bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical Properties of Doped Cd1−xMnxTe
Authors: Lansari Y. , Giles N. C. , Schetzina J. F. , Becia P. , Kaiser D.
Published: March 2011

The introduction of phosphorus and arsenic dopants into bulk Cd1−xMnx Te crystals grown by the Bridgman-Stockbarger technique has been studieA-with respect to the resulting optical properties. Samples with a Mn composition in the range 0.10 < x < 0.30, both as-grown and annealed, were investigated. A combination of room temperature transmittance and reflectance measurements over the spectral range from the ultraviolet to the far infrared has been used to gain information concerning the structural quality of the samples. Low temperature photoluminescence measurements (1.6−5 K) were used to determine optical quality and excitonic energies.
Raman Scattering by Magnetic Excitations in Diluted Magnetic Semiconductors
Authors: Ramdas A. K. , Rodriguez S.
Published: March 2011

Raman scattering provides significant information on the nature of the magnetic excitations of diluted magnetic semiconductors (DMS). Transitions involving an exchange of a quantum of angular momentum between the system and the radiation field result in Raman lines with shifts equal to the energy of the magnetic excitations in which the total spin of the crystal changes by ħ. Such transitions have a one-to-one correspondence with those seen in magnetic resonance but observed in the optical rather than in the microwave region of the electromagnetic spectrum. In the Mn-based II-VI DMS a variety of magnetic excitations have been observed in their Raman spectra. Microscopic models underlying these phenomena provide powerful insights into the nature of the magnetism of DMS.
Magnetic Contribution to the Energy Gap of Zn1−xMnxTe
Authors: Gaj J. A. , Golnik A. , Lascaray J. P. , Coquillat D. , Dejardins-Deruelle M. C.
Published: March 2011

Systematic masurements of energy gap and magnetic susceptibility of Zn1−xMnxTe in a composition range from 0 to 0.703 Mn mole fraction were performed as a function of temperature. The results show that the magnetic contribution to the energy gap variation is proportional to a product of temperature and susceptibility.
Ion - Carrier Exchange Interaction in Cd1−xMnxS
Authors: Nawrocki M. , Lascaray J. P. , Coquillat D. , Demianiuk M.
Published: March 2011

Magnetoreflectivity measurements in Faraday and Voigt geometry in the free exciton region (A, B and C excitons are visible) were performed on Cd0.07Mn0.13S at T=1.6 K for magnetic field up to 5.5 T. An analysis of the results in terms of a mean field model for wurtzite type crystal enabled to determine band parameters Δ1, Δ2, Δ3 and exchange integral for valence band N0ß = -1.8 ± 0.08 eV
Time-Resolved Photoluminescence of CD(1-x)MN(x)SE and CD(1-x)MN(x)TE as a Function of Temperature
Authors: zayhowski J. J. , Kershaw R. N. , Ridgley D. , Dwight K. , Wold A. , Galazka R. R. , Giriat W.
Published: March 2011

The characteristics of the photoluminescence of Cd(1-x)Mn(x)Se (x = 0.05, 0.10, 0.20, 0.30) and Cd(1-x)Mn(x)Te (x = 0.20, 0.30, 0.45) change considerably as the sample temperature is reduced below the exciton-magnetic polaron (EMP) threshold temperature. At low temperatures the formation of magnetic polarons has large effects on the luminescence energy, the radiative lifetime, the radiative efficiency, and the spectral half-width of the luminescence. It is also observed that the formation time of the EMP increases almost linearly with temperature. All of these effects are explained with a simple model for the EMP.
Effects of Exchange Interaction in Diluted Magnetic Semiconductor Quantum Wells
Authors: Kossut Jacek , Furdyna Jacek K.
Published: March 2011

The presence of transition metal ions (typically Mn2+) in diluted magnetic semiconductors (DMS) results in a strong spin-spin coupling between localized magnetic moments and band electrons. This leads to considerable modifications of the semiconductor band structure in the presence of strong magnetic fields, e.g., to large spin-dependent shifts of the electronic states at the band edge. This feature is of particular interest in the context of quantum wells involving DMS. Starting with the original idea of a “spin-superlattice”, we concentrate on various opportunities which arise due to the tunability of the depth of the quantum wells by the magnetic field and/or temperature associated with the aforementioned spindependent effects. Thus, we discuss boil-off and freeze-out of electrons to and from quantum wells, selective spin tunneling across the barriers, tunable infrared emitters, enhancement of electronic g-factors in shallow non-magnetic wells surrounded by DMS barriers, the possibility of transition from a type-1 to a type-il superlattice induced by the magnetic field, and quantum oscillations anomalies in DMS quantum wells.
Steady Thermocapillary Flows and Their Stability
Authors: Sen A. K. , Smith M. K. , Davis S. H. , Homsy G. M.
Published: March 2011

Thermocapillarity can drive steady flows in float-zone configurations. These flows can become unstable and exhibit time-periodic oscillations. In order to obtain insight into such phenomena, studies are made of a simplified system consisting of liquid in a two-dimensional slot. Steady flows and their instabilities are examined.
Measurement of Surface Rheological Effects on a Rotating Flow
Authors: Gans Roger F. , Singler Timothy J.
Published: March 2011

