Reduced critical thickness and photoluminescence line splitting in multiple layers of self-assembled Ge/Si islands

We present a transmission electron microscopy and photoluminescence (PL) study of stacked Ge island layers embedded in Si. We systematically varied the thickness ts of the Si interlayer and the number of island layers to explore the effect of the interlayer thickness on the growth of Ge islands. For sufficiently thin ts, the Ge islands align in a vertical direction. Analytic transmission electron microscopy reveals that, in the regime of vertical island alignment, the wetting layer (WL) thicknesses decrease in upper Ge layers. This effect is explained by a reduced critical thickness for island nucleation caused by the strain fields of buried islands. At the same time, PL experiments show that the WL energy transition is split into multiple lines, as would be expected due to the different WL thicknesses. The line splitting is dependent on the intermediate Si spacer thickness, reflecting the degree of strain field interaction between the stacked layers. The island-related optical transition is blue-shifted with increasing number of island layers. We attribute this effect to enhanced material intermixing in the second and proceeding island layers.