bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of Pb0.97Nd0.02(Zr0.55Ti0.45)O3 thin films prepared by pulsed laser ablation
Authors: Frantti J. , Lantto V.
Published: July 2002

Pulsed laser ablation was used to deposit Nd-doped lead zirconate titanate (PZT) thin films with thickness 100–200 nm from a Pb0.97Nd0.02(Zr0.55Ti0.45)O3 target. The films were ablated onto sapphire substrates with R-plane surfaces, without heating and at a pressure of 5×10−5 mbar using a XeCl excimer laser (pulse energy about 50 mJ, wavelength 308 nm, pulse duration about 20 ns). Energy dispersive spectroscopy (EDS) of x rays was used for the composition analysis of both annealed and unannealed (amorphous) films deposited using various substrate-target distances and laser-beam fluences. It was found that the distance between target and substrate together with the laser-beam energy density on the surface of the target had a significant effect on the composition of the films. From the composition analysis, a suitable range of values was found for the target-substrate distance and fluence to deposit films with correct composition of each component. Lead deficiency was also found in the films in the case of high fluence (≳1.5 J cm−2 with the target-substrate distance <40 mm), probably due to resputtering, while low fluence values (<1.0 J cm−2 with the target-substrate distance ≳40 mm) gave an excess of lead and too high Ti/(Zr+Ti) ratios. The films were annealed 2 h at 750 °C with and without PZT powder. The composition analysis revealed that the films with an excess of lead before annealing had almost the correct amount of lead after annealing. On the other hand, it was not possible to compensate the lead deficiency through annealing in a lead atmosphere. EDS and x-ray diffraction were used to determine the crystal structure of annealed films. Trigonal and tetragonal perovskite structures were the major and minor phases of the films, respectively. Some Raman spectra measured from annealed films were found to be typical to PZT.
Synthesis of Ge nanocrystals embedded in a Si host matrix
Authors: Ngiam Shih-Tung , Jensen Klavs F. , Kolenbrander K. D.
Published: July 2002

The synthesis of a composite material consisting of Ge nanoclusters (≥2 nm in diameter) embedded in a Si host matrix is reported. The Ge nanoparticles are produced by pulsed laser ablation and are codeposited in a Si film simultaneously grown by chemical beam epitaxy using disilane. Scanning transmission electron microscopy, combined with energy-dispersive x-ray measurements, show that discrete Ge particles (≥2 nm diameter) are deposited within a polycrystalline Si host matrix. High-resolution transmission electron microscopy reveals that the particles are crystalline with a lattice spacing corresponding to that of Ge. The enhancement of Si deposition rates from silanes in the presence of Ge, previously demonstrated in chemical vapor deposition of Si1−xGex alloys, is shown to facilitate the growth of a Si layer around the Ge nanocrystals. The overall composition of the Ge cluster/Si host composite material is determined by Rutherford backscattering measurements.
Compositional inhomogeneities in sputtered Co-Cr magnetic thin films studied by atom probe field ion microscopy
Authors: Hono K. , Maeda Y. , Babu S. S. , Sakurai T.
Published: July 2002

Compositional inhomogeneities in Co-22 at. % Cr sputtered thin films have been studied quantitatively using atom probe field ion microscopy. For comparison, a Co-22 at. % Cr bulk alloy specimen has also been examined. As predicted from the recently published phase diagram, a solution treated Co-Cr bulk alloy specimen does not contain detectable chemical inhomogeneities. On the other hand, the Co-22 at. % Cr thin film sputter deposited at 200 °C consists of two distinct phases with different Cr concentrations. The size of each phase is in the order of 8 nm which is significantly smaller than the grain size of the specimen; however, the film that was sputter deposited at ambient temperature exhibited a significantly lower level of compositional inhomogeneity. Based on these results, it is concluded that the phase separation progresses when thin films are deposited on heated substrates.
A multiband Monte Carlo approach to Coulomb interaction for device analysis
Authors: Abramo A. , Brunetti R. , Jacoboni C. , Venturi F. , Sangiorgi E.
Published: July 2002

This paper presents an extension of the theoretical approach for both the short- and long-range components of the Coulomb interaction among carriers in semiconductors to the case of an arbitrary isotropic multiband model, devised for Monte Carlo simulation of silicon devices. The analytical and numerical aspects of the model are discussed in detail. Results for the effect of the Coulomb interaction on the carrier distribution function and on the energy-loss properties of the carrier gas are presented for the case of electrons in homogeneous and inhomogeneous silicon structures.
Structural analysis of erbium sheet-doped GaAs grown by molecular-beam epitaxy, with ion channeling followed by Monte Carlo simulation
Authors: Nakata Jyoji , Jourdan Nicolas , Yamaguchi Hiroshi , Takahei Kenichiro , Yamamoto Yasuich , Kido Yoshiaki
Published: July 2002

