bluuebunny/crossref_metadata_2025 huggingface dataset:
Tunable diode laser based spectroscopic system for ammonia detection in human respiration
Authors: Lachish Uri , Rotter Shlomo , Adler Eli , El-Hanany Uri
Published: July 2002

A high-resolution infrared optical absorption spectroscopy system based on lead-salt tunable diode lasers as light sources has been developed for detection and monitoring of trace gases by resolving their narrow-width absorption bands. The system is a double beam setup, where a reference cell containing the monitored gas is used for tuning and locking the laser light frequency to that of a desired absorption line and a measurement cell where the examined gas flows continuously at a reduced pressure. Frequency modulation of the laser light is achieved by square waveform modulation of its current source and both the frequency locking and signal analysis devices are based on sample-and-hold techniques. The system capability is demonstrated by the monitoring of ammonia traces in human breath. Gas trace monitoring in respiration may prove to be an effective medical diagnostic method.
Electronic noise of submicron n+nn+ diodes under near-oscillatory macroscopic behaviors
Authors: Starikov E. , Shiktorov P. , Gružinskis V. , Nougier J. P. , Vaissiere J. C. , Varani L. , Reggiani L.
Published: July 2002

We present a theoretical investigation on the electronic noise of submicron n+nn+ GaAs diode under the conditions when the electrical characteristics of the diode exhibit a pronounced near-oscillatory macroscopic behavior caused by velocity fluctuations of single particles at the microscopic level. Two kinds of spontaneous oscillations, related to transit time and plasma oscillations, and their contributions to the diode noise are investigated by a Monte Carlo simulation. A simple analytical model which provides an excellent fit of the macroscopic features of the noise obtained by the Monte Carlo simulation is developed.
Effect of lattice mismatch on surface morphology of (110) thin films of 214-type superconductors
Authors: Sato H. , Naito M. , Kinoshita S. , Arima T. , Tokura Y.
Published: July 2002

We characterized the surface morphology of (110) thin films of 214-type superconductors epitaxially grown on substrates of 214-type materials with various degrees of lattice mismatch. By reactive co-evaporation, (110) thin films of La1.85Sr0.15CuO4 and Pr1.85Ce0.15CuO4 are grown on (110) substrates of La2CuO4 and Pr2CuO4. Reflection high energy electron diffraction, scanning electron microscopy, and atomic force microscopy show the roughness of the film surface is reduced by decreasing mismatch. The surface roughness of La1.85Sr0.15CuO4 (110) thin films on La2CuO4 substrates is comparably small to that of (001) films on SrTiO3(001) substrates.
Fabrication of 0.1 μm metal oxide semiconductor field-effect transistors with the atomic force microscope
Authors: Minne S. C. , Soh H. T. , Flueckiger Ph. , Quate C. F.
Published: July 2002

Using the atomic force microscope (AFM), we have fabricated a metal oxide semiconductor field-effect transistor (MOSFET) on silicon with an effective channel length of 0.1 μm. The lithography at the gate level was performed with the scanning tip of the AFM. The gate was defined by electric-field-enhanced selective oxidation of the amorphous silicon gate electrode. The electrical characteristics were reasonable with a transconductance of 279 mS/mm and a threshold voltage of 0.55 V.
The Synchrotron Radiation Facility at Frascati
Authors: Cimino R. , Burattini E. , Mobilio S.
Published: July 2002

In this paper we give a brief description of the Synchrotron Radiation Facility at Frascati. Particular emphasis is given in describing the most significant instrument developments during the last few years.
Nanofabrication of thin chromium film deposited on Si(100) surfaces by tip induced anodization in atomic force microscopy
Authors: Wang Dawen , Tsau Liming , Wang K. L. , Chow Peter
Published: July 2002

Writing of nanostructures on thin metal films using atomic force microscopy (AFM) was demonstrated. The writing experiments were done in a nitrogen ambient having variable humidity. Using a p-type heavily doped silicon AFM tip, a bias voltage was independently applied between the tip and the surface of a thin chromium layer deposited on a Si(100) substrate. Protruded patterns of various shapes were formed only on the water-adsorbed chromium surface when applying a negative bias on the tip. Their sizes were found to be dependent on the writing time, the bias voltage, and the humidity. The smallest feature size obtained is about 20 nm. From Auger electron spectroscopy (AES) analysis, the products are shown to be Cr oxides. The surface modification mechanism appears to be tip-induced local oxidation, i.e., anodization.
The thermocoax cable as the microwave frequency filter for single electron circuits
Authors: Zorin A. B.
Published: July 2002

We have analyzed the possibility of using the resistive coaxial cable, Thermocoax(R) Philips as a microwave cryofilter for single electron experiments performed in a top-loading system dilution refrigerator. The biasing and signal lines made of this cable are assumed to link a single electron device kept at the lowest temperature Td to the rf filter having a temperature Tf of several kelvin. The attenuation in a wide frequency range has been calculated and it turned out that a 40-cm-long piece of this cable with an outer diameter of 0.5 mm could drastically reduce the effect of noise from the 50 Ω source anchored at Tf (below 25 K), making its effect comparable with the effect of the thermal fluctuations in the device for Td≊40 mK. The calculated attenuation is in reasonable agreement with the data measured at frequencies up to 18 GHz.
A 5-MHz bandwidth SQUID magnetometer with additional positive feedback
Authors: Drung D. , Matz H. , Koch H.
Published: July 2002

