bluuebunny/crossref_metadata_2025 huggingface dataset:
Stabilisation of Hg1-xCdxTe:Hg1-yCdyTex ≠ y) Heterointerfaces and Applications in Infra Red Devices
Authors: Clifton Paul A. , Brown Paul D.
Published: March 2011

The interface between Hg1-xCdxTe(0 ≦ x ≦ 1) and Hg1-yCdyTe(0 ≦ y ≦ 1) epitaxial layers of different composition (x ≠ y) is unstable with regard to the intermixing of the Hg and Cd cations within the Group II sublattice. This phenomenon may give rise to long-term stability problems in HgTe-(Hg,Cd)Te superlattices and composition grading between (Hg,Cd)Te absorber layers and CdTe buffer or passivation layers in epitaxial infra red detectors. In this paper, a novel approach to the inhibition of interdiffusion in these systems is discussed. This involves the growth of an intervening ZnTe barrier layer at the heterointerface between two (Hg,Cd)Te layers. Initial results are presented which indicate the effectiveness of this technique in reducing interdiffusion in an experimental heterostructure grown by MOVPE. Some possible applications in a variety of HgTe-based long wavelength devices are discussed.
Development of Infrared Detectors Based on Type II, InAsSb Strained-Layer Superlattices
Authors: Kurtz Steven R.
Published: March 2011

An overview is provided of long wavelength, photovoltaic detectors constructed with type II (also known as “staggered”), III-V superlattices. Specifically, the electronic properties of InAsSb strained-layer superlattices and prototype detectors utilizing these structures are described.
Characterization of Gasb-Based Alloy Semiconductors
Authors: Uekita H. , Kitamura N. , Ichimura M. , Usami A. , Wada T.
Published: March 2011

GaSb, AlxGa1-xSb, and AlxGa1-xSb epitaxial layers were grown by the liquid-phase epitaxy and characterized by photoluminescence, Raman spectroscopy, and double-crystal X-ray diffraction. The concentration of residual acceptors which are related to structural defects decreased with lowering growth temperature, but the GaSb epitaxial layer grown at an extremely low temperature of 270°C had poor crystalline quality. The AlxGa1-xSb (x≥0.15) and AlxGa1-xSb (x=0.02) epitaxial layers grown at 270 °C, however, had much better quality than the GaSb epitaxial layer grown at the same temperature.
Optical Properties of MBE-Grown Gainassb
Authors: Bi Wengang , Li Aizhen , Tan Songsheng
Published: March 2011

The infrared optical absorption properties near and above the fundamental absorption edge of MBE grown undoped GaInAsSb quaternary semiconductor alloy deposited on GaSb and GaAs substrates have been measured and analyzed at room temperature by means of a Fourier Transform Infrared Spectrometer, and were found to be fully characterized by the interband transition theory and Urbach's rule. The optical band gap of MBE-GaInAsSb has been determined using a linear extrapolation of (ɑhv )2 as a function of the photon energy hv, and the refractive index n deduced from the interference pattern, which shows good agreement with the theory of Sadao Adachi's.
Interface Properties of Heterovalent InSb Multilayer Structures
Authors: Kobayashi M. , Gunshor R. L. , Glenn J. L. , O Sungki , Han J. , Menke D.R. , Kolodziejski L. A.
Published: March 2011

Multilayer InSb/CdTe and InSb/MnTe heterostructures are grown by molecular beam epitaxy. The analysis of multilayer structures by transmission electron microscopy (TEM) and x-ray rocking curves confirms the presence of periodic heterointerfaces. TEM, x-ray, and x-ray photoelectron spectroscopy (XPS) measurements are found to provide a sensitive indicator for the presence of ultra-thin interfacial layers. Double barrier structures in the MnTe/InSb/MnTe configuration exhibit the negative differential resistance which is characteristic of resonant tunneling diodes.
Rbs and Pixe Analysis Of Hg1-xCdxTe grown by MOCVD.
Authors: Russo S.P. , Elliman R.G. , Johnston P.N. , Pain G.N.
Published: March 2011

The techniques, Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering Spectrometry (RBS) have been used to investigate compositional and thickness uniformity of Hg1-xCdxTe (MCT) grown on GaAs substrates by Metal Organic Chemical Vapour Deposition (MOCVD). Composition and thickness variations are reported for orientations perpendicular and parallel to gas flow in the MOCVD reactor. Crystalline quality of the MCT layer was also determined by RBS channelling analysis.
Raman and Secondary Ion Mass Spectroscopy of Epitaxial CdTe/InSb Interfaces Grown by Low-Energy Bias Sputtering
Authors: Das Suhit R. , Lockwood David J. , Rolfe Stephen J. , McCaffrey John P. , Cook John G.
Published: March 2011

Heteroepitaxial (100)CdTe || (100)InSb structures have been fabricated by growing CdTe epilayers, at growth temperatures below 200°C, on single crystal InSb substrates by low-energy bias sputtering. Controlled low-energy ion bombardment at the substrate was employed to clean the growth surface in-situ just prior to film deposition and to modify the growth kinetics and enhance adatom mobility during deposition. Raman spectroscopy of the interface revealed no evidence of In2Te3 and secondary ion mass spectroscopy showed the interface to be chemically abrupt.
Reactive Ion Etching of GaSb, (Ai,Ga)Sb, and InAs for Novel Device Applications.
Authors: La Tulipe D.C. , Frank D.J. , Munekata H.
Published: March 2011

