Celia Castro

Scaling size of the interplay between quantum confinement and surface related effects in nanostructured silicon

Si nanocrystals (NCs) embedded in a SiO2 matrix provide an exemplar curved nanostructured interface to evidence the competition between surface states and quantum confinement (QC) effects. The study of the energy band alignment as a function of NCs size (<5 nm) clarifies their interplay and identifies, with subnanometric resolution, three different regimes. Primarily QC affects the conduction band, then surface effects pin the conduction states, and finally QC starts to modify the valence band. A way to study how different nanoscale configurations compete with pure quantum properties is established.

The fabrication of tunable nanoporous oxide surfaces by block copolymer lithography and atomic layer deposition

Patterned nanoscale materials with controllable characteristic feature sizes and periodicity are of considerable interest in a wide range of fields, with various possible applications ranging from biomedical to nanoelectronic devices. Block-copolymer (BC)-based lithography is a powerful tool for the fabrication of uniform, densely spaced nanometer-scale features over large areas. Following this bottom-up approach, nanoporous polymeric films can be deposited on any type of substrate. The nanoporous periodic template can be transferred to the underlying substrate by dry anisotropic etching. Nevertheless the physical sizes of the polymeric mask represent an important limitation in the implementation of suitable lithographic protocols based on BC technology, since the diameter and the center-to-center distance of the pores cannot be varied independently in this class of materials. This problem could be overcome by combining block copolymer technology with atomic layer deposition (ALD): by means of BC-based lithography a nanoporous SiO2 template, with well-reproducible characteristic dimensions, can be fabricated and subsequently used as a backbone for the growth of perfectly conformal thin oxide films by ALD. In this work polystyrene-b-poly(methylmethacrylate) (PS-b-PMMA) BC and reactive ion etching are used to fabricate hexagonally packed 23 nm wide nanopores in a 50 nm thick SiO2 matrix. By ALD deposition of Al2O3 thin films onto the nanoporous SiO2 templates, nanostructured Al2O3 surfaces are obtained. By properly adjusting the thickness of the Al2O3 film the dimension of the pores in the oxide films is progressively reduced, with nanometer precision, from the original size down to complete filling of the pores, thus providing a simple and fast strategy for the fabrication of nanoporous Al2O3 surfaces with well-controllable feature size.

Quantitative analysis of doped/undoped ZnO nanomaterials using laser assisted atom probe tomography: Influence of the analysis parameters

In the last decade, atom probe tomography has become a powerful tool to investigate semiconductor and insulator nanomaterials in microelectronics, spintronics, and optoelectronics. In this paper, we report an investigation of zinc oxide nanostructures using atom probe tomography. We observed that the chemical composition of zinc oxide is strongly dependent on the analysis parameters used for atom probe experiments. It was observed that at high laser pulse energies, the electric field at the specimen surface is strongly dependent on the crystallographic directions. This dependence leads to an inhomogeneous field evaporation of the surface atoms, resulting in unreliable measurements. We show that the laser pulse energy has to be well tuned to obtain reliable quantitative chemical composition measurements of undoped and doped ZnO nanomaterials.

Rubidium distribution at atomic scale in high efficient Cu(In,Ga)Se2 thin-film solar cells

The introduction of a rubidium fluoride post deposition treatment (RbF-PDT) for Cu(In,Ga)Se2 (CIGS) absorber layers has led to a record efficiency up to 22.6% for thin-film solar cell technology. In the present work, high efficiency CIGS samples with RbF-PDT have been investigated by atom probe tomography (APT) to reveal the atomic distribution of all alkali elements present in CIGS layers and compared with non-treated samples. A Scanning Electron Microscopy Dual beam station (Focused Ion Beam–Gas Injection System) as well as Transmission Kikuchi diffraction is used for atom probe sample preparation and localization of the grain boundaries (GBs) in the area of interest. The analysis of the 3D atomic scale APT reconstructions of CIGS samples with RbF-PDT shows that inside grains, Rb is under the detection limit, but the Na concentration is enhanced as compared to the reference sample without Rb. At the GBs, a high concentration of Rb reaching 1.5 at. % was found, and Na and K (diffusing from the glass subs...

Extraction of the characteristics of Si nanocrystals by the charge pumping technique

In this paper, the characteristics of silicon nanocrystals used as charge trapping centers in memory devices are examined using the two-level charge pumping (CP) technique performed as a function of frequency and energy filtered transmission electron microscopy (EFTEM). The parameters extracted from the two methods such as the depth location, density and effective diameter of the nanocrystals are in good quantitative agreement. These results validate the charge pumping approach as a non-destructive powerful technique to access most of the properties of nanocrystals embedded in dielectrics and located at injection distances from the substrate surface not limited to the direct tunneling regime.

