bluuebunny/crossref_metadata_2025 huggingface dataset:
All solid state laser source for tunable blue and ultraviolet radiation
Authors: Zimmermann C. , Vuletic V. , Hemmerich A. , Hänsch T. W.
Published: July 2002

Tunable blue and ultraviolet single mode laser light has been generated by frequency quadrupling the output of a semiconductor laser with two successive frequency doubling stages. The laser source is based on a commercial high power semiconductor laser near 972 nm which combines a low power single mode master oscillator with a high power amplifier. The doubling stages consist of nonlinear crystals which are placed inside compact optical buildup resonators. Up to 156 mW tunable blue radiation near 486 nm and 2.1 mW ultraviolet light near 243 nm have been produced.
An FT-IR based instrument for measuring spectral emittance of material at high temperature
Authors: Markham J. R. , Solomon P. R. , Best P. E.
Published: July 2002

For surfaces at high temperature the emitted spectrum of electromagnetic radiation is the most convenient probe of temperature, if the spectral emittance (emissivity) of the sample at that temperature is known. A bench top instrument has been developed that, when coupled to a Fourier transform infrared (FT-IR) spectrometer, allows for the measurements of emission, directional-hemispherical reflection, and directional-hemispherical transmission from materials at elevated temperatures. From these radiative property measurements, the temperature at the measurement point and the spectral emittance of the surface can both be obtained. The method has been applied to a variety of materials (i.e., metals, dielectrics, coal slags) with a variety of surface properties (i.e., partially transmitting and nontransmitting; specularly reflecting and diffusely reflecting) at temperatures up to 2226 K. The article describes the technique, presents the results for several materials, and compares results with those for other investigators.
Hyper-Rayleigh scattering in solution
Authors: Clays Koen , Persoons André
Published: July 2002

The experimental setup for the implementation of a new technique to determine the hyperpolarizability of nonlinear optical molecules in solution is presented. The new technique, hyper-Rayleigh scattering in solution [K. Clays and A. Persoons, Phys. Rev. Lett. 66, 2980 (1991)], has the advantage over the electric-field-induced second-harmonic generation technique that the dipole moment μ and the second hyperpolarizability γ do not have to be independently determined to obtain the first hyperpolarizability β. No electric field is needed to lower the intrinsic symmetry of the isotropic solution, leading to a simpler cell design and a simpler local field factor. The internal reference method, when applicable, completely eliminates the local field factor. With the technique presented, values of the first hyperpolarizability β of 23×10−30 esu for para-nitroaniline (PNA), 105×10−30 esu for 4-methoxy-4′-nitrostilbene (MONS), and 95×10−30 esu for 4-hydroxy-4′-nitrostilbene (HONS) dissolved in chloroform have been obtained.
Photoinduced intersubband transition in undoped HgCdTe multiple quantum wells
Authors: de Oliveira C. R. M. , de Paula A. M. , Cesar C. L. , West L. C. , Roberts C. , Feldman R. D. , Austin R. F. , Islam M. N. , Marques G. E.
Published: July 2002

We present photoinduced intersubband absorption measurements in HgCdTe undoped quantum wells. The transition energies and the linewidths are well described by a full 8×8 k⋅p Kane model calculation. Also, based on this model we show that different in-plane effective masses for the first and second electron subbands should be considered in order to properly fit the low energy side of the experimental spectra. The experimental results can be explained using the calculated intersubband oscillator strength with no exciton enhancement.
Investigation of longitudinal-optical phonon-plasmon coupled modes in highly conducting bulk GaN
Authors: Perlin P. , Camassel J. , Knap W. , Taliercio T. , Chervin J. C. , Suski T. , Grzegory I. , Porowski S.
Published: July 2002

We report a cross-correlated investigation, performed by means of Raman scattering and infrared spectroscopy, of coupled LO phonon-plasmon modes in bulk GaN. Using different samples with different (high) residual concentrations of free carriers, we find that the high-energy Raman mode follows closely the plasma frequency resolved from the infrared data. On the opposite, the low-frequency modes appears down shifted, with respect to the standard TO phonon frequency, by about 11 cm−1. Both findings agree satisfactorily with predictions of the linear response theory for undamped phonon-plasmon modes and establish Raman scattering as a powerful and nondestructive tool to investigate the residual doping level of GaN up to about 1020 cm−3 .
A lateral modulation technique for simultaneous friction and topography measurements with the atomic force microscope
Authors: Göddenhenrich T. , Müller S. , Heiden C.
Published: July 2002

A lateral sample modulation technique is presented to measure simultaneously lateral friction forces and topological features with an atomic force microscope (AFM). The employed technique allows one to use an AFM without any additional displacement sensor. This dynamic detection scheme is well suited for AFMs equipped with a fiber-optic displacement sensor. The technique and the mechanism of the contrast formation are discussed. The performance of the microscope is demonstrated by imaging flat surfaces and large corrugated films as well as low friction samples.
A laser diode fluorometer for field measurements of the F685/F730 chlorophyll fluorescence ratio
Authors: Mazzinghi P.
Published: July 2002

This article presents a new fluorometer developed for the simultaneous measurement of the two peaks of chlorophyll fluorescence (685 and 730 nm). The instrument is compact, battery operated, and easy to carry for field use. A diode laser at 635 nm excites the fluorescence through a single optical fiber. The same fiber also collects fluorescence, which is detected by two photodiodes with wavelengths selected by interference and long pass filters. A microprocessor controls the instrument and stores data in static random access memory, for a later backup on an external PC. The pre-processing software allows real time display of the F685/F730 fluorescence ratio and of the fluorescence induction parameters. Some typical examples of applications on plant physiology and stress detection are discussed, together with proposed experimental procedures and their interpretation.
An improved CO2 laser intracavity photoacoustic spectrometer for trace gas analysis
Authors: Fink T. , Büscher S. , Gäbler R. , Yu Q. , Dax A. , Urban W.
Published: July 2002

