bluuebunny/crossref_metadata_2025 huggingface dataset:
Bias current dependent resistance peaks in NiFe/Ag giant magnetoresistance multilayers
Authors: Kirschenbaum L. S. , Rogers C. T. , Beale P. D. , Russek S. E. , Sanders S. C.
Published: July 2002

We show that thin-film Ni82Fe18/Ag multilayer structures display multiple peaks in their magnetoresistance curves when biased at current densities above 106 A/cm2. These peaks appear for annealed and unannealed structures, and their number is correlated with the number of NiFe layers. At high bias currents, the peak positions shift linearly with the internal magnetic field created by the bias current. The peak positions extrapolate to nonzero fields at zero bias currents, providing an upper bound on the magnetic layer-layer coupling strength of J0≊10−20 J (kB×700 K). The peak positions do not shift with temperature over the range 200–375 K; their widths narrow with increasing temperature. The single-domain magnetic moment μ is estimated as 10−17 J/T (106 μB) from the peak widths of ∼0.8 kA/m.
Conversion of step configuration induced by strain in Si1−xGex layers deposited on vicinal Si(001) surface
Authors: Zhou J. M. , Guo L. W. , Cui Q. , Peng C. S. , Huang Q.
Published: July 2002

Reflection high energy electron diffraction (RHEED) intensity measurements reveal that the strain in a Si1−xGex layer on a vicinal Si(001) surface converts a (1×2) domain dominated step configuration to a (2×1) domain dominated one. The dependence of the effect on the Ge content is similar to the dependence of the critical thickness of the pseudomorphic growth of Si1−xGex layers on Ge content. No conversion effect has been observed on exact Si(001) substrates.
Study of light induced instability in intrinsic hydrogenated amorphous silicon films by the photomixing technique
Authors: Tang Yi , Dong S. , Braunstein R. , von Roedern B.
Published: July 2002

By using the photomixing technique we have found that the drift mobility (μd) of intrinsic hydrogenated amorphous silicon (a-Si:H) films produced by both glow discharge and hot wire techniques increases with increasing electric field, while the lifetime (τ) decreases with increasing electric field, and the μdτ product is essentially independent of the electric field. We have also found an empirical relationship that a greater field dependence of the drift mobility of an a-Si:H film in the annealed state indicates a poorer stability of the photoconductivity upon light soaking.
Scanning near-field optical microscope using an atomic force microscope cantilever with integrated photodiode
Authors: Akamine S. , Kuwano H. , Yamada H.
Published: July 2002

A combined atomic force and scanning near-field optical microscope is presented. The critical component of the instrument is a single crystal silicon, microfabricated force-sensing cantilever with an integrated photodiode. Near-field optical images are obtained by monitoring variations in the optical power detected by the photodiode while the cantilever tip is scanned in an evanescent optical field created by illuminating the sample by total internal reflection. Near-field optical power was detected at tip-sample spacings of one-quarter wavelength. Atomic force and scanning near-field optical microscope images of the same samples show corresponding features as small as 25 nm.
Determination of losses, gain, and pumping-beam mode overlap for Q-switched end-pumped lasers
Authors: Ozygus B. , Ziegler K.
Published: July 2002

A method to determine the losses and the gain of a Q-switched four-level laser is presented. For an end-pumped system with an additional measurement of relaxation oscillation frequencies the pumping-beam and laser mode overlap can be determined. Since no resonator changes are necessary, the measurement is very easy to perform. With losses, gain, and overlap determined, everything is known to optimize the system.
All-optical synchronization of self-pulsating laser diodes
Authors: Egan A. , Harley-Stead M. , Rees P. , Lynch S. , McEvoy P. , O’Gorman J. , Hegarty J.
Published: July 2002

We examine the behavior of self-pulsating laser diodes when injected with periodic optical signals. We experimentally and theoretically investigate the phase difference between the injected optical signal and the synchronized self-pulsating laser diode emission. We explore the phase difference dependence on detuning between the laser free-running self-pulsation frequency and the applied signal frequency, and on the injected signal power. The determined sensitive dependence of the phase difference on these factors has important consequences when self-pulsating lasers are used as optical signal processing elements in all-optical communication networks, where such sensitivity may lead to timing problems.
Fivefold femtosecond pulse compression by sum frequency generation
Authors: Nisoli M. , De Silvestri S. , Valiulis G. , Varanavicius A.
Published: July 2002

Pulse compression by sum frequency generation is demonstrated in femtosecond time domain with a type II phase matching BBO crystal. By frequency mixing in BBO crystal infrared signal and idler pulses of 160 fs duration generated by an optical parametric amplifier, sum frequency pulses as short as 34 fs were obtained. Experiments and computer simulations demonstrate the potential of this technique to produce ultrashort pulses with high conversion efficiency.
Electron energy loss in thin metallic films
Authors: Marrian C. R. K. , Perkins F. K. , McCarthy D. , Bass R.
Published: July 2002

Schottky diodes with patterned overlayers of thin metal films have provided a method to measure the energy loss of high energy electrons in the metal films. The technique avoids the problems of surface contamination and electron collection which complicate measurements on freestanding films. The patterned overlayer creates a modulation in the reverse biased diode current as a beam of focused electrons is moved across the diode surface. The amplitude of the modulation has been measured with overlayers of aluminum and gold for incident electron energies of 5 to 30 keV. The results have been compared to that predicted from a Monte Carlo calculation of the inelastic and elastic scattering of the incident electrons. Significantly better agreement between experiment and simulation is observed when the screened Rutherford cross section in the Monte Carlo code is replaced by an empirical expression for the Mott scattering formula.
Electrical characterization of low-temperature Al0.3Ga0.7As using n-i-n structures
Authors: Verma A. K. , Tu J. , Smith J. S. , Fujioka H. , Weber E. R.
Published: July 2002