A fast SQUID (superconducting quantum interference device) magnetometer system using the latest multiloop magnetometer W7A with additional positive feedback has been built. A 3-dB bandwidth of 5 MHz has been achieved in a simple direct-coupled flux-locked loop. The feedback range is ±620 Φ0 or ±290 nT, the white-noise level 3.4×10−6 Φ0/√Hz or 1.6 fT/√Hz, and the 1/ f corner frequency ≂7 Hz. Above 1 MHz the noise level increases slightly, approximately with √SΦ ∝f1/6. At high signal frequencies a slew rate of about 8×105 Φ0/s has been measured. Below the pole frequency of the second integrator (480 kHz) the slew rate increases with decreasing frequency, having a maximum of 3×107 Φ0/s at 7.6 kHz. The total harmonic distortion (THD) decreases strongly with the signal amplitude below the slew rate limit. For a sinusoidal signal with half the maximum amplitude, the THD is ≂4% at 1 MHz, ≂0.04% at 100 kHz, and ≂2.7 ppm (1 ppm=10−6) at 10 kHz. Below about 2 kHz the THD becomes almost frequency independent and lies between 0.4 and 7 ppm for signal amplitudes between 2.5 and 620 Φ0.
High performance polarization insensitive electroabsorption modulator based on strained GaInAs–AlInAs multiple quantum wells
Authors: Chelles S. , Ferreira R. , Voisin P. , Harmand J. C.
Published: July 2002

We report on the polarization independent operation of an electroabsorption guided-wave modulator based on strained GaInAs–AlInAs multiquantum well structure. The device operates in the 1.5–1.6 μm wavelength range and exhibits very high static performances as illustrated by the measured 15 dB/100 μm extinction ratio for a drive voltage of only 1.25 V. We show that the observed polarization insensitivity of this device is in good agreement with the calculation of the electroabsorption curves. The detailed analysis indicates that significant improvement of the performance is still possible.
Novel design for high pressure, integral, differential, absolute, and relative multicomponent permeability measurements
Authors: Steriotis T. A. , Katsaros F. K. , Mitropoulos A. Ch. , Stubos A. K. , Galiatsatou P. , Zouridakis N. , Kanellopoulos N. K.
Published: July 2002

A high pressure apparatus of novel design, used for the performance evaluation of ceramic and polymeric membranes, is presented. The apparatus can test the performance of several membranes simultaneously and provide significant time saving by reducing the time required for the equilibration and mixing of gases. The apparatus is capable of providing measurements of single and multicomponent permeability for pressures up to 70 bar, under integral or differential modes of operation.
A compact device for highly efficient dispersion compensation in fiber transmission
Authors: Peschel U. , Peschel T. , Lederer F.
Published: July 2002

We show that the supermodes of two coupled waveguides may exhibit a giant tunable group-velocity dispersion, which allows for dispersion compensation in long-haul fiber transmission lines, pulse compression, as well as bright soliton propagation in materials with large normal dispersion.
Polarizer/analyzer filter for nuclear resonant scattering of synchrotron radiation
Authors: Toellner T. S. , Alp E. E. , Sturhahn W. , Mooney T. M. , Zhang X. , Ando M. , Yoda Y. , Kikuta S.
Published: July 2002

A silicon (840) channel-cut crystal with a large asymmetry is used to linearly polarize synchrotron radiation generated by an undulator source at the 14.413 keV nuclear resonance of 57Fe. The resulting σ̂-polarized radiation is then scattered from an 57Fe foil placed in a magnetic field that effects σ̂→π̂ polarization conversion within the resonant bandwidth. A second crystal of the same type is placed in a crossed position to suppress the nonresonant radiation while transmitting the π̂-polarized resonant radiation. A polarization suppression factor of 6×10−7 has been obtained. The suppression of the nonresonant radiation allowed monitoring the decay of the nuclear levels after 1 ns of their excitation.
Blue light-emitting diode demonstrated as an ultraviolet excitation source for nanosecond phase-modulation fluorescence lifetime measurements
Authors: Sipior Jeffrey , Carter Gary M. , Lakowicz Joseph R. , Rao Govind
Published: July 2002

We have produced amplitude-modulated near-ultraviolet light, centered at 390 nm, using an inexpensive, commercially available blue light-emitting diode (LED). The LED was amplitude modulated with the +13 dBm ac output from a frequency generator while biased through a bias tee with 60 mA of dc current. The LED produced 45 to 54 μW of UV light over the modulation bandwidth of 0.01 to 200 MHz, when measured after optical filters to remove the residual blue output. Since the filter attenuated the UV output about 3 dB, more than 100 μW of UV light was initially produced. Modulated UV light was available to approximately 200 MHz, with a −3 dB point of 31 MHz, allowing the measurement of ns fluorescence lifetimes. The fluorescence lifetimes of standard fluorophores (9-cyanoanthracene and green fluorescent protein) were measured in the frequency domain using the phase-modulation technique, producing lifetimes that closely agree with those reported in the literature, confirming that the UV-emitting blue LED is practical for spectroscopic and sensor applications. When compared to a laser modulated with a Pockels cell, the LED was smaller, less expensive, required less power, generated less heat, and required less alignment. The ability to modulate the LED at high frequencies, along with the UV output, allows its use as an inexpensive UV light source in fluorescence lifetime optical sensors and even frequency-domain fluorometers.
A novel single-crystal adsorption calorimeter and additions for determining metal adsorption and adhesion energies
Authors: Stuckless J. T. , Frei Nathan A. , Campbell Charles T.
Published: July 2002

