bluuebunny/crossref_metadata_2025 huggingface dataset:
Hyperpolarizabilities of the m-substituent phenyl amine based chromophores determined from the hyper-Rayleigh scattering and two photon absorption induced fluorescence
Authors: Hsu Chia-Chen , Huang Tzer-Hsiang , Zang Yi-Liang , Lin Jiunn-Lih , Cheng Ying-Yen , Lin Jiann T’suen , Wu Hui Hsien , Wang C. H. , Kuo Chie-Tong , Chen Chen-Hwa
Published: July 2002

The technique of hyper-Rayleigh scattering is used to determine and compare the first hyperpolarizabilities (β) of five structurally similar m-substituent phenyl-amine based chromophores using both internal and external standards. Contribution from the two photon absorption induced fluorescence to the observed signal is carefully removed. The chromophores with the m-substituted phenyl ring, that stabilizes the maximal charge-transfer state through resonance effect, are found to have larger static β values and redshifted electronic absorption peaks (λmax) compared with the para-nitroaniline (pNA) chromophore. The electron-withdrawing m-substituent of the chromophore is found to have the effect of reducing both the β and λmax values. Its effect on β is less pronounced when the π-electron reservoir is large. The enhancement of β value and the redshift λmax are observed to be consistent with the extent of the π electron delocalization.
Oxidation and sulfur passivation of GaInAsP(100)
Authors: Rajesh K. , Huang L. J. , Lau W. M. , Bruce R. , Ingrey S. , Landheer D.
Published: July 2002

The quaternary III–V compound semiconductor GaInAsP is an important material for many optoelectronic devices, the surface of which generally needs to be passivated in the fabrication of such devices. Thus understanding the surface chemistry and monitoring the surface band gap states after oxidation and sulphur passivation have become necessary. Further, understanding the effect of ion bombardment on the GaInAsP surface during dielectric deposition is also of importance for device fabrication. In this study, quaternary GaInAsP(100) surfaces were subjected to UV/ozone and wet chemical treatments, dilute HF etching, sulfur passivation, and Ar ion bombardment. The composition and the relative movement of the surface Fermi level (EFs) of the surfaces were measured by x-ray photoemission spectroscopy (XPS) after oxidation, HF etching, sulfur passivation, and ion bombardment of surfaces. It was found that oxidation by ozone exposure formed multiple oxide phases of all the constituent elements. Both HF etching and sulfur passivation treatments were effective in generating surfaces having almost no oxide. It was also found that while sulfur passivation combined with an ultrahigh vacuum annealing at 300 °C reduced the surface band bending on n-type GaInAsP(100), it inverted p-type GaInAsP to n-type. An L-edge absorption spectrum of the sulfur passivated surface confirmed the presence of a sulfur layer. Further, it was found that an Ar+ ion bombardment pins the EFs near the midgap for both n- and p-type GaInAsP surfaces.
Interlayer coupling in Co/Si multilayers (abstract)
Authors: Inomata K. , Saito Y.
Published: July 2002

Although the picture of antiferromagnetic (AF) coupling in metal/metal multilayers is well-established, it is not clear how the picture can be extensively modified for coupling across insulating or semiconducting spacer layers. Unusual temperature sensitive interlayer coupling has recently been observed in Fe/Si/Fe sandwiches and Fe/Si multilayers.123 The spacer has been claimed to be a metastable metallic FeSi with CsCl structure in the Fe/Si multilayers,4 and a transition from AF coupling to biquadratic coupling has been observed with decreasing temperature in Fe/FeSi multilayers.56 In this study, interlayer coupling has been investigated as a function of nominal Si layer thickness tSi in Co/Si multilayers. The multilayers were grown at ambient temperature on water cooled thermally oxidized Si substrates by ion beam sputtering at a base pressure of 3×10−7 Torr. The sputtering was conducted using 1.8×10−4 Torr Ar ions and an acceleration voltage of 400 V. Co was shown to have hcp structure with (002) texture by x-ray and transmission electron diffractions. From the magnetization curve, the coupling was ferromagnetic below tSi=0.8 nm, changed to antiferromagnetic above 0.8 nm Si and disappeared at tSi>1.7 nm, exhibiting nonoscillation. We have estimated interlayer coupling J between Co layers across the spacer from the saturation field Hs using the relation of Hs=4J/dMs, where d is the Co layer thickness and Ms is the saturation magnetization. The interlayer coupling was also investigated using FMR for Co/Si/Co sandwiches, which demonstrates an exponentially decay of the coupling with increasing tSi, and gives AF coupling of J=−3.6×10−2 erg/cm2 for a 10 nm Co/1.5 nm Si/10 nm Co sandwich. The cross-sectional transmission electron microscopy exhibited the spacer consisting of Co silicide with amorphous nature, formed in the interface. Temperature dependence of the coupling was investigated using FMR and compared with that of Fe/Si multilayers. Substrate dependence of the interlayer coupling was also investigated.
Microscopic nature of thermally stimulated current and electrical compensation in semi-insulating GaAs
Authors: Kuisma S. , Saarinen K. , Hautojärvi P. , Fang Z.-Q. , Look D.
Published: July 2002

