RESEARCH PAPER Morphology and growth of capped Ge/Si quantum dots Yizhak Yacoby • Naomi Elfassy • Samit K. Ray • Raj K. Singha • Samaresh Das • Eyal Cohen • Shira Yochelis • Roy Clarke • Yossi Paltiel Received: 10 October 2012 / Accepted: 25 March 2013 / Published online: 4 April 2013 Ó Springer Science+Business Media Dordrecht 2013 Abstract The morphology, atomic structure, and chemical composition of small (4 nm average height and 20 nm average diameter), dense capped MBE- grown Ge/Si quantum dots are studied using an energy- differential extension of the direct X-ray phasing method, COBRA. Our results lead to the following conclusions: (i) the quantum dot system has a partial wetting layer; (ii) in the lower parts of the dots, the Ge content is small and increases toward the top; and (iii) the contact angle between the dots and the substrate is acute, consistent with the presence of a wetting layer. A growth mechanism compatible with these findings is proposed. Keywords Nano-crystals  Quantum dots  X-ray crystallography  Self-assembly  MBE Introduction Charge carriers in quantum dots (QDs) are confined in all three spatial dimensions, resulting in discrete energy states that are highly favorable for device applications (Yoffe 2001; Ledentsov et al. 1998), including energy harvesting (Aroutiounian et al. 2001; Gra ¨tzel 2001; Robel et al. 2006). For example, Ge QDs grown on Si substrates are interesting for integrated optoelectronic systems (Alguno et al. 2003). In the bulk form, the indirect bandgap of group IV semicon- ductors such as Si and Ge can be problematic for some applications. On the other hand, the electronic states of Ge/Si QDs can be engineered by optimizing the growth parameters which govern their size and in-built strain (Zhang and Drucker 2003; Singha et al. 2008). The Ge/Si QD system is therefore a promising candidate to be used for monolithically integrated optoelectronic devices compatible with the present Si- based technology (Wang et al. 2007). Extensive experimental studies on the structure and composition of QDs grown on Si substrates have been carried out in recent years (Stangl et al. 2004; Biasiol and Heun 2011; Lang 2003; Drucker 2002; Baribeau Special Issue Editors: Juan Manuel Rojo, Vasileios Koutsos This article is part of the topical collection on Nanostructured Materials 2012 Y. Yacoby Racah Institute of Physics, Hebrew University, 91904 Jerusalem, Israel N. Elfassy  E. Cohen  S. Yochelis  Y. Paltiel (&) Department of Applied Physics, Hebrew University, 91904 Jerusalem, Israel e-mail: paltiel@cc.huji.ac.il; paltiel@mail.huji.ac.il S. K. Ray  R. K. Singha  S. Das Department of Physics and Meteorology, Indian Institute of Technology Kharagpur, Kharagpur 721302, India R. Clarke Applied Physics Program, and Center for Solar and Thermal Energy Conversion, University of Michigan, Ann Arbor, MI 48109, USA 123 J Nanopart Res (2013) 15:1608 DOI 10.1007/s11051-013-1608-3