-Although a variety of novel device proposals for GaSb/(Al,Ga)Sb/InAs heterostructures have been made, relatively little is known about processing these materials. We have studied the reactive ion etching characteristics of GaSb, (AI,Ga)Sb, and InAs in both methane/ hydrogen and chlorine gas chemistries. At conditions similar to those reported elsewhere for RIE of InP and GaAs in CH4/H2, the etch rate of (AI,Ga)Sb was found to be near zero, while GaSb and InAs etched at 200Å/minute. Under conditions where the etch mechanism is primarily physical sputtering, the three compounds etch at similar rates. Etching in Cl2 was found to yield anisotropic profiles, with the etch rate of (AI,Ga)Sb increasing with Al mole fraction, while InAs remains unetched. Damage to an InAs “stop layer” was investigated by sheet resistance and mobility measurements. These etching techniques were used to fabricate a novel InAs-channel FET composed of these materials. Several scanning electron micrographs of etching results are shown along with preliminary electrical characteristics.
Ion Implantation in InSb Grown on GaAs
Authors: Rao M. V. , Echard R. , Thompson P. E. , Berry A. K. , Mulpuri S. , Moore F. G.
Published: March 2011

Be, S, Si, and Ne implantations were performed at room temperature into InSb layers grown on undoped semi-insulating GaAs substrates. The implant damage in InSb is of ntype behavior. The implanted material was subjected to both isochronal and isothermal annealing schemes using a molybdenum strip heater. A maximum p-type activation of 90 % and si-type activation of 16 % was achieved for Be and S implants, respectively. Si implant has an amphoteric doping behavior.
The Epitaxial Growth by Movpe of (Hg,Mn)Te On (001) Gaas Substrates
Authors: Al-Allak H. M. , Brinkman A. W. , Clifton P. A. , Brown P. D.
Published: March 2011

Epitaxial thin films of the dilute magnetic semiconductor Hg1-xMnxTe have been grown by MOVPE on (100) GaAs substrate with or without buffer layers. Deposition took place in a horizontal, atmospheric pressure reactor at temperatures in the range 350–400° C, using tricarbonyl (methyl cyclopentadienyl) manganese, di-isopropyl tellurium and elemental mercury. Buffer layers consisted of thin layers of ZnTe and an upper thick layer (∼1μm) of CdTe. The good crystallinity was confirmed by RHEED and double crystal x-ray diffraction with rocking curve widths of 500–600 arc. sec. for non-buffered layers, and 315 arc. sec. for buffered layers. TEM investigations show that layers grown on buffered substrates had improved microstructure defect content. Electrical transport measurements revealed that as-grown layers were ptype with Hall mobilities in excess of 0.1 m2V−1s−1.
A Systematic TEM and Rheed Investigation of the MBE Growth of InxGa1−xAs a Function of Composition
Authors: Edirisinghe S.P. , Staton-Bevan A.E. , Pashley D.W. , Fawcett P. , Joyce B.A.
Published: March 2011

0.15µm epilayers of InxGa1−xAs grown on GaAs (001) by MBE, having In concentrations in the range x = 0.05 - 0.30, have been investigated using RHEED and TEM. RHEED patterns indicate a 2-D growth mode for low In concentrations changing to Stranski-Krastanov growth for x > 0.30. TEM showed misfit dislocations for x > 0.05 only, which were found to relieve only a small part of the misfit strain. Although threading dislocations were rarely found in the epilayers, dislocations originating at the interface and penetrating the buffer layer were observed for 0.1 < x < 0.25.
Photovoltaic Infrared Devices in Epitaxial Narrow Gap Lead Chalcogenides on Silicon Substrates
Authors: Zogg H. , Maissen C. , Masek J. , Hoshino T. , Blunier S.
Published: March 2011

We review MBE growth of epitaxial IV-VI layers on Si(111) substrates and fabrication of photovoltaic infrared devices in the layers. Cut-off wavelengths are chemically tailored from 3 µm up to above 12 µm by using PbS, PbTe, Pb1−xEuxSe and Pb1−xSnxSe. An intermediate epitaxial stacked CaF2-BaF2 bilayer of 200 nm thickness serves to overcome the large lattice- and thermal expansion mismatch, and device quality IV-VI layers are obtained with layer thicknesses of only 2–4 µm. The layers are untwinned single crystal, exhibit perfectly smooth surfaces with surface defect concentrations down to 103 cm−2, and x-ray rocking curve line-width of ≈150 arcsec. Despite the large thermal expansion mismatch, the (111)-oriented layers withstand multiple cooling cycles down to 15K without problems.

Although our IR-device fabrication technique is far from optimized, the sensitivities of our best photovoltaic sensors are comparable to MCT. IV-VI on Si IR sensors have the potential for a low cost technique of large IR focal plane arrays both for the 3–5 µm and 8–12 µm range because of the easy fabrication procedure and because uniformity problems are much less severe in IV-Vls due to the weaker dependence of the band-gap of Pb1−xSnxSe on composition x compared to MCT.
Experimental Method of Determination of Structural Correlations in Surface Layers of Oxide Glass
Authors: BochyŃski Zenon
Published: March 2011

A new method of X-ray diffraction analysis of structural inhomogeneities in the quartz/Si02/n based inorganic glasses is presented. The method enables the determination of structural changes occuring in the real nodal lattice in the regions of 10…20 Å or more as well as substructural changes in the regions 5…15 Å comparable to the molecular size of SiO2…SiO4. In consequence these changes can be correlated with approximate nodal lattice models of different degree of ordering. The applied method provided the possibility of constructing structural models of nodal lattices describing the surface and inner layers of the real glasses, changes in the local inhomogeneities as well as boundaries in water-gel associates.
Chemical Design and Structural Chemistry of Lwir Optical Materials
Authors: Lowe-Ma C.K. , Kipp D.O. , Vanderah T.A.
Published: March 2011

Some applications for long-wavelength infrared (LWIR) windows require environmental durability and mechanical strength in addition to infrared optical transparency; i.e., the windows must simultaneously serve as optical and as structural ceramics. The requirement of optical transparency at long IR wavelengths eliminates from consideration all ceramics based on oxides and other light-anion compounds, making this a particularly difficult materials problem. The structure-property relationships and chemical rationale used to guide both the screening of known compounds and the synthesis of new compounds likely to possess the desired properties rely on factors such as atomic mass, electronic configuration, coordination number, and crystal structure type.

