bluuebunny/crossref_metadata_2025 huggingface dataset:
Re-establishment of photoluminescence in Cu quenched porous silicon by acid treatment
Authors: Hilliard Joseph E. , Nayfeh Hasan M. , Nayfeh Munir H.
Published: July 2002

HCl, H3PO4, and H2SO4 baths have been used to re-establish the photoluminescence of porous Si whose initial luminescence had been quenched by immersion in CuCl2. HCl had the fastest effect, with luminescence beginning to reappear after typically a few minutes, while the other acids required typically one to two days for dim luminescence to start reappearing. Auger electron spectroscopy (AES) depth profiling showed that after quenching, the Cu profile typically penetrated 2000 to 3000 Å into the porous Si and that the O concentration was greatly enhanced in this region. After re-establishment of the luminescence by acid treatment, the AES depth profiles showed no Cu, but still had an enhanced O region of similar size and concentration. Photoluminescence emission spectra taken in situ during acid treatment with HCl showed a redshift of about 8% in the peak wavelength as the intensity increased.
Photoluminescence analysis of Er-doped GaAs under host photoexcitation and direct intra-4f-shell photoexcitation
Authors: Takahei Kenichiro , Taguchi Akihito
Published: July 2002

Er-doped GaAs shows a sharp and simple intra-4f-shell luminescence spectrum of Er3+ ions under host photoexcitation, when the crystal is grown with deliberate oxygen codoping by metalorganic chemical-vapor deposition. Photoluminescence-excitation measurements by direct intra-4f-shell excitation, however, revealed that there are many kinds of Er3+ centers in the same crystal. To clarify the differences between these Er centers, photoluminescence was measured under both host photoexcitation and direct intra-4f-shell photoexcitation. It was found that there are three types of Er centers with distinctly different characteristics. The first type of Er center shows a sharp and simple photoluminescence spectrum with a high intensity under host photoexcitation. This center had been assigned as an Er atom coupled with two oxygen atoms (ErGa–2O center). The second type of Er center show sharp and simple spectra but only under direct intra-4f-shell photoexcitation and not under host photoexcitation. In a sample with Er concentration of 1.4×1018 cm−3, several such centers with distinctly different atomic configurations were found at substantial concentrations, probably exceeding that of the ErGa–2O center. As there is no intra-4f-shell luminescence for this type of center under host photoexcitation, no energy transfer path exists between the host and the 4f shells of this type of Er center. The third type of Er center shows complicated spectra even under intra-4f-shell photoexcitation with a specific photon energy. Analysis of photoluminescence spectra from this type of Er center revealed evidence of energy migration among Er3+ ions, suggesting that Er-rich aggregates are the origin of this type of PL spectra. Under host photoexcitation, this type of Er center shows luminescence but with a much lower intensity than the ErGa–2O center. The luminescence from Er centers similar to the third type probably is dominant in the PL spectrum of Er-doped GaAs grown without deliberate oxygen codoping, which does not contain an appreciable concentration of the ErGa–2O centers.
Intensity-dependent absorption and photorefractive properties in cerium-doped BaTiO3 crystals
Authors: Yang Changxi , Zhang Yuheng , Yi Xianmin , Yeh Pochi , Zhu Yong , Hui Mengjun , Wu Xing
Published: July 2002

We report the optical absorption and photorefractive properties of a series of cerium-doped barium titanate crystals. The light-induced absorption and the intensity-dependent two-beam coupling gain coefficient indicate that there are at least two active trap levels in cerium-doped BaTiO3 crystals. The total effective trap density, the Debye screening wave vector, and the intensity-dependent factor increase with cerium-doping concentration as well as the pump beam intensity. The positive light-induced absorption and the negative absorption gain coefficient as the grating wave vector approaches zero indicate that the excitation cross section of shallow traps is larger than that of deep traps. One of these crystals exhibits a two-beam coupling coefficient of 8.06 cm−1 for ordinarily polarized beams, which is the highest ever reported to date. Both electro-optic index grating and absorption grating are contributing to the coupling coefficient. The response time of as-grown cerium-doped BaTiO3 with 15 ppm cerium is 220 ms at an intensity of 800 mW/cm2.
Carrier-density-independent radiative constant in 1.3 μm buried heterostructure lasers
Authors: Flynn E. J.
Published: July 2002

An important carrier dependence of the radiative coefficient in InGaAsP active material in semiconductor lasers which operate at wavelength 1.3 μm is a result that has appeared in the literature for more than a decade. Quantitative measurements of the radiative current in 1.3 μm buried heterostructure lasers with very low leakage characteristics are presented. These data show that the saturation of spontaneous emission versus current curves is caused by the large, voltage-dependent nonradiative component of the current, rather than by a ‘‘shrinking’’ radiative coefficient B. Moreover, the dependence of the radiative current on active-layer voltage Vj is well explained by the bimolecular law S∝Bnp, where S is the radiative current and n (p) is the electron (hole) concentration. Plots of log(S) vs Vj reflect both the Fermi statistics of the conduction-band occupation and motion in energy of the hole Fermi level. A carrier dependence of the radiative coefficient of the size reported in the literature cannot be reconciled with the fundamental attributes of the spontaneous emission data. The carrier dependence of B in the literature arises experimentally as a requirement of self-consistency between differential carrier lifetime and spontaneous emission versus current data. It is observed here that the carrier lifetime versus current curves, as measured, are distorted by the high impedance of the laser chip at low bias. If a correction to the lifetime data is made, consistency between lifetime and spontaneous emission data is achieved, with no need to invoke a carrier dependence of B. Furthermore, the nonradiative coefficient needed to fit the corrected lifetime data is three times larger than before.
A numerical model for the calculation of the growth velocity of nonisothermal parabolic dendrites
Authors: Mullis Andrew M.
Published: July 2002

