ALBERT

Notes: The long-term compositional stability of hydrogenated and deuterated amorphous germanium was studied by infrared absorption measurements. The results reveal for substrate temperatures below about 200 °C the growth of a void-rich material which is oxidized rapidly by ambient oxygen and water.

Notes: Single cyrstalline ZnTe layers have been successfully grown on (001) GaAs substrates by hot wall epitaxy at substrate temperatures between 280–370 °C. The vapor phase near the substrate surface was investigated by a quadrupole mass spectrometer. Transmission electron microscopy, reflectivity, and photoluminescence were used for growth optimization, impurity identification, and strain determination. The biaxial inplane strain is about ε(parallel)=−0.5% at the interface and ε(parallel) (320 °C)=−0.06% at the surface for thicknesses of 1–5 μm. Two luminescence peaks can be assigned to As and N.

Notes: We have studied the strain in ZnTe epilayers grown by atmospheric-pressure metalorganic vapor-phase epitaxy on (001) GaAs and GaSb substrates. Reflectivity and absorption measurements are performed at 2 K using single-crystalline layers with thicknesses of 0.2–2 μm. The biaxial strain in the samples caused by the lattice mismatch of layer and substrate is deduced from the splitting of the degenerate heavy- and light-hole exciton. A polariton model is used to describe the reflectivity structure at the E0 gap and to determine the transverse exciton energies. The deformation potentials obtained from an analysis of the absorption structures are a=−5.5 eV and b=−1.4 eV. The critical thickness for ZnTe/GaSb is lower than 0.8 μm near thermodynamic equilibrium. It also slightly depends on growth temperature which has its optimum at 345 °C.

Notes: A brief reply to the Comment is provided. Both the procedure I suggested and the tables presented by Carroll [Textile Res. J. 47, 56 (1977)] allow easy and fast estimation of contact angles.

Notes: Color detectors based on multilayers of amorphous–silicon alloys facilitate the detection of the three fundamental components of visible light in one single pixel of a sensor array. In order to achieve sensitivity for the blue, green, and red components of light, three different bias voltages are applied to the device. By switching them sequentially the detector is read out. n-i-p-i-i-n structures with a controlled band gap and mobility–lifetime product exhibit excellent stationary properties, namely: good color separation and have dynamic behaviors above 95 dB. Besides the stationary behavior the transient response of a color detector is a further optimization criterion. The experimentally found transient photocurrent response after switching on monochromatic light at different applied bias voltages showed reasonable delay times in the range of tens of milliseconds before reaching steady state. Numerical simulations have been carried out which reproduce this characteristic behavior and facilitate a study of time dependent processes within the device, such as charge transport and storage in localized states. The delay times can be explained by the recharging of electrical defect states in the amorphous material. Consequently, the electrical potential within the device changes, which remarkably affects the carrier transport. Based on these results optimization criteria for the transient behavior of the color detectors are discussed. © 1999 American Institute of Physics.

Notes: Investigations of the relationship between changes in climate and the variation of composition of the atmosphere can be performed most directly and with the highest resolution possible by analyzing the air sampled and preserved in the polar ice sheets for hundreds of thousands of years by nature itself. For the determination of CO2, CH4, N2O, and CO in air samples of 1–3 cm3 extracted from ice cores, a high-frequency modulated tunable diode laser absorption spectrometer has been developed. The instrument can measure CO2, CH4, N2O, and CO at ambient mixing ratios of 300 ppmv (1 ppmv=10−6 volume mixing ratio), 1700, 300, and 100 ppbv (1 ppbv=10−9 volume mixing ratio), respectively, with a precision of 1%–2%. The measurement of high CO2 mixing ratios is not constrained by the detection limit. For other gases mixing ratios of 20 ppbv of CH4, 0.9 ppbv of N2O, and 1.6 ppbv of CO are detectable with the instrument in 2 cm3 standard temperature and pressure. These detection limits and the measurement precision are sufficient for the determination of past changes in atmospheric composition. The technique is also suitable for other applications in which several infrared active trace gases have to be determined in the low ppbv and possibly even in the upper pptv range in air samples of a few cm3 or small samples of other matrices. The sensitivity or the reproducibility of the instrument can be improved by another order of magnitude by using more powerful lasers and by averaging over longer periods. Such improvement could result in a capability to detect sub-ppbv mixing ratios in samples of the present size or ppbv measurements in even smaller air samples. The higher reproducibility would also make the technique interesting for measurements of isotope ratios. © 1997 American Institute of Physics.

