bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow effect in the chiral-homeotropic liquid-crystal cell
Authors: Chen Shu-Hsia , Chen Li-Yi
Published: July 2002

We investigated the dynamics of the chiral-homeotropic liquid-crystal cell and found that the flow of liquid crystal causes an optical bounce after switching on a high applied voltage. We analyzed the behavior of the directors by computer simulation and found that the field-induced backflow effect results in the abnormal twist of the directors near the substrates. The abnormal twist slows down the rising speed of the chiral-homeotropic cell and produces an optical bounce during the rising period. Our results indicate that the backflow effect should be considered on the design of fast chiral-homeotropic liquid-crystal cells. The significant distinctions between this field-induced optical bounce and the well-known optical bounce of the twisted nematic liquid-crystal cell are also described in this letter.
Oxygen plasma effects on sol-gel-derived lead-zirconate-titanate thin films
Authors: Jang H. K. , Lee S. K. , Lee Cheol Eui , Noh S. J. , Lee W. I.
Published: July 2002

We studied effects of oxygen plasma treatment on the ferroelectric Pb(Zrx,Ti1−x)O3 (PZT) films prepared by a sol-gel method. Electrical characteristics of the films were found to be improved considerably by exposure to the O2 plasma, with enhanced remanent polarization and decreased leakage current densities. Chemical bonding analysis by means of x-ray photoelectron spectroscopy in the Pb 4f region indicated that the intensity from the perovskite PZT phase increased considerably after the O2 plasma treatment, giving rise to the improved performances of the PZT films.
Fusion bonding of Si wafers investigated by x ray diffraction
Authors: Weichel S. , Grey F. , Rasmussen K. , Nielsen M. , Feidenhans’l R. , Howes P. B. , Vedde J.
Published: July 2002

The interface structure of bonded Si(001) wafers with twist angle 6.5 ° is studied as a function of annealing temperature. An ordered structure is observed in x-ray diffraction by monitoring a satellite reflection due to the periodic modulation near the interface, which results from the formation of a regular array of screw dislocations. This satellite reflection first appears at an annealing temperature of 800 °C, and increases abruptly up to temperatures of 1000 °C. We propose that this transition occurs when there is sufficient mobility for the reorganization of atomic steps and terraces in the interface region.
Recombination-independent photogeneration of ultrashort electrical pulses
Authors: Holzman J. F. , Vermeulen F. E. , Elezzabi A. Y.
Published: July 2002

We report on the operation of a photoconductive (PC) switch, capable of generating ultrashort electrical pulses. The PC switch geometry employed utilizes the rising edge of an ultrafast optical excitation pulse to both turn “on” and later turn “off” an electrical transient. The generation mechanism is, therefore, independent of both the semiconductor material and carrier lifetime. It is found that electrical pulses as short as 2 ps can be formed in bulk GaAs and that the duration of these pulses is limited solely by the time constant constraints of the device. By reducing the device time constant, therefore, this technique can be applied to subpicosecond switching of electrical pulses.
Lifetime estimation due to imprint failure in ferroelectric SrBi2Ta2O9 thin films
Authors: Grossmann M. , Lohse O. , Bolten D. , Boettger U. , Waser R. , Hartner W. , Kastner M. , Schindler G.
Published: July 2002

Two different failure modes for a ferroelectric memory cell caused by imprint, the read failure due to the loss of polarization, and the write failure due to the shift of the hysteresis loop are investigated. The quasistatic hysteresis loop allows us to distinguish which failure mode is dominating in a ferroelectric random access memory application and, hence, it can also be used as a powerful tool for lifetime estimation of ferroelectric thin films limited by imprint failure under operating conditions. The experimental results show that the write failure is only decisive for very low voltage operation (Vp<1.25 V), whereas for the Pt/SrBi2Ta2O9/Pt under investigation the read failure is the dominant failure mode for operating voltages exceeding 1.25 V.
Large enhancement of second harmonic generation in polymer films by microcavities
Authors: Cao H. , Hall D. B. , Torkelson J. M. , Cao C.-Q.
Published: July 2002

We report significant enhancement of second-harmonic-generation intensity in a nonlinear optical polymer by strongly confining the fundamental light in a planar microcavity. Employing a microcavity formed by a distributed Bragg reflector and a silver layer, we have observed up to a factor of 50 increase of second-harmonic light intensity in polymer thin films.
Direct observation of the Gouy phase shift in THz impulse ranging
Authors: McGowan R. W. , Cheville R. A. , Grischkowsky D.
Published: July 2002

Here we present a direct observation of the Gouy effect through THz impulse scattering from cylindrical and spherical targets. A π/2 Gouy phase shift through a one-axis focus compared to the more common π phase shift of the two-axis focus is required to interpret the scattering results using a physical optics model.
Visible photonic band gap engineering in silicon nitride waveguides
Authors: Netti M. C. , Charlton M. D. B. , Parker G. J. , Baumberg J. J.
Published: July 2002

We demonstrate experimentally the tuning of complete photonic band gaps in patterned silicon nitride waveguides. Transmission measurements were performed using an ultrabroadband high-brightness white light laser continuum, extracting extinction ratios as low as 10−4 in the gap regions. Angle-resolved measurements show the perfect alignment of the gap around the Γ-J direction.
Photoinduced layer alignment control in ferroelectric liquid crystal with N*–C* phase transition doped with photochromic dye
Authors: Matsui Tatsunosuke , Nakayama Keizo , Ozaki Masanori , Yoshino Katsumi
Published: July 2002

