摘要:

Dynamic force microscopy, a technique also known as non-contact force microscopy, has proved to be a powerful tool for atomic resolution imaging. A number of schemes have been developed, but recently the oscillator method has become the preferred operating mode. Here, the force sensor acts as resonator in an active feedback circuit. A practical implementation of the method is described and the underlying key concepts are discussed. It is shown that a tracking oscillator excitation scheme is superior to the more standard direct feedback method for cases in which the force sensor exhibits only a weak resonance enhancement. Furthermore, the simultaneous measurement of dissipative interaction channels is an important extension of dynamic force microscopy. It allows one to differentiate between sample materials via their plasto-mechanical response. As an example, a Cr test grating has been imaged in the constant force gradient mode. The dissipation measured on Cr-covered areas is significantly lower than that on the bare quartz glass substrate, which enables one to distinguish between the two materials with a lateral resolution comparable to that of the topographic image. ©1997 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.365726

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The relaxation of an atomic force microscope microlever is investigated after the unsticking instability. We show that the different oscillating behavior can be explained by considering that polymer chains remain stuck at the tip even when the microlever is away from the sample. A simple rheological model is used which satisfactorily describes the different features observed. The approach described in the article provides useful additional information to understand the behavior of a microlever coupled to a polymer network. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365727

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Single crystals ofBeAl6O10,beryllium hexaaluminate, were grown by the Czochralski method. The optical, acousto-optical, elastic, and a number of thermo-mechanical properties of bulk crystals ofBeAl6O10were investigated in comparison with crystal ofBeAl2O4,chrysoberyl. It has been demonstrated that this material is the promising host for active media of tunable solid state lasers. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365728

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Although photorefractive thin films in the transverse geometry retain most of the basic features of the bulk photorefractive materials, the abrupt changes in the dielectric constant across the boundary regions always accompany polarized charge gratings. These gratings are generated by the perpendicular component of the space charge field and they are absent in the bulk transport equations. Our systematic theoretical calculation indicates that these boundary charge gratings will strongly influence device performance. Adjustment of the dielectric properties of media sandwiching the thin film can serve as a method by which control device performance. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365729

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

An exact solution of a heat conduction problem with the effect of latent heat of solidification (Stefan problem) is derived. The solution of the one dimensional Stefan problem for a finite liquid phase initially existing in a semi-infinite body is applied to evaluate temperature fields produced by laser or electron beam welding. The solution of the model has not been available before, as Carslaw and Jaeger [Conduction of Heat in Solids, 2nd ed. (Oxford University Press, New York, 1959)] pointed out. The heat conduction calculations are performed using thermal properties of carbon steel, and the comparison of the Stefan problem with a simplified linear heat conduction model reveals that the solidification rate and cooling curve over 1273 K significantly depend on which model (Stefan or linear heat conduction problem) is applied, and that the type of the thermal model applied has little meaning for cooling curve below 1273 K. Since the heat conduction problems with a phase change arise in many important industrial fields, the solution derived in this study is ready to be used not only for welding but also for other industrial applications. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365730

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Metal and carbon plasmas produced at nonstationary cathode spots are inherently contaminated with micron-size macroparticles, and their removal is typically done by guiding the plasma through curved magnetic macroparticle filters in order to spatially separate plasma and macroparticles. In this article, a possible alternative approach is discussed in which macroparticles are destroyed rather than removed. Mass and energy transfer from the plasma to macroparticles are considered. Both cold and warm ion approximations are discussed. It was found that the desired result—complete evaporation of macroparticles before arrival at the substrate—cannot be easily achieved in any case. Ion bombardment heating is reduced by thermionic electron emission when the macroparticle approaches the temperature region of strong evaporation. The heat loss mechanisms, thermal radiation and evaporation, increase strongly with temperature. Direct heating of the macroparticle seems to be most promising. Microwave heating was ruled out because of the plasma shielding effect. The use of infrared lasers has been proposed, but this solution is likely to be uneconomical. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365731

