bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation between radiative transitions and structural defects in zinc selenide epitaxial layers
Authors: Shahzad K. , Petruzzello J. , Olego D. J. , Cammack D. A. , Gaines J. M.
Published: July 2002

We present low-temperature photoluminescence and transmission electron microscopy data to show that two transitions I0V at ∼2.774 eV and Y0 at ∼2.60 eV, frequently observed in unintentionally doped zinc selenide epitaxial layers, are directly related to structural defects. It is shown that these transitions are strong in those samples which have very low background impurities and high density of structural defects and weak in those cases that have either high background impurities or low density of structural defects.
Laser desorption mass spectra of YBa2Cu3O7−x
Authors: Chen C. H. , Murphy T. M. , Phillips R. C.
Published: July 2002

Quantitative measurements of desorbed ions and neutral molecules by a quadrupole mass spectrometer from laser ablation of YBa2Cu3O7−x were obtained. The results indicate that the number of desorbed neutral atoms and molecules is in general greater than the number of corresponding ions.
The mechanism of electron capture by DX centers
Authors: Su Z. , Farmer J. W.
Published: July 2002

It has been found that the electron capture transients of the individual DX levels in Si-doped AlGaAs include two regimes: net capture and redistribution. In this letter, by analyzing both of these regimes, the electron capture kinetics of DX centers are determined for the first time. It is found that the capture rate for an empty state is proportional to the square of the free carrier concentration. These kinetics are predicted only by the negative U model in which two electrons are captured sequentially via an intermediate one-electron state.
Laser deposition of CdS/Y-Ba-Cu-O heterostructures
Authors: Shi L. , Hashishin Y. , Dong S. Y. , Zheng J. P. , Kwok H. S.
Published: July 2002

Pulsed laser deposition was used to grow high-quality Y-Ba-Cu-O (YBCO) and CdS thin films sequentially on MgO. A temperature window between 200 and 250 °C was found to be suitable for the deposition of CdS on YBCO. The transition temperature of the heterostructure was measured to be 72 K while the single YBCO film had a Tc of 85 K. The degradation was determined to be due to interface reaction, rather than the result of poor deposition conditions. X-ray measurements indicated that the CdS film was hexagonal and c-axis oriented. In situ resistivity measurement was used to monitor the film growth dynamics.
Negative light-modulation effect of boron-doped hydrogenated amorphous silicon
Authors: Maeda Yoshinobu , Yamamoto Shigeichi , Migitaka Masatoshi
Published: July 2002

Laser intensity dependence of transmission and the light-modulation effect of boron-doped hydrogenated amorphous silicon (a-Si:H) were investigated with 780 nm laser diodes. Negative intensity dependence, which causes the transmission to decrease as the laser intensity is increased, was observed for B-doped, but not for undoped a-Si:H. Moreover, a negative light-modulation effect was discovered where the constant signal light decreases with increasing control light. To explain these phenomena, a double-absorption model, which has two absorption processes from a level associated with the impurity and from a valence band, was considered.
Effect of collector-base valence-band discontinuity on Kirk effect in double-heterojunction bipolar transistors
Authors: Mazhari B. , Morkoç H.
Published: July 2002

The effect of valence-band discontinuity at the collector base heterojunction on the current gain and base charge storage is modeled. It is shown that the onset of the Kirk effect is accompanied by a sharp drop in the current gain and ft due to the formation of a potential barrier. The variation of barrier height with collector current density is determined and its effect on current gain and base transit time described. The results discussed here are applicable to Si/SiGe double-heterojunction bipolar transistors.
375-GHz-bandwidth photoconductive detector
Authors: Chen Yi , Williamson Steven , Brock Tim , Smith F. W. , Calawa A. R.
Published: July 2002

We report the development of a new, integrable photoconductive detector, based on low-temperature-grown GaAs, that has a response time of 1.2 ps and a 3-dB bandwidth of 375 GHz. The responsivity is 0.1 A/W. Signal amplitudes up to 6 V can be produced with virtually no degradation in response time.
High quality (111)B GaAs, AlGaAs, AlGaAs/GaAs modulation doped heterostructures and a GaAs/InGaAs/GaAs quantum well
Authors: Chin Albert , Martin Paul , Ho Pin , Ballingall Jim , Yu Tan-hua , Mazurowski John
Published: July 2002

We report the successful growth of high quality molecular beam epitaxy (MBE) GaAs, AlGaAs, AlGaAs/GaAs modulation doped heterostructures and a GaAs/InGaAs/GaAs quantum well on GaAs (111)B substrates. Modulation doped heterostructures show a 77 K mobility of 145 500 cm2/V s with a sheet density of 5.0×1011 cm−2. Photoluminescence of (111)B GaAs indicates a lower carbon incorporation than achieved on (100) substrates. The low growth temperature and high material quality obtainable in (111)B growth will provide advantages for laser diodes and heterostructure field effect transistors.
High-contrast optically bistable optoelectronic switch based on InGaAs/GaAs (100) asymmetric Fabry–Perot modulator, detector, and resonant tunneling diode
Authors: Chen Li , Kapre R. M. , Hu Kezhong , Madhukar A.
Published: July 2002

The realization at room temperature of a high contrast ratio (20:1) and an on-state reflectivity of 46.5% in an optically bistable switch involving strained InGaAs/GaAs (100) multiple-quantum-well-based asymmetric Fabry–Perot reflection modulator, detector, and InGaAs/AlAs-based resonant tunneling diode and an Si field-effect transistor is reported.
Laser ablation of aluminum at 193, 248, and 351 nm
Authors: Wang Hongxin , Salzberg A. P. , Weiner Brad R.
Published: July 2002

