bluuebunny/crossref_metadata_2025 huggingface dataset:
Diagnostics of a N2/Ar direct current magnetron discharge for reactive sputter deposition of fullerene-like carbon nitride thin films
Authors: Neidhardt J. , Hultman L. , Abendroth B. , Gago R. , Möller W.
Published: November 2003

Plasma probe measurements and energy selective mass spectrometry were used to characterize the composition and energetic conditions of the ion flux of a direct current, N2/Ar, unbalanced magnetron sputtering plasma in front of a graphite target, depending on the total pressure and gas composition. It is shown that the flux of ionized film-forming species consist predominantly of CxNy (x,y⩽2) species. The total number of C-containing ions strongly depends on the N2 fraction in the discharge and can be increased by almost one order of magnitude scaling with the availability of N2 as the discharge gas. This finding suggests that besides C and N atoms and ions, other species, like preformed CxNy particles, play an important role during growth of fullerene-like (FL) carbon nitride (CNx) thin films. It is proposed that such clusters have a direct effect as growth templates or nucleation sites on the evolution of curved and cross-linked FL structures. The carbon target was identified as the main source of preformed C-containing species, following from the pressure dependence of the ion energy distributions of the most abundant CxNy (x,y⩽2) ions.
Velocity angular dispersion of surface and bulk acoustic modes in SrTiO3
Authors: Huang J. D. , Kuok M. H. , Lim H. S. , Ng S. C.
Published: November 2003

The velocity angular dispersion of Rayleigh surface and bulk acoustic modes propagating parallel to the surfaces of (100), (110) and (111) SrTiO3 plates has been measured by surface Brillouin scattering. Transverse bulk acoustic modes propagating in certain directions are not detected in the scattering geometry employed. The nonobservation of these modes is consistent with results of a calculation of their scattering coefficient. A simultaneous fit to the experimental data on the bulk acoustic modes for the three crystal orientations yields elastic constants that are used to accurately predict the various velocity angular dispersion branches of the Rayleigh surface mode.
Arsenic incorporation and its influence on microstructure of wurtzite GaN grown by molecular-beam epitaxy
Authors: Kim H. J. , Andersson T. G. , Chauveau J.-M. , Trampert A.
Published: November 2003

We present a comprehensive study on the incorporation of As in a local area and its influence on the microstructures of wurtzite GaN grown by rf-plasma-assisted molecular-beam epitaxy. Using transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS), we found that the incorporation of As in GaN is correlated with the formation of intrinsic basal plane stacking faults. The stacking fault, analyzed by high-resolution TEM, can be regarded as a stacking sequence of cubic GaAs or Ga(N,As) inserted into hexagonal GaN. Indeed, it may be energetically favorable for the As incorporation to form a thin cubic layer in hexagonal GaN, resembling the thermodynamically stable cubic phase for GaAs. We found experimental evidence of As surface segregation and analyzed it by fitting the As SIMS profiles with a one-dimensional empirical surface segregation model. This result declares a significantly large tendency of As surface segregation with the surface segregation coefficient R∼0.99, which indicates the difficulties to incorporate As into GaN, and thus to grow the ternary GaNAs alloys.
Electronic structure and magneto-optical Kerr effect in UGa2
Authors: Antonov V. N. , Harmon B. N. , Yaresko A. N.
Published: November 2003

The optical and magneto-optical (MO) spectra of UGa2 are investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as with generalization of the LSDA+U method which takes into account that in the presence of spin-orbit coupling the occupation matrix of localized electrons becomes nondiagonal in spin indexes. Although, the magnetic moment at the U site of UGa2 is described better by LSDA+U than by LSDA, neither the localized LSDA+U model nor the spin-polarized LSDA 5f-itinerant band model can explain all the peculiarities of the experimental MO spectra. The LSDA gives a better description of the optical and MO spectra of UGa2 in the 0–1 eV energy interval, but for higher energies the experimental Kerr rotation curve is situated someplace in between the curves obtained by the LSDA and the LSDA+U approximations. The origin of the Kerr rotation in the compound is examined.
Sound absorbing system as an application of the active elasticity control technique
Authors: Mokrý P. , Fukada E. , Yamamoto K.
Published: November 2003

Active elasticity control has recently become a promising method used in acoustics to suppress the vibration of mechanical systems or to increase the noise transmission loss. The principle of this technique is based on the simultaneous utilization of both the direct and inverse piezoelectric effects, which is realized by connecting the piezoelectric sample to an external feed back circuit. The action of the feedback circuit results in an essential change of the vibrational response of the system, and a significant change of the resonant frequency in the mechanical system. This leads to the modification of the acoustic properties of the system. For that reason, this technique offers a very general approach to solve vibration and noise problems. To demonstrate this method, the design and theoretical analysis of an experimental realization of a sound absorbing system are presented. The system consists of a curved piezoelectric membrane and a passive sound absorber, which are attached to a rigid backing wall. The theoretical analysis of the system starts from the calculation of the absorption coefficient of an audible sound. This result is used for the determination of the optimal adjustment in the feedback control electronics. The efficiency and the frequency range of the active sound absorption, as well as aspects of the generalization of this method to solve other kinds of noise and vibration problem are discussed.
Carbon nitride thin films prepared by radio-frequency magnetron sputtering combined with a nitrogen radical beam source
Authors: Xu Wentao , Wang Li , Kojima Isao
Published: November 2003