This work describes an improved CO2 laser intracavity photoacoustic spectrometer. It can detect and measure small quantities of trace gases in air. The measured absorption sensitivity of 3.3×10−10 cm−1 corresponds to a mixture of 11 pptv (volume parts per trillion) of ethene in nitrogen. The sensitivity and reliability of the instrument were enhanced by reducing the level of background signals originating from optical and acoustical sources. The sensitivity reached is comparable with the best existing continuous measuring instruments while the background signal is two orders of magnitude smaller. The improvements are described in detail and a comparison to previous work is made. A first measurement demonstrates the suitability of the apparatus for biological applications.
Zeeman spectroscopy of the Fe3+ center in GaN
Authors: Heitz R. , Thurian P. , Loa I. , Eckey L. , Hoffmann A. , Broser I. , Pressel K. , Meyer B. K. , Mokhov E. N.
Published: July 2002

We report an optical investigation of the Zeeman behavior of the deep iron acceptor in GaN grown on 6H–SiC. The characteristic ground state splitting of the near-infrared luminescence transition at 1.2988 eV allows for an unambiguous assignment to Fe3+previously proposed on the basis of ODMR results. The observed luminescence lifetime of 8 ms as well as the fine structure of the excited state are consistent with a 4T1(G)–6A1(S) transition. The 4T1(G) state is found to couple only weakly to ε-type phonon modes.
Electrostatic adhesion testing of electronic metallizations
Authors: Yang H. S. , Brotzen F. R. , Callahan D. L. , Dunn C. F.
Published: July 2002

A novel technique is developed to measure quantitatively the adhesion strength of metallizations deposited on substrates such as silicon. Electrostatic adhesion testing employs electrostatic forces to generate delaminating stresses in thin metallic films. The interfacial adhesion strength is readily calculated from the electrode geometry and the applied electrostatic field at failure. Unlike other adhesion tests, this technique does not require any mechanical contact and is virtually independent of the plastic deformation of the film. Furthermore, this test provides direct strength measurements as opposed to work or energy of adhesion measurements obtained by the common peel test. The adhesion strengths of several metallizations (Cu, Al) are characterized using the electrostatic technique. The distribution of stress-at-failure data follows Weibull failure statistics. Field emission scanning electron microscopy reveals that the films are delaminated in a microblister-type mode. It is shown that electrostatic adhesion testing is effective in providing quantitative values for the adhesion strengths and failure probabilities of thin-film metallizations.
High temperature thermal diffusivity measurement by the periodic cylindrical method: The problem of contact thermocouple thermometry
Authors: Benigni P. , Rogez J.
Published: July 2002

The conditions required for the accurate measurement of the thermal diffusivity of solids at high temperature (800–1800 K) by the Angström method with cylindrical geometry are studied. It is shown that the main error in the measurement arises from the difficulty in measuring the temperature accurately with contact thermocouples. On the basis of new experimental results on Cecorite and polycrystalline alumina, the effect of the finite size of the sensors and the consequence of the imperfect thermal contact between the sample and the sensors are investigated. Two models which allow a greater insight into the simultaneous influences of both effects are developed.
Temperature and pressure distribution in the laser-heated diamond–anvil cell
Authors: Dewaele Agnès , Fiquet Guillaume , Gillet Philippe
Published: July 2002

Thermomechanical modeling of a sample assembly (sample plus pressure transmitting medium) in a laser-heated diamond–anvil cell (LHDAC) is presented. Finite elements numerical calculation afforded to obtain the temperature distribution and the induced thermal pressure field, showing that a non-negligible pressure increase (called thermal pressure) occurs in the laser-heated zone. When argon is used as a pressure transmitting medium, thermal pressure can reach 20%–30% of the normal pressure measured in the cold zone. This modeling is supported by experimental studies. It is shown that discrepancies between diamond–anvil cell and large volume press experiments on the coesite to stishovite transition are quantitatively explained by the thermal pressure effect. Moreover, thermal pressure also explains the anomalous low thermal expansion coefficient obtained by x-ray diffraction studies in LHDAC.
“Millikelvin-stabilized cell” for x-ray diffraction measurements with a function of sensing thermal anomalies
Authors: Tozaki K. , Ishii C. , Izuhara O. , Tsuda N. , Yoshimura Y. , Iwasaki H. , Noda Y. , Kojima A.
Published: July 2002

Based on our technique of fine temperature control, we developed a new “millikelvin (mK)-stabilized cell” having a function of sensing thermal anomalies for the detailed x-ray diffraction study of solid phase transitions. It is easily mounted on a commercially available goniometer head. The performance was tested using CsPbCl3 single crystal, in which a phase transition is known to take place at 320 K (47°C) from tetragonal to cubic. An abrupt variation in the distance between some two reciprocal points in the precession pattern was clearly detected in a 0.1 K temperature range centered at 47°C along with two thermal anomalies observed. The “mK-stabilized cell” will be very useful to investigate a mechanism of phase transitions occurring in a narrow temperature range.
X-ray crystal imagers for inertial confinement fusion experiments (invited)
Authors: Aglitskiy Y. , Lehecka T. , Obenschain S. , Pawley C. , Brown C. M. , Seely J.
Published: July 2002

