Notizen: In this work, we have numerically integrated in space and time the effective mass Schrödinger equation for an exciton in a semiconductor quantum-well structure. Considering a Coulomb interaction between the electron-hole pair and an external electric field, we have studied the excitonic tunneling escape process from semiconductor quantum wells. Our method of calculation has been applied to types-I, -II, and -III quantum-well superlattices. In addition, we present the calculated excitonic lifetimes for the GaAs/GaAlAs, InAs/GaSb, and HgTe/HgCdTe systems under an external electric field. In the HgTe/CdTe system, the possibility of having similar electron and hole lifetime values is also found if the applied electric field is large enough.

Notizen: A self-consistent two-dimensional (2D) model of large volume inhomogeneously preionized transient high-pressure glows as used in XeCl lasers is described. The basic concept is to use a curvilinear orthogonal coordinate system generated by conformal mapping of a cartesian system. The model uses the local field approximation and is based on a cartesian 2D model of J.-P. Boeuf and L. C. Pitchford [IEEE Trans. Plasma Sci. 19, 286 (1991)]. As only direct ionization is taken into account, applications are limited to the ignition phase. Application to a discharge between cylindrical rods predicts a high degree of E-field homogenization due to space charges and demonstrates the limits of parallel resistor models. Inside the cathode sheath an explosive formation of a very thin highly ionized layer is predicted. This sheath ignition is also seen in experiments.

Notizen: The fractal dimensions of five fractional Brownian motion (fBm) surfaces of 257×257 pixel size, with Hurst exponent H ranging from 0.1 to 0.9, were computed by profile, contour, and surface area analyses. A technique was deemed reliable if it demonstrated accuracy, consistency and sensitivity. Of all the techniques examined, surface area analysis methods, namely, two-dimensional pyramid and Peleg methods, were found to be most reliable and efficient for the data size studied. Hence, these were employed in a preliminary fractal analysis of poly(methyl methacrylate) and poly(styrene) fracture surfaces. The surfaces were imaged at scan sizes ranging from 1–6.5 μm by atomic force microscopy (AFM). The images indicated the existence of fractal structure and a high degree of roughness at microstructural scales for both the surfaces. These observations were supported by the results of the two surface area analysis techniques. A more conclusive study was prevented by severe scoring of the surfaces by the AFM tip at smaller scan sizes and the availability of only a narrow range of scan sizes.

Notizen: We have investigated in detail the influence of interlayer structures on nonalloyed ohmic contact resistance (ρc), in terms of the crystalline defects and the potential barrier at the interlayer/GaAs interface. The interlayer structures are a graded-band-gap InAs/GaAs strained-layer superlattice (graded SLS), a graded-band-gap InGaAs, and conventional SLSs without graded band gaps. A two-layer transmission line model indicates that the barrier resistance in the interlayer highly depends on the interlayer structure: ≤5×10−8 Ω cm2 for the graded SLS and graded InGaAs interlayers and 10−5–10−6 Ω cm2 for the conventional SLS interlayers. To explain the large dependence of the interlayer structure, first, the density and distribution of the misfit dislocations and stacking faults caused by the large lattice mismatch between InAs and GaAs have been investigated in detail by high-resolution transmission electron microscopy. In the graded SLS and conventional SLS interlayers, the influence of the high-density depletion regions spread near the crystalline defects is found to be negligible because of the high doping concentrations (∼1019 cm−3) in the interlayers. Second, the potential barrier at the interlayer/GaAs interface has been investigated by simulating the barrier resistance. The potential barrier profile is calculated self-consistently with Poisson's equation and the Schrödinger equation. Tunneling current through the barrier is analyzed using the Wentzel–Kramers–Brillouin approximation or the numerical wave solution to the Schrödinger equation. The graded SLS interlayer has the effectively smooth conduction band profile without the barriers, which is similar to that of the graded InGaAs interlayer, because of its short period SLS. In the conventional SLS interlayers, the reasonable barrier heights of 0.14–0.26 eV obtained by this simulation indicates that these barriers are the dominant factor which increases the contact resistances. For the low-resistance nonalloyed ohmic contact, therefore, a smooth conduction band profile without band discontinuity is more predominant than the reduction in the crystalline defect density.

Notizen: The fracture characteristics of metal/polymer line structures formed by depositing Au/Cr lines on a semiflexible polyimide, pyromellitic dianhydride-oxydianiline (PMDA-ODA), substrate have been investigated using a stretch deformation technique. The delamination behavior, fracture morphology, fracture energy, and energy dissipation rate have been determined as a function of line width and thickness. The metal dimension was found to influence the crack formation mode and morphology. The experimental studies were supplemented by finite-element analysis to evaluate the stress distribution and deformation energetics of the line structure, which takes into account the plastic deformation of the metal and the polymer. Results from this analysis show that the observed fracture characteristics can be attributed to the edge and thickness effects induced by metal confinement. Essentially, the deformation behavior is determined by the mechanical environment induced by metal confinement at the interface. Plastic deformation of both metal and polymer plays an important role in controlling the stress distributions as well as the deformation energetics. The fracture energy of the metal-polyimide interface determined by an overall energy balance method was consistent with that obtained from energy dissipation rate. The average value is 25 J/m2 for the Au/Cr/PMDA-ODA line structure.

