bluuebunny/crossref_metadata_2025 huggingface dataset:
A simplified hydrokinetic model for a steady-state microwave discharge sustained by traveling waves at atmospheric pressure conditions
Authors: Gordillo-Vázquez F. J. , Cotrino J.
Published: July 2002

The properties of a microwave-induced argon plasma produced by traveling surface wave at atmospheric pressure are investigated theoretically. A hydrokinetic model is elaborated to obtain the nonequilibrium one-dimensional profiles of electron temperature Te (average electron energy), electron density ne, and the first excited state population density n4s, along the axis of a steady-state discharge. A three-level atomic structure is assumed for the argon atom. A particle balance is included through the continuity equations for ne and n4s. These equations are coupled with an energy balance equation for the electrons. The effects of different parameters on the properties of the argon discharge are investigated: discharge tube radius, gas flow rate, resonant radiation-escape factor, and neutral gas temperature.
Silicide mediated low temperature crystallization of hydrogenated amorphous silicon in contact with aluminum
Authors: Ashtikar M. S. , Sharma G. L.
Published: July 2002

The reaction between hydrogenated amorphous silicon (a-Si:H) film and aluminum has been investigated by Transmission Electron Microscopy and Scanning Electron Microscopy (SEM). The formation of aluminum silicide was observed even at room temperature. Subsequent annealing of the sample showed [111], [110] and [311] orientations of aluminum silicide coexisting as revealed by the electron diffraction patterns. This silicide has a bcc structure with lattice parameter of 6.36 Å. The precipitation of Si at the interface was observed in SEM after selective etching of Al from the top surface of the sample. The silicide disappears completely at 300 °C with the formation of polycrystalline and porous silicon. The effect of temperature related structural changes on the electrical properties of Al/a-Si:H Schottky diode has also been studied.
Detecting subsurface hot electrons with a scanning probe microscope
Authors: Vázquez F. , Furuya K. , Kobayashi D.
Published: July 2002

We propose a new kind of scanning probe microscope, the scanning hot electron microscope, which allows the detection of hot electrons emitted from a subsurface semiconductor structure. The principle of operation corresponds to a reversely biased ballistic-electron-emitting microscope (BEEM) so that hot electrons flow from sample to tip, where they are collected after being modulated at an appropriate frequency. Theoretical conditions for hot electron detection are discussed. In particular, the hot electron-generated current should be high enough compared with the ac component of the current generated by thermalized electrons in a residual resistance, which forces some changes in the usual BEEM configuration. A suitable experimental setup is proposed. When implemented, this technique will allow the visualization of subsurface wave electron effects in semiconductor structures.
Dominant diffusing species during cobalt silicide formation
Authors: Comrie C. M. , Newman R. T.
Published: July 2002

The dominant moving species during cobalt monosilicide and cobalt disilicide formation has been examined using a thin tantalum layer as a metal marker. The marker data obtained following the formation of CoSi from Co2Si showed that monosilicide growth was essentially due Si diffusion only. When used to study CoSi2 formation, the data indicated that silicon was also the dominant moving species during disilicide formation, although a noninsignificant amount of cobalt diffusion was also observed to take place.
Reinvestigation and extension of the steady-state Nyquist theorem for multi-terminal semiconductor devices and its application to minimum noise figure in microwave field effect transistors
Authors: Lee J. B. , Min H. S. , Park Y. J.
Published: July 2002

The steady-state Nyquist theorem for multi-terminal semiconductor devices, which describes the short-circuit thermal noise currents in arbitrarily shaped multi-terminal semiconductor devices, is reinvestigated. The concept of the equipotential surfaces for small ac signals is found to be wrong, which forces us to introduce a new approach to find the equipotential surfaces for noise voltages. Using this new approach, we rederive the steady-state Nyquist theorem with the same results as in the previous derivation, and extend it to the hot carrier regime. Using the extended theorem, we express the minimum noise figure for microwave field effect transistors as a function of the measurable device parameters with no fitting constant. The derived minimum noise figure is shown to reduce to a simple form, which can also be used as an empirical relation with one fitting constant. This simple form can explain the experimental results very well in wide frequency ranges, and gives through a clear mathematical relation the empirically known requirements for the low noise design of microwave field effect transistors.
Photoluminescence studies of band-edge transitions in GaN epitaxial layers grown by plasma-assisted molecular beam epitaxy
Authors: Chen G. D. , Smith M. , Lin J. Y. , Jiang H. X. , Salvador A. , Sverdlov B. N. , Botchkarv A. , Morkoc H.
Published: July 2002

Continuous-wave and time-resolved photoluminescence spectroscopies have been employed to study the band-edge transitions in GaN epitaxial layers grown by plasma assisted molecular beam epitaxy. In addition to the neutral-donor-bound exciton transition (the I2 line), a transition line at about 83 meV below the band gap has been observed in an epitaxial layer grown under a lower plasma power or growth rate. This emission line has been assigned to the band-to-impurity transition resulting from the recombination between electrons bound to shallow donors and free holes D0, h+). Systematic studies of these optical transitions have been carried out under different temperatures and excitation intensities. The temperature variation of the spectral peak position of the D0, h+) emission line differs from the band gap variation with temperature, but is consistent with an existing theory for D0, h+) transitions. The dynamic processes of the D0, h+) transition have also been investigated and subnanosecond recombination lifetimes have been observed. The emission energy and the temperature dependencies of the recombination lifetime have been measured. These results have provided solid evidence for the assignment of the D0, h+) transition and show that the motions of the free holes which participated in this transition are more or less restricted in the plane of the epitaxial layer at temperatures below 140 K and that the thermal quenching of the emission intensity of this transition is due to the dissociation of neutral donors. Our results show that time-resolved photoluminescence spectroscopy can be of immense value in understanding the optical recombination dynamics in GaN.
Planar and channel waveguide fabrication in LiB3O5 using MeV He+ ion implantation
Authors: Davis G. M. , Zhang L. , Chandler P. J. , Townsend P. D.
Published: July 2002

