bluuebunny/crossref_metadata_2025 huggingface dataset:
A novel 45-channel electron spectrometer for high intensity laser-plasma interaction studies
Authors: Gahn C. , Tsakiris G. D. , Witte K. J. , Thirolf P. , Habs D.
Published: July 2002

We have developed a magnetic spectrometer to characterize the hot electrons generated in high-intensity (>1018 W/cm2) laser-plasma interaction experiments. It comprises a dispersive element consisting of a permanent dipole magnet and an electron detector incorporating 45 scintillating/light-guiding plastic fibers connected to a cooled charged-coupled device camera. The main features of this instrument are high spectral resolution, low sensitivity to x and γ rays, and versatility due to its compact design. Performance and operational capabilities are illustrated based on experimental results in which electron energy–spectra in the range of 520 keV to 12.6 MeV were obtained with an energy resolution of 10% and a detection threshold of 106 electrons per MeV.
Fabrication of a thin film of an inorganic:organic composite via laser assisted molecular beam deposition
Authors: Wijekoon W. M. K. P. , Lyktey M. Y. M. , Prasad P. N. , Garvey J. F.
Published: July 2002

Thin films of a hybrid organic:inorganic [silica: N-4-(4-nitrophenyl)-(s)-prolinol] composite were fabricated by mixing the constituents in the gas phase via the technique of laser assisted molecular beam deposition. UV visible and infrared spectra indicate that the organic chromophore is trapped within the silica matrix without degradation. This approach will enable the fabrication of thin films of a variety of hybrid organic: inorganic composite materials which would otherwise be difficult to fabricate.
Doppler-free two-photon excitation of Lyman-α fluorescence for the diagnostics of magnetically confined fusion plasmas
Authors: Voslamber D.
Published: July 2002

Doppler-free two-photon excitation of hydrogen Lyman-α fluorescence is investigated as a possible laser-induced fluorescence (LIF) technique for the diagnosis of magnetically confined fusion (MCF) plasmas. A formal analysis is presented of the underlying atomic and plasma physics as well as of various practical aspects, such as parameter optimization and experimental precision. The latter is analyzed with regard to the photon noise and to the sensitivity of the fluorescence signals to the plasma and laser parameters. The diagnostic potential of the LIF technique described relies on its high spectral resolution. Thus, the absorption lines of the hydrogen isotopes H, D, and T are clearly separated from each other and can serve for isotope-selective density measurements. In addition, using a tunable laser system with small bandwidth, various plasma parameters can be inferred from the spectral line shapes, such as the neutrals’ temperatures or the effective charge number Zeff. The polarization of the fluorescence can, under favorable circumstances, be exploited for magnetic field measurements. The photon statistics impose neutral densities above 1014 m−3 and thus make the diagnostic suitable primarily for the plasma edge. However, previous work has shown that it is applicable even in the plasma bulk of large machines if a neutral beam is used that generates sufficient neutral densities by charge exchange with the plasma ions. Variations or insufficient knowledge of the neutrals’ temperatures are found to seriously affect the precision of absolute isotope density measurements. They are rather uncritical, however, for the determination of the H/D/T density ratios, which are of prime importance for the burn control of large MCF devices. A notable sensitivity is also found to variations of the laser frequency detuning, which should be known and stable to better than 100 MHz. The perspectives demonstrated in the present study and the success of a first experiment on the plasma generator PSI 1 at IPP Berlin are considered to be sufficient motivation for testing the diagnostic on a tokamak or other MCF device.
Nanostructured Nd–Fe–B magnets with enhanced remanence
Authors: Wecker J. , Schnitzke K. , Cerva H. , Grogger W.
Published: July 2002

Nanostructured isotropic Nd–Fe–B magnets with enhanced remanence were produced by hot compaction of mechanically alloyed Nd–Fe–B powders at temperatures of about 600 °C. Phase formation occurred during hot pressing without significant grain growth. The microstructure is a two phase nanocomposite of hard magnetic Nd2Fe14B and soft magnetic α-Fe with an average grain size of about 20 nm. These small dimensions allow effective exchange interactions between hard and soft magnetic grains and result in an enhancement of the remanence by more than 30% above the Stoner–Wolfarth limit expected for noninteracting single domain particles. So far, the best results are a remanence of 1.0 T, an energy density of 121 kJ/m3, and a coercivity of 4.2 kA/cm for a Nd–Fe–Co–Si–B magnet.
Direct writing of nanostructures from silane on silicon (111)
Authors: Wong T. M. H. , O’Shea S. J. , McKinnon A. W. , Welland M. E.
Published: July 2002

Silicon based structures were fabricated using scanning tunneling microscope on a Si(111) surface by the localized decomposition of gaseous silane at pressures of 10−5–10−6 Torr. Continuous wires of width 5 nm could be produced while atomically resolved images of the nearby substrate were obtained. We argue that the fabrication process is effected by field-assisted decomposition of silane on the tip surface, which subsequently migrates to the tunneling region at the tip apex where it field desorbs to the silicon surface.
Room-temperature migration and interaction of ion beam generated defects in crystalline silicon
Authors: Privitera Vittorio , Coffa Salvatore , Priolo Francesco , Larsen Kim Kyllesbech , Mannino Giovanni
Published: July 2002