Through temperature-dependent conductivity measurements, we show evidence of a deep trap level in low-temperature (LT) Al0.3Ga0.7As layers with an activation energy of ∼0.96 eV. This energy is near that of EL2-like defects found previously in ‘‘normal’’ epitaxial Al0.3Ga0.7As. It is also considerably larger than the 0.70 eV value typically associated with defects in LT GaAs, which may explain the observed large resistivity (≳1011 Ω cm) in LT Al0.3Ga0.7As. Current transient spectroscopy (CTS) of these samples yields a deep level activation energy of 1.01 eV, in close agreement with the value obtained from conductivity measurements.
Thermally stimulated current trap in GaN
Authors: Look D. C. , Fang Z.-Q. , Kim W. , Aktas Ö. , Botchkarev A. , Salvador A. , Morkoç H.
Published: July 2002

A thermally stimulated current peak, occurring at 100 K for a heating rate of 0.4 K/s, has been found in semi-insulating GaN grown by molecular beam epitaxy. This peak has contributions from two traps, with the main trap described by the following parameters: emission thermal activation energy E≂90±2 meV, effective capture cross-section σ≂3±1×10−22 cm−2, and Nμτ≂3±1 × 1014 cm−1 V −1, where N is the trap concentration, μ the mobility, and τ the free-carrier lifetime. This trap is much deeper than the typical shallow donors in conducting GaN, but shallower than any of the centers reported in recent deep level transient spectroscopy measurements.
Reliability of Hall effect measurements on chemical vapor deposited polycrystalline B-doped diamond films
Authors: Huang J. T. , Guo W. H. , Hwang J. , Chang H.
Published: July 2002

In situ boron doped polycrystalline diamond films of very good quality were grown on both scratched n-Si(100) and Al2O3 substrates. Carrier concentration and Hall mobility are mainly contributed from the n-Si(100) substrate, rather than the boron doped polycrystalline diamond film, when n-Si(100) is chosen as the substrate. This indicates that the p-diamond/n-Si junction is unable to isolate current flowing from the boron doped polycrystalline diamond to the n-Si(100) substrate during Hall effect measurement. In contrast, carrier concentration and Hall mobility are reliable when Al2O3 is used as the insulating substrate for boron doped diamond deposition. The Hall mobility of the boron doped polycrystalline diamond film varies from 120 cm2/V s at a hole concentration of 1010/cm3 to 0.4 cm2/V s at 2×1015/cm3.
Far-infrared pump-probe measurements of the intersubband lifetime in an AlGaAs/GaAs coupled-quantum well
Authors: Heyman J. N. , Unterrainer K. , Craig K. , Williams J. , Sherwin M. S. , Campman K. , Hopkins P. F. , Gossard A. C. , Murdin B. N. , Langerak C. J. G. M.
Published: July 2002

We report pump-and-probe measurements of the electron intersubband lifetime (T1) in an AlGaAs/GaAs heterostructure using a picosecond pulsed far-infrared laser. The subband spacing (11 meV) is less than the optical-phonon energy. Time-resolved measurements yield intersubband lifetimes ranging from T1=1.1±0.2 ns to T1=0.4±0.1 ns depending on measurement conditions. Results are in agreement with previous lifetime measurements on the same sample using continuous excitation at intensities ≤1 W/cm2. The steady-state measurements yielded shorter lifetimes at high excitation intensities, possibly due to carrier heating leading to intersubband scattering by optical phonon emission.
Deposition of heteroepitaxial diamond film on (100) silicon in the dense plasma
Authors: Kim Yoon-Kee , Lee Ki-Young , Lee Jai-Young
Published: July 2002

Almost perfectly oriented heteroepitaxial diamond film was deposited on the (100)Si substrate by the carburization, bias-enhanced nucleation, and growth process in a dense plasma using a bell-jar-type microwave-plasma chemical vapor deposition (CVD) system with an ASTeX 1.5 kW magnetron plasma source. This dense plasma was a prerequisite for the perfect orientation and was not obtained simply by increasing microwave power, but was obtained by introducing a graphite block between the substrate and the substrate holder. The plasma was concentrated over the substrate especially at the four corners with a thickness of several millimeters.
Uncongruent laser ablation and electroless metallization of SiC
Authors: Shafeev G. A. , Bellard L. , Themlin J.-M. , Fauquet-Ben Ammar C. , Cros A. , Marine W.
Published: July 2002

Experimental results on the ablation of a SiC single crystal in air under irradiation with a XeCl excimer laser beam are presented. XPS analysis reveals the Si enrichment of the ablated areas under an energy density of the laser beam of 1 to 3.5 J/cm2 and the formation of SiOx in the ablated areas. The nonstoichiometric ablation of SiC allows one to activate selectively the SiC surface for metal deposition from an electroless plating solution. Both Ni and Cu deposits show a good adherence to the SiC surface (up to 0.5 N/mm2). Similar results are observed with SiC ceramics with a better adherence of the electroless metal deposit. The adherence of the deposit to the ablated SiC samples increases upon HF treatment.
Electroabsorption field imaging between coplanar metal contacts on semi-insulating semiconductor epilayers
Authors: Nolte D. D. , Chen N. P. , Melloch M. R. , Montemagno C. , Haegel N. M.
Published: July 2002