Multiple energy He+ ion implantation to increase the width of the implantation-induced refraction index barrier in LiB3O5 (LBO) is reported. Improvements in planar waveguide transmission (coupling and propagation) at the HeNe laser wavelength of 633 nm, from 5% to 45% for TE polarized light, and from 10% to 28% for TM polarized light in a ∼5-mm-long LBO planar waveguide are shown. Annealing is found to increase the transmission (coupling and propagation) at the wavelength of 633 nm of multiple-energy implanted planar LBO waveguides by as much as 24% for TE polarized light and 71% for TM polarized light. Channel waveguides in LBO fabricated by multiple-energy ion implantation through a gold wire mask and their use for second-harmonic generation are reported.
Magnetic and magnetoresistance studies on radio frequency sputtered La–Pb–Mn–O films
Authors: Srinivasan G. , Brusca T. E. , Fisher A. S. , Suresh Babu V. , Seehra M. S.
Published: July 2002

Results on structural and magnetic characterization of radio frequency sputtered thin films of La0.74Pb0.26MnOz on (100) Si are presented. Stoichiometric films with a thickness of 0.6 μm showing (110) textured growth were deposited in a mixed argon–oxygen atmosphere. The low temperature magnetization is 10% smaller than the expected value for a collinear ferromagnetic order in the oxide. Zero-field resistivity data show a metal-to-semiconductor transition centered at Tms=250 K. Data on magnetoresistance (MR)=[ρ(0)−ρ(H)]/ρ(0) versus temperature reveal the following important features: (i) a maximum value of 22% for H=2 T at the metal-to-insulator transition temperature, (ii) MR value of 15%–22% over the entire temperature range 4.2–300 K, and (iii) a substantial MR, ∼10% even at temperatures as high as 380 K, well above the Curie temperature. Data on the temperature dependence of ferromagnetic resonance linewidth at 9.4 GHz show a discontinuity at Tms.
Observation of perpendicular anisotropy in granular magnetic solids
Authors: Xiao John Q. , Chien C. L. , Gavrin A.
Published: July 2002

Granular Co-Ag materials with low Co contents exhibit giant magnetoresistance and single-domain magnetic properties. In this paper, we discuss unusual hysteresis behavior that we have observed in Co-rich samples, where the magnetization is approximately linear with the applied field up to saturation, and the coercivity and remnant magnetization are both nearly zero. We attribute this unusual behavior to a perpendicular anisotropy that depends on the nanostructure in the phase-segregated materials. These results are corroborated by magnetic domain imaging using scanning electron microscopy with polarization analysis.
Numerical analysis of charge transport in semi-insulating GaAs with two contacts
Authors: Zdansky K. , Jones B. K. , Santana J. , Sloan T.
Published: July 2002

The dc, steady state charge transport in 200-μm-thick semi-insulating GaAs samples with two large whole area metal contacts is calculated numerically. The material is assumed to have shallow donors and an excess of deep acceptors. The distributions of the space charge density and of electron and hole conductivities are calculated for different electron and hole-supplying contacts with different bias voltages. It is found that the contact which injects majority carriers into the semiconductor due to the voltage bias determines the distribution of the space charge density in a greater volume of the sample than the other contact. Consequently, this contact is decisive for the space distribution of the electric field and the shape of the current–voltage (I–V) characteristic. Comparison is made with experimental observations of the I–V characteristics of one of our detector of particles.
Trapping of radiation in a laser ablated YBa2Cu3O7 plume
Authors: Riley D. , Doyle L. , Al-Wazzan R.
Published: July 2002

A tunable dye laser has been used to selectively photo-pump transitions in a laser ablated YBa2Cu3O7 plume. The resultant fluorescence and scattered light has been monitored both with a photo-multiplier tube and a gated intensified CCD array camera. The spatial and temporal characteristics of the fluorescence are used to demonstrate that phonons emitted within the plume are strongly reabsorbed, and thus radiation is effectively trapped. Measurement of the degree of trapping is limited due to an alternative radiative decay channel, but a maximum escape probability of about 10% can be inferred, even several centimeters from the target surface, and more than 10 μs after the initial ablation pulse. The implications for plasma spectroscopy and laser induced fluorescence studies of ablated plumes are discussed.
X-ray diffraction study of GaAs/InAs/GaAs ultrathin single quantum well
Authors: Bai Jie , Liu Wenhan , Wu Ziqin , Wang Yutian , Xiu Lisong , Jiang Xiaoming
Published: July 2002

