A review of the experimental and theoretical results describing thin film growth modes is presented. Thermodynamic criteria for determining which growth mode might be expected to occur in a particular case along with some kinetic considerations are given. The characteristics of each of the three principal growth modes, namely Frank and van der Merwe (layer), Stranski-Krastanov (layer plus island), and Volmer-Weber (island), are discussed. Lastly, the requirements favoring the growth of epitaxial multilayers are briefly considered.

The equilibrium structure and metastable states of thin films have been investigated using molecular dynamics computer simulations. The energy as a function of coverage for a variety of film/substrate systems has been determined, and this information has been used to determine the growth mode and the conditions under which the film is dislocation free. In particular, the influence of the strength of the film-substrate binding will be discussed.

Pulsed ArF excimer laser (193 nm) photolysis has been used to deposit entirely amorphous and mixed amorphous/polycrystalline superlattice structures containing Si, Ge and Si3N4. High resolution in situ optical reflectivity measurements were used to monitor and/or control deposition. Transmission electron microscope cross-section views demonstrate that amorphous superlattice structures having highly reproducible layer thicknesses (from about 50 to several hundred A), and sharp interlayer boundaries, can be deposited at low substrate temperatures under laser photolytic control.

We describe the strategy that we are using to make precision metal multilayers. Differential scanning calorimetry has been used with the Ni/Zr system to study the abruptness of the interface as a function of the sputtering pressure. For 10nm period multilayers there is a monotonic increase in the width of the interface with increasing sputtering pressure. W/Si multilayers have been studied as a function of both the sputtering pressure and the relative thicknesses of tungsten and silicon. At reasonably low sputtering pressures a well textured (110) tungsten X-ray peak is seen which is compatible with the expected thickness of the tungsten layers.

Finally, we report some preliminary work on sputtering from tungsten and silicon targets which are at different distances from the substrate so that the degrees of bombardment on the growing layers of the film can be independently varied.

An ultra high vacuum system fitted with thermal effusion cells that heat 20cc of material to temperatures as high as 2000 °C is being used to synthesize thin film and multilayer ceramics from their constituent elements. A molecular source for oxygen used in conjunction with these effusion cells allows oxides of Al, Ti, Cu, and Y to be grown with compositional modulation controlling growth rate and stoichiometry.

Superlattices between ceramic materials, such as tungsten carbide, and transition metals have been synthesized for the first time. The growth and structure of these superlattices were investigated by low angle X-ray diffraction and TEM lattice imaging and microdiffraction. The data show that the low temperature process of forming these two dimensional composites leads to unique crystal structures and morphology in the nanoscale range.

I review recent theoretical studies designed to examine the effect of strain, facetting and growth conditions on multilayer stability. A phenomenological approach is used to investigate both morphological and crystal structural phase stability.

In multilayer structures, dislocation forces that arise from coherency strains are well understood in both the isotropic and anisotropic elastic cases. Image forces produced by elastic inhomogeneity have also been developed for single layers surrounded by extended phases. Image forces for the multilayers are shown to reduce to the simpler single embedded layer case to a good approximation. Image forces are shown to lead to a “stand-off” position of interface dislocations from the interface, with several implications for the properties of multilayer structures.

Using recent results from the thermodynamics of stressed solids, two-phase coexistence in a simple binary strained-layer superlattice is examined. We show that for a given temperature and overall composition of the superlattice, there can exist more than one linearly stable, equilibrium thermodynamic state. That is, there may exist several combinations of relative thickness of the phases and corresponding phase compositions that minimize the free energy of the system. The equilibrium state observed experimentally can, therefore, be influenced by the processing path.

Studies of atomic arrangements in amorphous and crystalline, short period Mo-Ge multilayers using synchrotron radiation X-ray diffraction (transmission and reflection), supplemented by EXAFS, are described. Differential anomalous scattering and EXAFS were utilized to determine the environment of each species. Intermixing, as well as a BCC epitaxial Ge structure, are among the observed structural characteristics. A wide range of structural variations is present indicating that specification of the layer thicknesses or composition profile alone is insufficient for characterization of the multilayers.

High-resolution X-ray diffraction (HRXRD) studies have been cardied out to determine the structural perfection and periodicity for a number of high-quality InGaAsfInP superlattices grown by gas source molecular beam epitaxy. X-ray scans were carried out with a compact four-crystal monochromator resulting in a resolution of one molecular layer (∼3,Å), which enables one to observe very small variations in the periodic structure. Sharp and strong higher-order satellite reflections in the XRD profiles were observed indicating smooth interfaces with well-defined modulated structures. Excellent computer simulated fits of the X-ray satellite pattern could be generated based on a kinematical XRD step model which assumes ideally sharp interfaces, and periodic structural parameters such as the strain in the well could be extracted. Our results3 demonstrate that HRXRD in conjunction with the kinematical step model is a very sensitive method to assess periodic structural modifications in superlattices as a result of the precise growth conditions in the gas source MBE system.

