Buckling and band gap of theGe(111)2×1surface studied by low-temperature scanning tunneling microscopy

Abstract

Low-temperature scanning tunneling microscopy is used to study the 2×1 reconstruction of cleaved Ge(111) surfaces. Buckling of the surface atoms is investigated by observations of the corrugation shift between filled and empty states. In the direction, the shift in corrugation maxima from filled to empty states is found to be negative, consistent with expectations for the “negatively model for this surface. A surface band gap of 0.54 ± 0.04 eV is measured by tunneling spectroscopy. The 2×1 reconstructions of Ge(111) and Si(111) surfaces have served as prototypical systems for the investigation of surface electronic and optical properties. It is well accepted that the geometry of the surfaces is described by the π-bonded chain model [1]. Many-body effects in the electronic states[2], as well as possible buckling of the surface chains, are still being investigated [3]. In particular, a recent paper by Rohlfing et al. [4] following work by Takeuchi et al. [5] has proposed a model with relatively large “negative buckling” of the chains, based on a comparison of theoretical band gaps with experimental results from photoemission and inverse photoemission spectroscopy (PES/IPES) [6,7] as well as a comparison of computed optical spectra with experimental data [8]. In this work we investigate the buckling and the band gap of Ge(111)2×1 surfaces using low-temperature scanning tunneling microscopy (STM). The STM provides a relatively direct means of determining surface buckling (at least for large buckling) [9,10], as illustrated in Fig. 1. Structural models for positive and negative buckled geometries are shown, taken from Ref. [4] In the π-bonded chains, the surface atoms have 1 electron per dangling bond. Some charge transfer is expected from the lower to the upper atoms, making them 3-fold or 4-fold coordinated, respectively. Thus, one expects the filled (empty) surface states to be located preferentially on the upper (lower) atoms. Recent theoretical results of Lee et al. confirm this expectation [3]. Thus, if one examines the shift in spatial location from filled to empty states, one expects a shift in the direction for positively buckled chains whereas a is expected for negatively buckled chains. In our results below we observe a shift in the location of corrugation maxima when going from filled to empty states, which is consistent with a negatively buckled surface. In addition, we present detailed results for tunneling spectroscopy, revealing a rich spectrum of surface states and a surface gap of 0.54 ± 0.04 eV. This gap is consistent with that from PES/IPES, and together with the theoretical results of Rohlfing et al. [4] also argues in favor of a negatively buck211