Annual Review of Materials Research - Volume 29, 1999

The recent availability of intense synchrotron sources with selectable X-ray energies permits high-precision measurements of chemically specific atomic-pair correlations in solid-solution alloys. Short-range chemical order can be accurately measured to identify one atom in a 100 for 10 or more shells, even in alloys with elements nearby in the periodic table, and chemically specific static displacements can be measured with 0.0001 nm resolution. This new information tests theoretical models of alloy phase stability and structure and gives new insights into the physical properties of alloys.

This review on magnetic force microscopy does not provide an exhaustive overview of the past accomplishments of the method but rather discusses the present state of the art. Magnetic force microscopy is a special mode of noncontact operation of the scanning force microscope. This mode is realized by employing suitable probes and utilizing their specific dynamic properties. The particular material composition of the probes and the dynamic mode of their operation are discussed in detail. The interpretation of images acquired by magnetic force microscopy requires some basic knowledge about the specific near-field magnetostatic interaction between probe and sample. The general magnetostatics as well as convenient simplifications of the general theory, which often can be used in practice, are summarized. Applications of magnetic force microscopy in the magnetic recording industry and in the fundamental research on magnetic materials are discussed in terms of representative examples. An important aspect for any kind of microscopy is the ultimately achievable spatial resolution and inherent restrictions in the application of the method. Both aspects are considered, and resulting prospects for future methodical improvements are given.

Recently there has been a resurgence of research efforts related to the investigation of new and novel materials for small-scale thermoelectric refrigeration and power generation applications. These materials need to couple and optimize a variety of properties in order to exhibit the necessary figure of merit, i.e. the numerical expression that is commonly used to compare one potential thermoelectric material with another. The figure of merit is related to the coefficient of performance or efficiency of a particular device made from a material. The best thermoelectric material should possess thermal properties similar to that of a glass and electrical properties similar to that of a perfect single-crystal material, i.e. a poor thermal conductor and a good electrical conductor. Skutterudites are materials that appear to have the potential to fulfill such criteria. These materials exhibit many types of interesting properties. For example, skutterudites are members of a family of compounds we call open structure or cage-like, materials. When atoms are placed into the interstitial voids or cages of these materials, the lattice thermal conductivity can be substantially reduced compared with that of unfilled skutterudites. These compounds exhibit electrical properties ranging from that of low-temperature superconductors to narrow gap semiconductors.

The scanning SQUID microscope (SSM) is a powerful tool for imaging magnetic fields above sample surfaces. It has the advantage of high sensitivity and bandwidth and the disadvantages of relatively modest spatial resolution and the requirement of a cooled SQUID sensor. We describe the various implementations of this type of instrument and discuss a number of applications, including magnetic imaging of short circuits in integrated circuits, corrosion currents in aluminum, and trapped flux in superconductors.

Combinatorial materials synthesis methods and high throughput evaluation techniques have been developed to accelerate the process of materials discovery and optimization. Analogous to integrated circuit chips, integrated materials chips containing thousands, possibly millions, of different compounds/materials, often in the form of high-quality epitaxial thin film can be fabricated and screened for interesting physical or chemical properties. Microspot X-ray methods, various optical measurement techniques, and a novel evanescent microwave microscope have been used to characterize the structural, optical, magnetic, and electrical properties of samples on materials chips. These techniques are routinely used to discover and optimize luminescent, ferroelectric, dielectric, and magnetic materials.

Heteroepitaxial structures with strained semiconductor thin films are widely used in electronic and optoelectronic devices. One of the more important defect creation processes in these films is related to a stress-induced morphological instability that tends to roughen the film surface by mass diffusion during film growth or annealing. Interestingly, the same mechanism of surface roughening can be utilized for fabrication of quantum dot devices. This article gives an overview of a series of theoretical and experimental studies on surface roughening in heteroepitaxial films. It is shown that the strain caused by lattice mismatch drives the diffusional atomic flux along the film surface in such a way that an initially flat film evolves into an undulating profile with cusp-like surface valleys with singular stress concentration near the cusp tip. The essential features of this evolution process are described by a family of mathematical curves called cycloids. The fundamental length and time scales associated with surface roughening can be obtained from thermodynamic and kinetic considerations. The stress concentration at cycloid-like surface valleys caused by roughening is found to create dislocations of various characters that participate in the overall strain relaxation of a heterostructure.

The synthesis of nanocrystalline diamond films from carbon-containing noble gas plasmas is described. The nanocrystallinity is the result of new growth and nucleation mechanisms, which involve the insertion of C2, carbon dimer, into carbon-carbon and carbon-hydrogen bonds, resulting in hetereogeneous nucleation rates on the order 10¹⁰ cm⁻² s⁻¹. Extensive characterization studies led to the conclusion that phase-pure diamond is produced with a microstructure consisting of randomly oriented 3–15-nm crystallites.

By adjusting the noble gas/hydrogen ratio in the gas mixture, a continuous transition from micro- to nanocrystallinity is achieved. Up to 10% of the total carbon in the nanocrystalline films is located at 2 to 4 atom-wide grain boundaries. Because the grain boundary carbon is π-bonded, the mechanical, electrical, and optical properties of nanocrystalline diamond are profoundly altered.

Nanocrystalline diamond films are unique new materials with applications in fields as diverse as tribology, cold cathodes, corrosion resistance, electrochemical electrodes, and conformal coatings on MEMS devices.

