bluuebunny/crossref_metadata_2025 huggingface dataset:
Intermediate temperature molecular beam-epitaxy growth for design of large-area metal-semiconductor-metal photodetectors
Authors: Nabet Bahram , Paolella Arthur , Cooke Paul , Lemuene Mary L. , Moerkirk Robert P. , Liou Liann-Chern
Published: July 2002

Large-area metal-semiconductor-metal (MSM) photodetectors are fabricated on molecular beam epitaxy (MBE) grown GaAs material at growth temperatures ranging from 250 to 500 °C. It is shown that materials grown at intermediate temperatures are a suitable choice for large-area, high photocurrent detectors. Particularly, MSM devices made from material grown at around 350 °C have a dark current of the same magnitude as those grown at lower temperatures while having a substantially larger photocurrent. Higher low-field mobility at intermediate temperatures should give these devices speed advantage as well. A change of close to 4 orders of magnitude in dark current and more than 2 orders of magnitude in light response is observed for this temperature range.
Picosecond photoresponse of epitaxial YBa2Cu3O7−δ thin films
Authors: Hegmann F. A. , Hughes R. A. , Preston J. S.
Published: July 2002

Photoresponse signals as fast as 16 ps (full width at half-maximum) have been observed from current-biased bridge structures of epitaxial YBa2Cu3O7−δ thin films on LaAlO3 using 5 ps, 820 nm laser pulses. Operating at liquid-nitrogen temperature (77.4 K), the amplitude of the fast response was found to be linear with current at low bias currents. At higher bias currents, a slow component appeared in the signal with a decay over several nanoseconds which could be attributed to a resistive bolometric response. Fast transients about 20 ps wide have been observed in films with thicknesses ranging from 47 to 200 nm. We believe the fast response is primarily due to a kinetic inductive bolometric mechanism associated with heating of the film by the laser pulse. There is some evidence of a nonbolometric contribution but it is not yet conclusive.
Radiation from concentric-circle grating, surface-emitting planar waveguides: The volume current method
Authors: Jordan Rebecca H. , Hall Dennis G.
Published: July 2002

This letter describes the volume current boundary perturbation method and uses this method to find the field radiated by a set of concentric rings located at the film-cover interface of a three-layer, planar optical waveguide. Waveguides with annular and full (extending from the origin) sets of circular gratings, and with both traveling and standing guided waves, are analyzed. The resulting analytical expression for the radiation field and far-field intensity patterns are compared with results of the field-expansion boundary perturbation method.
Depth distribution of reactive ion etching-induced damage in InAlAs/InGaAs heterostructures evaluated by Hall measurements
Authors: Agarwala Sambhu , Adesida Ilesanmi , Caneau Catherine , Bhat Rajaram
Published: July 2002

The depth distribution of defects induced by reactive ion etching in InAlAs/InGaAs heterostructures has been determined from the measurements of low-temperature mobilities of samples with varying two-dimensional electron gas (2DEG) depth. The sample with a 2DEG depth of 35 nm from the surface showed no change in mobilities or sheet carrier densities, whereas the one with a 2DEG depth of 12.5 nm, etched under the same conditions using HBr plasma at a self-bias of −100 V, showed a large change in both mobilities and sheet carrier densities. The defect distribution, which was estimated using the reciprocal of mobilities, has been found to be exponential. A 1/e penetration depth of about 7.8 nm has been obtained for HBr plasma for self-bias voltages between −100 and −200 V and has been found to be independent of etching time. The exponential distribution of defects suggests that either ion channeling or diffusion is the possible mechanism of defect production in regions deeper than the projected ion range.
Ballistic-electron-emission microscopy characteristics of reverse-biased Schottky diodes
Authors: Davies A. , Craighead H. G.
Published: July 2002

Ballistic-electron-emission microscopy (BEEM) is used to study Au/Si Schottky diodes under reverse-bias conditions. The reverse bias causes a reduction in the barrier height and an increase in the collection efficiency of the electron transport. The former phenomenon is well described by the effects of the image potential. The increase in collection efficiency is likely due to a reduction in the percentage of backscattered electrons as the field strength in the semiconductor is increased. The analysis further indicates that this back scattering is a significant energy dependent process that is absent from the accepted BEEM transport model.
Pressure-induced Γ-X crossover in the conduction band of ordered and disordered GaInP alloys
Authors: Uchida K. , Yu P. Y. , Noto N. , Weber E. R.
Published: July 2002

Pressure-dependent photoluminescence in both ordered and disordered Ga0.5In0.5P is reported. In ordered Ga0.5In0.5P, emissions are observed from both the direct band gap at the Brillouin zone center (Γ) and from the indirect band gap involving the conduction band at the X point of the Brillouin zone. The conduction-band minima at X and Γ in the ordered phase are found to be lowered in energy by the same amount with respect to those in the disordered phase. Our results support the existence of ordering along the [001] direction in GaInP as proposed recently by Kurtz.
GaAs metal insulator semiconductor capacitors and high transconductance metal insulator semiconductor field effect transistors
Authors: Reed J. , Fan Z. , Gao G. B. , Botchkarev A. , Morkoç H.
Published: July 2002

We have fabricated 3 μm gate length self-aligned, depletion mode GaAs metal insulator semiconductor field effect transistors exhibiting transconductances, typically in the vicinity of 160 mS/mm. This achievement is attributed to the use of Si3N4 as the gate dielectric with a few monolayers of a Si/Ge interface layer between the GaAs channel layer and the insulator. The Si3N4/Si/Ge insulator structure is grown in situ using a plasma-enhanced chemical vapor deposition system which is connected by an ultrahigh vacuum transfer tube to an adjacent III-V molecular beam epitaxy system. Nearly ideal capacitance-voltage curves (compared to previous publications) suggest the existence of a high quality insulator/semiconductor interface. The lowest interface trap density that has been measured, as determined from the magnitude of the conductance peak is ∼2×1011 eV−1 cm−2.
Growth of single crystal Ge films on GaAs and InGaP and highly oriented Au films on Ge
Authors: Dubey M. , Jones K. A. , Eckart D. W. , Casas L. M. , Pfeffer R. L.
Published: July 2002