In this work undoped semi-insulating (SI) GaAs grown by vertical gradient freeze and liquid encapsulated Czochralski methods was studied by near-infrared absorption (NIRA), thermally stimulated current (TSC) and positron annihilation techniques. The positron experiments reveal both gallium and arsenic vacancies, as well as gallium and arsenic antisites, in the samples. By comparing the results from the TSC and positron measurements, the following relations are found in the defect concentrations: trap T2 correlates with the arsenic antisite and trap T5 with the arsenic vacancy. The ionized fraction of the arsenic-antisite-related EL2 defect is obtained from NIRA measurements. The positive charge of these ionized EL2 defects correlates with the net negative charge, 3[VGa3−]+2[GaAs2−]−[VAs+], related to the gallium vacancies and antisites and arsenic vacancies detected in positron measurements. The intrinsic defects may thus contribute significantly to the electrical compensation in SI GaAs.
Hole mobilities and the effective Hall factor in p-type GaAs
Authors: Wenzel M. , Irmer G. , Monecke J. , Siegel W.
Published: July 2002

We prove the effective Hall factor in p-GaAs to be larger than values discussed in the literature up to now. The scattering rates for the relevant scattering mechanisms in p-GaAs have been recalculated after critical testing the existing models. These calculations allow to deduce theoretical drift and theoretical Hall mobilities as functions of temperature which can be compared with measured data. Theoretical Hall factors in the heavy and light hole bands and an effective Hall factor result. The calculated room temperature values of the drift mobility and of the effective Hall factor are 118 cm2/V s and 3.6, respectively. The fitted acoustic deformation potential E1=7.9 eV and the fitted optical coupling constant DK=1.24×1011 eV/m are close to values published before. It is shown that the measured strong dependence of the Hall mobility on the Hall concentration is not mainly caused by scattering by ionized impurities but by the dependence of the effective Hall factor on the hole concentration.
Optical characterization of real-space hot-electron transfer in a strained GaAs/In0.2Ga0.8As/GaAs quantum well heterostructure
Authors: Yeh Yung-Hui , Lai Jiun-Tsuen , Lee Joseph Ya-min
Published: July 2002

Real-space transfer (RST) light-emitting transistors are implemented with a strained GaAs/InGaAs/GaAs quantum well heterostructure fabricated on GaAs substrates. The device energy band diagrams are simulated by using the MEDICI program. Two optical measurements, electroluminescence (EL) and photocurrent, are performed to characterize the RST light-emitting transistors. The RST of hot electrons into the active region is measured by EL at 77 K. The light emitted from the RST device is detected by a separate photodetector and the photocurrent is found to be consistent with the RST current-voltage (IC−VD) characteristics. The 77 K EL emission peak wavelength is about 9000 Å. The EL spectra confirm that the emission is dominated by emission from the GaAs/InGaAs/GaAs quantum well. At VD=0 V, the EL emission intensity due to hole leakage current is about 2 orders of magnitude lower than that of the electron leakage current.
Analysis of transient photocurrents in Cu(In,Ga)Se2 thin film solar cells
Authors: Nishitani M. , Negami T. , Kohara N. , Wada T.
Published: July 2002

A transient photocurrent measurement with a zero-field time-of-flight configuration was carried out on ITO/ZnO/CdS(n-type)/Cu(In,Ga)Se2(p-type, Eg=1.15 eV)/Mo/glass solar cell to get the information about the minority carrier transport on the Cu(In,Ga)Se2 film. The transient photocurrent was recorded after the incidence of a subnanosecond pulse light (500 nm wavelength) from the back side through the Mo thin film (<0.5 μm). The analysis of the transient photocurrent was conducted by using the time-dependent diffusion current equation. Consequently, the values of 1.0 cm2/s, 100 ns, and 103 cm/s as the diffusion coefficient, the minority carrier lifetime, and the recombination velocity in the Cu(In,Ga)Se2/Mo interface, respectively, were obtained on the Cu(In,Ga)Se2 film. These data corresponded to the electron beam induced current line profile data, and the experimental data of J0, analyzed by current–voltage characteristics.
Dynamic force microscopy by means of the phase-controlled oscillator method
Authors: Dürig U. , Steinauer H. R. , Blanc N.
Published: July 2002

Dynamic force microscopy, a technique also known as non-contact force microscopy, has proved to be a powerful tool for atomic resolution imaging. A number of schemes have been developed, but recently the oscillator method has become the preferred operating mode. Here, the force sensor acts as resonator in an active feedback circuit. A practical implementation of the method is described and the underlying key concepts are discussed. It is shown that a tracking oscillator excitation scheme is superior to the more standard direct feedback method for cases in which the force sensor exhibits only a weak resonance enhancement. Furthermore, the simultaneous measurement of dissipative interaction channels is an important extension of dynamic force microscopy. It allows one to differentiate between sample materials via their plasto-mechanical response. As an example, a Cr test grating has been imaged in the constant force gradient mode. The dissipation measured on Cr-covered areas is significantly lower than that on the bare quartz glass substrate, which enables one to distinguish between the two materials with a lateral resolution comparable to that of the topographic image.
Role of the surface morphology in cement gel growth dynamics: A combined nuclear magnetic resonance and atomic force microscopy study
Authors: Papavassiliou G. , Fardis M. , Laganas E. , Leventis A. , Hassanien A. , Milia F. , Papageorgiou A. , Chaniotakis E.
Published: July 2002