Photoinduced layer alignment control has been proposed in an azo-dye-doped ferroelectric liquid crystal with a chiral nematic (N*)–chiral smectic C (SmC*) phase sequence. This layer alignment control is based on the N*–SmC* phase transition induced by the photoisomerization of the azo-dye. Trans-cis photoisomerization of the azo-dye upon UV irradiation in the SmC* phase induces the N* phase, and an opposite isomerization process upon sequentially stopping the UV irradiation induces the SmC* phase again. The photoinduced SmC*–N*–SmC* phase transition under an appropriate electric field allows us to control the smectic layer alignment. This photoassisted layer switching can be applied to the fabrication of the designed multidomain structure in electro-optic elements such as display, optical memory, and optical grating.
Comparison of experimental and theoretical gain-current relations in GaInP quantum well lasers
Authors: Smowton P. M. , Blood P. , Chow W. W.
Published: July 2002

We compare the results of a microscopic laser theory with gain and recombination currents obtained from experimental spontaneous emission spectra. The calculated absorption spectrum is first matched to that measured on a laser, ensuring that the quasi-Fermi levels for the calculation and the experiment (spontaneous emission and gain) are directly related. This allows us to determine the inhomogeneous broadening in our experimental samples. The only other inputs to the theory are literature values of the bulk material parameters. We then estimate the nonradiative recombination current associated with the well and waveguide core from a comparison of measured and calculated recombination currents.
Electrical and magnetic properties of La0.5Sr0.5CoO3 thin films
Authors: Luo G. P. , Wang Y. S. , Chen S. Y. , Heilman A. K. , Chen C. L. , Chu C. W. , Liou Y. , Ming N. B.
Published: July 2002

Highly conductive perovskite La0.5Sr0.5CoO3 thin films were grown on (012) LaAlO3 by pulsed-laser deposition at different substrate temperatures. Magnetoresistance (MR) measurements show only a negative effect for the films grown at a temperature of 800 °C. However, a positive MR effect was found for the films grown at lower temperatures of 400 and 500 °C. The sign of the magnetoresistance changed from positive to negative near 150 K under an applied magnetic field of 5.0 T, and the largest positive MR of 2.6% was obtained at a temperature of 4.2 K for the film grown at 500 °C. Low-temperature magnetic hysteresis of the films was also studied, and it is suggested that the scattering of the carrier at the domain boundary may lead to the positive-MR effect.
An integrated thin-film thermo-optic waveguide beam deflector
Authors: Tang Suning , Li Bulang , Han Xinghua , Taboada John M. , Jang Chiou-Hung , Kim Jin-Ha , Sun Lin , Chen Ray T.
Published: July 2002

We have demonstrated the operation of a thin-film thermo-optical beam deflector in a three-layer optical planar waveguide. The fabricated waveguide beam deflector consists of a thin-film SiO2 bottom cladding layer, a thin-film polymer top cladding layer, and alternatively positioned thin-film polymer and silica microprisms as the guiding layer. The beam deflection is achieved through the thermo-optic effect that results in opposed index changes in polymer and silica with respect to temperature changes. The measured deflection sensitivity is 0.06°/°C, for the fabricated device with a 7.0 mm interaction length at the wavelength of 632.8 nm.
Mechanically induced Si layer transfer in hydrogen-implanted Si wafers
Authors: Henttinen K. , Suni I. , Lau S. S.
Published: July 2002

Mechanically induced layer transfer of single-crystal silicon by hydrogen ion implantation, low-temperature wafer bonding, and subsequent mechanical splitting of the implanted wafer has been investigated. The bond strength measurements using the crack opening method in room environment yield a surface energy of ⩾2000 mJ/m2 after exposure to oxygen plasma and subsequent hydrophilic silicon/silicon dioxide bonding at 200 °C. Mechanically induced layer transfer was carried out for silicon wafers implanted to a dose of 5×1016 H2/cm2 at 100 keV and annealed for 2 h at 200 °C. No feature was observed by atomic force microscopy (AFM) measurements on the unbonded free surface after this heat treatment. For lower doses of implantation, annealing at higher temperatures is required to enable the mechanical transfer. AFM measurements on the split silicon surface indicate that low-temperature wafer bonding and mechanical transfer yield a root mean square surface roughness of 4 nm which is less than in the standard Smart-Cut® process.
Effect of GaAsyP1−y(0⩽y<1) interlayers on the structural, optical, and electrical characteristics of GaAs/InGaP heterojunction
Authors: Kwon Yong-Hwan , Jeong Weon G. , Cho Yong-Hoon , Choe Byung-Doo
Published: July 2002

The effect of GaAsyP1−y(0⩽y<1) interlayers on the characteristics of GaAs/InGaP heterojunction has been investigated. For samples having GaAsyP1−y interlayers in the range of 0<y⩽0.75 inserted in the GaAs-on-InGaP interface, sharp GaAs band-edge emissions are recovered. These results are attributed to smoothly grown InGaAs(P) interfacial layers with the band-gap energy higher than that of GaAs through transmission electron microscopy measurements. In addition, the amount of carrier depletion at the GaAs-on-InGaP interface is smaller with the use of GaAsyP1−y interlayers than that for no interlayer in capacitance–voltage measurements.
Sono implantation of hydrogen and deuterium from water into metallic fine powders
Authors: Arata Yoshiaki , Zhang Yue-Chang
Published: July 2002

