bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-dimensional intensity distribution of photoluminescence from porous silicon
Authors: Matsuda T. , Tanino K. , Ishii H. , Ikeshita T. , Ohzone T.
Published: July 2002

Photoluminescent (PL) intensity-distribution images and PL spectra of porous silicon (PS) have been analyzed. The PL intensity of the top surface of n-type PS is nearly uniform and is dependent upon illumination conditions during anodization. N-type PS fabricated under high power illumination has the peak of the PL spectra at shorter wavelength than that under low power illumination. On the other hand, the peak position of the PL spectra of n-type PS does not shift significantly with change of current density. The PL intensity of n-type PS is the maximum at the surface and decreases exponentially towards the Si substrate, P-type PS has a relatively broad intensity peak within the PS layer with or without illumination during fabrication. These results suggest that illumination during fabrication plays a role in the PL light emission of PS.
Dual driving of magnetostatic modes in yttrium–iron–garnet film experiments
Authors: Mar D. J. , Carroll T. L. , Pecora L. M. , Heagy J. F. , Rachford F. J.
Published: July 2002

We study experimentally the behavior of magnetostatic wave modes in a rectangular yttrium–iron–garnet film placed in an in-plane magnetic field and subject to multiple drive excitations. Patterned slotlines and coplanar waveguides are used to drive the film with microwave (2–4 GHz) excitations corresponding to magnetostatic wave modes. For a single drive, we observe linear transmission at low drive powers and saturation above the Suhl instability. For sufficiently large powers above the instability, the transmitted power through the film displays periodic low-frequency (kHz) auto-oscillations. A second microwave drive applied to the sample is used to excite additional spin-wave modes that interact with those arising from the original excitation. Such dual-drive experiments can probe the nature of spin-wave mode interactions at high amplitudes.
Conductivity of disordered polycrystals
Authors: Chinh Pham Duc
Published: July 2002

New upper and lower bounds are constructed for the macroscopic conductivity of polycrystals with random microstructure, given the principal conductivities of the constituent crystals (and the volume fractions of phases in case of a multiphase polycrystal). The new bounds lie inside the well-known Hashin–Shtrikman ones.
Third-order nonlinear optical properties of phthalocyanine and fullerene
Authors: Kanbara Hirohisa , Maruno Tohru , Yamashita Akira , Matsumoto Shiro , Hayashi Takayoshi , Konami Hideo , Tanaka Norio
Published: July 2002

We studied the third-order nonlinear optical properties of phthalocyanine and fullerene in solution and in thin film form through third-harmonic generation measurement. X-ray diffraction measurement and scanning electron microscopy observation were also conducted to investigate the relationship between nonlinear optical and structural properties of the thin film. The nonlinearities depend on the π-conjugated length and the introduction of a central metal atom. We clarify the factors for forming suitable thin films by organic molecular beam deposition. It is shown that both the crystal structure and molecular alignment can be controlled to enhance the nonlinearities of the phthalocyanine thin film, thereby providing the maximum third-order susceptibility of 3.0×10−10 esu. We show that nonlinearities of the thin films can be estimated experimentally from those of solutions.
Use of an active wire Bθ cell for electron beam conditioning
Authors: Murphy D. P. , Myers M. C. , Weidman D. J. , Antoniades J. A. , Fernsler R. F. , Meger R. A.
Published: July 2002

The propagation of intense, relativistic electron beams in air is subject to the resistive hose instability. Conditioning the beam prior to injecting it into the air can extend its range by reducing the hose growth rate and by reducing the initial spatial perturbations that seed the hose instability. Experiments have been performed using the SuperIBEX accelerator (Ipeak=10–30 kA, E=4.5 MeV, 40 ns full width at half-maximum) to develop conditioning cells that suppress the hose. This paper describes the performance of an active wire Bθ cell that is used in conjunction with an ion focused regime (IFR) cell. The IFR cell detunes the instability by producing a head-to-tail radius taper on the beam. The wire cell maintains this radius taper while producing an emittance taper that is necessary to suppress the hose growth. In addition, the wire cell reduces the initial beam perturbations through the anharmonic centering force associated with the wire current and its azimuthal magnetic field Bθ. The ability of the Bθ cell to reduce the beam offset with a minimal increase in the beam radius gives it several advantages over the use of a simple, thick scattering foil to perform the radius taper to emittance taper conversion. The SuperIBEX beam propagation distance, in terms of the betatron oscillation scale length, was extended to ∼10λβ using these cells.
Hydrogen evolution from strained SixGe1−x(100)2×1:H surfaces
Authors: Ku Ja-Hum , Nemanich R. J.
Published: July 2002

The evolution of hydrogen from a series of strained SixGe1−x(100)2×1:H surfaces were studied with angle resolved ultraviolet photoemission spectroscopy (ARUPS). A series of strained SixGe1−x alloys were grown on Si(100) wafers using electron beam evaporation in an ultra-high vacuum molecular beam epitaxy chamber. The growth was followed by an in situ H-plasma exposure. After the in situ H-plasma exposure, a diffuse double domain 2×1 reconstructed surface was obtained, which indicates a Si(Ge)–H monohydride surface termination. ARUPS spectra of the series of H-terminated SixGe1−x alloys were obtained as a function of annealing temperature. Hydrogen induced surface states/resonances were observed from the H-terminated surfaces of all samples, and as the annealing temperature was increased the states were gradually extinguished. The ARUPS spectra of the H-terminated alloy surfaces indicated that the monohydride started to dissociate at annealing temperatures ≤250 °C. The results show that, for all H-terminated SixGe1−x alloys and Ge, the surface state attributed to the dangling bond was visible after annealing at ∼250 °C. In contrast, annealing to ≳400 °C was required to desorb H from a pure Si surface. It is proposed that the hydrogen starts to desorb preferentially from the Ge sites on the strained SixGe1−x alloy surfaces.
Melt-drawn scanning near-field optical microscopy probe profiles
Authors: Williamson R. L. , Miles M. J.
Published: July 2002

