bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of the CdS/CdTe interface using free-electron laser internal photoemission technique
Authors: Nishi Kazuhisa , Ohyama Hideaki , Suzuki Toshiji , Mitsuyu Tsuneo , Tomimasu Takio
Published: July 2002

The CdS/CdTe interface was investigated by the free-electron laser (FEL) internal photoemission technique. This technique is based on photocurrent spectroscopy utilizing the tunability and intense peak power of the FEL operative in the infrared range. We found two thresholds in the photocurrent spectrum, which can be identified as steplike band discontinuities. It is demonstrated that there is a mixed crystal layer of CdS1−xTex at the CdS/CdTe interface.
Improvement of ultrathin gate oxide and oxynitride integrity using fluorine implantation technique
Authors: Chowdhury Prasenjit , Chou Anthony I. , Kumar Kiran , Lin Chuan , Lee Jack C.
Published: July 2002

The effects of fluorine on ultrathin gate oxide and oxynitride (∼40 Å) have been studied. The incorporation of fluorine was done by fluorine ion implantation into polycrystalline silicon (polysilicon) gate followed by a high-temperature drive-in step. It has been found that the integrity of oxide has been improved with the incorporation of fluorine as demonstrated by the reduction of stress-induced leakage current and interface trap generation. Furthermore, unlike thicker dielectrics (>100 Å) for which the charge-to-breakdown (QBD) values decrease with increasing fluorine concentration, QBD’s remain the same as those of the control samples for the ultrathin thickness regime. The mechanism for oxide quality improvement by F will also be discussed in the letter.
Single charge carrier type sensing with a parallel strip pseudo-Frisch-grid CdZnTe semiconductor radiation detector
Authors: McGregor D. S. , He Z. , Seifert H. A. , Wehe D. K. , Rojeski R. A.
Published: July 2002

An improvement in gamma-ray energy resolution is demonstrated for a room-temperature operated CdZnTe semiconductor detector that incorporates a parallel strip pseudo-Frisch-grid detector design. The device construction allows primarily for the measurement of electron charge carriers and not hole charge carriers. The detector is a three-terminal device that requires signal output to only one preamplifier, and its simplistic design offers an alternative method to single charge carrier type sensing to that of coplanar semiconductor gamma-ray detector designs.
Discovery of cross-tie walls at saw-tooth magnetic domain boundaries in permalloy films
Authors: Lee Y. , Koymen A. R. , Haji-Sheikh M. J.
Published: July 2002

Scanning electron microscopy with polarization analysis was used to image the surface magnetic domain structure of permalloy films (350 Å Ni83Fe17) in ultrahigh vacuum. These thin-film materials are used in a variety of magnetoresistive sensing applications, which depend on the state of the domains present in the film. In the demagnetized state of the films, saw-tooth domain boundaries separating two large approximately head-on domains were observed. At high magnification, cross-tie walls were seen with a periodic vortex structure along the straight edges of the saw-tooth domain boundaries.
Laser field enhancement at the scanning tunneling microscope junction measured by optical rectification
Authors: Bragas A. V. , Landi S. M. , Martı́nez O. E.
Published: July 2002

In this letter we report the measurement of the field enhancement at the tip of a scanning tunneling microscope, by means of the detection of the optical rectification current. A field enhancement factor between 1000 and 2000 is obtained for highly oriented pyrolytic graphite and between 300 and 600 for gold. Field enhancement factors found are strongly dependent on the particular tip used. The magnitude of the emitted light at the field enhanced region, calculated from the measured optical voltage, could be easily detected by a simple photodiode.
High sensitivity spin-valve strain sensor
Authors: Mamin H. J. , Gurney B. A. , Wilhoit D. R. , Speriosu V. S.
Published: July 2002

A technique for detecting strain has been demonstrated based on a spin-valve sensor. The 400 Å thick sensor has been integrated onto an atomic force microscope cantilever. An applied strain caused by bending of the cantilever changes the orientation of the free-layer magnetization due to magnetostriction. This in turn results in a change in the electrical resistance because of the giant magnetoresistance effect. With the proper magnetic bias, a base-line strain sensitivity of 10−10/Hz1/2 has been achieved. The corresponding gauge factor of 150 is roughly 1.6× that of similar silicon piezoresistive cantilevers. In the future, one might be able to enhance the sensitivity by another factor of 3–5.
Molecular level alignment at organic semiconductor-metal interfaces
Authors: Hill I. G. , Rajagopal A. , Kahn A. , Hu Y.
Published: July 2002

In order to clarify the electronic structure of metal-molecular semiconductor contacts, we use photoemission spectroscopy to investigate the energetics of interfaces formed by vacuum deposition of four different molecular thin films on various metals. We find that the interface electron and hole barriers are not simply defined by the difference between the work functions of the metals and organic solids. The range of interface Fermi level positions is material dependent and dipole barriers are present at all these interfaces. The results demonstrate the breakdown of the vacuum level alignment rule at interfaces between these organic molecular solids and metals.
Irreversible Carnot cycle under per-unit-time efficiency optimization
Authors: Hernández A. Calvo , Roco J. M. M. , Velasco S. , Medina A.
Published: July 2002

