摘要:

An analytic solution for the normalized intensity for powder x‐ray diffraction has been obtained for a simple two‐dimensional lattice using a linear approximation for the interference function. The analytic solution, where the Bragg peaks are strongly asymmetric, is compared to computer simulations using the Debye formula, and is shown to be in closer agreement than earlier numerical solutions by Warren and others. For a two‐dimensional structure consisting of more than one monolayer of atoms, the shape of the Bragg peaks is modulated by the structure factor. This structure factor modulation provides a continuous plot of the structure factor over the range of the diffraction tail and thus provides valuable information about the structure of the layer. It is demonstrated that because of structure factor modulation the Warren expression which relates the width of Bragg peaks to layer size cannot be used for a two‐dimensional sheet with more than one layer of atoms, and it is proposed in such cases that measuring the low‐angle side width of half‐maximum intensity can be used for determination of the layer size. Single molecular layer MoS2and WS2suspensions, prepared by exfoliation, provide excellent randomly oriented two‐dimensional systems for demonstrating the unique features of powder x‐ray diffraction patterns of two‐dimensional materials and for structure identification using Bragg peak profiles. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361165

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Silicon microcantilever beams are fractured and characterized. The specially designed beams, etched into two wafers, are loaded to fracture in bending using a unique measurement system. A finite element model of the beams is created, andABAQUSis used to calculate the displacements and stresses produced by an applied load force. A special testing scheme is devised to obtain certain model parameters:E⟨110⟩, the Young’s modulus along the length of the beam andLforce, the position of the applied force. With these parameters defined, the model is well correlated with that of the experimental data. The fracture stress (strength) of the beam is obtained from the stress produced in the model at the fracture location by a load equivalent to the experimental fracture force. This fracture stress can be used as a design parameter for silicon micromechanical structures. Numerous beams are fractured from both the front and back sides of the wafer, and statistical fracture strength results are compiled for each of these cases. The fracture strength of the front side, 3.3 GPa (average), is significantly greater than that of the backside, 1.0 GPa (average). This dissimilarity is attributed to the differences in the surface roughness of these sides. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361102

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An on‐chip aluminum interconnect carries an intense electric current at an elevated temperature, motivating atoms to diffuse in the solid state, and inducing voids that may cause an open failure. Recent observations have shown that a void sometimes collapses to a slit running nearly perpendicular to the electric current direction. Such a slit often lies inside a grain rather than along a grain boundary. An earlier calculation showed that diffusion on the void surface, driven by the electric current, can cause a circular void to translate in an infinite, isotropic interconnect. It was suggested recently that this solution may be unstable, and that two forces compete in determining the void stability: surface tension favors a rounded void, and the electric current favors a slit. A linear perturbation analysis, surprisingly, revealed that the translating circular void isstableagainst infinitesimal shape perturbation. Consequently, the slit instability must have resulted from finite imperfections. This article reviews the experimental and theoretical findings, and describes a numerical simulation of finite void shape change. We determine the electric field by a conformal mapping of complex variables, and update the void shape for a time step by a variational method. The simulation shows that a finite void shape imperfection or surface tension anisotropy can cause a void to collapse to a slit. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361166

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We have measured the subpicosecond optical response of a solid‐state, semiconductor‐to‐metal phase transition excited by femtosecond laser pulses. We have determined the dynamic response of the complex refractive index of polycrystalline VO2thin‐films by making pump‐probe optical transmission and reflection measurements at 780 nm. The phase transition was found to be largely prompt with the optical properties very close to the high‐temperature metallic state being attained within about 5 ps. The equilibration of the metallic state after femtosecond excitation was modeled by non‐exponentially decaying perturbations in the metallic state electron density and collision frequency. The decay of both these plasma parameters was well fit by a 1/&sqrt;ttime dependence. This indicated that a diffusion process governed the equilibration of the metallic phase of VO2. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361167

