bluuebunny/crossref_metadata_2025 huggingface dataset:
Preparation and characterization of Dy nanoparticles
Authors: Shevchenko N. B. , Christodoulides J. A. , Hadjipanayis G. C.
Published: July 2002

A technique of pure nanoparticle preparation has been explored by forming first small metal particles under high pressure sputtering and then embedding them into a sputtered film matrix. The tandem deposition method is used where the sputtered matrix is deposited at an argon pressure of 5 mTorr and the particles are formed within a specially designed gun at a pressure of 1 Torr and then deposited. The particle-matrix compositional limitations of reactivity and miscibility are thus reduced. The new technique was applied to dysprosium metal particles in an aluminum matrix. The absence of the antiferromagnetic transition for particles with sizes of 4–12 nm was observed. The 4 nm particles behaved superparamagnetically, while the larger 12 nm particles showed coercivities over twice as large as that of the bulk material at cryogenic temperatures. The success of the technique is demonstrated by having been able to carry out the study on the dysprosium-aluminum system that normally reacts, preventing the existence of pure Dy particles.
Microfabricated tip arrays for improving force measurements
Authors: Green John-Bruce D. , Novoradovsky Alexey , Park Doewon , Lee Gil U.
Published: July 2002

Variability in the coverage or usable lifetime of active molecules at the apex of an atomic force microscope (AFM) tip is a key limitation to biomolecular force measurements with AFM. Microfabricated tip arrays make it possible to measure molecular forces between large arrays of biological molecules with AFM. The forces are measured between a probeless microfabricated cantilever and a microfabricated array containing approximately 105 addressable probes with variable radii. We measure intermolecular forces between the model ligand–receptor pair streptavidin–biotin, to demonstrate that these tip arrays can circumvent these coverage and lifetime obstacles. Further development of these arrays promises to provide a means for measuring millions of different intermolecular interactions, paving the way for AFM to be realistically applied to screen combinatorial libraries.
The critical thickness of an epilayer deposited on a semiconductor-on-insulator compliant substrate
Authors: Zhang Tong-Yi , Su Yan-Jing
Published: July 2002

The critical thickness of an epilayer grown on a compliant substrate with the semiconductor-on-insulator configuration is investigated on the assumption that sliding boundary conditions hold along the interface between the thin crystal substrate and the amorphous underneath layer. An exact solution determining the critical thickness is formulated using both superposition and Fourier transformation. The results show that the critical thickness increases with the decreases in the thickness of the thin crystal substrate and the elastic constant of the insulator.
Stability of ultrathin semiconductor layers
Authors: Di Ventra Massimiliano
Published: July 2002

The thermodynamical stability of ultrathin semiconductor layers embedded in isovalent bulk materials has been investigated by ab initio pseudopotential techniques. The AlAs/GaAs and GaAs/GaP structures have been chosen as prototypical examples for this study. We show that independently of the thin layers thickness: (i) At 0 K, the lattice-matched structures are thermodynamically more stable in the ideal configuration than in the case in which some Al and Ga atoms are allowed to diffuse forming a mixed plane; it is the finite-temperature entropy which finally favors the mixed configuration. Conversely, (ii) the lattice-mismatched structures are thermodynamically more stable in the mixed configuration than in the ideal one. In the latter case, it is mainly the chemical mixing and not the relaxation effects which favors diffusion.
Photoluminescence study of cavity-polariton-mode splitting using high-contrast selectively oxidized AlAs/GaAs mirrors
Authors: Pratt A. R. , Takamori T. , Kamijoh T.
Published: July 2002

Photoluminescence (PL) has been used to study cavity-polariton-mode splitting in a strongly coupled microcavity that utilizes high-contrast oxidized AlAs/GaAs mirrors. The cavity structure was specifically designed to reduce nonradiative recombination at the oxide–semiconductor interfaces and takes full advantage of the high-contrast mirrors to reduce the effective cavity length, and increase the exciton–photon coupling strength. The cavity-polariton splitting measured at room temperature was 6.2 meV, which we believe to be the first ever reported PL splitting measured at room temperature using an oxide-based microcavity. The temperature dependence of the cavity-polariton splitting has also been measured and is well described within a semiclassical polariton model.
In situ electron-beam lithography on GaAs substrates using a metal alkoxide resist
Authors: Mitchell William J. , Hu Evelyn L.
Published: July 2002

Using Auger electron spectroscopy and scanning electron microscopy, we have shown that it is possible to pattern thin films of titanium oxide on GaAs substrates by first condensing multilayers of titanium isopropoxide [Ti(–OC3H7)4] on a cold (<−20 °C) GaAs(001) surface and then exposing the condensed precursor film to a scanning electron beam (incident energy and flux of 10 keV and 0.18 mC/cm2/s). Under these conditions, the electron-beam-induced deposition rate was found to be constant and equal to a high value of 5.5±1.5 Å/s. Deposition of thick films (i.e., greater than 50 Å) results after electron exposures above 2 mC/cm2; however, increased carbon incorporation was observed within these thicker oxide films. The remaining unexposed precursor was found to desorb upon annealing to room temperature, ensuring selective area pattern definition. Efficient transfer of the written patterns to the underlying GaAs substrate was observed after etching in chlorine (etch depths of 8500 Å were measured after etching for 5 min at 180 °C in 2×10−4 Torr of Cl2).
Application of a new airbridge-gate structure for high-performance Ga0.51In0.49P/In0.15Ga0.85As/GaAs pseudomorphic field-effect transistors
Authors: Liu Wen-Chau , Chang Wen-Lung , Pan Hsi-Jen , Yu Kuo-Hui , Feng Shung-Ching , Lour Wen-Shiung
Published: July 2002

