bluuebunny/crossref_metadata_2025 huggingface dataset:
Doping of GaAs epitaxial layers grown on (100) GaAs by close-spaced vapor transport
Authors: Koskiahde E. , Cossement D. , Paynter R. , Dodelet J. P. , Jean A. , Lombos B. A.
Published: April 2011

Using H2O as a transport agent, epitaxial GaAs layers were grown by the close-spaeed vapor transport technique (CSVT) on (100) heavily Si-doped GaAs substrates. Three kinds of GaAs sources were used for the deposition: (100) GaAs, heavily doped with Te or Si, and undoped semi-insulating (SI) (100) GaAs. The growth rates obtained with SI and Te-doped GaAs are quite similar and show a clear tendency to be superior to the growth rates measured for Si-doped GaAs sources. Uncompensated charge carrier density (ND – NA) profiles have been measured electrochemically for the layers grown with the three kinds of sources. When Te-doped GaAs is used, (ND – NA) obtained for the epitaxy is the same as that of the source, implying a complete transfer of the Te impurity. (ND – NA) values varying from 1016 to 1018 cm−3 are obtained from SI GaAs sources, depending upon the thickness of the epitaxial layer. (ND – NA) < 1015 cm−3 are measured for layers grown from Si-doped GaAs sources. In this case, layers thicker than 10 μm cannot be mesured electrochemically because of their excessively high resistance. The small (ND – NA) values obtained in that case are explained by the reaction of Si contained in the source with the transport agent (H2O), resulting in the formation, at the Si-doped GaAs surface, of a passivating SiOx layer revealed by Auger spectroscopy. This passivating layer also explains the smaller growth rates measured with these sources. p–n Junctions have been prepared by Zn diffusion in CSVT layers grown from SI GaAs sources. Their I–V characteristics show good rectification behavior, indicating that the CSVT layers could be used for photovoltaic purposes.
GaAs–AlAs heterojunctions under hydrostatic pressure
Authors: Beerens J. , Grégoris G. , Lavielle D. , Amor S. Ben , Portal J. C. , Alexandre F.
Published: April 2011

We report magnetotransport measurements made at 4.2 K on GaAs–AlAs modulation-doped heterojunctions under hydrostatic pressure. The two-dimensional electron-gas concentration is observed to decrease linearly from 1.5 × 1011 cm−2 to 0 cm−2 when the pressure is increased from 1 bar to [Formula: see text] (1 bar = 100 kPa). The relative movement, induced by pressure, between the X band in AlAs (dEX/dP = −1.7 meV/kbar) and the Γ band in GaAs (dEΓ/dP = 10.7 meV/kbar) can account for about 65% of the decrease. Different possibilities are examined in order to interpret the remaining 35%.
Characteristics of n-ZnSe – n-GaAs using metalorganic chemical-vapour desposition
Authors: Mazuruk K. , Benzaquen M. , Walsh D. , Makuc B. , Jayatirtha H. , Aharoni H.
Published: April 2011

Thin epitaxial films of ZnSe were grown on GaAs substrates by a pulsed metalorganic chemical-vapour disposition technique. The X-ray analysis of the layers showed a consistent monocrystalline structure with (100) orientation, and a photoluminescence indicating a low level of residual impurities. The n-ZnSe – n-GaAs heterojunction has been studied on the basis of I–V characteristics and photoresponse, the results being consistent with a staggered-band offset model proposed previously.
Laser and photoluminescence spectra of InGaAs–GaAs strained-layer superlattices
Authors: Hunt N. E. J. , Jessop P. E. , Garside B. K. , Devine R. L. S.
Published: April 2011

Photoluminescence (PL) and laser emission from optically excited InxGa1−xAs–GaAs strained-layer superlattices (SLS) grown by molecular-beam epitaxy have been examined. In low-intensity PL, a single, narrow (≈5 nm FWHM) emission peak was observed, corresponding to the n = 1 electron to heavy-hole transition. This feature, as well as a series of higher energy transitions, was also observed in absorption and photoreflectance spectra. High-intensity PL, using a pulsed nitrogen laser for excitation, yielded an additional peak, several millielectronvolts lower in energy than the dominant low-power PL feature. Optical-gain measurements were performed using the variable stripe-length method. The stimulated emission emerging from the single cleaved end was shifted to the long-wavelength side of the low-power PL peak, and in the case of the narrower well samples, the emission consisted of two peaks separated by about 8 meV. Lasing at these gain positions was observed when ≈400 μm long chips were prepared and their lengths were pumped with a stripe of nitrogen laser light. The observed gain spectrum has been attributed to a combination of a free-carrier-induced bandgap shift and self-absorption by unpumped material deeper within the SLS waveguide.
A photodetector array for acousto-optic signal processors
Authors: Inkol R. J. , Washkurak W. D. , Chamberlain S. G.
Published: April 2011

