S. Schamm

Structural characterization of amorphous SiCxNy chemical vapor deposited coatings

Chemical bonding and local order around the different atoms of thick amorphous SiCxNy deposits [0.03⩽x/(x+y)⩽0.67] prepared with chemical vapor deposition at 1000–1200 °C using TMS–NH3–H2 have been investigated using x-ray photoelectron spectroscopy (XPS), Raman spectrometry, Fourier transform infrared spectrometry (FT-IR), electron energy loss spectroscopy (EELS) and 29Si magic-angle spinning nuclear magnetic resonance spectrometry (MAS-NMR). XPS analyses have shown that the main bonds are Si–C, Si–N, and C–C, and have suggested the existence of C–N bonds. According to Raman analyses and complementary FT-IR absorption of thin films, the coatings are nonhydrogenated. Si, C and N atomic chemical environments are more complicated than in a mixture of pure Si3N4–SiC phases. The examination of the Si KL2,3L2,3 line shapes recorded by XPS have allowed one to state the existence of Si(C4−nNn) units. Mixed coordination shells around silicon have been confirmed by EELS analyses. Additionally, FT-IR reflection ana...

Oxidation of Si nanocrystals fabricated by ultralow-energy ion implantation in thin SiO2 layers

The effect of thermal treatments in nitrogen-diluted oxygen on the structural characteristics of two-dimensional arrays of Si nanocrystals (NCs) fabricated by ultralow-energy ion implantation (1 keV) in thin silicon dioxide layers is reported. The NC characteristics (size, density, and coverage) have been measured by spatially resolved electron-energy-loss spectroscopy by using the spectrum-imaging mode of a scanning transmission electron microscope. Their evolution has been studied as a function of thermal treatment duration at a temperature (900 °C) below the SiO2 viscoelastic point. An extended spherical Deal-Grove [J. Appl. Phys. 36, 3770 (1965)] model for self-limiting oxidation of embedded silicon NCs has been carried out. It proposes that the stress effects, due to oxide deformation, slow down the NC oxidation rate and lead to a self-limiting oxide growth. The model predictions show a good agreement with the experimental results. Soft oxidation appears to be a powerful way for manipulating the NC s...

Multi-dot floating-gates for nonvolatile semiconductor memories: Their ion beam synthesis and morphology

Scalability and performance of current flash memories can be improved substantially by replacing the floating polycrystalline-silicon gate by a layer of Si dots. Here, we present both experimental and theoretical studies on ion beam synthesis of multi-dot layers consisting of Si nanocrystals (NCs) embedded in the gate oxide. Former studies have suffered from the weak Z contrast between Si and SiO2 in transmission electron microscopy (TEM). This letter maps Si plasmon losses with a scanning TEM equipped with a parallel electron energy loss spectroscopy system. Kinetic Monte Carlo simulations of Si phase separation have been performed and compared with Si plasmon maps. Predicted and measured Si morphologies agree remarkably well, both change with increasing ion fluence from isolated NCs to spinodal pattern. However, the predicted fluences are lower than the experimental ones. We identify as the main reason of this discrepancy the partial oxidation of implanted Si by atmospheric humidity, which penetrates in...

Study of the dielectric properties near the band gap by VEELS gap measurement in bulk materials

Measuring the band gap of bulk materials by valence electron energy loss spectroscopy (VEELS) is not straightforward. Mathematical procedures used to recover the single scattering distribution from raw data introduce artefacts in the signal, which complicate the gap measurement. In this work, we propose a method to overcome this and measure the direct band gap energy with an accuracy of +/-0.1eV. The method is tested on six crystalline wide-band gap materials: MgO, Ga(2)O(3), SrTiO(3), ZnO, BN and GaN.

White electroluminescence from C- and Si-rich thin silicon oxides

White electroluminescence from carbon- and silicon-rich silicon oxide layers is reported. The films were fabricated by Si and C ion implantation at low energy in 40nm thick SiO2, followed by annealing at 1100°C. Structural and optical studies allow assigning the electroluminescence to Si nanocrystals for the red part of the spectrum, and to C-related centers for the blue and green components. The external efficiency has been estimated to 10−4%. Electrical characteristics show a Fowler-Nordheim behavior for voltages above 25V, corresponding to the onset of electroluminescence. This suggests that light emission is related to the impact ionization of radiative centers.