Single crystal germanium films were deposited on (100) GaAs and InGaP substrates, and highly oriented gold films were deposited on the germanium films by ultrahigh vacuum E-beam evaporation. They were characterized by double crystal x-ray diffraction (DXRD), transmission electron microscopy (TEM), Rutherford backscattering spectroscopy (RBS), Auger electron spectroscopy (AES), and scanning electron microscopy (SEM). The germanium film grew epitaxially with a smooth, abrupt interface, and the highly oriented gold film formed a smooth interface with the germanium and had a (100)Au∥(100)Ge and (001)Au∥[011]Ge or [001]Au∥[0-1 1]Ge orientation relationship. Large grains with one or the other orientation relationship could be distinguished in the SEM. TEM micrographs show that the grains have a periodic dislocation pattern indicative of heteroepitaxy, and the grain boundaries appear to have a low energy. No contamination was detected in the gold film away from the interface with the germanium, and there was significant channeling of the RBS beam when it was normal to the gold film.
Surface structure of (NH4)2Sx-treated GaAs (100) in an atomic resolution
Authors: Yokoi Naoki , Andoh Hiroya , Takai Mikio
Published: July 2002

The three-dimensional structure of a GaAs (100) surface, treated in a (NH4)2Sx solution and annealed at 200 °C, was studied in an atomic scale by x-ray photoemission spectroscopy (XPS), high-resolution Rutherford backscattering spectroscopy (RBS), and scanning tunneling microscopy (STM). XPS spectra showed that S termination could suppress oxidization of the surface in the air and that S atoms on a GaAs surface bonded As atoms. The disorder of atomic sites in the surface region of a S-terminated GaAs was found by RBS channeling spectra to be smaller than that of an untreated sample. The thickness of the sulfur layer on GaAs was found to be about 1.5 monolayers from RBS measurement. STM observation of the S-terminated surface revealed a 1×1 lateral structure of the sulfur layer on the GaAs (100) surface.
Formation and quenching mechanisms of the electron beam pumped (XeRb)+ ionic excimer in different buffer gases
Authors: Lankhorst F. T. J. L. , Bastiaens H. M. J. , Peters P. J. M. , Witteman W. J.
Published: July 2002

The ionic excimer molecule XeRb+ is formed in an electron beam excited gas mixture of Xe, Rb, and a buffer gas. The formation and quenching mechanisms of ionic excimers are investigated by measuring the XeRb+ fluorescence as a function of the gas composition and gas pressure. The formation of XeRb+ is achieved by a three-body association reaction between Xe+, Rb, and a buffer gas atom. For the buffer gases He, Ne, or Ar the values of the important formation rate constants are determined from the observed fluorescence signal decay.
Conjugated photorefractive polymer
Authors: Yu Luping , Chen Yongming , Chan Wai Kin , Peng Zhonghua
Published: July 2002

A novel type of photorefractive polymer, containing a conjugated backbone and a second order nonlinear optical chromophore, has been developed. It was expected that the conjugated backbone would absorb photons in the visible region and play the triple role of charge generator, charge transporter, and backbone. Several physical measurements demonstrated this expectation. Two beam coupling experiments clearly indicated an asymmetric optical energy exchange between two beams without applying an external electrical field. This is a conclusive signature for photorefractivity. Also, large optical gain, 5.7 cm−1, was observed under zero-field condition.
Phase conjugation of BaTiO3:Ce by backward stimulated photorefractive scattering
Authors: Zhu Yong , Yang Changxi , Hui Mengjun , Niu Xiaojuan , Zhang Jinfeng , Zhou Tang , Wu Xing
Published: July 2002

We present a self-pumped phase conjugator (SPPC) based on backward stimulated photorefractive scattering in a BaTiO3:Ce crystal in air without any external seed beam. Stable reflectivities of as much as 70% and good phase conjugate fidelity have been demonstrated in the 0°-cut crystals. Over 50% reflectivities at 515 nm have been obtained at an incident angle between 20° and 70°. In comparison with undoped 0°-cut and 45°-cut crystals, the buildup time of SPPC is much faster for the same intensity.
Enhanced electroluminescence of AlP/GaP disordered superlattice
Authors: Sasaki Akio , Wang Xue-Lun , Wakahara Akihiro
Published: July 2002

Electroluminescent diodes with active layers of Al0.5Ga0.5P bulk alloy (b-AL), (AlP)5/(GaP)5 ordered superlattice (o-SL), and (AlP)m/(GaP)n (m,n=3,6,9) disordered superlattice (d-SL) are fabricated by organometallic vapor phase epitaxy using tertiarybutylphosphine. Stronger injection electroluminescence is observed from the d-SL diode as compared with the b-AL and the o-SL diodes. The total light output of the d-SL diode is about 4–5 times stronger than that of the b-AL and the o-SL diodes. The result indicates the remarkable luminescence capability of the d-SL.
Transient ellipsometric surface photoreflectance applied to GaAs
Authors: Frankel Michael Y. , Carruthers Thomas F.
Published: July 2002

We apply a time-resolved reflection ellipsometric technique to study the dynamic properties of the surface dielectric index of GaAs after optical excitation. The technique allows the separation of the refractive and absorptive components of the dielectric index change and accounts for possible anisotropic effects. The carriers optically injected at the surface create electric field perturbations of ∼7 kV/cm for an average carrier density of ∼1018 cm−3 over a depth of ∼0.25 μm. The measured absorptive component change is shown to be qualitatively different from that typically measured via thin-film transmission and can be attributed to the Franz–Keldysh effect.
Growth of highly oriented ZrTiO4 thin films by radio-frequency magnetron sputtering
Authors: Chang De-An , Lin Pang , Tseng Tseung-Yuen
Published: July 2002