1H nuclear magnetic resonance and atomic force microscopy have been correlated in order to exploit the relation between the microscopic morphological properties and the growth dynamics of cement gels during hydration. Studies in a number of samples with different hydration kinetics show that consideration of percolation, gel surface roughness, and related scaling properties is necessary for a thorough understanding of cement hydration kinetics. It is further shown that, at late hydration times, the hydration rate is highly correlated with the cement gel surface roughness exponent χ. A model based on a random walk on a three-dimensional network of rough micropores is proposed in order to explain this effect.
Experimental and theoretical study of dissociation in the positive column of a hydrogen glow discharge
Authors: Amorim J. , Loureiro J. , Baravian G. , Touzeau M.
Published: July 2002

The positive column of a hydrogen glow discharge was studied under typical operating conditions: gas pressure from 0.3 up to 5.0 Torr and discharge current from 1 up to 50 mA. Optical emission spectroscopy, optical absorption spectroscopy, and laser induced fluorescence have been employed in order to determine the gas temperature (300<Tg<600 K), the density of ground state hydrogen atoms (1012<[H(1s)]<1013 cm−3), and the kinetic temperature of H atoms (336<Ta<1600 K), respectively. Langmuir probes were utilized to measure the electric field (8<E<61 V cm−1) in the positive column. A kinetic model based on the solutions to the homogeneous electron Boltzmann equation coupled to a set of rate balance equations for the vibrational levels H2(X 1Σg+,v), H atoms, and H− ions was developed in order to predict the concentrations of these species. From a comparison between the measured and calculated concentrations of H atoms, the reassociation probability on the wall, in Pyrex glass, is estimated to be ≅10−2.
Stability of electron emission current in hydrogen-free diamond-like carbon deposited by plasma enhanced chemical vapor deposition
Authors: Chung Suk Jae , Moon Jong Hyun , Park Kyu Chang , Oh Myung Hwan , Milne W. I. , Jang Jin
Published: July 2002

We have studied the electron emission characteristics of the hydrogen-free diamondlike carbon (DLC) and conventional DLC films. The electron emission current of conventional DLC increases at first and then decreases continuously with bias stress time. The emission current of the hydrogen-free DLC, deposited by the layer-by-layer technique, increases at first and then stabilizes with increasing stress time. The resistivity of the hydrogen-free DLC decreases after long bias stress, and that appears to be due to the changes in the density of states in the gap of the hydrogen-free DLC.
Electronic effects of sodium in epitaxial CuIn1−xGaxSe2
Authors: Schroeder David J. , Rockett Angus A.
Published: July 2002

Temperature dependent Hall effect measurements were made on Na-contaminated and uncontaminated epitaxial p-type CuIn1−xGaxSe2 films. Na was introduced by diffusion into as-deposited samples using both NaOH and Na2Se as sources. Na concentrations were measured using secondary ion mass spectroscopy by comparison to an implanted standard. Films contaminated with Na from either source were found to have much lower compensating donor densities than as-deposited films. Oxygen and other impurities were not found to be necessary to produce this effect although some changes with Se content were observed. These were independent of the Na effect.
A gliding discharge applied to H2S destruction
Authors: Dalaine V. , Cormier J. M. , Lefaucheux P.
Published: July 2002

The destruction of foul smelling gases is an important subject involving a number of industrial applications. Increased interest is being devoted to technologies based on the properties of cold plasmas, which are the subject of a large body of research. Among all possible techniques, the sliding discharge is one of the simplest to utilize. We present a study of the destruction of H2S diluted in air by this type of discharge. We first describe the electric power supply, its characteristics and the design of the reactor. The properties of the discharge and its changes are then described using measurements of voltage, current, and different characteristic parameters: temperatures, velocities, and length of the discharge. After describing the instrumentation used for chemical diagnoses, the main experimental results are presented. They show the changes in the conversion rate as a function of the flow rate of the gas to treat and the dimensions of the reaction chamber, enabling a model of discharge changes to be defined. The model defines a zone of transformation corresponding to a volume swept by the plasma. It was thus possible to determine how destruction efficiency is affected by construction parameters, reactor dimensions, voltage, and flow rates. The energy cost per molecule destroyed is relatively high because the pollutant is diluted. With several improvements, the possibility of processing high flow rates with a simplified implementation could enable immediate industrial application to be envisaged.
Investigation of mid-infrared intersubband stimulated gain under optical pumping in GaAs/AlGaAs quantum wells
Authors: Gauthier-Lafaye O. , Sauvage S. , Boucaud P. , Julien F. H. , Glotin F. , Prazeres R. , Ortega J.-M. , Thierry-Mieg V. , Planel R.
Published: July 2002

We have investigated the mid-infrared intersubband stimulated emission under optical pumping in GaAs/AlGaAs coupled quantum wells. The quantum wells exhibit four levels bound in the conduction band. The energy between the ground and first excited subband is close to the optical phonon energy enabling population inversion. Intersubband stimulated gain between subbands E3 and E2 is observed around 14 μm wavelength by optically pumping the E1−E3 intersubband transition at 10 μm. The gain measurement is performed by time-resolved pump-probe experiments using a two-color picosecond free-electron laser. The dependence of the intersubband stimulated emission is analyzed as a function of the pump intensity, and the pump and probe wavelengths. We show that very large intersubband stimulated gain can be achieved at liquid nitrogen temperature in a 2 mm thick waveguide. The stimulated gain is resonant with the pump wavelength with a broadening ≈25 meV. The experimental results are explained with a nonperturbative matrix density treatment of a three-level system.
Laser-induced suppression of photocrystallization rate in amorphous selenium films
Authors: Roy Anushree , Kolobov Alexander V. , Tanaka Kazunobu
Published: July 2002

