bluuebunny/crossref_metadata_2025 huggingface dataset:
Annealing of silicon implanted by a high dose of cobalt ions investigated by in situ x-ray diffraction
Authors: Müller M. , Bahr D. , Press W. , Jebasinski R. , Mantl S.
Published: July 2002

A silicon wafer was implanted by a high dose of cobalt ions. Afterwards it was annealed in situ while grazing incidence x-ray diffraction measurements in a temperature range up to 690 °C were carried out. The formation of cobalt disilicide (CoSi2) precipitates starts during implantation. The annealing dependence of the precipitate growth, of strain relaxation, and of improvements of the silicide crystallinity was determined. We got an activation energy of (0.47±0.08) eV for the observed annealing process. The result is a buried cobalt disilicide layer with very rough interfaces. The film quality can be improved by a subsequent annealing at about 1000 °C.
A new mechanism for superlinear photoconductivity with relevance to amorphous silicon
Authors: Bube Richard H.
Published: July 2002

The phenomenon of superlinear photoconductivity in which the photocurrent varies as a power of the photoexcitation intensity greater than one, corresponding to an increase in carrier lifetime with increasing intensity, has been well known in a variety of crystalline semiconductors for many years. Description of the phenomenon in these materials requires two kinds of competing recombination centers: (1) one with a very small capture cross section for the majority carrier and a large cross-section ratio for minority carrier (Coulomb attractive) to majority carrier (neutral or Coulomb repulsive) capture, and (2) another with a larger capture cross section for majority carriers. This article describes a new mechanism for superlinear photoconductivity that involves only a single multivalent defect, such as the metastable dangling bond defect in amorphous silicon, which can give rise to superlinear photoconductivity provided that the capture cross-section ratio of Coulomb attractive to neutral capture is sufficiently small. Careful examination of the variation of photoconductivity with photoexcitation intensity in samples suitably selected for dark Fermi level position is capable of providing additional information about the capture cross-section ratios for metastable defects in amorphous silicon.
Suppression of In surface segregation and growth of modulation-doped N-AlGaAs/InGaAs/GaAs structures with a high In composition by molecular-beam epitaxy
Authors: Toyoshima Hideo , Niwa Takaki , Yamazaki Jin , Okamoto Akihiko
Published: July 2002

A model is proposed for describing the origin of the growth mode transition from two to three dimensions during the molecular-beam epitaxial growth of InGaAs on GaAs [H. Toyoshima, T. Niwa, J. Yamazaki, and A. Okamoto, Appl. Phys. Lett. 63, 821 (1993)]. In this model the amount of In atoms on the surface arising from surface segregation is crucial in determining the growth mode transition, which provides the upper limit of the In composition and/or the thickness of InGaAs for device applications. The increase of these limited values for a modulation-doped structure with an InGaAs channel has been implemented on the basis of this model. A lower substrate temperature and a higher As4 pressure during the InGaAs growth greatly suppress the In segregation ratio leading to the decrease of the amount of In on the surface. These growth conditions enable the upper limit of the In composition and/or the critical thickness of the InGaAs channel to be increased, which further verifies the validity of the proposed model. The surface of the InGaAs with the higher In composition is, however, metastable and can be stabilized by depositing AlGaAs or GaAs more than 3 monolayers thick on it. Modulation-doped structures with high electron transport properties have been successfully grown as high as the In composition of 0.45 by controlling the growth kinetics of InGaAs.
Emission studies of the plume produced during YBa2Cu3O7 film production by laser ablation
Authors: Sakeek H. F. , Morrow T. , Graham W. G. , Walmsley D. G.
Published: July 2002

The optical plume emissions produced on excimer laser ablation of a YBa2Cu3O7 target are reported and identified with the various atomic, ionic, and molecular species present. The spatial and temporal distribution of these emissions were studied as a function of the laser fluence and oxygen pressure. At the laser fluences used (4–6 J/cm2) some target material is ablated or evaporated directly in molecular form. In addition efficient formation of molecular oxides is observed at the contact front of the expanding plume with the surrounding oxygen atmosphere. The intensity and spatial distribution of oxide emission in the visible plume therefore provides a sensitive diagnostic for optimization of substrate location and deposition conditions.
Characterization of optical waveguides in KTiOPO4 by second harmonic spectroscopy
Authors: Roelofs M. G. , Suna A. , Bindloss W. , Bierlein J. D.
Published: July 2002

Segmented optical waveguides were fabricated in KTiOPO4 by Rb/Ba ion exchange. The waveguides were characterized by measuring, as a function of input optical wavelength, the Bragg-reflected power, and the intensity of second harmonic (SH) radiation generated within the waveguides. Seven peaks in the SH spectra are assigned to different interactions involving the two lowest-order spatial modes at the fundamental wavelength and the three lowest-order spatial modes at the SH wavelength. ‘‘Combined mode’’ SH interactions are found which sum spectrally degenerate fundamental waves which are spatially nondegenerate. Analysis of the spectra is shown to be a convenient method of determining the effective indices of the propagating modes, and these then determine the refractive index depth profile of the waveguide. The relative intensities of the SH peaks are dependent on the depth of the region within the ion-exchanged area which has a reversal of the ferroelectric polarization direction (reversed domain). In some waveguides, the spectra imply that the area of reversed domain is buried, and does not extend completely to the surface.
Laser ablation of multilayer polymer films
Authors: Reyna Luis G. , Soběhart Jorge R.
Published: July 2002

