Ambient and High Temperature STM Investigations of the Growth of Titanium Suicide on Silicon Substrates
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developed family of scanning probe microscopes. Despite extensive studies of the formation and growth of titanium silicide, there are relatively few studies of the epitaxial nature of the titanium/silicon system. It is known that titanium silicide is capable of polycrystalline epitaxial growth on silicon and that, despite a high lattice mismatch, it forms more extensive epitaxial regions than many silicides with lattice parameters close to those of silicon. 1 C54-TiSi2 has been found to grow epitaxially on both (100)2-4 and (111)5-11 silicon regions of growth. The substrates, with surface coverage involving both epitaxial and non-epitaxial best epitaxy has been obtained on Si(l 11). Of previous studies 24 on Si(100), however, two of the studies,, primarily investigating the effects of backsputter cleaning and the presence of amorphous silicon capping layers and doping impurities, suffered from oxygen contamination of the titanium films which has been identified as an important factor in the irreproducibility of Ti/Si reactions. 12,13 Titanium silicide is most reliably formed through the process of solid phase epitaxy (SPE), which involves depositing a thin film of pure titanium on a carefully cleaned silicon substrate and subsequently annealing at high temperatures (>600*C). The actual physical mechanisms involved in silicide formation are the subject of some controversy. However, there is general agreement that several precursor phases are involved, and that two crystalline silicide phases can form upon high C49-TiSi2 and the lowest resistivity temperature annealing. They are the 1metastable thermodynamically favourable C54-TiSi 2 4, 15 Here, ultrathin titanium films were deposited in UHV and annealed by direct current heating of the silicon substrate. The growth morphology and evolution of the microstructure of the ultrathin titanium silicide films is reported and the results are discussed in detail. Methods for STM crystallography are used to identify the epitaxial orientation of silicide crystallites and their surface structures. 287 Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
EXPERIMENT Atomically clean 2x1-Si(100) and 7x7-Si(1 11) substrates were prepared by direct current heating in UHV. STM images of the clean Si(100) surface displayed a symmetric 2x1 reconstruction with a low residual defect 'density due to missing silicon atoms. No traces of extrinsic contamination were found on either substrate. Titanium deposition was performed at pressures below lxl0-9 Torr using an e-beam microevaporator and a titanium source (99.6% purity). STM images of as deposited ultrathin titanium films clearly showed the presence of atomic steps in the silicon substrates underneath the titanium films. Titanium coverage was uniform with no appreciable surface roughness. Estimated film thicknesses were of the order of 5 to 10 monolayers or approximately 20 A. Detailed Fourier analysis of large area images revealed no periodic structure, indicating that the silicon substrate had been obscured and that the
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