We observed the production of hydrogenated/deuterated metallic fine powders when various metals (Pd, Ag, Ta, Pt, and Au) were irradiated in normal and heavy water by ultrasonic waves. Mass analyses of remnant metal powders revealed substantial amounts of sono-implanted hydrogen and deuterium. The deuterium implantation (D implantation) in D2O was found to be much stronger than the hydrogen implantation (H implantation) in H2O.
Pulsed-laser deposition for organic electroluminescent device applications
Authors: Farrar S. R. , Contoret A. E. A. , O’Neill M. , Nicholls J. E. , Eastwood A. J. , Kelly S. M.
Published: July 2002

An anthracene-based compound has been synthesized and used in the preparation of organic light-emitting diode devices by pulsed-laser deposition. Spectrally resolved electroluminescence has been observed and combined with current–voltage measurements in an investigation of the electro-optical dependence on laser fluence and device conditioning history. The device performance has been assessed and the charge-transfer process characterized. A space-charge-limited conduction regime with an exponential distribution of traps is proposed for the elevated electric fields sufficient to produce electroluminescence. The suitability of the pulsed-laser deposition technique is evaluated for this application. Evidence for molecular degradation associated with deviation from optimum deposition parameters is provided by Fourier transform infrared spectroscopy and scanning electron microscopy observations, comparing pulsed-laser deposition with established spin-coating and physical-vapor deposition techniques.
Voltage divider operation using high-Tc superconducting interface-engineered Josephson junctions
Authors: Saitoh Kazuo , Soutome Yoshihisa , Fukazawa Tokuumi , Tarutani Yoshinobu , Takagi Kazumasa
Published: July 2002

A rapid-single-flux-quantum toggle-flip-flop logic gate was fabricated using high-temperature superconducting interface-engineered Josephson junctions. It was shown that the gate can operate as a voltage divider up to 155 GHz at 15 K and 19 GHz at 27 K. At the same time, the temperature dependence of the IcRn product and the maximum divided voltage was compared. As a result, it was found that the ratio of these values is 0.4–0.1 for 15 K>T>27 K. Circuit simulation with noise sources reveals this peculiar temperature dependence of the maximum divided voltage.
High-frequency electrical pulse generation using optical rectification in bulk GaAs
Authors: Graf S. , Sigg H. , Bächtold W.
Published: July 2002

We present the realization of a traveling-wave electrical pulse generator using pure nonresonant instantaneous optical rectification in bulk GaAs. The optical excitation was achieved by far-infrared pulses of 1–6 ps duration in the wavelength range from 8 to 15 μm, generated by a free-electron laser. The coupling of the optical rectification polarization into the fundamental mode of the microstrip transmission line is verified by angle-resolved measurements. Since optical femtosecond pulses are now becoming readily available, this alternative technique, which gains in efficiency at shorter pulses, may find growing importance for ultrafast pulse generation.
Interface structures in GaAs wafer bonding: Application to compliant substrates
Authors: Vanfleet R. R. , Shverdin M. , Silcox J. , Zhu Z. H. , Lo Y. H.
Published: July 2002

The interface of direct bonded GaAs to GaAs has been studied by scanning transmission electron microscopy and electron energy loss spectroscopy. Voids are seen along the boundary with most being partially filled with a gallium particle. Two general sizes of voids are seen. The large voids (d∼45 nm) are distributed in an approximately linear relationship and the smaller (d∼12 nm) randomly. In compliant substrates, one of the layers is made thin (⩽10 nm) and twisted ∼45°. The larger voids often extend past this thin compliant layer, but no evidence of granularity of the epitaxial film is observed.
Auger recombination as a probe of the Mott transition in semiconductor nanocrystals
Authors: Němec P. , Malý P. , Nikl M. , Nitsch K.
Published: July 2002

We report on picosecond dynamics of photoexcited carriers in CsPbCl3 nanocrystals in a CsCl host. For low carrier densities, photoexcited carriers form excitons, which decay with a characteristic time constant of 170 ps at 300 K. Under strong photoexcitation, we have observed the Auger recombination with the Auger coefficient C≈10−29 cm6 s−1. We have identified the onset of the Auger recombination at the carrier densities of ≈1020 cm−3 as the Mott-like transition from excitons to electron–hole plasma in nanocrystals.
The effect of chlorine on dopant activation in hydrogenated amorphous silicon
Authors: Payne Adam M. , Wagner Sigurd
Published: July 2002

The dark conductivity of hydrogenated amorphous silicon (a-Si:H) films deposited from dichlorosilane (SiCl2H2) and silane (SiH4), and doped with diborane (B2H6), increases by as much as a factor of 100 over the usual a-Si:H,B films deposited without SiCl2H2. The effect is observed at gas phase concentrations of diborane ranging from 0.006 to 0.5 vol %, and for both direct current (dc) and 13.56 MHz radio frequency plasma depositions, although it is more noticeable for the dc discharge. An increase in dark conductivity is also observed in boron doped hydrogenated amorphous silicon carbon alloys (a-SiC:H) deposited with dichlorosilane, albeit coupled with a change in the Tauc gap. Chlorine reduces the conductivity of undoped and phosphorus doped a-Si:H films. A B–Cl cluster acceptor that is not passivated by H is proposed as a possible mechanism for chlorine enhanced conductivity. This increase in p-layer conductivity translates into an increase of solar cell efficiency, but surprisingly by an increase in photocurrent rather than open circuit voltage.
Enhanced ballistic phonon production for surface events in cryogenic silicon detector
Authors: Clarke R. M. , Brink P. L. , Cabrera B. , Colling P. , Crisler M. B. , Davies A. K. , Eichblatt S. , Gaitskell R. J. , Hellmig J. , Martinis J. M. , Nam S. W. , Saab T. , Young B. A.
Published: July 2002

