Papers by Author: Lee Chow

Abstract: Ge and B diffusion was studied in nanocrystalline Si, and Pd and Si self-diffusion was studied in nanocrystalline Pd₂Si during and after Pd/Si reactive diffusion. These experiments showed that grain boundary (GB) diffusion kinetic is the same in micro-and nanoGBs, whereas triple junction (TJ) diffusion is several orders of magnitude faster than GB diffusion. In addition, GB segregation and GB migration can significantly modify atomic diffusion profiles in nanocrystalline materials, and atomic transport kinetics can be largely increased in nanograins compared to micro-grains, as well as during reactive diffusion, probably due to an increase of point defect concentration. These observations show that atomic transport in nanometric layers during reactive diffusion is complex, since GBs and TJs are moving and the proportion of GBs and TJs is changing during the layer growth.

Abstract: In this paper, we report investigations concerning the fabrication of a diluted Ge (Mn) solution using solid state Mn diffusion, and Mn/Ge reactive diffusion for spintronic applications. The study of Mn diffusion shows that the quasi-totality of the incorporated Mn atoms occupies Ge substitutional sites and probably exhibits two negative elementary charges. The solubility limit of Mn in Ge is comprised between 0.7 and 0.9 % (T  600 °C). We show that substitutional Mn atoms are not ferromagnetic in Ge and consequently that Ge (Mn) diluted magnetic semiconductor can not be produced. Beside the ferromagnetic signal from Mn₅Ge₃, ferromagnetic signals detected in the samples could be always attributed to surface or bulk Mn-Ge clusters. Furthermore, we show that the CMOS Salicide process is potentially applicable to Mn₅Ge₃ nanolayer fabrication on Ge for spintronic applications. During Mn (thin-film)/Ge reaction, Mn₅Ge₃ is the first phase to form, being thermally stable up to 310 °C and exhibiting ferromagnetic properties up to T_C ~ 300 K.

Abstract: The use of nanometric size materials as embedded clusters, nanometric films, nanocrystalline layers and nanostructures is steadily increasing in industrial processes aiming to produce materials and devices. This is especially true in today Si-based microelectronics with transistors made of a multitude of different thin film materials (B-, As-, and P-doped Si, NiSi (Pt), poly-Si, W, TiO_x, LaO, SiO₂, Al, H_fO₂), and exhibiting a characteristic lateral size of 32-22 nm. Size reduction leads to an increasing role of surfaces and interfaces, as well as stress and nanoscale effects upon important phenomena driving fabrication processes, such as atomic diffusion, phase nucleation, phase growth, and coarsening. Consequently, nanotechnology related to Material Science requires an investigation at the nanometric (or atomic) scale of elementary physical phenomena that are well-known at the microscopic scale. This paper is focused on nanosize effects upon diffusion in Si and Si reactive diffusion. We present recent results showing that the kinetic of lattice diffusion is enhanced in semiconductor nanometric (nano) grains, while grain boundary (GB) diffusion is not changed in nanoGBs. It is also shown that diffusion in triple-junction (TJ) is several orders of magnitude faster than GB diffusion, and that its effect cannot be neglected in nanocrystalline (nc) layers made of 40 nm-wide grains. Experimental results concerning Si sub-nanometric film reaction on Ni (111) substrate are also presented and compared to theoretical results giving new prospects concerning nanosize effects on reactive diffusion at the atomic scale.

Abstract: The Diffusion and Solubility of B Implanted in δ-Ni₂Si and Nisi Layers Is Studied by SIMS. it Is Observed that both Diffusion and Solubility Are Higher in δ-Ni₂Si than Nisi. the Redistribution of B during Ni Silicidation Is Also Studied. the SIMS Profiles Show the Presence of Concentration Step in the Middle of the Final Nisi Layer. this Profile Shape Is Explained in Light of the Results Obtained in Preformed Silicides. the Proposed Model Is Supported by Redistribution Simulations that Can Reproduce the Main Features of the Profile.