ZrTiO4 thin films on Si(100), metals [Al, Ti and Pt coated on Si(100)] and glass were prepared by radio-frequency magnetron sputter deposition. All films on crystalline substrates exhibited a highly preferred orientation in [020], which were evidenced by the full width at half-maximum (≤0.046°) of the associated rocking curves. The structure of the films on glass, depending on the substrate temperature, was either amorphous or random polycrystalline. Good stoichiometric quality and thermal stability have been observed in films on silicon. The X-ray diffraction and transmission electron microscopy selected-area diffraction studies indicated that, after a sequence of annealing up to 650–700 °C, the distributions of Zr and Ti ions in the octahedral cation sites of crystal structure of the films remained disordered.
Emission linewidth measurements of two-dimensional array Josephson oscillators
Authors: Booi P. A. A. , Benz S. P.
Published: July 2002

We have coupled emission from 10×10 arrays of Josephson junctions at 4 K to a room-temperature mixer through a fin-line antenna and a WR-12 waveguide. A single voltage-tunable peak was detected in the frequency range from 53 to 230 GHz. A stripline resonance in the antenna reduced the array’s dynamic resistance and thereby the emission linewidth to as low as 10 kHz. We extract an effective noise temperature of 14 K from the linewidth data.
Energy dissipation during nanoscale indentation of polymers with an atomic force microscope
Authors: Boschung E. , Heuberger M. , Dietler G.
Published: July 2002

Nanometer size indentations on polypropylene and polymethylmethacrylate were made with the atomic force microscope and were 200-nm wide and 20-nm deep for a tip penetration depth of 75 nm. To image the indentations with the same tip used for writing, the tip was retracted at high speed, thus detaching any polymer sticking on it. Nanomechanical properties of polymer are studied in the limit of shallow indentations and slow penetration speed. From the hysteresis of the force versus tip motion curve, the dissipated energy during the indentation and the inelastic deformation of the polymer surface can be measured. We find that the measured dissipated energy is proportional to the volume of the indentation times the activation energy needed to excite chain segments motion during the deformation.
Investigation of the bulk laser damage of lithium triborate, LiB3O5, single crystals
Authors: Furukawa Yasunori , Markgraf Steven A. , Sato Masayoshi , Yoshida Hidetsugu , Sasaki Takatomo , Fujita Hisanori , Yamanaka Tatsuhiko , Nakai Sadao
Published: July 2002

The bulk laser damage threshold of lithium triborate (LiB3O5) single crystals were determined to be 45.0 GW/cm2 at 1.064 μm, 1.1 ns pulse width. The damage threshold was dependent on pulse duration and wavelength: at 1.064 μm, 25 ns the damage threshold was 11.2 GW/cm2, at 0.532 μm, 1.0 ns the damage threshold was 26 GW/cm2; and at 0.355 μm, 0.9 ns the damage threshold was 21.9 GW/cm2. To the best of our knowledge this is the highest value reported among inorganic frequency conversion crystals, and approximately 1.8 times higher than that of fused silica.
Surface phosphidation of GaAs by the laser-induced dissociation of trimethylphosphine
Authors: Davidson Jack L. , John Phillip , Roberts Peter G. , Jubber Michael G. , Wilson John I. B.
Published: July 2002

The localized phosphidation of single-crystal GaAs, by the frequency doubled argon ion laser photolysis at 257 nm of P(CH3)3, is reported. Tracks were deposited by scanning the focused laser beam at a speed of 34.6 μm s−1 and were characterized by scanning electron microscopy and laser ionization mass analysis. Evidence is presented which suggests that a photochemical reaction is initiated by band gap absorption by GaAs.
Low operating current self-sustained pulsation GaAlAs laser diodes with a real refractive index guided structure
Authors: Takayama T. , Imafuji O. , Sugiura H. , Yuri M. , Naito H. , Kume M. , Yoshikawa A. , Itoh K.
Published: July 2002

We have developed a new real refractive index guided GaAlAs laser which satisfies both low operating current and low noise characteristics by self-sustained pulsation. The structure has a planar active layer and a GaAlAs optical confinement layer which give large saturable absorber, and small internal loss. As a result, as operating current as low as 19.7 mA at an output power of 3 mW is obtained under self-sustained pulsation, leading to a relative intensity noise level less than −135 dB/Hz.
Experimental study of implantation-induced disordering in InGaAsP strained multiple-quantum-well heterostructures
Authors: Elenkrig B. B. , Thompson D. A. , Simmons J. G. , Bruce D. M. , Si Yu. , Zhao Jie , Evans J. D. , Templeton I. M.
Published: July 2002

The effect of strain on F+ and Si+ implantation-induced compositional disordering in InGaAsP strained layer multiple-quantum-well (MQW) heterostructures has been studied by investigating the temperature dependence of the photoluminescence (PL) spectra and spatial distribution of degree of polarization of PL for both compressive and tensile strained, and unstrained MQW heterostructures. It was found that under similar implantation and anneal conditions a spectral blueshift occurs which is largest in the compressively strained structure and the smallest in the tensile one. This behavior is explained in terms of implantation-enhanced interdiffusion, by taking into account the composition differences of elements between the wells and barriers. The development of strain related to the process of interdiffusion has been experimentally observed.
Normally-on GaAs/AlAs multiple-quantum-well Fabry–Perot reflection modulators for large two-dimensional arrays
Authors: Lin Chih-Hsiang , Goossen K. W. , Sadra K. , Meese J. M.
Published: July 2002

