Cui et al. Nanoscale Res Lett 2011, 6:59 http://www.nanoscalereslett.com/content/6/1/59 In the growth of ordered SiGe QDs, lithography tech- nologies [8-10] are frequently used to fabricated nano- pits on Si substrate. The grown QDs located in the nanopits, rather than around them, which is contrary to our results. The nucleation modes on the nanopit- patterned substrates have been observed and discussed based on the growth temperature [24]. They found that at a low growth temperature of 550°C, nucleation at the nanopits is in metastable phase. At high growth tem- perature of 750°C, QDs are stable at the terrace between nanopits. At a intermediate temperature of 650°C, ran- dom nucleation is observed. They attribute the variation of nucleation modes to the kinetic limitation at high growth temperature. In another theoretical paper [25], only random and ordered (in the nanopits) nucleation modes are predicted. In our experiments, the growth temperature is 650°C. Nearly all the QDs located around nanopits. However, by growing buffer layers with increasing growth temperature from 500°C to 640°C on nanopit-patterned substrates, QDs nucleate in the nano- pits by growing at 640°C [11]. It can be deduced that different pre-deposited surface morphologies in the two cases result in different nucleation modes. It may be the key point that gradually increasing the growth tempera- ture during the growth of Si buffer layers can effectively decreases the depth of the nanopits as well as the aspect ratio. Nevertheless, the morphological detail of nanopits plays important roles in determining the nucleation whether in or out of nanopits along with growth temperature. Conclusion In summary, nanopits are obtained by in-situ thermal annealing of low temperature Si capping layer on QDs. The formation of the nanopits is discussed based on the strain energy relaxation and surface energy minimiza- tion. The strained Si mounds formed over the QDs become instable under thermal annealing, the Si atoms in the mounds migrate to the surrounding area and sub- sequently the nanopits are formed. The strain distribu- tion in the Si capping layer defines the lateral size of the nanopits, which is close to the lateral size of QDs. QD molecules are grown by a subsequent deposition of Ge on the nanopit-patterned surface. Acknowledgements The work was supported by the special funds for the Major State Basic Research Project No. 2006CB92155 and partial supported by NSFC under No. 10875144. Received: 10 August 2010 Accepted: 15 September 2010 Published: 2 October 2010 Page 5 of 5 References 1. Sturm JC, Manoharan H, Lenchyshyn LC, Thewalt MLW, Rowell NL, Noël J-P, Houghton DC: Phys Rev Lett 1991, 66:1362. 2. Wang X, Jiang ZM, Zhu HJ, Lu F, Huang DM, Liu XH, Hu CW, Chen YF, Zhu ZQ, Yao T: Appl Phys Lett 1997, 71:3543. 3. 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Bergamaschini R, Montalenti F, Miglio L: Nanoscale Res Lett . doi:10.1007/s11671-010-9811-y Cite this article as: Cui et al.: Formation of Nanopits in Si Capping Layers on SiGe Quantum Dots. Nanoscale Res Lett 2011 6:59. Submit your manuscript to a journal and beneﬁt from: 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the ﬁeld 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com