We present evidence of an enhanced ballistic phonon component resulting from surface events in a 100 g silicon cryogenic dark matter detector. Surface events, calibrated using a C14 electron source, were found to have faster rise times (∼5 μs) than bulk gamma and neutron events (∼7 μs). Using this effect, we were able to discriminate bulk nuclear recoil events from a surface electron background at better than the 97% level above 25 keV recoil energy. The phonon risetime for bulk gamma events was dependent on the applied voltage, confirming that phonons produced from electron-hole emission are ballistic.
Effect of Mg ionization efficiency on performance of Npn AlGaN/GaN heterojunction bipolar transistors
Authors: Monier C. , Pearton S. J. , Chang P. C. , Baca A. G. , Ren F.
Published: July 2002

A drift-diffusion transport model has been used to examine the performance capabilities of AlGaN/GaN Npn heterojunction bipolar transistors (HBTs). Simulations using experimental mobility and lifetime reported in the literature closely reproduce the Gummel plot from the GaN-based HBT structure recently demonstrated. Numerical results have been explored to study the effect of the p-type Mg doping and its incomplete ionization in the base. The high base resistance induced by the deep acceptor level is found to be the cause of limiting current gain values. Increasing the operating temperature of the device activates more carriers in the base. An improvement of the simulated current gain by a factor of 2–4 between 25 and 300 °C agrees well with the reported experimental results. A preliminary analysis of high-frequency characteristics indicates substantial progress of predicted rf performances by operating the device at higher temperature due to a reduced extrinsic base resistivity.
Doping-dependent optical gain in GaN
Authors: Ramvall Peter , Aoyagi Yoshinobu , Kuramata Akito , Hacke Peter , Domen Kay , Horino Kazuhiko
Published: July 2002

We report on an investigation of the optical gain in undoped and Si-doped metal-organic vapor phase epitaxially grown GaN. For carrier densities around the Mott transition (at approximately 8×1017 cm−3), the optical gain was observed to increase from around 102 cm−1 up to about 103 cm−1, suggesting a much larger optical gain for an electron-hole plasma than for excitons. A maximum optical gain of 2700 cm−1 at an excitation density of 9.2×1018 cm−3 was found for undoped GaN. This value is the highest achieved for epitaxially grown GaN, verifying the high potential for low-threshold lasing in this material. The optical gain was observed to decrease with increasing doping density. On the contrary, the luminescence efficiency was found to increase up to a doping density of approximately 1018 cm−2. Together these effects create an optimum doping density of around 5×1017 cm−2 for stimulated emission in bulk GaN.
Cluster formation by barrier discharge in simulated engine exhaust gas at high temperature
Authors: Hoard John W. , Kalashnikov Evgenii V. , Szente Joseph J. , Podsiadlik Diane H.
Published: July 2002

Experiments were performed to examine cluster formation when simulated engine exhaust gas passes through an atmospheric pressure plasma discharge cell at 180 °C. The gas composition included N2, NO, CO, CO2, C3H8, C3H6, Ar, H2, H2O, O2 and SO2. Aerosol particles or clusters were measured after the gas was treated with a dielectric barrier discharge, although the quantity of clusters generated was several orders of magnitude smaller than normal engine emissions. The clusters were not formed when water was not present in the mixture. The number and size of clusters increased when hydrocarbons were not present, although the quantity was still very small. Methods to describe cluster formation phenomena are discussed.
Defect in the circular-circularly rubbed liquid crystal cell with off-center alignment
Authors: Zhuang Zhizhong , Suh Seong-Woo , Kim Young Jin , Patel J. S.
Published: July 2002

In this letter, we investigate the behavior of the defects in a circular-circularly rubbed liquid crystal cell with an off-center alignment. We show that the line defect forms a circle that passes through the rubbing centers of the two surfaces. The size and the position of the defect circle depend on the cell gap and the pitch of the materials. We propose a simple model, based on an analysis of the free energy, to explain this interesting phenomenon. This technique of defect making is useful to confine the defect to a particular position by controlling the cell parameters and the material properties. It can also be applied to the pitch measurement, the generation of the space-variant polarized light, and the study of the dynamic properties of the defect.
Accumulation hole layer in p-GaN/AlGaN heterostructures
Authors: Shur M. S. , Bykhovski A. D. , Gaska R. , Yang J. W. , Simin G. , Khan M. A.
Published: July 2002

We present the results on piezoelectric and pyroelectric doping in AlGaN-on-GaN and GaN-on-AlGaN heterostructures and demonstrate p-GaN/AlGaN structures with accumulation hole layer. Our results indicate that polarization charge can induce up to 5×1013 cm−2 holes at the AlGaN/GaN heterointerfaces. We show that the transition from three-dimensional (3D) to two-dimensional (2D) hole gas can be only achieved for hole sheet densities on the order of 1013 cm−2 or higher. At lower densities, only 3D-hole accumulation layer may exist. These results suggest that a piezoelectrically induced 2D-hole gas can be used for the reduction of the base spreading resistance in AlGaN/GaN-based heterostructure bipolar transistors.
Dynamic strength of aluminum single crystals at melting
Authors: Kanel G. I. , Baumung K. , Singer J. , Razorenov S. V.
Published: July 2002

