Temperature-dependent low electric field charging of Si nanocrystals embedded within oxide-nitride-oxide dielectric stacks.

In this work we examine the current peaks and the negative differential resistance that appear in the low electric field regime of oxide-nitride-oxide structures with a two-dimensional band of silicon nanocrystals embedded in a nitride layer. The silicon nanocrystals were synthesized by low energy ion implantation (1 keV, 1.5 x 10(16) Si(+) cm(-2)) and subsequent thermal annealing (950 degrees C, 30 min). Electrical examination was performed at temperatures from 20 to 100 degrees C using constant voltage ramp-rate current measurements. This approach enables us to determine the origin of the observed current peaks as well as to extract the trapping location of the injected carriers within the dielectric stack. The results confirm that the carriers are trapped within the Si nanocrystal band, verifying that this region corresponds to energy minima of the dielectric stack.

Imaging Si nanoparticles embedded in SiO(2) layers by (S)TEM-EELS.

Fabrication of systems in which Si nanoparticles are embedded in a thin silica layer is today mature for non-volatile memory and opto-electronics applications. The control of the different parameters (position, size and density) of the nanoparticles population is a key point to optimize the properties of such systems. A review of dedicated transmission electron microscopy (TEM) methods, which can be used to measure these parameters, is presented with an emphasis on those relying on electron energy-loss spectroscopy (EELS). Defocused bright-field imaging can be used in order to determine topographic information of a whole assembly of nanoparticles, but it is not efficient for looking at individual nanoparticles. High-resolution electron imaging or dark-field imaging can be of help in the case of crystalline particles but they always provide underestimated values of the nanocrystals population. EELS imaging in the low-energy-loss domain around the Si plasmon peak, which gives rise to strong signals, is the only way to visualize all Si nanoparticles within a silica film and to perform reliable size and density measurements. Two complementary types of experiments are investigated and discussed more extensively: direct imaging with a transmission electron microscope equipped with an imaging filter (EFTEM) and indirect imaging from spectrum-imaging data acquired with a scanning transmission electron microscope equipped with a spectrometer (STEM-PEELS). The direct image (EFTEM) and indirect set of spectra (STEM-PEELS) are processed in order to deliver images where the contribution of the silica matrix is minimized. The contrast of the resulting images can be enhanced with adapted numerical filters for further morphometric analysis. The two methods give equivalent results, with an easier access for EFTEM and the possibility of a more detailed study of the EELS signatures in the case of STEM-PEELS. Irradiation damage in such systems is also discussed.

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.

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.