Acousto-optic (AO) signal processors are of interest for processing wide bandwidth signals in real-time. Unfortunately the performance of practical systems is often limited by the photodetector array, which must satisfy conflicting requirements concerning sensitivity, dynamic range, and response/readout time. A recently developed photodetector structure has important potential advantages for this application. It uses the exponential current to voltage relationship obtainable with a profile channel metal oxide semiconductor field-effect transistor to provide a steady state logarithmic relationship between incident light intensity and output voltage and a dynamic range exceeding 70 dB. This paper discusses the design considerations associated with the use of the new photodetector structure in the context of a 128 element photodetector array being developed for realtime processing of wide bandwidth signals. This design uses a series-parallel architecture to provide a minimum exposure time of 1 μs. The photodetector and readout electronics are optimized to provide high sensitivity and response speed for transient signals. Simulation results for the time response to pulse signals and the crosstalk performance are presented. These suggest that the dynamic range exceeds 50 dB for pulses less than 1 μs in duration.
Nonreflecting stratifications
Authors: Lekner John
Published: April 2011

Electromagnetic, acoustic, and particle waves are specularly reflected by planar inhomogeneities (stratifications). A brief review is given of the cases in which zero reflection is known to occur. In general these are rare, but stratifications that are symmetric with respect to inversion about their midplane have an analytic property of the reflection amplitude that makes zero reflection a frequent occurrence. Examples are given of the locii of zero reflection in the angle of incidence – thickness plane of some symmetric stratifications. For uniform layers the zero reflection locii can be classified into Malus–Brewster–Green type and interference type, but this distinction does not hold in general.
Nonhomogeneous three-dimensional structures described by the nonlinear Klein–Gordon equation
Authors: Otwinowski M. , Paul R. , Tuszynski J. A.
Published: April 2011

We present exact solutions to the nonlinear Klein–Gordon equation in (3 + 1) dimensions. Vortex rings, spirals, and other solutions are demonstrated. On the basis of our solutions we discuss various nonhomogeneous patterns, which can be formed at the critical point of systems described by the Ginzburg–Landau model.
Is the cosmic-ray hadron shower a fractal?
Authors: Ohnishi Teruaki
Published: April 2011

The possible fractal nature of trajectories drawn by cosmic-ray particles in the atmosphere is investigated. In the course of the propagation of a primary cosmic-ray particle from the top of the atmosphere to sea level, it develops a ramified hadron shower, which is constituted mainly of pions and kaons. These hadrons are multiply produced at every collision of the hadron with an air nucleus. If the cross section corresponding to a definite multiplicity of hadrons decreases with the increase of energy E, in such a manner as to be reciprocally proportional to some powers of E, the resultant pattern of the shower possibly becomes fractal. Since the total interaction cross section is considered to be a superposition of many fundamental cross sections each of which corresponds to a definite multiplicity, the actual hadron shower can be interpreted as a superposition of many fractals each of which has an unique fractal dimension. The total energy of the shower has appeared to obey Laplace's equation under a quasi-stationary condition. Some ramified patterns of the cosmic-ray hadron shower are derived with computers.
Electron thermalization-distance distributions in electron-attaching fluids
Authors: Gee Norman , Freeman Gordon R.
Published: April 2011

The electron ejected from a molecule by an energetic impact moves away from the resultant ion and loses energy to the molecules with which it collides. The distance such electrons travel away from their ions during thermalization can be estimated by measuring the free-ion yields as a function of electric-field strength. This was done in gaseous and liquid carbon disulfide and hexafluorobenzene over wide ranges of densities. The electron thermalization-distance distribution in C6F6 was the same as that in most other liquids; it was a Gaussian distribution with a power tail. However in liquid CS2 the distribution was different, an exponential with a power tail, as in liquid nitrogen and liquid carbon monoxide. The different distributions reflect differences in the thermalization processes. The thermalizing ability of both CS2 and C6F6 is less in the liquid than in the gas. There is an especially sharp decrease in the thermalizing ability of CS2 at the highest densities.
Organized monolayers and monolayer assemblies as potential components of molecular devices
Authors: Möbius Dietmar
Published: April 2011

The assembly of molecules to functional units and their interconnection in molecular dimensions to achieve processes like sensing, signal storage and processing, or catalysis of particular reactions is a challenge to modern science. Various phenomena are reviewed that may be considered relevant to the design and construction of molecular devices. The propagation of mechanical excitations of the monolayer-covered air–water interface as a model for signal transmission has been investigated using energy-transfer processes or fast formation of dye aggregates for optical detection. The lateral conductivity in specially designed monolayers can be modulated photochemically, thus providing a possibility of switching. Fast transfer of energy by incoherent exciton hopping can be used to harvest light by concentrating the energy absorbed in large area on a reactive center. Molecular fluorescent probes are sensors for electrical potentials, structure of the local environment, and average ion concentration. At the air–water interface, molecules may self-organize to larger units with new properties. Examples are the formation of extended two-dimensional aggregates of dyes and the reorganization of mixed monolayers to form a replica of adsorbed molecules.
Molecular and global structure and dynamics of membranes and lipid bilayers
Authors: Sackmann E.
Published: April 2011

