bluuebunny/crossref_metadata_2025 huggingface dataset:
Energy resolved noise measurements in quantum well infrared photodetectors
Authors: Yao J. , Chen C. J. , Choi K. K. , Chang W. H. , Tsui D. C.
Published: July 2002

In quantum well infrared photodetectors, the detector dark current usually is composed of a wide range of energies originated from thermionic emission as well as thermally assisted tunneling. Yet, it is a common practice to assign a single noise gain to all electrons irrespective of their energies. This assigned value can only represent the mean since both the hot-electron lifetime and the transit time, whose ratio determines the gain, are energy dependent. In this work, we have resolved the energy dependence of the noise gain using an electron energy filter. We find that although the noise gain increases initially with energy as expected, it reaches a maximum at 0.27 eV above the GaAs conduction band edge, and then decreases and forms a minimum at 0.31 eV. We attribute this decrease to the Γ-L intervalley scattering, which increases the transit time of the electrons.
Exchange-biased ferromagnetic tunnel junctions via reactive evaporation of nickel oxide films
Authors: Shang Chang He , Berera Geetha P. , Moodera Jagadeesh S.
Published: July 2002

Nickel oxide films, prepared by reactive evaporation at room temperature, have shown good properties to be used as an exchange biasing antiferromagnet on ferromagnetic tunnel junctions. For instance, in the case of Co/Al2O3/Co/NiO junction, NiO shows an exchange field of 17 Oe at 295 K, increasing to 174 Oe at 77 K, and the junction magnetoresistances are 17% at 295 K and 22% at 77 K. The temperature dependence of the exchange field and junction magnetoresistance are also investigated between 77 K and 400 K.
Low frequency negative capacitance behavior of molecular beam epitaxial GaAs n-low temperature-i-p structure with low temperature layer grown at a low temperature
Authors: Chen N. C. , Wang P. Y. , Chen J. F.
Published: July 2002

The GaAs sample under study is a n-low temperature-i-p structure grown by molecular beam epitaxy with a low-temperature (LT) layer grown at 300 °C and annealed at 620 °C for 1 h. Admittance measurements on this sample reveal a negative capacitance at low frequency. This work analyzes the origin of the negative capacitance and its corresponding frequency-dependent conductance by combining two current components: charging–discharging current and the inertial conducting current. Analysis results indicate that the activation energies and time constants of both current components closely resemble each other and should correspond to the same trap. Based on the results presented herein, we can conclude that the negative capacitance at low frequency provides evidence of a generation-recombination center with an activation energy of 0.77 eV in the LT layer.
Determination of wurtzite GaN lattice polarity based on surface reconstruction
Authors: Smith A. R. , Feenstra R. M. , Greve D. W. , Shin M.-S. , Skowronski M. , Neugebauer J. , Northrup J. E.
Published: July 2002

We identify two categories of reconstructions occurring on wurtzite GaN surfaces, the first associated with the N face, (0001̄), and the second associated with the Ga face, (0001). Not only do these two categories of reconstructions have completely different symmetries, but they also have different temperature dependence. It is thus demonstrated that surface reconstructions can be used to identify lattice polarity. Confirmation of the polarity assignment is provided by polarity-selective wet chemical etching of these surfaces.
Micro-electromagnets for atom manipulation
Authors: Drndić M. , Johnson K. S. , Thywissen J. H. , Prentiss M. , Westervelt R. M.
Published: July 2002

Micro-electromagnets for atom manipulation have been constructed, including magnetic mirrors (serpentine patterns) and traps (circular patterns). They consist of planar micron-scale Au wires on sapphire substrates fabricated using lithography and electroplating. At liquid nitrogen or helium temperatures in vacuum the wires support currents of several amperes with current density ∼108 A/cm2 and power dissipation ∼10 kW/cm2, and they produce magnetic fields to 0.3 T and gradients to 103 T/cm. The micro-electromagnet mirror was used to deflect a beam of metastable helium atoms at grazing angles ∼0.5 mrad.
Infrared studies of silicon oxide formation in silicon wafers implanted with oxygen
Authors: Ono Haruhiko , Ikarashi Taeko , Ogura Atsushi
Published: July 2002

The formation process of buried oxide in low-dose oxygen-implanted wafers was investigated using Fourier-transform infrared absorption spectroscopy. In the wafers as-implanted with oxygen, the peak position of the Si–O–Si asymmetric stretching mode was observed to be lower in wave numbers for the lower dose samples, in which the oxygen atoms are buried as substoichiometric silicon oxide with small stress. Therefore, we conclude that the frequency shift is not caused by compressive stress but by the substoichiometry of the buried oxide. After annealing at over 600 °C, the buried oxide starts to form stoichiometric silicon dioxide, and completes it at over 1200 °C. However, we also found that some amount of oxygen atoms diffuse out of the wafer at a temperature over 1000 °C.
Annealing effects on carbon-induced germanium dots in silicon
Authors: Schieker S. , Schmidt O. G. , Eberl K. , Jin-Phillipp N. Y. , Phillipp F.
Published: July 2002

The growth of 2.4 monolayers Ge on 0.2 monolayers of predeposited C on Si results in the formation of 15 nm size Ge islands. Fifty stacked layers of these C-induced Ge dots are grown on Si (001) at 460 °C. Different pieces of the wafer are annealed at temperatures between 460 and 950 °C and for times ranging from 1 to 20 min at 850 °C. For temperatures higher than 550 °C, a pronounced energy blueshift and an evolution from one broad photoluminescence peak to two well-resolved lines reflect a gradual transition from quantum dot states to quantum well-like states. As transmission electron microscopy images illustrate, diffusion processes completely smear out the sharp interfaces between the dots and the surrounding Si. An activation energy of only 1.6 eV and temperature-dependent diffusion coefficients are derived from simple model calculations.
Giant magnetoresistance in multilayers electrodeposited on n-Si
Authors: O’Keeffe A. P. , Kasyutich O. I. , Schwarzacher W. , de Oliveira L. S. , Pasa A. A.
Published: July 2002

