, it was found that the distortion due to the lattice mismatch between a β-FeSi2 thin-film and a Si substrate originates in the β-FeSi2/n-Si interface. Moreover, the fine crystals of β-FeSi2 existed in an amorphous thin-film which was grown on Si(111) substrate at room temperature (RT).

From van der Pauw measurements, conduction type, carrier concentration and Hall mobility were p-type, 1018-1021 cm−3 and 200-500 cm2/Vsec, respectively. The p-n diode characteristics of these heterostructure diodes were investigated by I-V and C-V measurements. The results indicate that the carrier distribution agrees with an ideal one-sided slope junction.
Mechanical, Adhesive and Thermodynamic Properties of Hollow Nanoparticles
Authors: warz U. S. Sch , Safran S. A. , Komura S.
Published: April 2011

When sheets of layeredmaterial like C., WS2 or BN are restricted to [ ]nite sizes, they generally form single- and multi-walled hollow nanoparticles in order to avoiddangling bonds. Using contin uum approaches to model elastic deformation and van der Waals in teractions of spherical nanoparticles, we predict the variation of mechanical stability, adhesive properties and phase behavior with radius R and thickness h. We find that mechanical stability is limited by forces in the nN range and pressures in the GPa range. Adhesion energies scale linearly with R, but depend only weakly on h. Deformation due to van der Waals adhesion occurs for single-walled particles for radii of few nm, but is quickly suppressed for increasing thickness. As R is increased, the gas-liquid coexistence disappears from the phase diagram for particle radii in the range of 1-3 nm (depending on wall thickness) since the in teraction rangedecreases like 1/R.
Li-insertion Behavior in NanocrystallineTiO2-(MoO3)z Core-Shell Materials
Authors: Moore Gregory J. , Guyomard Dominique , Elder Scott H.
Published: April 2011

A fundamental study of the Li insertion behavior of a series of materials consisting of a TiO2 core having MoO3 on the surface has been carried out in order to determine the influence of the shell. These TiO2-(MoO3)z materials, where (z) denotes the fraction of coverage from a partial to a double layer, range in diameter from 40-100 Å. Calculations have been done on their theoretical lithium capacity using a maximum of Li0.5TiO2 for the core, and Li1.5MoO3 at the TiO2/MoO3 interface, and they have been compared to that found experimentally. The reversible Li-insertion capacity was shown to increase from 0.34 per Ti for the pure TiO2 sample, to 0.91 Li per transition metal when the MoO3 coverage increased to one monolayer. There was only one plateau observed in the electrochemical scans for the samples showing that they function as a single-phase material making them interesting for electrodes. The redox voltage of the TiO2/Li0.5TiO2 biphasic transformation increased 60 mV from the pure TiO2 to the sample containing one monolayer of MoO3. This effect was interpreted as due to a change in TiO2 surface charge coming from an inductive effect of Ti-O-Mo bonds.
Vanadium Oxide Frameworks Modified with Transition Metals
Authors: Zavalij Peter Y. , Whittingham M. Stanley
Published: April 2011

It is well known that vanadium oxides readily form open structures that defines the potential use of vanadium compounds as cathode materials for lithium batteries. In order to stabilize open frameworks during electrochemical cycling and improve their performance, vanadium oxide can be modified with other transition metals. This work presents the structural analysis of vanadium oxide frameworks modified with Mn and Zn as well as reporting on several interesting new structures, such as TMA4[Zn4V21O58] and NaMn(VO3)4·2H2O.
Itinerant Vibrons and High-Temperature Superconductivity*
Authors: Goodenough John B.
Published: April 2011

The La2−xSrxCuO4 phase diagram is interpreted within the framework of a transition from localized to itinerant electronic behavior. In the underdoped region 0 < x < 0.1, holes in the x2 – y2 band are not small polarons; each occupies a mobile correlation bag of 5 to 6 copper centers at temperatures T > TF, a spinodal phase segregation into the parent antiferromagnetic phase and a polaron liquid is accomplished below TF by cooperative oxygen displacements. In the overdoped compositions > x > 0.25, holes are excluded from strong-correlation fluctuations within a Fermi liquid. In the intermediate range 0.1 < x < 0.25, the polaron liquid formed below room temperature changes character with increasing x and decreasing T. In the polaron liquid, mobile two-hole bags of four copper centers order with decreasing temperature into alternate CuO-Cu rows of a superconductive CuO2 sheet at a critical composition xc ≍ 1/6. It is argued that hybridization of itinerant electrons with optical-mode phonons propagating along the Cu-O-Cu rows produces heavy electrons responsible for high-temperature superconductivity.
(Rare Earth/Li) Titanates and Niobates as Ionic Conductors
Authors: Morata-Orrantia A. , García-Martín S. , Morán E. , Amador U. , Alario-Franco M. A.
Published: April 2011

