摘要:

By the use of ultraviolet laser pulses of microsecond and millisecond duration it is shown that the chemistry of the transformations of Kapton by UV laser radiation is strongly dependent on the intensity (power/unit area) of the laser beam. With these long pulses, the polymer was not ablated. The decomposition resulted in 51% of the polymer weight being converted to gaseous products consisting mostly of CO (67%), HCN (15%), C2H2(12%), and some (<5%) CO2. The major solid product that remained was ‘‘glassy’’ carbon which was identified from its Raman spectrum. This material can be viewed as the product of the secondary addition reactions of the residue that is left after the loss of the gaseous products listed above. With 20 ms pulses, the evolution of the gaseous products increased linearly with intensity and the product composition was constant within the experimental uncertainty over a 12‐fold range of intensity up to 50 kW/cm2(≡1 kJ/cm2). These results show that pulses of duration much greater than ns do not lead to ablation even at fluences that are 104greater than the threshold for ablation using nanosecond pulses. It is therefore more appropriate to view the ablation of this polymer by UV laser pulses of nanosecond duration as being due to the scaling of an intensity threshold rather than a fluence threshold as has become the practice. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359776

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The exact solutions for fully three‐dimensional electrostatic quantities due to a free charge in a nanoscale tip/base junction, such as the scanning‐tunneling microscope, are given for the problem as modeled in the prolate‐spheroidal coordinate system. These exact solutions consist of summation series of integrals of conical functions, and methods for calculating these exact solutions with high accuracy are described in detail in this paper. Calculated results for the surface‐charge distribution on the tip and the base due to a free electron in a nanoscale tip/base junction are also presented. This analysis has important implications for the ultimate objective of quantifying electron tunneling, from first principles, in nonplanar geometries such as sharp vacuum field emitters. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359777

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Structure in the electrically detected magnetic resonance signal from a common type of Si diode has been resolved and measured for different orientations of the magnetic field in the (111) plane. The center is found to haveC3vsymmetry with &Dgr;g∥≳0 and &Dgr;g⊥≊0. It is proposed that the signal is due to a broken bond, or combination of colinear broken bonds, in a vacancy cluster which acts as a recombination center. The most likely cluster is the divacancy which has captured an electron in the antibonding state formed from the most distant broken bonds and a hole in a bonding state associated with the divacancy. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360700

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The evolution of surface features during the growth of a coating is of vital importance for the resulting overall film structure and morphology. In the low substrate temperature regime (T/TM<0.3) the growth front of a single component material roughens due to fluctuations in the incoming particle beam (kinetic roughening) and relaxes via surface diffusion. The continuous deposition process thus leads to the evolution of surface features of large lateral extensions compared to the extension of the film‐forming particles. A detailed experimental study of the surface evolution of sputtered copper coatings with thicknesses ranging from 300 to 105A˚ for a working gas pressure of 0.4 Pa is presented. The roughening as well as the relaxation of the film growth front is studied by means of atomic force microscopy (AFM). The increasing lateral extension of the surface features during growth (coarsening) can well be described by a deterministic continuum model of surface diffusion. The model is capable of predicting the evolution of the surface profile’s Fourier transform for increasing deposition times (i.e., increasing film thickness). A retransformation fromkspace to real space allows for a direct comparison of surface profiles obtained from AFM scans with those resulting from the continuum model and gives good qualitative agreement of the profile shapes. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359778

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The laser‐induced crystallization behavior of GeTe‐based amorphous alloy thin films has been quantitatively studied by local reflection measurements with a focused 780 nm laser. The use of multiple laser pulse sequences enables the nucleation rate and crystal‐growth speed to be separately deduced, allowing the compositional variation of both these processes to be followed. This not only gives detailed information on the crystallization mechanism, but also allows the fine tuning of phase change alloy compositions for use in erasable optical recording. The differences between the as‐deposited and melt‐quenched amorphous phases are also discussed. In particular, it is shown that the crystallization speed of the as‐deposited layer can differ by over an order of magnitude from that of the melt‐quenched amorphous layer. The as‐deposited state can, however, be transformed into a modified amorphous state equivalent to that obtained by melt quenching a previously crystalline layer. This allows the determination of the optical constants and crystallization speeds of the amorphous state written during optical recording. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359779

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The laser‐induced crystallization behavior of GeTe‐based amorphous alloys has been measured with a novel multipulse laser technique. This enables the composition dependence of the nucleation rate and crystal growth speed to be independently followed. Two types of crystallization are investigated. The first involves single‐phase crystallization of quaternary alloys based on Ge39Sb9Te52, in which the composition dependence of nucleation and growth is followed as Se, S, Sn, and Si are included. Both the nucleation rate and crystal‐growth speed vary exponentially with the composition, and a correlation is found between crystallization behavior and bond strengths. The second involves multiphase crystallization in the GeSbTe ternary system. It is shown that the observed variations in crystallization behavior primarily arise from the composition dependence of nucleation rather than crystal growth. The implications of this finding for the importance of long range diffusion during crystallization in the GeSbTe system are discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359780

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Tortuosity, i.e., the high‐frequency limit of the squared propagation index, has been evaluated with narrow‐band piezoelectric transducers on reticulated plastic foams. The basic procedure was recently reported by J. F. Allard, B. Castagne`de, M. Henry and W. Lauriks [Rev. Sci. Instrum.65, 754 (1994)]. Further experimental work related to the anisotropic nature of these materials has been done by probing the propagation index versus angle. A simple numerical routine has been implemented in order to recover the propagation index along principal directions. These predicted values compare well with measurements taken directly along the principal geometric axes of the samples. Slight angular deviations of the principal axes themselves have been observed. This is the very first account of the anisotropy of tortuosity measured by ultrasonic methods in air‐saturated porous materials. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359781

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We have studied the incorporation of Ga in silicon during the fabrication of delta‐doping layers. The delta‐function doping profiles were grown by molecular beam deposition following a solid phase epitaxial growth method. Medium‐energy ion scattering, secondary ion mass spectrometry, and Rutherford backscattering spectrometry were used to determine the structure and composition of the grown films. The interface velocity of the crystallization front and the diffusion coefficient of the impurity atoms in the Si matrix, both relevant parameters of the growth process, were measured. Optimum growth conditions were found that yield Ga doping profiles of less than 1.0 nm (full width at half maximum), with more than 95% of the buried dopant atoms on lattice sites. For these optimum growth conditions, a model is derived explaining the observed incorporation of the Ga atoms. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359782

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We performed imaging of semiconductor nanostructures (quantum wells and quantum wires) by atomic force microscopy of the cleaved edge of the samples under ambient conditions. Selective etching was used to enhance the composition contrast of the semiconductor heterostructure layers, and a procedure for retrieving the nanostructure dimensions from the scanned image is presented. The simple sample preparation and the relatively large (up to ∼100×100 &mgr;m2) imaged areas offer advantages over more conventional nanometer resolution techniques such as transmission electron microscopy. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359783

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Thin films of the nonlinear optical material 4′‐nitrobenzylidene‐3‐acetamino‐4‐methoxy‐aniline have been prepared using the organic molecular beam deposition technique. High quality homoepitaxial layers have been grown at substrate temperatures of 80 °C and moderate growth rates of 0.1–0.5 A˚/s. The samples have been characterized by optical polarization and interference microscopy as well as atomic force microscopy. Growth experiments on inorganic substrates, including silicon and glass, have been performed in a substrate temperature range of −190 to 100 °C and led to amorphous films at low temperatures and polycrystalline films at temperatures above 50 °C. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359784

出版商:AIP    

年代:1995

数据来源: AIP