bluuebunny/crossref_metadata_2025 huggingface dataset:
Alterable temperature-dependent wavelength shifts in (InP)2/(GaP)2 quantum wells
Authors: Cheng Y. C. , Tai K. , Chou S. T. , Huang K. F. , Tu S. L.
Published: July 2002

Alterable temperature-dependent emission wavelength shifts were observed from (InP)2/(GaP)2 short-period superlattices (SPSs) quantum wells grown with different interruption periods. Photoluminescence spectra showed that the peak energy of the SPS samples is inversely proportional to the interruption time conducted between the two consecutive binary growth sequences in the SPS regions. It is believed that a lateral composition modulation occurred during the interruption period which resulted in periodic In-rich and Ga-rich regions in the SPS quantum wells along the growth plane. With the lateral compositional variation, temperature variation can induce a multiaxial strain effect which can either amplify or reduce the temperature-dependent wavelength shift of the SPS quantum wells with different interruption periods.
High photoresponsivity of a p-channel InGaP/GaAs/InGaAs double heterojunction pseudomorphic modulation-doped field effect transistor
Authors: Kim H. J. , Kim D. M. , Woo D. H. , Kim S.-I. , Kim S. H. , Lee J. I. , Kang K. N. , Cho K.
Published: July 2002

In this letter, we report the electrical and optical characteristics of p-channel In0.13Ga0.87As double heterojunction pseudomorphic modulation-doped field effect transistor (MODFET) structure grown by gas source molecular beam epitaxy. The Hall mobility and the density of 2-DHGs (two-dimensional hole gases) in the pseudomorphic In0.13Ga0.87As channel were measured to be 250 cm2/V s and 1.9×1012 cm−2 at 300 K, and 5800 cm2/V s and 1.5×1012 cm−2 at 23 K, respectively. The fabricated p-channel MODFET shows a good mobility property which is due to high valence band discontinuity of InGaP/GaAs/InGaAs double barriers. The peak energy in the photoluminescence spectrum from the p-channel pseudomorphic MODFET structure was found to be 1.4 eV (λ=881 nm). The photoresponsivity with this modified pseudomorphic MODFET structure shows outstandingly better than that of a pin photodiode, particularly at low incident optical power.
One-way image transmission through a thick dynamic distorter without a reference beam
Authors: Zhang Jiasen , Wang Huitian , Yoshikado Shin , Aruga Tadashi
Published: July 2002

We demonstrated a method to perform one-way image transmission through a dynamic distorter without a reference beam. In this method, a photorefractive four-wave mixing configuration was used to pick up the reconstructed image from the image-bearing signal beam, which acted as an erase beam. The fluctuation period of the dynamic distorter must be much shorter than the response time of the nonlinear material. Reconstructed images with high-fidelity have been obtained. Because use of a reference beam is unnecessary this method is simpler and more effective.
Characterization of strain relaxation in As ion implanted Si1−xGex epilayers grown by gas source molecular beam epitaxy
Authors: Zou Lyu-fan , Wang Z. G. , Sun D. Z. , Fan T. W. , Liu X. F. , Zhang J. W.
Published: July 2002

Strain relaxation in the As ion implanted Si0.57Ge0.43 epilayers was studied by double-crystal x-ray diffractometry and transmission electron microscopy, and was compared to that in the nonimplanted Si0.57Ge0.43 epilayers. Experimental results show that after rapid thermal annealing (RTA) the x-ray linewidth of the As+-implanted Si0.57Ge0.43 epilayers is narrower than that of the nonimplanted epilayers, and than that of the partially relaxed as-grown samples, which is due primarily to low density of misfit dislocations in the As+-implanted SiGe epilayers. RTA at higher than 950 °C results in the formation of misfit dislocations for the nonimplanted structures, and of combinations of dislocations and precipitates (tentatively identified as GeAs) for the As+-implanted epilayers. The results mean that the strain relaxation mechanism of the As+-implanted Si1−xGex epilayers may be different from that of the nonimplanted Si1−xGex epilayers.
Large near-resonance third-order nonlinearity in an azobenzene-functionalized polymer film
Authors: Rangel-Rojo R. , Yamada S. , Matsuda H. , Yankelevich D.
Published: July 2002

The third-order nonlinear optical response of a thin film containing the azobenzene dye Disperse Red 1 was studied using the z-scan technique with tunable picosecond pulses. A nonlinear refractive index of −5.0 cm2/GW, corresponding to a Re χ(3)=−3.0×10−15 m2/V2 (2.1×10−7 esu), has been measured at 570 nm. The observed nonlinearity is attributed to the change in refractive index induced by the trans-cis transition in the dye molecule.
Hole transport over heterobarriers in InP based multiple quantum well structures
Authors: Knorr C. , Riedl T. , Geiger M. , Scholz F. , Hangleiter A.
Published: July 2002

We present a systematic analysis of the hole transport over heterobarriers in the InGaAs(P)/InP material system. The experiments have been performed on our recently developed all-optical switching structures [C. Knorr et al., Appl. Phys. Lett. 69, 4212 (1996)], which offer an elegant access to hole transport rates. We have varied barrier thickness, barrier height, bias voltage, and temperature. The time constants vary from 30 μs to 30 ns. Our model calculations, including all heavy and light hole subbands, show that only thermally assisted tunneling can explain both the temperature and electric field dependence of the transport rates. We have extracted the activation energies. The hole capture time is determined as 250±50 fs.
Excitonic photoluminescence in a shallow quantum well under electric field
Authors: Tignon J. , Heller O. , Roussignol Ph. , Delalande C. , Bastard G. , Thierry-Mieg V. , Planel R. , Palmier J. F.
Published: July 2002