For a circular paraboloid of revolution growing in a shape preserving manner neither the surface curvature, nor the local interface velocity are constant on the freezing front. Yet within the widely quoted model [J. Lipton, W. Kurz and R. Trivedi, Acta Metall. 35, 957 (1987)] for the calculation of dendritic growth velocities the kinetic and GibbsThomson undercoolings evaluated at the dendrite tip are assumed to apply equally over the whole dendrite surface, approximating the non-isothermal dendrite as an isothermal dendrite with a reduced surface melting temperature. Reasons are discussed why this approach may seriously overestimate the growth velocity at high undercooling, where kinetic effects are important. Using a finite difference model the full, non-isothermal growth problem is solved for the solidification of pure Ni. The model shows that an undercooling of 175 K neglecting non-isothermal effects leads to a 35% error in the calculated growth velocity. Comparison with the available experimental data suggest an adequate fit to the data for Ni can be made without the need for adjustable parameters.
Measurements of charged particles in the laser ablation plume of polymers
Authors: Fujii T. , Inoue S. , Kannari F.
Published: July 2002

Charged particles in organic polymer plumes photoablated by ultraviolet lasers are measured with a Faraday cup assembly. In spite of a relatively low F2 laser (157 nm) fluence <1 J/cm2, relative charged fragment concentrations measured for polyimide, polytetrafluoroethylene, and polyethyleneterephthalate targets are ≳10−3. Charged particle concentrations in the ablation plumes generated by an F2 laser with polyethyleneterephthalate and polyimide targets are always higher than those in KrF laser (248 nm) ablation plumes at the same specific laser energy deposited on the target. Charged fragments have also higher velocities in the F2 laser ablation plumes. An exponential increase in the charged fragment concentration with increasing laser fluence suggests that the ions are mainly produced through electron-neutral collisions in the hot material core close to the target surface.
Aluminum and nickel contact metallizations on thin film diamond
Authors: Chan Simon S. M. , Peucheret Christophe , McKeag Robert D. , Jackman Richard B. , Johnston Colin , Chalker Paul R.
Published: July 2002

Aluminum and nickel contact metallizations have been investigated on polycrystalline, randomly oriented diamond films of varying dopant concentrations. Hall measurements have been used to characterize the diamond films to indicate good control of dopant incorporation with carrier mobility comparable with those of the highest reported in similar films. Rectifying characteristics have been observed for both Al and Ni contacts provided the sheet resistance of the films is greater than 1200 Ω/sq. The thermal stability of these contacts have been investigated to 400 °C and Al diodes have been found to be electrically stable to such treatments.
Study of end of range loop interactions with B+ implant damage using a boron doped diffusion layer
Authors: Listebarger J. K. , Robinson H. G. , Jones K. S. , Law M. E. , Sieloff D. D. , Slinkman J. A. , Sedgwick T. O.
Published: July 2002

A boron doped epilayer was used to investigate the interaction between end of range dislocation loops (formed from Ge+ implantation) and excess point defects generated from a low dose 1014/cm2 B+ implant into silicon. The boron doping spike was grown in by chemical vapor deposition at a depth of 8000 Å below the surface. The intrinsic diffusivity of the boron in the doped epilayer was determined by simply annealing the as-grown layer. The end of range (type II) dislocation loops were created using two overlapping room-temperature Ge+ implants of 75 and 190 keV each at a dose of 1×1015/cm2. Upon annealing the amorphous layer regrew and a layer of type II dislocation loops formed ∼2300 Å deep at a density of ∼8×1010/cm2. The enhancement in the buried boron layer diffusivity due to the type II loop forming Ge+ implant was observed to increase approximately between 2.5 and 5 min from 1500× to a value 2500× above the intrinsic diffusivity before dropping back to intrinsic levels after 30 min at 800 °C. A low-energy (8 keV) 1×1014/cm2 B+ (Rp=320 Å) implant into material without loops resulted in an average enhancement of 1540× in boron epilayer diffusivity after 2.5 min at 800 °C. The enhancement dropped down to intrinsic diffusivity levels after 5 min at 800 °C. When a layer of loops was introduced and annealed prior to and deeper than a subsequent low-energy B+ implant, annealing of the B+ implant produced no measurable enhancement in the buried B layer diffusivity. Taken together this imples that the interaction kinetics between the dislocation loop layer and the damage induced interstitials are primarily diffusion limited and the loops are absorbing a significant fraction of the interstitials produced by the low-energy B+ implant.
Magnetic-field effect in a two-dimensional array of short Josephson junctions
Authors: Filatrella G. , Wiesenfeld K.
Published: July 2002

The effect of a constant magnetic field on the dynamics of a two-dimensional Josephson array is studied analytically. The magnetic field induces spatially dependent states and coupling between rows, even in the absence of an external load. Numerical simulations support these conclusions.
Temperature dependence of the biaxial modulus, intrinsic stress and composition of plasma deposited silicon oxynitride films
Authors: Harding David R. , Ogbuji Linus U. T. , Freeman Mathieu J.
Published: July 2002

