ALBERT

Notes: Anisotropic optical, electrical, and structural properties have been observed in thick InxGa1−xAs/GaAs single heterostructures grown by low-pressure metal-organic vapor phase epitaxy on (001) and slightly misoriented GaAs substrates. The luminescence of the (001) samples is polarized and the electron mobility is higher along one of the 〈110〉 directions. Asymmetric distributions of surface ridges and misfit dislocations have been observed in secondary electron micrographs (SEMS) of the surfaces and in transmission electron micrographs. Strong anisotropy and a clear correlation between the anisotropic optical, electrical, and structural properties have been observed for samples grown on (001) oriented substrates. For (001) samples, the results indicate that the anisotropic properties are induced by nonuniform strain relaxation. For samples grown on misoriented substrates, SEM and micro-Raman spectroscopy indicate a better crystal quality. The anisotropic properties are found to be reversed and the degree of anisotropy is significantly reduced. The results suggest the improvement of interfacial coherency owing to the mechanism of nucleation on a substrate presenting a series of steps and ledges. The reduction of defects within the epilayer can be partly due to a faster strain relief in vicinal (001) epilayers.

Notes: Defect interactions among oxygen, carbon, and nitrogen during a three-step high-low-high intrinsic gettering process (1100 °C+750 °C+1000 °C) were studied using differential analysis of Fourier transform infrared (FTIR) absorption spectra of Czochralski-Si crystals intentionally doped with carbon and nitrogen. Two phenomena related to nitrogen were observed: (1) Nitrogen-enhanced oxide nucleation in the temperature range from 750 to 1100 °C, thus having a negative effect on the denuding step; and (2) defect interaction associated with carbon during oxygen precipitation was significantly affected by the co-existence of nitrogen. For the latter effect the first-step annealing at 1100 °C was crucial. The results presented are consistently interpreted assuming that nitrogen participates in creation of nucleation sites for heterogeneous oxygen precipitation, and also assuming that nitrogen aggregates at the strain regions surrounding precipitates and thus retards Si self-interstitial migration. This explanation was supported by observed differences in oxygen-precipitate-related FTIR absorption and by the behavior of stress-related photoluminescence D lines.

Notes: A systematic Fourier-transformed infrared-spectroscopy study of oxygen and carbon in isolated form and as complexes in the silicon lattice has revealed a direct correlation between the decrease of substitutional carbon concentration and the decrease of interstitial oxygen concentration during 750 °C annealing. At a concentration exceeding 2 ppma, carbon was also found to enhance oxide precipitate growth. After completing a three-step annealing (1100 °C+750 °C+1000 °C), an oxide-precipitate-related IR-absorption band was observed. The changes of the IR-absorption band were correlated with annealing-induced changes in the state of carbon. A direct incorporation of carbon into oxide precipitates, and/or carbon interaction with silicon self-interstitials generated during oxygen precipitation, are suggested to have an effect on reducing lattice strain associated with the oxygen-precipitation process.

Notes: The vibrational and electronic structures of p-phenylenediamine radical cation have been studied using time-resolved resonance Raman spectroscopy and molecular orbital calculations. The Raman spectra in aqueous solution show striking variations in intensity profile when excitation is varied from 340 to 480 nm. Excitation in resonance with the 460–480 nm absorption shows enhancement of only totally symmetric vibrations. Most prominent are the δNH2 scissor mode at 1658 cm−1 and the Wilson modes ν8a (CC stretch) at 1644 cm−1, ν7a (CN stretch) at 1423 cm−1, and ν9a (CH bend) at 1184 cm−1. Also observed are ν1 (ring breathe) at 840 cm−1 and ν6a (CCC bend) at 467 cm−1. With excitation in the 340–410 nm region the ν1 band becomes relatively stronger and an additional band at 1524 cm−1 appears. This latter band, which dominates the spectrum at 360 nm, is assigned to the nontotally symmetric vibration ν8b (CC stretch) having b3g symmetry that gains intensity through vibronic coupling. Raman spectra of ring- and amine-deuterated radicals allow distinction of overtone and combination bands enhanced by Fermi resonance from fundamentals due to δNH2 and ν8a (CC stretch) modes in the 1600–1700 cm−1 region. The theoretical calculations show reasonable agreement with the observed vibrational frequencies and lead to detailed information on the bond structure and normal modes of the radical. The calculated CN bond lengths of 1.33 A(ring) are intermediate between lengths typical of single and double bonds in aromatic amines. The calculations also allow interpretation of the experimental absorption spectrum. The broad medium intensity absorption peaked at 460 nm with a shoulder at 480 nm is assigned to a 2B3u ←2B2g transition, the very weak 360 nm absorption to 2Au ← 2B2g and the very strong ∼325 nm absorption to a higher 2B3u ← 2B2g.Calculations with the simple relation Ik∝ (V'k)2/ωk, which requires evaluation only of vertical excited state gradients, are found to satisfactorily describe the dramatic changes in Raman intensity profile that are observed with different excitation wavelengths. The 460 and 480 nm Raman spectra are governed by scattering from the lowest 2B3u state. Spectra taken in the 410–340 nm range are governed mainly by preresonance scattering from the higher 2B3u state, which is vibronically coupled to the weak 2Au state via the b3g symmetry ν8b (CC stretch) vibration. The relative intensities of totally symmetric modes calculated for excitation into the two 2B3u excited states agree well with those in the two distinct patterns observed experimentally.