We have investigated the room-temperature migration properties of ion generated defects in crystalline Si. The defects were injected into the bulk of a pure epitaxial Si layer by a low energy (40 keV) Si implant and monitored using a preexisting defect distribution, produced by a high energy He implant, as a marker. The depth of this defective marker layer was changed by varying the He implant energy in the range 1–3 MeV. Spreading resistance measurements show that the injected defects produce a partial annihilation of the pre-existing damage. The magnitude of the annihilation process is strongly dependent on the depth of the defective marker, being very large when this marker is confined within 5 μm from the surface and negligible when it lies beyond a depth of ∼10 μm. From these results, detailed information on the nature of ion-generated defects which are injected and on their migration properties is obtained. It is found that the observed phenomena are due to the annihilation of divacancies and phosphorus-vacancy defect complexes, generated by the He implant, by Si self-interstitials injected by the shallow Si implant.
Pulsed laser-induced desorption and optical imaging on a nanometer scale with scanning near-field microscopy using chemically etched fiber tips
Authors: Zeisel Dieter , Nettesheim Stefan , Dutoit Bertrand , Zenobi Renato
Published: July 2002

Laser-induced desorption on a nanometer scale using scanning near-field optical microscopy (SNOM) is demonstrated. Pulsed laser-induced desorption of anthracene with completely metallized SNOM fiber tips resulted in a lateral resolution of 70 nm. This became possible due to the use of chemically etched tips with a taper region smaller than 200 μm and cone angles varying from 6 to 30 degrees. These tips are further useful for optical imaging with a very high optical transmission coefficient (up to 10−3), achieving a lateral resolution of 80 nm.
Very low resistance multilayer Ohmic contact to n-GaN
Authors: Fan Zhifang , Mohammad S. Noor , Kim Wook , Aktas Özgür , Botchkarev Andrei E. , Morkoç Hadis
Published: July 2002

A new metallization scheme has been developed for obtaining very low Ohmic contact to n-GaN. The metallization technique involves the deposition of a composite metal layer Ti/Al/Ni/Au (150 Å/2200 Å/400 Å/500 Å) on n-GaN preceded by a reactive ion etching (RIE) process which most likely renders the surface highly n type. Of the several attempts and with annealing at 900 °C for 30 s, contacts with specific resistivity values of ρs=8.9×10−8 Ω cm2 or lower for a doping level of 4×1017 cm−3 were obtained. The physical mechanism underlying the realization of such a low resistivity is elucidated.
High zenithal directivity from a dipole antenna on a photonic crystal
Authors: Brown E. R. , McMahon O. B.
Published: July 2002

A strip dipole on a (111)-oriented face centered cubic photonic crystal is found to have a copolarized directivity and radiative gain of 9.1 and 7.2±1.4, respectively, along the zenith and at a frequency in the first stop band of 18.0 GHz. Under the same conditions, the maximum copolarized directivity and radiative gain were 10.0 and 8.1±1.6, respectively, in the H-plane 5° down from the zenith. The zenithal results correspond to a 5.5-times greater radiation intensity than displayed by the same dipole in free space. From power balance measurements, only 10% of this enhancement is attributed to superior power transfer between the generator and antenna. The remainder is associated with constructive interference between the primary antenna radiation and radiation reflected from the crystal.
Direct observation of competitive adsorption of NO, O, and N on Rh(111) surface by scanning tunneling microscopy
Authors: Xu Hongwei , Ng K. Y. Simon
Published: July 2002

NO absorption on Rh(111) surface has been studied by scanning tunnel microscopy. NO molecules were observed when coadsorbed with oxygen or nitrogen atoms. NO molecules were found to form ordered islands with (4×4) symmetry. Coadsorption of NO and oxygen or nitrogen atoms shows competitive behavior on Rh(111).
Correlation between photoluminescence and oxygen vacancies in ZnO phosphors
Authors: Vanheusden K. , Seager C. H. , Warren W. L. , Tallant D. R. , Voigt J. A.
Published: July 2002

By combining electron paramagnetic resonance (EPR), optical absorption, and photoluminescence (PL) spectroscopy, a strong correlation is observed between the green 510 nm emission, the free-carrier concentration, and the density of singly ionized oxygen vacancies in commercial ZnO phosphor powders. From these results, we demonstrate that free-carrier depletion at the particle surface, and its effect on the ionization state of the oxygen vacancy, can strongly impact the green emission intensity. The relevance of these observations with respect to low-voltage field emission displays is discussed.
Localized epitaxial growth of hexagonal and cubic SiC films on Si by vacuum annealing
Authors: Luo Jian-Shing , Lin Wen-Tai
Published: July 2002

Localized epitaxial growth of hexagonal (6H) and cubic (3C) SiC films on Si by annealing the Si substrates at 700–850 °C in a vacuum of 1–2×10−6 Torr is presented. The orientation relationships of epitaxial SiC films grown on (111)Si and (100)Si were (0001)6H-SiC//(111)Si and [112̄0]6H-SiC//[011̄]Si, (111)3C-SiC//(111)Si and [1̄10]3C-SiC//[1̄10]Si, and (100)3C-SiC//(100)Si and [011]3C-SiC//[011]Si, respectively. The amount of 6H-SiC epitaxy was less than that of 3C-SiC epitaxy. For the films grown at 750–850 °C for 0.5 h, the area fraction of SiC epitaxy ranged from 60% to 80% and the film thickness ranged from 0.02 to 0.15 μm. The main carbon source for the growth of SiC on Si during vacuum annealing was the carbon-containing residue present at the chamber walls.
Recombination dynamics in InGaN quantum wells
Authors: Jeon E. S. , Kozlov V. , Song Y.-K. , Vertikov A. , Kuball M. , Nurmikko A. V. , Liu H. , Chen C. , Kern R. S. , Kuo C. P. , Craford M. G.
Published: July 2002

