bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation between the structural and optical properties of polydispersed II–VI quantum dots in glass matrix
Authors: Cingolani R. , Moro C. , Manno D. , Striccoli M. , DeBlasi C. , Righini G. C. , Ferrara M.
Published: July 2002

We report the results of a systematic investigation of the structural and optical properties of polydispersed semiconductor microcrystals in glass. Transmission electron microscope experiments reveal the existence of microcrystals of different radii and size dispersion around the average radius. A simple method is proposed to extract the structural parameters by the Gaussian deconvolution of the size histogram. The presence of a polydispersed dot distribution accounts very well for the observed absorption spectra which exhibit different excitonic features corresponding to the estimated microcrystal sizes. The linewidth of the low temperature photoluminescence spectra is satisfactorily explained by a simple emission-broadening model based on the effective mass approximation which takes into account the dispersion of the quantum dot radius obtained by the transmission electron microscope experiments.
Structure and crystallization of low-pressure chemical vapor deposited silicon films using Si2H6 gas
Authors: Hong C. H. , Park C. Y. , Kim H.-J.
Published: July 2002

Microstructures of silicon films deposited on SiO2 substrates by low-pressure chemical vapor deposition using Si2H6 gas were investigated and compared to those using conventional SiH4 gas by transmission electron microscopy and x-ray diffraction. The deposition rate of the Si2H6 process was about ten times higher than that of SiH4 process at low temperatures (<550 °C). The transition deposition temperature from amorphous to polycrystalline film was found to be around 580 °C, which was similar to that of the SiH4 process. The film deposited at 600 °C was partially crystalline and had equi-axed grains with the largest average grain size of 0.3 μm while the films using SiH4 has needle-like columnar grains with smaller sizes (200 Å). The x-ray diffraction analysis showed that the structural disorder to amorphously deposited Si films increases as deposition temperature decreases. The grain size in the film after crystallization at 600 °C strongly depended on the deposition temperature and the deposition rate, producing a larger grain size at a lower deposition temperature and/or at a higher deposition rate (Si2H6 deposition compared to SiH4 deposition). The apparent increase in grain size can be explained as a result of the lowered number of crystal nuclei due to a decrease in the number of pre-existing microcrystallites serving as heterogeneous nucleation seeds. When the deposition rate was lower than the critical value (approximately 2–4 nm/min), the grain size in the crystallized film decreased for both SiH4 and Si2H6 films. The maximum grain sizes were 4.5 and 0.3 μm at the deposition temperatures of 485 and 550 °C for the films using Si2H6 and SiH4 gases, respectively.
Heat flow in substrates induced by a scanning laser beam
Authors: Lu Yong-Feng
Published: July 2002

A general model is derived for computing the heat flow induced by a scanning continuous-wave laser beam with a Gaussian intensity distribution in a semi-infinite substrate. Temperature-dependent thermal conductivity, thermal diffusivity, and surface reflectivity are incorporated in the model. The model is then applied to different substrate materials such as silicon, GaAs, and Mn–Zn ferrite. The numerical results show that the heat flow intensity in the substrate depends on the incident laser power, substrate temperature, scan speed, and beam radius. This study is expected to be useful in investigating the distributions of the laser-induced thermal stresses and lattice damages.
Raman scattering of coupled longitudinal optical phonon-plasmon modes in dry etched n+-GaAs
Authors: Wang P. D. , Foad M. A. , Sotomayor-Torres C. M. , Thoms S. , Watt M. , Cheung R. , Wilkinson C. D. W. , Beaumont S. P.
Published: July 2002

We have carried out extensive Raman scattering investigations of the damage caused by the dry etching in GaAs. The heavily doped n+-GaAs (2–3×1018 cm−3) allows the study of the coupled longitudinal optical (LO) phonon-plasmon mode as a probe to assess the dry etch-induced damage. Three etching techniques were used including conventional radio frequency (rf) reactive ion etching (RIE), ion beam etching (IBE), and electron cyclotron resonance radio frequency reactive ion etching (ECR-RIE). It is demonstrated that the etched damage is confined to a few tens of nanometers after 20 nm of material is etched away. ECR-RIE etching produces the smallest damage. It is found that in RIE etching, as etching proceeds, the depletion depth saturates while for purely physical etching (IBE) the depletion depth increases continuously, at least under the conditions used.
The magneto-optical Kerr effect of bilayers Fe/Au-Cu and Fe/Ag-Au
Authors: Zhai H. R. , Zhou S. M. , Lu M. , Miao Y. Z. , Tian P. B. , Wang H. , Xu Y. B. , Huang H. B.
Published: July 2002

Bilayers Fe/Au-Cu and Fe/Ag-Au made by vacuum evaporation were studied. The magneto-optical polar Kerr rotation and reflectivity spectra were measured in the region from 400 to 700 nm. In the magneto-optical Kerr rotation spectra of bilayers Fe/Au-Cu and Fe/Ag-Au with the thickness of Fe layer less than the light penetration depth, there is a broad enhancement of the magneto-optical polar Kerr rotation with a Kerr peak near the plasma edge in the reflectivity spectra of Au-Cu and Ag-Au alloys. The peak shifts toward the long wavelength side with increasing Cu and Au content in Au-Cu and Ag-Au alloys, respectively. Our results show that the composition of the reflector in our bilayer structures can shift the wavelength range of the magneto-optical Kerr rotation enhancement. Interpretation and discussions are given.
Enhancement of nucleation and adhesion of diamond films on copper, stainless steel, and silicon substrates
Authors: Narayan J. , Godbole V. P. , Matera G. , Singh R. K.
Published: July 2002