A new high-performance Ga0.51In0.49P/In0.15Ga0.85As/GaAs pseudomorphic heterostructure field-effect transistor, based on a novel airbridge-gate structure with multiple piers, has been fabricated successfully. Due to the employment of high Schottky barrier GaInP layer and the newly designed double delta-doped sheets (D3S) InGaAs channel, the high gate-to-drain breakdown voltage and broad and linear transconductance are obtained simultaneously. Moreover, the use of airbridge-gate technique not only suppresses the parasitic capacitance, but also exhibits a wide and flat operation regime of the current gain cutoff frequency fT and maximum oscillation frequency fmax.
Cubic GaN formation under nitrogen-deficient conditions
Authors: Oktyabrsky S. , Dovidenko K. , Sharma A. K. , Narayan J. , Joshkin V.
Published: July 2002

We have studied crystal structure and associated defects in GaN/α-Al2O3 (0001) films grown under nitrogen-deficient conditions by metalorganic chemical vapor deposition and pulsed laser deposition. N-deficient films exhibit polycrystalline structure with a mixture of cubic zinc-blende and wurtzite hexagonal GaN grains retaining tetragonal bonding across the boundaries and hence the epitaxial orientations and polarity. Renucleation of the wurtzite phase at different {111} planes of cubic GaN results in a rough and faceted surface of the film. We elucidate that the cubic phase is more stable under the nitrogen deficiency.
Radiation-induced order–disorder transition in p+–n InGaP solar cells
Authors: Romero M. J. , Araújo D. , Garcı́a R. , Walters R. J. , Summers G. P. , Messenger S. R.
Published: July 2002

The effects of electron and proton irradiation on p+–n InGaP solar cells grown on GaAs substrates are investigated using cathodoluminescence (CL) and transmission electron microscopy (TEM). The CL measurements confirm the higher radiation resistance and defect annealing properties of InGaP compared to those of GaAs. The CL measurements also indicate the occurrence of a radiation-induced sublattice order–disorder transition in InGaP. TEM reveals the presence of ordering domains in the as-grown cells, which are effectively removed by radiation-induced defects. The results should be useful in the fabrication of radiation-resistant single-junction and dual-junction InGaP solar cells for space.
Study of the magnetic phase transition in La0.7Ca0.3MnO3 using a magneto-optical method
Authors: Lin Z. W. , Cochrane J. W. , Russell G. J. , Wang X. L. , Dou S. X. , Liu H. K.
Published: July 2002

The colossal magnetoresistive material, La0.7Ca0.3MnO3, which has the perovskite structure, has been investigated by means of a magneto-optical method. Analysis of the magnetic field distributions within such a material under applied perpendicular fields clearly showed that the sample was paramagnetic above the Curie transition temperature Tc and ferromagnetic below it. The field distributions were also studied as a function of decreasing temperature through Tc. The results show the microscopic magnetic transition process as the material changes magnetic phase.
Temperature-insensitive operation of real index guided 1.06 μm InGaAs/GaAsP strain-compensated single-quantum-well laser diodes
Authors: Asano Hideki , Wada Mitsugu , Fukunaga Toshiaki , Hayakawa Toshiro
Published: July 2002

Temperature-insensitive characteristics have been demonstrated in single mode 1.06 μm InGaAs single-quantum-well laser diodes grown by metalorganic vapor phase epitaxy. The large band-gap AlGaInP current blocking layer is employed in the buried-ridge laser structure. Tensile-strained GaAsP barrier layers were used for strain compensation and electron barriers for reducing electron overflow from a quantum well. As a result, almost totally temperature-insensitive light output-current characteristics have been realized. The uncoated device shows the highest record characteristics temperature of 437 K for the threshold current in the range of 20–50 °C. In addition, the slope efficiency does not change over 20–80 °C.
Measurement of induced surface charges, contact potentials, and surface states in GaN by electric force microscopy
Authors: Bridger P. M. , Bandić Z. Z. , Piquette E. C. , McGill T. C.
Published: July 2002

We have studied molecular beam epitaxy grown GaN films of both polarities using electric force microscopy to detect sub 1 μm regions of charge density variations associated with GaN extended defects. The large piezoelectric coefficients of GaN together with strain introduced by crystalline imperfections produce variations in piezoelectrically induced electric fields around these defects. The consequent spatial rearrangement of charges can be detected by electrostatic force microscopy and was found to be on the order of the characteristic Debye length for GaN at our dopant concentration. The electric force microscope signal was also found to be a linear function of the contact potential between the metal coating on the tip and GaN. Electrostatic analysis yielded a surface state density of 9.4±0.5×1010 cm−2 at an energy of 30 mV above the valence band indicating that the GaN surface is unpinned in this case.
Spatial distribution of radiation-induced defects in p+-n InGaP solar cells
Authors: Romero M. J. , Araújo D. , Garcı́a R. , Walters R. J. , Summers G. P. , Messenger S. R.
Published: July 2002

The spatial distribution of radiation-induced, radiative recombination centers in single-junction p+-n InGaP solar cells irradiated by 1 MeV electrons or 3 MeV protons has been determined from cathodoluminescence (CL) spectra. The energy levels of the radiation-induced, nonradiative recombination centers were determined from the temperature dependence of the CL intensity.
Doping in cubic silicon–carbide
Authors: Gubanov V. A. , Fong C. Y.
Published: July 2002

