F.S. Tautz

ULTIMATE RESOLUTION ELECTRON ENERGY LOSS SPECTROSCOPY AT H/SI(100) SURFACES

We have re-investigated the well-studied hydrogen covered Si(100) surface using high resolution electron energy loss spectroscopy (HREELS) with a spectral resolution down to 13 cm1, representing an improvement by a factor of 3–4 compared to earlier work. For the first time in HREELS, it has been possible to clearly distinguish between the stretching vibrations of H-Si-Si-H units on monohydride surfaces with a 2×1 low energy electron diffraction pattern, on the one hand, and SiH2 units on dihydride surfaces with 1×1 symmetry on the other hand, the two being shifted by 5 cm-1u2009with respect to each other, in good agreement with results from infrared spectroscopy. Furthermore, we find trihydride units even for relatively low exposures beyond monohydride saturation coverage from their distinct stretching frequencies, and this points towards early etching stages. The question of the scattering mechanism applicable in our experiments is discussed. Since there are no spectral limitations in HREELS, we can analyze ...

A comparison of the chemisorption behaviour of PTCDA on different Ag surfaces

Abstract We have studied the adsorption of the molecular semiconductor 3,4,9,10-perylene-tetracarboxylic-acid-dianhydride (PTCDA) on a range of silver substrates, including Ag(1xa00xa00), Ag(1xa01xa00), Ag(1xa01xa01), and Ag(7xa07xa05). In all of these cases, special attention was placed upon the chemical bonding between the organic molecule and the metal surface, as this is of fundamental importance for the understanding of the organic–inorganic interface. We demonstrate that high resolution electron energy loss spectroscopy (HREELS) in the vibrational energy range is able to provide detailed insight into the intricacies of molecular chemisorption, including electron–vibron coupling effects. Specifically, by analogy with potassium-doped thin films of PTCDA we prove that the Ag(1xa01xa01) surface imparts metallicity to PTCDA monolayers.

Differences in vibronic and electronic excitations of PTCDA on Ag(111) and Ag(110)

We have investigated ultrahigh vacuum (UHV)-sublimated films of the model molecular semiconductor 3,4,9,10-perylene-tetracarboxylicacid-dianhydride (PTCDA) on Ag(110) and Ag(111) surfaces by high resolution electron energy loss spectroscopy (HREELS) in the loss energy ranges of both vibrational and electronic transitions for film thicknesses from submonolayers up to multilayers. Marked differences are observed not only between (sub)monolayer and multilayers but also between the monolayers on Ag(111) and Ag(110) which exhibit different layer-structures. With the aid of density-functional-calculations (DFT) of the vibrational modes details of the intermolecular and the molecule-substrate interaction can be derived. Electronic transitions similar to those found by optical spectroscopy are detectable only for molecules beyond the monolayer. In the (sub)monolayer range the electronic transition signature is drastically changed, showing much narrower gaps which, moreover, are different for the two surfaces.

Strong K-induced changes in perylene-tetracarboxylic-dianhydride films on Ag(1 1 0) studied by HREELS and LEED

Abstract Using high-resolution electron energy loss spectroscopy (HREELS) and low energy electron diffraction (LEED) we have studied the strong structural, electronic, and vibrational changes occurring in K-“doped” films of perylene-tetracarboxylic-dianhydride (PTCDA) on Ag(1xa01xa00) as a function of K concentration and annealing temperature. Two major phase transitions were observed in the vibrational and electronic excitations as well as in the LEED patterns. At low K-coverages (∼0.5 K/PTCDA) mainly structural changes are observed. Enormous changes occur for K-concentrations of 1 to 2 K atoms per PTCDA molecule which are tentatively attributed to the formation of a charge transfer complex leading to a semiconductor–metal transition. This state is characterized by a drastic intensity increase of vibrational modes at 392, 532, 1042, 1210 and 1540 cm−1 and two low-energy electronic excitations at 0.6 and 0.9 eV, which are tentatively assigned to K2(PTCDA) and K(PTCDA) complexes. The second transition, perhaps a metal–semiconductor transition, occurs at higher K concentration and/or annealing temperature and is correlated with the abrupt disappearance of the prominent spectroscopic features related to the charge transfer state.

