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).     

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).     

Spectroscopy of Ce3+-doped and Ce3+-concentrated scintillators

Institute of Physics, Czech Academy of Sciences, Cukrovarnicka 10, 16200 Prague 6, Czech Republic. Published in Zhurnal Prikladnoi Spektroskopii, Vo. 62, No. 5, pp. 71–77, September–October, 1995.     

Space-charge-limited transient photoconductivity in polymers

Space-charge-limited (SCL) photoconductivity in transient regime and its modification, T-SCLC (transient space-charge-limited currents), are readily observed in high resistivity materials with mobile carriers and thus may be with advantage used as a tool for the study of the density of localized states (DOS) in organic molecular semiconductors. The only prerequisite for the experimental observation of SCL photoconductivity is a reasonably long Maxwell relaxation time. This paper presents the new possibilities of the T-SCLC method for elucidation of the DOS in organic molecular solids with practical examples of modeling, reconstruction of post-transit data and its application for a typical polymer — poly(methylphenylsilyene). Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998. The support of the Grant Agency of the Academy of Sciences of the Czech Republic by the grant No. A 1050901 and the COST 518 project are acknowledged.     

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.     

Effect of configurational averaging on observables

Effect of the distribution of the structural parameters on the time evolution of the fluorescence intensity is discussed. The influence of this distribution in two limit regimes of the excitation energy transfer — coherent and incoherent one are compared. It is shown that in many cases the fluorescence intensity measurements do not enable to decide between the two regimes. Presented at the Czech-Israeli-German Symposium “Dynamical Processes in Condensed Molecular Systems”, Prague, Czech Republic, 26–30 May 1997.     

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.     

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).     

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.     

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.     

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).     

Er-doped twin-core fibre coupler as a saturable-absorber-based narrow-band filter for fibre lasers

We have proposed and demonstrated experimentally that an Er-doped twin-core fibre (ErTCF) can be employed as a tracking narrow-band filter in fibre ring lasers for line narrowing and enhancement of the spectral stability. The device mimics a saturable absorber with inhomogeneous broadening. The slow erbium ion saturation dynamics in the ErTCF prevents wavelength hopping whilst it enables the filter to track the operation wavelength on its tuning. We have improved the device operation by utilizing the inherent filtering effect of a twin-core fibre coupler. Presented at the 1st Czech-Chinesse Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998 This research has been in part supported by the Commission of the European Union in the frame of Copernicus programme, Project # 7882 and by the Ministry of Education of the Czech Republic under Contract OC 241.10/1998.     

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.     

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.     

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.     

First-passage time distributions for linear chain segments with general site-dependent waiting-time distributions

First-passage time distributions (FPTD) of the arrival of a walk at a terminal site of a finite linear chain are considered, where the individual transition processes between the sites are specified by rather general waiting-time distributions. An exact expression for the first-passage time distribution can be given in Laplace space in terms of finite continued fractions. The circumstances under which the calculation of FPTD by this technique offers advantages over conventional calculations via master equations are discussed and possible applications to systems of biophysical interest are indicated. Presented at the Czech-Israeli-German Symposium “Dynamical Processes in Condensed Molecular Systems”, Prague, Czech Republic, 26–30 May 1997.     

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.     

Preparation and properties of Er and Yb doped InP-based semiconductor compounds

In this paper we pursue the effect of the erbium and ytterium addition during the liquid phase epitaxial (LPE) growth on physical properties of thin InP layers. Series of InP layer samples were prepared by LPE from the melts containing 0–0.3 wt.% of Er and Yb. The grown layers were examined by the Hall effect andC-V measurements, photoluminescence spectroscopy and Rutherford backscattering spectrometry (RBS). We have found that only Yb impurity was incorporated into the lattice of InP layer. With increasing Yb content in the growth melt the layer’s conductivity smoothly changed from n to p type when Yb admixture exceeded certain limit. On the basis of these results we prepared p-n junctions in the InP layers directly doped by Yb, and tested then byC-V measurements. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998. This work has been supported by the Grant Agency of the Czech Republic, project No. 102/99/0341.