Amal Chabli

Catalyst preparation for CMOS-compatible silicon nanowire synthesis

Metallic contamination was key to the discovery of semiconductor nanowires, but today it stands in the way of their adoption by the semiconductor industry. This is because many of the metallic catalysts required for nanowire growth are not compatible with standard CMOS (complementary metal oxide semiconductor) fabrication processes. Nanowire synthesis with those metals that are CMOS compatible, such as aluminium and copper, necessitate temperatures higher than 450 degrees C, which is the maximum temperature allowed in CMOS processing. Here, we demonstrate that the synthesis temperature of silicon nanowires using copper-based catalysts is limited by catalyst preparation. We show that the appropriate catalyst can be produced by chemical means at temperatures as low as 400 degrees C. This is achieved by oxidizing the catalyst precursor, contradicting the accepted wisdom that oxygen prevents metal-catalysed nanowire growth. By simultaneously solving material compatibility and temperature issues, this catalyst synthesis could represent an important step towards real-world applications of semiconductor nanowires.

A hard x-ray nanoprobe for scanning and projection nanotomography

To fabricate and qualify nanodevices, characterization tools must be developed to provide a large panel of information over spatial scales spanning from the millimeter down to the nanometer. Synchrotron x-ray-based tomography techniques are getting increasing interest since they can provide fully three-dimensional (3D) images of morphology, elemental distribution, and crystallinity of a sample. Here we show that by combining suitable scanning schemes together with high brilliance x-ray nanobeams, such multispectral 3D volumes can be obtained during a single analysis in a very efficient and nondestructive way. We also show that, unlike other techniques, hard x-ray nanotomography allows reconstructing the elemental distribution over a wide range of atomic number and offers truly depth resolution capabilities. The sensitivity, 3D resolution, and complementarity of our approach make hard x-ray nanotomography an essential characterization tool for a large panel of scientific domains.

Medium resolution off-axis electron holography with millivolt sensitivity

Focused ion beam prepared silicon calibration specimens with boron doped layers have been examined using off-axis electron holography. By using a state-of-the-art FEI Titan microscope with unprecedented stability, we have been able to record holograms for time periods of 128s with contrast levels of almost 40% and an average signal on the charge coupled device camera of 30 000 counts. A consequence of this is a significant improvement of the signal-to-noise ratio in the phase images allowing steps in potential of less than 0.030±0.003V to be measured if sufficient care is taken during specimen preparation.

Direct quantification of gold along a single Si nanowire.

The presence of gold on the sidewall of a tapered, single silicon nanowire is directly quantified from core-level nanospectra using energy-filtered photoelectron emission microscopy. The uniform island-type partial coverage of gold determined as 0.42+/-0.06 (approximately 1.8 ML) is in quantitative agreement with the diameter reduction of the gold catalyst observed by scanning electron microscopy and is confirmed by a splitting of the photothresholds collected from the sidewall, from which characteristic local work functions are extracted using a model of the full secondary electron distributions.

Optical properties of Cd1−xMgxTe epitaxial layers: A variable‐angle spectroscopic ellipsometry study

The index of Cd1−xMgxTe ternary alloys was measured for the first time by variable‐angle spectroscopic ellipsometry on layers of different concentrations. The ellipsometer’s wide spectral range (0.7–5.6 eV) clearly reveals critical points beyond the gap. Self‐consistency of the permittivity measurements is investigated by Kramers–Kronig analysis. A transition layer is revealed at the top surface of the samples and is taken into account as a rough layer. In the transparent region Sellmeier’s law is applied to describe the index behavior as a function of the wavelength and the magnesium content.

Extending the detection limit of dopants for focused ion beam prepared semiconductor specimens examined by off-axis electron holography

Silicon specimens containing p-n junctions have been prepared for examination by off-axis electron holography using focused ion beam (FIB) milling. FIB milling modifies the surfaces of the specimens due to gallium implantation and the creation of defects which has the effect of reducing the active dopant concentration measured during electrical characterization. Here we show that although this damage can be removed by using low temperature annealing, the presence of surface charge will modify the electrical potentials in the specimens and limit the dopant concentration that can be measured.