We report on our continued development of high resolution monochromatic x-ray imaging system based on spherically curved crystals. This system can be extensively used in the relevant experiments of the inertial confinement fusion (ICF) program. The system is currently used, but not limited to diagnostics of the targets ablatively accelerated by the Nike KrF laser. A spherically curved quartz crystal (2d=6.687 03 Å, R=200 mm) has been used to produce monochromatic backlit images with the He-like Si resonance line (1865 eV) as the source of radiation. Another quartz crystal (2d=8.5099 Å, R=200 mm) with the H-like Mg resonance line (1473 eV) has been used for backlit imaging with higher contrast. The spatial resolution of the x-ray optical system is 1.7 μm in selected places and 2–3 μm over a larger area. A second crystal with a separate backlighter was added to the imaging system. This makes it possible to make use of all four strips of the framing camera. Time resolved, 20× magnified, backlit monochromatic images of CH planar targets driven by the Nike facility have been obtained with spatial resolution of 2.5 μm in selected places and 5 μm over the focal spot of the Nike laser. We are exploring the enhancement of this technique to the higher and lower backlighter energies.
Electrochemical cell system for voltammetry of high purity solvents
Authors: May Michael A. , Gupta Vijay K. , Hounsokou Karen
Published: July 2002

A two-compartment cell system was designed to carry out high purity electrochemical experiments. Features of this cell include a monolithic quartz body, quartz isolation frit, pretreated platinum electrode, hydrogen reference electrode, snap-on cell tops, purified “type I” water, high purity supply gases, fluoropolymer tubing, and gas-tight cell integrity. An advantage of this design is the allowance for internal pre-electrolysis of the test solution. The system was quality tested by obtaining cyclic voltammograms of 0.5 molar sulfuric acid solution. To demonstrate its utility for high purity work, novel cyclic voltammograms of type I water were obtained with a platinum microelectrode in the absence of electrolyte. The water voltammograms exhibited three electroactive features: reduction potential limit near E=−0.2 V versus r.h.e. (H2), oxidation potential limit near E=+1.6 V (O2), and one surface reduction peak at E=+0.4 V. Thus, the voltammetric “potential window” for water was determined to be 1.8 V.
Relationship between surface morphology and strain relaxation during growth of InGaAs strained layers
Authors: Lavoie C. , Pinnington T. , Nodwell E. , Tiedje T. , Goldman R. S. , Kavanagh K. L. , Hutter J. L.
Published: July 2002

The relationship between the surface morphology and strain relaxation is explored in strained InxGa1−xAs layers grown on GaAs by molecular beam epitaxy. In situ light scattering, detected simultaneously along [110] and [11̄0], reveals an asymmetric surface roughening which is consistent with ex situ scanning force microscopy. Transmission electron microscopy shows that strain relaxation by misfit dislocation formation occurs before the surface roughening is detected, for In0.18Ga0.82As films grown at 490 °C.
Electric field induced interband second harmonic generation in GaAs/AlGaAs quantum wells
Authors: Fiore A. , Rosencher E. , Berger V. , Nagle J.
Published: July 2002

We investigate the second order optical susceptibility χ(2) of symmetric GaAs/AlGaAs quantum wells under applied bias, in the frequency region near half the band gap. The second harmonic is generated in a transmission geometry, and is separated from bulk contribution by a lock-in detection technique. As opposed to previous reports, we find that χxzx(2) is much higher than χzxx(2). For 50 Å quantum wells, χxzx(2)≊25 pm/V and χzxx(2)≊0 at λ=1.66 μm under an applied electric field E=50 kV/cm.
Migration of compensating defects in p-type ZnSe during annealing
Authors: Chen Anthony L. , Walukiewicz Wladek , Duxstad Kristin , Haller Eugene E.
Published: July 2002

We annealed p-type, nitrogen-doped ZnSe grown by molecular-beam epitaxy and subsequently characterized the samples by photoluminescence spectroscopy, capacitance–voltage profiling, and secondary ion mass spectroscopy. We found that the decrease in active acceptor concentration upon annealing stems from compensation by defects that originate from the surface. By fitting the nitrogen acceptor profiles with solutions of the diffusion equation, we estimate the migration energy of the compensating defect to be 4.0±0.3 eV.
Temperature of quasi-two-dimensional electron gases under steady-state terahertz drive
Authors: Asmar N. G. , Černe J. , Markelz A. G. , Gwinn E. G. , Sherwin M. S. , Campman K. L. , Gossard A. C.
Published: July 2002

We use photoluminescence to study the time-average energy distribution of electrons in the presence of strong steady-state drive at terahertz (THz) frequencies, in a modulation-doped 125 Å AlGaAs/GaAs square well that is held at low lattice temperature TL. We find that the energy distribution can be characterized by an effective electron temperature, Te(≳TL), that agrees well with values estimated from the THz-illuminated, dc conductivity. This agreement indicates that under strong THz drive, LO phonon scattering dominates both energy and momentum relaxation; that the carrier distribution maintains a heated, thermal form; and that phonon drift effects are negligible.
Heteroepitaxy of CuInS2 on Si(111)
Authors: Hunger R. , Scheer R. , Diesner K. , Su D. , Lewerenz H. J.
Published: July 2002

Epitaxial layers of CuInS2 are grown on chemically hydrogen terminated Si(111) surfaces with 4° miscut by molecular beam epitaxy. The morphological and structural properties are determined by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and texture analysis. The data show growth in the 〈112〉 direction and substantial twinning of the 75-nm-thick films. High-resolution cross-sectional micrographs of the interface indicate semicoherent epitaxial growth via an interfacial indium-rich secondary phase. The pronounced faceting of the film surface is discussed in relation to twin lamellae.
Pulsed laser deposition of SBN:75 thin films with electro-optic coefficient of 844 pm/V
Authors: Tayebati Parviz , Trivedi Dhrupad , Tabat Martin
Published: July 2002