We report normal-incident normally-on 90 Å GaAs/21 Å AlAs multiple-quantum-well Fabry–Perot reflection modulators operating at the same wavelength with the same reverse bias over much of a 2 in. wafer. The fabricated modulators achieve ON/OFF reflectance changes larger than 48% and ON/OFF contrast ratios larger than 14.7 over a diameter of 31 mm at the wavelength of 845 nm with a fixed bias of 7 V. Over a diameter of 40 mm, these modulators achieve reflectance changes larger than 43.2% and contrast ratios larger than 5.
Synthesis of YNi2B2C thin films by magnetron sputtering
Authors: Arisawa Shunichi , Hatano Takeshi , Hirata Kazuto , Mochiku Takashi , Kitaguchi Hitoshi , Fujii Hiroki , Kumakura Hiroaki , Kadowaki Kazuo , Nakamura Keikichi , Togano Kazumasa
Published: July 2002

Thin films of YNi2B2C superconductor by the magnetron sputtering technique have been synthesized. The YNi2B2C phase was found to crystallize very uniformly after annealing at 1050 °C in vacuum. The structure was investigated and the electric and magnetic properties were characterized. The films were c-axis oriented and showed a Tc onset of ∼15 K by both magnetization and resistivity measurements. The results of the critical current measurement suggested the existence of anisotropies of pinning force and an upper critical field Bc2.
Exciton and biexciton recombination in semiconductor nanocrystals
Authors: Zheng J. P. , Kwok H. S.
Published: July 2002

The time-resolved photoluminescence of semiconductor nanocrystals was obtained as a function of laser excitation intensity at a low temperature. Exciton and biexciton recombination processes were clearly identified. At laser intensities lower than 3.3×105 W/cm2, only exciton recombination with a lifetime of 4 ns was obtained. At higher laser intensities, biexciton recombination with a lifetime of 1 ns appeared. At laser intensities above 106 W/cm2, recombination of electron-hole plasma occurred with a much shorter lifetime.
Damage induced by exposing AlGaAs layers to electron cyclotron resonance SF6/CHF3 plasma
Authors: Mitani Katsuhiko , Oda Hiroto , Imamura Yoshinori
Published: July 2002

We investigated the damage induced by exposing AlxGa1−xAs (x=0.30, 0.15) and GaAs layers to electron cyclotron resonance SF6/CHF3 plasma and the repair of this damage by annealing. After plasma exposure the sheet resistance of all these samples is higher because of carrier reduction near the surface. This kind of damage in AlxGa1−xAs (x=0.15) and GaAs can be restored by annealing at 450 °C for 30 min. The plasma-damaged AlxGa1−xAs (x=0.30), however, has a further increased sheet resistance after annealing, and the sheet resistance AlxGa1−xAs (x=0.3), not exposed to the plasma is unaffected by annealing. Capacitance-voltage measurements show that annealing causes the carrier reduction in plasma-exposed AlxGa1−xAs (x=0.30), to extend to a greater depth.
Growth and magnetic characterization of face centered cubic Co on (001) diamond
Authors: Wolf J. A. , Krebs J. J. , Prinz G. A.
Published: July 2002

We report the growth and characterization of face centered cubic (fcc) Co epitaxial films (t=10–130 nm) on diamond. Growth was carried out under ultrahigh vacuum conditions in a commercial molecular beam epitaxy machine and monitored during growth using reflection high energy electron diffraction which showed single crystal growth oriented with 〈100〉Co∥〈100〉C. X-ray diffraction studies of the 130 nm sample demonstrated the single crystal fcc growth throughout the entire sample. Vibrating sample and quantum interference magnetometry yields a magnetic moment of (1.05±0.1)×103 emu/cm3. Ferromagnetic resonance measurements carried out at 35 GHz yield a large cubic anisotropy K1/MS=(−480±30) Oe and linewidth of the order of 110 Oe. Spin waves were observed in the thicker samples and the exchange constant determined to be ACo=1.09×10−6 erg/cm.
X-ray diffractometric characterization of the GaAsP/GaAs and InGaAs/GaAsP superlattices grown on offcut GaAs(001) substrate by means of the reciprocal space mapping
Authors: Gaca J. , Wojcik M.
Published: July 2002

A technique of mapping a region of reciprocal space near a given substrate reflection using a standard powder diffractometer enabling (θ−ω/2θ) scan is applied to investigate superlattices grown on misoriented substrate. The parameters describing superlattice geometry are calculated on the basis of collected diffraction data. The accuracy of the results is compared with that of other techniques.
Compensation of the dangling-bond space charge in amorphous silicon solar cells by graded low-level doping in the intrinsic layer
Authors: Fischer D. , Shah A. V.
Published: July 2002

The compensation of the dangling-bond space charge in amorphous silicon p-i-n solar cells by graded, low-level doping in the intrinsic layer is discussed and demonstrated experimentally. Carrier collection in p-i-n cells without doping indicates that the degraded state space charge is largely positive, and thus that boron doping should be beneficial. Solar cells with linearly decreasing boron doping profiles are shown to yield a homogeneous collection in the intrinsic layer, and a red light conversion efficiency superior to that of undoped cells after light soaking. Also, the optimal doping concentration is shown to be a direct measure of the degraded state defect density.
Independent contacting to electron layers in a double quantum well system using Pd-Ge shallow ohmic contacts
Authors: Patel N. K. , Burroughes J. H. , Tribble M. J. , Linfield E. H. , Churchill A. C. , Ritchie D. A. , Jones G. A. C.
Published: July 2002

