bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical engineering with Fibonacci dielectric multilayers
Authors: Maciá Enrique
Published: July 2002

We study the resonant transmission of light through Fibonacci dielectric multilayers (FDM). Making use of a transfer matrix renormalization technique [E. Maciá and F. Domı́nguez-Adame, Phys. Rev. Lett. 76, 2957 (1996)] we obtain closed analytical expressions for the transmission coefficient under arbitrary incidence angle conditions. We analyze the relationship between the resonant wavelengths and the quasiperiodic structure of the substrate, suggesting the potential use of arrays containing FDMs of different sizes in the design of optical microcavities.
Photoconductivity mechanism of quantum well infrared photodetectors under localized photoexcitation
Authors: Ershov M.
Published: July 2002

The physics of photoconductivity of quantum well infrared photodetectors (QWIPs) under nonuniform illumination across QWIP structure (in the direction normal to QWs plane) is studied theoretically with the use of numerical simulation. Local responsivity is a strong function of a coordinate across QWIP; it decreases from emitter to collector, having a constant value in the bulk of QWIP. The mechanism of photoconductivity under localized photoexcitation is related to the formation of the dipole layer (DL), with positive charge located in the photoexcited QW, and negative charge located in a few next QWs towards collector. As a result, electric field outside the DL is increased to cause photocurrent. Parameters of the DL and local responsivity for QWIPs with multiple QWs is calculated using an analytical model.
Above-room-temperature optically pumped midinfrared W lasers
Authors: Bewley W. W. , Felix C. L. , Aifer E. H. , Vurgaftman I. , Olafsen L. J. , Meyer J. R. , Lee H. , Martinelli R. U. , Connolly J. C. , Sugg A. R. , Olsen G. H. , Yang M. J. , Bennett B. R. , Shanabrook B. V.
Published: July 2002

We report temperature-dependent pulsed lasing performance, internal losses, and Auger coefficients for optically pumped type-II W lasers with wavelengths in the range of 3.08–4.03 μm at room temperature. All lased to at least 360 K, and produced 1.5–5 W peak power at 300 K. Internal losses at 100 K were as low as 10 cm−1, but increased to 90–360 cm−1 at 300 K. Room temperature Auger coefficients varied from 5×10−28 cm6/s at the shortest wavelength to 3×10−27 cm6/s at the longest.
Diffusion and electromigration of copper in SiO2-passivated single-crystal aluminum interconnects
Authors: Srikar V. T. , Thompson C. V.
Published: July 2002

We have experimentally studied the diffusion and electromigration of Cu in passivated single-crystal Al interconnects, fabricated on oxidized Si wafers. The test structure consisted of parallel lines (0.4 μm thick and 5 μm wide), with alternating lines terminating in shared contact pads. Cu was locally added to the same regions in all the lines, and the effects of temperature and electric fields on diffusion along the lines were characterized by analyzing the Cu concentration profile measured by electron-probe microanalysis. The activation energy for diffusion was found to be 0.8±0.2 eV in the temperature range 320–400 °C, which suggests that the path for diffusion of Cu is along the Al/AlOx/SiOy interface for these single crystals. The apparent effective valence (Z*) characterizing electromigration was measured to be −5.4±1.2 at 350 °C.
Piezoelectric polarization associated with dislocations in wurtzite GaN
Authors: Shi Changchun , Asbeck Peter M. , Yu Edward T.
Published: July 2002

The piezoelectric polarization and its associated charge density are calculated for edge, screw, and mixed dislocations oriented parallel to the c axis in wurtzite GaN. It is shown that the polarization field generated by screw components of dislocations is divergence free, and thus does not generate electric fields. Edge dislocations produce polarization fields that have nonzero divergence only at interfaces. These characteristics minimize the electrical and optical effects of the dislocations mediated by the piezoelectric effect.
An AlAs/InGaAs/AlAs/InAlAs double-barrier quantum well infrared photodetector operating at 3.4 μm and 205 K
Authors: Lee Jung Hee , Chiang Jung-Chi , Li Sheng S. , Kannam P. J.
Published: July 2002

A high-performance n-type AlAs/InGaAs/AlAs/InAlAs double-barrier quantum well infrared photodetector grown on InP with photovoltaic (PV) and photoconductive (PC) dual-mode operation has been fabricated and characterized in this work. The PV and PC detection scheme employs the intersubband transition from the ground bound state to the quasibound excited state in the double-barrier well. The peak responsivities for the PV mode at zero bias and PC mode at Vb=−3 V were found to be 19 and 159 mA/W at λp=3.4 μm and T=77 K, respectively. The corresponding background limited performance (BLIP) detectivity (DBLIP*) was given by 8.9×1010 cm Hz1/2/W for the PV mode and non-BLIP D*=5.36×1010 cm Hz1/2/W for the PC mode. This device is capable of operating up to 205 K with good performance characteristics.
The influence of diffusion temperature and ion dose on proximity gettering of platinum in silicon implanted with alpha particles at low doses
Authors: Schmidt D. C. , Svensson B. G. , Godey S. , Ntsoenzok E. , Barbot J. F. , Blanchard C.
Published: July 2002

