J. L. Batstone

Mechanisms of buried oxide formation by ion implantation

We have studied the process of buried oxide formation as a function of implantation and annealing conditions. Concentrating on substoichiometric implants (<1×1018 O/cm2), we varied the implantation energies from 100 keV to 1 MeV. Some apparent precipitation of SiO2 similar to that observed in Czochralski‐grown silicon occurs on implantation. This means that formation of the buried oxide layer and perfection of the overlying crystalline Si layer depend more strongly on the substrate temperature during the implant than on the annealing temperature.

In situ study of the molecular beam epitaxy of CoSi2 on (111) Si by transmission electron microscopy and diffraction

The growth of CoSi2 has been studied by deposition and reaction of Co on clean Si (111) surfaces in situ in a modified ultrahigh vacuum transmission electron microscope. On deposition of nominally 20 A Co at room temperature, strong interaction between Si and Co occurs yielding an epitaxial film which is believed to be a hexagonally distorted form of the Co2Si structure. On heating to ∼350 °C the growth of epitaxial CoSi is observed, which transforms to CoSi2 at ∼450 °C. Silicon‐rich phases propagate by lateral motion of phase boundaries. Substantial pinhole density arises in films only after annealing at higher temperatures. The B orientation dominates under similar annealing conditions. In contrast to NiSi2, the growth of cobalt silicide films on Si is dominated by bulk phase formation, with interfacial energy minimized in all cases by epitaxy.

Control of pinholes in epitaxial CoSi2 layers on Si(111)

The growth of ultrathin (<50 A thick) uniform CoSi2 layers at low temperatures (<450 °C) has been reported recently. Pinholes are formed in these silicide layers when the temperature is raised to above ∼550 °C. An important driving force for the generation of pinholes has been identified as a change of the surface structure from CoSi2‐C, stable at low temperature, to the high‐temperature stable CoSi2‐S. Treatment of the surface of CoSi2 facilitates this transition and prevents the formation of pinholes. A few important parameters in the silicide reaction are shown to govern the morphology of the reacted CoSi2 layers.

Control of epitaxial orientation of Si on CoSi2(111)

Template techniques for Si epitaxy are designed based on the two structures, CoSi2‐C and CoSi2‐S, of the CoSi2 surface. The different stacking sequences of the two CoSi2 surfaces have led to the growth of single‐crystal epitaxial Si layers with either type A or type B orientation on CoSi2(111). The crystalline quality of these Si/CoSi2/Si structures far exceeds that of those reported previously. The orientation of the epitaxial Si overlayer is also found to depend on the strain in the epitaxial CoSi2 thin films.

Electrical and structural characterization of ultrathin epitaxial CoSi2 on Si(111)

We report the fabrication of epitaxial CoSi2 layers on Si(111) as thin as 1 nm. The crystalline lattice of these layers is coherent with the Si lattice, and the silicide is electrically continuous. There are pronounced structural differences between films which are less than 3 nm thick and those which are thicker. The resistivity of the layers increases sharply with decreasing thickness. This is the first report of the growth of coherent, electrically continuous CoSi2 layers on Si.

Subboundary‐free zone‐melt recrystallization of thin‐film silicon

Scanned zone‐melt recrystallization (ZMR) of amorphous Si 1‐μm films on SiO2 results in subboundary‐free material provided the thermal gradient along the scan is reduced to 4 K/mm or less. Below this value the usual ZMR subboundaries consisting of networks of in‐plane edge dislocations are replaced by rows of threading dislocations. We account for the two kinds of crystallization by extending the faceted solidification model that we developed previously. We consider profiles of the solidification front at the intersections of {111} facet pairs where subboundaries are known to form, and postulate that the profiles are aligned approximately normal to the scan in the high gradient case, but become tilted toward the plane of the SiO2 cap layer for the low gradient case. The tilting accounts in a natural way for subboundary removal and the transition from in‐plane to threading dislocations.

Radiation‐enhanced diffusion of Au in amorphous Si

The radiation‐enhanced diffusion of implanted Au markers in amorphous Si has been measured in the temperature range 77–693 K. Samples were irradiated with 2.5 MeV Ar ions. The diffusion coefficients show three well‐defined regions. For temperatures <400 K, diffusion is athermal and due to ballistic mixing. For temperatures in the range 400–700 K diffusion is Arrhenius‐type with an activation energy of 0.37 eV and is considerably enhanced over the normal thermal diffusion. The defects that cause the enhanced diffusion come from nuclear energy loss processes. Thermal diffusion, with an activation energy of 1.42 eV, dominates at temperatures greater than 750 K.

Coreless defects and the continuity of epitaxial NiSi2/Si(100) thin films

Epitaxial thin films of NiSi2 on Si(100) have been grown by room‐temperature deposition of Ni followed by a high‐temperature reaction. Initial stages of epitaxy revealed by transmission electron microscopy show nucleation of crystallographically equivalent islands related by a translation vector a/4〈111〉 via the underlying silicon substrate. Coalescence of islands thus requires the generation of a/4〈111〉 dislocations, which is energetically unfavorable. We find that very thin films (∼60 A) do not coalesce, but choose to remain as islands leaving trenches of exposed substrate 15±1.5 A in width between them. We propose that the trench left between islands can be described as a coreless defect in the silicide.

Twin formation and Au segregation during ion‐beam‐induced epitaxy of amorphous Si

Ion‐beam‐induced epitaxial recrystallization of Au‐implanted amorphous silicon at temperatures >400 °C has been studied. Crystallization was induced using a 2.5 MeV Ar beam. Segregation of Au at the moving crystal/amorphous silicon interface occurs with an interface velocity of 5 A/s. At high Au concentrations, the interface breaks down with the formation of twins. The twinned crystal/amorphous interface then propagates under further irradiation with a reduced interface velocity of 3 A/s. Unusual Au redistribution profiles are obtained as a result of the sudden change in interface morphology. The Au profiles are interpreted on the basis of classical segregation theory with the interfacial segregation coefficient changing from 0.0012 to 0.02 at the onset of twin formation.

Dependence of the structural and electrical properties of ultrathin cobalt silicide films on formation conditions

We have studied the dependence of the electrical and structural properties of ultrathin cobalt silicide films on the annealing temperature and deposited Co thickness. If less than 10 A of Co is deposited, epitaxial type B CoSi 2 forms immediately. As the deposited thickness approaches 10 A, small amounts of Co 2 Si are observed. If greater than 10 A of Co is deposited, epitaxial Co 2 Si forms at room temperature, which proceeds either via the reaction Co 2 Si ⇉ CoSi ⇉ CoSi 2 or via Co 2 Si ⇉ CoSi 2 during annealing. In these thicker films our results suggest that the formation of type A CoSi 2 is correlated with the presence of Co 2 Si; the presence of CoSi as an intermediate phase is correlated with the occurrence of type B CoSi 2 . Both film thickness and reaction temperature strongly influence the electrical transport in these films such that very high resistivities are encountered when films either become very thin or are reacted at low temperatures. In the former case the size effect is responsible whereas in the latter the transport properties are dominated by extensive atomic-scale disorder.