Carbon nitride (CNx) thin films were prepared by radio-frequency magnetron sputtering combined with a nitrogen radical beam source. X-ray photoelectron spectroscopy, Raman spectroscopy, and grazing-incidence x-ray reflectivity have been used to analyze the chemical composition and structure of the CNx films. An atomic force microscope equipped with a diamond tip was employed to measure the hardness of the films. The nitrogen concentration, composition, hardness, density, and surface roughness varied with the operating power and gas flow applied to the sputtering target and radical gun in the sample preparation. CNx films were fabricated with a maximum nitrogen concentration of ∼62 at. % on the surface and ∼44 at. % after Ar+ sputtering, as well as a maximum hardness of ∼33.7 GPa. Compared with N2+ and N2, excited molecular and atomic nitrogen were more active in the formation of carbon nitrides. With the combination of magnetron sputtering, the radical beam source enables an obvious increase in the nitrogen concentration but only a slight increase in the film hardness. A relationship between the Raman spectra and hardness was found: CNx films with a higher G peak frequency and lower ID/IG have higher hardness. Our results also suggested that a higher concentration of the sp3 phase produces a harder CNx film.
Spectra and energy levels of Er3+(4f11) in NaBi(WO4)2
Authors: Gruber John B. , Sardar Dhiraj K. , Russell Charles C. , Yow Raylon M. , Zandi Bahram , Kokanyan Edvard P.
Published: November 2003

Absorption and fluorescence spectra of Er3+(4f11) in crystals of NaBi(WO4)2 (NBW) are reported at temperatures between 15 K and room temperature. The absorption spectra include the details of the crystal-field splitting of 11 multiplet manifolds, LJ2S+1 of Er3+(4f11), spanning the wavelength range between 350 nm and 1550 nm. The crystal-field splitting of the ground-state I15/24, is obtained from an analysis of the fluorescence spectrum, S3/24→I15/24. Spectra are characterized by inhomogeneous broadening due to the disordered crystal structure in which different valency cations, Na+ and Bi3+, statistically fill the S4 symmetry sites. The Er3+ ions likely replace the Bi3+ ions in these sites. A quasi-center model has been chosen to interpret the crystal-field splitting of each manifold, using D2d rather than S4 symmetry as the site for the rare-earth ion in the lattice. To test the feasibility of the model, the splitting of the energy levels of Nd3+ in NBW was carried out first and compared with experimental levels reported in literature. A least-squares fitting analysis between 26 calculated-to-observed energy (Stark) levels gave a root-mean-square (rms) deviation of 8 cm−1 for the IJ4 and F3/24 multiplet manifolds of Nd3+ in NBW. Using the phenomenological lattice-sum parameters, Anm, obtained from the analysis of the Nd3+ energy levels, we predicted an initial set of crystal-field parameters, Bnm, for Er3+. With only a modest fitting of the multiplet centroids, these Bnm predict the observed splitting in the Er3+ spectra remarkably well. In a fitting of the energy levels in which both the Bnm and centroids are allowed to vary, we obtained a rms deviation of 6 cm−1 for 57 calculated-to-observed Stark levels. The results suggest that the quasi-center model has merit when used to calculate the crystal-field splitting of the energy levels of the trivalent rare-earth ions in crystal hosts having a disordered structure.
Modification of ferromagnetism in semiconductors by molecular monolayers
Authors: Kreutz T. C. , Gwinn E. G. , Artzi R. , Naaman R. , Pizem H. , Sukenik C. N.
Published: November 2003

We report that adsorption of monolayers of organic molecules onto the surface of ferromagnetic semiconductor heterostructures produces large, robust changes in their magnetic properties. The heterostructures have half a monolayer of MnAs embedded in GaAs, 50 Å beneath the surface. The molecules investigated are alkylphosphonic acids that bind to GaAs via a phosphonate group. The organization of the organic monolayer determines the reduction in the Curie temperature, with ordered monolayers producing nearly complete suppression of ferromagnetism. We attribute this striking chemical modulation of magnetic properties to electronic changes brought about by the binding of the molecules to the semiconductor surface.
Column buckling of multiwalled carbon nanotubes using nonlocal continuum mechanics
Authors: Sudak L. J.
Published: November 2003

A model, based on the theory of nonlocal continuum mechanics, on the column buckling of multiwalled carbon nanotubes is presented. The present analysis considers that each of the nested concentric tubes is an individual column and that the deflection of all the columns is coupled together through the van der Waals interactions between adjacent tubes. Based on this description, a condition is derived in terms of the parameters that describe the van der Waals forces and the small internal length scale effects. In particular, an explicit expression is derived for the critical axial strain of a double walled carbon nanotube which clearly demonstrates that small scale effects contribute significantly to the mechanical behavior of multiwalled carbon nanotubes and cannot be ignored.
Band offset measurements of the GaN (0001)/HfO2 interface
Authors: Cook T. E. , Fulton C. C. , Mecouch W. J. , Davis R. F. , Lucovsky G. , Nemanich R. J.
Published: November 2003