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The behavior of rf plasma sheaths has been the subject of much scientific study and also is technologically important for plasma etching and deposition in the manufacture of integrated circuits. This paper presents a semianalytic model of rf sheaths and describes an experiment that tested the model. An approximation to the first integral of the Poisson equation allows solving for the response of plasma sheaths to an imposed rf bias voltage. This approximation enables the plasma sheaths to be included within an electrical model of the plasma and external rf circuit components, and affords a prediction of the ion energy distributions impacting the electrodes, which are in contact with the plasma. The model is a significant advance beyond previous sheath models because it has no restriction on the ratio of the rf period to the ion transit time across the sheath. The model is applicable to those high-density, low-pressure plasmas in which the Debye length is a small fraction of the ion mean-free path, which itself is a small fraction of the plasma dimension. The experimental test of the model was conducted by comparing the predicted and measured rf potential, current, and power at the sheath adjacent to a capacitively coupled, rf-biased electrode in a plasma reactor with argon discharges sustained by an inductively coupled plasma source. The comparisons included both linear and nonlinear components of the rf electrical parameters. Results of the experiment were in substantial agreement with model predictions. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365732

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A self-consistent nonlinear theory of a relativistic electron beam propagating through a waveguide loaded with a sheath helix is developed by making use of Maxwell equations. A closed integrodifferential equation is obtained for the normalized beam current described in terms of the normalized time &thgr; and propagation distance &zgr;. An analytical investigation of the partial integrodifferential equation of the current modulation is carried out by Fourier decomposing the current modulation with harmonic numberl. The mode strengthcl(&zgr;)is obtained in terms of the spatial oscillation frequency&eegr;land growth rate&xgr;lof the mode amplitude, which are, in turn, determined in terms of the electrostatic and helix effects. The spatial oscillation frequency increases as the electrostatic effect(gl)increases. On the other hand, the growth rate increases with the strength of the helix effect. It is found that the mode strengthclis oscillatory and that it grows during propagation when the spatial frequency&eegr;l(the electrostatic effect) is considerably larger than the growth rate&xgr;l(the helix effect). Otherwise, the exponential growth dominates. Investigation of the helix mode must include the electrostatic effect for an intense beam.

ISSN:0021-8979    

DOI:10.1063/1.365733

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Pure and magnesium doped (2000 ppm) LiF crystals have been irradiated with &ggr; rays from241Amand60Cosources and the thermal stability of the resulting defects has been studied using photoluminescence (PL) and optical absorption (OA). The composite nature of theMband at 440 nm is highlighted by PL data which reveal emission attributed toF3+defects otherwise hidden in OA measurements. The following bands appear in excitation spectra;Z3(222 nm),Z2(285 nm),R1(329 nm),R2(375 nm),F3+(440 nm), andF2(440 nm). Notable differences are observed between pure and doped samples during thermal annealing relating to theF2andF3+bands in particular. Results support the idea ofZcenters as beingFandF−centers trapped at impurity ions in the crystal. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365734

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A microstructural analysis of silicon-on-insulator samples obtained by high dose oxygen ion implantation was performed by Raman scattering. The samples analyzed were obtained under different conditions thus leading to different concentrations of defects in the top Si layer. The samples were implanted with the surface covered withSiO2capping layers of different thicknesses. The spectra measured from the as-implanted samples were fitted to a correlation length model taking into account the possible presence of stress effects in the spectra. This allowed quantification of both disorder effects, which are determined by structural defects, and residual stress in the top Si layer before annealing. These data were correlated to the density of dislocations remaining in the layer after annealing. The analysis performed corroborates the existence of two mechanisms that generate defects in the top Si layer that are related to surface conditions during implantation and the proximity of the top Si/buried oxide layer interface to the surface before annealing. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365735

出版商:AIP    

年代:1997

数据来源: AIP