Platinum has been diffused into epitaxial n-type silicon at 600, 650, and 700 °C for 30 min following implantation with 3.3 MeV alpha particles. The doses employed were between 1×1011 and 1×1014 He+ cm−2. Thereafter the samples were characterized using deep level transient spectroscopy (DLTS). The samples diffused at 700 °C show only the deep level at 0.23 eV below the conduction band that is attributed to substitutional platinum. DLTS profiling reveals a decoration of the region of maximal damage by the platinum for lower doses while for higher ones the platinum concentration is observed to decrease or vanish in this region. In addition, other deep levels may appear (so-called K lines). As the implantation dose increases, so does the platinum concentration following diffusion at 700 °C at the shallow end of the DLTS working region. It is shown that, by controlling the amount of implantation induced defects and the diffusion temperature, one can steer the amount of platinum that arrives in the region of maximal damage.
Near-field photoluminescence of microcrystalline arsenic oxides produced in anodically processed gallium arsenide
Authors: Finnie Christine M. , Bohn Paul W.
Published: July 2002

Anodic processing of (100) GaAs in aqueous HCl results in the formation of a pitted surface hosting arsenic oxide microcrystals within a porous surface network. The composition of the microcrystalline features evolves from As(V) to As(III) with processing time. Spatially localized near-field photoluminescence (PL) spectroscopy of the microcrystalline and porous features demonstrates that the strong visible photoluminescence observed in the far field originates from the μm-sized crystalline features. The spatial localization of the PL on the arsenic oxide microcrystalline features argues that it does not arise from quantum confinement effects, but rather is due to luminescent features intrinsic to the arsenic oxide microcrystals on the pitted surface.
Studies of hydrogen-induced degradation processes in SrBi2Ta2O9 ferroelectric film-based capacitors
Authors: Im J. , Auciello O. , Krauss A. R. , Gruen D. M. , Chang R. P. H. , Kim S. H. , Kingon A. I.
Published: July 2002

It is known that the forming gas (N2–H2 mixture) annealing process required for microcircuit fabrication results in an unacceptable electrical degradation of SrBi2Ta2O9 (SBT) ferroelectric capacitors due mainly to the interaction of H2 with the ferroelectric layer of the capacitor. We have found a strong relationship between changes in the surface composition of the ferroelectric layer and the electrical properties of SBT capacitors as a result of hydrogen annealing. Mass spectroscopy of recoiled ions (MSRI) analysis revealed a strong reduction in the Bi signal as a function of exposure to hydrogen at high temperatures (∼500 °C). The Bi signal reduction correlates with Bi depletion in the SBT surface region. Subsequent annealing in oxygen at temperatures in the range of 700–800 °C resulted in the recovery of the MSRI Bi signal, corresponding to the replenishment of Bi in the previously Bi-depleted surface region. X-ray diffraction (XRD) analysis (probing the whole SBT film thickness) showed little difference in the XRD spectra of the SBT films before and after hydrogen and oxygen-recovery annealing. The combined results of the MSRI and XRD analyses can be interpreted as an indication that the degradation of the electrical properties of the SBT capacitors, after hydrogen annealing, is mainly due to the degradation of the near surface region of the SBT layer.
Growth and characterization of small band gap (∼0.6 eV) InGaAsN layers on InP
Authors: Gokhale Milind R. , Wei Jian , Wang Hongsheng , Forrest Stephen R.
Published: July 2002

We demonstrate the growth of small band gap (Eg∼0.6 eV) strained and lattice matched single crystal InGaAsN alloys on InP substrates. InGaAsN layers with N concentrations varying from 0.6% to 3.25% were grown by gas source molecular beam epitaxy using a radio frequency plasma nitrogen source. Lattice-matched, 0.5-μm-thick InGaAsN layers with smooth surface morphologies and abrupt interfaces were achieved. Low temperature photoluminescence measurements reveal a band gap emission wavelength of 1.9 μm (at 20 K) for lattice matched InGaAsN (N∼2%). Tensile strained In0.53Ga0.47As/In0.53Ga0.47As0.994N0.006 multiple quantum wells emitting at 1.75 μm at 20 K are also reported.
Time-domain dielectric constant measurement of thin film in GHz–THz frequency range near the Brewster angle
Authors: Li M. , Cho G. C. , Lu T.-M. , Zhang X.-C. , Wang S.-Q. , Kennedy J. T.
Published: July 2002

We present a free-space time-domain method to measure the dielectric property of thin film on substrate in the GHz–THz frequency range. The concept is based on the phase flip of the field wave form for near-Brewster angle reflection. Realizing this concept, we demonstrate the determination of the dielectric constant of a thin polymer film at a few micrometer thickness on the silicon wafer.
Low-field magnetoresistance of La0.7Sr0.3MnO3 thin films with gradually changed texture
Authors: Walter T. , Dörr K. , Müller K.-H. , Holzapfel B. , Eckert D. , Wolf M. , Schläfer D. , Schultz L. , Grötzschel R.
Published: July 2002

A gradual change of the grain orientation with the substrate temperature (TS) during deposition has been observed for La0.7Sr0.3MnO3 films on Y-stabilized ZrO2(100) substrates. Epitaxial growth has been found around TS≈800 °C. Both the low-field and the high-field magnetoresistance (MR) observed for the ferromagnetic state of these polycrystalline colossal MR manganite films were found to systematically depend on the degree of texture. The experimental results suggest a crucial role of a highly resistive region at the grain boundaries for this MR effect.
Critical built-in electric field for an optimum carrier collection in multiquantum well p-i-n diodes
Authors: Serdiukova I. , Monier C. , Vilela M. F. , Freundlich A.
Published: July 2002