Photoemission spectroscopy has been used to observe the interface electronic states as HfO2 was deposited on clean n-type Ga-face GaN (0001) surfaces. The HfO2 was formed by repeated deposition of several monolayers of Hf followed by remote plasma oxidation at 300 °C, and a 650 °C densification anneal. The 650 °C anneal resulted in a 0.6 and 0.4 eV change in band bending and valence band offset, respectively. The final annealed GaN/HfO2 interface exhibited a valence band offset of 0.3 eV and a conduction band offset of 2.1 eV. A 2.0 eV deviation was found from the electron affinity band offset model.
Point defects as result of surface deformation on a GaAs wafer
Authors: Zamponi C. , Männig U. , Staab T. E. M. , Maier K. , Eichler S. , Hammer R.
Published: November 2003

Scratches on the surface of an undoped semi-insulating GaAs wafer have been produced by a wedge-shaped single diamond grain. The subsurface damage has been analyzed by a positron microprobe. This instrument provides laterally resolved positron annihilation measurements, which are sensitive to lattice defects like vacancies and dislocations. We can clearly identify different regions of damage which have been characterized both by conventional scanning electron microscopy and the positron microbeam. The latter reveals indications of plastic deformation due to the trace of created defects observed. We discuss the possible implications of the observed ductile behavior of GaAs usually known to be brittle at room temperature and under atmospheric pressure.
Amorphous germanium recombination junctions in amorphous-silicon-based tandem solar cells
Authors: Ganguly G. , Carlson D. E. , Arya R. R.
Published: November 2003

The recombination junction between two cells in amorphous-silicon-based tandem devices can be improved by inserting doped nanocrystalline silicon layers, which have higher conductivities, between doped amorphous silicon layers. However, long deposition times (∼15 min) are required to nucleate and grow these thin nanocrystalline silicon layers on an amorphous-silicon layer. We show that by increasing the phosphorus doping in the amorphous silicon n layer, and replacing the nanocrystalline silicon layers with amorphous germanium layers, a similar performance is obtained with a shorter (∼1 min) deposition time.
Interwell intersubband electroluminescence from Si/SiGe quantum cascade emitters
Authors: Bates R. , Lynch S. A. , Paul D. J. , Ikonic Z. , Kelsall R. W. , Harrison P. , Liew S. L. , Norris D. J. , Cullis A. G. , Tribe W. R. , Arnone D. D.
Published: November 2003

The quantum cascade laser provides one potential method for the efficient generation of light from indirect materials such as silicon. While to date electroluminescence results from THz Si/SiGe quantum cascade emitters have shown higher output powers than equivalent III–V emitters, the absence of population inversion within these structures has undermined their potential use for the creation of a laser. Electroluminescence results from Si/SiGe quantum cascade emitters are presented demonstrating intersubband emission from heavy to light holes interwell (diagonal) transitions between 1.2 THz (250 μm) and 1.9 THz (156 μm). Theoretical modeling of the transitions suggests the existence of population inversion within the system.
Use of the final thermally stimulated discharge current technique to analyze space charge behavior in dielectrics
Authors: Neagu E. R.
Published: November 2003

The final thermally stimulated discharge current technique is proposed for analyzing space charge (SC) trapping/detrapping. The letter investigates the effects associated with SC behavior in polyamide 11 in an attempt to discriminate between dipolar and SC related effects. An apparent peak, located 5–13 °C above the charging temperature, and a second peak around 96 °C were observed. Both peaks scale linearly with the field revealing a similar behavior of dipolar and interfacial polarization. The peaks position and the apparent activation energy W do not change with the charging field, but are very sensitive to the previous charging/discharging times. A distribution of the apparent activation energies in a range from 0.45 to 2.20 eV is revealed for the apparent peaks. The isothermal decay of SC at 35 °C is proportional to log t, a behavior that may be characteristic for the interfacial polarization.
Long coherence times at 300 K for nitrogen-vacancy center spins in diamond grown by chemical vapor deposition
Authors: Kennedy T. A. , Colton J. S. , Butler J. E. , Linares R. C. , Doering P. J.
Published: November 2003

Electron-spin-echo experiments reveal phase-memory times as long as 58 μs at 300 K for nitrogen-vacancy centers in chemical vapor deposition (CVD) single crystals. The spins were optically polarized and optically detected. Two high-quality CVD samples were studied. From the current results, it is not clear whether these phase-memory times represent a fundamental limit or are limited by an external source of decoherence.
Threshold dependence on the spectral alignment between the quantum-well gain peak and the cavity resonance in InGaAsP photonic crystal lasers
Authors: Cao J. R. , Kuang Wan , Choi Sang-Jun , Lee Po-Tsung , O’Brien John D. , Dapkus P. Daniel
Published: November 2003

Lithographically defined multiwavelength photonic crystal laser arrays are reported. The dependence of the threshold pump power on the spectral alignment between the quantum-well gain peak and the cavity resonance wavelength is investigated. This is done at, and slightly above, room temperature.
Simultaneous generation of multiple pairs of transverse electric and transverse magnetic output modes from titania zirconia organically modified silicate distributed feedback waveguide lasers
Authors: Ye Chao , Shi Lei , Wang Jun , Lo Dennis , Zhu Xiao-lei
Published: November 2003

Output polarization and output wavelength of titania zirconia organically modified silicate distributed feedback waveguide lasers were found to be dependent on the polarization of the crossing pump beams. Output waves with transverse electric polarization were obtained when the pump beams were s-polarized. Transverse electric and transverse magnetic optical waves belonging to the same propagation mode were generated by crossing two circularly polarized or p-polarized pump beams. Tuning of the polarized laser output was also achieved by varying the crossing angle. Up to sixteen output wavelengths corresponding to eight pairs of polarized output from eight propagation modes were observed simultaneously for a planar waveguide with a thickness of 6.7 μm.
Time-resolved photoluminescence of the size-controlled ZnO nanorods
Authors: Hong Sangsu , Joo Taiha , Park Won Il , Jun Yong Ho , Yi Gyu-Chul
Published: November 2003

