12 To avoid wetting layer formation, an alternate growth technique is required to promote a 3D growth mode. In the phenomenological 3D Vollmer-Weber island growth mode, the interaction between neighboring layer atoms should be stronger than that between substrate and layer atoms. This means that a low-surface free energy of the substrate is generally required for island growth. Thus, a key concept to prevent wetting layer formation is the modiﬁcation of the surface free energy of a substrate. An inert gas that has normally very low-surface free energy was ﬁrst proposed as a buffer layer by Weaver’s group [7–9]. This process, in which a buffer layer is applied to interrupt the normal growth process and artiﬁcially create a necessary condition for the nanoclusters formation, has developed into a unique buffer layer- assisted growth (BLAG) method. A group at Oak Ridge National Laboratory has successfully employed this method to the growth of a variety of semicon- ducting and magnetic nanoclusters [10–12]. In this paper, we review the application of BLAG in the fabrication of Ge nanoclusters on Si substrates. The nanoclusters thus produced are characterized by absence of a wetting layer, quasi-0D with tunable sizes, and high-cluster density compared with Ge nanoclus- ters that are formed with the normal SK growth method. The novel morphology, structure, and photo- luminescence (PL) from Ge nanoclusters are discussed. Buffer layer-assisted growth method The general approach to the fabrication of self- assembled nanoclusters using the BLAG method con- tains the following steps: (1) Condense and freeze a thick buffer layer of an inert gas and maintain the system at a temperature below the sublimation point of the buffer gas. (2) Deposit atoms of a semiconductor on top of the buffer layer. These atoms will exhibit extremely high mobility on top of the buffer layer (compared with direct deposit without the buffer layer) and will diffuse to form a relatively uniform 3D clus- ters. (3) Continue the growth of the nanoclusters to the desired size. Each nanoclusters acts as a nucleus for further growth and since each may collect arriving adatoms from approximately the same areas, they will grow to approximately the same size. (4) Raise the substrate temperature above the sublimation temper- ature of inert gas to remove the buffer layer. This provides a gentle landing of the nanocluster array with essentially the same spacing and size distribution as the original array on top of the buffer layer. The utilization of a temporary buffer layer before deposition of a Nanoscale Res Lett (2006) 1:11–19 Fig. 1 Schematic of the BLAG of Ge nanoclusters on Si source material frees the system from kinetic and stress constraints when depositing a source material on the substrate and thus promotes cluster formation. The Ge nanoclusters were grown on a Si(100) substrate in an ultrahigh vacuum (UHV) environment. A Si(100) 2 · 1 surface reconstruction was obtained by ﬂashing Si sample up to 1200 C after degassing at 400 C. A schematic of sample preparation with the BLAG method is shown in Fig. 1. First, a clean Si(100) substrate was cooled to 10 K under UHV conditions. A buffer layer of condensed Xe was then formed by exposure to 20–200 Langmuirs of pure Xe gas (99.995%). According to the ion gauge sensitivity to Xe, 1 monolayer (ML) of Xe corresponds to 5.5 Langmuirs of Xe exposure (1 L = 10–6 Torr s) [8]. Then a ﬂux of pure Ge atoms was deposited on top of the Xe buffer layer. These atoms exhibit extremely high mobility on top of the buffer layer because of the low-surface free energy of Xe and diffuse to form 3D nanoclusters. Finally, the sample was warmed to room temperature to remove the buffer layer. This provides a gentle landing of the nanoclusters to the Si substrate. Morphology and structure of Ge nanoclusters As already mentioned foremost, without the presence of a Xe buffer layer, Ge growth on Si(100) is well known to form SK clusters [13–15] in the absence of a Xe buffer layer. Strain-driven SK growth proceeds in two steps. First, below a critical thickness, a layer-by- layer growth can be maintained with built-in strain. Beyond the critical thickness, a 3D island growth sets in to relieve the strain energy. Figure 2 shows the scanning tunneling microscopy (STM) image of a 2.0 ML Ge layer on Si without a buffer layer. This is below a critical thickness (3.0 ML) of Ge coverage, and a layer-by-layer growth mode is observed, leading to