The lithium ion conducting properties of materials of composition La0.58Li0.26TiO3, Nd0.58Li0.26TiO3, La0.67Li0.25Ti0.75Al0.25O3 and La0.29Li0.12NbO3 have been compared in relation with their microstructure. All the oxides have powder X-ray diffraction patterns characteristic of a perovskite-related structure with lattice parameters a∼√2ap, b∼√2ap, c∼2ap (p refers to cubic perovskite). However, some important differences are observed in their microstructure by SAED and HRTEM. Ordering between vacancies, Li+ and La3+ or Nd3+ and twinning of the NbO6 or TiO6 octahedra tilting system are shown in La0.29Li0.12NbO3 and Nd0.58Li0.26TiO3, which are the materials having a lower ionic conductivity. The La0.58Li0.26TiO3 and La0.67Li0.25Ti0.75Al0.25O3 oxides do not show ordering between cations.
Ferroelectric Tunability Studies in Doped BaTiO3 and Ba1−xSrxTiO3
Authors: Li Dong , Subramanian M. A.
Published: April 2011

Acceptor and Donor codoped BaTiO3 and Ba1−xSrxTiO3 are prepared. For Ba1−xLaxTi1−xFexO3,BaTiO3 remains as tetragonal phase up to about 5mol% LaFeO3. For x ≥0.06, the structure changes to cubic at room temperature. The phase change shifts the Curie temperature to lower value and increases the tunability at room temperature. Doping of other acceptor (Al, Cr) and donor (Sm, Gd, Dy) ions has the same effect although with varying levels of tuning. BaTiO3: 4%LaFeO3 has the highest tunability among the studied systems, which is even higher than Ba0.6Sr0.4TiO3. Co-doping of (La, Fe) and (La, Al) in Ba1−xSrxTiO3 also lowers the Curie temperature and increases the tunability of high Ba content samples at cryogenic temperature.
Electronic and Optical Studies of Coexisting 5- and 6-Atom Rings in Tetrahedral a-C
Authors: Valladares R.M. , Calles A.G. , Nelis M.A. Mc , Valladares Ariel A.
Published: April 2011

We simulate both the amorphous cluster a-C57H52 that contains 5-atom planar rings and 6-atom boat-type rings and the crystalline cluster c-C59H60 that contains only chair-type 6-atom rings, as in the tetrahedral crystal (diamond structure). We carry out ab initio calculations using the DMol code and report the total energy, the electronic (density of states) structure and optical properties of the two types of clusters, with and without relaxation of the structures, in order to see the effect of the types of atom rings found in the amorphous structures compared to those found in the crystal structure.
The Synthesis and Characterization of a Novel Gallium Diphosphonate
Authors: Attfield Martin P. , Harvey Howard G.
Published: April 2011

The gallium diphosphonate, Ga2[O3PC2H4PO3](H2O)2F2·H2O, has been prepared under solvothermal conditions from a HF/ pyridine solvent system. The structure of the material was determined by Rietveld refinement using the isostructural aluminium compound as the starting model for refinement. The structure consists of chains of corner sharing GaO4F2 octahedra with bridging F atoms linking the octahedra. The chains of octahedra are linked together in two directions perpendicular to the chain direction by the ethylenediphosphonate groups. The resulting structure contains a one-dimensional channel system in which extra-framework water molecules reside. Thermal gravimetric analysis of the material shows that 40 % of the extra- framework water can be reversibly absorbed. Thermodiffraction studies show that the material retains its structural integrity until all the extra-framework water is lost and then undergoes a considerable loss in crystallinity as the framework water molecules are lost. The crystallinity of the material then progressively decreases as the temperature is increased.
Microcrystalline Germanium Photodetectors
Authors: Krause M. , Stiebig H. , Carius R. , Wagner H.
Published: April 2011

For sensor applications in the detection of near infrared light we have prepared μc-Ge:H by plasma enhanced chemical vapor deposition (PECVD) with a mixture of germane and hydrogen, investigatedits structural and electronic properties and incorporated it into thin pin diodes. In order to ensure microcrystalline growth we had to use high hydrogen dilution. However, only the material prepared with a ratio of germane to hydrogen of 0.2% shows high crystallinity. The optical absorption is remarkably different from c-Ge and exhibits no indication of a direct gap at 0.8eV. When this material is implemented as part of a 110nm thin absorber, a short circuit current of 20mA/cm2 and a quantum efficiency of 15% at a wavelength of 1.1μm are achieved. Higher germane concentrations in hydrogen lead to poor electronic properties due to an increase of the amorphous phase and the short circuit current of the devices deteriorates. As for crystalline germanium photodiodes cooling of the devices is used to overcome the restrictionoriginating from the high free carrier concentration.
Hydrogen Diffusion Duringc Amorphous Silicon Growth and Its Consequences for the Transition to Nanocrystalline Growth
Authors: Crandall Richard S. , Thiesen Jack
Published: April 2011

Using a closed-form, time-dependent solution to the partial differential equation representing hydrogen diffusion in the frame of a growing silicon film we obtain considerable insight into processes controlled by mobile H in silicon. We apply this insight to quantitatively explain the limiting thickness for Si homoepitaxy and explain by analogy the incubation thickness before the transition from amorphous to nanocrystalline silicon growth.
p-type Window Layers for pin Solar Cells Entirely Fabricated by Hot-Wire CVD
Authors: Weber Urban , Koob Markus , Mukherjee Chandrachur , Chandrashekhar D. , Dusane Rajiv O.
Published: April 2011