We study the efficiency of using multilayer structures as an etch-stop mechanism in the ablation of polyimide films by ultraviolet lasers. The study is done using a photothermal model that includes the light absorption by the decomposed fragments, which shield the polymer from the laser beam, an intermediate zone in which the polymer is suffering a phase transition and the underlying unburned material. The layers are differentiated from each other through their optical properties. Variation in the optical properties of polyimide has been achieved by a proper selection of impurities. From our modeling work, we conclude that optically thin foils may be used as etch stop in the ablation process when the penetration depth of the middle layer is around three times larger than the penetration depth of the surrounding layers, this for fluences below 200 mJ/cm2. We also present some experimental results.
Vertical inhomogeneity of the magnetization reversal in antiferromagnetically coupled Co/Cu multilayers at the first maximum
Authors: Mattheis R. , Andrä W. , Fritzsch L. , Langer J. , Schmidt S.
Published: July 2002

Magnetization behavior in antiferromagnetically (AFM) coupled multilayer systems was calculated by using an atomic layer model. Comparisons with the experimental results obtained on sputtered Co/Cu multilayers reveal remarkable differences in the magnetization reversal and in the field dependence of the magnetoresistance. Kerr loops measured from both sides of the stack display strong vertical differences. At the lower side near the Fe seed layer the magnetization reversal is in good agreement with that of our calculations whereas near the surface in large portions of the stack the AFM coupling is destroyed or varied. These effects are presumably caused by magnetic short circuits at defects in the multilayer structure. Cross-section transmission electron microscopy reveals growth defects which seem to be responsible for the deviations from the calculated ideal behavior.
Codiffusion of arsenic and boron implanted in silicon
Authors: Solmi S. , Valmorri S. , Canteri R.
Published: July 2002

The codiffusion of As and B implanted in Si at high doses (1×1016 cm−2) and with energies corresponding to the same projected range has been investigated at 900 and 1000 °C on the basis of dopant and carrier profile measurements. The comparison of the codiffusion data with the corresponding ones obtained by the diffusion of each element alone revealed some anomalous effects which can be explained by assuming the formation of neutral donor–acceptor pairs. These complexes are mobile with a diffusion coefficient Dpair=17 exp(−4/kT) cm2/s, very close to the diffusion coefficient of As in intrinsic Si. Electrons are the majority carriers in the region where both dopants are present at high densities. On the basis of these features, a diffusion model that takes pairing into account is presented. A simulation program including this model allows one to foresee the anomalous phenomena occurring in the high-concentration codiffusion of donors and acceptors in Si and in general shows a good agreement with experimental profiles.
Comparative study of silicon nitride encapsulated and phosphine overpressure annealing on the interdiffusion of InxGa1−xAs-InxGa1−xAsyP1−y heterostructures
Authors: Gillin W. P. , Perrin S. D. , Homewood K. P.
Published: July 2002

The thermal interdiffusion on the group-V sublattice in InxGa1−xAs-InxGa1−xAsyP1−y quantum-well structures has been characterized for samples annealed either with silicon nitride encapsulation or under a phosphine overpressure in two different metalorganic vapor-phase-epitaxy reactors under different phosphine overpressures. Under all conditions the interdiffusion lengths were found to be comparable, with only small effects due to the phosphine overpressure. This suggests that interdiffusion results obtained from silicon nitride capped samples can be applied to the interdiffusion that occurs during growth conditions.
Structure, chemistry, and growth mechanisms of photovoltaic quality thin-film Cu(In,Ga)Se2 grown from a mixed-phase precursor
Authors: Tuttle J. R. , Contreras M. , Bode M. H. , Niles D. , Albin D. S , Matson R. , Gabor A. M. , Tennant A. , Duda A. , Noufi R.
Published: July 2002

The formation chemistry and growth dynamics of thin-film CuInSe2 grown by physical vapor deposition have been considered along the reaction path leading from the CuxSe:CuInSe2 two-phase region to single-phase CuInSe2. The (Cu2Se)β(CuInSe2)1−β (0<β≤1) mixed-phase precursor is created in a manner consistent with a liquid-phase assisted growth process. At substrate temperatures above 500 °C and in the presence of excess Se, the film structure is columnar through the film thickness with column diameters in the range of 2.0–5.0 μm. Films deposited on glass are described as highly oriented with nearly exclusive (112) crystalline orientation. CuInSe2:CuxSe phase separation is identified and occurs primarily normal to the substrate plane at free surfaces. Single-phase CuInSe2 is created by the conversion of the CuxSe into CuInSe2 upon exposure to In and Se activity. Noninterrupted columnar growth continues at substrate temperatures above 500 °C. The addition of In in excess of that required for conversion produces an In-rich near-surface region with a CuIn3Se5 surface chemistry. A model is developed that describes the growth process. The model provides a vision for the production of thin-film CuInSe2 in industrial scale systems. Photovoltaic devices incorporating Ga with total-area efficiencies of 14.4%–16.4% have been produced by this process and variations on this process.
Admittance spectroscopy of InAlAs/InGaAs single-quantum-well structure with high concentration of electron traps in InAlAs layers
Authors: Brounkov P. N. , Benyattou T. , Guillot G. , Clark S. A.
Published: July 2002

