bluuebunny/crossref_metadata_2025 huggingface dataset:
Semiconductor Photoelectrochemistry
Authors: Anz Samir J. , Fajardo Arnel M. , Royea William J. , Lewis Nathan S.
Published: October 2003

This article discusses methods and experimental protocols in semiconductor electrochemistry. The basic principles that govern the energetics and kinetics of charge flow at a semiconductor‐liquid contact are discussed. The principal electrochemical techniques of photocurrent and photovoltage measurements used to obtain important interfacial energetic and kinetic quantities of such contacts are then described in detail.

After this basic description of concepts and methods in semiconductor electrochemistry, methods for characterizing the optical, electrical, and chemical properties of semiconductors through use of the electrochemical properties of semiconductor‐liquid interfaces are described. The latter part of this unit focuses on methods that provide information primarily on the properties of the semiconductor surface and on the semiconductor‐liquid junction.

In some cases, the semiconductor‐liquid junction provides a convenient method for measuring properties of the bulk semiconductor that can only be accessed with great difficulty through other techniques; in other cases, the semiconductor‐liquid contact enables measurement of properties that cannot be determined using other methods. Due to the extensive amount of background material and the interdisciplinary nature of work in this field, the discussion is not intended to be exhaustive, and the references cited in the various protocols should be consulted for further information.

This article covers the following methods in semiconductor electrochemistry:

Photoconductor/photovoltage measurements

Measurement of semiconductor band gaps using semiconductor‐liquid interfaces

Diffusion length determination using semiconductor‐liquid contacts

Differential capacitance measurements of semiconductor‐liquid contacts

Transient decay dynamics of semiconductor‐liquid contacts

Measurement of surface recombination velocity using time‐resolved microwave conductivity

Electrochemical photocapacitance spectroscopy

Laser spot scanning methods at semiconductor‐liquid contacts

Flat‐band potential measurements of semiconductor‐liquid interface

Time‐resolved photoluminescence spectroscopy to determine interfacial charge transfer kinetic parameters

Steady‐state
J‐E
data to determine kinetic properties of semiconductor‐liquid interfaces.
Scanning Electrochemical Microscopy
Authors: Wipf David O.
Published: October 2003

Scanning electrochemical microscopy (SECM) is one of a number of scanning probe microscopy (SPM) techniques that arose out of the development of the scanning tunneling and atomic force microscopes. Scanning probe microscopes operate by scanning, or “rastering,” a small probe tip over the surface to be imaged. The SECM tip is electrochemically active, and imaging occurs in an electrolyte solution. In most cases, the SECM tip is an ultramicroelectrode (UME), and the tip signal is a Faradaic current from electrolysis of solution species. Some SECM experiments use an ion‐selective electrode (ISE) as a tip. In this case, the tip signal is usually a voltage proportional to the logarithm of the ion activity in solution. The use of an electrochemically active tip allows an extremely versatile set of experiments, with chemical sensitivity to processes occurring at a substrate surface as an essential aspect.

A requirement of SPM techniques is that the signal from the tip must be perturbed in some reproducible fashion by the presence of the surface. One of the two methods used in SECM to provide this signal change is known as the “feedback” mode. Feedback can provide topographic images of either electronically insulating or conducting surfaces.

A unique advantage of SECM is the ability to design experiments in which the mediator interacts with the substrate surface to provide chemical and electrochemical activity maps at micrometer and submicrometer resolution.
The Quartz Crystal Microbalance In Electrochemistry
Authors: Arbuckle‐Keil Georgia A.
Published: October 2003

The electrochemical quartz crystal microbalance (EQCM) has found wide acceptance as an analytical tool. Nanogram mass changes that occur on the surface of a quartz crystal can be correlated with the amount of charge passed during an electrochemical process. This technique has been widely reviewed.

That a mass change can be determined from the resonant frequency change of a quartz crystal has been extensively used in vapor deposition techniques for many years. However, the use of the quartz crystal microbalance (QCM) in contact with a liquid during electrochemical studies is more recent. Initial investigations were applied to the underpotential deposition of metals. The EQCM has since been applied to a wide variety of electroactive materials. Examples include conducting polymers, conducting polymer bilayers, redox polymers, charge‐transfer salts, hydrogen uptake by a metal, electrochromic behavior of metal oxide electrodes, polymerization of disulfides, self‐assembled ferrocene‐based monolayers, Prussian blue and nickel ferricyanide.

The fact that the mass change of any electroactive material during any electrochemical change can be evaluated with the EQCM has led to its widespread application. The ability to simultaneously determine the mass and the charge passed during an electrochemical process in an electroactive material is critical for characterization in many instances. For example, the polymerization mechanism of an electrochemically synthesized material can be studied as well as quantifying the amount of ion and/or solvent transport during oxidation/reduction. In addition, recent applications of the QCM include simultaneous QCM and ellipsometry, investigating solvent dynamics in polymer films and determining contact angles and surface tensions.

