Atomic structure of metal-free and catalyzed Si nanowires
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Atomic structure of metal-free and catalyzed Si nanowires Giuseppe Nicotra1, Corrado Bongiorno1, Annalisa Convertino2, Massimo Cuscunà2, Faustino Martelli2 and Corrado Spinella1 1
CNR-IMM of Catania Stradale Primosole 50 I-95121 Catania, Italy.
2
CNR-IMM of Rome via del Fosso del Cavaliere 100, 00133 Rome, Italy.
ABSTRACT Metal-free and Au-catalyzed silicon nanowires (Si-NWs) grown at low temperatures have been analyzed through transmission electron microscopy (TEM) and scanning electron microscopy (SEM), and their crystalline phase studied. All the observed nanowires are crystalline, grow along two different directions, or , and contain high density of planar defects, such as stacking faults (SFs) and twins. The defect size is comparable to the wire diameter for the metal-free process whilst it is much larger than the wire diameter for the Aucatalyzed Si-NWs. In this latter case parallel SFs may re-arrange and transform in a metastable rhombohedral 9R polytype structure whose formation mechanism is discussed. INTRODUCTION Silicon nanowires (Si-NWs) are continuously drawing attention in the scientific community due to their potential applications in semiconductor based technology. In recent years, significant progress has been made in controlling their size, morphology, and crystallinity. Different methodologies have been used to synthesize Si-NWs, and among them metals mediated growth trhough the vapor–solid–liquid method [1] is the most common utilized. However, the use of external elements, and in particular of gold, may result in detrimental material contamination. To overcome these drawbacks, we have recently reported on the possibility to obtain metal-free growth of Si NWs at low-temperatures [2], down to values (about 300°C), compatible with plastic substrates, also applicable to the fabrication of low-cost solar cells. Despite a very reliable control on the synthesis of the Si NWs has been achieved, a controversial debate on their crystalline structure is still open in the scientific community [3-5]. In this framework an accurate study on the Si NWs crystal structure assumes an important role since the implication on the final device performance. Here we present the detailed study of the crystalline structure of metal-free and metal-catalyzed Si NWs grown at low temperatures. The influence of the growth technique on the lattice structure will be pointed out. EXPERIMENTS All the SiNWs have been grown by plasma-enhanced chemical vapor deposition (PECVD). To grow the catalyst-free NWs, Si(100) wafers have been cleaved along {110} planes and immediately inserted into the growth chamber with the cleaved surface face up [2].
The substrates has been heated up to a temperature of 320 °C in a 1 Torr He atmosphere. The same pressure has also been used for the growth, using a 1:6 dilution of SiH4 in H2. The silane flow rate was 10 sccm. The Au-catalyzed Si NW were grown by PECVD on SiO2(1 µm)/Si(100) substrates. A 1 nm thick Au film was evaporated onto the substrates prior to growth. The PECVD growth was performe
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