The detailed structure of erbium (Er) sheet-doped GaAs grown by molecular-beam epitaxy is directly determined by analysis of the ion-channeling spectra in the [100], [110], and [111] directions with 2.0 MeV He+ beams followed by a recently developed Monte Carlo simulation. It is shown that Er atoms form NaCl-type crystalline ErAs clusters in the GaAs epitaxial layer at 500 and 580 °C. The clusters grow in the [100] direction with the common three principal axes of the cubic unit cell; the lattice constant of the clusters coincides exactly with that of the zinc-blende-type crystalline GaAs epitaxial layer. The shape and size of the clusters can be roughly deduced in the simulation from the dependence of the visible Er fraction on the cluster size for various cluster shapes. These results are consistent with those obtained by cross-sectional transmission electron microscopy. Almost all Er atoms are located precisely in tetrahedral interstitial sites with 0.1 Å standard deviation; only 2% of the Er atoms occupy random sites. These results mean that the lattice constant of the crystalline ErAs clusters is compressed to that for the crystalline GaAs host during the cluster formation.
Structural analysis of ZnS/GaAs heterostructures grown by hydrogen transport vapor-phase epitaxy
Authors: Giannini C. , Peluso T. , Gerardi C. , Tapfer L. , Lovergine N. , Vasanelli L.
Published: July 2002

The structural characterization of ZnS epilayers grown by hydrogen transport vapor-phase epitaxy on (100)-oriented GaAs substrates is reported. X-ray-diffraction measurements in both double-axis (DA) and single-axis modes were performed to determine the residual strain tensor and the strain temperature dependence of the epilayer in the range between 25 and 650 °C. From the analysis of the data obtained by DA measurements of several symmetric Bragg reflections at different azimuth angles and several asymmetric reflections recorded in different geometries, an orthorhombic distortion of the ZnS lattice was found. The crystallographic symmetry could be explained by an asymmetric distribution of the misfit dislocation density in the interface plane along the [011] and [01̄1] directions. The temperature dependence measurements of the strain tensor components between room temperature and the growth temperature (650 °C) allowed determination of the thermal misfit between ZnS and GaAs and the linear thermal-expansion coefficient of ZnS. Finally, triple-crystal diffractometry and secondary-ion-mass spectrometry were used to investigate the chemical nature of the ZnS-GaAs interface. The results indicate the presence of an interdiffused ZnS-GaAs interface region, whose occurrence turns out to be associated with the initial defect structure of the substrate surface before the growth.
Molecular motions and ordering of the interfacial, droplet and binder regions of polymer dispersed liquid crystal displays: A paramagnetic resonance spin probe study
Authors: Kim Y. C. , Lee S. H. , West J. L. , Gelerinter E.
Published: July 2002

The molecular dynamics and ordering of polymer-dispersed liquid-crystal displays are studied using the technique of paramagnetic resonance of spin probes. A complex spectrum is observed in which components are identified from probes dissolved in the droplet, binder, and interfacial regions of the display. In particular, the liquid-crystal curing, phase separation, and structure of displays with both thermoplastic and epoxy binders are investigated. Observations are also performed as the liquid-crystal droplet size is varied. Among the more interesting results is the demonstrated ability to observe the interfacial region. It appears to be quite fluid and has little or no average order. It is also observed that the compatibility between the binder and liquid crystal, seen from some of the formulations studied, correlates with the reported electro-optical properties. These include the magnitude and hysteresis of switching voltages and the magnitudes of switching times.
Spatial extent of the correlation between local diffusion noise sources in GaAs
Authors: Mateos Javier , González Tomás , Pardo Daniel
Published: July 2002

We present a microscopic analysis of the spatial correlation between local diffusion noise sources in homogeneous GaAs. The calculations are performed by using an ensemble Monte Carlo simulation. The spatial extent of the correlations is determined for different applied electric fields. It is confirmed that the space correlations do exist over short distances, ranging from 0.1 to 0.6 μm depending on the electric field. This correlation length is much longer in GaAs than in Si. It is shown that the diffusion coefficient can be decomposed into the contribution of cross-correlations between close cells inside a homogenous sample. Under far-from-equilibrium conditions, the forward correlations at low frequency are found to be spatially shorter than those with the backward positions due to the effect of the randomizing scattering mechanisms.
Polarized absorption of Nd3+ in LiNbO3: Effect of MgO codoping
Authors: Loro H. , Voda M. , Jaque F. , García Solé J. , Muñoz Santiuste J. E.
Published: July 2002

A systematic investigation is presented of the polarized optical absorption of the Nd3+ ion in LiNbO3 and LiNbO3(MgO) crystals. Energy levels of Nd3+ are identified and labeled by appropriate crystal quantum numbers μ=1/2 or μ=3/2, in agreement with C3 local symmetry. The anisotropic Judd–Ofelt theory is applied to estimate spectroscopic parameters relevant for laser applications (radiative lifetimes and branching ratios from the 4F3/2 laser state). Fluorescence lifetimes are also determined to estimate the quantum efficiency from the 4F3/2 state: η=0.6 for LiNbO3:Nd and η=0.5 for LiNbO3(MgO):Nd. The effect of MgO codoping in the energy levels as well as in the spectroscopic parameters of the Nd3+ ion is evaluated and discussed.
Ion-beam doping of GaAs with low-energy (100 eV) C+ using combined ion-beam and molecular-beam epitaxy
Authors: Iida Tsutomu , Makita Yunosuke , Kimura Shinji , Winter Stefan , Yamada Akimasa , Fons Paul , Uekusa Shin-ichiro
Published: July 2002

