bluuebunny/crossref_metadata_2025 huggingface dataset:
Broadening factor due to electron–phonon collisions in semiconductor quantum wells
Authors: Houng M. P. , Wang Y. H. , Chu C. H.
Published: July 2002

Electron–phonon collisions result in finite relaxation time and energy shift of electron’s states in semiconductors. These effects make the optical line shape broadened and shift its peak position. Hence these two quantities play an important role in semiconductor optical devices absorption spectrum. Here a theoretical model is presented to calculate the broadening factor and energy shift resulting from the interactions of phonons and two-dimensional electrons, including intra- and intersub-band processes. In this model, taken into account are the effective-mass discontinuity and finite barrier height for electron wave functions in order to derive exact momentum selection function of electron–phonon interactions. Due to the steplike nature of two-dimensional density of states, the broadening factor also appears steplike and strongly energy dependent. The influences of carrier density, temperature, and well width are also discussed.
Time analyzed transient spectroscopy and multiple DX related emission centers in silicon doped AlxGa1−xAs
Authors: Agarwal Sandeep , Mohapatra Y. N. , Singh Vijay A. , Sharan R.
Published: July 2002

The multiple DX related emission centers are studied using the isothermal time analyzed transient spectroscopy (TATS). Four distinct emitting centers have been obtained using higher order TATS in the same silicon doped Al0.33As0.67As sample. A systematic observation of these levels has helped in reducing the ambiguity regarding the emission parameters of the DX centers. Particularly the activation energies, the nature of nonexponentiality of emission, and the effect of charge redistribution for the DX center have been made explicit. Our results suggest that the activation energy of each of the four distinct centers have a Gaussian distribution with similar full width half maxima of about 20 meV. Limitations of temperature scanning techniques such as deep level transient spectroscopy are pointed out.
Effect of fluorine on boron diffusion in thin silicon dioxides and oxynitride
Authors: Aoyama Takayuki , Suzuki Kunihiro , Tashiro Hiroko , Toda Yoko , Yamazaki Tatsuya , Takasaki Kanetake , Ito Takashi
Published: July 2002

We investigated the effects of fluorine on boron diffusion in thin silicon oxides used for metal-oxide-semiconductor structures, including silicon dioxide and oxynitride. We used secondary ion mass spectroscopy to measure the boron penetration depth from p+-polycrystalline silicon to the silicon substrate through the silicon oxides for various fluorine doses. We compared experimental and simulated results and determined the boron diffusion coefficients in the silicon oxides. The diffusion coefficients have an Arrhenius relationship for each fluorine dose. The diffusion coefficients at a fluorine dose of 1×1016 cm−2 were 30 times larger than with no fluorine dose, and at a fluorine dose of 1×1015 cm−2, diffusion coefficients were 5 times larger. With oxynitride, fluorine has the same enhancing effect. With no fluorine, the diffusion coefficients were independent of oxide thickness. When fluorine is implanted into polycrystalline silicon, especially with a high dose, the diffusion coefficient is larger for thinner oxides because of the higher fluorine concentration.
Ion damage buildup and amorphization processes in AlxGa1−xAs
Authors: Tan H. H. , Jagadish C. , Williams J. S. , Zou J. , Cockayne D. J. H. , Sikorski A.
Published: July 2002

The nature of keV ion damage buildup and amorphization in AlxGa1−xAs at liquid-nitrogen temperature is investigated for various Al compositions using Rutherford backscattering channeling, transmission electron microscopy, and in situ time-resolved-reflectivity techniques. Two distinct damage buildup processes are observed in AlxGa1−xAs depending on Al content. At low Al content, the behavior is similar to GaAs whereby collisional disorder is ‘‘frozen in’’ and amorphization proceeds with increasing dose via the overlap of damage cascades and small amorphous zones created by individual ion tracks. However, some dynamic annealing occurs during implantation in AlGaAs and this effect is accentuated with increasing Al content. For high Al content, crystallinity is retained at moderate ion damage with disorder building up in the form of stacking faults, planar, and other extended defects. In the latter case, amorphization is nucleation limited and proceeds abruptly when the level of crystalline disorder exceeds a critical level. The amorphization threshold dose increases with increasing Al composition by over two orders of magnitude from GaAs to AlAs. Dynamic annealing and damage creation processes during implantation compete very strongly in AlxGa1−xAs even at liquid-nitrogen temperatures. This behavior is discussed in terms of both the availability of very fast mobile defects and bonding configurational changes related to the Al sublattice in AlxGa1−xAs of high Al content.
Shape anisotropy in antiferromagnetic superparamagnetic particles
Authors: Bocquet S. , Pollard R. J. , Cashion J. D.
Published: July 2002

The contribution of shape anisotropy to the total anisotropy of an antiferromagnetic particle is shown to be negligible compared with the contribution from the magnetocrystalline anisotropy of the material. It follows that the superparamagnetic behavior of small antiferromagnetic particles is independent of the particle shape.
Electrical properties of ferroelectric thin-film capacitors with hybrid (Pt,RuO2) electrodes for nonvolatile memory applications
Authors: Al-Shareef H. N. , Auciello O. , Kingon A. I.
Published: July 2002