Consistently obtaining super-resolution with scanning near-field optical microscopy depends almost entirely on the ability to manufacture reproducibly probes with aperture sizes smaller than 100 nm. The probe fabrication process usually involves heating an optical fiber using a CO2 laser and melt-drawing the glass to produce a taper. A number of variables ultimately define the taper shape but the actual effects these parameters have are still not clear. In this work, the physics behind the taper formation is examined in detail for the first time and equations describing the initial taper profile and the final aperture size are derived in terms of the experimental conditions. It is shown that the taper shape is primarily determined by the laser spot size. The pulling force, although important, has a lower significance. Continuum mechanics and Stefan’s law are used to show that the aperture size is closely related to the radius of the fiber at the start of the hard pull and the fiber temperature at that time. Further comparisons of experimental data with the expected taper profile exposes the heating effect of the CO2 laser. Further analysis is given using a form of Mie theory which describes the interaction of electromagnetic fields with cylindrical structures. These results give many significant insights into the fabrication process and the formation of the aperture.
Susceptibility and magnetization processes in Sm2Fe17N3 powders
Authors: Kobayashi Kurima , Rao Xiao-lei , Coey J. M. D. , Givord D.
Published: July 2002

Low field ac susceptibility and magnetization curves of milled Sm2Fe17N3 powders are measured for isotropic and aligned samples with average particle sizes ranging from 3.0 to 52 μm. Most of the particles are in a multidomain state before application of a magnetic field. The transverse ac susceptibility (χ⊥ac∼0.08) due to magnetization rotation against the anisotropy field is independent of particle size. The parallel ac susceptibility χ∥ac and the susceptibility of isotropic powders are dominated by almost free domain wall movement in multidomain particles for which 〈χ〉=1.35. In very low fields (<0.5 mT) χ∥ac is small (∼0.1), but it increases toward a constant value of order 1 when the field exceeds a value of order 1 mT which is inversely proportional to the particle size. There is a corresponding increase in loss angle which is attributed to weak pinning of domain walls at surface defects. The volume fraction of single-domain and multidomain powder are deduced as a function of average particle size both in the unmagnetized and remanent states, from an analysis of high-field magnetization curves on isotropic powder. For example, about 87% of the volume of powder with an average size of 6.7 μm is multidomain in the unmagnetized state, but only 43% of the volume is multidomain in the remanent state.
Ultrafast carrier dynamics at a metal-semiconductor interface
Authors: Christianen P. C. M. , van Hall P. J. , Bluyssen H. J. A. , Leys M. R. , Drost L. , Wolter J. H.
Published: July 2002

The ultrafast carrier dynamics in the high electric field at an Au-GaAs interface has been studied experimentally as well as theoretically. The photoluminescence decay time is related directly to the carrier sweepout from the GaAs depletion region, i.e., to the time needed for photoexcited electrons and holes to leave this region. This decay time has been found to increase drastically with laser input power, ranging from a few picoseconds at low excitation to values of 10–20 ps at high excitation. These results indicate a significant retardation of the sweepout, which cannot be explained by intervalley scattering and space-charge effects. From our Monte Carlo calculations it has been found that the applied electric field collapses totally almost instantaneously after laser excitation due to the enormous excess of photoexcited charges. The sweepout only recovers after some time needed to recharge the device.
Isothermal capacitance transient spectroscopy study of deep electron traps in low resistivity melt-grown ZnSe single crystals
Authors: Okada H.
Published: July 2002

Deep electron traps in ZnSe crystals grown by the vertical gradient freezing method using a sealed molybdenum capsule were studied by isothermal capacitance transient spectroscopy. These crystals have electron concentrations of 1015–1017 cm−3 in the as-grown state. Besides the deep traps caused by isolated impurities, two characteristic traps were found. One is the most dominant trap in almost all samples studied, with an apparent activation enthalpy of ∼0.3 eV accompanied by an electron capture barrier of 0.05–0.29 eV. The concentration of this trap is less than 3×1015 cm−3. By a deconvolution of its highly broadened spectrum, the nontheoretical emission behavior was recognized as an overlapped emission of a number of different traps with similar emission time constants. That probably corresponds to the difference of the impurity species combined with the native defect in this trap. The other trap has an activation enthalpy of 1.03–1.1 eV and the concentrations of electrons captured by the trap are found to depend logarithmically on the duration of the filling pulse. The origin of this trap is assumed to be an impurity atom associated with extended defects such as dislocations.
Photonic band gaps with layer-by-layer double-etched structures
Authors: Biswas R. , Özbay E. , Ho K.-M.
Published: July 2002

