UHV Surface Science and IR Spectroscopy

Ultra-High-Vacuum Surface Science

Dr. Victor Bermudez, victor.bermudez@nrl.navy.mil

Left Panel: Surface-sensitive ELS data (in first-derivative form for β-Si₃N₄ showing the removal of a surface-state feature at 4.6cV when the clean surface is given a small O₂ exposure (2x10^-5 Torr-sec). The interband peak at 6.4eV and the volume pasmon loss at 19.3eV are unaffected by O₂ exposure.
Right Panel: Computed surface band structure for the β-Si₃N₄ surface showing an empty surface at 4.3cV above the valence band maximum. This state is derived from the anti-bonding π* orbitals of Si=N groups formed on the clean surface.

Recent effort in the area of UHV surface science has focused on the wide-bandgap semiconductors GaN and SiC and insulators ß-Si₃N₄ and ß-Ga₂O₃. The interest is in the physical and electronic structure of the surfaces, chemisorption phenomena, metal contact formation and functionalization with organic species. The techniques used include ultraviolet and x-ray photo-emission spectroscopies, Auger electron spectroscopy, electron energy loss spectroscopy and low-energy electron diffraction. A fully-instrumented UHV chamber is available in-house for this work. Experiment and theory are closely coupled in this research. Ab initio calculations of surface energies, band structures, etc. are routinely performed to analyze the spectroscopic results.

2006

• The Structure of Low-Index Surfaces of β-Ga₂O₃, Chem. Phys. 323, 193 (2006).
• Functionalization of Indium Tin Oxide, Langmuir 22, 11113 (2006).

2005

• Theoretical Study of the Electronic Structure of the Si₃N₄ (0001) Surface, Surf. Sci. 579, 11 (2005).

2004

• Preparation and Properties of Clean Si₃N₄ Surfaces, Appl. Surf. Sci. 235, 406 (2004).
• Theoretical Study of Hydrogen Adsorption on the GaN(0001) Surface, Surf. Sci. 565, 89 (2004).

2003

• Adsorption of 1-Octanethiol on GaN(0001), Langmuir 19, 6813 (2003).

2002

• Functionalizing the GaN(0001)-(1x1) Surface: the Chemisorption of Aniline, Surf. Sci. 499, 109 (2002).
• Adsorption and Photodissociation of 4-Haloanilines on GaN(0001), Surf. Sci. 519, 173 (2002).
• Band Bending and Photoemission-Induced Surface Photovoltages on Clean n- and p-GaN(0001) Surfaces, Phys. Rev. B (Rapid Commun) 66, 121308 (2002).

Surface Infrared Spectroscopy

Dr. Victor Bermudez, victor.bermudez@nrl.navy.mil

Left Panel: Infrared reflection-absorption spectra for a polycrystalline film of Si grown on a CoSi₂ film on a Si(200) wafer. Data are shown for a series of XeF₂ doses at room temperature. The various Si-F bending and stretching modes associated with SiF, SiF₂ and SiF₃ groups are labeled. With increasing dose, first SiF, then SiF₂, and finally SiF₃ are seen.
Right Panel: Data obtained under steady-state conditions in a constant flux of XeF₂. Below ~500K all SiF_x species are present. The etching reaction proceeds via Si → SiF → SiF₂ → SiF₃ → SiF₄. The SiF₄ is volatile but can be detected at 300K. Above ~600K, SiF₂ desorbs rapidly and only SiF can be detected. Thus the etching mechanism is controlled by the thermal stability of adsorbed SiF₂.

Infrared spectroscopy is being used to study the vibrational spectra of adsorbed species in vacuum, non-vacuum and liquid environments. The focus is on the study of surface reactions and on identifying both strongly-adsorbed stable species and also reaction intermediates and weakly-adsorbed moieties that are present only under steady-state conditions. The materials of interest are primarily semiconductors and dielectric materials in bulk, thin-film or nano-structure form. Experiments in vacuum or in the presence of gas-phase reagents use primarily reflection-absorption spectroscopy. In the latter case polarization modulation is used to suppress the effects of IR absorption by the gas-phase ambient. For materials in contact with liquid reagents, attenuated total reflection is used together with a specially-designed flow cell. Two FTIR spectrometers are available, one mated with a UHV chamber and one equipped with external optics for the flow-cell experiments. This work is supported by a capability for performing ab initio quantum-chemical calculations of the adsorbate vibrational modes.

2011

• Effects of Molecular Adsorption on the Electronic Structure of Single-Layer Graphene, Langmuir, Vol 27 Issue 17, pp 11026-11036 (2011).

2010

• Effect of Humidity on the Interaction of Dimethyl Methylphosphonate (DMMP) Vapor with SiO₂ and Al₂O₃ Surfaces, Studied Using Infrared Attenuated Total Reflection Spectroposcopy, Langmuir 26, 23, 18144-18154 (2010).

2008

• Infrared Spectroscopy and Surface Chemistry of β-Ga₂O₃ Nanoribbons, Langmuir. 2008

2006

• Infrared Spectroscopic Study of O₂ Interaction with Carbon Nanotubes, Langmuir 22, 2258 (2006).

2005

• Wet-Chemical Treatment of Si₃N₄ Surfaces Studied Using Infrared Attenuated Total Reflection Spectroscopy, J. Elec-trochem. Soc. 152, F31 (2005).
• Adsorption on Carbon Nanotubes Studied Using Polarization-Modulated Infrared Reflection-Absorption Spectroscopy, J. Phys. Chem. B 109, 9970 (2005).

2003

• Computational Study of Cycloaddition Reactions on the SiC(100)-c(2x2) Surface, Surf. Sci. 540, 255 (2003).

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