Structural studies of alkali metal adsorption on silicon surfaces

Abstract

The knowledge of the location of the atoms at a metal/semiconductor interface is a prerequisite to the understanding of its electronic structure. Unlike some metals, alkali metals (AM's) form abrupt, ordered monolayer interfaces with semiconductors, since they neither react nor cluster. This ordered adsorption, as well as the simple AM electronic structure of one valence s-electron, allows simplifications in theoretical models and in interpretations of experimental data of the AM/Silicon interface which are not available for metal/semiconductor interfaces in general. This thesis will document the use of a combination of synchrotron experimental techniques to determine the geometrical structure of a number of AM/Si interfaces. Knowledge of adsorption sites and substrate geometry from experiment will give a better starting point for theoretical calculations to explain the electronic structure and properties of these interfaces. In addition, studying the changes in surface reconstruction geometries of these interfaces with different AM coverages and under annealing will provide information on the adsorption process. The substrate surface which was chosen to use in the AM/semiconductor interface studies is the Si(111)2x1 surface. The 2x1 surface reconstruction is formed by cleaving along the (111) plane of crystalline Si in vacuum. The Si(111)2x1 surface is semiconducting with important surfacemore » states in the energy gap that can be altered by AM adsorption. In addition, while the Si(111)2x1 reconstruction is energetically stable for the clean surface, the total energy difference between this reconstruction and the ideal bulk-terminated Si(111)1x1 surface is small enough that room-temperature AM adsorption can revert the Si(111) surface structure to bulk-terminated. As with the Si(111) wafer surface, annealing the cleaved AM/Si(111) interface produces a 3x1 Si surface reconstruction for Na and K adsorption. The AM adsorbates used for this thesis will be K, Na, and Cs. The authors have found a number of previously unobserved reconstructions for room-temperature adsorption of each of the three alkali metals on the Si(111)2x1 surface. The results observed for K and Cs adsorption have several similarities, and are different from those for Na. The three AM adsorbates span a range of atomic sizes, electronic structures, electronegativities, and interaction strengths, and from their adsorption behavior the importance of these factors can be determined. The different sizes of adsorbate atoms affect the choice of adsorption site as well as the adsorbate-adsorbate spacing and therefore the periodicity and electronic structure of the interface. In terms of the experimental approach, a two-step regimen of several synchrotron experimental techniques is applied to the above AM/Si systems. In the first step, the authors utilize core-level and valence-band photoelectron spectroscopy (PES) in conjunction with LEED and secondary electron cutoff (work function difference) measurements for a series of increasing AM coverages upon the cleaved Si(111) surface. This provides a detailed picture of the electronic structure changes of the interfaces with adsorbate coverage through a structural transition. The second step of the regimen is the structural study via back-reflection X-ray standing waves (XSW). XSW is a synchrotron technique which determines the distance of adatoms from a chosen diffracting plane. XSW is performed on both the 111 and 11{bar 1} diffracting planes of Si, which are normal to and mostly lateral to the Si(111) surface, respectively. The resulting distances from the two planes can be triangulated to determine the adsorption site of the adatoms.« less