We have found that a film of amorphous selenium exposed to simultaneous action of two different (Kr+ and Ar+) lasers whose photon energies are on different sides of the optical band gap, crystallizes more slowly than the one exposed to only one of the above. A decisive role of the polarization of the two laser beams has been demonstrated, namely, the suppression of the crystallization rate is observed only for the polarizations of the two light sources being parallel to each other. A microscopic model for the laser-induced suppression of the photocrystallization rate is proposed.
A study of nitrogenation of a NdFe12−xMox compound by in situ neutron powder diffraction
Authors: Loong C.-K. , Short S. M. , Lin J. , Ding Y.
Published: July 2002

The effects on the crystal lattice of a NdFe12−xMox (x≅1.7) during controlled nitrogenation over the 25–600 °C temperature range were studied by neutron powder diffraction. Prior to nitrogenation the sample contained a major phase of NdFe10.3Mo1.7 and a minor phase (∼12 vol %) of bcc-Fe. The sample inside the furnace was connected to a closed volume of ultrapure nitrogen gas while neutron data were collected over regular time intervals during sequential heating. Substantial nitrogen absorption occurred between 500 and 600 °C. During the nitrogenation process the NdFe12−xMoxNy lattice expanded while the bcc-Fe lattice contracted. An increasing decomposition of the compound into bcc-Fe at 600 °C was observed. The average size of the NdFe12−xMoxNy crystalline grains decreased starting at ∼300 °C, reaching a minimum at ∼500 °C and then increased markedly at higher temperatures. The development of lattice strains, on the other hand, showed an opposite trend, i.e., a maximum at 500 °C.
Magnetic and transport properties of Pb perovskites and Fe containing giant magnetoresistance perovskites
Authors: Gutiérrez J. , Barandiarán J. M. , Insausti M. , Lezama L. , Peña A. , Blanco J. J. , Rojo T.
Published: July 2002

The magnetic and magnetoresistive properties of the Ln0.7Pb0.3Mn1−xFexO3 (Ln=La, Nd, and x=0, 0.1) perovskites, prepared by the solgel low temperature method, have been studied. In all cases the exact stoichiometry is different from the nominal one probably due to the presence of cation vacancies. All the phases exhibit ferromagnetic behavior with Tc values ranging form 345 K observed for La0.7Pb0.3MnO3 to 75 K for Nd0.7Pb0.3Mn0.9Fe0.1O3. The 10% Fe contribution lowers the Tc by about 130 K in the La compound and 90 K in the Nd one. These results can be interpreted in terms of antiferromagnetic coupling between Mn and Fe. However, the influence of the Nd ions is not well known although it seems not ordered down to 10 K. The magnetoresistence [MR=ΔR/R(0)] measured at 6 T in pellets of pressed powder reaches values of about 80% around the magnetic transition.
Additional Coulomb blockade and negative differential conductance in closed two-dimensional tunnel junction arrays
Authors: Shin Mincheol , Lee Seongjae , Park Kyoung Wan , Lee El-Hang
Published: July 2002

We have theoretically investigated the transport properties of a two-dimensional array of small tunnel junctions having two different paths between the source and the drain electrodes. We have found that at zero temperature the Coulomb interaction between tunneling electrons in the upper and the lower branches of the array can bring about stationary configurations of electrons which completely block further influx of electrons from the source at voltages above the usual Coulomb blockade. This “additional” Coulomb gap is transformed into a negative differential conductance regime at finite temperature. We also discuss the effect of imperfection of arrays on the additional Coulomb blockade phenomena.
Photoluminescence study of deep levels in Cr-doped ZnSe
Authors: Bhaskar S. , Dobal P. S. , Rai B. K. , Katiyar R. S. , Bist H. D. , Ndap J.-O. , Burger A.
Published: July 2002

Single crystals of intrinsic ZnSe were grown by the seeded physical vapor transport method and the diffusion doping was utilized to incorporate Cr in these crystals. The radiative recombinations in these samples with Cr concentration in the range 1.0–10.2×1019 cm−3 were studied by the steady state photoluminescence technique. It was found that the Cr deep centers inhibit the band-to-band emission in Cr-doped ZnSe. Except in undoped single crystals, no emission corresponding to the band-to-band transition was observed from any of the doped samples. Instead, the higher wavelength emissions associated with Cr deep levels were obtained. This capture of photoexcited carriers by deep centers was verified using different excitation wavelengths. The role of chromium impurities in nonradiative recombination processes was also evidenced from the sharp decreases in the deep level emission intensity with increasing Cr concentration.
Threading dislocation reduction in strained layers
Authors: Romanov A. E. , Pompe W. , Mathis S. , Beltz G. E. , Speck J. S.
Published: July 2002

In this article, we have developed models for threading dislocation (TD) reduction due to the introduction of an intentionally strained layer. Three different types of dislocations have been considered in this model: misfit dislocations (MDs), mobile TDs, and TDs whose glide motion has been blocked by a MD crossing the glide path of the TD (immobile TDs). The models are based on MD formation by the process of lateral TD motion. The strain-induced TD motion leads to possible annihilation reactions of mobile TDs with either other mobile TDs or blocked TDs, or reactions in which a mobile TD is converted to an immobile TD by a blocking reaction with a MD. The evolution of the density of mobile and blocked TDs and the MD density is represented by three coupled nonlinear first order differential equations. When blocking of TDs by MDs is not considered, the equations have an analytical solution that shows that the final TD density should decrease exponentially where the argument of the exponent is proportional to the product of the reaction radius between TDs (the annihilation radius rA) and the nominal misfit strain εm. The no-blocking limit represents the maximum possible TD reduction through the introduction of a strained layer, regardless whether this layer has a discrete step in strain, step-grade, or continuous strain grading. When only blocking reactions are considered (no annihilation), again analytic solutions to the equations are obtained which show the maximum possible plastic strain relaxation for a discretely strained layer. Several examples of numerical solutions to the three coupled differential equations are described for cases that include both blocking and annihilation reactions.
The electrical properties of terbium ions in crystalline Si
Authors: Libertino Sebania , Coffa Salvatore , Mosca Roberto , Gombia Enos
Published: July 2002

