Ultraviolet Photoemission Measurements Research Articles

Ultrathin-film differential-thermal-analysis apparatus with simultaneous photoemission measurements

We developed an apparatus of differential thermal analysis (DTA) capable of simultaneous surface specific ultraviolet (UV) photoemission measurements to investigate thin-film phase transitions. The apparatus was installed in a vacuum chamber of 10−6 Torr range for thermal isolation and the measurements of UV photoemission. As a sample substrate, we used a thin (10 μm) copper sheet supported by two wires for optimal thermal resistivity. The performance of the apparatus was examined using a 650-Å-thick pentacontane (n-C50H102) film, which may exhibit a unique monolayer phase transition known as surface freezing. We observed two anomalies of DTA curve around the bulk melting temperature, one of which is apparently due to the bulk melting. Since the temperature dependence of the surface specific UV photoemission measurements showed corresponding changes in photoemission current, we could conclude that the other phase transition peak originates from the surface freezing effect. This demonstrates that our DTA-UV apparatus is sufficiently sensitive to examine such monolayer phase transitions.

Band alignment at the organic-inorganic interface

The band alignment of the bithiophene interface with a diverse range of substrates has been determined by a combination of ultraviolet photoemission and work function measurements. Not only is vacuum level alignment clearly shown to be invalid but also any sort of linear relationship between band alignment and substrate work function is shown not to be the case. Rather, the alignment is determined by the interface dipole, which is specific to the interaction at the inorganic-organic interface. The interface dipoles, which always appear, while dominated by the first monolayer interaction, are completed after two to three monolayers. As the ionization potentials of the films are shown to be constant, it is argued that a simple work function measurement, for an organic film on a particular substrate, quantifies the band alignment.

Differential thermal analysis and ultraviolet photoemission study on the surface freezing effect of n-alkane

Abstract We have performed differential thermal analysis (DTA) and surface-specific ultraviolet (UV) photoemission measurements to investigate the surface freezing effect of n -alkanes. The samples used in the experiments were pentacontane ( n -C 50 H 102 : 650 A) and tetratetracontane ( n -C 44 H 90 : 600A) films evaporated on a copper substrate. We have observed a monolayer phase transition by DTA and proved that the transition is of first order. Since the temperature dependence of the surface-specific UV photoemission measurement showed a stepwise increase and decrease of the photoemission current at the transitions observed in DTA, we could conclude that the monolayer phase transition peak originates from the surface freezing effect.

HOMO/LUMO Alignment at PTCDA/ZnPc and PTCDA/ClInPc Heterointerfaces Determined by Combined UPS and XPS Measurements

The offsets between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) at the 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)/chloroindium phthalocyanine (ClInPc) and PTCDA/zinc phthalocyanine (ZnPc) organic heterojunctions were estimated from a combination of X-ray and ultraviolet photoemission (XPS,UPS) measurements. This combined method allows an improved determination of the electronic structure of such organic/organic‘ interfaces due to the separate determination of the band bending (charge redistribution) following heterojunction formation. Both interfaces have large offsets in the onset for photoemission from their HOMO levels (PTCDA/ZnPc: 0.88 eV; PTCDA/CnlInPc: 0.93 eV). Using thin film absorbance data, the corresponding offsets in LUMO levels were estimated to be 0.66 eV (PTCDA/ZnPc) and 0.34 (PTCDA/ClInPc). The ZnPc/PTCDA interface showed a significant interface dipole (0.25 eV) while the ClInPc/PTCDA contact was essentially dipole free.

Determination of frontier orbital alignment and band bending at an organic semiconductor heterointerface by combined x-ray and ultraviolet photoemission measurements

The alignment of the highest occupied molecular orbitals (HOMO) at the tris (8-hydroxy quinoline) aluminum (Alq3)/N,N′-di-(3-methylphenyl)-N,N′diphenyl-4,4′-diaminobiphenyl (TPD) heterojunction, used in organic light-emitting diodes (OLED), was determined by growing a TPD layer in several steps on a thick Alq3 substrate layer. After each growth step the sample was characterized in situ by x-ray and ultraviolet photoemission spectroscopy. The offset of the HOMO maxima at the interface was determined to be −0.13 eV from Alq3 to TPD. By including the known HOMO–lowest occupied molecular orbital (LUMO) gaps for both molecules into the evaluation, the offset of the LUMO minima was determined to be −0.33 eV from Alq3 to TPD. These values are consistent with previous assumptions that this interface represents a higher barrier for electron injection from Alq3 to TPD than for hole injection from TPD to Alq3.