Co–Ni–Cu/Cu multilayers have been electrodeposited directly onto n-type Si substrates. This removes the need to use a seed-layer deposited by some other method as part of the growth process and makes electrodeposition a significantly more convenient method for fabricating films that exhibit giant magnetoresistance (GMR). A maximum GMR of over 10% and a sensitivity of over 0.04%/Oe were recorded. The GMR and sensitivity of the multilayers both increase with increasing Cu layer thickness.
High-speed photorefractive polymer composites
Authors: Wright D. , Dı́az-Garcı́a M. A. , Casperson J. D. , DeClue M. , Moerner W. E. , Twieg R. J.
Published: July 2002

Two photorefractive polymer composites are presented that exhibit the fastest response times reported to date by an order of magnitude (τg≈5 ms at 1 W/cm2), while maintaining large gain coefficients (Γ≈230 and 130 cm−1). These materials show promise for video-rate optical processing applications. The factors limiting the photorefractive speed in these materials are investigated.
Spectroscopic ellipsometry characterization of (InGa)N on GaN
Authors: Wagner J. , Ramakrishnan A. , Behr D. , Obloh H. , Kunzer M. , Bachem K.-H.
Published: July 2002

Pseudodielectric function spectra of hexagonal (InGa)N epitaxial layers on GaN were obtained by spectroscopic ellipsometry and compared with photoreflection spectra. Composition and thickness of the InxGa1−xN layers grown by metalorganic chemical vapor deposition, were varied between 0.04⩽x⩽0.10 and 15–60 nm, respectively. The pseudodielectric function exhibits a clear maximum at the fundamental gap energy of the (InGa)N, which allows a determination of the In content via the composition dependence of that gap energy. The pseudodielectric function spectrum of a complete GaN/(InGa)N/(AlGa)N/GaN light-emitting diode structure shows maxima arising from fundamental gap interband transitions of all constituent layers including the (InGa)N active region.
Compensation doping of Ba0.7Sr0.3TiO3 thin films
Authors: Copel M. , Baniecki J. D. , Duncombe P. R. , Kotecki D. , Laibowitz R. , Neumayer D. A. , Shaw T. M.
Published: July 2002

We have investigated the effects of Mn impurities on Ba0.7Sr0.3TiO3 thin films using x-ray photoemission spectroscopy. Mn acts as an electron acceptor, compensating for the charge density found in nominally undoped films. This causes a greatly increased depletion width in acceptor-doped films. We also present evidence that acceptor-doped films have an increased barrier to thermionic emission of electrons from Pt contacts into the dielectric. This may explain the decrease in leakage current observed in some acceptor-doped titanates.
Enhanced tunnel magnetoresistance at high bias voltage in double-barrier planar junctions
Authors: Montaigne F. , Nassar J. , Vaurès A. , Van Dau F. Nguyen , Petroff F. , Schuhl A. , Fert A.
Published: July 2002

Single Co/Al2O3/NiFe and double Co/Al2O3/Co/Al2O3/NiFe planar tunnel junctions were grown by sputtering and subsequently patterned in a four-step process using optical lithography. The Al2O3 barriers are formed by radio frequency plasma oxidation of 1.5 nm aluminum layers. The double junctions exhibit three clear resistance levels depending on the relative configuration of the magnetizations. Both single and double junctions exhibit maximum magnetoresistance (MR) ratios above 10% at room temperature and 20% at 30 K and a decrease of MR with increasing bias voltage. With regard to its low bias value, the MR is reduced by a factor of 2 at 0.26 V for the single junctions and at values above 0.8 V for the double junctions. The decay of the MR of double junctions with bias voltage is significantly slower than expected from two independent junctions in series.
Optically induced angular alignment of birefringent micro-objects by linear polarization
Authors: Higurashi E. , Sawada R. , Ito T.
Published: July 2002

Fluorinated polyimide micro-objects having large birefringence of Δn=−0.13 (refractive index n1=1.62, n2=1.49 for the wavelength λ=1.06 μm), which were fabricated by reactive ion etching and suspended in water (n=1.33), were trapped and angularly aligned in a single Gaussian laser beam (λ=1.06 μm, power P>3 μW). The optically induced angular alignment effect is caused by the transfer of spin angular momentum due to the interaction between their birefringence and the linearly polarized light.
Fabrication and room-temperature characterization of a silicon self-assembled quantum-dot transistor
Authors: Choi B. H. , Hwang S. W. , Kim I. G. , Shin H. C. , Kim Yong , Kim E. K.
Published: July 2002

A quantum-dot transistor based on silicon self-assembled quantum dots has been fabricated. The device shows staircases and oscillations in the drain current at room temperature. These data are interpreted as due to single electron tunneling through the dots located in the shortest current path between the source and the drain electrodes. The dot size calculated from the data is ∼7 nm, which is consistent with the size of the self-assembled dots incorporated in the transistor.
Staggered to straddling band lineups in InAs/Al(As, Sb)
Authors: Bhargava S. , Blank H.-R. , Hall E. , Chin M. A. , Kroemer H. , Narayanamurti V.
Published: July 2002