The cell plasma is a composite type of material that is made up of a two-dimensional liquid crystal (lipid–protein bilayer) to which a macromolecular network (the cytoskeleton) is loosely coupled. The latter may be approximately two dimensional as in the case of the erythrocytes or may extend throughout the whole cell cytoplasm. Owing to this combination of two states of matter, the membrane combines the dynamics and flexibility of a fluid with the mechanical stability of a solid. Owing to its low dimensionality, the local structure of the bilayer or the global shape of cells may be most effectively controlled and modulated by biochemical signals such as macromolecular adsorption. The present contribution deals with comparative studies of the local and global dynamic properties of biological and artificial membranes. In the first part the question of the physical basis of selective lipid–protein interaction mechanisms is addressed and the outstanding viscoelastic properties of plasma membranes and their role for local instabilities shape fluctuations of cells and the cell–substrate interaction are described. The second part deals with the molecular architecture and dynamics of composite membranes prepared by combining monomeric and macromolecular lipids. These model membranes open new possibilities to mimick complex mechanical processes of cell plasma membranes and to prepare low-dimensionality macromolecular solutions and gels. Finally, the use of such compound systems by nature to prepare the semipermeable protective layers of plant leaves, the so-called cuticle, is discussed. In analogy to plasma membranes, the local transport properties are modulated by variation of the liquid-crystalline state of the monomeric waxes.
Hole-burning kinetics in biomolecules and polymers
Authors: Kushida Takashi , Kurita Atusi , Kanematsu Yasuo , Touyama Yoshikatsu
Published: April 2011

Various characteristics of the change in the absorption spectrum induced by monochromatic laser-light irradiation were compared for a chromophore in a protein, a dye-doped polymer, and a dye-intercalated DNA. It was found that persistent spectral holes are burned in the absorption spectrum of Zn-substituted myoglobin (ZnMb) and of methylene blue intercalated into DNA as easily as of rhodamine 640 in polyvinyl alcohol when the lowest optical absorption bands are illuminated with laser light at low temperatures. In both ZnMb and dye-doped polymer, the hole depth has been found to grow almost logarithmically with burning time. This is explained well by a dispersive burning-kinetics model. A heat-cycle experiment using ZnMb has revealed that the conformational barrier height in the electronic ground state has a broad distribution. We conclude that the hole-burning characteristics are very similar among the systems examined.
Drying of solids wetted by thin liquid films
Authors: Wyart F. Brochard , Daillant J.
Published: April 2011

(i) A film of a nonwetting liquid is not stable. If it is thick (thickness e ≥ 1000 Å) it is metastable and evolves via nucleation and growth of a dry spot. If it is thin, it is unstable against spinodal decomposition (amplification of thermal undulations) and breaks into microscopic droplets of size ~e2/a (a = a molecular size), (ii) A film made with a wetting liquid is metastable and tends to shrink whenever e < eS (the equilibrium thickness of the wetting layer). Contrary to case (i) where the growth of a dry patch is controlled by capillary forces, the central role for drying in case (ii) is played by long-range forces, and the only process is nucleation and growth.
Randomly branched polymers
Authors: Daoud M.
Published: April 2011

We show that the synthesis of randomly branched polymers is correctly described by percolation in general. This is characterized by a very broad distribution of molecular weights, or polydispersity. Because of the latter, the average properties actually observed are quite different from those of any given polymer. We discuss the swelling properties of the macro-molecules when a good solvent is added. These are the bases of the swelling of gels, which are solids that may change their volume by orders of magnitudes when diluted.
The effects of deposition temperature on properties of SiO2 films fabricated by a new electron cyclotron resonance microwave plasma-enhanced chemical-vapor deposition process
Authors: Chau T. T. , Mejia S. R. , Kao K. C.
Published: April 2011

Silicon dioxide (SiO2) films were deposited by a new electron cyclotron resonance (ECR) microwave plasma enhanced chemical vapor deposition (PECVD) process at various deposition temperatures ranging from room temperaure (~25 °C) to 300 °C. The deposition rate increases with increasing deposition temperature and tends to become saturated as the deposition temperature approaches 250 °C, in contrast to the SiO2 films fabricated by the conventional ECR microwave PECVD process. The Fourier transform infrared results provide no evidence of hydrogen incorporation in the SiO2 films even when they were deposited at room temperature. Films deposited at temperatures higher than 270 °C exhibit an excellent electrical integrity that is comparable with high-quality SiO2 films grown thermally at 1000 °C. Films deposited at room temperature have a slightly higher refractive index and also high-leakage current though the films appear as good as those deposited at 300 °C. As the deposition temperature is increased, both the shoulder height of the Si—O stretching band and the leakage current decrease. The correlation between the film properties and the film growth mechanism is also discussed.
Measurements of ion energy and flux during ion-assisted deposition of CdTe epilayers by rf magnetron-sputtering
Authors: Cook J. G. , Das S. R.
Published: April 2011

Previously we found that the crystallographic structure of CdTe films deposited by means of ion-assisted magnetron rf sputter deposition was very sensitive to the substrate bias voltage and temperature (S. R. Das et al. Can. J. Phys. 65, 864 (1987)). In this work, the ion energy and flux incident on the growing film were determined by means of rf-compensated Langmuir diagnostics. It was found that control of the film phase in the previous work was achieved largely by adjustments of ion energy and substrate temperature; the ion flux changed relatively little. The work is extended to a study of the discharge parameters as a function of rf power, using a CdTe target. The ion and electron densities are found to be sensitive to rf power, whereas the plasma potential Vp and the electron temperature are not. A well-known equation for Vp in terms of the positive excursions of the target voltage gives a poor estimate for Vp because of the voltage drop across the rf impedance of the target disc.
Effects of thermal processing on (SimGen)p superlattices
Authors: Pascual R. , Saimoto S. , Baribeau J. M.
Published: April 2011

