Quasi-fivefold symmetric electron diffraction patterns due to multiple twinning in silicon thin films grown from hexamethyldisiloxane

Unusual quasi-fivefold symmetric electron diffraction patterns are observed for silicon thin films grown by plasma-enhanced chemical vapour deposition and containing oxygen and carbon impurities in the range of 0.3–5.5%. These films were grown on crystalline (100) silicon wafers using a liquid precursor, hexamethyldisiloxane (HMDSO), mixed with silane, hydrogen and diborane diluted in argon. The occurrence of this quasi-fivefold symmetry is explained by multiple twinning and imperfect epitaxy. A quantitative method performed on the diffraction patterns is developed to evaluate the number of twin operations. This method is also used to discriminate twin positions from random microcrystalline ones in the diffraction patterns and thus to estimate their respective ratios for different growth conditions. Quite remarkably, the random microcrystalline part remains in the range of a few per cent and the diffracted intensities are the sum of two main contributions: multiple (micro-) twinned and amorphous. Increasing the amount of HMDSO decreases the microtwinned part directly to the benefit of the amorphous part with no significant microcrystalline phase. The causes of twinning are presented and discussed by comparing the observations with the literature; dynamical considerations where the system tends to align {111} planes with the growth direction would explain multiple twinning and, in turn, the fivefold symmetry.

Tilting structures ininverseperovskites,M3TtO (M= Ca, Sr, Ba, Eu;Tt= Si, Ge, Sn, Pb)

Single-crystal X-ray diffraction experiments were performed for a series ofinverseperovskites,M3TtO (M= Ca, Sr, Ba, Eu;Tt= tetrel element: Si, Ge, Sn, Pb) in the temperature range 500–50 K. ForTt= Sn, Pb, they crystallize as an `ideal' perovskite-type structure (Pm\bar 3m,cP5); however, all of them show distinct anisotropies of the displacement ellipsoids of theMatoms at room temperature. This behavior vanishes on cooling forM= Ca, Sr, Eu, and the structures can be regarded as `ideal' cubic perovskites at 50 K. The anisotropies of the displacement ellipsoids are much more enhanced in the case of the Ba compounds. Finally, their structures undergo a phase transition at ∼ 150 K. They change from cubic to orthorhombic (Ibmm,oI20) upon cooling, with slightly tilted OBa6octahedra, and bonding angles O—Ba—O ≃ 174° (100 K). For the larger Ba2+cations, the structural changes are in agreement with smaller tolerance factors (t) as defined by Goldschmidt. Similar structural behavior is observed for Ca3TtO. SmallerTt4−anions (Si, Ge) introduce reduced tolerance factors. Both compounds Ca3SiO and Ca3GeO with cubic structures at 500 K, change into orthorhombic (Ibmm) at room temperature. Whereby, Ca3SiO is the only representative within theM3TtO family where three polymorphs can be found within the temperature range 500–50 K: Pm\bar 3m–Ibmm–Pbnm. They show tiny differences in the tilting of the OCa6octahedra, expressed by O—Ca—O bond angles of 180° (500 K), ∼ 174° (295 K) and 170° (100 K). For largerM(Sr, Eu, Ba), together with smallerTt(Si, Ge) atoms, pronounced tilting of the OM6octahedra, and bonding angles of O—M—O ≃ 160° (295 K) are observed. They crystallize in theanti-GdFeO3type of structure (Pbnm,oP20), and no phase transitions occur between 500 and 50 K. The observed phase transitions are all accompanied by multiple twinning, in terms of pseudo-merohedry or reticular pseudo-merohedry.