bluuebunny/crossref_metadata_2025 huggingface dataset:
Wafer-scale sub-micron lithography
Authors: Khang Dahl-Young , Lee Hong H.
Published: July 2002

Wafer scale imprint lithography is introduced and demonstrated. Pattern features down to 100 nm in size are shown to be lithographed over the entire area of a silicon wafer 4 in. in diameter. An asymmetric heating and quenching cycle is used for the pattern fidelity. Simple mechanical arrangements make the wafer-scale lithography possible.
Gas-assisted focused-ion-beam lithography of a diamond (100) surface
Authors: Datta A. , Wu Yuh-Renn , Wang Y. L.
Published: July 2002

A focused Ga-ion beam is used to conduct lithography on a diamond (100) surface with the assistance of various gases (Cl2, O2, and XeF2). The beam-induced dilation and sputtering of the surface are measured by atomic force microscope. The dilation is found to be insensitive to the presence of assisting gases at low doses, while the sputtering is enhanced by O2 and XeF2 at high doses. The topographic evolution as a function of the ion dose is well described by a proposed semiempirical equation. Combining physical sputtering and XeF2-assisted etching, the lithographic process has been used to fabricate submicron structures on diamond surfaces.
Interdiffusion and surface segregation in stacked self-assembled InAs/GaAs quantum dots
Authors: Lita B. , Goldman R. S. , Phillips J. D. , Bhattacharya P. K.
Published: July 2002

We have investigated interdiffusion and surface segregation in molecular-beam-epitaxially-grown stacked self-assembled InAs/GaAs quantum dots. Using high-resolution cross-sectional scanning tunneling microscopy, we observe lateral variations in the vertical positions of In atoms in both the wetting layers and dot stacks. In some regions, the wetting layer thickness is much less than the dot height, while in other regions, the dot is immersed in the wetting layer. Using In and Ga atom counting, we obtain vertical In–Ga interdiffusion and 1/e segregation lengths of 1.25 and 2.8 nm, respectively. In the dot stacks, significant In–Ga intermixing, primarily due to In surface segregation, is apparent.
Real-time electron-ion dynamics for photoinduced reactivation of hydrogen-passivated donors in GaAs
Authors: Miyamoto Yoshiyuki , Sugino Osamu , Mochizuki Yasunori
Published: July 2002

Photoassisted reactivation of H-passivated Si donors in GaAs has been demonstrated by first-principles calculations. In order to examine the survival of the excited state, we have applied our recently developed scheme of the ab initio molecular dynamics coupled with the time-dependent Schrödinger equation for electrons. We have found a possible electronic excitation which significantly lowers the dissociation-barrier height from 1.79 to 0.23 eV. This result explains well the recent experiment of carrier recovery by laser illumination [D. Loridant-Bernard, S. Mezière, M. Constant, N. Dupuy, B. Sombret, and J. Chevallier, Appl. Phys. Lett. 73, 644 (1998)].
Nucleation of single-crystal CoSi2 with oxide-mediated epitaxy
Authors: Kleinschmit M. W. , Yeadon M. , Gibson J. M.
Published: July 2002

Oxide-mediated epitaxy (OME) has shown promise as a technique for the formation of epitaxial CoSi2 on a variety of Si surfaces. With our in situ ultra-high-vacuum transmission electron microscope we have studied the phase formation sequence of the deposited Co during an anneal on both clean and oxide (OME) -covered Si (001) samples. The striking difference in OME is the absence of polycrystalline CoSi2 nucleation. We discuss the origin and consequences of this observation, and report other details of the phase evolution sequence.
Polarized optical absorption and photoluminescence measurements in single-crystal thin films of 4′-dimethylamino-N-methyl-4-stilbazolium tosylate
Authors: Bhowmik Achintya K. , Xu Jianjun , Thakur Mrinal
Published: July 2002

Single-crystal thin films of the anhydrous (red) and the hydrated (orange) phases of the organic salt 4′-dimethylamino-N-methyl-4-stilbazolium tosylate were grown by a modification of the shear method. The optical absorption coefficients of the films were measured with light polarized along and normal to the dipole/molecular axis at both resonant and off-resonant wavelengths, and a strong dichroism was observed at the resonant wavelengths. The absorption measurements are important considering potential applications of these films (red phase) in high-speed single-pass thin-film electro-optic modulators [M. Thakur, J. Xu, A. Bhowmik, and L. Zhou, Appl. Phys. Lett. 74, 635 (1999)] and other photonic devices. Highly polarized photoluminescence (PL) has been observed in these films. The PL efficiencies of the red- and orange-phase single-crystal films were measured to be about 12% and 14%, respectively, which are significantly higher than the maximum PL efficiency measured in solution (3%).
Nanostructural conductivity and surface-potential study of low-field-emission carbon films with conductive scanning probe microscopy
Authors: Zhang L. , Sakai T. , Sakuma N. , Ono T. , Nakayama K.
Published: July 2002

Simultaneous surface topography and conductivity/potential measurements were carried out on low-field-emission (1 V/μm) carbon films by combining conductive atomic force microscopy and Kelvin probe force microscopy. The current image showed that highly conducting sites and nonconducting regions coexisted on a micro- and/or nanoscale. Further, in situ I–V characteristics of both regions demonstrated that the conducting sites have an Ohmic property, whereas nonconducting regions have a degenerated Schottky property. When combined with the current image, the contact potential difference image showed that the conducting sites have a highest contact potential difference of 0.5 V, which implies the existence of a graphite phase. It is revealed that the conducting channels play an important role in the low-field-emission process. It is also suggested that the combination of conductivity and surface-potential measurements is an effective method for investigating complex-phase nanostructural surfaces.
Evanescent field imaging of an optical fiber Bragg grating
Authors: Mills J. D. , Hillman C. W. J. , Brocklesby W. S. , Blott B. H.
Published: July 2002