Pulsed laser ablation of an aluminum target at 193, 248, and 351 nm has been investigated by laser-induced fluorescence (LIF) spectroscopy. Velocity distributions of the ablated Al atoms have been measured directly by monitoring their flight times from the target to the probe laser beam, and by Doppler laser spectroscopy. The velocity distributions resulting from the three wavelengths can all be characterized as hyperthermal, with average velocities of between 4.5×105 and 6.5×105 cm/s over a fluence range of 0.3–6.5 J/cm2. Average values of the Al atom distribution can also be derived by measuring the AlO internal energy distribution of the ‘‘hot’’ Al atom reaction with low pressures of O2. For excimer laser ablation of aluminum at 248 nm, we have found an average Al atom velocity of ≳2×105 cm/s. Possible mechanisms for the production of these hyperthermal velocity distributions are discussed.
Optical phase modulator utilizing electroabsorption in a Fabry–Perot cavity
Authors: Pezeshki B. , Williams G. A. , Harris J. S.
Published: July 2002

By changing the absorption coefficient in a Fabry–Perot cavity from a point below to a point above the critical matching condition, the phase of the reflected beam at resonance can be changed. At a constant refractive index, the phase switches by 180°. We demonstrate a device fabricated with GaAs/AlGaAs quantum wells that uses a combination of index and absorption modulation to produce a 90° phase change in a vertical Fabry–Perot cavity with an insertion loss of about 6 dB. Similar phase measurements on a conventional normally on absorption modulator yielded a chirp parameter, αc<0.4, a value superior to that of waveguide electroabsorption devices.
Crystallization-induced stress in silicon thin films
Authors: Miura Hideo , Ohta Hiroyuki , Okamoto Noriaki , Kaga Toru
Published: July 2002

Residual stress change in silicon thin films during crystallization of amorphous silicon is discussed experimentally by detecting the wafer curvature change using a scanning laser microscope. The as-deposited amorphous-silicon film shows compressive stress of about 200 MPa. During a crystallization reaction at about 650 °C, a large tensile stress of about 1000 MPa develops in the film due to film shrinkage. The final residual stress of polycrystalline film depends on the film formation process.
Double wavelength selective GaAs/AlGaAs infrared detector device
Authors: Köck A. , Gornik E. , Abstreiter G. , Böhm G. , Walther M. , Weimann G.
Published: July 2002

We demonstrate the first GaAs/AlGaAs multiquantum well infrared detector device with double wavelength selectivity and high wavelength resolution. By applying an efficient light coupling mechanism, which is based upon the excitation and emission of surface plasmons, we have achieved a responsivity R of 2.20 A/W and a detectivity D* of 2.2×1011 cm √Hz/W for λ=7.38 μm at a temperature T=5 K for a 300 K background irradiance and a 90° field of view, which are one of the highest values reported to date.
Strain tensor elements for misfit-strained [hhk]-oriented cubic crystals
Authors: Caridi E. A. , Stark J. B.
Published: July 2002

We calculate the strain tensor components for (hhk)-oriented cubic materials with misfit-generated strain. We assume the strained layer to be free of crystalline defects, and derive the tensor elements via minimization of the crystal strain energy with the imposed constraint that the epitaxial film remains in registry with the substrate lattice. The strain components described, both in the crystalline basis and in the basis of the growing film, are written in terms of the lattice mismatch and the growth-axis Miller indices. We show that the accommodation of misfit-generated strain produces a tetragonal distortion for all (hhk)-grown layers, and use the above results to interpret x-ray diffraction data.
Real-time picosecond electro-optic oscilloscope technique using a tunable semiconductor laser
Authors: Galvanauskas A. , Tellefsen J. A. , Krotkus A. , Öberg M. , Broberg B.
Published: July 2002

The feasibility of using a tunable semiconductor laser for real-time electro-optic measurements of picosecond electrical signals is demonstrated. The method is based on real-time amplitude modulation of a relatively long optical probe pulse, whose frequency is linearly swept (chirped) to provide temporal information. Such chirped optical probe pulses were obtained by fast tuning of the emission wavelength of a three-sectional distributed-Bragg-reflector diode laser. An electrical-pulse rise time of 18 ps was measured. Temporal-resolution limitations of this type of electro-optic oscilloscope are discussed.
Molecular beam epitaxial growth of Si on Ga-activated Si(111) surface
Authors: Nakahara Hitoshi , Ichikawa Masakazu
Published: July 2002

Molecular beam epitaxial (MBE) growth of silicon on gallium-activated Si(111) surface is investigated by microprobe reflection high-energy electron diffraction. Improvement in crystallinity is observed for a film grown on Ga adsorbed surface compared with that grown on a clean Si surface. Substrate temperature dependence of denuded zone width of two-dimensional (2D) nuclei is measured for various growth rates. The measurements give the activation energy of surface diffusion and the critical 2D nucleus size. Due to gallium adsorption on the surface, the activation energy becomes lower and the size becomes larger than those on the clean Si surface. This improves the crystallinity of the grown film. MBE growth of Si on partially activated surface is also reported. It is found that a film with good crystallinity grows selectively on Ga-adsorbed area.
Material properties of Ga0.47In0.53As grown on InP by low-temperature molecular beam epitaxy
Authors: Künzel H. , Böttcher J. , Gibis R. , Urmann G.
Published: July 2002

