V. A. Volodin

Visible and near-infrared luminescence from silicon nanostructures formed by ion implantation and pulse annealing

Abstract Thermally-grown, 500 nm thick SiO2 films were implanted with 1.6 × 1016–1.6 × 1017 cm−2 Si+ ions at 100 and 200 keV. Then the samples were subjected to either rapid thermal annealing at 900–1200°C for 1s or flash-lamp annealing at 1050–1350°C for 20 ms. Photoluminescence (PL) measurements, Raman spectroscopy (RS) and high-resolution transmission electron microscopy (HREM) were employed for sample characterization. Weak room-temperature (RT) visible PL was observed before the transient anneals. RS revealed in these samples a broad band centered at 480 cm−1 indicating the presence of non-crystalline Si inclusions. The initial annealing steps decreased the PL intensity, but after 1200°C, 1 s or 1350°C, 20 ms the PL was found to increase considerably over the red and IR region with a maximum around 830 nm. Simultaneously, the Raman signal at 480 cm−1 diminished and additional scattering near 520 cm−1 arose pointing to the formation of Si nanocrystals. Formation of 2–6 nm Si nanocrystals was directly confirmed by HREM. It is difficult to explain their occurrence by diffusion-limited growth or solid-phase crystallization of α-Si phase inclusions, if any. A model is presented considering the Si nanocrystal formation via segregation of Si atoms from SiOx, rapid percolation-like formation of Si chains or fractals and finally their transformation to Si phase inclusions and nanocrystals. The dramatic increase in PL correlated with the Si nanocrystals formation is considered to support the idea of quantum-confined origin of PL.

Raman study of silicon nanocrystals formed in SiNx films by excimer laser or thermal annealing

Silicon nitride films of different stoichiometric composition were studied using Raman spectroscopy. A Raman signal due to Si–Si, Si–N bond vibrations in silicon nanoclusters was detected in as-deposited films. The appearance of Raman peaks in the range 493–514 cm−1 after thermal and pulse laser treatments was interpreted as formation of silicon nanocrystals with sizes from 1.3 up to 5 nm depending on treatment parameters. Thermal treatment at 1200 °C allowed Si atom diffusion and its gathering in Si nanocrystals, meanwhile 5 ns pulse laser irradiation leads to crystallization of preexisting silicon nanoclusters inside the as-deposited SiNx films.

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.

Improved model of optical phonon confinement in silicon nanocrystals

We develop a model for calculating the Raman scattering spectra from phonons confined in for silicon nanocrystals, which is based on the familiar approach taking into account the uncertainty in the quasi-momentum of phonons localized in the nanocrystals. The model is considerably improved by taking into account dispersion of phonons not only in the magnitude of the quasi-momentum, but also in its direction. A significant refinement of the model is also due to the fact that phonon dispersion is calculated using the widely approved Keating model instead of being approximated by empirical expressions as was done in earlier approaches. The calculations based on this model make it possible to determine the sizes of silicon nanocrystals more precisely from analysis of the experimental Raman spectra.

Ordered arrays of vertically correlated GaAs and AlAs quantum wires grown on a GaAs(311)A surface

We study GaAs–AlAs short-period superlattices (SPSLs) grown on a GaAs(311)A surface using plan-view transmission electron microscopy (TEM). A strong in-plane compositional modulation with a period of 3.2 nm along the [011] direction is revealed by TEM under chemically sensitive imaging conditions and in high-resolution TEM. Our results confirm the formation of highly ordered vertically aligned arrays of GaAs and AlAs quantum wires formed via self-organized growth. Bright photoluminescence (PL) at room temperature in the green and yellow spectral range is observed.

Determination of the composition and stresses in GexSi(1−x) heterostructures from Raman spectroscopy data: Refinement of model parameters

Raman spectroscopy is used to monitor the composition and strains of GexSi1−x alloy films with 0.17 ≤ x ≤ 1.0. The composition and strains in the films were also determined from the X-ray diffraction data. Both the position and intensity of the Raman peaks related to vibrations of the Ge-Ge, Ge-Si, and Si-Si bonds were analyzed. This analysis provided substantial refinement of certain model parameters for calculation of the composition and strains in GexSi1−x/Si(100) heterostructures on the basis of Raman spectroscopy data.

Excimer laser and rapid thermal annealing stimulation of solid-phase nucleation and crystallization in amorphous silicon films on glass substrates

The solid-phase crystallization process in thin amorphous silicon films on glass substrates was studied with application of excimer laser annealing (ELA) and rapid thermal annealing (RTA) for stimulation of nucleation. Use of ELA allowed us to create homogeneous polycrystalline silicon films on glass with grain sizes up to at temperatures below C. Use of RTA reduced the incubation time of nucleation from 100 to 6 h. The textured silicon films on glass with predominant orientation (110) and sizes of textured areas up to were manufactured using excimer laser stimulation of nucleation. The influence of the mechanical stress mechanism on grain orientation was suggested, and it was theoretically shown that internal stresses retard the nucleation process. The addition of deformation to the chemical potential difference was estimated for nucleation in amorphous silicon as 11.4 meV per nucleated atom.

Annealing effects in light-emitting Si nanostructures formed in SiO2 by ion implantation and transient preheating

A dose of 1.6 × 1017 cm−2 Si+ ions was implanted in 500-nm-thick SiO2 layers with subsequent transient annealing at different temperatures. After the highest temperatures light-emitting Si nanoclusters were found that were formed in SiO2. Then all the layers were subjected to isochronal (30 min) furnace anneals and their properties were controlled by room temperature photoluminescence (PL) and Raman spectroscopy. The PL intensity from Si nanocrystal-containing layers progressively decreased with an increase in the anneal temperature (Ta) up to 800–900°C, but rapidly arose again in the Ta range of 1000–1150°C. Raman scattering has shown that Si nanocrystals vanish at Ta ∼ 800°C and that the amorphous silicon signal reappears. When the initial transient annealing failed to form Si nanocrystals, the furnace heat treatment at Ta < 700°C gave rise in PL intensity followed by its drop at Ta ∼ 800–900°C and a strong increase at Ta ∼ 1000–1150°C. The disappearance of Si nanocrystals and PL is considered to result from low stability of the smallest crystallites quenched in SiO2 by transient processing. When Si nanocrystals were not induced by transient preheating, the increase in Ta supposedly led to percolation-like formation of Si inclusions, their transformation to amorphous Si phase nanoprecipitates and, finally, to Si nanocrystals. For all the samples the formation of nanocrystals at Ta = 1000–1150°C was provided by the increase in their stability due to diffusion-limited grain growth. The results obtained are considered to support the idea of quantum-confined origin of PL.

Applying an improved phonon confinement model to the analysis of Raman spectra of germanium nanocrystals

The improved phonon confinement model developed previously [11] is applied for definition of germanium nanocrystal sizes from the analysis of its Raman scattering spectra. The calculations based on the model allow determining the sizes of germanium nanocrystals more precisely from the analysis of their Raman spectra. In some cases, the comparative analysis of Raman data and electron microscopy data is carried out, and good agreement is observed.

Extremely high response of electrostatically exfoliated few layer graphene to ammonia adsorption

Extremely high gas sensing properties of p-type few layer graphene flakes exfoliated from highly oriented pyrolytic graphite have been demonstrated. The current response to ammonia adsorption is strongly dependent on film thickness and is higher than that for graphene by 1-8 orders of magnitude. A maximal response was found for sample thickness ∼ 2 nm. The effect is attributed to the formation of multiple p-n-p junctions at the grain boundaries in the polycrystalline graphene flakes exposed to ammonia-containing ambient.