Results of measurements of dynamic tensile strength (“spall strength”) of aluminum single crystals are presented. In the shock-wave experiments the load duration was about 40 ns, the initial temperature was varied from 20 to 648 °C that is only 12 °C less than the melting temperature of aluminum. Under these conditions the dynamic tensile strength of aluminum single crystals has been found practically independent on the temperature up to ∼630 °C. The spall strength slightly decreases at further increase in the initial temperature up to 648 °C. The high-temperature data exceed estimated stresses at which melting should start in a stretched material.
Improvement of quantum efficiency in polymer light-emitting diodes by a single-ion conductor
Authors: Lee Tae-Woo , Park O. Ok
Published: July 2002

Polymer light-emitting diodes were fabricated using poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] as an emissive material. Sodium ionic polyurethane (NaPU), a single-ion conductor (SIC), was employed as an electron-injecting material and compared with the case of sodium sulfonated polystyrene (SSPS). The quantum efficiency was greatly enhanced, compared with the SSPS-based device. The electron injection in the NaPU-based device is more favorable, compared with the SSPS-based device because of a larger ionic space charge accumulation near the cathode by the good chain segmental motion of the soft block in the polymer chains of the SIC. Thus, a balanced injection of an electron–hole pair can be achieved to greatly improve the quantum efficiency.
Photoluminescence studies of type-II self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots grown on (311)A GaAs substrate
Authors: Chen Y. , Li G. H. , Zhu Z. M. , Han H. X. , Wang Z. P. , Zhou W. , Wang Z. G.
Published: July 2002

We investigated the photoluminescence (PL) of self-assembled In0.55Al0.45As/Al0.5Ga0.5As quantum dots (QDs) grown on (311)A GaAs substrate. The PL peak at 10 K shifts to lower energy by about 30 meV when the excitation power decreases by two orders of magnitude. It has a red-shift under pressure, that is the character of X-like transition. Moreover, its peak energy is smaller than the indirect gap of bulk Al0.5Ga0.5As and In0.55Al0.45As. We then attribute that peak to the type-II transition between electrons in X valley of Al0.5Ga0.5As and heavy holes in In0.55Al0.45As QDs. A new peak appears at the higher energy when temperature is increased above 70 K. It shifts to higher energy with increasing pressure, corresponding to the transition from conduction Γ band to valence band in QDs. The measurements demonstrate that our In0.55Al0.45As/Al0.5Ga0.5As quantum dots are type-II QDs with X-like conduction-band minimum. To interpret the second X-related peak emerged under pressure, we discuss the X-valley split in QDs briefly.
Optical spectroscopy of GaN grown by metalorganic vapor phase epitaxy using indium surfactant
Authors: Pozina G. , Bergman J. P. , Monemar B. , Yamaguchi S. , Amano H. , Akasaki I.
Published: July 2002

The effect of In surfactant during metalorganic vapor phase epitaxial growth on sapphire substrates on the properties of GaN layers is studied using time-resolved photoluminescence, cathodoluminescence, and scanning electron microscopy. The samples are divided into two groups, where hydrogen and nitrogen, respectively, have been used as a carrier gas during growth. It is shown that In-doped samples have a lower dislocation density, a narrower photoluminescence linewidth, and a longer free exciton lifetime. The influence of indium is stronger for GaN layers grown in nitrogen-rich conditions. The improvements of structural and optical properties are attributed to the effect of In on dislocations.
Carrier and light trapping in graded quantum-well laser structures
Authors: Aichmayr G. , Martı́n M. D. , van der Meulen H. , Pascual C. , Viña L. , Calleja J. M. , Schafer F. , Reithmaier J. P. , Forchel A.
Published: July 2002

We investigated the carrier and light trapping in GaInAs/AlGaAs single-quantum-well laser structures by means of time-resolved photoluminescence and Raman spectroscopy. The influence of the shape and depth of the confinement potential and of the cavity geometry was studied by using different AlGaAs/GaAs short-period superlattices as barriers. Our results show that grading the optical cavity improves considerably both carrier and light trapping in the quantum well, and that the trapping efficiency is enhanced by increasing the graded confining potential.
Structure of Zn adsorption on GaAs(001)-(2×4)
Authors: Miotto R. , Srivastava G. P. , Ferraz A. C.
Published: July 2002

The atomic structures of four possible models for the adsorption of Zn on GaAs(001)–(2×4) are investigated by means of a first-principles pseudopotential technique. Our calculations suggest that Zn atoms adsorb preferentially in trench (third layer) sites. All structures are characterized by the breaking of the arsenic dimer and the formation of two mixed dimers in the vertical plane containing the original As–As dimer. The Zn atom lies 0.16 Å higher than the As atoms, and the mixed dimer has a bond length of 2.31 Å. All other structural features for the four models studied (e.g., minimum interplanar distance, and remaining As dimer bond lengths) retain the characteristics of the free surface.
One- and two-dimensional tunnel junction arrays in weak Coulomb blockade regime: Absolute accuracy in thermometry
Authors: Pekola J. P. , Taskinen L. J. , Farhangfar Sh.
Published: July 2002

We have investigated one- and two-dimensional (1D and 2D) arrays of tunnel junctions with various sizes and geometries in a partial Coulomb blockade regime. To interpret the observed data, we have utilized the phase correlation theory. We demonstrate, both by experiment and by theoretical calculations in simple geometries that, due to the weaker effect of the electromagnetic environment in 1D arrays, such structures are better than their 2D counterparts for Coulomb blockade thermometry. Yet in both 1D and 2D, the influence of the environment can be made arbitrarily small by making the array sufficiently large.
Enhanced nucleation in solid-phase crystallization of amorphous Si by imprint technology
Authors: Makihira Kenji , Asano Tanemasa
Published: July 2002