Undoped Ga0.47In0.53As layers were grown by molecular beam epitaxy lattice matched to InP at substrate temperatures, Ts, in the range from 100 to 600 °C. X-ray diffraction indicated a widening of the vertical lattice parameter and a simultaneous increase of the arsenic content at low growth temperature. The epitaxial layers were single-crystalline down to Ts=125 °C. The room-temperature residual carrier concentrations and the related mobilities for layers grown below 350 °C are strongly affected by Ts, whereas at 77 K an influence of Ts on these parameters is already visible at 450 °C. At the very low growth temperatures the epitaxial layers show highly conductive behavior attributed to defect induced ionized deep centers.
Enhanced carrier densities and device performance in piezoelectric pseudomorphic high-electron mobility transistor structures
Authors: Sánchez-Dehesa J. , Sánchez-Rojas J. L. , Lopez C. , Nicholas R. J.
Published: July 2002

The addition of a piezoelectric field in AlGaAs/InGaAs/GaAs HEMT structures is shown to lead to enhanced electron densities and hence improved device performance. Growth of a strained InxGa1−xAs layer is in [111]A direction causes a piezoelectric field to be built into the quantum well of a pseudomorphic HEMT, which opposes the electric field due to charge transfer and hence lowers the confinement energy. This leads to carrier densities 50% larger than in equivalent [100] structures, with the wave function also spaced further away from the dopant impurities and the well interfaces. We expect these factors to give improved device performance.
High-density optical storage based on nanometer-size arsenic clusters in low-temperature-growth GaAs
Authors: Nolte D. D. , Melloch M. R. , Ralph S. J. , Woodall J. M.
Published: July 2002

Nanometer-size arsenic clusters in low-temperature-growth molecular beam epitaxy GaAs provide the basis for a high-density optical storage medium. The material exhibits a large Franz–Keldysch electro-optic effect at room temperature. Charge storage on the clusters and the excitonic electro-optic properties of the material combine to make a low-power high-density photorefractive storage medium. The ultrafast lifetimes of the photogenerated carriers produce excellent spatial resolution during the writing of holographic space-charge gratings. Fringe spacings as small as 0.6 μm can be supported in this material, yielding an optical data density greater than 108 bits/cm2. The saturation intensity is 2 mW/cm2 with a storage time of 2 ms.
Paramagnetic centers of amorphous carbon thin films: Influence of hydrogen content and O2 permeation
Authors: Hoinkis M. , Tober E. D. , White R. L. , Crowder M. S.
Published: July 2002

Electron paramagnetic resonance (EPR) studies of sputtered amorphous hydrogenated carbon (a-C:H) films have been conducted in several gaseous ambients, at three microwave frequencies, and as a function of film H content. Unpaired electron concentrations are found to decrease with increasing H content while EPR linewidths are shown to be a function of H content; unresolved proton hyperfine interaction is the linewidth determining factor for H content greater than 24% while conductivity is suggested to be the most significant linewidth determining factor for H content less than 24%. Furthermore, the EPR linewidth of a-C:H films broadens in the presence of gaseous O2 and demonstrates that these films are permeable to O2 and other gases.
Switchable bicolor infrared detector using an electron transfer infrared modulator
Authors: Berger V. , Vodjdani N. , Bois P. , Vinter B. , Delaitre S.
Published: July 2002

In this study, we have realized a bicolor switchable detector, in which tunneling between two quantum well detectors of different widths is used to populate or deplete their ground level. The behavior of the bicolor detector is simulated using a self-consistent model, which shows that under electric field, only the downstream well detects. The performance of the sample as a modulator and as a bicolor detector are analyzed in a Fourier transform infrared spectrometer.
Observation of quantum wire formation at intersecting quantum wells
Authors: Goñi A. R. , Pfeiffer L. N. , West K. W. , Pinczuk A. , Baranger H. U. , Stormer H. L.
Published: July 2002

We report the first observation of a quantum bound state formed at the junction of two intersecting quantum wells in the shape of a T. The atomically precise T junctions are fabricated by a novel cleaved edge overgrowth process in the AlGaAs/GaAs system. The identification of bound states with energies in excess of 20 meV is made by optical emission and absorption spectroscopy. Such quantum wire states are caused by the unique confinement of the lowest state wave function to the region of the T junction.
Magnetoresistance effect of La0.72Ca0.25MnOz/YBa2Cu3Oy/La0.72Ca0.25 MnOz trilayered films
Authors: Chahara Ken’ichi , Ohno Toshiyuki , Kasai Masahiro , Kanke Yoko , Kozono Yuzoo
Published: July 2002

The magnetoresistance (MR) effects of La0.72Ca0.25MnOz (LCMO) magnetic films and LCMO/YBa2Cu3Oy (YBCO)/LCMO trilayered films were studied. The LCMO films have an MR ratio of −4% when applying 1 T at 77 K and this value is independent of the film thickness from 1500 to 4500 Å. In contrast to that, the MR ratio for LCMO/YBCO/LCMO films depends on the thickness of the YBCO layer and is more than 1.5 times as large as that for LCMO films when the thickness of the YBCO layer is less than 2500 Å. These results imply that the magnetic spin interaction between the two LCMO layers may exist through the normal-conductive state YBCO layer.
Enhanced electro-optic properties of low-temperature-growth GaAs and AlGaAs
Authors: Nolte D. D. , Melloch M. R. , Woodall J. M. , Ralph S. J.
Published: July 2002