The collection efficiency of carrier photogenerated in the intrinsic region of strained InAsxP1−x/InP multiquantum well p-i-n structures is analyzed. The existence of a critical threshold built-in electric field value above which total carrier collection becomes possible is demonstrated. Maximized carrier collection and high output voltage are systematically reached for built-in electric field exceeding the critical value while similar structures operating with a substantially lower built-in electric field (e.g., identical well characteristics but thicker i region) yields nonoptimized collection of carrier in this area and altered voltage output. The slight dependence of the critical electric field with the carrier confinement level is revealed, stressing out the importance of thermally activated escape energy. Finally, the results are discussed in the context of photovoltaic devices showing substantial efficiency improvement for devices designed with built-in electric fields in excess of the threshold value.
In situ x-ray diffraction study of an electric field induced phase transition in the single crystal relaxor ferroelectric, 92% Pb(Zn1/3Nb2/3)O3–8% PbTiO3
Authors: Durbin M. K. , Jacobs E. W. , Hicks J. C. , Park S.-E.
Published: July 2002

We have used in situ x-ray diffraction to measure strain curves of single crystals of 92% Pb(Zn1/3Nb2/3)O3–8% PbTiO3 poled in the 〈001〉 direction. We observed shifts in the (002) and (330) peaks as functions of applied voltage (in the 〈001〉 direction), thereby measuring the crystallographic strain. Our measurements confirm that the high strains observed using macroscopic methods reflect microscopic strains in the crystal lattice. By watching the disappearance of a rhombohedral splitting in the (330) peak, we also find direct evidence that the field-induced phase transition reported by S.-E. Park and T. R. Shrout [J. Appl. Phys. 82, 1804 (1997)] occurs between a pseudorhombohedral and tetragonal crystal structure.
Fabrication of 0.1 μm gate aperture Mo-tip field-emitter arrays using interferometric lithography
Authors: Choi J. O. , Jeong H. S. , Pflug D. G. , Akinwande A. I. , Smith H. I.
Published: July 2002

A fabrication process has been developed which enables us to make matrix-addressable Mo-tip field-emitter arrays (FEAs) with 0.1 μm gate aperture and 0.2 μm tip-to-tip distance. An interferometric lithography combined with a trilevel resist process which uses an imaging resist layer, a silicon oxide interlayer, and antireflective coating has been implemented to fabricate the periodic structure of the gated FEAs in an addressable matrix. The matrix-addressable FEAs have shown a turn-on voltage as low as 13 V and an emission current density of 17 mA/cm2 at a gate voltage of 30 V.
Improved operational stability of polyfluorene-based organic light-emitting diodes with plasma-treated indium–tin–oxide anodes
Authors: Kim J. S. , Friend R. H. , Cacialli F.
Published: July 2002

We report the influence of various surface treatments of indium–tin–oxide (ITO) anodes on the operational stability of high-efficiency (up to 8.2 lm/W) green-emitting polymer light-emitting diodes (PLEDs), employing a doped poly(3,4-ethylene dioxythiophene), hole transport layer, a polyfluorene based emissive layer, and Ca–Al cathodes. The anodes were modified by physical (oxygen-plasma), chemical (aquaregia), and combined treatments. Oxygen plasma improves the stability under constant current with respect to all other anodes, with half-brightness (100 cd/m2) lifetimes two to five times longer than for untreated samples, and 1000 times longer than for aquaregia samples. We derive two major indications for optimization of PLEDs. First, thermal management of the diode is of the uppermost importance and there is significant scope for improvement. Second, the ITO anode and in general the electrical properties of the hole-injecting contact are crucial to device operation, even in the presence of a hole transport layer.
Interface engineering in preparation of organic surface-emitting diodes
Authors: Hung L. S. , Tang C. W.
Published: July 2002

Surface-emitting organic light emitting diode (OLED) was prepared by sputter deposition of indium-tin-oxide on a buffered organic layer structure. Confirming a previous report, a thin film of copper phthalocyanine (CuPc) was found to be a useful buffer layer in preventing sputter damage to the OLED layer structure, particularly the underlying Alq emissive layer. However, the CuPc layer forms an electron-injection barrier with the Alq layer, resulting in increased electron-hole recombination in the nonemissive CuPc layer, and thus a substantial reduction in electroluminescence efficiency. Incorporation of Li at the CuPc/Alq interface was found to reduce the injection barrier at the interface and recover the overall device efficiency with good surface emission characteristics.
Soft x-ray measurements of z-pinch-driven vacuum hohlraums
Authors: Baker K. L. , Porter J. L. , Ruggles L. E. , Chandler G. A. , Deeney Chris , Vargas M. , Moats Ann , Struve Ken , Torres J. , McGurn J. , Simpson W. W. , Fehl D. L. , Chrien R. E. , Matuska W. , Idzorek G. C.
Published: July 2002

This letter reports the experimental characterization of a z-pinch-driven vacuum hohlraum. We have measured soft x-ray fluxes of 5×1012 W/cm2 radiating from the walls of hohlraums which are 2.4–2.5 cm in diameter by 1 cm tall. The x-ray source used to drive these hohlraums was a z pinch consisting of a 300 wire tungsten array driven by a 20 MA, 100 ns current pulse. In this hohlraum geometry, the z-pinch x-ray source can produce energies in excess of 800 kJ and powers in excess of 100 TW to drive these hohlraums. The x rays released in these hohlraums represent greater than a factor of 25 in energy and more than a factor of 3 in x-ray power over previous laboratory-driven hohlraums.
Potentiometric imaging of (La0.7Sr0.3)MnO3 thin films
Authors: Versluijs J. J. , Ott F. , Coey J. M. D.
Published: July 2002

Scanning tunneling potentiometry has been used to simultaneously image the surface topography and map the potential distribution on films of (La0.7Sr0.3)MnO3 deposited on polycrystalline and single crystal MgO substrates. Potential steps in the polycrystalline films coincide with the crystallite boundaries in the film. The grain boundary resistivity varies in the range 3×10−7–3×10−5 Ω cm2, with a typical value of 6×10−6 Ω cm2.
Epitaxial switchable yttrium-hydride mirrors
Authors: Nagengast D. G. , Kerssemakers J. W. J. , van Gogh A. T. M. , Dam B. , Griessen R.
Published: July 2002