We use excitonic electroabsorption field imaging to study the electric field distribution between coplanar gold Schottky contacts on semi-insulating photorefractive AlGaAs epilayers. The field imaging shows consistently large localized enhancements of the electric field adjacent to the anode, followed by a region of reduced field. Complex behavior occurs at the cathode, with high-field regions extending far from the contact. These inhomogeneous near-contact field profiles are determined by the superposition of both diffused and drifted charge which affect the performance of many optoelectronic devices that use planar contacts or striplines on semi-insulating substrates.
Room-temperature preparation of crystallized luminescent Sr1−xCaxWO4 solid-solution films by an electrochemical method
Authors: Cho Woo-Seok , Yashima Masatomo , Kakihana Masato , Kudo Akihiko , Sakata Tadayoshi , Yoshimura Masahiro
Published: July 2002

A complete series of well-crystallized solid-solution oxide films, Sr1−XCaXWO4 (0≤X≤1), has been prepared on a tungsten substrate in the electrolytic solution containing Sr2+ and Ca2+ ions by an electrochemical method at room temperature (25 °C). The composition of solid-solution oxide films could easily be controlled by the concentrations of Sr and Ca species in the starting solutions. The films showed only single blue emission at liquid nitrogen temperature (−196 °C), strongly suggesting that they consisted of well-crystallized defect-free crystals.
Measurement of linear nanometric distances between smooth plane parallel bodies by scattered total internal reflection
Authors: Sides Paul J. , Lo Jenny
Published: July 2002

Measurement of linear nanometric distances between perfectly smooth plane parallel surfaces by scattering evanescent waves of internally reflected light is shown to be possible if one of the two surfaces is optically disseminative, that is, it either possesses a heterogeneous index of refraction or it is capable of absorbing and re-emitting radiation. Good agreement was obtained between the nominal 100 nm fly height of a magnetic recording slider and the 92 nm fly height measured by scattered total internal reflection. This method is simpler and more informative than interferometry or methods that employ the internally reflected beam.
Improvement of optical transmission of KTiOPO4 crystals by growth in nitrogen ambient
Authors: Miyamoto Akio , Mori Yusuke , Sasaki Takatomo , Nakai Sadao
Published: July 2002

KTiOPO4 (KTP) crystals were grown from K6 (K6P4O13) flux in various ambients. A colorless KTP crystal was obtained in an oxygen deficient ambient. It has been found that the absorption coefficient in the region from 400 to 550 nm and Pt inclusion concentration increased with the oxygen concentration in the growth ambient. The strong correlation between the absorption coefficient and Pt impurity concentration suggests that the absorption of KTP crystal is attributed to incorporated Pt ions in the crystal.
Novel quantum box intersubband lasing mechanism based on image charges
Authors: Lee S. J. , Khurgin J. B.
Published: July 2002

We consider theoretically a novel lasing scheme in semiconductor quantum boxes. Ultralow threshold pumping power can be obtained due to the development of images charges in surrounding layers. The impact on the intersubband lasing is stressed.
Thermally activated interface shift in the tungsten/silicon multilayers
Authors: Jergel M. , Bochníček Z. , Majková E. , Senderák R. , Luby Š.
Published: July 2002

The in situ x-ray reflectivity measurements during linear and isothermal annealings of a W/Si multilayer (ML) were performed, and successive disappearance and reappearance of the second and third ML Bragg maxima between 400 and 500 °C were observed. Such behavior is direct evidence of a long-range interface shift, and was found to be connected with a substantial decrease of the electronic density of the expanding originally W layers and the ML period. Surprisingly, the changes take place without smearing-out the interfaces. The results are explained by the Si diffusion into the W layers.
Spectroscopic evidence for the excitonic lasing mechanism in ultraviolet ZnS/ZnCdS multiple quantum well lasers
Authors: Ozanyan K. B. , Nicholls J. E. , O’Neill M. , May L. , Hogg J. H. C. , Hagston W. E. , Lunn B. , Ashenford D. E.
Published: July 2002

We demonstrate optically pumped lasing at room temperature from ZnS/ZnCdS quantum-well structures grown on (100) GaP substrates by molecular beam epitaxy. In the structures with the lowest Cd composition, optically pumped lasing at wavelengths as low as 333 nm (at 8 K) is observed. We present spectroscopic evidence which suggests that the stimulated emission involves states in the low-energy tail of an inhomogeneously broadened excitonic resonance. While lasing is excitonic at low thresholds, a transition to an electron-hole plasma mechanism may occur if the pump power approaches 100 kW cm−2.
Efficient green light-emitting diodes from a phenylated derivative of poly(p-phenylene–vinylene)
Authors: Cacialli F. , Friend R. H. , Haylett N. , Daik R. , Feast W. J. , dos Santos D. A. , Brédas J. L.
Published: July 2002

We have used polyvinylcarbazole, PVK, in combination with poly(p-phenylene vinylene), PPV and a green-emitting, soluble derivative, poly(4,4′-diphenylene diphenylvinylene) or briefly PDPV, to fabricate triple-layer light-emitting diodes PPV/PVK/PDPV/metal, with indium tin oxide as the anode. We report luminances in excess of 1300 cd/m2 with Al cathodes, turn-on voltages of ∼8 V in structures 300 nm thick, and internal efficiencies in the range 0.4%–0.8% for Al and Ca cathodes; at least ten times higher than for single-layer PDPV devices. Quantum efficiencies for Ca or Al electrodes are only a factor 1.5 different. The improvements over single-layer PDPV devices are considered to be due to better hole injection by PPV and to the formation of a hole barrier at the PPV/PVK interface, which forces a better balance of the electron and hole currents.
Observation of ion gettering effects in high-temperature superconducting oxide material
Authors: Hong S. H. , Chen M. L. , Baniecki J. , Ma Q. Y. , Wang H. A. , Odom R. W.
Published: July 2002