For Pb(ZrxTi1−x)O3 (PZT) thin-film capacitors to be used in ferroelectric nonvolatile memories, they must have low polarization fatigue and low leakage currents. PZT capacitors fabricated in our laboratory with RuO2 electrodes exhibit excellent polarization fatigue characteristics, but they have large and variable leakage currents (typically 10−3–10−4 A/cm2 at 1 V). On the other hand, PZT capacitors with Pt electrodes have low leakage currents (typically <10−9 A/cm2 at 1 V), but they undergo severe polarization fatigue. New (Pt,RuO2) hybrid electrodes which result in PZT capacitors that combine the excellent fatigue behavior of RuO2/PZT/RuO2 with the low leakage currents of Pt/PZT/Pt capacitors have been developed. The hybrid electrodes studied are of two main types: one type consisted of Pt/RuO2 or RuO2/Pt double layers, while the other consisted of a codeposited Pt–RuO2 layer. All capacitors discussed here had an RuO2/PZT/hybrid electrode/MgO heterostructure. It will be shown that capacitors with negligible fatigue (up to 1011 switching cycles) and with leakage currents that are two to four orders of magnitude lower than those of RuO2/PZT/RuO2 capacitors can be achieved. In addition, the capacitors with hybrid electrodes have very small retention loss.
Photoluminescence characterization of InGaP/GaAs heterostructures grown by metalorganic chemical vapor deposition
Authors: Nittono Takumi , Sugitani Suehiro , Hyuga Fumiaki
Published: July 2002

Growth conditions of metalorganic chemical vapor deposition have been investigated for the purpose of obtaining abrupt InGaP/GaAs interfaces. Photoluminescence (PL) spectra of InGaP/GaAs quantum wells (QWs) are used to characterize these interfaces. The conventional gas switching sequence, i.e., simultaneously switching on group-III and -V gases, is found to provide only a broad peak at wavelengths longer than those of near-band-edge emissions from GaAs in the PL spectrum of the InGaP/GaAs QW. PL studies using QWs having an AlGaAs barrier, for example, AlGaAs/GaAs/InGaP and InGaP/GaAs/AlGaAs, show that the GaAs-on-InGaP interface is responsible for this broad peak. A novel gas switching sequence where group-III gas is switched on first results in sharp peaks corresponding to 5.7- and 2.8-nm-thick wells in the PL spectrum of InGaP/GaAs QW. Preflow of TMGa on InGaP surface is effective in suppressing the substitution of P atoms in InGaP to As atoms at the GaAs-on-InGaP interface.
Magnetovolume effects and magnetic anisotropy in Ce3(FeM)29 compounds (M=Ti, Cr)
Authors: Arnold Z. , Kamarád J. , Morellon L. , Algarabel P. A. , Ibarra M. R. , Pareti L. , Albertini F. , Paoluzi A. , Fuerst C. D.
Published: July 2002

The magnetovolume effects and magnetic anisotropy of the new Ce3(FeTi)29 and Ce3(FeCr)29 compounds have been studied by measuring their linear thermal expansion (LTE), pressure dependence of the Curie temperature Tc, compressibility, and anisotropy field. The value of Tc, its pressure dependence dTc/dp, and anisotropy field HA for the Ce(FeCr)12 phase are also reported. The linear thermal expansion of these compounds exhibits Invar-like behavior with large positive values of the spontaneous magnetostriction at Tc [e.g., ωs(0)≊1.2% in Ce3(FeTi)29]. Different values of the compressibility have been observed in the ferromagnetic and paramagnetic states of Ce3(FeTi)29 (κf=1.25×10−3 kbar−1 and κp=1.12×10−3 kbar−1) in qualitative agreement with the large magnetovolume effect observed in the LTE. Tc for all the phases studied decreases under pressure with initial slopes dTc/dp=−3.75, −3.6, and −2.7 K/kbar for Ce3(FeTi)29, Ce3(FeCr)29, and Ce(FeCr)12, respectively. Two anisotropy fields have been detected in Ce3(FeTi)29 associated with saturation along the b (HA) and c (H′A) axes of the monoclinic structure. The temperature dependence of H′A for Ce3(FeCr)29 and HA for Ce(FeCr)12 is also reported.
Gettering of metals by voids in silicon
Authors: Raineri V. , Fallica P. G. , Percolla G. , Battaglia A. , Barbagallo M. , Campisano S. U.
Published: July 2002

The gettering efficiency of copper and platinum by cavities formed in silicon after high dose helium implantation and thermal processing has been investigated. The formation of helium bubbles and their evolution into cavities has been investigated by transmission electron microscopy; the measured values of void density, diameter and the width of the void layer can be interpreted by assuming a simple coalescence model. Metal impurities intentionally introduced in silicon by ion implantation are efficiently gettered inside these cavities, probably due to the large amount of unsatured bonds at the void internal surface. Processing at temperatures higher than 1000oC causes a release of the trapped metal atoms which can be gettered again by repeating the process. The method is demonstrated on real devices such as large area diodes (a particle detector) and bipolar transistors. The capability to localize in depth and across the wafer surface on the gettering sites allows the development of a new gettering engineering.
Photoelectric phenomena in metal-insulator-semiconductor structures at low electric fields in the insulator
Authors: Przewlocki H. M.
Published: July 2002

Numerous photoelectric methods were used to determine parameters of metal-insulator-semiconductor (MIS) structures. A model of photoelectric phenomena taking place in MIS structures, at low electric fields in the dielectric, was found to be necessary to explain results of some of the experiments. Such a model has been worked out and is presented for the practically important case of zero net photoelectric current (J=0). Formulas are derived allowing calculation of MIS structure photoelectric characteristics. These characteristics have also been determined experimentally and are presented in comparison with the calculated ones. Good agreement between the measured and calculated characteristics supports the validity of the analysis presented. This analysis, together with the analysis for the case of nonzero net photoelectric current (J≠0), which will be presented separately, creates a basis for development of a new family of photoelectric MIS structure measurement methods.
Structural order in thin a-Si:H films
Authors: Berntsen A. J. M. , van Sark W. G. J. H. M. , van der Weg W. F.
Published: July 2002