Ballistic electron emission spectroscopy (BEES) has been used to study both the conduction and valence band offsets between InAs and AlAsSb. With the addition of As to AlSb, the conduction band offset between it and InAs has been found to decrease despite the increase in the band gap. The resulting increase in the valence band causes the InAs/Al(As, Sb) band lineup to change from a staggered (type II) to a straddling (type I). Both room-temperature and low-temperature (77 K) BEES spectra have been taken to determine the band offsets.
Mode spacing “anomaly” in InGaN blue lasers
Authors: Jiang H. X. , Lin J. Y.
Published: July 2002

An important experimental observation in InGaN laser diodes (LDs), which is not yet fully understood, is that the measured mode spacing of the lasing spectra could be one order of magnitude larger than that “calculated” from the known cavity length. The aim of this letter is to shed light on the nature of the mode spacing “anomaly” in InGaN LDs. We have derived a formula which accurately determines the mode spacing in InGaN LDs. Our analysis has shown that the discrepancy between the “expected” and observed mode spacing is due to the effect of carrier-induced reduction of the refractive index under lasing conditions and this discrepancy decreases and naturally disappears as the threshold carrier density required for lasing decreases. Since the carrier-induced reduction of the refractive index is expected only from an electron–hole plasma state, our results naturally imply that electron–hole plasma recombination provides the optical gain in InGaN LDs, like in all other conventional III–V semiconductor lasers. The implications of our results on the design of nitride optoelectronic devices are also discussed.
Mobility enhancement through homogeneous nematic alignment of a liquid-crystalline polyfluorene
Authors: Redecker M. , Bradley D. D. C. , Inbasekaran M. , Woo E. P.
Published: July 2002

Homogeneous alignment of poly(9,9′-dioctylfluorene) films on rubbed polyimide results in a more than one order of magnitude increase in time-of-flight hole mobility normal to the alignment direction. We find μ=8.5±1×10−3 cm2/V s at an electric field of E=104 V/cm. Hole transport is found to be nondispersive, indicating a low degree of energetic disorder.
Domain structure in Fe-implanted Nd2Fe14B magnets
Authors: Gao Youhui , Zhu Jinghan , Weng Yuqing , Han Baoshan
Published: July 2002

The surface microstructure of Nd2Fe14B magnets was modified by Fe ion implantation. Their domain structures were studied by magnetic force microscopy. Corrugation and spike pattern domains were found on the (001) plane of unimplanted Nd2Fe14B. After implantation, a domain structure transition from complex maze to stripe pattern was found. The width of the stripe domain increases with increasing Fe dose. It indicates that the magnetic moments in the implanted layer turn out of their easy axis. The magnetization configurations in the implanted magnets were calculated using a micromagnetics method. The theoretical results agree well with the experimental results.
Low-field magnetoresistance anisotropy in ultrathin Pr0.67Sr0.33MnO3 films grown on different substrates
Authors: Wang H. S. , Li Qi , Liu Kai , Chien C. L.
Published: July 2002

We have conducted a comparative study of the strain effect on the anisotropic low-field magnetoresistance (LFMR) in ultrathin Pr0.67Sr0.33MnO3 films epitaxially grown on LaAlO3(LAO), NdGaO3(NGO), and SrTiO3(STO) substrates. Distinctive LFMR effects have been observed on films with compressive (on LAO), tensile (on STO), and nearly zero (on NGO) strains. The films with compressive strain show very large negative LFMR and MR hysteresis when a magnetic field is applied perpendicular to the film plane, while those with tensile strain show positive LFMR for the same field orientation. All samples show negative MR in a parallel magnetic field. These results can be qualitatively interpreted based on the strain-induced magnetic anisotropy.
Deep levels in p-type InGaAsN lattice matched to GaAs
Authors: Kwon D. , Kaplar R. J. , Ringel S. A. , Allerman A. A. , Kurtz Steven R. , Jones E. D.
Published: July 2002

Deep-level transient spectroscopy measurements were utilized to investigate deep-level defects in metal–organic chemical vapor deposition-grown, unintentionally doped p-type InGaAsN films lattice matched to GaAs. The as-grown material displayed a high concentration of deep levels distributed within the band gap, with a dominant hole trap at Ev+0.10 eV. Postgrowth annealing simplified the deep-level spectra, enabling the identification of three distinct hole traps at 0.10, 0.23, and 0.48 eV above the valence-band edge, with concentrations of 3.5×1014, 3.8×1014, and 8.2×1014 cm−3, respectively. A direct comparison between the as-grown and annealed spectra revealed the presence of an additional midgap hole trap, with a concentration of 4×1014 cm−3 in the as-grown material. The concentration of this trap is sharply reduced by annealing, which correlates with improved material quality and minority-carrier properties after annealing. Of the four hole traps detected, only the 0.48 eV level is not influenced by annealing, suggesting this level may be important for processed InGaAsN devices in the future.
The relation of active nitrogen species to high-temperature limitations for (0001̄) GaN growth by radio-frequency-plasma-assisted molecular beam epitaxy
Authors: Ptak A. J. , Millecchia M. R. , Myers T. H. , Ziemer K. S. , Stinespring C. D.
Published: July 2002

A reduced growth rate for plasma-assisted molecular beam epitaxy of GaN often limits growth to temperatures less than 750 °C. The growth rate reduction can be significantly larger than expected based on thermal decomposition. Conditions producing a flux consisting predominantly of either atomic nitrogen or nitrogen metastables have been established using various radio-frequency sources. The use of atomic nitrogen, possibly coupled with the presence of low-energy ions, is associated with the premature decrease in growth rate. When the active nitrogen flux consists primarily of nitrogen metastables, the temperature dependence of the decrease is more consistent with decomposition rates. A significant improvement in electrical properties is observed for growth with molecular nitrogen metastables.
Photoluminescence of indium–oxide nanoparticles dispersed within pores of mesoporous silica
Authors: Zhou Huijuan , Cai Weiping , Zhang Lide
Published: July 2002

