Geometric and electronic structure of reconstructed semiconductor surfaces;high-resolution photoemission spectroscopy (PES)
The combination of high-resolution photoemission spectroscopy (PES) using synchrotron radiation and reflection high-energy electron diffraction (RHEED), along with other techniques, have been used to examine the atomic-scale geometric and electronic properties of clean and adsorbate-covered semiconductor surfaces. The surfaces studied have been probed via core level, valence band,. and angleresolved photoemission spectroscopy, and with the extended photoemission fme stlucture technique. The surface core levels and swface states on the Si( 111 )-(7 x 7) surface have been used to examine submonolayer deposits of Ge onto this surface. Knowledge of the initial stages of intetface formation in these systems is impmtant due to their application in heterostructure device physics. The decomposed Si-2p and Ge-3d core levels and angle-integrated valence band spectra have been analyzed as a function of Ge coverage and annealing temperature. The results suppmt the assignment of the lower binding energy component in both the Si-2p and Ge-3d cores to adatom emission. The implications with respect to adatom- to- rest-atom charge transfer are discussed. Lead adlayers on the ( 111) surfaces of Si and Ge have been examined as well. The surface phase diagram of Pb on these substrates exhibits interesting propetties which arise in patt due to the differing degrees of lattice matching ( - 4% for Pb/Si,<1% for Pb/Ge) and the nature of the clean surface reconstructions [(7 x7) vs. c(2 x8)]. Atomically abrupt interfaces are insured for these systems since Pb is insoluble in these surfaces even for temperatures well beyond the Pb melting point (340°C). Thus, they ru·e considered ideal systems to study with regru·d to metalsemiconductor intetface formation and intetface-dependent Schottky batTier studies, in conu·ast to reactive systems where intermixing usually occurs. This lack of interdiffusion has also allowed studies of the 2D melting of the Pb overlayers. The growth, desorption, Schottky-barrier heights, and atomic structure of the Ph-induced phases on Si and Ge are investigated via synchrotron radiation photoemission spectroscopy, along with other techniques such as reflection high-energy electron diffraction and Auger electron spectroscopy (AES). Examining the differences between these closely related systems nicely illustrates the complex interrelationship between the atomic and electronic structure of reconstructed surfaces. The photon energy dependence of the surface- and bulk-derived core level components of the Si 2p core level, acquired from the Si( 111 )-(7 x 7) smface, have been carefully examined using high-resolution synchrotron radiation photoemission. The intensities of the core levels are found to vary in a way not fully describable by the classical layer attenuation model. The physics responsible for the discrepancy is the extended photoemission fme stmctme (EPFS) above the Si 2p edge. Bulk EPFS is found to modulate the surface-to-bulk intensity ratios up to 50%. This result means that structural models based on a surface-to-bulk intensity ratio analysis using only a few photon energies may s~ffer large errors, and in fact this EPFS effect may be responsible for the ongoing debate in the literature over the assignment of the surface core levels to stmctmal features on both the ( 111) and (100) smfaces of Si and Ge. This dilemma is resolved for the Si(111)-(7 x7) smface by isolating the smface EPFS signals from the bulk, thereby corroborating previous work which assigns the S2 component to adatom emission.
geometric structure, electronic structure, reconstructed semiconductor surfaces, synchrotron radiation, reflection high-energy electron diffraction (RHEED)
University of Illinois - Urbana-Champaign
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