Evolution of shape, size, and areal density of a single plane of Si nanocrystals embedded in SiO2 matrix studied by atom probe tomography

Single planes of Si nanocrystals (NCs) embedded in a SiO2 matrix were synthesized by annealing SiO2/SiO/SiO2 multilayer structures deposited on Si (100) substrates by e-beam evaporation. The dependence of the shape, size, and areal density of Si NCs on the thickness of the initial SiO layer was investigated using atom probe tomography and validated by energy filtered transmission electron microscopy. Three kinds of samples were prepared with SiO layer thicknesses of 4, 6, and 10 nm. The size of Si NCs enlarged with increasing SiO layer thickness. The shape of Si NCs was mainly extended spheroid in all three kinds of samples. In the sample with the 4 nm-thick SiO layer, the Si NCs were more prolate than those in the other two samples. Moreover, many rod-shaped Si NCs appeared in the sample with the 10 nm-thick SiO layer. These rod-shaped Si NCs were found to be connected by small Si NCs. The areal densities of Si NCs were in the order of 1012 NCs per cm2 in all samples.

Atomic characterization of Au clusters in vapor-liquid-solid grown silicon nanowires

By correlating atom probe tomography with other conventional microscope techniques (scanning electron microscope, scanning transmission electron microscope, and scanning tunneling microscopy), the distribution and composition of Au clusters in individual vapor-liquid-solid grown Si nanowires is investigated. Taking advantage of the characteristics of atom probe tomography, we have developed a sample preparation method by inclining the sample at certain angle to characterize the nanowire sidewall without using focused ion beam. With three-dimensional atomic scale reconstruction, we provide direct evidence of Au clusters tending to remain on the nanowire sidewall rather than being incorporated into the Si nanowires. Based on the composition measurement of Au clusters (28% ± 1%), we have demonstrated the supersaturation of Si atoms in Au clusters, which supports the hypothesis that Au clusters are formed simultaneously during nanowire growth rather than during the cooling process.

Nanoscale control of Si nanoparticles within a 2D hexagonal array embedded in SiO2 thin films

In this work, we investigate the ability to control Si nanoparticles (NPs) spatially arranged in a hexagonal network of 20 nm wide nanovolumes at controlled depth within SiO2 thin films. To achieve this goal an unconventional lithographic technique was implemented based on a bottom-up approach, that is fully compatible with the existing semiconductor technology. The method combines ultra-low energy ion beam synthesis with nanostructured block-copolymer thin films that are self-assembled on the SiO2 substrates to form a nanoporous template with hexagonally packed pores. A systematic analytical investigation using time of flight-secondary ion mass spectroscopy and low-loss energy filtered transmission electron microscopy demonstrates that by adjusting few fabrication parameters, it is possible to narrow the size distribution of the NPs and to control the number of NPs per nanovolume. Experimental results are critically discussed on the basis of literature data, providing a description of the mechanism involved in the formation of Si NPs.

Layered tellurides: stacking faults induce low thermal conductivity in the new In2Ge2Te6 and thermoelectric properties of related compounds

A new ternary layered compound In2Ge2Te6, belonging to the hexatellurogermanate family has been synthesized from the reaction of appropriate amounts of the pure elements at high temperature in sealed silica tubes. In2Ge2Te6 crystallizes in the rhombohedral space-group R:H with lattice parameters a = 7.0863(3) A and c = 21.206(2) A and its structure is resolved using single crystal X-ray diffraction. The transport properties (Seebeck coefficient, resistivity and thermal conductivity) of compounds belonging to the family AMTe3 (A = In and Cr; M = Ge and Si) are reported. All compounds are p-type semiconductors. InSiTe3 and Cr2Si2Te6 are too resistive to be good thermoelectric materials, with maximal power factors of 10−6 and 10−5 W m−2 K−2 at 473 K, while In2Ge2Te6 and Cr2Ge2Te6 exhibit maximal values of about 10−4 and 10−3 W m−2 K−2 at 673 K, respectively. All compounds exhibit thermal conductivity below 2 W m−1 K−1, with values dropping to 0.35 W m−1 K−1 at 673 K for In2Ge2Te6. Transmission electron microscopy evidences stacking faults explaining such low thermal conductivities. The best ZT values are observed for Cr2Ge2Te6 with 0.45 at 773 K and In2Ge2Te6 with 0.18 at 673 K. Among these layered structures, a spark plasma sintered Cr2Ge2Te6 sample exhibits some thermal conductivity anisotropy but only weakly due to crystallite orientations.