Band offsets in pseudomorphically grown Si/Si1-xGex heterostructures studied with core-level x-ray photoelectron spectroscopy.

Abstract

Band offsets, \ensuremath{\Delta}${\mathit{E}}_{\mathit{c}}$ and \ensuremath{\Delta}${\mathit{E}}_{\mathit{v}}$, at heterojunction interfaces in Si/${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ strained-layer structures, grown by molecular-beam epitaxy in combination with ion-beam doping, have been investigated by in situ core-level x-ray-photoelectron-spectroscopy measurements. The magnitude of the band gaps and the position of the Fermi level relative to the band edges were determined from the binding-energy shifts of the Si 2p or the Ge 3d core level as the doping was changed in the range from ${\mathit{p}}^{+}$ to ${\mathit{n}}^{+}$. The band-offset values are crystallographic-orientation-dependent, while there is no crystal-orientation dependence of the binding-energy difference \ensuremath{\Delta}${\mathit{E}}_{\mathit{B}}$ between the Si 2p level in the silicon overlayer and Ge 3d level in the ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ layer. This is evidence for a negligible interfacial dipole for these covalent heterojunctions. The value of \ensuremath{\Delta}${\mathit{E}}_{\mathit{B}}$, however, is altered with changes in the Ge concentration or the strain configuration. This can be referred to as the contribution from the hydrostatic pressure in the strained films. In contrast to assumptions often made in the literature, we found no correlation between measured \ensuremath{\Delta}${\mathit{E}}_{\mathit{v}}$ and \ensuremath{\Delta}${\mathit{E}}_{\mathit{B}}$ values. The dominant contribution to the band offsets in Si/${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ strained heterojunctions is due instead to the splitting of the band edges, which is induced by misfit strain. This is also borne out by comparisons with theoretical calculations of the band offsets, which show good agreement between experiment and theory.