We have investigated the electrical properties of terbium ions incorporated in crystalline Si. Silicon p+-n junctions were realized in n-type epitaxial or Czochralski-grown Si, having an O concentration of ∼1015 and 7×1017/cm3, respectively. These junctions were implanted with 5 MeV Tb ions to fluences in the range 6×1011–6×1012/cm2. Some epitaxial Si samples were also coimplanted with O in order to provide a concentration of ∼1018 O/cm3 in the region where Tb sits. Annealing at temperatures between 800 and 1000 °C and times ranging from 5 s to 30 min was performed. Deep-level transient spectroscopy, current–voltage, capacitance–voltage, and carrier lifetime measurements were used to characterize the levels introduced by Tb ions in the Si band gap. It is found that in a highly pure epitaxial Si, Tb introduces several donor levels at energies comprised between 0.15 and 0.53 eV from the conduction band. Interaction between Tb and O produces severe modifications in the distribution of deep levels related to the rare-earth ions. In particular, in the presence of O the concentration of the deep levels is reduced by more than one order of magnitude and shallower levels, with energies in the range 0.07–0.16 eV from the conduction band, dominate the spectrum. As a result, O co-doping produces an enhancement in the donor activity of Tb, a decrease in the freeze-out temperature of the electrons stored in Tb-related levels, and an increase in the minority-carrier lifetime. We have found that these modifications are produced by the formation of Tb–O complexes. Analysis of the growth and dissolution kinetics of these complexes reveal that they are produced by O diffusion to Tb ions at temperatures around 900 °C and annealed by evolution into more complex structures upon increasing thermal budget.
The use of Kubo formula to examine low temperature transport limited by interface roughness and phonons in metal–oxide–semiconductor field effect transistors
Authors: Zhang Yifei , Singh Jasprit
Published: July 2002

An approach to study transport in semiconductors using the Kubo formula is developed and applied to Si metal–oxide–semiconductor field effect transistors (MOSFETs). It is known that interface roughness is an important source of scattering in a MOSFET device operating at high sheet charge concentration (∼1012 cm−2). However, in spite of its importance, due to the complexity of the problem very simple models based on the Born approximation are used to study transport. The Born approximation breaks down when the interface quality is poor and transport occurs in lower energy localized states (for example, at low temperature). In this article we present results of a numerical method based on a three-dimensional approach to examine the interface roughness effects on electronic spectrum as well as on transport. Using the approach suggested by the Kubo formula we solve for the electronic states in the presence of interface roughness. Kubo formula is then used to study the transport properties as a function of sheet charge density, interface roughness level, and temperature. The model can easily be applied to other problems where scattering effects are very strong, e.g., in amorphous semiconductor devices.
Optical characterization of strain-induced structural modification in SiGe-based heterostructures
Authors: Usami N. , Leo K. , Shiraki Y.
Published: July 2002

We demonstrate that strain-induced structural modifications in SiGe-based heterostructures can be nondestructively probed by photoluminescence spectroscopy with several different excitation wavelengths. Owing to the indirect band structure of SiGe-based semiconductors, an appropriate choice of excitation energy leads to drastic change of the generation depth of photocarriers and allows to probe different part of the samples in the growth direction. By using this technique, we show that thin strained quantum wells on relaxed SiGe are almost free from threading dislocations and most of them are confined in composition-graded SiGe buffer layer. Furthermore, we show that the accumulation of strain in pure Ge/Si multiple quantum wells causes breakdown of the layer-by-layer growth mode at certain critical number of wells.
Control of the bias tilt angles in nematic liquid crystals
Authors: Yablonskii S. V. , Nakayama K. , Okazaki S. , Ozaki M. , Yoshino K. , Palto S. P. , Baranovich M. Yu. , Michailov A. S.
Published: July 2002

The pretilt angle controlled by electric field was studied by the modulation ellipsometry technique. The easy direction of compensated nematic liquid crystals was controlled by surface flexoelectric torque created by the linear coupling of the director deformation and electric field. The weak anchoring energy necessary for the occurrence of flexoelectric distortion was produced by unidirectional rubbing of the clean indium–tin–oxide covered glasses with a cotton cloth. The pretilt angle was measured as a function of electric field. Long relaxation times of the optical response (hundreds of seconds) were observed. The rubbed thin polyvinyl alcohol and polyimide aligning layers were seen to promote strong anchoring energy (>0.5 erg/cm2) preventing any deviation of pretilt angle and, consequently, to suppress the optical response. The probable applications of the obtained results are discussed.
Relationship between oxide density and charge trapping in SiO2 films
Authors: Mrstik B. J. , Afanas’ev V. V. , Stesmans A. , McMarr P. J. , Lawrence R. K.
Published: July 2002