In this work we report an X-ray diffraction study of thermally induced interdiffusion and strain relaxation in molecular beam epitaxy grown (SimGen)p short-period superlattices. Both rapid and furnace thermal annealings in the range 500–700 °C were used to generate structural changes in the various samples. Strain relaxation was studied by measuring the shift of (400) superlattice peaks on annealing. About half of the strain was relieved in the first few minutes of annealing. The remaining strain was relieved at a much lower rate and residual strain persisted even after several hours of heating. Also, the decay of the first order (000) superlattice peak was monitored as a function of annealing time. An initial rapid nonexponential decrease in peak intensity was observed, coincident with the sudden strain relaxation found at short annealing times. A slower, exponential decay rate was observed at longer times, so that a wavelength-dependent interdiffusion coefficient Dλ could be calculated. The variation of Dλ with the superlattice period and the effect of different substrates and buffer layers was studied. Diffusion was faster in structures alternating thin Si and thick Ge layers suggesting that migration of Si into Ge is the dominant diffusion process.
Midgap electron traps in n-type GaAs epitaxial layers grown by the close-spaced vapor transport technique
Authors: Bretagnon T. , Jean A. , Silvestre P. , Bourassa S. , Mao R. Le Van , Lombos B. A. , Cossement D. , Bel C. Le , Dodelet J. P.
Published: April 2011

The deep-level transient spectroscopy technique was applied to the study of deep electron traps existing in n-type GaAs epitaxial layers that were prepared by the close-spaced vapor transport technique using three kinds of sources (semi-insulator-undoped, Zn-doped and Si-doped GaAs). Two midgap electron traps labelled ELCS1 and EL2 were observed in all layers regardless of the kind of source used. In addition, the effect of the electric field on the emission rate of ELCS1 is discussed and its identification to ETX2 and EL12 is suggested.
Time-resolved photoluminescence study of InGaAs–GaAs single heterostructures
Authors: Leonelli R. , Morris D. , Brebner J. L. , Jiaqi Duan , Roth A. P. , Lacelle C.
Published: April 2011

We report the results of time-resolved photoluminescence studies of the near band-gap emission from relaxed, nonpseudomorphic In0.06Ga0.94As layers grown by metal-organic vapour-phase epitaxy on GaAs substrates oriented exactly on the (001) plane and misoriented by 2° off (001) towards <110>. The layer grown on the misoriented substrate shows a narrow excitonic emission whose lifetime of 1.2 ns is indicative of the high quality of the material. The emission from the layer grown on the oriented substrate consists of four bands. Their time-resolved spectra indicate that they can be attributed to excitons bound to different distributions of levels located either in homogeneous, strain-free regions or in regions near the epilayer misfit dislocations. We also present evidence indicating that hydrogen is more readily incorporated in layers grown on oriented substrates.
Laser-compatible wave-guide electroabsorption modulators
Authors: Moss D. , Chatenoud F. , Charbonneau S. , Delâge A. , Landheer D. , Barber R.
Published: April 2011

We demonstrate efficient wave-guide quantum-well electroabsorption modulators that also show good performance as lasers, in GaAs–AlxGa1−xAs. Two structures, having one and three quantum wells in the wave-guide core, are considered. We show that, although the background-doping impurity level in the intrinsic region is high enough to quench the exciton absorption peaks, efficient electroabsorption can still be achieved. We obtain maximum changes in absorption of between 140 and 250 cm−1 for both samples, depending on the polarization. For the three-well sample we observe a dramatic reappearance of exciton-absorption peaks under an applied field that enhances the contrast ratio. For the single-well sample, however, no excitonic features are observed for any applied field. Finally, the wave-length dependent chirp parameters is calculated at various bias voltages via the Kramers–Kronig relations. Both samples show chirp parameters of less than unity throughout most of the wavelength region of interest.
High-frequency coherent plasma waves observed in the lower auroral ionosphere
Authors: Margot Joëlle , McNamara A. G.
Published: April 2011

Probe measurements of plasma irregularities in the lower E region during an auroral event showed the presence of coherent wave forms with a wavelength extending down to 35 cm that can be explained by the Farley–Buneman instability. Our results can be interpreted in the frame of recent theories of electron heating by unstable waves. Our data provide experimental evidence that coherent short-wavelength waves can be excited as is assumed in these theories. The measurements were made from rocket AAD-VIIIC-09 (ARIES-A) that formed part of the ARIES campaign for auroral modelling.
Radiations from space: Swift charged particles and neutrons
Authors: Potvin L. , Rioux C. , Slobodrian R. J.
Published: April 2011

