Nanoscale Res Lett (2006) 1:11–19 DOI 10.1007/s11671-006-9011-y N A N O R E V I E W Buffer layer-assisted growth of Ge nanoclusters on Si A. P. Li Æ J. F. Wendelken Published online: 26 July 2006 to the authors 2006 Abstract In the buffer layer-assisted growth method, a condensed inert gas layer of xenon, with low-surface free energy, is used as a buffer to prevent direct inter- actions of deposited atoms with substrates. Because of an unusually wide applicability, the buffer layer- assisted growth method has provided a unique avenue for creation of nanostructures that are otherwise impossible to grow, and thus offered unprecedented opportunities for fundamental and applied research in nanoscale science and technology. In this article, we review recent progress in the application of the buffer layer-assisted growth method to the fabrication of Ge nanoclusters on Si substrates. In particular, we emphasize the novel conﬁgurations of the obtained Ge nanoclusters, which are characterized by the absence of a wetting layer, quasi-zero dimensionality with tunable sizes, and high cluster density in comparison with Ge nanoclusters that are formed with standard Stranski- Krastanov growth methods. The optical emission behaviors are discussed in correlation with the morphological properties. Keywords Nanocluster Æ Buffer layer-assisted growth Æ BLAG Æ Ge nanocluster Æ Photoluminescence Æ Semiconductor growth PACS Codes 61.46.+w Æ 78.55.Ap Æ 68.37.Lp Æ 78.67.Hc A. P. Li (&) Æ J. F. Wendelken Center for Nanophase Materials Sciences and Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA e-mail: apli@ornl.gov Introduction Nanostructure self-assembly is currently a topic of considerable interest at both fundamental and applied levels. Semiconductor nanoclusters, in which electrons are conﬁned in all three dimensions, show novel physi- cal phenomena related to dimensionality, such as an atom-like energy spectrum and quantum size effects. Among self-assembled semiconductor nanostructures, Ge nanoclusters on Si, in particular, have been inten- sively studied as it could promote the development of Si-based optoelectronics. Although remarkable suc- cesses have been achieved [1–3] it remains challenging to create 0D Ge clusters on Si(100) with small, dense, and uniform dot formation. One of the dominant growth mechanisms is the Stranski-Krastanov (SK) growth mode, which leads to the formation of coher- ently strained, pseudomorphic 3D islands after the growth of a 2D wetting layer. Germanium SK dots grown on Si have lateral dimensions of 10–50 nm and are 1–5 nm in height. Their larger lateral size means that quantum conﬁnement effects are determined by the Ge size in the growth direction. Moreover, SK growth is initiated with the formation of a 2D Ge wetting layer beneath the dots [4–6]. Wetting layer formation is due to the lattice mismatch (4.2%) between Si and Ge, which leads to a layer with built-up strain (layer-by- layer growth) and a relief of strain energy (3D island formation) when the layer grows beyond a critical thickness. This wetting layer is electronically coupled to the Ge islands increasing the structures’ thickness fur- ther, thereby reducing quantum conﬁnement effects. It also couples the dots to 2D quantum well-like states, reducing the electronic isolation of the dots from one another, and thus affecting device performance.