Ge QDs encountering
the Si substrate Dramatic changes in the QD morphology and shape occur when the Ge QD encounters the Si substrate after penetrating through the Si3N4 buffer layer following a longer duration (90 min) oxidation process (Figure 1f). We discovered two new phenomena: first, the Ge QD and the Si substrate are separated by a thin layer of SiO2 that is not only conformal with the QD but also conformal with a cup-shaped depression that appears to be ‘scooped out’ of the Si substrate (Figures 1f and 2a). Further examination of the edges of the cup with CTEM-EDX mapping reveals that it is ‘lined’ with Ge (Figure 2a). Second, with further oxidation, the Ge QD appears to explode into a number of smaller Ge ‘dew NCT-501 nmr drops’ that appear to migrate away from the Si substrate (Figure 2b). The Ge dew drops are about 5 to 7 nm in size, similar in size to the Ge nuclei formed in
the as-oxidized SiGe nanopillars described in the ‘Ge QDs in SiO2 matrix’ section above. Figure 2 STEM and EDX images of 50-nm Ge QDs formed after thermal oxidation of Si 0.85 Ge 0.15 pillars. Si0.85Ge0.15 pillars with a diameter of 100 nm were thermally oxidized at 900°C for (a) 60 and (b) 90 min. Thus, we have shown PD184352 (CI-1040) that the Ge QD exhibits two distinct types of morphological Ferrostatin-1 and migrational behaviors depending on whether it encounters a Si3N4 layer or the Si substrate. As mentioned above, in a previous paper [9], we have provided a detailed explanation
for the behavior of Ge QDs penetrating Si3N4 buffer layers. In this paper, we propose a new explanation for the radically selleck products different behavior of the very same QDs now interacting with the Si substrate. Here, we draw parallels from previous studies on the oxidation rate of silicon showing a marked dependence of the oxidation on the Ge content in Si and the oxygen flux [21–25]. We begin by considering the two steps in which these changes occur in the migration and morphology of the Ge QDs. The two steps are the following: a. SiGe ‘Shell’ formation: Upon ‘contact’ with the Si substrate, i.e., with a thin oxide separating the QD from the substrate, it becomes thermodynamically and kinetically favorable for Ge atoms to migrate from the QD and dissolve within the Si substrate to form a thin, cup-shaped SiGe alloy shell (Figure 2). This is because of the release of the free energy of mixing for the SiGe alloy [26, 27].