A method to enhance crystal nucleation at controlled sites in solid-phase crystallization of amorphous Si is demonstrated. The method uses imprint with Ni-coated Si tips prior to conventional furnace annealing of amorphous Si films deposited on SiO2 substrates. The incubation time for crystallization is found to be greatly reduced at sites imprinted with the tips. This enhanced nucleation can be used to form large crystal grains up to about 7 μm in diameter at controlled sites. Results obtained from imprint with SiO2-covered Si tips suggest that the enhanced nucleation results not from physical effects of indentation but from a chemical effect of metal transfered from the tip to the film surface.
On-chip clock technology for ultrafast digital superconducting electronics
Authors: Gupta Deepnarayan , Zhang Yongming
Published: July 2002

An on-chip, low-jitter clock technology is essential for future multi-GHz digital electronics. In this article, we describe an on-chip clock technology for superconducting rapid single flux quantum (RSFQ) circuits built around an unshunted long Josephson junction (LJJ) oscillator that acts as a master clock source. The clock technology also includes clock frequency divider and selector circuits as well as a clock distribution network. These LJJ oscillators, implemented in linear and annular geometries using 3 μm Nb technology, have quality factors of 106. This clock technology potentially has widespread use in 10–100 GHz superconducting digital circuits and systems, including analog-to-digital converters, time-to-digital converters, correlators, and processors for ultrahigh performance computers.
Influence of short-range ordering on roughness of (AlGa)As interfaces studied with cross-sectional scanning tunneling microscopy
Authors: Reusch T. C. G. , Wenderoth M. , Heinrich A. J. , Engel K. J. , Quaas N. , Sauthoff K. , Ulbrich R. G. , Weber E. R. , Uchida K. , Wegscheider W.
Published: July 2002

Cross-sectional scanning tunneling microscopy was used to identify individual Al atoms on cleaved surfaces of two sets of (AlGa)As heterostructure samples grown with metalorganic vapor-phase epitaxy and molecular-beam epitaxy. We determined the average Al concentration profile perpendicular to the GaAs–(AlGa)As interfaces. Based on former investigations of short-range ordering in (AlGa)As bulk material grown with metalorganic vapor-phase epitaxy, we conclude that short-range ordering during growth of the interfacial layers contributes significantly to the observed interface roughness.
Near-field coherent excitation spectroscopy of InGaAs/GaAs self-assembled quantum dots
Authors: Toda Y. , Sugimoto T. , Nishioka M. , Arakawa Y.
Published: July 2002

Using near-field optical microscopy, we have performed coherent excitation spectroscopy of self-assembled quantum dots (SAQDs). A pair of coherent pulses with a time delay between them allows measurement of the temporal coherence of the carrier wave function in single quantum dots. The observed decoherence time is about 15 ps and is well explained by resonant Raman scattering of phonons. Furthermore, quantum beats originating from the superposition of two closely spaced coherent states have been observed. This opens up possibilities of quantum mechanical control of the carrier wave function in SAQDs.
Dominant iron gettering mechanism in p/p+ silicon wafers
Authors: Lin Wen , Benton J. L. , Pinacho R. , Ramappa D. A. , Henley W.
Published: July 2002

Fe gettering mechanisms in p/p+ epitaxial Si were investigated under controlled contamination and annealing cycles. The dominant Fe gettering mechanism is the Fermi level controlled coulomb attraction between Fe+ and B− in the p+ substrate of the p/p+ wafers. Oxygen precipitates do not appear to contribute when using normal cooling rates following heat treatments. The epi-substrate interfacial strain plays no role in Fe gettering.
Interplay between segregation, roughness, and local strains in the growth of Ga0.75In0.25P alloy
Authors: Wallart X. , Priester C. , Deresmes D. , Mollot F.
Published: July 2002

In this work, using reflexion high energy electron diffraction and atomic force microscopy, we compare the growth of strained Ga0.75In0.25P alloys on GaAs(001) and GaP (001) substrates. We show that although the absolute strain value is similar in both cases, the transition from a bidimensional to a three-dimensional growth mode occurs much faster in the tensile case than in the compressive one. We interpret this result with a microscopic theoretical model which takes into account the interplay between a weak surface roughness, In vertical and lateral segregation, and strain effects.
Edge pinning effect in single- and three-layer patterns
Authors: Popkov A. F. , Savchenko L. L. , Vorotnikova N. V. , Tehrani S. , Shi J.
Published: July 2002

Magnetization reversal of patterned magnetic structure consisting of single and double coupled ultrathin magnetic layers is simulated by solving Landau–Lifshitz equations. Barkhausen’s steps on the magnetization hysteresis loop originating from magnetization lateral edge pinning are predicted and analyzed in dependence on the size of rectangle patterns for Permalloy films.
Electric field-induced layer deformations in the subphases of an antiferroelectric liquid crystal device
Authors: Matkin L. S. , Gleeson H. F. , Baylis L. J. , Watson S. J. , Bowring N. , Seed A. , Hird M. , Goodby J. W.
Published: July 2002

The layer structure in the antiferroelectric, ferrielectric, and ferroelectric phases of a liquid crystal device is reported, together with its electric field-induced deformation. The field-free chevron angle is comparable to the steric tilt angle, but differs significantly from the optical tilt angle. A sharp field threshold is observed for the chevron to bookshelf transition in the antiferroelectric phase at 1.3 V/μm, while layer deformations occur at much lower fields (0.3 V/μm) in the other subphases. Models are proposed for the layer deformations.
Synthesis and properties of epitaxial thin films of c-axis oriented metastable four-layered hexagonal BaRuO3
Authors: Lee M. K. , Eom C. B. , Tian W. , Pan X. Q. , Smoak M. C. , Tsui F. , Krajewski J. J.
Published: July 2002