We have investigated the evanescent field associated with an optical fiber Bragg grating using the subwavelength imaging properties of scanning near-field optical microscopy. Imaging of either the field distribution within the grating, or the periodic refractive index changes along the grating can be performed by tuning the launched light on or off the grating resonance. These measurements reveal nonuniformity in the resonant standing-wave pattern that occur due to phase errors in the refractive index profile of the grating under study.
Photoemission spectroscopy and velocity analysis of sputtered carbon atoms, ions, and clusters Cm0,± (m⩽4)
Authors: Qayyum Abdul , Naeem Akhtar M. , Riffat Tasneem , Ahmad Shoaib
Published: July 2002

Diagnostics of the carbon atoms, ions, and clusters Cm0,± (m⩽4) sputtered from a graphite surface under 10 keV Xe+ bombardment reveals the presence of neutral Cm0 and positively charged Cm+ species in the photoemission spectra. The complete absence of Cm+ in the mass spectra by a velocity analyzer is supplemented by the presence of the negatively charged Cm−. The velocity spectra are dominated by C and C2− with peaks due to C3− and C4−. These results may help us to understand the contribution of neutral and charged species in the heavy-ion sputtering of graphite and the energetics of regenerative soot.
Crystal tilting in GaN grown by pendoepitaxy method on sapphire substrate
Authors: Kim Ig-Hyeon , Sone C. , Nam Ok-Hyun , Park Yong-Jo , Kim Taeil
Published: July 2002

Pendeoepitaxy of GaN on sapphire substrate with SiO2 mask is demonstrated and characterized by transmission electron microscopy and double crystal x-ray diffraction. A continuous layer of GaN with low dislocation density was achieved by this method. Parts of the GaN layer are tilted symmetrically toward [11-20] direction and have two kinds of coalesce and tilt boundaries. Each boundary was formed by a vertical array of piled up dislocations with the Burger’s vector of [11-20]. The tilting mechanism in pendeo-epitaxy is discussed in terms of surface interaction between the SiO2 mask and ELO-GaN.
Promising intracavity mode-locking device: A strained GaInAs/AlInAs saturable Bragg reflector grown by molecular-beam epitaxy
Authors: Chang Yongmao , Leonelli Richard , Maciejko Roman , SpringThorpe Anthony
Published: July 2002

A strained GaInAs/AlInAs saturable Bragg reflector (SBR) was fabricated from layers grown by molecular-beam epitaxy. The device consisted of nominally 25 periods of GaAs/AlAs Bragg structures incorporating double GaInAs/AlInAs quantum wells latticed matched to InP. The SBR was measured to be more than 99.5% reflective from 1410 to 1525 nm. The structural parameters for the sample were obtained from high-resolution x-ray diffraction rocking curves. The quantum wells exhibited partially relaxed interfaces where the relaxation was characterized by misfit dislocations. The photoluminescence measurements showed a lack of well-defined band edges and exciton structure at room temperature. Luminescence peaks were obtained at liquid-helium temperature, corresponding to heavy-hole and light-hole related exciton levels. The fast recovery of the SBR nonlinear response was explained by the misfit dislocations, which act as nonradiative recombination centers.
In situ electron microscopy studies of electromigration in stacked Al(Cu)/TiN interconnects
Authors: Lau J. T. , Prybyla J. A. , Theiss S. K.
Published: July 2002

In situ transmission electron microscopy experiments on electromigration in Al(0.5%Cu)/TiN interconnects show that the TiN barrier layer improves reliability not only by acting as a shunting layer, but also by dramatically influencing electromigration behavior within the Al(Cu) interlayer. One highlight of these experiments is a complete absence of void migration in such lines, as opposed to Al(Cu) lines on SiO2 without barrier layer [Al(Cu)/SiO2]. This remarkable immobility of voids in Al(Cu)/TiN lines has a profound impact on failure mechanisms, lifetimes, and reliability of interconnects. The TiN barrier layer also dramatically effects void nucleation and growth.
Self-limiting behavior of scanning-electron-beam-induced local oxidation of hydrogen-passivated silicon surfaces
Authors: Wei Y. Y. , Eres Gyula
Published: July 2002

The mechanism and the kinetics of electron-beam-induced local oxidation of an H-passivated Si surface in the electron energy range from 10 to 40 keV was investigated using scanning-electron-beam lithography. The volume expansion of Si upon oxidation produces a negative image surface pattern that can be imaged by atomic force microscopy. This latent pattern was used to study the dependence of the height and width of dot and line patterns as a function of the electron-beam exposure parameters. Patterns with minimum linewidth below 50 nm have been obtained. Similarly to atomic-force-microscope-induced local oxidation of Si, the height and linewidth saturate with electron dose for a given accelerating voltage. The saturation height roughly scales with the accelerating voltage, and depends more strongly on the accelerating voltage than the linewidth. The experimental results are interpreted by a mechanism that is based on charge generation and transport through the evolving insulating SiO2 layer.
Influence of interstitial carbon defects on electron transport in strained Si1−yCy layers on Si(001)
Authors: Osten H. J. , Griesche J. , Gaworzewski P. , Bolze K. D.
Published: July 2002

