Ordered CaSi Microwall Arrays on Si Substrates Induced by the Kirkendall Effect.

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.