We studied the energetics and the properties of impurity states that result from doping cubic silicon–carbide (3C–SiC) with aluminum (Al), boron (B), and nitrogen (N) atoms using the tight-binding linear combination of muffin-tin orbital atomic sphere approximation method. For Al doping, it is only favorable to substitute Al for Si atoms. The corresponding hole states contribute to a partially filled weak peak near the Fermi energy. For B doping, it is possible to replace either Si or C atoms in the crystal. When a B atom is at a Si site, the hole states exhibit behavior similar to the case of Al doping. However, when a B atom is at a C site, the hole states form a partially filled strong peak above the Fermi energy. This localized feature is explained in terms of the screening effect of the neighboring atoms. For n-type doping, a N atom can enter either the Si or C site. The latter site is more energetically favorable. Furthermore, the corresponding donor states form deep impurity states within the gap. In contrast, when a N atom is at a Si site, shallow donor states are formed.
A phosphorous-rich structure of InP (001) produced by metalorganic vapor-phase epitaxy
Authors: Li L. , Han B.-K. , Law D. , Li C. H. , Fu Q. , Hicks R. F.
Published: July 2002

A phosphorous-rich structure is generated on the InP (001) surface during metalorganic vapor-phase epitaxy. It consists of phosphorous dimers, alkyl groups, and hydrogen atoms adsorbed onto a layer of phosphorous atoms. The adsorbed dimers produce c(2×2) and p(2×2) domains, with total phosphorous coverages of 2.0 and 1.5 ML. The alkyl groups and hydrogen atoms adsorb onto half of the exposed phosphorous atoms in the first layer. These atoms dimerize producing a (2×1) structure. It is proposed that the first layer of phosphorous atoms is the active site for the deposition reaction, and that the organometallic precursors compete with phosphorous dimers, alkyl radicals, and hydrogen for these sites during growth.
Phosphorus and arsenic profile control for high performance epitaxial base bipolar junction devices
Authors: Bashir R. , Kabir A. E. , Westrom P.
Published: July 2002

In this study, we report on the incorporation behavior of n-type species in undoped layers grown subsequent to doped layers at low temperature (700–750 °C) reduced pressure chemical vapor deposition of silicon/silicon germanium (Si/Si1−xGex) performed in a single wafer epitaxial deposition system. Significant amounts of these species are observed even in an undoped layer grown subsequent to the doped layer. The presence of these dopants in the subsequently grown undoped layer is attributed to segregation at the moving growth interface. The arsenic segregation is confirmed to be significantly higher than phosphorus. In addition, an increase in both the phosphorus and arsenic incorporation is observed in the presence of a highly doped boron layer.
Direct observation of hot-electron energy distribution in silicon metal–oxide–semiconductor field-effect transistors
Authors: Sakamoto T. , Kawaura H. , Baba T. , Iizuka T.
Published: July 2002

We have directly observed the energy distributions of hot electrons in Si metal–oxide–semiconductor field-effect transistors. We used a lateral hot-electron transistor employing two potential barriers (an emitter barrier and a collector barrier) that divided the Si surface into three regions (the emitter, base, and collector). For an emitter-base voltage of −1.04 V, hot electrons with an excess energy of 0.7 eV were detected at the collector. The ratio of hot electrons to the injected electrons was 1.3%.
Ion beam synthesized Ru2Si3
Authors: Sharpe J. S. , Chen Y. L. , Gwilliam R. M. , Kewell A. K. , McKinty C. N. , Lourenço M. A. , Shao G. , Homewood K. P. , Reeson Kirkby Karen
Published: July 2002

In this letter we report the synthesis of the semiconductor Ru2Si3 by ion implantation into a silicon substrate. The formation of this compound has been confirmed by x-ray measurements and electron diffraction. The absorption coefficient has been determined directly by optical transmission measurements. The band gap is found to be direct with a value in the region of 0.9 eV.
Magnetic composite materials obtained by swift heavy-ion irradiation of yttrium iron garnet ceramics
Authors: Fnidiki A. , Juraszek J. , Teillet J. , Studer F.
Published: July 2002

High-energy heavy-ion irradiation of magnetic insulators provides a unique way to generate reproducible random composites of ferrimagnetic crystals and paramagnetic amorphous latent tracks. Moreover, high-energy heavy-ion irradiation of the garnet ceramics Y3Fe5O12 is able to induce a third domain exhibiting a rotation of the hyperfine magnetic field parallel to the latent track axis. An analysis of the Mössbauer spectra allowed the intensities of the three contributions to be determined. A damage model, based on two shells surrounding an amorphous core, is proposed to estimate the volumes of the three components.
Surface plasmon-enhanced photoluminescence from a single quantum well
Authors: Hecker N. E. , Höpfel R. A. , Sawaki N. , Maier T. , Strasser G.
Published: July 2002

We have dramatically enhanced the photoluminescence intensity emitted from a single quantum well (typically by factors of 3–6) by covering the sample surface with a thin semitransparent metallic film. Using a photolithographically prepared gold grating, we show that this enhancement is due to the excitation of surface plasmons on the metal. By selectively turning off the surface plasmon excitation via sample or light polarization rotation, the enhancement can be suppressed.
Thermal stability of longitudinal magnetic recording media
Authors: Moser A. , Weller D. , Doerner M. F.
Published: July 2002