A combined ion-beam and molecular-beam-epitaxy (CIBMBE) system has been developed. This system consists of an ion implanter capable of producing ions in the energy range of 30 eV–30 keV and conventional solid-source MBE. As a successful application of CIBMBE, low-energy (100 eV) carbon ion (C+) irradiation during MBE growth of GaAs was carried out at substrate temperatures Tg between 500 and 590 °C. C+-doped layers were characterized by low-temperature (2 K) photoluminescence (PL), Raman scattering, and van der Pauw measurements. PL spectra of undoped GaAs grown by CIBMBE revealed that unintentional impurity incorporation into the epilayer is extremely small and precise doping effects are observable. CAs acceptor-related emissions such as ‘‘g,’’ [g-g], and [g-g]β are observed and their spectra are significantly changed with increasing C+ beam current density Ic. PL measurements showed that C atoms were efficiently incorporated during MBE growth by CIBMBE and were optically well activated as an acceptor in the as-grown condition even for Tg as low as 500 °C. Raman measurement showed negligible lattice damage of the epilayer bombarded with 100 eV C+ with no subsequent heat treatment. These results indicate that contamination- and damage-free impurity doping without postgrowth annealing can be achieved by the CIBMBE method.
Visible luminescence from single crystal-silicon quantum wells
Authors: Saeta P. N. , Gallagher A. C.
Published: July 2002

Single crystal-silicon quantum wells with SiO2 barriers have been grown from SIMOX silicon-on-insulator substrates. Photoluminescence in the red and near-infrared is observed for average well width <8 nm, with peak signal for 2-nm average width. The luminescence spectrum is independent of well width for SiO2 barriers, but shifts 0.3 eV to higher energy upon removal of the upper oxide layer with HF. Both results suggest the importance of radiation from surface states.
Determination of AlAs optical constants by variable angle spectroscopic ellipsometry and a multisample analysis
Authors: Herzinger C. M. , Yao H. , Snyder P. G. , Celii F. G. , Kao Y.-C. , Johs B. , Woollam J. A.
Published: July 2002

Using variable angle spectroscopic ellipsometry, optical constants for AlAs (1.4–5.0 eV) are presented which are simultaneously compatible with measured data from four different samples. The below-gap index values are compatible with published prism measured values. The second derivative spectrum are compatible with published values above the direct band gap. The AlAs spectra is Kramers–Kronig self-consistent over the measured range and is compatible with published values from 0.6 to 1.4 eV. The optical constants for thin (<50 Å) GaAs caps on AlAs are shown to be different from bulk GaAs values and require special consideration when fitting ellipsometric data. For the thin GaAs caps, the E1 and E1+Δ1 critical-point structure is shifted to higher energies as previously observed for GaAs quantum wells. Bulk AlAs optical constants are shown to be different from those of a thin (∼20 Å) AlAs barrier layer embedded in GaAs. The thin barrier layer exhibits a highly broadened critical-point structure. This barrier broadening effect (AlAs) and the thin cap shifting effects (GaAs) have implications for in situ growth control schemes which make use of the E1 and E1+Δ1 critical-point region.
Thermal stress in GaN epitaxial layers grown on sapphire substrates
Authors: Kozawa T. , Kachi T. , Kano H. , Nagase H. , Koide N. , Manabe K.
Published: July 2002

Thermal stress in GaN epitaxial layers with different thicknesses grown on sapphire substrates by metalorganic vapor phase epitaxy using an AlN buffer layer was investigated. Biaxial compressive stress in the GaN layer, due to the difference in the thermal expansion coefficients between GaN and sapphire, was obtained by measuring the curvature of wafer bending, and the observed stress agreed with the calculated stress. In Raman measurements, the E2 phonon peak of GaN was found to shift and broaden with the stress as a consequence of the change of the elastic constants with strain. The frequency shift Δω (in cm−1) was obtained for the first time, given by the relation: Δω=6.2 σ, where the biaxial stress σ is expressed in GPa.
Simple computation using Coulomb blockade-based tunneling arrays
Authors: Ancona M. G. , Rendell R. W.
Published: July 2002

We present a theoretical study of locally interconnected tunneling arrays operating in the Coulomb blockade regime. In these arrays bits of information are represented by single electrons and useful behaviors can be obtained if the size of a ‘‘dressed’’ electron on the array, i.e., an excess electron plus its polarization cloud, is small. Under this condition, we demonstrate (using simple quasistatic tunneling theory) that linear tunneling arrays can be made to act as charge-coupled devices for single electrons. Such devices could potentially be applied to computational and/or signal processing applications. As a simplest example, we show that these linear arrays can be made to simulate a random walk. These initial results illuminate some of the requirements on locally interconnected quantum device arrays if they are to do useful computation. More speculatively, they suggest that Coulomb blockade-based arrays could be the basis for future very high-powered digital processors. For such hardware to be practical, however, ways of dealing with device nonuniformities (particularly associated with ‘‘polarization charge’’) must be found.
Diffusion of copper in porous silicon
Authors: Andsager D. , Hetrick J. M. , Hilliard J. , Nayfeh M. H.
Published: July 2002

We present a study on the nature of diffusion of copper in p-type porous silicon. The diffusion of evaporated copper in porous silicon and deposition of metal ions in aqueous solution through the porous network was measured by monitoring the metal concentration depth profile as a function of time using Auger electron spectroscopy. We observed that increasing metal penetration from copper evaporated samples correlates with quenching of photoluminescence, in agreement with previous ion quenching results. We extracted a diffusion coefficient from Auger concentration depth profiles which was seven orders of magnitude lower than that expected for diffusion of copper in bulk crystalline Si at room temperature. Deposition of ionic species cannot be characterized as a simple diffusion process. The observed deposition rates were strongly dependent on the solution concentration.
Neodymium-doped GaAs light-emitting diodes
Authors: Chang S. J.
Published: July 2002