Periodic layer-by-layer dielectric structures with full three-dimensional photonic band gaps have been designed and fabricated. In contrast to previous layer-by-layer structures the rods in each successive layer are at an angle of 70.5° to each other, achieved by etching both sides of a silicon wafer. Photonic band-structure calculations are utilized to optimize the photonic band gap by varying the structural geometry. The structure has been fabricated by double etching Si wafers producing millimeter wave photonic band gaps between 300 and 500 GHz, in excellent agreement with band calculations. Overetching this structure produces a multiply connected geometry and increases both the size and frequency of the photonic band gap, in very good agreement with experimental measurements. This new robust double-etched structure doubles the frequency possible from a single Si wafer, and can be scaled to produced band gaps at higher frequencies.
High-field thermal noise of holes in silicon: The effect of valence band anisotropy
Authors: Hinckley J. M. , Singh J.
Published: July 2002

The effects of valence band anisotropy on longitudinal and transverse high-field differential mobilities, diffusivities, and thermal noise temperatures were theoretically investigated. The effects were examined for holes in silicon and in several hypothetical materials having systematically varied degrees of valence band anisotropy. The results show a pronounced dependence of the transverse high-field differential mobility and of the longitudinal high-field noise temperature upon the degree of anisotropy. This suggests that thermal noise measurements may provide an alternative to magnetotransport methods as a valence bandstructure measurement technique. The results also imply that thermal noise in semiconductor devices can be mitigated by choosing a semiconductor with a low degree of valence band anisotropy and, for a given semiconductor, by properly aligning the device layout with respect to the material crystallographic axes.
Variation of the cell parameter of polycrystalline boron doped diamond films
Authors: Brunet F. , Deneuville A. , Germi P. , Pernet M. , Gheeraert E.
Published: July 2002

The lattice parameter of undoped and boron doped polycrystalline diamond films has been measured up to 8×1020 B cm−3. It varies slightly according to the three crystallographic directions [111], [220], and [311] investigated here. The cell parameters for the undoped films are within the published values for synthetic crystal and thin films. For the boron doped films, the cell parameter has a high expansion coefficient versus boron incorporation, with a mean value of Δa/a=3×10−24 [B], ([B] in cm−3), and a variation law in the [220] direction in striking agreement with a Russian work. The simplest Vegard model predicts a smaller variation, while both the deformation potential (but with a high deformation potential on the boron impurity band of +19.5 eV) and a defect model might explain the experimental results.
Studies on room temperature characteristics and mechanism of visible luminescence of Ge-SiO2 thin films
Authors: Yue Lanping , He Yizhen
Published: July 2002

Photoluminescence (PL) characteristics of Ge nanocrystallites of different grain sizes embedded in SiO2 thin films prepared by the ion-beam sputtering technique have been further studied at room temperature under different exciting radiations. The dominant fluorescent band, with a peak at ∼420 nm under the exciting radiation of λ=300 nm, and two new luminescence peaks at about 420 and 470 nm under λ=633 nm radiation are observed in the wavelength region of 380–520 nm for the first time. Results indicate that the PL intensity depends on the size of the Ge nanocrystallites, but the position of the PL peaks is not closely related to the size of the Ge nanocrystallites. It is suggested that the mechanism of the visible PL can be discussed by a quantum size effect which is also related to the new microstructure of the embedded Ge nanocrystallites.
Roughness effects on the thermal stability of thin films
Authors: Palasantzas George
Published: July 2002

In this work, we investigate interface roughness effects on the energetic terms that play a key role on the thermal stability of thin silicide films. The roughness is modeled as a self-affine structure with power spectrum ∼σ2ξ2(1+aq2ξ2)−1−H convoluted with a domain size distribution ∝e−πR2/ζ2 to account for grain finite size effects in polycrystalline films. The parameters σ, ξ, H, and ζ denote respectively the rms roughness, the roughness correlation length, the roughness exponent, and the average domain size. The roughness effect becomes significant for small H (<0.5), and large long-wavelength roughness or σ/ξ(∼0.1). Indeed, in systems where agglomeration occurs via thermal grooving, roughness may increase significantly the critical grain sizes.
Effect of Er dopant on the properties of In0.53Ga0.47As layers grown by liquid phase epitaxy
Authors: Dhar S. , Paul S. , Mazumdar M. , Banerjee S.
Published: July 2002

Detailed properties of In0.53Ga0.47As layers grown by liquid phase epitaxy from melts containing 0.04 and 0.1 wt % Er are reported. The carrier concentration in the material is reduced by almost two orders of magnitude as a result of Er doping. Low temperature photoluminescence measurements indicate that both the donor and the acceptor type impurities are gettered by Er and the full-width at half-maximum of the major peak is reduced to 4 meV for the layer with the highest Er doping. From deep level transient spectroscopy experiments on undoped layers, we confirm the presence of an electron trap with activation energy of 0.17 eV. Density of this trap is reduced by more than two orders of magnitude in the Er doped material and another electron trap with activation energy of 0.15 eV is revealed from the analysis of the experimental data. We associate the 0.17 eV trap with impurities in the material. From low temperature photoconductivity and photocapacitance experiments, we further confirm that Er creates a level located 40 meV above the valence band. Density of this center increases in the material after high temperature annealing. We suggest that the Er-related level is due to Er atoms occupying cation sites in the material and acting as an isoelectronic impurity.
Epitaxial growth of Gd silicides prepared by channeled ion implantation
Authors: Jin S. , Bender H. , Wu M. F. , Vantomme A. , Pattyn H. , Langouche G.
Published: July 2002

