<title>Ge nanoclusters in GeO<formula><inf><roman>2</roman></inf></formula>: formation and optical properties</title>

Abstract

Germanium nanocrystals in Ge02 films have been obtained with the use of two methods and have been studied. The first method of Ge nanocrystal formation is a film deposition from supersaturated GeO vapor with subsequent dissociation of metastable GeO on heterophase system Ge:Ge02. The second method is growth of anomalous thick native germanium oxide layers with chemical composition GeO(H2O) during catalytically enhanced Ge oxidation, x is close to 1 . The obtained films were studied with the use of photoluminescence, Raman scattering spectroscopy, highresolution electron microscopy. Strong photoluminescence signals were detected in Ge02 films with Ge nanocrystals at room temperature. "Blue-shift" of the photoluminescence maximum was observed with reducing of Ge nanocrystal size in anomalous thick native germanium oxide films. So, the correlation between reducing of the Ge nanoerystal sizes (estimated from position of Raman peaks) and photoluminescence "blue-shift" was observed. The Ge nanocrystals presence was confirmed by high-resolution electron microscopy data. The optical gap in Ge nanocrystals was calculated with taking into account quantum size effects and compared with the position of the experimental photoluminescence peaks. It can be concluded that a Ge nanocrystal in Ge02 matrix is a quantum dot of type I. It was shown, that "band gap engineering" approaches can lead to creation of Ge:Ge02 heterostructures with required properties. This heterostructures can be perspective for using in opto-electronics, for creation of elements of quasi-nonvolatile MOS memory using Ge nanocrystals as traps for electrons or holes, e.t.c.

Nanostructured multilayer TiO2–Ge films with quantum confinement effects for photovoltaic applications

“…A series of peaks are visible in the spectrum of the as-deposited film at about 306, 414, and 612 cm À1 . The peak at 306 cm À1 can be assigned to Ge [15], while the other two peaks belong to TiO 2 [16]. The TiO 2 peak at $414 cm À1 shifts toward smaller wavenumbers of about 408, 400, 397, and 394 cm À1 and the peak at $612 cm À1 shifts toward higher wavenumbers of about 615, 625, 629, and 630 cm À1 for the films annealed at 200, 400, 450, and 500°C, respectively.…”

“…2a. It appears that large interface (surface) areas in the multilayer films have resulted in strained (and dangling) bonds such that Raman peak positions appear at higher wavenumbers in comparison with the Ge film [15].…”

“…It is well-known that band gap energy of thin films changes with film thickness [14], deposition techniques [18], and annealing [19], which is because of the formation of nanoparticles [14], nanoporosity [20], and an oxide network [15,19] causing quantum confinement effects associated with nanostructures [14,15,19]. It has been shown earlier that nanostructured Ge films exhibit quan- tum confinement effects [14,15,19], and annealing of Ge films causes blue shift in photo-absorption (from 1.1 eV to about 1.35 eV) due to enhanced quantum confinement effects.…”

Synthesis of Nanoamorphous Germanium and Its Transformation to Nanocrystalline Germanium

“…The plots in Figure 5b and c were obtained using an approximation for the nanoparticle size that was calculated using the onset energies at the plateau, where the shift on the absorption edge almost stops, and the HOMO-LUMO energy and the nanoparticle size relation. [32] The effective mass model (EMM) predicts that the bandgap of semiconductors scales with the particle diameter according to the following equation:…”

“…The Raman spectrum of the nc-Ge/SiO 2 displays a relatively sharp peak at 284 cm −1 , characteristic of the transverse optical phonon in crystalline Ge, which is quite red-shifted from the bulk value at around 300 cm −1 due to the phonon confinement effect [35] (Figure 6b). Using a phonon confinement model [32] and the Raman transverse optical (TO) mode, the size of the nc-Ge was calculated to be less than 3.0 nm. However, the PXRD, UV-vis-NIR absorption spectroscopy, and TEM measurements show that the average particle size is around 4.0 nm.…”