X-rays have found a wide range of applications in chemistry, physics, and materials engineering since their discovery in 1895 by W. Roentgen. The materials science community uses x-ray-based techniques extensively for characterization of materials. In the 1970s a new tunable source of x-rays from the radiation produced by synchrotron accelerators emerged. Synchrotron radiation (SR) is an intense and forward-focused beam of radiation that is emitted when the path of an electron traveling at almost the speed of light is bent by a magnetic field. Figure 1 illustrates the evolution of radiation intensity provided by various x-ray sources. In situ SR techniques provide real-time observation of atomic arrangements with high spatial sensitivity and precision, which are important features not only in fundamental materials research, but also in the development of novel processing techniques and in the search for new exotic materials. A major advantage of SR is that it covers a wide range of wavelengths continuously from infrared to gamma rays. This feature is attractive since a wealth of detailed information on the electronic and structural properties of materials can be obtained by optimizing the wavelength of the radiation.

Since the establishment of “first generation” facilities in the early 1970s, the x-ray emittance from synchrotron storage rings, where electrons traveling at almost relativistic speed s are constrained by magnetic fields to follow curved trajectories, has shown dramatic improvements. See Table I for an extensive list of SR facilities presentiy in use throughout the world.

Vapor-phase processes such as chemical vapor deposition (CVD) and reactive ion etching are the primary methods for the production-scale synthesis and processing of many high-quality thin-film materials. For example, these processes are widely used in the microelectronics industry for synthesis and lithography of the various semiconducting, insulating, and conducting layers in devices. Understanding the means of controlling the microstructure and composition of these materials is of great technological interest. However a difficulty often encountered in developing vapor-phase processes is an undesirable dependence on trial-and-error methods for optimizing the many process parameters. These parameters include gas composition, flow rate, pressure, and substrate temperature, all of which are typically changing with time. This reliance on empirical methods can be attributed to the tremendous chemical and physical complexity of vapor-phase processes and the lack of appropriate in situ measurement techniques for the vapor-phase environment.

We have initiated a program to apply synchrotron x-ray analysis techniques as real-time probes of film and surface structure during vapor-phase processing. X-rays have a combination of properties which makes them particularly well-suited for these studies. Unlike electrons, x-rays have a sufficiently low absorption to penetrate vapor-phase processing environments and chamber walls. Unlike visible light, x-rays have wavelengths and energies suitable for study of atomic-scale structure and chemistry. A growing number of in situ synchrotron x-ray investigations of film growth and processing demonstrate the power of these techniques.

A substantial and continuous volume expansion from liquid to rarefied vapor occurs by the change of temperature and pressure surrounding the liquid-vapor critical point without crossing the saturated vapor pressure curve. Many investigations have been carried out with molecular fluids including supercritical fluids, and several variations of the equation of state have been advanced since the famous work by van der Waals. In the expansion process the mean interatomic distance increases by up to 10 times that found under standard conditions. However reconstitution of the molecular units or other major structure changes are rare in these fluids. On the other hand, in the expansion process of metallic or semiconducting liquids, physical properties can drastically change. Liquid Hg, well-known as a prototype of liquid metals, undergoes a metal-insulator transition when the density is reduced to about 9 g/cm3 near the critical point.Elemental Se, which units in its liquid form is a semiconductor with a twofold-coordinated chain structure in which atoms are covalently bonded, also experiences a semiconductor-metal-insulator transition around the critical point (critical temperature and pressure data of Se: Tc = 1615°C, Pc = 385 bar, pc = 1.85 g/cm3).

The first indication of the semiconductormetal (SC-M) transition in fluid Se was found in electrical conductivity data by Hoshino and co-workers. Figure 1 shows the contours of constant dc conductivity plotted on the pressure-temperature (P-T) plane. The high-conductivity region appears in the immediate vicinity of the critical point. Another indication of the SC-M transition was obtained from the measurement of the optical reflectivity by Seyer et al. Drude-like energy dependence of the reflectivity spectra was observed at elevated pressures and temperatures.

Amorphous chalcogenides, of which selenium is the simplest representative, exhibit a number of unique properties such as the ability to undergo various transformations under the action of the bandgap light. On illumination, the absorption edge shifts to lower energies, and subsequent annealing near the glass-transition temperature leads to a recovery of the initial parameters as demonstrated in Figure 1. Such a photo-induced change could not be observed either in amorphous group IV semiconductors or a-As or in crystalline chalcogenides. A good review of the initial stage of these studies was made by de Neufville.

Reversible photodarkening can also be observed in pure chalcogens, but this process can be achieved only at lower temperatures, which is understandable if one takes into account that the glass-transition temperature of selenium is just above room temperature. This result indicates that there is a correlation between the temperature at which photodarkening is annealed out and the flexibility of the glassy network.

Reversible changes in the optical absorption are accompanied by (reversible) changes in electrical and photoelectric properties, volume, microhardness, glass-transition temperature, and dissolution rate in various solvents, to name a few. The totality of these changes has led investigators to the conclusion that the photoinduced changes in optical absorption are caused by changes in structure.