We present experimental results on Hall mobilities of electrons in tensile strained Si1−yCy layers with a substitutional carbon yS=0.4%, but different concentrations of interstitial carbon. Although the lattice distortion due to misfit strain and hence, the band alignment are identical for all investigated samples, we find differences in electron mobility of nearly a factor 2 due to the varying concentration of interstitial carbon. For the highest interstitial C concentration (1×1020 cm−3), it was not even possible to obtain any reliable electrical data. We demonstrate that it is not sufficient to consider only strain in evaluating electrical properties of C containing layers. Specific growth conditions can lead to very different electrical properties due to the different amounts of interstitial C, even for pseudomorphically strained layers with the same lattice mismatch and band alignment.
Parametric phase conjugation of a wide-band acoustic pulse in supercritical mode
Authors: Pernod P. , Preobrazhensky V.
Published: July 2002

Parametric wave phase conjugation of wide-band ultrasonic pulses of 100% relative bandwidth is demonstrated. The giant amplification allowed by the supercritical mode of the magnetoacoustic technique used, leads to a relatively high efficiency of the conjugation process. In addition, the pulse response function of the magnetoacoustic conjugator in supercritical mode is derived by fitting the experimental results with the linear theory of phase conjugation. The latter provides the parameters that define the maximum value of acoustic power in the conjugate wave. The results are of interest for applications in nondestructive evaluation and acoustical imaging.
Investigation of polarization-pinning mechanism in deep-line-etched vertical-cavity surface-emitting lasers
Authors: Sargent L. J. , Rorison J. M. , Kuball M. , Penty R. V. , White I. H. , Corzine S. W. , Tan M. R. T. , Wang S. Y. , Heard P. J.
Published: July 2002

Recently, it has been shown that the etching of deep trenches in close proximity to GaAs vertical-cavity surface-emitting laser (VCSEL) apertures causes the linearly polarized TE emission to be pinned in a direction parallel to the line etch. In this letter, we show that etching introduces compressive strain or relaxes tensile strain through the creation of free interfaces. An anisotropic variation of strain is the origin of the polarization pinning effect. We report on the enhancement of polarization pinning by postannealing after etching. Photoluminescence and Raman measurements of the VCSEL wafer were taken before and after etching and annealing. The observed shift in the Fabry–Perot mode was used to model the strain, giving 4×108 dyn/cm2, or 0.05%, compressive strain perpendicular to the etch.
Direct measurement of optical phase in the near field
Authors: Phillips P. L. , Knight J. C. , Pottage J. M. , Kakarantzas G. , Russell P. St. J.
Published: July 2002

To fully characterize photonic crystal guided wave optical devices, one needs to measure the spatial variation of both the phase and amplitude of the electromagnetic field. In this work, we simultaneously measure the intensity and phase in the near field of both propagating and evanescent fields by incorporating a scanning near-field optical microscope into one arm of a Mach–Zehnder interferometer. We demonstrate the technique by imaging the phase fronts of an evanescent wave formed by total internal reflection and by measuring the phase variation in the LP11 mode in an overmoded optical fiber.
Effect of carrier confinement on photoluminescence from modulation-doped AlxGa1−xN/GaN heterostructures
Authors: Shen B. , Someya T. , Moriwaki O. , Arakawa Y.
Published: July 2002

Photoluminescence (PL) of modulation-doped Al0.22Ga0.78N/GaN heterostructures was investigated. The PL peak related to recombination between the two-dimensional electron gases (2DEG) and photoexcited holes is located at 3.448 eV at 40 K, which is 45 meV below the free excitons (FE) emission in GaN. The peak can be observed at temperatures as high as 80 K. The intensity of the 2DEG PL peak is enhanced significantly by incorporating a thin Al0.12Ga0.88N layer into the GaN layer near the heterointerface to suppress the diffusion of photoexcited holes. The energy separation of the 2DEG peak and the GaN FE emission decreases with increasing temperature. Meanwhile, the 2DEG peak energy increases with increasing excitation intensity. These results are attributed to the screening effect of electrons on the bending of the conduction band at the heterointerface, which becomes stronger when temperature or excitation intensity is increased.
Growth and superconducting properties of YBa2Cu3O7−δ films on conductive SrRuO3 and LaNiO3 multilayers for coated conductor applications
Authors: Aytug T. , Wu J. Z. , Cantoni C. , Verebelyi D. T. , Specht E. D. , Paranthaman M. , Norton D. P. , Christen D. K. , Ericson R. E. , Thomas C. L.
Published: July 2002

Conductive multilayers of YBa2Cu3O7−δ/SrRuO3/LaNiO3 (YBCO/SRO/LNO), YBCO/SRO, and YBCO/LNO were grown epitaxially on single-crystal LaAlO3 and SrTiO3 substrates. Property characterizations revealed that the YBCO films on SRO and SRO/LNO buffer structures have excellent structural and superconducting properties. The YBCO/LNO, however, suffers degradation in superconducting transition temperatures and critical current densities, despite a good crystalline structure. The SRO/LNO bilayer showed excellent electrical contact with YBCO. These results have demonstrated the structural and chemical compatibility of SRO/LNO multilayers with YBCO and their potential use as conductive buffer layers for YBCO-based coated conductors as well as for other high-temperature superconducting applications.
Geometric heat trapping in niobium superconductor–insulator– superconductor mixers due to niobium titanium nitride leads
Authors: Leone B. , Jackson B. D. , Gao J. R. , Klapwijk T. M.
Published: July 2002

We analyze the current–voltage characteristics of a Nb superconductor–insulator–superconductor mixer with NbTiN leads to identify the heating processes in this device. We argue that the electron–electron interaction is much faster than the electron–phonon interaction, and show that the heat flow to the bath temperature is limited by the electron–phonon interaction time in the junction. A solution is suggested to considerably reduce the heating.
In situ transmission electron microscopy observation of thin solid aluminum film thermal flow into submicron-sized periodic grooves fabricated on a silicon oxide substrate
Authors: Yasuda Hidehiro , Wada Jun-ichi , Furuya Kazuo
Published: July 2002