Ion gettering effect has been observed in high-temperature superconducting YBa2Cu3O7 material. Silicon ions were implanted into the material and subsequent high-temperature annealing produced ion movement from a low concentration region to a higher concentration region where the damage of the crystal structure is severe. This gettering effect could be used to make a superconductor-nonsuperconductor-superconductor trilayer structure within a single YBCO film.
Observation of Cu vacancies and their ordering in YBa2Cu4O8
Authors: Zakharov N. D. , Kästner G. , Hesse D. , Lorenz M.
Published: July 2002

Laser-deposited epitaxial thin films of YBa2Cu3O7−δ investigated by high-resolution transmission electron microscopy reveal narrow layers of the 1-2-4 phase YBa2Cu4O8. Within the CuO double layers of this phase, columns of double Cu(1) vacancies are identified to occur at periodic spacings of 2a with a, b, and c denoting the unit cell axes. In addition, an interchange of the a and b axes of the 1-2-4 phase is observed on traversing along c. A corresponding structure model is presented, and possible reasons of forming this structure by stress relaxation or by diffusional processes are discussed. A possible contribution to flux pinning is considered in the context of the supercurrent and high frequency properties.
HgCdTe and CdTe(1̄ 1̄ 3̄)B growth on Si(112)5° off by molecular beam epitaxy
Authors: Kawano M. , Ajisawa A. , Oda N. , Nagashima M. , Wada H.
Published: July 2002

CdTe(1̄ 1̄ 3̄)B epilayers were grown on Si substrates oriented (112)5° off toward the [1̄ 1̄ 1] direction by molecular beam epitaxy (MBE). A Zn irradiation process was developed in order to obtain a (1̄ 1̄ 3̄)B face. HgCdTe(1̄ 1̄ 3̄)B epilayers were grown on 20 μm-thick CdTe/Si(112) 5° off, and characterized. These layers have double-crystal x-ray rocking curves with full width at half-maximum as low as 64 arc s, and etch pit densities of 4.4×106 cm−2 and 2.6×105 cm−2 for as-grown and thermal-cycle annealed films, respectively. Photodiodes were also fabricated to demonstrate the capability of large-area MBE-HgCdTe/Si focal-plane arrays.
Voltage-flux bistability in damped direct current SQUIDs
Authors: Atkin I. L. , Abraham E. , Pegrum C. M.
Published: July 2002

We present an analysis and numerical study of a dc superconducting quantum interference device (SQUID) with a damped loop inductance. We find a previously unreported regime of voltage-flux bistability due to the damping resistor when the SQUID is biased below the zero-field critical current. We determine the region where this phenomenon can be found in the parameter space defined by the damping resistance and the inductance parameter. We show that switching between the bistable states can be controlled by external flux pulses and by adding thermal noise to the model, we have determined the maximum noise level at which the bistability can be seen.
New aspects of copper diffusion in semi-insulating gallium arsenide
Authors: Griehl St. , Herms M. , Klöber J. , Niklas J. R. , Siegel W.
Published: July 2002

The diffusion of copper in semi-insulating liquid-encapsulated-Czochralski-grown gallium arsenide at 800 °C was examined by photoluminescence, photoetching, secondary ion mass spectroscopy, and temperature dependent Hall measurements. A diffusion rate of 4.5×10−6 cm2 s−1 was obtained. It could be estimated that the diffusion takes place predominantly by substitution on lattice sites. Diffusing copper migrates preferentially along the walls of the dislocation cellular network.
High temperature polarization retention of a Pb(Zr0.52Ti0.48)O3/YBa2Cu3O7−x memory cell
Authors: Zomorrodian A. R. , Lin H. , Wu N. J. , Huang T. Q. , Liu D. , Ignatiev A.
Published: July 2002

Polarization retention for Au/Pb(Zr0.52Ti0.48)O3/YBa2Cu3O7−x ferroelectric capacitors has been studied at elevated temperatures. A device with an effective area of 4×10−3 cm2 and 1 μm thickness was fabricated, and its retention measured at 150 °C and 200 °C. The retention was studied for two polarization directions in Pb(Zr0.52Ti0.48)O3 (PZT) by the pyroelectric current and phase detection method as well as by the conventional double pulse method. A polarization degradation of less than 5.5% was detected for PZT at 200 °C for more than 24 h.
Quenching of giant magnetoresistance by interface roughening and alloying in annealed [(NixFe1−x)yAu1−y]/Au multilayers
Authors: Farrow R. F. C. , Parkin S. S. P. , Marks R. F. , Krishnan Kannan M. , Thangaraj N.
Published: July 2002

Antiferromagnetically coupled permalloy/Au multilayers display giant magnetoresistance (GMR) with large changes in resistance in very low fields. We show that thermal annealing of such structures, exhibiting GMR, leads to a quenching of the magnetoresistance. The detailed structure of the permalloy/Au interfaces has been probed using high-resolution cross-section transmission electron microscopy. On annealing, the Au layers interdiffuse into the permalloy layers, which leads both to rougher permalloy/Au interfaces and to ‘‘thinner’’ Au spacer layers. We infer that the latter results in ferromagnetic coupling of the permalloy layers, which accounts for the reduced GMR.
Optical effect on thermal emission of semiconductors
Authors: Chen G.
Published: July 2002

Emissivity is generally considered as a material property that depends on the temperature and the surface condition of the sample. This letter will demonstrate that the thermal emission and, thus, the emissivity of a semiconductor wafer heated by a radiation source depends not only on the wafer temperature but also on the radiation heater. The observed phenomenon is explained through electron-hole excitation in the wafer by the photons emitted from the heater. Such an optical effect may affect the interpretation of the measurement of the temperature dependence of the optical properties of semiconductors and limit the accuracy of temperature measurement of semiconductors by infrared pyrometry. The finding also has important implications for the thermal modeling of semiconductor manufacturing processes such as rapid thermal processing.
Dramatic improvement of performance of visible hydrogenated amorphous silicon carbide based p-i-n thin-film light-emitting diodes by two-step hydrogenation
Authors: Lee Jong-Wook , Lim Koeng Su
Published: July 2002

