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).     

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).     

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.     

Transient binary nucleation from ethanol-hexanol vapour

We study condensation of ethanol-hexanol vapour by numerical solution of kinetic equations. The number of droplets formed in unit volume is computed within self-consistent classical model. It is shown that formation of ethanol-rich droplets prevails at the initial stage of nucleation process, but in the stationary state formation of droplets near the saddle point (on cluster formation energy surface) plays the dominant role. Presented at the 6th Joint Seminar “Development of Materials Science in Research and Education”, Karlštejn, Czech Republic, 17–19 September 1996. This work was supported by Grant No. A1010615 of the Grant Agency of the Academy of Sciences of the Czech Republic.     

Modeling of the surface nucleation and growth on active centers from the vapor phase

The formation of nuclei of a new phase and their growth on the inhomogeneous substrate from a vapor phase are studied. Basic kinetic equations describing such a process are solved numerically. The effect of the depletion of active sites during the growth of nuclei is taken into account. Basic characteristics of the nucleation process, such as size distribution function, nucleation rate and number of nuclei formed on the unit surface are determined. It is shown that the size distribution of nuclei evolves by a nontrivial way as a function of time. This process is fully nonstationary from the viewpoint of nucleation rate. The total number of nuclei reaches the number of active centers for a sufficiently long time. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. This work was supported by Grant No. 202/99/0403 of the Grant Agency of the Czech Republic.     

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.     

Photo-induced charge transfer in poly(methyl-phenylsilylene)

Photoconductivity in rigid polymers is based on complex processes including photo-induced electron transfer, formation of ion pairs and their dissociation in an external electric field. Thus, the charge carrier photogeneration in macromolecules differs largely from the direct band-gap excitation and the formation of free electrons and holes in inorganic materials. The processes mentioned above were studied on a catena silicon polymer, namely poly(methyl-phenylsilylene). Presented at the Czech-Israeli-German Symposium “Dynamical Processes in Condensed Molecular Systems”, Prague, Czech Republic, 26–30 May 1997. This work was supported in part by the Grant Agency of the Academy of Sciences of the Czech Republic (Grant No. 45007). Additional support was provided in the framework of the Bilateral Scientific-Technical Cooperation between the Czech Republic and Federal Republic of Germany, project Photo/Electro-Respondent Materials (KONTAKT, No. ME 087 and FKZ-Nr. TSR-005-95).     

Model of free charge generation in polymers

A time solution of the excimer-ion pair system is presented here with a special attention to the time asymptote. Presented at the Czech-Israeli-German Symposium “Dynamical Processes in Condensed Molecular Systems”, Prague, Czech Republic, 26–30 May 1997. This work was supported by the grant No. C 1050601 of the Grant Agency of the Academy of Sciences of the Czech Republic.     

Testing of resonant tunneling double barrier heterostructures by BEEM/BEES

This work shows how ballistic electron emission microscopy and spectroscopy (BEEM/BEES) can be used for study of the tunneling double barrier heterostructures. From positions of resonant levels follows that roughness at the interfaces is present. The variation of the thickness of the well atributed to the existed roughness is estimated using simple calculation. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998. This work is partly supported by the Grant Agency of the Czech Republic under grant No. 102/97/0427.     

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.     

CVD diamond films for electronics

Mechanical, optical, thermal and electronic properties of diamond films and chemical vapour deposition (CVD) techniques are briefly reviewed. Some spectroscopical methods for the characterization of CVD diamond films are described (optical absorption, photothermal deflection spectroscopy, Raman spectroscopy and electron paramagnetic resonance), together with our recent results on investigation of electronic properties of the main defects in this material, relevant to the application of CVD diamond films for the future electronic devices. Presented at the 6th Joint Seminar “Development of Materials Science in Research and Education”, Karlštejn, Czech Republic, 17–19 September 1996. This work was supported by Grant Agency of the Czech Republic, Grant No. 202/06/0446 and by the NATO Scientific Exchange Programme, HTECH.LG 940890 and NFWO (National Fonds voor Wetenschappelijk Orderzoek, Brussel) Project G.0014.96.     