Thermal processes may limit the lifetime of stored data and ultimately the achievable areal density in magnetic recording. To quantify this important problem, we have studied a series of state-of-the-art thin film CoPtCr longitudinal recording media samples with thicknesses ranging between 5.5 and 13 nm. Static write/read tester experiments are used to measure the remanent coercivity of these films as a function of the applied magnetic field pulse width, which is varied in the range of nanoseconds to seconds. The data are analyzed in terms of an inverse slope parameter, 1/C, which in the framework of a Neel-Arrhenius model is equivalent to the familiar stability ratio EB/kBT. This ratio is then correlated with signal decay measurements, performed over a time range after writing of 2.8 s to ≈1 day. Both measurements are carried out at variable temperatures, T, between 300 and 390 K. The onset of strong signal decay in low density square wave bit patterns occurs when 1/C drops below about 36.
Lattice effects on the magnetic and transport properties of La0.67−xSmxSr0.33CoO3 perovskites
Authors: Sun J. R. , Wong H. K.
Published: July 2002

Structural, magnetic, and transport properties of La0.67−xSmxSr0.33CoO3 with x=0.0, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30 are studied. The system exhibits a rhombohedrally distorted perovskite structure for x⩽0.10. A rhombohedral–orthorhombic structure transition is detected in the doping range from x=0.15 to 0.25 and a pure orthorhombic structure appears in sample x=0.30. When cooled, the sample shows a metal-to-metal and a metal-to-semiconductor transition subsequently, with the upper one of magnetic nature. Both resistive transitions vary against doping level, with the first one from ∼235 to ∼200 K while the second one from ∼75 to ∼175 K corresponds to a change of x from 0.0 to 0.3. The introduction of Sm drives the system from a cluster-glass state in La0.67Sr0.33CoO3 into a spin-glass state in La0.37Sm0.30Sr0.33CoO3, accompanied by an enhancement of the semiconductive character of the compounds. The absence of strong Jahn–Teller effects and the spin-state transition may be responsible for the complex lattice effects in Co-based perovskites.
Sensitive detection method of dielectric dispersions in aqueous-based, surface-bound macromolecular structures using microwave spectroscopy
Authors: Hefti John , Pan Alfred , Kumar Amit
Published: July 2002

An approach for measuring the dispersions in the microwave regime of larger aqueous-based molecules such as proteins and nucleic acids is presented. This method allows for the identification of molecular structures, changes in structures, and various types of intermolecular interactions via the direct detection of the dispersion properties of the system, and further allows for the measurement in physiologic environments such as aqueous buffers. By placing the molecular system on a surface, the structures and interactions are reduced to a two-dimensional sample, and thus the sensitivity of the measurement strategy is greatly enhanced.
Controlled agglomeration of Tb-doped Y2O3 nanocrystals studied by x-ray absorption fine structure, x-ray excited luminescence, and photoluminescence
Authors: Soo Y. L. , Huang S. W. , Kao Y. H. , Chhabra V. , Kulkarni B. , Veliadis J. V. D. , Bhargava R. N.
Published: July 2002

Local environment surrounding Y atoms in Y2O3:Tb nanocrystals under various heat treatment conditions has been investigated by using the extended x-ray absorption fine structure (EXAFS) technique. X-ray excited luminescence (XEL) with the incident x-ray energy near Y K edge and Tb L edges has also been measured to investigate the mechanisms of x-ray-to-visible down conversion in these doped nanoparticles. The observed changes in EXAFS, XEL, and photoluminescent data can be explained on the basis of increased average size of the nanoparticles as confirmed by transmission electron microscopy studies. Our results thus demonstrate that the doped nanoparticles can agglomerate to a controllable degree by varying the heat treatment temperature. At higher temperatures, the local environment surrounding Y atoms in the nanoparticles is found to become similar to that in bulk Y2O3 while the XEL output still shows the characteristics of nanocrystals. These results indicate that appropriate heat treatment can afford an effective means to control the intensity and signal-to-background ratio of green luminescence output of these doped nanocrystal phosphors, potentially useful for some device applications.
Sub-poissonian intensity noise from vertical-cavity surface-emitting lasers
Authors: Wiedenmann D. , Kicherer M. , Jung C. , Grabherr M. , Miller M. , Jäger R. , Ebeling K. J.
Published: July 2002

The intensity noise of selectively oxidized vertical-cavity surface-emitting lasers (VCSELs) is compared to the shot noise limit. Different operation regimes where transverse single- or multimode emission occurs are investigated. At low-temperature operation, quantum efficiency of the VCSEL is increased leading to photon-number fluctuations 1.4 dB below the shot noise limit.
Influence of Si doping on the infrared reflectance characteristics of GaN grown on sapphire
Authors: Hou Y. T. , Feng Z. C. , Chua S. J. , Li M. F. , Akutsu N. , Matsumoto K.
Published: July 2002

Si-doped GaN films grown on sapphire are investigated by infrared reflectance. A damping behavior of the interference fringes is observed, and interpreted to be due to the presence of an interface layer between the film and the substrate. A theoretical calculation using a two-layer model to take into account the interface layer resulted in this damping in agreement with the experiment. The damping behavior and an improvement of interface properties by Si incorporation are demonstrated.
Local structures of Ga atoms in amorphous silicon and hydrogenated amorphous silicon before and after synchrotron x-ray irradiation
Authors: Yu Kin Man , Walukiewicz W. , Muto S. , Jin H.-C. , Abelson J. R. , Clerc C. , Glover C. J. , Ridgway M. C.
Published: July 2002