We report measurement of azimuthal velocity as a function of radius near the boundary between a liquid annulus and (a) a rigid freely floating cylinder and (b) an air core contained in a rapidly rotating horizontal cylindrical container. Case (a) agrees with previous theoretical calculations and verifies the method. Case (b) demonstrates (1) that the interface can support stress and (2) that there is a distinct difference in the scale of radial variation in the bulk liquid from that observed in case (a).
Flow Structure in Continuous Flow Electrophoresis Chambers
Authors: DEIBER J. A. , Saville D. A.
Published: March 2011

There are at least two ways that hydrodynamic processes can limit continuous flow electrophoresis. One arises from the sensitivity of the flow to small temperature gradients, especially at low flow rates and power levels. This sensitivity can be suppressed, at least in principle, by providing a carefully tailored, stabilizing temperature gradient in the cooling system that surrounds the flow channel. At higher power levels another limitation arises due to a restructuring of the main flow. This restructuring is caused by buoyancy, which is in turn affected by the electro-osmotic crossflow. Approximate solutions to appropriate partial differential equations have been computed by finite difference methods. One set of results is described here to illustrate the strong coupling between the structure of the main (axial) flow and the electro-osmotic flow.
The Effect of Small Temperature Gradients on Flow in a Continuous Flow Electrophoresis Chamber
Authors: Rhodes Percy H. , Snyder Robert S.
Published: March 2011

Continuous flow electrophoresis employs an electric field to separate biological cells suspended in a flowing liquid buffer solution. Good separations based on differences in electrophoretic mobility are obtained only when a unidirectional flow is maintained. The desired flow has a parabolic structure in the narrow dimension of the chamber and is uniform across the width, except near the edges where the no-slip condition prevails. However, because of buoyancy, very small lateral or axial temperature gradients deform the flow significantly. The results of experiments conducted with a specially instrumented chamber show the origin and structure of the buoyancy-driven perturbations. It is found that very small temperature gradients can disturb the flow significantly, as was predicted by earlier theoretical work.
A Theory of Electrophoresis of Emulsion Drops in Aqueous Two-Phase Polymer Systems
Authors: Levine Samuel
Published: March 2011

Two immiscible liquid phases form when an aqueous mixture of the electrically neutral polymers dextran and polyethylene glycol are equilibrated at sufficient concentrations. Certain supporting electrolytes which contain sulphate, phosphate or citrate ions partition unequally between the phases, and in their presence, electrophoresis of a drop of one phase suspended in the other is observed, with large mobilities. These mobilities depend linearly on the radius of the drop and the direction of the drop's motion is reversed when the disperse phase and the continuous phase are interchanged. When those ions which produce electrophoresis are present the potential Implied by the direction of electrophoresis is opposite to the Donnan potential observed between the two phases. To explain these results, we postulate an electric dipole layer associated with a mixture of oriented polymer molecules at the surface of a drop. In addition, a potential difference between the interiors of the two phases results from the unequal ion distribution. For the idealised model of a surface layer of point dipoles the inner and outer diffuse layers carry net charges equal in magnitude but opposite in sign. The classical theory of electrophoresis due to Henry, Overbeek and Booth is adapted to the motion of an emulsion drop under an electric field when diffuse ionic layers are present inside and outside the drop. Relaxation effects are treated for the case where the two diffuse layer thicknesses are small compared with the drop radius. An expression is obtained for the electrophoretic mobility of a drop which depends linearly on radius and also shows an increase with increase in salt concentration. The theory presented here is related to the work of Levich.
Thermocapillary Motion of Bubbles Inside Drops
Authors: Shankar N. , Cole Robert , Shankar Subramanian R.
Published: March 2011

The quasi-static thermocapillary migration of a bubble located inside a drop in free fall is considered for arbitrary axisymmetric temperature fields prescribed on the drop surface. Some results are presented, and an approximation based on the superposition of simpler solutions is discussed.
Experimental Observation of the Thermocapillary Driven Motion of Bubbles in a Molten Glass Under Low Gravity Conditions
Authors: Smith H. D. , Mattox D. M. , Wilcox W. R. , Subramanian R. S. , Meyyappan M.
Published: March 2011

Theory and ground based studies of bubble behavior in a fluid in the presence of a temperature gradient strongly indicate the action of a thermocapillary force which causes the bubbles to move. This'phenomenon been considered in the traditional treatments of glass fining. To demonstrate that the observed motion conformed to theoretical prediction it was necessary to perform the experiment under low gravity conditions. NASA's SPAR program provided an excellent opportunity to do this.

A sodium borate melt containing a specific bubble array was subjected to a well defined temperature gradient for more than 4 minutes. The sample was contained in a platinum/ fused-silica cell which permitted photographic coverage of the experiment. Photographs were taken at one second intervals during the course of the experiment. They clearly show that the bubbles move toward the hot spot on the platinum heater strip. The observed motion is consistent with the theoretical predictions for the temperature gradients parallel and perpendicular to the heater strip.
Materials Science in Space
Authors: Carruthers John R.
Published: March 2011

The preparation of solid materials involves the control and manipulation of fluids in ways that are sensitive to gravitational influences. Although these effects are pervasive, surprisingly little is understood about phenomena such as natural convection and containerless processes under boundary conditions of interest to materials processing. Recent emphasis has focused on process fundamentals involving areas such as fluid dynamics, heat flow, and thermophysical property measurement as can be seen in this Symposium. Such a knowledge base is essential to any sensible evolution of the space environment as a capability for studying materials processing and preparing limited quantities of materials under quiescent or containerless conditions for subsequent assessment on earth. A brief overview of current work will be presented, together with possible future directions.
Preliminary Study of the Effects of a Reversible Chemical Reaction on Gas Bubble Dissolution
Authors: Weinberg Michael C.
Published: March 2011

