R. W. Bené

A kinetic model for solid‐state silicide nucleation

We present a model for solid‐state silicide nucleation at the interface of a thin metal film and a silicon substrate, starting with the assumption that compound nucleation in these systems is a kinetically controlled process. We consider the work being done on other systems, such as dendrite nucleation and growth, and snowflake generation, where the processes are kinetically self‐limited by the structures that form. We show that the requirements for kinetically self‐limited systems can be formally satisfied at the interfaces for silicide forming systems. In particular, in analogy with the selection of metastable shapes for snowflake structures, we would in general expect metastable chemical structures to form at metal‐semiconductor interfaces under conditions of large supercooling. We discuss the selection criteria (maximum energy degradation rate), the probable forms of these structures, and the conditions required for their growth. We then discuss an interpretation of phenomenological rules for selectio...

Variation of the nucleated products in ultrathin films of Ti‐Co on Si substrates with processing changes

The purpose of this study is to investigate solid‐state nucleation and growth in Ti/Co/Si and Co/Ti/Si thin‐film ternary systems, and to compare results under a variety of preparation conditions. The deposition methods used were rf sputtering and e‐beam evaporation. The nucleated compounds are determined by transmission electron microscopy and diffraction. CoTi2 is the first nucleated compound in the Ti/Co/Si ternary systems prepared by e‐beam evaporation irrespective of substrate temperature, thickness, or other factors. This is in contrast to the films produced by rf sputtering where this phase was not observed. Experimental findings also show that in thin‐film ternary systems, the first‐phase nucleation temperature and subsequent compounds could be altered as the intermediate metal layer becomes less than 20 A. This is an indication that interference between interfaces begins to play a significant role at about this thickness, so that the reaction paths and the first nucleated phases are determined not...

Dependences of titanium silicide properties on the ratio of silicon to titanium used for electron-beam coevaporation onto heated substrates

Abstract Variations of the properties of titanium silicides prepared under various deposition conditions were investigated. An amorphous phase was observed to form when samples were prepared by codeposition of silicon and titanium at the substrate temperature T s of 120°C. However, it was found that considerable crystallization takes place at T s = 300° C , even though island-like amorphous regions are present at the silicide-silicon substrate interface. The film properties of samples prepared at relatively high T s were observed to change with the deposition conditions in a systematic manner. It was suggested that the amorphous phase is probably the first product of the intermixing even at elevated substrate temperatures. It was also postulated that the silicide formation process may involve evaporation of the amorphous phase. Thus, the film properties were said to be determined by silicon diffusion from the substrate, and crystallization as well as evaporation of the amorphous phase. Each of these mechanisms becomes dominant depending on the initial composition used for codeposition. This conjecture was shown to be successful in explaining the observed variations of properties such as voids, silicon precipitates, interface roughness, and resistivity.

Interfacial nucleation of compounds for thin‐film Co‐Hg1−xCdxTe contacts

We have determined the phases formed at the interfacial contact between deposited Co thin films prepared with both e‐beam evaporation and sputtering of Co onto CdTe (111) and Hg0.8Cd0.2Te substrates using transmission electron microscopy and diffraction. Sputter profiling with Auger electron spectroscopy has been used to determine the depth profiles of Co, Cd, Te, and Hg at the interface and throughout the Co overlayer on Hg0.8Cd0.2Te and CdTe. For the Co‐Hg0.8Cd0.2Te system, the γ phase is formed upon sputter deposition of Co films of 80 A or thicker at room temperature. This Co‐Te phase is also seen for thinner films upon annealing (e.g., 100 °C for a 40‐A Co film) after sputter deposition. However, after the deposition of 80‐A Co by e‐beam evaporation onto an Hg0.8Cd0.2Te substrate, the γ phase forms only after annealing at 150 °C in vacuum. Texturing of the γ phase was observed in the transmission electron diffraction patterns with the Hg0.8Cd0.2Te substrates, indicating at least a partial ordering of this phase with respect to the substrate. On the CdTe substrates, Te was observed after Co deposition in either deposition system. The Te generated by either sputter deposition or e‐beam evaporation with a high deposition rate (8–10 A/s) of Co onto CdTe (111) was polycrystalline, whereas the slow deposition rate (0.3–0.5 A/s) e‐beam evaporation resulted in amorphous Te. Auger electron spectroscopy results indicated that the Te generated is distributed throughout the Co film. The Co‐Te γ phase was formed upon annealing at 300 °C after sputter deposition and at 350 °C after e‐beam evaporation, but the crystallites did not have the relationship to the substrate (texturing) which was displayed by those on the Hg0.8Cd0.2Te substrates. Annealing of the Co overlayer on Hg0.8Cd0.2Te causes the growth of the crystals and an increase in ordering of the γ phase.

