We review recent measurements of the structure of amorphous solidsolid interfaces using compositionally modulated structures to increase the density of interfaces in thin film materials.

Amorphous semiconductor superlattice structures consisting of alternating n-type and p-type doped layers of hydrogenated amorphous silicon (a-Si:H) have been made by silane glow discharge in a single chamber system. These multilayered films show the novel phenomenon of light-induced excess conductivity (LEC) associated with a metastable state having a lifetime of order of days. This report shows that the LEC effect is quite dependent on the specific details of the deposition parameters, namely dilution of the silane with inert gas, substrate temperature and layer thickness. In order to investigate the origin of the LEC effect, argon dilution was used for specific regions of the structure. This experiment shows that the slow states are distributed throughout the layers, and are not concentrated at the interfaces.

The elastic properties of modulated structure materials are presented. An enhanced modulus effect has been observed in several composition modulated thin film systems containing short wavelength modulations.8-10nm. The foils were produced by vapor deposition using two or three source evaporator. As compared with homogeneous foils of the same average composition, the modulated foils exhibited an appreciable increase (up to 300%) in modulus. The dependence of various moduli on the modulation parameters (wavelength, composition and amplitude) are described. The plastic behavior, breaking and microhardness of these foils are also presented as a function of the modulation parameters. Current theories based on electronic and strain effects on the elastic constants of metals are also presented to explain the origin of the supermodulus effect.

Soft x-ray reflectivities up to a factor 104 larger than those of the best homogeneous materials have been obtained with multilayers coatings. Thickness control and boundary definition has been achieved on an atomic scale with amorphous films. We will review design, fabrication, performance and characterization of the coatings and discuss possibilities to extent these methods to the improvement of other material properties.

Periodic multilayer structures, consisting of largely amorphous alternating layers of heavy and light elements, have been fabricated by sputtering techniques. Investigations have been carried out into the effects of various types of layer imperfections on the x-ray optical properties of these multilayers. These have included extensive numerical modeling of real multilayers (using a computational scheme based on the complete dynamical theory) with simulations of diffused interfaces and deviations from constant d-spacing. Results are presented for examples of W–C and Hf–Si multilayers, with comparison of measurements at Cu–K to the theoretical model.

X-ray diffraction techniques are utilized to characterize the biaxial state of stress in thermal vapor deposited, composition modulated layered structures. Thin film systems, as Au-Ni, Cu-Pd, Ag-Pd, Cu-Ni and Cu-NiFe, have manifested the ‘supermodulus effect’ as a mechanical property. The concept of interlayer coherency gives rise to the notion of accommodating misfit strains between the layers. In turn, such large elastic strains have been shown to cause large increases in the elastic moduli (the ‘supermodulus effect’) of noble metals. In the present work, pole figures are used to determine the texture and crystallographic orientations of these foils, and standard x-ray methods are then used to quantify this strain with lattice parameter measurements and diffraction peak profiles.

Upper critical field and its anisotropy have been measured on two very short wavelength Mo-Ni heterostructures of different degrees of perfection, λ = 13.8Å (disordered structure) and X = 16.6Å (layered structure). In both cases the parallel critical field has an unexpected temperature dependence, a large and temperature dependent anisotropy, and over 60% enhancement over the Clogston-Chandrasekhar limit. Data are fit to the Werthamer-Helfand-Hohenberg theory and the spin-orbit scattering times are found to be 1.79 × 10−13 sec and 2 × 10−13 sec, respectively.

Graphite intercalation compounds (GIC's) are metal-semimetal superlattices which exhibit crystalline order, and have atomically perfect interfaces between the layers of the constituent species. From the standpoint of superconductivity, the KHg-GIC's are particularly interesting. The preparation and properties of these compounds are described, along with a series of recent experiments with hydrogen doping which have helped to elucidate their electronic properties. A density of states model suggested by the results of the hydrogen–doping experiments is presented and used to explain the variation of the superconducting transition temperature in these materials.

Because of the difference in charge transfer and superlattice formation of the various intercalant species, graphite intercalation compounds (GICs) exhibit a variety of interesting electronic properties and phonon properties. These compounds form monolayered metallic superlattices along the c—axis, in contrast to the multilayered metallic superlattices grown from MBE and sputtering synthesis methods. GICs are generally divided into donor—type and acceptor—type compounds, depending on whether the electrons are transferred to the graphite or from the graphite. The modification of the electronic energy bands of GICs by charge transfer is analogous to that of the nipi superlattices. Because of the strong electron affinity of hydrogen relative to that of graphite, the addition of hydrogen into donor—type GICs (e.g. K—GICs, KHg—GICs) modifies the charge transfer from the intercalates to the graphite л-bands. Therefore, studies of the donor—type KHx—GICs provide us with new understanding of the variation of the electronic properties of alkali—metal GICs as the as charge transfer is modified.

A simple and inexpensive technique is described, which makes novel use of electrolytic anodization to obtain an effective depth profile of artificial nultilayered structures based on a number of refractory metals. Successful MAPs (Multilayer Anodization Profiles) have been obtained for multilayer structures of Nb/Si, W/Ti, and a variety of other material pairs, with individual layer spacings ranging from less than 10 Å to greater than 50 Å. The spatial resolution of these MAPs annears to be less than about 10 Å, and may in some cases be comparable to the interface sharpness of the sputtered films. Comparison with conventional techniques such as Auger depth profiling and low–angle x-ray diffraction is discussed. Examples of semiquantitative analysis, using the MAP technique, of changes in interface structure brought about by thermal annealing, are also presented.