The influence of intense x-ray irradiation on the local bonding structure of Ga dopants in both hydrogen-free (a-Si) and hydrogenated (a-Si:H) amorphous Si thin films has been studied. Prior to x-ray exposure, extended x-ray absorption fine structure measurements revealed that H reduced the static disorder around the Ga atoms in amorphous Si. Thereafter, x-ray irradiation modified the local structure in the a-Si and a-Si:H samples. The Ga coordination number increased from <3.5 to ∼3.80 atoms for both types of amorphous material as consistent with a greater fraction of electrically active, tetrahedrally coordinated dopant atoms. Also, greater structural disorder was observed around Ga atoms after x-ray irradiation suggesting that tetrahedrally coordinated Ga atoms were less well ordered than in threefold-coordinated sites.
Crystallographic study of electromigration failure sites in submicron Al(Cu) interconnects
Authors: Chu X. , Prybyla J. A. , Theiss S. K. , Marcus M. A.
Published: July 2002

Crystallographic grain orientations at a large number of electromigration failure sites in Al(Cu) interconnects have been measured and compared to the grain orientations away from failure sites. Electron backscattered diffraction analysis reveals a preferred in-plane orientation at failure sites.
Electrically active defects in surface pre-amorphized Si under rapid thermal anneal and their removal by concurrent titanium silicidation
Authors: Chi D. Z. , Ashok S. , Theodore D.
Published: July 2002

The interstitial-type nature of electrically active defects observed in surface pre-amorphized and subsequently annealed p-type Si is established by comparing the thermal evolution of electrically active defects for rapid thermal anneals (RTA) with/without titanium films. Detailed analyses of the deep level transient spectroscopy (DLTS) and transmission electron microscopy results suggest that some of these defects are small interstitial clusters. The results of this study also suggest that the release of self-interstitials from EOR extended defects and their subsequent diffusion into bulk are involved in the formation of the hole trap levels observed after high temperature RTA anneals. The observation of the complete elimination of the implantation-induced electrically active defects by concurrent RTA Ti-silicidation should be of interest to silicon microelectronic technology since it demonstrates the possibility of eliminating implantation induced defects using low thermal budget.
Polarization control of vertical-cavity surface-emitting lasers by electro-optic birefringence
Authors: Park Min Soo , Ahn Byung Tae , Yoo Byueng-Su , Chu Hye Yong , Park Hyo-Hoon , Chang-Hasnain C. J.
Published: July 2002

Polarization of vertical-cavity surface-emitting lasers (VCSELs) grown on (001) GaAs substrate has been controlled by electro-optic birefringence. Birefringence was induced at the top distributed Bragg reflector by applying an electric field along the [001] direction. The cavity resonance of the polarized light along the [110] or [11̄0] direction shifted to shorter and longer wavelength, depending on the direction of the applied electric field. By varying the direction and strength of the electric field, we actively controlled the polarization of VCSELs. The dominant polarization mode occurred along the [110] direction for the negative electric field, and along the [11̄0] direction for the positive electric field.
Observation of stress relaxation via step formation in Co/Pt metallic superlattices
Authors: Shin Sung-Chul , Kim Young-Seok , Kim Hong-Seung , Lee Jeong-Yong
Published: July 2002

We report the direct observation of stress relaxation in lattice-mismatched Co/Pt superlattices with increasing Co-layer thickness, via in situ and real-time measurements using a submonolayer-sensitive stress measurement apparatus. A large tensile stress in the Co layers found to be relaxed when the Co layer thickness was larger than 2 monolayers, corresponding to the critical thickness expected for coherent-to-incoherent transition in this system. Cross-sectional high-resolution transmission electron microscopy studies revealed that stress relaxation was realized by formation of steps, rather than misfit dislocations, near the interfaces.
On the origin of the large piezoelectric effect in morphotropic phase boundary perovskite single crystals
Authors: Bell Andrew J.
Published: July 2002

The high-field behavior of perovskite ferroelectrics close to a morphotropic phase boundary is investigated using the thermodynamic theory of ferroelectrics. For Pb(Zr0.55Ti0.45)O3, a phase transition from the rhombohedral to the tetragonal state is predicted for a critical field of approximately 4 MV m−1 parallel to the 〈001〉 axis. The transition is characterized by a strain of 0.015 parallel to 〈001〉. It is postulated that this transition is the origin of the large field-induced strains observed in single crystals of complex perovskite solid solutions with morphotropic phase boundaries. It is proposed that the transition is diffuse with respect to the applied field due to the inhomogeneity of the critical field arising from the B-site disorder in the complex perovskites. A simple random-field model illustrates the hypothesis.
Intracavity lens for low-divergence high-power laser diode operation
Authors: Rashed A. M. , Williams K. A. , Heard P. J. , Penty R. V. , White I. H.
Published: July 2002

An intracavity lens has been integrated into a high-power tapered ridge-waveguide semiconductor laser diode. The lens is designed to compensate for the phase curvature of the incident mode on the output facet. This has led to improved linearity in the light–current characteristics of the device, showing an increase of up to 47% in power slope efficiency and a reduction of up to 17% in the full width at e−2 intensity of the far field pattern.
Step-bunching instability in strained-layer superlattices grown on vicinal substrates
Authors: Patriarche G. , Ougazzaden A. , Glas F.
Published: July 2002

Epitaxial structures containing either compressive or tensile InGaAsP layers separated by InP layers were grown on variously misoriented vicinal (001) InP substrates and studied by transmission electron microscopy. Except for zero misorientation, the compressive layers develop long-wavelength asymmetrical lateral thickness modulations. We demonstrate that the associated surface undulation consists of periodic bunches involving only the initial substrate steps. With increasing misorientation, the wavelength of the modulation and its phase shift between successive layers decrease, whereas its amplitude increases. On the other hand, all tensile layers develop short-wavelength localized symmetrical thickness variations often involving facets. For intermediate misorientations, they also display undulations similar to those observed in compressive layers.
Local modification of the thin YBa2Cu3O7−y microstrips by the voltage-biased atomic force microscope tip
Authors: Kim B. M. , Song I. S. , Sok J. H. , Song I. H. , Min Y. S. , Kim M. K. , Lee J. W.
Published: July 2002