Results are presented of admittance spectroscopy measurements on the lattice-matched In0.52Al0.48As/In0.53Ga0.47As single-quantum-well structures. It has been found that the perpendicular conductivity of the structure is controlled by the strong temperature dependence of the space-charge region width around the quantum-well layer. This process is governed by a high density of deep electron traps present in the layers adjacent to the quantum well. Therefore, the energy activation of perpendicular conductivity is determined by the deep-level defects rather than the thermionic emission of electrons from the quantum well. Because of this, it is impossible to extract the magnitude of the band offset between the quantum well and barrier layers from the admittance measurements performed in this study.
Ion energy distributions in dual- and single-mode microwave/radio-frequency plasma
Authors: Reinke P. , Bureau S. , Klemberg-Sapieha J. E. , Martinu L.
Published: July 2002

Ion energy distribution functions (IEDF) were measured at the surface of a grounded or a rf-powered electrode exposed to a microwave (MW, 2.45 GHz) or a rf (13.56 MHz) discharge in argon. The IEDFs measured on the grounded electrode in both the rf and the MW modes show a higher contribution of low-energy ions when the pressure is raised. The maximum ion energy Em decreased from 12.0 to 8.5 eV in the MW plasma, but remained at 26±0.5 eV in the rf plasma. The mean ion energy Ē decreased from 6.1 to 3.8 eV (MW) and from 16.0 to 12.0 eV (rf) when the pressure was raised from 30 to 210 mTorr. The IEDFs in the rf mode exhibit a single peak, while a bimodal structure is observed in the MW mode. When the electrode is rf powered to achieve a negative dc bias voltage VB, the Em values reach ∼1.2eVB (rf) and ∼1.1eVB (MW), respectively. The IEDFs in the rf mode display a multiple peaked structure which is associated with a rf-modulated sheath. In the dual-frequency MW/rf mode the IEDF is single peaked, narrow, and centered around eVB. A nearly tenfold increase in the ion flux is observed when increasing the MW input power, while the ion energy remains unchanged for a constant VB value. This allows an independent control of ion energy and ion flux, suitable for the control of material properties in plasma processing.
Effective medium treatment of laminar flow in magnetic filtration
Authors: Natenapit Mayuree
Published: July 2002

A theory for laminar flow through randomly distributed magnetic spheres which uses an effective medium treatment (EMT) is presented. In the EMT, the system of fluid and spheres is replaced by a composite sphere—a representative sphere of radius a enclosed by a fluid shell of radius b—embedded in an effective medium of different viscosity. The velocity fields in the fluid shell and in the effective medium as a function of sphere volume packing fraction (γ3=a3/b3) or density of spheres in liquid are determined by using Green’s theorem and proper boundary conditions. The results are applied to study the capture of paramagnetic and diamagnetic particles by an assemblage of spheres. The capture radius as a function of γ and applied magnetic field (H0) is obtained and compared with results from previous study based on Happel’s theory. The dependence of capture radius on H0 is also given for various fixed values of γ. Finally, filter efficiency is predicted for varying γ and H0.
Growth kinetics of ultrathin silicon dioxide films formed by oxidation in a N2O ambient
Authors: Koyama N. , Endoh T. , Fukuda H. , Nomura S.
Published: July 2002

The growth kinetics of ultrathin SiO2 films on silicon in a nitrous oxide (N2O) ambient have been investigated as a function of oxidation temperature and time. The results show that the overall growth follows the linear-parabolic law proposed by Deal and Grove [J. Appl. Phys. 36, 3770 (1965)]. The data analysis indicates that although the oxidation proceeds by surface-limited reaction in the initial stage, it rapidly changes into a diffusion-controlled reaction. This behavior is evidenced from the fact that the reaction of the N2O molecule with the silicon surface produces an interfacial nitrogen-rich layer which acts as a barrier to the oxidant passing through the SiO2/Si interface. From the Arrhenius equation for N2O oxidation, the activation energies for the linear rate constant B/A and for the parabolic rate constant B are determined to be 1.5 and 2.3 eV, respectively.
p-type ion-implantation doping of Al0.75Ga0.25Sb with Be, C, Mg, and Zn
Authors: Zolper J. C. , Klem J. F. , Howard A. J. , Hafich M. J.
Published: July 2002

p-type ion-implantation doping of Al0.75Ga0.25Sb is reported. The surface morphology and electrical properties of Al0.75Ga0.25Sb are shown by atomic force microscopy and Hall measurements to be degraded after rapid thermal annealing of 650 °C. Implantation of Be and Mg results in sheet hole concentrations twice that of the implanted acceptor dose of 1×1013 cm−2 following a 600 °C anneal. This is explained in terms of double acceptor or antisite defect formation. Implanted C acts as an acceptor but also demonstrates excess hole conduction attributed to implantation-induced defects. Implanted Zn requires higher annealing temperatures than Be and Mg to achieve 100% effective activation for a dose of 1×1013 cm−2 probably as a result of more implantation-induced damage created from the heavier Zn ion. Secondary ion mass spectroscopy of as-implanted and annealed Be, Mg, and C samples are presented. Diffusion of implanted Be (5×1013 cm−2, 45 keV) is shown to have an inverse dependence on temperature that is attributed to a substitutional-interstitial diffusion mechanism. Implanted Mg (1×1014 cm−2, 110 keV) shows dramatic redistribution and loss at the surface of up to 56% after a 600 °C anneal. Implanted C (2.5×1014 cm−2, 70 keV) displays no redistribution even after a 650 °C anneal. This work lays the foundation for using ion-implantation doping in high performance AlGaSb/InGaSb-based p-channel field-effect transistors.
Film quality in relation to deposition conditions of a-SI:H films deposited by the ‘‘hot wire’’ method using highly diluted silane
Authors: Molenbroek Edith C. , Mahan A. H. , Johnson E. J. , Gallagher A. C.
Published: July 2002