Spectroscopic ellipsometry was used to determine the density of oxides thermally grown on Si substrates as a function of the oxidation temperature, and the time and temperature of postoxidation anneals. All the oxides were found to be denser than fused silica. The density of the as-grown oxides was found to decrease as the growth temperature was increased. Postoxidation anneals were found to reduce the oxide density; high temperature or long-time anneals caused the greatest reduction in density. Holes alone, or holes and electrons, were injected into the oxides by irradiating with vacuum ultraviolet light or x rays under electric field bias. Using capacitance–voltage measurements, it was found that low-density oxides trap charge more efficiently than high-density oxides. Electron spin resonance measurements indicated that, for most of these oxides, the number of paramagnetic defects was substantially smaller than the number of trapped charges. It is hypothesized that the additional, nonparamagnetic, charge is in the form of protons trapped near network oxygen atoms that have large Si–O–Si bond angles. The number of these large-angle bonds in the near-interfacial oxide increases as the oxide density decreases, explaining the observed correlation between the charge trapping and the oxide density.
Interface roughness scattering-limited electron mobility in AlAs/GaAs and Ga0.5In0.5P/GaAs wells
Authors: Nag B. R. , Mukhopadhyay Sanghamitra , Das Madhumita
Published: July 2002

Interface roughness scattering-limited electron mobility is calculated for AlAs/GaAs and Ga0.5In0.5P/GaAs wells taking into account the effects of finite barrier potential, extension of the wave function into the barrier layer, energy band nonparabolicity, and screening of the scattering potential. The mobility in AlAs/GaAs wells varies with the well width L as Ln(n=4.7–4.8), but the variation cannot be fitted to a similar relation for Ga0.5In0.5P/GaAs wells. Experimental results for both the systems may be explained with an asperity height between 2.83 and 5.67 Å, and a correlation length smaller than 170 Å. It is concluded that the larger experimental mobility in Ga0.5In0.5P/GaAs wells is due to the smaller barrier potential in this system.
Excimer laser sputtering of mica surfaces: Mechanisms and applications
Authors: Rubahn K. , Ihlemann J. , Rubahn H.-G.
Published: July 2002

The influence of excimer laser irradiation (193 nm, 20 ns pulses and 248 nm, 20 ns and 500 fs pulses) on the structure and morphology of plain and gold-film coated mica surfaces has been investigated for fluences between 8 mJ/c2 and 19 J/cm2. Surfaces treated with laser fluences below the ablation threshold (λ=248 nm) are well suited for controlled growth of metallic films, whereas above-threshold treated samples (λ=193 nm) form roughened surfaces with close-packed arrays of cones. The cone generation can be suppressed by appropriate choice of laser parameters, resulting in the formation of ablation holes with micrometer precision. The same precision can be obtained in ultrathin metallic films without destroying the underlying mica surface. Low-energy electron diffraction is used to monitor laser-induced microscopic changes of the mica surface upon low-fluence irradiation. It is seen that the first step of the ablation process includes reorientation of dipole domains on the surface. Thereafter, scanning electron microscopy reveals melting of the surface and the onset of explosive sputtering processes. Studies of the ablated products by the use of quadrupole mass spectrometry, quartz microbalance, and photoimaging indicate large kinetic energies and a high directionality of the ablated products.
Multilayer thermoelectric refrigeration in Hg1−xCdxTe superlattices
Authors: Radtke R. J. , Ehrenreich H. , Grein C. H.
Published: July 2002

The thermoelectric figure of merit ZT of Hg1−xCdxTe superlattices (SLs) having currents along the growth axis is computed using a realistic SL band structure and the multisubband Boltzmann equation. For a narrow well and wide barrier, a heavy C1 and higher-lying light C2 subband combine to form a (nonoptimal) carrier-energy filter, enhancing the thermopower. The multilayer thermionic emission model accounts for this effect qualitatively but not quantitatively. However, for a narrow well and narrow-barrier system, ZT is 20% larger than that in the wide-barrier structure, indicating that devices based on carrier-energy filter/thermionic processes are not necessarily advantageous. ZT is almost three times larger than that in Bi2Te3 and is four times larger than that in an alloy with the average composition of the SL. This effect is associated with reduced lattice thermal conductivity in the SL rather than improved electronic transport.
Scanning force microscope observations of particle detachment from substrates: The role of water vapor in tribological debonding
Authors: Hariadi R. F. , Langford S. C. , Dickinson J. T.
Published: July 2002

The tip of a scanning force microscope was used to detach nanometer-scale, single crystal NaCl particles from a glass substrate under controlled atmospheres of known humidity. After characterizing a particle at low contact force, a single line scan at high contact force was used to apply stresses to the attached particle. The lateral force during the line scan showed a sharp discontinuity associated with detachment of the particle from the substrate. The peak lateral force during this procedure is a strong function of particle contact area and humidity. As the relative humidity is raised from low values, the strength of the particle-substrate bond decreases dramatically. We interpret these results in terms of detachment by chemically assisted crack growth along the NaCl-glass interface.
Frequency response and scaling of hysteresis for ferroelectric Pr(Zr0.52Ti0.48)O3 thin films deposited by laser ablation
Authors: Liu J.-M. , Li H. P. , Ong C. K. , Lim L. C.
Published: July 2002