The signal-bandwidth products of excitonic electroabsorption of low-temperature-growth (LTG) molecular beam epitaxial films of GaAs:As and Al0.25Ga0.75As:As are larger than in the related stoichiometric materials. The enhanced electro-optic properties of these composites may be caused by increased inhomogeneity of dc electric fields. The differential transmission in LTG Al0.25Ga0.75As:As annealed at 750 °C for 30 s is relatively broadband and approaches 60% for dc electric fields of only 1.5×104 V/cm.
Comparison of boron diffusion in Si and strained Si1−xGex epitaxial layers
Authors: Kuo P. , Hoyt J. L. , Gibbons J. F. , Turner J. E. , Jacowitz R. D. , Kamins T. I.
Published: July 2002

We have investigated boron diffusion in Si and strained Si1−xGex, in situ doped, epitaxial layers. During inert ambient annealing at 860 °C, boron diffusion is observed to be slower in Si0.83Ge0.17 than in Si for boron concentration levels between 5×1016 and 2.5×1019 cm−3. Computer simulations of the measured boron profiles for annealed samples indicate that the effective boron diffusivity Deff in Si0.83Ge0.17 is approximately an order of magnitude lower than that in Si. This disparity is found to increase with increasing boron concentration.
Determination of the conduction-band offset of a single AlGaAs barrier layer using deep level transient spectroscopy
Authors: Zhu Q. S. , Mou S. M. , Zhou X. C. , Zhong Z. T.
Published: July 2002

The tunneling from an AlGaAs confined thin layer to a GaAs layer in the GaAs/Al0.33Ga0.67As/GaAs structure during the trapped electron emission from deep level in the AlGaAs to its conduction band has been observed by deep level transient spectroscopy. With the aid of the tunneling effect, the conduction-band offset ΔEc was determined to be 0.260 eV, corresponding to 63% of ΔEg. A calculation was also carried out based on this tunneling model by using the experimental value of ΔEc=E2−E1=0.260 eV, and good agreement between the experimental and calculated curves is obtained.
Optical bistable responses based on self-electro-optic effect in a nonbiased asymmetric coupled quantum well p-i-n photodiode
Authors: Tokuda Yasunori , Abe Yuji
Published: July 2002

We demonstrated the optical bistable responses of the self-electro-optic effect device without applying any external bias. The function was realized by taking advantage of the significant electric-field dependence of both intensity and wavelength of an excitonic absorption peak, which is related to a resonance of electron levels in an asymmetric coupled quantum well structure. The successful result suggests that it is possible to create an all-optical bistable device with nonsegmented planar structure.
New degradation mechanism associated with hydrogen in bipolar transistors under hot carrier stress
Authors: Gopi P. K. , Li G. P. , Sonek G. J. , Dunkley J. , Hannaman D. , Patterson J. , Willard S.
Published: July 2002

Avalanche hot carrier induced bipolar device degradation as a function of temperature, current density, and time is reported. The observed drift in emitter-base breakdown voltage (Vebo) is found to be well correlated to changes in forward base (Ib) and collector (Ic) currents. The model of hydrogen release from the Si-SiO2 interface and its subsequent passivation of base dopants during hot carrier stress is proposed to account for such a correlation.
Determination of surface lattice strain in ZnTe epilayers on {100}GaAs by ion channeling and reflectance spectroscopy
Authors: Lovergine N. , Cingolani R. , Leo G. , Mancini A. M. , Vasanelli L. , Romanato F. , Drigo A. V. , Mazzer M.
Published: July 2002

We report on the direct measurements of surface lattice strain in ZnTe epitaxial layers on {100}GaAs substrates by ion channeling Rutherford backscattering spectrometry and low-temperature (10 K) reflectance spectroscopy measurements. The measured ZnTe strain is the superposition of the expected thermal (tensile) strain and a thickness-dependent residual compressive strain. Our data indicate that the removal of this residual strain is slower than the rate predicted by the equilibrium theory, following an apparent h−1/2 power-law dependence on the epilayer thickness h, above ∼100 nm.
Local vibrational mode spectroscopy of nitrogen-hydrogen complex in ZnSe
Authors: Wolk J. A. , Ager J. W. , Duxstad K. J. , Haller E. E. , Taskar N. R. , Dorman D. R. , Olego D. J.
Published: July 2002

Using infrared and Raman spectroscopy, we have observed two local vibrational modes related to H bonded to N acceptors in ZnSe samples grown by metalorganic vapor phase epitaxy. We assign the new mode seen at 3194 cm−1 to a N—H stretching vibrational mode and tentatively assign the mode found at 783 cm−1 to a N—H wagging vibrational mode. Polarized Raman spectroscopy was used to determine that the symmetry of the defect complex is C3 v, which implies that the H atom is in either a bonding or antibonding position.
Optical anisotropy in wire-geometry SiGe layers grown by gas-source selective epitaxial growth technique
Authors: Usami N. , Mine T. , Fukatsu S. , Shiraki Y.
Published: July 2002

We report on the successful fabrication of SiGe quantum wire structures on a V-groove patterned substrate by gas-source selective epitaxial growth technique, and their optical properties. Optical anisotropy, showing the realization of luminescent SiGe layers with wire geometry, was clearly observed in electroluminescence from the SiGe layers grown inside the groove.
Graphite thin film formation by chemical vapor deposition
Authors: Yudasaka M. , Kikuchi R. , Matsui T. , Kamo H. , Ohki Y. , Yoshimura S. , Ota E.
Published: July 2002

Well-ordered graphite thin films were obtained by chemical vapor deposition of 2-methyl- 1,2′-naphthyl ketone on Ni substrates at substrate temperatures higher than 600 °C. Values of interlayer spacing calculated from x-ray diffraction data were between 3.360 and 3.350 Å. Raman scattering spectra of the films showed a peak centering at 1580 cm−1.
Low-frequency excess noise in YBa2Cu3O7−x dc superconducting quantum interference devices cooled in static magnetic fields
Authors: Miklich A. H. , Koelle D. , Shaw T. J. , Ludwig F. , Nemeth D. T. , Dantsker E. , Clarke John , McN. Alford Neil , Button Tim W. , Colclough M. S.
Published: July 2002