Nd-doped semiconductor light-emitting diodes were fabricated by implanting Nd ions into GaAs epi-layer. The fabricated GaAs:Nd diodes show good current-voltage characteristics with a typical reverse breakdown voltage between 8 and 12 V. By injecting minority carriers into the diodes, Nd3+ related emissions were observed, at 77 K, in the 0.92, 1.11, and 1.3 μm regions. These electroluminescence signals correspond to the transitions from Nd3+ 4F3/2 state to the Nd3+ 4I9/2, 4I11/2, and 4I13/2 states, respectively. The measured external quantum efficiency of the GaAs:Nd diodes at 77 K, was 5×10−7.
Photoacoustic depth profiling of the thermal conductivity of an inhomogeneously aligned liquid crystal at a free surface
Authors: Glorieux C. , Bozoki Z. , Fivez J. , Thoen J.
Published: July 2002

Some approaches, both old and new, to the inverse calculation of continuous inhomogeneous thermal conductivity profiles from photoacoustic data are compared. The liquid crystal 8CB in its nematic phase and subject to a magnetic field is chosen as a starting point, since it allows easy control of the conductivity profile. In particular, a new modified multilayer approach seems capable of reliable profile reconstruction.
Depletion width and capacitance transient formulas for deep traps of high concentration
Authors: Look D. C. , Sizelove J. R.
Published: July 2002

We derive expressions for the depletion width and capacitance transient applicable to traps which may be deep and of high concentration. The new results are compared with those obtained from the commonly used formulas, and also from an exact analysis. Experimental deep level transient spectroscopic data for EL2 in GaAs are in good agreement.
Annealing dynamics of arsenic-rich GaAs formed by ion implantation
Authors: Fujioka H. , Krueger J. , Prasad A. , Liu X. , Weber E. R. , Verma A. K.
Published: July 2002

We have investigated the annealing temperature dependence of structural and electrical properties in heavily arsenic implanted GaAs which has a similar amount of excess arsenic to low temperature GaAs (LT-GaAs). The fundamental properties of this material are quite similar to those of LT-GaAs. High resolution x-ray diffraction measurements have revealed that it has an increased lattice constant, which is reduced to the value of bulk GaAs by annealing between 300 and 400 °C. Electrical conduction in this material is dominated by hopping between deep states, which is also reduced by annealing above 350 °C. In samples annealed at temperatures ranging from 600 to 850 °C, the dominant electron trap is EL2; it has been confirmed by resistivity measurements with n-i-n structures that the Fermi level is pinned by EL2. In samples annealed below 500 °C, the dominant electron trap is not EL2 but the U-band, although electron paramagnetic resonance measurements show the existence of a large concentration of the ionized arsenic antisite defect (AsGa+). This supports the notion that the U-band is formed by AsGa defects with slightly modified carrier emission properties compared with EL2.
Molecular-dynamics simulations of direct reactive ion etching of silicon by fluorine and chlorine
Authors: Barone M. E. , Graves D. B.
Published: July 2002

We report results from molecular-dynamics simulations of F+ and Cl+ impact of silicon surfaces, at normal incidence and over a range of energies (10, 25, and 50 eV). The halogen content of the silicon layer increases with halogen fluence, and the simulations are continued until an apparent, quasisteady state in halogen coverage has occurred. Although in some cases the quantitative results differ, F+ and Cl+ are qualitatively similar in steady-state halogen coverage, depth of penetration, etch mechanisms, and etch yield dependence on ion energy. In both cases, a mixed halogenated silicon layer forms, with a substantial degree of surface roughness (∼1–2 nm for 25 or 50 eV ions). At 10 eV for both F+ and Cl+, the apparent steady-state coverage is about 2 equivalent monolayers and the depth of F+ (Cl+) penetration is about 15 Å. For 25 and 50 eV ions, the corresponding coverage (approximately independent of ion energy and type) is about 3 monolayers. The corresponding depth of penetration is about 35 Å. The silicon etch yield is a function of ion composition and energy: These values ranged from 0.25 at 10 eV to 0.45 at 50 eV for F+ and from 0.06 at 10 eV to 0.14 at 50 eV for Cl+. Simulations revealed that the dominant Si etch mechanism and the silicon etch product stoichiometry changed with ion energy. In addition to physical and chemical sputtering, an additional etch mechanism has been observed in the simulations. We term this mechanism direct abstractive etching (DAE). In DAE, an incoming F+ reacts with a surface SiFx, creating a volatile SiFx+1 that leaves the surface with nonthermal energies, similar to physically sputtered products. At 10 eV F+, DAE accounts for ∼80% of the etched species, but at higher energies this mechanism is less likely. DAE is observed for Cl+ impact as well. Comparison of simulation predictions to available experimental results shows at least qualitative agreement, suggesting the approximations made in the simulations are reasonable.
Optical properties of Cd1−xMgxTe epitaxial layers: A variable-angle spectroscopic ellipsometry study
Authors: Luttmann Michel , Bertin François , Chabli Amal
Published: July 2002

