bluuebunny/crossref_metadata_2025 huggingface dataset:
Single-particle dynamics of the solid heliums from deep inelastic neutron scattering
Authors: Simmons R. O.
Published: April 2011

Previous neutron-scattering research on solid heliums has been restricted to small momentum transfers, Q, both by large Debye–Waller factors and by scientific interest being restricted to collective modes and their interactions. It has also been limited by insufficient sources of neutron beams of about an electronvolt in energy, which are required to apply the method of deep inelastic scattering to solids. Making use of a spallation neutron source and a suitable chopper spectrometer, one can now collect data in which the dynamic structure factor S(Q,E) directly reflects the atomic-momentum distribution n(p). For 4He, the major determining factor on this single-particle property is the density of the sample, not its microscopic structure. At a strictly constant number density, samples of hexagonal close-packed (hcp), body-centered cubic (bcc), and normal liquid show identical S(Q,E)'s for Q's near 200 nm−1, at current levels of precision. The case of bcc 4He is used to illustrate the method, and the derived kinetic energy is compared with theoretical predictions. The applicability of these results in other areas of solid-helium physics is indicated.
The momentum distribution of normal and superfluid liquid 4He
Authors: Ceperley D. M. , Pollock E. L.
Published: April 2011

Path-integral methods can be employed to calculate properties of boson systems at any temperature. We have performed a number of simulations of liquid helium and have calculated the momentum distribution as a function of temperature. We find that liquid helium has a condensate below 2 K but not above. The momentum distribution is found to be non-Gaussian even in normal liquid helium. We have also calculated the difference in the momentum distribution between a boson system and a distinguishable particle system.
Viscosity and thermodynamic evidence for liquid 3He being nearly metamagnetic
Authors: Archie Chas. N.
Published: April 2011

Study of the melting process in rapid melting of spin-polarized 3He reveals that the liquid's magnetic susceptibility is enhanced at moderate temperatures (above 30 mK) and moderate polarization (20–40%) but declines at higher polarization. The first transport measurement on the enhanced polarized liquid shows anomalous behavior. Together, these measurements favor a microscopic model in which, at moderate temperatures and as a function of polarization, the liquid approaches but does not achieve a new phase.
Neutron diffraction studies of two-dimensional quantum systems
Authors: Lauter H. J. , Schildberg H. P. , Godfrin H. , Wiechert H. , Haensel R.
Published: April 2011

The phases of D2 monolayers on graphite between the commensurate and the incommensurate phase have been investigated by neutron diffraction, revealing new features including domain-wall constructions. For the related systems, 3He and 4He adsorbed on graphite, the structure of the solid first and second layers and the interaction between them have been analyzed.
Superfluid 3He and heavy-fermion superconductors near surfaces and interfaces
Authors: Kurkijärvi J. , Rainer D. , Sauls J. A.
Published: April 2011

We discuss various model boundary conditions that supplement the quasi-classical equations of superconductivity, and we review their application to superfluid 3He and unconventional superconductors in contact with walls and interfaces.
Thermal and mechanical properties of solid hydrogen and deuterium
Authors: Manzhelii V. G.
Published: April 2011

Some results of the studies of thermal expansion, heat capacity, conversion, quantum diffusion, phase equilibrium, and plasticity of solid hydrogen and deuterium and their solutions are presented and discussed.
Limits to the stability of the uniform texture in flowing superfluid 3He-A
Authors: Bozler H. M. , Gould C. M.
Published: April 2011

A review of theory and experiments on flowing 3He-A is presented. We explore the question of the stability of the uniform texture against deformation in the presence of flow and magnetic fields. The texture probe used in these experiments is the attenuation of zero sound. We find a region of stability that appears to extend to greater velocities than expected. The limitations of the sound probe are discussed, and we suggest possible extensions of this work.
Generation of persistent currents and their detection in superfluid 3He by nuclear magnetic resonance
Authors: Brewer D. F.
Published: April 2011

Superflow of 3He–B in a channel can be observed by nuclear magnetic resonance (nmr) techniques, the simplest arrangement being one in which the magnetic field is perpendicular to the flow. Above a certain critical velocity, the transverse resonance frequency shifts away from the Larmor frequency, and the velocity gives a value of the ratio of the flow coupling constant to the dipole coupling constant, which is found to differ substantially from theoretical prediction. The nmr shifts can be used to detect persistent currents in a doubly connected geometry, and some preliminary observations are described as well as an analysis of the way in which currents with multiply quantized circulation may be set up.
Properties of superfluid 3He–B in the low-temperature limit
Authors: Guénault A. M. , Pickett G. R.
Published: April 2011

Several experiments are described in 3He–B at temperatures down to 125 μK and below. In this low-temperature regime, the normal-fluid density is negligible with the consequence that the quasi-particle gas is virtually noninteracting, because the mean free paths for quasi-particle–quasi–particle scattering are orders of magnitude greater than the size of the experiment. We have measured the boundary conductance across a liquid-to-silver sinter interface. The measured conductance shows an exp(−Δ/kT) dependence, with Δ being a superfluid energy gap that is apparently lower than that appropriate for the bulk liquid. We observe the onset of dissipation by pair-breaking induced by a moving wire, which also implies that the energy gap is depressed near a boundary. Finally, we have used such a supercritically driven wire as a ballistic quasi-particle source to observe a new thermomechanical effect in 3He–B, and hence to devise the elements of a quasi-particle spectrometer.
The roughening transition of a growing 4He crystal
Authors: Gallet F. , Nozières P. , Balibar S. , Rolley E. , Wolf P. E.
Published: April 2011