A continuous buried GdSi1.7 layer is formed by channelled implantation of 90 keV Gd ions into Si(111). In the case of (001) oriented silicon substrates, the silicide film is formed on the silicon surface. Its worse crystalline quality is due to the epitaxy occurring relative to all four {111}Si planes resulting in a textured GdSi1.7 layer. Annealing at a temperature of ⩾850 °C for 30 min results in the presence of only the orthorhombic GdSi2 phase on the silicon surface for both (111) and (001) silicon substrates. However, the precipitates embedded in the silicon substrate are still hexagonal GdSi1.7. The phase transformation temperature is higher for (111) than for (001) silicon.
Measurements and simulations of micron size coplanar waveguides
Authors: Zhang S. , Sokoloff J. B. , Vittoria C.
Published: July 2002

Measurements and simulations have been done on a coplanar waveguide, which consists of a pair of slots which taper down to a separation and width of micron size. The purpose of this device is to permit one to concentrate microwaves or millimeter waves on magnetic samples of the order of a micron in order to do ferrimagnetic resonance (FMR) studies on such small samples. The transmission coefficient as a function of frequency found in the simulations agrees quite well with the measurements. The simulations show that the magnetic field at the pinch is about a factor of several thousand larger than the field of the incident wave. Results for the circuit parameters found from the simulations will be compared to the values for these parameters measured for this device, and the prospects for using the device for FMR studies on micron and submicron magnetic particles will be discussed.
Evolution of magnetic microstructure in high-coercivity permanent magnets imaged with magnetic force microscopy
Authors: Babcock K. L. , Folks L. , Street R. , Woodward R. C. , Bradbury D. L.
Published: July 2002

Magnetic force microscopy (MFM) has been shown to give high-resolution imaging of magnetic domain structures in a variety of high-coercivity permanent magnets [Folks et al., J. Magn. Magn. Mater. (in press)]. We show that this technique can be extended by the application of external fields during imaging, thus allowing direct observation of submicron microstructural evolution as a function of field. Electromagnets mounted on the MFM supplied fields up to 7 kOe laterally and 3 kOe vertically. In sintered materials, submicron processes such as depinning of domain walls at grain boundaries, domain fragmentation, and hysteresis were observed. MFM tips having very low coercivity highlighted domain walls, whereas higher-coercivity tips suffered unpredictable rotation of their magnetic moment due to both the sample and applied fields, leading to images which are difficult to interpret. For imaging of the finer-grained melt-quenched magnets, however, relatively high-coercivity tips were superior. These results show promise for the direct observation of the submicron-scale processes that dictate bulk magnetic properties, and the quantification of their field dependence.
Spin-dependent tunneling between ferromagnetic metals in a new type of tunnel junction (abstract)
Authors: Schelp F. , Lee S. F. , Fettar F. , Nguyen van Dau F. , Petroff F. , Vaurès A. , Fert A.
Published: July 2002

We report on tunneling experiments in tunnel junctions with ferromagnetic electrodes (cobalt or permalloy) separated by a relatively thick (15 nm) Al2O3 layer in which small cobalt clusters are embedded. Electron transport is, thus, by successive tunnelings from electrode to cluster and from cluster to cluster. This structure for multichannel and multistep tunneling is less liable to metallic bridging by pinholes than junctions with an unique thin layer of insulator, and also makes easier the control of the total tunnel resistance. Layers of Al2O3 and discontinuous ultrathin layers of cobalt (“cluster layers”) are alternately deposited by sputtering through masks. The structure is characterized by transmission electron microscopy (Co clusters of 3 nm typical diameter, with about 2 nm of Al2O3 between successive cluster layers and between clusters and electrode). The total tunnel resistance can reach 25 kΩ at 4.2 K. Its variation with magnetic field reflects the magnetization reversal of the electrodes and cobalt clusters, with a maximum fractional change of about 21% at room temperature.
The longitudinal wall impedance instability in a heavy-ion fusion driver
Authors: Callahan Debra A. , Langdon A. Bruce , Friedman Alex , Haber Irving
Published: July 2002

For more than ten years [J. Bisognano, I. Haber, L. Smith, IEEE Trans. Nucl. Sci. NS-30, 2501 (1983)], the longitudinal wall impedance instability was thought to be a serious threat to the success of heavy-ion driven inertial confinement fusion. This instability is a “resistive wall” instability, driven by the impedance of the induction modules used to accelerate the beam. Early estimates of the instability growth rate predicted tens of e-folds due to the instability which would modulate the current and increase the longitudinal momentum spread and prevent focusing the ion beam on the small spot needed at the target. We have simulated this instability using an r−z particle-in-cell code which includes a model for the module impedance. These simulations, using driver parameters, show that growth due to the instability is smaller than in previous calculations. We have seen that growth is mainly limited to one head to tail transit by a space-charge wave. In addition, the capacitive component of the module impedance, which was neglected in the early work of Lee [E. P. Lee, Proc. Linear Accelerator Conference, (UCRL-86452), Santa Fe, NM, 1981] significantly reduces the growth rate. We have also included in the simulation intermittently applied axial confining fields which are thought to be the major source of perturbations to seed the longitudinal instability. Simulations show the beam can adjust to a systematic error in the longitudinal confining fields while a random error excites the most unstable wavelength of the instability. These simulations show that the longitudinal instability must be taken into account in a driver design, but it is not the major factor it was once thought to be.
Sub-picosecond resonant third-order nonlinear optical response of azobenzene-doped polymer film
Authors: Dong F. , Koudoumas E. , Couris S. , Shen Yuquan , Qiu Ling , Fu Xingfa
Published: July 2002