A new microcalorimeter for measuring heats of adsorption on clean single-crystal surfaces is described, and its operational characteristics are presented. The principle is similar to that pioneered by David King’s group: A pulse of gas from a molecular beam adsorbs on an ultrathin single crystal’s surface, causing a measurable transient heat input and temperature rise. Our novel heat detector is a 9 μm thick pyroelectric polymer ribbon, which is mechanically driven to make a gentle mechanical/thermal contact to the back of the single-crystal sample during measurements. Advantages include use of thicker samples (1 μm), sample preparation at very high temperatures, and potential measurements at cryogenic temperatures. A novel chopped molecular beam of metal vapor and a method of correcting for absorbed radiation from the hot effusion cell are also described. This system is applied to study the heats of adsorption of metals on clean, well-defined and single-crystalline surfaces as a detailed function of coverage, from which metal/substrate adhesion energies can also be extracted. We obtain pulse-to-pulse standard deviations of <2% in the heat of adsorption for pulses containing <0.03 ML of Cu and Pb, and absolute accuracy to within a few percent.
Novel laser gain media based on Cr3+-doped mixed borates RX3(BO3)4
Authors: Wang Guofu , Han T. P. J. , Gallagher H. G. , Henderson B.
Published: July 2002

This letter discusses the optical properties of Cr3+ ions in mixed borate crystals with the formula RX3(BO3)4, where R3+=Y3+ or Gd3+ and X3+=Al3+ or Sc3+. Measurements of the optical absorption spectra have been used to estimate the crystal field strength Dq and the Racah parameters B and C. These quantities confirm that in YAl3(BO3)4(YAB) and GdAl3(BO3)4(GAB)Cr3+ ions occupy strong crystal field sites 2E)<E(4T2)], whereas in YSc3(BO3)4(YSB) and GdSc3(BO3)4(GSB) the Cr3+ ions occupy weak field sites 4T2)<E(2E)]. The consequence is that the fluorescence spectrum of Cr3+:YAB and GAB reveals the sharp R lines at low temperature and R lines plus broadband at 300 K. Both Cr3+:YSB and GSB emit into the broadband 4T2→4A2 transition at all temperatures. The potential of these materials as laser gain media is discussed.
Kinetics of photoconductivity in n-type GaN photodetector
Authors: Kung P. , Zhang X. , Walker D. , Saxler A. , Piotrowski J. , Rogalski A. , Razeghi M.
Published: July 2002

High-quality ultraviolet photoconductive detectors have been fabricated using GaN layers grown by low-pressure metalorganic chemical vapor deposition on (11⋅0) sapphire substrates. The spectral responsivity remained nearly constant for wavelengths from 200 to 365 nm and dropped sharply by almost three orders of magnitude for wavelengths longer than 365 nm. The kinetics of the photoconductivity have been studied by the measurements of the frequency-dependent photoresponse and photoconductivity decay. Strongly sublinear response and excitation-dependent response time have been observed even at relatively low excitation levels. This can be attributed to redistribution of the charge carriers with increased excitation level.
Measurement and analysis of terahertz radiation from bulk semiconductors
Authors: Li M. , Sun F. G. , Wagoner G. A. , Alexander M. , Zhang X.-C.
Published: July 2002

We report the recent measurement and analysis of the transmitted and pseudoreflected optically induced terahertz (THz) beams emitted from a semiconductor wafer under femtosecond laser illumination, where the static electric field is either parallel or perpendicular to the surface. In general, the amplitude of the transmitted THz field is different from that of pseudoreflected THz field, except at the Brewster angle.
High quality Al(Ga)As/GaAs/Al(Ga)As quantum wells grown on (111)A GaAs substrates
Authors: Chin Albert , Lee K.
Published: July 2002

We report the growth of high quality molecular beam epitaxy (MBE) AlGaAs, AlGaAs/GaAs/AlGaAs, and AlAs/GaAs/AlAs multiple quantum wells (MQWs) on (111)A GaAs substrates. For (111)A AlGaAs/GaAs/AlGaAs MQWs, there is no detectable photoluminescence (PL) at a growth temperature of 640 °C, and narrow PL linewidth can only be obtained at growth temperatures higher than 680 °C. A PL linewidth of 13.8 meV is measured at the growth temperature of 720 °C. To understand such growth temperature dependence of (111)A MQWs, we have investigated the material quality of (111)A AlGaAs at different growth temperatures. The strong PL integrated intensity of 640 °C grown (111)A AlGaAs indicates good material quality and a low concentration of non-radiative recombination centers. However, the broad PL linewidth of 640 °C grown (111)A AlGaAs indicates the strong compositional modulation and a rough growth front. We have used AlAs instead of AlGaAs in order to reduce the compositional modulation and smooth the interface. A PL linewidth of 13.4 meV is measured for 640 °C grown (111)A AlAs/GaAs/AlAs MQWs, which is the narrowest value for (111)A MQWs.
Epitaxially oriented growth of diamond on silicon by hot filament chemical vapor deposition
Authors: Chen Qijin , Wang Li-Xin , Zhang Ze , Yang Jie , Lin Zhangda
Published: July 2002

Expitaxially oriented growth of diamond film on Si(001) was achieved using hot filament chemical vapor deposition. The epitaxial relationship between the film and the substrate was confirmed by the observation through scanning electron microscopy and high-resolution transmission electron microscopy (HRTEM) as follows: Dia(001)//Si(001) and Dia〈110〉//Si〈110〉 with a misorientation angle of 9° between Dia(001) and Si(001). This reports the HRTEM observation of the largest area of the diamond/Si interface (larger than 880 Å). It demonstrates that the intermediate β-SiC layer is unnecessary for achieving diamond epitaxy on Si. Discussion reveals that the value of the misorientation angle between Dia(001) and Si(001) is not unique and should be controlled to deposit single-crystal diamond films on Si.
Excitation power and barrier width dependence of photoluminescence in piezoelectric multiquantum well p-i-n structures
Authors: David J. P. R. , Sale T. E. , Pabla A. S. , Rodríguez-Gironés P. J. , Woodhead J. , Grey R. , Rees G. J. , Robson P. N. , Skolnick M. S. , Hogg R. A.
Published: July 2002

