Size-, shape-, and orientation-dependent properties of SiC nanowires of selected bulk polytypes
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SiC nanowires (NWs) designed from selected bulk polytypes are investigated using quantum mechanics-based simulations (both ab initio and semiempirical methods), and their structure– property relationships examined vis-à-vis their size, shape, and orientation. It is found that 2H–SiC NWs of hexagonal morphology oriented along the ,0001. direction are the most stable ones compared to NWs designed using other bulk polytypes (e.g., 3C, 4H, or 6H) and other morphologies (round, square, rhombus, etc.) for diameters with 1 nm , d , 14 nm. Based on the electronic density calculations, it is found that 2H–SiC ,0001. NWs exhibit semiconductor-like characteristics (akin to their bulk counterparts), even when their diameters approach 1 nm. On the other hand, SiC NWs designed from 3C, 4H, and 6H bulk polytypes, regardless of their morphology, exhibit gapless features for diameters less than 3.5 nm. Undoubtedly, these novel properties of SiC NWs can be exploited in the fabrication of nanoscale devices.
I. INTRODUCTION
Bulk SiC is a wide-gap semiconductor and it manifests in many forms.1 There are more than 200 known polytypes of SiC. These polytypes are attributed to different stacking sequences of Si and C atoms in different bilayers. Depending on its polytype, its energy gap ranges from 2.4 to 3.3 eV. Besides large energy gaps, SiC also exhibits a large thermal conductivity (three times that of silicon) and a high breakdown voltage (five times that of silicon) that make it a suitable material for high-power electronic devices and high frequency transistors.2,3 The large energy gaps of SiC have been exploited in the fabrication of blue light-emitting diodes or blue laser diodes. These interesting properties of bulk SiC have prompted us to pose the question how they would manifest themselves in nanostructured systems. Therefore, in this work, we have carried out a systematic theoretical study of SiC-based nanowires (NWs) to ascertain how the quasi-one-dimensional characteristics of NWs affect the properties of the bulk SiC and whether the resulting properties could lead to interesting and useful device applications. The synthesis of large quantities of well-oriented crystalline 3C–SiC NWs along ,111. and ,100. orientations,4–18 as well as that of 2H–SiC NWs along the ,0001. direction with an intergrowth of 3C–SiC structure along ,111. direction,19 have been reported in the literature. According to the reported experiments, the yield of SiC NWs is substantially greater than that of SiC whiskers and a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.237 J. Mater. Res., Vol. 28, No. 1, Jan 14, 2013
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bulk SiC.15,20,21 Some SiC NWs have been shown to exhibit high electron field emission with high stability.6,13,22 A strong broad photoluminescence peak around 450 nm has also been reported for 3C–SiC nanocrystallites.9 These experimental results have definitely given credence to the notion that SiC NWs could be used as building bloc
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