Atomic scale investigation of silicon nanowires and nanoclusters
© Roussel et al; licensee Springer. 2011
Received: 4 October 2010
Accepted: 30 March 2011
Published: 30 March 2011
In this study, we have performed nanoscale characterization of Si-clusters and Si-nanowires with a laser-assisted tomographic atom probe. Intrinsic and p-type silicon nanowires (SiNWs) are elaborated by chemical vapor deposition method using gold as catalyst, silane as silicon precursor, and diborane as dopant reactant. The concentration and distribution of impurity (gold) and dopant (boron) in SiNW are investigated and discussed. Silicon nanoclusters are produced by thermal annealing of silicon-rich silicon oxide and silica multilayers. In this process, atom probe tomography (APT) provides accurate information on the silicon nanoparticles and the chemistry of the nanolayers.
Low-dimensional nano-structured materials, such as carbon nanotubes , silicon nanowires (SiNWs) , and silicon nanoclusters (SiNCs) , have attracted much interest in recent years because of their special properties (electrical, optical, mechanical, etc.) compared to bulk materials. The morphology and number density of nanoclusters as well as the dopant or impurity concentration and their spatial distributions in nanowires can greatly affect their properties. Thus, a key issue that remains is to analyze and characterize nano-structured materials at the atomic scale.
In this study, we have used the laser-assisted wide angle Tomographic Atom Probe to characterize SiNWs and SiNCs, respectively. The atom probe tomography (APT) involves the use of a three-dimensional (3D) high-resolution analytic microscope that can map the spatial distribution of atoms in materials at the atomic scale. The principle of the APT is based on the field evaporation of atoms. A conventional APT relies on the basic principle of the field evaporation of atoms from the surface of a specimen under high-voltage (HV) pulses [4, 5]. The chemical nature of each evaporated ion is determined by the time of flight mass spectrometry using a position-sensitive detector (PSD). The set of information (position and chemical nature) allows for the 3 D reconstruction of the ionized volume. The specimen must be prepared in the form of a sharp needle with a diameter smaller than 100 nm to generate a sufficient electric field at the apex to favor the ionization and evaporation of atoms during HV pulses. In the case of poor conductive materials such as semiconductor NWs, HV pulses are replaced by femtosecond laser pulses to evaporate the semiconducting materials. This is the so called laser-assisted APT. The laser pulse frequency triggers the evaporation of atoms from the surface of the specimen toward the PSD. This information is used for the 3 D reconstruction of the material at the atomic scale as previously mentioned. A detailed description of the laser-assisted APT can be found in [6–8].
Several methods can be utilized for the preparation of a very sharp nano-tip with a diameter smaller than 100 nm. Considering the preparation of SiNWs, a two-step process can be proposed: (i) choosing under optical microscopy a suitable SiNW (diameter, direction, length, etc.) from the Si-growing substrate covered with SiNWs using a pre-prepared W tip; and (ii) welding the SiNW and the W tip in a dual beam system [scanning electron microscope (SEM)-gas injection system and focused ion beam (FIB)] using Pt as solder. As far as the preparation of SiNCs for APT investigation is considered, a conventional lift-out sample preparation can be proposed. The SiNC-based materials are sputtered on a Si substrate and formed by annealing treatments. Using a classical lift-out method, a multilayer (ML) chunk can be mounted on a pre-prepared steel needle . The APT tip is then prepared by annular milling using the FIB .
Investigation of SiNWs
SiNWs are 1 D nano-structures which can be applied in various domains, such as solar cells  and biosensor . Either of the two types of approaches can be used to elaborate SiNWs: "top-down" or "bottom-up". In this study, we have studied the SiNWs synthesized by bottom-up approach. Several mechanisms are proposed based on this approach. Among them, the vapor liquid solid mechanism, proposed by Wagner and Ellis in 1964 , is the most widely used. According to this mechanism, the growth process of SiNWs involves three elements: reactant, catalyst, and substrate, and three processing steps: supply of Si atoms from reactant, incorporation of Si atoms into the catalyst droplet, and crystallization of Si atoms at the catalyst/substrate interface. Considering step 1, two methods can be utilized: chemical vapor deposition (CVD) and molecular beam epitaxy. Here, CVD was used because it allows for a better control of the growth condition and doping incorporation (addition of diborane silane). However, the true concentration of doping, its spatial distribution, and the presence or absence of impurities (catalyst atoms...) remain as experimental bottlenecks. In this study, Au was used as catalyst to grow SiNWs. Au is chosen because of its low eutectic temperature when mixed with Si and its easy preparation in the form of a homogeneous distribution of droplets at the surface of the silicon substrate. However, owing to its atomic diffusion , Au atoms can create intense deep traps in SiNW and, therefore, decrease the electrical transport . Thus, the investigation of the presence and the location of Au atoms in SiNW remain an important issue. The laser-assisted APT has been widely used to investigate this aspect.