Silicon oxynitride films were deposited by plasma-enhanced chemical-vapor deposition. The elemental composition was varied between silicon nitride and silicon dioxide: SiO0.3N1.0, SiO0.7N1.6, SiO0.7N1.1, and SiO1.7N0.5. These films were annealed in air, at temperatures of 40–240 °C above the deposition temperature (260 °C), to determine the stability and behavior of each composition. The biaxial modulus, biaxial intrinsic stress, and elemental composition were measured at discrete intervals within the annealing cycle. Films deposited from primarily ammonia possessed considerable hydrogen (up to 38 at. %) and lost nitrogen and hydrogen at anneal temperatures (260–300 °C) only marginally higher than the deposition temperature. As the initial oxygen content increased a different mechanism controlled the behavior of the film: The temperature threshold for change rose to ≂350 °C and the loss of nitrogen was compensated by an equivalent rise in the oxygen content. The transformation from silicon oxynitride to silica was completed after 50 h at 400 °C. The initial biaxial modulus of all compositions was 21–30 GPa and the intrinsic stress was −30 to 85 MPa. Increasing the oxygen content raised the temperature threshold where cracking first occurred; the two film compositions with the highest initial oxygen content did not crack, even at the highest temperature (450 °C) investigated. At 450 °C the biaxial modulus increased to ≂100 GPa and the intrinsic stress was ≂200 MPa. These increases could be correlated with the observed change in the film’s composition. When nitrogen was replaced by oxygen, the induced stress remained lower than the biaxial strength of the material, but, when nitrogen and hydrogen were lost, stress-relieving microcracking occurred.
Experimental sensitivity analysis of pseudomorphic InGaAs/AlAs resonant-tunneling diodes
Authors: Moise T. S. , Kao Y.-C. , Katz A. J. , Broekaert T. P. E. , Celii F. G.
Published: July 2002

Through the use of a novel vertically integrated resonant-tunneling diode (RTD) heterostructure we have established experimentally the relationship between intentional variations in the structural parameters of the pseudomorphic In0.53Ga0.47As/AlAs resonant tunneling diode (i.e., barrier thickness, quantum-well thickness, quantum-well composition, and doping density) and the measured current–voltage characteristics of the device. Based upon the results of these experiments, we have determined that a 1 monolayer increase in AlAs barrier width, InGaAs quantum-well width, or InAs subwell width results in a peak current reduction of 56%±7%, 19%±2%, and 18%±3%, respectively. Further, a 1% decrease in indium mole fraction of the InGaAs quantum well has been found to increase the peak current by 10%±1%. Sensitivity parameters have been tabulated for both the peak current and the peak voltage of the RTD. Through the use of these parameters, the maximum allowed fluctuation in the RTDs structural parameters has been estimated for a given tolerance in the RTDs electrical characteristics. Further, these data can also be used to evaluate the feasibility of in situ epitaxial growth control of resonant tunneling devices.
Kinetics of phosphorus proximity rapid thermal diffusion using spin-on dopant source for shallow junctions fabrication
Authors: Grabiec P. B. , Zagozdzon-Wosik W. , Lux G.
Published: July 2002

Proximity rapid thermal diffusion (RTD) has been investigated using phosphorus spin-on dopants (SOD) as a function of SOD thickness, baking and diffusion temperatures, separation distance between wafers, and ambient. Experimental results, based on sheet resistance measurements, dopant distributions obtained by secondary ion mass spectroscopy, and electron microprobe analyses indicate fast phosphorus out-diffusion from the dopant source. Simplified modelling of phosphorus diffusion within the SOD and evaporation confirms its fast transport and limited evaporation. An analysis of the process kinetics suggest that at high RTD temperatures doping of the processed wafer is controlled by gas phase transport. At low diffusion temperature surface reactions control the process.
Epitaxial YBa2Cu3O7−δ air-bridge microbolometers on silicon substrates
Authors: Barth R. , Siewert J. , Jaekel C. , Spangenberg B. , Kurz H. , Prusseit W. , Utz B. , Wolf H.
Published: July 2002

Silicon micromachining technology is applied to fabricate free-standing YBCO structures for low-thermal-mass bolometer devices. The bolometric response of the devices is studied with chopped IR illumination at a wavelength of 1.3 μm. The micromachining process leads to an enhancement of sensitivity by more than one order of magnitude. The optical responsivity of the air-bridge bolometers measured at the midpoint of the resistive transition is 1.29 kV/W and the time constant is 81 μs. The optical noise equivalent power is 1.1×10−11 W/Hz1/2.
Static properties and current steps in one-dimensional parallel arrays of Josephson tunnel junctions in the presence of a magnetic field
Authors: Gambardella U. , Caputo P. , Boffa V. , Celentano G. , Costabile G. , Pace S.
Published: July 2002

Static and dynamical states in one-dimensional parallel arrays of small, all refractory Josephson tunnel junctions, are experimentally investigated at 4.2 K. The dependence of the maximum Josephson current on the applied magnetic field in parallel arrays shows asymmetric behavior around secondary maxima which are discussed in terms of a nonuniform applied magnetic field. Current steps and linear branches induced by the applied magnetic field are observed together in the current-voltage characteristics for the first time in the strongly underdamped regime. The resonant structures appearing in the current-voltage characteristics of the arrays show features similar to high-order Fiske modes in a long Josephson junction.
Magnetic studies of Ce4+ compensated Co-doped yttrium iron garnet thin film grown by chemical vapor deposition
Authors: Dhara Sandip , Rastogi A. C. , Das B. K.
Published: July 2002

