Abstract
The growth mechanism and morphology of titanium silicide nanostructures on Si substrate have been investigated by scanning tunneling microscope (STM), atomic force microscope (AFM), transmission electron microscope (TEM), and in situ ultrahigh-vacuum TEM (UHV-TEM). The growth of titanium silicide was investigated in sub-monolayer Ti deposited on Si(111). The formation of nanowire (NW) and cluster structures was observed for 600 and 700 °C deposited samples. In sample heated to 800 °C, titanium silicide structures transformed to clusters. C49-TiSi2 NWs were found to orient along three equivalent Si <2-20> directions with C49-TiSi2(200)//Si(2-20). Deposition of sub-monolayer Ti at a high temperature was found to contribute to the lowering of the formation temperature of C49-TiSi2. The lowering of C49-TiSi2 growth temperature on heated substrate is attributed to smaller lattice mismatch between C49-TiSi2(200) and Si(2-20). Understanding the growth mechanism of NWs is essential for their successful implementation in advanced devices applications. In situ UHV-TEM has been applied to elucidate the interaction mechanisms of TiSi2 NWs on Si(111) substrate. Two phenomena were observed: merging of the two NWs in the same direction, and collapse of one NW on a competing NW in different directions when they meet at the ends. On the other hand, as one NW encounters the midsection of the other NW in different direction, it receded in favor of the bulging of the other NW at the midsection. The crucial information has been fruitfully exploited to grow long and high-aspect-ratio titanium silicide NWs on Si(110) since the NWs grow only in the [2-20] direction. The formation of titanium silicide NWs was investigated on Si(110) samples at 600-700 °C. From STM and AFM observation, the average length and aspect ratio of NWs increased with substrate temperatures. These NWs were oriented along one Si(2-20) direction with C49-TiSi2(200)//Si(2-20). The density of NWs is dependent of substrate temperatures and the activation energy E* for titanium silicide NW growth is 0.75 ± 0.1 eV. The growth of titanium silicide NWs was found to be significantly affected by the deposition rate and growth time. The self-assembled titanium silicide NWs on Si(001) substrate were investigated. Titanium silicides in the 600 °C deposited sample are of islands nanostructure and form NW shape in the 700 °C deposited sample. These titanium silicide NWs are aligned in two perpendicular <220> directions of Si substrate with large aspect ratio. From the analysis of TEM, the NWs were identified to be of O-Ti5Si4 phase with the epitaxial relationships of O-Ti5Si4[400]//Si[220] and O-Ti5Si4(034)//Si(001) and resulted from the anisotropic lattice mismatches that is good in the long axis and poor in the short axis of the NW.