Scanning tunneling spectroscopy of methyl- and ethyl-terminated Si(111) surfaces

Abstract

Methyl-and ethyl-terminated Si͑111͒ surfaces prepared by a two-step chlorination/alkylation method were characterized by low temperature scanning tunneling spectroscopy ͑STS͒. The STS data showed remarkably low levels of midgap states on the CH 3 -and C 2 H 5 -terminated Si surfaces. A large conductance gap relative to the Si band gap was observed for both surfaces as well as for the hydrogen-terminated Si͑111͒ surface. This large gap is ascribed to scanning tunneling microscope tip-induced band bending resulting from a low density of midgap states which avoid pinning of the Fermi levels on these passivated surfaces. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2203968͔Alkylation of Si by a two-step chlorination/alkylation process 1 has been shown to produce surfaces that are passivated both chemically and electrically. 2,3 In common with H-terminated Si surfaces, the alkylated surfaces show extraordinarily low surface recombination velocities. 2,3 However, unlike H-Si͑111͒ surfaces, alkylated surfaces are not susceptible to rapid oxidation in ambient air and do not suffer concomitant degradation of their surface electrical properties. 2,3 Alkylation of Si͑111͒ with methyl groups has been shown by low-energy electron diffraction 4 ͑LEED͒ to produce surfaces with long-range order and by scanning tunneling microscopy ͑STM͒ to produce surfaces with shortrange order. 5,6 Additionally, photoemission and work function measurements 4 in ultrahigh vacuum have shown that CH 3 -terminated Si samples have relatively little band bending, consistent with a low density of band-gap states and in contrast to the fixed charge that is universally present at Si/ SiO 2 interfaces.We report herein scanning tunneling spectroscopy ͑STS͒ data for H-, CH 3 -, and C 2 H 5 -terminated Si͑111͒ surfaces. To date, the only wet chemically prepared Si surface reported to exhibit nearly ideal STS behavior is the H-Si͑111͒ surface. 7 Interestingly, in ultrahigh vacuum ͑UHV͒, this surface exhibits a tunneling gap of Ͼ1.6 eV in which little or no midgap conductance is observed, 8 whereas the band gap of Si is 1.1 eV. The tunneling behavior of CH 3 -Si͑111͒ and C 2 H 5 -Si͑111͒ is thus interesting in this context, given the low midgap density of states expected for such alkylated Si surfaces from macroscopic measurements of such systems. 2,3 Silicon surfaces were functionalized using a two-step chlorination/alkylation procedure. 1 Samples were obtained from ͑111͒-oriented, Sb-doped, 0.005-0.02 ⍀ cm resistivity, n-type Si wafers having a miscut error of ±0.5°. The samples were cleaned and oxidized for 5 min at 80°C in a solution of 1:1:5 ͑vol͒ 30% H 2 O 2 : 30% NH 3 : H 2 O and were then terminated with Si-H bonds by etching for 15 min in 40% NH 4 F͑aq͒. This etching method has been demonstrated to produce large atomically flat terraces. 5 Chlorination was performed by exposure of the samples to a solution of PCl 5 in chlorobenzene. 3,9 A small amount of benzoyl peroxide was added to initiate a radical reaction, and the samples were heated ...

Scanning Tunneling Microscopy of Ethylated Si(111) Surfaces Prepared by a Chlorination/Alkylation Process

“…22 Infrared absorption spectroscopy 12 and X-ray photoelectron spectroscopy 22 are thus consistent with the hypothesis that Si-H bonds terminate most of the nonalkylated Si surface sites. 10,12,22 Scanning tunneling spectroscopy 25 and ultraviolet photoelectron spectroscopy (UPS) 11 indicate no detectable molecular orbital states near the Fermi level of such surfaces, and thus relatively few dangling bonds are present on the surface. The bias used for the STM imaging experiments was -3 V. Under such conditions, geometric factors rather than electronic ones, are likely to dominate the surface images because the alkyl groups that are bonded to the silicon surface have no molecular orbital levels within the applied voltage window.…”

“…The bias used for the STM imaging experiments was -3 V. Under such conditions, geometric factors rather than electronic ones, are likely to dominate the surface images because the alkyl groups that are bonded to the silicon surface have no molecular orbital levels within the applied voltage window. 11,25,26 On the basis of the height difference between an ethyl group and a H atom, it is likely that no H-terminated sites are imaged by STM because they would be shadowed by surrounding ethyl groups. Thus, the bright features in the STM images can consistently be assigned to some part of the top methyl group of the surfacebound ethyl groups.…”

Helium atom diffraction measurements of the surface structure and vibrational dynamics of CH3–Si(111) and CD3–Si(111) surfaces

“…32,33 Furthermore, the low levels of midgap states measured by scanning tunneling spectroscopy attest to the high degree of electrical passivation that is provided by methyl termination. 37 To date, although the static properties of such surfaces have been studied rather thoroughly, the dynamical properties of such systems remain largely unexplored. An understanding of the vibrational dynamics can provide insight into energy accommodation, vibrational decay pathways, and heat flow at the surface.…”

The dependence of electronic transport on compressive deformation of C60 molecule

“…The conductance gap has been demonstrated by many previous works both experimentally [26,27] and theoretically [28,29]. In this work, we mainly investigate how its conductance gap depends on the spatial structure of the C 60 .…”