A preliminary investigation of the effects of a reversible chemical reaction upon the dissolution of an isolated, stationary gas bubble in a glassmelt is presented. The exact governing equations, for the model system, are formulated and discussed. The approximate quasi-steady state version of these equations are solved analytically and bubble dissolution rates are computed. These results are compared with numerical solutions obtained from the finite difference form of the exact governing equations.
Anomalous Phase Separation Behavior of Gel-Derived soda-silica Glasses
Authors: Neilson George F. , Weinberg Michael C.
Published: March 2011

The effects of retained bound hydroxyl groups on amorphous immiscibility behavior and on the kinetics of phase separation were studied in glasses containing from 10 to 19 percent sodium oxide prepared by the gel process. Differences in behavior as functions of preliminary thermal treatment of the gel precursor and of melting conditions were studied, employing IR spectroscopy, SAXS and WAXD to monitor the variation in glass microstructure. Both the initial gel treatment and the OH concentration in the prepared glasses were found to affect the immiscibility temperatures, and the magnitude of the maximum temperature increase was also a function of the sodium oxide concentration. It is suggested that the variation in thermodynamic behavior may be caused by the structural arrangement attained by the OH groups during the gel condensation process, which in turn affects the extent of hydrogen bonding to nonbridging oxygen ions.
Chemical Vapor Transport, Crystal Growth, Thermodynamic and Fluid Dynamic Analysis of the Hgl−xCdxTe System
Authors: Wiedemeier Heribert , Chandra Dipankar
Published: March 2011

EXTENDED ABSTRACT

The chemical vapor transport and crystal growth properties of the Hgl−xCdxTe iodine system (x=0.2, 0.5 and 0.7) are investigated as a function of transport agent pressure and orientation of the density gradient with respect to the gravity vector to understand the effects of convection on mass transfer and crystal morphology. A thermodynamic analysis of the solid-gas phase system yielded the composition of the vapor and the partial pressures of the gaseous species as a function of temperature and total pressure to assist in the elucidation of the dominant transport mode. Optical and scanning electron microscopy, electron microprobe, chemical etching, X-ray diffraction and energy dispersive spectroscopy techniques were employed to examine the surface and bulk morphology and compositional homogeneity of single crystals obtained under different conditions.
Response of Mnbi-Bi Eutectic to Freezing Rate Changes
Authors: Nair M. , Fu T-W. , Wilcox W. R. , Doddi K. , Ravishankar P. S. , Larson D.
Published: March 2011

Previously we reported on a theoretical treatment of the influence on freezing rate of sudden changes in translation rate in the Bridgman-Stockbarger technique [11]. This has now been extended to consideration of a linear ramped translation rate and an oscillatory freezing rate. Oscillations above a few hertz are found to be highly damped in smalldiameter apparatus.

An experimental test was made of the theoretical predictions for a sudden change of translation rate. MnBi-Bi eutectic was solidified with current induced interface demarcation.The experimental results correspond reasonably well with theory if the silica ampoule wall is assumed either (1) to contribute only a resistance to heat exchange of sample with the furnace wall, or (2) to transmit heat effectively in the axial direction by radiation.

In an attempt to explain the fact that a finer microstructure is obtained in space, MnBi-Bi microstructure is being determined when the freezing rate is rapidly increased or decreased. Preliminary results indicate that fiber branching does not occur as readily as does fiber termination.
Directional Solidification of Alloys in Systems Containing a Liquid Miscibility Gap
Authors: Grugel R. N. , Lograsso T. A. , Hellawell A.
Published: March 2011

The problem considered is the unidirectional growth of alloys close to monotectic composition and within the miscibility gap when the direction of gravity is down the temperature gradient (solidification upwards) or up the temperature gradient (solidification downwards). The systems Al-In and Al-Bi are taken as examples. With solidification upwards it is shown that bulk liquid composition adjusts to that of the monotectic while in the reverse situation, the bulk liquid gradually rises. In the former case it is possible to grow aligned fibrous structures of monotectic composition but in the latter the microstructures are irregular and globular.
A Qualitative Model for Predicting Alkyl Stability and its Relevance to Organometallic Growth of HgCdTe and GaAlAs
Authors: Hoke William E.
Published: March 2011

A qualitative stability model established for hydrocarbon molecules and extended to organometallic compounds is applied to alkyl organometallic compounds used in thin film growth. For hydrocarbon molecules the strength of a carbon-hydrogen bond is reduced by neighboring alkyl groups, The mechanism for this effect is delocalization of the unpaired electronic charge in the resultant free radical by neighboring alkyl groups. The delocalization effect is present in organometallic compounds and is illustrated here for several alkyl organometallic systems. Two applications of the delocalization effect in thin film growth are discussed. First the thermal stability of organotellurium compounds is substantially reduced in branched compounds which permits significantly lower HgCdTe metalorganic growth temperatures. Also carbon incorporation in GaAs and GaAlAs is reduced with ethyl instead of methyl organometallic compounds. An important factor contributing to this result is the weaker carbon-metal bonds in the ethyl compounds.
MBE Growth of Mercury Cadmium Telluride: Issues and Practical Solutions
Authors: Cook J. W. , Harris K. A. , Schetzina J. F.
Published: March 2011