Photoluminescence study was performed on indium–oxide (INO) nanoparticles dispersed within pores of mesoporous silica, annealed at different temperatures (from 500 to 850 °C). It was found that, for the dispersed INO nanoparticles, there exists a broad luminescence band, consisting of three peaks at about 430, 480, and 520 nm, spanning the whole visible region, in contrast to the aggregates of INO nanoparticles which exhibit no luminescence. This band increases with rise of annealing temperature up to 650 °C, at which the band reaches maximum. When annealing temperature is 700 °C, the whole band redshifts and peaks at about 545 nm. Annealing at higher temperatures leads to additional redshift and decrease of the luminescence band. It has been shown that the luminescence in this study is associated with the size and the structure of the INO particles within pores of porous silica. The peaks at 430, 480, and 520 nm originate from amorphous INO particles about 2, 4, and 6 nm in diameter. The 545 nm peak is attributed to crystallized INO particles with larger size.
Nucleation layer microstructure, grain size, and electrical properties in GaN grown on a-plane sapphire
Authors: Twigg M. E. , Henry R. L. , Wickenden A. E. , Koleske D. D. , Culbertson J. C.
Published: July 2002

We have investigated the microstructural origins of center-to-edge differences in the electrical properties of GaN films grown on a-plane sapphire via metal–organic vapor-phase epitaxy. Using cross-sectional transmission electron microscopy, we have observed that the grain size at the wafer edge is approximately 1 μm, whereas the grain size in the wafer center ranges from 0.1 to 0.5 μm. The smaller grain size at the wafer center is traced to a higher density of extended defects in the AlN nucleation layer: defects which, in turn, act as nucleation sites for GaN grain growth.
Effect of external feedback on lasing in random media
Authors: Cao H. , Zhao Y. G. , Liu X. , Seelig E. W. , Chang R. P. H.
Published: July 2002

We have studied the effect of external feedback on random laser action in ZnO polycrystalline thin films. Reinjection of light into scattering-formed cavities strongly influences modes, intensity, and threshold of random lasers. We have compared the effect of external feedback from the side of the film and that from the film surface. Our study opens the possibility of controlling random laser frequencies by external feedback.
GaAs/AlGaAs superlattice quantum cascade lasers at λ≈13 μm
Authors: Strasser G. , Gianordoli S. , Hvozdara L. , Schrenk W. , Unterrainer K. , Gornik E.
Published: July 2002

We report the realization of an injection laser based on intraband transitions in a finite AlGaAs/GaAs superlattice. The active material is a 30 period sequence of injectors/active regions made from AlGaAs/GaAs quantum wells. By an applied electric field, electrons are injected into the second miniband of a chirped superlattice and relax radiative to the lowest miniband. At a heat-sink temperature of 10 K, the laser emission wavelength is 12.9 μm with peak optical powers exceeding 100 mW and a threshold current density of 9.8 kA/cm2. The maximum operating temperature is 50 K. For this device, a waveguide consisting of heavily doped GaAs cladding and low doped core layers has been used as a plasma-enhanced confinement.
Adaptive all-order dispersion compensation of ultrafast laser pulses using dynamic spectral holography
Authors: Ding Y. , Weiner A. M. , Melloch M. R. , Nolte D. D.
Published: July 2002

The time-varying dispersion of ultrafast laser pulses can be self-adaptively stabilized using real-time dynamic spectral holography in semiconductor photorefractive quantum wells. Dispersion of all orders is compensated by forming a dynamic spectral-domain hologram of a signal pulse (that has a time-varying dispersion) referenced to a stable clock pulse. The hologram is read out using forward-scattering phase conjugation to remove phase distortion to all orders, including drift in the time of flight. We have achieved adaptive cancellation of time-of-flight excursions up to ±15 ps to an accuracy of ±15 fs with a compensation bandwidth of 1 kHz.
2.6 μm optically pumped vertical-cavity surface-emitting laser in the CdHgTe system
Authors: Roux C. , Hadji E. , Pautrat J.-L.
Published: July 2002

We present results of a midinfrared vertical-cavity surface-emitting laser operating at 2.63 μm made of CdHgTe alloys. The resonator is constituted of two dielectric YF3/ZnS mirrors deposited after growth and after removal of the substrate. The top mirror reflectivity is increased step by step by deposition of an additional stack period allowing us to study the effect of mirror loss on lasing properties. With a reflectivity of 98.1% lasing is observed up to 190 K. The pulsed threshold power is 1.7 kW/cm2 at 80 K. The characteristic temperature is T0=113 K.
Raman spectroscopy of electrochemically self-assembled CdS quantum dots
Authors: Balandin A. , Wang K. L. , Kouklin N. , Bandyopadhyay S.
Published: July 2002

We report a Raman spectroscopy investigation of electrochemically self-assembled quasiperiodic arrays of CdS quantum dots with characteristic feature size of 10 nm. The dots were synthesized using electrochemical deposition of CdS into a porous anodized alumina film. Polarization-dependent Raman scattering study over an extended frequency range reveals the quantization of electronic states in the conduction band and intersubband transitions. Raman peaks observed at 2919 and 3050 cm−1 are attributed to transitions between the lowest two subbands. The results suggest that quantum dot arrays, produced by inexpensive robust electrochemical means, may be suitable for infrared detector applications.
Investigations into local piezoelectric properties by atomic force microscopy
Authors: Durkan C. , Chu D. P. , Migliorato P. , Welland M. E.
Published: July 2002