The index of Cd1−xMgxTe ternary alloys was measured for the first time by variable-angle spectroscopic ellipsometry on layers of different concentrations. The ellipsometer’s wide spectral range (0.7–5.6 eV) clearly reveals critical points beyond the gap. Self-consistency of the permittivity measurements is investigated by Kramers–Kronig analysis. A transition layer is revealed at the top surface of the samples and is taken into account as a rough layer. In the transparent region Sellmeier’s law is applied to describe the index behavior as a function of the wavelength and the magnesium content.
New III–V semiconductor lasers emitting near 2.6 μm
Authors: Baranov A. N. , Sherstnev V. V. , Alibert C. , Krier A.
Published: July 2002

Lasing has been obtained near 2.6 μm in double-heterostructure InGaAs/InAlAs diode lasers grown by metalorganic vapor phase epitaxy on InAs substrates. At 80 K threshold currents are in the range of 40–200 mA for lasers of deep mesa geometry. The characteristic temperature of the temperature dependence of the threshold current T0 is 21–23 K and lasing was achieved up to 150 K in a pulsed regime. A blueshift of up to 3.0 nm with increasing current is observed. A blueshift is also observed with increasing temperature, which is attributed to refractive index change due to the strong temperature dependence of the threshold carrier density in narrow gap III–V semiconductor lasers.
Temperature dependence of the indirect energy gap in crystalline silicon
Authors: Alex V. , Finkbeiner S. , Weber J.
Published: July 2002

The photoluminescence spectra of crystalline silicon samples are measured for temperatures below 1000 K. The optical transitions are analyzed in terms of excitonic and band-to-band transitions. From the modeling of the line shape we are able to determine the fundamental indirect band gap for temperatures up to 750 K. The temperature dependence follows the Varshni equation with Eg(0)=1.1692 eV, α=(4.9±0.2)×10−4 eV/K and β=(655±40) K.
Amorphous and microcrystalline silicon films deposited by hot-wire chemical vapor deposition at filament temperatures between 1500 and 1900 °C
Authors: Brogueira P. , Conde J. P. , Arekat S. , Chu V.
Published: July 2002

The optical, electronic and structural properties of thin films deposited by Hot-wire chemical vapor deposition with filament temperatures, Tfil, between 1500 and 1900 °C from silane and hydrogen are studied. The substrate temperature, Tsub, was kept constant at 220 °C. Amorphous silicon films (a-Si:H) are obtained at high filament temperatures, low deposition pressures and low hydrogen-to-silane flow rate ratio (Tfil∼1900 °C, p<30 mTorr and FH2/FSiH4≤1). At these deposition conditions, high growth rates are observed (rd≥10 Ås−1) both with and without hydrogen dilution, and no silicon deposition was observed on the filaments. However, if a lower filament temperature is used (Tfil∼1500 °C) a transition from a-Si:H to microcrystalline silicon (μc-Si:H) occurs as the pressure is decreased from above 0.3 Torr to below 0.1 Torr. The highest dark conductivity and lowest activation energy, of ∼1 Scm−1 and <0.1 eV, respectively, were observed for μc-Si:H deposited at p∼50 mTorr. In this Tfil regime, μc-Si:H growth is achieved without hydrogen dilution, for substrate temperatures as low as ∼150 °C, and for very thin films (∼0.05 μm). Silicon growth on the filaments is observed. For both Tfil regimes, an amorphous to microcrystalline transition is also observed when the hydrogen dilution is increased (FH2/FSiH4≳4). A kinetic growth model is developed, which suggests that the transition from amorphous to microcrystalline can be explained by considering a balance between the concentration of atomic hydrogen and the concentration of the precursor to silicon deposition (SixHz with z≤3x) near the sample. This concentration ratio is shown to be controlled both by the deposition pressure, p, and the filament temperature, Tfil.
Study of electrical properties of rapid thermally processed lead iron niobate films synthesized by a sol gel method
Authors: Sedlar M. , Sayer M.
Published: July 2002

Lead iron niobate thin films were synthesized using a modified sol gel method. Films processed at temperatures as low as 400 °C had a dielectric constant K≊400. K of ≊1000 and a low loss factor tan δ≊0.02 were obtained for films rapid thermally processed between 650–700 °C. Films processed at ≊400 °C showed a more linear behavior and were able to endure higher electric fields up to 1.6 MV/cm. The remanent polarization Pr and coercive field Hc for films processed at 650 °C/1 s were 4.5 μC/cm2 and 40 kV/cm, respectively. Leakage current densities of films increased with processing temperature and were of the order of 4 and 10 nA/cm2 for films multiply processed at 425 °C/30 s and 650 °C/1 s, respectively.
Raman spectra of ultrathin YBaCuO7−δ films
Authors: Zhang P. , Haage T. , Habermeier H.-U. , Ruf T. , Cardona M.
Published: July 2002

Thin films of YBa2Cu3O7−δ (YBCO) on LaSrAlO4 (001) substrates with various thicknesses have been investigated by Raman spectroscopy. The Raman spectra of the films have been obtained by properly subtracting the spectrum of the substrate from the recorded spectra. This data anaysis enables us to determine the Raman spectra of YBCO films with thicknesses down to 12 nm (about 10 unit cells). The evolution of the peak positions and the linewidths of the B1g mode with thickness allow us to conclude that the residual strains in these films are negligibly small within the measured thickness range. The influence of the birefringence on measurement of the degree of epitaxy by Raman has been studied. With the reduction of film thickness the degree of epitaxy is slightly increased in these films.
Scanning force microscopy of organic thin-film amorphous hole transport materials
Authors: Han Eun-Mi , Do Lee-Mi , Fujihira Masamichi , Inada Hiroshi , Shirota Yasuhiko
Published: July 2002