Elaboration by spray pyrolysis and characterization in the VUV range of phosphor particles with spherical shape and micronic size

Y2O3 : Eu and Zn2SiO4 : Mn are the red and green phosphors actually used in plasma display panels. These phosphors have been prepared by spray pyrolysis synthesis and thermal post-treatments. Their optical properties in the vacuum ultraviolet (VUV) energy range have been investigated. The absorption coefficients have been extracted from electron energy loss spectroscopy. The luminescence properties of phosphors from spray pyrolysis have been characterized under UV or VUV excitations with different dedicated experimental set-ups: excitation spectroscopy, measurements of luminescence efficiencies under Hg lamp, pure Xe and Ne–Xe discharges, determinations of luminescence decay times under plasma excitation. The characteristics have been compared with those of corresponding commercial phosphors.

Contamination and the quantitative exploitation of EELS low-loss experiments

Quantitative exploitation of the low-loss domain of electron energy loss spectra is based on an accurate determination of the corresponding signal intensity profile. This signal can be erroneous and contains artefacts as a result of sample contamination in the microscope, for example. The consequences of contamination on the signal intensity of the low-loss spectra are discussed. In the case of a carbonaceous contamination, a simple additional spurious signal can be considered, as has been demonstrated in the case of a Si single crystal, a highly oriented pyrolytic graphite (HOPG) and a strontium titanate single crystal (SrTiO3). The linear variation of the rate of contamination with time allows the implementation of a simple method based on the subtraction of the spurious signal in order to correct for the contamination effect. The relative errors induced by the carbonaceous contamination on the determination of the optical properties of SrTiO3 are estimated.

Element and phase identification via fine structure analysis in EELS : application to MOCVD-Y1Ba2Cu3O7-δ thin films

Abstract Electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM) is a very useful tool for identifying small secondary phases, for example, in YBCO off-stoichiometric thin films. Among these phases are the CuO and Y 2 BaCuO 5 submicrometric precipitates and the Y 2 O 3 nanometric inclusions. Beyond the classical elementary analysis based on the quantitative exploitation of core-level signals, we propose a new method of identification based on the interpretation of the fine structures in the EELS spectrum such as the near-edge structures (ELNES) at the O–K edge and the low-loss signatures. We also propose a method for determining the parameter δ in Y 1 Ba 2 Cu 3 O 7− δ from the volume plasmon energy, which is more direct and has the same accuracy as the method proposed by Browning, based on modelling of the O–K edge.

Nanocrystals in High-k Dielectric Stacks for Non-Volatile Memory Applications

The possibility to use semiconducting or metallic nanocrystals (ncs) embedded in a SiO2 matrix as charge storage elements in novel non volatile memory devices has been widely explored in the last ten years. The replacement of the continuous polysilicon layer of a conventional flash memory device by a 2-dimensional nanoparticle array presents several advantages but the fundamental trade-off between programming and data retention characteristics has not been overcome yet. The main problem is the limited retention time basically due to charge loss by leakage current through the ultra-thin SiO2 tunnelling dielectric. A longer retention time can be achieved by increasing the tunnel oxide thickness. This however implies higher operating voltages and consequently a reduced write/erase speed. Using high-k materials for tunnel and/or gate oxide it is in principle possible to achieve the goal of a low voltage non volatile memory device. The high dielectric constant of these materials allows using thicker tunnel oxide reducing leakage current. Several approaches have been explored to synthesise ordered arrays of ncs in SiO2 but the transfer of these methodologies to the synthesis of 2-d array of ncs in high-k materials is not trivial. In this work we address the material science issues related to the synthesis of metallic and semiconducting ncs in high-k materials using different techniques. A detailed review of the state of the art in the field is presented and further research strategies are suggested.

Preparation and microstructures of BaTi1?xZrxO3 hetero-epitaxial thin films on SrTiO3 substrates

Zr-substituted BaTiO3 thin films (BaTi1−xZrxO3: BTZ) were prepared by rf-sputtering on SrTiO3 (STO) substrates or STO substrates buffered by pulsed laser deposited SrRuO3 (SRO) electrodes. The epitaxial growth of BTZ was monitored in situ by reflective high energy electron diffraction. X-ray reflectivity measurement demonstrated that a smooth surface with a roughness of 0.5 nm could be achieved under optimized deposition parameters. Film composition and the change in chemical environment due to Zr substitution are discussed based on electron energy-loss spectroscopy. The high quality of the interfaces was confirmed by transmission electron microscopy. The epitaxial relation between BTZ/SRO and SRO/STO was studied by selected area diffraction and high resolution transmission electron microscopy. All the results show that high quality epitaxial BTZ thin films were successfully deposited and ready for further electrical studies.