Aqueous-solution/solid interfaces are ubiquitous in modern manufacturing environments as well as in our living environment, and studies of such interfaces are an active area of science and engineering research. An important area is the study of liquid/solid interfaces under active electrochemical control, which has many immediate technological implications, for example, corrosion/passivation of metals and energy storage in batteries and ultracapacitors. The central phenomenon of electrochemistry is the charge transfer at the interface, and the region of interest is usually wider than a single atomic layer, ranging from a monolayer to thousands of angstroms, extending into both phases.

Despite the technological and environmental importance of liquid/solid interfaces, the atomic level understanding of such interfaces had been very much hampered by the absence of nondestructive, in situ experimental techniques. The situation has changed somewhat in recent decades with the development of the largely ex situ ultrahigh vacuum (UHV) surface science, modern spectroscopic techniques, and modern surface microscopy.

However in situ experiments of electrochemical interfaces are difficult, stemming from the special nature of these interfaces. These are so-called buried interfaces in which the solid electrode surface is covered by a relatively thick liquid layer. For this reason, the probe we use in the structural investigation must satisfy simultaneously two conditions: (1) the technique must be surface/interface sensitive, and (2) absorption of the probe in the liquid phase must be sufficiently small for penetration to and from the interface of interest without significant intensity loss.

Circularly polarized x-ray radiation is attracting increasing interest as a tool for the characterization of the electronic, magnetic, and chiral properties of low-dimensional structures. Using circular light (with electric field vector parallel to the orbital plane), a dependence of the measured quantity by changing either the orientation of the light polarization or the magnetization is indicative of the existence of magnetic circular dichroism. It can be observed in x-ray absorption spectroscopy (XAS), in which the photon energy is scanned through an absorption threshold exciting a core electron into an unoccupied valence state using circularly polarized light. Synchrotron radiation sources have made this technique possible. It can also be observed in photo-emission spectroscopy from core and valence levels. Here we focus on magnetic circular x-ray dichroism (MCXD) in XAS as an element-specific tool to investigate magnetic properties of ultrathin films in situ. The application of magneto-optical sum rules enables the determination of the orbital and spin magnetic moments per atom from XAS spectra, as well as the easy magnetization direction.

MCXD-based magnetometry in XAS is extensively used by measuring the L absorption edges of 3d-transition metals, where large intensity changes (up to 60%) of the L-edge white lines are observed upon reversal of either the sample magnetization or the light helicity. The high magnetic contrast obtained, combined with the elemental specificity of the technique, allows for the study of very dilute samples such as ultrathin films. We first concentrate on the selection rules governing MCXD in XAS.

The study of processing stimulated by synchrotron radiation (SR) started about 13 years ago. Unfortunately, for semiconductor processing, the reaction rate of this technique was considered to be too slow for practical applications. However the situation is rapidly changing as device dimensions shrink to the nanometer range, which requires high controllability and low defect densities, but is not so reliant on quick processing since much less material is involved. SR etching—which has a typical etching rate of 1–10 nm/min, capability of high-resolution etching due to the short wavelength of SR photons, high controllability, low damage production, and a clean reaction system—is considered to be extremely suitable for nanometer processing.

The benefit of SR etching is that it has unique material selectivities. However the mechanisms for the selectivity are not yet clarified. Consideration of the future application of the SR process to nanometer structure fabrication is dependent upon the development of in situ observation techniques that can evaluate the reaction surface at an atomic or molecular level. Recently we have developed an in situ observation technique: infrared reflection absorption spectroscopy using buried metal layer substrates (BML-IRRAS), which is especially suitable for in situ monitoring of beam-induced reactions on the semiconductor surface in high vacuum.

Every theory of superconductivity can be disproved! This tongue-in-cheek theorem struck a chord when Felix Bloch announced it in the early 1930s. Virtually every major physicist then working on theory—including Bloch, Niels Bohr, Wolfgang Pauli, Werner Heisenberg, Lev Landau, Leon Brillouin, W. Elsasser, Yakov Frenkel, and Ralph Kronig—had tried and failed to explain the mysterious phenomenon in which below a few degrees kelvin certain metals and alloys lose all their electrical resistance. The frequency with which Bloch's theorem was quoted suggests the frustration of the many physicists who were struggling to explain superconductivity.

Neither the tools nor the evidence were yet adequate for solving the problem. These would gradually be created during the 1940s and 1950s, but bringing them to bear on superconductivity and solving the long-standing riddle required a special set of talents and abilities: a deep understanding of quantum mechanics and solid-state physics, confidence in the solubility of the problem, intuition about the phenomenon, a practical approach to problem-solving, patience, teamwork, and above all refusal to give up in the face of repeated failures. When John Bardeen took on the problem of superconductivity in the late 1930s, he held it like a bulldog holds a piece of meat, until he, his student J. Robert Schrieffer, and postdoctoral candidate Leon Cooper solved it in 1957.