This article provides a general overview of the EQCM technique and provides details about particular interpretative precautions. The QCM has also been called a thickness‐shear‐mode (TSM) oscillator. Related techniques involve the surface‐acoustic wave and flexural‐plate wave devices.
Reflected‐Light Optical Microscopy
Authors: Connell Richard G.
Published: October 2003

Reflected‐light microscopes were developed for imaging the surfaces of opaque specimens. Microscopy, regardless of the instrumental technique, has as its sole objective the production of visual images that can be further analyzed by counting measurements to quantify any of a number of microstructural properties including grain size, the volume fractions of phases, count (or identification) of microconstituents or microstructural features such as twins, number of inclusions, surface area of grains and second phases, and more esoteric microstructural measurements such as the total mean curvature of the grain boundary in a given specimen. Quantification of microstructural features may be accomplished by manual counting while viewing the image through the microscope, taking measurements from photographic or digital images, or using a computer for digital image analysis. Obtaining quantitative microstructural information constitutes the specialized field of quantitative microscopy or stereology, based upon geometric and topological considerations and statistics. Stereology is a branch of quantitative microscopy that provides for the inference of three‐dimensional structure information from two‐dimensional measurements.

While reflected‐light microscopy alone is not intended for the measurement of material properties, coupled with the techniques of quantitative microscopy it does provide the connecting links among properties, chemical composition, and processing parameters. However, an experienced microscopist can obtain much useful qualitative information related to processing and properties. For instance, one may assess the degree of annealing of a metal or alloy by the amount of the microstructure that appears to consist of recrystallized grains. In a production setting, a microscopist may quickly determine if a material meets specified properties merely by observing the microstructure and judging it on the basis of experience.

It is the intent of this article to describe the more common techniques of reflected‐light microscopy, but not to delve into microstructural property measurement. Emphasis is placed on the use of the higher‐power instruments (metallographs), because of the variety of useful illumination modes and image‐forming techniques that are available on these instruments.
Photoluminescence Spectroscopy
Authors: Toney James E.
Published: October 2003

Photoluminescence (PL) spectroscopy is a powerful technique for investigating the electronic structure, both intrinsic and extrinsic, of semiconducting and semi‐insulating materials. When collected at liquid helium temperatures, a PL spectrum gives an excellent picture of overall crystal quality and purity. It can also be helpful in determining impurity concentrations, identifying defect complexes, and measuring the band gap of semiconductors. When performed at room temperature with a tightly focused laser beam, PL mapping can be used to measure micrometer‐scale variations in crystal quality or, in the case of alloys and superlattices, chemical composition.

The information obtainable from PL overlaps with that obtained by absorption spectroscopy, the latter technique being somewhat more difficult to perform for several reasons. First, absorption spectroscopy requires a broadband excitation source, such as a tungsten‐halogen lamp, while PL can be performed with any above‐band‐gap laser source. Second, for thin‐film samples absorption spectroscopy requires that the substrate be etched away to permit transmission of light through the sample. Third, for bulk samples the transmitted light may be quite weak, creating signal‐to‐noise ratio difficulties. On the other hand, absorption spectroscopy has the advantage of probing the entire sample volume, while PL is limited to the penetration depth of the above‐band‐gap excitation source, typically on the order of a micrometer.

The areas of application of PL also overlap somewhat with Raman spectroscopy, but the latter is much harder to perform because of the inherent weakness of nonlinear scattering processes and because of its sensitivity to crystallographic orientation. On the other hand, this sensitivity to orientation gives Raman spectroscopy a capability to detect variations in crystallinity that PL lacks.

There are also more elaborate techniques, such as photoluminescence excitation (PLE), time‐resolved PL, and resonant Raman scattering, but these techniques are more useful for basic scientific investigations than for routine crystal characterization.
Ultraviolet and Visible Absorption Spectroscopy
Authors: Tissue Brian M.
Published: October 2003

Ultraviolet and visible (UV‐Vis) absorption spectroscopy is the measurement of the attenuation of a beam of light after it passes through a sample or after reflection from a sample surface. This article uses the term UV‐Vis spectroscopy to include a variety of absorption, transmittance, and reflectance measurements in the ultraviolet (UV), visible, and near‐infrared (NIR) spectral regions. These measurements can be at a single wavelength or over an extended spectral range. This article provides an overview of the technique and does not attempt to provide a comprehensive review of the many applications of UV‐Vis spectroscopy in materials research. In this regard, many of the references were chosen to illustrate the diversity of applications rather than to comprehensively survey the uses of UV‐Vis spectroscopy. A unifying theme is that most of the measurements discussed in this article can be performed with simple benchtop spectrometers and commercially available sampling accessories.