Hydrogenated amorphous silicon (a-Si:H) films were grown in an rf-plasma deposition system on various substrates. The thickness of the films ranged from 11 to 579 nm. The structural properties of the films were studied by means of ex situ Raman spectroscopy. The width of the transverse-optic peak in the Raman spectrum was used as a measure for the amount of bond-angle variation in the films. In contrast to earlier reports, it is found that bond-angle variation in glow-discharge-deposited a-Si:H does not depend on the film thickness, nor on the type of substrate material.
The distribution of charge concentration in InP/Si
Authors: Bartels A. , Peiner E. , Klockenbrink R. , Schlachetzki A.
Published: July 2002

The spatial distribution of the charge concentration of InP layers grown on Si substrates by metalorganic vapor-phase epitaxy was investigated. The concentration near the surface and within the bulk of the layer was found to be governed by Si doping out of the ambient gas. Diffusion of Si across the heterointerface which may be partially assisted by dislocations is dominant in a region near the InP/Si interface. In the vicinity of the heterointerface the charge concentration in the InP layer is determined by strong compensation, which is attributed to defects caused by the mismatch of lattice parameter and thermal-expansion coefficient of InP and Si.
Rapid flux motion and critical state dynamics in a superconducting disk
Authors: Gupta D. , Donaldson W. R. , Kadin A. M.
Published: July 2002

A disk of superconducting thin film partially screens an externally applied magnetic field. If the film temperature is increased, the depression of the critical current density allows the excluded flux to move in. When the film is driven to its resistive state by rapid heating, the fast decay of the screening currents results in rapid flux motion. We have developed a numerical technique for analyzing this transient flux motion along with the temporal variation of current density and magnetic field. Our theoretical model has been verified experimentally by heating a high-temperature superconducting film with short laser pulses and by measuring the induced voltage across a coil coupled closely to the film. This concept can be used to make a fast contactless opening switch for high-power applications, such as energy extraction from inductive energy storage systems.
A proposal for determination of band offset at a semiconductor heterojunction
Authors: Lan Sheng , Yang Cheng-Qing , Xu Wan-Jing , Liu Hong-Du
Published: July 2002

We propose a method to determine the band offset of a heterojunction based on the elimination of the diffusion potential. In0.5Ga0.5P/GaAs heterojunction samples were used for demonstration of this method. As many error sources related to the determination of diffusion potential are avoided in our case, the more accurate value of 137±5 meV has been obtained for the conduction-band discontinuity.
Influence of domain size on optical properties of ordered GaInP2
Authors: Ernst P. , Geng C. , Hahn G. , Scholz F. , Schweizer H. , Phillipp F. , Mascarenhas A.
Published: July 2002

Using dark-field transmission electron microscopy images of ordered GaInP samples, we show how the ordering domain size depends on the growth temperature. Samples with different average domain sizes are compared with regard to their photoluminescence (PL) and excitation spectra. We find a close correlation between the size of the ordered domains and the relative intensity of the PL peak from band–band recombination compared with the rapidly shifting, below-band-gap luminescence emission.
Dynamic work function shift in cold cathode emitters using current carrying thin films
Authors: Mumford P. D. , Cahay M.
Published: July 2002

We analyze a new cold cathode emitter which consists of a thin wide band gap semiconductor material sandwiched between a metallic material, and a low work function semimetallic thin film. We show that under forward bias operation the electrons captured in the low work function material are responsible for an effective reduction of the semimetallic film work function, together with a substantial increase of the cathode emitted current. The dynamic work function shift is shown to increase with the amount of injected current. Potential material candidates are suggested to achieve low-voltage (<20 V), room-temperature cold cathode operation with emission currents approaching several hundred A/cm2 and large efficiencies.
Quantum correction to the BKT transition for 2D easy-plane antiferromagnets
Authors: Biagini Cristiano , Cuccoli Alessandro , Tognetti Valerio , Vaia Ruggero , Verrucchi Paola
Published: July 2002

We consider the quantum antiferromagnet with easy-plane exchange anisotropy, namely, the antiferromagnetic XXZ model, on the square lattice. Its classical counterpart, compared to the planar model shows a reduction of the critical temperature TBKT of the Berezinskii–Kosterlitz–Thouless phase transition, that is a consequence of the thermal out-of-plane fluctuations. For the quantum system we use the pure-quantum self-consistent harmonic approximation to calculate how much the effective exchange interaction is weakened as an effect of the pure-quantum part of the fluctuations. One can then predict the further reduction of TBKT with respect to the corresponding classical system. The theory works well in a wide range of values of the easy-plane anisotropy. In the extreme case of the spin-1/2 model, the result is compatible with the estimate of TBKT obtained by previous quantum Monte Carlo simulations. When the anisotropy is weak the theory leads to an unphysical ‘‘isotropization’’ due to the use of the Villain spin-boson transformation.
Rapid loss of magnetic order in Ni on alloying with Cr, Mo, Re, and Si
Authors: Suresh Babu V. , Pavlovic A. S. , Seehra Mohindar S.
Published: July 2002