The ferroelectric hysteresis response against periodically varying electric field over frequency range of 10−2–105 Hz and amplitude range of 2–45 kV/cm for YBa2Cu3O7 (YBCO)/ Pb(Ti0.48Zr0.52)O3 (PZT)/YBCO thin film capacitors prepared by laser ablation is measured by utilizing the Sawyer–Tower circuit. Given amplitude Δ of the field, the hysteresis area 〈A〉 first grows and then decays as a function of frequency φ. At low and high ranges of frequency, 〈A〉 can be scaled as 〈A〉∝φ1/3Δ2/3 and 〈A〉∝φ−1/3Δ, respectively. It is established that the dynamic hysteresis at the high frequency range for a PZT thin film capacitor does not follow the theoretically predicted scaling law. An empirical scaling law 〈A〉∝φ1/3(Δ−Δ0)2/3/(1+bφ2/3Δ−1/3) with Δ0 the critical field and b a constant, is proposed to characterize the frequency and amplitude dependence of the hysteresis area over all the frequency range. In addition, the remnant polarization Pr and coercive field Ec as functions of φ are investigated, respectively, revealing a single-peaked pattern of both Pr and Ec.
Abnormal junction leakage characteristics in titanium-capped cobalt disilicide
Authors: Kang Chang-Yong , Kang Dae-Gwan , Lee Joo-Wan
Published: July 2002

Abnormal junction leakage characteristics in titanium-capped cobalt disilicide were investigated. The cobalt silicide n+-p junctions, fabricated with different capping layers, were characterized by current–voltage measurements and transmission electron microscopy. The reverse junction leakage currents of Ti-capped 13.5-nm-thick cobalt disilicide (CT) are higher than those of TiN-capped samples. The activation energy of CT at temperatures below 80 °C is 0.41 eV, and its dominant leakage mechanism is consistent with phonon-assisted tunneling. Transmission electron microscopy analysis indicates the existence of island phases and precipitates located at the silicide/Si interface, which are titanium disilicide and CoxTi1−xSi2 phases, as evidenced by energy dispersive spectroscopy. As a result, for the case of CT, the reasons for higher junction leakage currents and their field dependence appear to be the result of the diffusion of Ti atoms into CoSi2 grain boundaries and the resulting formation of TiSi2 and CoxTi1−xSi2 phases, which gives rise to a rougher silicide interface and a close spacing between silicide and the junction.
Growth and study of antiferroelectric lead zirconate thin films by pulsed laser ablation
Authors: Bharadwaja S. S. N. , Krupanidhi S. B.
Published: July 2002

Antiferroelectric lead zirconate (PZ) thin films were deposited by pulsed laser ablation on platinum-coated silicon substrates. Films showed a polycrystalline pervoskite structure upon annealing at 650 °C for 5–10 min. Dielectric properties were investigated as a function of temperature and frequency. The dielectric constant of PZ films was 220 at 100 kHz with a dissipation factor of 0.03. The electric field induced transformation from the antiferroelectric phase to the ferroelectric phase was observed through the polarization change, using a Sawyer–Tower circuit. The maximum polarization value obtained was 40 μC/cm2. The average fields to excite the ferroelectric state, and to reverse to the antiferroelectric state were 71 and 140 kV/cm, respectively. The field induced switching was also observed through double maxima in capacitance–voltage characteristics. Leakage current was studied in terms of current versus time and current versus voltage measurements. A leakage current density of 5×10−7 A/cm2 at 3 V, for a film of 0.7 μm thickness, was noted at room temperature. The trap mechanism was investigated in detail in lead zirconate thin films based upon a space charge limited conduction mechanism. The films showed a backward switching time of less than 90 ns at room temperature.
Copper complexes in silicon
Authors: Nakamura Minoru , Iwasaki Hitoshi
Published: July 2002

In order to investigate the exchange between copper complexes (Cu centers) in silicon crystal, the change of the photoluminescence (PL) intensity of the Cu center (Cu PL center; no-phonon peak: 1.014 eV) with annealing time was measured for p-type float-zone grown silicon crystals diffused with various concentrations of Cu at 700 °C followed by rapid cooling to room temperature. It was essential to assume the presence of at least one preceding species (precursor) of the Cu PL center to explain the annealing behavior of the PL intensity. Almost the same quantity of the precursor as the Cu PL center was estimated to remain in the as-cooled crystal. The formation energy of the Cu PL center was 0.57±0.05 eV, and dissociation energy was 0.63±0.05 eV. A positive correlation between the precursor of the Cu PL center and the deep level transient spectroscopy center located at Ec−0.15 eV was suggested.
Thermal conductivity of composite material with coated inclusions: Applications to tetragonal array of spheroids
Authors: Mercier S. , Molinari A. , El Mouden M.
Published: July 2002

The aim of this article is to determine the effective thermal conductivity of a composite material made of ellipsoidal coated inclusions bonded to a homogeneous isotropic matrix. The distribution of inclusions is considered to be periodic, nevertheless complex pattern can be investigated. An analytical approximate method, which takes into account interactions between inclusions at finite concentration, is proposed to solve steady state heat conduction problems. This method is an extension of the cluster scheme proposed, for elastic materials, by A. Molinari and M. E. Mouden Int. J. Solids Struct., 33, 3131 (1996). The morphological and spatial distribution of inclusions is accounted for in the present approach. Through different examples, results are compared to existing analytical or numerical solutions.
Laser coloration and bleaching of amorphous WO3 thin film
Authors: Lu Y. F. , Qiu H.
Published: July 2002