The dynamics and the sub-picosecond resonant third-order nonlinearity of azobenzene-doped polymer films are investigated by means of transient degenerate four-wave mixing and z-scan techniques. The third-order susceptibility χ(3) and the dephasing time T2 are measured to be (6.3±0.6)×10−12 esu and (1.0±0.1)×10−12 psec, respectively. The creation of a new intermediate state related to a conformational deformation mechanism is supported by experimental observations.
Dendritic crystallization of amorphous germanium by in situ thermal pulse annealing
Authors: Lui K. M. , Chik K. P. , Xu J. B.
Published: July 2002

Phase transformation of amorphous-germanium thin film has been carried out on amorphous substrate by in situ thermal-pulse annealing in a high vacuum chamber directly after evaporation. The microstructure of the resultant film was shown to depend markedly both on the annealing ambient and on the time of exposure (te) for the as-deposited films under full-powered (<102 W/cm2) incoherent broadband irradiation. The heating rate was estimated to be not less than 102 K/s. The surface morphology of the sample was examined by atomic force microscopy (AFM), scanning electron microscopy, and optical microscopy. For samples annealed in air, hillock growth mode was observed, while for samples annealed in vacuum, a transition from the microgranular to dendritic grain growth, depending sensitively on te, was clearly evident. Surprisingly, the length of the crystallized dendrites could be as long as ≃104 μm, being at least ≃104 times larger than the thickness of the film. The dendritic morphology, the implied growth rate, and the condition of crystallization lead us to suggest that the Ge film may exist in a supercooled semiconductive liquid phase just before crystallization. X-ray diffraction analysis revealed that grains were crystallized dominantly with a random orientation for te<2.84±0.05 s, while a sharp transition to a preferred 〈110〉 crystal orientation occurred at the critical te of 3.22±0.05 s, corresponding to a maximum temperature (Tm) of 577 °C reached by the system. This transition is consistent with the appearance of dendrites in AFM micrographs. More interestingly, an anomalous lateral size effect of the substrate on the misalignment of the 〈110〉 crystal direction of different grains with respect to the substrate normal was observed from the x-ray rocking curves. Careful inspection of the AFM images found that the giant dendrites broke up into individual columnar grains as the substrate width went down in size.
A diffusional model for the oxidation behavior of Si1−xGex alloys
Authors: Kilpatrick S. J. , Jaccodine R. J. , Thompson P. E.
Published: July 2002

We have developed a kinetic model to describe the oxidation behavior of Si1−xGex alloys during Ge segregation, which compares the Deal–Grove flux of oxidant diffusing through the oxide to the maximum flux of Si diffusing through the Ge-rich layer. This is motivated by thermal oxidation experiments on Si1−xGex alloys (x<0.17) using a fluorine-containing ambient (O2 and 200 ppm of NF3). The fluorine is known to modify point defect generation during oxidation of pure Si toward vacancy production, which is also the case for Ge in Si. We demonstrate that fluorinated oxidation of Si1−xGex enhances the oxidation rate by 25%–40% in the temperature range of 700–800 °C. Oxides formed at these temperatures were SiO2, while those formed at 600 °C exhibited a transition from SiO2 to mixed oxide growth at some point during the very early phase of oxidation, depending on the alloy composition. Consideration of these data suggests that other factors in addition to oxidation temperature must be considered in predicting which oxide type will be produced, in contrast to most previous reports. Our model, indeed, shows that alloy composition, oxide thickness, and oxidant partial pressure are also important parameters. We believe that the model is very useful in predicting the oxide type that should result from a given set of growth conditions, and in particular, it suggests that a changeover from SiO2 to mixed oxide formation is likely at some point during the oxidation process, particularly if carried to larger thicknesses.
Study of damage induced by room-temperature Al ion implantation in Hg0.8Cd0.2Te by x-ray diffuse scattering
Authors: Renault P. O. , Declémy A. , Lévêque P. , Fayoux C. , Bessière M. , Lefebvre S. , Corbel C. , Baroux L.
Published: July 2002

Ion-implantation is a widely used doping technique in II–VI semiconductors. Nevertheless, ion-implantation damage has to be better understood to properly control this process. In order to investigate the implantation-induced defects in such compounds, room-temperature implantations of 320 keV Al ions have been performed on crystalline samples of [111] Hg1−xCdxTe (x≈20%) for doses ranging from 1013 to 1015 cm−2. We report the first measurements of x-ray diffuse scattering close to different Bragg reflections on such as-implanted samples. The evolution of the diffuse intensity as a function of the dose has been observed. The defect-induced diffuse intensity arises mainly from interstitial dislocation loops. Nevertheless, vacancy loops are observed above 3×1014 Al/cm2. The mean radius of the dislocation loops increases in size by three to four times when the dose rises from 1013 to 1015 cm−2. Finally, the saturation of point defects has been observed independently of their clustering at about 5×1013 Al/cm2, that is in the same range as the saturation dose of the sheet electron concentration.
Hot-phonon effects on electron runaway from GaAs quantum wires
Authors: Paulavičius G. , Mickevičius R. , Mitin V. , Stroscio M. A.
Published: July 2002