Size dependence of the time-resolved photoluminescence (TRPL) has been investigated for the ZnO nanorods fabricated by catalyst-free metalorganic chemical vapor deposition. The nanorods have a diameter of 35 nm and lengths in the range of 150 nm to 1.1 μm. The TRPL decay rate decreases monotonically as the length of the nanorods increases in the range of 150 to 600 nm. Decrease of the radiative decay rate of the exciton-polariton has been invoked to account for the results.
Localization of intrinsic defects in CaF2–BaF2 superlattices
Authors: Kagimura Ricardo , Chacham H. , Schmidt T. M. , Miwa R. H.
Published: November 2003

We perform first-principles calculations for the formation energies of fluorine self-interstitials (Fi−) and fluorine vacancies (VF+) in CaF2–BaF2 superlattices and in the corresponding bulk compounds. In dislocation-free (strained-layer) superlattices, the calculations show that both the Fi− and the VF+ defects are energetically more stable in the CaF2 layers than in the BaF2 layers. If the misfit stress in the superlattices is released (by dislocations, for instance), our results indicate that charge transfer between layers becomes energetically favorable with Fi− in the CaF2 layers and VF+ in the BaF2 layers. This is consistent with recent experiments.
Narrow commensurate states induced by a periodic array of nanoscale antidots in Nb superconductor
Authors: Zhukov A. A. , de Groot P. A. J. , Metlushko V. V. , Ilic B.
Published: November 2003

We examine the interactions of the vortex lattice with a periodic square array of holes in a superconducting Nb film. Using high-resolution magnetic-field measurements of electrical losses, extremely narrow states with a magnetic field width reaching 1% of the matching field value are found at the commensurate points. They are accompanied by pronounced harmonic generation in response to ac modulation of the magnetic field or current. We relate these sharp anomalies to a locked commensurate state with characteristics of a Mott insulator. This offers opportunities for applications of superconducting films with periodic hole arrays as sensitive magnetic field detectors.
Far-infrared multilayer mirrors
Authors: Schiwon Robert , Schwaab Gerhard , Bründermann Erik , Havenith Martina
Published: November 2003

We present a concept for highly reflecting broadband mirrors in the far-infrared. We report on the fabrication of dielectric multilayer mirrors consisting of 1–6 layers of silicon wafers with vacuum gaps in between. By comparison to gold mirrors, we can demonstrate the high reflectivity (>99%) over the frequency range between 40 and 130 cm−1. The measurements are in very good agreement with theoretical predictions.
Electrospun polyaniline/polyethylene oxide nanofiber field-effect transistor
Authors: Pinto N. J. , Johnson A. T. , MacDiarmid A. G. , Mueller C. H. , Theofylaktos N. , Robinson D. C. , Miranda F. A.
Published: November 2003

We report on the observation of field-effect transistor (FET) behavior in electrospun camphorsulfonic-acid-doped polyaniline/polyethylene oxide (PEO) nanofibers. Saturation channel currents are observed at surprisingly low source–drain voltages. The hole mobility in the depletion regime is 1.4×10−4 cm2/V s, while the one-dimensional (1-D) charge density (at zero gate bias) is calculated to be approximately 1 hole per 50 two-ring repeat units of polyaniline, consistent with the rather high channel conductivity (∼10−3 S/cm). Reducing or eliminating the PEO content in the fiber is expected to enhance device parameters. Electrospinning is thus proposed as a simple method of fabricating one-dimensional polymer FETs.
Mid-infrared external-cavity two-segment quantum-cascade laser
Authors: Peng C. , Zhang H. L. , Le H. Q.
Published: November 2003

An external-cavity mid-infrared quantum-cascade laser allows broad and continuous wavelength tuning and modulation when controlled with a combination of grating position and cavity phase on a two-segment optical amplifier chip. The electrically controlled phase segment allows fine wavelength tuning to complement the coarse grating tuning. The 4.8-μm laser was tested with CO gas absorption and wavelength modulation spectroscopy, showing a spectral resolution limited by the laser linewidth of ∼550 MHz, but also allowing sublinewidth modulation of ⩽200 MHz. The tuning range from 4.855 to 4.9 μm (0.57 THz) was limited by the device intrinsic gain.
Patterned growth of single-walled carbon nanotube arrays from a vapor-deposited Fe catalyst
Authors: Peng H. B. , Ristroph T. G. , Schurmann G. M. , King G. M. , Yoon J. , Narayanamurti V. , Golovchenko J. A.
Published: November 2003

Single-walled carbon nanotubes have been grown on a variety of substrates by chemical vapor deposition using low-coverage vacuum-deposited iron as a catalyst. Ordered arrays of suspended nanotubes ranging from submicron to several micron lengths have been obtained on Si, SiO2, Al2O3, and Si3N4 substrates that were patterned on hundred nanometer length scales with a focused ion beam machine. Electric fields applied during nanotube growth allow the control of growth direction. Nanotube circuits have been constructed directly on contacting metal electrodes of Pt/Cr patterned with catalysts. Patterning with solid iron catalyst is compatible with modern semiconductor fabrication strategies and may contribute to the integration of nanotubes in complex device architectures.
Fluctuation and localization of acoustic waves in bubbly water
Authors: Kuo Chao-Hsien , Wang Ken Kang-Hsin , Ye Zhen
Published: November 2003

