S.S. Lau

Ion-beam-induced reactions in metal-semiconductor and metal-metal thin film structures☆

Abstract Ion-induced reactions in thin film systems have been used to form equilibrium and metastable compounds, amorphous layers, and solid solutions. Samples were prepared by depositing metal films on Si or Ge substrates or by depositing multiple-layer films on inert substrates. Ar, Kr, or Xe ions at energies between 100 and 300 keV and dose of a few times 1015 ions/cm2 were used. In compound-forming systems, such as silicides, where chemical driving forces are significant, the substrate temperature during ion bombardment plays an important role. At substrate temperatures above 25–50°C, equilibrium phases are generally formed at growth rates that depend strongly on substrate temperature. At lower temperatures, intermixed layers are formed; the thickness of the layers is relatively insensitive to temperature. In both temperature regimes, the thickness of the intermixed layer depends on ion dose and species. In Auue5f8Si and Auue5f8Ge eutectic systems, metastable phases were formed. With Ague5f8Si and Alue5f8Ge eutectic systems, only limited intermixing was found. In metal-metal systems, solids solutions of extended solubility have been formed. Single-phase fcc solid solutions were formed across the entire composition range of Ague5f8Cu and Auue5f8Co multiple-layer structures. With heat treatment, the single-phase solutions transform to equilibrium two-phase mixture. In near-immiscible systems, such as Ague5f8Ni, an extension of the solubities was found. In general, ion-beam mixing provides a wider range of composition of solid solutions that can be obtained by splat-cooling or high dose plantation.

Continuous series of metastable Ag‐Cu solid solutions formed by ion‐beam mixing

Metastable Ag‐Cu solid solutions have been formed by ion‐beam mixing of thin deposited Ag and Cu layers of various compositions. X‐ray diffraction measurements indicated that the lattice parameters of the ion‐induced Ag‐Cu alloys vary almost linearly with composition, with a slight positive deviation from Vegard’s law for ideal solid solutions. The microstructures of the alloyed layers were studied by transmission electron microscopy and their stability was examined by thermal annealing up to 250u2009°C. The present results are compared with those obtained previously by rapid quenching techniques.

Microalloying by ion-beam mixing

Abstract We have investigated ion-induced atomic mixing process as an alternative approach to conventional rapid quenching techniques for producing thin film metastable alloys. Multi-layered samples consisting of thin alternate layers of two elements were bombarded with energetic Xe ions. Formation of metastable phases such as supersaturated solid solutions and amorphous alloys were obtained as a result of atomic mixing. The systems under investigation are binary couples of Au and one of the fourth-period transition metals, Ni, Co, Fe or V. For samples irradiated at R.T., metastable crystalline phases were usually formed. In the Auue5f8Ni and Auue5f8Co systems, single-phase f.c.c. solid solutions over an entire range of composition have been produced. In the Auue5f8Fe and Auue5f8V systems, the Au-rich alloys showed a f.c.c. structure while the Fe (or V)-rich alloys showed a b.c.c. structure. The simultaneous presence of a f.c.c. phase and a b.c.c. phase was found in alloys with compositions Au 38 Fe 62 and Au 40 V 60 . For samples irradiated at LN 2 temperature, the structures of the alloys were found to be more random in nature. Amorphous phases of compositions Au 25 Co 75 and Au 40 V 60 were obtained. Comparisons of results observed on various systems indicate that the formation and structure of metastable phases are strongly influenced by the equilibrium nature of the system.

Sequence of phase formation in planar metal‐Si reaction couples

A correlation is found between the sequence of phase formation in thin‐film metal‐Si interactions and the bulk equilibrium phase diagram. After formation of the first silicide phase, which generally follows the rule proposed by Walser and Bene, the next phase formed at the interface between the first phase and the remaining element (Si or metal) is the nearest congruently melting compound richer in the unreacted element. If the compounds between the first phase and the remaining element are all noncongruently melting compounds (such as peritectic or peritectoid phases), the next phase formed is that with the smallest temperature difference between the liquidus curve and the peritectic (or peritectoid) point.

