One of the dominant trends in current research in materials science and related fields is the fabrication, characterization, and application of materials and device structures whose characteristic feature sizes are at or near the nanometer scale. Achieving an understanding of—and ultimately control over—the properties and behavior of a wide range of materials at the nanometer scale has therefore become a major theme in materials research. As our ability to synthesize materials and fabricate structures in this size regime improves, effective characterization of materials at the nanometer scale will continue to increase in importance.

Central to this activity are the development and application of effective experimental techniques for performing characterization of structural, electronic, magnetic, optical, and other properties of materials with nanometer-scale spatial resolution. Two classes of experimental methods have proven to be particularly effective: scanning-probe techniques and electron microscopy. In this issue of MRS Bulletin, we have included eight articles that illustrate the elucidation of various aspects of nanometer-scale material properties using advanced scanningprobe or electron-microscopy techniques. Because the range of both experimental techniques and applications is extremely broad—and rapidly increasing—our intent is to provide several examples rather than a comprehensive treatment of this extremely active and rapidly growing field of research.

As characteristic dimensions in semiconductor devices continue to shrink and as advanced heterostructure devices increase in prominence, the ability to characterize structure and electronic properties in semiconductor materials and device structures at the atomic to nanometer scales has come to be of outstanding and immediate importance. Phenomena such as atomic-scale roughness of heterojunction interfaces, compositional ordering in semiconductor alloys, discreteness and spatial distribution of dopant atoms, and formation of self-assembled nanoscale structures can exert a profound influence on material properties and device behavior. The relationships between atomic-scale structure, epitaxial growth or processing conditions, and ultimately material properties and device behavior must be understood for realization and effective optimization of a wide range of semiconductor heterostructure and nanoscale devices.

Cross-sectional scanning tunneling microscopy (STM) has emerged as a unique and powerful tool in the study of atomic-scale properties in III-V compound semiconductor heterostructures and of nanometer-scale structure and electronic properties in Si micro-electronic devices, offering unique capabilities for characterization that in conjunction with a variety of other, complementary experimental methods are providing new and important insights into material and device properties at the atomic to nanometer scale. In this article, we describe the basic experimental techniques involved in cross-sectional STM and give a few representative applications from our work that illustrate the ability, using cross-sectional STM in conjunction with other experimental techniques, to probe atomic-scale features in the structure of semiconductor heterojunctions and to correlate these features with epitaxial-growth conditions and device behavior.

Near-field scanning optical microscopy (NSOM) provides a means to study optical and optoelectronic properties of materials at the nanometer scale. The key to achieving resolution higher than the diffraction limit is to place a subwavelength-sized light source—e.g., an aperture—within the near-field zone of the sample. In this case, the area of the sample illuminated is determined by the aperture size and not by the wavelength (see Figure 1). An image can then be formed by moving the sample and light source with respect to each other. While the principle of near-field optics is straightforward, its realization at visible-light wavelengths was not achieved until the invention of scanning-probe techniques in the 1980s. Since Betzig et al. demonstrated in 1991 that bright subwavelength apertures can be made by tapering and metal-coating single-mode optical fibers, research activities involving NSOM have increased tremendously. The later incorporation of shear-force feedback to regulate tip-sample separation adds another strength to NSOM. Using this distance regulation, a topographic image similar to that obtained by a conventional scanning force microscope is acquired simultaneously with the optical image. This provides a way to correlate structural and physical properties at the same sample positions and greatly simplifies interpretation of the NSOM data.

Heteroepitaxial pseudomorphic thin-film growth allows the stabilization of solids in structures—that is, with crystalline symmetries and lattice constants that are far from those of the corresponding bulk material. This facilitates the tailoring of materials properties that do not exist in nature. Usually a strong correlation is found between the atomic, electronic, and magnetic structure of thin films, which leads to a drastic dependence of their physical properties on the details of growth conditions such as substrate temperature, evaporation rate, and background pressure. To learn more about the fundamental relationship between structural, electronic, and magnetic properties of thin films as well as to improve the reproducibility of their physical properties in view of technical applications, it is mandatory to apply experimental techniques with sufficient, ultimately atomic spatial resolution combined with high sensitivity. The class of scanning-probe microscopies and spectroscopies is ideally suited for that purpose as schematically illustrated in Figure 1. Scanning tunneling spectroscopy (STS) establishes the correlation between atomic structure and local electronic properties, while spin-polarized scanning tunneling spectroscopy (SPSTS) relates local electronic and magnetic properties. Additionally the correlation

between topographical and magnetic structure can be addressed by magnetic force microscopy (MFM) though the spatial resolution is limited to about 10–50 nm. Since magnetic thin films are usually sensitive to oxidation, their preparation and characterization have to be performed in situ under ultrahigh vacuum (UHV) conditions in order to achieve reproducible experimental conditions. While STS is now routinely applied in UHV, it has only been recently that the first MFM studies under UHV conditions have been reported. In the following, representative examples of the application of STS, SPSTS, and MFM in ultrathin Fe, Gd, and Co films will be presented.

