J T. Armstrong

High-accuracy determination of the dependence of the photoluminescence emission energy on alloy composition in AlxGa1−xAs films

In an effort to improve the accuracy of photoluminescence (PL) measurements of the Al mole fraction (x) of AlxGa1−xAs alloys, the PL peak emission energy, EPL,peak, was measured at room temperature for molecular-beam epitaxy-grown AlxGa1−xAs films with 0⩽x<0.37, and correlated with independent measurements of x by in situ reflective high-energy electron diffraction (RHEED) and also by ex situ wavelength-dispersive x-ray spectroscopy in an electron microprobe analyzer (WDS/EMPA). The measurement uncertainty of EPL,peak was minimized through the following procedures: Accurate calibration of the photon energy (or wavelength) scale, correction of the measured spectra for the spectrometer response function, fitting the data with a well-chosen line shape function, and compensation for the effect of ambient temperature drift. With these procedures, the 2σ measurement uncertainty of EPL,peak was of the order 5×10−4 eV for most samples. From correlation of the PL and WDS/EMPA composition data, the slope ∂EPL,peak/...

Electron beam induced x-ray emission: An in situ probe for composition determination during molecular beam epitaxy growth

An in situ, x-ray emission measurement technique is developed. This technique is demonstrated to measure composition at the monolayer level for the InGaAs/GaAs heterojunction system. This electron beam induced x-ray emission technique is a powerful method for in situ compositional analysis during molecular beam epitaxy (MBE) growth. Unlike the reflection high-energy electron diffraction (RHEED) method, this electron beam stimulated x-ray emission technique affords a new method for the real-time monitoring of the elemental composition while the sample is rotating during growth. The technique exhibits long term reproducibility, and in addition, compares reasonably well with RHEED. Well-characterized standards are required for quantitative analysis of the composition, and knowledge about the electron sampling trajectories is required for accurate elemental analysis at the monolayer level.

Effect of Film Composition on the Orientation of (Ba,Sr)TiO3 Grains in (Ba,Sr)yTiO2+y Thin Films

Thin films of composition (Ba,Sr) y TiO 2+ y with 0.43 ≤ y ≤; 1.64, were deposited by metalorganic chemical vapor deposition on (100) MgO substrates at various growth conditions. X-ray diffraction and transmission electron microscopy studies showed that the films were composed of epitaxial Ba 1– x Sr x TiO 3 ( x ≈0.06) grains and an amorphous phase. The orientation of the tetragonal Ba 1– x Sr x TiO 3 grains (pure a axis, pure c axis, or a mix of the two) was found to be strongly dependent upon film composition. This composition dependence is explained for the majority of the Ti-rich films by an analysis of average strains in the two-phase films, assuming a compressive strain of ≈1% in the amorphous phase.

Characterization of SiGe Bulk Compositional Standards with Electron Probe Microanalysis

Bulk SiGe wafers cut from single‐crystal boules were evaluated with the electron probe microanalyzer (EPMA) for micro‐ and macroheterogeneity for use as primary standards for future characterization of SiGe thin films on Si that are needed by the microelectronics industry as reference standards. Specimens with nominal compositions of 14 at. %, 6.5 at. %, and 3.5 at. % Ge were rigorously tested with wavelength dispersive spectrometers (WDS) using multiple point, multiple sample, and duplicate data acquisitions. The SiGe14 is a good bulk reference material for evaluation of SiGe thin films.

Improvements in Electron-Probe Microanalysis: Applications to Terrestrial, Extraterrestrial, and Space-Grown Materials

Improvement in the accuracy of electron-probe microanalysis (EPMA) has been accomplished by critical assessment of standards, correction algorithms, and mass absorption coefficient data sets. Experimental measurement of relative x-ray intensities at multiple accelerating potential highlights errors in the absorption coefficient. The factor method has been applied to the evaluation of systematic errors in the analysis of semiconductor and silicate minds. Accurate EPMA of Martian soil stimulant is necessary in studies that build on Martian rover data in anticipation of missions to Mars.

A Simple Method for Determining Optimum Corrections for High-Accuracy EPMA in Difficult Chemical Systems

Increasingly, analysts are called to perform high accuracy quantitative electron microprobe analyses using x-ray lines for which the available correction procedures have significant uncertainties. Conventional evaluation of microprobe corrections typically involves analyzing a series of standards as ‘unknowns’ (often at several different beam energies), correcting the data through the algorithms and parameters being tested, and plotting the results as error histograms. The problem with this method is that often there are simply not enough multi-element, homogeneous materials available to provide the necessary suite of secondary standards. Even if there are, independent determination of the exact compositions and degree of homogeneity of the candidate standards requires so much time, effort, and money as to make it impractical.

Fabrication and electron microprobe characterization of barium-strontium-titanate (BST) films

Barium strontium titanate (BST) thin films of varying composition and thickness (5 nm to 400 nm) on (100)Si or Pt/(100)Si substrates were measured using an electron microprobe analyzer with wavelength dispersive x-ray spectrometers. Most of the films were fabricated at NIST by a metalorganic deposition approach. Several films from external sources that were fabricated by sputtering or metalorganic chemical vapor deposition were also measured. For some of the films, the compositions determined by electron microprobe were in good agreement with the nominal compositions provided by the film suppliers. However, for other films, including the thinnest ones, there were discrepancies between the measured and nominal compositions and thicknesses.

AlGaAs composition measurements from in situ optical reflectance

We describe preliminary determinations of AlGaAs layer composition using in situ optical reflectance spectroscopy (ORS) data. RHEED oscillations are used to independently determine the composition of the AlGaAs layers. The results are compared with ex situ measurements. Although additional work is needed to refine the uncertainty estimates and reduce sources of error, we find that growth rate as measured by ORS agrees with RHEED oscillation data to within 2%. The ultimate goal of this project is to produce standard reference materials of certified alloy composition to mole-fraction uncertainty of 0.002 for a range of important III-V alloys.

Development of a Titanium Nitride Thin Film Standard

Titanium nitride films are used in a variety of applications in the semiconductor industry, but the stoichiometry and homogeneity of the films can be difficult to control and characterize. The National Institute of Standards and Technology (NIST) is developing a TiN thin film on silicon to be used as a physical and chemical standard for the semiconductor industry. A prototype 100 nm thick film has been manufactured by dual ion beam deposition. NIST is characterizing the film by x-ray interferometry for thickness and density, neutron depth profiling for nitrogen content and homogeneity as a function of depth, neutron activation analysis for titanium content, and x-ray fluorescence spectromety, electron probe microanalysis, grazing incidence x-ray photoelectron spectroscopy, and transmission electron microscopy for homogeneity and composition. The intent of this research is to create a TiN film of known thickness, density, stoichiometry, and homogeneity that can be used by the semiconductor industry as a primary, traceable standard for characterization of industry thin films. The intent of the standard is to promote more accurate characterization and improved TiN manufacturing capabilities as well as to aid manufactures in more easily employing ISO 9000 related traceability and quality assurance practices.