Abstract: With the development of nanotechnologies, the number of industrial processes dealing with the production of nanostructures or nanoobjects is in constant progress (microelectronics, metallurgy). Thus, knowledge of atom mobility and the understanding of atom redistribution in nanoobjects and during their fabrication have become subjects of increasing importance, since they are key parameters to control nanofabrication. Especially, todays materials can be both composed of nanoobjects as clusters or decorated defects, and contain a large number of interfaces as in nanometer-thick film stacking and buried nanowires or nanoislands. Atom redistribution in this type of materials is quite complex due to the combination of different effects, such as composition and stress, and is still not very well known due to experimental issues. For example, it has been shown that atomic transport in nanocrystalline layers can be several orders of magnitude faster than in microcrystalline layers, though the reason for this mobility increase is still under debate. Effective diffusion in nanocrystalline layers is expected to be highly dependent on interface and grain boundary (GB) diffusion, as well as triple junction diffusion. However, experimental measurements of diffusion coefficients in nanograins, nanograin boundaries, triple junctions, and interfaces, as well as investigations concerning diffusion mechanisms, and defect formation and mobility in these different diffusion paths are today still needed, in order to give a complete picture of nanodiffusion and nanosize effects upon atom transport. In this paper, we present recent studies dealing with diffusion in nanocrystalline materials using original simulations combined with usual 1D composition profile measurements, or using the particular abilities of atom probe tomography (APT) to experimentally characterize interfaces. We present techniques allowing for the simultaneous measurement of grain and GB diffusion coefficients in polycrystals, as well as the measurement of nanograin lattice diffusion and triple junction diffusion. We also show that laser-assisted APT microscopy is the ideal tool to study interface diffusion and nanodiffusion in nanostructures, since it allows the determination of 1D, 2D and 3D atomic distributions that can be analyzed using diffusion analytical solutions or numerical simulation.

Abstract: The measurement of diffusion coefficients in today’s materials is complicated by the down scaling of the studied structures (nanometric effects in thin films, nano-crystalline layers, etc.) and by the complex production process conditions of industrial samples or structures (temperature variations, complex solute and point defect distributions, stress gradients, etc.). Often diffusion measurements have to be performed in samples for which initial experimental conditions do not offer the possibility of using conventional diffusion analytical solutions. Furthermore, phenomena involved with diffusion are sometimes so numerous and complex (stress, matrix composition inhomogeneities, time dependence of point defect generation sources, electrical effects, clustering effects, etc…) that the use of analytical solutions to solve the observed diffusion behavior is difficult. However, simulations can be of use in these cases. They are time consuming compared to the use of analytical solutions, but are more flexible regarding initial conditions and problem complexity. The use of simulations in order to model physical phenomena is quite common nowadays, and highly complex models have been developed. However, two types of simulations have to be considered: i) simulations aiming to understand and predict phenomena, and ii) simulations for measurement purposes, aiming to extract the (average) value of a physical parameter from experimental data. These two cases have different constrains. In the second case, that is the subject of this article, one of the most important stress is that the simulation has to precisely scale the experiment (sample size, experiment duration, etc.), sometimes preventing the measurement due to computational time consumption. Furthermore, the simpler the model (small number of parameters) used in the simulation, the more relevant the measurement (minimum error). In this paper, examples of recent works using two- and three-dimensional finite element simulations for diffusion coefficient measurements in thin polycrystalline films and nano-crystalline layers are presented. The possible use of simulations for diffusion coefficient measurements considering GB migration, GB segregation, or triple junctions is also discussed.

Abstract: Nanostructured materials were synthesized by thermal evaporation process using silicon dioxide and carbon from coconut shell charcoal or graphite mixed with GeO2 by the ratio of 5:1:1 at temperature 1200 oC in one atmosphere of nitrogen for 3 hours. The nanostructured materials were characterized by the stereo microscope (image analyzer) and scanning electron microscope. The diameters of nanowires vary from 10 nm to 50 nm and length of several 10 micrometers. Length of nanorods was around 15 micrometers and diameters vary from 10 nm to 100 nm.

Abstract: ZnO and other nanomaterials were synthesized by thermal evaporation process by carbon assisted method using powder ZnO as a precursors at temperature 1200 oC in one atmosphere of nitrogen for 3 hours. The diameter of nanofibers and nanowires vary from 50 nm to 200 nm and length of several ten micrometers. The size of nanorods vary from 20 nm to 100 nm and length of a few micrometers. The stereo microscope with an image analyzer and scanning electron microscope instruments are used to characterize these nanostructured materials.

Abstract: The effect of hydrostatic argon pressure equal to 105 Pa and 1.1 GPa applied to processing at up to 1270 K (HT) of Si:Cr samples prepared by Cr+ implantation (dose 1x1015 cm-2, 200 keV) into (001) oriented Czochralski silicon, has been investigated by Secondary Ion Mass Spectrometry, photoluminescence, X-ray and SQUID methods. Cr+ implantation at this energy and dosage produces amorphous silicon (a-Si) near the implanted ions range. Solid phase epitaxial re-growth (SPER) of a-Si takes place at HT. The Cr profile does not depend markedly on HP applied during processing at 723 K. Si:Cr processed at up to 723 K indicates magnetic ordering. Annealing under 105 Pa at 873 K, 1070 K and 1270 K results in a marked diffusion of Cr toward the sample surface. In the case of processing under 1.1 GPa this diffusion is less pronounced, SPER of a-Si is retarded and the a-Si/Si interface becomes enriched with Cr. The Cr concentration in Si:Cr sample processed at 1270 K under 1.1 GPa forms two distinct maxima, the deeper one at 0.35 μm depth.