In this letter we report on the pulsed laser epitaxial deposition of thin films of Sr0.75Ba0.25Nb2O6 on MgO substrates with measured diagonal Pockel’s coefficient r33=844 pm/V in these films. This is the largest linear electro-optic coefficient reported to date in a thin film material. In order to measure this coefficient, we developed a modulated diffraction technique. This method is widely applicable to cases where the substrate is insulating and the optical axis is normal to the substrate.
Universal curves for optical power degradation of II–VI light-emitting diodes
Authors: Chuang S. L. , Ukita M. , Kijima S. , Taniguchi S. , Ishibashi A.
Published: July 2002

We propose a transient recombination-enhanced defect-generation model to analyze the degradation of optical output powers of blue-green II–VI light-emitting diodes (LEDs). We find an analytical solution and discover a set of universal curves for the time dependence of optical output power, which agree very well with the experimental data for strained CdZnSe quantum-well structures. Our model shows that the optical power can be non-exponential in character and its long-time behavior has a 1/t dependence. This 1/t dependence is also related to the growth of the defect density, which should behave as a t1/2 dependence. The generation of new defects due to electron-hole recombination at the defect sites is found to be the dominant degradation mechanism for II–VI LEDs.
Growth of polycrystalline silicon on glass by selective laser-induced nucleation
Authors: Toet D. , Koopmans B. , Santos P. V. , Bergmann R. B. , Richards B.
Published: July 2002

Polycrystalline silicon on glass substrates was grown by a method based on the creation of nucleation sites using laser crystallization of amorphous silicon followed by thermal annealing at temperatures below 600 °C. Annealing induces the crystallization of the material around the seeds, eventually leading to coalescence of adjacent domains before spontaneous nucleation sets in. Micro-Raman spectroscopy shows that the seeds experience a tensile stress, which causes a radial birefringence in the surrounding amorphous silicon, detected by optical anisotropy measurements. We conjecture that this stress facilitates the crystallization of the material around the seed upon thermal annealing.
Epitaxial growth of CuInS2 on sulphur terminated Si(111)
Authors: Metzner H. , Hahn Th. , Bremer J.-H. , Conrad J.
Published: July 2002

We demonstrate the direct heteroepitaxial growth of the chalcopyrite semiconductor CuInS2 on silicon (111) substrates by means of three-source molecular beam epitaxy. The pretreatment of the silicon wafers includes sulphur termination which leads to a new surface structure defining the starting condition for successful epitaxy. All stages of the growth process were controlled in situ using Auger electron spectroscopy and low energy electron diffraction. Furthermore, the epitaxial layers were characterized by means of x-ray diffraction methods and by Rutherford backscattering spectrometry including channeling. X-ray rocking curves showed a typical width of 0.2° while the minimum yield due to the channeling effect was found to be 56%.
Blue-light second-harmonic generation in ion-implanted KNbO3 channel waveguides of new design
Authors: Fluck D. , Pliska T. , Günter P. , Bauer St. , Beckers L. , Buchal Ch.
Published: July 2002

We report phase-matched second-harmonic generation in KNbO3 channel waveguides. The guides were fabricated by one single homogeneous He+ ion irradiation and specially structured photoresist on the KNbO3 crystals serving as an implantation mask. The mask enabled the formation of channel guides with a trapezoidal-shaped cross section providing simultaneous confinement of both TE and TM modes. 2.6 mW second-harmonic blue-light at 441 nm was generated in a 5.8-mm-long guide for an incident fundamental power of 280 mW.
Experimental characterization and modeling of a ferroelectric bulk channel field effect transistor with nonvolatile memory characteristics
Authors: Chen F. Y. , Fang Y. K. , Sun M. J. , Chen Jiann-Ruey
Published: July 2002

A ferroelectric bulk channel field effect transistor has been developed using a thin film of lead titanate (PbTiO3) deposited on a n/p+ Si substrate by rf sputtering, in which the drain conductance changes in proportion to the remanent polarization of PbTiO3 thin film induced by the gate pulse voltage. Since the remanent polarization will stay a long time after the gate voltage is removed, the device possesses the memory characteristics. The device is a bulk channel structure and different from the reported surface channel type ferroelectric field effect transistor. Therefore, it possesses higher mobility. In this letter, current–voltage curves have been shown to compare with the theoretical analysis. The fit is quite good.
Reduction of structural defects in II–VI blue green laser diodes
Authors: Chu C. C. , Ng T. B. , Han J. , Hua G. C. , Gunshor R. L. , Ho E. , Warlick E. L. , Kolodziejski L. A. , Nurmikko A. V.
Published: July 2002

Early blue/green laser diodes based on ZnSe exhibited room temperature, continuous wave (cw) lifetimes of the order of a minute. Similar to the history of (Al,Ga)As lasers, the source of the degradation was the presence of extended crystalline defects. The dominant extended defects in the early room temperature cw lasers originated as stacking faults generated at the ZnSe/GaAs heterovalent nucleation event, and exhibited densities of the order of 106 cm−2. In this letter, a procedure is described which will ensure a consistent run to run reduction of the density of such extended defects to the mid to low 103 cm−2 over a 3 in. wafer.
Curved crystal lattice in resolidified submicron Al lines
Authors: van den Homberg M. J. C. , Alkemade P. F. A. , Radelaar S. , Hurd J. L. , Dirks A. G.
Published: July 2002