Substrate influence on the ordering of organic submonolayers: a comparative study of PTCDA on Ag(110) and Ag(111) using HREELS

Vibrational spectra of ultrathin films of perylene-tetracarboxylic-dianhydride (PTCDA) on the more open Ag(110) surface and the close-packed Ag(111), as recorded by high-resolution electron energy loss spectroscopy (HREELS), are reported. By comparison to density functional calculations of the vibrational frequencies most experimental peaks can be reliably assigned. The internal vibrations of the molecules turn out to be very sensitive towards the effects of the interaction with the substrate. This leads to pronounced differences in the submonolayer spectra from both surfaces. Thus, the results bear some importance in the context of organic epitaxy, which relies on the formation of highly ordered interfaces.

Reassessment of core-level photoemission spectra of reconstructed SiC(0001) surfaces

Abstract In this letter, we present core-level photoemission spectra recorded from the (√3×√3)R30° and the (3×3) reconstructions of the SiC(0xa00xa00xa01) surface. The results are in agreement with published structure determinations by dynamical low energy electron diffraction analysis for these surfaces, yielding the T4-adatom and twisted-adlayer models, respectively. Specifically, we show that one of the surface-shifted core-level C 1s peaks for the (√3×√3)R30° phase is contamination induced. Our high-resolution electron energy loss spectroscopy experiments, which show a chemical shift between the SiH stretch vibrations on the hydrogen exposed (√3×√3)R30° and (3×3) surfaces, are also consistent with the most recent structural models for these two surfaces, since they prove exclusive silicon termination of both surfaces.

Polarity, Morphology and Reactivity of Epitaxial GaN Films on Al2O3(0001)

The surfaces of basal plane oriented GaN films heteroexpitaxially grown on sapphire, Al2O3(0001), were investigated using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS). The orientation of facets induced by annealing at elevated temperatures was determined from the extra LEED diffraction spots emerging from the surface. Based on angle dependent XPS data the orientation of our samples could be determined to be GaN(0001¯), the nominally N terminated face. This observation is corroborated by the chemical reactivity of the surface which was investigated by HREELS. Water from the residual gas adsorbs dissociatively with the hydrogen binding exclusively to N-sites as indicated by the vibrational spectrum. Similarily, upon exposing the surface to atomic hydrogen primarily N–H stretching vibrations are observed initially, while the Ga–H vibration becomes prominent only after the N–H peak has saturated. A reconstructed (3 × 3) phase develops upon heating the sample at moderate temparatures under simultaneous Ga exposure which again backs our orientation assessment as this (3 × 3) phase had previously been attributed to GaN(0001¯).

Surface plasmons at MOCVD-grown GaN

We report on the first observation of surface plasmons at a GaN surface by high-resolution electron energy loss (HREEL) spectroscopy. Dipole scattering theory using self-consistent electron density profiles is employed to calculate the surface energy loss function and hence the expected energy loss spectra. The characteristic features of both the surface plasmons and the Fuchs-Kliewer (FK) surface optical phonons are well reproduced in the framework of this model. Fitting measured HREEL spectra allows us to give estimates of important semiconductor parameters such as electron density, electron mobility and band bending. Moreover, it is found that the plasmon damping strongly depends on primary beam energy. Similarly, the FK phonon damping and frequency are also affected by the variation of primary beam energy.

Investigation of modified 3C SiC(100) surfaces by surface-sensitive techniques

Abstract We studied 3C-SiC (100) surfaces with various reconstructions by high-resolution electron energy-loss spectroscopy (HREELS), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). After annealing at 1325 K oxygen desorbs and a Si-terminated (2 × 1) surface is obtained. Subsequent annealing at higher temperatures leads to the sublimation of Si, which results in a carbon-rich c(2 × 2) surface. We exposed (2 × 1) and c(2 × 2) surfaces to atomic hydrogen, and were able to observe the Hue5f8Siue5f8H stretching vibration on silicon carbide.

Structural, vibrational and electronic properties of faceted GaN (0001̄) surfaces

Abstract We report on our investigations of the structural, vibrational and electronic properties of GaN epilayers grown on sapphire(0001). By a combination of high-resolution electron energy-loss spectroscopy (HREELS) and angular-resolved X-ray photoelectron spectroscopy (ARXPS) the orientation of the layers is determined to be (0001). Parts of the surface contain facets that are made of (1012)-type planes, as found using low-energy electron diffraction (LEED). We observe the dissociative adsorption of water from the residual gas at the GaN(0001) surface. Nue5f8H and Gaue5f8H stretching vibrations observed in hydrogenation experiments can be correlated to adsorption on terraces and facets, respectively. Finally, we report the observation of the conduction-band surface plasmon and use theoretical simulation of the HREEL spectra to extract important semiconductor parameters such as band bending, doping level and electron mobility.