Notes: New experimental rate coefficients are reported for the H2(D2)+CN→H(D)CN+H(D) reactions over the temperature range 209 to 740 K for H2 and 250 to 740 K for D2. Previous reduced dimensionality reaction probabilities for the reaction with H2, and new ones for the reaction with D2 are used to obtain analogous rate coefficients. In addition, reaction probabilities and rate coefficients for vibrationally excited reactants H2(v=1), D2(v=1), or CN(v=1) are presented. Comparisons of the calculated rate coefficients are made with the new and previous experiments, especially those of Sims and Smith [Chem. Phys. Lett. 149, 565 (1988)].

Notes: A study is described of the impact process and the corresponding surface damage to PMMA (polymethylmethacrylate) targets impacted by ice and nylon spheres. A gas gun system was used to project the spheres and the impacts were recorded using a high-speed image converter camera (Imacon 792). Special attention was paid to the conditions causing projectile failure and the surface damage to the target. The experimental results show that although the deformation and failure modes for ice and nylon are different, the surface damage to the PMMA target has a similar failure pattern, i.e., a central, circular undamaged region surrounded by an annular damaged region containing many short circumferential cracks. Analysis shows that the diameter d of the central undamaged region and the annular damaged area Am can be used to characterize the surface damage which depends on the projectile material, sphere radius R, as well as the impact velocity V0. For a given projectile, two critical impact velocities exist: One is the threshold velocity (V0)th, below which no visible surface damage is observed, and another is the fracture velocity (V0)f, above which a plate target is broken. Damage in the annular region is caused by the Rayleigh surface wave in the present experiments. The conditions for the Rayleigh surface wave to pass ahead of the expanding edge contact velocity are analyzed. Comparisons between the theoretical predictions and experimental results, including the present study and earlier data, are made for PMMA plates impacted by projectiles of different materials (ice, nylon spheres, and water drops) and at different impact angles. © 1995 American Institute of Physics.

Notes: The effects of substrate misorientation on anisotropic electrical properties of thick InGaAs layers grown on GaAs by low-pressure metalorganic vapor phase epitaxy are studied. Hall bar samples oriented along [110] and [1¯10] were cut from layers grown simultaneously on substrates oriented (a) exactly on (001) and (b) at 2° off (001) towards [011]. In layers grown on (001) oriented substrates a very large mobility anisotropy (μ[110]/μ[1¯10]≈ 1000) is observed and attributed mainly to a lattice mismatch induced anisotropy. For the misoriented substrates, the average electron mobility increases by an order of magnitude and shows only a small residual anisotropy in the opposite direction (μ[110]/μ[1¯10] ≈ 0.7).

Notes: From the capacitance–voltage dependence of Pt/PbZr0.5Ti0.5O3/Pt capacitors, we mathematically separated the capacitance into two parts corresponding to two regions of the samples, the uniform electric-field region and the nonuniform region. They are correlated with the bulk ferroelectric film region and the Pt/PbZr0.5Ti0.5O3 Schottky barrier interface region, respectively. The calculations based on the in-series capacitor model show a slight decrease of dielectric permitivity for the fatigued bulk films. By assuming a much smaller dielectric permitivity of the interface region than that of bulk films, it was found that the interface capacitance decreased remarkably compared with that of the bulk ferroelectric film after fatigue. This decrease was attributed to the lowering of ferroelectricity in the interface layer, which suggests that the fatigue is mainly an interface state controlled process. The asymmetricity in the interface capacitance–voltage curve is attributed to the different defect concentration levels in the top and bottom interface regions. © 2001 American Institute of Physics.