D. V. Marin

Effect of quantum confinement on optical properties of Ge nanocrystals in GeO2 films

Germanium dioxide films containing Ge nanocrystals are studied. The films have been prepared by two methods: (i) deposition from supersaturated GeO vapors with subsequent decomposition of metastable germanium monoxide into a heterophase Ge:GeO2 system, and (ii) formation of anomalously thick native germanium oxides with a GeO2(H2O) chemical composition by a catalyst-accelerated oxidation of germanium. The films, which have been prepared on various substrates, are studied using the photoluminescence technique, Raman spectroscopy, spectral ellipsometry, and high-resolution electron microscopy. In the GeO2 films with built-in Ge nanoclusters, intense photoluminescence is detected at room temperature. The nanocluster sizes are estimated from the position of the Raman peak related to localized optical phonons. The correlation between a decrease in the nanocluster size and the shift of the photoluminescence peaks to the blue spectral region as the relative Ge content decreases is revealed. The presence of nanoclusters is confirmed by the data obtained from high-resolution electron microscopy. The correlation of the optical gap calculated taking into account the quantum confinement of electrons and holes in the nanoclusters with the experimentally observed luminescence peak is established. It can be concluded from the data obtained that the Ge nanoclusters constructed in the GeO2 matrix represent type I quantum dots.

Formation of Ge and GeSi nanocrystals in GeOx/SiO2 multilayers

Amorphous GeOx films and GeOx/SiO2 multilayers were deposited by evaporations of GeO2 and SiO2 powders onto Si substrates maintained at 100??C. The GeOx layer thickness in multilayer structures was varied from 4 to 1?nm, the thickness of SiO2 layers was 4?nm, all samples contain 10 periods. Evolution of structure and optical properties of the samples was investigated by Raman spectroscopy, infrared (IR)-absorption spectroscopy and ellipsometry for annealing temperatures up to 800??C. Decomposition of the GeOx and appearance of amorphous Ge nanoclusters, Ge and GeSi nanocrystals (NCs) were observed. Correlations between the GeOx layer thicknesses and the structure of Ge and GeSi NCs were studied. The comparative analysis of evolution of the photoluminescence with the structure of the nanostructured GeOx layers and GeOx/SiO2 multilayers was carried out.

Electrical properties and photoluminescence of SiOx layers with Si nanocrystals in relation to the SiOx composition

The photoluminescence and electrical properties are compared for silicon-oxide layers containing Si nanocrystals and having different Si content. The oxide was deposited by co-sputtering of silicon dioxide and silicon with the subsequent annealing for the formation of nanocrystals. Excess Si content in the layer varies along the sample from 6 to 74 vol %. It is found that a charge magnitude determined from the flat-band voltage has a pronounced peak for the excess Si content of about 26%, the largest charge correlating with the highest photoluminescence intensity. The further increase in the excess Si content in oxide leads to a decrease in both the oxide charge and the photoluminescence intensity and to the appearance of percolation conductivity.

Photoluminescence of GeO2 films containing germanium nanocrystals

Germanium nanocrystals were formed in a GeO2 film during the process of germanium monoxide gas-phase deposition onto a sapphire substrate and studied by photoluminescence (PL) and Raman scattering spectroscopy. A PL peak in this heterosystem was observed in the visible region at room temperature. The sizes of Ge nanocrystals were estimated from the position of a Raman peak corresponding to scattering by localized optical phonons in germanium. The PL peak position calculated with allowance for the electron and hole size quantization in Ge nanocrystals coincides well with the experimentally observed position of this peak.

The formation of silicon nanocrystals in SiO2 layers by the implantation of Si ions with intermediate heat treatments

The effect of heat treatments at 1100°C on an ion-beam synthesis of Si nanocrystals in SiO2 layers is studied. The ion-implanted samples are subjected either to a single heat treatment after the total ion dose (1017 cm−2 has been implanted, two heat treatments (a heat treatment after the ion implantation of each half of the total dose), or three heat treatments (a heat treatment after each third of the dose). The total duration of the heat treatments is maintained at 2 h. It is found that the intermediate heat treatments lead to a shift of the Raman spectrum of the nanocrystals to longer wavelengths and to a shift of the photoluminescence spectrum to shorter wavelengths. Study using electron microscopy shows that the size of the nanoprecipitates decreases, which is accompanied by the disappearance of the characteristic features of crystallinity; however, the features of photoluminescence remain characteristic of the nanocrystals. The experimental data obtained are accounted for by a preferential drain of Si atoms to newly formed clusters, which is consistent with the results of a corresponding numerical simulation. It is believed that small nanocrystals make the main contribution to photoluminescence, whereas the Raman scattering and electron microscopy are more sensitive to larger nanocrystals.

