Photoluminescence from epitaxial Si/Si_0.95Ge_0.05 heterostructures as probed by optically active deep levels

Abstract

A detailed study of near-gap photoluminescence (PL) from strained Si1-xGex alloy layers (x = 0.01-0.05) and Si/Si0.95Ge0.05 multi-quantum-wells (MQWs) has failed to show either free- or dopant-bound excitons in as-molecular-beam-epitaxy-prepared epitaxial layers. Low-temperature PL was, however, successfully induced in these same heterostructures by the selective introduction of relatively shallow radiation-damage bound-exciton centers, I1 and G (137 and 186 meV deep, respectively). The I1 center, in particular, produced broadened spectra which are shown clearly to emanate from the epitaxial layers, with separate and distinguishable components originating from both the Si and the Si1-xGex (x = 0.01-0.05) layers. This is among the first such reports of luminescence verifiably originating from within a Si/Si1-xGex multiple heterostructure. Importantly, we find that this defect-induced PL is not significantly diminished (or even significantly influenced) by the addition of multiple heterointerfaces, thus leading to the conclusion that heterointerface quality may be good. This indicates that these heterointerfaces are not the source of the competing nonradiative recombination which likely prevents the observation of near-gap PL, which is instead possibly due to bulk defects.