Strained InGaAs/GaAs multiquantum well (MQW) structures grown on (111)B GaAs sub- strates incorporate strong internal piezoelectric fields. In contrast to (100)-oriented MQW p-i-n structures, the room temperature photoluminescence (PL) peak position and lineshape depend on the barrier width, which controls the ‘‘envelope field’’ across the MQW. The complex behavior of the PL peak position with excitation power results from a competition between screening of the well and envelope fields, and the photovoltaic effect.
Be doping of (311)A and (100) Al0.24Ga0.76As grown by molecular beam epitaxy
Authors: Galbiati N. , Pavesi L. , Grilli E. , Guzzi M. , Henini M.
Published: July 2002

The use of Be as a p-type dopant in AlxGa1−xAs and in modulation doped heterostructures grown by molecular beam epitaxy on (311)A oriented substrates is investigated. Photoluminescence and electrical measurements reveal that Be is incorporated as an acceptor in (311)A oriented AlxGa1−xAs with a slightly greater efficiency than in (100) oriented AlxGa1−xAs. The optical properties of the (311)A samples are better than those of the (100) oriented samples. An estimate of the Be binding energy yields 32±4 meV at x=0.240±0.004. A two-dimensional hole gas with a very high mobility (250 000 cm2/Vs at 100 mK) and with clear fractional quantum hall effect is formed in the modulation of Be doped heterostructures grown on the (311)A orientation.
Structure and formation mechanisms of AlGaAs V-groove vertical quantum wells grown by low pressure organometallic chemical vapor deposition
Authors: Biasiol G. , Reinhardt F. , Gustafsson A. , Martinet E. , Kapon E.
Published: July 2002

The structure of AlGaAs vertical quantum well (VQW) structures grown by low-pressure organometallic chemical vapor deposition on V-grooved GaAs substrates was analyzed as a function of growth temperature and Al mole fraction using transmission electron microscopy and atomic force microscopy (AFM). The low-pressure growth yields several, extremely narrow (a few nm wide) branches of Ga-enriched VQWs at the bottom of the grooves. The variation in Al content across the VQW was evaluated by measuring the AlGaAs oxide thickness on a cleaved edge of the structure using AFM in air. The transmission electron microscopy analysis demonstrates that the different VQW branches originate from distinct nanofacets that self-order at the bottom of the V-groove, probably due to facet-induced segregation of group III species.
Nucleation and bulk film growth kinetics of nanocrystalline diamond prepared by microwave plasma-enhanced chemical vapor deposition on silicon substrates
Authors: Lee Joungchel , Hong Byungyou , Messier R. , Collins R. W.
Published: July 2002

Real time spectroscopic ellipsometry has been applied to characterize the substrate temperature (T) dependence of the deposition rates for nanocrystalline diamond thin films prepared by microwave plasma-enhanced chemical vapor deposition on seeded Si substrates. With the real time capability, it is possible to determine the rates at which the diamond mass thickness (i.e., volume per area) increases during the early nucleation and bulk film growth regimes. The increases in the nucleating and bulk diamond growth rates with T for 400<T<800 °C are consistent with activation energies of ∼17 and 8 kcal/mol, respectively. The results reported here provide insights into the nature of the low-T growth rate limitations for diamond films on nondiamond substrates.
Vacuum arc discharges preceding high electron field emission from carbon films
Authors: Gröning O. , Küttel O. M. , Schaller E. , Gröning P. , Schlapbach L.
Published: July 2002

Field emission measurements on chemical vapor deposition diamond and laser ablated a-C films show an activation step after reaching a certain critical electric field. At this field a vacuum arc of some hundred ns duration initiates. While high current arcing leads to the evaporation of the spot surface melting, amorphization or cracking of the film is encountered for lower currents. In any case, much higher electron emission can be observed after this activation procedure due possibly to tip formation resulting in an electric field enhancement. By using a 1 GΩ resistance the discharge current can be limited nevertheless, an activation is observed.
Photoluminescence studies of GaAs quantum wires with quantum confined Stark effect
Authors: Arakawa T. , Kato Y. , Sogawa F. , Arakawa Y.
Published: July 2002

We investigated the quantum confined Stark effect in GaAs quantum wires formed in a V-groove structure, demonstrating observation of a blueshift of the photoluminescence peak with the increase of electric fields at 50 K. This blueshift is attributed to the fact that the change in enhanced binding energy of excitons due to the electric field is larger than that in quantized energy levels of electrons and holes. Time-resolved photoluminescence was also measured. The photoluminescence decay time is decreased in small quantum wires of 8 nm width with the increase of electric fields, while the decay time is increased in the quantum wires with a size of 35 nm. These results indicate that the escaping of carriers is more dominant in smaller structures than reduction of the oscillator strength due to the electric fields.
Electrical stress-induced variable range hopping conduction in ultrathin silicon dioxides
Authors: Okada Kenji , Taniguchi Kenji
Published: July 2002

Electrical stress-induced variable range hopping conduction is reported in the ultrathin silicon dioxides. The conduction is mediated by the localized states, including various trap sites and interface states induced by the electrical stressing. Based on the results, a model for the conduction mechanism of the “B-mode” stress-induced leakage current (B-SILC) is proposed. This model well explains the temperature dependence and large fluctuation of the B-SILC, which cannot be explained by the previous models for the B-SILC. Furthermore, an empirical expression for the current–voltage characteristics of the B-SILC is also proposed.
Photosensitivity and grating writing in hydrogen loaded germanosilicate core optical fibers at 325 and 351 nm
Authors: Atkins Robert M. , Espindola Rolando P.
Published: July 2002

