bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoexcited carrier transfer in InGaAs quantum dot structures: Dependence on the dot density
Authors: Marcinkevic̆ius Saulius , Leon Rosa
Published: July 2002

Carrier dynamics has been measured by time-resolved photoluminescence in self-assembled InGaAs/GaAs quantum-dot structures with dot density of the order of 108–1010 cm−2. The time of carrier transfer into a dot, which ranges from 2 to 20 ps, has been found to decrease with increasing quantum dot density. The temperature and photoexcited carrier density dependencies of the carrier transfer times suggest that potential barriers at wetting layer and quantum-dot interfaces hinder carrier capture in low-density quantum-dot structures.
Shock-induced transformation of β-Si3N4 to a high-pressure cubic-spinel phase
Authors: Sekine T. , He Hongliang , Kobayashi T. , Zhang Ming , Xu Fangfang
Published: July 2002

β- Si 3 N 4 powders were shock compressed and quenched from 12 to 115 GPa. β-Si3N4 transforms to the spinel-type Si3N4 (c-Si3N4) by a fast reconstructive process at pressures above about 20 GPa. The yield of c-Si3N4 recovered from 50 GPa and about 2400 K reaches about 80% and the grain sizes are about 10–50 nm. It is proposed that the fast transformation to c-Si3N4 occurs by rearrangement of nitrogen stacking layers, which initiates partial breakup of the SiN4 tetrahedra and formation of SiN6 octahedra at high density. Because of the advantages of massive production and the nanometer characteristics of shock-synthesized c-Si3N4, it is possible to investigate the mechanical properties experimentally and to develop new industrial applications.
Adsorption of a correlated random copolymer chain at a liquid-liquid interface
Authors: Denesyuk N. A. , Erukhimovich I. Ya.
Published: July 2002

Theoretical study of adsorption of an ideal correlated random copolymer chain at a liquid-liquid interface (ICRC-LL) is presented in this paper. Introduction of a certain referent system (a homopolymer chain in external attracting field) allows us to develop a principle of free energy calculation for ICRC-LL and to construct its phase diagram. Exponentially decaying (short-range) and oscillating correlations are considered in parallel that reveals easier adsorption for the first type of correlations. It is shown that chains with short-range and oscillating correlations undergo continuous (second-order) adsorption transition. For both types of correlations the transition temperature diverges when composition of a chain becomes symmetric.
Structure and luminescence of annealed nanoparticles of ZnS:Mn
Authors: Dinsmore A. D. , Hsu D. S. , Qadri S. B. , Cross J. O. , Kennedy T. A. , Gray H. F. , Ratna B. R.
Published: July 2002

Structural and light-emitting properties of nanoparticles of ZnS:Mn annealed in vacuum at temperatures up to 525 °C are presented. Annealing the 3.5 nm particles at temperatures up to 350 °C caused growth of some particles without substantial change in the luminescence or ZnS lattice. After annealing at 400–525 °C, the high-temperature wurtzite phase of ZnS appeared, accompanied by an increase of the average particle diameter to approximately 100 nm and a rearrangement of the Mn ions. Dramatic increase in cathodoluminescence emission was also observed and is compared to the structural information obtained from electron microscopy, x-ray diffraction, x-ray absorption fine structure, and electron paramagnetic resonance measurements.
Yttrium silicate formation on silicon: Effect of silicon preoxidation and nitridation on interface reaction kinetics
Authors: Chambers J. J. , Parsons G. N.
Published: July 2002

The effects of oxygen and nitrogen pretreatments on interface reaction kinetics during yttrium silicate formation on silicon are described. X-ray photoelectron spectroscopy (XPS) and medium energy ion scattering (MEIS) are used to determine chemical bonding and composition of films formed by oxidation of yttrium deposited on silicon. Capacitance–voltage testing is used to determine the quality of the dielectric and the electrical thickness. The effect of ultrathin silicon oxide, nitrided oxide, and nitrided silicon interfaces on metal oxidation kinetics is also described. When yttrium is deposited on clean silicon and oxidized, XPS and MEIS indicate significant mixing of the metal and the silicon, resulting in a film with Y–O–Si bonding and composition close to yttrium orthosilicate (Y2O3⋅SiO2). A thin (∼10 Å) in situ preoxidation step is not sufficient to impede the metal/silicon reaction, whereas a nitrided silicon interface significantly reduces the silicon consumption rate, and the resulting film is close to Y2O3. The mechanisms described for yttrium are expected to occur in a variety of oxide and silicate deposition processes of interest for high-k dielectrics. Therefore, in addition to thermodynamic stability, understanding the relative rates of elementary reaction steps in film formation is critical to control composition and structure at the dielectric/Si interface.
Volmer–Weber and Stranski–Krastanov InAs-(Al,Ga)As quantum dots emitting at 1.3 μm
Authors: Tsatsul’nikov A. F. , Kovsh A. R. , Zhukov A. E. , Shernyakov Yu. M. , Musikhin Yu. G. , Ustinov V. M. , Bert N. A. , Kop’ev P. S. , Alferov Zh. I. , Mintairov A. M. , Merz J. L. , Ledentsov N. N. , Bimberg D.
Published: July 2002

