ALBERT

Notes: An averaging procedure is applied to inelastic acoustic–phonon scattering which leads to lattice-temperature-dependent constants for the phonon energy and the square of the phonon wave vector. The resulting scattering rate depends on energy only thus facilitating the search of after-scattering states in full-band Monte Carlo simulations. The model still accurately reproduces the velocity–field characteristics over a wide range of lattice temperatures, but in silicon the hot-hole tail of the energy distribution is strongly enhanced compared with the elastic equipartition approximation. © 2001 American Institute of Physics.

Notes: The tunnel-assisted recombination current of a thin GaAs pn junction delta doped with Ti deep-level defects is modeled by 1D device simulation using a generalized Shockley–Read–Hall rate. The tunneling between band states and multiphonon sublevels of the recombination center, and the Poole–Frenkel effect are studied, varying the position of the delta–doped sheet and the junction width. Both field effects decisively influence the calculated current-voltage characteristics.

Notes: Coherent anti-Stokes–Raman scattering (CARS)-excitation profiles (CEPs) of the fundamental vibrational mode of I2 dissolved in cyclohexane and dibromomethane are determined with excitation wave numbers ranging from 17 000 to 23 000 cm−1. The experiments were performed at room temperature and with laser beams polarized parallel to each other. Calculations were performed for both the absorption spectrum and the CEPs of iodine in both solvents and compared with experimental results. Good qualitative agreement of the major features of the CEPs was found using gas phase potentials for the excited states 1Π1u and 3Π0+u and the ground state 1Σ+g. The excited state potential functions were slightly shifted to match the measured absorption spectra. © 1996 American Institute of Physics.

Notes: The vibrational infrared and Raman spectra of a series of t-butyl substituted oligomers of para-phenylenevinylene and of the corresponding polymer in the pristine and potassium-doped states have been recorded for the first time and interpreted on the basis of the theory of the effective conjugation coordinate. The Raman spectra of selectively deuteriated materials have also been analyzed. Evidence is found of strong phonon confinement. The information derived from the spectra is that the charge carrier is confined over a few monomer units and that when the polymer is doped with electron donors the benzene ring is preferentially perturbed. The data collected in this work are compared with the results from UV spectroscopy and ESR experiments which indicate a larger dimension of the charge carrier. The information which can be derived by using in a complementary way the data from these spectroscopies are presented and discussed. © 1996 American Institute of Physics.

Notes: Benzene and (1,4)-dimethyl-cyclohexane monolayers were physisorbed on graphite covered Pt(111) surfaces. Exposure of benzene monolayers at 125 K to D atoms (1700 K) initially hydrogenates sp2 hybridized C atoms with a cross section of ca. 8 A(ring)2 producing C6H6D intermediates. Additional D atom reactions either transform this intermediate via a second hydrogenation reaction to cyclohexadiene-d2, C6H6D2, or restore benzene, C6H5D, via H abstraction. Once the aromaticity is broken, successive hydrogenation of the diene occurs rapidly generating the saturated cyclohexane-d6, C6H6D6. The C6H5D reaction product can undergo further H/D exchange reactions and, at any level of deuteration, the benzene species might get hydrogenated. Monolayers of the saturated hydrocarbon (1,4)-dimethyl-cyclohexane (DMCH) that are exposed to D atoms produce deuterated DMCH via successive abstraction/hydrogenation reactions. Thermal desorption mass spectra revealed that H atoms at the ring were exchanged with an apparent cross section of 1.7 A(ring)2. Methyl groups H atoms were exchanged much more slowly than ring H atoms. It was also observed that D exposed molecules/radicals exhibit a tendency to desorb from the surface, which is ascribed to the exothermicity of the reactions which lead to these species. © 1996 American Institute of Physics.

Notes: An analytical bulk mobility model for hydrodynamic transport equations is developed from a microscopic level and designed for silicon device simulation. Applying Kohler's variational method extended to the regime of nonlinear transport yields the general expression for the mobility as function of carrier temperature, lattice temperature, and doping. Assuming a nonparabolic, isotropic band model and a heated Maxwellian allows for the analytical calculation of the collision integrals. A nonelastic approximation for intravalley acoustic-phonon scattering is proposed, which improves the model in the low-temperature range. Intervalley scattering is treated in a one-mode, equipartition approximation. Here, an accurate analytical approach for all carrier temperatures is derived. For impurity scattering the Brooks-Herring theory is used including Fermi statistics and the effect of dispersive screening. The influence of other effects like anisotropic valleys and perturbation of the density of states by heavy doping are discussed quantitatively. Despite the oversimplified band structure, all essential features of the measured mobility in silicon can be reproduced except in the heavy doping range. The adjusted deformation potentials coincide with estimated sums of the corresponding sets used in full-band Monte Carlo simulation. The method has the potential of an extension to the Si−SiO2 system. © 1996 American Institute of Physics.