Characterization of SiNCs embedded in silica
SiNCs have been extensively studied since the discovery of photoluminescence (PL) of porous silicon . SiNCs show very interesting electrical and optical properties. Their ability to trap charges could be exploited to create new generations of memory devices . As for their optical properties, they could be employed in solar cells , and light amplification [3, 19]. Optical and electrical properties are strongly dependent on the SiNCs structural characteristics: their size, their distribution, and the nature of their cluster/matrix interfaces. Consequently, a perfect control of the SiNCs growth during the elaboration and an accurate characterization of these parameters are crucial toward developing future potential applications and optimization of elaboration processes.
Si nanoclusters elaboration
SiNCs are usually grown from a silicon-rich silicon oxide (SRSO). SRSO can be produced with many techniques such as ion implantation , CVD  or magnetron sputtering . SRSO is then annealed to induce phase separation between silicon excess and silica. An efficient way to control the size of SiNCs is to produce SRSO thin layer between two SiO2 layers. These two silica layers prevent the Si excess from diffusing out the SRSO layer and restrict the diameter of produced SiNCs. Thus, a ML configuration is commonly adopted. In this study, SRSO/SiO2 MLs have been synthesized by reactive magnetron sputtering. A SiO2 pure target is sputtered on 1 cm2  Si substrates maintained at 500°C. Silica films are deposited under Ar plasma, while for the SRSO ones, a mixture of 50% H2 + 50% Ar gas is used. In such conditions, SRSO layers contain approximately 50 at.% of silicon, i.e., Si excess is estimated to be 25 at.%. The thickness of each layer is tuned by the sputtering time. This deposition process is fully described in the reference . MLs are subsequently annealed at 900°C for 1 h under N2 to favor the phase separation and the SiNCs growth. The thickness of each layer was accurately measured by high-resolution transmission electronic microscope (HRTEM) after the deposition process. It was estimated to be 4 nm for SiO2 layers, and 3.8 nm for SRSO layers. Specimens are then mounted on tip-shaped stainless steel needles for being analyzed with APT.
In summary, the ATP has been used for the investigation of SiNWs and SRSO/SiO2 MLs containing SiNCs. An efficient sample preparation for APT is presented in both cases. The Au droplet on top of SiNW is reconstructed, and the core concentration of Au atoms is less than 5 ± 0.5 × 1017 at./cm3. It is shown that B atoms can be uniformly distributed in the core of SiNW with a measured high concentration value of 1.4 ± 0.1 × 1020 B/cm3. We have shown that APT also provides information on SiNCs which are inaccessible with the conventional analytic methods and particularly with HRTEM. The local composition of each layer and each phase as well as the structural properties of SiNCs can be investigated in a very accurate manner. It is shown that in the case of a 3.8-nm-thick SRSO layer with 25% of silicon in excess, a 1-h annealing treatment at 900°C induces the precipitation of SiNCs with a mean diameter of 2.9 nm. After this annealing treatment, the SRSO layers still contain 13% of silicon excess, evidencing an incomplete phase separation. These measurements show that APT is an efficient technique for the investigation of the phase separation in SiO2/SRSO MLs and the structural properties of SiNCs embedded in silica. Such a study is crucial to correlate electrical and optical properties with structural properties.
atom probe tomography
chemical vapor deposition
focused ion beam
high-resolution transmission electronic microscope
position sensitive detector
scanning electron microscope
silicon-rich silicon oxide.
This study was supported by the upper Normandy Research and the French Ministry of Research in the framework of Research Networks of Upper-Normandy. The authors acknowledge also "Le Fond Européen de Développement Régional" (FEDER) for his support. This study was supported partly by the DGA (Direction Générale de l'Armement) under the contract REI - No. 2008.34.0031.
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