Strained In x Ga1−x As/GaAs multiple quantum wells grown by MOVPE

Luminescence properties of strained In _x Ga_1−x As/GaAs multiple quantum wells of different thickness and In content, prepared by metal organic vapour phase epitaxy were studied. The influence of the quantum well material composition on the shape of luminescence spectra was investigated. The experimental results were fitted by the Model Solid Theory. This fit was improved by the use of adjustedQ parameter. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998. This work was supported by Grant Agency of Czech Republic under grants numbers 202/98/0074, 102/99/0414 and Grant Agency of Academy of Sciences No. A 10110807/1998.     

Nanocrystalline titanium carbide thin films deposited by reactive magnetron sputtering

We investigated the structure and composition of titanium carbide thin films deposited by the reactive magnetron sputter ion plating process as a function of deposition parameters. The films were sputtered onto unheated glass substrates by means of an unbalanced planar d.c. magnetron equipped with a titanium target using a mixture of argon and methane. The deposition parameters ranged from 0.05 Pa to 2 Pa for total working gas pressurep _T, from 10% to 60% (volume) for relative methane concentration in the working gas mixture, from 45 mm to 85 mm for the substrate-to-target distanced _s-t and from −50V to −800V for the substrate biasU _S. It was found that the crystallinity of the thin films strongly decreases with increasingp _T,d _s-t andU _S. The experiments described show the conditions necessary to obtain sputter-deposited nanocrystalline titanium carbide films. This work has been partially supported by the Grant Agency of the Czech Republic under Grant No. 106/96/K245 and by the Ministry of Education of the Czech Republic under Grant No. VS96 059.     

Study of the plasmas produced during the deposition of TiC/SiC thin films in a hybrid magnetron-laser system

Time- and spatially-resolved optical emission spectroscopy was performed to characterize the plasma produced in a hybrid magnetron-sputtering-laser deposition system, which is used for TiC or SiC thin films preparation. A graphite target was ablated by a KrF excimer laser (λ=248 nm,τ=20 ns) and either Ti or Si targets were used for DC magnetron sputtering in argon ambient. Spectra were measured in the range 250–850 nm. The evolution of the spectra with varying magnetron powers (0–100 W) and argon pressures (0.3–10 Pa) was studied. Spectra of the plasmas produced by a) the magnetron alone, b) the ablation laser alone, and c) the magnetron and the ablation laser together, were recorded. Spectra (a) were dominated by Ar atoms and Ar⁺ ions. Emission lines of Ti and Si were detected, when Ti target and Si target was used, respectively. Spectra (b) revealed emission of C, C⁺, C₂, Ar, Ar⁺. Spectra (c) showed presence of all previously mentioned species and further of Ti⁺ ions emission was detected. The research was supported by Grant Agency of the Czech Republic No. 202/06/02161, GA ASCR project number A1010110/01 and Institutional Research Plan AV CR No. AV0Z 10100522.     

Modeling 1/f noise using moving averages

A model of 1/f noise is considered, based on moving averages of ordern. The coefficientsα _k defining the model are calculated numerically using Seidel iteration which turns out to converge rapidly. The convergence is independent ofn which seems to be caused by the fact that the nonlinear problem solved is defined by a self-similar matrix. The coefficientsα _k appear to approach, with indefinitely growingn, valuesα _k =1/√k and thus the model has a kind of Fourier invariance. Physical interpretation of the invariance is suggested as well as of coefficientsα _k describing long-range correlations. Fractal sets of dimensiond=2.5 are proposed to play certain role in explaining the latter. This work was partly supported by the Scientific Grant Agency of the Ministry of Education of Slovak Republic (VEGA) under the Grant No. 1/3143/96 and by the Slovak Literary Fund.     

Density of states of a terminated superlattice with periodically arrangedδ-defects

Surface electronic structure and the local density of states (LDOS) of a periodicallyδ-doped GaAs/Al _x Ga_1−x As superlattice (SL), terminated by an Al _y Ga_1−y As substrate, are investigated using the wave-function multi-matching procedure within an effectivemass approximation and a Green-function formalism based on factorization and direct techniques. The considered system is described in terms of a semi-infinite Kronig-Penneytype of model interleaved with a periodic array ofδ-function-like potentials, representing deep-center defect sheets. It is shown that the spectrum of surface-localized states and the corresponding space-charge distributions within particular SL minibands can be tailored by varying the terminating conditions (i.e., the substrate parameters) as well as the properties of inserted defects (i.e., the weight and position ofδ-potentials). In this respect, importance of a symmetric/nonsymmetric arrangement ofδ-dopants inside SL barriers and the related effect of miniband crossing/anticrossing are emphasized. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. One of us (B.B.) gratefully acknowledges the support of the University of Wrocław via the project for young researchers No. 2314/W/IFD/99, while two of us (R.K. and M.S.) have been supported within the Grant No. 2016/W/IFD/98.     