A refractive index grating has been side written in the core of a hydrogen loaded germanosilicate optical fiber using a phase mask and 351 nm light from an Ar ion laser, the longest wavelength yet reported, to the best of our knowledge, for a grating of this type. The grating strength was ∼1.25 dB, indicating an index change of ∼2×10−4. The fiber was sensitized by low temperature hydrogen loading followed by a brief exposure to a CO2 laser, resulting in the formation of high levels of germanium–oxygen deficiency centers. Fluorescence changes and UV transmission changes in hydrogen loaded germanosilicate glasses exposed to 325 nm and combined 351/364 nm radiation are also reported.
Phononless radiative recombination of indirect excitons in a Si/Ge type-II quantum dot
Authors: Fukatsu S. , Sunamura H. , Shiraki Y. , Komiyama S.
Published: July 2002

Phononless radiative recombination is observed in luminescence spectra of Ge quantum wells decorated by self-organized Stranski–Krastanow (S–K) dots grown on Si (100). External uniaxial tensile stress along [011] allows the discrimination of phonon-missing optical transitions. The phononless recombination is attributed to a dipole-allowed k diagonal Δ1-Γ25′ interband transition involving the hole in the Ge wetting layer and the electron in a Si quantum dot encompassed by large S–K dots. The weak oscillator strength of the phononless recombination is evidenced by its slow decay kinetics. The results indicate that low-lying broad band features due to S–K dots are also of phononless origins.
Characterization of zinc blende InxGa1−xN grown by radio frequency plasma assisted molecular beam epitaxy on GaAs (001)
Authors: Müllhäuser J. R. , Jenichen B. , Wassermeier M. , Brandt O. , Ploog K. H.
Published: July 2002

Successful growth of a cubic In0.17Ga0.83N/GaN structure exhibiting blue luminescence at temperatures up to 500 K is reported. Atomic force microscopy and x-ray diffraction are used to analyze the morphological and crystalline properties of the sample. Photoluminescence measurements reveal broad, but well defined emission with a maximum at 440–450 nm in the temperature range of 5–500 K. A line-shape analysis of the spectra, as well as measurements of the absorption coefficient, allow an estimation of the band-gap energy of the cubic In0.17Ga0.83N epilayer.
Influence of the microstructure on the thermal properties of thin polycrystalline diamond films
Authors: Verhoeven H. , Flöter A. , Reiß H. , Zachai R. , Wittorf D. , Jäger W.
Published: July 2002

Highly oriented and columnar grained diamond layers only a few microns thick, deposited at different substrate temperatures (500, 550, and 800 °C) on silicon using microwave-plasma-assisted chemical vapor deposition, are investigated by special photothermal techniques and high-resolution transmission electron microscopy (HRTEM). Small effective diamond–silicon boundary resistances of <4×10−9 m2 K/W are determined for thermal conduction normal to the interface. Thermal conductivities normal to the interface, k⊥, are found to be about an order of magnitude greater than the conductivities parallel to the interface, k∥ (k⊥/k∥=9–18). The boundary resistances measured are in good agreement with limits estimated from the interface structure observed by HRTEM, which indicate a low near-interfacial disorder for the layers.
Photoluminescence spectroscopy of erbium implanted gallium nitride
Authors: Thaik Myo , Hömmerich U. , Schwartz R. N. , Wilson R. G. , Zavada J. M.
Published: July 2002

Results of a photoluminescence (PL) and photoluminescence excitation (PLE) study of Er implanted GaN are presented. Upon optical excitation at 325 and 488 nm, we observed strong 1.54 μm Er3+ PL which remained temperature stable from 15 to 550 K. At 550 K, the integrated PL intensity decreased by ∼10% for above gap excitation (λex=325 nm) and ∼50% for below gap excitation (λex=488 nm) relative to its value at 15 K. The excellent temperature stability makes GaN:Er very attractive for high temperature optoelectronic device applications. PLE measurements were conducted to gain insight into the Er3+ excitation mechanisms in the GaN host. The PLE results show that Er3+ can be excited continuously over a broad wavelength region spanning from 425 to 680 nm. In addition, sharp PLE features were observed at approximately 495, 525, 553, 651, and 980 nm. The PLE spectrum suggests that optically active Er3+ ions can be excited either through carrier-mediated processes involving defects in the host or through resonant pumping into Er3+ 4f energy levels. With respect to these two excitation schemes, distinct Er3+ PL properties were observed for resonant and off-resonant Er3+ excitation indicating the presence of different subsets of Er3+ ions in GaN.
Mechanisms of optical gain in cubic gallium nitrite
Authors: Holst J. , Eckey L. , Hoffmann A. , Broser I. , Schöttker B. , As D. J. , Schikora D. , Lischka K.
Published: July 2002

We report on the mechanisms of optical gain in cubic GaN. Intensity-dependent gain spectra allow a distinction of the processes involved in providing optical amplification. For moderate excitation levels, the biexciton decay is responsible for a gain structure at 3.265 eV. With increasing excitation densities, gain is observed on the high energy side of the cubic band gap due to band filling processes. For the highest pump intensities, the electron-hole plasma is the dominant gain process. Gain values up to 210 cm−1 were obtained, indicating the high potential of cubic GaN for device applications. The observed gain mechanisms are similar to those of hexagonal GaN.
Strong ultraviolet photoluminescence from silicon oxide films prepared by magnetron sputtering
Authors: Song H. Z. , Bao X. M. , Li N. S. , Wu X. L.
Published: July 2002