We report here enhancement of nucleation and adhesion of diamond films on nondiamond substrates such as copper, stainless steel, and silicon substrates. The enhancement of nucleation is accomplished by pulsed laser irradiation which converts some of the amorphous carbon on the surface into the diamond phase or forms a reaction product that facilitates nucleation of diamond phase. The laser can also be used to evaporate carbon preferentially, leaving behind diamond particles unaffected. By pulsed laser irradiation it is possible to melt the substrate and embed the diamond particles into it, thus improving the adhesion of the diamond film.
A new approach for laser particle acceleration in vacuum
Authors: Steinhauer Loren C. , Kimura Wayne D.
Published: July 2002

A new approach to laser particle acceleration in vacuum is proposed that incorporates three ingredients: (i) a radially polarized laser beam; (ii) an aligned axicon focus; and (iii) the approximate matching of particle and laser phase velocities. This combination has several important advantages which are discussed. The critical issue in any vacuum acceleration scheme is the inherent backward slippage of the particle relative to the laser beam phase, causing the particles to experience successive accelerations and decelerations. According to the Lawson–Woodward theorem, which applies for fairly general conditions, the net result is no energy gain. It is shown that this theorem does not apply to the present laser particle acceleration scheme, and this is verified by applying an exact solution for the electric field. Indeed, acceleration gradients approaching 1 GeV/m over an interaction length of a few centimeters are predicted for 1013 W laser peak power at 10.6 μm.
A generalized model of heat effects in surface reactions. II. Application to plasma etching reactions
Authors: Martz Joseph C. , Hess Dennis W. , Petersen Eugene E.
Published: July 2002

Part I of this paper presented a generalized model of heat effects operative during surface reactions. The enhancement in reaction rate due to autothermic effects was analyzed as a function of activation energy, bulk temperature, and a parameter termed the characteristic temperature. Application of this model to experimental plasma etching data is presented in part II of this work. Characteristic temperatures calculated from experimental data in numerous plasma etching systems agree closely with the critical characteristic temperature predicted by the heat of the reaction model. Possible reasons for this consistency are given. Further, the autothermic enhancement in the Ta-CF4/O2 etching system is accurately predicted as a function of reactant concentration by a heat of reaction model.
Metastable chlorine ion transport in a diverging field electron cyclotron resonance plasma
Authors: Nakano Toshiki , Sadeghi Nader , Trevor Dennis J. , Gottscho Richard A. , Boswell R. W.
Published: July 2002

For applications in ultralarge scale integration, low pressure, high density plasmas are being developed for etching and deposition of thin films. To control critical parameters such as the flux and energy distribution of ions impacting surfaces, it is necessary to understand how these parameters are influenced by physical and electromagnetic design. In this work, we extend previous measurements of ion velocity distributions in Ar/He electron cyclotron resonance plasmas to Cl2/He plasmas. Using Doppler-shifted laser-induced fluorescence spectroscopy, we measure metastable Cl ion velocity distributions parallel and perpendicular to the magnetic field as a function of magnetic field amplitude, pressure, and microwave power. We also examine the effects of the wafer platen on the distribution functions by repeating the measurements after removing the platen. Surprisingly, little qualitative difference is seen when chlorine and argon discharges are compared; this is most likely a result of the low pressures employed (≲0.15 Pa). As in Ar, we find nearly isotropic ion velocity distributions when the source is operated as a magnetic mirror. Downstream, we consistently observe bimodal ion velocity distributions: the fast component, created in the source, appears to follow magnetic flux lines into the reactor; the slow component, created mostly where the plasma expands from the source into the reaction chamber, is more isotropic. Despite the localized input of energy by cyclotron resonance heating, the spread in ion velocities is largely determined by distributed ionization and spatial variations in the plasma potential.
Static field electrical conductivity of dilute fiber suspensions
Authors: Sturman Philip C. , McCullough Roy L.
Published: July 2002

The static field electrical conductivities of several discontinuous fiber composites are reported. Novel fabrication techniques are employed to produce systems of highly aligned and well-dispersed carbon fiber and coated carbon fibers. The thermal and breakdown properties of the matrix resin and breakdown behavior of the composites are examined. The distribution of orientations of the included phase is determined and recorded as an average orientation descriptor. Degree of fiber aggregation is found to be the principal determinant of bulk conductivity, eclipsing the effects of orientation and fiber fraction for the dilute concentrations investigated. A conductivity scale dependence is found for materials with tightly constrained microstructures.
Influence of magnetic anisotropy on the superferromagnetic ordering in nanocomposites
Authors: Mo/rup Steen , Christiansen Gunnar
Published: July 2002

Magnetic interaction between ultrafine particles may result in superferromagnetism, i.e., ordering of the magnetic moments of particles which would be superparamagnetic if they were noninteracting. In this article we discuss the influence of the magnetic anisotropy on the temperature dependence of the order parameter.
On magnetic properties and domain structure of strontium ferrite particles
Authors: Saha Bijay , Yetter Wilson
Published: July 2002