Notizen: In situ resistance measurements, x-ray diffraction, Rutherford backscattering spectrometry, transmission electron microscopy, isothermal and constant heating rate differential scanning calorimetry and Auger electron spectrometry depth profiles have been used to investigate the interactions in copper and magnesium thin films leading to the growth of Cu2Mg and CuMg2 intermetallics. The effect of exposing the reacting interfaces to controlled exposure of oxygen on the nucleation and growth kinetics of such intermetallics was also investigated. It is found that the first phase to form is CuMg2, at about 200–215 °C. It is determined that the formation of CuMg2 occurs by a two step process consisting of nucleation and growth. The nucleation of CuMg2 takes place in a region composed of a Cu/Mg solid solution. The nuclei form at certain preferred sites and grow in directions both parallel and perpendicular to the surface, eventually leading to a continuous CuMg2 layer. The growth of CuMg2 nuclei in the plane of the original interface occurs at a constant rate, whereas the growth in a direction perpendicular to the original interface is found to be diffusion limited. In the presence of excess copper Cu2Mg forms at higher temperatures, with complete conversion to Cu2Mg occurring at about 380 °C. When the Cu surface is dosed with oxygen prior to Mg deposition, ramp rate differential scanning calorimetry (DSC) shows that the nucleation and growth of CuMg2 as well as the growth of Cu2Mg are not disturbed. Dosing the Mg surface with oxygen results in significant changes in the growth of the two phases. In this case a thin MgO layer is formed at the oxygen dosed surface, lateral growth of CuMg2 is unaffected, but vertical growth of CuMg2 across the oxygen dosed interfaces is delayed by 25–30 °C. The growth of Cu2Mg is also shown to be delayed, by 22–54 °C due to the interfacial oxygen dose.

Notizen: Thin-film reactions between nickel and silicon are investigated by differential scanning calorimetry and transmission electron microscopy on evaporated bilayers with different compositions and thicknesses. The thermograms are interpreted by a computer modeling, based on diffusion-controlled growth. Experiments and simulation reveal the simultaneous formation of crystalline Ni2Si and of an amorphous phase of composition probably near NiSi. The derived kinetic data confirm literature values from isothermal experiments.

Notizen: Polar angle distributions of core level photoemission intensities recorded on YbAs/GaAs(001) and ScAs/GaAs(001) heterostructures are presented. They allow first the surface roughness of thin YbAs overlayers to be seen, second to estimate the tetragonal distortion of a strained ScAs film and, third, the most interesting point, to demonstrate in a direct fashion that the mixed (Yb-As) (010) planes of YbAs grow in the prolongation of the As planes of GaAs. The results are compared to those obtained by other authors with various techniques. The main advantage of the photoelectron diffraction method over the other techniques is that it can be performed on very thin epitaxial films (some monolayers) directly in situ under ultrahigh vacuum.

Notizen: We attempt to establish the mathematical expression of the current and the magnetic field in a metallized capacitor. The expression of the impedance of this capacitor is also presented. The distribution of the current is discussed through the variation of the capacitor impedance and compared to experimental ones. There is good agreement between experimental and theoretical results. The analytical expressions of the magnetic field and the current also show a dependence on parameter δ2 akin to the skin depth in conductors. δ2 also depends on frequency. When δ2 is smaller than the outer radius of the capacitor, the current distribution becomes inhomogeneous.

Notizen: High purity InP layers have been grown by chemical beam epitaxy using H2 as the carrier gas for transporting the metal alkyl trimethylindium into the growth chamber. InP layers exhibiting Hall mobility as high as 238 000 cm2/V s at 77 K and with a peak value of 311 000 cm2/V s at 50 K and residual Hall concentration of 6×1013 cm−3 at 77 K were grown at 500 °C using a low V/III ratio (2.2) and a phosphine (PH3) cracking cell temperature of 950 °C. The 4.2 K photoluminescence spectra were dominated by donor bound exciton (D0,X)n up to n=6 and free exciton (X) transitions for InP layers grown above 500 °C. All the InP samples exhibited very weak acceptor related photoluminescence transitions indicating very low concentration of acceptors. The energy of these transitions suggests that Mg is the major residual acceptor. Donor impurity identification by high resolution magnetophotoluminescence indicated that S and Si are the major impurities. PH3 has been found to be the major source of S impurities in the present study.