The deposition parameter space has been extensively explored using the hot wire technique with 1% SiH4 in He as a source gas. To achieve reasonable deposition rates despite the high dilution, the filament was positioned at 1–2 cm from the substrate. This short distance introduced a large nonuniformity across the substrate in deposition rate as well as in film properties. These spatial variations were used to analyze which factors in the deposition determine film quality. Radiation from the filament as well as deposition rate cannot explain the large variation in film properties, leaving gas-phase reactions of Si and H from the hot filament as the primary cause. It is clear that radicals evaporated from the filament must undergo gas-phase reactions with SiH4 before deposition in order to produce high-quality material. Thus, conditions such as increasing the chamber pressure or going to a heavier carrier gas increase the fraction of radicals that can react before reaching the substrate and, therefore, improve the film quality. However, such conditions also enhance multiple radical reactions before such radicals reach the substrate and this can have a negative effect on film quality: this is attributed to gas-phase nucleation with incorporation of conglomerates. The gas-phase chemistry is quite different from that of plasma-enhanced decomposition in that no disilane or trisilane is formed in significant quantities. This, and the dependence on pressure, indicates that the pathway for formation of these heavier particles is radical–radical reactions.
Resonant tunneling in (001)- and (111)-oriented III–V double-barrier heterostructures under transverse and longitudinal stresses
Authors: Albrecht J. D. , Cong L. , Ruden P. P. , Nathan M. I. , Smith D. L.
Published: July 2002

Calculations of the effects of external stress on the current–voltage characteristics of double-barrier (001)- and (111)-oriented resonant tunneling devices are presented. Crystal strains arising from the application of external pressure and, in pseudomorphic structures, lattice mismatch cause shifts in the conduction and valence bands of the well and barrier layers with respect to the unstrained alignment. For certain stress orientations piezoelectric effects give rise to internal electric fields parallel to the current direction. The combined piezoelectric and band-structure effects modulate the transmission resonances which control the shape of the current versus voltage characteristics of the structures.
Magnetic properties of Ni2In type (Co1–xMnx)65Ge35 compounds
Authors: Shiraishi Hiroshi , Hori Tomiei , Iguchi Yasuo , Yamaguchi Yasuo , Ohashi Masayoshi , Kanematsu Kazuo
Published: July 2002

The high temperature phase of (Co1–xMnx) 65Ge35 is found to be of the hexagonal Ni2In type in the range of 0≤x≤0.6. Co2Ge is Pauli paramagnetic, and ferromagnetism is induced with substitution of Mn for Co. Magnetization measurement and x-ray diffraction experiments for these compounds have been performed. The x dependences of the Curie temperature Tc and magnetization show a maximum near composition x=0.25. X-ray studies show that the lattice constants c are minimum at x=0.25. The x dependence of the relative intensities of x-ray lines suggests that the Mn and Co atoms at 2(a) sites in the compound x=0.25 align alternately in the c direction. The Co atoms at 2(d) site and the Ge atoms at 2(c) site shift along the c direction are found. The neutron diffraction experiment of x=0.25 also support such crystal structure. Maximum of Curie temperature and magnetization of compounds near x=0.25 are discussed with the ordering of Mn and Co atoms in the crystal lattice.
Evidence for screening effects on the 1/f current noise in GaAs/AlGaAs modulation doped field effect transistors
Authors: Py M. A. , Buehlmann H.-J.
Published: July 2002

Using a δ-doped GaAs/AlGaAs heterostructure with a 10 nm spacer layer, we exploit the metastable nature of the DX centers at low temperatures to control electrostatically their net frozen charge density. The concentration of DX− centers at 77 K is only determined by the applied gate voltage Vc during the cool-down of the sample to 77 K, i.e., it is independent, on the time scale of noise experiments, of the subsequent change in the gate bias VGS. The sheet carrier concentration ns of the two-dimensional electron gas is varied through the application of VGS. Hall experiments performed at 77 K on gated Hall-bar structures show that Vc strongly affects both, the threshold voltage Vt and the exponent k, which enters into the observed power-law dependence of the Hall mobility μ on ns. These dependencies were also studied directly on modulation doped field effect transistors from the analysis of their transfer characteristics IDS–VGS and gme–VGS at low drain bias VDS. The 1/f drain-current noise was investigated and, after subtraction of the noise arising from the series resistances, quantified by the extracted value of the channel-associated Hooge parameter αch. This parameter is found to depend on ns and exhibits the same power-law dependence as the reciprocal mobility 1/μ(ns). This striking correlation was established for various values of k and reveals screening effects on the 1/f noise. This correlation qualitatively supports the idea that the dominant mechanism of 1/f noise in modulation doped field effect transistors, at 77 K, is due to mobility fluctuations induced by screened fluctuations of Coulomb scattering, generated either by fluctuations of charge-state and/or motion of defects.
Donor formation in nitrogen doped silicon
Authors: Yang Deren , Fan Ruixing , Li Liben , Que Duanlin , Sumino Koji
Published: July 2002