The fluctuation properties of acoustic localization in bubbly water is explored. We show that the strong localization can occur in such a system for a certain frequency range and sufficient filling fractions of air bubbles. Two fluctuating quantities are considered, that is, the fluctuation of transmission and the fluctuation of the phase of acoustic wave fields. When localization occurs, these fluctuations tend to vanish, a feature able to uniquely identify the phenomenon of wave localization.
Very low chemical disorder in epitaxial NiMnSb films on GaAs(111)B
Authors: Van Roy W. , Wójcik M. , Je̢dryka E. , Nadolski S. , Jalabert D. , Brijs B. , Borghs G. , De Boeck J.
Published: November 2003

Single-crystalline NiMnSb(111) films with negligibly low defect levels have been grown epitaxially on GaAs(111)B using molecular beam epitaxy and characterized by nuclear magnetic resonance. In a film with only 1% deviation from stoichiometry, 1.1% of all Mn atoms is involved in planar defects, ∼0.5% of all Sb sites is occupied by AsSb substitutional atoms, and ∼0.2% of all Sb atoms has a modified environment. Both the average concentration of defects and the interface orientation are compatible with maintaining a half-metallic band structure at the ferromagnet/semiconductor interface, making these films a good candidate for spin injection into a semiconductor.
Direct observation of magnetic bit transition and physical grain boundary in perpendicular recording media
Authors: Yu Jun , Ahner Joachim , Weller Dieter
Published: November 2003

The correlation between magnetic bit transition line and physical grain boundary in a multilayer perpendicular recording medium is examined by combined magnetic force and electrostatic force microscopy. Reducing the scan height results in stronger electrostatic interaction between the tip and the sample disk. This electrostatic interaction causes the topographical information to be superimposed onto the magnetic force image and allows simultaneous imaging of the columnar grains and the magnetic bits. It is found that the magnetic transition follows the physical grain boundaries with high accuracy. The correlation of both is quantified by image processing and a correlation coefficient of 0.92 has been determined.
Long-wavelength emission from nitridized InAs quantum dots
Authors: Kita Takashi , Masuda Yoshitaka , Mori Takayoshi , Wada Osamu
Published: November 2003

A technique to grow InAs quantum dots (QDs) to extend the emission wavelength into 1.3 μm range has been developed. We performed nitridation after growing InAs QDs by molecular-beam epitaxy. During nitridation, the reflection high-energy electron diffraction keeps chevron patterns, as well as streak rods, coming from the wetting layer. A longer-wavelength emission line with a narrower spectral linewidth compared with those of InAs QDs has been observed.
Formation and electron activation energy of self-assembled CdTe quantum wires grown on ZnTe buffer layers
Authors: Kim T. W. , Lee E. H. , Lee K. H. , Kim J. S. , Park H. L.
Published: November 2003

Self-assembled CdTe quantum wires (QWRs) were grown on ZnTe buffer layers by using molecular-beam epitaxy. Atomic force microscopy images showed that preferentially oriented CdTe QWRs were formed on ZnTe buffer layers. The activation energy of the electrons confined in the CdTe QWRs, as obtained from the temperature-dependent photoluminescence spectra, was higher than that of electrons in CdTe/ZnTe quantum wells. These results indicate that self-assembled CdTe QWRs grown on ZnTe buffer layers hold promise for potential applications in optoelectric devices operating in the blue–green region of the spectrum.
Imaging of emission patterns in a T-shaped quantum wire laser
Authors: Takahashi Yasushi , Watanabe Shinichi , Yoshita Masahiro , Itoh Hirotake , Hayamizu Yuhei , Akiyama Hidefumi , Pfeiffer Loren N. , West Ken W.
Published: November 2003

Spatially and spectrally resolved microscopic images of spontaneous and stimulated emissions are imaged at the mirror facets of a GaAs T-shaped quantum wire laser with high uniformity. Laser emission from the one-dimensional ground state reveals a circular image located at the core of a T-shaped optical waveguide but significantly smaller in area than the low power spontaneous emission from the same waveguide. These images unambiguously allow assignment of all spontaneous and laser emissions to the wire ground state and respective intersecting wells in the structure.
Fabrication of elastomeric stamps with polymer-reinforced sidewalls via chemically selective vapor deposition polymerization of poly(p-xylylene)
Authors: Suh Kahp Y. , Langer Robert , Lahann Jörg
Published: November 2003

We report on the preparation of polydimethylsiloxane stamps with selectively grown polymer sidewalls by chemical vapor deposition polymerization of poly(p-xylylene). Using a thin iron layer as an inhibitor, the deposition occurs only on the sidewalls of the features in relief, resulting in a polymer-reinforced stamp. The wetting properties of stamps can be restored after removing the thin iron layer with an acidic solution, which has been verified by pattern transfer to an underlying substrate using molding and microcontact printing.
Structural and optical properties of strain-compensated GaAsSb/GaAs quantum wells with high Sb composition
Authors: Zheng X. H. , Jiang D. S. , Johnson S. , Zhang Y. H.
Published: November 2003