The growth of thin films of mercury-based materials by molecular beam epitaxy (MBE) presents significant experimental problems which must be overcome in order to successfully grow infrared detector materials such as mercury cadmium telluride (MCT). Many of the problems associated with the use of Hg in MBE arise from its high room temperature vapor pressure (2 mTorr) and its low sticking coefficient. The MBE system must be designed for Hg usage by considering such things as the ultra high vacuum pumping system, the Hg source, Hg containment, and Hg removal. In addition, Hg is a toxic heavy metal and must be handled appropriately. Other problems involved with the growth of MCT are associated with the design of the MBE furnaces which are used to evaporate cadmium telluride and tellurium.
Bulk Crystal Growth of Hg1−xCdxTe for Avalanche Photodiode Applications
Authors: Duy T. Nguyen , Durand A. , Lyot J. L.
Published: March 2011

High gap Hg1−xCdxTe (MCT) crystal is grown by a solvent method using a travelling heater zone. The use of the solvent zone permits a low temperature and low mercury pressure growth of MCT in a large composition range. In the Cadmium rich alloy range the MCT material exhibits a large spin orbit coupling leading to a resonance at Eg = ΔO. Due to this particular resonance effect the ionization coefficient of hole is higher than that of the electron, resulting in a low exess noise factor in the avalanche photodiodes whose bandgap energy is close to the spin orbit splitting ΔO.
Structure-Property Relationships in Semiconductor Alloys
Authors: Sher A. , Berding M. A. , Krishnamurthy S. , Van Schilfgaarde M. , Chen A.-B. , Chen W.
Published: March 2011

We have demonstrated that the atomic distribution of constituents in semiconductor alloys is never truly random. There are always interactions causing correlations; the degree and nature of the correlations depend on which interactions dominate and on the growth conditions. While we have identified most of the interactions which are expected to cause correlations, not all of them have been treated completely to date. Therefore, some details remain unclear, but the principal effects can now be appreciated in broad terms.
Orientation Relationships between BCC Deposits and FCC Substrates
Authors: Smith David A. , Segmüller Armin , Taranko A. R.
Published: March 2011

Oriented deposits are commonplace in vapor deposited films. The origins of this behavior are not always clear. Various bcc metals (Fe, Nb, Mo, Ta, Cr) have been deposited onto single crystal fcc substrates (Au, Ni, MgO, Si, NaCI) which were selected with the intention of varying the character of the substrate-deposit interaction. The resulting structures have been characterized by transmission electron microscopy and grazing incidence x-ray diffraction. The observed variants of the cube-cube and Nishiyama-Wassermann orientation relationship can be understood in terms of minimization of misfit except when there is a weak interaction between substrate and deposit.
Photoemission Study of the α-Sn/Cdte(110) Interface Growth
Authors: Tang Ming , Niles David W. , Hernández-Calderón Isaac , Hóchst Hartmut
Published: March 2011

Angular Resolved Photoemission Spectroscopy with Synchrotron radiation has been used to study the MBE growth of α-Sn on CdTe(110). Sn grows epitaxially and the Fermi level pins at 0.72eV above the CdTe valence band maximum. Outdiffusion or segregation of Cd in the α-Sn layer is not observed. For small Sn coverages the Sn4d core spectra show a second component which may be due to the initial interfacial growth of SnTe.
Structural Analysis and Electronic Properties of in on Si(100) from Synchrotron Photoemission Studies
Authors: Rich D. H. , Samsavar A. , Miller T. , Lin H. F. , Chiang T. -C.
Published: March 2011

High-resolution photoemission spectroscopy was used to study the initial growth and interaction of In on Si(100). A quantification of the number of Si surface atoms selectively modified in the presence of an In adatom is enabled by decomposing the Si 2p core level into bulk and surface-shifted components; this analysis leads to an In-Si bonding coordination number determination. The In-to-Si bonding coordination number is 3 for very low In coverages and decreases to 2 for one half monolayer coverage. An analysis of the In 4d core levels indicates that the In adatoms occupy equivalent sites for the first 0.5 monolayers. Surface state emission near the top of the valence band persists for coverages exhibiting no Si 2p surface core level shift; implications regarding the different mechanisms I ehind the surface state and surface core level shift are addressed. The results are consistent with a structural model deduced from electron diffraction, Auger, and scanning electron microscopy studies.
Epitaxy, Oxidation and Surface Structure of Copper on Gold Films by High Resolution Electron Microscopy
Authors: Krakow William , Cast˜no Victor
Published: March 2011

High resolution electron microscope studies were undertaken of the epitaxy and oxidation of thin layers (<30Å) of vapor deposited Cu on single crystal Au films. The metal/metal and oxide/metal orientation relationships were analyzed for both the diffraction and imaging modes, before and after the Cu oxidation was significant. For the oxide overlayers two cases were studied: (111) Cu which transforms into three (110) variants of Cu2O, each rotated by 120° and (110) Cu transforming to a layered (211) oxide overgrowth. A lateral displacement of the surface top layer of Cu atoms was found for the (110) orientation of Cu2O. The (211) orientation of Cu2O had two structural variants of surface ordering rotated by 90° relative to the underlying metal. One of these cases directly exhibits a 3.01Å × 7.39Å supercell structure characteristic of a 0.58Å atomic step height of the (211) planar layers perpendicular to the surface.
Nucleation and Growth Processes in OMCVD of GaAs
Authors: Shastry S. K. , Hefter J.
Published: March 2011