The UV‐Vis spectral range is approximately 190 to 900 nm. This definition originates from the working range of a typical commercial UV‐Vis spectrometer. This article includes the use of UV‐Vis‐NIR instruments because of the importance of characterizing the NIR properties of materials for lasers, amplifiers, and low‐loss optical fibers for fiber‐optic communications and other applications. Wavelength and energy ranges of the UV‐Vis and related spectral regions are summarized.

Ultraviolet‐visible spectroscopy is one of the more ubiquitous analytical and characterization techniques in science. There is a linear relationship between absorbance and absorber concentration, which makes UV‐Vis spectroscopy especially attractive for making quantitative measurements. Ultraviolet and visible photons are energetic enough to promote electrons to higher energy states in molecules and materials. UV‐Vis spectroscopy is useful to the exploration of the electronic properties of materials and materials precursors in basic research and in the development of applied materials. Materials that can be characterized by UV‐Vis spectroscopy include semiconductors for electronics, lasers, and detectors; transparent or partially transparent optical components; solid‐state laser hosts; optical fibers, waveguides, and amplifiers for communication; and materials for solar energy conversion. The UV‐Vis range also spans the range of human visual acuity of approximately 400 to 750 nm, making UV‐Vis spectroscopy useful in characterizing the absorption, transmission, and reflectivity of a variety of technologically important materials, such as pigments, coatings, windows, and filters.

The use of UV‐Vis spectroscopy in materials research can be divided into two main categories: (1) quantitative measurements of an analyte in the gas, liquid, or solid phase and (2) characterization of the optical and electronic properties of a material.
Raman Spectroscopy of Solids
Authors: Devine Thomas M.
Published: October 2003

Raman spectroscopy is based on the inelastic scattering of light by matter and is capable of probing the structure of gases, liquids, and solids, both amorphous and crystalline. In addition to its applicability to all states of matter, Raman spectroscopy has a number of other advantages. It can be used to analyze tiny quantities of material (e.g., particles that are ∼1 μm on edge), as well as samples exposed to a variety of conditions such as high temperature and high pressure and samples embedded in other phases, so long as the surrounding media are optically transparent.

Succinctly stated, Raman scattering results from incident radiation inducing transitions in the atoms/molecules that make up the scattering medium. The transition can be rotational, vibrational, electronic, or a combination (but first‐order Raman scattering involves only a single incident photon). In most studies of solids by Raman spectroscopy, the transitions observed are vibrational and these are the focus of this article.
Ultraviolet Photoelectron Spectroscopy
Authors: Hill J.P.
Published: October 2003

Ultraviolet photoelectron spectroscopy (UPS) probes electronic states in solids and at surfaces. It relies on the process of photoemission, in which an incident photon provides enough energy to bound valence electrons to release them into vacuum. Their energy
E
, momentum ℏ
k
, and spin σ provide the full information about the quantum numbers of the original valence electron using conservation laws. Depicts the process in an energy diagram. Essentially, the photon provides energy but negligible momentum (due to its long wavelength λ=2π/|
k
|), thus shifting all valence states up by a fixed energy (“vertical” or “direct” transitions). In addition, secondary processes, such as energy loss of a photoelectron by creating plasmons or electron‐hole pairs, produce a background of secondary electrons that increases toward lower kinetic energy. It is cut off at the vacuum level
E
V
, where the kinetic energy goes to zero. The other important energy level is the Fermi level
E
F
, which becomes the upper cutoff of the photoelectron spectrum when translated up by the photon energy. The difference ϕ = 
E
V
‐
E
F
is the work function. It can be obtained by subtracting the energy width of the photoelectron spectrum from the photon energy.

Photoemission is complemented by a sister technique that maps out unoccupied valence states, called inverse photoemission or bremsstrahlung isochromat spectroscopy (BIS). Inverse photoemission represents the reverse of the photoemission process, with an incoming electron and an outgoing photon. The electron drops into an unoccupied state and the energy is released by the photon emission.

Both photoemission and inverse photoemission operate at photon energies in the ultraviolet (UV), starting with the work function threshold at ∼4 eV and reaching up to 50‐ to 100‐eV photon energy, where the cross‐section of valence states has fallen off by an order of magnitude and the momentum information begins to get blurred. At kinetic energies of 1 to 100 eV, the electron mean free path is only a few atomic layers, making it possible to detect surface states as well as bulk states.
Impulsive Stimulated Thermal Scattering
Authors: Rogers John A. , Maznev Alex , Nelson Keith A.
Published: October 2003

Impulsive stimulated thermal scattering (ISTS) is a purely optical, non‐contacting method for characterizing the acoustic behavior of surfaces, thin membranes, coatings, and multilayer assemblies, as well as bulk materials . The method has emerged as a useful tool for materials research in part because: (1) it enables accurate, fast, and nondestructive measurement of important acoustic (direct) and elastic (derived) properties that can be difficult or impossible to evaluate in thin films using other techniques; (2) it can be applied to a wide range of materials that occur in microelectronics, biotechnology, optics, and other areas of technology; and (3) it does not require specialized test structures or impedance‐matching fluids which are commonly needed for conventional mechanical and acoustic tests. Further, recent advances in experimental design have simplified the ISTS measurement dramatically, resulting in straightforward, low‐cost setups, and even in the development of a commercial ISTS photoacoustic instrument that requires no user adjustments of lasers or optics. With this tool, automated single‐point measurements, as well as scanning‐mode acquisition of images of acoustic and other physical properties, are routine.