The atomic force microscope (AFM) tip biased at around −15 V is found to be capable of locally modifying the entire thickness of 40-nm-thick semiconducting or superconducting YBa2Cu3O7−y microstrips in air. We show, using combined electrical and AFM measurements, that the local regions underneath the surface of the semiconducting or superconducting YBa2Cu3O7−y microstrips are transformed into either nonconducting or nonsuperconducting regions, respectively, upon applying the negatively biased AFM tip. The conductance of the nonsuperconducting regions is also found to be comparable to that of the superconducting regions before modification at 298 K.
Demonstration of III–V semiconductor-based nonvolatile memory devices
Authors: Pan Zhongwei , Shum Kai
Published: July 2002

Using the concept for nonvolatile memories recently proposed by K. Shum, J. Q. Zhou, W. Zhang, L. F. Zeng, and M. C. Tamargo [Appl. Phys. Lett. 71, 2487 (1997)], a promising nonvolatile memory device has been designed and demonstrated using a III–V semiconductor quantum structure. Preliminary data on the device’s stability and reliability reveals that further improvements are possible on the cycling endurance and retention time.
Implantation damage effect on boron annealing behavior using low-energy polyatomic ion implantation
Authors: Jin Jian-Yue , Liu Jiarui , van der Heide Paul A. W. , Chu Wei-Kan
Published: July 2002

We have studied ion-implantation damage effects on boron clustering and transient enhanced diffusion (TED) by using polyatomic boron (Bn−, n=1–3) ion implantation with the same atomic boron dose and energy. This Bn− series implantation can produce different amounts of damage with the same boron as-implanted profile and same amount of excess interstitials, hence a net effect of implantation damage can be extracted. Secondary ion mass spectrometry measurements indicate that for 1 keV boron atomic energy implantation and 10 s 1050 °C rapid thermal annealing, B1− implantation has less TED and less boron–interstitial clustering than B2− and B3− implantation. A boron trapping peak at the SiO2/Si interface is also speculated since the amount of boron trapped is correlated to the size of implanted ions.
Nanoscale magnetostrictive response in a thin film owing to a local magnetic field
Authors: Berger R. , Krause F. , Dietzel A. , Seo J. W. , Fompeyrine J. , Locquet J.-P.
Published: July 2002

Scanning probe microscope experiments are presented in which thin magnetostrictive films deposited on top of micrometer-sized magnetic write heads as used in magnetic hard disk drives, are used to visualize their emanating magnetic field. The magnetostrictive expansion owing to magnetic writing fields is discussed, together with the transduction mechanisms that lead to the vertical and lateral contrast observed. Experimental results verify that the techniques described have a lateral resolution in the realm of 100 nm.
Scanning tunneling microscopy on the formation of lipoamide-cyclodextrin monolayer on Au(111)
Authors: Yasuda Satoshi , Shigekawa Hidemi , Suzuki Iwao , Nakamura Tohru , Matsumoto Mutsuyoshi , Komiyama Makoto
Published: July 2002

β-cyclodextrin molecules modified with lipoamide residue (LP-β-CyD) were self-assembled on an Au(111) surface in ethanol solution, and the growth process was studied by scanning tunneling microscopy. At the initial stage, adsorption sites were not only random, but also partially linear ordering, which suggests the existence of some influence by the herringbone structure of the Au(111) surface. According to the macroscopic analysis, the subsequent growth process was explained by the Elovich model, which is based on the repulsive interaction between adsorbed molecules. However, when the immersion time increased, island structures began forming. This result suggests the interaction between LP-β-CyD molecules as attractive, which in fact is more probable in consideration of the possibility of the hydrophobic and the hydrogen bonding interactions between CyD molecules. Finally, formation of a single LP-β-CyD layer was clearly confirmed.
Blue emitting Sr2Ga2S5:Ce phosphor thin films grown by multisource deposition
Authors: Okamoto Shinji , Tanaka Kastu , Inoue Youji
Published: July 2002

Blue emitting Sr2Ga2S5:Ce phosphor thin film has been first grown by multisource deposition. The evaporating source materials are Sr metal and Ga2S3 compound, and the dopant is CeCl3. This polycrystalline thin film shows the single orientation of (004). The SrS thin film is a buffer layer underneath the Sr2Ga2S5 thin film and is necessary for growth. The photoluminescence spectrum by excitation of the UV light at 290 nm is a broad band with two peaks at 443 and 484 nm, that is originated from D–42F5/2,7/2 transitions of Ce3+ ion. The emission is a saturated blue color and the Commission International de l’Eclairage chromaticity coordinates are x=0.15 and y=0.12.
Band structure calculation of field emission from AlxGa1−xN as a function of stoichiometry
Authors: Chung M. S. , Miskovsky N. M. , Cutler P. H. , Kumar N.
Published: July 2002