Scanning-probe microscopies (SPMs) have experienced an impressive development in the last 10 years, such that obtaining atomically resolved images of surfaces is now routine. The scanning tunneling and atomic force microscopes (STM and AFM) are the most widely known representatives of the growing family of SPMs. With these two instruments, which were invented more than 15 years ago, the atomic structure of the surfaces of solid materials can be imaged in real space and, very importantly, in virtually any environment: vacuum, air, reactive gas atmospheres, and under liquid. These two circumstances (atomic resolution and lack of environmental constraints) make SPM unique among microscopies. One frontier remains unconquered however: the imaging of liquid surfaces. Even though liquids constitute a very large fraction of all materials, the study of their microscopic morphology has been limited to the diffraction limit of visible optical microscopy, a situation that has remained unchanged for decades. Techniques capable of breaking this resolution limit are welcomed. One such technique was recently developed in our laboratory, which we call scanning-polarization-force microscopy (SPFM). It did not result from the discovery of any new physical principle but from the simple application of well-known methods and techniques, in combination with the AFM, to the field of liquid surface structures. In this article, we will first summarize the working principle of SPFM. Then we will show how we have applied it to the investigation of liquid films and droplets of nanometer dimensions and their interactions with solid supports.

Metal-ceramic composite applications range from electronic devices such as electronic packaging, thin-film technology in microwave circuitry, and magnetic storage media to catalyst supports, protective coatings, and high-temperature structural components. These applications rely heavily on the ability to engineer the mechanical and electronic properties of metal-ceramic interfaces. To understand fundamental aspects of these interfaces, a variety of experimental and theoretical studies on “model systems” have been performed, correlating macroscopic material behavior and microscopic characteristics. The results serve as a guide to more complex systems, with a closer relationship to technological applications.

The behavior of solids in the nanometer size regime, as their dimensions approach the atomic scale, is of increasing fundamental and applied interest in materials research. Electronic, optical, magnetic, mechanical, or thermodynamic properties all may depend on the size and shape of the solid. As a result, in the nanoscale regime, size and shape may be used as design variables to tailor a material's properties such as giant magnetoresistance in multilayer films, or the optical properties in semiconductor nanocrystals. In most cases, the size dependence of properties is not well-understood. Nanophase materials constitute a new frontier in materials science, and accurate nanoscale characterization is extremely important in exploring this new frontier. In this area, transmission electron microscopy (TEM) plays a key role. Because of its unique ability to provide information on the structure and composition of internal interfaces in solids, TEM is particularly important in cases of buried nanophase structures such as small solid inclusions—that is, solid particles embedded within another solid.

Nanoscale inclusions have recently been shown to exhibit unusual melting behavior that depends strongly on their size and the embedding matrix. For example, small inclusions of Pb in SiO exhibit melting-point depressions of several hundred degrees, whereas similarsized Pb inclusions in aluminum have shown large increases in melting point. Although a full understanding of these effects is still lacking, it appears that they are related not just to inclusion size but also to their shape and interface structure.

Interest in semiconductor grain boundaries relates to the development of polycrystalline materials for photovoltaics and integrated-circuit interconnects. Although these structures are responsible for deleterious electrical effects, there are few experimental techniques available to study them at the required atomic scale. Therefore models of the physical processes occurring at grain boundaries have necessarily taken a macroscopic approach. Fortunately recent developments have resulted in tools that provide unprecedented glimpses into these interfaces and that will allow us to address anew the connection between grain-boundary structure and properties.

Z-Contrast Imaging

When exploring the unknown, we rely heavily on our eyes (incoherent imaging) to provide a direct image of a new object. In order to explore the unforeseen atomic configurations present at extended defects in materials, it again would be desirable if one could obtain a directly interpretable image of the unfamiliar structures present in the defect cores. Z-contrast electron microscopy provides such a view with both atomic resolution and compositional sensitivity.

This high-resolution imaging technique differs from conventional high-resolution phase-contrast imaging. The phase-contrast technique produces a coherent image, an interference pattern formed by recombining the waves diffracted by the specimen. In the Z-contrast technique, the image is incoherent; it is essentially a map of the scattering power of the specimen. Additionally as was first determined by Lord Rayleigh, the incoherent mode of image formation has double the resolving power of the coherent mode.

Nanoscale materials show a variety of interesting physical and chemical properties. Since the properties are mostly determined by mesoscopic- and atomicscale areas of the materials, characterization of nanoscale materials at the atomic level is crucial for understanding the origin of their properties as well as for tailoring them to industrial needs. High-resolution electron microscopy (HREM) is an effective method to observe atomic structures in nanophase materials. The “structure-imaging technique,” originated by Ueda et al., and Cowley and Iijima, contributed much to the development of the method. Innovations are anticipated in (1) three-dimensional analysis of nanophase materials in real space, (2) observation of dynamic phenomena, and (3) extraction of compositional and physical information from nanometer-sized areas. Using the structure-imaging technique, one can observe directly nanometer-scale particles and defect structures such as voids and line and plane defects in crystals, including surfaces and interfaces. There were previously some limitations on the study of the dynamic process on the atomic level since images were captured on films. The recent development of high-sensitivity silicon-integrated-target-television (SIT-TV) cameras has initiated a new stage of HREM.

Our time-resolved-high-resolution-electron-microscopy (TRHREM) system is composed of a 200-kV HREM (JEOL: JEM-2010) equipped with a high-sensitivity SIT-TV camera, various types of dedicated sample holders, an ultrahigh-vacuum (UHV) sample-preparation chamber, and a digital videotape recorder, as partly illustrated in Figure 5. The HREM images at 0.8–1.0-million magnification are recorded on the videotape recorder. In the U.S.-Japan TV system, one “frame image” of 1/30-s time resolution is composed of two interlaced “field images” of 1/60-s time resolution. The digital videotape recorder enables us to output the two “field images” separately from two frame memories.