(a) Schematic of sample structure, (b) cross-sectional bright-field Z-contrast TEM images of 5-nm-thick a-Ge QW sample, and (c) RBS spectra of a-Ge QWs. The filled areas are proportional to the Ge content of each QW (from

1.0×1016 Ge/cm3 to 13.6×1016 Ge/cm3) as reported in the figure. Results and discussion The structural characterization of a-Ge QWs is summarized in Figure 1. If relevant fractures occurred in the Ge film, the quantum confinement would change from one-dimensional (1D) regime to two-dimensional (2D) or three-dimensional (3D) regimes, as the unconfined feature of the electron wave functions in the plane parallel to the surface would be lost. Such circumstances have been denied by extensive TEM and HRTEM investigation performed both in plan and in cross-sectional NU7441 research buy view. As an example, a TEM image is reported in Figure 1b for the 5-nm a-Ge QW sample (grown on Si substrate), showing SiO2 films (brighter layers) embedding the Ge QW (thin darker layer). The measured thickness, d, and roughness of the a-Ge QW are 5.36 and 3.65 nm, respectively. This means that even if some sparse thinning of the Ge QW occurs, the electronic wave functions are still confined only in the growth direction, preserving the 1D confinement regime. Similar considerations can be done for all the a-Ge QW samples. Figure 1c reports the RBS data in the 0.88- to 1.09-MeV energy range

which is relative to He+ backscattered from Ge atoms. The peak area was BAY 57-1293 chemical structure converted into Ge atomic dose contained in each QW, as indicated in the figure. By combining these data with the thickness measured by TEM, we obtain a density of 4.35 × 1022 Ge atoms/cm3, which is in agreement with that of bulk Ge (4.42 × 1022 atoms/cm3) [18]. This last evidence clearly indicates the absence of low-density regions or voids in the as-deposited a-Ge films. To ascertain if quantum confinement affects the energy gap of a-Ge QWs, light absorption spectroscopy was performed in the samples grown on quartz substrates. Accurate

T and R measurements (some of which are reported in the inset of Figure 2a) have been performed at room temperature to extract the absorption coefficient (α) of such thin Ge films, as described in another study [19]. The overall indetermination on α, also including errors on d, Low-density-lipoprotein receptor kinase T, and R, is about 5%, while the dynamic range of the product αd was 1 × 10−3 to 2 × 10−1. Figure 2a shows the α spectra of the a-Ge QWs and of an a-Ge film (125-nm thickness) used as a reference in a bulk, unconfined film. The absorption coefficient of the 30-nm a-Ge QW is similar to that of the 125-nm a-Ge sample, both evidencing an absorption edge at about 0.8 eV, typical of an a-Ge bulk [20]. On the contrary, by decreasing the thickness of the a-Ge QW from 12 to 2 nm, an evident blueshift occurs in the onset of the absorption spectrum. Moreover, in the 12-nm a-Ge QW, the α spectrum is higher than in the 30-nm a-Ge QW sample, despite the similar onset.