The formation behavior of donors associated with oxygen in nitrogen-doped and undoped Czochralski silicon has been investigated by means of electrical and infrared measurements at low temperatures (8 K). It is found that donors are not generated in N-doped silicon annealed at 650 °C up to 100 h. We have also studied the effect of preannealing at 450 and 1050 °C on the formation of donors in the N-doped silicon. The 450 °C preannealing enhances the formation of donors in the subsequent annealing at 650 °C, while the 1050 °C preannealing has no effect. The donor concentration increases at 650 °C with the 450 °C preannealing time. The formation of nitrogen–oxygen complexes in the N-doped silicon is believed to suppress the generation of nuclei of donors, and, consequently, the formation of donors.
Detection of particles in rf silane plasmas using photoemission method
Authors: Fukuzawa Tsuyoshi , Obata Kouji , Kawasaki Hiroharu , Shiratani Masaharu , Watanabe Yukio
Published: July 2002

Nucleation and subsequent initial growth processes of particles in rf silane plasmas are investigated using a newly developed photoemission method. In this method, electrons produced due to interactions of particles with photons (photodetachment, photoionization) are measured. Since threshold energies for the interactions are dependent on particle size, size ranges of the detected particles are determined by wavelengths of the irradiated light. Using this method, small particles such as SinHx (n≲10), SinHx (n≲200) and SinHx (n≳20) can be detected. The experiments show that even in a very early phase of their growth, particles nucleate and grow principally around the plasma/sheath boundary near the rf electrode, where short lifetime radicals are produced actively. This suggests that the short lifetime radicals are the key species contributing to the particle nucleation and its subsequent initial growth processes. Moreover, it is found that small particles exist even in the bulk plasma. Since some of them are considered to be neutral, they are likely to be transported to the substrate on the grounded electrode.
Diffractive phase elements by electron-beam exposure of thin As2S3 films
Authors: Nordman Nina , Salminen Olli , Kuittinen Markku , Turunen Jari
Published: July 2002

Variable-dose electron-beam exposure of thin amorphous As2S3 films is studied as a potential fabrication technique of index-modulated diffractive optical elements for the near infrared. The relationship between the electron dose and the phase delay is determined using a coarse grating structure, which eliminates the influence of volume diffraction and proximity effects. The effective refractive index change is determined by comparing experimental and calculated zeroth- and first-order diffraction efficiencies, taking into account the small but detectable surface modulation.
Photothermal ionization identification of residual donors in high purity InP grown by gas source molecular beam epitaxy
Authors: Shi X. H. , Liu P. L. , Shen S. C. , Chen J. X. , Xin H. P. , Li A. Z.
Published: July 2002

Residual donors in the high purity InP epitaxial layer grown by gas source molecular beam epitaxy (GSMBE) have been identified by a photothermal ionization technique in a magnetic field. The dominant residual donors in GSMBE InP are identified to be silicon and sulfur. The Zeeman effect of the hydrogenic donors has also been investigated. The 1s–3p photothermal ionization transitions of the hydrogenic donors are observed even at zero magnetic field. The photoconductivity transition related to the LO phonon is observed and discussed. Furthermore, the photothermal ionization transitions arising from different chemical donors are also clearly resolved in a magnetic field. From the spectral peaks in magnetic field, the electron effective mass m* in GSMBE InP is estimated to be (0.0817±0.0002)m0.
The role of surface roughness in the fabrication of stacked Nb/Al–AlOx/Nb tunnel junctions
Authors: Kohlstedt H. , König F. , Henne P. , Thyssen N. , Caputo P.
Published: July 2002

The surface roughness of sputtered Nb films was determined with high precision using x-ray specular reflectivity measurements in the 10 keV range. The roughness of Nb films increased from 0.9 nm for a 70-nm-thick film to 1.8 nm for a 210-nm-thick film. The roughness of the Nb surface strongly influences the tunnel barrier formation and the electrical properties of that barrier. For stacked tunnel junctions each thermal Al oxidation has to be adjusted as a function of the underlying Nb film thickness. Alternatively, one can use an Al or Al/AlOx underlayer in order to obtain stacks with small spread in critical currents.
Ultra-low values of the absorption coefficient for band–band transitions in moderately doped Si obtained from luminescence
Authors: Daub E. , Würfel P.
Published: July 2002

The absolute value of the absorption coefficient αbb(ℏω) for band–band transitions near the band edge was determined in moderately doped silicon by photoluminescence spectra analysis. The major advantage of this method in determining αbb(ℏω) is the lack of interference with free carrier absorption, in contrast to conventional methods like transmission or photothermal deflection measurements. We deduce values for αbb(ℏω), which are nearly five orders of magnitude smaller than the absorption coefficient αfc(ℏω) for free carrier absorption. With this method it is possible to examine in detail the influence of doping on the absorption coefficient for band–band transitions near the absorption edge. The appearance of band tails and band-gap narrowing are very well reflected. With conventional methods, which can only detect the overall absorption of the incident radiation, the determination of αbb(ℏω) in the vicinity of the band edge is impossible for moderately and heavily doped silicon, because it is completely masked by the free carrier absorption.
Hamiltonian plasma-harmonic oscillator theory: Generalized depth profilometry of electronically continuously inhomogeneous semiconductors and the inverse problem
Authors: Salnick Alex , Mandelis Andreas
Published: July 2002