We have grown epitaxial thin films of c-axis oriented metastable four-layered hexagonal BaRuO3 on a (111) SrTiO3 substrate by 90° off-axis sputtering techniques. X-ray diffraction and transmission electron microscopy reveal that the films are single domains of c-axis four-layered hexagonal structures with an in-plane epitaxial arrangement of BaRuO3 [21̄1̄0]∥SrTiO3[110]. Surfaces with smooth terraces having a step height of a half unit cell (∼4.7 Å) have been observed by scanning tunneling microscopy. The in-plane electrical resistivity of the films is metallic, with a room temperature value of 810 μΩ cm and slightly curved temperature dependence. Their magnetic susceptibility is Pauli paramagnetic. The metastable layered metallic oxide can be used for understanding new solid-state phenomena and device applications.
Temperature-dependent retardation effect of dopants on oxygen diffusion in heavily doped Czochralski silicon
Authors: Takeno Hiroshi , Sunakawa Ken , Suezawa Masashi
Published: July 2002

The influence of boron (B), arsenic (As), and antimony (Sb) on oxygen diffusivity at 500–800 °C was investigated in heavily doped Czochralski silicon wafers with resistivities below 0.02 Ω cm. The oxygen diffusivity was determined from the outdiffusion profile measured by secondary ion mass spectrometry after prolonged heat treatments. It was found that the heavily doped As and Sb reduce the oxygen diffusivity more at lower temperature. The increases in the activation energy for diffusion were found to be about 0.64–0.68 and 1.40 eV for As and Sb doping, respectively. Heavy B doping, however, exhibited anomalous temperature dependence showing a reduction rate peak around 600–700 °C, supposedly due to enhanced formation of immobile oxygen aggregates.
Nanoscale characterization of stresses in semiconductor devices by quantitative electron diffraction
Authors: Demarest J. , Hull R. , Schonenberg K. T. , Janssens K. G. F.
Published: July 2002

By using transmission electron microscopy and focused ion beam technology in conjunction with finite element and electron diffraction contrast simulations, we have been able to quantitatively measure stresses in semiconductor devices with a spatial resolution on the order of tens of nanometers and a sensitivity on the order of tens of mega pascals. Examples of measuring stresses around shallow isolation trenches in semiconductor device structures are presented.
Cathodoluminescent properties at nanometer resolution through Z-contrast scanning transmission electron microscopy
Authors: Gao H.-J. , Duscher G. , Kim M. , Pennycook S. J. , Kumar D. , Cho K. G. , Singh R. K.
Published: July 2002

We report the observation of porous structures in laser-ablation-deposited Y2O3:Eu thin films and their correlation with luminescent properties by a combination of transmission electron microscopy and Z-contrast scanning transmission electron microscopy (Z-STEM). Depending on growth conditions, a large density of voids is incorporated into the films, which leads to a much increased surface area. Cathodoluminescence imaging in the STEM directly reveals a 5 nm “dead layer” around each void, which is responsible for the observed reduction in luminescence efficiency.
Multiply warped products with nonsmooth metrics
Authors: Choi Jaedong
Published: July 2002

In this article we study manifolds with C0-metrics and properties of Lorentzian multiply warped products. We represent the interior Schwarzschild space–time as a multiply warped product space–time with warping functions and we also investigate the curvature of a multiply warped product with C0-warping functions. We give the Ricci curvature in terms of f1, f2 for the multiply warped products of the form M=(0,2m)×f1R1×f2S2.
Substitutional C fraction and the influence of C on Si dimer diffusion in Si1−yCy alloys grown on (001) and (118) Si
Authors: Croke E. T. , Grosse F. , Vajo J. J. , Gyure M. F. , Floyd M. , Smith David J.
Published: July 2002

The dependence of substitutional C fraction on growth temperature and substrate orientation is measured for Si1−yCy alloy films grown on (001) and (118) Si by molecular-beam epitaxy. Secondary ion mass spectrometry and high-resolution x-ray diffraction were used to measure the total C and the substitutional C concentrations, respectively, in several samples prepared at temperatures between 450 and 650 °C. The substitutional C fraction decreased rapidly with increasing temperature in this range, regardless of orientation, and was slightly lower for growth on (118) Si. Cross-sectional transmission electron microscopy on (118)-oriented samples revealed a tendency for C to concentrate periodically on (001) facets which formed immediately after initiation of Si1−yCy growth. A kinetic Monte Carlo simulation based upon enhanced diffusion of Si dimers in the presence of subsurface C predicted a step instability leading to step bunching and the formation of periodic surface features, as well as the accumulation of high C concentrations on nearly (001) planes.
Bias-dependence of the thermal time constant in diffusion-cooled superconducting hot-electron bolometer mixers
Authors: Wilms Floet D. , Klapwijk T. M. , Gao J. R. , de Korte P. A. J.
Published: July 2002

We present an experimental study of the intermediate frequency bandwidth of a Nb diffusion-cooled hot-electron bolometer mixer for different bias voltages. The measurements show that the bandwidth increases with increasing voltage. Analysis of the data reveals that this effect is mainly caused by a decrease of the intrinsic thermal time of the mixer and that the effect of electrothermal feedback through the intermediate frequency circuit is small. The results are understood using a qualitative model, which takes into account the different effective diffusion constants in the normal and superconducting domains.
Potential energy surfaces and coordinate dependence
Authors: Wales David J.
Published: July 2002