Notes: In zinc-blende II–VI alloys the thermal-expansion coefficient for low temperatures is negative and becomes positive at higher temperatures. We investigated the luminescence properties of molecular-beam-epitaxy-grown (CdMnMg)Te layers in the temperature range from 2 up to 200 K and show that the anomalous temperature dependence of the lattice constant is reflected in the luminescence properties of the excitonic recombination and the internal transition of manganese (ITM). The temperature behavior of the ITM energy is nonmonotonic and the existence of a minimum in the photon energy (at the temperature TMn) can be correlated to the change of sign of the thermal-expansion coefficient. The decay constants of the ITM begins to decrease drastically at TMn, too. Considering a lattice constant dependent energy transfer rate to the infrared emitting state (1.2 eV) of the manganese ion the variation of the lifetimes can also be explained by the temperature dependence of the lattice constant. Furthermore, we have measured the ITM in bulk layers and observed a different dependence of the ITM properties on the temperature, demonstrating the influence of the growing conditions on the microscopic surrounding of the manganese ions. © 1996 American Institute of Physics.

Notes: Several monolayers thick hydrogenated carbon films, C:H, were prepared by ion beam deposition from hydrocarbon process gases onto Pt and monolayer C covered Pt single crystal surfaces and investigated with Auger electron and thermal desorption spectroscopies in an UHV environment. Efficient deposition was achieved at ion energies in the 160–300 eV range. The deposited thickness and H/C ratio of the films depend on both, target temperature and H/C ratio of the process gas. It is shown that the C monolayer is crucial for efficient on-top deposition. Irrespective of the process gas used for deposition, the films grow as a C network and assume a constant H/C ratio at thicknesses greater than ∼ 3 monolayers. The H/C ratio of the films scale with the H content of the hydrocarbon process gas, a H/C ratio of 0.4 was obtained for ethane at 350 K substrate temperature. Upon thermally activated decomposition the films release molecular hydrogen as the major gaseous species and various hydrocarbons as minority species. The latter products signal chemical erosion of the film. It is shown that the rate determining step towards erosion via methane is a C–C bond breaking event which releases methyl radicals from the C network in the film. The activation energies for this step are determined as a 10 kcal/mol wide Gaussian distribution centered at 56 kcal/mol. Transport through the film is found to be so fast that it does not contribute to the observed gas release rates. © 1995 American Institute of Physics.

Notes: Few monolayers thick hydrogenated carbon films doped with boron (C/B:H) were prepared and investigated in an ultrahigh-vacuum environment by high-resolution electron energy loss, Auger electron, electron energy loss, and thermal desorption/decomposition spectroscopies with specific emphasis on their chemical erosion behavior as compared to their undoped C:H counterparts. Films of thicknesses ranging from 1 to about 10 monolayers, with a maximum B/C ratio of 0.5, were grown by ion-beam deposition at room temperature on a carrier consisting of a Pt(100) single-crystal surface covered with a graphite monolayer. The process gas used was a mixture of ethane and trimethylboron of varied compositions. While at zero boron concentration the films exhibit a graphiticlike structure with about equal amounts of carbon atoms in the sp2 and sp3 hybridization state, with increasing boron concentration the film structure becomes increasingly sp3 dominated. This is evidenced by decreasing HREELS loss intensities of the vibrational modes related to graphitic hydrogenated carbon, i.e., C=C, sp2 CH stretches, and aromatic CH deformations, but enhanced C–C and sp3 CHn stretch mode intensities. No BH vibrational modes have been observed at any doping level. In accordance with these observations, the C-272 eV Auger peak line shape underwent a change characteristic for a sp3-dominated network upon B doping. The π-plasmon energy was found to shift toward lower energies at C/B:H films which also is in line with a decrease of the carbon sp2 concentration, giving further support for a change to a less graphitic structure. The observed enhanced capacity for hydrogen in the films was found to correlate in a linear fashion with the increase of the fraction of carbon atoms in the sp3 configuration. The relative hydrogen content of the films, H/C, starting at 0.4 at zero boron content, was observed to increase, saturating at 0.75 for boron concentrations greater than 10%.This in turn coincides with a substantial growth of the film hydrogen capacity, as judged from the amount of H2 desorbing from the films between 500 and 1100 K upon thermal decomposition of the films. Although hydrogen originating from sp3 CH groups increased significantly, the amount of chemically eroded species, monitored by CnHm production in the thermal decomposition spectra, was unaffected by boron doping. However, the desorption maxima for either species, hydrogen and hydrocarbons, shift to lower temperatures at boron doped films. As reasons for the effect of B doping on the chemical constitution of C/B:H films and the resulting chemical erosion behavior, the capability of B to block the formation of aromatic structures is proposed. © 1995 American Institute of Physics.

Notes: We calculated the steady-state recombination rate for two coupled defect levels and implemented the model into a device simulator. This model generalizes the familiar single-level Shockley–Read–Hall (SRH) formula. If the intercenter transition probability vanishes, it reduces to the sum of two individual SRH rates, which are only linked via the band occupancies. The cases, where one of the levels or even both behave like traps in carrier capture, and the case of a rate-limiting intercenter transition are derived from the general expression. The important feature of the model is a possible increased field effect which might lead to large excess currents. The field effect is discussed in terms of tunnel-assisted multiphonon capture or direct tunneling into the levels, respectively. We show by means of numerical simulation that the large ideality factors found for liquid phase epitaxy grown diodes with weak intrinsic fields can be the result of a rapid direct charge transfer between donors and acceptors and the high probability of tunneling into the hydrogenic states. © 1995 American Institute of Physics.