We present a careful study of the growth of hexagonal close-packed 4He crystals near the roughening transition of their (0001) surfaces, at TR = 1.28 K. We confirm that the dominant growth mechanism in the smooth state (T < TR) is the two-dimensional nucleation of terraces of atomic height. Moreover, we discovered a new regime of growth, which takes place slightly below TR, for a surface state intermediate between smooth and rough. It exists in a temperature domain whose width increases with the applied driving force. All our experimental results are consistent with a new calculation based on the critical theory of roughening, which predicts a completely continuous or infinite-order transition. Precise values are deduced for the roughening temperature (TR = 1.28 ± 0.01 K) and the relative amplitude of the lattice potential (tc = 0.63 ± 0.13).
Liquid and solid 4He near a graphite wall
Authors: Polturak E. , Eckstein Y. , Lipson S. G. , Shechter H.
Published: April 2011

We describe two experiments on the properties of 4He near the surface of graphite. In the first experiment, we measured the growth of solid 4He on the surface as a function of the pressure in the liquid. In a second, recent experiment we attempted to observe the melting of this solid layer as the temperature was raised. In addition to melting, we observed another, more dominant effect, which we interpret as a possible penetration of 4He into the graphite.
Velocity of propagation of the 3He A → B transition
Authors: Yip S. , Leggett A. J.
Published: April 2011

The first-order phase transition between the superfluid A and B phases of liquid 3He is strongly hypercooled, so that its propagation velocity cannot be determined by the usual thermal considerations. We present the essentials of a theory in which the limiting friction is due to the momentum change suffered by Bogoliubov quasi particles on Andreev reflection from, or transmission across, the moving interface, and we show that it gives reasonable agreement with the existing experimental data.
Mobility oscillations of electrons on thin He films
Authors: Cieslikowski D. , Leiderer P. , Dahm A. J.
Published: April 2011

We have studied electrons above liquid films of 3He and 4He condensed on various solid substrates. For helium films on solid hydrogen, pronounced oscillations of the electron mobility are observed as the film thickness is changed. The oscillations are related to the formation of layers of the liquid close to the wall. Our results indicate substantial layering of the films up to nearly 10 atomic distances.
Finite-size Kosterlitz–Thouless transition of 4He films in packed powders
Authors: Cho Hyung , Kotsubo Vincent , Williams Gary A.
Published: April 2011

The Kosterlitz–Thouless transition of 4He films adsorbed in packed powders is studied using a third-sound technique. We have extended the powder-grain diameter to a larger size (50 μm Al2O3) and to a smaller size (150 Å Carbolac II) than in our previous work. The results show that finite-size effects are very pronounced in the Kosterlitz–Thouless transition and can be observed even up to micrometre-length scales.
Properties of multielectron bubbles in liquid helium
Authors: Albrecht U. , Leiderer P.
Published: April 2011

We report charge and velocity measurements on multielectron bubbles in liquid helium. Oscillations and fissioning of bubbles are observed.
Analysis of the specific heat of body-centred cubic 3He in a high magnetic field using high-temperature series expansion with a multiple-exchange Hamiltonian
Authors: Suaudeau E. , Roger M. , Bernier M. E. R.
Published: April 2011

We have extended to order four in β = 1/kBT the high-temperature series expansion for the body-centred cubic phase of solid 3He with a Hamiltonian including two-, three-, and four-particle exchange. The results are used to analyze recent experimental measurements of the specific heat at low temperature and high magnetic fields.
Nuclear magnetism of 3He and vacancies
Authors: Kumar P. , Sullivan N. S.
Published: April 2011

A summary is given of new results for the effect of vacancies on the nuclear-spin susceptibility of solid 3He for very high molar volumes close to the melting curve.
Magnetism in 3He: measurements in a very restricted geometry
Authors: Engel B. N. , Ihas G. G. , Spencer G. F.
Published: April 2011

The nuclear magnetic susceptibility, χ, of 3He is seen to nearly obey a pure Curie Law, χ = C/T, over the temperature range 0.5 mK < T < 2 K when contained in the 20 Å, multiply connected pores of Vycor glass at low external pressure. Raising the external pressure to 5 bar (1 bar = 100 kPa) causes the susceptibility to obey the Curie–Weiss law, χ = C′/T−θ whose θ = 0.3 mK. The longitudinal relaxation time, T1, varies monotonically with T between 17 and 90 ms at both pressures. At 5 bar, the transverse relaxation time, T2*, is nearly temperature independent (about 600 μs) above 2 mK. Below 2 mK, T2* drops precipitously. Preliminary measurements indicate that the magnetism is produced by atoms that can flow through the pores of the Vycor glass.
Ultrasonic studies of liquid helium in porous media
Authors: Beamish J. R. , Warner K.
Published: April 2011

