Ordered CaSi₂ Microwall Arrays on Si Substrates Induced by the Kirkendall Effect

Abstract

We have specified the synthetic conditions to obtain one-directionally ordered CaSi microwall arrays vertically grown on a Si substrate. Our basic concept is based on the utilization of the Kirkendall effect for reactive deposition epitaxy (RDE). We found for the first time that: 1) a much larger Ca vapor supply on the Si substrate than the conventional RDE, 2) the adoption of a two-step heating process, and 3) the selection of the crystal axis of the Si surface are the keys to control the microstructures of CaSi on the Si substrate. The CaSi phase was first formed on Si, then the CaSi phase was formed at the CaSi/Si interface. Based on the Kirkendall effect, the interdiffusion of Ca and Si was enhanced in the vertical direction rather than in the parallel direction to the Si surface. CaSi tends to grow along four equivalent Si{111} planes, however, the specific orientation of the Si surface resulted in CaSi microwalls grown along its Si(111‾ ) plane, the only plane directing nearly vertical to the surface among the Si{111} planes. These results suggest that the Kirkendall effect under asymmetric growth of target materials would be a rational strategy to obtain their ordered microstructures.

Chemical potential gradient induced formation of Kirkendall voids at the epitaxial TiN/MgO interface

“…While diffuse composition changes at the interface are consistent with imbalanced diffusion, rectangular faceted voids suggest concurrent nucleation and growth of vacancy clusters at the interface that is influenced by anisotropy. On the other hand, we note that Kirkendall voids at epitaxial interfaces have been observed in a number of different material systems, including Cu(In,Ga)Se 2 / GaAs, 43 CaSi 2 /Si, 44 and ZnO/Si. 45 Alternatively, considering that the microscopic mechanism(s) for atom migration in the TiN-MgO system is likely vacancy mediated, asymmetries in vacancy migration at or across the interface will generate imbalanced atom fluxes that will lead to the formation of Kirkendall voids.…”

“…While diffuse composition changes at the interface are consistent with imbalanced diffusion, rectangular faceted voids suggest concurrent nucleation and growth of vacancy clusters at the interface that is influenced by anisotropy. On the other hand, we note that Kirkendall voids at epitaxial interfaces have been observed in a number of different material systems, including Cu(In,Ga)Se 2 /GaAs, 43 CaSi 2 /Si, 44 and ZnO/Si. 45…”

Formation of Si-based Nanosheet Bundles and Morphological Modication of CaSi₂Crystals by Thermal Treatment Using Chloride Compounds

“…the CaSi 2 crystals are described elsewhere [9]. Figure 2(e) shows an STEM image and corresponding EDS mappings of the FeCl 3 -treated nanosheets formed on the Si substrate with the thermal treatment condition of 550…”

“…The CaSi 2 micro-walls were initially grown on Si(111) substrates [9]. Commercially-available CaSi 2 crystal powders were also used as the source material.…”

“…1(b). Further detailed information about the treatment conditions of the micro-walls and nanosheet bundles is described elsewhere [6,7,9]. Throughout this paper, for example, these nanosheets treated under the FeCl 3 vapor are denoted here as "FeCl 3 -treated" nanosheets, etc.…”