We report a study of electrophotoluminescence in a biased shallow GaAs/AlxGa1−xAs (x=0.04) quantum well. It is shown that photocarriers escape from the well via direct tunneling, resulting in a drastic quenching of the photoluminescence at remarkably low fields (F<10 kV/cm). We develop a simple method to measure the field-induced escape time from a set of cw photoluminescence, photocurrent, and time-resolved photoluminescence experiments. Comparison with a semiclassical model shows that, in this regime, Coulomb interaction affects significantly the single electron direct tunneling scheme.
Thermal donors in silicon-rich SiGe
Authors: Hild E. , Gaworzewski P. , Franz M. , Pressel K.
Published: July 2002

We have investigated thermal donor formation at 450 °C in oxygen-rich SiGe with more than 1% Ge content by Hall measurements and infrared spectroscopy. These measurements prove the presence of double donors in annealed Si-rich SiGe samples. The ionization energies of the donors correspond to those of the thermal double donors in Si, only the formation rate is reduced. Due to alloy effects, we observe only broadbands in the low-temperature infrared absorption spectra in SiGe instead of individual lines of neutral and singly ionized thermal donors in Si. The shift of these absorption bands to lower energies with longer heat treatment and a corresponding decrease of the ionization energy found from Hall measurements indicates the formation of shallower donor species.
Polarization-insensitive four-wave mixing in a semiconductor optical amplifier
Authors: Mecozzi A. , Contestabile G. , Graziani L. , Martelli F. , D’Ottavi A. , Spano P. , Dall’Ara R. , Eckner J.
Published: July 2002

We propose and demonstrate a method to obtain polarization-insensitive four-wave mixing in a semiconductor optical amplifier. The method makes use of two pumps with orthogonal polarization placed at the two sides of the signal spectrum. Unlike a previously proposed scheme, a large bandwidth of the signal does not set a lower limit to the degree of polarization insensitivity obtained. Using a commercially available pig-tailed semiconductor optical amplifier, we have obtained less than 0.4 dB of intensity variation of the conjugate field for arbitrary changes of the signal polarization.
Manipulation and writing with Ag nanocrystals on Si(111)-7×7
Authors: Chey S. Jay , Huang Lin , Weaver J. H.
Published: July 2002

Manipulation of Ag nanocrystals derived from up to 200 000 atoms was performed using the tip of a scanning tunneling microscope. By varying the scanning conditions, it was possible to move them laterally on contamination-free Si(111)-7×7 surfaces or to remove them. In both cases, thin Ag tracks were left behind. This demonstrates the concepts of nano patterning and nano painting with metals on clean semiconductor surfaces.
Direct evidence of the indirect energy gap in InAlAs/AlAsSb multiple quantum wells by time-resolved photoluminescence
Authors: Ringling J. , Kawamura Y. , Schrottke L. , Grahn H. T. , Yoshimatsu K. , Kamada A. , Inoue N.
Published: July 2002

Time-resolved photoluminescence spectroscopy has been applied to determine the nature of the energy gap of InAlAs/AlAsSb multiple quantum well structures. While the InAlAs buffer layer exhibits a decay time of the order of 1 ns, which is typical for direct gap semiconductors, the decay time of the InAlAs/AlAsSb multiple quantum well structures is prolonged by more than two orders of magnitude. This observation is direct evidence for the presence of an indirect energy gap. The decay time increases with increasing InAlAs layer thickness indicating the decreasing overlap of electron and hole wave functions.
Unusual stabilization of Si surfaces during roughening in fluoride solution
Authors: Dittrich Th. , Timoshenko V. Yu. , Rappich J.
Published: July 2002

Surface recombination, Si–Hx bonds and initial formation of luminescing porous Si are investigated in situ by infrared and photoluminescence spectroscopy during electrochemical roughening of Si in fluoride solution. The concentration of nonradiative surface defects remains unchanged even during the formation of a rough hydrogenated Si surface. The SiH/SiH2 ratio of such a surface is constant and independent of type of doping, surface orientation, and formation of porous Si. This effect is explained by a decrease of the polarizability of Si backbonds in a disordered Si–Hx surface layer.
Fractional dissociation efficiency measurements of D2 in a helical resonator D2–N2 gas mixture discharge
Authors: Ganguly B. N. , Bletzinger P.
Published: July 2002

Very high fractional dissociation efficiency of D2 has been measured in an inductive-mode helical resonator discharge in a % D2–% N2 gas mixture at 1 Torr pressure. For high N2 dilution, nearly 100% D2 dissociation is obtained compared to 13% for pure D2 at a constant power. The high D2 dissociation efficiency appears to be achieved through the multiquantum vibrational energy transfer from N2 to D2 populating the dissociative D2 (b 3Σu+) state at a large internuclear distance.
Minimization of detrimental effect of air in HgBa2CaCu2O6+δ thin film processing
Authors: Kang B. W. , Gapud A. A. , Fei X. , Aytug T. , Wu J. Z.
Published: July 2002

We have studied the effects of halogenizing HgBa2CaCu2O6+δ (Hg-1212) thin films and have found that the addition of chlorine/fluorine significantly enhances (1) the reproducibility and stability of Hg-1212 thin films, (2) the phase purity of Hg-1212 to over 90%, and (3) the magnetic critical current density (Jc,mag) both at zero field and in-field. A possible mechanism of preventing contamination of precursor materials by chlorine/fluorine doping is proposed.
Optical data storage in semi-insulating GaAs
Authors: Alex V. , Weber J.
Published: July 2002

