Ballistic-electron-emission microscopy of strain nonuniformities in Si1-xGex/Si structures.

Abstract

Ballistic-electron-emission microscopy (BEEM) is used to probe local conduction-band structure in strained ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ layers of pseudomorphic ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$/Si heterostructures. The strain variation produced by a roughened ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ surface is seen as a variation of splitting between thresholds in BEEM spectroscopy. This splitting is directly related to the strain-induced conduction-band splitting in the ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ layer, enabling BEEM to directly measure local strain variations. Elasticity calculations for a roughened ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ surface predict variations in strain that are consistent with BEEM observations. For the case of a smooth ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ surface, a uniform conduction-band splitting is observed that is in good agreement with calculations.