Intense ultraviolet photoluminescence centered at 370 nm was observed from magnetron-sputtered silicon oxide films after they were annealed at about 1000 °C in N2 atmosphere. This photoluminescence is found to be associated with the formation of nanocrystal silicon particles in the specially structured SiO2, which highly resembles the oxide layer of porous silicon. The luminescence centers at the interface between the nanocrystal silicon particles and the SiO2 matrix are responsible for the strong ultraviolet luminescence.
Exciplex formation at the organic solid-state interface: Yellow emission in organic light-emitting diodes using green-fluorescent tris(8-quinolinolato)aluminum and hole-transporting molecular materials with low ionization potentials
Authors: Itano Koji , Ogawa Hiromitsu , Shirota Yasuhiko
Published: July 2002

The bilayer organic light-emitting diodes using green-fluorescent tris(8-quinolinolato)aluminum (Alq3) as an emitting material and hole-transport materials with low ionization potentials, 1,3,5-tris(3-methylphenylphenylamino)triphenylamine and 4,4′,4″-tris[bis(4-tert-buthylbiphenyl4-yl)amino]triphenylamine, emitted bright yellow light instead of green light. The yellow emission is attributed to exciplex formation at the solid interface between Alq3 and the hole-transport material. The exciplex formation was evidenced by the measurement of the photoluminescence spectra and lifetimes of the mixture of an equimolar amount of Alq3 and each of the hole-transport materials. The emission color can be tuned by varying the applied voltage.
Direct formation of self-assembled quantum dots under tensile strain by heteroepitaxy of PbSe on PbTe (111)
Authors: Pinczolits M. , Springholz G. , Bauer G.
Published: July 2002

Direct growth of tensile-strained PbSe quantum dots by molecular beam epitaxy on 5.5% lattice mismatched PbTe (111) is investigated by atomic force microscopy and in situ reflection high energy electron diffraction. After wetting layer formation, two types of PbSe islands are formed distinguishable in size and ordering behavior. All islands exhibit a well defined pyramidal shape with triangular base and steep (100) side facets. In addition, the dots exhibit a remarkably narrow size distribution with a relative variation of height and width as low as ±7%.
Mobility of two-dimensional electrons in AlGaN/GaN modulation-doped field-effect transistors
Authors: Oberhuber R. , Zandler G. , Vogl P.
Published: July 2002

We present quantitative calculations of the electron drift mobility in wurtzite (WZ) and zincblende (ZB) structure n-type AlGaN/GaN modulation-doped field-effect transistors. The two-dimensional character of the quantum confined carriers as well as spontaneous and piezoelectric electric field effects are fully taken into account. For given doping concentration, we find that the internal electric fields lead to a much stronger carrier confinement and higher channel densities than in standard III–V materials. For high quality n-type heterostructures, we predict a room temperature mobility at high densities close to 2000 cm2/V s.
Intermixing at the tantalum oxide/silicon interface in gate dielectric structures
Authors: Alers G. B. , Werder D. J. , Chabal Y. , Lu H. C. , Gusev E. P. , Garfunkel E. , Gustafsson T. , Urdahl R. S.
Published: July 2002

Metal oxides with high dielectric constants have the potential to extend scaling of transistor gate capacitance beyond that of ultrathin silicon dioxide. However, during deposition of most metal oxides on silicon, an interfacial region of SiOx can form that limits the specific capacitance of the gate structure. We have examined the composition of this layer using high-resolution depth profiling of medium ion energy scattering combined with infrared spectroscopy and transmission electron microscopy. We find that the interfacial region is not pure SiO2, but is a complex depth-dependent ternary oxide of Si–Tax–Oy with a dielectric constant at least twice that of pure SiO2 as inferred from electrical measurements. High-temperature annealing crystallizes the Ta2O5 film and converts the composite oxide to a more pure SiO2 layer with a lower capacitance density. Using low postanneal temperatures, a stable composite oxide structure can be obtained with good electrical properties and an effective SiO2 thickness of less than 2 nm with ∼10 nm of composite oxide.
Temperature dependence of the growth rate for nanocrystalline diamond films deposited from an Ar/CH4 microwave plasma
Authors: McCauley Thomas G. , Gruen Dieter M. , Krauss Alan R.
Published: July 2002

We have investigated the effect of substrate temperature on the growth rate and properties of nanocrystalline diamond thin films prepared by microwave plasma-assisted chemical vapor deposition on (100) Si from a 1% methane (CH4) precursor in argon (Ar). In previous work we have shown that the carbon dimer C2 is the dominant growth species for this CH4/Ar system without the addition of molecular hydrogen. In the present work, the apparent activation energy for this growth process from C2 was determined from a standard Arrhenius-type analysis of the growth rate data for substrate temperatures between 500 and 900 °C. The measured value of 5.85±0.438 kcal/mol (0.254±0.019 eV/atom) is shown to be in close agreement with the results of recent modeling studies of the energetics of C2 addition to the diamond (110)–(1×1):H surface. These results have important implications for low-temperature diamond coating of nonrefractory materials such as glasses.
Al
x
Ga
1−x
N:Si
Schottky barrier photodiodes with fast response and high detectivity
Authors: Monroy E. , Calle F. , Muñoz E. , Omnès F. , Gibart P. , Muñoz J. A.
Published: July 2002