We have prepared multilayer films of W-Si with bilayer repeat spacing from approximately 1.5 nm to 9 nm and performed high resolution electron microscopy and low angle x-ray scattering on them. Average composition estimates as inferred from deposition conditions, x ray scattering and electron microscopy are compared. Determinations of the individual layer thickness ratios by electron microscopy and x ray scattering vary significantly from expectations as the bilayer thickness approaches 1.5 nm. Layer intermixing to increase as the bilayer thickness decreases. Composition profiles as inferred from the Cuk x ray profile are compared to those inferred from the high resolution electron micrographs. Visual observations from melectron microscopy are presented indicating that the interface roughness is rapidly damped in the W-Si multilayer system. Estimates of the layer uniformity are made from the high resolution images.

The tungsten-carbon multilayer system has been studied using x-ray scattering techniques as a function of multilayer period and annealing. Grazing incidence x-ray scattering shows that interatomic arrangements in the W-rich layers depend strongly on their proximity to the interface with the C-rich layers. Amorphous interface layers are stabilized by intermixing of W and C during deposition into local structures similar to those in the tungsten carbides. Further intermixing and structural relaxation in the amorphous state occurs on annealing these structures. This interatomic structural interpretation is consistent with trends in the observed x-ray optical properties.

Results are presented from glancing angle EXAFS studies of three tungsten-carbon multilayer systems, of different W/C thickness ratios, by monitoring the fine structure above the tungsten L3-edge. The purpose of the investigations was to determine the structural changes occurring in the multilayer as a result of moderate annealing. Surprisingly, dramatic changes are observed in the structural environment of the W atoms as a result of heating to temperatures as low as 350°C for 3 hours. It is found that, when the W layer is sufficiently thick, and the W/C layer thickness favorable, W2C is preferentially formed and crystallization is extensive.

The experiment demonstrates the suitablility of the EXAFS technique for the study of multilayer systems since, 1) contribution to the signal from the interface is significant and 2) the phenomenon is not dependent on the existence of long range order.

X-ray diffraction, transmission electron microscopy (TEM), and photoluminescence measurements have been made on strained InxGa1-xAs/GaAs quantum well structures. The well widths measured from TEM are 187, 115 and 69 Å for an interrupted growth, and 218, 126, 60 Å for a non-interrupted growth. In the measured x-ray diffraction patterns, the Pendellosung fringes due to GaAs barriers are modulated by a broad weak peak mostly coming from the thickest InxGa1-xAs well layer and is fairly symmetric for the noninterrupted sample. For the interrupted quantum well, the x-ray diffraction pattern is less symmetric, since there is further modulation by another broader and weaker peak. This results show that the In content in the InxGa1-xAs well layers are not well controlled for the interrupted quantum well. Using actual thickness measured from TEM, x-ray diffraction patterns are calculated and good agreement is obtained between the measured and the calculated x-ray diffraction patterns. The three strained InxGa1-xAs/Gaks quantum wells grown without interruption produce high intensity and narrow full-width at half-maximum (FWHIM) of 2.9 meV of the photoluminescence peak. The photoluminescence peaks for the interrupted quantum well are relatively broad and asymmetric, and have lower intensities, indicating that better quality InxGa1-xAs/GaAs quantum wells can be grown without interruption.

Thin film bilayers of ferromagnetic Fe and nonmagnetic W have been deposited by sputtering on flat glass substrates according to a sequence of gradually varying bilayer thicknesses which in effect extends the critical angle for external mirror reflection for neutrons of one spin state when the Fe is magnetized in the plane of the film. The measured reflectivity of this Fe-W multilayer system is compared with that of other supermirror polarizers consisting of different materials and layer sequences.

T.E.M. methods are described for the quantitative characterisation of the compositional and structural changes at interfaces and in homo- and hetero-phase multilayer structures. Many of the newer approaches described including the Fresnel and Centre Stop Dark Field Imaging Methods were developed specifically for such characterisations. The range of applications of each of the techniques is assessed as is the importance of delineating the limiting effects of inelastic and inelastic/elastic multiple scattering.

We have used transmission electron microscopy to study the structure of Pb1-xEuxTe alloys grown by molecular beam epitaxy. We have observed ordered solid solutions of the Pb1-x EuxTe alloys as well as spinodal decomposition for 0.35≤x<0.75. The spinodal decomposition corresponds to a modulation of both the composition and the lattice spacing of the Pb1-x EuxTe alloy. These modulated structures have periodicities of ∼18Å along the <111> and <110> directions and indicate that the solid solution of Pb1-x Eux Te is unstable in this range of compositions.

The physics governing stability properties and relaxation of mismatch strain in semiconductor strained-layer structures is reviewed. Experimental data on stability and rates of strain relaxation are examined. We conclude that essentially all observations on structural relaxation of semiconductor strained-layer structures can be explained by standard models of plastic deformation adapted to the special conditions controlling dislocation dynamics in these structures.

For bulk materials, plastic deformation mechanisms and rates depend in a complex way on resolved shear stress and temperature, but can be succinctly described using the deformation mechanism maps pioneered by Ashby and Frost [1]. In this paper, we describe the use of such maps to demarcate the various stability and metastability regimes of single and multilayered strained epitaxial structures, and to interpret experimental work in the SiGe system.