We investigate a-SiC:H p-layer deposition for a-Si:H-based solar cells by Hot-Wire CVD using alternatively methane, ethane, and acetylene. Carbon incorporation in the film results from gas-phase reactions and not from direct dissociation at the hot filament for all hydrocarbon gases. Ethane can be dissociated more easily than methane allowing less extreme deposition conditions. With all types of materials the requirements of high dark conductivity and high band gap for the use as window layers in solar cells can be fulfilled. Highest conductivity is observed with ethane indicating a better network structure, which is supported by the IR signatures. A larger band gap (>2 eV) can be obtained at a similar conductivity with the use of acetylene. We compare these results with the utilization of [.proportional]c-Si:H p-layers. All types of p-layers are incorporated into pin solar cells. Methane- and ethane-based a-SiC:H-p-layers yield similar Voc and FF (∼850 mV and 72%). Acetylene-p-layer-based solar cells yield higher current and higher Voc (890 mV) but lower fill factor (∼67%). Microcrystalline p-layers improve Voc and FF up to 900 mV and 72%, respectively, however higher absorption leads to lower short circuit current and prevents an increase of initial efficiency beyond 8%. Using ethane for p-layer deposition, a significant improvement of the stability of all-Hot-Wire CVD pin solar cells is achieved.
Low-Temperature (630°C) Epitaxy of Silicon on Seeded Glass by Ion-Assisted Deposition
Authors: Aberle Armin G. , Harder Nils-Peter , Oelting Stefan
Published: April 2011

Thin polycrystalline silicon (poly-Si) films on glass are of great interest for electronic and photovoltaic applications. In this paper we report the successful realisation of thick (5 microns), large-grained (5 microns), continuous and smooth poly-Si films on glass at a temperature below 700°C. This significant technological milestone was achieved by using ion-assisted deposition (IAD) of silicon at 630°C for thickening a thin poly-Si seeding layer made on the glass at about 500°C by aluminium-induced crystallisation of amorphous silicon. Transmission electron micro-graphs reveal an epitaxial relation between about 50% of the grains of the IAD-grown layer and the seeding layer, while the remainder grew with a different orientation than the seed grains. The grain size of the IAD-grown poly-Si film is determined by the grain size of the underlying polySi seeding layer, a clear experimental proof of the “seeding layer concept”.
The Effects of Cu on Field Aided Lateral Crystallization (FALC) of Amorphous Silicon (a-Si) Films
Authors: Lee Jae-Bok , Kim Chul-Ho , Kwon Se-Youl , Choi Duck-Kyun
Published: April 2011

A novel concept of field aided lateral crystallization (FALC) and the effects of Cu on FALC of amorphous silicon (a-Si) were investigated. Cu was found to induce the lateral crystallization toward a metal-free region as well as the crystallization of a-Si in contact with Cu. In particular, the lateral crystallization caused by Cu was noticeably accelerated at the negative electrode side in every pattern with an aid of electric field, while it was retarded at the positive electrode side. The occurrence of Cu-FALC phenomenon was interpreted in terms of dominant diffusing species (DDS) in the reaction between metal and Si. The FALC velocity increased with the applied field intensity and the annealing temperature. The crystallization of a-Si was achieved at temperatures as low as 375°C when the annealing time increased in the presence of high electric field, above 30V/cm. Therefore, we could demonstrate the possibility of low temperature (<500°C) polycrystalline silicon (poly-Si) crystallization using Cu as a mediator in FALC technology.
Post-Deposition Sulfur Incorporation into CuInSe2 Thin Films
Authors: Titus Jochen , Schock Hans-Werner , Birkmire Robert W. , Shafarman William N. , Singh Udai P.
Published: April 2011

The effect of initial film composition and substrate in the sulfurization of CuInSe2 was investigated. CuInSe2 films deposited on either soda-lime glass (SL) or Corning 7059® borosilicate glass (7059) substrates were reacted in flowing H2S for times from 1 to 8 hours. Films with Cu-rich composition, Cu/In > 1, reacted for 1 hour had nearly all the Se replaced by S. For Cu-poor films the incorporation of S was significantly reduced. In addition, in Cu-poor films on SL glass CuInS2 and NaInS2 were found at the film surface. These phases were not detected in films on 7059 substrates or in Cu-rich films. A phenomenological model is proposed to explain the formation of segregated surface phases in Cu-poor films on SL substrates.
Thermochemical and Kinetic Aspects of Cadmium Telluride Solar Cell Processing
Authors: McCandless Brian E.
Published: April 2011

A chemical and kinetic basis underlying processing strategies for thin film polycrystalline CdTe/CdS solar cell fabrication is presented. The processing conditions employed for moderate and high conversion efficiency CdTe/CdS solar cells fall within a consistent framework based on temperature and concentration of CdCl2 and O2 species during film deposition or thermal treatment. Detailed microstructural and compositional results are compared for thin-film CdTe/CdS structures with CdTe deposited by physical vapor deposition, electrodeposition and close-space sublimation. X-ray diffraction coupled with a model for diffusion of CdS into CdTe is used to determine the effect of treatment conditions on bulk and grain boundary diffusion coefficients. The effects of CdS diffusion, oxides, recrystallization and grain growth with respect to device efficiency, stability and process control over large area are discussed.
Molecular Level Approach for Low-Temperature Synthesis of CuInSe2 Films
Authors: Menezes Shalini
Published: April 2011