Thin solid aluminum film thermal flow into submicron-sized periodic grooves fabricated on a silicon oxide substrate has been studied in situ by transmission electron microscopy (TEM). Preparation of an aluminum film on the substrate was carried out using an evaporator installed in the specimen chamber of a TEM. The aluminum film was then gradually heated in situ in the microscope. At temperatures around 878 K, the polycrystalline aluminum film on the surface of the substrate started to flow into the grooves, and eventually the grooves were filled with polycrystalline aluminum. At 933 K, the aluminum in the grooves melted. It was confirmed that thin-film thermal flow into the grooves took place in the solid state. It is suggested that a creep deformation of the film driven by surface tension is responsible for this flow.
The initial stages of growth of CuPtB ordered Ga0.52In0.48P/GaAs and Ga0.47In0.53As/InP
Authors: Hanna Mark C. , Cheong Hyeonsik M. , Mascarenhas Angelo
Published: July 2002

Atomic force microscopy and Raman spectroscopy were used to investigate the development of the surface morphology and ordering of Ga0.52In0.48P and Ga0.47In0.53As. A series of lattice-matched highly ordered Ga0.52In0.48P/GaAs and Ga0.47In0.53As/InP samples ranging in thickness from 2 to 50 nm were grown by low-pressure metalorganic vapor phase epitaxy on (001) direct and vicinal substrates. Raman spectroscopy provided direct evidence of CuPtB-type ordering in layers as thin as 10 nm for Ga0.52In0.48P and 5 nm for Ga0.47In0.53As. We find that the morphology of both Ga0.52In0.48P and Ga0.47In0.53As on (001)6B substrates consists of ridges with heights ranging from 2 to 10 nm, which are aligned predominately along the [110] direction. For Ga0.52In0.48P growth on (001) direct substrates, ridges similar to those obtained on 6B vicinal substrates form with no preferential orientation, while Ga0.47In0.53As growth on (001) direct substrates proceeds by a combination of two-dimensional-island and step-flow growth. The average roughness of the GaInP layers is approximately twice that of GaInAs layers. These findings suggest it may be difficult to produce abrupt large-area heterointerfaces in structures containing ordered Ga0.52In0.48P and Ga0.47In0.53As alloys.
Microvolume field-effect pH sensor for the scanning probe microscope
Authors: Manalis S. R. , Cooper E. B. , Indermuhle P. F. , Kernen P. , Wagner P. , Hafeman D. G. , Minne S. C. , Quate C. F.
Published: July 2002

A pH sensitive scanning probe is realized by integrating a micron-sized field-effect sensor onto a cantilever designed for an atomic force microscope. The hybrid device, called a scanning probe potentiometer (SPP), is capable of measuring pH gradients over a sample surface. The device was used to profile the pH across a reservoir of laminar streams created by fluid flow in an array of microfluidic channels of varying pH. When a single SPP scanned, a 1.5 mm reservoir in a 10-channel array, the pH profile was measured in less than 1 min with a spatial resolution of 10 μm and sensitivity of less than 0.01 pH units.
Anisotropic growth of single-crystal graphite plates by nickel-assisted microwave-plasma chemical-vapor deposition
Authors: Badzian Teresa , Badzian Andrzej , Roy Rustum , Cheng Shang-Cong
Published: July 2002

Growth of single-crystal graphite free-standing plates has been achieved by a microwavehydrogen-plasma etching of graphite powder and nickel mesh. The plates resemble a knife blade and grow in the 〈11̄00〉 direction with long crystals exceeding 100 μm. Hexagonal growth features at the edges and electron diffraction patterns confirm the single-crystal nature of these ultrathin plates. Electron microprobe and Raman spectroscopy indicate the presence of graphite. Diamond crystals nucleate on these plates and they grow simultaneously. We suggest that the paradoxical growth of graphite in a hydrogen plasma, under conditions in which graphite is usually etched away, is possible because of a protective coating by a Ni–C–H phase. This thin coating allows for transport of carbon atoms from the gas phase to the growing graphite surface.
Electron energy control in an inductively coupled plasma at low pressures
Authors: Shindo Haruo , Urayama Takuya , Fujii Takashi , Horiike Yasuhiro , Fujii Syuitsu
Published: July 2002

A method of electron energy control was studied in an inductively coupled plasma that employed a multimode antenna, for high performance in device fabrication plasma processes. The electron energy was reduced by changing the azimuthal mode of a one-turn antenna from m=0 to 2 with no notable change in electron density. Langmuir probe measurements showed that the electron energy reduction was more pronounced in the higher modes and at lower pressures. The antenna size was also a critical parameter. These results were confirmed by optical emission spectroscopy. It was found to be essential for the energy control that the distance between the induction-field reverse points is shorter than the electron free path.
4.8 μm vertical emitting PbTe quantum-well lasers based on high-finesse EuTe/Pb1−xEuxTe microcavities
Authors: Springholz G. , Schwarzl T. , Aigle M. , Pascher H. , Heiss W.
Published: July 2002

Vertical laser emission at 4.8 μm from PbTe quantum wells in high-finesse Pb0.95Eu0.05Te/EuTe microcavity structures at temperatures between 35 and 85 K is reported. The vertical-cavity laser structure was grown by molecular-beam epitaxy on BaF2(111) substrates, and consisted of a 2λ cavity with four 20 nm quantum wells at the cavity antinodes. Laser emission was excited by optical pumping with a pulsed Nd:YVO4 laser. The comparison of the cavity mode positions with envelope function calculations of the quantum-well energy levels indicates that, in this temperature range, lasing is due to transitions between the ground level of the oblique valleys in the conduction and valence bands.
Deposition rate in modulated radio-frequency silane plasmas
Authors: Biebericher A. C. W. , Bezemer J. , van der Weg W. F. , Goedheer W. J.
Published: July 2002