A Carnot-like irreversible power cycle is analyzed under maximum per-unit-time efficiency conditions. The model includes finite-rate heat transfers between the working fluid and the external heat reservoirs, heat leak between heat reservoirs, and internal dissipations of the working fluid. We present the results for the optimum distribution of the external and internal thermal heat conductances and the optimum area allocation ratio in terms of a dimensionless price parameter, the engine temperature ratio, and the internal irreversibility factor. The calculated optimized efficiencies agree with observed values for real power plants.
AlGaN/GaN high electron mobility field effect transistors with low 1/f noise
Authors: Levinshtein M. E. , Rumyantsev S. L. , Gaska R. , Yang J. W. , Shur M. S.
Published: July 2002

Low-frequency noise in the frequency region of 20 Hz to 20 kHz is investigated in AlGaN/GaN high electron mobility transistors (HEMTs) grown on SiC substrates. The noise spectra have the form of the 1/f (flicker) noise. The measured Hooge parameter is as low as 0.0001. This value is comparable with Hooge parameter values for commercial GaAs field effect transistors and approximately two orders of magnitude smaller than Hooge parameter value measured for AlGaN/GaN heterostructures grown on sapphire. The level of noise depends on the gate leakage current; the noise is much higher in devices with a high gate leakage current. The small measured values of the Hooge parameter are related to a smaller leakage current and to a better material quality of the devices on SiC substrates and to a high electron sheet density. The low levels of the 1/f noise in the AlGaN/GaN HEMTs on SiC substrates make them suitable for applications in communication systems.
Effect of hydrogen on Pb(Zr,Ti)O3-based ferroelectric capacitors
Authors: Aggarwal S. , Perusse S. R. , Tipton C. W. , Ramesh R. , Drew H. D. , Venkatesan T. , Romero D. B. , Podobedov V. B. , Weber A.
Published: July 2002

The properties of ferroelectric films are known to degrade when subjected to hydrogen in forming gas anneals. Earlier studies have attributed this degradation to the loss of oxygen from these films during these anneals. In this study, we show that though oxygen is lost during forming gas annealing, hydrogen incorporation is the primary mechanism for the degradation of ferroelectric properties. Raman spectra obtained from the forming gas-annealed films show evidence of polar hydroxil [OH−] bonds in the films. The most probable site for hydrogen ions is discussed based on ionic radii, crystal structure, electrical properties, and Raman spectra. We propose that the hydrogen ion is bonded with one of the apical oxygen ions and prevents the Ti ion from switching. Pyroelectric measurements on forming gas-annealed capacitors confirm that the capacitors no longer possess spontaneous polarization.
Electrochemical fabrication of n-Si/Au Schottky junctions
Authors: Oskam G. , van Heerden D. , Searson P. C.
Published: July 2002

We report on the electrochemical deposition of gold films onto n-type silicon. Gold deposition occurs through progressive nucleation and diffusion limited growth. A high density of gold nuclei was obtained by using a short potential pulse to −1.6 V(Ag/AgCl), and subsequent growth was performed at about −1.1 V(Ag/AgCl) where the growth rate is kinetically limited. Transmission electron microscopy showed that high quality, continuous gold films were formed with an average grain size on the order of 50–70 nm. The electrical properties of the electrochemically deposited Si/Au Schottky junctions are comparable to junctions prepared by evaporation or sputtering techniques.
Blue-light laser source by sum-frequency mixing in Nd:YAl3(BO3)4
Authors: Jaque D. , Capmany J. , Molero F. , García Solé J.
Published: July 2002

Sum-frequency mixing in neodymium-doped YAl3(BO3)4 of its fundamental laser line at 1062 nm and the pump light from a tunable Ti:sapphire laser has been used to generate blue radiation from 435 to 465 nm in an end-pumped configuration. In nonoptimal preliminary experiments 22 μW at 458 nm was obtained out of 200 mW absorbed power at 807 nm. Nd:YAB, therefore, emerges as a promising laser material for diode-pumped solid-state lasers in the blue spectral region.
Enhanced hole injection in a bilayer vacuum-deposited organic light-emitting device using a p-type doped silicon anode
Authors: Zhou X. , He J. , Liao L. S. , Lu M. , Xiong Z. H. , Ding X. M. , Hou X. Y. , Tao F. G. , Zhou C. E. , Lee S. T.
Published: July 2002

We report the fabrication of a vacuum-deposited light-emitting device which emits light from its top surface through an Al cathode using p-type doped silicon as the anode material. Enhanced hole injection is clearly demonstrated from the p-Si anode as compared to the indium–tin–oxide (ITO) anode. The mechanisms of hole injection from both the p-Si and ITO anodes into the organic layer are investigated and a possible model based on anode surface band bending is proposed. During the operation of the organic light-emitting device, the surface band bending of the anode plays a very important role in modifying the interfacial barrier height between the anode and the organic layer.
Detection of copper contamination in silicon by surface photovoltage diffusion length measurements
Authors: Henley Worth B. , Ramappa Deepak A. , Jastrezbski Lubek
Published: July 2002

Surface photovoltage minority carrier lifetime/diffusion length analysis of copper contaminated silicon was performed. It was observed that copper and copper associated defects degrade minority carrier lifetime more in n-type than in p-type silicon. This finding is explained by analysis of copper related defect levels identified by other deep level transient spectroscopy studies. In copper contaminated p-type silicon, an optical or thermal activation procedure significantly degrades the diffusion length. A process similar to that of Fe–B in p-type silicon is proposed. The activation process dissociates the Cu–Cu pairs, a weak recombination center in p-type silicon, and the copper forms extended substitutional defects in silicon, which have much greater recombination activity. No recovery of diffusion length was observed following such an activation procedure. The difference in copper and iron diffusion length recovery properties after activation can be used to differentiate iron contamination from copper contamination.
Hysteresis variations of (Pb, La)(Zr, Ti)O3 capacitors baked in a hydrogen atmosphere
Authors: Tamura T. , Matsuura K. , Ashida H. , Kondo K. , Otani S.
Published: July 2002