This paper presents a new approach for relatively rapid and inexpensive synthesis of morphologically homogenous CuInSe2 absorber layers without high-temperature treatment. The approach, molecular layer electrodeposition, combines the low-cost, scale-up and environmental advantages of electrodeposition with the atomic level control of epitaxial deposition. This patented method entails building a superlattice of CuInSe2 monolayers by electro-deposition from a single electrolyte. The monolayer deposition approach extends the utility of epitaxial methods to device fabrication with simpler apparatus and practical deposition rates. The paper outlines the concepts underlying molecular layer electrodeposition, describes the special deposition apparatus and presents the results of initial investigations involving the synthesis of a ternary compound in a flow cell, controlled at molecular level.
Improvements in Non-vacuum Deposition of Particulate Materials
Authors: Eberspacher Chris , Pauls Karen , Serra Jack
Published: April 2011

One of the most promising strategies for lowering the cost of photovoltaics (PV) is the use of thin-film technologies in which thin films are formed from particulate precursor materials deposited by non-vacuum techniques. The quality of particle-based PV thin films and in turn the sunlight-to-electricity conversion efficiencies and cost structures of devices based these films are strongly affected by the planarity and packing density of the layers of particles from which the films are formed and by the materials use efficiency of the processes used to deposit the layers. This paper reports progress made in depositing planar, well-packed layers of particles using non-vacuum spray deposition techniques.
Composition dependent doping and transport properties of CuGaSe2
Authors: Siebentritt Susanne , Gerhard Andreas , Brehme Stephan , Lux-Steiner Martha Ch.
Published: April 2011

Chalcopyrites are doped by intrinsic defects, therefore their doping behavior depends on their composition. The doping and transport properties of epitaxial CuGaSe2 layers prepared under varying Cu excess have been investigated by temperature dependent Hall effect and conductivity measurements. Two acceptors, 134 meV and 80 meV deep, and a high degree of compensation, increasing with decreasing Cu excess, are found. The temperature dependence of the mobility indicates scattering with phonons, demonstrating high quality material. Defect scattering dominates at lower temperatures for CuGaSe2 grown under moderate Cu excess. CuGaSe2 grown under little or no Cu excess shows transport in a defect band at lower temperatures.
Preparation and Characterization of CdSxTe1−x Alloys and Films
Authors: Dhere R.G. , Albin D.S. , Asher S.E. , Moutinho H.R. , Compton D. , Gessert T.A.
Published: April 2011

Intermixing at the CdS/CdTe interface is considered crucial for the performance of CdTe-based solar cells. High-efficiency devices have been fabricated in spite of large lattice mismatch between CdS and CdTe. Intermixing and formation of CdSxTe1−x alloys in present-generation devices result from high-temperature CdTe deposition and post-deposition CdCl2 treatment. In this paper, we present our work on preparation of CdSxTe1−x-alloy powders and films. In this process, the CdS and CdTe powders are sized and mixed, isostatically pressed into a bar, sealed into closed-quartz ampoules, annealed at temperatures >1000°C, and water quenched. X-ray diffraction (XRD) analysis showed that they contained only distinct phases on S-rich and Te-rich sides of the phase diagram. Recently, two new batches of CdSxTe1−x material, produced by an improved process have been obtained (x =0.71 and x= 0.35 in CdSxTe1−x alloy). Initial XRD results from the films deposited by close spaced sublimation (CSS) using these powders reveal that both samples of bulk powders (prior to CSS deposition) are single-phase and demonstrate CdS-like characteristics (hexagonal). We have deposited CdSxTe1−x-alloy films using these powders. These alloy powders evaporate more congruently as opposed to mixed powders of CdS and CdTe. We will also present results on the structural properties of thin CdSxTe1−x alloy films deposited using these powders by CSS.
Characterization of the CuGaSe2/ZnSe Interface Using Kelvin Probe Force Microscopy
Authors: Sadewasser S. , Glatzel Th. , Rusu M. , Meeder A. , Fuertes Marrón D. , Jäger-Waldau A. , Lux-Steiner M.Ch.
Published: April 2011

To improve the efficiency of heterostructure solar cells based on chalcopyrite semiconductors a good understanding of the interface properties is crucial. By Kelvin Probe Force Microscopy it is possible to obtain laterally resolved images of the work function of semiconductor surfaces in addition to the topographical information usually obtained by noncontact atomic force microscopy. We studied the CuGaSe2/ZnSe interface prepared by growth of CuGaSe2 onto the (110) face of freshly cleaved ZnSe single crystals using chemical vapor deposition. We observed different work function values for different crystal facets on single CuGaSe2 grains. From the obtained work function data and surface photovoltage measurements a schematic band diagram for the CuGaSe2/ZnSe heterostructure is proposed.
Properties of RF Sputtered ZnTe:N films for Back Contact to CdS/CdTe Solar Cells
Authors: Drayton Jennifer , Gupta A. , Makhratchev K. , Price K. J. , Bohn R. G. , Compaan A.D.
Published: April 2011

