M. von Allmen

Ion-beam mixing of metal-semiconductor eutectic systems

Abstract A study has been made of the ion-beam mixing of four metal-semiconductor eutectic systems (AuSi, AuGe, AlGe and AgSi). Two sample configurations were used: (1) unlimited supply where a single metal layer was deposited on a semiconductor substrate, and (2) limited supply where multiple layers of metal and semiconductor of a fixed composition were deposited onto an inert substrate. Inert gas ions were used to cause mixing between the metal and semiconductor. The ion-beam mixing behavior of the four systems can be categorized into two groups.For AuSi and AuGe (group A), the mixing is efficient. For Al-Ge and AgSi (group B), the mixing is much less compared with that of the group A systems. In the case of samples with an unlimited supply configuration, group A systems exhibited a mixed layer of well-defined composition (Au 71 Si 29 and Au 50 Ge 50 ) which is formed initially at the metal-semiconductor interface. The thickness of the mixed layer increases with the square root of the ion dose. The structure of the mixed layer is metastable in nature. The efficiecy of mixing and the uniformity of the mixed layer improve with an increase in the substrate temperature from cryogenic to room temperatures. For group B systems, the mixing is limited to the interfacial regions even at elevated substrate tempertures (up to ∼ 200°C). At higher substrate temperatures, solid-phase reactions (similar to solid-phase epitaxial growth) take place even without the influence of an ion beam. In the case of multiple layered samples (limited supply), mixing is pronounced for group A systems. For group B systems, relatively uniform mixed layers can be obtained if the individual layer thickness is similar to the interfacial concentration spread caused by ion beams in samples with a single deposited metal layer (unlimited supply configuration). These results are compared with those obtained by laser melting aand quenching experiments as well as splat-cooling experiments.

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.

Solid‐state epitaxial growth of deposited Si films

Epitaxial growth by furnace annealing of amorphous Si layers deposited onto 〈100〉 Si substrates is demonstrated. Substrate cleaning prior to the evaporation includes only conventional chemical procedures without any attempt to achieve an atomically clean substrate layer interface. The crystalline quality of the grown layers near the surface is comparable to that of 〈100〉 Si regrown layers amorphized by Si implantation. Residual damage is usually found near the substrate‐layer interface. The growth mechanism is believed to be vertical growth of isolated epitaxial columns which subsequently grow laterally to consume the remaining amorphous Si.

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.

Properties of laser‐assisted doping in silicon

Ohmic contacts and p‐n junctions in p‐ and n‐type silicon are generated with the aid of a laser. Doping was achieved by covering the surface of the silicon with a layer of dopant and melting locally with pulses from either a Nd : YAG or a CO2 laser. Typical residual resistances of the Ohmic contacts are of the order of 0.1–1 Ω cm2 and backward/forward resistance ratios of 104 were measured for the diodes. A model which takes account of segregation during the cooling process is discussed and shown to agree with the resulting distribution of dopant. Highly doped material was found in a surface layer of a thickness less than 0.5 μm. This thickness was independent of laser parameters.

Metastable phases in laser‐irradiated Pt‐Si and Pd‐Si thin films

Amorphous phases of Pt‐Si and Pd‐Si covering a wide range of compositions are produced by laser‐induced melting and quenching of vapor‐deposited thin films. It is concluded that with the present cooling rates all compositions except those close to the congruently melting phases and the pure elements lead to amorphous films. Thermal decomposition of the Si‐rich amorphous films reveals the formations of at least two metastable crystalline phases in the case of Pt‐Si, and a low‐temperature nucleation of the equilibrium PdSi phase in the case of Pd‐Si.

Solar furnace annealing of amorphous Si layers

We demonstrate that a simple Al solar reflector can be used to induce solid-phase epitaxy of amorphous Si layers obtained either by ion-implantation or ion-deposition techniques. The annealing can be accomplished in air and takes a few seconds for a 1-cm^2 sample area. For ion-implanted samples, the regrown layers are defect free on substrates, and contain microtwins on substrates. For deposited layers on substrates the degree of epitaxy is not as good as that obtained by furnace annealing (550 followed by 950°C annealing).

Anisotropic melting and epitaxial regrowth of laser‐irradiated silicon

Using Nd‐YAG laser pulses in the 100‐μs regime, melting and regrowth of monocrystalline silicon is studied. It is shown that surface melting proceeds preferentially along the crystallographic axes. This leads to a characteristic surface pattern if uniform irradiation slightly above melting threshold is applied. Regrowth under these circumstances is epitaxial.

STRUCTURE OF LASER-PRODUCED SILICIDE LAYERS

The occurrence of solidification instabilities is demonstrated in laser reacted films of Pd-Si, Pt-Si and W-Si. The conditions for such instabilities are discussed.

CW LASER ANNEALING STUDIES OF DEPOSITED FILMS

CW laser annealing of thin (0.05, 0.1, 0.2, 0.5 μ m) silicon films deposited in UHV on silicon produces high quality epitaxial layers when the annealing is conducted in ultrahigh vacuum (UHV). However, incomplete crystallization occurs when cw laser annealing of these films is carried out in air, and a layer of polycrystalline silicon is formed at the surface. The depth of this disordered region increases monotonically with the thickness of the original deposited layer. Auger analysis of deposited films after air exposure shows the presence of significant amounts of oxygen. We postulate that the rate of solid phase epitaxy (SPE) is reduced sufficiently by oxygen to allow spontaneous nucleation of polycrystallites to compete with SPE and eventually totally inhibit planar epitaxial crystallization of the deposited films.