The initial stages of GaAs growth on mirror-polished metal (W, Mo), semiconductor (Si, GaAs), and insulator (alumina) substrates have been experimentally studied in the organometallic chemical vapor deposition (OMCVD) process. Growth on the metallic substrates is often polycrystalline and nonepitaxial, while epitaxial GaAs layers with preferred orientations can be grown relatively easily on the nonmetallic substrates. The rate of nucleation of GaAs on all the substrates is found to increase with increasing arsine flow rate and with decreasing growth temperature. Our experimental results suggest that the GaAs nucleation process on all the substrates consists of an initial condensation and nucleation of gallium and subsequent formation of GaAs by arsenic supersaturation. In addition, following the nucleation of GaAs, a decrease in the substrate temperature is observed until further GaAs nucleation occurs, suggesting continued supersaturation. The nonepitaxial growth on the metallic substrates is attributed to the high rate of GaAs nucleation on these substrates.
Effects of Dopants on the Nucleation and Growth of NiSi2 on Silicon
Authors: Lu S. W. , Nieh C. W. , Chu J. J. , Chen L. J.
Published: March 2011

The influences of implantation impurities, including BF2, B, F, As and P on the formation of epitaxial NiSi2 in nickel thin films on ion-implanted silicon have been investigated by transmission electron microscopy.

The presence of BF2, B, and F atoms was observed to promote the epitaxial growth of NiSi2 at low temperatures. Little or no effect on the formation of NiSi2 was found in samples implanted with As or P ions.

The results indicated that the influences of the implantation impurities are not likely to be electronic in origin. Good correlation, on the other hand, was found between the atomic size factor and resulting stress and NiSi2 epitaxy at low temperatures.
The Early Stages of the Spinel-Alumina Phase Transformation
Authors: Simpson Y. Kouh , Carter C. B.
Published: March 2011

The initial stage of topotactic growth of Ni-Al spinel into Al2O3has been examined using transmission electron microscopy. A new experimental approach to the study of solid-state reactions, which may be adapted for in-situ experiments for low-temperature systems, has been used in this study. In its present form, the technique involves heating a thin film of one oxide in the presence of a vapor of the second oxide. In the study of the growth characteristics of Ni-Al spinel phase, the orientation of the Al2O3substrate has been found to influence greatly both the structural and morphological aspects of the spinel growth. In particular, the topotactic relationship between the spinel and the alumina are very different for (0001) and {1120} substrate orientations. The very early stage of the kinetics of the spinel growth, in which the length and the width of the spinel particles are only a few hundred angstroms, is illustrated with the results obtained from the re-heating experiments. The structure of the spinel-alumina interface has also been studied using high-resolution electron microscopy. These results are discussed in relation to the different models proposed for the spinel-alumina phase transformation.
Initial Stages of Interface Formation: The Influence of Surface Reconstruction
Authors: Gossmann H.-J.
Published: March 2011

The initial stages of interface formation are crucial for the growth of an overlayer, particularly for ultra-thin films. While growth rate, growth temperature and lattice match are important parameters in molecular beam epitaxy, growth of semiconductor heterostructures always begins on a reconstructed surface. The effect of this reconstruction has to be known before a complete understanding of the growth process is possible. Two examples will be discussed: (1) A necessary condition for perfect growth is the reordering of the substrate surface, i.e. those atoms at the interface which are originally displaced due to the reconstruction have to be brought back to bulk positions. Si(111)7×7 and Si(100)2×1 surfaces behave differently in this respect. The 2×1 orders upon deposition of Si, Ge or Sn at room-temperature whereas the 7×7 does not. This result will be correlated with the different classes of reconstruction to which these surfaces belong. The possible rela-tionship with the difference in substrate temperatures required to achieve homo-epitaxial growth on Si(100) and Si(111) surfaces will be discussed. (2) Perfect growth also relies on proper control of the growth-mode. As an example, the island-ing behavior found for deposition of Sn on Si will be discussed. While surface diffusion over macroscopic distances is observed, dramatic changes in the affinity for clustering occur depending on surface reconstruction and preparation, suggesting that clustering in this system may be controlled by appropriate choices of growth parameters.
Drift Mobility Measurements Under Single and Double Injection in a-Si:H
Authors: Xu L. , Winborne G. , Silver M. , Cannella V
Published: March 2011

Drift mobility measurements under high level double injection in p/i/n and single injection in M/i/n devices have been made as a function of d.c. voltage and temperature using a small step voltage technique. For comparable ratio's of V/L2 the transit time of injected electrons was of order one decade shorter under double injection than under single injection. The room temperature drift mobility for single injection was approximately 1cm2−1s−1. We attribute the difference in drift transit time to a higher extended state mobility with double injection than with single injection due to the neutralization of charged defects.
Hydrogen Diffusion and Thermal Equilibrium of Electronic States in a-Si:H
Authors: Street R. A.
Published: March 2011