We describe nanoscale characterization of sol-gel produced ferroelectric thin films of lead–zirconate–titanate. We have performed quantitative localized measurements of surface polarization charge density using atomic force microscopy techniques in conjunction with electric field calculations. We show that domains with radii of 40 nm may by written and subsequently characterized, and we analyze the dependence of domain size on write voltage and write time, and show that surface contaminants influence the formation of domains.
Patterning 100 nm features using deep-ultraviolet contact photolithography
Authors: Goodberlet James G.
Published: July 2002

The extension of contact photolithography to pattern features at the 100 nm level, and below, is described. Isolated lines, nested L’s, and gratings with 100 nm linewidths are patterned using a conformable embedded-amplitude mask, a trilayer resist stack on the substrate, and a deep-ultraviolet radiation source having a wavelength of λ∼220 nm. A broad exposure latitude of ±21% for a linewidth control of ±10% is measured. Preliminary calculations indicate a practical resolution limit for the lithographic process. The patterning process is reliable and repeatable.
Interband optical absorption in free standing layer of Ga0.96In0.04As0.99N0.01
Authors: Perlin Piotr , Wiśniewski Przemek , Skierbiszewski Czesław , Suski Tadeusz , Kamińska Eliana , Subramanya Sudhir G. , Weber Eicke R. , Mars Dan E. , Walukiewicz Wladek
Published: July 2002

We have measured the interband optical absorption of a free-standing sample of Ga0.96In0.04As0.99N0.01 in a wide energy range from 1 to 2.5 eV. We found that the fundamental absorption edge is shifted by 150 meV towards lower energies, and the absorption coefficient measured at higher energies exhibits substantial reduction comparing to that of GaAs. By removing the GaAs substrate, we were able to get an experimental insight into the interband optical transitions and the density of state in this material. The changes can be understood within the band anticrossing model predicting the conduction band splitting. New absorption edges associated with optical transitions from the spin-orbit split off band to the lower conduction subband (1.55 eV) and from the top of the valence band to the upper subband (1.85 eV) are observed.
Comparison of spontaneous and piezoelectric polarization in GaN/Al0.65Ga0.35N multi-quantum-well structures
Authors: Hogg R. A. , Norman C. E. , Shields A. J. , Pepper M. , Iizuka N.
Published: July 2002

We report a luminescence study of GaN/Al0.65Ga0.35N multi-quantum-well structures. The surface of the samples exhibits microcracking allowing the same quantum well to be measured under two different strain conditions. We can accurately describe the emission energies in the two strain conditions by considering piezoelectric polarization alone in contrast to the theoretical prediction that spontaneous polarization effects should dominate.
Exciton-related photoluminescence in homoepitaxial GaN of Ga and N polarities
Authors: Kirilyuk V. , Zauner A. R. A. , Christianen P. C. M. , Weyher J. L. , Hageman P. R. , Larsen P. K.
Published: July 2002

A photoluminescence (PL) study of GaN homoepitaxial layers grown by metal–organic chemical-vapor deposition demonstrates the high optical quality of N-face layers deposited on vicinal (0001̄) GaN substrates. In contrast to broad PL emission in exact (0001̄) layers, narrow-bound (0.9 meV) and free- (A and B) excitonic transitions are observed. By following the PL spectra as a function of temperature and excitation power, the main optical transitions in the Ga- and the misoriented N-face layers are found to be the same. Observed differences are related to the distinct creation of donor and acceptor states in the samples of different polarities.
Field emission from quasi-aligned SiCN nanorods
Authors: Tarntair F. G. , Wen C. Y. , Chen L. C. , Wu J.-J. , Chen K. H. , Kuo P. F. , Chang S. W. , Chen Y. F. , Hong W. K. , Cheng H. C.
Published: July 2002

We report on the preparation and field emission properties of quasi-aligned silicon carbon nitride (SiCN) nanorods. The SiCN nanorods are formed by using a two-stage growth method wherein the first stage involves formation of a buffer layer containing high density of nanocrystals by electron cyclotron resonance plasma enhanced chemical vapor deposition and the second stage involves using microwave plasma enhanced chemical vapor deposition for high growth rate along a preferred orientation. It should be noted that growth of the SiCN nanorods is self-mediated without the addition of any metal catalyst. Scanning electron microscopy shows that the SiCN nanorods are six-side-rod-shaped single crystals of about 1–1.5 μm in length and about 20–50 nm in diameter. Energy dispersive x-ray spectrometry shows that the nanorod contains about 26 at. % of Si, 50 at. % of C, and 24 at. % of N. Characteristic current–voltage measurements indicate a low turn-on field of 10 V/μm. Field emission current density in excess of 4.5 mA/cm2 has been observed at 36.7 V/μm. Moreover, SiCN nanorods exhibited rather stable emission current under constant applied voltage.
Interband luminescence and absorption of GaNAs/GaAs single-quantum-well structures
Authors: Sun B. Q. , Jiang D. S. , Luo X. D. , Xu Z. Y. , Pan Z. , Li L. H. , Wu R. H.
Published: July 2002

We have investigated the interband electron transitions in a GaNAs/GaAs single quantum well (QW) by photoluminescence and absorption spectra. The experimental results show that the dominant photoluminescence at low temperature and high excitation intensity originates from transitions within the GaNAs layer. The interband transition energy for QWs with different well widths can be well fitted if a type-II band line up of GaNAs/GaAs QWs is assumed.
Heterogeneously integrated organic light-emitting diodes with complementary metal–oxide–silicon circuitry
Authors: Mathine D. L. , Woo H. S. , He W. , Kim T. W. , Kippelen B. , Peyghambarian N.
Published: July 2002