Quantum dots (QDs) formed on GaAs(100) substrates by InAs deposition followed by (Al,Ga)As or (In,Ga,Al)As overgrowth demonstrate a photoluminescence (PL) peak that is redshifted (up to 1.3 μm) compared to PL emission of GaAs-covered QDs. The result is attributed to redistribution of InAs molecules in the system in favor of the QDs, stimulated by Al atoms in the cap layer. The deposition of a 1 nm thick AlAs cover layer on top of the InAs–GaAs QDs results in replacement of InAs molecules of the wetting layer by AlAs molecules, leading to a significant increase in the heights of the InAs QDs, as follows from transmission electron microscopy. This effect is directly confirmed by transmission electron microscopy indicating a transition to a Volmer–Weber-like QD arrangement. We demonstrate an injection laser based on this kind of QDs.
Anticontrol of chaos in continuous-time systems via time-delay feedback
Authors: Wang Xiao Fan , Chen Guanrong , Yu Xinghuo
Published: July 2002

In this paper, a systematic design approach based on time-delay feedback is developed for anticontrol of chaos in a continuous-time system. This anticontrol method can drive a finite-dimensional, continuous-time, autonomous system from nonchaotic to chaotic, and can also enhance the existing chaos of an originally chaotic system. Asymptotic analysis is used to establish an approximate relationship between a time-delay differential equation and a discrete map. Anticontrol of chaos is then accomplished based on this relationship and the differential-geometry control theory. Several examples are given to verify the effectiveness of the methodology and to illustrate the systematic design procedure.
Phonons in strained In1−xGaxAs/InP epilayers
Authors: Shin H. K. , Lockwood D. J. , Lacelle C. , Poole P. J.
Published: July 2002

Raman spectroscopy has been used to assess the concentration dependence of optical phonons in In1−xGaxAs epilayers grown by chemical beam epitaxy on InP(100). The alloy composition was varied from x=0.325 to x=0.55 to cover the technologically important and physically interesting range where the strain changes sign at x=0.468 from negative to positive. The Raman spectra were curve resolved to reveal the GaAs-like longitudinal optic (LO), disorder induced, InAs-like LO, and InAs-like transverse optic phonons. An examination of the concentration dependence of the phonon frequencies showed that the GaAs-like LO mode varied as ω(cm−1)=252.77+58.643x−50.108x2 for 0.325⩽x⩽0.55. A comparison of these results with previous infrared and Raman work on In1−xGaxAs has revealed that the concentration dependence of the optical phonon frequencies in the unstrained system is still not accurately known.
Silver nanowires grown in the pores of a silica gel
Authors: Bhattacharyya S. , Saha S. K. , Chakravorty D.
Published: July 2002

Silver nanowires of diameter ∼40 nm and length ∼0.3 mm have been grown by electrodeposition within the pores of silica gels which were heat treated in the temperature range 523 to 823 K and, subsequently, soaked in a silver nitrate solution. A staircase current–voltage characteristic was observed in the direction of electrodeposition after nanowires were disrupted by the application of a dc voltage pulse. Such gels containing interrupted nanowires of silver showed a dielectric constant value ∼104 both in directions parallel and perpendicular to that of electrodeposition.
Threefold enhancement of the thermoelectric figure of merit for pressure tuned Sr8Ga16Ge30
Authors: Meng J. F. , Chandra Shekar N. V. , Badding J. V. , Nolas G. S.
Published: July 2002

Semiconducting germanium clathrates are thermoelectric materials with cage-like structures that exhibit “phonon glass, electron crystal” behavior. We report an improvement by at least a factor of 3 in the dimensionless thermoelectric figure of merit for the clathrate semiconductor Sr8Ga16Ge30 upon compression to 7 GPa. These results suggest that there is potential for further improving the thermoelectric properties of this material at ambient pressure by chemical or other methods.
Production of radioactive nuclides by energetic protons generated from intense laser-plasma interactions
Authors: Santala M. I. K. , Zepf M. , Beg F. N. , Clark E. L. , Dangor A. E. , Krushelnick K. , Tatarakis M. , Watts I. , Ledingham K. W. D. , McCanny T. , Spencer I. , Machacek A. C. , Allott R. , Clarke R. J. , Norreys P. A.
Published: July 2002

Nuclear activation has been observed in materials exposed to the ablated plasma generated from high intensity laser–solid interactions (at focused intensities up to 2×1019 W/cm2) and is produced by protons having energies up to 30 MeV. The energy spectrum of the protons is determined from these activation measurements and is found to be consistent with other ion diagnostics. The possible development of this technique for “table-top” production of radionuclides for medical applications is also discussed.
Oxide nanostructures through self-assembly
Authors: Aggarwal S. , Ogale S. B. , Ganpule C. S. , Shinde S. R. , Novikov V. A. , Monga A. P. , Burr M. R. , Ramesh R. , Ballarotto V. , Williams E. D.
Published: July 2002

A prominent theme in inorganic materials research is the creation of uniformly flat thin films and heterostructures over large wafers, which can subsequently be lithographically processed into functional devices. This letter proposes an approach that will lead to thin film topographies that are directly counter to the above-mentioned philosophy. Recent years have witnessed considerable research activity in the area of self-assembly of materials, stimulated by observations of self-organized behavior in biological systems. We have fabricated uniform arrays of nonplanar surface features by a spontaneous assembly process involving the oxidation of simple metals, especially under constrained conditions on a variety of substrates, including glass and Si. In this letter we demonstrate the pervasiveness of this process through examples involving the oxidation of Pd, Cu, Fe, and In. The feature sizes can be controlled through the grain size and thickness of the starting metal thin film. Finally, we demonstrate how such submicron scale arrays can serve as templates for the design and development of self-assembled, nanoelectronic devices.
Ultraviolet photoenhanced wet etching of GaN in K2S2O8 solution
Authors: Bardwell J. A. , Webb J. B. , Tang H. , Fraser J. , Moisa S.
Published: July 2002