In this article, the variation of the Curie temperature Tc and the magnetic moment μ of Ni in the fcc alloys Ni1−xMx (M=Cr, Mo, Re, and Si) are reported for concentrations x between 0 and 15 at. %. The decrease of Tc and μ with x reported earlier for NiCr alloys are also observed for the NiMo, NiRe, and NiSi alloys. The initial rates of decreases are about 60 K/at. % for Tc and 0.05 μB/at. % for μ, leading to critical concentration xc≂13% above which ferromagnetism disappears. Qualitatively, these decreases may be understood in terms of transfer of outer shell electrons of M atoms to the 3d↓ band of Ni in the alloys. However, a quantitative calculation of xc is not yet available.
On the soft wall guiding potentials in realistic quantum waveguides
Authors: Xia Jian-Bai , Sheng Wei-Dong
Published: July 2002

A transfer matrix method is presented for the study of electron conduction in a quantum waveguide with soft wall lateral confinement. By transforming the two-dimensional Schrödinger equation into a set of second order ordinary differential equations, the total transfer matrix is obtained and the scattering probability amplitudes are calculated. The proposed method is applied to the evaluation of the electron transmission in two types of cavity structure with finite-height square-well confinement. The results obtained by our method, which are found to be in excellent agreement with those from another transfer matrix method, suggest that the infinite square-well potential is a good approximation to finite-height square-well confinement for electrons propagating in the ground transverse mode, but softening of the walls has an obvious effect on the electron transmission and mode-mixing for propagating in the excited transverse mode.
Vertical drift of P–E hysteresis loop in asymmetric ferroelectric capacitors
Authors: Zheng Lirong , Lin Chenglu , Xu W.-Ping , Okuyama Masanori
Published: July 2002

One kind of distortion in P–E hysteresis of ferroelectric thin-film capacitors, namely vertical drift of P–E hysteresis loop, is discussed. This kind of distortion, usually observed in asymmetric ferroelectric thin-film capacitors, originates from the asymmetric conduction of the capacitor at different polarities. The shapes of the distorted hysteresis loops are studied by applying a simple circuit model with respect to the measuring frequency, applied voltage amplitude, and different leakage conductance, etc. and are verified with experiment.
Surface photovoltage spectroscopy of thin films
Authors: Leibovitch M. , Kronik L. , Fefer E. , Burstein L. , Korobov V. , Shapira Yoram
Published: July 2002

The surface photovoltage (SPV) spectrum due to subband-gap illumination of thin films is theoretically studied. It is shown that this SPV is inherently sensitive to buried interfaces just as it is sensitive to the external semiconductor surface. The different contributions to the SPV from all the optically active gap states present within a sample, consisting of a bulk substrate covered by a thin film, are analyzed. Analytical expressions are obtained in the low illumination intensity and the depletion approximation regime. The evolution of the SPV spectrum with film thickness is examined and is found to depend on both site and population of the gap states. Three modes of evolution are found, according to the relative importance of gap state population changes with film thickness. These modes are confirmed by a numerical simulation of a thin film of pseudomorphic InAlAs on InP substrates and by experiments conducted on the same system. The approach is also applied to the InP/In2O3 system, revealing gap state formation, followed by filling with electrons, thereby explaining previous observations of nearly ideal I–V behavior at this junction.
Effect of barrier material on the two-dimensional electron gas in δ-doped GaInAs-based quantum wells
Authors: Lo Ikai , Mitchel W. C. , Yu P. W. , Fathimulla A. , Hier H.
Published: July 2002

We have observed two oscillations due to the first two subbands of the two-dimensional electron gas in δ-doped AlAs0.56Sb0.44/Ga0.47In0.53As and Al0.48In0.52As/Ga0.47In0.53As quantum well by Shubnikov–de Haas measurements. From the electron densities of the two subbands we determined the energy difference between the first two subbands, ΔE01, to be 72 meV for the AlAs0.56Sb0.44/Ga0.47In0.53As, and 54 meV for the Al0.48In0.52As/Ga0.47In0.53As quantum well. By comparing with the data from an Al0.24Ga0.76As/Ga0.78In0.22As quantum well, we found that the larger band offset sample gives the higher ΔE01, but when the band offset is relatively small, the well width may determine the energy difference ΔE01.
Simulating device size effects on magnetization pinning mechanisms in spin valves
Authors: Oti J. O. , Cross R. W. , Russek S. E. , Kim Y. K.
Published: July 2002

The effects of magnetostatic interactions on the giant magnetoresistive (GMR) response of NiFe/Cu/NiFe spin valves are studied using an analytical model. The model is applicable to devices small enough for the magnetic layers to exhibit single-domain behavior. Devices having lengths in the track-width direction of 10 μm and interlayer separations of 4.5 nm are studied. Stripe heights are varied from 0.5 to 2 μm. The magnetization of one magnetic layer is pinned by a transverse pinning field that is varied from 0 to 24 kA/m (300 Oe). GMR curves for transverse fields are calculated. At zero external field the magnetization of the layers shows a tendency to align themselves antiparallel in the transverse direction. This results in an offset from the ideal biasing of the device. Broadening of the curves due to shape anisotropy occurs with decreasing stripe height and increasing magnetic layer thickness, and the magnetization in the pinned layer becomes less stable.
Infrared spectroscopic and electronic transport properties of polycrystalline semiconducting FeSi2 thin films
Authors: Tassis D. H. , Mitsas C. L. , Zorba T. T. , Dimitriadis C. A. , Valassiades O. , Siapkas D. I. , Angelakeris M. , Poulopoulos P. , Flevaris N. K. , Kiriakidis G.
Published: July 2002

