It is proposed that interfacial reaction barriers in binary A/B diffusion couples lead to the absence of phases predicted by the equilibrium phase diagram, provided that the diffusion zones are sufficiently thin (thin‐film case). With increasing thickness of the diffusion zones the influence of interfacial reaction barriers decreases and the simultaneous existence of diffusion‐controlled growth of all equilibrium phases is expected (bulk case). Selective growth of the first and second phases and the effect of impurities are discussed with the influence of interfacial reaction barriers and with references to the known cases of silicide formation.

Growth kinetics of planar binary diffusion couples: ’’Thin‐film case’’ versus ’’bulk cases’’

The kinetics of silicide growth are classified into three different categories: (a) diffusion controlled, (b) nucleation controlled, (c) others (reaction rate controlled). These are analyzed with the aim of understanding both the phenomenology of growth and the specific atomic mechanisms of phase formation. Diffusion-controlled growth is discussed with respect to the Nernst-Einstein equation. Stress relaxation is considered as a possible cause of reaction-rate control. The relative merits of two different types of marker experiments are compared. A few silicides are discussed in terms of what can be inferred about diffusion mechanisms. The competition between reaction-rate and diffusion control phenomena is shown to have specific effects on the sequence of phase formation; it is also related to the formation of some amorphous compounds. Reactions between silicon and alloyed metal films are used to illustrate the respective influences of mobility and driving force factors on the kinetics of silicide growth; they can also be used to underline the dominance of nucleation over diffusion in some silicide formation processes.

Kinetics of formation of silicides: A review

The formation of NiSi films from the reaction of Ni2Si with (100) and (111) silicon substrates was found to be controlled by a lattice diffusion process with an activation energy of 1.70 eV. In order to correlate kinetic information obtained by Rutherford backscattering with x‐ray diffraction data, ‘‘standard’’ diffraction powder patterns for both Ni2Si and NiSi have been established. The existence of a metastable hexagonal form of NiSi has been confirmed. Observations on the formation of Ni2Si confirm previous investigations. The diffusion process at work during the formation of NiSi is discussed in terms of the crystalline anisotropy of this compound and compared to what is known about diffusion in other silicides.

Formation of thin films of NiSi: Metastable structure, diffusion mechanisms in intermetallic compounds

Interactions in the Co/Si thin‐film system were investigated by MeV backscattering and x‐ray‐diffraction techniques. It was found that Si diffuses through the Co layer and accumulates at the sample surface at about 300 °C. Increasing the temperature causes the growth of the Co2Si phase, followed by the simultaneous growth of the CoSi phase. The growth rates for both phases have a square root of time dependence. The activation energies of growth are 1.5 and 1.9 eV for the Co2Si and CoSi phase, respectively. A model for the growth of multiple phases is suggested. The transformation between CoSi2 and CoSi is found to be a reversible reaction.

Interactions in the Co/Si thin‐film system. I. Kinetics

The modified embedded atom method (MEAM), an empirical extension of the embedded atom method (EAM) that includes angular forces, has been used to examine the interface between a silicon substrate and a thin overlayer of nickel. A brief review of the MEAM is given and parameters are determined for the Si-Ni system. As verification of the reliability of the model, the geometry, energy and elastic constants of a number of ideal Si-Ni compounds are calculated and are found to agree reasonably well with experiment and first-principles calculations. Planar defect energies are also presented. Calculations of the relaxed energy and geometry of a coherent 10 AA overlayer of Ni on Si(001) yield two similar structures, both of which were typified by a slightly rippled Ni structure relative to perfect FCC Ni. The lower-energy interface also contained rows of slightly shifted Ni atoms. It is found that significant differences occur between the energetics of a rigid or relaxed separation of the overlayer. Separation of the overlayer with a monolayer of Si atoms attached to the Ni yields a significantly lower-energy structure than separation exactly at the interface. The relaxed brittle fracture energy of this interface is found to be 1.5 J m-2, which is significantly lower than the unrelaxed fracture energy of 4.8 J m-2.

https://iopscience.iop.org/article/10.1088/0965-0393/2/3A/006/pdf

Atomistic calculations of composite interfaces

Rutherford backscattering of energetic 4He+ ions has been used to study the composition and depth profile of silicide layers formed when thin nickel films evaporated on (111) and (100) silicon surfaces are heated in a nitrogen-hydrogen ambient. In the temperature range 297 to 430 degrees C two phases are seen to form sequentially. The growth of the first phase, Ni2Si, is parabolic with time, whilst the second, NiSi, grows linearly with time. The simultaneous presence of both diffusion and reaction-rate-limiting mechanisms is implied by the parabolic and linear growth laws. An investigation of the electrical characteristics of diodes which were metallized and heat-treated simultaneously shows little change in the barrier height.

https://iopscience.iop.org/article/10.1088/0022-3727/9/6/009/pdf

D J Coe

Papers

Silicide formation in Ni-Si Schottky barrier diodes