Amy S. Gandy

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.

Crystal structure and non-stoichiometry of cerium brannerite: Ce{sub 0.975}Ti{sub 2}O{sub 5.95}

The crystal structure of cerium brannerite, Ce{sub 0.975}Ti{sub 2}O{sub 5.95}, was determined from Rietveld analysis of high resolution neutron powder diffraction data (space group C2/m, a=9.8320(1) A, b=3.75287(6) A, c=6.8852(1) A, {beta}=119.230(1) Degree-Sign ). The presence of both cerium and oxygen vacancies was confirmed from Rietveld analysis and defect energy calculations, combined with careful investigation of the phase diagram. Formation of oxygen vacancies at the O1 site, charge compensated by Ce vacancies, relieves considerable Coulombic repulsion and hence structural strain associated with short O1-O1 contacts forming the shared edge of neighbouring TiO{sub 6} polyhedra. Consideration of the bond valence sum of Ce{sup 4+} in Ce{sub 0.975}Ti{sub 2}O{sub 5.95} afforded a revised value of the r{sub 0}=2.098{+-}0.039 A for the reference bond length, based on analysis of 21 crystal structures mined from the ICSD. - Graphical abstract: Neutron diffraction and atomistic defect energy calculations reveal the mechanism of nonstochiometry in cerium brannerite, Ce{sub 0.975}Ti{sub 2}O{sub 5.95}. Highlights: Black-Right-Pointing-Pointer The defect brannerite Ce{sub 0.975}Ti{sub 2}O{sub 5.95} was prepared by solid state synthesis. Black-Right-Pointing-Pointer Neutron diffraction confirmed the presence of Ce vacancies charge compensated by O vacancies. Black-Right-Pointing-Pointer Defect energy calculations show oxygen vacancies relieve structural strain associated with short O-O contact distances.