We have investigated growth and overgrowth of Ge hut cluster islands. In a single island layer, free-standing hut clusters intermix only slowly with Si. When the islands are grown in a vertical stack, the critical thickness for island formation is reduced and intermixing is strongly increased where buried islands are present. Only one island layer contributes to the photoluminescence (PL) signal, since carriers tunnel to the island layer which is the lowest in energy before recombining. Furthermore, hut cluster islands which were overgrown with Si at reduced temperatures are investigated. We show that at low capping temperatures, the island-related photoluminescence peak shifts below the band gap of Ge and saturates at around 0.616 eV (2.01 mum). (C) 2003 Elsevier B.V. All rights reserved.