We observe a light-induced on–off switching of the selective donor–acceptor pair excitation in bulk-grown semi-insulating GaAs. The spectral dependence of the switching process is related to the metastability of the EL2 defect. In the ground state, this As-antisite related midgap donor compensates the shallow acceptors and is responsible for the semi-insulating properties of the material. The loss of the shallow acceptor compensation, that accompanies the transfer of the EL2 to its metastable state leads to the observed absorption and luminescence quench of the shallow donor–acceptor pairs. We exploit these effects in demonstrating optical data storage in semi-insulating GaAs.
Conduction in nonstoichiometric molecular-beam epitaxial GaAs grown above the critical thickness
Authors: Kordoš P. , Marso M. , Luysberg M.
Published: July 2002

Different conduction behavior is observed in nonstoichiometric (NS) molecular-beam epitaxial GaAs grown at 200 °C below and above the critical thickness. In the low-field Ohmic region only the monocrystalline part of the layer contributes to the room-temperature resistivity, but at higher temperatures the resistivity scales with the total layer thickness. In NS GaAs grown above the critical thickness, a superlinear J–Vn (n=2–3) dependence is found at intermediate fields. The prebreakdown voltage is proportional to the total thickness. This indicates that different defects control the electrical properties of the polycrystalline and monocrystalline parts of the NS GaAs. These results can be useful in the design of NS GaAs based devices, which operate at higher temperature and/or higher electric fields.
Imaging spatial variations in resistance along electrical conductors
Authors: Wen Q. , Clarke D. R.
Published: July 2002

A nondestructive, noncontact method of imaging resistance variations along electrical conductors is described. It is especially suitable for locating resistance variations in interconnect lines buried beneath layers of passivation. In its implementation, an intensity modulated laser beam is scanned over the circuit carrying a dc current and the voltage induced in the circuit at the modulation frequency is used to vary the intensity of a synchronously scanned display. Resistance variations, such as caused by defects, produce variations in the image thereby identifying their spatial location. Under certain circumstances, other features in the immediate vicinity of the conductors, such as remanent photoresist and decohered interfaces, can also be imaged. An advantage of the method is that the electrical circuitry in the device or package can be used.
A multistate external cavity laser diode
Authors: Kanjamala A. P. , Levi A. F. J.
Published: July 2002

A semiconductor laser diode in a photon cavity containing a mirror whose optical bandwidth is less than the classical cavity mode spacing can have multiple lasing states at the same bias current. The physics determining the behavior of this nonlinear multistate device is self-consistency between photon number, carrier number, and refractive index.
Measurement of the dynamic error rate of a high temperature superconductor rapid single flux quantum comparator
Authors: Ruck B. , Oelze B. , Dittmann R. , Engelhardt A. , Sodtke E. , Booij W. E. , Blamire M. G.
Published: July 2002

The application of high temperature superconductor (HTS) Josephson junctions in digital rapid single flux quantum circuits requires a careful study of the influence of thermal noise on the bit error rate (BER). We have determined experimentally, for the first time, the BER of a HTS rapid single flux quantum circuit. A comparator, formed by two Josephson junctions, was integrated in a Josephson transmission line ring oscillator, allowing us to perform high speed testing of the comparator at GHz frequencies. For fabrication, focused-electron-beam-irradiated junctions have been used because of their small parameter spread and excellent alignment possibilities. A BER of less than 10−11 was obtained at 39 K.
Channeled ion beam synthesis of heteroepitaxial Nd0.32Y0.68Si1.7 layers
Authors: Wu M. F. , Vantomme A. , Hogg S. , Pattyn H. , Langouche G.
Published: July 2002

Heteroepitaxial Nd0.32Y0.68Si1.7 layers with good crystalline quality (χmin of Nd and Y is 3.5% and 4.3%, respectively) have been prepared by high dose 65 keV Y and 80 keV Nd implantation into a Si(111) substrate using channeled ion beam synthesis. Although the disilicide of Nd only exists in a tetragonal or an orthorhombic phase which cannot be grown epitaxially on a Si(111) substrate, our results show that the addition of Y to the Nd–Si system forces the latter into a hexagonal structure. Rutherford backscattering/channeling and x-ray diffraction studies reveal that the lattice parameters of the Nd0.32Y0.68Si1.7 epilayer are aepi=0.3915 nm and cepi=0.4152 nm and that the epilayer is stable up to 950 °C. Annealing at 1000 °C results in partial transformation into other unidentified phases.
The influence of cavities and point defects on boron diffusion in silicon
Authors: Wong-Leung J. , Williams J. S. , Petravić M.
Published: July 2002

Cavities, formed in Si by hydrogen implantation and subsequent annealing, can provide strong sinks for metal interstitials and are ideal gettering sites for metal impurities. This letter reports the effect of cavities on the transient enhanced diffusion (TED) of boron. Boron implantation was carried out into wafers containing pre-formed cavities and TED of boron was suppressed during subsequent annealing. In some cases, the boron was introduced into an amorphous layer and the presence of cavities was also observed to reduce the amount of the transient enhanced diffusion occurring.
Fine structure of near-band-edge photoluminescence in He+-irradiated GaN grown on SiC
Authors: Joshkin V. A. , Parker C. A. , Bedair S. M. , Krasnobaev L. Y. , Cuomo J. J. , Davis R. F. , Suvkhanov A.
Published: July 2002

The effect of He ion implantation on the optical properties of epitaxial GaN-on-SiC was studied. We observed that He+ irradiation increases the relative intensity of the “blue emission” and resistivity of GaN films and decreases the intensity of the near-band-edge photoluminescence. Because the intensity of the main peak is drastically decreased, the fine structure of the near-band-edge photoluminescence in GaN after He+ irradiation was observed. From a comparison of observed sharp lines with photoluminescence peaks of GaN doped with oxygen, we conclude that oxygen can produce a complex, which is characterized by a strong localization of free carriers and a large lattice distortion. The zero-phonon line of this defect has energy close to the band-gap energy of GaN.
High resolution imaging microellipsometry of soft surfaces at 3 μm lateral and 5 Å normal resolution
Authors: Albersdörfer A. , Elender G. , Mathe G. , Neumaier K. R. , Paduschek P. , Sackmann E.
Published: July 2002