Polycrystalline semiconducting FeSi2 thin films were grown on (100) Si substrates of high resistivity by electron beam evaporation of amorphous Si/Fe ultrathin multilayers in an ultrahigh vacuum system, followed by conventional vacuum furnace (CF) or rapid thermal annealing (RTA). Infrared reflectance and transmittance measurements were employed for optical characterization of the samples at room temperature. The results indicate a direct transition at about 0.85 eV, an indirect transition at about 0.79 eV, and exponential band tail states within the band gap. The quality of the silicide is improved by increasing the annealing temperature from 600 to 800 °C in the RTA process, while the opposite is observed in the CF annealed samples. Transport measurements were performed on a typical β-FeSi2 layer of high quality grown by CF at low temperature. The measured mobility is about 97 cm2/V s and the hole concentration is about 1×1017 cm−3. The mobility is a factor of 10 higher and the hole concentration a factor of 100 lower than the corresponding published data, indicating a significantly improved quality of β-FeSi2 layers. Temperature-dependent measurements indicate that carrier transport is dominated by impurity conduction.
The effect of anisotropic flux pinning microstructure on the sample length dependence of the magnetization critical current density in niobium–titanium superconductors
Authors: Nunes C. Bormio , Heussner R. W. , Larbalestier D. C.
Published: July 2002

Magnetization measurements of the critical current density Jc in Nb 47 wt % Ti with Nb artificial pinning centers revealed that the shape and magnitude of the field dependent magnetization hysteresis ΔM(H) was a strong function of the sample length and that ΔM(H) for short wire samples was up to six times smaller than for long wires. This is caused by the strong anisotropy of the critical current density Jc. The magnitude of Jc flowing perpendicular to the wire axis J⊥ was deduced to be 50–175 times smaller than the longitudinal current density J∥. The source of the anisotropy lies in the anisotropic flux pinning microstructure of the wires. When the magnetization current crosses perpendicular to the filament axis at each end of the wire, the Lorentz force is parallel to the pinning center axis. The pinning force is weak in this direction and J⊥ is correspondingly small. The technologically important critical current density is the longitudinal current density J∥. It can be extracted from magnetization measurements only in the case of large length to diameter filaments, as is quantitatively analyzed here.
Initial stages of GaN/GaAs(100) growth by metalorganic chemical vapor deposition
Authors: Wang Kun , Pavlidis Dimitris , Singh Jasprit
Published: July 2002

A study of GaN buffers grown by metalorganic chemical vapor deposition on (001) GaAs substrates was performed. Nucleation images obtained by atomic force microscopy (AFM) were employed to investigate the growth temperature, growth time, and growth rate dependence of the nucleation mechanisms. The growth mode corresponds to two-dimensional (2D) island nucleation at low temperatures, while three-dimensional (3D) island growth is observed at high temperatures. Large grain sizes and good surface coverage was obtained for 3 min growth. Higher growth rates help nucleation on mismatched substrate and promote the 2D growth mode. GaN films 0.5 μm thick were grown on different buffers using a two-step technique as suggested by the AFM analysis and their morphology was found to improve when grown on optimized buffers.
Valence-band offsets and different band-gap behaviors of (β-GaN)/(β-AlN) superlattice and (α-GaN)/(α-AlN) superlattice
Authors: Ke San-huang , Zhang Kai-ming , Xie Xi-de
Published: July 2002

The valence-band offsets at (β-GaN)/(β-AlN) (001), (110), (111) interfaces and strained (α-GaN)/(α-AlN) (0001) interface are determined systematically by ab initio calculations using supercells of up to (6+6) layers, and by an average bond energy model. The results are in very good agreement with experimental data. It is found that the variation of the band gap in (α-GaN)n/(α-AlN)n (0001) system with n is completely different from that in the (β-GaN)/(β-AlN) system. These different band-gap behaviors are shown to be related to the internal electric fields in α-GaN and α-AlN slabs, which are induced by the difference of the spontaneous polarizations in these slabs.
Fitting of x-ray or neutron specular reflectivity of multilayers by Fourier analysis
Authors: Li Ming , Möller M. O. , Landwehr G.
Published: July 2002

The dependence of x-ray or neutron specular reflectance on the scattering density has been linearized by modifying the Born approximation. This makes it possible to analyze the reflectivity curves by the Fourier transform method by using the box refinement technique. Thus, the phases of the scattered waves are iteratively obtained, by which the scattering density profile in layered systems can be directly evaluated. The validity of these modifications is demonstrated by some numerical examples. The box refinement technique requires fewer constraints to obtain the physically realistic scattering density than the least-squares-fitting method does.
Competitive evolution of the fine contrast modulation and CuPt ordering in InGaP/GaAs layers
Authors: Diéguez A. , Peiró F. , Cornet A. , Morante J. R. , Alsina F. , Pascual J.
Published: July 2002

We use transmission electron microscopy to characterize the morphology of InGaP epitaxial layers grown by metal-organic vapor-phase epitaxy over misoriented GaAs (001) substrates, with a cutoff angle in a range from 0° to 25°. The occurrence of phase separation and CuPt-type ordered superstructures has been observed. The most ordered configuration has been found to appear in layers grown on 2° off substrates, and the strength of order decreases with increasing the misorientation angle beyond α=2°. Conversely, whereas the phase separation is less evident in the layer grown at 2°, the sample grown with a misorientation of 25° exhibits the most phase separated configuration. The completion between these two phenomena is discussed depending on the misorientation angle.
Photoluminescence studies of strained CdTe/Cd0.633Mn0.367Te single quantum wells
Authors: Shen W. Z. , Shen S. C.
Published: July 2002