Structure and optical properties of SiNx: H films with Si nanoclusters produced by low-frequency plasma-enhanced chemical vapor deposition

The SiNx: H films with compositions differently deviating from the stoichiometric proportion are produced by low-frequency plasma-enhanced chemical vapor deposition at the temperatures 100 and 380°C. Deviations from the stoichiometric composition are varied by varying the ratio between the ammonia and monosilane fluxes from 0.5 to 5. The films are studied by ellipsometry, Raman spectroscopy, infrared absorption spectroscopy, and luminescence measurements. In the SiNx: H films (x < 4/3), amorphous silicon clusters were found. According to estimations, only a small fraction of excess silicon coalesces into clusters, and an increase in the substrate temperature stimulates clustering. It is found that, with increasing the content of excess silicon in the films, the photoluminescence peak shifts to longer wavelengths.

The modification of Si nanocrystallites embedded in a dielectric matrix by high energy ion irradiation

We have followed the effects of heavy ion irradiation on the structural, electrical, and photoluminescence properties of ensembles of silicon nanocrystallites embedded in a dielectric (SiO(2)) matrix. This was done as a function of the irradiation dose and the content of the Si phase. The results obtained can be accounted for self-consistently assuming that a relatively small dose of the irradiation enhances the crystallization while for higher doses the irradiation enhances the amorphization. The corresponding processes suggest that tuning of the above properties can be achieved by swift heavy ion irradiation.

Visible photoluminescence from silicon nanopowders produced by silicon evaporation in a high-power electron beam

Silicon nanopowders produced by electron-beam-induced evaporation of a bulk silicon sample in an argon atmosphere are studied by the photoluminescence technique and Raman scattering spectroscopy. A photoluminescence peak in the visible region of the spectrum has been detected at room temperature in powders consisting of silicon nanocrystals. The strong short-wavelength shift of the photoluminescence peak can be attributed to the quantum size effect of electrons and holes in small silicon nanocrystals (about 2 nm). The size of silicon nanocrystals is determined by analyzing Raman spectra, and it is consistent with estimates obtained from photoluminescence data.

High Volume Synthesis of Silicon Nanopowder by Electron Beam Ablation of Silicon Ingot at Atmospheric Pressure

The evaporation of high purity silicon ingot was performed in Ar, N2, and air atmospheres using a power electron accelerator. The obtained powders with primary particle sizes of 10–500 nm were investigated using Brunauer–Emmett–Teller analysis (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence measurement, and Raman spectroscopy. The structure and photoluminescence properties of Si(Ar) nanopowder obtained at a large quenching rate differ substantially from those of Si(Ar) and Si(N2) obtained at a smaller quenching rate. Photoluminescence peaks in the visible region of the spectrum are detected at room temperature for the Si(Ar) nanopowders.

Anomalous temperature dependence of photoluminescence in GeOx films and GeOx/SiO2 nano-heterostructures

The optical properties of GeOx film and GeOx/SiO2 multilayer heterostructures (with thickness of GeOx layers down to 1 nm) were studied with the use of Raman scattering and infrared spectroscopy, ellipsometry and photoluminescence spectroscopy including temperature dependence of photoluminescence. The observed photoluminescence is related to defect (dangling bonds) in GeOx and interface defects for the case of GeOx/SiO2 multilayer heterostructures. From analysis of temperature dependence of photoluminescence intensity, it was found that rate of nonradiative transitions in GeOx film has Berthelot type, but anomalous deviations from Berthelot type temperature dependence were observed in temperature dependences of photoluminescence intensities for GeOx/SiO2 multilayer heterostructures.