We have investigated the performance of YBa2Cu3O7−x dc superconducting quantum interference devices (SQUIDs) cooled in static magnetic fields, B0, of 0.01–1 mT. For fields less than the earth’s ambient field, about 0.05 mT, the white noise of the devices at 77 K is not materially affected. However, at a frequency f of 1 Hz the spectral density of the 1/f noise, SΦ (1 Hz), at 0.05 mT increases by an order of magnitude over that for zero field. Furthermore, SΦ (1 Hz) scales approximately linearly with B0, suggesting strongly that the noise originates in the motion of vortices in the YBCO film. This increase in noise is likely to be an issue for SQUIDs operated in the earth’s field.
Single electron transport and current quantization in a novel quantum dot structure
Authors: Nagamune Y. , Sakaki H. , Kouwenhoven L. P. , Mur L. C. , Harmans C. J. P. M. , Motohisa J. , Noge H.
Published: July 2002

We report on single electron transport via a novel quantum dot structure fabricated by a combination of mesa etching and gate formation. In this device electrons are confined in an etched submicron wire and squeezed further by two barrier gates. The resulting dot is of a very small size, and the number of confined electrons can be tuned down to the few electron limit. This novel structure has a large charging energy and an improved current quantization during turnstile operation. In small dots, containing only a few electrons, we found Coulomb oscillations with an unexplained multiple peak structure.
Light induced electron spin resonance in porous silicon
Authors: Carlos W. E. , Prokes S. M.
Published: July 2002

We report on the observation of a new metastable light induced defect center in porous silicon which has a g tensor with 〈110〉 symmetry and which is similar to the thermal donor, NL8, seen in thermally annealed Si. The optical response has an onset in the vicinity of the Si indirect band gap. The time dependence of the response has a sharp rise followed by a nearly saturated regime. Changing to a higher photon energy then results in another marked increase in the response. We suggest that this dependence of the absorption on excitation energy reflects the distribution of crystallite sizes. We observe that near ultraviolet light significantly bleaches the signal.
Slow conductance oscillations in nanoscale silicon clusters of quantum dots
Authors: Tsu Raphael , Li Xiao-Lei , Nicollian Edward H.
Published: July 2002

At fixed reverse bias voltages of a diode structure consisting of nanoscale silicon particles embedded in an amorphous SiO2 matrix, conductance oscillation in time is observed on some samples at room temperature. Possible mechanisms of the conductance oscillations include the exchange of electrons between the quantum confined states coupled to localized defects and the charge state of these defects. The precise origin of the observed oscillations has not been identified.
Deep levels in 6H-SiC wafers and step-controlled epitaxial layers
Authors: Jang Seongjoo , Kimoto Tsunenobu , Matsunami Hiroyuki
Published: July 2002

We have investigated deep levels in 6H-SiC wafers grown by a modified Lely method and step-controlled epitaxial layers by transient capacitance methods. Several deep electron traps, of which concentrations were on the order of 1015 cm−3, located at 0.39–0.69 eV below the conduction band edge were observed in the 6H-SiC wafers. However, the epitaxial layers by step-controlled epitaxy had very few traps of which concentrations were below the detection limit (<1013 cm−3).
Time-resolved decay of the blue emission in porous silicon
Authors: Harris C. I. , Syväjärvi M. , Bergman J. P. , Kordina O. , Henry A. , Monemar B. , Janzén E.
Published: July 2002

The technologically interesting blue emission from porous silicon has been studied using time-resolved photoluminescence. In as-etched samples prepared using the standard electrochemical method, the decay time is shown to be extremely fast (τ=0.86 ns). This decay time remains unchanged from room temperature down to 77 K. The porous material giving the blue emission is unstable and degrades rapidly under UV photoexcitation. A corresponding increase in red intensity with decreasing blue intensity is observed, demonstrating a correlation between the two types of emission. Under ambient light conditions at room temperature the material is more stable and remains emissive for a number of days.
Magnetic properties of exchange coupled NiFe/CoO/NiFe trilayers
Authors: Ambrose T. , Chien C. L.
Published: July 2002

A trilayer system consisting of an antiferromagnet (CoO) sandwiched by two ferromagnetic (NiFe=Ni81Fe19) layers of unequal thicknesses has been studied. Four different spin structures have been observed. The temperature dependencies of the exchange fields (HE) and coercivity (Hc) have been determined. The layer order between the antiferromagnet and the ferromagnet dictates the exchange field temperature behavior.
Thermal stability of implanted dopants in GaN
Authors: Wilson R. G. , Pearton S. J. , Abernathy C. R. , Zavada J. M.
Published: July 2002

Results are reported of measurements of depth profiles and stability against redistribution with annealing up to 800 or 900 °C, for implanted Be, C, Mg, Si, S, Zn, Ge, and Se as dopants in GaN. The results confirm the high-temperature stability of dopants in this material up to temperatures that vary from 600 to 900 °C. S redistributes for temperatures above 600 °C, and Zn and Se, for temperatures above 800 °C. All of the other elements are stable to 900 °C. These results indicate that direct implantation of dopants rather than masked diffusion will probably be necessary to define selective area doping of III–V nitride device structures based on these results for GaN.
Uniform and efficient GaAs/AlGaAs quantum dots
Authors: Bestwick Tim D. , Dawson Martin D. , Kean Alistair H. , Duggan Geoffrey
Published: July 2002