We report the photoluminescence studies of the strained CdTe/CdMnTe single quantum wells (SQWs) with large manganese of 0.367. The conduction band-offset ratio Qc of this structure is found to be 0.92±0.01. The band filling effect in this SQW is discussed in detail. Our experiments show that the excitonic transition dominates the whole measured temperature (20–200 K) in the well. The broadening of linewidth with temperature can be well explained by exciton-longitudinal-optical (LO) phonon coupling model and a quantitative approach is presented to explain the experimental fact that the strength of the exciton-LO phonon coupling is quite stronger in SQWs than that of multiple quantum wells. We demonstrate the main mechanism of the decrease in radiative QW recombination efficiency is due to the thermal excitation of excitons out of CdTe well, followed by nonradiative recombination in the CdMnTe barriers.
Numerical analysis of the coherent radiation emission by two stacked Josephson flux-flow oscillators
Authors: Wallraff A. , Goldobin E. , Ustinov A. V.
Published: July 2002

The numerical investigation of the radiation emission by a system of two magnetically coupled, long Josephson junctions is reported. Time-dependent synchronized voltage response in the flux-flow regime is analyzed for the case of in-phase and out-of-phase oscillations in the junctions. Simulations show that Josephson junctions operating in the in-phase flux-flow mode may generate rf radiation power by a factor of more than 4 larger than that of a single Josephson junction. The radiation in the out-of-phase flux-flow mode is characterized by nearly completely suppressed amplitudes of odd harmonics and considerably damped even harmonics as compared to that of a single barrier junction. The dependence of the radiation power on the parameter spread between the junctions is investigated. The advantages of using stacked Josephson junctions as oscillators for the sub-mm wave band are discussed.
Charge transfer-originated large Faraday rotation in Rh4+-substituted magnetic garnets
Authors: Shinagawa K. , Tobita E. , Ando K. , Saito T. , Tsushima T.
Published: July 2002

A large Faraday rotation is reported for Rh4+-substituted magnetic garnets in the near-infrared wavelength region. The Faraday rotation at 0.9 μm is about seven times larger than that in yttrium iron garnet by substituting Rh4+ ions of only 0.13 per formula unit at room temperature. From the electronic state calculations of a (Rh4+O62−)8− octahedral cluster performed by using an unrestricted self-consistent field–scattering wave–X α method, the origin of the Faraday rotation is attributable to the charge transfer transitions of an electron from O 2p to Rh 4d orbitals, or t1un(2p)→t2g*(4d) and t2un(2p)→t2g*(4d) transitions. Based on this assignment, the Faraday rotation spectrum is calculated by taking into account the spin–orbit interaction and the molecular field from iron magnetizations in the magnetic garnet.
Structural and optical characteristics of SiO2 thin film containing GaAs microcrystallites
Authors: Shi Wangzhou , Lin Kuixun , Lin Xuanying
Published: July 2002

Structure of GaAs-SiO2 composite thin films prepared by magnetron rf co-sputtering technique were investigated by transmission electron microscopy, x-ray diffraction, and x-ray photoelectron microscopy. The results show that GaAs microcrystallites with an average size of 10 nm were uniformly dispersed in SiO2 thin film. Quantum confinement effect was observed in the film by the measurement of absorption spectrum, and the blueshift of absorption edge was measured to be 0.5 eV. Nonlinear optical absorption and nonlinear optical refraction were studied by Z-scan technique using single Gaussian beam of He-Ne laser (632 nm). An enhanced third-order nonlinear optical absorption coefficient and nonlinear optical refractive index were achieved in the thin film, and measured to be 2×10−1 and 4×10−6 cm2/W, respectively.
Dependence on gate work function of oxide charging, defect generation, and hole currents in metal–oxide–semiconductor structures
Authors: DiMaria D. J.
Published: July 2002

Oxide charge trapping and current sensing techniques are used to test for enhanced anode hole generation from metal–oxide–semiconductor devices with gate electrodes of increasing work function during hot electron stress. In this study, the gate work function variation was obtained by comparing devices with polycrystalline silicon gates doped degenerately with either phosphorus or boron. Contrary to expectation, significant differences were not observed in hole currents carried in the silicon substrate valence band of these devices. However, differences were seen in oxide charging, interface-state formation, and neutral trap generation. The enhanced positive charging observed on devices with larger work function boron-doped gates is discussed in terms of mechanisms related to slow donorlike state formation caused by hydrogen release and reaction at energies of less than 5 eV.
On the study of the magnetic domain pattern via the initial magnetization curve
Authors: Wu Te-ho
Published: July 2002

This study inquires into the relationships between the initial magnetization curve and the magnetic domain pattern in the demagnetized states for amorphous TbFeCo as well as multilayered Co/Pd thin film samples. This was done specifically through an investigation of different demagnetized states of samples demagnetized by a variety of methods. The magnetic domain pattern for the sample demagnetized by an in-plane magnetic field and for the sample demagnetized by a perpendicular magnetic field was found to be quite different even though both states have zero magnetization. The former state has denser and finer domains than the latter. In addition, both states were studied in light of the initial magnetization curves obtained by measurements of the magneto-optic Kerr effect and the extraordinary Hall effect. Moreover, the initial magnetization for the fine domains increases with an increase in magnetic field, while for the coarse domains, the initial magnetization remains at zero for magnetic field below coercivity Hc, then rises sharply to saturated magnetization when magnetic field is nearly equal to Hc.
Monte Carlo and spin dynamics study of the anisotropic Heisenberg model in two dimensions
Authors: Costa B. V. , Costa J. E. R. , Landau D. P.
Published: July 2002

