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Hard-coating materials range from ultrahard materials such as ‘‘diamondlike carbon’’ through refractory compounds to alloys. However, transition-metal carbides and nitrides have achieved by far the highest level of commercial success. Titanium nitride and titanium carbide are the most studied and used. In this paper a review of the hard coating literature is given and includes in addition to nitrides and carbides also oxides, borides, mixed compounds, metals and alloys, and ‘‘diamondlike’’ carbon coatings. Only coatings grown from the vapor phase are discussed. Some considerations involved in selecting coating/substrate combinations as well as basic concepts of hardness and hardness measurements are also given. For example, it is shown that in order to measure the hardness of the coatings correctly the ratio between the film thickness and the depth of the indentation has to exceed a critical value, which depends on the coating/substrate combination. For TiN on steel, the coating thickness has to be a factor of 5 larger than the indentation depth. For all types of hard coatings there is still a lack of knowledge on how nucleation and growth processes are affected by processing parameters and how the resulting film microstructure correlates to physical properties. Based on results presented in the literature, the existing knowledge about relationships between the microstructure and physical properties of hard coatings is discussed. Particular emphasis is placed on the role of microstructural features such as grain boundaries, nonequilibrium structures, impurities, and texture in controlling the film hardness. For example, voids and weak grain boundaries give rise to low film hardnesses whereas dense films with a high defect concentration can have hardnesses far above bulk values. Because the coatings in many cases are grown at high rates, low temperatures, and under the influence of an impinging ion flux, supersaturated solid solutions and entrapment of noble-gas impurities are common features. Such coatings exhibit high stress levels (most commonly of a compressive nature) and high hardness values.

The crystallization of amorphous Si induced by Al during heat treatment has been investigated by cross section and plan view transmission electron microscopy. The lowest temperature of Al induced crystallization of amorphous Si was found to be 440 K. The crystallization temperature, however, depends on the thickness of Al layers in layered structures and on the concentration of Al in co-deposited layers below 1-nm-layer thickness and 15 at.% of Al concentration, respectively. Al-induced crystallization in layered structures starts at the Al/amorphous Si interfaces and is located close to them. The amount of crystallized Si depends on the quantity of Al and on the temperature and increases with them. The mechanism of crystallization involves intermixing of Al with Si and the formation of an alloy of high metal concentration in the amorphous/crystalline interface. When the formation of this alloy is not assured due to low Al concentration, then crystallization does not start or the process of crystallization stops. In Al induced crystallization the nucleation of polycrystalline Si grains rather than their crystal growth is affected.

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We report the preparation and properties of higher nitrides of Hf, Zr, and Ti synthesized by dual ion beam deposition. For Hf and Zr, evidence is given for the existence of a metastable nitride phase with composition of approximately Hf 3 N 4 and Zr 3 N 4 . These two materials are insulating and transparent straw colored, in contrast to the well-known mononitrides, which are shiny, gold colored, and highly conducting. For Ti-N we do not reach as high an N content and do not obtain an insulating, transparent phase. The higher nitrides of Hf and Zr are synthesized under energetic nitrogen ion bombardment (200 eV) of a growing film and do not form in the presence of molecular nitrogen gas alone. Several variations of the ion beam deposition process are used to obtain a wide range of film composition and to study the transition from the mononitride to the higher nitride phase. Transmission electron diffraction shows the structure of Hf 3 N 4 and Zr 3 N 4 to be very close to the Bl (NaCl) structure of the mononitrides, but with a slight rhombohedral distortion. Additional evidence from noble gas incorporation (Ne, Ar, and Xe) supports a model qf these higher nitrides as containing a large number of vacancies on the metal atom sites.442

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Thin-film multilayer structures of a-Si/Al/a-Si and a-Si/Sb/a-Si were deposited by electron-beam evaporation. The microstructure and the electrical properties of as-deposited and annealed (T<1370 K) thin films were determined. A p-n junction was formed between polycrystalline silicon (poly-Si) doped with Sb and a p-type Si substrate. Al and Sb were found to induce crystallization of a-Si at 600 and 700 K, respectively. After annealing to 1370 K for 60 min, the resistivities 7.0×10−3 Ω cm for the Al-Si sample and 1.4×10−2 Ω cm for the Sb-Si sample were obtained. Passivation of poly-Si grain boundaries by Sb is proposed.

We describe a quantitative ion beam technique for the deposition of compound thin films. The metal atom flux is supplied by inert ion beam sputtering, and the reactive flux is supplied by a low-energy ion beam directed at the growing film, allowing the fundamental deposition parameters of arrival rates, ion energy, and direction to be measured and controlled. Analysis gives the sputtering yields and incorporation probabilities as a function of film composition, arrival rate ratios, and ion energy. Results are presented for Al films deposited under a range of N2+ ion bombardment (100–500 eV) up to arrival rate ratios exceeding the value needed to form AlN. Nitrogen ions are almost fully incorporated into Al films, and excess N is rejected above the composition N/Al=1. The microstructure is shown to depend on the N/Al arrival rate ratio and the nitrogen ion energy used during deposition.