Plasma-enhanced chemical-vapor deposition of amorphous silicon by a square-wave amplitude-modulated radio-frequency excitation has been studied by optical emission spectroscopy and plasma modeling. By the modulation, the deposition rate is increased or reduced, depending on the plasma parameters. The increase in the deposition rate in powder-free (α-regime) plasmas is explained by the behavior of the electrons. High-energy electrons cause a large production of radicals at the onset of the plasma, as evidenced by an overshoot in optical emission. This is confirmed by a one-dimensional fluid model. An optimum in the deposition rate at a modulation frequency of about 100 kHz is determined by the decay time of the electron density.
Depletion region in thermally poled fused silica
Authors: Triques A. L. C. , Cordeiro C. M. B. , Balestrieri V. , Lesche B. , Margulis W. , Carvalho I. C. S.
Published: July 2002

The depletion-layer width and the recorded electric field in thermally poled fused silica are investigated experimentally as a function of the applied voltage. The depletion-layer width is observed to vary linearly with the poling voltage. The average electric field recorded in the depletion region was found to be (5.3±0.3)×108 V/m for all samples, independently of the poling voltage.
Fabricating conductive microstructures by direct electron-beam writing on hydrogenated n-type Si-doped GaAs
Authors: Silvestre S. , Constant E. , Bernard-Loridant D. , Sieber B.
Published: July 2002

We study the changes in electronic properties of Si-doped GaAs epilayers exposed to a hydrogen plasma and then submitted to electron beams of 20 keV injection energy. Using Hall-effect measurements, we have observed the formation of Si–H complexes and their dissociation due to the electron-beam irradiation. As this last effect increases the conductivity of the epilayer, we have been able to fabricate, with an electron-beam lithography system, various conductive microstructures. Characterizations have been achieved by cathodoluminescence (CL) imaging. Actually, due to the CL reading method which has been used, we have only fabricated micronic-size structures. However, taking into account the high spatial resolution of electron-beam writing, such a process could, possibly, be used as a way of fabrication of mesoscopic structures.
Reflection-type 2×2 optical waveguide switch using the Goos–Hänchen shift effect
Authors: Sakata Tomomi , Togo Hiroyoshi , Shimokawa Fusao
Published: July 2002

We have designed a structure for a 2×2 silica-based optical waveguide switch that is based on a thermocapillarity effect. This switch can use the reflection walls on both sides of the slit, because the Goos–Hänchen shift effect was taken into account when the structure of the waveguides and the slit was designed. This switch can provide a cross/bar function through a single element, and the measured reflection losses in the reflection walls on both sides of the slit were consistent. The loss was comparable to the insertion loss of a Mach–Zender-interferometer-type thermo-optic switch.
Influence of Cs and Ag metal vapor on output characteristics of a HyBrID copper vapor laser
Authors: Ohzu Akira , Kato Masaaki , Maruyama Yoichiro
Published: July 2002

Effects of Cs and Ag metal vapor additives on the lasing performance of a HyBrID copper vapor laser have been experimentally investigated. By adding Cs to the discharge plasma of the laser, the optimum HBr gas flow rate is changed to be smaller than that without the additives. And the average output power increases by about 20%. With the addition of Ag, the output power increases by 30%. And the output power characteristics at high-pulse repetition frequency from 16 to 20 kHz are improved. Based on a relaxation of the laser lower level of Cu atoms in the metastable state and a reduction in the remanent free electron density in the interpulse period, the mechanisms for the remarkable improvement are discussed.
Fabrication of nanometer size gaps in a metallic wire
Authors: Lefebvre J. , Radosavljević M. , Johnson A. T.
Published: July 2002

We present a simple shadow mask method to fabricate electrodes with nanometer scale separation. Metal wires with gaps are made by incorporating multiwall carbon nanotubes or single-wall carbon nanotube (SWNT) bundles into a trilayer electron beam lithography process. The simple, highly controllable, and scaleable method has been used to make gaps with widths between 20 and 100 nm and may be extended to gap sizes of 1 nm. We report electron transport measurements of individual SWNTs bridging nanogaps with electrode spacings of approximately 20 nm. Metallic SWNTs exhibit quantum dot behavior with an 80 meV charging energy and a 20 meV energy level splitting. We observe a strong field effect behavior in short semiconducting SWNT segments, evidence for diffusive electron transport in these samples.
Microstructure and magnetic properties of FePt–Al–O granular thin films
Authors: Watanabe M. , Masumoto T. , Ping D. H. , Hono K.
Published: July 2002

We have investigated the microstructures and magnetic properties of L10 ordered FePt granular thin films prepared by ion beam sputtering and subsequent annealing. As-deposited FePt–Al–O films are composed of fine spherical FePt particles of ∼2 nm diameter with disordered face-centered-cubic structure. These particles are embedded in the amorphous aluminum oxide matrix with average spacing of ∼1.3 nm, and the film exhibits superparamagnetism. By annealing above 650 °C, the nanoparticles transform to the L10 ordered phase and the grains are coarsened to ∼20 nm, which results in a high coercivity up to 10 kOe. In the films with a high Al content, most of the particles are isolated by the amorphous aluminum oxide matrix, while the grains are interconnected in the films with a low Al content. The changes in magnetic properties are discussed based on the microstructural observation results.
Modal frequencies of vertical-cavity lasers determined by an effective-index model
Authors: Serkland Darwin K. , Hadley G. R. , Choquette K. D. , Geib K. M. , Allerman A. A.
Published: July 2002