Gold and nickel Schottky barrier photovoltaic detectors have been fabricated on Si-doped AlxGa1−xN layers (0⩽x⩽0.22) grown on sapphire by metalorganic vapor phase epitaxy. Responsivity is independent of the Schottky metal or diode size, and also of the incident power in the range measured (10 mW/m2–2 kW/m2). A higher visible rejection has been observed in the spectral response of Au photodiodes (>103). Time response is resistance-capacitance limited, with time constants as short as 14 ns in Al0.22Ga0.78N diodes. Low frequency noise studies are also presented, and detectivities of 6.1×107 and 1.2×107 mHz1/2 W−1 are determined in GaN/Au and Al0.22Ga0.78N/Au detectors, at −2 V bias.
Very high-speed metal-semiconductor-metal ultraviolet photodetectors fabricated on GaN
Authors: Carrano J. C. , Li T. , Brown D. L. , Grudowski P. A. , Eiting C. J. , Dupuis R. D. , Campbell J. C.
Published: July 2002

We report on the temporal and the frequency response of metal-semiconductor-metal ultraviolet photodetectors fabricated on single-crystal GaN. The best devices show a fast 10%–90% rise time of ∼23 ps implying a bandwidth of >15 GHz. These time domain data have been corroborated by direct measurement of the power spectral content. From this a cutoff frequency, f3 dB, of ∼16 GHz has been obtained. Analysis in terms of reverse bias and geometric scaling indicates that the photodetectors are transit-time limited. Modeling of the temporal behavior indicates that a slow component in the time and frequency response data is a consequence of the hole drift velocity.
Contrast reversal in scanning capacitance microscopy imaging
Authors: Stephenson Robert , Verhulst Anne , De Wolf Peter , Caymax Matty , Vandervorst Wilfried
Published: July 2002

We have investigated the quantification properties of scanning capacitance microscopy (SCM) by using two dedicated test structures and highlight the response of SCM to changes in dopant density. Our results indicate that contrast reversal occurs and that the SCM output is not always a monotonically increasing signal with decreasing dopant density. Two epitaxially grown staircase structures covering the doping ranges 1014–1020 cm−3 p type and 5×1014–5×1019 cm−3 n type were produced for this study as the turning point in the response function typically occurs at a doping level of around 1017 cm−3. Through the use of a simple simulation model we see that contrast reversal is expected due to a relative shift between the dC/dV curves for different doping levels. The onset of contrast reversal can be adjusted by changing the dc sample bias leading to a shift in the operating position of the SCM, and the significance of this point will be discussed here.
Spontaneous lifetime control in a native-oxide-apertured microcavity
Authors: Graham L. A. , Huffaker D. L. , Deppe D. G.
Published: July 2002

Spontaneous lifetime control is demonstrated using very small apertured microcavities, with quantum-dot light emitters used to obtain electronic confinement within the aperture. A factor of 2.3 increase in the averaged spontaneous emission rate is achieved due to the optical confinement. The enhancement/inhibition ratio of the spontaneous emission rate tracks the optical mode size and spectral response of the apertured microcavity.
Stable dynamic avalanche in Si power diodes
Authors: Domeij Martin , Breitholtz Bo , Östling Mikael , Lutz Josef
Published: July 2002

A stable dynamic avalanche at a maximum power density of about 2.4 MW/cm2 was measured in small areas of 3.3 kV Si power diodes, using an optical measurement technique, and very good dynamic ruggedness was verified in a conventional turn-off measurement. Device simulations of a diode with a shallow n+ emitter indicate that impact ionization at the nn+ junction can result in negative differential resistance (NDR) and current filamentation, whereas a deep n+ emitter in the experimentally studied diode suppresses NDR. It is, therefore, proposed that the deep n+ emitter is important for the stable dynamic avalanche.
Violet and blue emitting (In,Ga)N/GaN multiple quantum wells grown on γ-LiAlO2(100) by radio frequency plasma-assisted molecular beam epitaxy
Authors: Waltereit P. , Brandt O. , Ploog K. H.
Published: July 2002

We study the growth of GaN and (In,Ga)N/GaN multiple quantum wells on γ-LiAlO2 by rf plasma-assisted molecular beam epitaxy. The GaN layers exhibit promising optical and electrical properties, whereas the structural quality is significantly influenced by inferior substrate morphology. The (In,Ga)N/GaN multiple quantum well structures show intense room-temperature photoluminescence in the violet and blue spectral range.
In-plane gate single-electron transistor in Ga[Al]As fabricated by scanning probe lithography
Authors: Lüscher S. , Fuhrer A. , Held R. , Heinzel T. , Ensslin K. , Wegscheider W.
Published: July 2002

A single-electron transistor has been realized in a Ga[Al]As heterostructure by oxidizing lines in the GaAs cap layer with an atomic force microscope. The oxide lines define the boundaries of the quantum dot, the in-plane gate electrodes, and the contacts of the dot to source and drain. Both the number of electrons in the dot as well as its coupling to the leads can be tuned with an additional, homogeneous top gate electrode. Pronounced Coulomb blockade oscillations are observed as a function of voltages applied to different gates. We find that, for positive top-gate voltages, the lithographic pattern is transferred with high accuracy to the electron gas. Furthermore, the dot shape does not change significantly when in-plane voltages are tuned.
Site-controlled InAs single quantum-dot structures on GaAs surfaces patterned by in situ electron-beam lithography
Authors: Ishikawa Tomonori , Nishimura Tetsuya , Kohmoto Shigeru , Asakawa Kiyoshi
Published: July 2002

We studied a site-control technique for InAs quantum dots (QDs) on GaAs substrates using a combination of in situ electron-beam (EB) lithography and self-organized molecular-beam epitaxy. In small, shallow holes formed on prepatterned mesa structures by EB writing and Cl2 gas etching, QDs were selectively formed, without any formation on the flat region between the patterned holes. The density of the QDs in each hole was dependent on the hole depth, indicating that atomic steps on the GaAs surfaces act as migration barriers to In adatoms. In an array of holes including 5–6 monolayer steps, a single QD was arranged in each hole.
Micromachined convective accelerometers in standard integrated circuits technology
Authors: Milanović Veljko , Bowen Edwin , Zaghloul Mona E. , Tea Nim H. , Suehle John S. , Payne Beverly , Gaitan Michael
Published: July 2002