Amorphous organic thin films of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)- 4,4′-diamine (TPD) and 4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) which function as hole transport materials were fabricated on slide glass plates by vapor deposition. By heating the films in air, the change in photoluminescence (PL) spectra for TPD was little, while that for m-MTDATA was significant and a new broad peak appeared in the longer-wavelength range. The latter change was pronounced in an oxygen atmosphere, but was suppressed in a nitrogen atmosphere. To clarify the difference in the effect of the storage conditions on the PL characteristics between TPD and m-MTDATA, these two films were also studied by the scanning force microscopy techniques of atomic force microscopy (AFM) and friction force microscopy (FFM). It was found from the observation by AFM that the m-MTDATA films showed no change in topography, while the TPD films were crystallized easily at room temperature in the air. However, it was found by FFM that the m-MTDATA film stored at 80 °C above Tg showed crystal-like structure, which was difficult to detect by topographic AFM imaging due to little morphological change. It was finally concluded from the above results together with FFM after scratching the film surface with the AFM tip(i) that the crystal-like structure was not due to crystallization of m-MTDATA itself, but due to the oxidation product of m-MTDATA, because the crystal-like structure was observed only near the surface; (ii) that m-MTDATA is better than TPD in terms of difficulty in crystallization of their amorphous films by heating, but is inferior to TPD in terms of oxidation and the film requires protection from atmospheric oxygen; and (iii) that FFM can be used to observe the reaction products over the film surface on the nano scale. Interdiffusion for m-MTDATA by heating in two layered films with tris(8-hydroxyquinoline) aluminum was more suppressed than that for TPD due to higher Tg and lower thermal structural change of the m-MTDATA film.
Area-selective disordering of multiple quantum well structures and its applications to all-optical devices
Authors: Kan’an Ayman M. , LiKamWa Patrick , Mitra-Dutta  , Pamulapati Jagadeesh
Published: July 2002

A technique of impurity-free vacancy-induced disordering of GaAs/AlGaAs multiquantum wells (MQW) that is area selective, very reliable, and highly reproducible, has been developed. The localized compositional disordering is induced by rapid thermal annealing of the sample after it has been coated with a thin film of ‘‘spin-on’’ glass and prebaked at 400 °C in a high purity nitrogen:oxygen (78:22) atmosphere. In order to self-consistently determine the diffusion coefficient of the Al and Ga atoms, the photoluminescence peak is fitted to the n=1 electron to heavy hole transition that corresponds to an error function potential profile caused by the diffusion. The process has been used to integrate two optical devices on a MQW structure. One is a nonlinear directional coupler all-optical switch, and the other is an integrated Mach–Zehnder all-optical modulator. The switching characteristics of the devices were measured using the conventional pump-probe measurement technique.
Electron tunneling from the edge of thin single-crystal Si layers through SiO2 film
Authors: Ono Yukinori , Takahashi Yasuo , Horiguchi Seiji , Murase Katsumi , Tabe Michiharu
Published: July 2002

To investigate the electron tunneling from the nanometer-scale Si, we fabricated tunnel devices with an extremely thin Si-on-insulator layer as a cathode. The devices consisted of a 5-nm-thick Si layer sandwiched between thick SiO2 films, a thin (≊2 nm) SiO2 film (an edge oxide) terminating the Si layer on one side, and a polycrystalline Si (poly-Si) electrode covering the edge oxide. Using this Si/edge-oxide/poly-Si tunnel device, we measured the tunneling current and found that when the poly-Si electrode was positively biased with respect to the Si layer, the current versus voltage characteristics exhibited a steplike feature at 42 K. The current decreased with increasing edge-oxide thickness, indicating that the tunneling at the edge limited the current. The step structure was never observed in the current directly flowing into the poly-Si electrode for devices without the edge oxide. These results indicate that the observed steps reflect the electronic nature of the electron system at the edge of the Si layer. Analysis of the substrate-bias dependence of the tunneling current strongly suggests that electrons are localized at the edge of the Si layer because of the band bending. The effect of one-dimensional subbands on the present tunneling properties is discussed as an origin of the observed steps. Although these steps become less distinct at higher temperatures, they are evident even above 200 K. This indicates that the strong electron confinement occurs at the edge.
Observation of room temperature surface-emitting stimulated emission from GaN:Ge by optical pumping
Authors: Zhang X. , Kung P. , Saxler A. , Walker D. , Razeghi M.
Published: July 2002

Optically pumped surface-emitting stimulated emission at room temperature was observed from GaN:Ge grown by metalorganic chemical vapor deposition. The sample was optically pumped perpendicularly on the top surface while the stimulated emission was collected from the back colinearly with the pump beam. The cavity was formed by the GaN/air and GaN/sapphire interfaces without any other structure. The stimulated emission was gain guided by the pump beam. The threshold optical pump density for stimulated emission was approximately 2.8 MW/cm2 and the linewidth was 2.5 nm. The emission from GaN:Ge showed a redshift as the pump density increased. The comparison between theoretical calculations and experimental results suggested that many-body interactions can account well for the redshift.
Microstructure and magnetic properties of inert gas atomized rare earth permanent magnetic materials
Authors: Sellers C. H. , Hyde T. A. , Branagan D. J. , Lewis L. H. , Panchanathan V.
Published: July 2002