Transient photoluminescence measurements are reported on a thin InGaN single quantum well, encompassing the high injection regime. The radiative processes that dominate the recombination dynamics, especially at low temperatures, show the impact of localized electronic states that are distributed over a large energy range (∼100 meV). We suggest that these states originate from microstructural disorder in the InGaN/GaN system.
Transient response of wavelength switching in multicavity mode-locked laser diodes
Authors: Kanjamala A. P. , Levi A. F. J.
Published: July 2002

We report the transient response of wavelength switching in multicavity laser diodes. Spatially separated Bragg gratings embedded in a single fiber are used to map optical emission wavelength to photon round-trip time in an external cavity mode-locked laser diode. Transient emission wavelength and mode-locked pulse formation are explored by switching applied radio frequency modulation. Initial conditions are found to dominate transient response. A hot photon cavity has a characteristic rise time corresponding to approximately two photon cavity round trips. A cold photon cavity exhibits significant turn-on delay and rise time that depends on applied radio frequency signal power and is independent of above threshold steady-state current bias.
Very high breakdown voltage and large transconductance realized on GaN heterojunction field effect transistors
Authors: Wu Y.-F. , Keller B. P. , Keller S. , Kapolnek D. , Kozodoy P. , Denbaars S. P. , Mishra U. K.
Published: July 2002

We report record high breakdown voltages up to 340 and 230 V realized on unintentionally doped (1.5 μm gate length) and Si doped (1 μm gate length) AlGaN/GaN modulation doped field effect transistors (MODFETs), respectively. The devices also have large transconductances up to 140 mS/mm and a full channel current of 150–400 mA/mm. The Si doped MODFET sample demonstrated a very high room temperature mobility of 1500 cm2/Vs. With these specifications, GaN field effect transistors as microwave power devices are practical.
Electroluminescence in vertically aligned quantum dot multilayer light-emitting diodes fabricating by growth-induced islanding
Authors: Solomon G. S. , Larson M. C. , Harris J. S.
Published: July 2002

A molecular beam epitaxy (MBE) growth-induced islanding process has been used to form self-organized multiple quantum dot layers, in which quantum dots from different layers are vertically aligned in columns and are electronically coupled. These structures are used as the active region in light-emitting diodes operating at room temperature. Light-emitting diodes are investigated using quantum dot columns containing single, 5 and 10 InAs quantum dots. These diodes emit light over a broad band with typical spectral linewidths of 120 nm, peaked between 1000 and 1100 nm. In addition to the quantum dot spectral feature, a spectral feature from a thin quantum well region, integral to the quantum dot formation process, is seen in the single quantum dot diode, but is eliminated in diodes with active regions containing columns of multiple quantum dots.
Influence of carrier flow on the temperature-dependent capacitance-voltage profiles of heterojunction structures
Authors: Kwon S. D. , Lim H. , Shin H. K. , Choe Byung-Doo
Published: July 2002

We suggest a model which can explain the shifting of carrier concentration peaks in the temperature-dependent capacitance-voltage carrier profiles of heterojunction (HJ) structures. The shift of concentration peaks, which was frequently observed in the inverted isotype HJs was previously attributed to the traps at the heterointerface. The main feature of our model is the role of band offset as a limiter to the test signal current. The model can explain the difference of the peak shift in the carrier profiles of the normal and inverted type HJs. According to this model, the peak shifts at low temperatures occur naturally for the inverted type HJs.
Band gap of Ge rich Si1−x−yGexCy alloys
Authors: Orner B. A. , Olowolafe J. , Roe K. , Kolodzey J. , Laursen T. , Mayer J. W. , Spear J.
Published: July 2002

Si1−x−yGexCy films ( x≊0.90, y⩽0.02) were grown by molecular beam epitaxy on Si substrates. Infrared optical absorption was used to obtain the band gap energy at room temperature. Biaxial strain obtained from x-ray diffraction measurements verified the presence of nearly relaxed films, and the total and substitutional C contents were obtained from channeling C-resonance backscattering spectrometry. We show by direct measurements that interstitial C had a negligible impact on the band gap, but substitutional C was found to increase the band gap with respect to equivalently strained Si1−xGex alloys. While strain decreases the band gap, the effect of substitutional C on the band gap depends on the Si and Ge fractions.
Strained InGaAsP/InGaAsP/InAsP multi-quantum well structure for polarization insensitive electroabsorption modulator with high power saturation
Authors: Ougazzaden A. , Devaux F.
Published: July 2002

In this study we report a novel strained InGaAsP/InGaAsP/InAsP multiquantum well structure for electroabsorption modulators giving high power saturation together with polarization insensitivity. A careful design of the structure in terms of band-gap engineering has been performed to fulfil both requirements. The polarization sensitivity is less than 0.5 dB for the on-state and for a wide range of wavelengths. The carrier escape time, even at very low field, is estimated at lower than 20 ps. The transmitted power for different wavelengths is linear up to 16 dBm.
In-plane optical anisotropy of GaAs/AlAs multiple quantum wells probed by microscopic reflectance difference spectroscopy
Authors: Koopmans B. , Richards B. , Santos P. , Eberl K. , Cardona M.
Published: July 2002

We present a technique, microscopic reflectance difference spectroscopy (μRDS), for the measurement of optical anisotropy with sub-micron resolution. The technique is applied to the determination of the in-plane anisotropy of GaAs/AlAs multiple quantum well structures in a phase resolved way, both below and above the fundamental gap. Confinement and local field effects are discussed, and a comparison is made with microscopic calculations based on a tight-binding Hamiltonian for the electronic states.
Room-temperature operation of InTlSb infrared photodetectors on GaAs
Authors: Kim J. D. , Michel E. , Park S. , Xu J. , Javadpour S. , Razeghi M.
Published: July 2002