Ab-initio theory of the CPP-magnetoconductance

The current-perpendicular-to-plane (CPP) magnetoconductance of a trilayer consisting of a spacer sandwiched between two ideal leads is described on anab initio level. We employ the transmission matrix formulation of the conductance within the framework of the spin-polarized surface Green function technique as formulated in terms of the tight-binding linear muffin-tin orbital method. The formalism is extended to the case of lateral supercells in each layer with random arrangements of atoms which allows to treat both the ballistic and diffusive transports on equal footing. The application is made to fcc-based Co/Cu/Co(001) trilayers. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. Financial support for this work was provided by the Grant Agency of the Czech Republic (Project No. 202/97/0598), the Grant Agency of the Academy of Sciences of the Czech Republic (Project A1010829), the Center for the Computational Materials Science in Vienna (GZ 45.442 and GZ 45.420), the Austrian BMWV (AKTION WTZ-?sterreich-Tschechien I.23), MŠMT ČR (Project COST P3.70), and the TMR Network ‘Interface Magnetism’ of the European Commission (Contract No. EMRX-CT96-0089).     

Electron yield per ion charge-state correction for an ion collector with unsuppressed secondary electron emission

The electron yield per ion charge-state γ/q was measured for emission of electrons from clean polycrystalline gold induced due to impact of Ta ^q+ (11≤q≤41) ions with kinetic energy per chargeE _i/q from 15 keV/q to 150 keV/q. The dependence of γ on angle of incidence was analyzed with use of relation γ(ϑ)=γ₀ cos^−f ϑ. The fitting of experimental data gives a range of γ₀/q from 1 to 1.75 for Ta¹³⁺ and from 1.5 to 1.73 for Ta³⁹⁺. The dependence of γ₀/q onq andE _i is discussed with respect to measurement of ion currents emitted from laser-produced plasmas with an ion collector with unsuppressed secondary electron emission. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy, and by grant A1010819 from the Grant Agency of the Academy of Sciences of the Czech Republic.     

Possible relevance of lattice effects in high-T c superconductors

The relevance of the lattice-mediated superconducting pairing in a system of quasilocalized polarons dressed by local lattice deformations is considered. The spin correlations are taken into account using thet-J model expressed in terms of holon and spin operators. The Holstein-like Hamiltonian for holons with the transport term depending on spin correlations is transformed by the generalized Lang-Firsov transformation which implies the spread of the charge and the deformations to the nearest neighbours. The analytical formula for the superconducting transition temperatureT _c is deduced using the assumption of an extremely narrow polaron band. The hole-concentration dependence ofT _c and the isotope exponent are discussed using the classical approximation for incommensurate spiral spin correlations. This work was supported by the Grant Agency of the Czech Republic, project No. 202/96/0864.     

Conformational properties of poly(p-Phenylene vinylene)

We investigate the quasiparticle conformational defects (excitons, polarons and bioplarons) in the phenylene vinylene oligomers. The conformations are determined by means of the minimization of the total Hartree-Fock energy calculated at 3–21G level. The infrared vibrational transitions are calculated and the types of vibrations are assigned. The theoretical spectra are in good agreement with the experimental vibrational spectrum of poly(p-phenylene vinylene). Presented at the Czech-Israeli-German Symposium “Dynamical Processes in Condensed Molecular Systems”, Prague, Czech Republic, 26–30 May 1997. The quantum mechanical calculations were performed on the following computers: Power Challenge XL at Prague Supercomputing Centre, Charles University, IBM Power 2 at Supercomputing Centre of the Czech Technical University and Power Challenge XL at Supercomputing Centre Brno. The work is supported by Projects No. 202/94/0453 and 202/97/1016 of the Grant Agency of the Czech Republic and by Project No. 155/96 of the Grant Agency of the Charles University.     