Several permanent magnet alloys based on the ternary Nd2Fe14B (2-14-1) composition have been prepared by inert gas atomization (IGA). The microstructure and magnetic properties of these alloys have been studied as a function of particle size, both before and after heat treatment. Different particle sizes have characteristic properties due to the differences in cooling rate experienced during solidification from the melt. These properties are also strongly dependent on the alloy composition due to the cooling rate’s effect on the development of the phase structure; the use of rare earth rich compositions appears necessary to compensate for a generally inadequate cooling rate. After atomization, a brief heat treatment is necessary for the development of the optimal microstructure and magnetic properties, as seen from the hysteresis loop shape and improvements in key magnetic parameters (intrinsic coercivity Hci, remanence Br, and maximum energy product BHmax). By adjusting alloy compositions specifically for this process, magnetically isotropic powders with good magnetic properties can be obtained and opportunities for the achievement of better properties appear to be possible.
Detection statistics of deep levels in minority carrier transient spectroscopy
Authors: Davidson J. A. , Evans J. H.
Published: July 2002

A theoretical treatment of the minority carrier transient spectroscopy (MCTS) experiment is presented. We have modeled the minority carrier flux through the depletion region of an illuminated Schottky diode held under reverse bias, and used these data to calculate the occupancy of minority carrier traps as a function of energy, capture cross section, and temperature. The model shows that the capacitance transient monitored in the MCTS experiment decreases in intensity as the temperature is raised. It is demonstrated that this causes inaccuracies in the measured deep level activation energy Ea derived from an Arrhenius plot of the data. Simulated MCTS spectra have been compared with measured MCTS spectra of hole emission from the gold donor in silicon, and very good agreement between modeled and experimental spectra is observed. The model explains the commonly observed phenomenon of a reduction in MCTS peak heights for increasing temperature, which is the opposite effect from that commonly observed in conventional deep level transient spectroscopy data. It is shown that the correct choice of rate window can significantly reduce errors in the measured value of Ea.
Single electron tunneling and level spectroscopy of isolated C60 molecules
Authors: Porath Danny , Millo Oded
Published: July 2002

The interplay between single electron tunneling effects and the discrete molecular levels of C60 molecules is studied using scanning tunneling microscopy. Isolated C60 molecules were deposited onto a gold substrate, covered by a thin insulating layer. The tunneling current–voltage characteristics of isolated molecules, both at room temperature and at 4.2 K, exhibit rich structures, resulting from the interplay between charging effects and the electronic spectrum. In particular, we observe degeneracy lifting within the C60 molecular orbitals, probably due to the Jahn–Teller effect and local electric fields.
Magnetic and magnetoelastic response of stress cycled Terfenol-D
Authors: Prajapati K. , Greenough R. D. , Wharton A.
Published: July 2002

The changes in magnetic and magnetomechanical properties of grain-oriented Terfenol-D have been monitored after stress cycling. Measurements of B–H loops and differential permeabilities under uniaxial preloads (⩽20 MPa) applied along the [112̄] axis show that magnetic anisotropy is induced after ∼106 cycles of ±100 MPa about a mean preload of 120 MPa. According to the permeability data, magnetic moments lie preferentially along the 〈111〉 axes, normal to the [112̄] grain growth direction, and with the subsequent application of field along the [112̄] axis magnetic moment rotation is enhanced. This magnetic response is virtually identical to that induced by the application of measurement preloads and to results of earlier work on magnetic annealing. The magnetomechanical coupling data show little variation from the value prior to stress cycling (k33∼60%) provided the measurement preloads and bias fields are adjusted accordingly. The reasons for the observed modified behavior after stress cycling are discussed in the light of hysteresis loss and elastic modulus prior to and after the cycling.
Interlayer coupling in NiFe/Mo multilayers and effect of Fe underlayer
Authors: Yan M. L. , Shan Z. S. , Sellmyer D. J. , Lai W. Y.
Published: July 2002

We report our studies of exchange coupling in NiFe/Mo multilayers with and without Fe underlayer. The interlayer coupling of NiFe layers across the Mo spacer as well as the magnetic exchange coupling between Fe underlayer and the NiFe/Mo multilayers were observed: The interlayer coupling in NiFe/Mo multilayers exhibits the typical oscillations as a function of the Mo spacer with a period of about 11 Å, and the magnetic exchange coupling between Fe underlayer and the NiFe/Mo multilayers is strongly dependent on the thickness of the Mo spacer tMo and the bilayer number N of NiFe/Mo multilayers. For the [NiFe(20 Å)/Mo(tMo)]30 samples with a 100 Å Fe underlayer, the hysteresis loops with thinner Mo spacer (tMo<4 Å) are single loops indicating that the Fe underlayer is strongly magnetic exchange coupled with the NiFe/Mo multilayers; whereas hysteresis loops with thicker Mo spacer (tMo⩾5 Å) show a two-magnetic-phase feature. For the [NiFe(20 Å)/Mo(10 Å)]30 sample with 100 Å Fe underlayer, the hysteresis loops are single loops for bilayer number N<15, while hysteresis loops show the two-phase feature for N⩾15.
Electroreflectance study of effects of indium segregation in molecular-beam-epitaxy-grown InGaAs/GaAs
Authors: Chattopadhyay K. , Aubel J. , Sundaram S. , Ehret J. E. , Kaspi R. , Evans Keith R.
Published: July 2002