Our research has included the directed synthesis and characterization of a number of ternary indium sulfides as well as ternary calcium yttrium sulfides. Ternary indium sulfides feature both tetrahedral and octahedral coordination of indium. The crystal structure of KInS2 and its relationship to structures observed in other systems such as AIn2S4, A = Ca,Sr,Ba, is described. The crystal structure of CaY2S4 along with studies of yttrium-doped CaS are also described. The AIn2S4 compounds are more fully described in references [1] and [2].
Tem Studies of CD:ZN:TE-Based II-VI Superlattices and Epitaxial Layers
Authors: Brown P.D. , Kelly H. , Clifton P.A. , Mullins J.T. , Simmons M.Y. , Durose K. , Brinkman A.W. , Golding T.D. , Dinan J.
Published: March 2011

A TEM study is presented charting the development of a MOVPE growth process for the deposition of CdTe//ZnTe superlattices. In addition, MBE grown (Cd,Zn)Te//CdTe superlattices deposited onto GaAs and InSb substrates are compared.
A Study of the Interface Between THM Grown MCT Crystals and Cdte Seeds.
Authors: Shilo I. , Kedar E. , Szafranek D.
Published: March 2011

HgCdTe crystals have been grown from CdTe seeds by the travelling heater method (THM). Three different kinds of interfaces between the growing crystal and the seed were found: a sharp planar interface; a non-planar interface which is caused by meltback near the ampoule walls; a diffuse zone, where an interface cannot be discerned. A correlation was found between the initial growth parameters and the interface structure. Microprobe analysis revealed a boundary layer which, in the case of the planar interface, had an unusual shape. This layer had a thickness varying from 400 to 800 microns. It consisted mostly of HgTe, and contained holes as defects.
GaAs Quantum Well Infrared Photodetectors Grown by OMVPE
Authors: Hobson W. S. , Zussman A. , Levine B. F. , Pearton S. J. , Swaminathan V. , Luther L. C.
Published: March 2011

We have grown, fabricated, and measured GaAs quantum well infrared photodetectors (QWIPs) using organometallic vapor phase epitaxy (OMVPE). The epitaxial layers were characterized by electrochemical capacitance-voltage profiling, double-crystal X-ray diffraction, cathodoluminescence, and infrared absorption. Dark current, responsivity spectra, and detectivity were measured for the QWIP devices. The performance of these QWIPs was comparable to detectors grown using MBE. This is of importance since OMVPE has advantages for wafer throughout and cost.
Double Implantation in GaAs
Authors: Kozlov I.P. , Odjaev V.B. , Prosolovich V.S. , Yankovsky Yu.N.
Published: March 2011

High mobility of electrons, the possibility of formation of semi-insulating layers excites an interest to GaAs. The implantation of donor impurities into semi-insulating GaAs:Cr has a great practical application, allowing to create the conducting suhnicrcn layers. To obtain the functional suhmicron n+-layers one uses mainly the implantation of ions of Si, that accounts for its small mass and the lack of the necessity for irradiation at higher teMperatures. Silicon is the amphoteric impurity in GaAs. At low doses ≤1013 Cm−2 it behaves as donor inpurity with the high level of activation, though at the increase of the implantation dose some of the siliccn atoms begin to substitute arsenic and in this position they display the properties of acceptor impurity, leading to self-compensation. Its amphoteric properties were shown with the help of As+Si and Ga+Si double implantation [1]. The increase of activation of Si, as donor impurity, can be obtained by double implantation of 31p+28Si [2]. Phosphorus has atomic mass close to Si, it is isoelectronic to As and does not cause compensation effect in GaAs up to the doses ≃1015 cm−2. Moreover double implantation allows to vary the magnitude of the diffusion coefficients of impurities.
The Role of Rare Earths in Narrow Energy Gap Semiconductors
Authors: Partin D.L. , Heremans J. , Morelli D.T. , Thrush C.M.
Published: March 2011

Narrow energy band gap semiconductors are potentially useful for various devices, including infrared detectors and diode lasers. Rare earth elements have been introduced into lead chalcogenide semiconductors using the molecular beam epitaxy growth process. Europium and ytterbium increase the energy band gap, and nearly lattice-matched heterojunctions have been grown. In some cases, valence changes in the rare earth element cause doping of the alloy. Some initial investigations of the addition of europium to indium antimonide will also be reported, including the variation of lattice parameter and optical transmission with composition and a negative magnetoresistance effect.
Large Area (100)CdTe and CdZnTe Layers on (100)GaAs Grown by Hot Wall Beam Epitaxy for Hgcdte Photovoltaic MWIR-Detectorarray Application
Authors: Humenberger J. , Gresslehner K.H. , Schirz W. , Lischka K. , Sitter H.
Published: March 2011