Galactic and solar cosmic radiations received on Earth are reviewed briefly together with a discussion of proposed new detection techniques. During transient periods associated with flares the Sun emits high intensities of energetic charged particles and radiations due to subatomic processes. Neutrons are also emitted as proven recently. They should provide a nondistorted picture of hadronic processes in the outer layers of the Sun when analyzed with a dedicated neutron spectrometer, which is described in detail.
Propagation of bremsstrahlung X rays through the atmosphere
Authors: Vij K. K. , Venkatesan D.
Published: April 2011

To understand auroral phenomena and associated magnetospheric processes, balloon-borne measurements of bremsstrahlung X rays produced by electrons precipitating in our atmosphere have been made by various scientists. Measurements made by balloon-borne payloads have an advantage over direct measurements of electrons by satellites or rockets as these measurements provide an intense coverage at one location. These observations are usually made between 30–35 km altitude, and to relate them to the parent electron flux at the top of the atmosphere, it is necessary to understand the effect of propagation of X rays through the terrestrial atmosphere from ~100 km to the detection altitude (~30 km).
Detection of very soft X-ray spectra by balloon-borne detectors
Authors: Varga L. , Venkatesan D.
Published: April 2011

A balloon payload launched from Cold Lake, Alta., Canada, on October 14, 1982, measured the spectrum of X rays (E = 20–160 keV) with a hard and a very soft component. The latter contributes significantly only to the 20–40 keV channel; thus a "flapping motion" at the lower energy end of the composite spectrum is observed. We have simulated, using the Monte-Carlo technique, the propagation of X rays through the atmosphere and determined the e-folding energy of the very soft component, which we estimate to be 5 keV, at a particular time (10:45 UT). The electron spectrum producing this soft X-ray source agrees well with in situ satellite and rocket-borne measurements of electron spectra during active auroras. Simulataneous auroral optical emissions (5577 Å (1 Å = 10−10 m)) by a ground-based all-sky charge-coupled device camera are also available for comparison. We also determine the count rate ratio between 20–40 and 60–80 keV channels as a parameter of the variability of the intensity of the very soft component. The time-changes of this ratio and that of the auroral optical emission intensity near the vicinity of the balloon are correlated. The lack of correlation at some other times is ascribed to differences in the behavior of the low- and high-energy ends of the elecron spectrum.
Two-dimensional high-latitude thermospheric modeling: A comparison between moderate and extremely disturbed conditions
Authors: Chang C. A. , St.-Maurice J.-P.
Published: April 2011

A set of two-dimensional nonlinear calculations has been done to simulate the auroral region electrojet and to examine the effect of the electric field on the dynamics and thermodynamics of the thermosphere. A large number of physical and dynamical processes in the ionosphere have been considered, including the ion-drag force, the Coriolis force, gravitation, Joule heating, viscous heating and viscous work, solar extreme ultraviolet heating, thermal conduction, and cooling to space owing to infrared radiation of different species. Navier–Stokes equations for a compressible, viscous and thermal conducting fluid flow with source terms have been solved by a MacCormack explicit, alternative forward-backward finite differencing scheme in spherical coordinates. Results have been recorded at various time intervals for three hours simulation time, for altitudes between 80 and 450 km, and from the north pole to the equator. In addition to a strong zonal drift motion and to the basic upward and meridional motion away from the heated region, we obtain a complex structure of waves involving meridional and vertical winds, as well as the density and temperature fields. This computation suggests that waves play a much more important role than ordinary diffusion of energy and momentum is spreading the effects of the disturbances away from the electrojet region. The net result is that there is, strictly speaking, no steady state reached by the neutrals except for the bulk of the zonal flow. A second major feature that we obtain is that nonlinear terms can often dominate the momentum equation, which can reduce the magnitude of the zonal flow by a considerable amount, and can displace the region of maximum neutral flow away from where the electrojet is. The nonlinear terms are also responsible for the formation of a neutral density 'hole' at nonelectrojet latitudes. This hole is found below the region where Joule heating reaches its peak value and is used to enhance the neutral densities at high altitudes on a global scale.
Inversion techniques for recovering two-dimensional distributions of auroral emission rates from tomographic rocket photometer measurements
Authors: McDade Ian. C. , Llewellyn Edward J.
Published: April 2011

In this paper we demonstrate how the spatial distribution of optical emission rates within an auroral arc may be recovered from rocket photometer measurements made in a tomographic spin scan mode. We describe the tomographic inversion procedures required to recover this information and the implementation of two inversion algorithms that are particularly well suited for dealing with the problem of noise in the observational data. These algorithms are based upon the algebraic reconstruction technique and utilize "least-squares" and "maximum-probability" iterative relaxation methods. The performance of the inversion algorithms and the limitations of the rocket tomography technique are assessed using various sets of simulated rocket measurements that were generated from "known" auroral emission-rate distributions. The simulations are used to investigate how the quality of the tomographic recovery may be influenced by various factors such as noise in the data, rocket penetration of the auroral form, background sources of emission, smearing due to the photometer field of view, and temporal variations in the auroral form.
Rapid motion of the 1989 Arctic ozone crater as viewed with TOMS data
Authors: Bunn F. E. , Thirkettle F. W. , Evans W. F. J.
Published: April 2011