SrFe12O19 is used commercially as carrier. Magnetic properties, crystal structure, particle morphology, grain structure, and magnetic domain structure were studied as a function of processing temperature (T). The particle properties were found to depend critically upon T. Excessively low T (e.g., 1000 °C) yields an incomplete reaction, with poor magnetics and impurity phases. Excessively high T (e.g., 1350 °C) alters the structure and is detrimental to the magnetic properties. Within the acceptable temperature range, low T produces fine grain and submicron magnetic domains, whereas high T produces coarse grain and a micron-size domain structure.
Ab initio analysis of the electron-phonon interaction in silicon
Authors: Yoder P. D. , Natoli V. D. , Martin Richard M.
Published: July 2002

A Harris functional approach is used to investigate the electron-phonon interaction in silicon, within the rigid ion approximation. The necessary lattice dynamics are solved via the valence shell model. The electron-phonon matrix elements for transitions between selected electronic states are calculated, and equivalent deformation potentials are presented and compared with results of other models. The resulting deformation potentials exhibit significant dispersion throughout much of the Brillouin zone, though remain nearly constant for intervalley transitions between states close to the conduction band minima. The overall value of the deformation potentials is somewhat higher than found in previous models and thus in better agreement with experiment.
Self-interstitial mechanism for Zn diffusion-induced disordering of GaAs/AlxGa1−xAs (x=0.1−1) multiple-quantum-well structures
Authors: Ky Nguyen Hong , Ganière J. D. , Gailhanou M. , Blanchard B. , Pavesi L. , Burri G. , Araújo D. , Reinhart F. K.
Published: July 2002

GaAs/AlxGa1−xAs multiple-quantum-well (MQW) structures with identical well thicknesses but with different Al contents x in the barrier (x≊0.1, 0.2, 0.45, and 1) were grown by molecular-beam epitaxy to study the impurity-induced disordering mechanism. The disordering of the structures is observed directly by transmission electron microscopy on cleaved wedges of the sample, by the secondary electron imaging mode of scanning electron microscopy, and by secondary-ion-mass spectroscopy after Zn diffusions at 575 °C during different times (1, 4, 9, and 16 h). The results show that the totally and partially disordered regions are always behind the Zn diffusion front. The partially disordered extent depends on x. As x increases, the disordering rate increases due to the increase in Zn diffusivity. The effect of high Zn concentration is investigated by photoluminescence and by Raman scattering measurements. The systematical analysis of the photoluminescence spectra of the MQW structures diffused for different times and of the photoluminescence spectra taken on different depths below the sample surface makes it possible to describe the physical processes occurring during Zn diffusion. The column-III vacancies are created at the sample surface. They diffuse into the bulk of the sample where they are filled by other defects. Using the x-ray-diffraction technique, an expansion of the lattice constant in the region behind the Zn diffusion front was observed. This is due to a supersaturation of column-III interstitials. During the incorporation of Zn into the crystal lattice, column-III interstitials are generated. These interstitials could be responsible for the enhancement of the Al-Ga interdiffusion. The important role of the electric field at the p-n junction formed by Zn diffusion is discussed. The negatively charged column-III vacancies and the positively charged column-III interstitials are confined, respectively, on the n and p sides of the p-n junction. The results give evidence for the self-interstitial mechanism of Zn diffusion-induced disordering in GaAs/AlGaAs MQW structures.
Passivation of carbon-doped GaAs layers by hydrogen introduced by annealing and growth ambients
Authors: Kozuch D. M. , Stavola Michael , Pearton S. J. , Abernathy C. R. , Hobson W. S.
Published: July 2002

Carbon acceptors in GaAs epitaxial layers grown from metalorganic sources are often partially passivated by hydrogen following growth. Here we examine heavily C-doped GaAs epilayers grown by metalorganic molecular beam epitaxy and metalorganic vapor phase epitaxy by infrared absorption, secondary ion mass spectrometry, and Hall measurements. The concentration of passivated C has been determined by calibrating the intensity of infrared absorption due to C-H complexes. We have investigated the sources of H in the layers and have found that H2 in the growth and annealing ambients is especially effective in passivating C. A brief anneal in an inert ambient at temperatures above 550 °C is sufficient to activate C acceptors that are passivated by H.
Frequency doubling in ion-implanted KTiOPO4 planar waveguides with 25% conversion efficiency
Authors: Zhang L. , Chandler P. J. , Townsend P. D. , Alwahabi Z. T. , Pityana S. L. , McCaffery A. J.
Published: July 2002

Second-harmonic generation (SHG) has been successfully performed in ion-implanted planar waveguides in KTiOPO4. The waveguides were formed by helium implantation, and SHG was achieved using type 2 zero order mode phase matching at a wavelength of ∼1.07 μm. The results indicate that the high nonlinearity of the material remains in the guiding region after ion implantation. The conversion efficiency in a typical guide is estimated at ∼25% for ∼1 μJ pulsed excitation. At a lower input power level, the harmonic green output from the waveguide is 20 times higher than that from the bulk for the same amount of fundamental power. This clearly demonstrates the advantage of using waveguides in achieving frequency doubling for integrated optical devices.
A comparison of reducing the dislocation densities in GaAs grown by the vertical gradient freeze and liquid-encapsulated Czochralski techniques
Authors: Jordan A. S. , Monberg E. M. , Clemans J. E.
Published: July 2002