Ultrathin single quantum well (about one monolayer) grown on GaAs(001) substrate with GaAs cap layer has been studied by high resolution x-ray diffractometer on a beamline of the Beijing Synchrotron Radiation Facility. The interference fringes on both sides of the GaAs(004) Bragg peak are asymmetric and a range of weak fringes in the higher angle side of the Bragg peak is observed. The simulated results by using the kinematical diffraction method shows that the weak fringe range appears in the higher angle side when the phase shift introduced by the single quantum well is very slightly smaller than mπ (m:integer), and vice versa. After introducing a reasonable model of single quantum well, the simulated pattern is in good agreement with the experiment.
Time-temperature-transformation study of a nanocrystalline Fe91Zr7B2 soft magnetic alloy
Authors: Suzuki K. , Cadogan J. M. , Sahajwalla V. , Inoue A. , Masumoto T.
Published: July 2002

The structure, soft magnetic properties, magnetostriction, grain size and volume fraction of the residual amorphous phase have been studied for an amorphous Fe91Zr7B2 alloy annealed for periods of 60 s–1080 ks, at temperatures of 823–973 K. The highest permeability (at 1 kHz and 0.4 A/m) 31,000 and smallest coercivity 4.2 A/m are obtained for the sample annealed at 923 K for 60 s, where a small grain size (≊12 nm) and about 35% of the residual amorphous phase are measured. The coercivity of the nanocrystalline alloy varies as the 3rd power of the grain size, corresponding to the case where the magnetically coupled region has a dimensionality between two and three. This slightly low dimensionality is presumed to be due to the alignment of the magnetization in the sample plane.
Magnetic hardening of melt-spun nanocomposite Nd2Fe14B/Fe magnets
Authors: Panagiotopoulos I. , Withanawasam L. , Murthy A. S. , Hadjipanayis G. C. , Singleton E. W. , Sellmyer D. J.
Published: July 2002

Coercivity optimization studies were done on melt-spun nanocomposite Nd4R2Fe87−xNbTxB6 (R=Nd,Y,Dy; T=Ag,Cu) isotropic ribbon samples. The maximum attainable coercivities, after adjusting the annealing time, were found to be very sensitive to the annealing temperatures. The optimum magnetic properties [HC=3.9 kOe, (BH)max=10 MGOe] were obtained by annealing at 750–775 °C for a few minutes. Optimization by flash annealing gave similar results. Microstructural studies show that the grain size is greater than the theoretically predicted grain size for optimum coupling between the hard and the soft phase. With the annealing conditions used, Nd4Dy2Fe87NbB6 samples gave moderate coercivities and in Nd4Y2Fe87NbB6 samples the coercivity was reduced more than the expected reduction in the anisotropy field due to the presence of Y.
Studies of anisotropic and giant magnetoresistance in Co/Cu(111) epitaxial multilayers
Authors: Huang J. C. A. , Lee Y. H. , Hu Y. M. , Chang T. C.
Published: July 2002

Co/Cu(111) multilayers, [Co(17 Å)/Cu(8 Å<tCu<14 Å)]30, have been prepared on Co(70 Å) buffer layers on Al2O3(0001) substrates by molecular beam epitaxy. From the longitudinal and transverse magnetoresistance (MR) measurements, it is observed that MRs consist of two components with a small anisotropic MR (<2%) component at low field sitting on top of the giant MR (up to 22%) component at higher field. The AMR effect strongly correlates with the abundance of hcp stacking of Co, which tends to decrease with the increasing of Cu spacer thickness. The AMR saturation fields (1–3 kOe) coincides with those of the magnetization. It is suggested that the observed AMR effect is due to scattering from the hcp-phase Co layers in the multilayers. This together with the large saturation field (30–40 kOe) obtained from the entire MR curves indicate that the observed GMR effect may result from the Co-Cu interfacial spin-dependent scattering.
Magnetoresistance in NiFe/Au multilayers and spin-valve structures (abstract)
Authors: Wu K. T. , Gambino R. J.
Published: July 2002

Permalloy has been widely studied and used in industry for magnetic recording heads and sensors. This article presents a study of the magnetoresistance of (NiFe/Au)n multilayer films and NiFe/Au/NiFe sandwich structures. Gold is used as the spacer layer because of its low solid solubility in NiFe and the tendency of NiFe and Au to phase separate. Films were grown by dc sputtering with a series of different permalloy and Au thickness. In the multilayer films, the thickness of permalloy varies from 20 to 40 Å and Au varies from 8 to 25 Å. A difference in the sign and magnitude of magnetoresistance for transverse and longitudinal field directions with respect to current indicates the presence of anisotropic magnetoresistance (AMR) effect. The magnitude of Δρ/ρ is small because of shunting by Au layers. In the sandwich structures, the thickness of permalloy layers is different on each side of the Au and varies from 150 to 800 Å. The Au spacer layer varies from 20 to 60 Å. The measurements show mainly anisotropic magnetoresistance. There was a small opposite jump at ∼8 Oe on resistivity measurement, which indicates spin valve effect superimposed on the AMR effect. After annealing, the change in magnetoresistance is much more sensitive in low field ∼15 Oe, a property which is critical to many applications.
Temperature dependence of the pinning field and coercivity of NiFe layers coupled with an antiferromagnetic FeMn layer
Authors: Fujiwara H. , Nishioka K. , Hou C. , Parker M. R. , Gangopadhyay S. , Metzger R.
Published: July 2002