Electronic and chemical properties of the TaN/a-SiOC:H stack studied by photoelectron spectroscopy for advanced interconnects

Thin TaN metallic barriers are used to prevent copper diffusion into porous low-k dielectrics such as a-SiOC:H for advanced interconnects. We investigate the detailed electronic properties of the TaN/a-SiOC:H stack. Here we combine ultraviolet and x-ray photoelectron spectroscopy to measure the chemical composition and the whole band diagram of the TaN/a-SiOC:H stack. An original interpretation based on the image-force model used for internal photoemission is suggested to explain the electric field effect induced by negative bias of a-SiOC:H. This model is used to extrapolate the unbiased electron affinity of the dielectric. TaN work function, a-SiOCH band gap, valence band maximum and electron affinity of 4.6, 7.7, 4.0, and 3.8 eV are respectively obtained. Kelvin force microscopy and spectroscopic ellipsometry confirm TaN work function and a-SiOC:H band gap measurements, respectively. From the full band diagram of the TaN/a-SiOC:H stack, an interfacial barrier height of 0.8 eV is deduced.

Drying nano particles solution on an oscillating tip at an air liquid interface: what we can learn, what we can do

Evaporation of fluid at micro and nanometer scale may be used to self-assemble nanometre-sized particles in suspension. Evaporating process can be used to gently control flow in micro and nanofluidics, thus providing a potential mean to design a fine pattern onto a surface or to functionalize a nanoprobe tip. In this paper, we present an original experimental approach to explore this open and rather virgin domain. We use an oscillating tip at an air liquid interface with a controlled dipping depth of the tip within the range of the micrometer. Also, very small dipping depths of a few ten nanometers were achieved with multi walls carbon nanotubes glued at the tip apex. The liquid is an aqueous solution of functionalized nanoparticles diluted in water. Evaporation of water is the driving force determining the arrangement of nanoparticles on the tip. The results show various nanoparticles deposition patterns, from which the deposits can be classified in two categories. The type of deposit is shown to be strongly dependent on whether or not the triple line is pinned and of the peptide coating of the gold nanoparticle. In order to assess the classification, companion dynamical studies of nanomeniscus and related dissipation processes involved with thinning effects are presented.

The influence of strain on dewetting of silicon films

The influence of strain on the thermally induced dewetting mechanism of silicon films is reported. This study shows that the initial strain level in the silicon film significantly affects the final size and shape of the silicon agglomerates resulting from the film dewetting. With the increase of the biaxial strain up to 1.6%, the size of the silicon agglomerates is significantly reduced while their density increases. Moreover, the shape of the agglomerates becomes elongated when the strain favors one of the in-plane crystallographic directions to minimize the total energy of the system. A quantitative analysis of the dewetting mechanism is presented in terms of the agglomerates size and density versus the strain level. Finally, phenomenological laws are extracted, which predict the size and shape of the agglomerates.

Characterization of geometrical factors for quantitative angle-resolved photoelectron spectroscopy

For conventional angle-resolved x-ray photoelectron spectroscopy (ARXPS), the area under the core-level peaks depends mainly on the in-depth distribution of chemical species at the top surface of a specimen. But the x-ray photoelectron spectroscopy (XPS) intensity is also affected by tool-related geometrical factors such as the shape of the x-ray beam, the spectrometer analysis volume, and the manipulator rotation axis. Data analysis is therefore typically based on normalization with respect to the signal from the substrate. Here, we present an original method to perform quantitative ARXPS without normalization, involving evaluation of these geometrical factors. The method is illustrated for a multiprobe XPS system using a methodology based on a specific software (XPSGeometry®), but is a general process that can be adapted to all types of XPS equipment, even those not specifically designed for ARXPS. In that case, this method enables bringing the sample as close as possible to the manipulator axis of rota...