We have recently applied the quasi-steady state heat transfer/thermal stress model for dislocation generation to the vertical gradient freeze (VGF) process for GaAs, permitting a direct comparison with the original treatment of liquid-encapsulated Czochralski (LEC) growth. Very recent high temperature critical resolved shear stress (CRSS) data on undoped VGF and In-doped LEC specimens were used. We show that the ∼threefold increase in CRSS with In is sufficient to inhibit defect formation in the central ∼75% of 3 in. diameter LEC wafers grown in a high ambient temperature gradient, duplicating the etch-pit density (EPD) data. Undoped VGF wafers are predicted to be nearly dislocation-free. The theoretical results on 3 in. material track the low EPD counts in both the 〈100〉 and 〈110〉 directions in a 5 K/cm gradient imposed on the crystals’ surface. We also discuss the origin of dislocations in regions free of thermal stresses and propose their suppression by the addition of a small amount of In in VGF experiments.
Correlation between stress voiding of Al(Si)(Cu) metallizations and crystal orientation of aluminum grains
Authors: Kordić S. , Wolters R. A. M. , Troost K. Z.
Published: July 2002

The stress voiding behavior of Al(Si)(Cu) alloys has been investigated. The grain structure of the metal lines has been analyzed using the electron backscatter diffraction technique in a scanning electron microscope. A [111] fiber texture of aluminum has been observed perpendicular to the wafer surface. A correlation has been found between a high density of voids and the amount of misalignment of the [111] grain directions with respect to the wafer normal.
Electrical and optical properties of thermally evaporated LiBO2 films
Authors: Benkhelifa F. , Ashrit P. V. , Bader G. , Girouard Fernand E. , Truong Vo-Van
Published: July 2002

Optical and electrical properties of thermally evaporated lithium borate (LiBO2) films have been studied. This work is especially aimed towards the application of such films as lithium ion conductors (IC) in electrochromic (EC) devices. LiBO2 films exhibit a high degree of transmission (over 90%) in the solar spectral range and a lithium ion conductivity of the order of nearly 10−7 S cm−1. Hence, such films have a good potential for use as ICs in EC devices.
The barrier to misfit dislocation glide in continuous, strained, epitaxial layers on patterned substrates
Authors: Watson G. Patrick , Ast Dieter G. , Anderson Timothy J. , Pathangey Balu
Published: July 2002

In a previous report [G. P. Watson, D. G. Ast, T. J. Anderson, and Y. Hayakawa, Appl. Phys. Lett. 58, 2517 (1991)] we demonstrated that the motion of misfit dislocations in InGaAs, grown by organometallic vapor phase epitaxy on patterned GaAs substrates, can be impeded even if the strained epitaxial layer is continuous. Trenches etched into GaAs before growth are known to act as a barrier to misfit dislocation propagation [E. A. Fitzgerald, G. P. Watson, R. E. Proano, D. G. Ast, P. D. Kirchner, G. D. Pettit, and J. M. Woodall, J. Appl. Phys. 65, 2220 (1989)] when those trenches create discontinuities in the epitaxial layers; but even shallow trenches, with continuous strained layers following the surface features, can act as barriers. By considering the strain energy required to change the length of the dislocation glide segments that stretch from the interface to the free surface, a simple model is developed that explains the major features of the unique blocking action observed at the trench edges. The trench wall angle is found to be an important parameter in determining whether or not a trench will block dislocation glide. The predicted blocking angles are consistent with observations made on continuous 300 and 600 nm thick In0.04Ga0.96As films on patterned GaAs. Based on the model, a structure is proposed that may be used as a filter to yield misfit dislocations with identical Burgers vectors or dislocations which slip in only one glide plane.
Wavelength conversion of quadrupled Nd:YAG laser radiation to the vacuum ultraviolet by anti-Stokes stimulated Raman scattering
Authors: Moriwaki Hiroki , Wada Satoshi , Tashiro Hideo , Toyoda Koichi , Kasai Akinari , Nakamura Akira
Published: July 2002

Up-converted beams of a conventional quadrupled Nd:YAG laser by anti-Stokes Raman scattering in hydrogen are characterized as a vacuum ultraviolet light source. The beams diverge with the increase of the hydrogen pressure and also with the increase of the order of anti-Stokes scattering. Although the profiles of anti-Stokes beams vary from a Gaussian-like shape to a typical ring shape, the maximum energy conversion is always obtained with a near-Gaussian beam profile. The output energy ranges from 5.9 mJ for the first-order anti-Stokes wave (240 nm) to 8 μJ for the ninth-order anti-Stokes wave (133 nm). The energy of the sixth-order anti-Stokes wave (160 nm) fluctuates ±45% of the average output energy. The beam characteristics are described well by a model on the basis of phase matching between four waves related to each Raman process.
Surface roughness and the scattering of glancing-angle x rays: Application to x-ray lenses
Authors: Kimball J. C. , Bittel D.
Published: July 2002

Hollow glass fibers can guide x rays because glancing-angle collisions with a smooth glass surface are highly reflective. Surface roughness decreases this reflectivity. We have developed relatively simple expressions for the effects of surface roughness on x-ray scattering, and we relate our results to the theoretical efficiency of x-ray lenses formed from bundles of hollow glass fibers.
Heavy ion beam pumped amplified spontaneous emission on the 172 nm xenon excimer transition
Authors: Busch B. , Ulrich A. , Krötz W. , Ribitzki G. , Wieser J. , Winkler M.
Published: July 2002