Previously, an effective-index optical model was introduced for the analysis of lateral waveguiding effects in vertical-cavity surface-emitting lasers. We show that the resultant transverse equation obtained in that model is almost identical to the one typically obtained in the analysis of dielectric waveguide problems, such as a step-index optical fiber. In this letter, we extend the effective-index model to obtain predictions of the discrete frequencies of the microcavity modes. As an example, we apply the analysis to vertical-cavity lasers that contain thin-oxide apertures. The model intuitively explains our experimental data and makes quantitative predictions in good agreement with a highly accurate numerical model.
Drastic increase of the density of Ge islands by capping with a thin Si layer
Authors: Usami N. , Miura M. , Ito Y. , Araki Y. , Shiraki Y.
Published: July 2002

Capping Ge islands with a thin Si layer was found to be very effective in increasing the density of islands. An increase of the density by a factor of 3 was observed in 4.5 monolayers (MLs) of Ge islands by capping with a 6 Å Si layer. In addition, the formation of islands was confirmed even for 2.7 ML Ge and a thin Si cap layer although the amount of Ge is smaller than the critical coverage of the island formation. Possible explanations for these phenomena are discussed based on coarsening of the islands and the intermixing effect.
Ion-beam synthesis of epitaxial silicon carbide in nitrogen-implanted diamond
Authors: Heera V. , Fontaine F. , Skorupa W. , Pécz B. , Barna Á.
Published: July 2002

Natural IIa diamond was implanted at 90 keV to 1×1015 N+/cm2 and subsequently at 150 keV to 3×1017 Si+/cm2 at a temperature of 900 °C. The structure of the implanted diamond region was investigated by high-resolution cross-sectional transmission electron microscopy, Raman, and infrared absorption spectrometry. A buried layer with crystalline 3C–SiC domains in perfect epitaxial relation to the diamond substrate was detected. Amorphization and graphitization were completely prevented by the elevated temperature during the implantation. Resistance measurements demonstrated low electrical resistivity in the implanted regions.
Single-phase hexagonal GaN grown on AlAs/GaAs(001)
Authors: Funato Mitsuru , Ishido Teruki , Hamaguchi Atsushi , Fujita Shizuo , Fujita Shigeo
Published: July 2002

This letter describes successful growth of single-phase hexagonal GaN (h-GaN) layers on cubic GaAs(001) nominally singular substrates with the assistance of thin AlAs intermediate layers. The crystallographic relationship between h-GaN and GaAs is extracted from a pole figure to be h-GaN[0001]∥GaAs[001] in the growth direction and h-GaN[101̄0]∥GaAs[11̄0] in the in-plane direction. In a photoluminescence spectrum measured at 20 K, excitonic emission from the h-GaN layer is detected at 3.47 eV.
Characterizing the hard x-ray diffraction properties of a GaAs linear Bragg–Fresnel lens
Authors: Li Youli , Wong Gerard C. L. , Case Ryan , Safinya Cyrus R. , Caine Ernie , Hu Evelyn , Fernandez Partricia
Published: July 2002

We investigated the diffractive focusing properties of (111) GaAs linear Bragg–Fresnel lenses (BFLs) developed for hard x-ray microscopy and microdiffraction of complex materials in confined geometries. We demonstrated that the use of GaAs yields significant processing advantages due to the reduced zone depth. Focal plane diffraction patterns of linear BFLs measured at the advanced photon source using 8–40 keV x rays were compared to a simple model based on Kirchhoff–Fresnel diffraction theory. Good agreement was obtained between experimental data and model calculations using only zones within an effective aperture defined by the transverse coherence of the source.
Electron drift mobility in pyrazolo[3,4-b]quinoline doped polystyrene layers
Authors: Tameev A. R. , He Z. , Milburn G. H. W. , Kozlov A. A. , Vannikov A. V. , Danel A. , Tomasik P.
Published: July 2002

Pyrazolo[3,4-b]quinoline derivatives are found to have electron transport properties. A field dependent electron drift mobility, in the range of 10−6–3×10−5 cm2/(V s) at an electric field of (1–7)×105 V/cm, has been found from the pyrazolo[3,4-b]quinoline doped polystyrene specimen, which was measured using a conventional time-of-flight method. The results are fit well into a correlated disorder model for a dipole medium when energetic features of the transport molecules are also taken into account.
Independent determination of Ge content in thin Si1−xGex quantum wells by spectroscopic ellipsometry
Authors: Broschwitz M. , Dettmer K. , Schoenes J.
Published: July 2002

The pseudodielectric functions of Si1−xGex multi-quantum-well samples have been measured by spectroscopic ellipsometry in the photon energy range from 1.1 to 4.5 eV at room temperature. A sharp structure near 3.2 eV, obtained in all our spectra, is identified as an interband transition of the Si1−xGex well, whose exact energy depends on x. By fitting line shapes to the second derivative of the spectra it is possible to determine the Ge content x of the thin Si1−xGex wells. The detailed analysis of the transition energies in comparison with x-ray diffraction data then allows the exact determination of the thicknesses of barriers and wells. The theoretically calculated shift of the transition energies caused by the strain of the Si1−xGex wells was taken into account in our analysis.
Superconducting device with transistor-like properties including large current amplification
Authors: Pepe G. P. , Ammendola G. , Peluso G. , Barone A. , Parlato L. , Esposito E. , Monaco R. , Booth N. E.
Published: July 2002