ISTS, which is based on transient grating (TG) methods, is a spectroscopic technique that measures the acoustic properties of thin films over a range of acoustic wavelengths. This article, focuses only on ISTS measurement of acoustic motions in thin films. It begins with an overview of other related measurement techniques. It then describes the ISTS acoustic data and demonstrates how it can be used to determine: (1) the stress and flexural rigidity in thin membranes; (2) the elastic constants of membranes and supported films; and (3) the thicknesses of single or multiple films in multilayer stacks.
Nuclear Quadrupole Resonance
Authors: Herreros Bruno , Harbison Gerard S.
Published: October 2003

Nuclear quadrupole resonance (NQR) was once written off as a “dead” field, but has recently had a modest rebirth, and has been applied with success to several areas of materials science, most notably the high‐Tcsuperconductors. The name itself is a misnomer—NQR is really (NMR) at zero field. For that reason, it has many of the same disadvantages as the more familiar NMR spectroscopy e.g., poor sensitivity and complications arising from molecular dynamics, but because it does not require a superconducting magnet, NQR is generally cheaper and can be applied to a far greater number of nuclei. It is inherently noninvasive and nondestructive, and can be applied both to pure substances and to materials in situ. It is currently, for example, being used as an antiterrorist technique to screen persons and baggage for the presence of explosives, using receiver coils that are of the order of 1 m in radius. The principal disadvantage of NQR, other than sensitivity, is that, as a radiofrequency (RF) technique, it is incompatible with conducting or ferromagnetic materials; however, for the somewhat narrow range of materials to which it can be applied, NQR gives information which is otherwise unavailable.

The NQR properties of a material are primarily defined by a transition frequency or set of transition frequencies, which can be used to determine two parameters—the quadrupole coupling constant, and the asymmetry parameter. These quantities are often quite characteristic of particular materials—as when they are used to detect drugs or explosives—but are also sensitive to temperature and pressure, and can therefore be used as a noninvasive probe of these properties. Moreover, by applying external spatially varying magnetic or radiofrequency fields, the NQR signal can be made position‐dependent, allowing the distribution of the NQR‐active species in the material to be imaged. Finally, the relaxation times of NQR nuclei are highly sensitive to dynamics, and so can be used as a local probe of molecular mobility.

The internal structure of materials can also be imaged by x‐ray tomography and ultrasonic methods, which depend on the existence of inhomogeneities in the distribution of x‐ray absorbers, or acoustic properties, respectively. The stress and pressure dependences of the NQR frequency are generally much stronger than those of NMR imaging, and therefore NQR is a superior method for mapping the temperature or stress distribution across a sample. Such stress distributions can be determined for optically transparent material by using polarized light; for opaque material, there may be no alternative methods.

This article aims to be a general and somewhat cursory review of the theory and practice of modern one‐dimensional (1D) and two‐dimensional (2D) NQR and NQR imaging.
Cyclotron Resonance
Authors: Kono J.
Published: October 2003

Cyclotron resonance (CR) is a method for measuring the effective masses of charge carriers in solids. It is by far the most direct and accurate method for providing such information. In the simplest description, the principle of the method can be stated as follows. A particle of effective mass

and charge

in a DC magnetic field

executes a helical motion around

with the cyclotron frequency

. If, at the same time, an AC electric field of frequency

is applied to the system, perpendicular to

, the particle will resonantly absorb energy from the AC field. Since

and/or ω can be continuously swept through the resonance and known to a very high degree of accuracy,

can be directly determined with high accuracy by

.

As a secondary purpose, one can also use CR to study carrier scattering phenomena in solids by examining the scattering lifetime τ (the time between collisions, also known as the collision time or the transport/momentum relaxation time), which can be found from the linewidth of CR peaks.

Although this article is mainly concerned with the simplest case of free carrier CR in bulk semiconductors, one can also study a wide variety of FIR magneto‐optical phenomena with essentially the same techniques as CR. These phenomena (“derivatives” of CR) include: (a) spin‐flip resonances, (b) resonances of bound carriers, (c) polaronic coupling, and (d) 1‐D and 2‐D magneto‐plasmon excitations.

It is important to note that all the early CR studies were carried out on semiconductors, not on metals.

Many techniques can provide information on effective masses, but none can rival CR for directness and accuracy. Effective masses can be estimated from the temperature dependence of the amplitude of the galvanomagnetic effects.

