Diamond‐hexagonal silicon ribbons in silicon fins

The formation of diamond‐hexagonal silicon [1‐3] or Ge [4] is recently reported under different experimental conditions. The dh‐phase has potential for application in optoelectronic devices. A transformation of diamond‐cubic (dc) to diamond‐hexagonal (dh) Si can occur during the wet oxidation treatment applied to densify the oxide fillings between silicon fins in nano‐electronic devices [3]. The phase change is induced by the compressive stress due to the expanding oxide. A similar transformation is reported in Ge nanowires [4] under non‐oxidizing conditions and related to tensile stress of the shrinking oxide filling. In the finfet structures, the transformation occurs at the base of the outer fins where the lateral stress is the largest and unbalanced between the wide and narrow oxide spacings (Fig. 1). The outer fin moves outward and typically a step and bulge are formed at inner and outer side respectively. In about half of the fins a thin dh‐Si ribbon is formed across the full or partial width of the fin. In the other cases only steps/bulges are present with defect‐free dc‐silicon in between. To increase the volume of the dh‐Si, in this work the oxidation time is doubled compared to the conditions in [3]. The phase transformation is investigated by high resolution HAADF‐STEM at 120 kV in order to minimize the beam damage during the observations. The longer treatment results in larger steps/bulges (Fig. 1b vs 1a). They are also formed for fins with larger spacings which are not modified in case of the standard oxidation time. The average thickness of the dh‐Si in the outer fins increases by a factor 2‐3 while the transformed material in the outer fins generally becomes a mixture of several Si‐polytypes (Fig. 2b). Additionally the transformation also occurs in the second outer fins with thinner transformed slabs that are pure dh‐Si phase (Fig. 2a). The dh‐Si is epitaxial to the silicon substrate with its c‐axis horizontally across the fins i.e. (110) dc //(0001) dh and [‐110] dc //[2‐1‐10] dh . Both (001) dc‐Si and (115) dc‐Si interfaces are present which are characterized by stepped (Fig. 3a) and flat (Fig. 3b) interfaces. In the latter case the dh‐lattice is ~4º rotated. The doubling of the oxidation time does not result in further consumption of the silicon on the fin sidewalls (Fig. 1), i.e. the oxidation rate is reduced by the stress which is therefore also not further increasing. The continued transformation of dc to dh‐phase during the prolonged oxidation is therefore a time related phenomenon and indicates a relatively slow process. Stress‐retarded oxidation of Si sidewalls is previously reported in [5]. Although dh‐Si is a metastable phase, once present, it remains stable during subsequent high temperature treatments even up to 1050ºC. As the dh‐Si is situated at the base of the fins it does not affect the transistor structures.