E. Ettedgui

Energy level bending and alignment at the interface between Ca and a phenylene vinylene oligomer

We have studied the interface formation of a vinylene phenylene oligomer with a Ca substrate using photoemission spectroscopy. The evolution of core and valence spectra during the deposition of the oligomer on Ca has indicated the molecular energy level bending. The total energy level bending was 0.5 eV and the thickness of the level bending region was about 100 A. We propose an energy level diagram of the oligomer‐Ca interface based on the information obtained from the photoemission spectra.

Band bending modified tunneling at metal/conjugated polymer interfaces

Metal/polymer interfaces play an important role in polymeric light‐emitting diodes. Recent transport measurement studies by Parker promote a rigid band model with triangular barrier tunneling at the metal/polymer interface. This finding, however, stands in contradiction to the band bending and Schottky barrier formation observed using surface analytical techniques. We found that this apparent contradiction can be nicely reconciled if a modification of the tunneling model by band bending is included in the interpretation of the transport data. Band bending modified tunneling gives a clear physical picture of the tunneling process across the metal/polymer interface and is important for low field tunneling processes.

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...

The surface species of poly (p-phenylene vinylene) and their effects on metal interface formation

Abstract We used X-ray photoemission spectroscopy (XPS) to investigate the surface species of poly ( p -phenylene vinylene) (PPV) and its interface formation with Ca and Al. PPV surface compositions varied with sample preparation. For relatively ‘clean’ surfaces with 4–5% O, analysis of the O( Is) peak revealed four types of oxygen species, namely carbonyl (C=0), hydroxyl (C-OH), ether (C-O-C) and the carboxylic groups (HO-C=O). The oxygen groups, excluding ether, reacted with Al or Ca to form the corresponding metal oxides. Chemical interactions between the metals and the phenylene and vinylene units to yield new species were not detected. For sulfur-free surfaces, a C(Is) peak shift of +0.5 eV followed the deposition of 15-30 A of Ca on PPV. For sulfur-containing surfaces, the C(Is) peak shift was -0.5 eV. We attribute this difference to the interaction of metal atoms with the sulfur impurities. For Al/PPV, a C(Is) peak shift occurred at

Schottky barrier formation at the Ca/poly(p-phenylene vinylene) interface and its role in tunneling at the interface

Abstract We have studied the formation of the Ca/poly( p -phenylene vinylene) (PPV) interface using X-ray photoelectron emission spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS). The rigid shift seen by XPS in the C(ls) core level spectra of PPV points clearly to charge transfer and Schottky barrier formation at the interface as a result of Ca deposition. The NEXAFS spectra suggest that charge transfer at the metal/polymer interface induces the formation of a bipolaron lattice at the interface. Taken together, these results imply that the metal/polymer interface is not rigid and that triangular barrier tunneling fails to take into account the effect of barrier formation. We propose a band bending modified tunneling model to explain charge transfer at the Ca/polymer interface.

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.

X‐ray photoemission study of the interface formation between calcium and self‐assembled alkanethiol monolayers

We have investigated the interface formation of Ca with an alkanethiol CH3(CH3)17SH monolayer self‐assembled onto gold‐coated silicon. Deposited Ca neither diffused into nor interacted chemically with the monolayer. Core‐level shift and work function changes were observed in the monolayer as soon as Ca was deposited and they were stabilized after 8 A of Ca coverage. The shifts are interpreted in terms of charge transfer from Ca to the monolayer, creating a dipole layer at the interface.

Comparative Study of Light-Emitting Porous Silicon Anodized with Light Assistance and in the Dark

In this study, we compare two different types of light emitting porous silicon (LEpSi) samples: LEpSi anodized in the dark (DA) and LEpSi anodized with light assistance (LA). On the basis of photoluminescence (PL), Raman, FTIR, SEM, spatially resolved reflectance (SRR) and spatially resolved photoluminescence (SRPL) studies, we demonstrate that the luminescence in LA porous silicon is strong, easily tunable, very stable and originates from macropore areas. These attractive properties result from passivation by oxygen in the Si-O-Si bridging configuration that takes place during electrochemical anodization. In addition, we have been able to correlate light emission with the presence of crystalline silicon nanograins.

Post-Anodization Implantation and CVD Techniques for Passivation of Porous Silicon

Proper surface passivation is critical for achieving stable, efficient PL from light-emitting porous silicon (LEPSi). As-anodized LEPSi is passivated by hydrogen which desorbs at a temperature as low as 400 °C. For device purposes, it is necessary that porous Si can tolerate at least 450 °C for post anodization annealing/metallization steps. We have established that, if the hydrogen at the surface is substituted by oxygen, the resulting Si-O x passivation is significantly more stable. One way of achieving this is to implant low energy/low dose oxygen to form a thin coating of SiO 2 on the surface. Post implantation FTIR data report the absence of Si-H peaks. XPS data indicate the formation of nearly stoichiometric SiO 2 at the surface. Similar results were achieved by implanting with nitrogen to form Si 3 N 4 . As an alternative to implantation, we have deposited thin capping layers of SiO 2 , Si 3 N 4 and SiC by plasma-enhanced chemical vapor deposition (PECVD) which resulted in a similar degree of passivation. Wafers were pre-treated at 400 °C to remove hydrogen from the surface. Finally, we carried out a low-pressure CVD (LPCVD) oxide deposition on LEPSi. Post implantation/CVD annealing was done at temperatures up to 600 °C. In most cases, little or no change was observed in the resultant PL intensities.

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...