A low pressure metalo-organic chemical vapor deposition (LPMOCVD) technique to form Co2+ and Ce4+ doped yttrium iron garnet (Y3−xCexFe5−xCoxO12: Co,Ce:YIG) films is described. A large concentration of Co2+ doping with x=0.3–0.7 results in uniaxial anisotropy perpendicular to the plane of the film with high coercivity values and sufficiently high saturation magnetization values. This has been possible by an alternate doping scheme where commonly used compensator, Ge4+ at Fe3+ sites has been replaced by a new compensator Ce4+ at Y3+ sites in the Co-doped YIG films. The structural and compositional aspect of stabilized garnet phase in Co2+,Ce4+:YIG thin films and optical, thermomagnetic and magnetic hysteresis properties are presented.
A copper-related acceptor complex in vacuum grown germanium crystals
Authors: Sirmain G. , Dubon O. D. , Hansen W. L. , Olsen C. S. , Haller E. E.
Published: July 2002

A copper-related shallow acceptor complex has been discovered in germanium single crystals grown in vacuum and doped with arsenic and copper. Photothermal ionization spectroscopy of samples quenched from 673 K reveals two sets of hydrogenic lines with ground state binding energies of 9.15 and 10.05 meV. The line intensity ratios between corresponding transitions (1s-np) of the two hydrogenic series follow a Boltzmann dependence. This shows that the two series belong to the same impurity complex with a split ground state. Taking into account the crystal growth conditions together with the changes in the donor concentration deduced from variable temperature Hall effect measurements, we conclude that arsenic and substitutional copper form the new acceptor complex A(Cus,As) since copper is the only fast-diffusing species at this low annealing temperature. This complex is expected to have C3v symmetry in agreement with preliminary piezospectroscopy measurements.
Observation of the nanoscale epitaxial growth of diamond on Si (100) surface
Authors: Song S. G. , Chen C. L. , Mitchell T. E. , Hackenberger L. B. , Messier R.
Published: July 2002

Epitaxial growth of noncrystalline diamond thin film on Si(001) surface has been observed using high-resolution transmission electron microscopy. The epitaxial lattice relationship at diamond/Si interface has been described based on a cube-cube orientation with a≊7° tilt plus a rotation of the diamond lattice from the substrate lattice. The observed epitaxial behavior can be explained by the 3:2 lattice coincidence and the introduction of interfacial misfit dislocations.
Metalorganic molecular beam epitaxy of strain-compensated InAsP/InGaAsP multi-quantum-well lasers
Authors: Sugiura Hideo , Ogasawara Matsuyuki , Mitsuhara Manabu , Oohashi Hiromi , Amano Toshimasa
Published: July 2002

Strain-compensated InAsP/InGaAsP multi-quantum-wells (MQWs) grown by metalorganic molecular beam epitaxy (MOMBE) are characterized by conventional photoluminescence (PL), micro-PL, transmission electron microscopy, and x-ray diffraction measurements and applied to fabricate 1.3 μm wavelength laser diodes. These methods reveal that there is no deterioration in the optical properties or structure of strain-compensated MQWs having up to 25 wells, which means that the critical thickness of InAsP grown by MOMBE exceeds 1000 Å. The critical conditions of strain and thickness over which misfit dislocations are generated are determined for the MQWs. The MQW lasers with ten wells (Lz=55 Å) have no misfit dislocations and have uniform threshold current densities of 0.9±0.1 kA/cm2. The maximum operating temperature Tmax of the lasers increases with increasing well number, the highest Tmax is 155 °C, which is obtained for MQW lasers with 15 wells. The lasers have no problems in terms of long-term reliability.
Quadratic electro-absorption and electro-optic effects in a guest/host nonlinear optical polymeric system
Authors: Goodson T. , Wang C. H.
Published: July 2002

Quadratic electro-absorption (QEA) and quadratic electro-optic measurements were performed with a guest/host system consisting of 4(4’-N,N-Dimethylaminophenyl)-1,1-bis(trifluoromethyl sulfonyl)-1,3-butadiene doped in poly(ethyl)methacrylate host polymer. The imaginary part of the QEA response was measured over a wavelength range of 450–600 nm. The real and imaginary parts of the electric field induced optical Kerr coefficient were measured at wavelengths near and far from resonance. Under different polarizations of the optical field in the QEA measurement, the two contributing coefficients of the third order susceptibility (χ(3)) were deduced. A distinctive temperature variation of the third order response was observed. Temperature dependence measurements from ambient to well above the glass transition of the guest-host system were carried out. The same chromophore in a different polymer host, poly(vinyl)chloride, was also investigated. Correct expressions for the real and imaginary parts of the Kerr coefficient are provided. The present results confirm the importance of the electronic contribution to χ(3) at room temperature where molecular reorientation is insignificant. At high temperatures (above the glass transition temperature of the polymer) the ratio of χ(3)3333/χ(3)1133 deviates from −2, which is predicted by the elastic model assuming molecular reorientation to be the dominant mechanism.
A comparative study of mechanically alloyed and mechanically milled Nd10Fe84B6
Authors: Miao W. F. , Ding J. , McCormick P. G. , Street R.
Published: July 2002

A comparative study has been made of the structures and magnetic properties of Nd10Fe84B6 prepared by mechanical alloying using elemental powders as starting materials and by mechanical milling of the alloy. Both milling procedures resulted in a mixture of α-Fe and an amorphous phase, with the fraction of α-Fe phase being significantly higher in the as-milled mechanically alloyed powder than in the mechanical milled counterpart. Annealed mechanically alloyed powders showed a somewhat coarser and nonuniform grain structure in comparison with annealed mechanically milled powders. The structural differences of mechanically milled and mechanically alloyed powders resulted in higher remanences and coercivities being exhibited by the mechanically milled powders after annealing.
On the decay of the trapped holes and the slow states in metal–oxide–semiconductor capacitors
Authors: Meinertzhagen A. , Petit C. , Yard G. , Jourdain M. , Salace G.
Published: July 2002