Long-wavelength InTlSb photodetectors operating at room temperature are reported. The photo- detectors were grown on (100) semi-insulating GaAs substrates by low-pressure metalorganic chemical vapor deposition. Photoresponse of InTlSb photodetectors is observed up to 11 μm at room temperature. The maximum responsivity of an In0.96Tl0.04Sb photodetector is about 6.64 V/W at 77 K, corresponding to a detectivity of about 7.64×108 cm Hz1/2/W. The carrier lifetime in InTlSb photodetectors derived from the stationary photoconductivity is 10–50 ns at 77 K.
Positron annihilation studies of silicon-rich SiO2 produced by high dose ion implantation
Authors: Ghislotti G. , Nielsen B. , Asoka-Kumar P. , Lynn K. G. , Di Mauro L. F. , Corni F. , Tonini R.
Published: July 2002

Positron annihilation spectroscopy (PAS) is used to study Si-rich SiO2 samples prepared by implantation of Si (160 keV) ions at doses in the range 3×1016–3×1017 cm−2 and subsequent thermal annealing at high temperature (up to 1100 °C). Samples implanted at doses higher than 5×1016 cm−2 and annealed above 1000 °C showed a PAS spectrum with an annihilation peak broader than the unimplanted sample. We discuss how these results are related to the process of silicon precipitation inside SiO2.
Properties of SrBi2Ta2O9 ferroelectric thin films prepared by a modified metalorganic solution deposition technique
Authors: Joshi P. C. , Ryu S. O. , Zhang X. , Desu S. B.
Published: July 2002

Polycrystalline SrBi2Ta2O9 thin films having a layered-perovskite structure were fabricated by a modified metalorganic solution deposition technique using room temperature processed alkoxidecarboxylate precursor solution. It was possible to obtain a complete perovskite phase at an annealing temperature of 650 °C and no pyrochlore phase was observed even up to 600 °C. In addition, the SrBi2Ta2O9 thin films annealed at 750 °C exhibited better structural, dielectric, and ferroelectric properties than those reported by previous techniques. The effects of postdeposition annealing on the structural, dielectric, and ferroelectric properties were analyzed. The electrical measurements were conducted on Pt/SrBi2Ta2O9/Pt capacitors. The typical measured small signal dielectric constant and dissipation factor at 100 kHz were 330 and 0.023 and the remanent polarization and the coercive field were 8.6 μC/cm2 and 23 kV/cm, respectively, for 0.25-μm-thick films annealed at 750 °C. The leakage current density was lower than 10−8 A/cm2 at an applied electric field of 150 kV/cm. The films showed good switching endurance under bipolar stressing at least up to 1010 switching cycles.
Micromachined aperture probe tip for multifunctional scanning probe microscopy
Authors: Noell W. , Abraham M. , Mayr K. , Ruf A. , Barenz J. , Hollricher O. , Marti O. , Güthner P.
Published: July 2002

A novel micromachined aperture tip has been developed for near-field scanning optical microscopy. The advantages of the new probe over commonly used fiber probes are illustrated. The aperture tip is fabricated in a reliable batch process which has the potential for implementation in micromachining processes of scanning probe microscopy sensors and therefore leads to new types of multifunctional probes. For evaluation purposes, the tip was attached to an optical fiber by a microassembly setup and subsequently installed in a near-field scanning optical microscope. First measurements of topographical and optical near-field patterns demonstrate the proper performance of the hybrid probe.
Structural and magnetic properties of epitaxial (0001) MnSb thin films grown on (111) B GaAs: Influence of interface quality
Authors: Akinaga H. , Miyanishi S. , Van Roy W. , Kuo L. H.
Published: July 2002

We have succeeded in growing epitaxial (0001) MnSb/(111)B GaAs films with an atomically flat heterointerface using a GaAs buffer layer. For MnSb films with a thickness of 2–20 Å, the in-plane strain in the films arising from a lattice mismatch between MnSb and GaAs is compressive, which was observed by in situ reflection high-energy electron diffraction measurement. On the other hand, a rough interdiffused interface was obtained in samples grown without a GaAs buffer layer. In contrast to the case of samples with flat interfaces, the in-plane strain is tensile, which is attributed to interdiffusion of Sb into GaAs substrates. The magnetization of the MnSb films with the thickness of this range shows a strong dependence on the interface quality. The saturation magnetization and the saturation magnetic field of the samples with the rough interface are smaller than those of the samples with the atomically flat interface.
Electron emission from nitrogen-doped pyramidal-shape diamond and its battery operation
Authors: Okano K. , Yamada T. , Ishihara H. , Koizumi S. , Itoh J.
Published: July 2002

Although the low-threshold emission from polycrystalline diamond heavily doped with nitrogen has been proposed in our previous report, this is the first report on a practical diamond cold cathode, having pyramidal-shape array structure, promising for battery-driven flat panel displays. We have used urea [(NH3)2CO] as a dopant and grown diamond using hot filament chemical vapor deposition technique, and the mold-based technique is used in order to obtain the pyramidal-shape array structure. The obtained pyramids are identified as diamond from the results of Auger electron spectroscopy and Raman spectroscopy. The turn-on field as low as 0.2–0.3 V/μm and the remarkably high stability in the emission properties have been confirmed. In addition, the battery operation of the cathode has been demonstrated.
Microphotoluminescence of single disks comprising buried quantum wells fabricated by in situ electron-beam lithography
Authors: Kasai Jun-ichi , Kawanishi Hidenori , Katayama Yoshifumi
Published: July 2002