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”.     

Kinetic Monte Carlo simulations of Si/Si(111)7×7 homoepitaxy

Kinetic processes that influence epitaxial growth of thin films are being more and more explored. Recent observations of growth on reconstructed surfaces suggested that material attachment to existing islands may be hindered by a barrier to attachment. We present a kinetic Monte Carlo model of Si/Si(111)7×7 homoepitaxy with the barrier to attachment implemented. We show that this model successfully reproduces available experimental results obtained during initial stages of growth and during surface relaxation. We show that the high scaling exponents experimentally observed are due to specific growth kinetics of the Si/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 the Czech Republic, project GAČR 202/97/1109 and by the Volkswagen Stiftung.     

Laser/sol–gel synthesis: a novel method for depositing nanostructured TiN coatings in non-vacuum conditions

In this work results of experiments on the in situ production of titanium nitride by the reaction of titania sol–gel with a nitrogenous admixture under laser irradiation are reported. A diode laser beam at different powers and traverse speeds was applied to the mixture placed on EN43 mild steel and 316L stainless steel substrates. Composite coatings of titanium nitride and titanium oxide with a hardness of 17–21 GPa have been achieved by this new method. Surface morphology and microstructure of the deposited coatings and substrate surface layers were examined using optical microscopy, scanning electron microscopy, and field-emission gun scanning electron microscopy. Chemical composition was determined by energy-dispersive X-ray analysis. The phases were identified by X-ray diffraction. Results of microhardness and nanohardness at the top surface were evaluated. PACS 81.15.Fg; 81.20.Fw; 81.05.-t     

On the chaining dynamics of inclusions in SmC* free standing films

A simplified model of an inclusion represented by (+1)-wedge disclination and an accompanying hyperbolic defect ((−1)-wedge disclination) in smectic C* free standing films is used to describe the early stage of the ordering process of inclusions into chains. The elastic interaction between inclusions and their associated hyperbolic defects is used to discuss the dynamics observed experimentally during the inclusion chaining when inclusions are at distances much larger than their radii. This work was also supported by Grant No. 202/02/0840 of the Grant Agency of the Czech Republic and by the research project AV0Z1-010-914.     

Effect of growth conditions on InAs nanoislands formation on Si(100) surface

In this paper we have studied the island formation during InAs/Si(100) three dimensional (3D) heteroepitaxial growth using RHEED, SEM and TEM methods. We have found the strong influence of the growth conditions on the surface morphology. Both kinetic and energetic parameters play an important role during the growth in InAs/Si system. Dislocation-free InAs islands are formed at the Si(100) when lateral size is less then 5 nm. Presented at VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. This work was partially supported by INTAS Grant No 96-0242, RFBR Grant No 98-02-18317 and Grant No 99-02-16799, Scientific Programmes “Physics of Solid State nanostructures” Grant No 98-2029 and “Methods and Devices of Micro- and Nanoelectronics” Grant No 02.04.5.1.40.E.46. G.E. Cirlin and B.V. Volovik are grateful to the INTAS Young Scientists Fellowship (Grant No 98-54 and No 98-41, respectively).     

Effect of holmium addition during LPE growth on the properties of InP and GaInAsP layers

The influence of holmium addition during the liquid phase epitaxial growth (LPE) on the optoelectronic properties of InP and GaInAsP semiconductor layers has been studied. Series of InP and GaInAsP layer samples have been prepared by LPE from the melt containing 0–0.5 wt.% of the Ho. The samples were examined by different diagnostic techniques. Hall effect, capacitance-voltage curves and photoluminescence spectra showed the dramatic impact of the Ho admixture on the carrier and residual donor concentrations. The carrier density was reduced by two-and-a-half orders of magnitude and photoluminescence spectra became markedly narrowed and their fine features were resolved. The observed effects are attributed to holmium acting as a very efficient gettering agent with regard to shallow donors. Presented at the 6th Joint Seminar “Development of Materials Science in Research and Education”, Karlštejn, Czech Republic, 17–19 September 1996. This work has been supported by the Grant Agency of the Czech Republic, project No. 102/96/1238.