The mechanism of the UV photoenhanced wet etching of GaN is determined. The UV photoenhanced wet etching does not require an electrical contact to be made to the sample, and nitrides deposited on insulating substrates (such as sapphire) can be etched, unlike photoelectrochemical (PEC) wet etching. The present technique relies on adding an appropriate oxidizing agent, in this case, peroxydisulfate (S2O82−), to KOH solutions. In a similar mechanism to PEC wet etching, the regions of low defect density are preferentially etched, leaving regions of high electron recombination such as threading dislocations relatively intact. The threading dislocations may be physically broken off, either by stirring or by a postetch sonication of the sample in KOH solution. Smoothly etched surfaces can be obtained under the proper conditions. A noble metal mask acts in a catalytic manner, yielding etch rates approximately one order of magnitude greater than those observed using inert masks. The essential role of the free radicals, originating from the peroxydisulfate ion, in the etching reaction is confirmed. The etching reaction is more rapid for more heavily n-type doped samples, and insulating C-doped layers act as an etch stop layer.
Height dispersion control of InAs/InP quantum dots emitting at 1.55 μm
Authors: Paranthoen C. , Bertru N. , Dehaese O. , Le Corre A. , Loualiche S. , Lambert B. , Patriarche G.
Published: July 2002

A procedure of InAs/InP quantum dots elaboration emitting at 1.55 μm by gas source molecular beam epitaxy is described. It is based on a modification of the capping layer growth which is deposited in two steps, separated by a growth interruption under phosphorus flux. The main effect of this two step capping layer growth is to reduce the height of the biggest islands and thus to decrease the photoluminescence linewidth of the quantum dots emission line. Transmission electron microscopy and photoluminescence experiments show that quantum dots structure are still present after growth interruption under phosphorus flux and that the photoluminescence linewidth at 1.55 μm is reduced from 120 to 50 meV, thanks to this procedure.
Spontaneous Hall effect in amorphous Tb–Fe and Sm–Fe thin films
Authors: Kim T. W. , Lim S. H. , Gambino R. J.
Published: July 2002

The spontaneous Hall effect in amorphous Tb–Fe and Sm–Fe thin films, which possess excellent magnetic softness, is investigated to seek a possibility of practical applications of these thin films. The resistivity of Tb–Fe thin films ranges from 180 to 250 μΩ cm as the Tb content varies from 35 to 46 at. %. Tb–Fe thin films show negative Hall resistivity ranging from −7.3 to −5.0 μΩ cm in the same composition range, giving the normalized resistivity ratio from −4.1% to −2.0%. On the other hand, the resistivity of Sm–Fe thin films ranges from 150 to 166 μΩ cm as the Sm content varies from 22 to 31 at. %. Sm–Fe thin films show positive Hall resistivity which varies from 7.1 to 2.8 μΩ cm in the same composition range, giving the normalized resistivity ratio from 4.8% to 1.7%. Between the two different sets of samples, Tb–Fe thin films with perpendicular anisotropy are considered to be more suitable for practical applications, since saturation is reached at a low magnetic field.
Proton transport polarization and depolarization of hydroxyapatite ceramics
Authors: Nakamura Satoshi , Takeda Hiroaki , Yamashita Kimihiro
Published: July 2002

Polarization of sintered hydroxyapatite (HAp) ceramics by application of an external dc field at higher temperature was analyzed by thermally stimulated depolarization current (TSDC) measurements. The mechanisms for the polarization and depolarization of HAp were discussed in relation to the instability of the protons in the hydroxide groups. The TSDC spectra consisted of broad peaks, while the ferroelectric substances such as the BaTiO3 ceramics exhibited a sharp peak. Although the maximum current density of 7.87 nA cm−2 for the HAp polarized at 400 °C under 1.0 kV cm−1 was approximately 1/12 lower than that of BaTiO3, the polarization charge of 14.9 μC cm−2 was almost twice as large as that of BaTiO3. Considering the activation energy of 0.72–0.89 eV for the depolarization, it was revealed that the polarization of HAp was ascribed to the migration of protons in the columnar OH− channels with a micrometer-order distance. It was also found that the polarization charge was large and long enough to enhance the biological reactivity of HAp ceramics for biomedical implants.
Transient growth: A factor in bypass transition
Authors: Reshotko Eli
Published: July 2002

Transient growth arises through the coupling between slightly damped, highly oblique (nearly streamwise) T–S and Squire modes leading to algebraic growth followed by exponential decay in a region that is subcritical with respect to the T–S neutral curve. A weak transient growth can also occur for two dimensional or axisymmetric modes since the Orr–Sommerfeld operator and its compressible counterpart are not self-adjoint, therefore their eigenfunctions are not strictly orthogonal. So transient growth is a candidate mechanism for many examples of bypass transition. The original transient growth theories were all temporal. However spatial growth formulations are now emerging including their extension to compressible flow with pressure gradient and heat transfer. The relevance to bypass transition is examined through several examples including Poiseuille pipe flow, the hypersonic blunt body paradox and distributed roughness effects.
Formation of the TiSi2 C40 as an intermediate phase during the reaction of the Si/Ta/Ti system
Authors: La Via F. , Mammoliti F. , Corallo G. , Grimaldi M. G. , Migas D. B. , Miglio Leo
Published: July 2002