By means of x-ray scattering and scanning probe microscopy it is shown that high-quality epitaxial Y films can be deposited on (111)-CaF2 substrates. The films can reversibly be switched from metallic YH2 to transparent insulating YH3−δ. Although hydrogen absorption involves an expansion of the lattice and a symmetry change from hcp to fcc, the epitaxiality of the film remains intact during the switching process. The transparency and the insulating nature of the substrate opens unique possibilities to investigate electrically and optically these switchable mirror films in the single crystalline state.
Large-scale synthesis of single crystalline gallium nitride nanowires
Authors: Cheng G. S. , Zhang L. D. , Zhu Y. , Fei G. T. , Li L. , Mo C. M. , Mao Y. Q.
Published: July 2002

Large-scale synthesis of single crystalline GaN nanowires in anodic alumina membrane was achieved through a gas reaction of Ga2O vapor with a constant flowing ammonia atmosphere at 1273 K. X-ray diffraction, Raman backscattering spectroscopy, scanning electron microscopy, and transmission electron microscopy indicated that those GaN nanowires with hexagonal wurtzite structure were about 14 nm in diameter and up to several hundreds of micrometers in length. The growth mechanism of the single crystalline GaN nanowires is discussed.
Operation of quantum cellular automaton cells with more than two electrons
Authors: Girlanda M. , Governale M. , Macucci M. , Iannaccone G.
Published: July 2002

We present evidence that operation of quantum cellular automaton (QCA) cells with four dots is possible with an occupancy of 4N+2 electrons per cell (N being an integer). We show that interaction between cells can be described in terms of a revised formula for cell polarization, which is based only on the difference between diagonal occupancies. We validate our conjectures with full quantum simulations of QCA cells for a number of electrons varying from 2 to 6, using the configuration–interaction method.
The magnetism of a buried La0.7Sr0.3MnO3 interface
Authors: Stadler S. , Idzerda Y. U. , Chen Z. , Ogale S. B. , Venkatesan T.
Published: July 2002

Using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) techniques, we have investigated the electronic structure and magnetic properties of La0.7Sr0.3MnO3 (LSMO) as a function of YBa2Cu3O7−δ (YBCO) cap layer thickness. The Mn L2,3 XAS and XMCD data clearly show that the magnetic properties and electronic structure of the LSMO are adversely affected by the YBCO overlayer caused by cation displacement/exchange that effectively reduces the concentration of the La atoms in the LSMO near the YBCO/LSMO interface.
Mg-doped GaN: Similar defects in bulk crystals and layers grown on Al2O3 by metal–organic chemical-vapor deposition
Authors: Liliental-Weber Z. , Benamara M. , Swider W. , Washburn J. , Grzegory I. , Porowski S. , Lambert D. J. H. , Eiting C. J. , Dupuis R. D.
Published: July 2002

Defects were observed in GaN:Mg grown on sapphire substrates using metal–organic chemical-vapor deposition (MOCVD) with Mg-delta doping similar to those previously observed in bulk GaN:Mg grown from Ga solution under high hydrostatic pressure of nitrogen. Pyramidal defects (pinholes) with (11̄00) hexagonal facets on the (0001) base plane and six {112̄2} side facets, and defects with a rectangular shape also delineated by planar facets on the basal (0001) planes, were observed for growth with Ga polarity for both of these very different growth methods. The Mg dopant is apparently responsible for their formation since the oxygen concentration in the MOCVD-grown samples was orders of magnitude lower than in the bulk samples. Mg segregation on these planes apparently does not allow uniform continuous growth on these planes leading to these hollow defects. Some defects in the heterolayers also develop into longer nanotubes elongated along the c axis. Change of polarity from Ga to N followed by a change back to Ga polarity also resulted in formation of planar defects previously observed in bulk samples for growth with N polarity.
Room-temperature imprint lithography by solvent vapor treatment
Authors: Khang Dahl-Young , Lee Hong H.
Published: July 2002

We demonstrate room-temperature nanoimprint lithography using solvent vapor treatment of the polymer film on a substrate. In this method, the film treated with the solvent vapor is pressed with a mold at room temperature, requiring no heating that has been needed for the lithography. We show that the mold or mask patterns down to 60 nm can well be transferred onto the polymer film without any problem of the polymer adhering to the mold. The vapor treatment of the dried polymer film results in lowering both the viscosity and the glass transition temperature, thereby allowing for the room-temperature lithography. This room-temperature imprinting should make the imprint lithography more valuable for practical applications.
Recessed gate AlGaN/GaN modulation-doped field-effect transistors on sapphire
Authors: Egawa T. , Ishikawa H. , Umeno M. , Jimbo T.
Published: July 2002

A recessed gate AlGaN/GaN modulation-doped field-effect transistor (MODFET) has been grown on a sapphire substrate by metalorganic chemical vapor deposition. The two-dimensional electron gas mobility as high as 9260 cm2/V s with the sheet carrier density 4.8×1012 cm−2 was measured at 4.6 K for the AlGaN/GaN heterostructure on the sapphire substrate. The recessed gate device showed the maximum extrinsic transconductance 146 mS/mm and drain-source current 900 mA/mm for the AlGaN/GaN MODFET with a gate length 2.1 μm at 25 °C. At an elevated temperature of 350 °C, the maximum extrinsic transconductance and drain-source current were 62 mS/mm and 347 mA/mm, respectively.
High-power quantum-dot lasers at 1100 nm
Authors: Heinrichsdorff F. , Ribbat Ch. , Grundmann M. , Bimberg D.
Published: July 2002

