Chemical reaction and Schottky-barrier formation at V/Si interfaces

The behavior of the V/Si interface has been studied under atomically clean interface conditions for a variety of Si surfaces [Si(111)-(7×7), Si(111)-(2×1), and $\mathrm{Si}\mathrm{}\left(100\right)\mathrm{}-c(4\mathrm{}\times 2)$] as a function of annealing used to promote the silicide formation reaction. Interface chemistry (composition, reactivity, and electronic structure) was revealed by angle-integrated ultraviolet (UPS) and x-ray photoemission spectroscopy and by Auger-electron spectroscopy (AES); transmission electron microscopy (TEM) and low-energy electron diffraction (LEED) were used to assess interface and surface atomic structure and microstructure; and the Schottky-barrier height ${\phi }_{\mathrm{bn}}$ was monitored by synchrotronradiation-photoemission measurements of $\mathrm{Si}\mathrm{}\left(2p\right)\mathrm{}$ core-level band bending. In contrast to the behavior for near-noble metals, the interface reactivity between the refractory metal V and the Si substrate is sufficiently low that even under atomically clean conditions no spontaneous intermixing reaction involving atomic motion across the interface occurs at room temperature. Instead, as seen by UPS and AES, simple chemisorption of V atoms on the Si substrate occurs, which is followed by the growth of polycrystalline V metal at higher V coverage. As expected from previous thin-film studies, the stable high-temperature product of the reaction at 500-550°C is identified to be V${\mathrm{Si}}_2$ by TEM phase analysis, by AES composition analysis, and by its characteristic UPS spectrum. However, surprising new information is obtained upon annealing at lower temperatures (≤350°C): Strong intermixing of V and Si atoms across the initially abrupt interface proceeds even through a relatively thick layer (∼ 100 Å or more). This low-temperature intermixed overlayer is characterized by an average surface composition of VSi, by UPS spectra which are different from those of V${\mathrm{Si}}_2$ and unreacted V, and by the presence of the V${\mathrm{Si}}_2$ compound phase in a matrix of V metal. This low-temperature intermixing behavior represents a new and important aspect of the metal/Si interfacial reaction, which has now been observed because of the high sensitivity of surface-spectroscopy techniques and the preparation of atomically clean, well-ordered Si surfaces and interfaces. Its implications become even more striking in view of the observation that the Schottkybarrier height ${\phi }_{\mathrm{bn}}\sim 0.64$ eV of the fully reacted V${\mathrm{Si}}_2$/Si contact is already attained once this low-temperature intermixing process occurs. Although the character of the interfacial reaction is more complex in this case, it is clear that the basic interface reactivity associated with silicide-compound formation dominates the microscopic chemistry and properties of this refractory-transition-metal/Si interface and is strongly correlated with the Schottky-barrier electrical characteristics as well.