We have investigated the optical properties of single disks comprising buried quantum wells fabricated by in situ electron-beam lithography by using a low-temperature microphotoluminescence method. Photoluminescence images of a disk array showed the distribution due to emissions from single disks with diameters down to 0.1 μm. In the photoluminescence and photoluminescence excitation spectra of single disks, the peak positions were almost constant irrespective of disk diameter. Moreover, the photoluminescence spectra had almost equal peak intensities when corrected for the disk diameter and the distribution width in the photoluminescence image. These optical properties indicate successful fabrication of buried quantum-well structures.
Long wavelength (1.3 μm) vertical-cavity surface-emitting lasers with a wafer-bonded mirror and an oxygen-implanted confinement region
Authors: Qian Y. , Zhu Z. H. , Lo Y. H. , Huffaker D. L. , Deppe D. G. , Hou H. Q. , Hammons B. E. , Lin W. , Tu Y. K.
Published: July 2002

We proposed and demonstrated a novel design for long wavelength (1.3 μm) vertical-cavity surface-emitting lasers (VCSELs). In this design, oxygen-implanted current-confinement regions were formed in a GaAs/AlGaAs Bragg reflector which is the bottom mirror wafer bonded to an AlGaInAs/InP cavity consisting of nine strain-compensated quantum wells. Room- temperature continuous-wave (cw) operation of 1.3 μm-VCSELs with a record low cw threshold current density of 1.57 kA/cm2 and a record low cw threshold current of 1 mA have been realized.
Growth mechanisms of epitaxial metallic oxide SrRuO3 thin films studied by scanning tunneling microscopy
Authors: Rao R. A. , Gan Q. , Eom C. B.
Published: July 2002

We report the deliberately controlled growth of epitaxial metallic oxide SrRuO3 thin films in three distinctly different growth modes. Scanning tunneling microscopy and x-ray diffraction indicate that the growth mechanism for films on exact (001) SrTiO3 substrates is two-dimensional nucleation, which results in a two domain in-plane structure. As the miscut angle of vicinal (001) SrTiO3 substrates is increased, the growth mechanism changes to step flow which leads to single domain thin films. Films on (001) LaAlO3 substrates have an incoherent three-dimensional island growth due to the large lattice mismatch, resulting in a bulk-like lattice. The vast difference in the growth mechanisms of these films leads to a corresponding difference in their electrical transport and magnetic behavior. Such nanoscale control of growth mechanism, surface morphology, and domain structure can be very important in the fabrication of novel perovskite oxide devices.
New equations for domain wall dynamics with nonconservation of magnetization modulus
Authors: Sobolev V. L. , Chen S. C. , Huang H. L.
Published: July 2002

New Slonczewski-type equations taking into account the nonconservation of magnetization modulus are applied to study the steady state motion of a planar domain wall in the presence of an external field normal to the anisotropy axis. It is shown that calculations of the wall velocity in the frames of traditional Landau-Lifshitz-Gilbert equation may lead to essentially the same results in ferromagnets with a wide ferromagnetic resonance line in the case where the relaxation constant in the Landau-Lifshitz-Gilbert equation is deduced from the domain wall mobility experimental data, instead of the data on ferromagnetic resonance linewidth.
Misfit dislocations and stresses in GaN epilayers
Authors: Kang Junyong , Ogawa Tomoya
Published: July 2002

GaN epilayers nitridated initially for different times have been investigated by light scattering tomography (LST) and Raman scattering. In the LST images of the plan-view epilayers, the light scattering defects mainly distribute in 〈112_0〉 directions. The density of the defects is lower in epilayer nitridated initially for a longer time. The defects are considered to be the straight threading edge dislocations on {11_00} planes. The Raman shift of E2 mode is larger in the sample initially nitridated for a longer time. Our results show that the misfit between the GaN epilayer and the Al2O3 substrate is more unfavorably accommodated by the threading edge dislocations in the epilayers initially nitridated for a longer time.
Investigations of double capillary discharge scheme for production of wave guide in plasma
Authors: Kaganovich D. , Sasorov P. V. , Ehrlich Y. , Cohen C. , Zigler A.
Published: July 2002

Double capillary scheme is proposed for high performance electrical discharge in a capillary. This scheme allows more than 103 shots without significant destruction. It is shown that a thin main capillary can be filled with the plasma formed in a trigger capillary discharge so that the discharge in the main capillary can be excited and a hollow distribution of electron density can be obtained with concentration of up to a few 1019 cm−3. Main properties of this scheme are reported.
Spontaneous formation and photoluminescence of ZnSe dot arrays
Authors: Zhang B. P. , Wang W. X. , Yasuda T. , Segawa Y. , Edamatsu K. , Itoh T.
Published: July 2002

Highly luminescent ZnSe quantum dot arrays (QDAs) are spontaneously formed on cleavage-induced GaAs (110) surfaces. The QDAs are configured for their preferred growth on the step top. The confinement on carriers results from the difference in the band gaps of the strained ZnSe layer and the strain-relaxed ZnSe QDA. In contrast to other emissions from the ZnSe layer, the linewidth of the QDA emission is dependent neither on temperature nor on excitation intensity. Moreover, the energy position of the QDA emission is stable even at high excitations. These results reflect the δ functionlike density of states of the QDAs. This letter suggests a novel approach to semiconductor QDs and QDAs.
Hydrogen-implant induced exfoliation of silicon and other crystals
Authors: Varma C. M.
Published: July 2002