This paper reviews the recent evidence for thermal equilibrium effects in the electronic behavior of hydrogenated amorphous silicon, and relates the thermally induced changes to the motion of bonded hydrogen. The electronic properties are studied through measurements of d.c conductivity and-.weep out, and the role of hydrogen is explored through its diffusion. The magnitude and doping dependence of the diffusion coefficient DH matches the data on the equilibration of the electronic states. Furthermore both the diffusibn and the relaxation can be described by the same dispersive time dependence. It is argued that the diffusion of hydrogen is the rate limiting step in the the equilibration mechanisms and determines the kinetics.
Hydrogen Microstructure in Amorphous Semiconductors.
Authors: Gleason Karen K. , Petrich Mark A. , Reimer Jeffrey A.
Published: March 2011

The effects of deposition parameters on the H microstructure of plasma deposited amorphous silicon (a-Si:H) and amorphous silicon carbide (a-SiC:H) are measured via multiple quantum nuclear magnetic resonance (MQ NMR). These studies indicate clusters of 5 to 7H atoms exist in a-Si:H films prepared at temperatures ranging from 113 to 324°C. In the range from 270 to 324°C, only these small clusters exist, but lower temperature films also contain larger clusters. Annealing studies indicate H rearranges in a-Si:H prior to evolution. Deposition temperature and annealing temperature have similar effects on H concentration in a-Si:H, but deposition temperature control the density and microstructure of the film. The addition of dopant atoms also affects the H microstructure, with phosphorous causing larger H clusters to form, and boron reducing clustering in a-Si:H films. This perturbation of the film's microstructure suggests that the effects of dopant addition are more complex in amorphous than in crystalline semiconductors. The concentration of carbon atoms also effects H microstructure of a-SiC:H in a complex way. We surmise that H microstructure, rather than H content, determines amorphous semiconductors properties.
A Structural Analysis of Thin Amorphous Silicon Films
Authors: Erkoç Ş. , Halicioglu T. , Tiller W. A.
Published: March 2011

Using a semiempirical potential energy function for Si, thin amorphous films of Si have been simulated on crystalline Si by a partial melting and subsequent quenching process. After relaxation at 450°K, the dominant structural feature was a dense free surface skin with a void layer underneath for both c-Si(100) and c-Si(111) substrates and for film thicknesses up to 16Å. Tetrahedral coordination was maintained throughout the amorphous region and structural differences were noted in the α-Si/c-Si interface region for the two orientations considered.
Film Growth Mechanisms of Amorphous Silicon in Diode and Triode Glow Discharge Systems
Authors: Tsai C. C. , Shaw J. G. , WACKER B. , Knights J. C.
Published: March 2011

By simulating the growth morphology of plasma-deposited amorphous silicon films on patterned substrates using a Monte-Carlo technique, we find the effective sticking coefficient for the dominating mass transporting species to be quite different depending on the film growth mechanism. The physical-vapor-deposition-like growth regime producing defective materials is associated with species like SiH2 and SiH which have a large effective sticking coefficient, while the chemical-vapor-deposition-like process producing “device quality” materials is linked to species like SiH3 which has a much smaller effective sticking coefficient. In a triode reactor the physical-vapor-deposition-like process is suppressed since species with small effective sticking coefficients are selected. However, the growth rate decreases rapidly as the substrates are moved away from the plasma. There seems to be an incompatibility between good material quality and fast growth rate in the plasma deposition of amorphous silicon in either diode or triode reactors.
The Role of Lattice Relaxation in the Competition Between Optical and Thermal Transitions from Gap States in Hydroacenated Amorphous Silicon
Authors: Gelatos A. V. , Cohen J. D. , Harbison J. P.
Published: March 2011

We have carried out a detailed study of the competition between optical and thermal transitions from mobility gap states in hydrogenated amorphous silicon films. By employing transient photocapacitance and capacitance DLTS techniques on n-type doped films, we have observed the dangling bond optical transitions:D− → D0 + e and D0 → D+ + e, and the thermal transitions: D− → D0 + e and D0 → D0 + h. From the transition energies we have determined that the degree of lattice relaxation for dangling bonds does not exceed 0.1 eV and that the value Ueff of the D−/D0 splitting is 0.3 ± 0.1 eV. Voltage pulse photocapacitance studies on undoped films revealed the existence of the optical transition D−/D0 + e. In addition, in all films we have observed a photocapacitance signal due to optical transitions from the valence band tail states (over the energy range 1.4 to 1.9 eV) at temperatures as high as 440 K, in spite of the expectation that such shallow photoexcited holes would immediately be thermally emitted yielding no photocapacitance signal. We discuss possible explanations for this result by considering two different trapping models.
Predictions on the Physical Properties of Amorphous Carbon-Tin Semiconducting Alloys: II Photoconductivity Properties
Authors: Mpawenayo Prosper
Published: March 2011

It is intended to throw some high lights on the photoconductivity properties of narrow band semiconducting CSn alloys. The feasability, the structure and the usual optical properties of dielectric constants and optical gap have been predicted in a previous paper /1/. Starting with a number of well acknowledged models for semiconductors, properties, the values of the ratio between the light and the dark conductivities of a material having an o9tifal gap of 1.2 eV at 300 °K, under an illumination of 1016 photons cm−2s−1 are found between 1 and 103, but a density of gap states lower than 1018 cm−3 eV−1 is required.
Why the Photoconductivity Decreases in a-SiC:H and a-SiGe:H when the Amount of Alloying Increases
Authors: Mahan A. H. , Raboisson P. , Menna P. , Tsut R.
Published: March 2011