We report on the design of an imaging microellipsometer enabling the generation of maps of the two ellipsometric angles Δ and Ψ. Areas of 60×200 μm2 are imaged at a rate of 1–2 images per minute. By working at angles (45°) much smaller than the Brewster angle (≈73° for Si/SiO2/air) a lateral resolution of 3 μm and a height resolution of 5 Å is achieved. The performance is demonstrated by thickness measurement of a laterally structured polymer film and a transient thickness measurement of dewetting fluid film of n-hexadecane on a Si/SiO2 wafer.
Optical depth profiling of band gap engineered interfaces in amorphous silicon solar cells at monolayer resolution
Authors: Fujiwara H. , Koh Joohyun , Wronski C. R. , Collins R. W. , Burnham J. S.
Published: July 2002

Real time spectroellipsometry and a two-layer optical analysis have been applied to obtain alloy composition (x) and optical gap (Eg) depth profiles with ∼3 Å resolution and sensitivities better than ±0.01 in x and ±0.02 eV in Eg for graded amorphous semiconductor alloy thin films prepared by plasma-enhanced chemical vapor deposition. Graded amorphous silicon–carbon alloy (a-Si1−xCx:H) layers incorporated at the i–p interfaces of a-Si:H n-i-p solar cells have been studied using these methods, and the layer characteristics have been related to improvements in solar cell performance.
Coherent control of terahertz radiation from semiconductor nanostructures
Authors: Pötz Walter
Published: July 2002

Interference between single-photon and two-photon absorption is investigated theoretically to demonstrate coherent control of the emission of THz radiation from semiconductor double wells. Variation of the relative phase between the two concurrent pump pulses controls both the final density of photogenerated electron-hole pairs, predominantly excitons, and the intensity of microwave emission associated with coherent charge oscillations in the double well.
Optically induced domain waveguides in SrxBa1−xNb2O6 crystals
Authors: Bekker Alexander , Peda’el Aviad , Berger Naum K. , Horowitz Moshe , Fischer Baruch
Published: July 2002

Optically induced refractive index patterns, which can be used for waveguides, were formed in SrxBa1−xNb2O6 crystals using an erasable fixing mechanism, based on domains formation by the screening effect. The sign and strength of the refractive index change of the fixed waveguides was controllable by an applied electric field. Fixed patterns are shown to allow the storage of many spatially multiplexed holograms.
Electrical evaluation of damage on the sidewalls of InP mesa structures fabricated by reactive ion etching with methane and hydrogen
Authors: Yamamoto Norio , Mawatari Hiroyasu , Kishi Kenji
Published: July 2002

We propose a method for evaluating the electrical properties of damage on the sidewalls of mesa structures. In the method, current flowing through the mesa sidewalls (Imesa) is obtained from the forward current–voltage characteristics for Schottky barriers formed on both the sidewalls of the mesa structures and (100) surface. In applying the method, to evaluate the damage on the sidewalls of InP mesa structures fabricated by reactive ion etching (RIE) with methane (CH4)/hydrogen (H2), we found that Schottky barrier height φ on the mesa sidewalls is increased by RIE. This suggests that an n-type damage layer is induced by RIE on the sidewalls. We also found that the extent of the damage on the mesa sidewalls is lower than that on (100) surface.
Suppression of sequential tunneling current by a perpendicular magnetic field in a three-barrier, two-well heterostructure
Authors: Ji Yang , Chen Yuanzhen , Luo Kejian , Zheng Houzhi , Li Yuexia , Li Chengfang , Cheng Wenchao , Yang Fuhua
Published: July 2002

When an intersubband relaxation is involved in vertical transport in a tunneling heterostructure, the magnetic suppression of the intersubband LO or LA phonon scattering may also give rise to a noticeable depression of the resonant tunneling current, unrelated to the Coulomb correlation effect. The slowdown of the intersubband scattering rate makes fewer electrons able to tunnel resonantly between two adjacent quantum wells (QWs) in a three-barrier, two-well heterostructure. The influence of the magnetic field on the intersubband relaxation can be studied in an explicit way by a physical model based on the dynamics of carrier populations in the ground and excited subbands of the incident QW.
Resistivity of boron-doped diamond microcrystals
Authors: Jaeger M. D. , Hyun S. , Day A. R. , Thorpe M. F. , Golding B.
Published: July 2002

We describe measurements of the electrical resistivity of micron-size crystallites of boron-doped diamond. Electron-beam lithography was employed for writing sample-specific contacts on small, well-faceted diamond crystals grown by chemical-vapor deposition on silicon substrates. After generating a three-dimensional computer model of the crystallite, a finite-element analysis was used to calculate the internal electrostatic potential distribution. Multiterminal resistance measurements, in conjunction with a computed geometrical factor, enabled the absolute resistivity to be determined. We find that the resistivities obtained from two different crystallites agree to better than 10%. The results are compared with transport measurements on a large-area homoepitaxial diamond film grown simultaneously with the crystallites. This method can be generalized to obtain electrical transport properties of other small, irregularly shaped samples.
Direct measurement of ballistic electron distribution and relaxation length in InP-based heterojunction bipolar transistors using electroluminescence spectroscopy
Authors: Teissier R. , Pelouard J.-L. , Mollot F.
Published: July 2002