A novel recess technique is used to produce reliable Pd-Ge ohmic contacts to GaAs/AlGaAs high mobility two-dimensional electron gas (2DEG) systems, operating down to cryogenic temperatures. By altering the depth of the recess the diffusion length of the contacts is found to be less than 20 nm. Pd-Ge shallow ohmic contacts were also used for forming independent contacts to two 2DEGs in a double quantum well structure where the 2DEGs are separated by only a 20 nm AlxGa1−xAs(x=0.3) barrier. Tunnel current measurements and magnetoresistance analysis confirmed that the layers could be probed individually and that the carrier densities and mobilities of the separate layers could be accurately determined.
Short-wavelength room-temperature continuous-wave laser operation of InAlP-InGaP superlattices grown by metalorganic chemical vapor deposition
Authors: Chelakara R. V. , Islam M. R. , Neff J. G. , Fertitta K. G. , Holmes A. L. , Ciuba F. J. , Dupuis R. D. , Richard T. A. , Holonyak N. , Hsieh K. C.
Published: July 2002

The growth and laser operation of high-quality InAlP-InGaP superlattice-active-region quantum-well heterostructure lasers on GaAs substrates are reported. These heterostructures exhibit cw room-temperature (300 K) optically pumped laser operation at wavelengths as short as λ∼586 nm (yellow, Ehν∼2.11 eV). This is the shortest wavelength room-temperature cw laser operation reported to date for any III-V material system.
Domain structures of die-upset melt-spun NdFeB
Authors: Folks L. , Street R. , Woodward R. C.
Published: July 2002

Domain patterns at the surface of thermally demagnetized MQIII, anisotropic Nd2Fe14B permanent magnet produced by hot pressing and die-pressing and die-upsetting melt-spun ribbons have been obtained by Kerr microscopy. Patterns have been observed on surfaces perpendicular, parallel, and at an angle of ∼25°, to the direction of easy magnetization. The magnetic domains are shown to have a mazelike structure on a scale of approximately 0.3–0.5 μm in the plane perpendicular to the c axis. In the plane parallel to the c axis a much finer magnetic structure was observed. There is evidence that domains extend across the interfaces between neighboring ribbons.
Thermally stimulated current related to Cu antisite-vacancy complex defects in Cu-diffused semi-insulating GaAs
Authors: Kuriyama K. , Tomizawa K. , Uematsu S. , Takahashi Hirokazu
Published: July 2002

The evaluation of the copper related complex defects in copper-diffused semi-insulating GaAs was studied using thermally stimulated current (TSC) method. Two new TSC traps, T1 (ionization energy 0.25 eV) and T2 (0.52 eV), are observed. These traps are consistent with two acceptor levels evaluated by van der Pauw method and samples change p as the T2 trap appears with increasing copper content. These results suggest that the shallower T1 trap compensates not only the midgap electron trap (EL2 defect) but also the deep T2 trap. The relative photoionization cross sections, which have a maximum at around 940 nm (1.32 eV) for the T1 trap and at around 1400 nm (0.89 eV) for the T2 trap, respectively, are obtained. Origins of these traps are also discussed.
Thermodynamically stable p-channel strained-layer AlGaAs/InGaAs/GaAs heterostructure field effect transistor
Authors: Baca A. G. , Zipperian T. E. , Howard A. J. , Klem J. F. , Tigges C. P.
Published: July 2002

Device characteristics of a thermodynamically stable p-channel, strained quantum-well heterostructure field effect transistor (HFET) are reported. The AlGaAs/InGaAs/GaAs material system was used to fabricate the p-channel HFETs with Al and In mole fractions of 0.20 and 0.18, respectively. Transconductances of 32 and 94 mS/mm were achieved at 300 and 77 K, respectively, for devices with 1.2 μm recessed gates. These numbers are comparable to InGaAs quantum-well, recessed gate pHFETs whose quantum-well thicknesses exceed the thermodynamic stability limit. These results have important implications for high performance self-aligned devices which require high-temperature processing.
Artificial two-phase Nb-Ti nanostructures using powder metallurgy techniques
Authors: Jablonski P. D. , Lee P. J. , Larbalestier D. C.
Published: July 2002

Several techniques to form artifical high critical current density superconducting Nb-Ti nanostructures have been reported. A drawback to virtually all of these techniques is that they require fabrication strains of 30 or more to reduce the second phase pinning center to the optimum 1–10 nm size at which critical current densities of 103–104 A/mm2 are obtained. Here we describe a powder metallurgy process that yields 6020 A/mm2 at 2 T and 1470 A/mm2 at 5 T for an alloy with an upper critical field of ∼8 T within the same strain space (∼13) employed in the conventional Nb-Ti fabrication process.
Vertically tapered InGaAsP/InP waveguides for highly efficient coupling to flat-end single-mode fibers
Authors: Brenner T. , Bachmann M. , Melchior H.
Published: July 2002

We report on InGaAsP/InP optical waveguide tapers for efficient coupling to flat-end single-mode fibers. These integrated tapers are realized by an optoelectronic integrated circuit compatible diffusion limited etch process for the vertically tapered core layer and a selective area regrowth for the upper cladding. The smoothly etched vertical tapers and the excellent quality of the regrown layers show low scattering and radiation losses. Polarization insensitive fiber to waveguide coupling losses are below 1 dB and relaxed alignment tolerances are ±2.5 μm in the horizontal and vertical, and 28 μm in longitudinal directions for 1 dB in addition.
Scanning tunneling microscopy of step bunching on vicinal GaAs(100) annealed at high temperatures
Authors: Skala S. L. , Chou S. T. , Cheng K.-Y. , Tucker J. R. , Lyding J. W.
Published: July 2002