By using Monte Carlo and molecular dynamics we study the static as well as the dynamical behavior of the classical two dimensional anisotropic Heisenberg model. Our results show that the vortices developed in such a model exhibit two different behaviors depending if the value of the anisotropy λ lies below or above a critical value λc. The in-plane and out-of-plane correlation functions were obtained numerically for λ<λc and λ>λc. We found that the out-of-plane dynamical correlation function exhibits a central peak for λ>λc but not for λ<λc at temperatures above TKT.
Magnetic and magnetoresistance measurements on iron-based nanoclusters in In0.53Ga0.47As
Authors: Pekarek T. M. , Crooker B. C. , Li S. , McElfresh M. , Chang J. C. P. , McInturff D. , Harmon E. S. , Melloch M. R. , Woodall J. M.
Published: July 2002

We have obtained magnetoresistance data on low iron-concentration samples (∼1%) showing a large negative magnetoresistance (3.2% at 5 K in 0.5 T) attributed to imbedded superparamagnetic clusters in In0.53Ga0.47As. The samples were prepared by ion implanting a p-In0.53Ga0.47As layer with iron followed by a rapid thermal anneal. Magnetic measurements confirm the formation of a cluster size distribution with a mean diameter of 6.2 nm and effective moment of 7000 bohr magnetons. The magnetization of these single domain ferromagnets is 50% saturated in a field of only 0.2 T even at room temperature which is important for device applications.
Microstructural study of ion-beam deposited giant magnetoresistive spin valves
Authors: Tang Li , Laughlin David E. , Gangopadhyay Sunita
Published: July 2002

Detailed microstructural investigation of ion beam deposited giant magnetoresistance (GMR) spin valves has been carried out using various techniques of transmission electron microscopy (TEM) and x-ray diffraction. Two fcc phases, i.e., FeMn and NiFe/Co/Cu/Co/NiFe have been identified in ion-beam deposited Ta (50 Å)/FeMn (80 Å)/NiFe(30 Å)/Co(15 Å)/Cu(33 Å)/Co(15 Å)/NiFe(60 Å)/Ta(25 Å)/Si(001) spin valves. The Ta buffer layer is amorphous, while the 50-Å-thick Ta cap layer consists of a 25-Å-thick amorphous layer and on top of which a Ta oxide layer. The lattice constants of the fcc FeMn and the fcc NiFe/Co/Cu/Co/NiFe increase with the ion-beam voltage. Both the FeMn and the NiFe/Co/Cu/Co/NiFe layers are (111) textured. The misfit strain between the FeMn layer and the pinned NiFe layer is released by the formation of dome shape FeMn surface rather then by the formation of misfit dislocations at the interface between the two layers. The peak to valley height of the domes seems to have little effect on the GMR properties of the ion-beam deposited spin valves. It was found that large columnar grain width gives large exchanged field and ΔR/R of the spin valves.
Grain structure and magnetic clustering in SiO2 added CoNiPt granular films
Authors: Murayama Akihiro , Hyomi Kyoko , Ohshima Kiyoshi , Miyamura Masao , Maekawa Mikako , Kondoh Shinji
Published: July 2002

Grain structure and its relation to the magnetic properties in SiO2 added CoNiPt granular films have been studied, with the SiO2 content varied from 0 to 8 mol %. With SiO2 more than 4 mol %, significant grain boundaries with a width of a few nm are formed by the segregated SiO2. The average grain size in the film plane decreases from 16 to 9 nm with the SiO2 increased up to 8 mol %. The media noise significantly decreases with increasing SiO2 and remanent magnetized domains observed by magnetic force microscopy (MFM) also become finer. With 8 mol % SiO2, MFM shows that weak magnetic clustering spreads over the grains, with a diameter of about 100 nm. This clustering indicates the collaborative behavior of the magnetization between grains is responsible both for the steep magnetization reversal as well as the slight increase in the media noise as the recording density is increased.
High-coercivity (NdDy)2(FeNb)14B–α–Fe nanocrystalline alloys
Authors: Wang Zuocheng , Zhang Maocai , Li Fubiao , Zhou Shouzeng , Wang Run , Gong Wei
Published: July 2002

High coercivity, high remanence, and high energy product (NdDy)2(FeNb)14B–α–Fe nanocrystalline alloys containing 0 to 30 wt % α–Fe have been prepared by melt spinning and subsequent annealing. The best magnetic properties of remanence (Br), coercivity (Hci), and maximum energy product [(BH)max] are 1.02 T, 702 kA/m, and 134 kJ/m3, respectively, for Nd8.16Dy1Fe85.26Nb1B4.58. The microstructure consists of a two phase nanocomposite of hard magnetic (NdDy)2(FeNb)14B and soft magnetic α–Fe with an average size of about 30 nm. These small dimensions allow effective exchange coupling between hard and soft magnetic grains and result in the simultaneous enhancement of Br, Hci, and (BH)max. A systematic study on the effect of annealing temperature and time on the microstructure and magnetic properties has been carried out.
Acetone transport in poly(ethylene terephthalate)
Authors: Ouyang Hao , Chen Che-Chen
Published: July 2002