Uniform arrays of approximately 57 nm diam free-standing quantum dots have been produced from GaAs/AlGaAs single quantum well material by electron-beam lithography and low damage electron cyclotron resonance plasma etching. Low-temperature (5 K) photoluminescence and photoluminescence excitation spectroscopy were used to characterize the material before and after processing into quantum dots. Clear free-exciton features of linewidth comparable to that obtained from the unprocessed material have been observed in the excitation spectra of the quantum dots. As expected for this size of dot, no significant shift in the wavelength of the luminescence was observed, however, there is an apparent enhancement of the external luminescence efficiency when the geometric fill factor is taken into account. The results also show that luminescence efficiency measurements seeking to identify and elucidate intrinsic 0D effects (i.e., those due to quantum confinement in the active region) should be performed with photoexcitation directly into the active region energy states.
Near field probe microscopy of porous silicon: Observation of spectral shifts in photoluminescence of small particles
Authors: Rogers J. K. , Seiferth F. , Vaez-Iravani M.
Published: July 2002

Imaging of topography and locally induced photoluminescence of anodically etched porous silicon is performed using a force regulated near field scanning optical microscope. The photoluminescence signal on as-prepared wafers shows strong evidence for inherent, rather than purely geometrically induced, variations. Images of small particles display distributions of regions of relatively strong luminescence on the 100 nm lateral scale. Highly localized photoluminescence spectra obtained on these particles show spectral shifts as high as 50 nm compared with those obtained on as-prepared porous silicon wafer.
Thickness modulation of InGaAs/GaAs superlattices studied by large angle x-ray scattering
Authors: Ming Z. H. , Soo Y. L. , Huang S. , Kao Y. H. , Stair K. , Devane G. , Choi-Feng C.
Published: July 2002

Superlattices of 100-period InxGa1−xAs (15 Å)/GaAs(100 Å) grown on GaAs (100) substrates by molecular beam epitaxy were studied by using large angle x-ray scattering techniques. In contrast to the usual superlattice satellite peaks corresponding to structural periodicity along the growth direction, unusual satellite peaks in the lateral direction parallel to the sample surface were observed in a sample with x=0.535 grown at 480 °C, indicating an in-plane ordering. This result is confirmed by high resolution transmission electron microscopy observations that thickness modulation in the InxGa1−xAs layers gives rise to long-range lateral periodic arrays of clusterlike microstructures with spacing on the order of a few hundred angstroms. This thickness modulation is found to occur only in the [110] direction, thus the material can be viewed as a somewhat disordered array of grown-in parallel quantum wires.
Differential Al–Ga interdiffusion in AlGaAs/GaAs and AlGaInP/GaInP heterostructures
Authors: Beernink K. J. , Sun D. , Treat D. W. , Bour B. P.
Published: July 2002

Interdiffusion of Al and Ga in Al0.4Ga0.6As/GaAs and Al0.3Ga0.2In0.5P/Ga0.6In0.4P quantum wells has been investigated by measuring the photoluminescence of samples annealed at temperatures from 850 to 1065 °C with and without an SiO2 cap. At 1000 °C under an SiO2 cap, the Al–Ga interdiffusion coefficient is found to be at least two orders of magnitude larger for an AlGaAs/GaAs quantum well compared to an AlGaInP/GaInP quantum well within the same sample. By comparing calculated photoluminescence shifts with measured values, an activation energy of 4.5 eV is found for the Al–Ga interdiffusion in an AlGaAs/GaAs quantum well under an SiO2 cap.
Three-axis sample manipulator with X
Y
Z translation for use in UHV
Authors: Engel Thomas , Braid Donald , Conrad Edward H.
Published: July 2002

A three-axis manipulator suitable for atom diffraction experiments has been constructed by combining three modules with translation, polar rotation, and azimuthal-tilt rotation capabilities. No magnetic materials are used and materials have been chosen to minimize the possibility of galling in vacuum. Azimuthal rotations of up to ±110°, tilt rotations of up to ±30°, and polar rotations of ±180° are possible with the manipulator. The Z motion is ±2 in. and XY motions of 10 mm lead to a change in azimuthal and tilt angles of less than one degree. The sample is heated with electron bombardment and can be cooled to 80 K.
A quantitative luminescence imaging system for biochemical diagnostics
Authors: Batishko Charles R. , Stahl Kurt A. , Erwin David N. , Kiel Johnathan
Published: July 2002

A prototype quantitative low-light-level imaging system, capable of providing calibrated gray-scale imagery of radio-frequency (rf) stimulated chemiluminescence distributions in biological samples, has been developed by these authors. The quantitative luminescence imaging system (QLIS) represents a significant advance in the experimental instrumentation used for the study of such reactions, in that the system allows time-resolved and spatially resolved quantitative analyses to be performed. Measurements of chemiluminescence have traditionally been made by using individual photon counting photomultiplier tubes, integrating over the spatial volume and over time. The QLIS provides imagery at video rates, in which individual or integrated multiple frames can be calibrated to give a quantified image of the two-dimensional distribution of chemiluminescence in the sample. The system has a temporal resolution of 1/30th of a second, spatial resolution of 0.25 mm, a 40-mm-diam linear field of view, and a radiometric sensitivity range of 10−3–10−8 W/m2 sr. Radiometric calibration of the video imagery is accomplished with a self-luminous light source of known radiance and spectral content. The concept can be implemented for measurement at three scales: using a microscope for microscopic-scale measurements; using a wire mesh rf waveguide for medium-scale samples contained in a flat, cylindrical sample cell; or in an anechoic chamber for small- to human-body-sized samples. The prototype system has been implemented at the medium scale, with a wire mesh waveguide. Efforts are under way for development of the other two systems.
A proposal of a ring-type vortex flowmeter
Authors: Miau J. J. , Chiu E. G. , Chou J. H.
Published: July 2002