The structural and optical properties of GaAsSb/GaAs quantum wells (QWs) and strain-compensated GaAsP/GaAs/GaAsSb/GaAs/GaAsP QWs grown on a GaAs substrate by molecular beam epitaxy are investigated using high-resolution x-ray diffraction and photoluminescence (PL) measurements. We demonstrated that the insertion of tensile GaAsP layers into the active region of GaAsSb/GaAs QWs effectively improves the structural and optical quality. Even the Sb composition is as high as 0.39. The PL spectra at 11 K and room temperature indicate that the PL peak of strain-compensated QWs has a narrower linewidth and higher intensity in comparison to the sample without strain compensation. The results of PL peak blueshift with increasing excitation show the strain-compensated GaAsSb/GaAs interface characteristic of type-I band alignment.
Intrinsic compensation of silicon-doped AlGaN
Authors: Wagener M. C. , James G. R. , Omnès F.
Published: November 2003

The silicon doping characteristics of AlxGa1−xN were investigated over the x=0.2–0.5 composition range. A combination of Hall and capacitance–voltage measurements indicated a significant deepening of the Si level, as well as a systematic increase in carrier compensation with increasing compositions. Optical isothermal capacitance transient spectroscopy also revealed the presence of two midgap states with concentrations in the low 1017 cm−3 range. The two levels, which are thought to be responsible for the observed compensation, have been assigned to the third and second ionization states of the aluminum vacancy.
Reduction of threading dislocations in crack-free AlGaN by using multiple thin SixAl1−xN interlayers
Authors: Akasaka T. , Nishida T. , Taniyasu Y. , Kasu M. , Makimoto T. , Kobayashi N.
Published: November 2003

Crack-free AlGaN thin films were directly grown on SiC substrates by metalorganic vapor phase epitaxy, and their threading dislocation density was reduced by one order of magnitude using 1–2 nm thick, heavily Si-doped AlN multiple interlayers. The interlayers form SixAl1−xN ternary alloys, where the Si molar fraction ranges typically from 0.07 to 0.17. This technique enables us to grow crack-free AlGaN films, since the film thickness of about 1 μm is much smaller than that required in conventional epitaxial lateral overgrowth techniques. Both termination and looping of threading dislocations were observed near the interlayers using cross-sectional transmission electron microscopy. Light emitting devices with the SixAl1−xN multiple interlayers showed a remarkable improvement in the intensity and spectral width of electroluminescence and the series resistance.
Spatial distribution of yellow luminescence related deep levels in GaN
Authors: Hsu J. W. P. , Schrey F. F. , Ng H. M.
Published: November 2003

Using two-photon excitation, we study the excitation power density dependence and spatial variation of photoluminescence (PL) in GaN films grown by molecular beam epitaxy. Under our experimental conditions, the excitation power density dependence is quadratic for near band-gap emission (NBE) and linear for yellow luminescence (YL), consistent with the YL process being saturated. The PL mapping reveals NBE fluctuations at the domain-size scale while YL is uniform. These results provide strong evidence that the spatial distribution of deep levels associated with YL is uniform; hence, YL is unrelated to dislocations.
Mapping of strain fields about thin film structures using x-ray microdiffraction
Authors: Murray C. E. , Noyan I. C. , Mooney P. M. , Lai B. , Cai Z.
Published: November 2003

Substrate distortions were mapped near pseudomorphically grown SiGe thin film etched lines of various widths from 1.5 to 20 μm on Si(001) and 190 μm diameter Ni dots on Si(111) using reflection x-ray microdiffraction topography. The strain field extended 30–120 times the thickness of the film away from the feature edge. The profile of the enhanced diffracted intensity was found to follow a characteristic curve when the distance from the feature edge is normalized by a mean interaction distance that depends on the feature size. This normalization and the observed strain decay profiles cannot be predicted or modeled using existing micromechanical models.
Improved coherent terahertz emission by modification of the dielectric environment
Authors: Zedler M. , Janke C. , Bolivar P. Haring , Kurz H. , Künzel H.
Published: November 2003

We present a generally applicable approach to enhance the conversion efficiency from optical input to usable coherent output power for THz emitters based on optically excited charge carriers. Guided by numerical simulations, the dielectric environment of the emitter is modified to improve radiation rate and spatial emission characteristics. The modifications comprise a sapphire layer applied on the semiconductor surface and a gold backside metallization. Comparison between a standard InGaAs semiconductor surface field emitter and an optimized version yields an increase of one order of magnitude in coherently emitted THz power. This result is in close agreement with the numerical simulations.
Dielectric characteristics of low-permittivity silicate using electron beam direct patterning for intermetal dielectric applications
Authors: Liu Po-Tsun , Chang T. C. , Tsai T. M. , Lin Z. W. , Chen C. W. , Chen B. C. , Sze S. M.
Published: November 2003

A direct patterning technology of low-permittivity silicate-based polymer is investigated with electron-beam lithography for multilevel interconnections. The smallest feature size of 60 nm for damascene lines can be directly patterned in the silicate film. In this direct patterning, dielectric regions exposed by electron beam are crosslinked and form desirable patterns, while the others are dissolvable in an aqueous solution containing 2.38% tetramethylammonium hydroxide. With an optimum condition of electron-beam lithography, the electron-beam-irradiated silicate exhibits superior dielectric properties than that of the furnace-cured silicate film, due to minimizing the break of Si–H bonds and moisture uptake. The explanation is in agreement with the analyses of Fourier transform infrared spectroscopy and thermal desorption spectroscopy.
Quantitative scanning capacitance spectroscopy
Authors: Brezna W. , Schramboeck M. , Lugstein A. , Harasek S. , Enichlmair H. , Bertagnolli E. , Gornik E. , Smoliner J.
Published: November 2003

