18 likely result from defect centers at the Ge/Si interface or in the clusters since the structure is not pseudo- morphic. Figure 12 compares the spectra obtained for both a single-layer structure and a 3-layer structure. For the 3-layer structure, spacer layers of p-doped Si are grown on top of the nanoclusters at room temperature then the system is cooled again for the BLAG process for each layer. The spectra obtained for both structures have two principal peaks at the same energies. This is remarkable in view of recent results obtained for a 2-layer nanocluster structure of SK grown dots in which the spectrum shows a blueshift and an additional peak resulting from the second layer [5]. This layer dependence stems from the increased stress at the growth front of the second layer that results in increased intermixing of the Si with the Ge, a problem which is clearly not present with the BLAG. The observed spectrum with two principal peaks is not consistent with the published single-layer SK dot spectra that show only one peak. In the conventional SK growth method, the Ge dots possess a highly strained interface with the surrounding silicon resulting in a neighboring conﬁnement structure and a type II band alignment with transitions from electrons trapped in the interface to holes in the germanium dots. The low-temperature deposition of the silicon used in the present work to avoid alloying of the Ge with the Si results in nanoclusters surrounded by amorphous Si and little interface strain. The PL behaviors show that the Ge nanoclusters fabricated with the BLAG method are not type-II and the radiative recombination is not associated with band edges. To clarify the role of these amorphous Ge nanoclusters in the PL process, a reference sample was fabricated without the mediation of a Xe buffer layer. In this way, clustering was avoided and a uni- form Ge amorphous layer of 6 ML equivalent cov- erage was deposited directly on Si at 10 K, and then capped with an amorphous Si layer at room temper- ature. An amorphous nature of the deposited Ge layer was conﬁrmed with STM and low energy-elec- tron diffraction analysis. Under the same measure- ment conditions, only a weak emission band from the Si substrate (1.1 eV) appeared, and P1 and P2 bands were not observed from the uniform Ge amorphous layer or from the amorphous Si cap layer in the ref- erence sample. The exclusive presence of P1 and P2 emission bands in the samples containing the amor- phous Ge nanoclusters indicates that these amorphous clusters must play a critical role in the PL process. A possible model of the PL origin is that the excitons are generated in the Ge nanoclusters or in bulk Si and Nanoscale Res Lett (2006) 1:11–19 then decay to defect centers located either within the dots or at the dot/Si interface to recombine. The presence of Ge nanoclusters provides high high-en- ergy excitons, higher than the bulk Ge bandgap en- ergy, and a high density of interface states that lead to PL with band energies higher than the bulk Ge bandgap. This model has been extensively used to explain luminescence properties from Si and Ge nanocluster systems [25–29]. We have noticed that our observed P1 and P2 bands are in similar energy positions to the dislocation centers D1 and D2, to which are attributed the emission lines of 0.808 (D1) and 0.875 (D2) meV [20, 30, 31]. A temperature- dependent study of the D1 and D2 emission bands indicated that these bands have an association with Si band edges, showing a Varshni dependence, and thus possess a free-to-bound recombination nature [12]. However, as shown in Fig. 11b, the temperature dependence of the P1 and P2 bands depart signiﬁ- cantly from the Varshni relationship, and a bound-to- bound recombination behavior was demonstrated. Thus, the D1 and D2 dislocation centers are probably not the responsible recombination centers for the P1 and P2 bands, and the identiﬁcation of these defects is not clear. Summary A BLAG method has been applied in the growth of Ge nanoclusters on Si. STM images indicate the absence of a Ge wetting layer. These nanoclusters are orders of magnitude smaller and spatially denser than the Ge nanoclusters that are formed with the normal SK growth mode. The nanoclusters sizes are tunable in a range of 2–8 nm by changing the Ge coverage. And the nanocluster morphology changes signiﬁcantly when varying the Xe buffer thickness. A thicker Xe buffer leads to formation of larger nanoclusters, and at 40 ML of Xe, signiﬁcant aggregation occurs with the forma- tion of nanocluster Ge chains. Samples with different cluster sizes show strong, sharp PL bands in the near infrared spectral region. PL bands do not shift when tuning cluster sizes. The excitation power and temperature dependences of PL spectra suggest a bound-to-bound nature for the PL transition, which is distinguished from the type-II band alignment struc- tures of SK dots. Although the PL is probably associ- ated with defect centers, the Ge nanoclusters play a role in the PL process. High-resolution microscopy indicates that these Ge nanoclusters crystallize and adopt a pseudomorphic structure with the Si substrate under increased electron beam irradiation.