Shear-driven phase transformation in silicon nanowires

Abstract

We report on an unprecedented formation of allotrope heterostructured Si nanowires by plastic deformation based on applied radial compressive stresses inside a surrounding matrix. Si nanowires with a standard diamond structure (3C) undergo a phase transformation toward the hexagonal 2H-allotrope. The transformation is thermally activated above 500 °C and is clearly driven by a shear-stress relief occurring in parallel shear bands lying on {115} planes. We have studied the influence of temperature and axial orientation of nanowires. The observations are consistent with a martensitic phase transformation, but the finding leads to clear evidence of a different mechanism of deformation-induced phase transformation in Si nanowires with respect to their bulk counterpart. Our process provides a route to study shear-driven phase transformation at the nanoscale in Si.

Crystalline, Phononic, and Electronic Properties of Heterostructured Polytypic Ge Nanowires by Raman Spectroscopy

“…29,30 The case of hexagonal Ge, Si, and SiGe seems to be particularly challenging, as so far the optical studies are only theoretical,16 despite the availability of high quality NW samples. 10,31,32,33 Moreover, the information on the band alignment in crystal-phase heterostructures that emission and absorption spectroscopies would provide is restricted to the point of the Brillouin zone where radiative recombination takes place (usually at Γ). For applications related to heat transport and management, the phononic properties of the material in the two crystal phases play a crucial role.…”

Si and Ge allotrope heterostructured nanowires: experimental evaluation of the thermal conductivity reduction

“…The structure is stable at least up to 600°C. As the heterostructuration is not performed during growth but by phase transformation after the nanowire top down etching, we point out that the orientation relationship between the 3C and 2H bands is [1][2][3][4][5][6][7][8][9][10]3C//[-2110]2H and (110)3C//(0001)2H. It is clearly not the usual one obtained by VLS growth for III/V nanowires for instance that is (111)3C//(0001)2H.…”

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 NWs are embedded in a hardening hydrogen silsesquioxane resist (HSQ) followed by a baking at 500°C. With this process described in [8,9], a phase transformation occurs heterogeneously along the NW length, on (115) shear bands, i.e. at 39° from the (111) surface plane.…”

“…In the case of Si or Ge NWs, the polytype synthesis cannot be obtained during the growth. Indeed, recently we have developed an original method based on top-down etching to achieve phase transformation in Si and Ge nanowires under external Page 2 of 33 AUTHOR SUBMITTED MANUSCRIPT - NANO-120817 .R2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 A c c e p t e d M a n u s c r i p t 3 stress [8,9]. <111>-oriented NWs with standard diamond structure 3C undergo a phase transformation toward the 2H hexagonal phase.…”

“…Indeed, the number of 2H domains for the different samples cannot be compared if the NWs have different lengths, which is the case after polishing. NW structural properties for each annealing temperature.The sample processed at 500°C is not presented in this table since it presents almost no phase transformation and most of the NWs (more than 90%) are then homogeneous 3C NWs.Very few transformed NWs only exhibit 1 or 2 domains which are very thin and do not extend over the full diameter of the NWs (see images in[9]). This 500°C-sample can then be considered as a reference.…”