We have studied the propagation of 12 MHz transverse ultrasonic waves in a porous ceramic containing liquid 4He. Both the sound velocity and the attenuation clearly show the superfluid nature of helium. The helium in the pores increases the system's effective density by an amount proportional to the normal-fluid density and so decreases the sound speed. The decoupling of the superfluid fraction below the lambda transition allows us to use the shear wave essentially as a "high-frequency torsional oscillator" to determine the superfluid density and pore tortuosity. The sound attenuation in this system is due to the same mechanism as for fourth sound, namely, viscous losses due to motion of the normal-fluid component. We observed an attenuation proportional to the normal-fluid density and compare this result to predictions of the Biot theory of sound propagation in fluid-filled porous media.
Measurements of the nuclear magnetic resonance T2 for 3He–4He mixture films as a function of 3He coverage and 4He film thickness
Authors: Valles Jr. J. M. , Higley R. H. , Johnson B. R. , Hallock R. B.
Published: April 2011

We report measurements of the nuclear magnetic resonance (nmr) T2 for 3He–4He mixture films on Nuclepore substrates for 39 mK < T < 180 mK. 3He coverages up to a 0.091 atomic monolayer have been studied on 4He films with thickness up to 18.1 Å. We find the surprising result that the nmr T2 is not a strong function of either the 3He coverage or the 4He film thickness.
Structures in the Raman spectra of superfluid 4He
Authors: Ohbayashi Kohji , Udagawa Masayuki , Watabe M.
Published: April 2011

A high-accuracy determination of the fine structures in the Raman spectrum of superfluid 4He at 1.3 K under saturated vapor pressure has been done with a multichannel monochromator. The experimental Raman structure cannot be explained by conventional theories. Comments on our recent studies of the pressure dependence of the Raman structure are given.
Effect of annealing on polysilicon emitter transistors
Authors: Keyes E. P. , Tarr N. G.
Published: April 2011

The effect of 900 °C furnace annealing on transistors with in situ phosphorus-doped polysilicon emitters has been investigated. For devices with chemically grown interfacial oxides, annealing is essential to give acceptable emitter resistance and emitter Gummel numbers. For devices lacking an intentionally grown interfacial oxide, annealing is necessary to reduce the emitter resistance to a tolerable level, but it simultaneously lowers the emitter Gummel number.
Fabrication of microbridges in standard complementary metal oxide semiconductor technology
Authors: Parameswaran M. , Ristic Lj. , Dhaded A. C. , Baltes H. P. , Allegretto W. , Robinson A. M.
Published: April 2011

Complementary metal oxide semiconductor (CMOS) technology is one of the leading fabrication technologies of the semiconductor integrated-circuit industry. We have discovered features inherent in the standard CMOS fabrication process that lend themselves to the manufacturing of micromechanical structures for sensor applications. In this paper we present an unconventional layout design methodology that allows us to exploit the standard CMOS process for producing microbridges. Two types of microbridges, bare polysilicon microbridges and sandwiched oxide microbridges, have been manufactured by first implementing a special layout design in an industrial digital CMOS process, followed by a postprocessing etching step.
Series resistance calculations for polysilicon tunnel junction emitter transistors
Authors: Tsou B. P. C. , Chu Kan M. , Pulfrey D. L.
Published: April 2011

Calculations are reported of the series resistance caused by the thin insulating layer which separates the polycrystalline and monocrystalline regions of the emitter in certain types of polysilicon emitter transistors. It is demonstrated that the series resistance depends on the shape of the potential barrier which characterizes the thin insulator. For a triangular barrier the series resistance may be low enough for the transistors to be acceptable for very large scale integrated circuit applications.
Application of data screening to the characterization of integrated circuits
Authors: Ghaemi Zarrin
Published: April 2011

Large amounts of integrated-circuit test data may be too great to be analysed by (human) visual graph inspection, and must be inspected by an automatic method. In this paper, a method called data screening is described. Data screening automatically identifies the presence of abnormalities (unexpected clusters and (or) outliers) in sets of test data, and has been implemented in a program in a statistical software package called Enhansys. The method has been used on actual test data with good results.
Instability and gate voltage noise in GaAs metal-semiconductor field-effect transistors
Authors: Day D. J. , Trudeau M. , McAlister S. P. , Hurd C. M.
Published: April 2011

GaAs metal-semiconductor field-effect transistor devices grown on semi-insulating substrates are shown to display noise spectra with a f−3/2 high-frequency roll-off. This is interpreted as diffusion noise from generation-recombination events at traps in the semi-insulating substrate. This interpretation is confirmed by the instabilities and oscillations that occur when the devices are operated under a large drain bias.
Optical techniques for characterizing SI GaAs
Authors: Steiner T. , Zhang Yu , Charbonneau S. , Villemaire A. , Thewalt M. L. W. , Maciaszek M. , Bult R. P.
Published: April 2011

A number of optical techniques that can be used for the nondestructive characterization of semi-insulating GaAs are described. These techniques include regular photoluminescence, photoluminescence excitation spectroscopy, selective donor–acceptorpair luminescence, electronic Raman scattering, and local vibrational mode absorption for the identification and concentration determination of acceptors. For donors, magneto-photoluminescence can determine the chemical identity and relative concentration of the shallow donors, while for the important deep donor EL2, absorption and bleaching experiments determine the neutral EL2 concentration. The utility of these techniques for routine and reliable characterization of production and developmental GaAs wafers is discussed. The possible extension of these techniques to spatially resolved 2D mapping of GaAs wafers is also discussed.
CF4–Ar reactive ion etching of gallium arsenide
Authors: Lussier P. , Bélanger M. , Meunier M. , Currie J. F.
Published: April 2011