The pinning field Hp (the amount of the shift of the hysteresis loops) and the coercivity Hc of the samples of the form glass/Ta 120 A/(Cu 100 A)/NiFe 75 A/FeMn 150 A/Ta 50 A increase almost linearly with decreasing temperature down to 20 K, below which Hc increases sharply. The observed strong positive correlation between Hp and Hc, seems to be reasonably explained by a combination of a newly developed model in which a directional distribution of the pinning field caused by a random distribution of the crystalline orientations in the antiferromagnetic FeMn layer is taken into account and Hoffmann’s ripple theory in which the local anisotropy is assumed to be proportional to Hp, although the sharp increase in Hc at very low temperatures remains to be explained.
Experimental and analytical properties of 0.2-μm-wide, end-on, multilayer, giant magnetoresistance, read head sensors
Authors: Pohm A. V. , Beech R. S. , Daughton J. M. , Chen E. Y. , Durlam M. , Nordquist K. , Zhu T. , Tehrani S.
Published: July 2002

Sensing elements which are 0.20 μm wide have been fabricated from giant magnetoresistance material that has four magnetic layers separated by 16–17 Å copper alloy layers. The material has a bulk magnetoresistive coefficient of 12%–15%, and when etched into the 0.2-μm-wide stripes, the stripe material has a shifted linear response range of about 8%, extending from −140 to +140 Oe, for example. Empirical models have been developed for both the bulk material and the stripe material. Using the stripe material model, a numerical analysis was performed for end-on, 0.2-μm-wide, read head sensors in a shielded read head with a 0.25 μm head gap. The analysis shows that with input fields of ±100 Oe from transitions, a peak to peak output of 0.85 mV can be achieved in the assumed read head structure with a 0.20-μm-wide tip. If a 0.25 μm flux gathering tip is used, the peak to peak output can be increased to 1.3 mV.
Calculation of the wave-vector-dependent interband impact-ionization transition rate in wurtzite and zinc-blende phases of bulk GaN
Authors: Kolnik Jan , Oguzman Ismail H. , Brennan Kevin F. , Wang R. , Ruden P. Paul
Published: July 2002

We present calculations of the wave-vector-dependent interband impact-ionization transition rate in wurtzite and zinc-blende phases of bulk GaN. The transition rate is determined by integrating Fermi’s golden rule for a two-body, screened Coulomb interaction over the possible final states using a numerically generated dielectric function and pseudowavefunctions. The full details of all relevant conduction and valence bands in zinc-blende and wurtzite GaN are included from an empirical pseudopotential calculation. It is found that the transition rate is consistent with a relatively ‘‘soft’’ threshold energy.
Formation of EL2, AsGa and U band in irradiated GaAs: Effects of annealing
Authors: Jorio A. , Carlone C. , Parenteau M. , Aktik C. , Rowell N. L.
Published: July 2002

Gallium arsenide (GaAs) which was grown by metallorganic chemical vapor deposition, doped n with silicon to 2×1015 cm−3, and irradiated at room temperature with 1 MeV neutrons in the fluence range 1012 cm−2 to 3×1015 cm−2, has been studied by deep level transient spectroscopy (DLTS) and by far infrared photoluminescence (PL) spectroscopy. We report the effect of annealing at 550 °C for 30 min, which, in irradiated GaAs, is to introduce the gallium vacancy (VGa). The DLTS signal at 780±40 meV, attributed to the EL2 deep level, has an introduction rate of about 3×10−2 cm−1 in the unannealed case, and 0.19±0.02 cm−1 in the annealed case. The PL signal at 702 meV has been attributed to a phonon-assisted transition of the EL2 defect, a defect which has been identified as the isolated arsenic antisite (AsGa+). The PL peak increases in intensity up to fluences of 3×1013 cm−2, before decreasing at higher fluences. After annealing, its intensity increases up to fluences of 1015 cm−2, before decreasing at the highest fluence studied. The similar behavior of the DLTS and PL signals with respect to irradiation and annealing confirms that they have the same origin. It also supports the AsGa point defect model of EL2. The decrease in PL intensity at high fluences is attributed to radiationless transitions involving radiation-induced complexes such as the U band, EL6, and EL14 observed by DLTS. After annealing, the concentration of these defects is reduced to a large extent, but weaker sharper peaks remain in the DLTS spectrum. We suggest that the U band consists of complexes involving AsGa and acceptors like VGa.
Analysis of thin CdS layers on InP for improved metal–insulator–semiconductor devices
Authors: Dauplaise Helen M. , Vaccaro Kenneth , Davis Andrew , Ramseyer George O. , Lorenzo Joseph P.
Published: July 2002

Cadmium sulfide (CdS) layers were deposited from an aqueous solution of thiourea, cadmium sulfate, and ammonia on (100) n-InP at 60–95 °C. X-ray photoelectron spectroscopy showed that the deposition process effectively removes native oxides on InP and forms a protective layer for subsequent dielectric deposition. Surface analysis also showed that the InP surface is not P deficient following oxide deposition on CdS-treated InP. Capacitance–voltage and conductance–voltage measurements of metal–insulator–semiconductor (MIS) capacitors were used to compare samples with and without CdS films between InP and a deposited insulator. Capacitance–voltage response of CdS-treated MIS structures showed well-defined regions of accumulation, depletion, and inversion. The interface-state density at midgap was reduced from 5×1011 to 6×1010 eV−1 cm−2 with CdS treatment. Depletion-mode MIS field-effect transistors made using this new passivation technique exhibited superior device performance to that of untreated samples.
On the contribution of vacancy complexes to the saturation of the carrier concentration in zinc doped InP
Authors: Mahony J. , Mascher P. , Puff W.
Published: July 2002