The newly developed Hot Wall Beam Epitaxy was used for the growth of (100)CdTe and (100)Cd1-yZnyTe layers (y ∼ 0.04) on 1 inch (100)GaAs wafers. The structural properties of the epilayers were investigated by means of high resblution x-ray rocking curve measurements. The results demonstrate the high crystalline perfection of the layers. The CdTe and Cd1-yZnyTe/GaAs wafers were used as substrates for the growth of Hg1-xCdx Te by close space vapor phase epitaxy. The layers show excellent homogeneity. Across a 1 inch wafer the composition x varies by ±0.0026, and the width of the rocking curve (FWHM) by ±10arcsec. Values of the FWHM as low as 50arcsec for Hg1-xCdxTe (x = 0.32) on Cd1-yZnyTe/GaAs were obtained. Linear arrays of planar photovoltaic detectors were fabricated. Arrays of 32 elements show high uniformity of the cutoff wavelength and the responsivity. The variation of the cutoff wavelength is ±1.5% (±0.04μm) and the variation of the responsivity is ±3%.
Tem Assessment of Defects in (CdHg)Te Heterostructures
Authors: Lawson-Jack S.G. , Jones I.P. , Williams D.J. , Astles M.G.
Published: March 2011

Transmission electron microscopy has been used to assess the defect contents of the various layers and interfaces in (CdHg) Te heterostructures. Examination of cross sectional specimens of these materials suggests that the density of misfit dislocations at the interfaces is related to the layer thicknesses, and that the high density of dislocations which are generated at the GaAs/CdTe interface are effectively prevented from penetrating into the CdHgTe epilayer by a 3um thick buffer layer. The majority of the dislocations in the layers were found to have a Burgers vector b = a/2<110> and either lie approximately parallel or inclined at an angle of ∼ 60° to the interfacial plane.
Two-Layer LPE Hgcdte P-on-n 8-18μm Photodiodes
Authors: Krueger E. E. , Pultz G. N. , Norton P. W. , Mroczkowski J. A. , Weiler M. H. , Reine M. B.
Published: March 2011

This paper reports recent results on two-layer P-on-n LPE HgCdTe heterojunction photodiodes with cutoff wavelengths beyond 19μm. These results demonstrate the potential of photovoltaic HgCdTe detectors to satisfy the detector requirements of advanced NASA satellite instruments out to wavelengths of 17μm.
Detection of Free Liquid in Cement-Solidified Radioactive Waste Drums Using Computed Tomography
Authors: Steude J.S. , Tonner P.D.
Published: March 2011

Acceptance criteria for disposal of radioactive waste drums require that the cement-solidified material in the drum contain minimal free liquid after the cement has hardened. Free liquid is to be avoided because it may corrode the drum, escape and cause environmental contamination. The DOE has requested that a nondestructive evaluation method be developed to detect free liquid in quantities in excess of 0.5% by volume. This corresponds to about 1 liter in a standard 208 liter (55 gallon) drum. In this study, the detection of volumes of free liquid in a 57 cm (2′) diameter cement-solidified drum is demonstrated using high-energy X-ray computed tomography (CT). It is shown that liquid concentrations of simulated radioactive waste inside glass tubes imbedded in cement can easily be detected, even for tubes with inner diameters less than 2 mm (0.08″). Furthermore, it is demonstrated that tubes containing water and liquid concentrations of simulated radioactive waste can be distinguished from tubes of the same size containing air. The CT images were obtained at a rate of about 6 minutes per slice on a commercially available CT system using a 9 MeV linear accelerator source.
Imaging of Diffusion Processes in Differently Crosslinked Polystyrenes by 1H and 19F-Microscopy
Authors: Pfleiderer M. Ilg. B. , Albert K. , Rapp W. , Bayer E.
Published: March 2011

Polymeric beads are valuable tools for a variety of applications in chemistry, e.g. separation and synthesis support. A common property of all applications is, that diffusion processes and mass transport balances play an important part and are heavily dependent upon matrix parameters such as crosslinking. We have applied 1H and 19F-microscopy to monitor diffusion processes in differently crosslinked polystyrene beads (CL-PS). MRI is used primarily as a medical diagnostic method, but has begun to find increasing application in material science, including studies of fluids in polymers [1,2].
Characterization of Stressed and Crosslinked Polymers by 13C-CP/MAS and NMR Imaging
Authors: Kuhn Winfried , Theis Isolde , Koeller Elmar
Published: March 2011

Aging of polymers and elastomers is a time dependent alteration of the chemical composition and physical properties depending on the aging conditions. Chemical reactions and hence changes in the chemical composition result in many cases from influence by oxygen or solvents and treatment of the bulk material at high temperatures. Aging of the materials by mechanical strain as well as in an oxidative manner can alter the ratio of amorphous to crystalline portions in the polymer, break chemical bondings and hence can alter the physical behavior of the materials.
Characterization of Biomaterials with NMR
Authors: Garrido Leoncio , Pfleiderer Bettina , Ackerman Jerome L. , Moore John
Published: March 2011

Silicone based biomaterials are characterized with NMR. Bulk spin-lattice (T1) and spin-spin (T2) relaxation times are measured in polydimethylsiloxane (PDMS) model networks and various types of implants. The T2 results seem to indicate that crosslink densities of these biomaterials are lower than those of the PDMS model networks studied. 1H chemical shift NMR imaging techniques are developed to investigate the aging (e.g., migration of free polymer, rupture due to mechanical stress, etc.) of biomaterials in vivo.
A Study of Changes in Poly(Methyl Methacrylate) (pMMA) Resulting from the Adsorption of Methanol using NMR Imaging
Authors: Schuff Norbert , Hornung Phillip A. , Williams Evan H.
Published: March 2011