STM study of nucleation of Ag on Si(111)−(7 × 7) surface

Initial stages of Ag on Si(111)−(7 × 7) surface nucleation were studied at submonolayer coverage. Samples were prepared by thermal evaporation of Ag from tungsten wire under UHV conditions (p<2.5 × 10⁻⁸ Pa). Various deposition rates (0.002–0.1 ML s⁻¹) were used to prepare Ag island films with coverages (0.002–2) ML (1 ML ≈ 7.58 × 10¹⁴ atoms cm⁻²) at room temperature. We observed preferential growth on faulted half unit cells (F cells). At constant coverage both the island density and ratio of occupied F and U (unfaulted) cells are independent of the deposition rate, which is an evidence for dominant influence of substrate structure. The preference of nucleation in the F cells against U cells decreases with the coverage until the ratio is 1:1 for 1 ML Ag film. We have observed that presence of an Ag island in any type of the half unit cell (F or U) considerably reduces nucleation probability in neighbouring cells. This results in forming of structural patterns observed among randomly grown Ag-islands which is a new feature found for Ag/Si(111)−(7 × 7) system. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. This work was supported by the Grant Agency of Charles University — projects GAUK 34/97 and 147/99, by the Grant Agency of Czech Republic — project GAČR 202/97/1109 and by the Ministry of Education grant VS 97116.     

Speed and shape of solitary waves in relativistic warm plasma

Large-amplitude solitary waves are investigated in a relativistic plasma with finite ion-temperature. The mass of electron is also considered. The Sagdeev’s pseudopotential is determined in terms ofu, the ion speed. It is found that there exists a critical value ofu ₀, the value ofu at which (u′)²=0, beyond which the solitary waves cease to exist. The critical value also depends on the parameters likeν, the soliton velocity;μ, the electronion mass ratio orσ, the temperature ratio of ion to electron. This result reproduces our previous result [Czech. J. Phys., Vol. 54 (2004), No. 4, 489–496] when the ion temperature is neglected.     

Poly(organylsilylene)s — perspective materials for optoelectronics

Poly(organylsilylene)s with their uninterrupted chains of silicon atoms are a new class of materials with significant delocalization of electrons along the polymer chain. Their electronic structure, optical properties, photoconductivity, electroluminescence, and photorefractivity are discussed on the model compound poly[methyl(phenyl)silylene]. Their unusual electrical and optical properties, such as high quantum generation efficiency, high charge-carrier mobility, efficient luminescence, and optical non-linearity, can be utilized in some optoelectronic devices. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998. This work was suported by the Grant Agency of the Academy of Sciences of the Czech Republic (grant No. A1050901) and by the Grant Agency of the Czech Republic (grant No. 106/98/0700).     

Oxygen incorporation in polyethylene implanted with 150 keV Sb+ ions

Samples of polyethylene (PE) loaded with carbon black up to 8 wt.% and implanted with 150 keV Sb⁺ ions to the doses from 2×10¹³–2×10¹⁵ cm^–2 were studied using standard Rutherford Back Scattering (RBS) technique. On the PE samples implanted to the doses above 2×10¹⁴ cm^–2, a considerable surface carbonization is observed. The measured parameters of the Sb depth profile are compared with theoretical TRIM estimations. The projected range is by 25% lower than the theoretical one and the range straggling is about twice of that predicted. The differences are explained by stepwise polymer degradation during the ion bombardment. Strong oxidation of the ion implanted polymers is also observed. The oxygen depth profiles from the sample surface up to the depth comparable with Sb⁺ ion range evolve from nearly uniform one for low ion doses to highly non-uniform one for doses above 1×10¹⁵ cm^–2. The total oxygen content in the sample surface layer 300 nm thick reaches a maximum for the doses of (1–2)×10¹⁴ cm^–2.On leave from Belorussian State University, 220050 Minsk, Belarus.The authors thank the member of electrostatic accelerator group at INP for help during RBS measurements. The work was partly (V.H. and J.K.) supported by the Internal Grant Agency of the Academy of Sciences of Czech Republic under the grant No. 14805 and by the Internal Grant Agency of the Ministry of Education of Czech Republic under the grant No. 1002 (V.., V.R., V.H. and J.K.). In the final stage, the work was also supported by the Grant Agency of Czech Republic under the grant No. 202/93/0121.     

