Shape preservation of self-assembled SiGe quantum rings during Si capping

Self-assembled SiGe quantum rings (QRs) on Si(001) are capped with Si layers at temperatures varying from 200 to 550 ◦C; their shape changes after Si capping are investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The SiGe QR shape can be well preserved in the Si capping layer when the substrate temperature at Si capping is lower than 350 ◦C, whereas the SiGe QR shape transforms from a ring to a mound when the substrate temperature is higher than 480 ◦C. The SiGe QR shape could also be well preserved in the Si capping layer with an initial low temperature (300 ◦C) Si capping followed by a relatively high temperature (550 ◦C) Si capping. A comparison of the photoluminescence (PL) spectra of the SiGe QRs and the SiGe quantum dots (QDs) is also reported.