The understanding of the adsorption of solvents into materials is of great interest in different fields of material science. NMR imaging techniques can be used to follow the process of solvent adsorption by obtaining spatial and chemical information in a nondestructive way. We report here a study of the uptake of methanol (a non-solvent) into poly(methyl methacrylate) beads produced by suspension polymerization and into compression mouldings. In both cases, the uptake is far faster than the rate of diffusion measured elsewhere. We also illustrate the nonreversible nature of this uptake with respect to temperature cycling.
Oxygen-17 and Proton Mr Microscopy in Materials Analysis
Authors: Mateescu G.D. , Kinsey R.A. , Yvars G.M.
Published: March 2011

Preliminary results of two applications of 17O/lH NMR microimaging (MRμI) are described. In the first case, it is shown that oxygen-17 MRμI presents a unique advantage in situations where a strong proton background hinders the analysis of water ingress in polymeric materials. This is illustrated with 17O-enriched water diffusion into an agar gel from a central well. While proton imaging cannot readily distinguish between exogenous and endogenous water, 17O imaging makes a clear differentiation, due to the fact that the natural abundance gel has virtually no 17O background. In the second case, proton MRμI was used to analyze the structure and performance of commercial time-release pellets (ProMacR) containing an active ingredient (surfactant) dispersed into an inert matrix. This was done after leaching the detergent with natural abundance water or after drying the leached sample and refilling the pores with water. Pores ranging in size from 25 to 325 μm diameter are clearly visualized. The 1H images have been quantitated with commercial image analysis software in terms of the total porosity and the individual pore characteristics. Two types of water can be separately imaged: “bound” water with restricted mobility (interacting with the polymer matrix), and “free” water (in large pools). The majority of water is in the bound state. It is possible to evaluate the control-release performance of the product from the size of the pores, the uniformity of dispersion and the time course of detergent-water exchange.
NMR Imaging of Industrial Flow Processes
Authors: Sinton S. W. , Iwamiya J. H. , Chow A. W.
Published: March 2011

The application of nuclear magnetic resonance flow imaging to the study of Poiseuille flows of solid/liquid suspensions is described. The basic imaging method is presented along with results on suspensions with solid particle loading levels up to 52 vol. % and viscosities up to 7kPoise. The most concentrated suspensions are non-Newtonian, and the NMR flow data show evidence for deviations from a simple parabolic velocity profile in straight pipes. Cross-sectional intensity images derived from the flow image data on concentrated suspensions are non-uniform. Possible explanations for these results are discussed in terms of current rheological understanding of suspension flow and relaxation weighting phenomena in the NMR imaging experiments.
Coupled Effects of Light and Temperature on Degradation of a-Si:H
Authors: Benatar Lisa E. , Grimbergen Michael , Redfeeld David , Bube Richard H.
Published: March 2011

The effects of excitation rate and temperature on the kinetics and steady-state behavior of metastable defect formation in hydrogenated amorphous silicon (a-Si:H) have been studied. The dependences on temperature of the lifetime, τ, and stretching parameter, β, from a stretched exponential description of the kinetics were measured for one sample. We do not see a linear dependence of β on temperature over die entire temperature range studied (270K–370K), and τ increases monotonically with decreasing temperature. Steady-state results show defect density to be dependent on bodi temperature and excitation rate over the ranges measured (from 395K to 470K and from 6 × 1020 to 2 × 1022 s-l cm-3). The gradual change in temperature dependence is explained by a distribution of barrier heights between the ground and metastable states.
An Alternative Model for the Kinetics of Light-Induced Defects in A-Si:H
Authors: Santos Paulo V. , Jackson W. B. , Street R. A.
Published: March 2011

The kinetics of light-induced defect generation in a-Si:H was investigated over a wide range of illumination intensities and temperatures. The defect density around 1016cm-3 exhibits a power-law time dependence Ns ∼ G2εfε with ε = 0.2 to 0.3, where G is the photo-carrier generation rate. A model for the kinetics of defect generation is proposed based on the existence of an exponential distribution of defect formation energies in the amorphous network, associated with the valence band tail states. The model reproduces the observed time dependence of the defect density with an exponent e determined by the exponential width of the valence band tail. The temperature dependence of the defect generation rate is well-reproduced by the model, which provides a connection between the Stabler-Wronski effect and the weak-bond model.
Optically Addressable Input Circuit for Two-Dimensional Image Sensing
Authors: Hack M. , Lewis A. G. , Bruce R. H. , Lujan R.
Published: March 2011

Using the combination of a novel amorphous silicon (a-Si) photosensing element and four a-Si thin-film transistors (TFTs) we have developed a circuit which has been incorporated into a two-dimensional image sensor to provide gain at the pixel level. Our circuit only requires two global connections and the complete array is fully process compatible with standard a-Si TFT fabrication procedures. This contrasts with conventional a-Si imaging arrays which have no gain at the pixel level, require an additional global power line, and necessitate a separate sensor deposition.
Vibrational Spectra of a-Si:H Obtained by Pds
Authors: Lotter E. , Schubert M. B. , Heintze M. , Bauer G. H.
Published: March 2011

In the energy range below mid-gap, absorption spectra obtained by photothermal deflection spectroscopy (PDS) show much higher values than data evaluated from photo-current measurements. This difference can now be explained as an additional absorption arising from combined vibrational excitation of molecular hydrogen and Si-H stretching modes, which can occur when the H2 molecule is deformed in a collision with Si-H. Down to lower energies, our samples absorb even stronger than in mid-gap. Extension of spectral scans down to photon energies of 0.4 eV shows that this results from absorption of the high energy wings of the collision induced H2 absorption, which is superimposed by the first overtone of the Si-H and Si-H2 stretching modes. The analysis of the H2 absorption profile and strength on samples as grown and annealed gives evidence that molecular hydrogen is enclosed in small cavities and polarized by anisotropic environment in as-grown material while it is concentrated in larger voids under high pressure after annealing above 350°C.
Hydrogen Density of States Model for Amorphous Silicon and Alloys
Authors: Street R. A.
Published: March 2011