A systematic study of the etch rate of GaAs and of positive photoresist for different mixtures of argon and carbon tetrafluoride was conducted over radio frequency powers (from 0.06 W/cm2 to 0.55 W/cm2), pressures (from 6 to 35 mTorr (1 Torr = 133.3 Pa)), and concentrations of CF4 in Ar (0–60% with a constant mass flow of 10 sccm). Capacitance–voltage and I–V measurements on GaAs diodes made after reactive ion etching were carried out to estimate possible etching damage through thin dielectric film and surface state creation. Etch rates up to 200 Å/min were obtained on GaAs with low damage in a 40% CF4, 20 mTorr, and 0.55 W/cm2 plasma while the etch rate of the patterning photoresist was 600 Å/min. These results are in good agreement with those reported in literature. The ideality factors and Schottky barrier heights of reactive ion etching of GaAs are comparable to those obtained by sulphuric and peroxide acid etching. However, reactive ion etched samples seem to suffer from higher surface state densities as measured by C–V techniques.
High resolution electron microscopy characterization of interfaces in group III–V semiconductors
Authors: Mallard R. E.
Published: April 2011

The technique and application of high resolution electron microscopy (HREM) to the study of group III–V semiconductor multilayers is described. The theory of HREM contrast is briefly reviewed, emphasizing the need to bear in mind resolution and information limits to which the images may be interpreted. The role of image simulations in this interpretation is stressed. Examples are given of the application of HREM to the study of the morphology and chemical abruptness of GaInAs–AlInAs interfaces and the defect structure at GaSb–GaAs strained interfaces.
Photoluminescence studies of defects in annealed Czochralski silicon
Authors: Steele A. G. , Thewalt M. L. W.
Published: April 2011

Photoluminescence spectroscopy (PL) is becoming a standard technique for characterizing defects and impurities in high-purity semiconductors. Further information can be obtained by combining high-resolution continuous-wave PL with time-resolved measurements and excitation spectroscopy. It is well known that low-temperature annealing, near 450 °C, produces a wide variety of electrically active defects in Czochralski-grown Si containing high concentrations of O, including the well-known thermal donors. The nature of these heat-treatment centers strongly depends on the annealing time, the prior thermal history of the sample, and the concentration of O (and C) in the starting material. We present the results of extensive photoluminescence studies on such annealed Si samples. Both thermal-donor bound excitons and two isoelectronic centers are discussed.
Defect profiling of semiconductor epilayers using positron beams
Authors: Tandberg Erik , Schultz Peter J. , Aers Geof C. , Jackman T. E.
Published: April 2011

Over the last 10 years, positrons have been used as a tool to study and profile dilute concentrations of defects in solids. The two basic techniques involve the measurement of the positron lifetime and the Doppler broadening of the annihilation γ-rays. Only recently have positrons been used to profile defects in multilayered structures. We review the theory of positron–defect interactions and positron transport in silicon, and introduce a model used to profile defects and electric-field effects using the steady-state diffusion equation. An example is discussed that involves the modelling of defects and electric fields in homoepitaxial layers of silicon grown by molecular-beam epitaxy on Si(100) substrates. Finally we discuss the limitations of the modelling technique and future prospects.
Deep-level transient spectroscopic investigation of the EL2 deep-level in metalorganic chemical vapour deposited Ga1−xInxAs epilayers
Authors: Lang R. V. , Leslie J. D. , Webb J. B. , Roth A. P. , Sacilotti M. A. , Masut R. A.
Published: April 2011

The thermal activation energy of the EL2 deep level in low-pressure metalorganic chemical vapour deposited Ga1−xInxAs epilayers has been determined by deep-level transient spectroscopy. Variation of the deposition conditions, which included a change in the substrate orientation, resulted in different dependencies of the EL2 thermal activation energy upon the epilayer indium content. In all cases a decrease in this deep-level property was observed for increasing indium content in the epilayers. Although the cause of this variation in the dependence of the thermal activation energy upon the epilayer indium content could not be identified in this work, it can be shown that it is not associated with different amounts of residual strain or impurities in the epilayers.
Emission spectroscopy as a noninvasive probe for process control during radio frequency sputter deposition of PbTe
Authors: Cook J. G. , Aouadi M. S. , Das S. R.
Published: April 2011

An rf magnetron discharge used to sputter PbTe in Ar is analyzed using emission spectroscopy. The intensity (I) of the strong Pb, Te, and Ar emission lines is determined near the target and near the substrate as the rf power or gas pressure is varied. It is shown that as the rf power is varied at pressures below 0.5 Pa, the electron temperature is not affected, so that the sputtered atom density NPb is proportional to IPb/IAr. The electron temperature is, however, sensitive to pressure changes. Application of a substrate bias voltage has a large effect on the emission spectra which are not understood.
Crystal growth of CuInSe2 by the Bridgman method
Authors: Weng W. S. , Yip L. S. , Shih I. , Champness C. H.
Published: April 2011