Effects of amorphous titanium silicide on subsequently formed crystalline compound prepared by two‐step thermal process

Titanium silicides prepared by codeposition at intermediate substrate temperatures with subsequent high temperature annealing were studied. It was found that coevaporation with Si/Ti ratio of 2.5 at the substrate temperature (Ts) of 120 °C results in the formation of an amorphous phase. On the other hand, samples prepared at Ts of 300 °C resulted in crystalline TiSi2(C49). However, the amorphous phase was still present at the silicide/substrate interface. Considerable grain growth was observed to start at the silicide/Si interface even though the interfacial amorphous phase was the last to crystallize. It was suggested that Si diffusion is much more rapid through the amorphous silicide than crystalline TiSi2. This difference in kinetics was shown successful in explaining the dependency of the film properties of annealed samples on the structure of the product of the initial reaction.

Investigation of excess noise in ultrathin metal films deposited onto single‐crystal silicon

We present a silicide nucleation process in planar metal‐silicon interfaces in terms of excess noise properties. The excess noise power spectral density has been measured in systems composed of ultrathin metal films deposited onto silicon substrates as a function of metal thickness. The metal films have been prepared by electron‐beam evaporation and rf sputtering, and structural information of the films has been obtained by using transmission electron microscopy and diffraction. Surface resistance measurements for near‐noble metal films deposited onto silicon generally show that the increase of metal thickness reduces the surface resistance of the film, which undergoes a transition from a semiconducting to a metallic state immediately prior to the onset of the first nucleation of the crystalline silicide. Results of excess noise measurements show that some of the noise parameters of the power spectral density for cobalt and nickel film on silicon have similar trends along the nucleation reaction path. A l...

An analysis of pyromagnetic phenomena: uniaxial ferromagnet in the molecular field approximation

We present a thermodynamic analysis of the pyromagnetic effect for the case of a ferromagnet with uniaxial anisotropy using the molecular field approximation (MFA). We have divided the response function into a product of terms which we believe gives the best insight into the requirements of a material for a good pyromagnetic detector. In particular, we find that one should use a material with the largest magnetic heat capacity possible. Also, for small fields, we may realize the best detector with H along the hard axis and the pickup along the easy axis. Our analysis suggests that certain of the critical point exponents, as well as the anisotropy power law behavior, may be more easily measured using the pyromagnetic technique.

High frequency pyromagnetic measurements of CoS2−xSex

In this paper we present the results of the pyromagnetic measurements on CoS2−xSex single crystals in the frequency range of 100 Hz to 10 MHz. The high frequency response measurements reveal in these compounds the presence of a surface magnetic state with a Curie temperature higher than that of the bulk material. The surface Curie temperature, as well as the thickness of the surface region, depends on the level of selenium doping: the depth of the surface region increases with higher level of selenium doping and varies from about 0.5 to 25 microns. The cause for the existence of the surface magnetic state is believed to be a uniform selenium deficiency in a surface region.

Stabilization of C‐49 Ti(AlxSi1−x)2 observed from the reaction between TiSi2 and Al

The solid state reaction between aluminum and titanium silicide was studied over the temperature range of 450–800 °C. The phase formed from the reaction was different from Ti7Al5Si12, which has been frequently reported as the reaction product. Also, two other phases were reported nearly identical to those observed in terms of the similarity in the structure type and lattice parameters, namely, ternary C‐49 Ti2AlSi3 and binary C‐49 Ti(AlxSi1−x)2 which is the metastable C‐49 TiSi2 with Al solid solution. Based on the detailed microscopic and spectroscopic analysis, it was suggested that the observed phase is likely the metastable C‐49 Ti(AlxSi1−x)2 stabilized by substitution of Si with Al. Even though we did not find the maximum value of x, approximately 10% of Si was found to be substituted with Al for the sample annealed at 800 °C for 30 min.

High‐frequency pyromagnetic effect in CoS2−xSex compounds

In this paper we present the results of the pyromagnetic measurements on CoS2−xSex in the frequency range 100 Hz–10 MHz. The effect of the eddy current losses on the frequency response behavior is calculated and seems to explain the experimental data fairly well. The high‐frequency response measurements also reveal in these compounds the presence of a surface magnetic state with a Curie temperature higher than that of the bulk material. The surface Curie temperature as well as the thickness of the surface region depend on the level of selenium doping. The depth of the surface region increases with higher level of selenium doping and varies from about 0.5 to 25 μ. The cause for the existence of the surface magnetic state is believed to be the selenium deficiency near the surface. A possible selenium doping profile near the surface consistent with our experimental data is also suggested.