Institut des
NanoSciences de Paris

Silicene : structure and growth mechanism

Romain Bernard,   Yves Borensztein,   Geoffroy Prévot

Recent years have shown an increasing infatuation for the quest of silicene, the silicon-equivalent of graphene. Calculation have shown that silicene, i.e. Si atoms arranged in a honeycomb structure, could possibly exist and display electronic, transport or magnetic properties similar to graphene’s one. Moreover, this system would present the advantage of a good compatibility with silicon-based microelectronics.

Because of its immiscibility with silicon, Ag appears as a good candidate for the growth of a silicene layer. Silicon films grown on Ag (110) and Ag (111), and identified as silicene, have been actually observed by several authors but all these studies rely on the unchecked hypothesis that the substrate is not modified during the deposition of Si.

Fig 1 : STM images of nanoribbons grown on Ag(110) upon Si deposition at (a) room temperature and (b) 190°C

In order to get a better understanding of the structure of this so-called silicene layer, we have performed in situ scanning tunneling microscopy (STM) and grazing incidence X-ray diffraction (GIXD) measurements during the growth of this system on a Ag(110) substrate. The strong point of our approach is that we have been able to follow by STM imaging the evolution of a well defined region of the sample during the growth process.

Fig 2 : sequence of STM images (330 × 310 nm2) of the same area showing the evolution of the silver surface upon Si deposition at room temperature. (a) bare silver surface. (b), (c) and (d) after Si deposition corresponding respectively to 0.1, 0.2 and 0.3 monolayer.

Our results clearly show an important modification of the substrate during the growth. Indeed, STM images recorded during the deposition of Si show the growth of new Ag islands, either on the step edges or on the terraces. The analysis of the STM images proves that the Ag atoms composing these new terraces have been released from the substrate. This result was confirmed by the evolution of the GIXD pattern during the growth, whichis not compatible with an unreconstructed substrate.

Fig 3 : (a-b) STM images (75 × 80 nm2) showing the growth of fingers from steps perpendicular to the NRs upon Si deposition at RT. (a) 0.2 ML. (b) 0.3 ML. (c) Height profile along the transverse line in (b).

The results that we have obtained strongly question the validity of all the analysis done under the asumption of an unreconstructed substrate and show that the reconstruction of the substrate and the possible incorporation of Ag atoms in the growing layer have to be considered in order to determine the structure of this system.