The effect of a thin Ta layer at the Si/Ti interface on the intermediate phase formation has been studied in detail by in situ sheet resistance, x-ray diffraction, transmission electron microscopy and Rutherford backscattering spectroscopy of partially reacted samples. When a Ta layer is deposited at the Si/Ti interface, a new intermediate phase has been detected, i.e. the hexagonal TiSi2 C40. This phase grows on the C40–TaSi2 that is formed at the interface with silicon. The lattice parameters of the C40–TiSi2 obtained by ab initio calculations agree quite well with the experimental ones.
Improved extraction of the activation energy of the leakage current in silicon p–n junction diodes
Authors: Poyai A. , Simoen E. , Claeys C. , Czerwinski A. , Gaubas E.
Published: July 2002

An accurate method is proposed for the extraction of the activation energy ET from the volume generation current density JgA in silicon p–n junctions. It combines temperature-dependent current–voltage (I–V) and capacitance–voltage measurements on an array of diodes with different geometry, in order to separate the peripheral from the volume components. The JgA can be found from the volume leakage current by subtraction of the volume diffusion current JdA, which is calculated from the forward I–V characteristic. To derive the correct slope from an Arrhenius plot of the JgA, several additional corrections have been applied. One is the temperature dependence of the depletion width, which is derived from the corrected volume capacitance. The most important ET change is shown to come from the temperature dependence of the recombination lifetime.
Temperature-dependence of interlayer exchange bias coupling in NiO/Cu/NiFe
Authors: Lin Minn-Tsong , Ho C. H. , Chang Ching-Ray , Yao Y. D.
Published: July 2002

The trilayers 10 nm NiO(AF)/X Cu/10 nm NiFe(FM) were prepared for the study on the temperature effect in interlayer exchange bias coupling. The characteristic behavior of the interlayer exchange bias coupling as a function the spacer thickness was shown to strongly depend on the temperature. A monotonic decrease of the exchange bias field with increasing Cu spacer layer was observed at low temperature around 20 K. At higher temperatures (about 145 K), a clear oscillatory evolution of the exchange bias field with the Cu thickness was found even without background subtraction. The temperature-dependent feature of the interlayer exchange bias coupling was also found to vary significantly with different Cu thickness.
Near-room-temperature operation of an InAs/GaAs quantum-dot infrared photodetector
Authors: Tang Shiang-Feng , Lin Shih-Yen , Lee Si-Chen
Published: July 2002

A ten-stacked self-assembled InAs/GaAs quantum-dot infrared photodetector operated in the 2.5–7 μm range by photovoltaic and photoconductive mixed-mode near-room-temperature operation (⩾250 K) was demonstrated. The specific peak detectivity D* is 2.4×108 cm Hz1/2/W at 250 K. The use of high-band-gap Al0.3Ga0.7As barriers at both sides of the InAs quantum-dot structure and the long carrier recombination time are the key factors responsible for its near-room-temperature operation.
Large magneto-optical effect in an oxide diluted magnetic semiconductor Zn1−xCoxO
Authors: Ando K. , Saito H. , Jin Zhengwu , Fukumura T. , Kawasaki M. , Matsumoto Y. , Koinuma H.
Published: July 2002

Strong magneto-optical effect of Zn1−xCoxO near the optical band gap is reported. Magnetic circular dichroism (MCD) and Faraday rotation of Zn1−xCoxO (x=0.012 and 0.016) at 5 K are as high as 2°/cm Oe at 3.4 eV, which are about two orders larger than those of ZnO. Absence of strong Co2+ d–d* transition near the optical band gap makes Zn1−xCoxO an useful material for short-wavelength magneto-optical applications. Large magneto-optical effects and paramagnetic temperature dependence of MCD show a strong mixing of the sp bands of the host ZnO with Co2+ d orbitals. The polarity of the MCD peak is negative indicating the antiferromagnetic p–d exchange interaction.
A critical note on extrapolated helium pair potentials
Authors: Klopper Wim
Published: July 2002

It is difficult, if not impossible, to extrapolate the helium pair potential to the limit of a complete basis to within the accuracy needed to improve significantly on existing, directly computed potentials. Even though the basis-set convergence of calculations in a correlation-consistent basis with cardinal number X is dominated by the X−3 term, it is important to account for energy terms that converge more rapidly than ∝X−3. The electron-correlation contribution to the potential will be overestimated noticeably when these terms are not properly taken into account.
Current transport at the p-InP|poly(pyrrole) interface
Authors: Jones Frank E. , Daniels-Hafer Carrie , Wood Ben P. , Danner Robert G. , Lonergan Mark C.
Published: July 2002

The interface between the inorganic semiconductor p-type InP and the conjugated polymer poly(pyrrole) exhibits the electrical characteristics of a Schottky diode. Capacitance–voltage measurements yield an average barrier height of 0.62 ± 0.01 eV at temperature T =298 K. At the same temperature, the empirical quality factor, extracted from current–voltage measurements, is near unity. However, the current–voltage measurements show a deviation from thermionic emission theory as the temperature is reduced, as witnessed by the increase of the quality factor and the curvature in the Richardson plot. Such deviation is best explained by the barrier inhomogeneity model, in which the barrier becomes voltage dependent due to the interaction of a small low-barrier region with a higher surrounding potential, termed the “pinch-off” effect. Traditional current–voltage models, including image force lowering or an interfacial layer, cannot predict the temperature dependence of the current–voltage data, although thermionic field emission may facilitate current transport in the interfaces with a higher doped InP substrate. Furthermore, the probability of sufficiently energetic incident charge carriers crossing the interface, termed the transmission coefficient, is smaller than that observed in metal Schottky diodes.
Influence of substrate roughness on spin reorientation transition of ultrathin Co films on Pd(111)
Authors: Kim Jonggeol , Lee Jeong-Won , Jeong Jong-Ryul , Kim Sang-Koog , Shin Sung-Chul
Published: July 2002

