Ken T. Park

Interface formation of Ca with poly(p‐phenylene vinylene)

We have investigated the interface formation of Ca with poly(p‐phenylene vinylene) (PPV) using x‐ray photoemission spectroscopy. The most significant part of our findings was the very late occurrence of band bending at the Ca/PPV interface and the lack of strong reaction between the Ca overlayer with the PPV substrate, indicating that the Schottky barrier formation in Ca/PPV was a slow process. The late barrier formation at the Ca/PPV interface may be due to the shielding by surface oxygen impurities, part of which underwent calcium oxide formation at the interface.

X-ray photoemission investigations of the interface formation of Ca and poly(p-phenylene vinylene)

We have investigated the interface formation of Ca with poly(p‐phenylene vinylene) (PPV) using x‐ray photoemission spectroscopy (XPS). The most astonishing result of the investigations is that the Schottky barrier formation in Ca/PPV is a slow process possibly caused by the oxygen and sulfur impurities segregated on the PPV surface.

An x‐ray photoemission spectroscopy study of the role of sample preparation on band bending at the interface of Al with poly(p‐phenylene vinylene)

We report on our recent x‐ray photoemission spectroscopy investigations of the interface formation of Al with poly(p‐phenylene vinylene) (PPV) prepared under various conditions. We have found that during deposition Al reacts with residual hydroxyl groups in the polymer. In addition, we have found that Schottky barrier formation and the associated band bending depend strongly on surface preparation. Samples converted in situ, containing 5% surface oxygen, show band bending that depends on the thickness of the Al overlayer, with effects arising after as little as 1 A Al. By contrast, a sample converted ex situ, with 10% surface oxygen, is insensitive to aluminum deposition. In view of the results obtained, we feel that surface impurities and adsorbed species may delay Schottky barrier formation by acting as a buffer layer which prevents the PPV substrate from interacting with the growing layer of Al. In the in situ samples where band bending occurs, we find that it takes place after the formation of metalli...

Photon‐assisted oxidation of the GaAs(100) surface using water at 90 K

Photoelectron spectroscopy is used to study the interaction of H2O with GaAs(100) at 90 K and to assess its use as a photon‐assisted oxidizing agent. The condensation of H2O at 90 K produces a thin physisorbed layer on GaAs(100). We found that intense synchrotron radiation causes most of the water to desorb in a sequential manner, without reacting with the substrate. The fraction of water that does not desorb reacts with Ga, giving rise to a Ga oxide. The As present in the substrate does not react with H2O during the process, thereby avoiding the formation of volatile As compounds.

Photon‐assisted nitridation of GaAs(100) at liquid‐nitrogen temperature

We show that nonmonochromatic synchrotron radiation promotes reactions at liquid‐nitrogen temperature (∼80 K) between NH3 and molecular beam epitaxy grown GaAs(100) which produces a thin nitride film. The photon‐assisted reaction causes changes in the valence band and core level photoemission spectra from GaAs(100) which are similar to those reported for nitridation by a nitrogen plasma.

The Surface Species of Poly(p-Phenylene Vinylene) and Their Effects on Calcium Interface Formation

Abstract X-ray photoemission spectroscopy (XPS) was used to investigate the cespecies of poly(phenylene vinylene) (PPV). PPV samples with 1% S and 5-10% O impurities were obtained whenever the corresponding sulfonium precursor polymers were subjected to XPS scans prior to thermal conversion. Relatively “clean” PPV surfaces with 4-5% oxygen atoms as the only detectable impurities were obtained by performing the thermal conversion in an argon circulating oven or in ultra high vacuum at 320°C, without prior XPS scan. Therefore the S impurities were resulted from x-ray induced chemical reactions which might involve the formation of sulfur ylides. By resolving the O 1s peak, four types of oxygen species were identified, namely carbonyl (C =O), hydroxy (C-OH), ether (C-O-C) and the carboxylic groups (HO-C=O). The oxygen groups reacted With Ca to form CaO. A reversed surface band bending was found for the S containing PPVs with respect to that for the S free PPVs during the Ca interface formation. Our overall re...

Growth of Al nitride layers on GaAs(100) by reaction with condensed ammonia

An Al nitride‐GaAs(100) bilayer formed by deposition of Al into a solid ammonia overlayer on GaAs(100) at T=100 K is studied using synchrotron radiation photoemission. Al does not react with NH3 to an appreciable amount at 100 K, but stable AlN layers are formed as the temperature of the substrate is raised to room temperature. The Al1−xNx layer formed is stable upon annealing up to 600 K, and detectable amounts of AlAs are not observed during deposition of 20 A of Al into NH3 or after heating the sample to 300 K and higher.

Photoemission spectroscopy study of thin Cr overlayers on NH3/GaAs(100)

The interface formation of Cr/NH3/GaAs(100) and its temperature dependence using synchrotron radiation photoemission and resonance photoemission spectroscopies have been investigated. We observed that at T=90 K the initial deposition of Cr stimulated nitridation of the GaAs substrate and subsequent interface reaction was characterized by Cr‐induced substrate disruption. Annealing of the thus formed interface to room temperature and above caused further Cr–GaAs reaction. The metallicity of the Cr overlayer was examined using constant initial‐state spectroscopy (CIS) of Cr 3d with Ei=1.7 eV below the Fermi level. The CIS spectra showed a maximum at the photon energy hν=50.0 eV followed by a second broad maximum centered at hν=56–58 eV depending on the stage of the interface formation. As the nominal Cr coverage increases from 1 to 10 A, the valence band photoemission spectra show a shift of the Cr 3d band toward the Fermi level. At the same time, the second maximum in CIS gradually dominates over the first,...

Temperature-dependent x-ray photoelectron diffraction of Pb(100) : experimental results and the single-scattering cluster model

An angle‐resolved x‐ray photoelectron diffraction study of the temperature‐dependent surface structure of Pb(100) is performed. Below 550±11 K, the decrease in forward‐scattered intensity attenuation is exponential with temperature. The intensity attenuation of the forward‐scattered peaks is much larger than predicted by both single‐scattering and multiple‐scattering x‐ray photoelectron diffraction models for a bulklike lattice. Corrections for surface relaxation and thermal expansion in the single‐scattering cluster model result in an increase in the predicted intensity attenuation; however, sufficient agreement between experiment and calculation is not achieved.

Growth of a Cr oxide layer on GaAs(100) by oxidation with condensed water

A Cr oxide layer produced by the deposition of Cr onto a condensed H2O overlayer on a GaAs(100) wafer at T=90 K was studied using synchrotron radiation photoemission. Cr reacted with H2O immediately upon deposition at low temperature resulting in the synthesis of hydrated Cr oxide compounds, and then formed a stable dry oxide as the temperature of the substrate was raised toward room temperature and eventually 400 °C. The oxide was stable up to a temperature of at least 400 °C, although the substrate did show changes indicative of reaction.