Infrared, photoconductivity, and photothermal deflection spectroscopy measurements have been used to probe the relationship between material quality and the amount of microstructure in glow discharge deposited a-SiC:H and a-SiGe:T films. We find that the microstructure is directly responsible for the decrease in photoconductivity observed in both alloys as a function of increased alloy content. The microstructure does this by causing a decrease in steepness of the Urbach tail, thus allowing for an increase in both carrier trapping at the wider band edges and carrier recombination at or near the band tails.
Carrier Transport in Compositionally Modulated a-Si:H Based Superlattice Structures
Authors: Wronski C. R. , Persans P. D. , Abeles B.
Published: March 2011

Results are presented on the electron transport, parallel and perpendicular to the layers, in a Ge:H/a-Si:H superlattices. The dependence of this carrier transport on the optical gap shifts, the heterojunction band alignment, as well as the bulk properties and thickness of the Ge,Si layers is established. No significant change in the transport mechanisms at the band edge or Ge with dr was detected.
Interface Passivation in Amorphous Silicon TFTs by Various Gate Dielectric Materials
Authors: Fryea R. C. , Wong C. C. , Kornfeld C.
Published: March 2011

We have used photothermal deflection spectroscopy to examine deep gap absorption in amorphous silicon films deposited on silicon oxide and silicon nitride. Variations in the interface state density deduced from PDS correlate well with the performance characteristics of thin-film transistors. We have demonstrated processes which degrade the interfacial abruptness also increase the interface state density. In transistors this leads to a degradation in device stability. We found devices with oxide gates to be more stable and show lower interface state density than devices with nitride gates for a specific set of deposition conditions. The correspondence between deep gap absorption and transistor characteristics shows that PDS is a valuable technique for characterizing and optimizing fabrication processes.
The Influence of Bulk and Interface Gap States on the Performance of Amorphous Silicon Thin Film Transistors
Authors: Bohm M. , Houghton J. , Salamon S.
Published: March 2011

Amorphous silicon thin film transistors with on/off ratios >108 and on currents >100 μA for supply voltages <30 V have been manufactured. The transistors are of the structure glass/Cr/SiNx/intrinsic a-Si/n+a-Si/Al. The field effect electron mobility is typically 0.24 cm2 /Vs. Evaluation of quasistatic CV measurements yields a gap state density of 3.2*1016 cm−3eV−1 at midgap. A one-dimensional model calculation describing the influence of bulk and interface gap states and various other material and geometry parameters on transistor performance is outlined. Calculated data include space charge density, spatial distribution of band bending, band bending vs gate source voltage, static capacitance, and channel conductance. A fit calculation for the experimental sub pinch-off transfer characteristic of a thin film transistor is presented.
The Effects of Coordination and Local Disorder on Impurity States in Hydrogenated Amorphous Silicon
Authors: Nichols C. S. , Fong C. Y.
Published: March 2011

The self-consistent pseudopotential method has been used to calculate the electronic structure of a periodic model of hydrogenated amorphous silicon containing, independently, P and B impurities. We have investigated both impurities in threefold- and fourfold-coordinated sites. Both types of sites studied also include the realistic distortion inherent in disordered systems. In fourfold coordination, both P and B introduce gap states, the energy and spatial extent of which depend upon the local geometrical distortion. In threefold coordination, the lone pair state of P is found below the valence band edge, while the B non-bonding state is found above the conduction band edge. The effects of local geometrical distortion on both of these states is also discussed. Comparisons with other calculations and experimental results are made.
An Investigation of Photo-Induced Degradation by Means of Photothermal Deflection Spectroscopy
Authors: Bennett M. S. , Wiedeman S. , Newton J. L. , Rajan K.
Published: March 2011

Absorption measurements of as deposited and photodegraded intrinsic amorphous hydrogenated silicon films were made using photothermal deflection spectroscopy (PDS). The films were light-soaked in situ using HeNe laser light to simulate AM1 illumination. An increase in subbandgap absorption occurred predominantly near energies of 1.2eV. A simple model was developed in which a density of states function is hypothesized and the resulting optical absorption at subgap energies is calculated. The measured absorption could be well matched in all cases by assuming a single peak of defect states at or slightly below the Fermi level. Further, the observed changes in optical absorption due to degradation could be modeled by increasing the density of the single peak of defect states and moving the Fermi level towards the valence band.
Amorphous Silicon-Carbide Multilayered Visible-Light Emitting Diode
Authors: Kruangam Dusit , Deguchi Masahiro , Hattori Yutaka , Toyama Toshihiko , Okamoto Hiroaki , Hamakawa Yoshihiro
Published: March 2011

A series of systematic trials to improve the light emission efficiency has been made on thin film visible a-SiC LEDs. A wide variety of the approaches, such as a superlattice structure, p-i-n/p-i-n tandem structure, an efficient injection electrode with a wide-gap layer etc., have been done. From the results, two practically available new technologies for bright LEDs have been developed, that is; a) a hot carrier injection structure by inserting a highly- resistive layer between p-and i- or i- and n-layers and b) a wide-gap highly-conductive p-type injector layer prepared by ECR (Electron Cyclotron Resonance) CVD deposition. With these trials, the brightness of yellow LEDs have been increased by more than one order of magnitude to be about 5cd/m2
Binary Synaptic Connections Based on Memory Switching in a-Si:H for Artificial Neural Networks
Authors: Thakoor A. P. , Lamb J. L. , Moopenn A. , Khanna S. K.
Published: March 2011