The electroluminescence signal emitted from operating InP-based heterojunction bipolar transistors (HBT) is analyzed, giving clear evidence of ballistic electron transport through the base. The luminescence signal is studied as a function of base thickness, providing direct access to the ballistic electron mean free path. This continuous wave technique proves to be very efficient to measure ballistic electron scattering rates: 30 fs at 77 K and 19 fs at 300 K for a base Be doped to 1019 cm−3.Quasiballistic transport range is found to be comparable to base width accessible by state of the art device technology, demonstrating its importance for ultrafast HBT operation.
Emission process in bilayer organic light emitting diodes
Authors: Hochfilzer C. , Leising G. , Gao Y. , Forsythe E. , Tang C. W.
Published: July 2002

Efficient organic light emitting devices (OLEDs) using methyl substituted ladder-type poly (paraphenylene) (m-LPPP) and tris(8-hydroxy) quinoline aluminum (Alq3) as active materials are presented. For bilayer OLEDs the emissive region is found to be in both layers adjacent to the m-LPPP/Alq3 interface. The performance of these hybrid devices with constant m-LPPP thickness and varying Alq3 thickness is compared to that of a single layer m-LPPP device by investigating the relative spatial distribution of the light emission. The relation between the intensity of the light emitted from each layer and the distance to the cathode metal is discussed. Furthermore, the m-LPPP emission is also affected by the internal electric field at the interface.
The complex formation of ripples during depth profiling of Si with low energy, grazing oxygen beams
Authors: Jiang Z. X. , Alkemade P. F. A.
Published: July 2002

Surface roughening of Si under low-energy (0.5–2.0 keV) O2+ bombardment at incidence angles between 45° and 80° has been studied. Surface roughening occurred already at an erosion depth of only a few tens of nanometers. It was found that there were distinctly two angular ranges for sub-keV beams where roughening was strong, and two ranges where it was insignificant. The transition between the different ranges can be very sharp. These observations cannot be explained by the current models for surface roughening. Instead, it is believed that it is the combined sputtering rate dependence on both the surface topography and the oxygen content that determines the occurrence of roughening.
Study of microstructure of high stability hydrogenated amorphous silicon films by Raman scattering and infrared absorption spectroscopy
Authors: Sheng Shuran , Liao Xianbo , Kong Guanglin , Han Hexiang
Published: July 2002

The microstructure, hydrogen bonding configurations and hydrogen content of high quality and stable hydrogenated amorphous silicon (a-Si:H) films prepared by a simple “uninterrupted growth/annealing” plasma enhanced chemical vapor deposition technique have been investigated by Raman scattering and infrared absorption spectroscopy. The high stability a-Si:H films contain small amounts of a microcrystalline phase and not less hydrogen (10–16 at. %), particularly, the clustered phase hydrogen. Besides, the hydrogen distribution is very inhomogeneous. Some of these results are substantially distinct from those of conventional device-quality a-Si:H film or stable a-Si:H films prepared by the other techniques examined to date. The stability of a-Si:H films appears to have no direct correlation with the hydrogen content or the clustered phase hydrogen concentration. The ideal a-Si:H network with high stability and low defect density is perhaps not homogeneous.
Piezoelectric surface barrier lowering applied to InGaN/GaN field emitter arrays
Authors: Underwood Robert D. , Kozodoy P. , Keller S. , DenBaars S. P. , Mishra U. K.
Published: July 2002

A method of lowering the surface barrier for field emission by using the piezoelectric effect is presented. The piezoelectric effect produces a surface dipole that decreases the surface barrier, which in turn decreases the turn-on voltage of the field emitter. Calculations show that significant reduction of the tunneling barrier can be effected with relatively thin layers of strained InGaN on GaN field emitter arrays. Dramatic reduction of the turn-on voltage from 450 V (GaN field emitter array) to 70 V (InGaN/GaN field emitter array) was observed and can be attributed partly to surface barrier lowering.
Hot carrier induced picosecond dynamics of a vertical cavity surface emitting laser: Influence of transverse effects
Authors: Mücke O. D. , Wegener M.
Published: July 2002

The interplay of carrier heating effects and transverse variations of the carrier distribution functions has a significant influence on the ultrafast emission dynamics of semiconductor vertical cavity surface emitting lasers. This interplay can lead to dynamic overshooting and oscillations (here at 200 GHz) even for lasing on a single transverse and single longitudinal mode. A simple equivalent model is a cavity containing the gain medium and a saturable absorber. The numerical results can directly be compared with recently published experiments.
Photogeneration and modification of birefringence in crosslinked films of liquid crystal/polymer composites
Authors: Morino Shin’ya , Kaiho Aki , Ichimura Kunihiro
Published: July 2002

A binary composite consisting of a low-mass nematic liquid crystal and a crosslinked polymer bearing azobenzene chromophores exhibited thermally stable birefringence upon linearly polarized light irradiation owing to the angular-selective photoexcitation of the chromophores. The extent of photoinduced birefringence was critically dependent on temperatures during the photoirradiation. The largest photoinduced birefringence of 0.05 was obtained at 75 °C—slightly lower than TNI of the liquid crystal. The photogenerated birefringence was overwritten on linearly polarized light irradiation.
Effect of in-plane epitaxy on magnetotransport properties of (La0.5Sr0.5)CoO3 thin films
Authors: Kwon C. , Gim Y. , Fan Y. , Hundley M. F. , Roper J. M. , Arendt P. N. , Jia Q. X.
Published: July 2002