The basic theory and experimental methods of cyclotron resonance are presented in this article. Basic theoretical background is presented. A detailed description is given of the actual experimentation procedures. Finally, typical data analysis procedures are presented.
Mössbauer Spectrometry
Authors: Fultz Brent
Published: October 2003

Mössbauer spectrometry is based on the quantum‐mechanical “Mössbauer effect,” which provides a nonintuitive link between nuclear and solid‐state physics. Mössbauer spectrometry measures the spectrum of energies at which specific nuclei absorb γ rays. Curiously, for one nucleus to emit a γ ray and a second nucleus to absorb it with efficiency, the atoms containing the two nuclei must be bonded chemically in solids.

Most applications of Mössbauer spectrometry in materials science utilize “hyperfine interactions,” in which the electrons around a nucleus perturb the energies of nuclear states. Hyperfine interactions cause very small perturbations of 10−9to 10−7eV in the energies of Mössbauer γ rays.

Interpretations of Mössbauer spectra have few parallels with other methods of materials characterization. A Mössbauer spectrum looks at a material from the “inside out,” where “inside” means the Mössbauer nucleus. Hyperfine interactions are sensitive to the electronic structure at the Mössbauer atom, or at its nearest neighbors. The important hyperfine interactions originate with the electron density at the nucleus, the gradient of the electric field at the nucleus, or the unpaired electron spins at the nucleus. These three hyperfine interactions are called the “isomer shift,” “electric quadrupole splitting,” and “hyperfine magnetic field,” respectively.

The viewpoint from the nucleus is sometimes too small to address problems in the microstructure of materials. In general, it is found that Mössbauer spectrometry is best for identifying the electronic or magnetic structure at the Mössbauer atom itself, such as its valence, spin state, or magnetic moment. The Mössbauer effect is sensitive to the arrangements of surrounding atoms, however, because the local crystal structure will affect the electronic or magnetic structure at the nucleus. Different chemical and structural environments around the nucleus can often be assigned to specific hyperfine interactions.

Other applications of the Mössbauer effect utilize its sensitivity to vibrations in solids, its timescale for scattering, or its coherence. To date these phenomena have seen little use outside the international community of a few hundred Mössbauer spectroscopists. Nevertheless, some new applications for them have recently become possible with the advent of synchrotron sources for Mössbauer spectrometry.

This article is not a review of the field, but an instructional reference that gives the working materials scientist a basis for evaluating whether or not Mössbauer spectrometry may be useful for a research problem.

A few representative applications to materials studies are presented.
X‐Ray Techniques
Authors: Goldman Alan I.
Published: October 2003

This article explores the wide variety of applications of x‐ray scattering and spectroscopy techniques to the study of materials. Materials, in this sense, include not only bulk condensed matter systems, but liquids and surfaces as well. The term “scattering,” is generally used to include x‐ray measurements on noncrystalline systems, such as glasses and liquids, or poorly crystallized materials, such as polymers, as well as x‐ray diffraction from crystalline solids. Information concerning long‐range, short‐range, and chemical ordering as well as the existence, distribution and characterization of various defects is accessible through these kinds of measurements. Spectroscopy techniques generally make use of the energy dependence of the scattering or absorption cross‐section to study chemical short‐range order, identify the presence and location of chemical species, and probe the electronic structure through inelastic excitations.

The energy dependence of the x‐ray absorption cross‐section is punctuated by absorption edges or resonances that are associated with interatomic processes of constituent elements. These resonances can be used to great advantage to investigate element‐specific properties of the material and are the basis of many of the spectroscopic techniques described in set of articles.
X‐Ray Powder Diffraction
Authors: Stephens Peter W. , Bendele Goetz M.
Published: October 2003

X‐ray powder diffraction is used to determine the atomic structure of crystalline materials without the need for large (∼100‐μm) single crystals. “Powder” can be a misnomer; the technique is applicable to polycrystalline phases such as cast solids or films grown on a substrate. X‐ray powder diffraction can be useful in a wide variety of situations. A number of questions that can be effectively addressed by this technique are listed. This is an attempt to illustrate the versatility of x‐ray powder diffraction and not, by any means, a complete list. Six experiments are described as concrete examples and constitute an assortment of problems of varying difficulty.

X‐ray powder diffraction is an old technique, in use for most of this century. The capabilities of the technique have recently grown for two main reasons: (1) development of x‐ray sources and optics (e.g., synchrotron radiation, Göbel mirrors) and (2) the increasing power of computers and software for analysis of powder data. This article discusses the fundamental principles of the technique, including the expression for the intensity of each diffraction peak, gives a brief overview of experimental techniques, describes several illustrative examples, gives a description of the procedures to interpret and analyze diffraction data, and discusses weaknesses and sources of possible errors.
XAFSSpectroscopy
Authors: Heald Steve
Published: October 2003

X rays, like other forms of electromagnetic radiation, are both absorbed and scattered when they encounter matter. X‐ray scattering and diffraction are widely utilized structural methods employed in thousands of laboratories around the world. X‐ray diffraction techniques are undeniably among the most important analysis tools in nearly every physical and biological science. As this article shows, x‐ray absorption measurements are achieving a similar range of application and utility. Absorption methods are often complementary to diffraction methods in terms of the area of application and the information obtained.