A configuration of a ring-type bluff body situated in a circular pipe is suggested as a design of a vortex flowmeter. Experiments were carried out to test a series of ring-type bluff models, whose geometrical parameters are expressed in terms of the gap ratio G/W and the slenderness ratio D̄/W, where G denotes the gap width between the outer edge of the ring and the pipe wall, W denotes the width of the ring, and D̄ denotes the mean diameter of the ring. Major efforts are focused upon a group of the ring-type vortex shedders whose vortex shedding process is strongly influenced by the presence of the wall. Results obtained suggest the appropriate sizes of rings for which the vortex shedding frequency can be clearly measured either on the pipe wall or at the center of the pipe.
InGaAs/InP quantum wires grown by gas source molecular beam epitaxy onto V-grooved InP substrates with (111)A facet sidewalls
Authors: Wang J. , Robinson B. J. , Thompson D. A. , Simmons J. G.
Published: July 2002

InGaAs/InP layers have been grown under optimized conditions by gas source molecular beam epitaxy on (100) InP substrates patterned with V grooves having (111)A facet sidewalls. Transmission electron microscopy shows that InGaAs/InP quantum wires are obtained with well thickness variation as high as a factor of 6 and that all epilayers are defect-free. Lateral subband separations are estimated by a simple one-dimensional parabolic potential model with the thickness determined by transmission electron microscopy. Photoluminescence from the InGaAs quantum wires is resolved with a selective etching technique. The quantum wire emission has a significant red shift compared to the adjacent quantum wells on the groove sidewalls and the (100) surface region between grooves. The red shift results from both the increased well thickness and compositional change due to adatom diffusion from sidewalls.
Relationship between self-organization and size of InAs islands on InP(001) grown by gas-source molecular beam epitaxy
Authors: Ponchet A. , Le Corre A. , L’Haridon H. , Lambert B. , Salaün S.
Published: July 2002

Using the strained-induced 2D–3D transition, InAs dots have been grown on InP(001) and examined by transmission electron microscopy. Two different modes of island size and spatial distribution have been identified. For deposit of 1.5 and 1.8 monolayers, the islands are about 7 nm high and randomly distributed. Above 2 monolayers, they are about five times smaller in volume and locally self-organized, with a typical distance of 40 nm independent of the island density. It is suggested that the strong dependence of the island size on the total amount of deposited InAs is mainly due to long range interactions through the substrate.
Diffraction and diffuse scattering measurements in material science: Improvement brought to a six-circle goniometer on a synchrotron beam line
Authors: Elkaim E. , Lefebvre S. , Kahn R. , Berar J. F. , Lemonnier M. , Bessiere M.
Published: July 2002

The six-circle diffractometer D23 set up in LURE-DCI (bending magnet) at Orsay has been improved since its first presentation in 1987 [M. Bessiere, G. Bessenay, J. Frouin, M. Jouvin, and S. Lefebvre, Nucl. Instrum. Methods A 261, 591 (1987)]. We present a new design for the sagittal focusing monochromator, a two-mirror device for harmonics rejection and detectors for adjustments. All these modifications are now operational and can be used together with a crystal analyzer for powder studies.
A canal surface viscometer for the in-plane steady shear viscosity of monolayers at the air/water interface
Authors: Sacchetti Mark , Yu Hyuk , Zografi George
Published: July 2002

A canal surface viscometer has been designed and constructed to measure the in-plane steady shear viscosity (ηs) of monolayers at the air/water interface. The technique involves forcing a monolayer to flow through a canal to another monolayer under a constant surface pressure gradient and measuring the area flow rate. The instrument incorporates a feedback electronic system to maintain the surface pressure on each side of the canal to about 0.1 dyn/cm during monolayer flow. The canal design is modified from that of an earlier design in the literature to eliminate artifacts that give rise to systematic variabilities as reported. Specifically, it is demonstrated that the glass-paraffin canal design used in the past produces ill reproducible results in contrast to an all Teflon canal, which gives rise to consistently reproducible surface viscosity data. A comparison is made between ηs values obtained from the canal viscometer and the knife-edge ring surface viscometer, another technique reported in the literature. The comparison reveals the importance of controlling key experimental variables which can have an order-of-magnitude effect on the surface viscosity data.
Density measurements of a pulsed supersonic gas jet using nuclear scattering
Authors: Pronko J. G. , Kohler D. , Chapman I. V. , Bardin T. T. , Filbert P. C. , Hawley J. D.
Published: July 2002

A method using nuclear scattering with an ion beam microprobe for characterizing the spatial and temporal density profile of a supersonic gas jet is described. The technique was applied to Ar and N2 gas jets from an axially symmetric DeLaval nozzle and the results are presented.
Examination to eliminate undesirable phase delay of an avalanche photodiode (APD) for intensity-modulated light
Authors: Yokoyama Shuko , Okamoto Akito , Araki Tsutomu , Suzuki Norihito
Published: July 2002