Laser-induced chromism of amorphous WO3 thin films prepared by pulsed laser deposition (PLD) has been investigated. The original films could be colored from light brown to purple by a single pulse of KrF excimer laser irradiation at 248 nm and subsequently bleached to brown by a single pulse of Nd–yttrium–aluminum–garnet laser at 1.06 μm in air. Spectroscopic investigations were applied to the films at three different states: original, colored, and bleached. The measurements by ellipsometry spectroscopy showed an increase in the refractive index (n) and decrease in extinction coefficient (k) in the luminous range of films from the colored state to the bleached state. Scanning tunneling microscopy and Raman spectroscopy showed slight crystallization in the films after coloration, with both the grain dimension and the surface roughness around tens of nanometers. The films were very stable to maintain the same color after long-time exposure in air, or in oxygen. X-ray photoelectron spectroscopy was used to study the chromism mechanisms. While some W5+ states were introduced in the original films by PLD, W4+ states were produced when the films were colored by the KrF excimer laser, along with the decrease of W5+ states and the increase of W6+ states. In contrast, laser bleaching was accompanied with increasing W4+ states in the films. Therefore, it is believed that the purple color is due to the polaron transition between the W4+ and W5+ states. Photochemical activation and photothermal oxidation are ascribed to the coloring and the bleaching processes, respectively.
An exact solution of the van der Waals interaction between two ground-state hydrogen atoms
Authors: Koga Toshikatsu , Matsumoto Shinya
Published: July 2002

A momentum space treatment shows that perturbation equations for the H(1s)–H(1s) van der Waals interaction can be exactly solved in their Schrödinger forms without invoking any variational methods. Using the Fock transformation, which projects the momentum vector of an electron from the three-dimensional hyperplane onto the four-dimensional hypersphere, we solve the third order integral-type perturbation equation with respect to the reciprocal of the internuclear distance R. An exact third order wave function is found as a linear combination of infinite number of four-dimensional spherical harmonics. The result allows us to evaluate the exact dispersion energy E6R−6, which is completely determined by the first three coefficients of the above linear combination.
Viscoelastic relaxation of segment orientation in dilute polymer solutions
Authors: Perico Angelo , Guenza Marina
Published: July 2002

Time autocorrelation and memory functions for segment orientation are derived for a general diffusion model of a linear polymer chain in solution. Exact analytical results for the orientation and alignment memories P1(t), P2(t) (averages of the first and second order Legendre polynomial of the cosine of the angle of rotation of a segment) are obtained as a function of the time autocorrelation function M1(t) of the segment vector. These expressions significantly depart from the results for the diffusional rotation of a sphere: P1=M1, P2=M31.
Direct inversion in the iterative subspace (DIIS) optimization of open-shell, excited-state, and small multiconfiguration SCF wave functions
Authors: Hamilton Tracy P. , Pulay Peter
Published: July 2002

The direct inversion in the iterative subspace (DIIS) method is applied to several simple SCF wave functions in an effective Fock matrix formulation. The following cases are treated: high-spin-restricted open shell, open-shell singlet, and two-configuration wave functions. Open-shell singlet states are described by a three-determinant 2×2 CAS expansion which is equivalent to Davidson’s nonorthogonal SCF method in the case of the first open-shell singlet. Very sharp convergence is usually obtained in less than 20 cycles. The method is applicable to slowly convergent or even inherently divergent cases, and able to enforce convergence to excited states not the lowest of their symmetry. For these simple wave functions, the present first order method is asymptotically more efficient than second-order methods. Examples are presented for H2O, H2O2, C2H4, F2, several states of NO2, C2H5, formaldehyde, and ketene.
The vibrational predissociation lifetime of the HF dimer upon exciting the ‘‘free-H’’ stretching vibration
Authors: Huang Z. S. , Jucks K. W. , Miller R. E.
Published: July 2002

A computer controlled F-center laser has been used, in conjunction with a molecular beam apparatus employing optothermal detection, to obtain an accurate measurement of the vibrational predissociation lifetime of the HF dimer following ν1 vibrational excitation. The lifetime obtained in this study is 24±2 ns. This is to be compared with a lifetime for ν2 excitation of 1.0±0.1 ns.
Intrinsic framework electronegativity: A novel concept in solid state chemistry
Authors: Van Genechten Karin A. , Mortier Wilfried J. , Geerlings Paul
Published: July 2002

The charge-dependent ‘‘effective’’ electronegativity χα of an atom α in a molecule or crystal, i.e., χα=(χ0α+Δχα)+2(η0α +Δηα)qα +∑β≠α qβ/Rαβ differs from the expression for the isolated atom (χ0α+2η0αqα) because of corrections to the electronegativity (Δχα) and hardness (Δηα), respectively, and due to the external potential generated by the surrounding charges qβ at distances Rαβ (new quantum-mechanical proof given). After calibration of Δχα and Δηα for all atom types, ab initio charges can be reproduced to within a few hundredths of an electron by the electronegativity equilization method (using χav=χα=χβ⋅⋅⋅ and ∑β qβ=constant). Application of this algorithm to the solid state (i.e., charges and external potential being generated in a self-consistent way and using Ewald’s method for determining the Madelung potential) leads to a novel method for determining ‘‘ab initio quality’’ charges and the value of the average electronegativity. If applied to SiO2 polymorphs, a relation between the refractive index and the average electronegativity is discovered, indicating for the first time quantitatively the importance of the structure type for the charge distribution and the average electronegativity in particular, and for the electronic properties in general.