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The effects of microstructure on electromigration behavior were evaluated in three nominally 1 &mgr;m thick pure aluminum films, which were tested at temperatures from 423 to 523 K. The three Al films had essentially the same grain structure but different variants of an ⟨111⟩ texture. Texture had a very strong effect on the electromigration behavior in ∼2 &mgr;m wide polycrystalline lines, where both a reduced fraction of randomly oriented grains and a tighter ⟨111⟩ distribution increased the electromigration lifetime. The apparent activation energy for electromigration decreased as the texture strengthened. The near bamboo microstructure of 0.5 &mgr;m narrow lines showed extensive orientation clustering with an unusually high proportion of low angle boundaries in the most strongly ⟨111⟩ textured film. The electromigration damage in both 2 and 0.5 &mgr;m wide lines was correlated with the types of flux divergence sites in each film. The texture impacts the character of the grain boundaries and interfaces which control the mass transport during electromigration. A weaker texture has more juxtaposed ‘‘fast’’ and ‘‘slow’’ diffusivity grain boundaries and interfaces, which results in faster mass transport, more flux divergence sites, and a more rapid accumulation of damage. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361168

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In this article, the results of a study of the diffusion mechanism of boron in silicon based on the Monte Carlo method are presented. The kickout mechanism has been examined for the case of a delta function impurity profile under both inert and oxidation conditions. It is shown that the initial conditions play a significant role in obtaining the mean migration path lengths of the atoms. The kickout mechanism in the case of an initial delta function interstitial impurity profile has been analytically examined. The atomic level computational experiments carried out in this article validate Cowern’s results of ‘‘intermittent diffusion’’ of boron in silicon and yield, the values for &lgr;0, the prefactor for the mean migration path length, which are found to lie between 0.024 and 0.035 nm. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361169

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Measurements of dc critical current as a function of temperature, and spontaneous voltage transients, are reported for superconducting thin films of Nb, Mo, and Ti near their critical temperatures and currents. The form of the temperature dependence is found to reflect the presence of stable and/or metastable normal regions in the films, as well as the different temperature scaling laws for the intrinsic and thermal‐runaway critical currents. Voltage transients (pulses) occur in the nominally superconducting samples when the current is below both the intrinsic and thermal‐runaway values. Thermal coupling between film and substrate is reported for a wide variety of samples fabricated by different methods. The coupling in all cases follows a thermal boundary resistance law with power flux proportional to the difference of fourth powers of film and substrate temperatures. Applications to minimum‐ionizing particle detection above liquid helium temperatures are discussed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361170

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Nanometer scale features on the semiconductor p‐PbTe single crystal surface were created and studied at 4.2 K by means of a scanning tunneling microscope. Local tunneling spectra were obtained simultaneously with the surface topographic image. The local tunneling density of states and the magnetotransport data reveal the amorphous state of the modified regions on the surface. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361171

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The details of epitaxial growth and microstrictural characteristics of AlN films grown on sapphire (0001), (101¯2) and Si (100), (111) substrates were investigated using plan‐view and cross‐sectional high‐resolution transmission electron microscopy and x‐ray diffraction techniques. The films were grown by metalorganic chemical vapor deposition using TMA1+NH3+N2gas mixtures. Different degrees of epitaxy were observed for the films grown on &agr;‐Al2O3and Si substrates in different orientations. The epitaxial relationship for (0001) sapphire was found to be (0001)AlN∥(0001)sapwith in‐plane orientation relationship of [011¯0]AlN∥[1¯21¯0]sap. This is equivalent to a 30° rotation in the basal (0001) plane. For (101¯2) sapphire substrates, the epitaxial relationship was determined to be (112¯0)AlN∥(101¯2)sapwith the in‐plane alignment of [0001]AlN∥[1¯011]sap. The AlN films on (0001) &agr;‐Al2O3were found to contain inverted domain boundaries anda/3⟨112¯0⟩ threading dislocations with the estimated density of 1010cm−2. The density of planar defects (stacking faults) found in AlN films was considerably higher in the case of (101¯2) compared to (0001) substrates. Films on Si substrates were found to be highly texturedcaxis oriented when grown on (111) Si, andcaxis textured with random in‐plane orientation on (100) Si. The role of thin‐film defects and interfaces on device fabrication is discussed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361172

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The thermal stability of an &agr;‐Ti film in contact with a &dgr;‐TiN film in the structure of a TiN/Ti/TiN film stack on SiO2substrates was studied byinsitusheet resistance (Rs) measurement, Auger electron spectroscopy, glancing angle x‐ray diffractometry, cross‐sectional transmission electron microscopy, scanning electron microscopy, and atomic force microscopy. It was found that nitrogen dissolves from TiN into Ti between 405 and 474 °C and a significant reaction between Ti and TiN results in the formation of &egr;‐Ti2N in the temperature range of 505–548 °C. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.361103

出版商:AIP    

年代:1996

数据来源: AIP