Hydrogen-mediated structural changes of amorphous and microcrystalline silicon

Abstract

Amorphous $(a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H})$ and microcrystalline silicon $(\ensuremath{\mu}c\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H})$ samples were exposed to a hydrogen plasma in a clean electron-cyclotron-resonance system. We present unequivocal experimental evidence for hydrogen-induced crystallization of $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}.$ A 60-min post-hydrogenation at 325 \ifmmode^\circ\else\textdegree\fi{}C resulted in an increase of the crystalline fraction by 10--15 %. Similar H plasma treatments performed on $\ensuremath{\mu}c\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ caused a decrease of the crystalline fraction ${X}_{C}$ by up to 20%. The lack of an amorphous phase in posthydrogenated c-Si shows that the presence of grain boundaries is required to observe hydrogen-induced conversion of crystalline to amorphous silicon. We propose that the driving force for the decrease of ${X}_{C}$ is the minimization of the lattice-strain energy.