Single crystals of CuInSe2 have been fabricated by the vertical Bridgman method. A conventional Czochralski crystal-pulling system was adapted for this purpose. An accelerated crucible-rotation technique was employed for a better mixing of the melt during the growth. Void- and crack-free crystal grains with an area as large as 50 mm2 and a thickness of more than 5 mm could be selectively cut from the ingots. From room-temperature Hall-effect measurements, mobility values as large as 73 cm2 ∙ V−1 ∙ s−1 were obtained for the present samples. X-ray diffraction studies suggested that abrasive polishing might create an amorphous layer on the surface of the CuInSe2 crystals.
The study of carbon incorporation during growth in epitaxial indium antimonide layers prepared by metalorganic magnetron sputtering
Authors: Rao T. Sudersena , Webb James B. , Noad J. P. , Jackman J.
Published: April 2011

Secondary-ion mass spectrometry and Auger electron spectroscopy measurements combined with Hall data have indicated high levels of electrically active carbon impurities in InSb films grown by metalorganic magnetron sputtering. Background impurity concentrations in the range of 5 × 1018 cm−3 were observed for films grown at optimum deposition conditions. While it was also observed that the level of carbon in the layers decreased with increasing substrate temperature and V-III ratio, the addition of small amounts of molecular hydrogen (H2) to the sputtering gas resulted in an order of magnitude decrease in the carbon content of the as-grown InSb layers. Mass spectrometric studies suggest that the major source of carbon contamination in the layers is the methyl radicals produced from the pyrolysis of trimethylindium.
Physics of microstructures
Authors: Movaghar B.
Published: April 2011

A brief review is presented of the novel physics associated with submicrometre structures produced by molecular beam epitaxy and electron beam lithography. In low-temperature transport, the most important effect is the achievement of extraordinary long coherence lengths (10 μm) or high mobilities for carriers moving in two dimensions and constructed topologies. Ballistic motion, fractional quantum Hall effect, and Bohm–Aharonov interference are direct consequences. When barriers are present in multiple quantum well and superlattice structures, we have resonant tunnelling, Stark localization, and magneto-Stark transport. In optics, the main novelty is the consequence of the quantum well confinement, giving rise to sharp inter-subband transitions and excitonic effects. The strongly wavelength-dependent absorption can be of considerable importance in photodiodes, wavelength demultiplexers, and infrared detectors.
Transient photoluminescence studies of coupled double-quantum wells
Authors: Charbonneau S. , Thewalt M. L. W. , Koteles Emil S. , Elman B.
Published: April 2011

We have undertaken a detailed experimental study of the electric-field-dependent coupling between two identical GaAs quantum wells separated by a narrow AlGaAs barrier using photoluminescence (PL), transient PL, and PL excitation spectroscopy. It was found that the coupling of the wave functions in a coupled double-quantum well (CDQW) structure produces a splitting of the single-well levels. The influence of an electric field on the exciton states in such a structure showed clear evidence that both intrawell and interwell exciton transitions occur. Furthermore, the effect of rapid thermal annealing (RTA) on these CDQW structures with and without SiO2 cap layers was also studied. While RTA without a capping layer produced only minor changes in the PL spectrum, the combination of a 300 nm SiO2 overlayer plus RTA caused a shift to lower energy of the main PL peak. This difference was attributed to a change in the shape of the CDQW structure caused by capped RTA.
Photothermal ionization spectroscopy of selectively boron-doped GexSi(1–x)Si strained-layer heterostructures
Authors: Jang H. F. , Datars W. R. , Timusk T. , Berezin A. A. , Houghton D. C.
Published: April 2011

Far-infrared photothermal ionization spectroscopy has been used to investigate the impurities in selectively boron-doped GexSi(1–x)–Si strained-layer heterostructures. The spectra are obtained under various experimental conditions: with and without band-edge light and by varying the temperature of, and voltage applied to, the sample. The transport properties, resistivity, sheet charge density, and Hall hole mobility of these samples are presented. It is found that a sample with a two-dimensional hole gas (2-DHG) behavior at the GexSi(1–x)–Si interface has a weak photoresponse. This is due to a small number of photo-generated carriers in comparison with the residual hole carriers. For samples that do not show 2-DHG behavior, the photo-response from the substrate plays the dominant role for low applied voltages. At higher voltages, the response from the epitaxial layers becomes evident. The majority impurity is identified as the intentional dopant boron, and the dominant minority impurity is found to be phosphorus. It is concluded that although the free carriers are generated in the heavily B-doped layer, those carriers conducted through the low-energy high-mobility GexSi(1–x)–Si interface dominate the photoresponse.
Selective contacts for silicon-doping superlattices
Authors: McKinnon G. H. , McMullin J. N. , Landheer D. , Buchanan M. , Janega P. , Denhoff M. W.
Published: April 2011