The microstructure of Al lines resolidified in an amorphous groove pattern, is investigated by backscatter Kikuchi diffraction, x-ray diffraction, and transmission electron microscopy (TEM). If a temperature gradient is present during cooling, the Al lines do not have the common (111) fiber texture. Instead, the (111) crystal planes are parallel to the vertical sidewalls of the grooves and the crystal lattice shows a remarkable, constant curvature over the entire line length of 1 cm. TEM revealed parallel dislocations in the bulk of the line and small crystallites at the interfaces. It is suggested that the (111) sidewall texture is a consequence of the sidewall area being larger than the bottom area. The observed lattice curvature is explained by a nonisotropic movement of dislocations during cooling.
Lateral stiffness of the tip and tip-sample contact in frictional force microscopy
Authors: Lantz M. A. , O’Shea S. J. , Hoole A. C. F. , Welland M. E.
Published: July 2002

In atomic force and frictional force microscopy, quantitative interpretation of lateral stiffness at the tip-sample contact requires a detailed understanding of all factors contributing to the frictional force as measured in a typical experiment. We used a scanning transmission electron microscope to image and determine the geometry of the tip apex of a variety of atomic force microscope cantilevers. On the basis of this measured structure, we then used finite element analysis to model the lateral stiffness of the tip and found that the tip stiffness is often smaller than the lateral stiffness of the cantilever. Furthermore, we analyzed the stiffness of the tip sample contact and found that for sharp tips the contact stiffness can also be comparable to the lateral stiffness of the cantilever. If these two effects are ignored, significant errors can result in the calculation of lateral forces. We demonstrated the effects of lateral tip and contact stiffness experimentally and used the measurements to calculate the radius of the tip-sample contact.
Self-assembled island formation in heteroepitaxial growth
Authors: Barabási Albert-László
Published: July 2002

We investigate island formation during heteroepitaxial growth using an atomistic model that incorporates deposition, activated diffusion, and stress relaxation. For high misfit the system naturally evolves into a state characterized by a narrow island size distribution. The simulations indicate the existence of a strain assisted kinetic mechanism responsible for the self-assembling process, involving enhanced detachment of atoms from the edge of large islands and biased adatom diffusion.
Optical phonons in laser-deposited CdSxTe1−x films
Authors: Fischer A. , Feng Z. , Bykov E. , Contreras-Puente G. , Compaan A. , de Landa Castillo-Alvarado Fray , Avendaño Jaime , Mason Alice
Published: July 2002

We report the longitudinal optical (LO) phonon frequencies for thin films of the ternary alloy CdSxTe1−x and their composition dependence for the full range of x values from pure CdTe to pure CdS. Pulsed laser deposition was used to prepare the polycrystalline thin films including compositions well inside the miscibility gap. We find that this ternary system exhibits a “two-mode” behavior with CdS- and CdTe-like longitudinal optic phonon modes. The modified random-element isodisplacement model yields a good description of the composition dependence of the LO phonon frequencies.
Metallization induced band bending of SrTiO3(100) and Ba0.7Sr0.3TiO3
Authors: Copel M. , Duncombe P. R. , Neumayer D. A. , Shaw T. M. , Tromp R. M.
Published: July 2002

We have investigated the interaction of Pt with single-crystal SrTiO3(001) and polycrystalline Ba0.7Sr0.3TiO3 thin films using photoemission spectroscopies. Significant band bending is caused by interface formation, determining the Schottky barrier height. We have depth profiled the band bending for Ba0.7Sr0.3TiO3 thin films, giving a direct measurement of the depletion depth and built-in potential.
Integrated intracavity quasi-phase-matched second harmonic generation based on periodically poled Nd:LiTaO3
Authors: Abedin Kazi Sarwar , Tsuritani Takehiro , Sato Manabu , Ito Hiromasa
Published: July 2002

Integrated intracavity quasi-phase-matched second harmonic generation (QPM-SHG) has been demonstrated in a periodically domain inverted Nd:LiTaO3 crystal which functions simultaneously as a gain and nonlinear medium. Lasing at 1.082 μm and simultaneous QPM-SHG at 541 nm has been established by pumping near 806 nm. The effect of doping on the domain reversal voltage of LiTaO3 and the photorefractive effect have also been investigated. It is found that optical damage can be avoided by heating above 80 °C.
Origin of optical nonlinearity for PbO, TiO2, K2O, and SiO2 optical glasses
Authors: Zhu Xinhua , Li Qi , Ming Naiben , Meng Zhongyan
Published: July 2002

The nonlinear optical properties for the PbO, TiO2, K2O, and SiO2 system have been measured by the Z-scan method. The magnitude and sign of the nonlinear refractive index n2 were determined, as was the negative sign, which indicated a self-defocusing optical nonlinearity. Two optical absorption bands at 540 and 660 nm, respectively, are observed in the optical absorption spectra. The sources of the absorption bands are attributed to the 3d-shell electronic transitions of Ti3+ ions from the ground state to the excited states. The origin of the negative nonlinear refractive index was the contribution of resonant electronic transition processes, which can cancel the positive nonresonant refractive index that mainly resulted from the hyperpolarizabilities of the Pb–O and Ti–O pairs.
Strong blue and violet photoluminescence and electroluminescence from germanium-implanted and silicon-implanted silicon-dioxide layers
Authors: Rebohle L. , von Borany J. , Yankov R. A. , Skorupa W. , Tyschenko I. E. , Fröb H. , Leo K.
Published: July 2002