The field emission current density j from the ternary alloys AlxGa1−xN is theoretically calculated as a function of stoichiometry. The material parameters of AlxGa1−xN are obtained as weighted averages of those of AlN and GaN. Using the method of W. W. Lui and M. Fukuma [J. Appl. Phys. 60, 1555(1986)], the transmission coefficients are numerically calculated using Airy functions that are solutions of the Schrödinger equation in a piecewise linear potential region. Band structure effects in the calculation of j are included using the projection of the energy ellipsoids on the emission surfaces. The results for j show a strong dependence of the field emission on the stoichiometric composition, which reflects the composition dependence of the electron affinity. The Fowler–Nordheim plots and calculated field electron energy distribution curves both exhibit structures which suggest different field-dependent transmission probabilities in the low and high field regimes.
Ultrasonic study on charge ordering, magnetic, and structural changes in La0.25Ca0.75Mn0.93Cr0.07O3
Authors: Li X.-G. , Chen H. , Zhu C. F. , Zhou H. D. , Zheng R. K. , Zhang J. H. , Chen L.
Published: July 2002

The longitudinal and transverse ultrasonic velocities and attenuations, the transport and magnetic properties, as well as the structural changes of polycrystalline La0.25Ca0.75Mn0.93Cr0.07O3 have been studied systematically. A velocity minimum and an attenuation peak for both longitudinal and transverse waves occur around the temperature Tco of charge ordering transition accompanied by the structural change from a cubic to a tetragonal phase. Near temperature TN (where the c-axis length shows a maximum), the sharp decrease of velocities for both waves and the appearance of a longitudinal attenuation peak are attributed to the antiferromagnetic transition together with the anomalous change in c-axis length of the tetragonal phase. The simultaneous occurrence of the ultrasonic, lattice, charge ordering, magnetic, and transport anomalous characteristics indicates the strong electron–phonon and spin–phonon interactions in the system.
Modulation of the second-order susceptibility in GaAs/AlAs superlattices
Authors: Hutchings D. C.
Published: July 2002

The modulation of the bulk-like second-order susceptibility with quantum well disordering is calculated for a GaAs/AlAs superlattice. The calculation is based on the A⋅p form of the susceptibility, which is the more appropriate form for semiconductors, and includes the influence of the Γ15c upper conduction band set which ensures the necessary absence of inversion symmetry. The undisordered structure has the degeneracy broken between the χxyz(2) and χzxy(2) tensor elements which is restored upon disordering. The shift of the half-band gap resonance gives a maximum modulation depth Δχxyz(2)≈60 pm V−1 in the transparency window for this structure.
Copper thin films used as transmission remoderators for slow positron beams
Authors: Brusa R. S. , Deng W. , Checchetto R. , Karwasz G. P. , Zecca A.
Published: July 2002

The positron remoderation efficiency of self-supported copper films 1000–5000 Å thick was measured by a slow positron beam at impinging positron energies between 1 and 20 keV. A maximum efficiency of 11.5 (−1+3)% was found. It has been shown that copper films are easily produced. A relatively low temperature annealing (≃450 °C) is needed to remoderate positrons. These properties make them good candidate for positron beams with brightness enhancement stage in transmission geometry.
Electro-optic effect in Ba1−xPbxTiO3 films
Authors: Fuflyigin V. , Wang F. , Jiang H. , Zhao J. , Norris P.
Published: July 2002

High quality Ba1−xPbxTiO3 (x=0–0.25) films were grown on R-Al2O3 in a wide thickness range of 0.5–3 μm. Significant improvement of the films’ crystallinity and optical quality was observed in the presence of lead oxide for the films prepared at 650–700 °C. Strong texture of (110) type was observed in these films. The material is transparent at 350–2000 nm, indicating the possibility of its application in light controlling devices at wavelengths used in optical communication: 1300 and 1500 nm. Maximum field induced relative phase shift of 0.22 rad was measured in the film with composition of Ba0.9Pb0.1TiO3 under a field strength of 3×106 V/cm.
Simplified-antiresonant reflecting optical waveguide-type vertical-cavity surface-emitting lasers
Authors: Zhou D. , Mawst L. J.
Published: July 2002

A two-step metalorganic chemical vapor deposition growth process is used to fabricate antiguided vertical-cavity surface-emitting lasers (VCSELs) incorporating a simplified-antiresonant reflecting optical waveguide (S-ARROW) design. Preliminary results show single-mode cw operation up to 1 mW output power from a 12 μm-diam (λ=930 nm) S-ARROW VCSEL with a large lateral index step (Δn=0.1). Modal discrimination in the S-ARROW-VCSEL is calculated using a fiber-mode approximation and device optimization for high-single-mode powers is discussed.
Phase-modulated beams technique for thin photorefractive films characterization
Authors: Barmenkov Yu. O. , Kir’yanov A. V. , Starodumov A. N. , Kozhevnikov N. M. , Lemmetyinen H.
Published: July 2002

The phase-modulated beams technique is developed for nonlinear thin photorefractive films characterization. In the Raman–Nath diffraction approximation, the formulas are deduced, allowing us to measure the amplitude of phase grating recorded in a film and its nonlinear refractive index n2. The method is applied for studying Langmuir–Blodgett multilayer thin (∼0.6 μm) films of Bacteriorhodopsin at wavelength 633 nm.
Highly ordered films of quaterthiophene grown by seeded supersonic beams
Authors: Iannotta S. , Toccoli T. , Biasioli F. , Boschetti A. , Ferrari M.
Published: July 2002

Seeded supersonic beams of oligothiophenes have been developed and used to grow very high quality films on CaF2 and SiO2. Optical characterizations show features previously observed only on single crystals and very thin films. In particular, the vibronic progression of the 0–0 transition is well resolved at 10 K. Highly supersonic beams give best results. High quality films are produced over a range of thicknesses ⩾500 nm. The correlation between properties of the films and beam parameters are discussed. The energy state of oligomers is envisaged to play a key role in the growth.
Temporal evolution of resonant Raman-scattering in ZnCdSe quantum dots
Authors: Kaschner A. , Strassburg M. , Hoffmann A. , Thomsen C. , Bartels M. , Lischka K. , Schikora D.
Published: July 2002