We have compared the charge created in p-metal–oxide–semiconductor capacitors by Fowler–Nordheim injection from the gate and from the substrate. We have shown that an injection from the gate creates a negative charge, trapped holes, and positively charged slow states whereas an injection from the substrate creates a negative charge, slow states, and amphoteric neutral traps; once charged these neutral traps are discharged irreversibly, as are the trapped holes, by an appropriate gate bias. We have observed that the discharge of the trapped holes, and the charge or discharge of the slow states, obey the same general law, but the time response of the trapped holes is always shorter than the time response of the slow states. This general law is equivalent to the so-called ‘‘universal law,’’ which is the law which describes the time dependence of current observed in any dielectric in response to a step-function field.
Incorporation and optical activation of erbium in silicon using molecular beam epitaxy
Authors: Serna R. , Shin Jung H. , Lohmeier M. , Vlieg E. , Polman A. , Alkemade P. F. A.
Published: July 2002

Erbium is incorporated in crystalline silicon during molecular beam epitaxy on Si(100) at 600 °C, either in vacuum (6×10−11 mbar) or in an O2 ambient (4×10−10 mbar). Strong Er segregation takes place during growth in vacuum, and only 23% of the total deposited Er is incorporated in the epitaxial layer. Films grown in an O2 ambient show no Er segregation, and an Er concentration of 1.5×1019 Er/cm3 is incorporated in the crystal. The O content is 4×1019 O/cm3. Photoluminescence spectra taken at 10 K show the characteristic intra-4f luminescence of Er3+ at 1.54 μm for both samples, grown with and without O2. Differences found in the spectral shape indicate a difference in the local environment (presumably O coordination) of Er for the two cases. The O codoped film shows a 7 times higher Er luminescence peak intensity than the film grown without O. This is due to the higher incorporated Er concentration as well as an increased luminescence efficiency (lifetime without O: 0.33 ms, with O: 1.81 ms). The Er excitation efficiency is lower in the O codoped film than in the O-undoped film, which is attributed to the lower minority carrier lifetime in the O-doped material. Thermal annealing of the O codoped film at 1000 °C increases the excitation efficiency and hence the Er luminescence intensity.
High recording performance of Co–Cr medium sputter-deposited at high Ar pressure and high substrate temperature
Authors: Honda Naoki , Yanase Satoshi , Ouchi Kazuhiro , Iwasaki Shun-ichi
Published: July 2002

Co–Cr films were prepared by sputter deposition at a high Ar pressure of 70 Pa with elevated substrate temperatures up to 400 °C. Films of 100 nm thickness were deposited using a 19 at. % Cr–Co target onto a well c-axis oriented Ti underlayer prepared on glass disk substrates. The perpendicular coercivity, Hc⊥, of the films increased from 660 to 1940 Oe with increasing temperature. The films with high Hc⊥ exhibited a dense fine microstructure with distinct grain boundaries. The recording performance of the disk samples were measured by using a metal in gap type ring head, comparing with that of conventional Co–Cr films deposited at a low Ar pressure of 0.2 Pa. It was found that the high pressure deposited Co–Cr film exhibited higher output by more than 2 dB at the densities below 320 kFRPI (flux reversals per inch) compared with the conventional film with the same high coercivity. The D50* of as high as 250 kFRPI and the highest recordable density of over 600 kFRPI were confirmed for the new type of Co–Cr film deposited at 70 Pa and at 400 °C. The noise level of the film, however, was slightly higher than that of conventional method films, disagreeing with the suggestion of the microstructure.
Very high carbon δ -doping concentration in AlxGa1−xAs grown by metalorganic vapor phase epitaxy using trimethylaluminum as a doping precursor
Authors: Li G. , Petravić M. , Jagadish C.
Published: July 2002

Using trimethylaluminum (TMAl) or trimethylgallium (TMGa) as a doping precursor, carbon δ-doped AlxGa1−xAs has been grown in metalorganic vapor phase epitaxy. Compared to TMGa, TMAl exhibits very high carbon δ-doping efficiency. The best hole profile of carbon δ-doped Al0.3Ga0.7As grown at 580 °C using TMAl as a doping precursor has a peak hole density of 1.6×1019 cm−3 for a full width at half-maximum of 85 Å with most of the incorporated carbon atoms being electrically active. When TMGa is used as the doping precursor, the hole density of carbon δ-doped AlxGa1−xAs significantly increases with an increase of the Al mole fraction. By comparison, the use of TMAl almost induces independence of the hole density on the Al mole fraction. The hole density of carbon δ-doped Al0.3Ga0.7As weakly increases when reducing the δ-doping temperature regardless of the doping precursors. The hole density of carbon δ-doped Al0.3Ga0.7As grown at 580 °C is proportionally associated with the moles of TMGa or TMAl totally input during a δ-doping step. Using heavily carbon δ-doped layers in Al0.3Ga0.7As, a carbon δ-doped pipi doping superlattice possessing a bulk-doped-like hole profile with an average hole density of 1.1×1019 cm−3 is therefore demonstrated as an alternative with unique advantages over other conventional carbon bulk-doping approaches.
Catalytic activity of catalase under strong magnetic fields of up to 8 T
Authors: Ueno S. , Iwasaka M.
Published: July 2002