Amplified spontaneous emission on the second xenon excimer continuum at 172 nm wavelength was observed in dense gas evaporated from a solid xenon surface and pumped by heavy ions. A pulsed 3.78 MeV Kr+ beam from a radio-frequency quadrupole accelerator was used for the experiment. A series of 800 μJ particle pulses each 10 ns long was focused to a small, elliptical beam spot leading to a pump power density of the order of 1 GW/g. Optical gain in the target was deduced from the angular distribution of light emitted along the target axis, and from a variation of optical intensity with gain length. A maximum gain of 26%/cm was determined using a model calculation which reproduced the experimental data.
The degradation behavior of LaOBr:Tb under cathode-ray excitation
Authors: Ronda C. R. , Bechtel H. , Kynast U. , Welker T.
Published: July 2002

Investigations on the degradation behavior of LaOBr:Tb phosphors under cathode-ray excitation are presented. The microscopic nature of defects created under prolonged electron bombardment has been identified using extended optical analyses on aged LaOBr:Tb phosphor screens and supported by ion desorption experiments performed in an ultrahigh vacuum system. The degradation behavior of LaOBr:Tb can be improved considerably by codoping this phosphor with chloride or iodide.
An investigation of the electrical and chemical properties of intimate metal-InyAl1−yAs(100) interfaces
Authors: Clark S. A. , Wilks S. P. , Morris J. I. , Woolf D. A. , Williams R. H.
Published: July 2002

The electrical and chemical properties of the interfaces formed at room temprature, between the surface of epitaxial n-type InyAl1−yAs(100) and a selection of metals have been studied. Highly ideal Au, Ag, Cu, and In diodes exhibiting the highest reported barriers (0.78–0.91 eV), measured by the current-voltage (I-V) technique, have been obtained by forming intimate contacts on atomically clean, lattice matched, molecular beam epitaxy grown InyAl1−yAs/InP(100). The formation of Au- and In-InyAl1−yAs interfaces has been investigated using x-ray photoemission spectroscopy, showing that in both cases the Fermi level is pinned at the surface prior to metal deposition. The deposition of both In and Au overlayers initiated the selective removal of As from the interface to segregate on the metal surface; however the presence of these metals on the semiconductor surface produced no further Fermi shift. These observations, in conjunction with the barrier heights measured by the I-V technique, are discussed in the context of currently supported models of Schottky barrier formation.
Transient electroluminescence behavior and mechanism of a Schottky-type porous silicon diode
Authors: Wang Jian , Zhang Fu-Long , Wang Wen-Cheng , Zheng Jia-Biao , Hou Xiao-Yuan , Wang Xun
Published: July 2002

The transient electroluminescence behavior of a Au/porous silicon/silicon/aluminum structure is investigated by a doubly injected short voltage pulse method. It is found that the electroluminescence response of porous silicon diode is very similar to that of an ordinary p-n junction light-emitting diode. A delay time and a rise time at the ‘‘on’’ side and a fall time at the ‘‘off’’ side are observed. The mechanism of electroluminescence is explained as the carrier injection through the Au/porous silicon Schottky barrier and the porous silicon/p-Si heterojunction into the corrugated Si nanostructures, where the radiative recombination of carriers occurs. From the fall edge of electroluminescence transient response, the recombination lifetime of the injected carriers is found to be in the order of 30 ns. The major time constant of the diode structure is the delay time which ranges from 10−7 to 10−6 s and is determined by the quality of the Au/porous silicon contact.
Barrier potential design criteria in multiple-quantum-well-based solar-cell structures
Authors: Mohaidat Jihad M. , Shum Kai , Wang W. B. , Alfano R. R.
Published: July 2002

The barrier potential design criteria in multiple-quantum-well (MQW) -based solar-cell structures is reported for the purpose of achieving maximum efficiency. The time-dependent short-circuit current density at the collector side of various MQW solar-cell structures under resonant condition was numerically calculated using the time-dependent Schrödinger equation. The energy efficiency of solar cells based on InAs/GayIn1−yAs and GaAs/AlxGa1−xAs MQW structures were compared when carriers are excited at a particular solar-energy band. Using InAs/GayIn1−yAs MQW structures it is found that a maximum energy efficiency can be achieved if the structure is designed with barrier potential of about 450 meV. The efficiency is found to decline linearly as the barrier potential increases for GaAs/AlxGa1−xAs MQW-structure-based solar cells.
Carbon doping and growth rate reduction by CCl4 during metalorganic chemical-vapor deposition of GaAs
Authors: Lee Jeong-Seok , Kim In , Choe Byung-Doo , Jeong Weon Guk
Published: July 2002

The electrical, structural, and optical properties of GaAs grown by metalorganic chemical-vapor deposition using CCl4 have been studied and the growth rate reduction by CCl4 under various growth conditions has been investigated. Hole concentrations ranging from 2×1016 to 1.8×1020 cm−3 have been obtained by varying V/III ratio and growth temperature. From Hall, x-ray, and low-temperature photoluminescence measurements, a low compensation is ensured. A growth rate reduction up to 50% has been observed. The dependence of the growth rate reduction on the growth temperature, the V/III ratio, and the CCl4 mole fraction was investigated. It is believed that the growth rate reduction is caused not by etching of solid GaAs but by reduction of Ga species through the formation of GaCl in gas phase.
Inhomogeneous pulsed laser melting of high-dose Ge-implanted silicon
Authors: Calcagnile L. , Grimaldi M. G. , Baeri P.
Published: July 2002