Investigation of radial profiles of vibrational and rotational temperatures in nitrogen DC glow discharge

Spatially resolved measurements of vibrational and rotational temperature determined from the N₂(C) nitrogen bands intensities have been performed by means of optical scanner of original construction. It has been found that radial variations of studied bands are independent of pressure and discharge current under our experimental conditions, i.e. in the pressure range (100–300) Pa and for discharge current up to 40 mA. Moreover, it has been found that vibrational as well as rotational temperatures stay almost constant in the radial direction. No radial changes of both temperatures can be explained by good thermal conductivity of the positive column of DC glow discharge. This research was supported by grants: Charles University No. GAUK 194/01, Ministry of Education of Czech Republic MSM 11320002, and Grant Agency of Czech Republic GAČR 202/03/0827. The theme of presented article was included in the EU project No. G1RT-CT-2002-05083 “Plasmatech”.     

Discontinuity walls terminated by twist disclinations in smectic a liquid crystals

A discontinuity wall in smectic A liquid crystals was discussed in M. Kléman:Points, Lines and Walls in Liquid Crystals, Magnetic Systems and Various Ordered Media (J. Wiley, Chichester, 1983). Possible solutions describing a singularity — twist disclination which terminates the discontinuity wall in an infinite model smectic A liquid crystal are discussed. One of solutions proposed can be used to describe the so-called zigzag defect line in chiral smectic C liquid crystals on which opposite oriented chevrons are terminated. This work was supported by Grant No.: 202/96/1687 of the Grant Agency of the Czech Republic.     

A diagrammatic method for analysis of GaAs and InP based optoelectronic devices

A modified version of the signal flow graphs method can be applied to reveal the topological structure of physical models describing the operation of optoelectronic devices based on heterostructures comprising A^IIIB^V semiconductor compounds. In particular, this kind of analysis is apt to reveal the presence of closed paths (feedback loops) in the causal make-up of the phenomena underlying function of the devices. The analytical apparatus associated with the diagrams affords a new formulation of criteria for the occurrence of such physical conditions as the bistability or the threshold behaviour. The approach is illustrated on the instances of injection semiconductor laser, nonlinear Fabry-Perot resonator, self-electro-optic effect device and semiconductor laser optical amplifier. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998 Support of the Grant Agency of the Czech Republic (project No. 102/99/0341) is gratefully acknowledged.     

Time-resolved emission spectra of plasma produced by excimer laser ablation of Pb-Bi-Sr-Ca-Cu-O

Time-resolved emission spectroscopy was employed to detect excited species formed in the laser ablation of superconductor Pb-Bi-Sr-Ca-Cu-O by an excimer laser at various background gas pressures. A simple model has been created to explain the chemical kinetics of reactions in a plasma plume by its expansion. We explained the time dependence of plasma emission on the wavelengths of lead, strontium and their oxides. For the purpose of this paper, we determined reaction rates of lead deexcitation and the rate of recombination of Pb⁺ ion. This work was supported by the Grant Agency of Charles University (Grant No. 28/97) and Grant Agency of Academy of Sciences of the Czech Republic (Grant No. A4040708).     

A contribution to the characterization of laser-hardened steel surfaces

The article deals with the possibility to characterize the laser heat-treatment of steel surface by means of X-ray diffraction and microhardness measurements. For tensometric analysis the X-ray one-tilt method with no reference substance was used. It is shown that hardened surface layers of the carbon steel are affected by compressions reaching in the middle of the laser beam track up to ≈ 350 MPa. The microhardness increased by as much as 350 %. Presented at the 6th Joint Seminar “Development of Materials Science in Research and Education”, Karlštejn, Czech Republic, 17–19 September 1996. This research is a part of the research project supported by Grant Agency of the Czech Republic (Grant No. 106/95/0080).     

Some properties of a-C:N films prepared by KrF laser ablation

Thin a-C:N films were prepared by pulsed laser ablation using a KrF excimer laser and a graphite target. The influence of the substrate temperatureT _s (27°C and 330°C), and the nitrogen gas ambient pressurep n (1.5 Pa, 13.3 Pa and 40 Pa) on the film properties was studied. The films were characterized by scanning electron microscopy, Rutherford backscattering, spectroscopic ellipsometry and by Raman scattering. The nitrogen content in the films was in the range from 20 to 31 at.%, and those of oxygen from 5.6 to 10.7 at.%. For lowerp n andT _s the film delamination was observed.The authors wish to thank J. Sobota from the Institute of Scientific Instruments of ASCR, Brno, for valuable discussions.This work was supported by Grant Agency of the Academy of Sciences of the Czech Republic under Grant No. 202/93/0464.