Baking (Pb, La)(Zr, Ti)O3 capacitors in a hydrogen atmosphere causes a significant loss of remanent polarization even at 150 °C. The hysteresis variations depend on the polarization states during baking. The hysteresis loop showed voltage shifts when the capacitor was polarized before baking, whereas it became a cramped shape when the baking was carried out on a virgin capacitor. Although remanent polarization diminished in all cases, saturation polarization was not suppressed. The clamped hysteresis loop can be described as an average of two loops shifted to positive and negative voltages. The results indicate that the loss of remanent polarization is not due to the suppression of switching, but due to the shift of the hysteresis of each domain larger than the coercive voltage.
Continuous refinement of the microstructure during crystallization of supercooled Zr41Ti14Cu12Ni10Be23 melts
Authors: Schroers J. , Busch R. , Masuhr A. , Johnson W. L.
Published: July 2002

Isothermal crystallization studies are performed on Zr41Ti14Cu12Ni10Be23 melts between the liquidus and the glass transition temperature. The resulting time–temperature-transformation diagram is in good agreement with the data obtained from containerlessly processed samples. The various solidification products are examined by electron microprobe. The investigations reveal the morphology and typical length scale of the microstructure as well as the primarily solidified phases after crystallizing at different degrees of undercooling. The typical length scale decreases continuously with increasing supercooling over five orders of magnitude. The number density of nuclei during primary crystallization is estimated from the microstructure. The results are discussed within several models, such as steady-state nucleation, spinodal decomposition, and a nonpolymorphic nucleation model.
The effect of As passivation on the molecular beam epitaxial growth of high-quality single-domain CdTe(111)B on Si(111) substrates
Authors: Xin Y. , Rujirawat S. , Browning N. D. , Sporken R. , Sivananthan S. , Pennycook S. J. , Dhar N. K.
Published: July 2002

The effect of As passivation of Si(111) substrates on the subsequent molecular beam epitaxial growth of CdTe(111) is investigated through a detailed comparison of the microstructures of two types of films. The film grown on a substrate treated with a Te flux is found to exhibit a rough film-substrate interface and has very poor crystalline quality with a (111)A orientation. In contrast, a CdTe film grown under identical conditions except for the Si substrate treated with an As flux is observed to have an atomically abrupt film-substrate interface and a single-domain structure in the technologically more relevant (111)B orientation. A growth mechanism for the formation of these high-quality single-domain CdTe(111)B films is proposed.
Reduced oxygen diffusion through beryllium doped platinum electrodes
Authors: Stumpf R. , Liu C.-L. , Tracy C.
Published: July 2002

Using first principles electronic structure calculations we screen nine elements for their potential to retard oxygen diffusion through polycrystalline Pt (p-Pt) films. We determine that O diffuses preferentially as interstitial along Pt grain boundaries (GBs). The calculated barriers are compatible with experimental estimates. We find that Be controls O diffusion through p-Pt. Beryllium segregates to Pt GBs at interstitial (i) and substitutional (s) sites. i-Be is slightly less mobile than O and it repels O, thus stuffing the GB. s-Be has a high diffusion barrier and it forms strong bonds to O, trapping O in the GB. Experiments confirm our theoretical predictions.
Metal–organic atomic-layer deposition of titanium–silicon–nitride films
Authors: Min Jae-Sik , Park Hyung-Sang , Kang Sang-Won
Published: July 2002

Titanium–silicon–nitride films were grown by metal–organic atomic-layer deposition at 180 °C. When silane was supplied separately in the sequence of a tetrakis(dimethylamido) titanium pulse, silane pulse, and ammonia pulse, the Si content in the deposited films and the deposition thickness per cycle remained almost constant at 18 at. % and 0.22 nm/cycle, even though the silane partial pressure varied from 0.27 to 13.3 Pa. Especially, the Si content dependence is strikingly different from the conventional chemical-vapor deposition. The capacitance–voltage measurement revealed that the Ti–Si–N film prevents the diffusion of Cu up to 800 °C for 60 min. Step coverage was approximately 100% even on the 0.3 μm diam hole with slightly negative slope and 10:1 aspect ratio.
Raman scattering enhancement in porous silicon microcavity
Authors: Kuzik L. A. , Yakovlev V. A. , Mattei G.
Published: July 2002

We have measured an enhancement factor of Raman signal up to 30 times using a Fabry-Pérot structure made of porous silicon (PS) layers of different porosity. The obtained enhancement was due to the coupling of the laser radiation and Stokes photons of porous silicon with the microcavity mode at the optimal laser beam incidence and scattering angles. Our results provide a way to increase the sensitivity of Raman spectroscopy for studying the species inside porous silicon which can considerably influence the properties of this material and hence of PS based devices.
Low-frequency noise of improved direct-coupled high-Tc superconducting quantum interference device magnetometers in ac and dc magnetic fields
Authors: Ludwig F. , Drung D.
Published: July 2002