Phase transformations in laser‐irradiated Au‐Si thin films

Laser‐pulse‐induced melting, interdiffusion, and rapid resolidification are applied to deposited Au‐Si thin films of various compositions. It is found that, if 30‐ns pulses are used, amorphous Au‐Si films can be produced over a compositional range 9–91 at.% Au. The stability of the amorphous phases varies with their composition. Thermal decomposition involves the formation of a single‐metastable silicide with a hexagonal structrue. Application of 300‐μs laser pulses directly leads to formation of the same compound.

Ion-beam induced epitaxy of silicon

Abstract Epitaxial regrowth of an amorphous Si layer on a 〈100〉 Si crystal held at 200–400°C is achieved under bombardment with Si, Kr, or Xe ions. Channeling measurements with MeV He ions show the regrowth proceeds from the amorphous-crystalline interface, and has an initially linear dose dependence. The annealing beam, however, introduces additional damage centered at or beyond the ion range. Amorphous layers obtained by low-temperature self-ion bombardment regrow much more readily than amorphous deposited layers.

Epitaxial growth of Si deposited on (100) Si

Epitaxial growth of deposited amorphous Si on chemically cleaned (100) Si has been found and layer‐by‐layer growth occurred at rates comparable to those in self‐ion‐implanted‐amorphous Si. There is no evidence for appreciable oxygen penetration into the deposited layer during storage in air. The critical factors in achieving epitaxial growth are fast (∼50 A/sec) deposition of Si onto a surface cleaned with a HF dip as a last rinse before loading into the vacuum system. Channeling and transmission electron microscopy measurements indicated that the epitaxial layers are essentially defect free. Secondary‐ion mass spectroscopic analysis showed about 1014 oxygen/cm2 at the amorphous/crystal interface. With either higher interfacial oxygen coverage or slow (∼2 A/sec) deposition, epitaxial growth rates are significantly slower.

Ion‐beam‐induced formation of the PdSi silicide

Formation of PdSi has been obtained by implanting energetic Xe ions through a thin Pd (or Pd2Si) film on a Si substrate. The PdSi phase was found to form near the Pd2Si‐Si interface from Rutherford backscattering measurements. Phase formation was confirmed by glancing‐angle x‐ray‐diffraction analysis. Subsequent thermal annealing at 300–400u2009°C resulted in successive growth of the phase. A uniform PdSi layer was obtained at the final stage of the annealing and exhibited a sheet resistivity of 18 μΩu2009cm.

Epitaxial growth of the nickel disilicide phase

Abstract The nickel disilicide (NiSi 2 ) phase which forms epitaxially on silicon single-crystal substrates in many ways exhibits an interesting behavior. A regular network of the silicon-rich phase was observed to originate at or near the Si-silicide interface using the Berg-Barrett X-ray topographic technique. The regularity, the thickness and the density of the network increase as the epitaxial layer increases in thickness. For (100)-oriented silicon substrates the network runs along two perpendicular 〈110〉 directions; for (111)-oriented silicon substrates the network runs along 〈112〉 directions. Large voids on the surface of the epitaxial layer and fracturing of the layer along the edges of the network for thick epitaxial layers were also observed. The results are explained in terms of the corrugated morphology of the NiSi 2 -Si structure.

Solid phase epitaxy in silicide-forming systems

Abstract We investigated nine metal-silicon systems for solid phase epitaxy with a sample configuration of a layer of amorphous silicon deposited onto a silicon crystal substrate interposed with a metal layer (where the metal was palladium, platinum, nickel, chromium, iron, cobalt, titanium, vanadium or rhodium, all of which form silicides). It is shown that solid phase epitaxial growth is a rather general phenomenon in these systems. We further observed that the temperature at which the solid phase epitaxy process took place at a given rate generally increased with the heat of formation, the melting point and the formation temperature of the silicide.