Most of the low resistance back contacts formed on high efficiency CdS/CdTe solar cells involve copper either in elemental form (such as Cu/Au back contacts) or as dopant in other material (such as Cu-doped ZnTe). But copper is also suspected to be a cause of degradation of devices in long-term stability tests due to its high diffusion coefficient in polycrystalline CdTe. In this paper, we present results on the development of nitrogen-doped ZnTe back contacts for CdS/CdTe solar cells. Reproducible N-doped p-ZnTe films were prepared using reactive RF magnetron sputtering with Ar/N2 gas mixtures. The conductivity of the doped ZnTe films was more than five orders of magnitude higher than that of intrinsic films. We find that annealing in air can further increase the conductivity. Efficiencies near 10% have been achieved with a ZnTe:N/Ni back contact.
Interface Investigation of ZnO/CdS/CuIn1−xGaxSe2/Mo Solar Cells
Authors: Hasoon F.S. , Al-Thani H.A. , Jones K.M. , Yan Y. , Moutinho H. R. , Young M. , Asher S. E. , Alleman J. , Bhattacharya R. , Ramanathan K. , Keane J. , Al-Jassim M.M.
Published: April 2011

Graded-band-gap CuIn1−xGaxSe2 (CIGS) absorbers with Ga/Ga+In value in the 20%-30% range have a demonstrated efficiency of 18.8%. For CdS-containing devices, the shortcircuit current density (Jsc) has almost reached its expected maximum. However, the open-circuit voltage of CIGS solar cells is limited by the surface microstructure and chemistry. In this work, we examine the microstructural properties and chemistry of CIGS. We also attempted to correlate the above observations and device performance.
Cu(In,Ga)Se2 Thin-Film Evolution During Growth from (In,Ga)2Se3 Precursors
Authors: AbuShama Jehad , Noufi Rommel , Yan Yanfa , Jones Kim , Keyes Brian , Dippo Pat , Romero Manuel , Al-Jassim Mowafak , Alleman Jeff , Williamson Don L.
Published: April 2011

We examined the Cu(In,Ga)Se2 thin-film growth from (In,Ga)2Se3 precursors and found that the evolution of the microstructure and intrinsic native defects depends on the compositional changes that occur as the film transitions from being Cu rich to In(Ga) rich.
Characterization and Photoelectrochemistry of Layer-by-Layer Self Assembled Films of CdS Quantum Dots in Polyelectrolyte Matrix
Authors: Halaoui Lara. I.
Published: April 2011

Photoelectrochemical studies of polyacrylate-capped CdS nanoparticles embedded in a polyelectrolyte matrix on electrode surfaces are presented. Multilayered films of polyacrylatecapped Q-CdS are assembled layer-by-layer on solid surfaces modified with a cationic polyelectrolyte, poly(diallyldimethylammonium chloride). The characterization of the layer-bylayer growth by means of UV-visible absorption, photoluminescence, FT-IR spectroscopy and ellipsometry measurements revealed the incorporation of the nanoparticles in the matrix and the reproducible surface-charge reversal taking place. In this paper, we present initial photocurrentvoltage studies of films thus formed, where the feasibility of charge conduction through the polyelectrolyte matrix and the dependence of photocurrent magnitude on the number of bilayers are demonstrated. In addition, the dependence of photocurrent direction on the electrode potential, and the observation of anodic photocurrent spikes attributed to surface states in the presence of dissolved oxygen are reported.
When, Why and Where are CdTe/CdS Solar Cells Stable?
Authors: Dobson K. D. , Visoly-Fisher Iris , Jayakrishnan R. , Gartsman K. , Hodes G. , Cahen David
Published: April 2011

The role of Cu in CdTe/CdS solar cell instability remains the subject of much debate. The investigation of a range of ‘Cu’-contacted CdTe/CdS cells, which had received various thermal stress treatments, is described. Cells that were stressed in air exhibit strong current-voltage (I-V) rollover and junction degradation. No such degradation was observed for ‘Cu’-contacted cells that had been stressed in dry-N2 atmosphere. Cu is found to diffuse rapidly through the cell structure during back contact annealing and to accumulate in the CdS layer. With stress, significant levels of Cu dope the grain bulk, producing (with Cl) high resistance, photoconducting CdS. This behavior is independent of stress atmosphere and is, therefore, unlikely to (initially) be a dominating mechanism for cell degradation. Our results suggest simple air oxidation of the back contact interface to be a likely origin of I-V rollover in CdTe/CdS cells.
Ultra-Thin Zirconium Oxide Films Deposited by Rapid Thermal Chemical Vapor Deposition (RT-CVD) as Alternative Gate Dielectric
Authors: Chang Jane P. , Lin You-Sheng
Published: April 2011

