16 This experiment has demonstrated that high-energy electron beam bombardment can induce the recrys- tallization of Ge nanoclusters to align to Si substrate lattices. Photoluminescence of Ge nanoclusters PL studies on these nanoclusters have revealed intri- guing new features in comparison with SK dots. Fig- ure 8 displays PL spectra from Ge nanoclusters grown with the BLAG method with equivalent Ge deposition of 0.3, 0.6, and 6 ML, where the Xe buffer-layer thickness is 6 ML for each case. The dominant PL bands are centered at 0.806 eV (P1) and 0.873 eV (P2) with a full width at the half maximum (FWHM) of 20 meV [11], which are much sharper as compared to those from SK dots (FWHM of 60–100 meV) [2–5]. However, the energies of P1 and P2 are independent of cluster size. Since the clusters are much smaller than SK dots, and should therefore, exhibit much stronger quantum conﬁnement, the absence of size effects on the PL peak energy suggests that the recombination probably is not directly associated with the band structures of the clusters. The emission band at around 1.1 eV is the phonon-assisted recombination of the free-exciton in the Si substrate [2, 4, 19, 20]. The PL bands at around 0.94 and 1.00 eV, which appear on the 6 ML Ge sample and also on other high Ge coverage samples, correspond to those from the excitonic no- phonon and transverse-optical-phonon-assisted transi- tions of 2D Ge on Si [21, 22]. This implies the possi- bility that 2D Ge starts to form when Ge coverage becomes higher. However, TEM analysis shows no evidence of a pseudomorphic Ge layer. Thus, if this Fig. 8 PL spectra of Ge nanoclusters with nominal Ge thickness of (a) 0.3 ML, (b) 0.6 ML, and (c) 6 ML. The spectra were measured at 15 K Nanoscale Res Lett (2006) 1:11–19 Fig. 9 Excitation power dependence of PL spectra from Ge nanocluster formation on Si(100) with a BLAG method. The nominal Ge thickness is 0.5 ML, and the Xe buffer thickness is 6 ML. The inset shows the excitation dependence of the integrated PL intensity for the same sample layer indicated by the PL spectra is associated with a 2D layer, it not the conventional Ge wetting layer. The excitation power dependence of the PL from a sample with 0.5 ML Ge deposited with 6 ML Xe as a buffer layer is shown in Fig. 9. In contrast to the PL behavior from SK dots, the peak energies of both P1 and P2 do not shift with increasing excitation power from 1 to 5 W/cm2 [11]. Coherent strain in SK-grown dots gives them a type-II band alignment, in which the electrons and holes occupy two spatially separate regions [2–5, 19]. Band bending occurs at the interfaces of type-II structures because of the Hartree potential, blueshifting the PL signal as greater excitation power densities increase the carrier density [20–22]. Such a blueshift is not observed in the nanoclusters prepared with the BLAG method, and it is thus probable that these clusters do not possess a type-II band alignment at the Ge/Si interface. This would also be consistent with the fact that negligible strain exists at the Ge nanocluster/Si interface because of the BLAG process. The excitation curves, shown in the inset of Fig. 9, reveal sublinear (~0.50) power exponents for both P1 and P2 bands. Sublinear power exponentfs with a power exponent of ~0.78 have previously been attrib- uted to a type-II band alignment with a limited density of localized states for excitons [4, 15]. An increase of localization of the radiative centers would lead to a smaller power exponent. A power exponent of ~0.66, which is closer to our value, has been observed from SiGe dots at high excitation power density and attrib- uted to direct competition between Auger and radia- tive recombination channels in the dots [2, 20, 23]. Coupled with the fact that the peak energy does not blueshift at high excitation power, it is plausible to