The photoluminescence (PL) and electroluminescence (EL) properties of Ge-implanted SiO2 layers thermally grown on a Si substrate were investigated and compared to those of Si-implanted SiO2 films. The PL spectra from Ge-implanted SiO2 were recorded as a function of annealing temperature. It was found that the blue-violet PL from Ge-rich oxide layers reaches a maximum after annealing at 500 °C for 30 min, and is substantially more intense than the PL emission from Si-implanted oxides. The neutral oxygen vacancy is believed to be responsible for the observed luminescence. The EL spectrum from the Ge-implanted oxide after annealing at 1000 °C correlates very well with the PL one, and shows a linear dependence on the injected current. The EL emission was strong enough to be readily seen with the naked eye and the EL efficiency was assessed to be about 5×10−4.
Strain-induced change in the elastically soft direction of epitaxially grown face-centered-cubic metals
Authors: Ozoliņš V. , Wolverton C. , Zunger Alex
Published: July 2002

The theory of epitaxial strain energy is extended beyond the harmonic approximation to account for large film/substrate lattice mismatch. We find that for fcc noble metals (i) directions 〈001〉 and 〈111〉 soften under tensile biaxial strain (unlike zincblende semiconductors) while (ii) 〈110〉 and 〈201〉 soften under compressive biaxial strain. Consequently, (iii) upon sufficient compression 〈201〉 becomes the softest direction (lowest elastic energy), but (iv) 〈110〉 is the hardest direction for large tensile strain. (v) The dramatic softening of 〈001〉 in fcc noble metals upon biaxial tensile strain is caused by small fcc/bcc energy differences for these materials. These results can be used in selecting the substrate orientation for effective epitaxial growth of pure elements and ApBq superlattices, as well as to explain the shapes of coherent precipitates in phase separating alloys.
Epitaxial growth of CuInS2 on sulphur terminated Si(001)
Authors: Hahn Th. , Metzner H. , Plikat B. , Seibt M.
Published: July 2002

Using three-source molecular beam epitaxy, we demonstrate the direct heteroepitaxial growth of the direct semiconductor CuInS2 on silicon (001) substrates. The pretreatment of the silicon wafers includes a high-temperature exposure to the sulphur beam which leads to an ideal (1×1) sulphur-terminated surface defining the starting condition for successful epitaxy. All stages of the growth process were controlled in situ using Auger electron spectroscopy and low energy electron diffraction. Furthermore, the epitaxial layers were characterized by means of x-ray diffraction methods and by transmission electron microscopy. It is shown that the CuInS2 epilayers grow with a tetragonal structure which is clearly distinct from chalcopyrite.
A new type of dissociated misfit dislocation in [001] ZnTe/GaAs strained-layer heterostructures
Authors: Han P. D. , Zou J.
Published: July 2002

A new type of dissociated misfit dislocation in [001] ZnTe/GaAs strained-layer heterostructures was studied using high-resolution electron microscopy. These dislocations can relieve the localized misfit strain, since they belong to either dissociated screw dislocations and/or partial dislocation dipoles. Their generation depends upon a localized negative strain.
Self-organized InAs quantum dots formation by As/P exchange reaction on (001) InP substrate
Authors: Wang Benzhong , Zhao Fanghai , Peng Yuheng , Jin Zhi , Li Yudong , Liu Shiyong
Published: July 2002

In this letter, we present the results of InAs quantum dots (QDs) prepared on a (001) InP substrate. As/P exchange reaction at the surface of InP buffer was used to form the InAs islands in the reactor of low pressure metalorganic chemical vapor deposition at 600 °C. Preliminary characterizations of the InAs QDs have been investigated by using atomic force microscopy and photoluminescence (PL). Room temperature PL emission from the 0-dimensional system centers at 1520 nm and the full width at half maximum of the PL is 92 meV.
High performance α-Fe/Nd2Fe14B-type nanocomposites
Authors: Chang W. C. , Chiou D. Y. , Wu S. H. , Ma B. M. , Bounds C. O.
Published: July 2002

The α-Fe/R2Fe14B-type exchange-coupled NdFeB nanocomposites with a coercivity (iHc) of more than 11 kOe and energy product, (BH)max, of more than 16 MGOe have been successfully obtained on melt spun (Nd0.95La0.05)7.5+xFebalCr2B10 alloy powders, where x ranged from 3 to 3.5. It was found that a slight substitution of Cr for Fe suppresses the formation of the R2Fe23B3 and Fe3B phases during crystallization and results in the formation of an α-Fe/R2Fe14B mixture. Increasing the total rare earth content was found to enhance the remanence, and the Hic of (Nd0.95La0.05)7.5+xFe80.5−xCr2B10 ribbons was also increased drastically with increasing values of x. A Br of 9.2–9.7 kG, Hic of 11.1–13.2 kOe, and (BH)max of 16.5–18.0 MGOe have been obtained on (Nd0.95La0.05)7.5+xFe80.5−xCr2B10 (x=3–3.5).
Enhancement of the intensity of the short-wavelength visible photoluminescence from silicon-implanted silicon-dioxide films caused by hydrostatic pressure during annealing
Authors: Tyschenko I. E. , Rebohle L. , Yankov R. A. , Skorupa W. , Misiuk A.
Published: July 2002