We have fabricated and studied a stacked superconducting double tunnel junction device with transistor-like properties. The intermediate electrode is a bilayer consisting of a Nb film together with an Al film that acts as a quasiparticle trap. Large current gains of more than 50 are observed at 4.2 K when the Al layer is normal. The operation is highly directional. Results are explained on the basis of trapping of quasiparticles from a superconductor into a normal metal, together with a conversion of relaxation energy into electronic excitations. Similar devices should have wide applications in low-temperature measurement and detection systems.
Stacking fault energies in Si doped GaN: A first principles study
Authors: Chisholm J. A. , Bristowe P. D.
Published: July 2002

The variation of stacking fault energy in GaN as a function of silicon doping is calculated using density functional theory. It is found that the stacking fault energy reduces with increasing silicon concentration in agreement with experimental observations of higher stacking fault densities in Si doped GaN compared to undoped GaN. The result is explained in terms of a decrease in the magnitude of the Mulliken charges on the atoms as silicon is incorporated into the lattice and an increase in the overlap populations. We propose that Mulliken charges can be used to predict how any substitutional impurity will influence the stacking fault formation energy.
Transfer function and noise properties of YBa2Cu3O7−δ direct-current superconducting-quantum-interference-device magnetometers with resistively shunted inductances
Authors: Kahlmann F. , Booij W. E. , Blamire M. G. , McBrien P. F. , Tarte E. J. , Peng N. H. , Jeynes C. , Romans E. J. , Pegrum C. M.
Published: July 2002

We have investigated resistively shunted direct-coupled high-Tc dc superconducting quantum interference device (SQUID) magnetometers with four different inductances (50, 100, 150, and 200 pH). The SQUIDs were based on 200 nm thick YBa2Cu3O7−δ films deposited on bicrystal substrates with a 24° misorientation angle, and the shunt resistors were fabricated by masked ion damage. At T=77 K, good quantitative agreement was observed between the measured maximum voltage modulation depth ΔV and calculated values based on the theoretical predictions by Enpuku et al., whereas the white magnetic flux noise SΦ at 10 kHz of all four devices was found to be a factor of 2.3 higher than predicted. The lowest white magnetic field noise of 153 fT/Hz was obtained for the magnetometer with a SQUID inductance of 100 pH with an outer dimension of the pickup loop of just 2 mm.
Ultrafast dynamics microscopy
Authors: Dyba M. , Klar T. A. , Jakobs S. , Hell S. W.
Published: July 2002

We report the three-dimensional imaging of the vibrational, solvent, orientational, and electronic relaxation in organic fluorescent samples at 200–500 nm spatial resolution. This is achieved in steady-state recordings by exciting the fluorophore with a femtosecond pulse and subsequent quenching with a time-delayed, redshifted femtosecond pulse through stimulated emission. Temporal resolution of 380 fs is solely determined by the pulse widths and is further reducible. Images of submicron structures revealing vibrational and solvent relaxation gradients are shown. Furthermore, we introduce contrast modes based on stimulated emission depletion and apply them to cellular imaging.
2.79 μm erbium laser with lead–lanthanum zirconate titanate ceramics electro-optic Q-switching output coupler
Authors: Ozolinsh Maris , Eichler Hans J.
Published: July 2002

Highly refractive and transparent lead–lanthanum zirconate titanate (PLZT) ceramics are used for an output coupler of the erbium yttrium scandium gallium garnet laser (λ=2.79 μm) that simultaneously serves as the laser electro-optic Q switch. The optical aperture of such a dual-function PLZT element was 6×6 mm2 and the thickness was 3 mm. For Q switching, short ≈3 μs negative voltage pulses (1500–1800 V) were applied to the positively direct current biased (≈1400 V) PLZT element. Single-pulse lasing with a pulse half width of 160–170 ns was observed at short and long delays of the Q switch with respect to the pump flash. The highest output energy of 20–22 mJ within 1.5–2.5 μs, consisting of two or three pulses with 130 ns half width was obtained at an intermediate delay of 90–110 μs.
The current–voltage dependence of nominally undoped thin conjugated polymer films
Authors: Freire José A. , da Luz M. G. E. , Ma Dongge , Hümmelgen I. A.
Published: July 2002

The charge transport in conjugated polymer films sandwiched between metallic electrodes is described by means of a one-dimensional master equation where the hopping rates are given by thermally activated jumps over barriers separating trapping sites of same energy. An applied electric field along the chain reduces the barrier height leading to a field-dependent mobility, μ(E)∼ sinh(E)/E. We exactly derive the current–voltage characteristics of such a system assuming that the injected carriers generate space charge effects inside the polymer. We find an excellent agreement with the experimental data for different types of conjugated materials and compare our model with other models that predict exp(E) for the field dependence of the mobility, discussing the essential differences between them.
Single-electron detector and counter
Authors: Stone N. J. , Ahmed H.
Published: July 2002

An electron detector, constructed with highly doped silicon nanowires, is described. It is shown that, at a temperature of 4.2 K, the presence or absence of a single excess electron on a storage node can be recognized. The detector can also be used to count the precise number of electrons transferred to the node.
Gasochromic mechanism in a-WO3 thin films based on Raman spectroscopic studies
Authors: Lee Se-Hee , Cheong Hyeonsik M. , Liu Ping , Smith Dave , Tracy C. Edwin , Mascanrenhas Angelo , Pitts J. Roland , Deb Satyen K.
Published: July 2002