We report a drastically different behavior in spin reorientation transition of ultrathin Co films grown on smooth versus rough Pd(111) single crystal substrates. The morphology and magnetic anisotropy of epitaxially grown Co films have been studied with in situ scanning tunneling microscopy (STM) and surface magneto-optical Kerr effects. On an atomically flat substrate, a smooth transition from perpendicular to in-plane magnetization occurs in a thickness interval of over 1.5 ML, beginning at ∼4.5 ML. In contrast, rough substrate causes the transition to begin quite earlier at ⩽3 ML and complete abruptly in less than 1 ML range. Morphology difference of the Co films obtained with STM suggests that on rough substrate, nonuniform coverage of Co leads to locally thicker regions triggering earlier spin reorientation transition.
Anomalous barium filling fraction and n-type thermoelectric performance of BayCo4Sb12
Authors: Chen L. D. , Kawahara T. , Tang X. F. , Goto T. , Hirai T. , Dyck J. S. , Chen W. , Uher C.
Published: July 2002

Barium-filled skutterudites BayCo4Sb12 with an anomalously large filling fraction of up to y=0.44 have been synthesized. The lattice parameters increase linearly with Ba content. Magnetic susceptibility data show that Ba0.44Co4Sb12 is paramagnetic, which implies that some of the Co atoms in BayCo4Sb12 have acquired a magnetic moment. The presence of the two different valence states of Co (Co3+ and Co2+) leads to the anomalously large barium filling fraction even without extra charge compensation. All samples show n-type conduction. The electrical conductivity increases with increasing the Ba filling fraction. The lattice thermal conductivity of BayCo4Sb12 is significantly depressed as compared to unfilled Co4Sb12. The dimensionless thermoelectric figure of merit, ZT, increases with increasing temperature reaching a maximum value of 1.1 for Ba0.24Co4Sb12 at 850 K.
Magic number behavior for heat capacities of medium-sized classical Lennard-Jones clusters
Authors: Frantz D. D.
Published: July 2002

Monte Carlo methods were used to calculate heat capacities as functions of temperature for classical atomic clusters of aggregate sizes 25⩽N⩽60 that were bound by pairwise Lennard-Jones potentials. The parallel tempering method was used to overcome convergence difficulties due to quasiergodicity in the solid-liquid phase-change regions. All of the clusters studied had pronounced peaks in their heat capacity curves, most of which corresponded to their solid-liquid phase-change regions. The heat capacity peak height and location exhibited two general trends as functions of cluster size: for N=25 to 36, the peak temperature slowly increased, while the peak height slowly decreased, disappearing by N=37; for N=30, a very small secondary peak at very low temperature emerged and quickly increased in size and temperature as N increased, becoming the dominant peak by N=36. Superimposed on these general trends were smaller fluctuations in the peak heights that corresponded to “magic number” behavior, with local maxima found at N=36, 39, 43, 46, and 49, and the largest peak found at N=55. These magic numbers were a subset of the magic numbers found for other cluster properties, and can be largely understood in terms of the clusters’ underlying geometries. Further insights into the melting behavior of these clusters were obtained from quench studies and by examining rms bond length fluctuations.
Simultaneous experimental evaluation of In and N concentrations in InGaAsN quantum wells
Authors: Grillo V. , Albrecht M. , Remmele T. , Strunk H. P. , Egorov A. Yu. , Riechert H.
Published: July 2002

We propose a method that solves the problem of the independent determination of the indium and nitrogen concentrations in a strained quaternary InGaAsN superlattice. The method is experimentally based on the simultaneous measurement: (i) of the tetragonal lattice distortion of the unit cell from high resolution micrographs and (ii) of the intensity of the chemically sensitive (002) reflection from dark field images. As an example, we evaluate InGaAsN quantum wells with a nominal N concentration of 1.7% and with In concentrations of 10%, 20%, or 35%. We reveal local fluctuations of the In and N concentrations over distances down to 4 nm with a sensitivity of 0.1% for N and 1% for In fluctuations in this distance range.
Covalent attachment of acetonitrile on Si(100) through Si–C and Si–N linkages
Authors: Tao Feng , Wang Zhong Hai , Qiao Ming Hua , Liu Qin , Sim Wee Sun , Xu Guo Qin
Published: July 2002

The covalent binding and adsorption states of acetonitrile on Si(100) have been investigated using temperature programmed desorption (TPD), x-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and density function theory (DFT) calculation. XPS and HREELS results show that acetonitrile chemisorbs on Si(100) in a side-on di-σ binding configuration, forming Si–C and Si–N σ bonds. TPD measurements reveal the presence of two desorption states, β1 and β2 with desorption energies of 29.8 and 24.6 kcal mol−1, respectively. Based on DFT calculations, the β1 state is possibly assigned to di-σ bonded acetonitrile on top of a dimer and/or in an in-row bridging chemisorption, while the β2 state is related to acetonitrile bonded in a cross-row bridging configuration.
Interstitial nitrogen induced by low-energy ion beam nitridation of AIII–BV semiconductor surfaces
Authors: Hecht J.-D. , Frost F. , Hirsch D. , Neumann H. , Schindler A. , Preobrajenski A. B. , Chassé T.
Published: July 2002