Positron annihilation spectroscopy on Zn-doped InP has revealed the presence of a defect with a positron lifetime of ∼330 ps in samples in which the carrier concentration has saturated. This lifetime is attributed to a complex involving vacancies and Zn atoms. A model is proposed in which this complex has a (−/0) level near the bottom of the band gap, and undergoes a large inward lattice relaxation upon the transition to the neutral charge state, causing a reduction in the positron lifetime to ∼281 ps. This model explains the positron annihilation results on annealed samples and at low temperatures, and is supported by Hall effect measurements. The concentration of these complexes is less than 1017 cm−3. Therefore, these complexes cannot solely account for the observed discrepancy between the carrier concentration and the Zn concentration in very heavily Zn-doped InP.
Super band-gap time-resolved luminescence study of degenerate electron–hole plasma in thin GaAs epilayers
Authors: Poles E. , Goldberg S. Y. , Fainberg B. , Huppert D. , Hanna M. C. , Rosenwaks Y.
Published: July 2002

Super band-gap time-resolved photoluminescence is employed to measure the transport properties of degenerate electron–hole gas in thin GaAs epilayers. It is found that the luminescence decay at wavelengths shorter then the energy gap wavelength is much faster then expected from a simple diffusion–reabsorption model. The results are explained by using a transport model based on Fermi–Dirac carrier statistics and nonparabolic band structure. We have found that only by introducing the above two phenomena the photoluminescence spectra at all energies can be fitted. The importance of the results in studies of hot carrier energy loss rates is discussed.
Impact ionization rate and high-field transport in ZnS with nonlocal band structure
Authors: Reigrotzki Martin , Redmer Ronald , Lee Insook , Pennathur Shankar S. , Dür Manfred , Wager John F. , Goodnick Stephen M. , Vogl Peter , Eckstein Harald , Schattke Wolfgang
Published: July 2002

The impact ionization rate in ZnS is calculated using a nonlocal empirical pseudopotential band structure and compared to previous results using a local calculation. The two resulting rates are then compared and simple fit formulas are presented. These are included in an ensemble Monte Carlo simulation of electron transport in bulk ZnS. The calculated impact ionization rate is then compared to experimental impact ionization coefficient data; reasonable agreement between the experimental data and the calculated impact ionization rate is obtained with an appropriate choice of optical deformation potentials.
Linearity of high-T
c dc superconducting quantum interference device operated in a flux-locked loop
Authors: Nichols D. G. , Dantsker E. , Kleiner R. , Mück M. , Clarke John
Published: July 2002

Measurements have been made of the linearity of a high transition temperature dc superconducting quantum interference device (SQUID) operated at 77 K with 130 kHz flux modulation in a flux-locked loop. The degree of nonlinearity was determined from harmonic generation. A sinusoidal magnetic flux with harmonic content less than −130 dB was applied to the SQUID, which was cooled in a magnetic field below 10−7 T, and the harmonics at the output of the flux-locked loop were measured with a spectrum analyzer. For input signals at frequencies up to 248 Hz and amplitudes up to 20Φ0 rms (Φ0 is the flux quantum), the second, third, and fourth harmonics were each at least 115 dB below the fundamental. At higher frequencies the harmonic content began to increase because of the reduction in the open-loop gain of the flux-locked loop. The magnitude of the harmonics was not measurably changed when the SQUID was cooled in a field of 100 μT. The amplitudes of the even harmonics depended critically on the amplitude of the 130 kHz flux modulation, and became zero when its peak-to-peak value was precisely Φ0/2.
Optical characterization of Al0.5Ga0.5As1−xSbx buffer layers with modulation doped AlInAs/InGaAs structures
Authors: Svensson Stefan P. , Gill David M. , Uppal Parvez N.
Published: July 2002

We present results of photoluminescence studies of Al0.5Ga0.5As1−xSbx grown by molecular beam epitaxy on GaAs substrates. We compared the photoluminescence (PL) peak position as a function of composition with the band gap prediction using the established formalism developed for other quaternaries. We found a discrepancy of up to 0.2 eV indicating that an extra bowing factor needs to be included in the band gap composition equation for this material system. InGaAs/AlInAs structures grown on top of the Al0.5Ga0.5As1−xSbx were also characterized and compared with similar structures grown on InP and were shown to exhibit similar PL intensities and Hall mobilities, indicating that the quaternary buffer layer can be used for growth of high-In concentration InGaAs device structures on GaAs.
Modeling of time resolved x-ray diffraction from laser-shocked crystals
Authors: Woolsey N. C. , Wark J. S.
Published: July 2002

The rapid laser driven shock compression of solids is modeled using a hydrodynamic computer code. By postprocessing output from these simulations and using a new, efficient x-ray diffraction algorithm we calculate time-dependent x-ray diffraction images which can be directly compared with experimental x-ray diffraction data. The measurement of shock compression of crystalline solids by x-ray diffraction is illustrated with three examples, one a square wave, and two triangular waves. The development of these waves and the associated x-ray diffraction patterns are followed as the waves propagate through the crystalline solid and interact with a free surface. Simulated x-ray streak camera data created with this hydrodynamic-x-ray diffraction model are presented and are compared to experimental data.
High temperature optical properties of cadmium telluride
Authors: Mullins J. T. , Carles J. , Brinkman A. W.
Published: July 2002