ZrO2 ([.kappa]∼ 18) was deposited on Si(100) wafers by rapid thermal chemical vapor deposition (RT-CVD) process to replace SiO2 as the gate dielectric material in metal-oxide- semiconductor devices for its high dielectric constant, good thermal stability on silicon, and large bandgap. The deposited films are nearly stoichiometric, amorphous, uniform, and highly smooth, as determined by X-ray photoemission spectroscopy, X-ray diffraction, ellipsometry, and atomic force microscopy. The high resolution transmission electron microscopy (TEM) image shows an interfacial ZrSiO4 layer between ZrO2 and the silicon substrate, and this interfacial layer is verified by thermodynamic calculations and etching resistance measured at the interface. Excellent step coverage was observed for depositing ZrO2 on nanometer scale features with an aspect ratio of 4. The dielectric constant of RTCVD ZrO2 was 15-18, with small C-V hysteresis and low leakage current.
Annealing Behaviour of WSix Films Prepared By CVD
Authors: Katiyar M. , Samal G. S. , Deepak R. K. Gupta , Sahoo P. K. , Kulkarni V. N. , Adetutu O.
Published: April 2011

Tungsten silicide (WSix) films, deposited by chemical vapour deposition are normally amorphous in nature, and need to be annealed at high temperature to obtain low resistivity required for interconnections and metallization layers in VLSI circuits. In this paper, we focus on this annealing process for films deposited on Si and SiO2 substrate, and having Si/W ratio of 2.4. The characterization methods used were time-resolved X-ray diffraction, Resistivity measurement, and Rutherford backscattering spectroscopy analysis. We observe that 30 minutes is not sufficient for complete transformation of the WSi2.4 films on Si substrate. We also report on the dependence of annealing behaviour of nonstoichiometric WSixfilm on the substrate type.
A Study on Solid Phase Reactions of Ni and Pt on Si-Ge Alloys as Contacts to Ultra-Shallow P+N Junctions for CMOS Technology Nodes Beyond 70nm
Authors: Liu Jing , Mo Hongxiang , Ötürk Mehmet C.
Published: April 2011

Recently, selectively deposited SiGeB alloys have been proposed to form ultra-shallow source/drain junctions for 35-70 nm CMOS. The technology provides super-abrupt junctions with above equilibrium dopant activation at temperatures lower than 800°C. In addition to their low resistivities, the lower bandgap of SiGeB provides the potential advantage of reducing the Schottky barrier height and therefore, the junction contact resistance. This is a critical concern for future CMOS technology nodes since the contact resistance will dominate the MOSFET series resistance unless new technologies yielding contact resistivities near 10−8 ω−cm2 are developed. This paper examines the solid phase reactions of Ni and Pt with SiGeB alloys in order to form self-aligned low resistivity contacts. The results show that both Ni and Pt can form germanosilicides with low sheet resistances. Furthermore, both metals can form self-aligned contacts with a contact resistivity near 10−8 ω−cm2.
Mechanical behavior of thin Cu films studied by a four-point bending technique
Authors: Weihnacht Volker , Brückner Winfried
Published: April 2011

Four-point bending experiments in combination with thermal cycling of thin films on substrates were performed in a dedicated apparatus. Strains up to ±0.8% could be imposed into Cu films of 0.2, 0.5, and 1.0 μm thickness on Si substrates by bending the substrates at various temperatures in high vacuum. After relief of the bending, the residual stress was measured by the wafer-curvature method. At temperatures below 250°C, the yield behavior is asymmetric in tension and compression. The amount of plastic strain introduced by external bending increases with film thickness, but the absolute values of the introduced plastic strains are very low throughout. At higher temperatures, there is no clear thickness dependence and no asymmetry in tension and compression. The results are discussed in connection with the formation of misfit dislocations during plastic deformation of thin films.
Mechanism for the Reduction of Threading Dislocation Densities in Si0.82Ge0.18 Films on Silicon on Insulator Substrates
Authors: Rehder E.M. , Kuan T.S. , Kuech T.F.
Published: April 2011

We have made an extensive study of Si0.82Ge0.18 film relaxation on silicon on insulator (SOI) substrates having a top Si layer 40, 70, 330nm, and 10[.proportional]m thick. SiGe films were deposited with a thickness up to 1.2[.proportional]m in an ultrahigh vacuum chemical vapor deposition system at 630°C. Following growth, films were characterized by X-ray diffraction and a dislocation revealing etch. The same level of relaxation is reached for each thickness of SiGe film independent of the substrate structure. Accompanying the film relaxation is the development of a tetragonal tensile strain in the thin Si layer of the SOI substrates. This strain reached 0.22% for the 1.2[.proportional]m film on the 40nm SOI and decreases with SOI thickness. The Si thickness of the SOI substrate also effected the threading dislocation density. For 85% relaxed films the density fell from 7×106 pits/cm2 on bulk Si to 103pits/cm2 for the 40, 70, and 330nm SOI substrates. The buried amorphous layer of the SOI substrate alters the dislocation dynamics by allowing dislocation core spreading or dislocation dissociation. The reduced strain field of these dislocations reduces dislocation interactions and the pinning that results. Without the dislocation pinning, the misfit dislocations can extend longer distances yielding a greatly reduced threading dislocation density.
Dislocation Core Spreading at Interfaces between Crystalline and Amorphous Solids
Authors: Gao Huajian , Zhang Lin , Baker Shefford P.
Published: April 2011