Organic solvents like acetone can penetrate into poly(ethylene terephthalate) (PET). The model of case I (Fickian) and case II (swelling) is employed to study the phenomenon of mass transport. This model is successful in explaining the behavior of mass transport in an amorphous polymer, for example, poly(methyl methacrylate) (PMMA). The characteristic parameters, diffusivity D and velocity v, can be obtained from the analysis of experimental data. The mass transport in PET is different from that in PMMA. It is accompanied by a large-scale structural rearrangement, which leads to induced crystallization of the original amorphous state. This is the so-called “solvent-induced crystallization.” Acetone-induced crystallization was confirmed by x-ray diffraction. The differential scanning calorimetry thermograms of acetone-treated PET show that the crystallization peak disappears and the glass transition temperature decreases.
A new method of developing ultralow-loss glasses
Authors: Saito K. , Ikushima A. J. , Ito T. , Itoh A.
Published: July 2002

A new and effective method for developing ultralow-loss glasses for long-distance optical fiber systems is presented. The idea is based on the observed strong effect of a very small amount of alkaline or alkaline-earth impurities on structural relaxation of “frozen-in” density fluctuations. Efficiency of this method is demonstrated in the case of silica glass with Na2O of less than 50 wt. ppm, where the light scattering loss was reduced by 13% by the addition of only 10 wt. ppm (≈1×10−3 mol %) Na2O. Various problems in developing ultralow-loss fiber materials in multicomponent glasses are avoidable in this method, since concentrations of dopants are very small.
Wavefunction engineering for enhanced quantum well intermixing and integrated infrared spectrometers
Authors: Fafard S.
Published: July 2002

Quantum well potentials are engineered to control the energy level shifts induced by semiconductor alloy intermixing. A few monolayers of a semiconductor with a different band gap can be inserted at the node or at the crest of wavefunctions with different parities to enhance the interdiffusion-induced interband transition energy-shifts, or to manipulate the intersubband transition energies.
Giant magnetoresistance in Co1−xCux/Cu multilayers: A new approach to reduced magnetoresistive hysteresis
Authors: Kubinski D. J. , Holloway H.
Published: July 2002

Co 1−x Cu x / Cu multilayers have been made by sputtering using codeposition of Co and Cu to obtain Co1−xCux alloy layers that are separated by 20 Å Cu spacers. As with Co/Cu multilayers, this Cu spacer thickness corresponds to the second antiferromagnetic maximum. At ambient temperatures, the Co1−xCux/Cu multilayers with x≈0.5 exhibit an absence of magnetoresistive hysteresis resembling that reported previously for Co/Cu multilayers at the second antiferromagnetic maximum when the Co layers are very thin (∼3 Å). The multilayers with Co1−xCux alloys differ significantly from the low-hysteresis Co/Cu multilayers by exhibiting low hysteresis over a larger range of ferromagnetic layer thickness. This is practically significant because it reduces the demands for thickness control during manufacturing.
Amorphous magnetic films produced by pulsed laser deposition
Authors: Nikitin P. I. , Beloglazov A. A. , Toporov A. Yu. , Valeiko M. V. , Konov V. I. , Perrone A. , Luches A. , Mirenghi L. , Tapfer L.
Published: July 2002

Amorphous magnetic Fe–Co–Ni–Si–B films have been produced on silicon and glass substrates by means of XeCl excimer laser ablation of amorphous magnetic ribbons. Laser fluence, the number of pulses, and target–substrate distance were the variable parameters. The films were studied with x-ray specular reflectivity, x-ray diffraction, x-ray photoelectron spectroscopy, profilometry, and ellipsometry. Laser ablation has been found to provide a good transfer of composition and structure from the ribbon targets to the deposited films, with excellent mechanical and optical quality of the films. Magnetic properties of the films were examined by a technique based on the magneto-optical Kerr effect. The films revealed well-defined in-plane uniaxial anisotropy. They demonstrated coercivity and anisotropy field values comparable to those of the initial ribbons, and much higher steepness of substantially rectangular hysteresis loops. The films can have a number of prospective applications in magnetic sensors, switches, and concentrators.
Inversion in x-ray Bragg diffraction: A practical technique to compensate for dynamical scattering features
Authors: Nikulin A. Yu. , Petrashen’ P. V.
Published: July 2002

A new numerical technique to compensate for the dynamical effects in experimental x-ray intensity profiles is proposed and implemented to enhance the crystal-lattice strain determination algorithm which is applicable to x-ray diffraction data collected from nearly perfect crystals. A practical procedure for the preliminary treatment of experimental data exhibiting dynamical features was tested using three sets of data representing different cases of near-surface distorted layers. It is shown that the influence of the substrate peak on the overall diffraction pattern can be removed by an iterative modification of the very vicinity of the Bragg region in experimental intensity profile.
Measurement of depth-dependent atomic concentration profiles in CdTe/Hg1−xCdxTe structures
Authors: Mainzer N. , Shilo D. , Zolotoyabko E. , Bahir G. , Sher A. , Cytermann K. , Brener R.
Published: July 2002

A novel method to obtain diffusion-controlled atomic concentration profiles in II–VI semiconductor heterostructures has been developed using high-resolution x-ray diffraction. Measured diffraction spectra are compared with simulations based on direct summation of scattered waves across the heterostructure. In this approach, short-range variations of structural parameters, including the concentrations of the components, interface roughness, etc., can be easily introduced into the simulation routine. The application of the fitting procedure to the experimental spectra taken from variously annealed CdTe/Hg1−xCdxTe heterostructures grown by metal organic chemical vapor deposition, allowed determination of the Hg (or Cd) concentration depth profile as a function of the annealing temperature. As a result, the activation energy and the pre-exponential coefficient for mercury diffusion was found. The diffusion profiles derived from x-ray diffraction spectra were compared with secondary-ions mass spectrometry results, and the advantages of the new method are discussed.