The question of whether or not magnetic fields affect enzymatic activity is of considerable interest in biomagnetics and biochemistry. This study focuses on whether magnetically related enzymatic activities can be affected by magnetic fields. We examined the effect of magnetic fields of up to 8 T on catalytic decomposition of hydrogen peroxide (H2O2). We observed changes in absorbance of reaction mixture of hydrogen peroxide and catalase at 240 nm, during and after magnetic field exposures. When the reaction mixture was not treated with nitrogen-gas bubbling, it was observed that the initial reaction rate of the reaction which was exposed to magnetic fields of up to 8 T was 50%–85% lower than the control data. This magnetic field effect was not observed, however, when the reaction mixture was bubbled with nitrogen gas to remove the dissolved oxygen molecules which were produced in the solution. We also measured concentration of dissolved oxygen which was produced by the decomposition of hydrogen peroxide. Dissolved oxygen concentration in the reaction mixture which was exposed to magnetic fields increased 20%–25% compared to the control solution. The results of the present study indicate that magnetic fields affect dynamic movement of oxygen bubbles which are produced in the reaction mixture by the decomposition of hydrogen peroxide, but not the catalytic activity of catalase itself.
Space-charge-limited conduction in Si n+–i–n+ homojunction far-infrared detectors
Authors: Yuan H. X. , Perera A. G. U.
Published: July 2002

An analytic model is presented to describe the space-charge-limited (SCL) conduction in Si homojunction interfacial work-function internal photoemission far-infrared detectors. The basic detector unit is a thin n+–i–n+ structure, which is operated at low temperatures and characterized by an interfacial work function at the n+–i interface. The unique aspects of this case lead to simple analytic expressions for all variables of interest. The barrier shape and free-carrier concentration distribution in the i layer, and their dependence on the applied bias, i layer thickness, and compensating acceptor concentration, are calculated. The SCL current–voltage characteristic is also investigated as a function of i layer parameters. The results obtained are useful for the IR detector design and performance optimization.
Theoretical predictions of the polar Kerr effect in Fe and Co
Authors: MacLaren J. M. , Huang W.
Published: July 2002

We present theoretical calculations of the magneto-optic polar Kerr effect in bcc iron, fcc cobalt, and hcp cobalt. The Kerr angle and ellipticity are derived from the optical conductivity tensor, which is computed using wave functions obtained from full potential local spin-density electronic structure calculations. Predicted Kerr angles and ellipticities are computed as a function of magnetization direction. In the case of iron, the calculated Kerr spectra are found to be in good agreement with experimental results and previous calculations. However, in the case of cobalt, the results of previous theoretical studies differ significantly. Our results are in close agreement with experiment and with one of these previous theoretical studies.
Soliton bunching in annular Josephson junctions
Authors: Vernik I. V. , Lazarides N. , So/rensen M. P. , Ustinov A. V. , Pedersen N. F. , Oboznov V. A.
Published: July 2002

By studying soliton (fluxon) motion in long annular Josephson junctions it is possible to avoid the influence of the boundaries and soliton-soliton collisions present in linear junctions. A new experimental design consisting of a niobium coil placed on top of an annular junction has been used to insert individual fluxons or antifluxons into the junction in a controllable way. The dynamical behavior of different numbers of trapped fluxons was investigated. In addition, we were able to change the junction parameters by changing temperature. In some of the zero-field steps, the experiments reveal a small jump to higher values of the voltage at the top of the steps. This phenomenon can be caused by a crossover from a nonbunched state to a bunched state of the fluxon motion. By performing direct numerical simulations using the perturbed sine-Gordon equation with parameter values determined from the experiments we have been able to confirm the above explanation.
Bi12(GaxBi1−x)O19.5 optical waveguides grown by pulsed laser deposition
Authors: Alfonso J. E. , Martín M. J. , Mendiola J. , Ruiz A. , Zaldo C. , da Silva M. F. , Soares J. C.
Published: July 2002

Bi12(GaxBi1−x)O19.5 (BGaO) films with thickness in the range 100–1000 nm have been deposited on (100) Y-stabilized zirconia (YSZ) and (100) Bi12GeO20 (BGO) substrates using a KrF excimer pulsed laser and polycrystalline targets with composition x=0.63–0.72. The laser power density threshold for ablation of the targets has been determined to be J0=4.8 J/cm2. A deposition rate of 0.03 Å/pulse was found for the substrate-target distance (6 cm) and laser fluence (J=7 J/cm2) used. Transparent and crystalline films were obtained heating the substrate in the 450–550 °C range under 1.5×10−1 mbar of oxygen pressure. Films deposited on hot substrates have a Ga stoichiometry x=0.5 but a Bi content slightly lower than that corresponding to sillenite. Films deposited on YSZ show preferential orientation. From the x-ray diffraction results and the comparison of the ionic distributions, it has been concluded that the most likely orientation between both lattices is 〈310〉{130}BGaO∥〈011〉{100}YSZ. A {100}BGaO∥{100}BGO epitaxy has been inferred from Rutherford backscattering analysis. On both substrates the films behave as step waveguides with refractive index close to the value determined in bulk BGaO.
Two-photon absorption and photoconductivity in photosensitive glasses
Authors: Weitzman Peter S. , Österberg Ulf
Published: July 2002