The basic utility of x‐ray absorption arises from the fact that each element has characteristic absorption energies, usually referred to as absorption edges.

The widespread use of x‐ray absorption methods is intimately connected to the development of synchrotron radiation sources. The measurements require a degree of tunability and intensity that is difficult to obtain with conventional laboratory sources. Because of this need and of advances in the theoretical understanding that came at about the same time as the major synchrotron sources were first developed, the modern application of absorption methods for materials analysis is only ∼25 years old. However, it has a long history as a physical phenomenon to be understood, with extensive work beginning in the 1930s.

This article concentrates on the modern application of XAS at synchrotron sources. Conducting experiments at remote facilities is a process with its own style that may not be familiar to new practitioners. Some issues related to working at synchrotrons are discussed.
Magnetic X‐Ray Scattering
Authors: Hill J.P.
Published: October 2003

There are two regimes of importance in magnetic x‐ray scattering, characterized by the incident photon energy relative to the positions of the absorption edges in the system under study. These are the nonresonant and resonant limits to the cross‐section.

This article provides an overall background to the theoretical and experimental principles of nonresonant and resonant magnetic x‐ray scattering. The experimental techniques are very similar to standard synchrotron x‐ray diffraction experiments; therefore discussion of the beamlines and spectrometers are limited, with only those elements particular to magnetic scattering, such as polarization analyzers, discussed in any detail. At high incident photon energy, it is possible to perform inelastic experiments utilizing the nonresonant magnetic scattering terms in the cross‐section. This allows for the measurement of magnetic Compton profiles (projections of the momentum density of the magnetic electrons, along a given direction). Magnetic Compton scattering may only be applied to ferromagnets, because of the incoherent nature of the inelastic scattering, and the technique differs in many aspects of its practical implementation from the diffraction experiments discussed here.
Resonant Scattering Techniques
Authors: Finkelstein Kenneth D.
Published: October 2003

This article will describes resonant (anomalous) x‐ray diffraction as it is used to obtain information about the roles of symmetry and bonding on the electronic structure of selected atoms in a crystal. These effects manifest themselves as crystal orientation‐dependent changes in the diffracted signal when the x‐ray energy is tuned through the absorption edges for those atoms. Applications of the method have demonstrated it is useful in: (1) providing site‐specific electronic structure information in a solid containing the same atom in several different environments; (2) determining the positions of specific atoms in a crystal structure; (3) distinguishing electronic from magnetic contributions to diffraction; (4) isolating and characterizing multipole contributions to the x‐ray scattering that are sensitive to the local environment of atoms in the solid; and (5) studying the ordering of 3
d
electron orbitals in the transition metal oxides.

This article describes resonant x‐ray scattering by presenting its principles, illustrating them with several examples, providing background on experimental methods, and discussing the analysis of data. Principles of the method presented are the classical picture of x‐ray scattering, including anomalous dispersion, followed by a quantum‐mechanical description for the scattering amplitudes, and ends with a discussion on techniques of symmetry analysis useful in designing experiments. Two examples from the literature are used to illustrate the technique. This is followed by a discussion of experimental aspects of the method, including the principles of x‐ray polarization analysis. A list of criteria is given for selecting systems amenable to the technique, along with an outline for experimental design. A general background on data collection and analysis includes references to sources of computational tools and expertise in the field.
X‐Ray Microprobe for Fluorescence and Diffraction Analysis
Authors: Ice Gene E.
Published: October 2003

X‐ray diffraction and x‐ray‐excited fluorescence analysis are powerful techniques for the nondestructive measurement of crystal structure and chemical composition. X‐ray fluorescence analysis is inherently nondestructive, with orders‐of‐magnitude lower power deposited for the same detectable limit as with fluorescence excited by charged particle probes. X‐ray diffraction analysis is sensitive to crystal structure with orders‐of‐magnitude greater sensitivity to crystallographic strain than electron probes. When a small‐area x‐ray microbeam is used as the probe, chemical composition, crystal structure, crystalline texture, and crystalline strain distributions can be determined. These distributions can be studied both at the surface of the sample and deep within the sample. Current state‐of‐the‐art can achieve an ∼1‐μm diameter x‐ray microprobe and an ∼0.1 μm diameter x‐ray microprobe has been demonstrated.