High-power semiconductor laser diodes based on multiple InGaAs/GaAs quantum-dot layers grown by metal–organic chemical-vapor deposition are demonstrated. The devices exhibit a peak power of 3 W (4.5 W) at 1100 nm (1068 nm), respectively, during pulsed operation at room temperature and show slope efficiencies of 57% (66%).
Effect of silicon doping on the optical and transport properties of InGaN/GaN multiple-quantum-well structures
Authors: Wang T. , Saeki H. , Bai J. , Shirahama T. , Lachab M. , Sakai S. , Eliseev P.
Published: July 2002

Temperature-dependent photoluminescence and transport measurements were performed on the In0.13Ga0.87N:Si/GaN:Si multiple-quantum-well (MQW) structures with different doping levels. By fitting the temperature-dependent emission energy of these samples using the band tail model, an obvious localization effect is observed in lightly doped MQW structures. Correspondingly, the electron mobilities in these structures are significantly higher than those of undoped and heavily doped MQW structures. Furthermore, when the localization effect is stronger, the mobility is higher.
Evidence of an oxygen recombination center in p+–n GaInNAs solar cells
Authors: Balcioglu A. , Ahrenkiel R. K. , Friedman D. J.
Published: July 2002

We have studied deep-level impurities in p+–n GaInNAs solar cells using deep-level transient spectroscopy (DLTS). These films were grown by atmospheric- and low-pressure metalorganic vapor-phase epitaxy. The base layer is doped with silicon, and the emitter layer is zinc doped. Two types of samples have been studied: samples were grown with and without the addition of oxygen impurity. Two electron traps were found in all samples. These are designated as: E1, at EC−0.23–EC−0.27 eV, E2 at EC−0.45 eV, and E2* at 0.77 eV. With the addition of oxygen impurity, DLTS showed additional traps designated as E3 (electron) at EC−0.59 eV and H3 (hole) at EV+0.59 eV. Using secondary ion mass spectroscopy, the oxygen concentration was found to be about 2–3×1019 and 1×1017 cm−3 in two sets of samples. However, only samples containing oxygen contained the two near-midgap levels (E3 and H3). We present evidence that these levels are associated with the oxygen defect. As we change the dc bias voltage, the E3 trap disappears in unison with the appearance of the H3 trap. Furthermore, E3 and H3 trap levels have comparable capture cross sections. This oxygen-related trap is an effective recombination center. The measured Shockley–Hall–Read lifetime for this center is about 0.6 μs.
Evidence for valence fluctuation of Fe in Sr2FeMoO6−w double perovskite
Authors: Lindén J. , Yamamoto T. , Karppinen M. , Yamauchi H. , Pietari T.
Published: July 2002

In this letter evidence for the formation of a valence-fluctuation state of iron, formally denoted as Fe2.5+, is presented. The system under study is the Sr2FeMoO6−w double perovskite, known for exhibiting a very large magnetoresistance. Samples of Sr2FeMoO6−w were synthesized by means of an encapsulation technique utilizing an Fe getter technique and characterized by Fe57 Mössbauer spectroscopy. From 5 K to room temperature the Mössbauer spectrum is dominated by a component with hyperfine parameter values between those expected for high-spin Fe3+ and high-spin Fe2+.
The He–OCS van der Waals potential from model calculations: Bound states, stable structures, and vibrational couplings
Authors: Gianturco F. A. , Paesani F.
Published: July 2002

A large number of relative distances and orientations between the linear OCS molecule and a helium atom are computed using density functional theory at the post-Hartree–Fock level to treat the repulsive and the short-range well regions, while the dispersion interaction and the attractive parts are included via perturbative corrections. The bound states of the van der Waals (vdW) complex are obtained and the geometries of the most stable structures discussed. The interaction dependence on the intermolecular coordinates is also computed and the corresponding vibrationally adiabatic and nonadiabatic terms obtained and analyzed for the ν3 stretching motion of the nuclei.
Superparamagnetic magnetization equation in two dimensions
Authors: Jiles D. C. , Lee S. J. , Kenkel J. , Metlov K. L.
Published: July 2002

An equation for the dependence of magnetization on magnetic field in the case of two-dimensional (base plane) anisotropy has been derived. The resulting equation is expressed as an infinite series of modified Bessel functions, unlike the elementary function expressions that are applicable to the one-dimensional (axially anisotropic) and three-dimensional (isotropic) cases. Nevertheless, in the low-field limit, the series can be effectively truncated to give an approximate solution, while, in the high-field limit, an alternative expression has been derived which represents the limiting function as the field strength tends to infinity. The resulting expressions can be used to describe the superparamagnetic magnetization and susceptibility as a function of magnetic field in situations where the magnetic moments are constrained to lie in a plane, with no preferred direction within the plane. This can therefore be applied to two-dimensional structures, such as magnetic thin films, where magnetostatic energy confines the moments to the plane of the film, or to three-dimensional structures with planar magnetocrystalline anisotropy.
Continuous, dense, highly collimated sodium beam
Authors: DeGraffenreid W. , Ramirez-Serrano J. , Liu Y.-M. , Weiner J.
Published: July 2002

We have developed a slow, highly collimated, and bright sodium atom beam suitable for orientation and alignment studies in cold collisions. A combination of transverse-optical collimation, longitudinal cooling, rapid decoupling from the longitudinal cooling cycle, and a final “optical-force extrusion” stage produces an atom density of 1×1010 cm−3 within a beam-divergence solid angle of 2×10−6 sr. Rapid Zeeman-cooler decoupling results in a narrow laboratory velocity distribution of 5 m/s full width at half maximum and a cold binary intrabeam collision temperature of 4 mK.
Coherence properties of a smoothed laser beam in a hot plasma
Authors: Riazuelo Gilles , Bonnaud Guy
Published: July 2002