We have developed direct-coupled high-Tc superconducting quantum interference device (SQUID) magnetometers based on 30° bicrystal junctions with superior noise performance in dc and ac magnetic fields. These magnetometers are composed of a 100 pH SQUID loop with a linewidth of 4 μm and a pickup coil consisting of 16 parallel, 50-μm-wide loops. The magnetic field noise down to 1 Hz did not increase when the magnetometers were exposed to ac fields with peak-to-peak amplitudes of up to 54 μT or cooled in static magnetic fields above that of the Earth. The best noise at 1 Hz of such a device cooled in 64 μT was 65 fT Hz1/2. The increased noise below 1 Hz when the devices were cooled in a static magnetic field can be quantitatively described by the measured temperature fluctuations assuming a temperature coefficient of the pickup loop area of about 1×10−4/K.
Giant magnetic-field-induced strains in Heusler alloy NiMnGa with modified composition
Authors: Wu G. H. , Yu C. H. , Meng L. Q. , Chen J. L. , Yang F. M. , Qi S. R. , Zhan W. S. , Wang Z. , Zheng Y. F. , Zhao L. C.
Published: July 2002

A giant magnetic-field-induced strain (MFIS) of −3100 ppm has been obtained in Heusler alloy Ni52Mn22.2Ga25.8 single crystal in the [001] direction at a temperature from 23 to 31 °C. This MFIS reaches saturation in an applied field about 6 kOe, and exhibits the same amplitude with an opposite sign while the field is perpendicular to the samples. According to a previous model, this MFIS associates with the twin boundary motion. The martensitic self-strain has been found to be 2%, implying a preferential orientation of martensite variants. Results related to the magnetic properties are discussed.
Relationship between film thickness and the critical current of YBa2Cu3O7−δ-coated conductors
Authors: Foltyn S. R. , Jia Q. X. , Arendt P. N. , Kinder L. , Fan Y. , Smith J. F.
Published: July 2002

During the development of YBa2Cu3O7−δ (YBCO) coatings on flexible metal tapes, it has become evident that the achievable critical current (Ic) reaches a maximum value of about 200 A per cm of conductor width at a coating thickness of 1–2 μm. Additional YBCO beyond this thickness can actually reduce Ic. To investigate, critical current density (Jc) has been measured for samples with YBCO ranging from 0.39 to 6.3 μm in thickness. Several films were thinned by ion milling and remeasured with two significant results: almost no supercurrent is carried at thickness levels above 2 μm; and for films thicker than 3 μm, Jc is drastically reduced near the substrate as well.
Ortho-molecular hydrogen in hydrogenated amorphous silicon
Authors: Su Tining , Taylor P. C. , Chen Shenlin , Crandall R. S. , Mahan A. H.
Published: July 2002

Using a Jeener–Broekaert three-pulse sequence to measure directly the concentration of o-H2 by H1 nuclear magnetic resonance (NMR), we find that this concentration is one order of magnitude larger than that previously inferred from spin-lattice relaxation time (T1) measurements. At 300 K, this concentration of o-H2 contributes at most 30% to the narrow H1 NMR line attributed to hydrogen bonded to silicon. For a plasma-enhanced-chemical-vapor-deposition (PECVD) sample, two distinct values of T1 for o-H2 are found, only one of which contributes to the T1 for bonded hydrogen. In hot-wire-chemical-vapor-deposition samples, the line shape of o-H2 exhibits motional narrowing at lower temperatures, suggesting a more ordered structure than in a typical PECVD sample.
1.6 W continuous-wave coherent power from large-index-step (Δn≈0.1) near-resonant, antiguided diode laser arrays
Authors: Yang H. , Mawst L. J. , Botez D.
Published: July 2002

Near-diffraction-limited-beam continuous-wave (cw) operation has been achieved to high powers from antiguided arrays with a large effective-index step between element and interelement regions. InGaAs/InGa(As)P/GaAs 40-element arrays (λ=0.985 μm) emit in beams 2×diffraction-limit (0.67°) at 1.6 W and 9×threshold in cw operation. 1 W of the coherent cw power resides in the central lobe. The external differential quantum efficiency and the threshold current are 40% and 0.4 A, respectively, for 1-mm-long devices of 191 μm emitting aperture. The overall electrical to optical power conversion efficiency at 1.6 W output power is 23%. Modeling of the thermal effects in cw operation on the array modes reveals that for high-index-step (∼0.1) near-resonant antiguided arrays thermal lensing hardly affects high-order modes, and as a consequence, 2×diffraction-limited beams can be maintained to watt-range cw powers.
Atomic resolution on Si(111)-(7×7) by noncontact atomic force microscopy with a force sensor based on a quartz tuning fork
Authors: Giessibl Franz J.
Published: July 2002

Atomic resolution by noncontact atomic force microscopy with a self-sensing piezoelectric force sensor is presented. The sensor has a stiffness of 1800 N/m and is operated with sub-nanometer amplitudes, allowing atomic resolution with relatively bluntly etched tungsten tips. Sensitivity and noise are discussed.
Room-temperature Al single-electron transistor made by electron-beam lithography
Authors: Pashkin Yu. A. , Nakamura Y. , Tsai J. S.
Published: July 2002

We present a lithographically made Al single-electron transistor that shows gate modulation at room temperature. The temperature dependence of the modulation agrees with the orthodox theory, however, energy-level quantization in a tiny metallic island affects the device characteristics below 30 K. The charge-equivalent noise of the device at 300 K was measured to be ∼4×10−2 e/Hz1/2 at 1 Hz and is expected to be 1000 times lower in the white-noise regime at higher frequencies.
Realization of a complex-coupled InGaN/GaN-based optically pumped multiple-quantum-well distributed-feedback laser
Authors: Hofstetter Daniel , Romano Linda T. , Paoli Thomas L. , Bour David P. , Kneissl Michael
Published: July 2002