The effects of two-step hydrogenation on the performance of visible p-i-n thin-film light-emitting diode (TFLED) have been investigated. The TFLEDs were fabricated by a photochemical vapor deposition (CVD) method. A hydrogenation process was performed in two steps: One was an in situ hydrogenation process using a photo-CVD system and the other an ex situ hydrogenation process using a plasma apparatus after TFLED fabrication. It was found that the performance of visible a-SiC:H-based p-i-n TFLEDs was drastically improved by a two-step hydrogenation process. The threshold voltage decreased by about 3 V, the electroluminescence peak shifted towards a shorter wavelength, from 680 to 590 nm, and the brightness increased from 1.3 to 128 cd/m2.
Molecular field theory analysis of magneto-optic sensitivity of gallium-substituted yttrium iron garnets
Authors: Deeter Merritt N. , Bon Silvia Milián
Published: July 2002

The temperature dependence of the magneto-optic sensitivity of gallium-substituted yttrium iron garnets was measured at 1.3 μm and compared with a model based on molecular field theory. The model incorporates results of measurements of both the saturation magnetization and saturation Faraday rotation versus temperature. These measurements were analyzed in the context of molecular field theory to extract the fundamental molecular field coefficients and the magneto-optical coefficients as functions of gallium content. The model and direct sensitivity measurements both indicate that the magneto-optic sensitivity of garnet compositions with gallium substitution levels near 0.8 should exhibit a vanishing first-order temperature sensitivity.
Local structural modification in ion damaged InGaAs
Authors: Yu Kin Man , Hsu Leonardo
Published: July 2002

The indium nearest-neighbor environment in InGaAs thin films damaged by Ar ion implantation has been studied by extended x-ray absorption fine structure spectroscopy. We find that before the material turns amorphous by Ar irradiation, the In–As nearest-neighbor distance remains close to its crystalline value even when the layer is heavily damaged but not entirely amorphous. Once the Ar dose exceeds the threshold for amorphizing the InGaAs layer, the In–As bond distance relaxes to that of pure crystalline InAs. This sudden change in local structure as the material transforms from crystalline to amorphous suggests that the transition is due to simultaneous amorphous nucleation rather than the accumulation and overlapping of isolated amorphous regions. Moreover, this change in the local structure in a ternary alloy can be used as a criterion for determining the crystalline-to-amorphous transition of the alloy.
Electric field domains in intentionally perturbed semiconductor superlattices
Authors: Schwarz G. , Prengel F. , Schöll E. , Kastrup J. , Grahn H. T. , Hey R.
Published: July 2002

Simulations based on a rate equation model for high-field transport through a doped semiconductor superlattice are presented for the case that one barrier is chosen significantly wider than the others. The distinct impact of that local perturbation on the overall shape of the current–voltage characteristic is discussed and related to the spatial field distribution. The measured current–voltage characteristic of a superlattice, which was intentionally grown with one thicker barrier, confirms the strong asymmetry predicted by the model calculations.
High field flashover strength of intrinsic gallium nitride and aluminum nitride in vacuum
Authors: Khan M. A. , Chen O. , Sudarshan T. S. , Gradinaru G.
Published: July 2002

The behavior of intrinsic materials (i-GaN and i-AlN) at high fields and room temperature, using small gap lateral test devices in vacuum ambient, is analyzed. Unprecedented hold-off fields of more than 375 kV/cm were obtained for intrinsic nitride layers grown on sapphire. The superior high field performance of intrinsic nitride materials indicate a great potential of nitride semiconductors for the next generation high power/high voltage/high frequency/high temperature electronic devices.
High efficiency diode pumping of a saturable Bragg reflector-mode-locked Cr:LiSAF femtosecond laser
Authors: Tsuda S. , Knox W. H. , Cundiff S. T.
Published: July 2002

We pump a saturable Bragg reflector (SBR)-mode-locked Cr:LiSAF laser with a 0.5 W diffraction-limited 670 nm laser diode and obtain average CW mode-locked power of 100 mW of 70 fs pulses at λ=868 nm. The low loss of the SBR-mode-locking technique combined with a nearly diffraction limited pumping results in the highest optical conversion efficiency obtained with this type of laser to date. We discuss overall electrical efficiency of femtosecond lasers.
Evidence for acoustic waves induced by focused ion beams
Authors: Teichert Jochen , Bischoff Lothar , Köhler Bernd
Published: July 2002

Acoustic waves induced by an intensity modulated focused ion beam have been measured. The experiments were performed with Ga+ ions of 35 keV at a current of 3 nA and variable chopping frequency up to 10 MHz in a common focused ion beam system. The acoustic signals were detected by means of a piezoelectric sensor with integrated preamplifier. Frequency and position sensitivity of the sensor has been measured by laser excitation. Ion acoustic measurements have been carried out at the resonance frequencies of the sensor. A dependence on the sample material was found. The results show that the ion acoustic effect can be utilized for imaging and material analysis in focused ion beam systems.
Analysis of parameters of multilayer carbon interference structures in the soft x-ray range
Authors: Kondrashov P. E. , Smirnov I. S. , Novoselova E. G. , Baranov A. M.
Published: July 2002