Nonequilibrium (hot) optical phonon effects on electron runaway from GaAs quantum wires embedded in AlGaAs have been investigated by Monte Carlo technique. We have simulated the carrier runaway kinetics in the 0<E<1000 V/cm electric-field range for a lattice temperature of 30 K. Due to optical phonon mode confinement by GaAs/AlGaAs heterointerfaces, the buildup of generated hot phonons is strongly pronounced in the quantum wires. Even at moderate electron concentrations and electric fields, the accumulation of these phonons may become significant and substantially affect all transport properties in the structure. As a result of reduced hot electron cooling rates in the presence of nonequilibrium optical phonons, the high-energy tail of the carrier distribution function extends above the potential barriers at the quantum wire boundaries. This may eventually lead to significant electron escape from the potential well, even at relatively low electric fields, what significantly affects the performance of such nanoscale systems.
Carbon-related platinum defects in silicon: An electron paramagnetic resonance study of high spin states
Authors: Scheerer O. , Höhne M. , Juda U. , Riemann H.
Published: July 2002

In this article, we report about complexes in silicon investigated by electron paramagnetic resonance (EPR). In silicon doped with C and Pt we detected two different complexes: cr-1Pt (cr: carbon-related, 1Pt: one Pt atom) and cr-3Pt. The complexes have similar EPR properties. They show a trigonal symmetry with effective g-values geff,⊥=2g⊥≈4 and geff,‖=g‖≈2 (g⊥, g‖ true g-values). The g-values can be explained by a spin Hamiltonian with large fine-structure energy (electron spin S=3/2) and smaller Zeeman interaction. The participation of platinum in the complexes is proved by the hyperfine interaction. From experiments with varying carbon concentration we conclude that the complexes contain carbon. Atomistic models based on the Watkins vacancy-model for substitutional Pt were developed.
Temperature mapping of reactive gas layer in thermal plasma chemical vapor deposition
Authors: Larjo Jussi , Walewski Joachim , Hernberg Rolf
Published: July 2002

Two-dimensional temperature maps of reactive gas layers were produced using pulsed laser Rayleigh scattering thermometry. The measurements were made in conditions of diamond film chemical vapor deposition (CVD) using a thermal inductively coupled plasma. In these conditions, the reactive gas layer is typically a few millimeters thick and the temperature drops across the layer from 4000 K at the free stream boundary to about 1200 K at the substrate. The reactive layer exhibits strong chemical nonequilibrium. Rayleigh scattering was induced using a pulsed laser sheet at 532 nm wavelength. The scattered radiation was detected and calibrated with an intensified charge coupled device camera. Temperature maps and axial profiles obtained under parametric variation of the gas flow conditions demonstrate how the technique can be employed to measure the reactive layer thickness and its radial distribution across the substrate. The results demonstrate that imaging Rayleigh scattering thermometry can be employed as a nonintrusive diagnostic tool to obtain useful experimental information pertinent to the gas phase chemistry in diamond CVD under conditions of extremely large temperature gradients.
X-ray diffraction and x-ray photoelectron spectroscopy study of partially strained SiGe layers produced via excimer laser processing
Authors: Martelli S. , Vianey I. , Larciprete R. , Borsella E. , Castro J. , Chiussi S. , León B.
Published: July 2002

Structural properties of graded Si(1−x)Gex layers obtained on Si(100) by pulsed laser induced epitaxy were investigated by means of conventional powder x-ray diffraction and x-ray photoelectron spectroscopy. The Si(1−x)Gex epitaxial layers were formed by pulsed KrF-laser driven rapid melting and crystallization of thin amorphous Ge layers deposited onto the Si(100). The experimental results showed that, by increasing the number of laser pulses, good quality and partially strained epitaxial layers could be attained. A Monte Carlo data evaluation algorithm is proposed, which is capable to determine, by the simultaneous fit of data obtained by x-ray diffraction and x-ray photoelectron spectroscopy, the strain level as a function of Ge concentration.
Strain effects on optical gain in wurtzite GaN
Authors: Jeon J. B. , Lee B. C. , Sirenko Yu. M. , Kim K. W. , Littlejohn M. A.
Published: July 2002

Strain effects on optical gain in hexagonal bulk GaN are calculated and explained in terms of the change in the effective hexagonal crystal field component. Qualitatively, even unstrained wurtzite structures correspond to cubic crystals with a proper biaxial stress applied. Such biaxial stress results in effective tensile deformation along the c axis ([111] direction in cubic crystals) and compressive strain in the perpendicular plane. Therefore, the light mode with a polarization vector parallel to the c axis is suppressed, while the mode with a perpendicular polarization is enhanced in wurtzite structures. Thus, compared to cubic structures with similar material parameters, a strong optical anisotropy of wurtzites results in enhanced gain for certain light polarizations, which make wurtzite structures superior for lower-threshold lasing. These qualitative arguments are illustrated by numerical calculations of optical gain in biaxially strained wurtzite GaN, based on a 6×6 envelope-function Hamiltonian.
Properties of electron traps in In1−xGaxAsyP1−y grown on GaAs0.61P0.39
Authors: Kwon Ho Ki , Choe Byung-Doo , Lim H.
Published: July 2002