It is proposed that hydrogen reactions near the growing surface control the structure of plasma-deposited amorphous silicon and its alloys. The model explains how the plasma influences the dangling bond density and silicon weak bond distribution, which are the main parameters influencing the electronic properties. The hydrogen interactions account for the dependence of the electronic structure on growth conditions, the transition to microcrystalline growth in hydrogen-rich plasmas, and the different electronic properties of a-Si:H alloys. The proposed growth mechanisms are closely related to the metastability phenomena which occur in bulk a-Si:H films.
Auger and EELS Study of a-Si1−x Cx:H Alloys
Authors: Seta M.De , Narducci P. , Evangelisti F.
Published: March 2011

It is shown that Auger and electron energy loss spectroscopies can provide a well defined picture of the local bonding and structure of a-Si1−x Cx:H alloys. The samples were found macroscopically homogeneous but microscopically heterogeneous on a scale of few Angstroms. A simple model where the tetrahedral network is chemically ordered at low x values and carbon tends to segregate in C-C microclusters at high x values seems to reproduce the data very well. By fitting the C and Si Auger spectra it was possible to have a quantitative determination of the percentage of different configurations.
A New Self-Aligned A-SI TFT Using Ion Doping and Chromium Silicide Formation
Authors: Nishida S. , Uchida H. , Kaneko S.
Published: March 2011

A new self-aligned a-Si TFT has been developed. Ion doping and chromium silicide (CrSix) formation technique was used to fabricate source and drain, which are self-aligned to the gate electrode, instead of using the previously reported lift-off process. The fabricated TFT mobility is about 0.5cm2/V' sec and threshold voltage is about 3V. The ON/OFF ratio is over 106. The actions of as short as 2μm channel TFT have been confirmed, using a large area TFT process. These results show that this technique can be applicable to manufacturing high quality TFT-LCDs in a large area.
High Temperature off Current in a-Si TFTS - Effect of Process and Structure
Authors: Possin George E.
Published: March 2011

The OFF current in a-Si TFTs is an important parameter, especially for applications such as active matrix liquid crystal displays. In some demanding applications operation at 70°C or higher is required. This paper reports studies of the OFF current limiting mechanisms at room temperature and above. It is shown that the limiting mechanisms are hole injection from the drain junction at room temperature and electron conduction at higher temperatures. The importance of silicon thickness and interface state density at the passivation interface is stressed.
Effects of Temperature and Electrical Stress on the Characteristics of Amorphous Silicon Thin-Film Transistors
Authors: Park Jin S. , Oh Chang H. , Choi Hong S. , Han Min K. , Choi Yearn I. , Han Chul H.
Published: March 2011

The experimental and analytical results regarding to the effects of temperature and electrical stress on the characteristics of amorphous silicon thin-film transistors (a-Si TFT's) have been presented. The variations in the device parameters of a-Si TFT, such as threshold voltage and field-effect mobility, have been examined under various operating temperatures and electrical stress conditions. The hysteresis in the transfer characteristics and the trapped charges at the a-Si/silicon nitride interface were measured at the operating temperature ranges. From the experimental results, it has been found out that the increase of the interface charge trapping may be responsible for the degradation in the a-Si TFT characteristics. Also, an analytical formulation, employing the interface charge trapping, is presented to clarify the instability phenomena and verified successfully with the experimental results.
Temperature Dependence of a-Si:H Nip Diodes
Authors: van Berkel C. , Powell M. J. , French I. D.
Published: March 2011

We analyse the temperature dependence of the forward characteristics of a-Si:H nip diodes, which are characterised by a well defined exponential region at low bias and space charge limited current at higher bias. At each bias, the temperature dependence of the diodes shows a well defined activation energy and a linear dependence on bias exists for both the activation energy and exponential prefactor. We describe these phenomena with a first order temperature expansion of the diode quality factor n and present a physical interpretation in terms of electron and hole recombination in the i-layer.
Studies of MOS and Heterojunction Devices Using Doped μc-Si and a-Si
Authors: Lee D. R. , Bjorkman C. H. , Wang C. , Lucovsky G.
Published: March 2011

Current-voltage voltage characteristics of heterojunctions formed by remote plasma enhanced chemical vapor deposition (PECVD) of heavily doped μc-Si onto doped c-Si have been studied, as well as capacitance-voltage characteristics of MOS capacitor structures using heavily doped remote PECVD μc-Si and a-Si films as gate electrodes on thermally oxidized crystalline Si. Shifts in the flat-band voltages of MOS devices using the μc-Si and a-Si as gate electrodes relative to that of a reference Al/SiO2/c-Si structure are measured and explained in terms of a band structure model for the μc-Si and a-Si. Rectification and a photovoltaic effect observed in the pn heterojunctions are also explained in context of the same model.
Bias Stress Induced Instabilities in Amorphous Silicon Nitride / Crystalline Silicon and Amorphous Silicon Nitride / Amorphous Silicon Structures
Authors: Kanicki J. , Godet C. , Gelatos A. V.
Published: March 2011