A new type of x-ray interference mirrors using hydrogenated carbon films of various densities is described. The reflection coefficient and resolution of multilayer carbon structures obtained in the soft x-ray range (1.54–44.7 Å) are investigated experimentally and theoretically. As is shown, hydrogenated carbon films can be used to create mirrors with high resolution.
Formation and control of phosphorus buried layers in silicon using a pulsed XeCl excimer laser
Authors: Verma G. , Talwar S. , Sigmon T. W.
Published: July 2002

In this letter, we report fabrication of pulsed-shaped phosphorus doping profiles in Si, using a XeCl (λ=380 nm) pulsed excimer laser. Phosphorus buried layers as shallow as 750 Å and as deep as 3200 Å, with peak full width at half-maximums as narrow as 100 Å and as broad as 900 Å are demonstrated. It is also demonstrated that the sample is free of point defects and full electrical activation is achieved by using a rapid thermal anneal at 800 °C for 30 s after the laser processing. This process, we demonstrate, afford simplicity, versatility, and independent control over the depth, width, and height of the phosphorus buried layer. Phosphorus buried layers have potential application in the fabrication of metal–oxide–semiconductor field effect transistors and BiPOLAR electronic devices.
Surface morphology of silicon–tin alloys grown by molecular beam epitaxy on Si and Si1−xGex substrates
Authors: Fyhn M. F. , Shiryaev S. Yu. , Lundsgaard Hansen J. , Nylandsted Larsen A.
Published: July 2002

The surface morphology and bulk structure of both strained (0.01≤x≤0.053) and strain-free Si1−xSnx (x=0.055) layers grown by molecular beam epitaxy on (001) Si and Si1−yGey substrates have been investigated by atomic force and transmission electron microscopy. The surfaces of all Si1−xSnx layers were found to have a mazelike structure composed of rectangular features faceted primarily by {113} planes. Transmission electron microscopy images of the alloys revealed columnar structures originating at the interface between the alloys and the substrates. The formation of the observed surface morphologies and the columnar structure is attributed to the surface-growth processes modified by the Sn atoms as compared to the growth of pure Si.
Microstructure and subpicosecond photoresponse in GaAs grown by molecular beam epitaxy at very low temperatures
Authors: Chin Albert , Chen W. J. , Ganikhanov F. , Lin G.-R. , Shieh Jia-Min , Pan Ci-Ling , Hsieh K. C.
Published: July 2002

Cross-sectional transmission electron microscopy (TEM) and femtosecond (fs) reflectivity are used to study (Ga,As) compound grown at low temperatures from 160 to 70 °C. A columnar polycrystalline GaAs is observed for samples grown at 160 °C while it appears amorphous for samples grown at temperatures below 120 °C. The successful growth of amorphous (Ga,As) at 70 °C, instead of a mixture of metallic Ga droplets and As solids, suggests that the decomposition of As4 molecules may be catalyzed by the surface Ga. Upon annealing, all samples become polycrystalline before epitaxial solid state regrowth eventually dominates and the whole sample becomes single crystal for long enough annealing. Carrier lifetime of 230 fs is measured for the as-grown amorphous sample. For the annealed sample, the photoresponse exhibits a fast initial decay of 120 fs and a much slower secondary decay of 33 ps. The initial decay is attributed to the formation of fine polycrystalline grains (∼500 Å in size) whose grain boundaries provide effective carrier traps and recombination centers.
Self-screening of the ferroelectric polarization in smectic C* cells in the bookshelf geometry
Authors: Galerne Yves
Published: July 2002

Surface stabilized ferroelectric liquid crystal (SSFLC) displays made of chiral smectic C* liquid crystals often present screening effects generally discussed in terms of the depolarizing field produced by ions attracted close to the electrodes. Here, we analyze the screening properties of distortions in the director field that form close to the electrodes because of the anchoring conditions. We show that the ferroelectric polarization effectively measured in such SSFLC cells is smaller than the real one, and depends on the spacing distance between the plates. Corrections are, therefore, necessary for samples of thickness <10 μm. This screening mechanism, which moreover increases the bistability of the cell, is as fast as the polarization reversal itself contrarily to the screening due to ion migrations.
Defect reduction in ZnSe grown by molecular beam epitaxy on GaAs substrates cleaned using atomic hydrogen
Authors: Yu Zhonghai , Buczkowski S. L. , Giles N. C. , Myers T. H.
Published: July 2002

Atomic hydrogen is demonstrated to effectively clean GaAs substrates for subsequent growth of ZnSe by molecular beam epitaxy. Optical fluorescence microscopy is shown to be a useful technique to image nonradiative defects related to stacking faults. While the density of stacking faults in ZnSe films grown using conventional thermal cleaning is greater than 107 cm−2, stacking fault densities lower than 104 cm−2 are obtained using atomic hydrogen cleaning. Low-temperature photoluminescence spectra of undoped ZnSe are dominated by excitonic transitions for the low defect density samples in contrast to the high level of defect-related emission from high defect density samples.
Interdiffusion as a means of fabricating parabolic quantum wells for the enhancement of the nonlinear third-order susceptibility by triple resonance
Authors: Li E. Herbert
Published: July 2002

Interdiffusion of a standard AlGaAs/GaAs quantum well is proposed as a viable alternative to the complex techniques necessary to fabricate parabolic quantum wells. The extent of the linear diffusion is optimized in order to produce an energy-level ladder if four almost equally spaced eigenstates. The calculated third-order susceptibility of 2700 (nm/V)2 is comparable with that of the parabolic quantum well, which is more than six orders of magnitude compared to that of the bulk GaAs.
Photoinduced synthesis of amorphous SiO2 with tetramethoxysilane
Authors: Awazu Koichi , Onuki Hideo
Published: July 2002