The properties of electron traps in nominally undoped In1−xGaxAsyP1−y layers grown on GaAs0.61P0.39 substrates by liquid phase epitaxy were investigated by deep level transient spectroscopy measurements. Only one kind of electron trap with the activation energy of 0.38 eV was observed in In0.32Ga0.68P. A new kind of electron trap with the activation energy of 0.15 eV began to appear and its concentration increased as the As composition was increased. But this trap was not detected in GaAs0.61P0.39. This 0.15 eV trap was found to have a capture barrier causing the persistent photoconductivity phenomenon in In1−xGaxAsyP1−y at low temperatures. This defect was also found to have a charge-state controlled stability.
Ensemble Monte Carlo study of the influence of plasmon–phonon coupled modes on hot electron transport in GaAs
Authors: Borowik Piotr
Published: July 2002

The ensemble Monte Carlo method is used to calculate the velocity–field characteristics in highly doped GaAs. A new technique is developed in order to include into Monte Carlo simulations electron interactions with coupled plasmon–polar optical phonon modes. Calculations are performed for an electron concentration level equal to 7×1017 cm-3. Results of simulations are compared with model when coupling is neglected. Various models of screening length and screening by nonequilibrium electron gas are also considered.
Characterization of a-Si:H based metal/insulator/semiconductor structures by feedback charge capacitance-voltage measurements and charge deep-level transient spectroscopy
Authors: Thurzo I. , Teramura S. , Durny R. , Nádaždy V. , Kumeda M. , Shimizu T.
Published: July 2002

We have measured feedback charge capacitance-voltage (C–V) characteristics and spectra of the charge deep-level transient spectroscopy (QDLTS) on hydrogenated amorphous silicon (a-Si:H) based metal/insulator/semiconductor structures with different a-Si:H and insulator layer thicknesses. The measured QDLTS spectra are quite complicated ones, pointing to an inhomogeneous density of states in a-Si:H which is confirmed by the obtained distribution of bulk traps in energy and space. Despite this fact we are able on the basis of the feedback charge C-V method to determine the flatband voltage and the electron threshold voltage of a thin-film transistor based on a-Si:H without subjecting it to thermal bias stressing.
Finite-element Maxwell’s equations modeling of etched air/dielectric Bragg mirrors
Authors: Berger V. , Pavel I. , Ducloux E. , Lafon F.
Published: July 2002

The importance of diffraction losses in etched air/GaAs Bragg mirrors in waveguides is theoretically evaluated. A complete three-dimensional resolution of the Maxwell equations by a finite element method is performed, and shows the great importance of diffraction into the substrate. Two cases are distinguished: the weak guiding case, where the index contrast between waveguide and substrate is low (example: GaAs/AlAs), and the strong guiding case where this contrast is high, thanks to the use of lower refractive index substrates, such as oxidized AlAs. The latter solution is found to be very promising since the diffraction losses into the substrate are practically eliminated.
Study of the morphology of the InAs-on-AlSb interface
Authors: Wong K. C. , Yang Celeste , Thomas Mason , Blank Hans-Richard
Published: July 2002

Using an atomic force microscope, we studied various InAs-on-AlSb interface structures grown by molecular beam epitaxy. We found marked differences between the effects of the two interface bond configurations—InSb-like and AlAs-like—on the morphology of the subsequent InAs layer. In general, InSb-like interfaces lead to a much smoother InAs overgrown layer with clearly resolvable monolayer terraces. AlAs-like interfaces, on the other hand, lead to increasingly rougher InAs growth with longer As exposure. Previous studies have demonstrated a strong correlation between the interface configuration and the electron mobility in the InAs quantum well. The morphology and transport results we obtained indicate one reason for the influence of the interface configuration—a rough InAs layer.
Observation of laser-induced cellular organization of vortex filaments, spatial period doubling, and transition to chaos
Authors: Lugomer S.
Published: July 2002

Laser-induced vaporization of tantalum on the nanosecond time scale, in the nonstationary regime generates the vortex filaments which show the organization of a cellular type with remarkable degree of two-dimensional order. The adjacent hexagonal cells (deformed into the rectangularlike ones) are linked by the helical pairing (double helix), thus giving rise to a chain-link-fence vortical structure. This type of vortical flow was shown to originate from the spanwise perturbation characterized by the perturbation phase Δφ. The change of Δφ causes the change of two-dimensional lattice period, thus causing the cellular period doubling and transition to chaos. This process is accompanied with a gradual decrease of the “linking number”, ℒ, from 0.65 to 0, in the direction from the near-field (cellular) to the far-field (chaotic) region. Various types of vortex filament dynamics found in the far-field region, were shown to result from the change of Δφ, as well as from the interaction of filaments with the background shear. These experiments indicate the presence of very complex dynamics in the vaporizing surface layer in laser–matter interaction.
Self-organized Ge clustering on partially Ga-terminated Si(111) surfaces
Authors: Maruno S. , Fujita S. , Watanabe H. , Kusumi Y. , Ichikawa M.
Published: July 2002

We demonstrated that spatially controlled self-organization in Ge clustering occurs on partially Ga-terminated Si(111) surfaces with both Ga-adsorbed 3×3 and Ga-desorbed 7×7 striped areas on them. Ge growth on Si(111) 3×3 surfaces proceeds in a modified layer-by-layer mode and in a Stranski–Krastanov mode on 7×7 surfaces. The difference in growth causes confinement of Ge clustering in the 7×7 striped areas. The 7×7 areas on the partially Ga-terminated surface do not play a role in determining Ge cluster size but in adjusting the alignment of Ge clusters.
The Fermi level of annealed low-temperature GaAs on Si-δ-doped GaAs grown by molecular beam epitaxy
Authors: Lee W. C. , Hsu T. M. , Wang S. C. , Chang M. N. , Chyi J.-I.
Published: July 2002