The Hamilton–Jacobi formalism of the propagation of an electron-hole photoexcited plasma in continuously inhomogeneous semiconductors with arbitrary depth profiles in carrier diffusivity and/or minority-carrier lifetime is presented. The theoretical model is based on the variational formulation of the canonical Hamiltonian for the evolution of carrier plasmas and shows that propagating plasma waves can be formally described by a plasma-harmonic oscillator, thus generalizing existing theoretical treatments of photoexcited carrier diffusion in electronic solids. Simple analytical expressions for the free-carrier diffusion magnitude and phase frequency dependencies in the case of exponential carrier diffusivity and minority carrier lifetime profiles are obtained. The effect of continuously varying electronic properties on the surface plasma density magnitude and phase frequency behavior is demonstrated through computer simulations and very good quantitative agreement is obtained with photothermal radiometric data from an ion-implanted Si wafer allowing the reconstruction of the lifetime depth profile.
Transport modeling of multiple-quantum-well optically addressed spatial light modulators
Authors: Smith S. L. , Hesselink L.
Published: July 2002

A transient, two-dimensional drift-diffusion model is developed for optically addressed spatial light modulators made with quantum-well materials. The transport of free and well-confined carriers is considered along with nonlinear transport effects such as velocity saturation, field-dependent carrier escape from quantum wells, and resonant absorption. In addition to full numerical solutions to the transport equations, analytical and simplified numerical solutions are developed to describe basic screening behavior and to give estimates of speed and resolution performance. In particular, a self-consistent small signal model is developed to justify the surface-charge picture often used to describe device operation. This model is also used to simulate grating formation and decay. It is found that the maximum screening rate and peak grating amplitude are achieved using vertical drift lengths much longer than the device length. A detailed analysis of resolution performance is also given in which the effects of transit time, carrier lifetime, and free and confined transport along the wells are simulated. For typical device parameters, the two main limitations to resolution performance are found to be anisotropic drift in the interior due to the quantum wells and transverse drift along the device interfaces. Two device designs are compared to assess the ability to optimize device performance by changing experimentally accessible parameters such as carrier lifetime and quantum-well escape rates. Resolutions down to 7 μm and frame rates of 100 kHz at 10 mW/cm2 are achieved.
Hall mobility lowering in undoped n-type bulk GaAs due to cellular-structure related nonuniformities
Authors: Siegel W. , Schulte S. , Kühnel G. , Monecke J.
Published: July 2002

In undoped bulk-grown GaAs single crystals, which show a wide variation of the resistivity, a characteristic dependence of the Hall mobility on the carrier concentration with a pronounced minimum at about 1×1010 cm−3 is observed. By applying a standard effective medium theory it is shown that this minimum is caused by mesoscopic nonuniformities of the charge carrier concentration and not by increased scattering rates or additional scattering mechanisms as would be the standard interpretation in the case of homogeneous samples. These nonuniformities observed by high-resolution point-contact measurements are connected with the cellular structure of dislocations.
Moving Preisach model analysis of nanocrystalline SmFeCo
Authors: Cornejo D. R. , Bue M. Lo , Basso V. , Bertotti G. , Missell F. P.
Published: July 2002

Nanocrystalline SmFeCo, prepared by mechanically alloying the elementary powders and subsequent annealing (600 °C/15 min), showed remanence enhancement (η=Ir/Is=0.67) and Hc=18.6 kOe. Starting from the ac-demagnetized state, minor hysteresis loops were determined for magnetic fields up to 70 kOe for numerical calculation of the Preisach function p(hc,hu), where hc and hu are the elementary loop coercive field and interaction field. The numerically determined irreversible part of p(hc,hu) had a form indicating that it could be factored: pirr(hc,hu)=f(hc)g(hu), where f(hc) and g(hu) are log–normal and Gaussian distributions. An analytic expression for p(hc,hu)=f(hc)g(hu)+prev was fitted to experimental hysteresis curves. The analytic expression for p(hc,hu) was used with the moving Preisach model to calculate minor loops and Henkel plots. Agreement between theory and experiment was very good.
Structure and magnetism of epitaxial rare-earth–transition-metal films
Authors: Fullerton Eric E. , Sowers C. H. , Pearson J. E. , Bader S. D. , Patel J. B. , Wu X. Z. , Lederman D.
Published: July 2002

Epitaxial rare-earth–transition-metal thin films have been grown by magnetron sputtering onto buffered single-crystal MgO substrates. Results are presented for SmFe12(001) and magnetically hard Sm2Co7 and Sm2Co17 (110) films using W (100) buffer layers. The methodology of utilizing epitaxial W buffer layers combined with cosputtering from elemental sources provides a general approach to tailor the orientation and phase of the films.
Spin-valve giant magnetoresistive films with antiferromagnetic Ir-Mn layers
Authors: Fuke Hiromi Niu , Saito Kazuhiro , Kamiguchi Yuzo , Iwasaki Hitoshi , Sahashi Masashi
Published: July 2002