Pulsed laser melting of Si substrates implanted either with 74Ge+ ions at a nominal dose of 1017 cm−2 or 28Si+ ions at a dose of 1016 cm−2 has been investigated by time-resolved reflectivity using visible and infrared probe laser beams. In Si-implanted samples one reflectivity peak is observed during irradiation with 25 ns ruby laser pulses due to the formation and solidification of a liquid layer nucleated at the sample surface. Instead, Ge-implanted Si samples show different reflectivity spectra during irradiation in the same experimental conditions. Two reflectivity peaks have been observed in Ge-implanted samples for laser energy densities below 0.5 J/cm2 whose origin cannot be explained with the same melting dynamics as in pure Si samples. The results are explained assuming that during laser irradiation an inhomogeneous melting process occurs in Ge-implanted samples. On the basis of time-resolved reflectivity, electron microscopy, and Rutherford backscattering measurements, a melting model is suggested for the melting process of Ge-implanted samples. According to this model the liquid phase is nucleated just below the sample surface because of the reduced melting temperature caused by the Gaussian Ge implantation profile. The two reflectivity peaks originate from a time modulation of the liquid-solid ratio at the sample surface.
Choice of a range-energy relationship for the analysis of electron-beam-induced-current line scans
Authors: Luke Keung L.
Published: July 2002

The electron range in a material is an important parameter in the analysis of electron-beam-induced-current (EBIC) line scans. Both the Kanaya–Okayama (KO) and Everhart–Hoff (EH) range-energy relationships have been widely used by investigators for this purpose. Although the KO range is significantly larger than the EH range, no study has been done to examine the effect of choosing one range over the other on the values of the surface recombination velocity sT and minority-carrier diffusion length L evaluated from EBIC line scans. Such a study has been carried out, focusing on two major questions: (1) When the KO range is used in different reported methods to evaluate either or both sT and L from EBIC line scans, how different are their values thus determined in comparison to those using the EH range?; (2) from EBIC line scans of a given material, is there a way to discriminate between the KO and the EH ranges which should be used to analyze these scans? Answers to these questions are presented to assist investigators in extracting more reliable values of either or both sT and L and in finding the right range to use in the analysis of these line scans.
Optical dispersion of AlxGa1−xAs at crystal growth temperature
Authors: Shin Jae-Heon , Lee Yong-Hee
Published: July 2002

We propose a scheme to calculate the optical dispersion of III-V compounds at crystal growth temperature, based on the Afromowitz’ formula and the shift of the lowest direct band gap energy. The thermal behaviors of the major parameters are extracted from their respective physical meanings. This model is applied to AlxGa1−xAs materials and the calculated results compare quite well with the experimental data. The validity of our model is also tested by comparing the static dielectric constant of GaAs with that of known empirical formula in the temperature range from 200 to 600 K.
Coercivity of Ti-modified (α-Fe)-Nd2Fe14B nanocrystalline alloys
Authors: Yao J. M. , Chin T. S. , Chen S. K.
Published: July 2002

Coercivity of Ti-modified nanocrystalline (α-Fe)-Nd2Fe14B alloys with soft iron particles embedded in hard Nd2Fe14B matrix was studied. The alloys were prepared by melt spinning and annealing. The grain size of Nd2Fe14B is from 20 to 60 nm, while that of the soft iron particles is 11 to 30 nm. The coercivity of the annealed flakes decreased dramatically with increasing particle size of free iron, following the same tendency of a theoretical calculation by Schrefl et al. For the Ti-alloyed flakes, the 1 at. % Ti-containing ones show the highest coercivity of 11.1 kOe. Its (BH)m value is about 16 MGOe despite lower Nd and B contents than stoichiometric Nd2Fe14B. More Ti addition would deteriorate the coercivity.
Magnetic phase diagrams of YCo4B-based components
Authors: Zhao Z. G. , de Boer R. , Buschow K. H. J. , Ge Y. P. , Wang J. Y.
Published: July 2002

The magnetic properties of (Y,Nd)Co4B, Y(Co,Fe)4B, and YCo4(B,C) compounds have been investigated. The spin-reorientation temperature of YCo4B increases with substitution of Nd for Y and Fe for Co, but decreases with the replacement of B for C. The Curie temperature of YCo4B decreases upon substitution of Nd for Y and C for B, whereas it increases upon the introduction of Fe. The magnetic phase diagrams are given for (Y,Nd)Co4B, Y(Co,Fe)4B, and YCo4(B,C). The results show that the temperature range of the easy-axis anisotropy in YCo4B is increased by C substitution, and decreased by Nd and Fe substitution. The saturation magnetization of YCo4B increases by the introduction of Nd and Fe. Substitution of C for B changes the magnetic moment of Co only slightly. The magnetic anisotropy is increased by substitution of C for B.
The electrochemical hydrogenation of NdFeB sintered alloys
Authors: Chang Kuo En , Warren Garry W.
Published: July 2002