We have systematically studied the temperature- and magnetic field-dependent resistance in (La0.5Sr0.5)CoO3 (LSCO) films with various degrees of in-plane texturing using the biaxially oriented LSCO on CeO2/YSZ/SiO2/Si substrates. We find that the ferromagnetic transition temperature, Tc, is much the same in all samples while the resistive transition exhibits metallic behavior (dR/dT>0) in the epitaxial LSCO to semiconducting behavior (dR/dT<0) in the poorly textured LSCO. The magnetoresistance (MR) in the epitaxial LSCO on a LaAlO3 substrate displays two distinct regions; the low field hysteretic MR related to the magnetization hysteresis and the high field linear negative MR due to the suppression of spin fluctuations. In contrast, the MR in the biaxially textured LSCO sample shows only the hysteretic behaviors at low fields and no linear MR at high fields. Compared to the polycrystalline-doped LaMnO3 which shows an enhanced MR at low fields, the MR in the biaxially oriented LSCO does not exhibit such characteristics. We attribute this to the differences in the spin polarization between LSCO and the doped LaMnO3.
Noise characteristics of 850 nm single-mode vertical cavity surface emitting lasers
Authors: Wiedenmann D. , Schnitzer P. , Jung C. , Grabherr M. , Jäger R. , Michalzik R. , Ebeling K. J.
Published: July 2002

We have measured the noise characteristics of single-mode oxide confined surface emitting vertical cavity laser diodes. For pumping levels of more than 3.5 times threshold current, the relative intensity noise is below −150 dB/Hz up to 5 GHz at output powers near 1 mW. For various frequencies, we observe sub-Poissonian noise.
Experimental evidence for high-yield C74 production in an arc periphery plasma
Authors: Hatakeyama R. , Hirata T. , Ishida H. , Sato N.
Published: July 2002

Spatial profiles and buffer-gas pressure dependences of fullerenes contained in primary carbon soots are measured in order to investigate production ratios of higher fullerenes to C60 in an arc-discharge fullerene generator. It is found that C74 is efficiently produced in the arc periphery-plasma region while C60 is mainly produced in the core-plasma region, being the most dominant higher fullerene under the condition of a higher helium-gas pressure (>100 Torr).
Three-dimensional orientational control of molecules by slantwise photoirradiation
Authors: Ichimura Kunihiro , Morino Shin’ya , Akiyama Haruhisa
Published: July 2002

When a film of poly[2-(4-phenylazophenyloxy)ethyl methacrylate] is subjected to irradiation with nonpolarized 436 nm light for n-π* excitation at an oblique incident angle with respect to a film surface, dichroism is induced as a result of the reorientation of azobenzene chromophores. The orientational direction is determined reversibly by an incident plane of the light while azobenzene residues reorient in a direction parallel with the light propagation. This procedure is applicable to the generation of tilt angles of nematic liquid crystals assisted by obliquely photoirradiated films as a function of exposure doses, maintaining homogeneous alignment during photoirradiation.
The role of zinc pre-exposure in low-defect ZnSe growth on As-stabilized GaAs (001)
Authors: Miwa S. , Kuo L. H. , Kimura K. , Yasuda T. , Ohtake A. , Jin C. G. , Yao T.
Published: July 2002

Zinc coverage and the structures of Zn-exposed As-stabilized GaAs(001)-(2×4) and -c(4×4) surfaces have been studied using x-ray photoelectron spectroscopy and scanning tunneling microscopy in order to clarify the role of the Zn pre-exposure process in ZnSe growth on GaAs(001). Since Zn atoms stick on the GaAs-(2×4) surface even though their interaction is very weak, Zn may act as a balancer to form a neutral ZnSe/GaAs interface. Zn can also remove excess As atoms and make a “pure” (2×4) structure that is the only possible starting surface for low-defect ZnSe heteroexpitaxy on a GaAs(001) surface.
High-performance (Hg, Re)Ba2CaCu2Oy grain-boundary Josephson junctions and dc superconducting quantum interference devices
Authors: Tsukamoto A. , Takagi K. , Moriwaki Y. , Sugano T. , Adachi S. , Tanabe K.
Published: July 2002

High-performance grain-boundary Josephson junctions and dc superconducting quantum interference devices (SQUIDs) have been fabricated using c-axis oriented (Hg, Re)Ba2CaCu2Oy thin films grown on bicrystal SrTiO3 substrates with a 24° misorientation angle. The junctions exhibit supercurrent up to a temperature close to the film Tc and resistively shunted junctionlike current–voltage characteristics. A rather high IcRn product of 400–460 μV is observed at 77 K in some junctions. The dc SQUIDs show a field-induced periodic voltage up to 111 K. They also exhibit a voltage modulation depth as large as 90, 27, and 1–2 μV at 77, 97, and 111 K, respectively.
Correlation between in situ optical emission spectroscopy in a reactive Ar/O2 rf magnetron sputtering discharge and Pb(ZrxTi1−x)O3 thin film composition
Authors: Ayguavives F. , Ea-kim B. , Aubert P. , Agius B. , Bretagne J.
Published: July 2002

Lead zirconate titanate Pb(ZrxTi1−x)O3 (PZT) thin films have been deposited by rf magnetron sputtering on Si substrates from a metallic target of nominal composition Pb1.1(Zr0.4Ti0.6) in a reactive argon/oxygen gas mixture. During plasma deposition, in situ optical emission spectroscopy measurements show clearly a correlation between the evolution of characteristic atomic emission line intensities Zr—386.4 nm, Ti—399.9 nm, Pb—405.8 nm, and O—777.2 nm and the thin-film composition determined by a simultaneous use of Rutherford backscattering spectroscopy and nuclear reaction analysis.
Structured luminescent porous silicon layers produced with laser assisted chemical etching
Authors: Starovoitov A. , Bayliss S.
Published: July 2002