We demonstrate an optically pumped complex-coupled InGaN/GaN-based multiple-quantum-well distributed-feedback laser in the violet/blue spectral region. The third-order grating providing feedback was defined holographically and dry etched through a portion of the active region by chemically assisted ion-beam etching. Epitaxial overgrowth of the GaN waveguide completed the device structure without introducing dislocations, as shown by transmission electron microscopy. The laser emitted light at 392.7 nm with high side-mode suppression and a narrow linewidth of 1.5 Å. In contrast to Fabry–Pérot lasers fabricated from the same piece of material, only a very minor change in emission wavelength was observed when operating the device at higher pump intensities.
Effect of band gap renormalization on threshold current and efficiency of a distributed Bragg reflector laser
Authors: Wenzel H. , Klehr A. , Erbert G. , Sebastian J. , Tränkle G. , Pereira M. F.
Published: July 2002

The threshold current and the external efficiency of a three-section distributed Bragg reflector laser are investigated as a function of a forward electrical bias applied to the passive sections. Within a well-determined range, the threshold current increases and the external efficiency decreases with increasing bias. This effect is attributed to band gap renormalization.
Electronic structure of kinked multiwalled carbon nanotubes
Authors: Tekleab D. , Czerw R. , Carroll D. L. , Ajayan P. M.
Published: July 2002

Changes in the local density of states (LDOS) of kinked multiwalled carbon nanotubes have been studied using scanning tunneling microscopy and spectroscopy (STM)/(STS). The measured STS spectra in the inelastically kinked region show that the LDOS is asymmetric about the Fermi level, while the spectra from regions away from the kink exhibit the usual symmetric LDOS that corresponds to perfect tubes. In the kinked region, the distribution of the unoccupied states is suppressed; this suppression of the conduction band states becomes unnoticeable ∼1.66 nm from the kink center.
Atomic-force-microscopy investigation of the formation and evolution of Ge islands on GexSi1−x strained layers
Authors: Huang C. J. , Li D. Z. , Yu Z. , Cheng B. W. , Yu J. Z. , Wang Q. M.
Published: July 2002

A constant amount of Ge was deposited on strained GexSi1−x layers of approximately the same thickness but with different alloy compositions, ranging from x=0.06 to x=0.19. From their atomic-force-microscopy images, we found that both the size and density of Ge islands increased with the Ge composition of the strained layer. By conservation of mass, this implies that these islands must incorporate material from the underlying strained layer.
Structural, dielectric, and ferroelectric properties of compositionally graded (Pb,La)TiO3 thin films with conductive LaNiO3 bottom electrodes
Authors: Bao Dinghua , Mizutani Nobuyasu , Yao Xi , Zhang Liangying
Published: July 2002

We report structural, dielectric, and ferroelectric properties of compositionally graded (Pb,La)TiO3 thin films on LaNiO3/SiO2/Si substrates, where all the films were prepared by the sol-gel technique. The crystalline orientation and surface morphology of the graded films were closely related to the deposition sequence of film layer. Upgraded film exhibited (100) preferentially oriented growth, whereas downgraded film showed a randomly oriented growth, where the films with La content increasing or decreasing gradually along film thickness from the substrate to the top surface are called “upgraded” or “downgraded” films, respectively. The dielectric constants, for upgraded and downgraded films annealed at 650 °C for 60 min, were found to be 659 and 641, respectively. The thin films had large polarization offsets in hysteresis loops when driven by an alternating electric field. The magnitude of the offsets displayed a power law dependence on the electric field, and the direction of the offsets depended on the direction of the composition gradient with respect to the substrate. The leakage current of the upgraded film was 3.43×10−8 A/cm2 at the voltage of 3 V. These results showed that the compositionally graded (Pb,La)TiO3 thin films had excellent dielectric properties and abnormal ferroelectric properties which can be used in various microelectronic devices.
Hydrogen barriers for SrBi2Ta2O9-based ferroelectric memories
Authors: Yang B. , Suh C. W. , Lee C. G. , Kang E. Y. , Kang Y. M. , Lee S. S. , Hong S. K. , Kang N. S. , Yang J. M.
Published: July 2002

We report on a hydrogen barrier necessary for a conventional passivation process of integrated SrBi2Ta2O9 (SBT)-based memories. The passivation process significantly degraded electrical properties of the memories, resulting from hydrogen damage in the SBT capacitors. Metallic films (Ti, TiN, and Al) were investigated as a hydrogen barrier during the passivation process. The Ti(>500 Å) hydrogen barrier only showed the electrical properties of memories free from hydrogen damage. The formation of stable hydrides and the suppressed diffusion of hydrogen through the Ti films during the passivation processes resulted in sufficient switching polarization, low leakage current, and good reliabilities at high temperature.
Dynamics of electrohydrodynamic laminar plumes: Scaling analysis and integral model
Authors: Vázquez P. A. , Pérez A. T. , Castellanos A. , Atten P.
Published: July 2002