Results are presented that show that selective n- and p-type contacts with low resistance have been formed on silicon-doping superlattices. The superlattices were grown by molecular-beam epitaxy through a silicon mask to create built-in blocking regions at the edges of the mesas. Contacts of Mg2Si for the n-side and PtSi for the p-side were formed along the edges of the mesa, resulting in a successful diode-like behavior. A surface etch to remove doping impurities from the region surrounding the superlattice mesa was required to maintain contact–substrate isolation.
The effects of substrate orientation on the optical properties of InGaAs epitaxial layers grown by low-pressure metal organic vapour-phase epitaxy
Authors: Roth P. , Sacilotti M. A. , Masut R. A. , Morris D. , Young J. , Lacelle C. , Fortin E. , Brebner J. L.
Published: April 2011

The effect of substrate orientation on the optical properties of nonpseudomorphic InxGa1–xAs epitaxial layers grown by metal organic vapour-phase epitaxy has been investigated in two series of samples. Series (a) samples were grown on GaAs substrates cut 2° off (001) towards [Formula: see text], and those of series (b) were grown on GaAs substrates cut exactly on (001). The low-temperature photoluminescence spectra of the two series of samples differ drastically. Those of the (a) series are characteristic of highly uniform, fully relaxed ternary layers and are used to determine the composition dependence of the alloy band gap and of the carbon acceptor binding energy. In contrast, those of the (b) series are dominated by an intense, low-energy emission, which is attributed to exciton recombinations in small regions of small band gap. The reduction of the band gap is tentatively assigned to the presence of In-rich regions created by the large strain fields present along dislocations. The density of dislocations is much larger in the samples of the (b) series than in those of the (a) series owing to the combination of three-dimensional growth and slow strain relaxation for layers grown on (001) substrates.
Extended X-ray absorption fine-structure analysis of vacuum-deposited amorphous germanium
Authors: Johnson G. W. , Brodie D. E. , Crozier E. D.
Published: April 2011

In this study, thin films of germanium have been vacuum deposited in four regimes. Care was taken to prepare reproducible films, which required that the partial pressure of water be below 10−8 Torr during deposition (1 Torr = 133.3 Pa). First, films deposited onto substrates held during deposition at a temperature Ts that is below 473 K are amorphous. Once annealed above 423 K, their electrical conductivity and optical band gap are independent of deposition temperature and rate, and of whether or not low-energy electron irradiation of the substrate is used during deposition. This suggests that a well-defined and reproducible structure is being prepared. Second, a "precrystallization regime" is obtained when Ts is between 473 and 513 K. Extended X-ray adsorption fine-structure and X-ray diffraction confirm that this regime is a two-phase mixture of amorphous material and crystallites. Third, films deposited with Ts near 513 K, while using low-energy electrons to bombard the substrate, are amorphous, but these films have different electrical and optical properties from the films m the first regime. From this, we infer that a second well-defined amorphous structure exists. Fourth, films deposited with Ts above 513 K are polycrystalline. Extended X-ray adsorption fine-structure and X-ray adsorption near-edge structure could not distinguish between the two amorphous materials in the first and third regimes.
Spatially resolved lifetimes in polycrystalline silicon
Authors: Atkinson M. R. , Dixon A. E.
Published: April 2011

Spatially resolved carrier lifetimes have been obtained for polycrystalline silicon samples using a modified scanning laser microscope. Photoconductivity as a function of illumination modulation frequency was measured at a set of positions across a grain boundary. Improved response models for light focussed both at a grain boundary and in the bulk have been introduced and applied to the measured frequency responses. The spatially resolved lifetimes were obtained from these frequency response spectra. Since frequency response measurements employ lock-in amplification techniques, they allow for lower illumination levels, smaller illuminated areas, and lower applied biases than can be used for photoconductivity decay measurements of lifetimes. The variation of grain boundary lifetime with illumination intensity has also been investigated.
A study of hydrogen diffusion in crystalline silicon by secondary-ion mass spectrometry
Authors: Tong B. Y. , Wu X. W. , Yang G. R. , Wong S. K.
Published: April 2011

The diffusion coefficient of hydrogen in crystalline silicon, obtained from recent profiling experiments such as nuclear resonance retention and secondary-ion mass spectroscopy, is 3–9 orders of magnitude smaller than the previously accepted value measured by Van Wieringen and Warmoltz in 1956. Here we point out several items often overlooked in the analysis of profiling measurements. A limited flux model is proposed to explain the observed results. Predictions by the model are supported by further experiments.
Photoluminescence for characterization of commercial semi-insulating GaAs
Authors: Maciaszek M. , Rogers D. W. , Bult R. P. , Steiner T. , Zhang Yu , Charbonneau S. , Thewalt M. L. W.
Published: April 2011

Photoluminescence (PL) and selective pair luminescence (SPL) techniques were used to evaluate a large number of GaAs crystals. It was found that the results of these photoluminescence measurements, combined with data on carbon concentration obtained by local vibrational mode absorption spectroscopy, make possible an analysis of shallow impurities. Their relative concentrations were compared from seed to tail and from crystal to crystal and correlated to material resistivity. Impurities segregation in a crystal was analysed. Results of polariton line shape and intensity analysis were used in surface-related studies. Advantages of using both PL and SPL as a routine production analytical technique will be enhanced when the equipment for two-dimensional mapping of a whole GaAs wafer is completed.
Optoelectronic matrix switching
Authors: MacDonald R. I.
Published: April 2011