The gasochromic mechanism in amorphous tungsten oxide films has been investigated using Raman scattering measurements. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 cm−1 due to vibrations of the O–W6+–O and W6+=O bonds, respectively. When the Pd/a-WO3 films are gasochromically colored by exposure to diluted hydrogen gas, extra Raman peaks due to O–W5+–O and W5+=O bonds appear at 330 and 450 cm−1, respectively. The Raman spectra of a-WO3 films bleached with isotopic heavy oxygen (18O) show that the gasochromic coloration in a-WO3 films is directly related to the double injection of hydrogen ions and electrons.
Hydrogen effects on ZnSe/GaAs interface: A photoreflectance study
Authors: Sim E. D. , Song J. H. , Chang S. K. , Kim H. K.
Published: July 2002

ZnSe/GaAs heteroepitaxial structures were prepared by metalorganic chemical vapor deposition with the use of thermal cleaning treatments at varying hydrogen flow rates in preparing the substrate. The photoreflectance spectra for the samples prepared revealed Franz–Keldysh oscillation (FKO) signals with the superimposition of free exciton transition features. The FKO signals changed depending on the hydrogen flow rate while the exciton transition features remained unchanged. Fitting through Aspnes’ and Franz–Keldysh’s model showed that the built-in electric field at the interface increased as the hydrogen flow rate decreased. These results imply that the hydrogen flow rate in the course of thermal etching plays a crucial role in the change of the energy-band profile at the interface.
Scanning near-field optical microscopy in the near-infrared region using light emitting cantilever probes
Authors: Heisig S. , Rudow O. , Oesterschulze E.
Published: July 2002

We present an application of an active emitting cantilever probe for scanning near-field optical microscopy and scanning force microscopy. A vertical cavity surface emitting laser (VCSEL) integrated in a galliumarsenide (GaAs) cantilever serves as a light source at 980 nm emission wavelength that is below the band gap energy of the GaAs substrate material. The VCSEL of 8 μm diameter is centered with respect to the metalized GaAs tip and illuminates a small near-field aperture at its apex. Aperture fabrication is accomplished by a proper thermal metal evaporation process. Optical measurements on a Fischer projection pattern revealed an edge resolution of about 80 nm.
Hot hole relaxation dynamics in p-GaN
Authors: Ye Hong , Wicks G. W. , Fauchet P. M.
Published: July 2002

The hot hole relaxation dynamics is studied in a Mg-doped p-type GaN film grown by molecular-beam epitaxy on sapphire. A nondegenerate femtosecond pump-probe technique is used, in which the holes are excited by an infrared pump and the hole dynamics is monitored by a tunable near ultraviolet probe. Complex transients, showing bleaching or induced absorption, are observed. A hot hole energy relaxation time of 0.6 ps has been obtained. Modeling suggests that longitudinal optical phonon emission modified by hot phonon effects is the dominant energy relaxation process.
High-resolution microbeam x-ray diffractometry applied to InGaAsP/InP layers grown by narrow-stripe selective metal-organic vapor phase epitaxy
Authors: Kimura Shigeru , Kimura Hidekazu , Kobayashi Kenji , Oohira Tomoaki , Izumi Koich , Sakata Yasutaka , Tsusaka Yoshiyuki , Yokoyama Kazushi , Takeda Shingo , Urakawa Masafumi , Kagoshima Yasushi , Matsui Junji
Published: July 2002

We measure the diffraction peaks of InGaAsP selective metal-organic vapor-phase epitaxial layers on 1.7-μm-wide InP stripe regions between a pair of SiO2 mask stripes. This is achieved by using an x-ray microbeam with low angular divergence and a narrow energy bandwidth that was produced through two-dimensional condensation of undulator radiation x rays from a synchrotron light source using successive asymmetric diffraction. The lattice strain is investigated by changing the SiO2 mask width from 4 to 40 μm. The rocking curves reveal clear peak shifts in the InGaAsP layers from the higher angle side to the lower angle side of the InP substrate peaks as the mask width increases.
Controlling Fe nanocrystallization in amorphous Fe86Zr7Cu1B6 by linear varying current Joule heating
Authors: da Silva F. C. S. , Ferrari E. F. , Knobel M. , Torriani I. L. , dos Santos D. R.
Published: July 2002

Amorphous melt-spun Fe86Zr7Cu1B6 ribbons were annealed using the linear varying current Joule heating method. Experimental curves of resistance (R) and temperature (T) versus applied current (I) allow one to follow precisely the crystallization of α−Fe nanoparticles during annealing. This result proves that the applied current can be considered a reliable parameter to control the crystalline fraction in this alloy. A comparison between structural and magnetic measurements shows that the R(I) curve can be used as a guide to identify a condition for optimum soft magnetic properties of this alloy.
Photon-induced modifications in cadmium telluride/mercury cadmium telluride heterostructure interfaces
Authors: Agnihotri O. P. , Sehgal H. K. , Pal R. , Gopal Vishnu
Published: July 2002

We report on photon-induced modifications in CdTe/MCT (cadmium telluride/mercury cadmium telluride) heterostructure interfaces formed by flash evaporated CdTe on to the bulk p-Hg0.8Cd0.2Te. Metal–insulator–semiconductor (MIS) test structures were processed and their electrical properties measured by capacitance–voltage (C–V) and current–voltage characteristics. The effect of preannealing the MCT wafers under photon excitation and in Hg environment as well as the postannealing of CdTe/MCT under photon excitation were investigated. By optimizing the preanneal and postanneal processes, a significant improvement in the interface characteristics was obtained as evidenced by C–V measurements. The ultraviolet (UV) photons seem to play an important role in the process. The Hg atoms in the vapor phase absorb UV photons. In the presence of excited Hg atoms, hydrogen radicals are formed by direct collisions in the vapor phase and passivate the MCT surface.