Spectroscopy of Metal Adsorbates on the GaAs(110) Surface Studied with the Scanning Tunneling Microscope
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SPECTROSCOPY OF METAL ADSORBATES ON THE GaAs(1 10) SURFACE STUI)IED WITH TIlE SCANNING TUNNELING MICROSCOPE R. M. FEENSTRA, P. MARTENSSON, AND R. LUDEKE IBM Research Division, T. J. Watson Research Center, Yorktown Hleights, New York 10598
ABSTRACT The geometric and electronic structure of metal adsorbates on cleaved GaAs(110) surfaces is studied with the scanning tunneling microscope. For the metals Sb and Bi, an ordered monolayer is formed, although in the case of Bi a series of misfit dislocations appear in the overlayer. In the case of Au, individual atoms are observed on the surface, forming clusters at higher metal coverage. Spectroscopic measurements reveal the presence of a state within the GaAs band gap. This state is observed near individual metal adsorbates (for Au and Sb), and near the edge of metal terraces (for Sb and Bi). The observed state is responsible for determining the position of the Fermi-level at the surface.
I. INTRODUCTION The properties of thin metal films on semiconductor surfaces have been studied for many years.' Of particular interest for practical applications is the relationship between the structure of the film and the ensuing electronic properties. In most previous studies, these two aspects - structural and electronic - have been studied separately, with techniques such as electron or ion diffraction yielding structural information, and photoemission or inverse photoemission providing electronic information. With the development of the scanning tunneling microscope (STM), 2 we have a tool which combines these two types of measurement in a single experiment. Atomic resolution imaging, together with spatially resolved spectroscopy, enables us to directly observe the spectrum of states (within a few eV on either side of the Fermi-level) induced by the metal adsorbates, and to determine the origin of the states in terms of the geometric structure of the surface. For the case of semiconductor surfaces, a major point of interest is the occurrence of electronic states within the band gap. Deposition of metal on the semiconductor surfaces may induce states within the band gap, and these states then determine the position of the Fermilevel at the surface. For an entire monolayer of metal, or some definite fraction thereof, the metal sometimes forms a 2-dimensional ordered overlayer on the semiconductor. Such ordered metal overlayers occur on many semiconductor faces, and in recent years their properties have been extensively studied. Depending on the details of the bonding between the metal and the semiconductor, the 2-dimensional bands of the metal overlayer may overlap into the band gap region of the semiconductor. In most cases, such bands will themselves have a band gap in their energy spectrum, and disorder or defects on the surface will induce further states within this band gap. Thus, to fully determine the complete spectrum of states on a surface, and thus determine the position of the surface Fermi-level, it is necessary to go beyond the states of the ordered metal overlayer. I
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