In this work, a setup for quantitative scanning capacitance spectroscopy is introduced, where an ultrahigh precision, calibrated capacitance bridge is used together with a commercially available atomic force microscope (AFM). We show that capacitance data measured with this setup are of comparable quality as data obtained on macroscopic metal oxide semiconductor capacitors. In addition, our setup is sensitive enough to resolve the energy distribution of interface traps with the spatial resolution of an AFM. This is an advantage compared to conventional scanning capacitance microscopes, which have a limited energy resolution and only yield qualitative results due to large modulation voltages.
Influence of substitutional carbon incorporation on implanted-indium-related defects and transient enhanced diffusion
Authors: Tan Chung Foong , Chor Eng Fong , Liu Jinping , Lee Hyeokjae , Quek Elgin , Chan Lap
Published: November 2003

It has been demonstrated that, by incorporating a thin ∼20 nm Si1−yCy (with y as low as 0.1%) layer at the deep indium implant end-of-range (EOR) region, the EOR defects and enhanced diffusion behavior associated with indium implant can be eliminated. The Si1−yCy layer was grown epitaxially followed by a silicon epitaxy cap of 60 nm. Indium implantations were performed at 1×1014 cm−2 at 115 keV followed by spike annealing at 1050 °C. The experimentally observed EOR defect and enhanced diffusion elimination are explained based on the undersaturation of implantation-induced silicon interstitials with the presence of substitutional carbon at the Si1−yCy layer.
Reversible axial-strain effect and extended strain limits in Y-Ba-Cu-O coatings on deformation-textured substrates
Authors: Cheggour N. , Ekin J. W. , Clickner C. C. , Verebelyi D. T. , Thieme C. L. H. , Feenstra R. , Goyal A.
Published: November 2003

The dependence of transport critical-current density Jc on axial tensile strain ε was measured at 76 K and self-magnetic field for YBa2Cu3O7−δ (YBCO) coatings on buffered, deformation-textured substrates of pure Ni, Ni–5-at. %-W, and Ni–10-at. %-Cr–2-at. %-W. Expectations have been that the strain tolerance of these composites would be limited by the relatively low yield strains of the deformation-textured substrates, typically less than 0.2%. However, results show that the irreversible degradation of Jc(ε) occurs at a strain equal to about twice the yield strain of the substrate. Therefore, YBCO/Ni-alloy composites may satisfy axial-strain performance requirements for electric devices, including the most demanding applications, motors and generators in which a strain tolerance exceeding 0.25% is needed. Furthermore, the YBCO/Ni–5-at. %-W conductors showed a reversible strain effect, which may be induced by a reversible strain-field broadening around mismatch dislocations at the grain boundaries. This effect may contribute to the unexpectedly large usable strain range of these conductors.
Thermal conductivity in strain symmetrized Si/Ge superlattices on Si(111)
Authors: Chakraborty S. , Kleint C. A. , Heinrich A. , Schneider C. M. , Schumann J. , Falke M. , Teichert S.
Published: November 2003

We have presented systematic cross-plane thermal conductivity (λ) data for the undoped strain-symmetrized Si/Ge superlattices grown on Si(111) with superlattice (SL) period thickness varying from 3.6 to 16 nm. In thin SL period (L⩽7 nm) samples, the data have shown considerable reductions of λ, by more than 50% and 30% compared to the SiGe alloy and to the earlier reported values in (100)-oriented Si/Ge superlattice structures (SLS), respectively. For the thick SL period samples (L>10 nm), λ has shown a tendency to saturate at the SiGe alloy value. This is understood as, with increasing L, the SLS breaks and the SiGe alloying starts to grow. This structural behavior is clearly observed in the cross-plane transmission electron microscope images as well. In addition to these, for the thin SL period (L⩽7 nm) samples, the data have shown a shallow minimum which is attributed to the competing behavior of the wave nature and the classical particle nature of the localized phonons. Nevertheless, the present study of thermal conductivity on undoped strain-symmetrized Si/Ge SLs in (111) orientation suggests that an enhancement of thermoelectric figure-of-merit Z is possible.
Accurate polymer polarizabilities with exact exchange density-functional theory
Authors: Mori-Sánchez Paula , Wu Qin , Yang Weitao
Published: November 2003

The long-standing problem of the large overestimation of polymer polarizabilities in density-functional theory is reexamined and largely solved using an exact exchange method. We have built an accurate optimized effective potential as the sum of a fixed potential and a linear combination of basis sets based on our direct optimization method. This effective potential properly develops a linear counteracting depolarization field, and it significantly improves recent results from approximate optimized potentials. The controversial case of hydrogen chains is now correctly described and the failure of the local density approach is attributed to the large self-interaction error in systems with a non-integer number of electrons.
The Effect of Preheating of Work Material on Chatter During End Milling of Medium Carbon Steel Performed on a Vertical Machining Center (VMC)
Authors: Amin A. K. M. N. , Abdelgadir M.
Published: November 2003