We succeeded in developing CoFe spin valves with an antiferromagnetic Ir-Mn film. Ir-Mn single-layer films and spin valves of Ta(5 nm)/Ir-Mn(8 or 9 nm)/Co90Fe10(x nm)/Cu(3 nm)/Co90Fe10(3 nm)/NiFe(2 nm)/CoZrNb(10 nm)/ (x=2, 2.3, 2.6 nm), prepared by the sputtering method, showed the crystal structure of a fcc (111) preferred orientation. As-deposited CoFe spin valves with Ir-Mn exhibited an interfacial exchange coupling energy of J=0.192 erg/cm2 (Hua∼640 Oe at tCoFe=2 nm), that was the highest ever reported for as-deposited antiferromagnetic films, such as NiO, NiMn, and FeMn. Furthermore, CoFe spin valves with Ir-Mn exhibited a higher blocking temperature of 260 °C, and a higher MR ratio of 6.37% than the spin valves with FeMn film. After annealing, the MR ratio increased to 7.82%. On the other hand, the Hua decreased about 100 Oe after annealing. The Hua-T curve was, however, improved and the Hua at 100 °C increased to 400 Oe. The decrease in Hua was not observed after second annealing and seems to be stabilized by first annealing.
Photoluminescence measurement of InGaN and GaN grown by a gas-source molecular-beam epitaxy method
Authors: Yoshida Seikoh
Published: July 2002

GaN and InxGa1−xN were grown using a gas-source molecular-beam epitaxy method. Sapphire substrates were used for GaN growth. Ammonia (NH3) gas was used as the nitrogen source gas. As a result, GaN and InxGa1−xN crystals with mirror surfaces were obtained using uncracked ammonia gas. Photoluminescence measurements of InxGa1−xN and GaN were investigated. No deep-level emission was observed and only the band-edge emission spectra of InxGa1−xN and Si-doped GaN crystals were observed. It was confirmed that, in the case of undoped GaN growth, the yellow luminescence disappeared upon increasing the V/III flux ratio.
Comment on “Schottky contact and thermal stability of Ni on n-type GaN” [J. Appl. Phys. 80, 1623 (1996)]
Authors: Duxstad K. J. , Haller E. E.
Published: July 2002

J. D. Guo et al. [J. Appl. Phys. 80, 1623 (1996)] have studied the thermal stability of Ni on GaN and have deduced by x-ray diffraction measurements that new phases form at the interface between Ni and GaN. This determination, based solely on x-ray diffraction data, is ambiguous, as the peaks assigned to the Ga4Ni3 and Ni–N phases can also be attributed to substrate related x-ray peaks.
Absolute density measurement of cyanogen fluoride in CHF3/N2 electron cyclotron resonance plasma using infrared diode laser absorption spectroscopy
Authors: Miyata Koji , Arai Hiroyoshi , Hori Masaru , Goto Toshio
Published: July 2002

The absolute density of the cyanogen fluoride (FCN) molecule has been measured in a CHF3/N2 electron-cyclotron-resonance (ECR) plasma using infrared diode laser absorption spectroscopy. The R(22) rotational-vibrational line at 1060.340 cm−1 in the ν1 fundamental of F19C12N14 was used for the spectroscopy. The extinction process of FCN in the afterglow was discussed on the basis of the decay rate after discharge termination. Moreover, the absolute FCN density in a CHF3 ECR plasma during etching of silicon nitride has been calculated on the basis of the data shown in our previous study [K. Miyata et al., J. Vac. Sci. Technol. A 15, 568 (1997)]. It was found that approximately 10% of nitrogen atoms coming from silicon nitride formed FCN.
Shape and stability of a floating liquid zone between two solids
Authors: Saitou M.
Published: July 2002

The shape and stability of a floating liquid zone between two solids are analytically investigated using the principle of variation. The formulas for its shape and stability are obtained.
Interaction of light with a transversely moving nonlinear medium: Beyond Doppler laser velocimetry
Authors: Tabiryan N. V. , Nersisyan S. R. , Warenghem M.
Published: July 2002

Interaction of a laser beam with a transversely moving nonlinear medium has been studied both experimentally and theoretically. With the mesophase of a nematic liquid crystal (NLC) as the nonlinear medium, transverse motion of the NLC at speeds of ∼1 μm/s relative to the laser beam, causes remarkable changes in its orientational state. The response of the medium to the transverse motion is enhanced considerably in the case of threshold nonlinearity where the reorientation of the LC takes place as a second order phase transition. The diffusive transport of light induced perturbations of the nonlinear medium, give rise to a far field projection pattern that consist of a system of concentric arcs. The observed phenomena can lead to potential applications for measurement or visual sensing of motion. Alternatively it can provide a means for the characterization of nonlinear materials.
Particle formation in SiOx film deposition by low frequency plasma enhanced chemical vapor deposition
Authors: Yamaguchi Tomoyo , Sakamoto Naoshi , Shimozuma Mitsuo , Yoshino Masaki , Tagashira Hiroaki
Published: July 2002