Top-emitting arrays of organic light-emitting diodes (OLEDs) have been fabricated and demonstrated on complementary metal–oxide–silicon (CMOS) circuitry. The 8×8 array of OLEDs is composed of 90 μm micropixels with a 55 μm separation. The OLEDs are based on an emitting layer of tris-(8-hydroxyquinoline)aluminum (Alq3) doped with coumarin 6 to provide green light emission. A layer of N,N′-diphenyl-N, N′-bis(3-methylphenyl)1-1′-biphenyl 1-4, 4′-diamine (TPD) was used as a hole transport layer and poly(ethylenedioxythiophene) doped with polystyrenesulfonate was used as a buffer layer between the TPD and the CMOS anode metal. Bright light was emitted through a semitransparent Mg:Ag cathode when the micropixel was driven by an individual current source.
Silicon monolithic microchannel-cooled laser diode array
Authors: Skidmore J. A. , Freitas B. L. , Crawford J. , Satariano J. , Utterback E. , DiMercurio L. , Cutter K. , Sutton S.
Published: July 2002

A monolithic microchannel-cooled laser diode array is demonstrated that allows multiple diode-bar mounting with negligible thermal cross talk. The heat sink comprises two main components: a wet-etched Si layer that is anodically bonded to a machined glass block. The continuous wave (cw) thermal resistance of the 10 bar diode array is 0.032 °C/W, which matches the performance of discrete microchannel-cooled arrays. Up to 1.5 kW/cm2 is achieved cw at an emission wavelength of ∼808 nm. Collimation of a diode array using a monolithic lens frame produced a 7.5 mrad divergence angle by a single active alignment. This diode array offers high average power/brightness in a simple, rugged, scalable architecture that is suitable for large two-dimensional areas.
Experimental demonstration of clocked single-electron switching in quantum-dot cellular automata
Authors: Orlov Alexei O. , Amlani Islamshah , Kummamuru Ravi K. , Ramasubramaniam Rajagopal , Toth Geza , Lent Craig S. , Bernstein Gary H. , Snider Gregory L.
Published: July 2002

A device representing a basic building block for clocked quantum-dot cellular automata architecture is reported. Our device consists of three floating micron-size metal islands connected in series by two small tunnel junctions where the location of an excess electron is defined by electrostatic potentials on gates capacitively coupled to the islands. In this configuration, the middle dot acts as an adjustable Coulomb barrier allowing clocked control of the charge state of the device. Charging diagrams of the device show the existence of several operational modes, in good agreement with theory. The clocked switching of a single electron is experimentally demonstrated and advantages of this architecture are discussed.
Diffraction of guided optical waves by surface acoustic waves in GaN
Authors: Rimeika R. , Ciplys D. , Gaska R. , Yang J. W. , Khan M. A. , Shur M. S. , Towe E.
Published: July 2002

We report on acousto-optical diffraction of the guided optical modes (λ=633 nm) in GaN layers grown on (0001) sapphire substrates. The diffraction of both transverse electric (TE) and transverse magnetic (TM) modes has been observed. The dependence of the diffraction efficiency on the surface acoustic wave (SAW) power is in good agreement with Raman-Nath theory. The values of acousto-optic figure of merit, M2, and effective photoelastic constants p have been determined. For the SAW propagating in [11̄00] direction, the extracted values are M2=1.54×10−15 s3/kg, p=0.061 (TE modes) and M2=1.95×10−15 s3/kg, p=0.066 (TM modes). These values of M2 correspond to 1 and 1.3 of the acousto-optic figure for fused quartz. The obtained results demonstrate the potential of GaN-based structures for the development of blue–ultraviolet acousto-optical devices.
Simultaneous liquid viscosity and density determination with piezoelectric unimorph cantilevers
Authors: Shih Wan Y. , Li Xiaoping , Gu Huiming , Shih Wei-Heng , Aksay Ilhan A.
Published: July 2002

We have examined both experimentally and theoretically a piezoelectric unimorph cantilever as a liquid viscosity-and-density sensor. The fabricated piezoelectric unimorph consisted of a PbO⋅ZrO2⋅TiO2 (PZT) layer on a thin stainless-steel plate. In addition to a driving electrode, a sensing electrode was placed on top of the PZT layer, permitting the direct measurement of the resonance frequency. The cantilever was tested using water–glycerol solutions of different compositions. In all three of the tested modes, the resonance frequency decreased while the width of the resonance peak increased with increasing glycerol content. To account for the liquid effect, we consider the cantilever as a sphere of radius R oscillating in a liquid. By including the high and low frequency terms in the induced mass and the damping coefficient of the liquid, we show that for a given liquid density and viscosity the oscillating-sphere model predicts a resonance frequency and peak width that closely agree with experiment. Furthermore, the viscosity and the density of a liquid have been determined simultaneously using the experimentally measured resonance frequency and peak width as inputs to the oscillating-sphere model. The calculated liquid viscosity and density closely agreed with the known values, indicating that our cantilever-based sensor is effective in determining viscosity and density, simultaneously. We also show that scaling analysis predicts an increase in the width of the resonance peak with decreasing cantilever size, an observation in agreement with the large peak widths observed for microcantilevers.
Magnetopolaron effect in CdTe cylindrical quantum dot
Authors: Charrour R. , Bouhassoune M. , Fliyou M. , Bria D. , Nougaoui A.
Published: July 2002