Undesirable phase delay in avalanche photodiode (APD) between input light and output current is noticeable when modulation frequency of the incident light is over several hundreds of MHz. The degree of the delay increases with a decrease of the incident intensity. To clarify this phenomenon, relationships among the phase delay, output current, and the multiplication factor (current gain) of APD have been investigated. From this investigation, we found that the multiplication factor of APD increased with a decrease of the incident intensity, resulting in a decrease of the cutoff frequency of the APD. Supposing that the cutoff frequency defines the signal delay, the amount of the phase delay has been quantified with respect to the multiplication factor. Such a phase delay can be eliminated by superimposing a bias light on the modulation light so that the total intensity remains constant. To confirm this, a servo control system using an intensity modulated laser diode and unmodulated laser diodes (LDs) was constructed. The drive current of the unmodulated LD was changed automatically with respect to the modulated LD intensity, thus keeping the APD output at a constant value. The resulting APD phase delay was within 0.05°, which is only 0.05% of that of the uncompensated system.
Soft x-ray emission spectrometer for undulator radiation
Authors: Shin S. , Agui A. , Fujisawa M. , Tezuka Y. , Ishii T. , Hirai N.
Published: July 2002

An experimental system for high-resolution soft x-ray emission spectroscopy was made. The experimental system is designed to be attached to the undulator beamlines. Spectrometer uses the Rowland circle geometry in which input slit, spherical grating, and multichannel detector lie on the focal circle. Three blazed holographic gratings are changeable in the vacuum. The Rowland circle radii of gratings are 5, 7, and 10 m and their line densities are 600, 1200, and 2400 lines/mm, respectively. The available photon energy is from 30 to 1200 eV. The resolution ΔE is achieved to 0.06 eV by 50 μ-input-slit width for hν=120 eV. Several experimental results measured by this system are presented.
GaAs quantum well distributed Bragg reflection laser with AlGaAs/GaAs superlattice gratings fabricated by focused ion beam mixing
Authors: Steckl A. J. , Chen P. , Cao Xuelong , Jackson Howard E. , Kumar M. , Boyd J. T.
Published: July 2002

GaAs quantum well (QW) lasers with distributed Bragg reflection (DBR) Al0.3Ga0.7As/GaAs superlattice gratings have been fabricated by the single-step, maskless focused ion beam (FIB) mixing. 200 keV Si++ FIB implantation with a beam diameter of ∼60–70 nm and a dose of 1014 cm−2 was used to obtain localized compositional mixing. The DBR grating period was 350 nm, corresponding to a third order grating matched to the emission from the 30 nm wide QW. Lasing operation was examined by optical pumping. With a pumping power 1.6× the threshold value, lasing modes were observed near 827 nm, with a spacing of 3 Å and a linewidth of 1.5 Å.
Reactive magnetron co-sputtered antiferroelectric lead zirconate thin films
Authors: Yamakawa K. , Trolier-McKinstry S. , Dougherty J. P. , Krupanidhi S. B.
Published: July 2002

Antiferroelectric lead zirconate thin films were formed on platinum coated silicon substrates by a reactive magnetron co-sputtering method. The films showed (240) preferred orientation. The crystallization temperatures and the preferred orientation were affected by the lead content in the films. The electric field forced transformation from the antiferroelectric phase to the ferroelectric phase was observed at room temperature with a maximum polarization value of 36 μC/cm2. The average field to excite the ferroelectric state and that for the reversion to the antiferroelectric state were 267 and 104 kV/cm respectively.
A 0.1–500 Hz analog thin film BH hysteresis loop tracer with automatic Y-axis drift correction
Authors: Horn James L. , Grimes Craig A.
Published: July 2002

An analog magnetic hysteresis loop tracer is described that simply and effectively corrects for the Y-axis integrator offset drift inherent in hysteresis loop tracers. In order to keep a small input offset of about 30 μV from being integrated over time into an output offset of volts, the integrator is reset once every cycle: A comparator is used to trigger a one-shot that engages a relay that discharges the integrator capacitor. The circuit was built with standard available and inexpensive parts, and allows stable BH hysteresis loop measurements to be made without Y-axis drift or distortion over a frequency range of 0.1–500 Hz.
Observation of Josephson effect in YBa2Cu3O7−x/Nd1.85Ce0.15CuO4−y bilayer junctions
Authors: Takeuchi I. , Mao S. N. , Xi X. X. , Petersen K. , Lobb C. J. , Venkatesan T.
Published: July 2002

Josephson junctions were fabricated at the interface of YBa2Cu3O7−x (YBCO) and Nd1.85Ce0.15CuO4−y (NCCO) in heteroepitaxially grown superconducting bilayers. Devices of various configurations and sizes were fabricated, and they display a resistively shunted junction like I–V characteristics with hysteresis at low temperatures. A clear ac Josephson effect was observed under microwave irradiation, and critical currents were completely suppressible by external magnetic fields. Oxygen diffusion or charge diffusion at the interface are possible origins for the barrier formation.
Band-gap tuning of InGaAs/InGaAsP/InP laser using high energy ion implantation
Authors: Charbonneau S. , Poole P. J. , Feng Y. , Aers G. C. , Dion M. , Davies M. , Goldberg R. D. , Mitchell I. V.
Published: July 2002

The technique of ion-induced quantum well intermixing using broad area, high energy (1 MeV P+) ion implantation has been used to tune the emission wavelength of an InGaAs/InGaAsP/InP multiple quantum well (MQW) laser operating at 1.5 μm. The optical quality of the band-gap shifted material is assessed using low-temperature photoluminescence (PL). The band-gap tuned lasers are characterized in terms of threshold current density and external quantum efficiency and exhibit blue shifts in the lasing spectra of up to 63 nm. This approach offers the prospect of a powerful and relatively simple fabrication technique for integrating active as well as passive optoelectronic devices.