ALBERT

Notizen: High-temperature charge transport across an oxide-nitride-oxide sandwich of erasable programmable read only memories is mainly governed by the oxide conductivity as experimentally determined. It was verified in the examined devices that charge loss is not due to mobile ions. Since hole injection from the control gate into the nitride can be blocked by a 70-A(ring)-thick top oxide we conjecture that charge loss is due to leakage of electrons; however, the observed leakage current is too large to be explained by pure electrode-limited charge transport (Richardson emission and direct tunneling). It was also verified that field gain on asperities and along edges cannot increase the charge loss current to the required range. Numerical evaluation of trap tunneling and resonant tunneling indicated that both mechanisms are weakly temperature dependent while charge loss has a typical activation energy of 1.2 eV in the range of 250–350 °C. Consequently, a multiphonon-assisted tunneling mechanism is proposed where electrons stored on the floating gate tunnel to oxide traps, then are emitted into the nitride. The coupling of the trap level to oxide phonons results in virtual energy levels in the oxide which allow for more effective transition paths. As a consequence of the electron-phonon coupling, the emission occurs close to the oxide conduction-band edge at temperatures between 250 and 350 °C, producing a strong temperature dependence for the mechanism. © 1995 American Institute of Physics.

Notizen: 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.

Notizen: 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.

Notizen: 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.

Notizen: Direct and Fowler-Nordheim tunneling through ultra-thin gate dielectrics is modeled based on an approach for the transmission coefficient (TC) of a potential barrier that is modified by the image force. Under the constraint of equal actions the true barrier is mapped to a trapezoidal pseudobarrier resulting in a TC very close to the numerical solution of the Schrödinger equation for all insulator thicknesses and for all energies of the tunneling electron. The barrier height of the pseudopotential is used as a free parameter and becomes a function of energy in balancing the actions. This function can be approximated by a parabolic relation which makes the TC of arbitrary barriers fully analytical with little loss of accuracy. The model was implemented into a multidimensional device simulator and applied to the self-consistent simulation of gate currents in metal-oxide-semiconductor (MOS) capacitors with gate oxides in the thickness range 15 Å–42 Å. Excellent agreement with experimental data was obtained using a thickness-independent tunnel mass mox=0.42 m0. Thanks to the CPU-time efficiency of the method the simulation of a complete MOS-field-effect-transistor with dominating gate current becomes possible and shows the potential for further applications. © 1997 American Institute of Physics.

Notizen: 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.

Notizen: 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.