We have measured the two-photon absorption coefficent at 0.527 μm to be 1.8±0.4×10−3 cm/GW in a germanium-doped silica fiber. Using absorption and luminescence measurements we estimated the two-photon absorption coefficient at 1.053 μm to be smaller than 6×10−7 cm/GW. Using a capacitive technique we measured the photoconductivity to be approximately 6×10−9 1/Ω cm in the visible. The photoconductivity inreased linearly with intensity. A qualitative discussion of the defect dynamics related to photosensitive phenomena in optical glass fibers is given based on the experimental results.
Photoluminescence excitation spectroscopy of GaAs:Er,O in the near-band-edge region
Authors: Hogg R. A. , Takahei K. , Taguchi A.
Published: July 2002

We report a spectroscopic study of GaAs:Er,O samples grown by metalorganic chemical vapor deposition, with excitation in the near-band-edge region, 1.22–1.62 eV. Photoluminescence under host-excitation is dominated by luminescence due to the Er-2O center (an erbium atom at a gallium site coupled with two adjacent oxygen atoms) in these samples. The characterization of different Er centers is demonstrated by the observation of 4I15/2→4I11/2 intra-4f-shell transitions by photoluminescence excitation (PLE) spectroscopy. The transitions between these crystal-field-split 4f-shell levels are observed at ∼1.26 eV (∼980 nm) which is at an energy attainable by PLE spectroscopy with a Ti:Sapphire laser. Two lines are assigned to the Er-2O center. This allows a semi-quantitative measurement of the relative concentrations of different centers to be made. PLE spectroscopy was also employed to study the trap levels related to the Er-2O center. However, we find no evidence for such a trap level in this energy range. The expected position of this trap is discussed in the light of these results and recent calculations.
Temperature-dependent transport properties of n+ GaAs/low-temperature GaAs/n+ GaAs structures grown by molecular beam epitaxy
Authors: Chen Jenn-Fang , Chen Nie-Chuan , Chiu Shih-Yang , Wang Pie-yong , Lee Wei-I , Chin Albert
Published: July 2002

The temperature-dependent I–V characteristics of n+ GaAs low-temperature GaAs(AlGaAs) n+ GaAs structures in which the low-temperature layers were grown at 250, 350, and 450 °C were analyzed. Band conduction with an activation energy of 0.72 eV dominates at T≳250 K. Hopping conduction dominates at T<250 K, where the resistivity was found to be insensitive to temperature. From this analysis, it is shown that Fermi level is pinned to an acceptorlike deep level of about 1017 cm−3, which lies at 0.72 eV below the conduction band. Measured capacitance can be described in terms of a parallel-plate capacitance with separation being equal to the expected growth thickness. Majority traps (electrons) were observed by deep-level transient spectroscopy with an activation energy about 0.72 eV, confirming the result of the resistivity analysis. In addition, the I–V characteristics were fitted to the simulated curves based on a simplified space-charge limited theory and the result was found to be consistent with the resistivity analysis.
Reaction mechanism of cobalt with silicon dioxide
Authors: Nguyen Tue , Ho Herbert L. , Kotecki David E. , Nguyen Tai D.
Published: July 2002

The reaction mechanism of thin cobalt (Co) films with silicon dioxide (SiO2) substrate under rapid thermal annealing conditions has been investigated. Reaction of thin cobalt film (12.5 nm) with a SiO2 substrate is observed in an inert ambient (N2) and in vacuum (∼10−8 Torr). The reaction is manifested by the formation of craterlike depressions on the SiO2 substrate and by the presence of a Co2SiO4 reaction product determined by transmission electron microscopy diffraction patterns. Much less damage is observed with no reaction product observed if the samples are annealed in a forming gas ambient (90% N2/10% H2), the cobalt film is much thicker (150 nm), or the cobalt film is in situ cleaned (e.g., 5 min in 400 °C, forming gas ambient) prior to annealing in either inert or vacuum ambient. It is proposed that the presence of oxygen is required in order to initiate the reaction between cobalt and SiO2. The source of the oxygen contaminant, in our studies, is the oxygen on the surface of the cobalt film. The proposed reaction is 2 Co+SiO2+2〈0〉→Co2SiO4 where 〈0〉 represents oxygen contaminant at the surface.
Synchrotron x-ray scattering measurements of bulk structural properties in superconducting (Bi,Pb)2Sr2Ca2Cu3O10–Ag tapes
Authors: Thurston T. R. , Wildgruber U. , Jisrawi N. , Haldar P. , Suenaga M. , Wang Y. L.
Published: July 2002

The structural properties of superconducting (Bi,Pb)2Sr2Ca2Cu3O10–Ag (2223) tapes have been measured using synchrotron x-ray scattering techniques. The x-ray photon energy was tuned just below the silver K absorption edge so the penetration depth was large, which allowed the measurements to be performed in a transmission geometry without removing the silver cladding. Analysis of the peaks in 2θ scans indicates that residual (Bi,Pb)2Sr2CaCu2O8 (2212) superconductor starting material is present in all samples studied. The amount of 2212 varied widely among the tapes, and was not homogeneous along the length of each individual tape. Residual 2212 content increased near the ends of most samples, suggesting that 2223 phase development is sensitive to whether the superconducting material is encased in silver or not. The bulk c-axis alignment was measured in ∼100 mono- and multifilament samples, and correlations between c-axis alignment and current carrying capacity at 77 K were found. Multifilament samples generally had better alignment than monofilament samples. The c-axis alignment along the length of the tapes was uniform, and the superconducting material within ∼1 μm of the Ag was better textured than the bulk of the sample. Intermediate pressings were directly shown to have an adverse affect on c-axis alignment. Finally, the evolution of texture and phase development was examined in a series of samples annealed for varying times. The 2212 starting material acquired the final c-axis alignment state after brief heating times, and only after much longer heating times did the 2212 transform into the 2223 phase. These results and their implications for improving processing procedures are discussed.
Growth modes of ZnSe on GaAs
Authors: Qiu Y. , Osinsky A. , El-Emawy A. A. , Littlefield E. , Temkin H. , Faleev N.
Published: July 2002