Photoinduced fabrication of amorphous SiO2 (a-SiO2) film is examined with tetramethoxysilane (TMOS) at room temperature. The photoinduced reactions consist of two steps: photoexcitation of silicon substrate and photoinduced synthesis of a-SiO2. The surface of the irradiated silicon with the Xe2 excimer lamp peaking at 7.2 eV is covered with silanol groups. Although the TMOS is difficult to coat on a silicon substrate because of its high volatility, the TMOS can be coated on the irradiated silicon wafer. The increase of the cohesive power is thought to originate from silanol groups on silicon generated with irradiation. Infrared absorption related to the methyl group in TMOS disappeared with illumination in the second step. The line shape of IR absorption turns to be similar to that of thermal SiO2 on silicon. The dependence of photon energy for illumination is also examined. The refractive index of SiO2 synthesized from TMOS is 1.453 at 633 nm, which is almost the same as the refractive index of thermal SiO2. Finally, a model of photochemical reaction is proposed.
The ring-hexavacany in silicon: A stable and inactive defect
Authors: Estreicher S. K. , Hastings J. L. , Fedders P. A.
Published: July 2002

Molecular dynamics simulations as well as ab initio and near ab initio Hartree-Fock calculations in crystalline silicon predict that the configuration of the hexavacancy that has a hexagonal ring missing from the crystal is remarkably stable. The energetics imply that it does form and is more likely to grow than to dissociate during heat treatments. Further, the energy eigenvalues and the charge distribution imply that it has no electrical or optical activity. However, it is a large void in the crystal and could be an efficient gettering center and a precursor of extended defects.
Optical linewidths of InGaN light emitting diodes and epilayers
Authors: O’Donnell K. P. , Breitkopf T. , Kalt H. , Van der Stricht W. , Moerman I. , Demeester P. , Middleton P. G.
Published: July 2002

A comparative study of the optical linewidths of photo- and electroluminescence from high-quality InGaN epilayers and commercial single quantum well light emitting diode structures was undertaken. Optical linewidths in both cases are temperature insensitive and increase systematically with increasing indium concentration. We assess the contribution of three mechanisms to the luminescence linewidth: alloy fluctuations, well width fluctuations, and strain effects. It is found that the broadening of the emission line is an intrinsic property of InGaN alloys. The piezoelectric effect in wurtzite semiconductor is proposed as a novel line-broadening mechanism.
Direct and indirect excitation of Er3+ ions in Er: AIN
Authors: Wu X. , Hömmerich U. , Mackenzie J. D. , Abernathy C. R. , Pearton S. J. , Schwartz R. N. , Wilson R. G. , Zavada J. M.
Published: July 2002

Photoluminescence (PL) excitation and time-resolved PL measurements were conducted on erbium doped AlN. The Er:AlN film was grown by metal organic molecular beam epitaxy with a solid Er effusion source. The resulting Er concentration was 2–5×1019 Er/cm3. Photoluminescence excitation measurements revealed that Er3+ in Er:AlN can be excited either through direct optical pumping into Er3+ 4f levels or through an indirect carrier-mediated process. With respect to these two Er3+ excitation schemes, distinct Er3+ PL decay patterns were observed. The Er3+ PL excited by direct optical excitation was longer lived than that excited via a carrier-mediated process. Time-resolved PL measurements support that at least two classes of Er3+ PL centers exist in Er:AlN, and that direct optical excitation of Er3+ 4f levels primarily excites a set of longer-lived Er3+ sites which are not excited through carrier-mediated processes.
Investigation of the current path of Pb(Zr,Ti)O3 thin films using an atomic force microscope with simultaneous current measurement
Authors: Fujisawa Hironori , Shimizu Masaru , Horiuchi Toshihisa , Shiosaki Tadashi , Matsushige Kazumi
Published: July 2002

The current path of Pb(Zr,Ti)O3 thin films was investigated using an atomic force microscope with simultaneous observations of the surface topography and current measurements using a conductive cantilever. From the current images, it was found that there were some spots where the current flowed relatively easily. By comparing this with the topographic images, these spots were found to be distributed along the grain boundaries—the current barely flowed through the flat surface within the bulk grains. Moreover, the position of these distributed spots did not change even if the value and polarity of the applied voltages were changed. These experimental results indicated that the current flowed through the grain boundaries and that the grain boundary was the current path.
Stress mapping of chemical-vapor-deposited diamond film surface by micro-Raman spectroscopy
Authors: Vlasov I. I. , Ralchenko V. G. , Obraztsova E. D. , Smolin A. A. , Konov V. I.
Published: July 2002

A confocal Raman spectroscopy was used to measure intrinsic stress distribution on the growth surface within individual grains of chemical-vapor-deposited diamond film. Polarization analysis of the Raman line shape revealed that even in high quality (2.8 cm−1 linewidth), free-standing film of 0.6 mm thickness, small regions exist where high local stresses (both compressive and tensile) develop. The stressed regions tend to appear near crystal edges and grain boundaries. A strong gradient in defect or impurity concentrations is supposed to cause the stress fluctuations observed.
Luminescence enhancement in AlN(Er) by hydrogenation
Authors: Pearton S. J. , Abernathy C. R. , MacKenzie J. D. , Hömmerich U. , Wu X. , Wilson R. G. , Schwartz R. N. , Zavada J. M. , Ren F.
Published: July 2002