The nonlinear paraxial propagation of a laser beam inside a hot plasma has been simulated by three-dimensional simulations, where the laser beam is smoothed by spectral dispersion and the plasma response modeled by ion acoustic waves. The typical coherence times and transverse correlation sizes of the beam pattern are provided as a function of the propagation distance, the incident irradiance, the initial laser bandwidth, and the plasma density damping.
Very long wavelength infrared type-II detectors operating at 80 K
Authors: Mohseni H. , Tahraoui A. , Wojkowski J. , Razeghi M. , Brown G. J. , Mitchel W. C. , Park Y. S.
Published: July 2002

We report a demonstration of very long wavelength infrared detectors based on InAs/GaSb superlattices operating at T=80 K. Detector structures with excellent material quality were grown on an optimized GaSb buffer layer on GaAs semi-insulating substrates. Photoconductive devices with 50% cutoff wavelength of λc=17 μm showed a peak responsivity of about 100 mA/W at T=80 K. Devices with 50% cutoff wavelengths up to λc=22 μm were demonstrated at this temperature. Good uniformity was obtained over large areas even for the devices with very long cutoff wavelengths.
Coherent population transfer in coupled semiconductor quantum dots
Authors: Hohenester Ulrich , Troiani Filippo , Molinari Elisa , Panzarini Giovanna , Macchiavello Chiara
Published: July 2002

We propose a solid-state implementation of stimulated Raman adiabatic passage in two coupled semiconductor quantum dots. Proper combination of two pulsed laser fields allows the coherent carrier transfer between the two nanostructures without suffering significant losses due to environment coupling. By use of a general solution scheme for the carrier states in the double-dot structure, we identify the pertinent dot and laser parameters.
Atomic-scale study of GaMnAs/GaAs layers
Authors: Grandidier B. , Nys J. P. , Delerue C. , Stiévenard D. , Higo Y. , Tanaka M.
Published: July 2002

Cross-sectional scanning tunneling microscopy was used to study GaMnAs diluted magnetic semiconductors grown by low temperature molecular beam epitaxy. The Ga1−xMnxAs layer, containing a concentration of x=0.005, shows that the dominant defect in the material is the arsenic antisite. Mn ions can also be resolved and show a signature distinct from the arsenic antisites. Spectroscopic measurements are perfomed to study the variation of the Fermi level between the Ga0.995Mn0.005As and GaAs layers. The Mn ions act as acceptor dopants. However, for x=0.005, the Mn concentration in comparison with the As antisite concentration is too small to induce a significant change of the Fermi level from the midgap position, preventing the layer from being ferromagnetic.
Effects of high-temperature annealing on the dielectric function of Ta2O5 films observed by spectroscopic ellipsometry
Authors: Nguyen N. V. , Richter C. A. , Cho Yong Jai , Alers G. B. , Stirling L. A.
Published: July 2002

Postdeposition annealing of high-k dielectric Ta2O5 films to eliminate contaminations can adversely cause the films to crystallize, which can be detrimental to their complementary metal–oxide–semiconductor device performances. In this letter, we will show that spectroscopic ellipsometry can be used to quickly and nondestructively detect such crystallization by identifying the two relatively sharp absorption peaks at 4.7 and 5.2 eV in the complex dielectric function of the films. Such peaks are absent in amorphous Ta2O5 films. In general, these sharp structures in the dielectric function are expected from the presence of long-range order in materials, which produces singularities in their interband density of states. Using this approach, we will show that Ta2O5 films become crystalline when annealed at or above 750 °C and remain amorphous below 700 °C.
Analyses of electroluminescence spectra of silicon junctions in avalanche breakdown using an indirect interband recombination model
Authors: Lahbabi M. , Ahaitouf A. , Abarkan E. , Fliyou M. , Hoffmann A. , Charles J.-P. , Bhuva Bharat L. , Kerns S. E. , Kerns D. V.
Published: July 2002

Light emission from a p-n junction biased in avalanche breakdown has been modeled over the range 1.4–3.4 eV. The model emphasizes indirect interband processes and Si self-absorption. Comparisons between measured and simulated spectra for sample junctions from multiple devices demonstrate that the model is simple, accurate, and consistent with fundamental physical device characteristics.
Long-range ordered lines of self-assembled Ge islands on a flat Si (001) surface
Authors: Schmidt O. G. , Jin-Phillipp N. Y. , Lange C. , Denker U. , Eberl K. , Schreiner R. , Gräbeldinger H. , Schweizer H.
Published: July 2002

Self-assembled growth in combination with prepatterning yields ordered lines of Ge islands on a planar Si (001) surface. The self-assembled Ge nanostructures are grown on top of a 15-period Si/SiGe superlattice, which is deposited on a prepatterned Si substrate. The pattern consists of 10 nm deep trenches with a period of 250 nm. The superlattice translates the surface modulation of the substrate into a strain-field modulation at the growth front of the superlattice. This strain field modulation provides the template for the ordered nucleation of self-assembled Ge islands. Our method gives rise to the long-range ordering of perfectly passivated nanostructures and can in principle be applied to any other strained material system.
Comment on “Critical points and reaction paths characterization on a potential energy hypersurface” [J. Chem. Phys. 112, 4923 (2000)]
Authors: Quapp Wolfgang
Published: July 2002