A simple theory is formulated in which the pressure of bubbles containing hydrogen molecules in the implanted region drives exfoliation due to the anisotropy of the elastic forces and the evaporation or slow diffusion of atomic hydrogen to molecular hydrogen in the bubbles leading to a catastrophic growth in the bubble radius and the molecular fraction as temperature increases. The process of blistering and cratering at small implant depths is also considered. Comparison to recent experiments is made.
Optical properties of GaN grown over SiO2 on SiC substrates by molecular beam epitaxy
Authors: Torvik John T. , Pankove Jacques I. , Iliopoulos Eleftherios , Ng Hock M. , Moustakas Theodore D.
Published: July 2002

We investigate the optical properties of GaN grown over SiO2 on SiC substrates by electron cyclotron resonance assisted molecular beam epitaxy. The photoluminescence spectra and refractive index of GaN were compared for GaN/SiO2/SiC and GaN/SiC. Strong band-edge luminescence was observed at 3.40 eV from the GaN on both SiO2/SiC and on SiC. No defect-related yellow luminescence was observed. The refractive index of GaN at 1.96 eV (632.8 nm) was measured at 2.22 and 2.24 for GaN/SiO2/SiC and GaN/SiC, respectively.
A traveling wave-driven, inductively coupled large area plasma source
Authors: Wu Yaoxi , Lieberman M. A.
Published: July 2002

Measurements are reported on an inductively coupled large-area (71×61 cm2) plasma source driven by a 13.56 MHz traveling wave. Launching a traveling wave eliminates standing wave effects to obtain a uniformly excited processing plasma. The driving coil consists of a series connection of eight parallel rods, embedded in the plasma inside thin quartz tubes for efficient power delivery. The network required to launch a traveling wave through the driving coil is described. Our measurements confirm that a radio frequency traveling wave is launched and that a high-density plasma is produced.
Transmissive properties of Ag/MgF2 photonic band gaps
Authors: Bloemer Mark J. , Scalora Michael
Published: July 2002

We present results of transmittance measurements on periodic layers of Ag/MgF2 at optical and microwave frequencies. These one-dimensional, photonic band gap materials exhibit transparency bands at optical frequencies and a huge stop band that extends throughout the near-IR to microwave frequencies and beyond. A unique feature of these metal/dielectric photonic band gap (MD–PBG) materials is that the overall transmittance in the pass band may increase as more periods are deposited. The center frequency, width, and sharpness of the pass bands are adjustable and generally depend on the thickness of the layers and the number of Ag/MgF2 periods. These simple periodic structures have applications as sensor and eye protection devices, heat reflecting windows, ultraviolet blocking films, and transparent electrodes for light emitting diodes and liquid crystal displays.
The Schottky energy barrier dependence of charge injection in organic light-emitting diodes
Authors: Campbell I. H. , Davids P. S. , Smith D. L. , Barashkov N. N. , Ferraris J. P.
Published: July 2002

We present device model calculations of the current–voltage (I–V) characteristics of organic diodes and compare them with measurements of structures fabricated using MEH-PPV. The structures are designed so that all of the current is injected from one contact. The I–V characteristics are considered as a function of the Schottky energy barrier to charge injection from the contact. Experimentally, the Schottky barrier is varied from essentially zero to more than 1 eV by using different metal contacts. A consistent description of the device I–V characteristics is obtained as the Schottky barrier is varied from small values, less than about 0.4 eV, where the current flow is space-charge limited to larger values where it is contact limited.
Current–voltage characteristic of organic light emitting diodes
Authors: Ioannidis A. , Forsythe E. , Gao Yongli , Wu M. W. , Conwell E. M.
Published: July 2002

It has been claimed that the variation of current I with voltage V in an organic light emitting diode (LED), based on either metal chelate complexes or conducting polymers, is explained by shallow traps that trap carriers propagating in the conduction or valence band. However, because these are disordered materials all states are localized. We show that it is possible to fit the dependence of I on V and on film thickness without explicitly introducing traps, but taking their effect into account by including the mobility variation with the electric field that arises from the distribution in energy of the localized levels.
Spectroscopic light scattering for real-time measurements of thin film and surface evolution
Authors: Chason Eric , Sinclair Michael B. , Floro Jerry A. , Hunter John A. , Hwang Robert Q.
Published: July 2002

We describe a light scattering technique for measuring the real-time evolution of thin film and surface morphology. By using spectroscopic detection, the technique requires no motion of the sample during the measurement, which makes it compatible with many processing geometries. Results from the growth of strained heteroepitaxial layers of SixGe1−x on Si(001) are presented to demonstrate the technique.
Room-temperature 1.5 μm photoluminescence of Er3+-doped AlxGa1−xAs native oxides
Authors: Kou L. , Hall D. C. , Wu H.
Published: July 2002

Data are presented demonstrating 300 K, continuous wave (cw) photoluminescence near λ=1.53 μm from Er-implanted Al0.8Ga0.2As films oxidized in water vapor (N2+H2O, 500 °C) and annealed (1 h, 700 °C) in Ar+O2. The 40 nm full width at half-maximum (FWHM) spectra are 1.5× broader and ∼10× more intense relative to spectra from unoxidized but annealed samples. The fluorescence decay shows a τ=7 ms lifetime, with a faster τ=1.9 ms component characteristic of a cooperative upconversion mechanism. The data suggest that AlxGa1−xAs native oxides may provide a suitable host for rare-earth optical activity.
Phonon density of states of bulk gallium nitride
Authors: Nipko J. C. , Loong C.-K. , Balkas C. M. , Davis R. F.
Published: July 2002

