Theoretical STM images of Cu atoms on a Sið111Þ surface P. Mutombo * , V. Ch ab Institute of Physics, Academy of Science of the Czech Republic, Cukrovarnick a 10, CZ 162 53 Prague, Czech Republic Abstract We calculated scanning tunneling microscope images (STM) of Cu atoms adsorbed on a Si(1 1 1)-1  1 structure within the Tersoff–Hamann approximation. The following configurations were considered: CuSi substitutional, CuSi 2 interstitial and Cu 2 Si with Cu atoms residing both in H 3 sites and substituting Si in the top layer of the Si(1 1 1) double layer. Our images of the CuSi and Cu 2 Si show hexagonal arrays of deep crater-like features, surrounded by bright protrusions. Their shape depends on the bias voltage, in accordance with experimental findings. Additionally, we found shallow dark spots, whose contrast also changes with applied bias voltage. The interstitial CuSi 2 resembles a pure Si(1 1 1)-1  1 structure. Our calculations suggest that the origin of craters observed in the experiments cannot un- ambiguously be connected solely to the topographic features, but that the local density of states plays an important role in the image formation. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Copper; Silicon; Surface chemical reaction 1. Introduction The Cu–Si interface has attracted the attention of many researchers because of its technological importance for the semiconductor industry. When approximately 1.3 Cu monolayers are deposited on Si(1 1 1), followed by annealing at higher tem- perature, a quasi-5  5 phase forms [1,2]. This system has been investigated by means of various surface analytical techniques [1–11]. Based on early measurements, three structure models have been proposed in the literature. They claim that the 1  1 regions of the CuSi layer have one of the following stoichiometries: CuSi, Cu 2 Si, and CuSi 2 . Chambers et al. [4] suggested, based on Auger electron diffraction measurements, that Cu atoms are located on the H 3 sites at the surface, while the upper double layer of the Si(1 1 1) is compressed to give the sixfold diffraction pattern. This model corresponds to a CuSi 2 (Fig. 1(a)) atomic struc- ture. Chambliss and Rhodin [3] claimed that the quasi-5  5 phase can be obtained by replacing a Si atom by a Cu atom in the upper layer of the Si double layer, resulting in CuSi stoichiometry (Fig. 1(b)). This was found to show a better agreement between band structure calculation and their angle resolved ultraviolet photoemission spectroscopy. Using an X-ray standing wave study (XSW) and taking into account Mortensen STM results [6], Zegenhagen et al. [5] proposed that the Cu–Si overlayer has the stoichiometry of CuSi 2 (Fig. 1(c)). They based their reasoning on the fact that they could identify two inequivalent Cu atoms on * Corresponding author. Tel.: +420-2-20318528; fax: +420-2- 33343184. E-mail address: mutombo@fzu.cz (P. Mutombo). 0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00180-8 Surface Science 532–535 (2003) 645–649 www.elsevier.com/locate/susc