A fundamental question addressed here is concerned with the equilibrium structure of a dislocation core at an interface between a crystalline and an amorphous solid. This is motivated by experimental observations that the contrast of dislocations at an interface between a crystalline film and an amorphous substrate disappears under transmission electron microscopy. We have developed a mathematical moedl to describe the time-dependent behavior of dislocation core spreading as a function of the adhesive strength of the interface. The equilibrium core width and the rate of core spreading are determined in closed form solutions.
Dislocation Networks Strain Fields Induced By Si Wafer Bonding.
Authors: Eymery J. , Fournel F. , Rousseau K. , Buttard D. , Leroy F. , Rieutord F. , Rouvière J.L.
Published: April 2011

Buried dislocation superlattices are obtained by bonding ultra-thin single crystal Si (001) films on Si (001) wafers. The twist of two Si wafers induces a regular square grid of dissociated screw dislocations and the tilt a 1-D array of mixed dislocation. The Burgers vector is a/2 <110> for both types of dislocation. The atomic displacements and deformations of pure screw and edge dislocations are calculated with an isotropic elasticity approximation taking into account the free surface and the thickness of the upper crystal. It is shown by these calculations that the elastic strain field propagates up to the surface, and quantitative arguments are given to choose the network period / film thickness ratio.
Ge Layer Transfer To Si For Photovoltaic Applications
Authors: Zahler James M. , Ahn Chang-Geun , Zaghi Shahrooz , Atwater Harry A. , Chu Charles , Iles Peter
Published: April 2011

We have successfully used hydrophobic direct wafer bonding along with hydrogen-induced layer splitting of germanium to transfer 700 nm thick, single-crystal germanium (100) films to silicon (100) substrates without using a metallic bonding layer. The metal-free nature of the bond makes the bonded wafers suitable for subsequent epitaxial growth of layered solar cells at high temperatures without concern about metal contamination of the device active region. Contact mode atomic force microscopy images of the transferred germanium surface generated by the formation of micro-bubbles and micro-cracks along the hydrogen-induced layer-splitting interface reveals minimum rms surface roughness of between 10 nm and 23 nm. Electrical measurements indicated ohmic I-V characteristics for germanium layers bonded to silicon substrates with ∼400 Ω cm−2 resistance at the interface. Triple-junction solar cell structures grown on these Ge/Si heterostructure templates by metal-organic chemical vapor deposition show comparable photoluminescence intensity and minority carrier lifetime to a control structure grown on bulk Ge. The use of a molecular beam epitaxy Ge buffer layer to smooth the cleaved surface of the Ge heterostructure has been shown to smooth the rms surface roughness from ∼11 nm to as low as 1.5 nm with a mesa-like morphology that has a top surface roughness of under 1.0 nm giving a promising surface for improved solar cell growth on solar cell structures.
Direct Bonding of Silicon Wafers with Simultaneous Dopant Diffusion
Authors: Grekhov Igor V. , Agrunova Tatiana S. , Kostina Lioudmila S. , Shmidt Natalia M. , Föll Helmut , Kostin Konstantin B.
Published: April 2011

Bonding of silicon surfaces in aqueous solution of compounds containing III and IV impurities was performed for the first time. It was observed that the presence of aluminum at the bonding interface improved structural quality of the interface. This phenomenon is explained by the increase of the contact area due to Al-OH group sandwiched between the water molecules adsorbed at hydrophilic wafer surfaces at the first bonding stage. The incorporation of Al produces a p-type layer and the I/V characteristics of the resultant np+n diodes is shown not to be influenced by the presence of the bonding interface. The technique developed could be advantageous for the design of multi-layer large area semiconductor devices.
Integration of InGaN-based Optoelectronics with Dissimilar Substrates by Wafer Bonding and Laser Lift-off
Authors: Wong William S. , Kneissl Michael , Treat David W. , Teepe Mark , Miyashita Naoko , Johnson Noble M.
Published: April 2011

InGaN-based optoelectronics have been integrated with dissimilar substrate materials using a novel thin-film laser lift-off process. By employing the LLO process with wafer-bonding techniques, InGaN-based light emitting diodes (LEDs) have been integrated with Si substrates, forming vertically structured LEDs. The LLO process has also been employed to integrate InGaN-based laser diodes (LDs) with Cu and diamond substrates. Separation of InGaN-based thin-film devices from their typical sapphire growth substrates is accomplished using a pulsed excimer laser in the ultraviolet regime incident through the transparent substrate. Characterization of the LEDs and LDs before and after the sapphire substrate removal revealed no measurable degradation in device performance.
Fabrication of Three-Dimensional Photonic Crystal by Wafer Fusion Approach
Authors: Yamamoto Noritsugu , Tomoda Katsuhiro , Noda Susumu
Published: April 2011