This letter describes an implementation of micromachined accelerometers in standard complimentary metal–oxide–semiconductor technology. The devices operate based on heat convection and consist of microheaters and thermocouple or thermistor temperature sensors separated by a gap which measure temperature difference between two sides of the microheater caused by the effect of acceleration on free gas convection. The devices show a small linearity error of <0.5% under tilt conditions (±90°), and <2% under acceleration to 7g(g≡9.81 m/s2). Sensitivity of the devices is a nearly linear function of heater power. For operating power of ∼ 100 mW, a sensitivity of 115 μV/g was measured for thermopile configuration and 25 μV/g for thermistor configurations. Both types of devices are operable up to frequencies of several hundred Hz.
Time-resolved photoluminescence studies of AlxGa1−xN alloys
Authors: Kim H. S. , Mair R. A. , Li J. , Lin J. Y. , Jiang H. X.
Published: July 2002

The optical properties of AlxGa1−xN alloys with x varied from 0 to 0.35 have been investigated by picosecond time-resolved photoluminescence (PL) spectroscopy. Our results revealed that while the PL intensity decreases with an increase of Al content, the low-temperature PL decay lifetime increases with Al content. These results can be understood in terms of the effects of tail states in the density of states due to alloy fluctuation in the AlxGa1−xN alloys. The Al-content dependence of the energy-tail-state distribution parameter E0, which is an important parameter for determining optical and electrical properties of the AlGaN alloys, has been obtained experimentally. The PL decay lifetime increases with the localization energy and, consequently, increases with Al content. The implications of our findings to III-nitride optoelectronic device applications are also discussed.
Cooperative nucleation leading to ripple formation in InGaAs/GaAs films
Authors: Chokshi Nehal S. , Mirecki Millunchick Joanna
Published: July 2002

In 0.25 Ga 0.75 As epilayers were grown on GaAs (001) substrates (1.8% misfit strain) by molecular beam epitaxy to investigate the two-dimensional to three-dimensional transition as a function of thickness (t⩽30 MLs). Tapping-mode atomic force micrographs show the evolution of the morphology as a function of thickness. As the film is deposited, the nucleation of 3D islands followed by cooperative nucleation of pits is observed. As the thickness increases, both islands and pits continue to nucleate and grow until they coalesce, resulting in a fully formed ripple morphology running along the [11̄0]. The ripples also exhibit a secondary alignment roughly along the 〈310〉 which is attributed to the nucleation of islands with {136} faces.
Stark shift in electroluminescence of individual InAs quantum dots
Authors: Itskevich I. E. , Rybchenko S. I. , Tartakovskii I. I. , Stoddart S. T. , Levin A. , Main P. C. , Eaves L. , Henini M. , Parnell S.
Published: July 2002

We have fabricated light-emitting-diode heterostructure devices, in which a layer of InAs self-assembled quantum dots is embedded, with an active area of submicron size. In the electroluminescence spectra of these devices, we observed isolated narrow peaks due to emission from individual dots. From the shift of the peaks in an electric field (the quantum confined Stark effect), we show that the ground and excited states in the dots have different spatial alignments of the electron and hole.
Visible and ultraviolet Raman scattering studies of Si1−xGex alloys
Authors: Holtz M. , Duncan W. M. , Zollner S. , Liu R.
Published: July 2002

We report Raman studies of the Si–Si phonon band in Si1−xGex alloys, where the excitation is by visible and ultraviolet (351 nm) light. At a wavelength 351 nm, the optical penetration depth is extremely shallow (≈5 nm). By varying the excitation from 351 to 514 nm, the optical penetration depth spans from 5 to 300 nm. Two sets of samples were examined. Thin layers grown using molecular beam epitaxy were coherently strained to match the lattice constant of the silicon substrate. Thick layers grown using organo–metallic chemical vapor deposition were strain relaxed. For the thin, strained layers, visible excitation produces a spectrum, which is a superposition of the substrate and the epilayer phonon bands. Reducing the wavelength (and, consequently, penetration depth) allows us to isolate the epilayer spectrum. Phonon energies obtained using all excitation wavelengths agree. We conclude that Raman scattering from these alloys using 351 nm laser light gives us bulk alloy properties pertinent to the near-surface composition and strain. The epilayers show no evidence of compositional variance or strain relaxation near the surface.
An evaluation of the assumed beta probability density function subgrid-scale model for large eddy simulation of nonpremixed, turbulent combustion with heat release
Authors: Wall Clifton , Boersma Bendiks Jan , Moin Parviz
Published: July 2002

The assumed beta distribution model for the subgrid-scale probability density function (PDF) of the mixture fraction in large eddy simulation of nonpremixed, turbulent combustion is tested, a priori, for a reacting jet having significant heat release (density ratio of 5). The assumed beta distribution is tested as a model for both the subgrid-scale PDF and the subgrid-scale Favre PDF of the mixture fraction. The beta model is successful in approximating both types of PDF but is slightly more accurate in approximating the normal (non-Favre) PDF. To estimate the subgrid-scale variance of mixture fraction, which is required by the beta model, both a scale similarity model and a dynamic model are used. Predictions using the dynamic model are found to be more accurate. The beta model is used to predict the filtered value of a function chosen to resemble the reaction rate. When no model is used, errors in the predicted value are of the same order as the actual value. The beta model is found to reduce this error by about a factor of two, providing a significant improvement.