An approach to the problem of preparation of laterally structured luminescent porous silicon is proposed. The effect is based on the photosensitivity of chemical etching of silicon. Contrary to the other technique recently reported where the porous layer was modified with laser assisted dissolution, a one stage fast anodization-free process is used. Any desired 2D microstructure can be produced, depending on the illumination pattern, which is defined by optical imaging. The accuracy of the method as well as morphology and the luminescent properties of the prepared layers are studied.
Direct observation of the interaction between a vortex lattice and dislocations in a superconducting Nb crystal
Authors: Horiuchi Shigeo , Cantoni Marco , Uchida Masaya , Tsuruta Tadamasa , Matsui Yoshio
Published: July 2002

Using a cryo-Lorentz electron microscope, we have succeeded in observing directly the interaction between a vortex lattice and dislocations in a Nb film, to which magnetic fields were externally applied up to 200 G at 5 K. At 100 G, vortices were clearly pinned by dislocations and a vortex lattice was locally formed around the pinned sites. Its extension and the density of the vortices increase with the increase of magnetic-field strength to form “the Abrikosov lattice.” At 200 G, it is not perfect yet but consists of “domains” with disordered areas at their boundaries. When the specimen temperature was raised above the critical temperature (9.2 K), with keeping the magnetic field at 200 G, the vortex lattice disappeared. When the temperature was successively decreased back to 5 K, a similar vortex lattice appeared again but with a slightly changed orientation. The reorientation suggests that the interaction between vortices and dislocations pinning them is rather weak.
Open-volume defect tails in Ge-implanted Si probed by slow positrons
Authors: Knights A. P. , Nejim A. , Coleman P. G. , Kheyrandish H. , Romani S.
Published: July 2002

Positron annihilation spectroscopy has been used in conjunction with anodic oxidation and etching to profile the distribution of open-volume defects beyond the range of 120 keV Ge ions implanted into (100) Si at a dose of 1×1014 cm−2. For a time-averaged dose rate (Jt) of 0.02 μA/cm−2 and incident angle of 7°, open-volume defects are found to exist at concentrations exceeding 1016 cm−3 at depths up to 600 nm, whereas the peak of the depth distribution of the implanted Ge ions (Rp) is 76 nm, measured using secondary ion mass spectroscopy. An increase in the depth of the defects observed when the implant is intentionally channeled on the 〈100〉 axis is thought to be simply correlated with a corresponding increase in Rp to 79 nm. When the time-averaged current is increased by a factor of 10 (incident angle=7°), defects persist at concentrations in excess of 1017 cm−3 beyond 1 μm and the Rp increases to 101 nm; this extended tail is attributed primarily to increased defect diffusion.
The observation of the low-frequency acoustic phonon torsional modes in nanocrystalline silicon
Authors: Wu X. L. , Siu G. G. , Yuan X. Y. , Li N. S. , Gu Y. , Bao X. M. , Jiang S. S. , Feng D.
Published: July 2002

Low-frequency Raman spectra of the porous structure formed on C+-implanted silicon were examined using the 514.5 and 488 nm lines of Ar+ laser. The sharp low-frequency Raman peaks were observed with narrow linewidth and large intensity. According to usual vibration theory of the elastic body and related symmetry analysis, they are identified as the localized acoustic phonon torsional modes resulting from the surface vibrations of quasifree Si nanocrystals with nonspherical shapes.
Surface, stress, and impurity effects on room-temperature migration of ion-beam-generated point defects
Authors: Coffa S. , La Magna A. , Privitera V. , Mannino G.
Published: July 2002

We have analyzed the perturbations produced by recombination at surface, trapping at impurities, and stress fields on the room-temperature migration properties of point defects in Si. A stack consisting of a Si oxide (or a Si nitride) and a polycrystalline Si layer, deposited on Si samples, was patterned to open 2-μm-wide, 10-μm-spaced stripes. A 40-keV Si implantation to fluences of 1×1012–5×1013/cm3, performed through this mask at room temperature, was used to inject point defects into the bulk of the wafer. After implants, defect-induced dopant deactivation, in the cross section orthogonal to the direction of the stripes, has been monitored using two-dimensional spreading resistance profilometry. It has been found that, in highly pure epitaxial Si samples, dopant deactivation extends in depth to several microns beyond the region (∼0.4 μm) directly modified by the ions. Furthermore, the two-dimensional deactivation profiles exhibit a strong recess at the surface and a significant anisotropy, being markedly elongated in the lateral direction. Analysis of the data shows that long-range migration of defects is interrupted by trapping at impurities (C and O) or recombination at the surface, characterized by a coefficient of ∼100 μm−1. Moreover, the lateral elongation of the profiles is tentatively explained assuming an anisotropy in the defect diffusivity tensor produced by the strain field under the mask.
Photorefractive correlation filtering of time-varying laser speckles for vibration monitoring
Authors: Kamshilin Alexei A. , Raita Erik , Paivasaari Kimmo , Jaaskelainen Timo , Kulchin Yuri N.
Published: July 2002

We proposed and experimentally studied a technique of vibration object monitoring. The technique is based on self-diffraction of the speckle pattern on the adaptive correlation filter recorded in photorefractive Bi12TiO20 crystal. This filter is recorded owing to the light-induced scattering known as fanning effect. The proposed technique is as sensitive as interferometric systems and it is very simple for implementation: neither reference nor readout beam is needed for operation. It is shown that the system has a linear response on the lateral displacement of the object surface and its dynamic range can be easily varied depending on experimental conditions.
Single-crystal α-GaN grown on a α-Ga2O3 template layer
Authors: Kobayashi Nobuhiko P. , Kobayashi Junko T. , Choi Won-Jin , Dapkus P. Daniel
Published: July 2002

