ABSTRACT
The epitaxial NiSi2 NWs and can be grouped into two types. The two types of NWs are designated as Type-A′ and Type-B′ following the conventional nomenclature that Type-A and Type-B interfaces represent the interfaces between the silicide and the substrate with identical and twinning orientation relationships, respectively. Type-A′ NiSi2 NWs are of the identical crystal orientation to that of the Si substrates. Type-B′ NiSi2 NWs possess one Type-B NiSi2/(111) Si interface with a twinning orientation relationship to the Si substrates. Issues relating to Type-A and Type-B silicide/Si interfaces have been extensively discussed previously.5,69 The result is similar to that of the Co/Si system.42 The average aspect ratio of Type-B′ NWs is higher than that of Type-A′ NWs, i.e., about 3.5 for Type-A′ and 25 for Type-B′ NWs. The result indicated that the breaking of the symmetry with the substrate at the twinning interface would promote 1-D growth of silicide NWs. The formation mechanisms of Type-A′ and Type-B′ NiSi2 NWs are quite different. For Type-A′ islands with isotropic lattice mismatch, the formation of NWs could be attributed to the strain-driven “shape transition” mechanism, as represented by the theoretical models put forward by Tersoff and Tromp, as well as by Jesson et al.70,71 In contrast, the formation of Type-B′ NiSi2 NWs involved the mechanism of endotaxy.42 The Type-B interface breaks the symmetry of the surface and leads to the asymmetric growth of Type-B′ islands. NWs. The growth of both Type-A′ or Type-B′ NiSi2 NWs involves the rearrangement of Ni atoms from the edge of elongated NWs to the end since the Ni atoms impinge from all sides. Ideally, the deposition rate of Ni atoms during the RDE should be low enough to provide sufficient time for epitaxial islands to release the elastic strain by means of shape transition. However, the deposition rate cannot be arbitrarily decreased in the limited space of the deposition chamber. Thus a NME method which can efficiently diminish the flux of metal atoms to the Si surface was been utilized to further promote NW growth. Previous work revealed that for the Ni/Si3N4/Si system, nitride served as a pure physical barrier to diminish the Ni flux.72 In addition to the flux adjustment, one main mechanism of OME or NME is to allow the metal-rich phase to be skipped and lead to the formation of epitaxial NiSi2 phase directly.73,74 This mechanism is closely related to the NW growth since only the epitaxial phase could undergo the shape transition. Figure 4.2 shows a SEM image of epitaxial NiSi2 NWs on nitride-capped (001)Si substrates by RDE at
730 °C. The aspect ratios can be improved to 30 and 200 for Type-A′ or Type-B′ NiSi2 NWs, respectively.