We report the measured phonon density of states of a bulk GaN powder by time-of-flight neutron spectroscopy. The observed one-phonon excitation spectrum consists of two broad bands centered at about 23 and 39 meV corresponding to the acoustic and the first group of optical phonons; two sharp bands of upper optic modes at about 75 and 86 meV; and a gap of 45–65 meV. The phonon dispersion curves, lattice specific heat, and Debye temperature are calculated from fitting the data with a rigid-ion model.
The origin of persistent photoconductivity and its relationship with yellow luminescence in molecular beam epitaxy grown undoped GaN
Authors: Reddy C. V. , Balakrishnan K. , Okumura H. , Yoshida S.
Published: July 2002

The results of persistent photoconductivity (PPC) and photoluminescence measurements made on radio-frequency plasma assisted molecular beam epitaxy grown, undoped, GaN are reported in this work. Hexagonal GaN (h-GaN) epilayers grown on sapphire and cubic GaN (c-GaN) epilayers grown on GaAs and cubic SiC substrates, are employed in this study. Three clear experimental evidences are reported to claim that the commonly seen persistent photoconductivity and yellow luminescence (YL) are related to each other through the same defect. First, PPC is observed only in those samples which show YL. Second, the threshold (the minimum photon energy required) to observe PPC is determined as 1.6±0.2 eV, which is almost at the same energy at which the YL band starts raising. Third, the photocurrent increases monotonically from 1.8 to 2.2 eV, which is consistent with the broad nature of YL band.
Control of microcavity effects in full color stacked organic light emitting devices
Authors: Burrows P. E. , Khalfin V. , Gu G. , Forrest S. R.
Published: July 2002

We model a three-color stacked organic light emitting device (SOLED) and determine the influence of microcavity effects on the color saturation of the layered, light emitting elements. Using the model, we design and demonstrate a SOLED with good color saturation and minimal viewing angle effects. The Commission Internationale de L’Éclairage chromaticity coordinates for the red, green, and blue subpixels of the SOLED are (0.68, 0.32), (0.32, 0.53), and (0.14, 0.19), respectively. The full color SOLED opens a path to full color, lightweight displays utilizing vertically stacked color elements to maximize the resolution and aperture ratio of the display.
Kinetics of strain relaxation through misfit dislocation formation in the growth of epitaxial films on compliant substrates
Authors: Maroudas Dimitrios , Zepeda-Ruiz Luis A. , Weinberg W. Henry
Published: July 2002

A phenomenological mean-field theory is presented for the kinetics of strain relaxation due to misfit dislocation generation in the strained-layer growth of epitaxial semiconductor films on thin compliant substrates. The theory provides a generalized dislocation kinetic framework by coupling the mechanics of an epitaxial film on a compliant substrate with a well-known description of plastic deformation dynamics in semiconductor crystals. The theoretical results reproduce successfully recent experimental data for strain relaxation in the InAs/GaAs(110) heteroepitaxial system.
Microstructuring of silicon with femtosecond laser pulses
Authors: Her Tsing-Hua , Finlay Richard J. , Wu Claudia , Deliwala Shrenik , Mazur Eric
Published: July 2002

We report that silicon surfaces develop an array of sharp conical spikes when irradiated with 500 laser pulses of 100-fs duration, 10-kJ/m2 fluence in 500-Torr SF6 or Cl2. The spikes are up to 40-μm tall, and taper to about 1-μm diam at the tip. Irradiation of silicon surfaces in N2, Ne, or vacuum creates structured surfaces, but does not create sharp conical spikes.
Relativistic electron-beam generation in a gas-loaded foil-less diode
Authors: Qian Bao-Liang , Li Chuan-Lu , Liu Yong-Gui , Zhang Jian-De , Tan Qi-Mei , Liu Jin-Liang , Liu Che-Bo
Published: July 2002

Relativistic electron beams were generated in a gas-loaded foil-less magnetized diode. The helium gas was employed to fill the diode, and the experimental results have shown that the diode operated normally without the closure problem when the gas pressure was below a certain value (typically, 100 mTorr in the presence of an externally applied guide magnetic field). It has been observed that the current of the electron beam could increase with increasing the gas pressure, and that the presence of helium gas could reduce the operating voltage of the diode producing an electron beam with lower kinetic energy, which implies a decrease in the diode impedance. The externally applied magnetic field along the axial direction also exerted an influence on the electron-beam generation in the gas-loaded diode.
Solid-immersion photoluminescence microscopy of carrier diffusion and drift in facet-growth GaAs quantum wells
Authors: Yoshita Masahiro , Baba Motoyoshi , Koshiba Shyun , Sakaki Hiroyuki , Akiyama Hidefumi
Published: July 2002

Carrier diffusion and drift in facet-growth quantum wells (QWs) on mesa-patterned substrates by molecular beam epitaxy was studied by high-resolution microscopic photoluminescence spectroscopy and imaging using a solid immersion lens at low temperatures. Under point excitation, excitation-position-dependent anisotropic carrier migration was observed, which was explained by carrier diffusion and drift due to spatial change in the quantization energy in QWs.
Optical fiber temperature sensor using a gain-switched Fabry–Perot semiconductor laser self-seeded from a linearly chirped fiber Bragg grating
Authors: Li Shenping , Chan K. T.
Published: July 2002

