摘要:

Using a new ultrasonic method which relies on the measurement of the in‐phase and quadrature components of a continuous ultrasonic excitation as a function of position, we have measured the flexural modulus of composition modulated Cu‐Ni self‐supporting thin films. For some of the films, we have also observed the shear (Sz) modulus (allowing the calculation of the biaxial and Young’s moduli). No enhancement of these moduli was observed for composition wavelengths in the range of 17–40 A˚.

ISSN:0003-6951    

DOI:10.1063/1.103034

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Stress‐corrosion cracking—the phenomonon in which the initiation and propagation of cracks is enhanced by a chemically active environment—has previously not been observed in silicon. For example, extensive experiments have shown no effect of water on the fracture properties. However, using indentation cracks in the presence of a HF etch, we have been able to show stress‐corrosion cracking in silicon for the first time. This is attributed to the initial removal of the native silica layer, and the subsequent lowering of the fracture resistance by passivation of the crack surfaces.

ISSN:0003-6951    

DOI:10.1063/1.103035

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The fiber fragmentation phenomenon in the single‐filament composite test is currently among the most intensively researched aspects of composite micromechanics. By this method the interfacial shear strength, a physical parameter which reflects the quality of the fiber/matrix bonding, is calculated. In most studies various statistical models for the distribution of fragment lengths have been assumed without any justification other than a good fit to experimental data. Here we argue that if the flaws are assumed to be distributed along the fiber length according to a spatial Poisson process, then far from the saturation point the resulting fragment lengths must exactly follow a shifted exponential distribution. At the saturation limit, the distribution of fragment lengths is still approximately exponential. This is confirmed by single‐filament composite experiments with high strength carbon fibers embedded in epoxy. Cumulative distributions of interdefect spacings at various levels of stress (or strain) are made available by means of an experimental technique described here. It is shown that the resulting data may be used to unfold the average strength versus length dependence in the single fiber, and to calculate the relevant Weibull shape parameter.

ISSN:0003-6951    

DOI:10.1063/1.103012

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Clusters of uniform‐sized oriented cubic nucleations (OCNs) about 1–2 &mgr;m and local epitaxy crystals (LECs) of 120×150 &mgr;m2on silicon {100} have been obtained by pretreating the substrateinsitufollowed by a typical diamond growth process with a microwave plasma‐assisted chemical vapor deposition approach. The observations made under the scanning electron microscope reveal that the OCNs are not only oriented to one another, but also to the highly faceted LECs at an angle 45° off of the silicon substrate cleavage plane. The x‐ray diffraction data show {400} dominant diamond.

ISSN:0003-6951    

DOI:10.1063/1.103013

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A new discrete Monte Carlo technique suitable for simulations of the kinetics of heteroepitaxial crystal growth has been developed and tested on a 103atom system. The technique offers sizable speed advantages over previous simulation methods and allows for realistic three‐dimensional studies of the kinetics of both pseudomorphic and misfit growth modes and of the transformation between them. Elements of the method are of general utility and can also be used to substantially improve the efficiency of continuous‐space Monte Carlo and molecular dynamics simulations of growth and other atomic transformations.

ISSN:0003-6951    

DOI:10.1063/1.103014

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Voltage‐dependent images of liquid crystals on graphite were observed in air by scanning tunneling microscopy (STM). Molecular rows of liquid crystals and the atomic pattern of the graphite substrate were imaged with high (above 1 V) and low (below 0.1 V) bias voltages, respectively. Patterns of molecules, grain boundaries, and distinguishable defects of the liquid crystal arrangement were reproduced even after imaging the substrate in the same area. This indicates that the graphite lattice can be seen by STM without touching or disturbing the adsorbed molecules on it. A resonant tunneling model is proposed to explain the phenomenon.

ISSN:0003-6951    

DOI:10.1063/1.103015

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We analyze the linear stability of a Stefan‐like problem for moving steps in the context of W. K. Burton, N. Cabrera, and F. C. Frank’s theory of crystal growth [Philos. Trans. R. Soc. (London) A243, 299 (1951)]. Asymmetry and departures from equilibrium at steps are included. The stability criterion depends on supersaturation and average step spacing, both experimentally accessible, and on dimensionless combinations of surface diffusivity, surface diffusion length, and adatom capture probabilities at steps, which can be estimated from bond models. This stability criterion is analyzed and presented graphically in terms of these physical parameters.

ISSN:0003-6951    

DOI:10.1063/1.103016

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We have measured emission of Ga+ions and neutral Ga0atoms induced by irradiation of the GaP (110) surface with laser pulses of subband‐gap energies. It is found that the Ga+yield of partially annealed Ar+ion bombarded surfaces is reduced by thermal annealing and also by repeated irradiation with laser pulses, while the Ga0yield is not influenced to an important extent by these treatments. The Ga+emission, which occurs at a lower fluence than Ga0emission, is ascribed to emission from defect sites at surfaces. We suggest that measurements of laser‐induced particle emission for subband‐gap energies will be useful for detection and elimination of low‐density defects on surfaces.

ISSN:0003-6951    

DOI:10.1063/1.102995

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Correlations between midgap interface state density (Dit) and thickness‐averaged stress in thermally grown SiO2thin films have been investigated by infrared spectroscopy, an optical beam deflection technique, and capacitance‐voltage measurements. We find no correlations betweenDitand either (i) the maximum stress in the Si or SiO2at the Si/SiO2interface or (ii) the stress gradient in the SiO2film. By direct measurements of the strain‐induced bending of the Si wafer, and by calculating the microscopic strain from the SiOSi bond‐stretching vibrational frequency, we have established linear relationships betweenDitand the thickness‐averaged stress and strain in the oxide.

ISSN:0003-6951    

DOI:10.1063/1.103228

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Large Stark shifts of intersubband transitions in a step quantum well are observed for the first time. The Stark shifts are ∼8 and 7 meV at ∼18 kV/cm for the 1→2 and 1→3 intersubband transitions, respectively, while the Stark shift of a similar transition in a square quantum well is only about 0.5 meV under the same bias condition. The intersubband transitions in step quantum wells can be either red or blue Stark shifted depending on the direction of the applied electric field. The large Stark shifts of intersubband transitions in the step quantum wells can be exploited for the fabrication of optical modulators operating in the range from mid to far infrared.

ISSN:0003-6951    

DOI:10.1063/1.103193

出版商:AIP    

年代:1990

数据来源: AIP