The mechanism for growth of single-crystal GaN on oxidized AlAs (AlOx) formed on a Si(111) substrate by metalorganic chemical vapor deposition has been studied. Cross-sectional transmission electron microscopy (XTEM) indicates that the grown GaN is single-crystal α-GaN in spite of the fact that the AlOx on which the GaN is grown is found to contain predominantly polycrystal γ-Al2O3. Reflection high-energy electron diffraction (RHEED) shows that oriented crystallized α-Ga2O3 is formed between AlOx and the GaAs cap layer during the oxidation process. The α-Ga2O3 acts as a growth template and results in the crystalline orientation of α-GaN on polycrystal γ-Al2O3. Further support for this template is derived from energy dispersive x-ray spectroscopy that shows the existence of Ga atoms on AlOx. Combined XTEM/RHEED analysis suggests that α-GaN is oriented in the growth direction as [0001]α-GaN∥[0001]α-Ga2O3∥[111]Si and the in-plane direction as [211¯0]α-GaN∥[11_00]α-Ga2O3∥[011_]Si, which can be understood by considering the misfit in the in-plane atomic separation at each interface.
An electro-optic device based on field-controlled anchoring of a nematic liquid crystal
Authors: Jägemalm P. , Komitov L. , Barbero G.
Published: July 2002

A surface-controlled electro-optic device using a nematic liquid crystal enclosed between two thin spaced glass plates is presented. Modulation angles of the optic axis up to more than 70° can be achieved even for small applied voltages. The switching of the optic axis occurs with a large component in the plane of the sample even though the electric field is applied normal to the sample surfaces. The effect seems to be driven by the dielectric coupling of the liquid crystal molecules with the applied electric field. The field induced switching process is governed by the bounding surfaces that have been covered with obliquely evaporated SiOx. The mechanism of the phenomenon is briefly discussed.
Fabrication of high-temperature superconducting Josephson junctions on substrates patterned by focused ion beam
Authors: Chen C.-H. , Jin I. , Pai S. P. , Dong Z. W. , Lobb C. J. , Venkatesan T. , Edinger K. , Orloff J. , Melngailis J.
Published: July 2002

High aspect ratio trenches were patterned on (100) LaAlO3 substrates by focused ion-beam milling prior to film deposition. Subsequent YBCO films deposited by pulsed laser deposition were found to break naturally across these trenches. In situ Au was then deposited to connect the separated YBCO electrodes. Junctions were then patterned across the trenches to form a superconductor/normal/superconductor configuration. The resistively shunted junctions exhibited supercurrents up to 86 K. Under microwave irradiation, the junctions exhibited a number of clear Shapiro steps in the I–V characteristics. The transverse magnetic-field diffraction pattern indicted uniform current distribution across the junction.
Transport across conducting ferromagnetic oxide/metal interfaces
Authors: Mieville Laurent , Worledge Daniel , Geballe T. H. , Contreras Ramiro , Char K.
Published: July 2002

We have investigated transport properties across interfaces between the conducting ferromagnetic oxides SrRuO3, La0.5Sr0.5CoO3, La0.67Sr0.33MnO3, and the metals Nb, Al, Pd, and Au. The interface resistances range from smaller than 10−8 to 102 Ω cm2, depending on the choice of ferromagnetic oxide and metal. The three conducting metal oxides exhibit distinctly different behavior in their interface resistances with metals, which we believe is due to different processes taking place at the interface. By making the metals superconducting at 4.2 K, by proximity effect with a capping Nb layer, we show that the dominant transport mechanism through the resistive interface seems to be elastic tunneling. In the case of La0.67Sr0.33MnO3/Al interfaces, we found a way to control the magnitude of the tunneling resistance by adding a very thin Pd layer.
Ordering nanometer-scale magnets using bacterial thread templates
Authors: Field M. , Smith C. J. , Awschalom D. D. , Mendelson N. H. , Mayes E. L. , Davis S. A. , Mann S.
Published: July 2002

Nanometer-scale ferromagnetic particles (Fe2O3, Fe3O4) are dispersed within a mutant bacterial chain which is drawn into a macroscopic fiber “rope.” Cross-sectional scanning electron microscopy images reveal that the iron oxide particles are intercalated between the walls of the bacterial cells which are bundled into parallel threads. The field-dependent switching is seen to markedly sharpen when the synthesis is conducted within an applied magnetic field.
Barrier lowering in dye-sensitized porous- TiO2 solar cells
Authors: Dittrich Th. , Beer P. , Koch F. , Weidmann J. , Lauermann I.
Published: July 2002

The current–voltage characteristics of dye-sensitized porous-TiO2(por-TiO2) solar cells are investigated in the dark and under illumination at light intensities up to 1500 W/m2 and temperatures between −5 and 80 °C. In the dark, the barrier height and the ideality factor of the por-TiO2/electrolyte contact are 0.67 eV and 1.05, respectively. The very low effective Richardson constant indicates the importance of diffusion for transport. A current-dependent effective barrier height has been established under illumination of dye-sensitized por-TiO2 solar cells. The barrier lowering effect should be caused by the low neutralization rate of the positively charged dye radicals in the electrolyte.
Cleaved cavity stimulated emission from an optically pumped cubic GaN/AlGaN heterostructure grown on GaAs (100) substrate
Authors: Wu J. , Yaguchi H. , Onabe K. , Shiraki Y.
Published: July 2002

Cubic GaN/AlGaN double heterostructure was grown on semi-insulating GaAs (100) substrate by metalorganic vapor phase epitaxy. Strong stimulated emission was observed from the cleaved edge of the optically pumped cubic GaN/AlGaN heterostructure at 15 K. The cavity was formed simply by cleaving the substrate. The stimulated emission was demonstrated by the superlinear increase of the output intensity and the highly transverse electric polarized nature. The stimulated emission showed an obvious redshift compared with the spontaneous one.