We present a highly sensitive fiber temperature sensor based on a gain-switched Fabry–Perot semiconductor laser self-seeded from a linearly chirped fiber Bragg grating. A temperature resolution of better than 0.1 °C was demonstrated. This sensor not only has the advantage of robustness against fluctuating light levels, but also obviates the need for fine wavelength discrimination.
Integration of nonlinear dielectric barium strontium titanate with polycrystalline yttrium iron garnet
Authors: Jia Q. X. , Groves J. R. , Arendt P. , Fan Y. , Findikoglu A. T. , Foltyn S. R. , Jiang H. , Miranda F. A.
Published: July 2002

Biaxially oriented nonlinear dielectric Ba0.6Sr0.4TiO3 (BST) films have been grown on polycrystalline ferrite yttrium iron garnet (YIG) substrates. We use a structurally and chemically compatible MgO buffer to improve the crystallinity of the BST on polycrystalline YIG substrates, where the biaxially oriented MgO is deposited by an ion-beam assisted-deposition technique. The biaxially oriented BST has a dielectric loss of less than 0.01 and a capacitance tunability of greater than 25% at a direct current bias voltage of 40 V at room temperature.
Cathodoluminescence mapping of epitaxial lateral overgrowth in gallium nitride
Authors: Rosner S. J. , Girolami G. , Marchand H. , Fini P. T. , Ibbetson J. P. , Zhao L. , Keller S. , Mishra U. K. , DenBaars S. P. , Speck J. S.
Published: July 2002

The dislocation arrangements in gallium nitride (GaN) films prepared by lateral epitaxial overgrowth (LEO) have been studied by cathodoluminescence mapping and transmission electron microscopy. A very low density of electrically active defects (<10−6 cm−2) in the laterally overgrown material is observed. Individual electrically active defects have been observed that propagate laterally from the line of stripe coalescence into the overgrown material. Additionally, by mapping wavelength-resolved luminescence in an InGaN quantum well grown on top of the overgrown material, these defects are shown to be limited to the underlying material and do not propagate normal to the surface, as in other GaN films. In the seed region, threading dislocation image widths are seen to be nearly identical in the quantum well and the underlying GaN, indicating a comparable upper limit (∼200 nm) for minority carrier diffusion length in InGaN and GaN. Additionally, it is shown that, through processing variation, these lateral defects can be avoided in LEO films and that wavelength-resolved cathodoluminescence is an excellent large-area method for rapidly and quantitatively observing variations in process development.
Instant holography
Authors: Ramanujam P. S. , Pedersen M. , Hvilsted S.
Published: July 2002

Instant photography, epitomized by Polaroid films is characterized by the rapid appearance of an image after exposure. We describe here an instant off-axis holographic process, characterized by exposures as short as 5 ns and an instant display process, not requiring any wet chemical processing. The holograms are made in a side-chain azobenzene polyester and can be erased through a thermal treatment of the film enabling the film to be reused. Significantly, an atomic force microscopic scan of the film shows a strong surface relief after the 5 ns exposure paving the way for a cheap, mass replication of the holograms using a micromolding technique.
Microwave near-field polarimetry
Authors: Lann A. F. , Golosovsky M. , Davidov D. , Frenkel A.
Published: July 2002

We report a near-field microwave scanning probe which allows local phase-sensitive polarimetric measurements with a subwavelength spatial resolution (at least λ/16). The probe is a symmetrical transmitting/receiving antenna formed by two very narrow orthogonal slits fabricated at the end plate of a circular waveguide. A linearly polarized incident wave passes through one slit and is reflected from the sample mounted in the near field of the probe. A reflected wave with the incident polarization is received by the same slit, while the orthogonal polarization component is received by another, orthogonal slit. Our probe is very sensitive to local inhomogeneities and may be also useful for mapping Hall mobilities in semiconductors.
Observation of optical isolation based on nonreciprocal phase shift in a Mach–Zehnder interferometer
Authors: Fujita J. , Levy M. , Ahmad R. U. , Osgood R. M. , Randles M. , Gutierrez C. , Villareal R.
Published: July 2002

Measurements up to 45° nonreciprocal phase shift using the transverse magneto-optical effect have been made in a hybrid Mach–Zehnder waveguide/fiber interferometer. BiLu-IG waveguides of various thicknesses and lengths have been used to study the length and thickness dependence of the effect at λ=1.55 μm. This interferometric scheme is also employed to observe optical isolation; an extinction ratio of 11 dB is obtained for the 45° nonreciprocal phase shift, in agreement with theoretical predictions.
Storage capabilities of a four-junction single-electron trap with an on-chip resistor
Authors: Lotkhov S. V. , Zangerle H. , Zorin A. B. , Niemeyer J.
Published: July 2002

We report on the operation of a single-electron trap comprising a chain of four Al/AlOx/Al tunnel junctions attached, at one side, to a memory island and, at the other side, to a miniature on-chip Cr resistor (R≈50 kΩ), which served to reduce cotunneling. At appropriate voltage bias, the bistable states of the trap, with the charges differing by the elementary charge e, were realized. At low temperature, spontaneous switching between these states was found to be very infrequent. For instance, at T=70 mK the system was capable of holding an electron for more than 2 h, this time being limited by the time of the measurement.
Carrier-induced Er3+ luminescence quenching of erbium-doped silicon-rich silicon oxide
Authors: Seo Se-young , Shin Jung H.
Published: July 2002

The quenching of Er3+ luminescence induced by carriers in silicon nanoclusters of Er-doped silicon-rich silicon oxide (SRSO) is investigated. The dependence of Er3+ photoluminescence intensities and lifetimes upon the temperature, pump power, and the background illumination intensity shows that in SRSO, Auger-type interactions with carriers in the host matrix that can severely limit the Er3+ luminescence efficiency are greatly suppressed. The results also show that efficient optoelectronic devices using Er-doped SRSO may be feasible.