Heat conduction in novel electronic films influences the performance and reliability of micromachined transistors, lasers, sensors, and actuators. This article reviews experimental and theoretical research on heat conduction in single-crystal semiconducting and superconducting films and superlattices, polycrystalline diamond films, and highly disordered organic and oxide films. The thermal properties of these films can differ dramatically from those of bulk samples owing to the dependence of the material structure and purity on film processing conditions and to the scattering of heat carriers at material boundaries. Predictions and data show that phonon scattering and transmission at boundaries strongly influence the thermal conductivities of single-crystal films and superlattices, although more work is needed to resolve the importance of strain-induced lattice defects. For polycrystalline films, phonon scattering on grain boundaries and associated defects causes the thermal conductivity to be strongly anisotropic and nonhomogeneous. For highly disordered films, preliminary studies have illustrated the influences of impurities on the volumetric heat capacity and, for the case of organic films, molecular orientation on the conductivity anisotropy. More work on disordered films needs to resolve the interplay among atomic-scale disorder, porosity, partial crystallinity, and molecular orientation.

The chemical effects of ultrasound derive primarily from acoustic cavitation. Bubble collapse in liquids results in an enormous concentration of energy from the conversion of the kinetic energy of the liquid motion into heating of the contents of the bubble. The high local temperatures and pressures, combined with extraordinarily rapid cooling, provide a unique means for driving chemical reactions under extreme conditions. A diverse set of applications of ultrasound to enhance chemical reactivity has been explored with important uses in synthetic materials chemistry. For example, the sonochemical decomposition of volatile organometallic precursors in low-volatility solvents produces nanostructured materials in various forms with high catalytic activities. Nanostructured metals, alloys, oxides, carbides and sulfides, nanometer colloids, and nanostructured supported catalysts can all be prepared by this general route. Another important application of sonochemistry in materials chemistry has been the preparation of biomaterials, most notably protein microspheres. Such microspheres have a wide range of biomedical applications, including their use in echo contrast agents for sonography, magnetic resonance imaging, contrast enhancement, and oxygen or drug delivery. Other applications include the modification of polymers and polymer surfaces.

The electrophoretic deposition of materials is reviewed. Numerous applications of electrophoretic deposition are described, including production of coatings, free-standing objects, and laminated or graded materials, infiltration of porous materials, and fabrication of woven fiber preforms. The preparation of electrophoretic suspensions is discussed as are a number of mechanisms of deposition that have been proposed elsewhere. In discussing the kinetics of the process, primary attention is given to the relation between the evolution of the current and the electric field strength.

The electrostatic force microscope is one of many specialized tip sensors used in near-field microscopy. This type of microscope is realized by applying a voltage on a conducting AFM tip. It can be used to image samples that present a distribution of electrical properties on inhomogeneous materials as well as on nanostructures. This microscopy technique has been used to probe phase separation, chemical recognition, molecular orientation, and photo-induced charge separation in molecular photodiodes in Langmuir-Blodgett films.

Based on the spin conservation in electron tunneling across an insulator (I) and the spin polarization of conduction electrons in ferromagnets (FM) established by Meservey and Tedrow, Jullière put forward a quantitative model (1975) showing that tunneling in FM-I-FM junctions should lead to a large junction magnetoresistance (JMR). This conjecture was realized with repeatable results only in 1995, and since then JMR values >30% have been achieved at room temperature. This phenomenon has tremendous potential for applications as nonvolatile magnetic memory elements, read heads, and picotesla field sensors.

We review the experimental results and the current theoretical understanding of FM-I-FM tunneling and its dependence on bias, temperature, and barrier characteristics. The influence of inelastic tunneling processes and material properties on the JMR is extensively discussed. Early theories are reviewed and their relationship to the linear response theory is presented. Future directions, both from the point of fundamental physics as well as applications, are also covered.

Recent progress on the use of near-field scanning optical microscopy (NSOM) to characterize organic thin film materials is extensively reviewed. NSOM is leading to important new information on the morphology and spatially resolved optical properties of a variety of materials, complementing more widely available methods for thin film analysis. Materials described in this review include polymer thin films, molecular aggregates, molecular crystals, molecular semiconductor heterojunctions, biological materials, and molecular mono-, bi-, and multi-layer films.

The scanning capacitance microscope (SCM) provides a direct method for mapping the dopant distribution in a semiconductor device on a 10 nm scale. This capability is critical for the development, optimization, and understanding of future ULSI processes and devices. The basic elements of the SCM and its application to nanometer scale metal oxide semiconductor (MOS) capacitor measurements are described. Experimental SCM methods are reviewed. Basic measurements show that nanometer scale capacitance-voltage relations are understood. High-quality probe tips and surfaces are critical for obtaining accurate measurements of two-dimensional dopant profiles. Quantitative modeling of SCM measurement is described for converting raw SCM data to dopant density. An inverse modeling method is presented. Direct comparison between secondary ion mass spectroscopy (SIMS) and SCM-measured dopant profiles are made. Quantitative junction measurements and models are discussed and images of small transistors are presented.

This chapter presents a review of the technology of scanning thermal microscopy (SThM) and its applications in thermally probing micro- and nanostructured materials and devices. We begin by identifying the parameters that control the temporal and temperature resolution in thermometry. The discussion of SThM research is divided into three main categories: those that use (a) thermovoltage-based measurements, (b) electrical resistance techniques, and (c) thermal expansion measurements. Within each category we describe numerous techniques developed for (a) the method of probe fabrication, (b) the experimental setup used for SThM, (c) the applications of that technique, and (d) the measurement characteristics such as tip-sample heat transfer mechanism, spatiotemporal resolution, and interpretation of data for property measurements. Because most of the SThM techniques require fundamental knowledge of tip-sample heat transfer, all possible heat transfer mechanisms are discussed in depth, and relations for estimating the tip-sample conductance for each mechanism are provided. This is critical because tip-sample heat transfer controls spatial resolution, temperature accuracy and resolution, and imaging artifacts. Based on this discussion, a simple model is given for future design of SThM probes. The review concludes by describing some new developments on the applications of near-field optical microscopy for temperature measurements.