Based on a set of requirements identified for photonic crystals intended for use in optoelectronic devices, we have developed a method of fabricating three-dimensional photonic crystals that involves stacking air/semiconductor gratings by wafer fusion approach. Precise alignment of the stacked layers is achieved through the use of a laser beam assisted very precise alignment system, and three-dimensional photonic crystal has been successfully fabricated for the infrared and optical communication wavelength regions. We have also developed a photonic crystal waveguide providing sharp 90° bend.
Synthesis and Luminescent Properties of GaN and GaN-Mn Blue Nanocrystalline Thin-Film Phosphor for FED
Authors: Bondar Vyacheslav D. , Felter Thomas E. , Hunt Charles E. , Kucharsky Igor Yo. , Chakhovskoi Andrei G.
Published: April 2011

The technologies of fabrication of thin film phosphors based on gallium nitride using rf-magnetron sputtering are developed and the structural properties of these films are studied. Luminescence and electron spin resonance (ESR) spectra of GaN and GaN-Mn thin films are obtained. A correlation between cathodo-luminescence intensity and conductivity of GaN films is found. The nature of emission centers in GaN and GaN-Mn thin films is discussed and a mechanism of luminescence in these films is proposed.
Zinc Oxide Thin Films Deposited by RF Magnetron Sputtering on Mylar Substrates at Room Temperature
Authors: Fortunato Elvira , Nunes Patrícia , Marques António , Costa Daniel , Águas Hugo , Ferreira Isabel , Costa Maria E. V. , Martins Rodrigo
Published: April 2011

Aluminium doped zinc oxide thin films (ZnO:Al) have been deposited on polyester (Mylar type D, 100 μm thickness) substrates at room temperature by r.f. magnetron sputtering. The structural, morphological, optical and electrical properties of the deposited films have been studied. The samples are polycrystalline with a hexagonal wurtzite structure and a strong crystallographic c-axis orientation (002) perpendicular to the substrate surface. The ZnO:Al thin films with 85% transmittance in the visible and infra-red region and a resistivity as low as 3.6×10−2 ωcm have been obtained, as deposited. The obtained results are comparable to those ones obtained on glass substrates, opening a new field of low cost, light weight, small volume, flexible and unbreakable large area optoelectronic devices.
Image Acquisition Using Non-Pixeled Amorphous Silicon Based Sensors
Authors: Fernandes M. , Vygranenko Yu. , Martins J. , Vieira M.
Published: April 2011

We suggest to enhance the performance of image acquisition systems based on large area amorphous silicon based sensors by optimizing the readout parameters such as the intensity and cross-section of scanner beam, acquisition time and bias conditions. The main output device characteristics as image responsivity, signal to noise ratio and spatial resolution were analyzed in open circuit, short circuit and photodiode modes. The result show that the highest signal to noise ratio and best dark to bright ratio can be achieved in short circuit mode.

It was shown that the sensor resolution is related to the basic device parameters and, in practice, limited by the acquisition time and scanning beam properties. The scanning beam spot size limits the resolution due to the overlapping of dark and illuminated zones leading to a blurring effect on the final image and a consequent degradation in the resolution.
Long-Pulse Duration Excimer Laser Processing in the Fabrication of High Performance Polysilicon TFTs for Large Area Electronics
Authors: Fogarassy E. , Prévot B. , Unamuno S. de , Prat C. , Zahorski D. , Helen Y. , Mohammed-Brahim T.
Published: April 2011

In this work, was investigated both numerically and experimentally, the excimer laser processing of a-Si films deposited on SiO2-coated glass substrates, using the very large area (∼ 20 cm2) and long pulse duration (200 ns) excimer source from SOPRA Company. Experiments were carried out in air or in neutral atmosphere, using both the single- and multi-shot mode. From the microstructural analysis of the laser irradiated area the formation of a large-grained material through the so-called SLG regime was evidenced. In addition, the application of a multi-shot process was demonstrated to be very efficient to prepare uniform polysilicon layers with enlarged grain sizes (up to 1.5 µm after 20 shots). Finally, poly-Si TFTs prepared in the optimized conditions (multi-shot, neutral ambience) exibited field effect mobilities up to 235 cm2/V.s (for N-type) and 84 cm2/V.s (for P-type), with fairly uniform device characteristics over large area and excellent stability under electrical stress.
Strain Relaxation Mechanisms in He+-Implanted and Annealed Si1−xGex Layers on Si(001) Substrates
Authors: Christiansen S.H. , Mooney P.M. , Chu J.O. , Grill A.
Published: April 2011

Strain relaxation in He+-implanted and annealed Si(001)/Si1−xGex heterostructures was investigated using transmission electron microscopy techniques and x-ray diffraction. Depending on the implant conditions, bubbles and/or platelets form below the Si/Si1−xGex interface upon annealing and act as nucleation sources for dislocation loops. The dislocation loops extend to the interface and form a misfit dislocation network there, resulting in relaxation of 30-80% of the strain in layers as thin as 100-300 nm. When bubbles form close to the interface, dislocations nucleate by a climb loop mechanism. When smaller bubbles form deeper in the Si substrate an irregular three-dimensional dislocation network forms below the interface resulting in an irregular misfit dislocation network at the interface. When platelets form deeper in the Si substrate, prismatic punching of dislocation loops is observed and dislocation reactions of misfit dislocations at the interface result in Lomer dislocation formation.