The high temperature optical properties of cadmium telluride have been studied at temperatures up to 1104 K by measurement of the incandescence spectra from a wafer of that material. These measurements provided information equivalent to that of a transmission experiment and allowed the optical absorption spectra of the material to be determined using a theoretical expression for the refractive index and its dispersion with wavelength modified to account for the effect of temperature. This analysis required only a knowledge of the temperature dependence of the fundamental gap of cadmium telluride. Absorption by intrinsic carriers was not found to be significant at photon energies in the region of the fundamental gap even at such elevated temperatures and accordingly the near band-edge absorption was clearly discernable. As is typical in II-VI compound semiconductors, a disorder related exponential absorption (Urbach) tail was observed below the fundamental (mobility) gap and it was to such values of the absorption coefficient that this experiment was sensitive for the relatively thick sample used here. The absorption data were well fitted by assuming the energy gap to have a linear temperature coefficient of −0.34±0.02 meV K−1 and determining the absorption spectra self-consistently with that temperature dependence. At very low values of absorption, however, the condition of the sample surface dominated the spectra. Such measurements may have application in the in situ monitoring of the bulk growth of CdTe by vapour phase techniques, providing information regarding temperature, structural disorder, and surface stoichiometry.
Ferromagnetic resonance studies of amorphous and nanocrystalline FeCuNbSiB alloys
Authors: Schmool D. S. , Gorria P. , Barandiarán J. M. , Leccabue F. , Watts B. E.
Published: July 2002

Alloys with composition Fe73.5Cu1Nb3Si22.5−xBx, where x=6 and 9, have been studied from the as-cast state, through various stages of crystallization, in annealing range of 300–650 °C, by the technique of ferromagnetic resonance (FMR). The annealing was performed isothermally at preset temperatures to produce nanocrystalline structures in an amorphous matrix. Both the nanocrystalline structures and the surrounding amorphous matrix are ferromagnetic, and will therefore contribute to the FMR spectra. The spectral features, resonance field, intensity, and linewidth have been used to characterize the structurally related changes in the sample during the crystallization process. The major changes in the spectra are observed to occur in the region of the crystallization peak in the differential thermal analysis curves for these samples. The FMR spectra exhibit a complex in-plane angular variation, which is understood in terms of preferential orientation of the magnetization vector in the direction of the ribbon, and shape effects. The square cut samples give rise to multipeaked spectra when the external magnetic field is applied in an “off-square” direction. This is the first report of the appearance of a second resonance feature of this type. This is explained as arising from the magnetic confinement of the nanocrystallites in the amorphous matrix, producing spin waves localized at the interface of the two phases.
Surface magnetic microstructure of melt-spun magnetic ribbons
Authors: Seddon E. A. , Xu Y. B. , Greig D. , Wardell I. R. M. , Rubio-Temprano D. , Hardiman M.
Published: July 2002

Scanning electron microscopy with polarization analysis (SEMPA) and photon-excited spin polarized secondary electron spectroscopy have been used to study respectively the lateral variations and depth profiles of the surface magnetic microstructure of as-cast amorphous melt-spun Fe80B20 ribbons. The 5 μm resolution SEMPA images showed very similar complex domain patterns for both remanent and field-on states, and spot mode hysteresis loops showed significant differences across the sample. These data are consistent with a very wide distribution of strain-induced anisotropy fields. Energy resolved hysteresis loops (ERHL) measured using photoemitted 1 and 20 eV secondary electrons showed clear differences in form, attributable to changes in probing depth. A unidirectional anisotropy found in the 20 eV ERHL is linked tentatively to the inhomogeneities revealed by SEMPA.
Investigation of flux creep in high-Tc superconductors using Hall-sensor array
Authors: Abulafia Y. , Giller D. , Wolfus Y. , Shaulov A. , Yeshurun Y. , Majer D. , Zeldov E. , Peng J. L. , Greene R. L.
Published: July 2002

The details of a new method for studying thermally activated flux creep in thin superconducting samples is described. The method employs a linear array of microscopic Hall sensors to measure the time and spatial dependence of the magnetic induction across the sample. These data are analyzed on the basis of the local rate equation for thermally activated flux motion, taking into account both the in-plane and out-of-plane components of the induction field. Following this analysis, flux creep parameters, such as the flux-line current density, flux-line average velocity, and the activation energy for flux creep, can be directly determined as a function of position and time. New experimental data in a superconducting Nd1.85Ce0.15CuO4−δ crystal demonstrate this method.
Si
1−x−y
Ge
x
C
y
alloy band structures by linear combination of atomic orbitals
Authors: Orner B. A. , Kolodzey J.
Published: July 2002

We have applied a virtual crystal approximation to the linear combination of atomic orbitals method to calculate critical point energies of unstrained Si1−x−yGexCy alloys spanning the composition parameter space. Additionally, we have calculated the band structure across the Brillouin zone for a series of alloy compositions. We found the band energies had significant bowing departures from linearity throughout the system. In some cases, the energy band gap was not monotonically dependent on composition. Our theoretical results are compared with recent experimental results, and good agreement was found overall.
A Monte Carlo study on electron mobility in quantized cubic silicon carbide inversion layers
Authors: Gámiz F. , Roldán J. B. , López-Villanueva J. A.
Published: July 2002