In this paper electrohydrodynamic plumes are examined in the region far from the injecting electrode and the collector plate, for both two-dimensional and axisymmetric geometries. The relative importance of the conduction mechanisms (convection, drift and diffusion of electric charge) is analyzed. Diffusion turns out to be negligible compared to convection and drift for the experimental conditions. But the transverse drift (Coulomb repulsion) is of the same order of magnitude than convection. We find a set of three differential equations giving the evolution of the velocity at the center of the plume and the widths of the plume and the charged core inside.
Low-threshold and high power λ∼9.0 μm quantum cascade lasers operating at room temperature
Authors: Matlis A. , Slivken S. , Tahraoui A. , Luo K. J. , Diaz J. , Wu Z. , Rybaltowski A. , Jelen C. , Razeghi M.
Published: July 2002

We report a low threshold current density and high power for λ∼9 μmAlInAs/GaInAs quantum cascade lasers operating at room temperature. The threshold current density is 1.95 kA/cm2 at 300 K and 0.61 kA/cm2 at 80 K for 5 μs pulses at 200 Hz repetition rate. The peak output power is 700 mW at room temperature and 1.3 W at 80 K per two facets for cavity length is 3 mm with a stripe width of 20 μm. The characteristic temperature T0 is 185 °C. The slope efficiency is 450 and 800 mW/A at 300 and 80 K, respectively. In continuous wave operation, the output power is more than 150 mW at 80 K and 25 mW at 140 K. This high performance was achieved by improving the material growth and processing technology.
Capacitance and admittance spectroscopy analysis of hydrogen-degraded Pt/(Ba, Sr)TiO3/Pt thin-film capacitors
Authors: Liedtke R. , Grossmann M. , Waser R.
Published: July 2002

One of the problems occurring in conjunction with the integration of Ba0.7Sr0.3TiO3 (BST) thin film capacitors into the Si technology is the large increase of leakage current after a forming gas heat treatment. In order to reveal the underlying mechanism, we studied the electric properties of Pt/BST/Pt (metal–insulator–metal) (MIM) structures after annealing in atmospheres containing hydrogen (H2) or carbon monoxide by means of admittance spectroscopy. Frequency-dependent capacitance measurements on these MIM structures revealed a thermally activated relaxation step at low frequencies with an activation energy of 0.62 eV. Admittance spectroscopy, in which the conductance is monitored as a function of temperature and frequency, verifies the Schottky barrier heights at the Pt/BST interface revealed by dc measurements. We found that the Schottky barrier height decreased by 0.4 eV after annealing in a reducing atmosphere, independent of the presence of protons.
Nanometer-scale observation of ferroelectric domains using an apertureless near-field optical microscope
Authors: Orlik X. K. , Labardi M. , Allegrini M.
Published: July 2002

Optical observation of the polarization domain structure of a ferroelectric material (triglycine sulfate) is achieved using a reflection apertureless near-field optical microscope operating with an electrical ac polarization of the tip. By showing that there is no detectable contribution of the converse piezoelectric effect of the sample nor of the electrical forces acting on the tip in the optical signal detected, we explain the observed contrast between domains of opposite polarity by the electro-optic effect. Using the phase of the demodulated optical signal, we image the ferroelectric domain structure with a resolution better than 35 nm.
Influence of carbon incorporation on dopant surface segregation in molecular-beam epitaxial growth of silicon
Authors: Osten H. J. , Lippert G. , Liu J. P. , Krüger D.
Published: July 2002

We describe the effect of carbon incorporation into Si on dopant surface segregation during molecular-beam epitaxial growth. Low concentration of carbon can significantly reduce the surface segregation of boron and phosphorus. Combining the surface diffusion model with a two-state exchange process, we are able to model the experimental results over the whole temperature range between 350 and 800 °C. Each exchange process alone is not sufficient to describe surface segregation at all investigated temperatures. Our results show that the presence of carbon lowers the energy difference for boron in subsurface and surface states. The energy barriers for surface diffusion as well as for the two-state exchange process are not affected by carbon.
Chemical ordering around open-volume regions in bulk metallic glass Zr52.5Ti5Al10Cu17.9Ni14.6
Authors: Asoka-Kumar P. , Hartley J. , Howell R. , Sterne P. A. , Nieh T. G.
Published: July 2002

We provide direct experimental evidence for a nonrandom distribution of atomic constituents in Zr52.5Ti5Al10Cu17.9Ni14.6 bulk metallic glass using positron annihilation spectroscopy. The Ti content around the open-volume regions is significantly enhanced at the expense of Ni and Cu. Our results indicate that Ni and Cu atoms closely occupy the volume bounded by their neighboring atoms while Al, Ti, and Zr are less closely packed, and more likely to be associated with the open-volume regions. The overall distribution of elements seen by the positron is not significantly altered by annealing or by crystallization. Theoretical calculations indicate that the observed elemental distribution is not consistent with the known crystalline phases Zr2Cu and NiZr2, while Al3Zr4 shows some of the characteristics seen in the experiment.
The atomic scale origin of wear on mica and its contribution to friction
Authors: Kopta Susanne , Salmeron Miquel
Published: July 2002

Atomic force microscopy is used to show that production of surface defects at the interface of rubbing solids is an important mechanism of energy dissipation in friction. Using mica and Si-tips, we demonstrate that defects produced by the rupture of Si–O bonds at the surface, which are not visible in contact mode AFM images, have a noticeable contribution to friction. When defects accumulate beyond a critical concentration, they grow to form visible wear scars ∼2 Å deep at first and deeper holes later. The contribution of defect production to friction is explained by a simple model, which is based on the stress-induced enhancement of the rate of thermal defect production.
Boron diffusion in silicon in the presence of other species
Authors: Li Hong-Jyh , Kohli Puneet , Ganguly Swaroop , Kirichenko Taras A. , Banerjee Sanjay , Zeitzoff Peter
Published: July 2002