Because cross talk from electromagnetic coupling increases with frequency, it is difficult to design matrix switches for high-frequency electronic signals. Signals on optical carriers are easier to isolate but more difficult to switch. A hybrid technique exploiting passive optical-signal distribution and switching by optoelectronic effects shows good promise for high isolation matrices to handle signals in the dc to 10 GHz range. The key elements for such optoelectronic switches are optical detectors that have switchable sensitivity.Initial efforts in optoelectronic switching used silicon diode detectors and forward bias to establish the off-state. Recent work has turned primarily to photoconductors fabricated in GaAs or other compound semiconductors. Such photoconductors can have very wide response bandwidths and have the advantage of a zero-bias off-state. Efforts have also been made to develop bipolar detectors that do not need forward bias for the off-state.Experimental work has progressed to the demonstration of complete matrices capable of switching up to eight incoming signals to as many outgoing lines, at bandwidths up to 500 MHz and cross-talk levels below 50 dB. Results for individual switching devices give promise of matrices for many tens of lines and bandwidths to several gigahertz. Monolithic integration of GaAs photoconductor switch arrays has been demonstrated, and work progresses to include integrated control devices and amplifiers.Matrix switches of large dimension and very large bandwidth are expected to have significant applications in areas other than communications switching. In particular, such matrices can be used to set up patterns of delay and signal recombination, using fibre delay lines, to provide rapidly selectable wide-band delay for phasing and other applications, or for wide-band tapped delay-line filters that can be reconfigured rapidly.A review of the current status in optoelectronic matrix switching is given.
All-optical switching in a GaAs-based multiple quantum well directional coupler
Authors: Keyworth B. P. , Cada M. , Glinski J. M. , SpringThorpe A. J. , Mandeville P.
Published: April 2011

The nonlinear, all-optical switching characteristics of a GaAs-based directional coupler are investigated. The structure consists of two Al0.18Ga0.82As planar waveguides coupled through a GaAs–AlGaAs multiple quantum well (MQW) layer. Changing the refractive index of the MQW layer, through a Kerr-type nonlinearity, varies the coupling length of the element, which in turn determines the distribution of optical power at the output of the sample. Our theoretical analysis of the element predicted that a strong nonlinear switching effect should be observed near the critical power, Pc, for samples cleaved to the appropriate length. This has been verified experimentally, for the first time with such a structure, and reveals a full-transfer coupling length of approximately 160 μm and a critical launched power of approximately 750 μW.
High efficiency CuInSe2–(CdZn)S solar cells: fabrication, results, and analysis
Authors: Damaskinos S. , Phillips J. E. , Roy M. , Birkmire R. W.
Published: April 2011

The CuInSe2 thin films for solar application are grown by physical vapor deposition in a standard evaporation system. Indium-doped high conductivity (CdZn)S is evaporated onto the CuInSe2 to form the junction. The CuInSe2–(CdZn)S devices require air heat treatments to achieve the highest efficiency. High-efficiency small-area devices exceeding 11% (87.5 mW/cm2, ELH, T = 32 °C) have been fabricated this way. The behavior of the devices cannot be described as a classical heterojunction dominated by interface recombination. Instead, current–voltage and spectral-response analysis show that the diode current transport is dominated by a Shockley–Read type of recombination model, with space–charge being the dominant recombination mechanism via midgap states, with an estimated concentration of 1014 cm−3.
Screen printed Ta2Os and TiO2 antireflection coatings for crystalline and polycrystalline silicon solar cells
Authors: Thomas R. E. , Varma S. , Waechter D. , Dodd C. X. , Das S. R.
Published: April 2011

Screen printable inks for Ta2O5 and TiO2 antireflection coatings have been applied to silicon wafers and characterized. On polished crystalline wafers, both coatings give near-zero minimum reflectance. The wavelength of the minimum can be easily tuned to match the solar spectrum by changing the screen mesh size. The average reflectance over the AM1 spectrum can be further reduced by texturizing the wafer surface prior to coating. Similar reflectance results are obtained for both crystalline and polycrystalline wafers, although the average reflectance is not quite as low in the latter case due to the difficulty of texturizing polycrystalline material.
CdZnSO and ZnSO films for optoelectronic application
Authors: Qiu S. N. , Qiu C. X. , Shih I.
Published: April 2011

In the present work, thin films of CdZnSO and ZnSO have been deposited on glass substrates from targets of mixed ZnO and CdS (or ZnS) powder by using a radio frequency magnetron sputtering method. It was found that the crystalline quality of the pure materials (ZnO, CdS, or ZnS) was better than the crystalline quality for an intermediate composition. The lattice parameters of these materials were also found to vary with the composition. By using a target containing 40% ZnO and 60% CdS, we obtained films with lattice parameters very near those of the semiconductor CuInSe2.
Intrinsic and extrinsic factors limiting the time response of interdigital photodetectors
Authors: Seymour R. J. , Garside B. K.
Published: April 2011

Very high speed photodetectors can be produced using an interdigital electrode structure on the surface of a semiconductor-on-insulator material. The factors which limit the time response of such a structure are briefly discussed, with emphasis on the calculation of capacitance. The validity of an approximate analytical model to calculate capacitance is verified both theoretically and experimentally. This model is used to allow simple optimization of the design parameters of an interdigital photodiode. A comparison between the optimized area of an interdigital device and an equivalent p-type intrinsic n-type (pin) photodiode shows that the interdigital device can have an area up to four times larger.
Photovoltaic devices fabricated on electrodeposited CuInSe2 films
Authors: Qiu C. X. , Shih I.
Published: April 2011