The nitridation of GaAs, InAs, and InSb by low-energy N2+ ion bombardment at room temperature was studied by near-edge x-ray absorption fine structure (NEXAFS) and x-ray photoelectron spectroscopy measurements. The formation of thin surface nitride layers, consisting mostly of GaN or InN but also containing minor amounts of mixed nitrides, was observed. Besides the nitride-related features, sharp peaks in the NEXAFS due to π* resonance at 401.0 eV and correlated peaks at 403.8 eV in N 1s core level spectra were detected. Both spectral features could be assigned to the presence of interstitial nitrogen, most likely molecular nitrogen. It was found that the amount of interstitial nitrogen in the surface layer strongly depends on the AIII–BV semiconductor system and may be affected by modification of the conditions during low energy ion bombardment.
Ion beam technologies in the semiconductor world (plenary)
Authors: Poate J. M. , Saadatmand K.
Published: July 2002

Ion beam technologies, in particular ion implantation, have had a profound effect on the development of Si integrated circuits. We review the pertinent history of ion source and machine development within the constraints of Moore’s law. For ion sources, the critical roles of hot cathode and rf sources are discussed. Novel applications such as sources for finely focused beams for lithography and cluster beams for doping (decaborane) or smoothing (Ar) will be discussed. Future trends in terms of the next generation of devices and the required implantation machines will be reviewed.
Correlation for the Vapor Pressure of Heavy Water From the Triple Point to the Critical Point
Authors: Harvey Allan H. , Lemmon Eric W.
Published: July 2002

A new formulation has been developed to represent the vapor pressure of heavy water (D2O) from its triple point to its critical point. This work takes advantage of several developments since the publication of the best previous formulation: the availability of some new data in the lower part of the temperature range, the adoption of the ITS-90 temperature scale, and the adoption of recommended values for the critical constants by the International Association for the Properties of Water and Steam (IAPWS). The new formulation fits the available data within their scatter across the entire temperature range, and is constrained to pass through the values recommended by IAPWS for the critical point. The previous formulation can be made to provide a similarly good fit if its input temperatures are converted from ITS-90 to IPTS-68 before the vapor pressure is computed. The new formulation offers the advantage of computation without having to perform a temperature conversion.
Phase separation suppression in InGaN epitaxial layers due to biaxial strain
Authors: Tabata A. , Teles L. K. , Scolfaro L. M. R. , Leite J. R. , Kharchenko A. , Frey T. , As D. J. , Schikora D. , Lischka K. , Furthmüller J. , Bechstedt F.
Published: July 2002

Phase separation suppression due to external biaxial strain is observed in InxGa1−xN alloy layers by Raman scattering spectroscopy. The effect is taking place in thin epitaxial layers pseudomorphically grown by molecular-beam epitaxy on unstrained GaN(001) buffers. Ab initio calculations carried out for the alloy free energy predict and Raman measurements confirm that biaxial strain suppress the formation of phase-separated In-rich quantum dots in the InxGa1−xN layers. Since quantum dots are effective radiative recombination centers in InGaN, we conclude that strain quenches an important channel of light emission in optoelectronic devices based on pseudobinary group-III nitride semiconductors.
Thermoelectric properties of the n-type filled skutterudite Ba0.3Co4Sb12 doped with Ni
Authors: Dyck Jeffrey S. , Chen Wei , Uher Ctirad , Chen Lidong , Tang Xinfeng , Hirai Toshio
Published: July 2002

Synthesis and electrical and thermal transport properties are reported for several filled skutterudite compounds doped with Ni: Ba0.3NixCo4−xSb12 with 0<x<0.2. Divalent Ba readily fills the cages of the skutterudite structure and is effective in reducing the thermal conductivity of the structure. The presence of a small amount of Ni increases the electron concentration, further reduces the thermal conductivity, and enhances the thermoelectric power factor. Hall mobility studies indicate that the addition of Ni to the system has the effect of increasing the relative strength of ionized impurity scattering as compared to acoustic phonon scattering. These results suggest that doping with Ni is an attractive avenue to optimization of filled skutterudites. The dimensionless thermoelectric figure of merit ZT was observed to increase from a value of 0.8 at 800 K for Ba0.3Co4Sb12 to a value of 1.2 for the sample with x=0.05. These materials show considerable potential as n-type legs in thermoelectric power generation at elevated temperatures.
Nanoembossing of thermoplastic polymers for microfluidic applications
Authors: Studer V. , Pépin A. , Chen Y.
Published: July 2002

We present a method for the fabrication of plastic microfluidic devices based on nanoembossing and thermal bonding. By nanoembossing of thermoplastic polymer pellets, both microfluidic deep channels and high resolution features can be formed using a silicon mold fabricated by electron beam lithography and reactive ion etching. By thermal bonding with another plastic sheet, the fabricated microfluidic devices can be sealed without clogging. Observation of pressure driven and electrokinetic flows through high density pillar arrays indicates the feasibility of nanofluidic analysis using plastic devices.
Ionic/electronic mixed conductor tip of a scanning tunneling microscope as a metal atom source for nanostructuring
Authors: Terabe K. , Nakayama T. , Hasegawa T. , Aono M.
Published: July 2002