For coordinate systems with a nontrivial metric tensor covariant derivatives must be used to obtain properties that are coordinate independent. Applications to instantaneous normal mode theory and bifurcation points are presented as illustrations.
Microstructure of BaRuO3 thin films grown on (001) SrTiO3
Authors: Tian W. , Pan X. Q. , Lee M. K. , Eom C. B.
Published: July 2002

BaRuO 3 thin films with hexagonal 4H structure were grown on (001) SrTiO3 by a 90° off-axis rf-sputtering technique. The thin films were epitaxially grown on the (001) surface of SrTiO3, with (202̄3) planes parallel to the surface of the substrate. Within the growth plane, the film consists of four different crystallographic orientations with respect to the substrate, defined by the surface symmetry of the (001) SrTiO3 substrate. BaRuO3 grains of all four orientations show an anisotropic shape elongated along the [1̄21̄0] direction. The reason for the anisotropic growth is that the lattice mismatch between BaRuO3 and SrTiO3 is smaller along the [1̄21̄0] direction of SrTiO3 in comparison to that along its perpendicular direction. Stacking faults and intergrowths of the 9R structure were observed in small local regions of the film.
Design of thin-film photonic crystal waveguides
Authors: Silvestre E. , Pottage J. M. , Russell P. St. J. , Roberts P. J.
Published: July 2002

We present numerical designs for single-mode leak-free photonic crystal waveguides exhibiting strongly anisotropic spatial and temporal dispersion. These structures may be produced quite simply by drilling regular arrays of holes into thin films of high refractive index, and permit the realization of highly compact optical elements and wavelength division multiplexing devices.
Magnetic structure of the crystal CsDy(MoO4)2
Authors: Anders A. G. , Bondarenko V. S. , Naumenko V. M. , Feher A. , Orendaceva A.
Published: July 2002

The energy of the ordered state of the rare-earth subsystem of CsDy(MoO4)2, with a unit cell containing two magnetically inequivalent rare-earth ions, is calculated in the dipole approximation. The magnetic configuration of the rare-earth ions corresponding to the ground state of the magnetically ordered phase is determined. The field dependence of the magnetizations Ma, Mb, and Mc at T=0 K is calculated. The phase transitions caused by an external magnetic field are analyzed. The values of the equivalent G factors obtained from the calculated magnetization curves agree satisfactorily with the values determined from the experimental data on the heat capacity and magnetic susceptibility.
Piezoelectric mechanism for the orientation of stripe structures in two-dimensional electron systems
Authors: Fil D. V.
Published: July 2002

A piezoelectric mechanism for the orientation of stripes in two-dimensional electron systems in GaAs–AlGaAs heterostructures is considered. It is shown that when the anisotropy of the elastic constants and the influence of the boundary of the sample are taken into account, the theory gives an orientation of the stripes along [110] direction, in agreement with the experimental data. For a two-layer system an effect is found wherein a reorientation of the stripe structure along the [100] direction occurs when the period of the structure exceeds the distance between layers.
Dynamic characteristics of helium adsorbents. The influence of palladiation
Authors: Shcherbachenko R. I. , Grigor’ev V. N.
Published: July 2002

The equilibrium helium pressure is measured under static and dynamic conditions for a series of new adsorbents with various degrees of palladiation. It is confirmed that the helium pressure above the adsorbent in the region where it is independent of the degree of filling of the adsorbent is a universal function of the rate of helium admission. It is shown that the admixture of several percent Pd has practically no influence on the adsorption properties of the adsorbents studied.
Automatic feature selection for unsupervised image segmentation
Authors: Al-Nuaimy W. , Huang Y. , Eriksen A. , Nguyen V. T.
Published: July 2002

A computational bottleneck is often imposed by the volume of image data generated in disciplines such as remote sensing and medical imaging, especially in situations where automatic analysis or interpretation is required. Segmentation and classification tasks that utilize multivariate data can be impeded by this dimensionality. A general-purpose unsupervised image segmentation system is presented here for the automatic detection of image regions exhibiting different visual texture properties. A suboptimal feature selection procedure is proposed to automatically select the set of texture features best suited for the particular application. Results are presented for the segmentation of ground-penetrating radar images for generating automatic subsurface reports. The reduction in the size of the feature set both reduces the computation time and improves the accuracy.
Single-electron tunneling study of two-dimensional gold clusters
Authors: Wang Bing , Xiao Xudong , Huang Xianxiang , Sheng Ping , Hou J. G.
Published: July 2002

By using scanning tunneling microscopy/spectroscopy, we have studied the current–voltage characteristics of two-dimensional (2D) Au clusters, thermally deposited on self-assembled alkanethiol monolayer. The curves display Coulomb blockade and staircase with asymmetric behavior. The measured zero conductance gap as a function of cluster size is in excellent agreement with classical model calculations, in which the 2D Au island is treated as metallic in the planar direction but nonmetallic in the normal direction.
The effect of back channel hydrogen plasma treatment on the electrical characteristics of amorphous thin film transistors
Authors: Kang S. G. , Bae S. C. , Choi S. Y.
Published: July 2002

This letter reports that hydrogen plasma treatments on the back channel of thin film transistors are an effective way to improve the off-current characteristics. The effects of the hydrogen plasma treatments on the amorphous Si were checked using current–voltage plotter, atomic force microscopy, and secondary ion mass spectrometry. The surface of the amorphous Si was etched and passivated by the hydrogen radicals at various radio-frequency (rf) powers. The resistance of the modulated back channel increased and the off-current characteristic was improved from 530 to 1.68 pA, which was caused by the increase of roughness and hydrogen contents at back channel as a function of rf power.