C. E. Bouldin

Interfacial properties of ZrO2 on silicon

The interface of zirconium oxide thin films on silicon is analyzed in detail for their potential applications in the microelectronics. The formation of an interfacial layer of ZrSixOy with graded Zr concentration is observed by the x-ray photoelectron spectroscopy and secondary ion mass spectrometry analysis. The as-deposited ZrO2/ZrSixOy/Si sample is thermally stable up to 880 °C, but is less stable compared to the ZrO2/SiO2/Si samples. Post-deposition annealing in oxygen or ammonia improved the thermal stability of as-deposited ZrO2/ZrSixOy/Si to 925 °C, likely due to the oxidation/nitridation of the interface. The as-deposited film had an equivalent oxide thickness of ∼1.3 nm with a dielectric constant of ∼21 and a leakage current of 3.2×10−3 A/cm2 at −1.5 V. Upon oxygen or ammonia annealing, the formation of SiOx and SiHxNyOz at the interface reduced the overall dielectric constants.

Extended x-ray absorption fine structure study of AlxGa(1−x)N films

Extended x-ray absorption fine structure above the Ga–K edge has been used to study the local structure of AlxGa1−xN films grown by metal organic chemical vapor deposition. With increasing Al content, x, the Ga–N bond length decreases, but much less than the average bond length. On the other hand, the x dependence of the Ga–Ga and Ga–Al distances does follow the variation of the average cation–cation distance. We conclude that bond angle distortions accommodate the differences between the Ga–N and Al–N bond lengths.

MacXAFS: An EXAFS analysis package for the Macintosh

Abstract We present an EXAFS analysis package that runs on the Apple Macintosh. (Certain commercial products are identified in the article for the sake of completeness. This does not constitute and endorsement by the National Institute of Standards and Technology, the Naval Research Laboratory, or Broohaven National Lab.) The MacXAFS package is derived from a suite of EXAFS analysis programs originally written at the University of Washington. The MacXAFS package preserves the portable computational kernel of this well-tested group of programs and adds a graphical interface by using HyperCard. Communication between the core analysis programs and the HyperCard interface is done by using ASCII script files that direct the execution of the FORTRAN-based analysis programs. The user is largely insulated from the FORTRAN code, but instead interacts with the more intuitive Hypercard interface. This implementation preserves the portability of the ANSI FORTRAN programs, while taking full advantage of the platform-specific user interface features of the Macintosh. Computationally intensive programs, such as the ab initio XAFS calculation program FEFF can be executed remotely via a TCP/IP connection. Due to the clean separation of the interface and the computational kernel, this package is readily extensible by writing new programs, in FORTRAN or any other language, and adding the corresponding interface element in the HyperCard stack.

Silicon photodiode detector for fluorescence EXAFS

A large‐area silicon diode is used as a fluorescence detector for extended x‐ray absorption fine‐structure (EXAFS) measurements. A direct comparison of this diode detector relative to a gas ionization fluorescence detector is made. Advantages of the diode detector include: higher signal for a given photon flux (due to higher quantum efficiency), vacuum and cryogenic compatibility, freedom from microphonic noise, good linearity, extremely wide dynamic range, operation without high voltage or gas connections, very simple electronics, and low cost. A brief comparison with other detection methods for fluorescence EXAFS is given. Use of photodiodes for transmission EXAFS is discussed.

Accelerating Scientific Discovery Through Computation and Visualization II

The rate of scientific discovery can be accelerated through computation and visualization. This acceleration results from the synergy of expertise, computing tools, and hardware for enabling high-performance computation, information science, and visualization that is provided by a team of computation and visualization scientists collaborating in a peer-to-peer effort with the research scientists. In the context of this discussion, high performance refers to capabilities beyond the current state of the art in desktop computing. To be effective in this arena, a team comprising a critical mass of talent, parallel computing techniques, visualization algorithms, advanced visualization hardware, and a recurring investment is required to stay beyond the desktop capabilities. This article describes, through examples, how the Scientific Applications and Visualization Group (SAVG) at NIST has utilized high performance parallel computing and visualization to accelerate condensate modeling, (2) fluid flow in porous materials and in other complex geometries, (3) flows in suspensions, (4) x-ray absorption, (5) dielectric breakdown modeling, and (6) dendritic growth in alloys.

