Solid-phase epitaxial regrowth of amorphous silicon containing helium bubbles

Transmission electron microscopy has been used to study processes occurring when a layer of amorphous silicon (a-Si) containing helium-filled cavities buried in crystalline silicon (c-Si) recrystallizes by solid phase epitaxial growth (SPEG). The buried layer was formed in (100) silicon by means of bombardment with 150 keV Li ions with the bubbles resulting from subsequent implantation of 80 keV He ions; the energies being chosen to ensure that the resulting bubble distribution was entirely contained within the amorphous layer. The presence of bubbles in a-Si undergoing SPEG at a (100) interface with c-Si has previously been observed to give rise to the formation of microtwin lamellas, assumed to nucleate at the bubble surfaces; however, the present work indicates clearly that, in fact during SPEG, many microtwins nucleate remote from the bubbles. There is also an apparent interaction between the amorphous-crystalline (a-c) interface and the bubbles, in which the bubbles seem to be swept by the moving interface. The paper will discuss possible mechanisms for both phenomena, in terms of interstitial defects for the nucleation of microtwins and in terms of enhanced bubble mobility in a-Si for the apparent interaction between the a-c interface and the bubbles.