Electrolyte electroreflectance (EER) experiments were performed on In0.22Ga0.78As/GaAs single quantum wells grown by the conventional molecular-beam-epitaxy (MBE) shutter operation, and also by modified MBE shutter operation intended to form more compositionally abrupt normal and inverted interfaces. The latter included controlled thermal desorption of the surface segregated In at the InGaAs layer surface (flash off), and the deposition of In at the InGaAs/GaAs interface to eliminate compositional broadening (predeposition). The fundamental energy gap and subband transitions were determined experimentally, and compared with an accurate calculation of the potential well problem including strain. These results confirmed the segregation of In atoms near the interface. The segregation was maximum in the conventional (normal) MBE sample and least with the modified growth incorporating predeposition and flash off, as expected. The segregated atoms are observed to act as dopants and form junctions near the InGaAs/GaAs interface. This study shows that EER can be used as an effective tool for studying the segregation process in MBE growth.
Structural properties of Zn-diffused InP layers
Authors: Bocchi C. , Franzosi P. , Pelosi C. , Maslov A. V. , Mukhamedzhanov E. Kh. , Gambacorti N. , Simeone M. G. , Audino R.
Published: July 2002

A sealed tube method has been adopted to prepare Zn-diffused InP layers. Both Zn3P2 and Zn+InP have been used as sources. The samples were prepared at 500 °C. The diffusion time ranged from 5 up to 120 min. Both S- and Zn-doped InP crystals have been used as substrates. The Zn depth profile has been measured by secondary ion mass spectroscopy, while the lattice strain produced by diffusion has been carefully investigated by x-ray double crystal diffraction and the standing-waves method of recording photoelectrons. The results show that in the S-doped crystal the diffused/virgin interface is very sharp and the diffused layers are lattice contracted. The concentration of Zn, as well as the lattice strain, do not depend on the diffusion time, whereas the thickness of the diffused layer increases with time. The plot of diffused layer thickness versus the square root of the diffusion time showed different slopes depending on the diffusion sources. Both lattice strain and diffusion depth depend on the diffusion source, whereas the concentration of Zn is not influenced by the type of diffusion source.
Evidence for the formation of n+-GaAs layer in Pd/Ge ohmic contact to n-type GaAs
Authors: Lee Jong-Lam , Kim Yi-Tae , Kwak Joon Seop , Baik Hong Koo , Uedono Akira , Tanigawa Shoichiro
Published: July 2002

Microstructural reactions of PdGe ohmic contact to n-type GaAs were investigated using x-ray diffraction, Auger electron spectroscopy, and slow positron beam. The results were compared with electrical properties to interpret the ohmic contact formation mechanism for the Ge/Pd/n-type GaAs system. The lowest contact resistance of 1.7 Ω mm and the formation of a PdGe compound are observed at the annealing temperature of 240 °C. Slow positron beam results show that Ga vacancies are produced below PdGe during the formation of PdGe ohmic contact to n-type GaAs. This means the existence of n+-GaAs layer below PdGe because Ga vacancy concentration increases with n-type impurity concentration. This supports that the n+-GaAs layer is a regrown layer decomposed from PdxGaAs containing excess Ge atoms during annealing.
Effects of the DX center multiplicity on the transport properties of AlxGa1−xAs:Si
Authors: Rziga Ouaja F. , Mejri H. , Selmi A. , Gibart P.
Published: July 2002

Magnetotransport measurements were performed on a series of AlxGa1−xAs:Si samples having different alloy compositions in the temperature range 77–300 K. Multiple-peak structures were observed in the temperature-dependent resistivity, and free carrier concentration spectra during the heating process. A detailed analysis is made to explain the physical origin of these structures using the large lattice relaxation model. Two main features are revealed: the multicomponent structure depends on the alloy composition; and in contrast to the deep level transient spectroscopy data, the peaks of this structure appear in the same order than the thermal activation of the DX center capture barriers. An attempt to assign this multiple-peak structure to the nonresonant energy levels related to the DX configurations will be presented.
Structural investigations of GaN grown by low-pressure chemical vapor deposition on 6H–SiC and Al2O3 from GaCl3 and NH3
Authors: Koynov S. , Topf M. , Fischer S. , Meyer B. K. , Radojkovic P. , Hartmann E. , Liliental-Weber Z.
Published: July 2002

GaN films grown on (0001) 6H–SiC and (0001) Al2O3 substrates using low-pressure chemical vapor deposition with GaCl3 and NH3 as precursors are comparatively explored by optical, scanning tunneling, and transmission electron microscopy. Independent of the substrate material used, the surface of the GaN layers is covered by hexagonally shaped islands. For GaN on 6H–SiC, the islands are larger in diameter (≈50 μm) and rather uniformly distributed. An atomically flat interface is observed for GaN on Al2O3 in contrast to GaN grown on 6H–SiC, where the interface is characterized by large steps. For both substrates, faceted holes (named as pinholes) are observed in near-surface regions of the GaN layers occurring with a density of about 7×108 cm−2. No unequivocal correlation between the density of pinholes and the density of threading dislocations (≈1.6×1010 cm−2 for GaN/Al2O3 and ≈4×109 cm−2 for GaN/6H–SiC) can be found. Rather, different types of defects are identified to be correlated with the pinholes, implying a dislocation-independent mechanism for the pinhole formation. Despite the small lattice mismatch between GaN and 6H–SiC, the pronounced original surface roughness of this substrate material is believed to account for both the marked interfacial roughness and the still existing high density of threading dislocations.