Electron transport properties in cubic silicon carbide (β-SiC) quantized inversion layers have been studied and the results of electron mobility calculations at room and higher temperatures have been reported. To do so, we have developed a Monte Carlo simulator used in conjunction with the self-consistent solution of the Poisson and Schroedinger equations. We show that for a fixed inversion charge concentration, β-SiC inversion layer electrons spread less into the bulk than Si ones as a consequence of the effective mass values. Therefore, the defects of the SiO2/β-SiC (interface roughness, charged centers) will strongly affect electron transport properties. We present simulated mobility curves for quantized β-SiC inversion layers taking into account different scattering mechanisms which are then compared to Si mobility curves. Special attention has been paid to the effect of Coulomb scattering due to both interface- and oxide-trapped charges. Mobility curves obtained for different interface-trapped charge concentrations show that electrons in silicon carbide inversion layers are more affected by surface defects at room and higher temperatures than they are in silicon inversion layers.
X-ray photoelectron spectroscopy study of rapid thermal annealed silicon–silicon oxide systems
Authors: Choi W. K. , Poon F. W. , Loh F. C. , Tan K. L.
Published: July 2002

We present the results of an investigation on the effects of rapid thermal annealing (RTA) temperature (Tp) and time (tp) on the structural and electrical properties of silicon–silicon oxide systems. X-ray photoelectron spectroscopy (XPS) was used to provide information on the oxide composition of the annealed oxide sample. We found that a higher Tp and/or a longer tp will increase the percentage of SiO2 in the annealed oxide layer and thus improve the oxide quality. We also discovered that increasing Tp and/or tp will result in a thicker oxide layer. The suboxide density calculation based on the XPS results indicates that the Si–SiO2 interface of our RTA samples is not abrupt. We have used the conclusions obtained from the XPS study to provide satisfactory explanations for the different current versus voltage characteristics exhibited by our tunnel diodes.
Hole confinement and mobility in heterostructure Si/Ge/Si p-channel metal–oxide–semiconductor field effect transistors
Authors: Cyca B. R. , Robins K. G. , Tarr N. G. , Xu D. X. , Noel J.-P. , Landheer D. , Simard-Normandin M.
Published: July 2002

Heterostructure Si/Ge/Si p-metal–oxide–semiconductor field effect transistors (MOSFETs) with 1-nm-thick pure Ge channels grown pseudomorphically on Si substrates have been fabricated and characterized. Simultaneous solution of Schrodinger and Poisson’s equations reveals that the 1-nm-thick Ge region can effectively confine holes in a subsurface channel. This result is confirmed through the fabrication of test MOS capacitors and MOSFETs using a process with plasma enhanced chemical vapor deposition gate oxide and a peak thermal budget of just 5 s at 600 °C. (Raman spectroscopy shows that this thermal treatment does not significantly relax the strain in the Ge layer). However, transconductance measurements on the MOSFETs indicate that the mobility of holes in the buried channel is substantially less than at the Si surface. It is speculated that this poor mobility may result from hole scattering at the abrupt Si/Ge interface.
Domain phases in antiferromagnetically coupled sandwiches
Authors: Persat N. , van den Berg H. A. M. , Cherifi-Khodjaoui K. , Dinia A.
Published: July 2002

The impact of buffer stacks containing Cr/Fe sandwiches on the quality of the antiferromagnetic coupling in Co(1.2 nm)/Cu(0.83 nm)/Co(1.2 nm) sputtered trilayers has been investigated. Coupling strengths J larger than 0.4 erg/cm2 have been realized. The completeness of the antiferromagnetic alignment of both Co layers at zero field has been probed with the soft magnetic layer in the buffer stack. No remanence could be detected, demonstrating the completeness of the AF coupling across the sandwiches. The lack in remanence is partly due to the (111) texture imposed by the buffer and polycrystallinity of the Co layers, causing low rotational friction. The freedom in the sense of rotation of the Co layers causes a dense domain configuration with domain walls having their magnetization in the centers parallel to the original saturation field Hs. This causes an increment in the magnetization component in the direction of Hs and a reduction in the resistance. The state of high domain density converts into a low density one by annihilation of domains at positive field, so that no remanence due to these domains is detected.
Electronic structure calculations of doped organic materials for electroluminescent devices
Authors: Nilar Shahul H. , Dharma-wardana M. W. C.
Published: July 2002

Recent very promising fabrication advances in organic light emitting diodes have brought out the need for a microscopic understanding of the interaction of light emitters, hole blockers, hole transporters, dopants, and their electroluminescence. We report exploratory quantum calculations on spectra with and without the presence of an external electric field, with different “guest-host” arrangements of emitter and dopant molecules. These calculations, when suitably extended, could be used for predictive optimization of candidate materials, and for the design of optimal layer-structure and layer-ordering etc., in diodes and other proposed structures. The molecules studied are: Scandium 8-hydroxyquinoline as the light emitter, a 1,2,4-triazole molecule as a typical hole blocker, and triphenyl diamine derivatives as hole transporters. The dopant or “guest” molecule was selected to be Nile Red. Studies on the use of a highly conjugated system such as C60 as an element of layer structure are also presented here. The results give an insight into the physical processes at the molecular level and provide guidance in designing optimized material for use in “organic” light emitting diodes.
Laser power effects on the Raman spectrum of isolated diamond chemical vapor deposition particles
Authors: Laikhtman A. , Hoffman A.
Published: July 2002