Silver sulfide (Ag2S) which has Ag-ionic/electronic mixed conductivity is used for fabricating a tip used for a scanning tunneling microscope. The mixed conductor tip is capable of nanostructuring by depositing Ag atoms continuously on a sample as well as imaging the surface structure. To obtain the surface image, a nanoscale Ag protrusion is formed at an apex of the tip using a local solid electrochemical reaction, working as “a mini-tip.” We fabricate a nanoscale line structure on the sample by scanning the Ag2S tip with the protrusion under appropriate bias voltages and tunneling currents. The structuring is thought to be made up of two layers of Ag atoms deposited from the protrusion.
On noise reduction methods for chaotic data
Authors: Grassberger Peter , Hegger Rainer , Kantz Holger , Schaffrath Carsten , Schreiber Thomas
Published: July 2002

Recently proposed noise reduction methods for nonlinear chaotic time sequences with additive noise are analyzed and generalized. All these methods have in common that they work iteratively, and that in each step of the iteration the noise is suppressed by requiring locally linear relations among the delay coordinates, i.e., by moving the delay vectors towards some smooth manifold. The different methods can be compared unambiguously in the case of strictly hyperbolic systems corrupted by measurement noise of infinitesimally low level. It was found that all proposed methods converge in this ideal case, but not equally fast. Different problems arise if the system is not hyperbolic, and at higher noise levels. A new scheme which seems to avoid most of these problems is proposed and tested, and seems to give the best noise reduction so far. Moreover, large improvements are possible within the new scheme and the previous schemes if their parameters are not kept fixed during the iteration, and if corrections are included which take into account the curvature of the attracting manifold. Finally, the fact that comparison with simple low-pass filters tends to overestimate the relative achievements of these nonlinear noise reduction schemes is stressed, and it is suggested that they should be compared to Wiener-type filters.
Controlling chaos in a fast diode resonator using extended time-delay autosynchronization: Experimental observations and theoretical analysis
Authors: Sukow David W. , Bleich Michael E. , Gauthier Daniel J. , Socolar Joshua E. S.
Published: July 2002

We stabilize unstable periodic orbits of a fast diode resonator driven at 10.1 MHz (corresponding to a drive period under 100 ns) using extended time-delay autosynchronization. Stabilization is achieved by feedback of an error signal that is proportional to the difference between the value of a state variable and an infinite series of values of the state variable delayed in time by integral multiples of the period of the orbit. The technique is easy to implement electronically and it has an all-optical counterpart that may be useful for stabilizing the dynamics of fast chaotic lasers. We show that increasing the weights given to temporally distant states enlarges the domain of control and reduces the sensitivity of the domain of control on the propagation delays in the feedback loop. We determine the average time to obtain control as a function of the feedback gain and identify the mechanisms that destabilize the system at the boundaries of the domain of control. A theoretical stability analysis of a model of the diode resonator in the presence of time-delay feedback is in good agreement with the experimental results for the size and shape of the domain of control.
Characteristic length of domain structures and its effect on the coercive force of ferromagnetic materials
Authors: Ozimek E. J.
Published: July 2002

The magnetic properties of micrometer-size Permalloy thin films have been investigated for a variety of sample thicknesses, aspect ratios, and sizes. The formation of edge domains was an important factor in the magnetization response of the sample to an applied external magnetic field. Sample coercivity depended on the length of the domain wall structure. Small specimens (6.5 μm wide) have high coercivity, whereas large samples (104 μm wide) approach the bulk-film value. The reduced length, defined as the ratio of the edge-domain extent (or characteristic length) to the sample length, and its dependence on sample size, aspect ratio, and thickness were studied. Samples with low aspect ratio have wall structures throughout the specimen, giving rise to higher coercivities and correspondingly larger reduced lengths. Samples with large aspect ratio have wall structures confined to the edge areas, lower coercivities, and shorter reduced lengths. The absolute domain size increased as the sample increased, and reduced length approached a constant value for all samples with a given aspect ratio. The reduced length was approximately proportional to the reciprocal of the aspect ratio.
Temperature-induced frequency shifts in quartz resonators
Authors: Valentin J. P. , Théobald G. , Gagnepain J. J.
Published: July 2002

The thermal exchanges in a quartz crystal resonator are investigated. This leads to a temperature gradient distribution which is in the main plane of the crystal, and therefore does not exhibit any functional dependence (at least at the first order) along the plate thickness. Internal heat source due to the energy dissipation by the internal friction is considered. The corresponding in-plane thermal stresses are calculated in the isotropic approximation, and anisotropy is introduced by means of the stress-strain relations. Applying a perturbation method, the resulting frequency shifts are obtained for both static and dynamic thermal behaviors.
Band gaps and refractive indices of AlGaAsSb, GaInAsSb, and InPAsSb: Key properties for a variety of the 2–4-μm optoelectronic device applications
Authors: Adachi Sadao
Published: July 2002

The methods for calculation of material parameters in compound alloys are discussed, and the results for AlxGa1−xAsySb1−y, GaxIn1−xAsySb1−y, and InPxAsySb1−x−y quaternaries lattice matched to GaSb and InAs are presented. These quaternary systems may provide the basis for optoelectronic devices operating over the 2–4-μm wavelength range. The material parameters considered are: the lattice constant, the lowest direct- and indirect-gap energies, and the refractive index. The model used is based on an interpolation scheme, and the effects of compositional variations are properly taken into account in the calculations. Key properties of the material parameters for a variety of optoelectronic device applications are also discussed in detail.
Laser-induced damage in quartz: A study of the influence of impurities and defects
Authors: Bandyopadhyay Pradip K. , Merkle Larry D.
Published: July 2002