Step bunching and terrace widening are observed with scanning tunneling microscopy on GaAs(100), tilted 2° toward [1̄10] and [110], after annealing in ultrahigh vacuum at 600 °C. The resulting surface consists of two separate phases, c(8×2) reconstructed terraces and a bunched step region that is either amorphous or (2×6) reconstructed. Average terrace widths increase to 2 or 4 times the nominal average terrace width for a 2° miscut and steps are correspondingly compressed by 40% or 75% depending on the misorientation direction. The observed step bunching is explained by a thermodynamic instability against facetting resulting from a high step energy on c(8×2) reconstructed surfaces.
Evidence for fullerene with single chlorine anion inside
Authors: Zhu Lei , Wang Shenyi , Li Yufen , Zhang Zhenman , Hou Huiqi , Qin Qizhong
Published: July 2002

We present evidence for fullerenes with single chlorine anion inside by the laser desorption mass spectrometric technique in negative-ion channel. Coalescence reaction of fullerenes was also observed in the processes and the possible mechanism involved is briefly proposed.
Structures and electronic properties of misfit dislocations in ZnSe/GaAs(001) heterojunctions
Authors: Chen Y. , Liu X. , Weber E. , Bourret E. D. , Liliental-Weber Z. , Haller E. E. , Washburn J. , Olego D. J. , Dorman D. R. , Gaines J. M. , Tasker N. R.
Published: July 2002

The structures and electronic properties of misfit dislocations in ZnSe/GaAs(001) heterojunctions have been studied by transmission electron microscopy, cathodoluminescence (CL), and photoluminescence. In undoped ZnSe epilayers, irregular dislocation segments tending to lie roughly along [100] and [010] directions were observed as well as long straight 60° dislocations along [110] and [11̄0] directions. In N-doped ([N]≳1×1018 cm−3) ZnSe epilayers, the misfit dislocations were predominantly dissociated into partial dislocations which makes cross slip and formation of irregular dislocations more difficult; only the straight dislocations along [110] and [11̄0] were observed. The CL observations suggest that the irregular dislocations trap carriers more efficiently than the dislocations along 〈110〉.
Micropatterning of cultured cells on polystyrene surface by using an excimer laser
Authors: Lee Jae-Suk , Sugioka Koji , Toyoda Koichi , Offenhäusser Andreas , Knoll Wolfgang , Sasabe Hiroyuki
Published: July 2002

Micropatterning of cultured cells on the surface of polystyrene was formed using a 248 nm KrF excimer laser and less than 10 pulses. Adhesion and proliferation of NG 108-15 neuroblastoma x glioma hybrid cells were good on the oxygen plasma treated polystyrene (O-PSt) surface, but poor on the PSt surface. The surfaces were characterized with a surface profiler, scanning electron microscopy, contact angle microscopy, and x-ray photoelectron spectroscopy. When the laser ablation was carried out on a masked O-PSt surface, a patterned surface architecture and the micropattern of cultured cells were clearly obtained. A positive cell pattern was also achieved on the PSt surface by laser modification.
Time-resolved investigations of sidewall recombination and in-plane diffusivity in dry-etched InGaAs/GaAs air-post structures
Authors: Nambu Yoshihiro , Yokoyama Hiroyuki , Yoshikawa Takashi , Sugimoto Yoshimasa , Asakawa Kiyoshi
Published: July 2002

Nonradiative carrier recombination in dry-etched InGaAs/GaAs air-post structures is investigated by time-resolved photoluminescence measurement. Both linearly and squarely size-dependent regions are observed in the measured carrier lifetime. It is shown that in-plane diffusivity Dr and sidewall nonradiative recombination velocity Sr can be simultaneously evaluated using a carrier diffusion model. Comparing an as-etched sample and a sample wet etched after dry etching shows that Dr as well as Sr are changed by removing the subsurface layer at the sidewall by wet etching. Possible causes of this change and the mechanism determining Sr are discussed.
High peak power, burst mode operation of a multikilowatt CO2 laser for deep penetration welding
Authors: Scott S. W. C. , Rogozinski K. , Seguin H. J. J.
Published: July 2002

A significant advancement in laser welding of heavy metal sections is presented. Operation of a photoinitiated, impulse-enhanced, electrically excited multikilowatt continuous wave (cw) laser in the pulse periodic regime results in significant improvement in weld penetration depth. High average power pulsed operation is made possible through the use of a low level trigger for a digitally controlled hydrogen thyratron pulser circuit. Peak optical powers on the order of 3 times the cw level are produced while maintaining multikilowatt average output power. A comparison of cw and pulsed welding of X70 pipeline steel is included. Burst mode welding with this system is found to yield up to a 50% increase in weld penetration with a superior weld profile when compared to cw welding at the same average power level.
Reduction of zinc diffusion into the collector of InP-based double heterojunction bipolar transistors grown by organometallic chemical vapor deposition
Authors: Bhat R. , Koza M. A. , Song J.-I. , Schwarz S. A. , Caneau C. , Hong W.-P.
Published: July 2002

It is shown that the growth of emitter layers of InP/InGaAs/InP double heterojunction bipolar transistors can result in significant Zn diffusion from the base into the collector, with the extent of diffusion depending on the n-doping level of the emitter. This behavior is explained in terms of nonequilibrium point defects induced by a combination of surface pinning of the Fermi level and n doping. It is also shown that the Zn diffusion can be substantially reduced by using AlInAs, instead of InP, as the emitter layer. The difference in behavior is shown to be at least in part due to the lower diffusivity of group III interstitials in AlInAs. Furthermore, it is shown that the introduction of only 50 nm of AlInAs between the n-InP emitter and p+-InGaAs base resulted in a significant reduction of Zn diffusion into the collector.
Absolute active frequency locking of a diode laser with optical feedback generated by Doppler-free collinear polarization spectroscopy
Authors: Liu Zi Dong , Bloch Daniel , Ducloy Martial
Published: July 2002