The data from the NIMBUS-7 TOMS instrument were used to study the Arctic ozone layer in late winter and spring, 1989. This paper presents an analysis of TOMS total ozone values, to produce a picture of the morphology of the Arctic stratospheric ozone crater in winter–spring 1989. The Arctic crater formed in late January when the vortex moved off the pole to over Scandinavia. The TOMS data clearly show the Arctic ozone-crater feature over Scandinavia and the western Soviet Union, on February 2, 1989. It later moved south to Baffin Island and then, in March, down over Toronto, and eventually to western Canada, near Edmonton. A similar, unexpected, crater was present in the Antarctic fall, on March 15, 1989. This phenomenon is mainly produced by dynamic uplift, but there may be ozone depletion occurring as well owing to reduced temperatures.
Global ozone trends from a reanalysis of TOMS data
Authors: Evans W. F. J. , Bunn F. E. , Walker A. E.
Published: April 2011

Analyses of 10 years of daily average total ozone data for March 15 and October 10 were carried out. These days were chosen as they correspond, respectively, to the maxima in occurrence of the polar ozone thinning phenomena in the northern and southern polar regions. Several conclusions of significance arise from these analyses. The NIMBUS-7 TOMS data provide one of the best data sets available, with which to monitor global ozone depletion. Utilizing the assumption that equatorial ozone levels remain stable, a self-consistent compensation for instrumental drift indicates a −0.70% annual drift in the TOMS data values, which needs correction. Correcting for the TOMS drift in the 1979–1988 March 15 and October 10 data shows that a global ozone depletion is occurring at the rate of 0.224% per year. These analyses show that the maximum ozone-depletion rate occurs in the polar zone containing the ozone thinning phenomena. The southern zone depletion rate is some three times greater than the northern (1.06% compared with 0.39% per year), and the rate in the southern zone remains significantly above the global depletion rate, even during absence of the ozone thinning phenomenon (0.47% per year). This decrease is alarmingly greater than predicted by scenario models.
X-ray photoelectron spectroscopy studies of Ge epilayers on Si(100)
Authors: Lu Z. H. , Baribeau J.-M. , Jackman T. E.
Published: April 2011

High-energy resolution X-ray photoelectron spectroscopy, with full width at half maximum of 0.41 eV for the Si 2p and of 0.54 eV for the Ge 3d, has been used to study the valence band offsets of different strained Ge layers grown on Si (100). The fractional volume changes in Ge epilayers have been measured by X-ray photoelectron diffraction and are used to correct the valence band maximum shifts caused by strained-induced spin-orbit splitting at the maxima. Band offset values of 0.80, 0.76, and 0.71 eV are found for Si/(Ge5Si5)/Si (100), Si/(Ge4Si4)/Si (100), and Si/(Ge0.5Si0.5)/Si (100) epilayers, respectively.
Variation de la contrainte des verres de silice sous cycle thermique
Authors: Bouchard H. , Azelmad A. , Currie J. F. , Meunier M.
Published: April 2011

Using an in situ technique, stress was measured as a function of annealing temperature to investigate the effect of phosphorous and boron doping of silicon dioxide glass films deposited by low-pressure chemical vapor deposition (LPCVD). It was found that the initial stress of phosphosilicate glass is independent of the amount of phosphorus present, while the boron content influences the initial stress in borophosphosilicate glass. The stress increases to a maximum, σm, corresponding to a temperature Tm, above which the onset of viscous flow reduces the stress to zero at a temperature T0. All these parameters are dependant on dopant concentrations. The observed mechanical behavior is discussed in terms of film viscosity.
Fine structure in Raman spectroscopy and X-ray reflectometry and its uses for characterization of superlattices
Authors: Zhang P. X. , Lockwood D. J. , Baribeau J.-M.
Published: April 2011

Fine structure in the folded longitudinal acoustic (FLA) phonon peaks from high-resolution Raman spectra of Si/Si1−xGex superlattices has been observed. The observed FLA fine structure is attributed to fluctuations in the superlattice periodicity. A 0.2 cm−1 splitting in the FLA peaks is observed in the Raman spectrum of a 15-period superlattice. According to the Rytov theory for the FLA phonons, the peak splitting corresponds to a change in period of about four monolayers (~0.5 nm) during growth. X-ray reflectometry on these samples also shows a similar doublet formation on the main reflection peaks, consistent with a 2–3 monolayer period fluctuation. A test sample with a built-in period variation was also well characterized by both techniques. The good agreement between the two techniques, as well as between the theories and the experiments, demonstrates that both techniques are capable of high precision and allow detailed and quantitative characterization of the superlattice perfection. In addition to determining the phonon velocity, dispersion relation, layer thickness, period, and composition of the superlattices, these results demonstrate the possibility of measuring the fluctuation in the period and the interface quality, as well as the numbers of given periods in some special cases.
Investigation of interdiffusion in (SimGen)p superlattices and Gen buried layers by Raman and X-ray techniques
Authors: Lockwood D. J. , Baribeau J.-M. , Labbé H. J.
Published: April 2011