Growth modes of coherently strained ZnSe on GaAs are investigated as a function of the surface preparation and temperature. We find that the flattest and most uniform layers are grown at low temperatures, ∼300 °C, on surfaces exposed to high-temperature Se treatment. Nucleation rate depends on the surface coverage of Ga2Se3. High-temperature exposure of Ga-rich, (001)-oriented, GaAs to Se reduces the thickness required for the transition to a two-dimensional growth mode. The surface roughness increases with temperature as a result of three-dimensional island growth.
Unique x-ray diffraction pattern at grazing incidence from misfit dislocations in SiGe thin films
Authors: Jordan-Sweet J. L. , Mooney P. M. , Lutz M. A. , Feenstra R. M. , Chu J. O. , LeGoues F. K.
Published: July 2002

Grazing-incidence x-ray diffraction (GIXD) permits the direct measurement of in-plane lattice parameters of SiGe films that are too thin to yield good results from normal-geometry triple-axis techniques. A unique ‘‘X’’-shaped pattern has been seen in H–K reciprocal space maps of diffracted x-ray intensity from SiGe films that have relaxed via a modified Frank–Read mechanism. Contours of intensity are seen along the 〈110〉 directions from the (4̄00) reciprocal lattice peak with the introduction of the first dislocations. For higher dislocation densities the X-shaped contours are anisotropically distorted and a satellite peak, corresponding to the lattice parameter for a partially relaxed film, becomes identifiable at lower H. In contrast, H–K reciprocal-space contours from thin SiGe films that have relaxed by roughening and subsequent random nucleation of dislocations display broad, oval-shaped contours centered at the (4̄00) reciprocal lattice point for the film. Numeric simulations of GIXD from a variety of dislocation arrangements were performed in order to understand the origin of the X pattern. We show that this pattern arises from an array of long misfit dislocations running in the 〈110〉 directions. The anisotropic distortion of the X pattern arises at higher dislocation densities from orthogonal intersections of dislocations with equal Burgers vector, which are characteristic of dislocation networks generated by the modified Frank–Read mechanism. We also verify that the measured values of the in-plane lattice parameter, together with the out-of-plane lattice parameter determined from the symmetric (004) reflection, lead to accurate calculation of the composition and strain in these SiGe layers.
Proton-implantation-induced nanosized Ge crystal formation in SiO2:GeO2 glasses
Authors: Kawamura Ken-ichi , Hosono Hideo , Kawazoe Hiroshi
Published: July 2002

Nanometer-sized crystalline Ge colloid particles in 9SiO2–1GeO2 glasses were formed by implantation of protons at 1.5 MeV without post-thermal annealing. Although oxygen-deficient type point defects associated with Ge ions were primarily formed to fluences ≲1×1017 cm−2, the formation of Ge fine crystalline particles was observed for fluences ≳5×1017 cm−2. No formation of Ge colloids and the Ge-related point defects were noted for implantation of 1.5 MeV He+ to a fluence of 1×1018 cm−2. The depth of Ge colloid formation layers was 22–26 μm from the implanted surface. This depth region agreed well with the peak region of electronic energy deposition. Ge–OH groups were formed preferentially over Si–OH groups upon implantation of protons and the decay curve upon isochronal annealing was close to that of the optical absorption at ∼3 eV, which was attributed to nanometer-sized Ge. A red photoluminescence peaking at ∼1.9 eV was observed for all the implanted substrates. A tentative formation mechanism of Ge colloids in these glasses was proposed.
Electron transfer efficiency in quantum well waveguide couplers
Authors: Yang Rui Q.
Published: July 2002

Theoretical formulations based on a coupled-mode theory and a supermode theory are developed for discussing electron transfer efficiency in quantum well waveguide couplers. We apply a coupled-mode theory to electron waveguide couplers in the weak coupling regime and show that the transfer efficiency can be near but not higher than 100%. In the strong coupling regime, we can appropriately handle the electron waves in quantum well waveguide couplers by introducing a supermode theory, and demonstrate that the transfer efficiency is always lower than 100%. We will discuss the underlying physics in an attempt to gain an insight into the transfer efficiency related issues. Calculations for specific semiconductor electron waveguide couplers made of GaAs/AlGaAs quantum wells will be given for quantitative illustrations.
Interface state-induced shift of the oxide and semiconductor core levels for metal–oxide–semiconductor devices
Authors: Kobayashi H. , Namba K. , Yamashita Y. , Nakato Y. , Komeda T. , Nishioka Y.
Published: July 2002

Measurements of x-ray photoelectron spectra are performed for ∼3-nm-thick Pt/∼3.6-nm-thick silicon oxide/n-Si(100) devices under biases between the Pt layer and the Si substrate. It is observed that the oxide Si 2p peak as well as the substrate peaks is shifted upon applying biases. These shifts are caused by a bias-induced change of the potential drop across the oxide layer due to the change in the amount of the interface state charge. The amount of the shift of the oxide Si 2p peak is well correlated to that of the substrate Si 2p3/2 peak. The energy distribution of the interface states is obtained by analyzing the amount of the shift of the substrate Si 2p3/2 peak measured as a function of the bias voltage. The interface state spectrum has one peak near the midgap, and the peak is attributed to isolated Si dangling bond states.