Absolute stabilization of the emission frequency of a laser diode can be achieved in a nearly all-optical scheme involving a resonant optical feedback from an atomic signal. We propose here two novel schemes of collinear Doppler-free polarization spectroscopy and report on preliminary experimental demonstrations of diode frequency locking performed on the D2 line of Rb.
Low-temperature gettering of trace iron and copper by misfit dislocations in Si/Si(Ge) epitaxy
Authors: Lee D. M. , Rozgonyi G. A.
Published: July 2002

Two examples are presented to illustrate that transition metallic impurities can be extrinsically gettered during low-temperature processing, even when they are present at trace background levels. Samples of Si/Si(Ge)/Si epitaxial heterostructures containing interfacial misfit dislocations were studied using cross-sectional analytical transmission electron microscopy (TEM) after a 700 °C annealing in vacuo. In one example, a trace iron precipitate (∼6.5 nm in diameter) which segregated at an end-on misfit dislocation was analyzed by nanoprobe x-ray spectroscopy combined with high-resolution lattice imaging. This iron-containing precipitate was cubic and epitaxially aligned with the host Si crystal. In a second example, a trace copper precipitate (∼0.5 μm in diameter) with a disk-like morphology was identified along a misfit dislocation segment. These results demonstrate the effectiveness of misfit dislocations as extrinsic gettering sites at very low-impurity levels.
Effect of carrier charge imbalance on the threshold current in diode lasers with thin intrinsic quantum wells
Authors: Kosinovsky G. A. , Grupen M. , Hess K.
Published: July 2002

This study is based on the results of a two-dimensional self-consistent laser simulator MINILASE for the GaAs/AlGaAs system. It predicts that the electron/hole density ratio in thin quantum wells (QW’s) of laser diodes with intrinsic QW active regions (QW p-i-n lasers) can be significantly different from unity and depends on the doping density near the active region. These deviations from local charge neutrality can have significant effects on the laser threshold.
Electrical conductivity percolation in the (CdTe)1−xTex system
Authors: Ramírez-Bon R. , Espinoza-Beltrán F. J. , Pedroza-Montero M. , Ruíz F. , González-Hernández J. , Zelaya-Angel O. , Sánchez-Sinencio F.
Published: July 2002

The critical volume fraction for conductivity percolation in the di-phasic system (CdTe)1−x Tex has been determined using atomic force microscopy. The onset for rapid increase in the electrical conductivity is found at a volume fraction of Te of about 0.4. According to previous theoretical calculations this value is characteristic of a system with two-dimensional symmetry. It is also found that the Te phase grows in the form of columns, which explains the observed critical volume fraction value for percolation in the conductivity measurements.
Anisotropy and irreversibility line of iodine intercalated Bi2Sr2CaCu2O8+δ single crystals
Authors: Ricketts J. , Puzniak R. , Liu C.-J. , Gu G. D. , Koshizuka N. , Yamauchi H.
Published: July 2002

A comparative study of the superconducting properties of pristine and stage 1 iodine intercalated Bi2Sr2CaCu2O8+δ single crystals is presented. We find the anisotropy of the penetration depth to be significantly reduced after intercalation. Specifically, the penetration depth perpendicular to the CuO2 planes is dramatically decreased, by about 60%. Reducing the anisotropy improves the position of the irreversibility line in the low field region where pinning is weak. However, in high fields the position of the irreversibility line is depressed. This can be explained by the influence of strong flux pinning.
Observation of ballistic conductance and Aharonov–Bohm oscillations in Si/SiGe heterostructures
Authors: Gao W. X. , Ismail K. , Lee K. Y. , Chu J. O. , Washburn S.
Published: July 2002

We have fabricated quantum devices from remotely doped Si/SiGe heterostructures. The devices are interferometers (loops) similar in plan to those used in experiments on ballistic GaAs/AlxGa1−xAs devices. The loops are approximately 2r=0.8 μm in diameter with linewidths of w=0.4 μm. We have observed clear Aharonov–Bohm (AB) oscillations that vanish systematically as the carrier temperature increases. Response of up to the second harmonic of the fundamental AB frequency e/h implies a phase coherence length of around Lφ=1.2 μm. In some samples, we see steps in conductance G(Vg) as a function of gate voltage similar to the ballistic mode steps seen in GaAs/AlxGa1−xAs point contacts.
Gain coupled distributed feedback lasers realized by masked implantation enhanced intermixing
Authors: Kaden C. , Gauggel H.-P. , Hofsäss V. , Hase A. , Schweizer H. , Künzel H.
Published: July 2002

Long wavelength 1.3 μm gain coupled distributed feedback (GC-DFB) lasers were realized by masked implantation enhanced intermixing. On the basis of this full planar technology GC-DFB lasers with first and second order gratings were fabricated in the InGaAs/InGaAlAs system. The lasers show clear GC-DFB laser emission with a single mode yield of 69% and remain single mode up to several times laser threshold. The results are in good agreement with the theoretical description of our gain coupled lasers by optical matrix theory.
Thermodynamic guidance for implantation treatments to improve coating–substrate adhesion
Authors: Singer I. L. , Fayeulle S. , Ehni P. D. , Vardiman R. G.
Published: July 2002