The absorption of hydrogen by NdFeB alloys in aqueous solution has been investigated using dc electrochemical methods and x-ray diffraction. Immersion of the NdFeB alloy in 0.1 M H2SO4 results in the formation of the Nd2Fe14BH2.7 hydride phase as indicated by x-ray diffraction analysis. The hydrogen entering the NdFeB lattice is believed to result from the preferential acid dissolution or etching of the Nd-rich phase. The hydrogen absorption and desorption behavior of sintered NdFeB alloys, (NdDy)15Fe78.3Al0.7B6 and (NdDy)15Fe67Co5V4Al2B7, have been compared with that of a LaNi5 alloy utilizing an electrochemical technique. LaNi5 is capable of absorbing extremely high quantities of hydrogen as a metal hydride. The level of hydrogen absorbed by the three alloys was compared by ‘‘charging’’ electrodes of each alloy in 6 M KOH at a constant cathodic current, followed by a constant anodic ‘‘discharge’’ current. An arrest in the resulting potential versus time curve during discharge indicates the oxidation of the hydride, i.e., of absorbed hydrogen, formed during the cathodic charging process. Results indicate that both NdFeB alloys absorb hydrogen in aqueous solution. Similar experiments performed on a set of samples with varying total rare-earth (TRE) content showed that the amount of hydrogen absorbed is proportional to TRE content. Anodic potentiodynamic polarization after charging shows that the corrosion rate increases as the level of absorbed hydrogen increases.
Study of wall coatings for vapor-cell laser traps
Authors: Stephens M. , Rhodes R. , Wieman C.
Published: July 2002

Efficient collection of atoms into a vapor-cell laser trap requires a special wall material for the cell that minimizes the interactions between the vapor and the wall. Tests of several different wall coatings and materials are reported, and measurements of adsorption energies, outgassing, and chemical reaction rates between the alkali vapor and the walls are described. It is demonstrated that each of these parameters affects the collection efficiency.
High-energy-ion damage in semicrystalline polyvinylidene fluoride
Authors: Musumeci P. , Calcagno L. , Percolla R. , Foti G.
Published: July 2002

Damage produced in semicrystalline polyvinylidene fluoride by high-energy ions (0.3–0.8 MeV/u) is studied. The samples, 9 μm thick, are irradiated with proton and carbon ions in the electronic stopping power regime with a fluence ranging from 1011 to 1014 ions/cm2. Irradiated samples are analysed by differential scanning calorimetry, x-ray diffraction, and solubility measurements. Differential scanning calorimetry curves show a decrease of the melting enthalpy and the appearance of a melting peak whose temperature decreases with increasing the ion fluence and x-ray diffraction analysis shows a decrease of the main diffraction peak intensity. All these effects are associated with a change in the crystalline morphology and with an increase of the polymer disordered regions. Solubility measurements suggest that the change in the polymer morphology is related to the formation of cross links between the polymeric chains produced by ion irradiation and to the production of a gel material. The data of amorphous fraction versus ion fluence are well fitted by using a track overlap model based on an amorphyzation process through the overlap of damaged clusters. The efficiency of this process does not depend on the nature of the incident ion, showing that the average energy deposited in electronic excitations and/or ionizations is the key parameter in the polymer amorphyzation. Moreover, the change in the polymer morphology is not directly related to the average deposited energy, but to the spatial distribution of energy inside the ion track.
Surface waves in an anisotropic periodically multilayered medium: Influence of the absorption
Authors: Potel Catherine , de Belleval Jean-François
Published: July 2002

The influence of absorption upon behavior of the reflection coefficient in water for an anisotropic periodically multilayered medium is studied. From observing a trough of the experimental reflection coefficient of a carbon/epoxy composite immersed in water, the propagation modes of the Floquet waves in an infinite anisotropic periodically multilayered medium in vacuum have been studied. This was explained by a mode that we called the multilayered Rayleigh mode. This mode results from a combination of the Floquet waves which propagate in a periodically multilayered medium. This occurs when all the Floquet waves are inhomogeneous. Analyzing the results obtained in the isotropic case permits the more complicated cases to be explained. By analogy with the isotropic case, the multilayered Rayleigh wave is related to the poles and the zeros of the reflection coefficient. The existence of a critical attenuation becomes evident when the reflection coefficient in water for the multilayered medium reaches zero.
Mechanism of low-temperature polycrystalline silicon growth from a SiF4/SiH4/H2 plasma
Authors: Kakinuma H. , Mohri M. , Tsuruoka T.
Published: July 2002

A model for the low-temperature growth of poly-Si by plasma-enhanced chemical-vapor deposition using SiF4/SiH4/H2 gases is presented. The model is based on the existing so-called etching model in which growth and etching take place simultaneously. In this model a set of chemical reactions are postulated. The crucial factors to obtain high-quality poly-Si films are (1) the flux of precursors, (2) the concentration of F radicals in the vicinity of the growing surface which determines the etching rate, and (3) the H-covered surface which ensures long diffusion length of precursors. The flow rate of SiH4 [factor (1)] determines whether the film becomes crystalline or amorphous, and variation in the other gas flow rates and plasma parameters affect factors (2) and (3). According to the model the electrode spacing and rf power predominantly determine the concentration of F radicals diffused to the growing surface, while the gas pressure changes the residence time of radicals which predominantly affects the etching reaction. Natural consequences of the model are that an excess supply of F radicals will in turn deteriorate the crystallinity by stripping the hydrogen covering the surface and increasing nucleation sites. The crystallinity of poly-Si films prepared by changing the above plasma parameters are characterized by x-ray diffraction, and their dependence on the above parameters are found to be consistent with the model. A high degree of hydrogen exchange between the growing surface and the plasma is observed by secondary-ion-mass spectroscopy for the film prepared with SiF4/SiH4/D2 gases.
Optical band gap of the α−mercuric iodide
Authors: Ferreira da Silva A. , Veissid N. , An C. Y. , Caetano de Souza J. , Batista da Silva A. V. , César Farias P. , da Cruz M. T. F.
Published: July 2002