This study analyzes the role of the magnetic field and the polaron effect on the binding energy of shallow donor impurities in a cylindrical quantum dot (QD), where the magnetic field is applied in the axial direction. The interactions of carrier charges (electron and ion) with the confined longitudinal optical phonons as well as the side surface and top surface optical phonons are considered. Calculations are made using the effective-mass approximation within the variational approach for an infinite confinement potential at all surfaces of the system. Results are obtained as a function of the dot sizes (radius and height) and the donor impurity position for several magnetic field strengths. Emphasis is put on the dependence of the polaronic correction on the magnetic field and the impurity position. The presented results can be useful to control the spatial distribution of the donor center in the QD.
Self-assembled Ge nanostructures on polymer-coated silicon: Growth and characterization
Authors: Das Amal K. , Kamila J. , Dev B. N. , Sundaravel B. , Kuri G.
Published: July 2002

Self-assembled Ge nanoparticles have been grown on polymer-coated Si substrates by thermal evaporation under high vacuum utilizing the nonwetting condition given by the surface free-energy relation σGe≫σpolymer. The nanostructures have been characterized by Raman spectroscopy, atomic-force microscopy (AFM), and optical microscopy. Raman spectrum shows a prominent Ge–Ge vibration peak at 302 cm−1. AFM and optical microscopy show the formation of isolated Ge islands (≲100 nm base, ≲25 nm height), nanowires (160 nm base, 25 nm height), and islands in linear chains. The possibility of embedding such nanostructures in waveguide structures are discussed.
Critical behavior of lattice polymers studied by Monte Carlo simulations
Authors: Yan Qiliang , de Pablo Juan J.
Published: July 2002

A newly developed expanded grand-canonical formalism is applied to locate the critical point of systems of long polymeric molecules. Two polymer systems are investigated in this work; the first consists of chains in a simple cubic lattice, the second consists of bond-fluctuating molecules. For the former we simulate molecules of up to 16 000 sites, and for the latter we study molecules of up to 500 sites. These chain lengths are well above those investigated by all prior simulation studies of critical phenomena in polymer solutions. Critical parameters are determined as a function of chain length by means of field-mixing finite-size scaling techniques. Our results for the scaling behavior of the critical temperature are consistent with literature values. Our results for the scaling of the critical density, however, indicate that the corresponding critical exponent is higher than that reported by previous authors. The leading logarithmic term of the finite-chain-length correction to the critical density is confirmed by our results.
GaN homoepitaxy by metalorganic chemical-vapor deposition on free-standing GaN substrates
Authors: Miskys C. R. , Kelly M. K. , Ambacher O. , Martı́nez-Criado G. , Stutzmann M.
Published: July 2002

Lattice-matched GaN layers are grown by metalorganic chemical-vapor deposition on free-standing GaN substrates, which were fabricated by laser-induced liftoff of 300-μm-thick films grown by hydride vapor-phase epitaxy. Pretreatment of the free-standing films before the homoepitaxial growth of GaN involved mechanical polishing of the Ga-face surface and a final Cl-based plasma etch. By a combination of high-resolution x-ray diffraction, atomic-force microscopy, as well as Raman and photoluminescence spectroscopy, the structural and optical properties of the lattice-matched GaN layers are characterized. X-ray diffraction patterns of (0002), (0004), and (0006) reflexes with a full width at half maximum (FWHM) of as low as 20 arcsec are obtained. The dislocation density is determined to be 2×107 cm−2 and the surface morphology is dominated by bilayer steps with terraces of about 200 nm. The lattice mismatch between the GaN substrate and the homoepitaxial layer is below 3×10−5, resulting in a very narrow FWHM of the excitonic luminescence of 0.5 meV.
Formation and characterization of neutral krypton and xenon hydrides in low-temperature matrices
Authors: Lundell J. , Khriachtchev L. , Pettersson M. , Räsänen M.
Published: July 2002

A family of rare-gas-containing hydrides HXY (where X=Kr or Xe, and Y is an electronegative fragment) is described. These molecules are experimentally prepared in low-temperature matrices by photodissociation of a hydrogen-containing HY precursor and thermal mobilization of the photodetached hydrogen atoms. The neutral HXY molecules are formed in a concerted reaction H+Y→HXY. Experimental evidence for the formation of these species is essentially based on strong infrared absorption bands that appear after annealing of the photolyzed matrices and are assigned to the H-X stretch of the HXY molecules. Computationally, the formation of these HXY molecules decreases the H-X distance by a factor of ⩾2 from its van der Waals value, which emphasizes their true chemical bonding, possessing both covalent and ionic contributions. The estimated dissociation energies vary from 0.4 to 1.4 eV and hold promise for forthcoming observation of these molecules in the gas phase. The experiments with the HXY molecules widen our knowledge on solid-state photolysis dynamics of hydrogen-containing species. In particular, the photolysis of small HY hydrides in solid Xe seems to be a quite local process, and the accompanying losses of H atoms play a minor role.
Successive generation of sounds by shock–strong vortex interaction
Authors: Inoue Osamu , Takahashi Yasuo
Published: July 2002

The development of a flow field and the generation of sound due to the interaction between a vortex ring and a shock wave are studied numerically. The axisymmetric, unsteady, compressible Navier–Stokes equations are solved by a finite difference method. The results show that, when the vortex ring moves in the opposite direction to the shock wave, the interaction produces reflected shock waves, first. Then, the reflected shock waves interact with the vortex ring, and new rarefaction and compression waves are produced. The new compression waves interact with the vortex ring again, resulting in the further generation of rarefaction and compression waves. As the strength of the vortex ring is increased, this process is repeated and the pressure waves are generated successively.
Electron magnetohydrodynamic turbulence in a high-beta plasma. II. Single point fluctuation measurements
Authors: Urrutia J. M. , Stenzel R. L.
Published: July 2002