It has been observed that samples of high-quality cultured quartz can differ markedly from sample to sample in their resistance to laser-induced catastrophic damage. The bulk damage properties of quartz from several sources have been investigated using both 1064- and 532-nm laser pulses. It is found that the single-pulse damage ‘‘threshold’’ is ill defined, indicating considerable inhomogeneity in even the best samples. Neither resistance to damage by a single-pulse nor damage behavior under multiple-pulse irradiation exhibit any correlation with the concentrations of Al or OH, two impurities important to other properties of quartz. Further, the dependence of damage resistance on temperature and on electron irradiation is much weaker than would be expected if Al were important to the damage process. However, there does appear to be a correlation with the density of coarsely spaced defects revealed in the damage morphology and in chemical etching. Thus, it appears that even in high-quality quartz-laser-induced damage is initiated on low-concentration defects, but that the relevant defects are not related to aluminum impurities.
Large grain size CdTe films grown on glass substrates
Authors: Zelaya O. , Sánchez-Sinencio F. , Mendoza-Alvarez J. G. , Farías M. H. , Cota-Araiza L. , Hirata-Flores G.
Published: July 2002

Polycrystalline films of CdTe (with grain size up to 100 μm) were grown by the hot wall close-spaced vapor transport technique in an Ar atmosphere. In most cases they grow on a zinc-blende type of structure with a preferential orientation of growth along the 〈111〉 directions, as shown by x-ray diffraction studies. The grain size in the films presents a maximum around 3.5 Torr, as well as a minimum of preferential orientation. The grain size shows an exponential increase in the substrate temperature (Ts) 450–650 °C range. For Ts≤600 °C, the films grow with a 〈111〉 preferential crystallographic orientation, whereas for Ts>600 °C the preferential orientation changes to the 〈220〉 directions. For Ts>600 °C, the films show a mixture of zinc-blende and wurtzite phases, according to the x-ray analysis. A Te crystalline phase is detected for Ts>550 °C.
Point defect sinks in self-ion-irradiated nickel: A self-diffusion investigation
Authors: Müller A. , Naundorf V. , Macht M.-P.
Published: July 2002

The diffusion coefficient of 63Ni in pure nickel after irradiation with 300 keV Ni ions has been measured directly using a secondary ion mass spectrometer. The calculated displacement rate for this irradiation ranged from 1.2×10−5 to 3.1×10−2 dpa/s, the dose from 0.2 to 102 dpa, and the temperatures from 293 to 950 K. Between 293 and 650 K the irradiation-induced diffusion coefficient is temperature independent for a displacement rate of 1.2×10−2 dpa/s. In this temperature range mass transport by atomic mixing prevails over diffusion via point defects (radiation-enhanced diffusion). Normalized to the displacement rate K′ the diffusion coefficient describing atomic mixing attains a value of Dmix /K′=1.3×10−18 m2 /dpa. The temperature and displacement rate dependence of the radiation-enhanced diffusion coefficient is discussed in the framework of a rate equation model. The analysis yields a production rate of freely migrating defects of about 1.5% of the calculated displacement rate. The effective concentration of point defect sinks was derived for nickel under heavy ion irradiation. This effective sink concentration Cs showed no dependence on displacement rate but a considerable temperature dependence, e.g., Cs =2×10−6 at 850 K and Cs =2.5×10−7 at 950 K. The conditions for a stationary effective sink concentration in self-ion irradiated nickel are discussed.
A novel organic electro-optic crystal: 3-(1,1-dicyanoethenyl)-1-phenyl- 4,5-dihydro-1H-pyrazole
Authors: Allen S. , McLean T. D. , Gordon P. F. , Bothwell B. D. , Hursthouse M. B. , Karaulov S. A.
Published: July 2002

Details are given of a new organic electro-optic crystal, 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole, having a reduced half-wave voltage of 370 V (approximately 8 times lower than lithium niobate). The molecular design, synthesis, and molecular characterization of this compound are outlined and a description is given of growth of 1-cm dimension crystals. These crystals have been characterized in terms of their structure, linear optical properties (refractive index, orientation of optical axes, absorption spectra), and also of their linear electro-optic (Pockels) effect. The compound crystallizes in the noncentrosymmetric space group Cc, with the molecules showing a high degree of parallelism. This structure is discussed in terms of the applicability of the crystals to different nonlinear optical effects. Calculated molecular hyperpolarizabilities are used, in conjunction with the determined crystal structure, to estimate the coefficients of the crystalline χ(2) tensor, which is dominated by the single coefficient χ333. The related dominant electro-optic coefficient, r333, is measured to be 8.7×10−11 m/V.
Magnetomechanical damping in iron
Authors: Coronel V. F. , Beshers D. N.
Published: July 2002

We have carried out a complete, detailed study of magnetomechanical damping on a set of annealed and deformed iron specimens. The results for macroeddy-current and microeddy-current damping are consistent with previous data on iron. The macroeddy peak gives a good indicator of achieving saturation. The microeddy damping offers a usable method for estimating domain size. The slope of damping versus amplitude in the hysteretic Rayleigh region is inversely proportional to the square of a measure of internal stress, as predicted by Smith and Birchak (SB). At higher amplitudes, in annealed and slightly deformed specimens, the damping increases more rapidly than in the Rayleigh region, a phenomenon not previously recognized, and contrary to existing models. The very long straight line in the Rayleigh region, implying a lack of any linear scale for the hysteresis, suggests that there is a fractal nature to the process. This suggestion is consistent with the SB theory. A possible model for the elementary hysteretic event is the reorientation of a quadrupole as the local stress changes sign; if this process involves a random sampling of the internal stress field, the lack of scale can be accounted for.