Two thin pseudomorphic (SimGen)p superlattices with m = n = 4 and p = 5 and vice versa were prepared at 350 °C by molecular beam epitaxy on (100) Si to investigate interdiffusion upon annealing. A Raman scattering and X-ray reflectometry study of the as-grown specimens indicated significant interdiffusion at the Si–Ge interfaces. The Raman investigations of specimens annealed for 20 s at temperatures up to 750 °C showed that atomic diffusion across the Si–Ge interfaces had occurred at an anneal temperature of 600 °C. After a 20 s 700 °C anneal. Raman and X-ray techniques showed that the superlattices modulation was destroyed consistent with a diffusion coefficient of about 10−21 m2/s. Both techniques revealed that 100 s anneals at temperatures of 750–950 °C introduced a slower diffusion rate (10−20 m2/s at 950 °C), but also strain relief, in Gen (n < 12) epilayers buried in Si. The differences in the effective diffusion rates for the two types of heterostructure are attributed to a different degree of interface mixing during growth.
Phase formation in Fe–Si thin-film diffusion couples
Authors: Ivey Douglas G. , Wang Dashan
Published: April 2011

The formation of FeSi2, as well as other iron silicides, from solid-state reactions of Fe thin films on Si substrates has been investigated. Iron layers, approximately 50 nm thick, were deposited by electron beam evaporation on <100> oriented Si substrates. Silicon (≈35 nm) and SiO2 (≈170 nm) layers were deposited on top of the Fe layer in the same evaporator without breaking the chamber vacuum. SiO2 acted as a protective layer during subsequent annealing in a nitrogen ambient. All annealed samples were examined using X-ray diffraction and transmission electron microscopy (TEM). Both plan view and cross section specimens were prepared for TEM. Detailed phase analysis was accomplished through the various electron diffraction and X-ray microanalysis techniques available with the TEM. Silicon dissolved readily in Fe, at temperatures lower than 300 °C, up to the solubility limit of ≈26 at%Si. FeSi formation followed (350 °C), with semiconducting FeSi2 forming at 500 °C. The Fe–amorphous Si interface was more reactive, with silicide formation occurring at lower annealing temperatures (300 °C). There was also evidence that FeSi2 formed directly from α-Fe and amorphous Si.
Strain-induced compositional shift in the growth of InAsyP1−y onto (100) InP by gas-source molecular beam epitaxy
Authors: Qiu C. , Kruzelecky R. V. , Thompson D. A. , Comedi D. , Balcaitis G. , Robinson B. J. , Streater R. W.
Published: April 2011

The growth of InAsyP1−y onto (100) InP by gas-source molecular beam epitaxy was examined systematically, focusing on control of the resulting As/P incorporation ratio. The group V fluxes were obtained by passing phosphine and arsine through a dual-input low-pressure gas cracker. For a given flow ratio of the source gases, the arsenic fraction y of the resulting InAsyP1−y films is seen to increase with the film thickness over the first 1500 Å (1 Å = 10−10 m) as indicated by secondary ion mass spectroscopy, Auger depth profiling, and by Rutherford backscattering spectroscopy. Thin, strained InAsyP1−y layers (0.30 < y < 0.70, corresponding to a compressive strain of about 1.0–2.2%) contain about 5–20% less As than similarly grown thicker, relaxed layers. For a given growth rate and substrate temperature, the relative compositional shift is found to be linearly proportional to the effective strain corresponding to y. Substrate temperatures above 475 °C further reduce the incorporation ratio of As into both strained and relaxed InAsyP1−y layers, initially enhancing the strain-induced compositional shift. However, strain minimization via a compositional shift competes with a greater rate of relaxation of the InAsP lattice with film thickness at higher substrate temperatures.
Epitaxial growth of gallium arsenide prepared by low-pressure metal-organic chemical vapour deposition at low temperatures
Authors: Aktik C. , Beerens J. , Blain S. , Bsiesy A.
Published: April 2011

The low-pressure metal-organic chemical vapour deposition (LPMOCVD) technique has been investigated previously as a growth method for compound semiconductors, offering the possibility of selective epitaxy and the potential advantage of better controllability for changing the doping level and the alloy composition. Low-temperature growth is also desirable to reduce the carbon incorporation generated by the decomposition of the organic radicals. In this article we report for the first time the epitaxial growth of gallium arsenide (GaAs) by LPMOCVD at temperatures as low as 510 °C. The vertical reactor that was developed by the authors employs conventional precursors such as trimethylgallium and arsine. By carefully choosing the growth parameters, we were able to grow high-quality GaAs epilayers with good surface morphology at temperatures as low as 510 °C. The carbon incorporation is shown to decrease with decreasing growth temperature without deterioration of the film quality. By carefully controlling the purity of the sources and the gas flow dynamics, we reduced the deep level impurity concentration and obtained reproducible n-type material with residual net donor concentration of 4.4 × 1014 cm−3 and mobility of 92 000 cm2 V−1 s−1 at 77 K.
Determination of the trap energy levels and the emitter area dependence of noise in polyemitter bipolar junction transistors from generation–recombination noise spectra
Authors: Ng A. , Deem M. J. , Ilowski John
Published: April 2011