We have studied the influence of the hydrostatic pressure during annealing on the intensity of the visible photoluminescence (PL) from thermally grown SiO2 films irradiated with Si+ ions using double-energy implants at 100 and 200 keV and ion doses ranging from 1.2×1016 to 6.3×1016 cm−2. Postimplantation anneals have been carried out in an Ar ambient at temperatures Ta of 400 and 450 °C for 10 h at both atmospheric pressure and hydrostatic pressures of 0.1, 10, 12, and 15 kbar. It has been found that the intensity of the ultraviolet (∼360 nm), blue (∼460 nm), and red (∼600 nm) PL emission bands increases with raising hydrostatic pressure whereby the PL peaks retain their wavelength positions. The results obtained have been interpreted in terms of enhanced, pressure-mediated formation of ≡Si–Si≡ centers and small Si clusters within metastable regions of the ion-implanted SiO2.
Forward-transfer laser implantation of pyrene molecules in a solid polymer
Authors: Karnakis D. M. , Goto M. , Ichinose N. , Kawanishi S. , Fukumura H.
Published: July 2002

Excimer laser- (248 nm) induced pyrene implantation in a solid polymer has been investigated in the forward-transfer configuration. Atomic force microscopy provides detailed topological images of the implanted surface, the geometry of which remains mostly unaltered following low-intensity laser radiation in contrast to a laser ablation transfer case. The dopant host polymer thickness appears to strongly influence the efficiency of the process. The experimental results are interpreted on the basis of a thermally activated ejection and implantation mechanism.
Atomic force microscopy studies of ZnSe self-organized dots fabricated on ZnS/GaP
Authors: Ma Z. H. , Sun W. D. , Sou I. K. , Wong G. K. L.
Published: July 2002

ZnSe self-organized dot structures on ZnS thin films were fabricated by the molecular beam epitaxy technique. In situ reflection high-energy electron diffraction studies reveal that growth interruption is required for the formation of the dot structure. Atomic force microscopy (AFM) images of the dots taken within the same day of growth reveal that the dot density increases with increasing ZnSe coverage. A density of 18 μm−2 was achieved with a coverage of 8.0 ZnSe monolayers. AFM images taken at later times (up to six months later) show ripening effects. The average dot size measured at various times after growth is consistent with the prediction of the Ostwald ripening model with a growth time constant of 4±1 days for the structure with a coverage of 8.0 ZnSe monolayers. The dot size and density in the fully ripened state are essentially independent of the initial ZnSe coverage.
Domain wall pinning contribution to the nonlinear dielectric permittivity in Pb(Zr, Ti)O3 thin films
Authors: Taylor D. V. , Damjanovic D.
Published: July 2002

The ac field dependence of dielectric response of sol-gel derived Pb(Zr, Ti)O3 thin films is presented. The analysis of amplitude and phase angle of first and third harmonic of the polarization at subswitching fields shows that a description of the dielectric nonlinearity by the classical polynomial approximation is inadequate. A qualitatively better description of many features of the experimental data is obtained using the Rayleigh relationship, indicating that the nonlinear behavior is associated with the pinning of domain walls on randomly distributed pinning centers.
A photoelectron spectroscopy study on the indium tin oxide treatment by acids and bases
Authors: Nüesch F. , Rothberg L. J. , Forsythe E. W. , Le Quoc Toan , Gao Yongli
Published: July 2002

We report on the chemical adsorption of acids and bases on indium tin oxide (ITO). Ultraviolet photoelectron spectroscopy was used to measure the work function of the treated ITO and atomic surface concentrations were determined by x-ray photoelectron spectroscopy. The acid treatments yield work-function shifts as high as 0.7 eV compared to the nontreated ITO. Huge shifts in the work function are also obtained for the treatments with bases and are opposite to those obtained with the acids. These dramatic shifts are indicative of a double ionic surface layer. The importance of an appropriate plasma treatment prior to the chemical adsorption of acids or bases is discussed in terms of surface acido-basicity.
Storage of electrons and holes in self-assembled InAs quantum dots
Authors: Bödefeld M. C. , Warburton R. J. , Karrai K. , Kotthaus J. P. , Medeiros-Ribeiro G. , Petroff P. M.
Published: July 2002

We report spectroscopic measurements of charge-tunable quantum dots. The samples contain vertically aligned double dots which we can fill with electrons from a back contact. We show how we can also accumulate holes in the dots by illuminating the samples with below band gap radiation when a large negative bias is applied. We argue that this is possible through a large disparity in the electron and hole tunneling times. Interband spectroscopy reveals a strong reduction in the quantization energy for the dots in the second layer.
Growth and fabrication of GaN/AlGaN heterojunction bipolar transistor
Authors: Han J. , Baca A. G. , Shul R. J. , Willison C. G. , Zhang L. , Ren F. , Zhang A. P. , Dang G. T. , Donovan S. M. , Cao X. A. , Cho H. , Jung K. B. , Abernathy C. R. , Pearton S. J. , Wilson R. G.
Published: July 2002

A GaN/AlGaN heterojunction bipolar transistor structure with Mg doping in the base and Si doping in the emitter and collector regions was grown by metalorganic chemical vapor deposition on c-axis Al2O3. Secondary ion mass spectrometry measurements showed no increase in the O concentration (2–3×1018 cm−3) in the AlGaN emitter and fairly low levels of C (∼4–5×1017 cm−3) throughout the structure. Due to the nonohmic behavior of the base contact at room temperature, the current gain of large area (∼90 μm diameter) devices was <3. Increasing the device operating temperature led to higher ionization fractions of the Mg acceptors in the base, and current gains of ∼10 were obtained at 300 °C.
A metal/insulator tunnel transistor with 16 nm channel length
Authors: Sasajima Ryouta , Fujimaru Kouji , Matsumura Hideki
Published: July 2002