We investigate by photoacoustic spectroscopy the optical band-gap energy of mercuric iodide, α-HgI2, grown by sublimation in a sealed ampoule. Due to its importance as a detector material operating at ambient temperature, the physical properties of α-HgI2 have been recently studied. We found, by two different methods, the band-gap energies EG=2.32 and 2.39 eV, respectively. These results are in good agreement with recent measurements based on reflection and absorption spectra.
Thermal properties of the novel nonlinear optical crystal N-methylurea
Authors: Bailey R. T. , Cruickshank F. R. , Kerkoc P. , Lochran S. , Pugh D. , Sherwood J. N. , Blake A. J. , Parsons S.
Published: July 2002

The heat capacity of crystalline N-methylurea has been measured in the range from 223–303 K by differential scanning calorimetry, and was found to obey the relationship Cp(T)=(5.44±0.04) ×10−3 T−(0.02821±0.0016) J g−1 K−1. Thermal expansion data have also been measured in the range from 100–353 K, allowing the temperature dependent density to be calculated. From these data, the principal thermal expansion coefficients were found to vary linearly, e.g., over the range from 100–250 K. In this range α1=−(5.84±0.42)×10−5 K−1 and α2 and α3 were found to be (1.31±0.14)×10−4 K−1 and (1.04±0.10)×10−4 K−1, respectively. The thermal conductivity tensor of this orthorhombic crystal was then calculated from values of the thermal diffusivity in the directions normal to the (100), (010), and (001) planes determined by the laser flash method. The thermal conductivity coefficients at 295 K were found to be k1=0.47 W m−1 K−1, k2=0.26 W m−1 K−1, and k3=0.30 W m−1 K−1.
Stress dependence of the magnetization process in amorphous wires and ribbons
Authors: Turtelli R. Sato , Sinnecker J. P. , Grössinger R. , Vázquez M.
Published: July 2002

Magnetization processes in Fe75Si10B15 and (Co92.5Fe6.3Nb1)77.5Si7.5B15 amorphous wires were studied by measuring the stress dependence of the hysteresis loops as well of the pinning field. Disaccommodation measurements were also used to characterize quenched-in stresses and to study magnetization processes. The results were compared with those of ribbons with similar compositions. It is shown that the geometry of the samples (wires and ribbons) due to the different cooling conditions determines the different domain structures and consequently the magnetization process.
Luminescence properties of SrS:Ce3+
Authors: Hüttl B. , Troppenz U. , Velthaus K. O. , Ronda C. R. , Mauch R. H.
Published: July 2002

SrS:Ce is an intensively investigated phosphor due to its blue-green electroluminescence, which shows efficient blue emission after filtering. Recently reported devices based on this material have demonstrated a luminous efficiency of 1.6 lm/W. The luminescence properties of SrS:Ce,X (X=Na or Cl) have been studied on powders and thin films. It is shown that a high density of traps in SrS:Ce,X occurs. The interaction of Ce3+ with traps gives rise to a phosphorescence. An energy transfer from Ce3+ to traps is responsible for an observed luminescence quenching in the presence of high electric fields. Moreover, the traps are electrically active and are involved in the electroluminescence process. The observed energy transfer is proposed to be the dominant excitation mechanism of Ce3+ in electroluminescence. It is demonstrated for thin films that the defect density increases with doping; therefore, the luminescence yield is already limited at doping concentrations below the onset of the concentration quenching. Thus, the prepared SrS:Ce,Cl thin films show a lower photoluminescence yield than powders. It is concluded that an undisturbed Ce incorporation into SrS thin films has not been achieved so far, although high electroluminescence efficiencies (1.6 lm/W) have been obtained.
Absorption of the laser beam by the plasma in deep laser beam welding of metals
Authors: Tix C. , Gratzke U. , Simon G.
Published: July 2002

We present a simple model for the keyhole plasma in deep penetration welding with a laser beam. Simplifying the complete set of transport equations investigated by C. Tix and G. Simon [Phys. Rev. E 50, 453 (1994)] for high laser powers yields an energy balance for the plasma, which can be treated analytically. The mean plasma temperature is calculated accounting for higher charge states of the ions. For high laser intensities double and higher charged ions dominate in the plasma bulk in the case of iron. The degree of absorption remains approximately constant for increasing laser power. The critical Péclét number for which the absorbed laser energy is able to maintain a keyhole increases strongly with increasing laser power. The energy absorption by the plasma is enhanced for higher modes of the laser intensity distribution.
Giant magnetostriction in amorphous (Tb1−xDyx)(Fe0.45Co0.55)y films
Authors: Duc N. H. , Mackay K. , Betz J. , Givord D.
Published: July 2002

The magnetization and magnetostriction have been studied in amorphous (Tb1−xDyx)(Fe0.45Co0.55)y films. A well-defined easy axis is created by magnetic field annealing and the sperimagnetic cone structure, characteristic of these amorphous alloys, is reduced. The anisotropy has a minimum at x=0.73 as in the R Fe2 Laves phase. This points to the similarity between the local environments in the amorphous and crystalline states. A giant magnetoelastic coupling coefficient b of 60 MPa is developed at 300 K in low applied field for x=0, optimally annealed films. Assuming a Young’s modulus and a Poisson ratio of 80 GPa and 0.31, respectively, the magnetostriction is evaluated at 1020×10−6. This is much larger than previously reported values in other amorphous films.