In the present work causes and after effects of chatter have been discussed and attempts have been made to control the occurrence of chatter during end milling operation conducted on a vertical machining center (VMC). Chatter has been treated as a resonance effect and preheating of the incoming work material layers using high frequency induction heating has been tried as a method of controlling chatter. It has been found that preheating results in substantial lowering down of acceleration amplitude of chatter in a wide range of cutting speed. This has facilitated low tool wear intensity, improved machined surface finish and low noise level during machining. Work material properties were practically unaffected due to low preheating temperature.
Engineering Surface Analysis With Different Wavelet Bases
Authors: Fu Shengyu , Muralikrishnan B. , Raja J.
Published: November 2003

Traditional surface texture analysis involves filtering surface profiles into different wavelength bands commonly referred to as roughness, waviness and form. The primary motivation in filtering surface profiles is to map each band to the manufacturing process that generated the part and the intended functional performance of the component. Current trends in manufacturing are towards tighter tolerances and higher performance standards that require close monitoring of the process. Thus, there is a need for finer bandwidths for process mapping and functional correlation. Wavelets are becoming increasingly popular tools for filtering profiles in an efficient manner into multiple bands. While they have broadly been demonstrated as having superior performance and capabilities than traditional filtering, fundamental issues such as choice of wavelet bases have remained unaddressed. The major contribution of this paper is to present the differences between wavelets in terms of the transmission characteristics of the associated filter banks, which is essential for surface analysis. This paper also reviews fundamental mathematics of wavelet theory necessary for applying wavelets to surface texture analysis. Wavelets from two basic categories—orthogonal wavelet bases and biorthogonal wavelet bases are studied. The filter banks corresponding to the wavelets are compared and multiresolution analysis on surface profiles is performed to highlight the applicability of this technique.
Analysis of Heat Transfer Between the Gas Torch and the Plate For the Application of Line Heating
Authors: Shin Jong Gye , Woo Jong Hun
Published: November 2003

Line heating (LH) is a method of forming curved thick plates. Residual deformation of a steel plate is obtained by applying heat to the plate. Heat transfer analysis between a torch and the steel plate is essential to control the residual deformation of the steel plate in LH. This paper presents an analysis of the heat transfer during LH. The mechanism of combustion flame from a torch is simplified as an impinging gas jet. A turbulent thermal fluid flow analysis is employed to determine the temperature field. Results of numerical and experimental approaches are compared to verify the proposed analysis procedure. The parametric analysis shows that the nozzle diameter and the tip clearance are the important factors of the heat flux induced by the flame.
Developments in Microelectromechanical Systems (MEMS): A Manufacturing Perspective
Authors: Tadigadapa Srinivas A. , Najafi Nader
Published: November 2003

This paper presents a discussion of some of the major issues that need to be considered for the successful commercialization of MEMS products. The diversity of MEMS devices and historical reasons have led to scattered developments in the MEMS manufacturing infrastructure. A good manufacturing strategy must include the complete device plan including package as part of the design and process development of the device. In spite of rapid advances in the field of MEMS there are daunting challenges that lie in the areas of MEMS packaging, and reliability testing. CAD tools for MEMS are starting to get more mature but are still limited in their overall performance. MEMS manufacturing is currently at a fragile state of evolution. In spite of all the wonderful possibilities, very few MEMS devices have been commercialized. In our opinion, the magnitude of the difficulty of fabricating MEMS devices at the manufacturing level is highly underestimated by both the current and emerging MEMS communities. A synopsis of MEMS manufacturing issues is presented here.
Thermophysical-Property-Based Selection of Tool Protective Coatings for Dry Machining of Steels
Authors: Grzesik Wit , Nieslony Piotr
Published: November 2003

A method is proposed for determining the optimum cutting conditions in dry turning of carbon and stainless steels using multilayer coated tools for physical criteria such as the maximum temperature of a cutting tool or maximum heat flux. The modified thermal number describing the tool-chip behavior, is introduced. The correlation between the thermal number and the average tool-chip interface temperature and frictional heat flux is examined for both flat-faced and grooved rake configurations. Although not shown here directly, the thermal barrier effect observed for multilayer coatings with an intermediate Al2O3 layer increases the heat partition to the chip at substantially reduced cutting temperatures. The experimental results indicate that thermal and tribological outputs from the interface are sensitive to changes in the thermophysical properties of the workpiece and coating materials, in particular the thermal conductivity, thermal diffusivity and the heat transmission coefficient. It was proven that the described approach makes it possible to select the appropriate cutting conditions based on process constraints such as the heat transfer intensity and temperature on the tool face.
Modeling Forces Including Elastic Recovery for Internal Thread Forming
Authors: Chowdhary Sameer , DeVor Richard E. , Kapoor Shiv G.
Published: November 2003

This paper presents a model that incorporates the effects of elastic recovery for the prediction of thrust and torque experienced by a forming tap during an internal thread forming process. The tap tooth geometry has been parametrized to facilitate a generic representation of the geometry. The model relates the normal and frictional forces experienced by the teeth on both the entry taper and the back taper portions of the tap to contact areas between the teeth and the workpiece. Experiments to calibrate and validate the model are conducted on taps with different tooth geometries. The model-predicted thrust and torque values compare favorably with the experimental value. Elastic recovery is found to significantly increase the thrust and torque produced during form tapping.