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) st udy of stacked Ge island layers embedded in Si. We systematically varied th e thickness t(s) of the Si interlayer and the number of island layers to ex plore the effect of the interlayer thickness on the growth of Ge islands. F or sufficiently thin t(s), the Ge islands align in a vertical direction. An alytic transmission electron microscopy reveals that, in the regime of vert ical island alignment, the wetting layer (WL) thicknesses decrease in upper Ge layers, This effect is explained by a reduced critical thickness for is land nucleation caused by the strain fields of buried islands. At the same time, PL experiments show that the WL energy transition is split into multi ple lines, as would be expected due to the different WL thicknesses. The li ne splitting is dependent on the intermediate Si spacer thickness, reflecti ng the degree of strain field interaction between the stacked layers. The i sland-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. (C) 2000 Elsevier Science S.A. Al l rights reserved.