Photoreflectance has been used to study the Fermi level of annealed low-temperature GaAs in sample structures composed of low-temperature GaAs on top of Si-δ-doped GaAs. The diffusion of As precipitates across the interface between low-temperature GaAs and normal GaAs is observed by cross-sectional imaging via transmission electron microscopy. We have calculated the Fermi-level pinning in low-temperature GaAs by including the Si-δ-doped carrier concentration correction due to the accumulation of As precipitates. The Fermi level is found to decrease from 0.7 to 0.5 eV below the conduction band when the annealing temperature is increased from 600 °C to 900 °C. This may be explained with the buried Schottky barrier model.
Influence of cationic stoichiometry of La1−xSrxCoO3 electrodes on the ferroelectric properties of lead based thin film memory elements
Authors: Aggarwal S. , Song T. K. , Dhote A. M. , Prakash A. S. , Ramesh R. , Velasquez N. , Boyer L. , Evans J. T.
Published: July 2002

In earlier publications, we have demonstrated that La0.5Sr0.5CoO3 electrodes dramatically improve the phase stability and electrical properties of lead based ferroelectric capacitors. This study evaluates the influence of deviation from the cationic stoichiometry, La/Sr=1, on the ferroelectric properties. Polycrystalline Pb(Nb0.04Zr0.18Ti0.78)O3 based capacitors were fabricated with La0.5Sr0.5CoO3 as the bottom electrode and either La0.5Sr0.5CoO3 or La0.85Sr0.15CoO3 as the top electrode. The as-grown capacitors with La0.85Sr0.15CoO3 as the top electrode were slightly asymmetric about the voltage axis. However, the asymmetry did not increase when the capacitors were subjected to single side pulses and temperature. Both capacitor structures showed good fatigue (no fatigue up to 1011 cycles), retention, and imprint characteristics. Detailed pulse width and voltage dependent measurements were also carried out to further understand the impact of the change in electrode composition. The polarization values at 1 μs pulse width were as large as 13 μC/cm2, though the dependence was steeper for capacitors with asymmetric electrodes. The resistance to switching during polarization reversal, formally termed activation field, α, was measured from the switching current dependence of the applied field. These values were slightly larger for the capacitors with asymmetric electrodes. The data indicate that the ferroelectric properties of the capacitor are almost not influenced by a change of the top electrode from La0.5Sr0.5CoO3 to La0.85Sr0.15CoO3.
Near-band-edge optical properties of molecular beam epitaxy grown ZnSe epilayers on GaAs by modulation spectroscopy
Authors: Tu R. C. , Su Y. K. , Li C. F. , Huang Y. S. , Chou S. T. , Lan W. H. , Tu S. L. , Chang H.
Published: July 2002

A study of near-band-edge optical properties of ZnSe epilayers grown on GaAs substrates using various modulation techniques is presented. We compare the contactless electroreflectance (CER) and piezoreflectance spectra to ascertain that our ZnSe epilayers of 1.2 μm in thickness grown on GaAs substrates are under a biaxial tensile strain. The defect-related transitions near the ZnSe/GaAs interface are also compared by identifying the photoreflectance and other spectra. In addition, in order to observe the temperature-dependent energy splitting and strains, we present a detailed investigation of the heavy-hole and light-hole related transition energies as a function of temperature in the 15–200 K range by identifying the excitonic signatures in the CER spectra. We have also calculated the energy splitting between heavy-hole and light-hole valence bands by utilizing the temperature-dependent elastic constants for ZnSe and the thermal expansion coefficients for ZnSe and GaAs. Both the experimental result and the theoretical calculation have shown a similar trend that the biaxial tensile strains decrease in magnitude with increasing temperatures in the 1.2 μm ZnSe epilayer grown on a GaAs substrate.
Low temperature photoluminescence characteristics of Zn-doped InP grown by metalorganic chemical vapor deposition
Authors: Moon Youngboo , Si Sangkee , Yoon Euijoon , Kim Sung June
Published: July 2002

Zn-doped InP layers were obtained by two different doping techniques: in situ doping by low pressure metalorganic chemical vapor deposition, and thermal diffusion from a Zn-containing film. Their low temperature photoluminescence (PL) characteristics were studied, and compared. In Zn-diffused InP, the deep donor to acceptor transition was the most dominant transition and other transitions such as the band edge transition and the band to band or shallow donor to acceptor transition were not observed at the excitation power of 10 mW. On the other hand, well resolved band edge peaks and the band or shallow donor to acceptor transition peak were observed for in situ Zn doped InP, implying that less interstitial Zn atoms were generated during in situ doping. Saturation of the hole concentration at 1.5×1018 cm3 was observed in in situ Zn doped InP, and the changes in PL characteristics at the saturation level were extensively studied. Two new deep bands at 0.88–1.0 eV and 1.21–1.27 eV were observed, and the intensity of the lower energy band increased with diethylzinc flow rate. The lower energy band was observed even at room temperature, and it is presumed to be related with the saturation of hole concentration.