Stabilization of silicon honeycomb chains by trivalent adsorbates

The atomic structure of self-assembled quasi-one-dimensional Gd chains on Si(111) has been investigated by low-energy electron diffraction and scanning tunneling microscopy. Based on comparison between Gd and Ca chains we show that this Gd induced surface reconstruction belongs to the class of honeycomb chain-channel structures. This clearly demonstrates that, besides monovalent and divalent adsorbates, also trivalent adsorbates such as Gd stabilize silicon honeycomb chains. Consequently silicon honeycomb chains emerge as an universal building block in adsorbate induced silicon surface reconstructions. Introduction. – Self-assembled atomic chains on silicon surfaces have been the focus of intense research because of their quasi-one-dimensional (1D) electronic properties and their interesting physics. Recently the fluctuation and condensation phenomena at the metal insulator phase transition of the In/Si(111) system could directly be visualized via scanning tunneling microscopy (STM) [1–4]. Competing periodicities in fractionally filled bands lead to the coexistence of different Peierls distortions for the gold induced reconstructions [5–7]. Another important class of 1D systems are the alkali metal (AM=Li, Na, K, Rb, Cs) and Ag induced, insulating (3×1) reconstructions formed by the deposition of 1/3 monolayer (ML) onto the Si(111) surface. The AM/Si(111) systems adopt the so-called honeycomb chain-channel (HCC) structure [8–10] shown in Fig. 1a) which is stabilized by the transfer of one electron from the monovalent AM adsorbate into the Si surface states. A very similar reconstruction with (3×”2”) periodicity is formed by adsorption of alkaline-earth metals (AEM=Mg, Ca, Sr, Ba), where the ×”2” notation stands for a ×2 periodicity along the adsorbate chains but missing coherence between adjacent chains [11]. Due to the divalency of the adsorbate only 1/6 ML, i.e. half the AM coverage, is required to stabilize the HCC structure [12]. At 1/2 ML divalent adsorbates induce a (2×1) phase which was proposed to be formed of π-bonded Seiwatz chains shown in Fig. 1b) [13, 14]. For intermediate coverages, a series of [111]