V. Paillard

Improved one-phonon confinement model for an accurate size determination of silicon nanocrystals

In this article, we show how the well-known one-phonon confinement model can be improved to determine the diameter of silicon nanocrystalline spheres from the optical phonon wave-number shift, even using a physical-meaning weighting function. We show that the fundamental parameter is the knowledge of the phonon dispersion. The accuracy of our approach is supported by experimental data obtained by selective UV Raman scattering on nanocrystalline silicon thin films produced by size-selected silicon cluster beam deposition.

Stress measurements of germanium nanocrystals embedded in silicon oxide

Ge nanocrystals embedded in thermal SiO2 on top of a Si substrate are investigated using Raman spectroscopy and transmission electron microscopy. We observe that the Raman peak frequency of the Ge nanocrystals is strongly affected by compressive stress. In the case of large particles for which the phonon confinement-induced Raman shift can be neglected, the stress is measured taking into account isotopic composition effects induced by the ion implantation process used to produce the nanocrystals. The stress is proposed to originate from a liquid–solid phase transition in Ge.

Resonant Raman scattering in polycrystalline silicon thin films

In this letter, we report the results obtained on polycrystalline silicon thin films using Raman spectrometry in resonance with the silicon direct band gap. First, we show that accurate information about crystallites can be obtained in these experimental conditions, without any deconvolution of Raman spectra. Second, we apply the technique to estimate the mechanical stress of polycrystalline silicon thin films.

Experimental study of LO phonons and excitons in ZnO nanoparticles produced by room-temperature organometallic synthesis

We report here the study by optical microspectroscopy of crystalline ZnO nanoparticles produced by room-temperature organometallic synthesis. We present resonant Raman scattering spectrum obtained from different sized nanostructures from which the longitudinal-optical (LO) phonon frequency has been found to be very weakly dependent on their size. Low-temperature photoluminescence measurements reveal that (i) the band-edge PL of ZnO nanoparticles is dominated by weakly bound localized exciton, and that (ii) its energy does not exhibit any spatial confinement effect. These results enlighten the need for surface passivation of the nanoparticles to improve their UV emission potential.

Measurement of the in-depth stress profile in hydrogenated microcrystalline silicon thin films using Raman spectrometry

Raman spectrometry is used to measure stress in hydrogenated microcrystalline silicon thin films. Moreover, by the use of different excitation wavelengths, from red to near ultraviolet, we can probe different film depths and get information on the stress distribution along the growth direction. For films deposited by standard rf glow discharge at different substrate temperatures, on glass substrates, we found large stress gradients. Indeed, the high compressive stress (up to 1 GPa) in the bulk of the film, close to the glass substrate, reduces and becomes tensile as the film free surface is approached. Moreover, the higher the substrate temperature, the higher the stress gradient.

Structured films of light-emitting silicon nanoparticles produced by cluster beam deposition

Crystalline silicon nanoparticles (quantum dots) with diameters around 4 nm are produced via CO2-laser-induced decomposition of SiH4 in a flow reactor and subsequently transferred into a freely propagating cluster beam. Thin structured films are then obtained by shaping the beam with a mask and depositing the nanoclusters at low energy on a sapphire substrate. Upon illumination with ultraviolet radiation, the nanoparticles exhibit strong photoluminescence in the red.

Raman study of E2 and surface phonon in zinc oxide nanoparticles surrounded by organic molecules

Using Raman spectrometry, we obtained results showing the influence of organic ligands on the vibrational properties of small zinc oxide nanocrystals (2.1–6.8nm). It is shown that it is possible to distinguish both mechanical and dielectric effects from the E2 nonpolar phonon mode and from a surface mode, theoretically predicted but rarely observed. It has been found that E2 phonon is not dependent on the nanocrystal size, but its frequency decreases with increasing ligand length, characteristic of a tensile stress on the nanocrystal. We report also the observation of a surface optical mode, the experimental frequency of which is in reasonable agreement with available calculations.

Laser production and deposition of light-emitting silicon nanoparticles

Abstract Silicon clusters and nanoparticles are produced by CO2-laser-induced decomposition of silane in a flow reactor. In contrast to conventional techniques, the particles are expanded, directly after production, through a conical nozzle into a high vacuum chamber and then transferred into a molecular beam apparatus where they are analyzed in situ by time-of-flight mass spectrometry (TOF-MS). The analysis reveals that the flow reactor emits, besides small clusters, also high-purity silicon crystallites with diameters between 2 and 20 nm. It is found that the particles velocity strongly correlates with their mass. This feature and the fact that the particles are produced in the pulsed mode enable us, by introducing a chopper into the cluster beam, to considerably reduce the dispersion of their size distribution and to perform size-selected low-energy cluster deposition on various substrates. High-resolution electron transmission micrographs demonstrate the capabilities of the new apparatus and reveal interesting details of the crystalline structure of silicon nanoparticles as a function of their size. The monodispersed silicon films have been further characterized by studying their luminescence and Raman scattering behavior. As predicted by theoretical models, the peak of the luminescence curve shifts with decreasing particle size to smaller wavelengths (higher energies). Structured thin films are obtained by shaping the cluster beam with a mask and depositing the nanocrystals at low energy on a sapphire substrate. Upon illumination with ultraviolet radiation, the structured film exhibits strong photoluminescence (PL) in the red.

Photoluminescence of Si nanocrystal memory devices obtained by ion beam synthesis

In this letter, we propose an original method to investigate Si nanocrystal-based nonvolatile memory devices, taking benefit of the photoluminescence (PL) spectroscopy and the specific optoelectronic properties of Si nanocrystals (Si-NCs). Ordered two-dimensional-arrays of Si-NCs were synthesized by ultralow-energy ion implantation in 7-nm-thick SiO2 and subsequent annealing. The Si-NCs population characteristics (size and density) were adjusted by different oxidizing annealing. This allowed, at the same time, the progressive healing of the oxide matrix. The analysis of the spectra revealed the presence of two PL bands, one due to quantum confinement effects in Si-NCs, and the other one attributed to silicon-rich oxide. Therefore, the evolution in energy and intensity of the PL bands was correlated to the oxidizing conditions, thus to the change of the Si-NCs size and density, and to the formation of stoichiometric SiO2. These results are of great interest as being the first step in using PL spectroscopy ...

Resonant Raman scattering of a single layer of Si nanocrystals on a silicon substrate

We report Raman spectra of a single layer of silicon nanoparticles, spatially ordered in SiO2 at a tunneling distance from a silicon substrate. This is achieved by exploiting effects which enhance the nanocrystal signal, while suppressing the substrate one. The method is applied to investigate the structure of ion-implantation-produced Si nanoparticles annealed under different conditions. The results, which are in good agreement with transmission electron microscopy data, are used to explain photoluminescence measurements.