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. This paper reports on quasi-fivefold symmetric electron diffraction patterns observed in multiply twinned silicon thin films after epitaxy breakdown; the weight of twinned orientations compared to that of random microcrystalline orientations is determined. The origins of twinning and multiple twinning during epitaxy breakdown are discussed.