The effects of positive and negative bias stress on hydrogenated amorphous silicon nitride / crystalline silicon and hydrogenated amorphous silicon nitride / hydrogenated amorphous silicon (a-Si:H) structures are investigated as a function of stress time, stress temperature and stress bias. It is shown that in both structures bias stress induces a parallel shift of the C-V (capacitance-voltage) characteristics. For a given stress bias the direction of the C-V shift depends on the sign of the applied stress voltage, while the magnitude of the C-V shift depends on stress time and temperature. In addition, it is shown that positive bias stress slightly increases the number of localized states in the a-Si:H mobility gap, but negative bias stress does not. These results lead us to conclude that the C-V shift is not induced by dangling bond defects in a-Si:H but rather by carrier trapping in the insulator.
Critical Assessment of Sub-Bandgap Primary Photocurrent in a-Si:H Based Solar Cells.
Authors: Zhou T. X. , Hegedus S. S. , Fortmann C. M.
Published: March 2011

The sub-bandgap primary photocurrent and the solar cell performance of a-Si:H p-i-n devices have been studied before and after light induced degradation. The results indicate significant discrepancy between the two methods when used to estimate the degree of degradation and the defect density in the i-layers. A preliminary explanation is proposed.
Simulation of Amorphous Silicon Hydride Solar Cells Using a Personal Computer
Authors: Shapiro Finley R.
Published: March 2011

The simulation of a-Si:H solar cells has the potential to make a major contribution to the development of improved devices. However, to achieve this potential, simulations must run quickly on personal computers. This will require simplified approaches which are justified by the correspondence of their results to more elaborate but slower running simulations.
Role of Sio at TCO/P Interface on the Electrical Properties of the P/I Junction
Authors: Carvalho C. , De Nijs J. M. M. , Martins R. , Guimarāes L.
Published: March 2011

The reduction of the ITO (Indium Tin Oxide) by the deposition of an a-Si:C:H p layer by means of a Plasma Enhanced Chemical Vapour Deposition (PECVD), is considerably diminished if a thin silicon monoxide layer (estimated to be between 5–50Å thick) is applied as a diffusion barrier. The amount of reduced indium diminishes (its concentration is two times lower), while the amount of silicon oxide is less although silicon monoxide was added on purpose. The influence of the silicon monoxide layer on the electrical properties of the p/i junction is shown by its J-V characteristics (in the dark and under illumination). We can see that with a suitable silicon monoxide thickness we can improve the short circuit current density (Jsc) and the rectifying ratio.
Electron Transport in a-Si:H at Low Temperatures
Authors: Nebel C. E. , Kocka J.
Published: March 2011

Time-of-flight and charge collection experiments performed on a-Si:H p-i-n structures in the temperature range 40–300K reveal electron drift mobilities which continuously decrease with decreasing temperature and electric field. At 40K, fields ≥ 2×105 V/cm are necessary to collect almost all carriers. A discussion of the data on the basis of a nearest neighbour hopping model shows the significant influence of the applied field on the hopping down process of electrons. Field-induced re-emission of deep trapped carriers back into shallow tail states establishes a narrow band of energy levels where electrons accumulate and propagate. The band width and energy position of the accumulation levels in the tail depends strongly on the applied field and the distribution of localized states. With increasing electric field, it shifts up in energy closer towards the conduction band edge and changes the hopping mobility for orders of magnitude. The calculated and experimental data are in reasonable agreement with the model of a hybrid distribution of localized states.
States in the Gap of Improved a-Ge:H Studied by Photomodulation Spectroscopy
Authors: Chen L. , Tauc J. , Pang D. , Turner W. A. , Paul W.
Published: March 2011

The photomodulation spectra of a-Ge:H of average photoelectronic quality(ημπ = 1 × 10-10cm2/V) and of improved quality (ημπ = 3 × 10-7cm2/V), produced under different plasma conditions in an r.f. diode reactor by glow discharge, were measured at 80K and are analyzed in analogy with earlier studies of a-Si:H. The spectra of the poorer material are dominated by transitions between dangling bond states and the conduction and valence bands. By contrast, the spectra of the better material require contributions of transitions from the band tail states, indicating that the reduced defect density has resulted in pump-beam induced quasi-Fermi levels reaching near the conduction and valence band edges. A very acceptable fit between plausible density-of-states distributions and the experimental spectra has been found.
In Situ Measurements of Hydrogen Flux, Surface Coverage, Incorporation and Desorption During Magnetron Sputter-Deposition of A-SI:H.
Authors: Abelson J. R. , Maley N. , Doyle J. R. , Feng G. F. , Fitzner M. , Katiyar M. , Mandrell L. , Myers A. M. , Nuruddin A. , Ruzic D. N. , Yang S.
Published: March 2011

High quality a-Si:H films are deposited by d.c. magnetron reactive sputtering of a Si target in an (Ar + H2) plasma. This paper reports the first comprehensive understanding of the growth process. The incident flux, surface H coverage, H2 release, and bulk H incorporation are determined using four in situ, real time techniques: double modulation mass spectroscopy, isotope replacement experiments, reflection absorption infra-red spectroscopy, and spectroscopic ellipsometry. In addition, the sputtered particle transport is simulated using Monte-Carlo techniques. For conditions which produce electronic quality a-Si:H, the total H flux arriving at the surface varies between 0.5 – 2 times the depositing Si flux; approximately half of this flux appears to reflect from the surface without interaction. The growth surface has excess H varying between 0.5 – 2 × 1015/cm2, and this surface H coverage is uniquely related to the bulk H incorporation.
Preparation of High-Quality a-SiGe:H films with Low Impurity Oncentration by the Hydrogen Dilution Method
Authors: Sayama Katsunobu , Haku Hisao , Dohjoh Hiroshi , Isomura Masao , Nakamura Noboru , Tsuda Shinya , Kishi Yasuo , Nakano Shouichi , Kuwano Yukinori
Published: March 2011