Nonvolatile, associative electronic memory based on neural network models promises high (−109 bits/cm2 ) information storage density since the information is stored in a matrix of only two-terminal, passive interconnections (synapses). An electronic memory switch at each interconnect would be ideal as a programmable synapse. The massive parallelism in the architecture, however, requires that the ‘ON’ state of a synaptic connection must be unusually ‘weak’ (i.e., highly resistive). For example, a binary synapse should be 106Ω in its ‘ON’ state and >109Ω in the ‘OFF’ state for a 1024 × 1024 matrix (−256 K bits of programmable read only memory, PROM). The small deliverable switching energy dictated by the resistive ‘ON’ state requirement is a new constraint for switching in thin films. Memory switching in hydrogenated amorphous silicon (a-Si:H) along with ballast (current limiting) resistors patterned from resistivity-tailored, amorphous Ge-metal alloys are investigated for a binary PROM matrix. A lμm2 area of a-Si:H could be switched from a −1010Ω (OFF state) to −105Ω (ON state) by a voltage pulse of lpsec duration, with the switching energy of −1 nanojoule, delivered through a 106Ω ballast resistor. Programmable, read-only, 1600 synapse (40x40) test arrays of uniform connection strengths (variation ±2%) and a-Si:H switching elements have been fabricated. Suitability of the memory switching in a-Si:H for high-density neural networks is discussed.
Preparation of Boron-Rich Refractory Semiconductors
Authors: Aselage T. L.
Published: March 2011

Boron-rich refractory solids based on the rhombohedral structure of α-B exhibit electrical properties that range from a hopping-type semiconductor (boron carbide) to wide bandgap room temperature insulators (the boron pnictides B6P and B6As). As such, they are of interest for a variety of high temperature semiconductor applications. Preparation techniques for these unusual materials are reviewed, and new results on the crystal growth of boron carbides and B6As and on thin film deposition of B6P are presented.
On the Pressureless Sintering of Lanthanum Hexaboride Materials
Authors: Knoch Heinrich , Lipp Alfred
Published: March 2011

LaB6 powders of a particle size less than 2 μm can readily ge sintered to nearly pore free microstructures, when boron and carbon are used as additives. The microstructure consists of LaB6 B4C and C as seperate, finely divided phases. The preparatton of starting powders, the sintering process, and the resulting microstructures are discussed.
Silicon Carbide: Progress in Crystal Growth
Authors: Powell J. Anthony
Published: March 2011

Silicon carbide (SiC), with a favorable combination of semiconducting and refractory properties, has long been a candidate for high temperature semiconductor applications. Research on processes for producing the needed large-area high quality single crystals has proceeded sporadically for many years. Two characteristics of SiC have aggravated the problem of its crystal growth. First, it cannot be melted at any reasonable pressure, and second, it forms many different crystalline structures, called polytypes. Recent progress in the development of two crystal growth processes will be described. These processes are the modified Lely process for the growth of the alpha polytypes (e.g. 6H SiC), and a process for the epitaxial growth of the beta polytype (i.e. 3C or cubic SiC) on single crystal silicon substrates. A discussion of the semiconducting qualities of crystals grown by various techniques will also be included.
Highly Mismatched Hetero-Epitaxial Growth of Cubic Sic on Si
Authors: Matsunami Hiroyuki
Published: March 2011

Single crystals of cubic SiC were hetero-epitaxially grown on Si by chemical vapor deposition (CVD) method. A carbonized buffer layer on Si is utilized to overcome the large lattice mismatch of 20 %. Optimum conditions to make the buffer layers and those structures are discussed. Crystal quality of the CVD grown cubic SiC is analyzed by using X-ray analyses and microscopic observations. Electrical properties controlled by impurity doping during epitaxial growth are described together with fundamental electronic devices.
Beta Silicon Carbide Growth with Device Applications
Authors: Harris G. L. , Spencer M. G. , Jackson K. , Jones A. , Osborne K. , Fekade K. , Wongchotigul K.
Published: March 2011

Epitaxial thin films of cubic-SiC (100) have been grown at temperatures ranging from 1340–1400°C by using a reduced pressure chemical vapor deposition system. In this work, we will report on the doping of SiC using trimethylalumium (TMA), phosphine, and nitrogen. The electrical measurements on the SiC layers have been performed (Hall, CV, DLTS, etc.) and will be discussed in reference to SiC MESFETs and other SiC device structures.
Investigation on the Gap Density of States in Amorphous Semiconducting Carbon Silicon and Carbon Tin Alloys
Authors: Mpawenayo P. , Nsabimana M.
Published: March 2011

The density of states in a-Six C1-x:H and a-Cy Sn1-y:H (F) semiconducting materials has been investigated by phototheTmal deflection spectroscopy (PDS) and their photoconductive properties have been related to the gap states distribution parameters, the Urbach energy Eo and the localized states density gm. We found that an Eo of 200 meV is the highest value above which no photoconductivity can be detected. This threshold corresponds to a density of gap states of about 1019 eV−1 cm−3 for optical gaps ranging between 1.1 and 2.5eV. Fourier spectral analysis of the long photoresponse decay time observed in our fluorinated a-Cy, Sn1-y samples shows the existence of well defined trapping levels and randomly distributed defect states.