Despite their great chemical and crystallographic sensitivities, x‐ray microprobe techniques have until recently been restricted by inefficient x‐ray focusing optics and weak x‐ray sources; x‐ray microbeam analysis was largely superseded by electron techniques in the 1950s. However, interest in x‐ray microprobe techniques has now been revived by the development of efficient x‐ray focusing optics and ultra‐high‐intensity synchrotron x‐ray sources. These advances have increased the achievable microbeam flux by more than 12 orders of magnitude; the flux in a tunable 1 μm‐diameter beam on a so‐called third‐generation synchrotron source such as the Advanced Photon Source (APS) can exceed the flux in a fixed‐energy mm2beam on a conventional source. These advances place x‐ray microfluorescence and x‐ray microdiffraction analysis techniques among the most powerful techniques available for the nondestructive measurement of chemical and crystallographic distributions in materials.

This article reviews the physics, advantages, and scientific applications of hard x‐ray (E> 3 keV) microfluorescence and x‐ray microdiffraction analysis. Because practical x‐ray microbeam instruments are extremely rare, a special emphasis will be placed on instrumentation, accessibility, and experimental needs which justify the use of x‐ray microbeam analysis.
Scanning Electron Microscopy
Authors: Chandler Gary W. , Seraphin Supapan
Published: October 2003

The scanning electron microscope (SEM) is one of the most widely used instruments in materials research laboratories and is common in various forms in fabrication plants. Scanning electron microscopy is central to microstructural analysis and therefore important to any investigation relating to the processing, properties, and behavior of materials that involves their microstructure. The SEM provides information relating to topographical features, morphology, phase distribution, compositional differences, crystal structure, crystal orientation, and the presence and location of electrical defects. The SEM is also capable of determining elemental composition of micro‐volumes with the addition of an x‐ray or electron spectrometer and phase identification through analysis of electron diffraction patterns. The strength of the SEM lies in its inherent versatility due to the multiple signals generated, simple image formation process, wide magnification range, and excellent depth of field.

Lenses in the SEM are not a part of the image formation system but are used to demagnify and focus the electron beam onto the sample surface. This gives rise to two of the major benefits of the SEM: range of magnification and depth of field in the image. Depth of field is that property of SEM images where surfaces at different distances from the lens appear in focus, giving the image three‐dimensional information. The SEM has more than 300 times the depth of field of the light microscope. Another important advantage of the SEM over the optical microscope is its high resolution. Resolution of 1 nm is now achievable from an SEM with a field emission (FE) electron gun. Magnification is a function of the scanning system rather than the lenses, and therefore a surface in focus can be imaged at a wide range of magnifications from 3× up to 150,000×. The higher magnifications of the SEM are rivaled only by the transmission electron microscope (TEM), which requires the electrons to penetrate through the entire thickness of the sample. As a consequence, TEM sample preparation of bulk materials is tedious and time consuming, compared to the ease of SEM sample preparation, and may damage the microstructure. The information content of the SEM and TEM images is different, with the TEM image showing the internal structure of the material.

Due to these unique features, SEM images frequently appear not only in the scientific literature but also in the daily newspapers and popular magazines. The SEM is relatively easy to operate and affordable and allows for multiple operation modes, corresponding to the collection of different signals. This article reviews the SEM instrumentation and principles, its capabilities and applications, and recent trends and developments.
Low‐Energy Electron Diffraction
Authors: Gerken Craig A. , Somorjai Gabor A.
Published: October 2003

Low‐energy electron diffraction (LEED) is a powerful method for determining the geometric structure of solid surfaces. It is similar to x‐ray diffraction (XRD) in the type of information that it provides, and XRD will be frequently referred to for analogy throughout this unit. The most obvious difference is that a beam of electrons, rather than x rays, is used. Since electrons have a mean free path (or attenuation length) measured in angstroms (Å) as opposed to microns (μ) for x rays, LEED is particularly sensitive to surface geometry.

LEED is a structural technique that can provide essentially two levels of information. In both cases, it is most commonly used to determine the structure of a solid surface when the bulk structure of the material is already known by other means (e.g., XRD). It is possible, and indeed straightforwardly simple, to use LEED to determine both the absolute dimensions of the surface unit cell and the unit cell symmetry. One can easily deduce that, for example, a surface has a two‐dimensional (2D) unit cell twice as large as that of the bulk; this often allows a reasonable guess at the true structure. Sometimes it is sufficient to know that a particular phase is present without knowing the details of the structure. Determination of the exact atomic positions is more difficult, but quantitative experiments can elucidate this second level of information. Sophisticated calculations, generally run on a workstation, can provide atomic coordinates with a typical precision of ±0.05 Å, which is generally more than adequate to determine the adsorption site of a molecule or the atomic positions in a reconstructed surface.

Many different surfaces can be examined by LEED, and their terminology must be understood.