Using both Al–CuInSe2 Schottky junctions and CdS–CuInSe2 heterojunctions fabricated on electrodeposited p-type CuInSe2 films, we have investigated some properties of CuInSe2. From current–voltage and capacitance–voltage measurements carried out on these devices, it was found that the electronic quality of the CuInSe2 film surface was improved after the deposition of the CdS layer. In addition, it was observed that the electronic properties of the heterojunctions were dependent on the In/Cu ratio in the electrodeposited CuInSe2 films. Using a film with In/Cu = 1.1, we successfully fabricated a cell with an active-area conversion efficiency of 5.2%. Post-fabrication air heat treatment at 180 °C on the CdS–CuInSe2 devices was found to result in an increase in the charge-carrier concentration in the electrodeposited CuInSe2 films.
Preparation and electrical properties of heterojunctions of ZnO on Zn3P2 and CdTe
Authors: Ginting M. , Leslie J. D.
Published: April 2011

"Heterojunctions" have been fabricated by the reactive evaporation of thin film n-type ZnO onto p-type single crystal Zn3P2, polycrystalline films of Zn3P2, and single crystal CdTe. The photovoltaic response of the n-ZnO – single crystal p-CdTe devices was good, that of the n-ZnO – single crystal p-Zn3P2 devices was poor, and that of the n-ZnO – p-Zn3P2 polycrystalline film devices was nonexistent. The ideality factor n of all devices studied was greater than two. On the basis of 1/C2 vs. V results, the n-ZnO – single crystal p-Zn3P2 devices behaved most like Schottky barrier devices, whereas the n-ZnO – p-Zn3P2 polycrystalline film devices, and the n-ZnO – p-single crystal CdTe "heterojunctions" behaved most like metal–insulator–semiconductor devices. The high series resistance of all devices had to be considered in the measurement and analysis, and it limited the photovoltaic performance. Deep-level transient spectroscopy measurements indicated majority (hole) traps in the CdTe and Zn3P2 with activation energies in agreement with previous measurements in the literature.
KNP89: Kinetics of nonhomogeneous processes (KNP) and nonlinear dynamics
Authors: Freeman Gordon R.
Published: April 2011

Most systems in nature are nonhomogeneous: at least one component is not distributed homogeneously. A nonhomogeneous process is one that occurs in a nonhomogeneous system. The new subject Kinetics of Nonhomogeneous Processes (KNP) deals with processes in all aspects of nature. Examples range from quantum mechanics to membranes and to the evolution of the large scale structure of the universe. Models dealing with molecules and galaxies have common features: each has translational and rotational motions as an entity, and it has internal motions that are governed by the internal masses, fields and energies. To make progress towards understanding behaviour of a complex system, simplifications are needed. The formulation of a model requires identification of essential features of the behaviour, finding correlations between the features, and then representing them by equations. There is interplay between the structure of a system and the kinetics of processes that occur in it. To identify essential elements of any process we visualize it. Visualization is a powerful tool, especially the visualization of behaviour represented by equations. Models of KNP are of two general kinds: deterministic and stochastic. Deterministic models involve an assumption that events are the inevitable result of preceding conditions. The mathematical description of KNP involves nonlinear equations. Part of the physics community has therefore come to speak of nonlinear dynamics, which is a subdivision of KNP. An example of a deterministic model is the time dependent Landau–Ginzberg equation and modifications of it, which apply to pattern formation, self-localization, instabilities, and chaos. A stochastic model involves a step-by-step process and considers the probability of events as a function of time. In a system that contains many zones and each zone contains a relatively small number of entities, for example two ion–electron pairs or a group of seven galaxies, the probable reaction or deflection of any given entity is strongly affected by the actual number of entities initially in the zone, by their relative motions and separation distances, and by the forces that act between and within them. Scientific literacy in the future will require an understanding of both deterministic and stochastic models.
Laws and events: The dynamical basis of self-organization
Authors: Petrosky Tomio Y. , Prigogine Ilya
Published: April 2011

Traditionally, self-organization has been studied using macroscopic phenomenological descriptions. Recent computer simulations show that "dissipative structures" may be obtained through dynamical programs without any macroscopic assumptions. Self-organization is rooted in dynamics. This leads to the question, "what type of dynamical laws permit self-organization?" The answer refers to unstable dynamical systems, which were widely studied after the pioneering work of Kolmogorov. We concentrate our work on large Poincaré systems with continuous spectrum of the Liouville operator, a generalization of Poincaré's nonintegrable systems. Examples are collisions or quantum jumps. We show that resonances lead to a dynamics of correlations, which can be analyzed using our recent theory of subdynamics. We decompose the dynamical evolution into a set of independent processes. As an example, we study radiation processes as well as the transformation of classical dynamics information into order or disorder processes. We show that in quantum mechanics, our approach leads to a reinterpretation of the collapse of the wave function, owing to the dynamical nature of the system and not to human measurement.