Notes: The effect of particulate volume fraction vp and diameter dp on the composite Young's modulus Ec is studied both experimentally, using a silica bead/epoxy system, as well as with the help of computer simulations. The experimental and simulation results show that for a given particulate size, the overall Ec vs vp curve displays a concave upward shape and not a linear shape. This superlinear trend of the data implies that the average strain normalized to the applied strain λ=ε¯p/εc transferred to the particulates increases with volume fraction. The above finding is explained in terms of a mean-field picture, where a single particle interacts with an effective medium consisting of the remaining particles embedded in the matrix. As the modulus of the effective medium surrounding a reference particle increases with vp, the modulus mismatch between the reference particulate and the medium is consequently reduced. This leads to an overall increase in the normalized average strain λ transferred to each particulate as vp is increased. The experimental results using silica particulates with various sizes dp, as well as the simulation results, show that smaller particulates provide an increased composite modulus as compared to larger particulates, at constant vp. General equations are developed, which relate the composite modulus to the average particle stress or strain, given only information about the volume fraction and the Young's modulus of each of the phases present.Through the application of these relations, it is found that smaller particulates display a greater amount of normalized average strain λ transferred than larger particulates. The effect of particulate Young's modulus Ep in combination with particulate size on the resulting Ec is also studied using simulations only. It is found that for a low particulate to matrix modulus ratio Ep/Em, the particulate size has very little influence on Ec. Moreover, the shape of the Ec vs vp curve can be well approximated by a straight line up to large values of vp. On the other hand, as the ratio Ep/Em is increased, the superlinear trend of the composite modulus Ec vs vp data is more apparent. This results in a smaller range of the Ec vs vp curve, which can be approximated by a linear function. It is also found that the extent of this linear region also decreases with particle size.

Notes: Wide gap II-VI semiconductors have strong second order susceptibilities χ(2) and are therefore promising materials for efficient second harmonic generation. We have grown high quality single crystalline ZnSe/ZnTe/ZnSe/GaAs (001) waveguides by metalorganic-vapor-phase-epitaxy. Using end-fire coupling we observe a phase matched signal of the 1170.5 nm fundamental wave. The fundamental beam is generated by a tuneable KTP optical-parametric-oscillator pumped by a ps-Ti:sapphire laser system. Phase matching is achieved by coupling the TE0 fundamental mode with the TM2 second harmonic mode within the symmetric ZnTe waveguide. © 1995 American Institute of Physics.

Notes: A new approach is proposed for assessing the stress concentration factors in two-dimensional undirectional composite materials containing broken fibers. A closed-form expression is derived for the stress concentration factor profiles as a function of materials and geometrical parameters. The model differs significantly from earlier schemes as the local effect of a fiber break on nearest neighbors is much milder than previously assumed, both as a function of the interfiber distance (or fiber content) and of the number of adjacent broken fibers. Comparison with experimental results for SiC/epoxy composites demonstrates the validity of the proposed scheme. The model may help shed light on the (multifilament) fragmentation process [H. D. Wagner and A. Eitan, Appl. Phys. Lett. 56, 1965 (1990)] and on fracture nucleation and growth in composites.

Notes: Hydrogenated amorphous silicon based nipiin three color detectors with a bias voltage controlled spectral response have been fabricated. These band-gap and mobility-lifetime product engineered structures employed as two terminal devices exhibit a dynamic range above 95 dB. The maximum of the spectral response shifts by variation of the applied voltage. Three linearly independent spectral response curves can be extracted to generate a red-green-blue signal. Conventional spatial color separation with optical filters is transferred into a voltage multiplexed read out sequence. Bias voltage switching under different monochromatic illumination and illumination switching-on transients for different bias voltages are carried out to investigate the time dependent behavior of the photocurrent. Based on these results optimization criteria to accelerate the transient behavior and to determine the maximum frame rate for color detection are presented. © 1998 American Institute of Physics.