A magnetic void is created by high electron pressure in a large nonuniform laboratory plasma. A strong instability is observed in regions of high pressure and magnetic field gradients. It is associated with the electron diamagnetic drift through the essentially unmagnetized ions. Its spectrum is broad and peaks near the lower hybrid frequency. The coupled fluctuations in density, electron temperature, plasma potential, and magnetic field are measured with probes and cross-correlated. The temporal correlation extends only over 1–2 oscillations. The fluctuations propagate in the direction of the electron diamagnetic drift but at the lower ion acoustic speed. In the saturated regime of the instability, the fluctuation waveforms are highly nonlinear. Density cavities with δn/n≃−40% are formed with steepened density rise at the trailing edge. The associated high pressure gradient forms a diamagnetic current sheet. Positive density perturbations are smaller (δn/n⩽20%), broader, and produce regions of weak magnetic fields where the electrons become nearly unmagnetized. Amplitude distributions of nonlinear density, magnetic field, and current waveforms are evaluated. The three-dimensional magnetic field fluctuations are analyzed with hodograms. The direction of the average wave vector points essentially across the mean field in the direction of the diamagnetic drift. The magnetic fluctuations can be interpreted as highly oblique electron whistlers, the density fluctuations as sound waves, but both modes are coupled in a high-beta plasma. Fluctuations in the electric and magnetic fields lead to a time-averaged electron drift, i.e., anomalous transport, across the mean field.
Electron magnetohydrodynamic turbulence in a high-beta plasma. III. Conditionally averaged multipoint fluctuation measurements
Authors: Stenzel R. L. , Urrutia J. M.
Published: July 2002

A large discharge plasma is generated whose electron pressure exceeds that of an external magnetic field. A magnetic cavity exists in the plasma interior. The ions are unmagnetized while the electron magnetization varies from complete to none. In the region of pressure and field gradients a strong instability is observed. It is a cross-field instability driven by the electron diamagnetic drift through the unmagnetized ions, creating large density and magnetic field fluctuations near the lower hybrid frequency that propagate at the sound speed in the diamagnetic drift direction. The basic plasma parameters leading to the instability have been presented in Part I of three companion papers. Spectra, correlations, amplitude distributions, and magnetic hodograms derived from single-point fluctuation measurements have been presented in Part II. These led to the discovery of density cavities and current sheet formation by nonlinear wave steepening. The present Part III deals with multipoint fluctuation measurements using on-line conditional averaging, which resolves the structure of typical fluctuations in space and time. Propagation velocity and coherence of the flutelike density perturbations is measured. The topology of the magnetic fluctuations and associated current density is investigated and shown to consist of flux ropes of negative self-helicities. Superposition of the nonuniform mean field and fluctuating fields yields the net field and current density. Instantaneous field lines and magnitude distributions in three-dimensional space are presented for a turbulent high-beta plasma. The results suggest that the demagnetization of electrons in large positive density fluctuations provides a new saturation mechanism for the instability.
Enhancement of cathodoluminescent and photoluminescent properties of Eu:Y2O3 luminescent films by vacuum cooling
Authors: Kumar D. , Sankar J. , Cho K. G. , Craciun V. , Singh R. K.
Published: July 2002

This letter investigates the role of postdeposition oxygen treatments upon their luminescent properties by synthesizing Eu:Y2O3 films by pulsed laser deposition under different conditions and subjecting them to various postdeposition oxygen treatments. Our results have shown that Eu:Y2O3 films grown in 300 mTorr oxygen ambient and cooled in vacuum (10−6 Torr) after the termination of growth exhibit better photoluminescent and cathodoluminescent brightness than those grown in (i) vacuum and cooled in vacuum and (ii) 300 mTorr oxygen and cooled in 2 Torr oxygen. Two possible mechanisms have been proposed to explain these results.
Simulation and extrapolation of coexistence properties with single-phase and two-phase ensembles
Authors: Escobedo Fernando A.
Published: July 2002

A unified framework is developed to describe extrapolations from simulations performed in either a single system box or two subsystems at equilibrium (i.e., Gibbs ensembles). It is shown that the Gibbs ensemble can be used in conjunction with histogram reweighting and pseudo-ensemble techniques in order to map out more effectively different kinds of phase diagrams, in particular for binary and ternary systems. These extrapolation schemes allow the use of different phase-equilibrium specifications, some of which could not be simulated by conventional approaches. Novel semi-open and osmotic Gibbs ensembles are also described as counterparts of single-phase open and osmotic ensembles, respectively. Applications of the proposed methods are presented to the simulation of pressure-composition diagrams, bubble lines, and isoenthalpic partitioning.
Fixed blueshift of high intensity short pulse lasers propagating in gas chambers
Authors: Koga J. K. , Naumova N. , Kando M. , Tsintsadze L. N. , Nakajima K. , Bulanov S. V. , Dewa H. , Kotaki H. , Tajima T.
Published: July 2002

An explanation is given for the fixed blueshift observed of an intense short pulse laser propagating in a gas chamber filled with helium. The observed shift is found to be independent of the laser power and gas pressure above some critical power. The results are explained by the formation of filaments, which occur due to nonlinear gas polarization effects. The blueshift is in qualitative agreement with a one dimensional particle-in-cell simulation including ionization of the gas.