Near-band-gap photoluminescence from pseudomorphic Si1−<i>x</i>Ge<i>x</i> single layers on silicon

Abstract

The systematic study of band-edge luminescence in pseudomorphic Si/Si1−xGex/Si double-heterostructure layers is reported for a wide composition range, 0.12&amp;lt;x&amp;lt;0.24, for the first time. An analytical expression for the exciton energy gap at 4.2 K valid for x&amp;lt;0.24 is derived from the no-phonon line energies: ESX(x) = 1.155−0.874x+0.376x2 eV. Addition of an expression for the exciton binding energy provides an approximation for the energy difference between the alloy valence band and the lowest conduction-band edge at low temperature. An exciton upshift of 16.9 meV due to quantum confinement is observed in a 6.3-nm Si0.83Ge0.17 alloy well. This is consistent with either type-I or type-II band alignment for which the conduction-band offset has a magnitude ‖ΔEc‖ ≤ 10 meV. The excitonic hole is closely confined in the alloy but the spectra suggest that the electron density in the silicon barriers is increased for the thin layer.