J. T. McKinley

Thiophene on Si(111)2×1: Synchrotron radiation study of a desulfurization process

Abstract The adsorption and desulfurization of thiophene on cleaved silicon was studied at different temperatures. For substrate temperatures of 60–85 K, we found the co-existence of two different adsorption states at low exposures, which yield to a condensed thiophene multilayer at high exposures. For room-temperature substrates, we observed a desulfurization process. The process is probably followed by further fragmentation, and the fragmentation path depends on the substrate preparation process.

Mo(CO)6 on Si(111)2 × 1: a synchrotron radiation-excited photoemission study

A synchrotron radiation-excited photoemission study of the adsorption behaviour of Mo(CO)6 on Si(111)2 × 1 is reported. The compound has been investigated in the temperature range 50 K-room temperature (RT), and in the exposure range 0.1-200 L (1 Langmuir = 10-6 Torr s-1). Both core (Mo 3d, C 1s, Si 2p, O 1s) and valence levels were investigated. At T ≤ 100 K and exposures ≥0.5 L, the adsorption of Mo(CO)6 on cleaved Si is molecular, as clearly indicated by a 1: 1 correspondence of the features in the valence band (VB) spectrum with those already reported for free Mo(CO)6, at the same photon energy. At T < 100 K and very low exposures (0.1 L) we found evidence of a decomposition process taking place. At T ≥ 150 K, the adsorption is dissociative at exposures < 120 L and molecular at higher exposures. As the temperature increases up to 200 K, only dissociative processes are detected, also at high exposures. At RT, no adsorption at all was detected. Reacted surfaces were exposed to zero-order light to promote metal deposition on Si. A metal layer on top of Si was obtained after short exposures. CO adsorbs on this layer and is partially dissociated. Molecular CO is removed by further exposure to zero-order light. The presence of atomic C and O from CO partial dissociation and of carbide species was detected.

From heterojunction interfaces to metal-semiconductor interfaces

Abstract Systematic studies of Schottky barrier heights and heterojunction band discontinuities have revealed a clear correlation between these two fundamental semiconductor interface parameters. The correlation is qualitatively predicted by all major semiconductor interface models, i.e., the defect model, theories based on metal-induced gap states (MIGS), and Schottky-like models. However, the experimentally observed correlation substantially deviates from the common prediction of all these theories. We investigated the causes of this discrepancy by measuring band line-ups at semiconductor-semiconductor interfaces with metal intralayers of thickness ranging from zero to back-to-back Schottky barrier configurations. The photoemission experiments discussed here identify the chemical and morphological properties of the CdS/Al/Ge system, and indicate that the cause of the discrepancy is a Schottky-like correction term.

Metal-GaAs(110) Interfaces Formed at Low Temperature: From Adsorbate- to Metal-Induced Gap States

After two decades of research on metal-semiconductor interfaces, the problem of the formation of Schottky barriers is still very much present (and controversial) 1. The origin and nature of the interface states which pin the Fermi level (EF) at metal-semiconductor interfaces remain uncertain. The difficulty in resolving this issue stems from the diversity of metal-semiconductor interfaces and from the complexity of their chemistry, morphology and atomic structure. Progress has been made in the understanding of the microscopic structure of some of these interfaces with low energy electron diffraction (LEED)2,3, ion channeling4, total energy minimization calculations5,6 and with the recent development of scanning tunneling microscopy7. Enough remains unknown, however, that interface structure, interdiffusion, defects or metallization still raise unanswered questions about the pinning process. In addition, Vitturo et al. have recently indicated that deep levels in the bulk of the materials used for most Schottky barrier formation studies, ie. liquid-encapsulated Czochralski GaAs, might be responsible for the narrow range of EF pinning in the gap, and that pinning occurs in a much wider range on (100) surfaces of substrates grown by molecular beam epitaxy8. This result adds another dimension to an already difficult and overly discussed subject.

Effects of Al and Ti Interlayers on Sb/(Hgcd)Te Interface Behavior

The effects of 0.1 and 0.5‐nm Al and 0.02‐nm Ti interlayers on the Sb/(HgCd)Te system have been investigated with photoelectron spectroscopy using synchrotron radiation. With no interlayer, the Sb forms an abrupt, uniform overlayer with a stoichiometric interface and causes no change in the band bending induced during the cleaving process. With the two Al interlayers Sb exhibits less uniform deposition and diffuses into the semiconductor enough to reverse the additional band bending caused by Al in‐diffusion. It also reacts with the elemental Al of the 0.5‐nm interlayer to form AlSb. The increased disruption of the (HgCd)Te surface by the Ti interlayer leads to enhanced out‐diffusion of Te in addition to Sb clustering and in‐diffusion. In this case, Sb is able to compensate for the inversion occurring during cleavage and returns the surface to a nearly flat‐band condition.

Low-Temperature Pyridine Adsorption on Cleaved Silicon - a Synchrotron Radiation Photoemission-Study

Note: Univ rome la sapienza,dept chem,i-00185 rome,italy. univ wisconsin,ctr synchrotron radiat,madison,wi 53706. univ wisconsin,dept phys,madison,wi 53706. Piancastelli, mn, univ rome tor vergata 2,dept chem sci & technol,i-00173 rome,italy.ISI Document Delivery No.: DR088 Reference LSE-ARTICLE-1990-018doi:10.1016/0038-1098(90)90897-K Record created on 2006-10-03, modified on 2017-05-12

Search for the 1+ State of Copper in the Electron Superconductor Nd2-Xcexcuo4

We used the resonant photoemission technique to search for Cu1+ resonances in the spectra of superconducting and non‐superconducting compounds in the Nd2−xCexCuO4 family. No evidence for such resonances was found. The possible implications of this somewhat suprising result are discussed.