Anomalous photoluminescence of pure-Ge/Si type-II coupled quantum wells (II-CQWs)

Anomalies have been observed in photoluminescence (PL) of pure-Ge/Si type-II coupled quantum wells (II-CQWs). Although the II-CQWs (LGe=1.5 ML) showed clear energy red-shift with decreasing Si spacer thickness (LSi) due to interwell coupling of the hole wave-function, an anomalous energy lowering was observed for smaller LSi (≤6 A). Moreover, PL intensity of the II-CQWs increased linearly with increasing excitation power, as opposed to a sublinear increase in type-II QWs due to exciton localization at the interfaces. These results seem to be the result of enhanced electron capture by the II-CQW potential profile. Furthermore, correlation of thickness fluctuation among the layers was observed by high-resolution transmission electron microscopy, which may be connected to the PL anomalies.