Fivefold symmetry, like any kind of n-fold rotational symmetry, can be identifiable when rotating a crystalline configuration 5 times (or n times) around a certain axis and realizing that the structure is transformed into a configuration that is equivalent to the initial one. The occurrence of this specific symmetry, forbidden by the conventional periodic crystallography, was attributed in the literature to the presence of a new state of matter “the quasicrystals” [1] [2] or simply to an effect of multiple twinning. Particularly, the tendency of multiply twinning in a fivefold symmetry has been widely reported in small particles having a special morphology like the decahedral [3] or icosahedral [4] structures, usually called multiply twinned particles. In this study, we will highlight on the fivefold symmetry observed in the electron diffraction patterns of two types of materials elaborated in different growth conditions, originating from multiple twinning and not from the presence of multiply twinned particles.

The first case concerns the fivefold symmetry on p-type doped silicon thin films containing a non-negligible amount of carbon and oxygen. These films were deposited in a plasma enhanced chemical vapor deposition reactor (PECVD) at 0.2 W/cm² using silane, hydrogen, diborane and hexamethydisiloxane (C₆H₁₈OSi_2, HMDSO) diluted in argon. Since all the diffraction patterns recorded on different regions of these films exhibit a fivefold symmetry along [0-11] zone axis (Figure 1), it is clear that this symmetry is real and characteristic of our films. Further diffraction measurements reveal that there is a relation of epitaxy with the (100) crystalline silicon substrate. This is also confirmed by high resolution TEM images, where {111} planes are continuing from the substrate to the film across the interface. Moreover, energy filtered TEM images were correlated with SIMS measurements to provide elemental mapping of silicon, carbon and oxygen with absolute values.

The second case illustrates a quasi-fivefold symmetry recorded on intrinsic silicon thin films deposited by PECVD using silicon tetrafluoride, hydrogen and argon chemistry at a purposely high power density of 0.3 W/cm². After few hundred nanometers of epitaxial growth, a high density of defects appears, followed by a multiply twinned part (as shown in Figure 2a). Fourier Transforms recorded on the first part reveal a monocrystalline structure (Figure 2c), and on the second part a fivefold symmetry (Figure 2b), which is, in this case also, linked to an epitaxial growth.

It has been proved in some references [5] [6] that a high power density is responsible for a high ion energy impinging on the substrate and causing some surface or even bulk damage. Thus, the twin defects present in our films are most probably caused by the application of a high power density. However, to obtain a fivefold symmetry, it is necessary to have at least three orders of twinning that contribute to 10 spots in the diffraction pattern, i.e, if there only exist two orders of twinning, some additional diffraction spots appear without giving rise to a fivefold symmetry as it is the case of Figure 3. Detailed investigation of the multiple twinning in a fivefold symmetry fashion will be presented.

1.      Shechtman, D., et al., Metallic Phase with Long-Range Orientational Order and No Translational Symmetry. Physical Review Letters, 1984. 53(20): p. 1951-1953.

2.      Pauling, L., Apparent Icosahedral Symmetry Is Due to Directed Multiple Twinning of Cubic-Crystals. Nature, 1985. 317(6037): p. 512-514.

3.      Iijima, S., Fine Particles of Silicon. II. Decahedral Multiply-Twinned Particles. Japanese Journal of Applied Physics, 1987. 26(3R): p. 365.

4.      Yang, C.Y., Crystallography of decahedral and icosahedral particles: I. Geometry of twinning. Journal of Crystal Growth, 1979. 47(2): p. 274-282.

5.      Rosenblad, C., et al., Silicon epitaxy by low-energy plasma enhanced chemical vapor deposition. Journal of Vacuum Science & Technology A, 1998. 16(5): p. 2785-2790.

6.      Ohmi, T., et al., Study on further reducing the epitaxial silicon temperature down to 250 °C in low‐energy bias sputtering. Journal of Applied Physics, 1991. 69(4): p. 2062-2071.

Figures:

Figure 1. Diffraction pattern along [0-11] zone axis revealing a clear quasi fivefold symmetry on p-type silicon thin films containing 4% of oxygen and 4 % of carbon.

Figure 2. Cross-section TEM image on intrinsic silicon thin film deposited on Si wafer (a) and Fast Fourier Transforms performed on the part presenting a quasi-fivefold symmetry (b) and on the epitaxial part of the film (c).

Figure 3. Diffraction pattern of a monocrystalline silicon thin film presenting twin defects, not revealing a fivefold symmetry.

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