We model B diffusion in the category of equilibrium diffusivity in the presence of other species using ab initio calculations. The result shows that in the presence of other atoms X (X=F, N, C, Al, Ga, In, and Ge), the migration energy for B along the migration pathway is larger than the case if X=Si (0.33 eV). This suggests the reduction of B diffusion. If another B occupies that position (X=B), then a smaller migration energy is observed and enhanced diffusion is expected. The simulation results are consistent with experiments. The reason for the migration energy difference can be understood in terms of electronic and strain effects. Estimates of the enthalpy of bond formation agree with the simulation except for the low-enthalpy cases where the strain effect caused by atom-size misfit dominates.
Resonant two-photon ionization and laser induced fluorescence spectroscopy of jet-cooled adenine
Authors: Kim Nam Joon , Jeong Gawoon , Kim Yung Sam , Sung Jiha , Keun Kim Seong , Park Young Dong
Published: July 2002

Electronic spectra of the jet-cooled DNA base adenine were obtained by the resonant two-photon ionization (R2PI) and the laser induced fluorescence (LIF) techniques. The 0–0 band to the lowest electronically excited state was found to be located at 35 503 cm−1. Well-resolved vibronic structures were observed up to 1100 cm−1 above the 0–0 level, followed by a slow rise of broad structureless absorption. The lowest electronic state was proposed to be of nπ* character, which lies ∼600 cm−1 below the onset of the ππ* state. The broad absorption was attributed to the extensive vibronic mixing between the nπ* state and the high-lying ππ* state.
Observation of coherent zone-folded acoustic phonons generated by Raman scattering in a superlattice
Authors: Hawker P. , Kent A. J. , Challis L. J. , Bartels A. , Dekorsy T. , Kurz H. , Köhler K.
Published: July 2002

We have used pulse time-of-flight techniques to examine the phonon emission from an optically excited GaAs/AlAs superlattice structure. For laser excitation wavelengths shorter than 767 nm (the energy of E1HH1 transition), we detect a significant longitudinal acoustic phonon component directed in a narrow beam normal to the structure. Under identical excitation conditions, generation of coherent longitudinal acoustic phonons has previously been observed in this structure. We suggest that the excitation wavelength and angular characteristics of the longitudinal acoustic emission is consistent with those of propagating modes produced as coherent phonons “leak” from the superlattice structure.
Elastic constants of Pd39Ni10Cu30P21 bulk metallic glass under high pressure
Authors: Wang Li Min , Sun L. L. , Wang W. H. , Wang R. J. , Zhan Z. J. , Dai D. Y. , Wang W. K.
Published: July 2002

The pressure-dependent acoustic velocities of a Pd39Ni10Cu30P21 bulk metallic glass (BMG) have been measured up to 0.5 GPa by using an ultrasonic technique with the pulse echo overlap method. The elastic constants, the Debye temperature, and their pressure dependence are obtained. The isothermal equation of state (EOS) of the BMG is established in terms of the Murnaghan form. The atomic configurations of the BMG are discussed by comparing the elastic constants and the EOS with those of its metallic component and of other amorphous materials.
Carrier transport and luminescence in inverted-pyramid quantum structures
Authors: Leifer K. , Hartmann A. , Ducommun Y. , Kapon E.
Published: July 2002

Cathodoluminescence spectroscopy and wavelength-dispersive imaging were employed for investigating the carrier transport and recombination in GaAs/AlGaAs inverted-pyramid quantum dot (QD) heterostructures grown on patterned (111)B GaAs substrates. The spectra and images clearly evidence carrier recombination in quantum wells and quantum wires (QWR) and show potential variations in these structures. Luminescence from the lens-shaped QDs was identified and characterized as a function of the GaAs layer thickness. Furthermore, we show a tapering of the GaAs QWR that self-forms at the corners of the pyramids. Application of such tapered QWRs as “exciton accelerators” is discussed.
Magnetic-field-enhanced generation of terahertz radiation in semiconductor surfaces
Authors: Weiss C. , Wallenstein R. , Beigang R.
Published: July 2002

A comparative study of magnetic-field-enhanced THz generation in semiconductor surfaces of InSb, InAs, InP, GaAs, and GaSb is reported. Applying an external magnetic field, the power of the generated THz radiation is increased for all examined semiconductor materials. The use of time-resolved measurements of the THz waveform allows to distinguish between the fraction of radiation originating from the surface depletion field and the fraction that is additionally generated by the magnetic field. It turns out that the power enhancement factor due to the magnetic field is inversely proportional to the effective electron mass.
Extracting information from cDNA arrays
Authors: Herzel Hanspeter , Beule Dieter , Kielbasa Szymon , Korbel Jan , Sers Christine , Malik Arif , Eickhoff Holger , Lehrach Hans , Schuchhardt Johannes
Published: July 2002