LEED is a mature area of surface science and it is useful to consider the state of the art in this field. A great deal of work has been done on clean metal and semiconductor surfaces, and much of this is considered “done”; current work in this area consists mostly of refinements.
High‐Energy Ion Beam Analysis
Authors: Doyle Barney , McDaniel Floyd Del
Published: October 2003

It is very hard to identify the earliest application of high‐energy ion beam analysis (IBA), which was a natural outgrowth of accelerator‐based atomic and nuclear physics that started in the 1940s with the development of particle accelerators. In these early experiments, the composition of the target to be studied was well known, and it was the nuclear scattering or reaction cross‐section that was measured. It would not be surprising if one of the very first nuclear physics pioneers turned this emphasis around and used their beams to measure the compositional uniformity of an unknown sample. Perhaps the earliest known proposal for IBA was that of E. Rubin in 1949 at the Stanford Research Institute, which spelled out the principle of Rutherford backscattering spectrometry (RBS). Another very early application of IBA was performed by Harald Enge at MIT. Enge used mega‐electron‐volt deuterons from one of Van de Graaff's first accelerators with a magnetic spectrometer to measure the constituents of aluminized Formvar in perhaps the earliest materials analysis application of nuclear reaction analysis (NRA). It is interesting to note that the use of magnetic spectrometers to obtain ultrahigh depth resolution is now common.

High‐Energy IBA is generally thought to apply when the ion beam has energy greater than 0.5 MeV. There are several attributes of high‐energy IBA, including (1) the ability to measure virtually any element in the periodic table, at times with isotopic specificity; (2) the ability to determine concentration as a function of depth; (3) that trace element sensitivity is not uncommon; and (4)that the technique is nondestructive with respect to the chemical makeup of the sample.

The periodic table given provides a convenient overview of high‐energy IBA capabilities.
Auger Electron Spectroscopy
Authors: Grant John T.
Published: October 2003

Auger electron spectroscopy (AES) is a method for determining the elemental composition of the several outermost atomic layers of materials. The surface layers often have a composition that is quite different from the bulk material due to contamination, oxidation, or processing. In AES, a specimen is probed with an energetic electron beam with an energy fixed between 3 and 30 keV, resulting in the ejection of core‐level electrons from atoms. The resulting vacancy in a core level can be filled by an outer‐level electron, with the excess energy being used to emit either an x‐ray (electron probe microanalysis) or another electron from the atom (AES). This emitted electron is called an Auger electron. AES is a surface‐sensitive technique due to the strong inelastic scattering of low‐energy electrons traveling within specimens. Auger electrons from only the outermost several atomic layers are emitted from the specimen without energy loss, and contribute to the Auger peak intensities in a spectrum. The Auger electron kinetic energies are characteristic of the emitting atoms, and the measurement of their energies is used to identify the elements that produce them. The concentrations of elements detected can be determined from the intensities of the Auger peaks. Variation of composition with depth can be determined by depth profiling, which is usually accomplished by removing atomic layers by sputtering with inert gas ions and monitoring the Auger signals from the newly created surfaces. For most elements, the detection limit with AES is between 0.1 and 1 at.%.
Radiation Effects Microscopy
Authors: Breese Mark B.H.
Published: October 2003

Radiation effects microscopy is a general name that includes both ion‐beam‐induced charge (IBIC) microscopy and single‐event upset (SEU) imaging. Ion‐beam‐induced charge microscopy was initially developed as a means of imaging the distribution of buriedpnjunctions in microelectronic devices and dislocation networks in semiconductors. More recently it has found a wider range of applications, e.g., in the study of solar cells, segmented silicon detectors, and chemical vapor deposition (CVD) diamond films.

Ion‐beam‐induced charge microscopy uses a beam of mega‐electron‐volt light ions, mainly protons or helium, to create electron‐hole pairs in semiconductor material. This energy transfer from the incident ions to the material causes them to lose energy and eventually stop in the material at a well‐defined distance called the ion range. The number of these electron or hole charge carriers that are collected at a junction within the sample is measured and displayed as a function of position of the focused beam within the scanned area.

As microelectronic device features continually shrink, the amount of charge that defines the different logic levels becomes smaller. This means that the charge generated by the passage of ionizing radiation through the device is more likely to cause a change in its logic state, called a soft upset, or a SEU. This phenomenon is particularly important for satellite‐based devices, which are exposed to a high flux of ionizing cosmic radiation. It is a serious problem in the design of high‐density semiconductor memories. The lack of spatially resolved information led to difficulties in determining which part of the device caused the upsets. Single‐event upset analyses with collimated heavy‐ion beams revealed the benefits of spatially resolving the results. This prompted the development of SEU microscopy at Sandia. This has many similarities to IBIC, which is usually performed in conjunction with SEU microscopy to image the device components present within the area analyzed for upsets. The two techniques use almost all the same nuclear microprobe hardware and electronics. Whereas IBIC mainly uses mega‐electron‐volt light‐ion beams for analysis, SEU analysis uses heavier ions, such as carbon or silicon, to create a plasma density of electron‐hole charge carriers high enough to cause upsets.

A major advantage of IBIC and SEU microscopy over competing electron beam techniques is their ability to give high‐spatial‐resolution analysis in thick layers. This is due to the different shapes of the charge carrier generation volume of mega‐electron‐volt ions compared with kilo‐electron‐volt electrons.