A nanometer transistor, metal/insulator tunnel transistor (MITT), which consists of only metal and insulator is experimentally studied. In the MITT, the Fowler–Nordheim tunneling currents through an insulator in lateral metal/insulator/metal structure are controlled by changing a voltage at a gate electrode upon the middle insulator, due to variation of tunnel-barrier thickness at the insulator. It is demonstrated that the MITT with 16 nm channel length fabricated by conventional photolithography can operate similarly to the conventional metal/oxide/semiconductor field-effect transistor with on/off ratio of current larger than 105. The result indicates that the MITT is a promising candidate for future switching transistors in ultralarge scale integrated circuits.
Structured films of light-emitting silicon nanoparticles produced by cluster beam deposition
Authors: Huisken Friedrich , Kohn Bernhard , Paillard Vincent
Published: July 2002

Crystalline silicon nanoparticles (quantum dots) with diameters around 4 nm are produced via CO2-laser-induced decomposition of SiH4 in a flow reactor and subsequently transferred into a freely propagating cluster beam. Thin structured films are then obtained by shaping the beam with a mask and depositing the nanoclusters at low energy on a sapphire substrate. Upon illumination with ultraviolet radiation, the nanoparticles exhibit strong photoluminescence in the red.
Ballistic electron magnetic microscopy: Imaging magnetic domains with nanometer resolution
Authors: Rippard W. H. , Buhrman R. A.
Published: July 2002

A variation of ballistic electron emission microscopy has been developed to image magnetic structure in thin-film multilayers with nanometer resolution. In studies of nominally uncoupled Co/Cu/Co trilayer films, magnetic domains and domain-wall motion are readily observable with this technique. In the Co/Cu/Co trilayer system magnetic domains are found to occur on an ∼500-nm-length scale and less, while smaller-scale fluctuations in the ballistic electron transport properties of the system are found on an ∼10 nm length scale.
Quantum-well infrared photodetectors with digital graded superlattice barrier for long-wavelength and broadband detection
Authors: Lee Jung-Hee , Li Sheng S. , Tidrow M. Z. , Liu W. K. , Bacher K.
Published: July 2002

We report two high-performance quantum-well infrared photodetectors (QWIPs) using GaAs/AlGaAs digital graded superlattice barriers and InGaAs quantum wells for long-wavelength infrared and broadband (BB) detection. The compositionally digital graded superlattice barriers (DGSLBs) of the QWIP structures were grown using GaAs/AlGaAs digital graded superlattices to form a staircase-like composition-graded barrier layer without adjustment of the source temperature (510 °C) and the AlGaAs composition (15% Al). In the BB DGSLB QWIP, a broad spectral response from 7 to 16 μm wavelength range was obtained under positive bias condition, while a normal spectral response with peak wavelength at 11 μm was obtained under the negative bias condition. In addition, a double-barrier (DB) DGSLB QWIP structure adding a thin (20 Å) undoped Al0.15Ga0.85As on each side of the InGaAs quantum well to form a DB structure for the confinement of electron wave functions in the E4 states was also studied. A peak responsivity of 3 A/W was obtained at Vb=1 V, T=35 K, and λp=12 μm, and normal spectral response was observed in this device.
Controlling the performance of GaAs–AlGaAs quantum-cascade lasers via barrier height modifications
Authors: Wilson L. R. , Keightley P. T. , Cockburn J. W. , Skolnick M. S. , Clark J. C. , Grey R. , Hill G.
Published: July 2002

A series of GaAs/AlGaAs quantum-cascade lasers has been studied in which the confinement of the upper lasing level is systematically varied. This is achieved by modifying the aluminum composition, and hence the height, of a single barrier in each active region. Increasing the height of the barrier increases the upper laser level lifetime, while decreasing the optical transition matrix element. We find an optimum barrier composition (Al0.4Ga0.6As), with the sample containing this barrier exhibiting a significantly improved low-temperature threshold current density (3.8 kA/cm2) and peak power output (∼800 mW) relative to previously reported GaAs-based quantum-cascade lasers. The temperature performance of all the samples is very similar, indicating that thermal activation of electrons from the upper laser level is not the dominant factor limiting high-temperature operation.
Improved characteristics of InGaN multiple-quantum-well laser diodes grown on laterally epitaxially overgrown GaN on sapphire
Authors: Hansen M. , Fini P. , Zhao L. , Abare A. C. , Coldren L. A. , Speck J. S. , DenBaars S. P.
Published: July 2002

InGaN multiple-quantum-well laser diodes have been fabricated on fully coalesced laterally epitaxially overgrown (LEO) GaN on sapphire. The laterally overgrown “wing” regions as well as the coalescence fronts contained few or no threading dislocations. Laser diodes fabricated on these low-dislocation-density regions showed a reduction in threshold current density from 10 to 4.8 kA/cm2 compared to those on conventional planar GaN on sapphire. The internal quantum efficiency also improved from 3% for laser diodes on conventional GaN on sapphire to 22% for laser diodes on LEO GaN on sapphire.
Potential profile between boron-doped diamond electron emitter and anode electrode
Authors: Yamada Takatoshi , Sawabe Atsuhito , Koizumi Satoshi , Itoh Junji , Okano Ken
Published: July 2002

The voltage drop of vacuum and boron (B)-doped diamond electron emitter bulk has been discussed in order to understand the electron emission mechanism of B-doped diamond. It is confirmed that the electron emission from B-doped diamond depends on its film thickness and the localized electron pass/channel is formed in the film. From the results of the threshold voltage versus anode-diamond spacing characteristics, it is found that most of the anode voltage is applied in vacuum, and the high electric field near the B-doped diamond surface is required for the electron emission from B-doped diamond regardless of the film thickness.