摘要:

An analytical and computational study has been made of the influence of surface impurity on the normal impact response of two identical crystal lattices. Identical monolayers of adsorbed atoms are considered to cover each host lattice. The incoming lattice is assumed to move with uniform initial speed, impacting an initially motionless lattice. A nonlinear differential‐integral equation governing the impact response of the surface impurity is derived and then solved analytically by asymptotic expansion for compressive interaction. For numerical calculations, oxygen surface impurity and a copper host lattice are considered, with surface interaction force determined from quantum‐mechanical results. Because the surface interaction force exhibits a gradual long‐range change in force with distance, surface oscillations do not occur during impact. The time required for the surface of an impacted lattice to accelerate to the classical continuum shock velocity is shown to depend on impact velocity and two parameters from the quantum‐mechanical calculations. The subsequent distance at which breakdown of weak shock‐wave motion far into the interior of the lattice occurs is determined by a simple approximation. Shock responses in the lattice beyond the breakdown distance are determined by numerical integration of both the lattice equations of motion, with quadratic nonlinear force interaction between atoms, and the Korteweg–de Vries (KdV) equation. The asymptotic solution for velocity response of the surface impurity provides the initial condition for numerical integration of the KdV equation. The calculations for both sets of equations show a good correspondence in the structure of the shock wave far into the lattice.

ISSN:0021-8979    

DOI:10.1063/1.345696

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Photoconductivity of semi‐insulating gallium arsenide illuminated with monochromatic photons in the 0.7–1.8 eV range has been studied. It has been found that photoconductivity strongly depends on the occupancy of deep traps present in the material, so that the photoconductivity measured for the full trap regime is several orders of magnitude larger than one taken when the traps are empty. By selective emptying or filling of traps it was possible to identify the contribution of each particular trap to the photoconductivity enhancement in different temperature intervals.

ISSN:0021-8979    

DOI:10.1063/1.345697

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The nucleation and disappearance of current filaments in homogeneously slightly doped semiconductors during avalanche breakdown at low temperatures is quantitatively evaluated in terms of a minimum and maximum value of the filament current. Both values arise from a model based on the power balance in the system. Experimental results obtained by the two‐dimensional imaging of the filament configurations inn‐GaAs andp‐Ge agree quantitatively with the predictions of our power balance model.

ISSN:0021-8979    

DOI:10.1063/1.345698

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Platinum silicide Schottky barrier infrared diodes have been formed onp‐type silicon substrates having both ⟨100⟩ and ⟨111⟩ orientations. The potential barrier to optically generated hot carriers has been measured and found to be 0.219 eV for ⟨100⟩ substrates and 0.313 eV for ⟨111⟩. Platinum‐layer thickness was varied from 1 to 10 nm. The nearly 0.1 eV difference in Schottky barrier potential appears to depend only on the orientation of the silicon substrates.

ISSN:0021-8979    

DOI:10.1063/1.345699

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We calculate the effects of the coupling between the eigenstates of two symmetric GaAs/ AlxGa1−xAs quantum wells separated by a thin barrier with an applied electric field using a very accurate numerical method that we have developed. Energy shifts with field and overlap integrals for the symmetric and antisymmetric electron and hole eigenstates are calculated. These results are compared to a single quantum well of equivalent dimensions. As suggested in the literature, the quantum‐confined Stark effect is shown to be significantly increased by the use of coupled quantum wells. Comparison with the experimental results of Chenetal. [Y. J. Chen, E. S. Koteles, B. S. Elman, and C. A. Armiento, Phys. Rev. B36, 4562 (1987)] is made with surprisingly good agreement.

ISSN:0021-8979    

DOI:10.1063/1.345672

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In order to improve the capabilities of the electron‐beam‐induced current method, a technique based on scanning transmission electron‐beam‐induced current has been developed. It is shown that it enables the direct correlation of structural defects with their electrical activity. It implies the fabrication of ultrathin Schottky diodes (thickness ≤600 nm). From an approximate theoretical model it was inferred that the spatial resolution reaches about 200 nm in our experimental conditions. Experimental data are obtained on electron‐grade and on upgrade metallurgical grade polycrystalline silicon, grown by the heat exchange method on which the behavior of carbon at grain boundaries and defects has been studied by transmission electron microscopy, high‐resolution electron microscopy, and electron energy loss. There is a good agreement between the experimental data on electrical activity and the calculated approximation. The present method shows that the electrical activity is mainly related to the presence of impurities. Carbon seems to trap recombining impurities and specifically oxygen. Isolated dislocations are always electrically active whereas ‘‘clean’’ twin boundaries are not active. The activity at boundaries is always localized on extrinsic dislocations or on precipitates. Asymmetric profiles are also observed on boundaries and stacking faults. This was related to the existence of segregated zones lying on one side of these defects which are likely to act as diffusion barriers.

ISSN:0021-8979    

DOI:10.1063/1.345673

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

60‐Hz ac losses at 4.2 and 77 K for sintered YBa2Cu3O7cylinders having a large difference in the grain size were measured as a function of applied magnetic field. The results are discussed in terms of the intra‐ and transgranular critical current densities in the specimens.

ISSN:0021-8979    

DOI:10.1063/1.345674

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Magnetic force microscopy (MFM) images of rapidly quenched FeNdB are presented. The magnetic domain structure as observed by MFM consists of elongated, polyhedral‐shaped domains imaged by measuring forces smaller than 10−9N at distances ranging from 20 to more than 200 nm. The domain transition regions, which are quite sharp and well defined, often show a double‐peaked structure with a peak‐to‐peak distance of 10 nm. At force sensor tip‐to‐sample separations larger than 90 nm only a single peak is observable. It has to be assumed that the relevant effective magnetic volume of the force sensing tip is considerably smaller than the geometric dimensions as determined by scanning electron microscopy in order to understand this 10‐nm resolution.

ISSN:0021-8979    

DOI:10.1063/1.345675

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The influence of preparation conditions on the morphology and the resulting oxidation behavior of amorphous GdTbFe films was studied. The morphology was characterized by transmission electron microscopy and mass‐density measurements, the magnetic properties by torque and vibrating sample magnetometer measurements, and the oxidation kinetics byinsitumeasurements of electrical conductivity and Hall hysteresis loops. For the first time a quantitative comparison of the aging kinetics of evaporated and magnetron sputtered films prepared at different Ar sputter gas pressures and substrate‐target distances is reported. With increasing the pressure‐distance product from 36 to 192 Pa mm the always‐observed columnar structure becomes much more pronounced and the mass density, electrical conductivity, and corrosion resistance of the films decrease significantly. The pressure‐distance product for thermalization plays an important role in determining the microstructure of the films. Evaporated films show similar features to dc magnetron sputtered films prepared at pressure‐distance products of about 100 Pa mm. The oxidation rate is proportional to the porosity of the films and shows, independent of the preparation conditions, an activation energy of 0.3 eV.

ISSN:0021-8979    

DOI:10.1063/1.345676

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Fe‐C/Ni‐Fe multilayered films have a pattern of stripes 10 or 20 &mgr;m wide. Their magnetic properties and domain structures are examined. For certain film geometries, insertion of nonmagnetic BN layers eliminates 90° closure domains in the stripe‐patterned Fe‐C/Ni‐Fe films. The disappearance of the closure domains increases the relative permeability of the films. The stripes, with a width of 10 or 20 &mgr;m, display a relative permeability of 1500 and 1700, respectively, when closure domains have disappeared.

ISSN:0021-8979    

DOI:10.1063/1.345650

出版商:AIP    

年代:1990

数据来源: AIP