A review is given of the evidence available on the mechanism of growth and the structure of thin deposited films. The main attention is devoted to the growth of metals onto single crystal substrates, particularly those formed by the evaporation technique. Some consideration of other types of deposit is also given, including chemically formed films and epitaxial silicon layers. A survey of the various experimental techniques which can be used for structural studies on thin films is given, but the main evidence presented is based upon electron diffraction and transmission electron microscopy. The mode of nucleation of the layers is described, and the evidence for the way in which the nuclei develop into a continuous deposit film is outlined. This includes the evidence obtained from growing films inside the electron microscope, which reveals two important processes. These are the ‘liquid-like’ coalescence of nuclei and islands, and recrystallization of islands. The various imperfection structures ob...

The nucleation, growth, structure and epitaxy of thin surface films

Transmission electron microscopy and diffraction have been used in a study of single crystal and polycrystalline gold films prepared by evaporation. The specimens include films in (001) orientation containing many {111} microtwins, and films in (111) orientation having a pronounced twin related double positioning structure. The electron diffraction patterns from these films contain some well-known anomalous spots which are not immediately interpreted in terms of the twinning. Dark field electron microscopy reveals that in the (001) film the extra spots arise from the twin boundaries, and that double diffraction is an adequate and likely explanation of their origin. Similarly, the anomalous spots from the (111) films arise from the double positioning boundaries, but double diffraction does not provide an adequate explanation in this case, and the effect of the imperfect nature of the double positioning boundary is considered to be important. Extra rings observed on polycrystalline diffraction patt...

Electron microscopy and diffraction of twinned structures in evaporated films of gold

Interface microstructural development during solid state diffusion bonding of Ni to single-crystal α-Al2O3 has been studied by electron microscopy. Nickel aluminate spinel (NiAl2O4) interphase layers ∼ 1 μm thick formed under high vacuum bonding conditions. Very high vacuum (VHV) annealing caused the spinel to disappear, indicating that its stability depends critically upon the oxygen activity. High vacuum diffusion bonding utilizing initially oxygen-free Ni and oxygen-containing Ni established that spinel formation requires a threshold oxygen activity, and furthermore, that the source of the required oxygen can be oxygen initially dissolved in the Ni. Thermodynamic calculations confirm that the threshold oxygen level necessary to stabilize the spinel increases from 0.006 at.% (60 at.ppm) at 1273 K to 0.025 at.% (250 at. ppm) at 1663 K. Further analysis indicates the spinel exhibits a maximum thickness determined by the difference between the initial and threshold oxygen concentrations and the Ni thickness. Considering the solubility limit of oxygen in solid Ni, the spinel thickness is limited to ∼0.005 times the Ni thickness. The reaction is explored further in the context of diffusion path concepts with a calculated NiAlO phase diagram.

The thermodynamics of spinel interphase formation at diffusion-bonded Ni/Al2O3 interfaces

The condensed-state reaction between single crystals of Al2O3 and MgO was conducted in air at 1560°C. Spinel formed as two product layers which were shown to agree with the proposed mechanism. The crystallographic orientation of the parent crystals had no effect on the rate of spinel growth. The orientation of the spinel grown from sapphire was the same as that found by other investigators for isomorphic crystals. The orientation of spinel grown from periclase was largely dependent on the transport mechanism across the periclase-spinel interface.

Epitaxial Growth of Spinel by Reaction in the Solid State

In a hydrogen atmosphere, MgO was vaporized by heating MgO (periclase) in the range 1500° to 1900° C, and the vapor products diffused into Al2O3 (sapphire) to form a uniform outer layer of spinel on all surfaces. At 1900°C. a spinel layer 48 mils thick could be obtained in 16 hours. The rates of spinel formation were determined and the activation energy was calculated to be about 100 kcal. per mole. In the spinel layer, the lattice constant, Vickers hardness, and the refractive index varied from the outer surface to the inner boundary in a fairly uniform manner, indicating a continuous change in composition from 1MgO: 1Al2O3 to 1MgO: 2–3Al2O3. In conversion to spinel a volume increase of 47% over that of the original sapphire took place. Transparent clear shapes of spinel such as disks and rods were obtained from clear sapphire and from translucent polycrystalline alumina. Clear rods of spinel with polished ends acted as thin lenses, round rods as convex lenses, and flat rods as planocylindrical lenses owing to the increase in refractive index from the outer surface to the inner central portion. Objects made of opaque polycrystalline alumina were also converted to spinel. The MgO periclase blocks were etched in the hydrogen atmosphere, and the vapor products of Al2O3 entered the MgO to form tiny spinel droplets in an opalescent border. In an oxidizing atmosphere, spinel was formed only on the surface of the sapphire directly in contact with periclase, in the range 1500° to 1900°C.

H. R. Thirsk

Papers

An electron diffraction study of the surface reaction between nickel oxide and corundum