The pathway of steepest descent on a potential-energy surface is independent of the choice of the coordinate system. The same can be shown from the zero eigenvalue of the Hessian matrix. Thus, the valley-ridge-inflection points of the potential-energy surface does not move under a coordinate transformation.
Theoretical and experimental analysis on photothermal gratings in guest-host liquid crystal film
Authors: Ono Hiroshi , Saito Isao
Published: July 2002

Holographic grating originating from photothermal effects in guest-host liquid crystals was characterized using heat-conduction analysis and Fraunhofer’s diffraction theory. The refractive index distributions, which are caused by interference light illumination and photothermal optical nonlinearity, were determined by solving the heat-conduction equation numerically. The diffraction efficiencies, including the dependence of the sample temperature and the grating spacing, were determined from the diffraction theory, and those values showed good quantitative agreement with the experimental values.
Vertical correlation of SiGe islands in SiGe/Si superlattices: X-ray diffraction versus transmission electron microscopy
Authors: Stangl J. , Roch T. , Bauer G. , Kegel I. , Metzger T. H. , Schmidt O. G. , Eberl K. , Kienzle O. , Ernst F.
Published: July 2002

We have investigated the vertical correlation properties of SiGe islands in a series of Si/SiGe multilayers using grazing incidence x-ray diffraction. The degree of island correlation is found to strongly depend on the thickness of the Si spacer layer separating subsequent SiGe layers. A comparison with results obtained from transmission electron microscopy demonstrates the feasibility of the x-ray diffraction method for the investigation of sample series, and an improved statistical accuracy of the obtained parameters: with x-ray diffraction, the statistical average of typically 106–107 island columns is obtained, compared to only few in the case of transmission electron microscopy.
Preferential amorphization and defect annihilation at nanocavities in silicon during ion irradiation
Authors: Williams J. S. , Zhu Xianfang , Ridgway M. C. , Conway M. J. , Williams B. C. , Fortuna F. , Ruault M.-O. , Bernas H.
Published: July 2002

Si containing a band of nanocavities has been irradiated with Si+ ions at elevated temperatures to study interactions of irradiation-induced defects with open volume defects. For irradiation at 100 °C, nanocavities are shown to be preferential nucleation sites for amorphization. It is proposed that this behavior occurs to minimize the local free energy, whereby less dense amorphous Si is free to expand into the cavity open volume. Furthermore, for irradiation at 300 °C, cavities are very efficient sinks for Si interstitials during irradiation, leaving a region denuded of interstitial-based clusters surrounding each nanocavity.
Pressure-resisting cell for high-pressure, high-resolution nuclear magnetic resonance measurements at very high magnetic fields
Authors: Yamada H. , Nishikawa K. , Honda M. , Shimura T. , Akasaka K. , Tabayashi K.
Published: July 2002

A pressure-resisting cell system has been developed for high-pressure high-resolution nuclear magnetic resonance (NMR) measurements up to a maximum pressure of 600 MPa. This cell system is capable of performing high-pressure experiments with any standard spectrometer, including modern high field NMR machines. A full description of the high-pressure NMR assembly mounted on a 750 MHz spectrometer is presented along with a detailed explanation of the procedure for preparing the pressure-resisting quartz and glass cells.
Quantitative analysis of scanning microwave microscopy on dielectric thin film by finite element calculation
Authors: Lee J. H. , Hyun S. , Char K.
Published: July 2002

Using numerical calculations based on a finite element method, we present a quantitative analysis of the dielectric constant of thin films on thick substrates in order to fit the experimental data measured by a scanning microwave microscope. The shift of the resonance frequency of the probe was calculated for the case of dielectric thin films on LaAlO3 (ε=24) and MgO (ε=10) substrates and, at the same time, the fitting functions were derived as a function of the tip–sample distance. The experimental data were found to agree well with our fitting functions, rendering our quantitative analysis reasonable. We have also discussed the effect of the anisotropy of dielectric constants.
Effect of residual stress on the Raman-spectrum analysis of tetrahedral amorphous carbon films
Authors: Shin Jin-Koog , Lee Churl Seung , Lee Kwang-Ryeol , Eun Kwang Yong
Published: July 2002

Tetrahedral amorphous carbon (ta-C) films deposited by the filtered vacuum arc process have large compressive residual growth stresses that depend on the atomic-bond structure. We observed that the G peak of the Raman spectrum shifts to higher frequency by 4.1±0.5 cm−1/GPa due to the residual compressive stress. This value agrees well with the calculated Raman-peak shift of the graphite plane due to applied stress. By considering the effect of residual stress on the G-peak position, we also observe a similar dependence between the G-peak position and the atomic-bond structure in both ta-C and hydrogenated amorphous carbon (a-C:H) films; namely, that a higher sp2 bond content shifts the G-peak position to higher frequency.
Structure of defects in silicon oxynitride films
Authors: Hasegawa S. , Sakamori S. , Futatsudera M. , Inokuma T. , Kurata Y.
Published: July 2002

Amorphous silicon oxynitride (a-SiNxOy) films were deposited at 300 °C using a plasma-enhanced chemical vapor deposition technique, which was carried out with variation in the nitrogen-flow-rate ratio (RN) and the oxygen-flow-rate ratio (RO) to control the nitrogen and oxygen contents, x and y, respectively. Changes in the structural properties and the characteristics of defects in the films were examined based on the electron spin resonance as well as infrared and optical absorption measurements. An increase in either RN or RO was found to decrease the deposition rate, the density of charged or neutral defects, and the randomness of the bonding network. Thus the incorporation of O or N atoms into Si nitride or Si oxide films, respectively, acts to improve the qualities of the insulating films. Furthermore, the nearest N and O atom neighboring to a Si site would be randomly bonded to the Si atom. Our results also showed that N-related dangling bonds, other than so-called K centers, are favorably created after annealing, but that the formation of Si–O bonds in the films makes the formation of such defects difficult.
Epitaxial growth and optical characterization of InAs/InGaAsP/InP self-assembled quantum dots
Authors: Jeong Weon G. , Dapkus P. Daniel , Lee U. H. , Yim J. S. , Lee D. , Lee B. T.
Published: July 2002

