| Semiconductor nano-islands (i.e., Ge and InAs nano-islands) play an important role in the field of high-efficiency photoelectronic and microelectronic devices due to its special properties of Quantum confinement effect, Coulomb blockade effect, Phonon bottleneck effect, etc. For IV-group semiconductor materials, the investigation of SiGe nano-islands fabricated by Stranski-Krastanov (S-K) growth mode on Si substrate has become one of the research hotspots in the past decade due to its advantage that it can integrate with Si based CMOS readout circuit directly.The study of the growth of the co-sputtering SiGe islands on microcrystalline Si (μc-Si) can contribute to understand the evolution mechanism of SiGe nano-islands fabricated on Si buffer layer with different crystallinity. On the other hand, the combination of the μc-Si material with the infrared Ge material can provide some methods for the development of μc-Si based devices. The examination results of atomic force microscope (AFM) indicate that the co-sputtering SiGe nano-islands grown on μc-Si buffer layer (700℃) show a bi-model distribution.～75%of the islands are short islands (h<3nm) and～25%of the islands are high islands (3<h<6nm). For the sample grown at750℃, the co-sputtering SiGe nano-islands grown on crystalline Si (c-Si) buffer layer show a multi-model distribution. The island size becomes larger and the uniformity of the islands is improved. The related Raman spectrum indicates that the Si buffer layer grown at700℃exhibits a microcrystalline structure. We propose that the vibration mode with its center at381cm-1is derived from the Si-Ge vibration mode of amorphous SiGe (a-SiGe) which is formed on the a-Si region and the one located at395cm-1stems from the Si-Ge mode of SiGe nano-islands. However, the Si buffer layer grown at750℃exhibits crystalline state, the Si-Ge vibration mode of a-SiGe is not observed. Thus, it suggests that the co-sputtering SiGe layer grown on μc-Si exhibits a mixed-phase structure, which contains a-SiGe thin film and SiGe islands.Based on the surface structure of the SiGe co-sputtering layer, we further deposit a thin Ge layer with different thickness on this SiGe layer in order to investigate the secondary-grown behavior and the surface atomic migration of SiGe nano-islands on μc-Si buffer layer. This may provide some guidance for the fabrication of some μc-Si based devices. The experiment results indicate that the secondary-grown SiGe islands fabricated at700℃exhibit a preferential growth model, that is, the deposited Ge atoms preferentially aggregate at the original high islands to form the super islands. On the other hand, a class of islands with lower diameter and higher height emerges on the surface during the secondary growth. It suggests that these islands are the new formed islands based on the4.2%lattice mismatch between the deposited Ge layer and Si buffer layer. For the sample grown at730℃, the secondary-grown SiGe islands exhibit an Ostwald ripening model. Almost all of the islands grow up together when Ge layer is deposited on the SiGe layer. With the increase of the Ge coverage, larger islands grow up at the expense of the small islands. The volume and aspect ratio of a part of the islands decrease. One interesting feature is that with the increase of the Ge coverage, the a-SiGe thin film disappears gradually. This is ascribed to the fact that the atoms in the a-SiGe thin film laterally migrate into the SiGe islands due to the chemical potential difference and the thermal diffusion. This lateral atomic migration also leads to the stronger Si-Ge intermixing effect in SiGe nano-islands. On the other hand, for the sample grown at730℃, the a-SiGe thin film is absent during the secondary growth process. Thus, the intermixing effect at730℃is attributed to the traditional uphill diffusion of Si atoms into SiGe islands.After the secondary growth, the aspect ratios of the preferential-grown super islands are larger than that of the Ostwald-ripening ones. One reason is that the Si-Ge intermixing effect in Ostwald-ripening islands is stronger. This leads to the decrease of the aspect ratios of islands. The other reason is that the Si buffer layer grown at700℃exhibits a mixed-phase structure, the surface energy of a-Si (1.05±0.14N/m) is larger than that of Ge (～0.75N/m), but smaller than that of c-Si (～1.4N/m). The Ge atoms firstly nucleate on the c-Si at the initial Ge deposition process. With the increase of the Ge coverage, when the base edges of the islands extend to interface of the c-Si and a-Si, the sizes of the islands seem to be more difficult to increase in lateral direction due to the difficult wetting behavior of Ge on a-Si. This results in the higher aspect ratio of the islands grown at700℃. |
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