We investigated ZnCdSe/ZnSe quantum-dot structures which include planar and coherently strained three-dimensional islands with different sizes. Optical excitation of these islands well below the ZnSe band gap leads to a resonant enhancement of the Zn0.7Cd0.3Se longitudinal-optical (LO) phonon-scattering efficiency and makes the 2LO and 3LO multiphonon emission observable. Resonant excitation with a power density of about 1.3 MW/cm2 using a micro-Raman setup results in an exponential decrease of the 1LO, 2LO, and 3LO intensity with irradiation time. This decay behavior is not observed for pure ZnSe crystals and can be avoided for the ZnCdSe/ZnSe structures using much lower excitation densities. The decrease in intensity is accompanied by a shift of the LO mode to higher frequencies resulting from a lower cadmium concentration in the alloy. From these experimental findings, we conclude that resonant excitation at a certain power density leads to cadmium out-diffusion from the planar quantum dots, which shifts the resonance away from the excitation energy.
Dependence of radiative lifetimes of porous silicon on excitation wavelength and intensity
Authors: Moyer Patrick J. , Pridmore Andy , Martin Timothy , Schmidt Jan , Hasche Tom , Eng Lukas , Gole James L.
Published: July 2002

Excited state radiative lifetime measurements are made on porous silicon as a function of excitation wavelength and excitation intensity. The results indicate that a simple quantum confinement model for the light absorption and emission mechanism is not suitable. We support our results by suggesting that a cascading energy transfer process among surface molecule-like states is most likely active and we provide a general indication of the density of energy transfer states.
An enhanced resolution technique for determination of the distribution of localized states in semiconductors from transient photocurrents
Authors: Main C. , Reynolds S.
Published: July 2002

We describe a simple variation in the analysis of the transient photocurrent i(t) in semiconductors to determine localized state distributions g(E), which affords a remarkable improvement in the energy resolution available, without special computing requirements. The initial stage of the analysis uses numerical Fourier or Laplace transformation of i(t), and this results in an “ill-posed” integral equation for g(E), which has been hitherto approached by approximate low resolution techniques, or by specialized numerical methods. Our proposal allows solution of this implicit equation for a fine discrete version of g(E) using a simple least squares fitting algorithm with an exact model function. The technique is demonstrated by application to i(t) data, computer generated for several representative g(E) distributions.
Time-resolved photoluminescence study of isoelectronic In-doped GaN films grown by metalorganic vapor-phase epitaxy
Authors: Huang H. Y. , Shu C. K. , Lin W. C. , Chuang C. H. , Lee M. C. , Chen W. K. , Lee Y. Y.
Published: July 2002

Time-resolved photoluminescence spectra were used to characterize isoelectronically doped GaN:In films. Our results indicate that the recombination lifetime of the donor-bound-exciton transition of undoped GaN exhibits a strong dependence on temperature. When In is doped into the film, the recombination lifetime decreases sharply from 68 to 30 ps, regardless of the measured temperature and In source flow rate. These observations might be related to the isoelectronic In impurity itself in GaN, which creates shallow energy levels that predominate the recombination process.
Reliable contacts to two-dimensional conduction layers
Authors: Souw Victor , Li Shi , McElfresh M. , Duan Zhan , McInturff D. , Yulius Aristo , Chen E.-H. , Woodall J. M.
Published: July 2002

For many experiments and device applications, electrical contacts to a two-dimensional conduction layer must remain reliable under repeated temperature cycling between 300 and 77 K or lower. This work introduces the use of a silicon-doped InAs contact to the AlGaAs/GaAs two-dimensional electron gas which demonstrates exceptional reliability under such temperature cycling. The noise spectrum of AlGaAs/GaAs contacted with silicon-doped InAs shows almost no dependence on bias current; this fact can be used to improve the performance of device applications such as Hall sensors. In addition, this work introduces an alternative two-dimensional conduction structure, highly mismatched InAs/GaP. InAs/GaP contacted with Ti/Au shows reliability equal to AlGaAs/GaAs contacted with silicon-doped InAs. The InAs/GaP material may be more desirable for some applications because of the lower temperature dependence of its electronic properties and potentially easier integration with silicon-based microelectronics.
Wavelength monitor based on two single-quantum-well absorbers sampling a standing wave pattern
Authors: Kung H. L. , Miller D. A. B. , Atanackovic P. , Lin C. C. , Harris J. S. , Carraresi L. , Cunningham J. E. , Jan W. Y.
Published: July 2002

We demonstrate a wavelength monitor and a two-wavelength detector based on two single-quantum-well absorbers that sample a standing wave created by a distributed Bragg reflector. As a wavelength monitor, our device is power independent over a 15 dB range. Wavelength discrimination is linear over a 12 nm range.
High-gain, reflection-double pass, Ti:sapphire continuous-wave amplifier delivering 5.77 W average power, 82 MHz repetition rate, femtosecond pulses
Authors: Liu Zhenlin , Murakami Hidetoshi , Kozeki Toshimasa , Ohtake Hideyuki , Sarukura Nobuhiko
Published: July 2002

A confocal, reflection-double-pass, Ti:sapphire continuous-wave (cw) amplifier with a small signal gain of 4.2 has been invented. Femtosecond pulses with an 82 MHz repetition rate from a mode-locked Ti:sapphire laser are amplified to 5.77 W average power with a slightly saturated gain of 3.7 through an amplifier pumped by three cw green lasers, and the extraction efficiency reaches 10.6%.