High-density DNA arrays allow measurements of gene expression levels (messenger RNA abundance) for thousands of genes simultaneously. We analyze arrays with spotted cDNA used in monitoring of expression profiles. A dilution series of a mouse liver probe is deployed to quantify the reproducibility of expression measurements. Saturation effects limit the accessible signal range at high intensities. Additive noise and outshining from neighboring spots dominate at low intensities. For repeated measurements on the same filter and filter-to-filter comparisons correlation coefficients of 0.98 are found. Next we consider the clustering of gene expression time series from stimulated human fibroblasts which aims at finding co-regulated genes. We analyze how preprocessing, the distance measure, and the clustering algorithm affect the resulting clusters. Finally we discuss algorithms for the identification of transcription factor binding sites from clusters of co-regulated genes.
Resolution limits for infrared microspectroscopy explored with synchrotron radiation
Authors: Carr G. L.
Published: July 2002

The spatial resolution for infrared microspectroscopy is investigated to determine the practical limits imposed by diffraction or optical aberrations. Quantitative results are obtained using high brightness synchrotron radiation, which serves as a diffraction-limited infrared “point source” for the microscope. The measured resolving power is in good agreement with diffraction theory, including a ∼ 30% improvement for a confocal optical arrangement. The diffraction calculation also shows how the confocal setup leads to better image contrast. The full width at half maximum of the instrument’s resolution pattern is approximately λ/2 for this arrangement. One achieves this diffraction limit when the instrument’s apertures define a region having dimensions equal to the wavelength of interest. While commercial microspectrometers are well corrected for optical aberrations (allowing diffraction-limited results), the standard substrates used for supporting specimens introduce chromatic aberrations. An analysis of this aberration is also presented, and correction methods described.
Telecentric viewing system for light collection from a z-pinch plasma
Authors: Hartog D. J. Den , Golingo R. P.
Published: July 2002

As part of a Doppler spectroscopy system to measure the radial variation of ion flow and temperature, a pair of telecentric viewing telescopes has been installed on the ZaP z-pinch plasma device. Each telescope simultaneously collects 20 chords of light (200–1200 nm) emitted by impurities in the plasma, and images the chords on a fiber optic bundle for transport to a spectrometer. The center-to-center spacing of adjacent chords in the plasma is 1.24 mm, thus radial variation across the r=10–15 mm ZaP plasma is completely recorded. In this telecentric imaging system, all object chords and image points, including those laterally displaced from the optical axis, are formed by ray bundles whose chief ray is parallel to the optical axis. Thus all 20 light collection chords passing through the ZaP plasma are parallel, and all 20 image points fill the optical fibers with an identical cone. This maximizes system efficiency and measurement precision, and simplifies calibration and data analysis.
Ground state normal mode analysis: Linking excited state dynamics and experimental observables
Authors: Kurtz Lukas , Hofmann Angelika , de Vivie-Riedle Regina
Published: July 2002

Aspects of the photoinduced ring opening reaction of cyclohexadiene with subsequent relaxation to cZc-hexatriene or back to cyclohexadiene are investigated theoretically. In this ultrafast reaction the S1 state plays a central role, providing the possibility for the molecule to rapidly decay from the excited to the ground state via multiple conical intersections. To better understand the dynamics of the molecule on its way toward the product states, we perform a normal mode analysis of its vibrations in the vicinity of two prominent conical intersections and find several eigenmodes to characterize its motion. This raises hope that our findings could lead to an experimental scheme of detection for the wave packet motion in the hot product states by a time resolved transient absorption measurement. In the end this might be useful in designing an input signal for an optimal control experiment to selectively steer the molecule through a specific conical intersection and influence the product yield.
Low-temperature resistivity minima in single-crystalline and ceramic La0.8Sr0.2MnO3: Mesoscopic transport and intergranular tunneling
Authors: Auslender M. , Kar’kin A. E. , Rozenberg E. , Gorodetsky G.
Published: July 2002

A slight minimum in the zero-field resistivity of a single crystalline La0.8Sr0.2MnO3 and shallow one for ceramic La0.8Sr0.2MnO3 samples were observed at T∼4 K and at T∼25–30 K, respectively. The minimum for the ceramic shifts towards lower temperatures, flattens with increasing magnetic fields (H), and vanishes at some critical H. The above effects are accompanied by an appreciable negative magnetoresistance (MR). On the other hand, the minimum for the single-crystalline sample is almost field independent and the MR in the relevant temperature range is very small. Two different mechanisms were found to account for the results observed in the single crystal and the polycrystalline samples: (i) mesoscopic corrections to the bulk resistivity that include Coulomb interaction and weak localization and (ii) intergranular tunneling. The resistivity of the large-grain ceramic sample comprises both types of behavior. The minimum of the bulk contribution becomes clearly seen under H, which suppresses the ceramic-type minimum.
Structure and electronic properties of carbon onions
Authors: Tomita Satoshi , Sakurai Takahiro , Ohta Hitoshi , Fujii Minoru , Hayashi Shinji
Published: July 2002

Spherical and polyhedral carbon onions prepared from diamond nanoparticles are investigated by high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and electron spin resonance (ESR). HRTEM and Raman studies show that, as a result of thermal annealing, diamond nanoparticles are transformed into spherical onions, and finally into polyhedral onions by the progress of graphitization. ESR spectra for spherical onions show only a narrow signal corresponding to dangling bond spins associated with structural defects. In contrast, for polyhedral onions, an additional broad signal due to conduction electron spins emerges. These results combined with previous results of electron energy-loss spectroscopy suggest that the spherical onions consist of small domains of the graphitic sp2 sheets with dangling bond defects in the peripheries. π electrons in spherical onions are thus localized in the small domains and do not act as conduction electrons. In the polyhedral onions, the graphitization proceeds further, resulting in the decrease in the number of dangling bonds and the delocalization of π electrons.