A-SiGe: H film properties with a low impurity concentration were investigated using a super chamber in the range of Ge content of 0% to 36%. These a-SiGe films can maintain a high photosensitivity of about 106 for Ge content up to 13%, which is comparable to high-quality a~Si films. It was found that impurity incorporation deteriorates optoelectronic properties in the range of a small amount of Ge content, and film rigidity in the range of a large amount of Ge content. Using high-quality a-SiGe films with a low impurity concentration in solar cells, the highest conversion efficiency of 3.15% was obtained for an a-SiGe single-junction cell under red light (AM-1.5, 100mW/cm2 light through the optical filter, in which the wavelength of transmitted light is longer than 650nm). A stacked cell of a-Si/a-Si/a-SiGe has a conversion efficiency of 11.9%.
Kinetics of Gas-Phase Chemical Reactions and Growth of a-SiC:H Films from Silane and Acetylene in a Remote Hydrogen Plasma Reactor
Authors: Johnson N. M. , Santos Paulo V. , Walker J. , Stevens K. S.
Published: March 2011

Gas-phase chemical reactions of interest for the deposition of amorphous silicon carbide in a remote hydrogen plasma reactor have been quantitatively characterized with electron spin resonance, and the deposition of a-SiC:H from silane and acetylene is demonstrated.
Substrate Induced Crystallinity in Reactive Sputter Deposition of Hydrogenated Silicon
Authors: Feng G. F. , Katiyar M. , Abelson J. R. , Maley N.
Published: March 2011

We use in situ multi-wavelength ellipsometry to study growth of hydrogenated amorphous silicon and crystalline silicon deposited by reactive DC magnetron sputtering. Crystal silicon covered with native oxide and oxide free crystal silicon are used as substrates. Deposition on c-Si/ox over a wide range of conditions yields amorphous films, and the growth proceeds with formation of small islands, regardless of hydrogen partial pressure. Growth on oxide free c-Si, on the other hand, is hydrogen dependent. Even at a low hydrogen pressure, a layer with mixed phase of c-Si and a-Si:H begin to appear near the c-Si substrate. This layer thickness and its c-Si fraction increase with hydrogen pressure. Bombardment of growing surface by energetic particles, both silicon and hydrogen species, appears to lead to the crystallization.
ArF Excimer Laser Doping into Amorphous Silicon thin films
Authors: Elliq M. , Slaoui A. , Fogarassy E. , Pattyn H. , Stuck R. , Siffert P.
Published: March 2011

Doping of amorphous silicon (a-Si or a-Si:H) coated by a spin-on oxide film containing the dopant (phosphorus or boron) using an ArF excimer laser has been investigated as a function of laser fluence, number of pulses and dopant film thickness. From these results, it is found that the surface concentration and the junction depth vary with the number of pulses, and that the doping process is rate limiting. Sheet resistance lower than 10 kΩ/□ have been obtained. It is also shown that for a-Si:H films, laser irradiation produces exodiffusion of hydrogen from the molten layer resulting in rough surface. This one-step process seems suitable for polycrystalline silicon thin film transistors (TFT's) fabrication.
Wide Bandgap Fluorinated Silicon-Carbide-Nitride Films Using NF3
Authors: Goh H. C. , Tan S. M. , Naseem H. A. , Ang S. S. , Brown W. D.
Published: March 2011

Amorphous hydrogenated silicon carbide has been studied extensively because of its properties as a wide bandgap material. However, a large amount of methane is needed to deposit the material. Also, the high carbon content of these films poses some problems. The addition of NF3 to the gas stream results in wide bandgap films with a substantial reduction in the required CH4 flow for deposition. Amorphous SiCx Ny :H:F films were prepared using rf glow discharge decomposition of silane, methane, and nitrogen trifluoride in a parallel-plate stainless steel reactor. Gas flow rate and power density were varied. For a gas mixture containing 6% NF3 and 78% CH4, FTIR measurements reveal a reduction in C-H peak heights at 2960 cm-1 and 2880 cm-1 with respect to the Si-H peak at 2080 cm-1 indicating a smaller carbon content in the film. The C-H peaks shift to higher wavenumbers with increasing NF3. The use of NF3 increases the bandgap from 2.6 to 3.14 eV while reducing the refractive index from 2.12. to 1.87. A maximum deposition rate of 625 A/min was achieved. This should be compared to the very low deposition rate of 18 A/min for comparable bandgap Si-C films deposited using 97% methane in silane. Increasing the deposition power density resulted in a larger bandgap and a smaller refractive index.
Methods of Improving Glow-Discharge-Deposited a-Si1−xGex:H
Authors: Tsuo Y. S. , Xu Y. , Ramsay E. A. , Crandall R. S. , Salomon S. J. , Balberg I. , Nelson B. P. , Xiao Y. , Chen Y.
Published: March 2011

We have studied methods of improving glow-discharge-deposited a-Si1−x Gex :H alloys deposited using silane and germane gas mixtures. Material processing methods studied include (1) varying the substrate temperature from 170° to 280°C, (2) varying the process gas composition and pressure, (3) dilution of the feed gas by hydrogen, argon, or helium, (4) enhancing etching during deposition by adding small amounts of XeF2 vapor into the process gas, and (5) postdeposition annealing and/or hydrogenation.
Effects of Substrate Parameters on Amorphous-Crystalline Silicon Interface
Authors: Besi Vetrella U. , Cohen J. D.
Published: March 2011

Capacitance vs. temperature, deep-level transient spectroscopy (DLTS), and transient photocapacitance spectroscopy have been used to investigate the amorphous-crystalline silicon interface region of a device made of hydrogenated amorphous silicon deposited on a lightly doped n-type crystalline silicon.

By comparing our results between substrates with and without oxide contamination with those in a earlier study, we have been able to correlate the effects of substrate preparation on the density of interface states.