Analysis of DAFS fine structure and background

Abstract The oscillatory structure in the diffraction anomalous fine structure (DAFS) spectrum contains at least as much local structural information as X-ray absorption fine structure (XAFS). We present three XAFS analysis programs, atoms, autobk , and feffit , which are easily extended to background removal and fine structure analysis in DAFS. The background subtruction and curve-fitting of DAFS data require the crystallographic structure factor, F , for the material. atoms calculates F for an arbitrary crystal from tables of anomalous scattering factors. autobk , an automated background removal program, estimates the shape of the DAFS background and correctly normalizes the resulting ξ(k). feffit , an XAFS curve-fitting program using multiple scattering theory and sophisticated model-building capabilities, is easily extended to handle the DAFS oscillatory structure. We show the succes of these programs in handling DAFS spectra.

Thermal Expansion Coefficients of Low-K Dielectric Films From Fourier Analysis of X-Ray Reflectivity

We determine the thermal expansion coefficient of a fluorinated poly(arylene ether) low-k dielectric film using Fourier analysis of x-ray reflectivity data. The approach is similar to that used in Fourier analysis of x-ray absorption fine structure. The analysis compares two similar samples, or the same sample as an external parameter is varied, and determines the change in film thickness. The analysis process is very accurate and depends on no assumed model. We determine a thermal expansion coefficient of 55±9×10−6 K−1 using this approach.

Extended x-ray absorption fine structure and x-ray standing wave study of the clean InP(110) surface relaxation

Through a unique combination of surface sensitive extended x‐ray absorption fine structure (EXAFS) and x‐ray standing waves (XSW), we have determined the pertinent structural parameters of the clean InP(110) surface reconstruction. We find a rotation angle of 27° between the P–In chains, no change of the first neighbor P–In bond length, and a small but measurable, ∼0.1 A, expansion of the average P–P second neighbor distance. The general application of the EXAFS and XSW techniques to the study of clean surfaces will be discussed.

Silicon photodiode detector for a glancing-emergence-angle EXAFS technique

We have constructed a silicon photodiode detector for use with a glancing‐emergence‐angle (GEA) geometry useful for obtaining fluorescence EXAFS spectra from thick specimens with concentrated absorbing species. We present a description of the detector and the results of tests, including dark‐noise tests, EXAFS spectra from a standard sample, and a comparison to an ion chamber also in the GEA configuration. Data obtained from the two detectors are comparable in quality, making the diode detector a preferable choice for this application due to factors such as simplicity of construction and compact size. The diodes also have potential for significant further improvement in the quality of the signal due to their high quantum efficiency if the dark noise can be reduced. We present suggestions for achieving this in future generations of the detector.

Surface extended x‐ray adsorption fine structure studies of the Si(001) 2×1–Sb interface

Surface extended x‐ray adsorption fine structure (SEXAFS) has been used to investigate the structure of Sb on the Si(001) 2×1 surface. The coverage of Sb which remains after annealing thick layers at 375 °C, previously reported to be one monolayer (ML), is found in this work to form a disordered overlayer with three dimensional Sb clusters. This finding is concluded from the Sb L3 absorption spectra which are similar for this coverage to that of bulk Sb. After a 550 °C anneal, Auger electron spectroscopy, and scanning tunneling microscopy (STM) show that about 1 ML of Sb remains. Phase and amplitude analysis of the Sb L3 edge SEXAFS shows that the remaining Sb atoms occupy a modified bridge site with a Si–Sb bond length of 2.63±0.04 A. The Sb atoms form dimers with a Sb–Sb bond length of 2.91±0.04 A, which is almost identical to the bulk Sb–Sb bond length of 2.90 A. The Sb atoms lie 1.74±0.06 A above the Si(001) surface. STM confirms the dimer structure of the Sb overlayer.