A noise model for bipolar junction transistors based on generation–recombination (GR) noise originating from the traps located in the space-charge region of the emitter–base junction is derived. The model indicates that the current noise power owing to traps located inside the base–emitter depletion region should be proportional to the square of the collector current and inversely proportional to the emitter area. In addition, the model shows how the activation energy of a trap and the quasi-Fermi level can be related to the rising and falling edges of a GR noise spectrum by plotting the noise power against the reciprocal absolute temperature. Predictions from the noise model were compared with experimental data using seven bipolar transistors with emitter areas varying from 1.6 to 144 μm2. The noise measurements were performed at 10 temperatures between 10 and 100 °C and at different biasing currents. From the measurements, the base current noise power spectra [Formula: see text] is found to be proportional to the square of the base current (ib) and inversely proportional to the square of the emitter area. By plotting the noise power against the reciprocal temperature on a log–log graph and performing a linear least square fit on part of the data, a trap energy level of 821 meV above the valence band was determined.
On transconductance in the p-channel Si/SiGe heteroj unction field effect transistor
Authors: Kovacic S. J. , Ojha J. J. , Swoger J. H. , Simmons J. G. , Noël J-P. , Houghton D. C.
Published: April 2011

Three factors affecting the transconductance characteristics of inversion-channel heterojunction field effect transistors are discussed. In each case, the effect on the hole population in the channel and, thereby, on the transconductance of the device is hypothesized. Experimental evidence which corroborates the effect of the various phenomena is also presented. These factors are (i) saturation of the hole population in the channel arising from dopant de-ionization in the charge sheet, (ii) parallel gate to source and drain current conduction arising from the physical structure of the device, and (iii) the effect of gate leakage on the non-equilibrium surface potential of the n–n heterojunction.
Ultrafast electron tunnelling in a reverse-biased, high-efficiency quantum well laser structure
Authors: Halliday D. P. , Moss D. , Charbonneau S. , Aers G. C. , Landheer D. , Chatenoud F. , Conn D.
Published: April 2011

We have performed a detailed series of photoconductivity (PC) and photoluminescence (PL) measurements on a reverse-biased GaAs/AlGaAs single quantum well graded index separate confinement heterostructure laser. The PC was performed, as a function of bias, at room temperature on a high-speed ridge waveguide structure. The PL was performed at low temperatures (20, 70, and, 150 K) on a ring mesa sample as a function of bias. The measurements show that this device behaves as an extremely efficient high-speed photodetector with an internal quantum efficiency of 100% and a FWHM response time of 35 ps. The data is fitted using a simple model based on electron recombination in the quantum well or escape out of the well. The escape occurs by one of three possible routes: direct tunnelling out of the lower electron level, thermally assisted tunnelling through the upper electron level, or thermionic emission over the barrier. Each of these three terms is calculated theoretically. A comparison of theory and experiment leads us to the conclusion that the theories explaining thermal emission of carriers from a quantum well underestimate the lifetimes.
Hysteresis in resonant tunneling diode based multiple-peak driver device for multivalued SRAM cells: analysis, simulation, and experimental results
Authors: Raychaudhuri A. , Yan Z. X. , Deen M. J. , Seabaugh A. C.
Published: April 2011

This paper describes results of our investigation on the hysteresis phenomenon observed in multiple-peak resonant tunneling devices based on a series combination of resonant tunneling diodes (RTDs). We have modelled this hysteresis by assuming that when one of the diodes in the series combination is switching from its prenegative differential resistance (NDR) region to its post-NDR region or back, the others are acting as a combined load to it. Our analytical model based on a load-line analysis with a piecewise-linear approximation for the RTD I–V curve brings out the salient features of hysteresis as measured on a three-peak device based on discrete RTDs. Also, the model provides a first-order quantitative picture that is particularly useful in the context of difficulties in reliable dc circuit simulation on these devices. We have further shown results of our transient circuit simulation on a four-state memory cell that uses the tree-peak device as its driver and constant current metal semiconductor field effect transistor load device. The simulation results clearly bring out the importance of considering hysteresis in the driver device and its implications on important cell parameters such as logic levels, noise margins, and power dissipation. Finally, we verify the simulation scheme with experimental results from an actual memory cell constructed with discrete RTDs and a constant-current load device.
Excess noise, gain, and dark current in Ge avalanche photodiodes
Authors: Scansen D. , Kasap S. O.
Published: April 2011

Measurements have been carried out on a p+ n type Ge avalanche photodiode (typical guard ring structure with channel stoppers) to obtain the excess noise factor, F; multiplication, M; and the dark current, Id over a wide range of temperatures (143–323 K). The multiplication initially increases with the illumination wavelength and peaks in the 1550 nm region because of more holes being injected into the avalanche region as the absorption depth becomes much longer than the width of the p+ region. The excess noise factor, F, at À = 1550 nm was typically slightly less than multiplication, M, and showed very little dependence on temperature. The effective ratio of electron to hole ionization coefficients was approximately 0.7–0.8, the dark current activation energy close to room temperature was about Eg, indicating that the dark current is controlled by the diffusion of minority carriers. Below ~ 232 K, dark current activation energy is about Eg/2, indicative of carrier generation in the depletion region. M varied approximately as T−n, with n depending strongly on the reverse bias. In the low-frequency range below ~ 200 Hz, the noise current followed a 1/f behavior.