Room-temperature Er3+ photoluminescence increases of a factor of 5 are observed for AlN(Er) samples treated in a H2 plasma at 200 °C for 30 min. The atomic deuterium passivates defects in the AlN, which normally provide alternative carrier recombination routes. Postdeuteration annealing at 300 °C for 20 min removes the luminescence enhancement by depassivating the nonradiative centers. The AlN(Er) provides a high degree of resistance to thermal quenching of luminescence as a function of temperature because of its wide band gap (6.2 eV), and hydrogenation is a simple method for maximizing the optical output in this materials system.
Observation of Ga vacancies in silicon δ-doping superlattices in (001) GaAs
Authors: Laine T. , Saarinen K. , Mäkinen J. , Hautojärvi P. , Corbel C. , Ashwin M. J. , Newman R. C.
Published: July 2002

Positron annihilation experiments have been performed to investigate the compensating defects in silicon δ-doping superlattices in (001) GaAs. The results reveal vacancies and ion-type defects, which are located between the delta planes in undoped GaAs. The vacancy defect is identified as the Ga vacancy and the negative ion is attributed to the Ga antisite. The concentrations of these defects increase strongly, when the areal concentrations of free carriers are reduced at the delta planes.
Low threshold 1.55 μm wavelength InAsP/InGaAsP strained multiquantum well laser diode grown by chemical beam epitaxy
Authors: Carlin J. F. , Syrbu A. V. , Berseth C. A. , Behrend J. , Rudra A. , Kapon E.
Published: July 2002

By using chemical beam epitaxy at growth temperatures as low as 460–480 °C, we have overcome strain relaxation problems that prevented so far the successful use of InAsP quantum wells in 1.55 μm lasers. Five quantum well InAsP/InGaAsP horizontal cavity lasers showed 88% internal efficiency, 1.6 cm−1 losses per well, and 33 A/cm2 transparency current density per well, which equal or even surpass the best published characteristics for 1.55 μm wavelength lasers based on any material system. Moreover, up to 17 quantum wells were integrated in a strain-balanced laser, which showed equally good characteristics.
Evolution of interstitial- and vacancy-type defects upon thermal annealing in ion-implanted Si
Authors: Libertino S. , Benton J. L. , Jacobson D. C. , Eaglesham D. J. , Poate J. M. , Coffa S. , Kringho/j P. , Fuochi P. G. , Lavalle M.
Published: July 2002

We have quantitatively analyzed the structure and the annealing behavior of the point defects introduced by ion implantation in Si. We used deep-level transient spectroscopy to monitor and count interstitial-type (e.g., carbon–oxygen complexes) and vacancy-type (e.g., divacancies) defects introduced by MeV Si implants in crystalline Si and to monitor their annealing behavior for temperatures up to 400 °C. A small fraction (∼4%) of the initial interstitial–vacancy pairs generated by the ions escapes recombination and forms equal concentrations of interstitial- and vacancy-type room-temperature stable defect pairs. At T⩽300 °C, vacancy-type defects dissociate, releasing free vacancies, which recombine with interstitial-type defects, producing their dissolution. This defect annihilation occurs preferentially in the bulk. At temperatures above 300 °C, all vacancy-type defects are annealed and the residual damage contains only ∼3 interstitial-type defects per implanted ion. This imbalance between vacancies and interstitials is not observed in electron-irradiated samples, demonstrating that it is the direct consequence of the extra ion introduced by the implantation process.
Formation of InAs/GaAs quantum dots by molecular beam epitaxy: Reversibility of the islanding transition
Authors: Lee Hao , Lowe-Webb Roger R. , Yang Weidong , Sercel Peter C.
Published: July 2002

We report a study of the dynamics of coherent island formation in InAs/GaAs films grown by molecular beam epitaxy. A comparison of the temperature dependence of the critical layer thickness for islanding between the migration-enhanced and the continuous growth modes confirms that surface adatom diffusion and indium desorption are the controlling processes which determine the variation of the measured critical layer thickness with temperature. We find that under conditions in which indium desorption is significant, the islanding transition is reversible, which provides a new way to study the dynamics of the islanding transition. Applying this technique, we find that the size distribution of the three-dimensional islands evolves into a bimodal distribution during the reverse process.
High efficiency guided-wave magnetooptic Bragg cell modulator using nonuniform bias magnetic field
Authors: Tsai C. S. , Lin Y. S. , Su J. , Calciu S. R.
Published: July 2002

A technique for large enhancement of the diffraction efficiency of a guided-wave magnetooptic (MO) Bragg cell modulator using a nonuniform bias magnetic field in bismuth-doped yttrium iron garnet-gadolinium gallium garnet waveguide is reported. Since the velocity of propagation of the magnetostatic forward volume wave (MSFVW) varies with the bias magnetic field, a bias magnetic field of proper spatial distribution will modify its wave front and, thus, create a lensing effect upon the MSFVW. This lensing effect in turn reduces the angular beam spread of the MSFVW, and results in a higher MO Bragg diffraction efficiency. The experimental data obtained for the cases with uniform and nonuniform bias magnetic fields at the carrier center frequency range of 2.0–3.5 GHz have demonstrated a diffraction efficiency enhancement by two to six times. A diffraction efficiency as high as 70% has also been accomplished. Furthermore, the measured frequency responses indicate that the nonuniform bias magnetic field has had only a mild effect on the bandwidth of the MO Bragg cell modulator.
A model of bonding and band-forming for oxides and nitrides
Authors: Sun Chang Q.
Published: July 2002

Correlation between chemical bonds, energy bands, and the corresponding properties of oxides and nitrides is established. It is proposed that an oxygen or nitrogen atom can hybridize and form a tetrahedron with its four neighbors through bonding orbitals and nonbonding lone pairs. As a result, the energy states of the host material are modified with four additional features, namely, sp3-hybrid bonding, nonbonding (lone pair), antibonding (dipole), and hole states. Therefore, oxygen and nitrogen possess the special ability of not only enlarging the band gap by hole production but also adding an antibonding subband above the Fermi level.