Five stacks of InAs quantum dots (QDs) with InGaAsP barriers were grown on (100) InP and luminescence characteristics were analyzed. Cross-sectional transmission electron microscopy shows that small dots with a lateral size of ∼30 nm and a height of ∼3 nm are formed with an areal density of ∼5×1010 cm−2. The QDs emit strong photoluminescence (PL) peaks in the range of 1.4–1.6 μm that can be controlled by nominal InAs thickness. The integrated PL intensity from QDs stays very high at room temperature as much as 20% of that at 10 K. At weak excitation, the carrier lifetimes are measured to be almost the same across the whole PL band at low temperature with a value of ∼4 ns and they remain at that value at room temperature. These characteristics strongly evidence that individual QDs are well isolated and have a strong carrier confinement at room temperature.
Plasma induced smoothing of a spatially incoherent laser beam and reduction of backward stimulated Brillouin scattering
Authors: Maximov A. V. , Ourdev I. G. , Pesme D. , Rozmus W. , Tikhonchuk V. T. , Capjack C. E.
Published: July 2002

The angular divergence and spectral bandwidth of a spatially incoherent laser beam propagating in an underdense plasma are shown to correspond to additional spatial and temporal incoherence in the regime where the average laser intensity in a speckle approaches the self-focusing threshold. The transverse and longitudinal sizes of laser speckles inside the plasma are related in a way that makes it possible to define a local effective beam f-number. The effective f-number decreases as light propagates through the plasma, and the effective f-number at the outgoing boundary is consistent with the transmitted light angular spreading. The spectral broadening and red shift of the transmitted light are interpreted as a result of multiple near-forward stimulated Brillouin scattering of the propagating laser beam. The reduction of backward stimulated Brillouin scattering follows from decrease of the effective speckle length and from the plasma induced temporal incoherence.
Highly sensitive and tunable detection of far-infrared radiation by quantum Hall devices
Authors: Kawano Y. , Hisanaga Y. , Takenouchi H. , Komiyama S.
Published: July 2002

We studied far-infrared (FIR) response due to cyclotron resonance (CR) of two-dimensional electron gas systems in GaAs/AlGaAs heterostructures by using cyclotron radiation from n-InSb devices as the illumination source. We examined the dependence of the FIR response on different experimental parameters, including the aspect ratio of Hall bars, electron mobility, bias current, illumination intensity, magnetic field, and lattice temperature. A strong photoresponse emerges only in the vicinity of integer quantum Hall effect (IQHE) regimes. Time-resolved measurements show that the recombination lifetime of excited carriers depends largely on the electron mobility, ranging from 5 μs to 1 ms at 4.2 K. The temporal decay of photoresponse is nonexponential in higher-mobility samples, whereas it is exponential with a single time constant in lower-mobility samples. This, together with the relatively large time constants, suggests that the FIR response is induced through multitrapping processes, in which excited carriers in Landau levels are repeatedly captured by localized states and reexcited to delocalized states. This multitrapping process is suggested to be promoted by the CR-induced heating of the electron system. Theoretical calculation based on the electron heating model reasonably reproduces characteristic dependence of the photoresponse on the magnetic field in the vicinity of IQHE plateaus. The IQHE Hall bars serve as a high-sensitive narrow-band FIR detector, where the highest sensitivity reaches ∼108 V/W. Tunability of the detector is demonstrated by varying the electron density. We discuss briefly the design of high-sensitive FIR detectors using the IQHE Hall bars.
Experimental demonstration of a latch in clocked quantum-dot cellular automata
Authors: Orlov Alexei O. , Kummamuru Ravi K. , Ramasubramaniam Rajagopal , Toth Geza , Lent Craig S. , Bernstein Gary H. , Snider Gregory L.
Published: July 2002

We present an experimental demonstration of a latch in a clocked quantum-dot cellular automata (QCA) device. The device consists of three floating micron-size metal dots, connected in series by multiple tunnel junctions and controlled by capacitively coupled gates. The middle dot acts as an adjustable barrier to control single-electron tunneling between end dots. The position of a switching electron in the half cell is detected by a single-electron electrometer. We demonstrate “latching” of a single electron in the end dots controlled by the gate connected to the middle dot. This ability to lock an electron in a controllable way enables pipelining, power gain and reduced power dissipation in QCA arrays.
Normal incidence InAs/AlxGa1−xAs quantum dot infrared photodetectors with undoped active region
Authors: Chen Zhonghui , Baklenov O. , Kim E. T. , Mukhametzhanov I. , Tie J. , Madhukar A. , Ye Z. , Campbell J. C.
Published: July 2002

We have performed a comprehensive investigation of n-type quantum dot infrared photodetectors (QDIPs) based on InAs/GaAs epitaxical island quantum dots (QDs) grown via the innovative punctuated island growth technique. The structural properties of the QDs were investigated with cross-sectional transmission electron microscopy and atomic force microscopy. The electronic properties of the QDs inserted in QDIP devices were investigated with photoluminescence (PL), PL excitation, and intra- and inter-band photocurrent spectroscopy. The influence of AlGaAs layers inserted into the QDIP active regions on the performance of dark current and inter- and intra-band photocurrent was examined. Initial results on intra-band responsivity and detectivity of these QDIPs at 77 K with undoped active region show promise for application.