Dust particle formation dynamics in the process of SiOx film deposition from a SiH4 and N2O gas mixture by a low frequency plasma enhanced chemical vapor deposition have been investigated using scanning electron microscopy and laser light scattering. The deposited films are confirmed to be SiOx from the measurements of Auger electron spectroscopy, x-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. It is observed by scanning electron microscopy that particles are deposited on Si substrate at the plasma power frequency f=5 kHz and above both with and without substrate heating (400 °C), while no particle is deposited below f=1 kHz. Moreover, the laser light scattering indicates that particles are generated at the plasma power frequency of f=3 kHz and above in the gas phase, and that they are not generated in the gas phase at below f=3 kHz. Properties (the refractive index, resistivity, and Vickers hardness) of the films with particles are inferior to those of the films without particles. This article has revealed experimentally the effect of plasma power frequency on SiOx particle formation and makes a contribution to the explication of the particle formation mechanism. We suggest that high-quality film deposition with the low frequency plasma enhanced chemical vapor deposition method is attained at f=1 kHz or less without substrate heating.
Monte Carlo calculation of electron transport properties of bulk AlN
Authors: Albrecht J. D. , Wang R. P. , Ruden P. P. , Farahmand M. , Brennan K. F.
Published: July 2002

The Monte Carlo method is used to simulate electron transport in bulk, wurtzite phase AlN using a three valley analytical band structure. Spherical, nonparabolic conduction band valleys at the Γ, K, and U symmetry points of the Brillouin zone are fitted to a first-principles band structure. The electron drift mobility is calculated as a function of temperature and ionized donor concentration in the ranges of 300–600 K and 1016–1018 cm−3, respectively. The effect of compensation on ionized impurity scattering and the associated change in the mobility are considered. The simulated electron steady-state drift velocity and valley occupancy for electric fields up to 600 kV/cm are presented for 300, 450, and 600 K. Our calculations predict that AlN will exhibit a much smaller negative differential mobility effect than GaN, and that the drift velocity versus electric field curve will show a very broad peak.
High-frequency permeability of soft-magnetic Fe–Hf–O films with high resistivity
Authors: Huijbregtse Jeroen , Roozeboom Fred , Sietsma Jilt , Donkers Johan , Kuiper Ton , van de Riet Erik
Published: July 2002

A series of Fe–Hf–O films has been prepared by means of rf sputter deposition. The composition for optimal permeability was found to be Fe55Hf17O28. These films exhibit a combination of soft-magnetic properties (μ>103, Hc<50 A/m, μ0Ms⩾1.0 T) and a high resistivity (ρ>1 mΩ cm) after rapid thermal annealing in flowing argon at 400 °C in a dc magnetic field. The structure of the films consists of α-Fe nanocrystallites, randomly dispersed in an amorphous matrix. The amorphous phase is essential in achieving the advantageous combination of properties. In contrast to conventional soft-magnetic films, the properties of the Fe55Hf17O28 films do not show a strong thickness dependence for thicknesses up to 10 μm. Therefore, the roll-off frequency of the initial relative permeability for 10 μm thick films is high, but not as high as expected from only the resistivity. It is shown that, besides electromagnetic induction, ferromagnetic resonance is also essential in explaining the observed frequency dependence of the permeability.
Nonlinear dynamics of excitons in asymmetric coupled quantum wells
Authors: Cruz H.
Published: July 2002

In this work, we have numerically integrated in space and time the effective-mass Schrödinger equation for an excitonic wave packet in a coupled quantum well system. Considering an electronic many-body interaction and an external electric field, we have obtained a nonlinear dynamical evolution for the excitonic wave function. At large electronic sheet densities, it is found that the charge dynamically trapped in both wells produces a reaction field which modifies the system resonant condition.
Photoemission study of pristine and photodegraded poly(methyl methacrylate)
Authors: Okudaira Koji K. , Hasegawa Shinji , Sprunger Phillip T. , Morikawa Eizi , Saile Volker , Seki Kazuhiko , Harada Yoshiya , Ueno Nobuo
Published: July 2002

Degradation of poly(methyl methacrylate) (PMMA) thin films by vacuum ultraviolet (VUV) monochromatic synchrotron radiation was investigated by ultraviolet photoelectron spectroscopy. The photodegradation reaction was analyzed, for the first time, by different spectrometry techniques and ab initio molecular orbital calculations. It is concluded that the main degradation mechanism in PMMA by VUV photons is ascribed to the disappearance of ester groups and formation of double bonds in the polymer chain. The final product of the degradation seems to possess a relatively rich conjugation of unsaturated bonds. The rate constant of the degradation by VUV photons is evaluated to be 2.4×10−17 photons−1 cm2.
Thin layers and multilayers of porous silicon: X-ray diffraction investigation
Authors: Buttard D. , Bellet D. , Dolino G. , Baumbach T.
Published: July 2002

Porous silicon is a unique example of a porous material exhibiting the properties of a nearly perfect single crystal. High resolution x-ray diffraction has been used to investigate thin p− and p+ type porous silicon layers in the 100–1000-nm-thickness range. Since several thickness fringes are observed, the comparison between experimental results and simulations enables one to deduce information about the main structural parameters such as porosity, lattice parameter, thickness, and heterotransition width. Porous silicon multilayers have also been investigated: some satellites are clearly observed. The obtained results are then compared and discussed with the literature.
Imaging of magnetic domains with scanning tunneling optical microscopy
Authors: Bertrand P. , Conin L. , Hermann C. , Lampel G. , Peretti J. , Safarov V. I.
Published: July 2002

Near field magneto-optical images of magnetic domains in ferromagnetic soft garnet films have been obtained with a scanning tunneling optical microscope working in total reflection geometry with shear-force control of the tip-to-sample distance. In this geometry a magneto-optical contrast is observed for the first time between domains of opposite magnetization without using modulation techniques. When applying a static or alternating external magnetic field, the magneto-optical images provide the location of domain wall pinning points.