A novel ‘‘seeding’’ method is disclosed, in which ion implanted Al atoms promote bonding between SiC and a subsequently deposited Al2O3 coating. Al+ ions were implanted in SiC substrates near room temperature to a fluence of 4×1017 Al/cm2 at an energy of 110 keV. The substrates were coated with 100–200 nm of aluminum oxide and then heated in air (i.e., oxidized) for 4 hours at temperatures from 500 to 1000 °C. Scratch tests demonstrated that coatings spalled from nonimplanted substrates but not from Al+-implanted substrates. Auger sputter depth profiles of substrates oxidized at 850 °C showed that Al diffused out of the implanted layer and an Al-Si-O layer formed at the coating-substrate interface. Cross-section and plan TEM of a substrate oxidized at 1000 °C identified the interface phase as mullite and found fine-grained SiC and mullite (Al6Si2O13) in the implanted layer. The composition and phase changes were explained in terms of the thermochemical reactions expected of the Al-Al2O3-SiC-O2 system.
Enhancement of transport critical current densities in (Bi,Pb)2Sr2Ca2Cu3O10/Ag tapes at 77 K following fast neutron irradiation
Authors: Hu Q. Y. , Weber H. W. , Sauerzopf F. M. , Schulz G. W. , Schalk R. M. , Neumüller H. W. , Dou S. X.
Published: July 2002

We report on significant enhancements of the transport critical current densities Jc in (Bi,Pb)2Sr2Ca2Cu3O10/Ag tapes at 77 K and higher magnetic fields after neutron irradiation. This enhancement is attributed to an improvement in the flux pinning capability of this material by the neutron-induced defects. The angular dependence of Jc is still consistent with two-dimensionality, i.e., flux pinning of pancake and/or Josephson vortices is directly confirmed by this transport measurement. Fast neutron irradiation also affects the weak links in a way which agrees with previous results on unaligned ceramics. However, there are some peculiarities which need further investigation.
Phase equilibria and melt processing of Bi2Sr2Ca1Cu2O8+x tapes at reduced oxygen partial pressures
Authors: MacManus-Driscoll Judith L. , Wang Pin-Chin , Bravman John C. , Beyers Robert B.
Published: July 2002

A coulometric titration technique has been employed to melt process Ag-clad Bi2Sr2Ca1Cu2O8+x (Bi-2212) tapes in a region of the phase diagram corresponding to binary melting. This region occurs over the temperature range 798–883 °C and the oxygen partial pressure range 2.8×10−4–7.1×10−2 atm. Phase-pure, well-aligned tapes with magnetization Jc’s of 7×104 A cm−2 have been obtained at 4.2 K in zero field.
Importance of steps in heteroepitaxy: The case of aluminum on silicon
Authors: Sosnowski Marek , Ramac Samuel , Brown Walter L. , Kim Young O.
Published: July 2002

We have observed dominant epitaxial growth of Al(100) films on chemically cleaned, hydrogen terminated, off-oriented Si(111). The films were deposited by thermal evaporation at room temperature in ultrahigh vacuum. X-ray diffraction shows sharp and intense Al(200) diffraction, enhanced by postdeposition annealing. Crystal quality and the dominance of Al(100) structure depend strongly on the substrate treatment and the off-cut angle, both of which control the steps on the Si(111) surface. The steps were found responsible for the epitaxial alignment of the film and the substrate lattices. Details of this alignment were observed in transmission electron microscopy cross-sectional images of the interface. Our findings are in contrast to previously published results which indicate epitaxial growth of Al(111) on Si(111).
Electrically induced uniform planar alignment of S*C liquid crystals on homeotropically treated plates
Authors: Jákli A. , Saupe A
Published: July 2002

Chevron textures of SC* films are usually obtained with the conventional procedure where a planar alignment coating (e.g., buffed polyimide) is used. We describe a new technique which enables us to obtain uniform bookshelf textures which show bistability and can be switched with low fields (∼1 V/μm). This is achieved with a homeotropic surface coating. With a homeotropic coating the samples form a uniform texture with horizontal layers on cooling. The homeotropic texture becomes unstable under sufficiently strong electric fields and transforms to a birefringent texture with smectic layers oriented vertically. Without an additional bias the texture has domains of different layer orientations. By simultaneous application of a vertical field and horizontal shear stable uniform bookshelf textures can be obtained. Here we show that it is possible to obtain uniform bookshelf alignment by the simultaneous action of a horizontal field gradient and ac fields. The horizontal gradient leads to a more or less well-defined propagating front of the texture transformation. It results in a biased domain texture with vertical layers. The effect of the strong ac field (∼100 Hz) is to smooth out the domain texture.
Plasma etched polycrystalline hot-filament chemical vapor deposited diamond thin films and their electrical characteristics
Authors: Liaw B. Y. , Stacy T. , Zhao G. , Charlson E. J. , Charlson E. M. , Meese J. M. , Prelas M. A.
Published: July 2002

Etching of hot-filament, chemical vapor deposited, diamond thin films utilizing low energy ion irradiation was investigated. The films used in this study were boron doped polycrystalline diamond, deposited on p-type (100) oriented silicon substrates. A low voltage dc corona discharge with an oxygen plasma was used to sputter etch the films. Surfaces were investigated by scanning electron microscopy and profilometry. Etch rates were approximately 500 Å/min, depending on the various processing conditions. Characteristics of In/diamond/Si Schottky diodes were used to evaluate the electrical properties of diamond surfaces with various treatments. Results indicate that plasma etching can significantly affect Schottky device characteristics.
Identification and correlation of microstructural defects with flux pinning in Ni-doped melt textured YBa2Cu3O7−δ
Authors: Pavate V. , Williams L. B. , Kvam E. P. , Kozlowski G. , Endres W. , Oberly C. E.
Published: July 2002

Microstructural examination using transmission electron microscopy has been conducted on melt textured Ni-doped and pristine YBaCu3O7−δ. Magnetization hysteresis suggests that the observed microstructural imperfections strongly influence flux pinning behavior. The investigation reveals a considerable difference in the density and distribution of stacking faults, especially near the grain boundaries. Dislocation nets and tangles were also observed in the Ni-doped samples. It is proposed that the addition of Ni creates an excess of Cu which diffuses into the subgrains during oxygenation to be incorporated as stacking faults. Fine inclusions (25–50 nm) of Y2BaCuO5 were observed, suggesting that Ni refines the microstructure.