In the present work the effect of incident laser power on the Raman spectra of diamond isolated particles and continuous films deposited on silicon and glassy carbon (GC) substrates by the chemical vapor deposition method is investigated. It is shown that the Raman line position measured for diamond particles shifts to lower wave numbers as a function of incident laser power. These shifts were most drastic for single particles deposited on GC that were examined using a Raman microprobe. In this case the diamond peak displayed a negative shift of ∼18 cm−1 when the laser power output was increased from 1 to 15 mW. The laser beam diameter was ∼2 μm and the diamond particle measured was 3–6 μm in diameter. Micro-Raman measurements of diamond particles deposited on a silicon substrate or continuous diamond films on GC display very small changes in the diamond Raman peak wavelength for the same laser power range. From our studies it is concluded that the negative shift of the Raman peak position is caused by laser-induced local heating of the irradiated diamond particles. The temperature under the laser spot was calculated from the intensity ratio of Stokes to anti-Stokes Raman lines measured as a function of laser power output. The Raman peak wavelength calculated for each temperature showed excellent agreement with our experimental results. The local temperature of an isolated diamond crystal on GC rises to ∼1000 K at 15 mW laser power output, whereas the temperature change of the continuous film on GC and of a single particle on silicon was in the 0–30 K range above room temperature for the same laser power output range. This difference in heating is explained on the basis of efficient heat dissipation through a large contact area between the deposited particles and the substrate surface in the case of single particles deposited on silicon or through grain boundaries in the case of the continuous film on GC. The inefficient heat dissipation from the isolated diamond particles on GC is related to the small contact area between the diamond crystals and the GC substrate as a result of etching during the deposition process and possibly to the presence of an amorphous component in the diamond crystals deposited.
Interfacial reaction and formation mechanism of epitaxial CoSi2 by rapid thermal annealing in Co/Ti/Si(100) system
Authors: Kim Gi Bum , Kwak Joon Seop , Baik Hong Koo , Lee Sung Man
Published: July 2002

A ternary compound of Co3Ti2Si is suggested as a reaction barrier for the formation of epitaxial CoSi2 in the Co/Ti/Si system when adopting the rapid thermal annealing process. It controls Co diffusion to the Si substrate, followed by formation of epitaxial CoSi2. After the epitaxial CoSi2 was formed, the interfacial morphology of the upper layer/ CoSi2 interface was very different according to silicidation temperature, that is, the interface was planar at 800 °C, but rough at 900 °C. This was attributed to the reaction between the upper layer consisting of Co–Ti–Si and the CoSi2 layer at 900 °C, which resulted in Ti-rich precipitates at the surface. The Ti-rich precipitates acted as a diffusion sink of dopant, thus, the leakage current density for the silicidation temperature of 900 °C was much higher than that for the temperature of 800 °C. These results suggest that the silicidation temperature is one of the most critical factors in determining the leakage current of the p+n junction diode.
Transient enhanced diffusion of boron in presence of end-of-range defects
Authors: Bonafos C. , Omri M. , de Mauduit B. , BenAssayag G. , Claverie A. , Alquier D. , Martinez A. , Mathiot D.
Published: July 2002

The presence of a supersaturation of Si self-interstitials in ion implanted silicon has been shown to be the origin of several physical phenomena such as transient enhanced diffusion (TED) of boron, the formation of extended defects at the projected range of implanted atoms at doses below the amorphization threshold, and the formation of end-of-range (EOR) defects in the case of a preamorphization stage. In this article, we discuss the relation between boron anomalous diffusion and end-of-range defects. Modeling of the behavior of these defects upon annealing allows one to understand why and how they affect dopant diffusion. This is possible through the development of the Ostwald ripening theory applied to extrinsic dislocation loops. This theory is shown to give access to the variations of the mean supersaturation of Si self-interstitial atoms between the loops and also to be responsible for anomalous diffusion. This initial supersaturation is, before annealing, at least five decades larger than the equilibrium value and exponentially decays with time upon annealing with activation energies that are the same as the ones observed for TED. It is shown that this time decay is precisely at the origin of the transient enhancement of boron diffusivity through the interstitial component of boron diffusion. Side experiments shed light on the effect of the proximity of a free surface on the thermal behavior of EOR defects and allow us to quantitatively describe the space and time evolutions of boron diffusivity upon annealing of preamorphized Si layers.
In situ reflectance difference spectroscopy and reflection high-energy electron diffraction observation of nitridation processes on GaAs(001) surfaces
Authors: Jung H. D. , Kumagai N. , Hanada T. , Zhu Z. , Yao T. , Yasuda T. , Kimura K.
Published: July 2002

Nitridation processes on GaAs(001) surfaces exposed to N2 microwave plasma were investigated by in situ reflectance-difference spectroscopy, reflection high-energy electron diffraction, and in-line Auger electron spectroscopy. We have found that a stable GaN layer is formed only when the As background pressure is sufficiently low. Nitridation is significantly suppressed under a high background pressure of As. A possible mechanism and its implication to GaN growth on GaAs surfaces are discussed.