Energy level alignment at Alq/metal interfaces

The energy level alignment for both Mg/8-hydroxyquinoline aluminum (Alq) and Au/Alq interfaces has been determined by the ultraviolet photoemission measurements. For both interfaces, the difference between the Fermi level and the low-energy edge of the highest occupied molecular orbital (HOMO) is around 1.7 eV. This implies that the Fermi level with respect to the HOMO edge of Alq is independent of the work function of Mg and Au despite a large difference in the metal work function. A Fermi level alignment model is proposed, invoking a charge transfer between the metal and Alq and the formation of a dipolar layer at the metal/Alq interface.

Characterization of cobalt-diamond (100) interfaces: electron affinity and Schottky barrier

UV photoemission measurements were used to relate the electron affinity and Schottky barrier of thin Co layers on diamond (100) surfaces. Cobalt films of 2 Å thickness were deposited on natural single crystal diamond (100) substrates by hot filament evaporation in ultra-high vacuum (UHV). The surfaces were characterized with auger electron spectroscopy and atomic force microscopy. The study explores the properties of the cobalt-diamond interface as a function of different surface cleaning procedures. Prior to deposition the diamond samples have been cleaned by UHV anneals at either 500°C or 1150°C. Following either of these anneals a positive electron affinity was deduced from the ultraviolet photoemission measurements. The measurements indicate that the surface annealed at 500°C is terminated with oxygen while the surface annealed at high temperature is free of adsorbates. Upon deposition of Co on the surface heated to 1150°C, a negative electron affinity (NEA) was detected, and a Schottky barrier height of 0.35 eV was measured. However, for Co films deposited on substrates annealed to 500°C a positive electron affinity and a Schottky barrier height of 1.45 eV were observed. The results are discussed in terms of a model that relates the electron affinity to the metal workfunction and the Schottky barrier.

Theoretical study of optical and ultraviolet photoemission spectra of SrTiO 3

We present the results of the self-consistent ELAPW calculation of the band structure of SrTiO 3. The ground-state eigenvalues and wavefunctions obtained within the local-density approximation (LDA) are used to calculate optical spectra in the photon energy range up to 30 eV and the photoelectron energy distribution curves (EDC's) for ℏ ω=21.2 and 40.8 eV. The results are compared with optical, X-ray absorption and ultraviolet photoemission measurements. A good agreement with experiment is obtained.

Surface electronic structure of epitaxial √3 × √3R30° Er silicide on Si(111)

High-resolution angle-resolved ultraviolet photoemission measurements of ultra-thin (12 Å) epitaxial √3 × √3R30° ErSi1.7 films on Si(111) are presented. Band dispersions of typical surface states or resonances are followed along the high-symmetry Γ̅gGK̄ and K̄M̄ lines of the (1 × 1) surface Brillouin zone (SBZ). Extensive studies of two-dimensional p(1 × 1) Er silicide allowed us to fold back its band structure into the reduced √3 × √3R30° SBZ. A comparison of these data to the surface bands recorded on the ultra-thin lacunary ErSi1.7 indicates that this “bulk” silicide is terminated with a buckled Si layer quite similar to the surface silicide termination. Of particular interest are the conspicuous effects of folding back on relative intensities of the spectral features recorded with He I photon energy at equivalent high-symmetry points of the √3 × √3R30° SBZ.

Thermal effects on the dynamics and electronic states of Na3C60.

Raman-scattering and ultraviolet-photoemission measurements on ${\mathrm{Na}}_{3}$${\mathrm{C}}_{60}$ thin films indicate a transition at elevated temperatures to a system with increased metallic character. This is observed by a substantial increase in line widths and peak shifts of selected Raman ${\mathit{H}}_{\mathit{g}}$(i) modes accompanied by an increase in the density of states at the Fermi energy as measured by photoemission. Changes in these spectra with temperature, as well as differences with ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$, suggest that low-temperature nonmetallic behavior is due to octahedral Na site disorder.

Structural and electronic properties of K/Si(100)2 x 1.

The interface K/Si(100)2\ifmmode\times\else\texttimes\fi{}1 has been investigated by using a variety of surface techniques sensitive to both structural and electronic properties. The adsorption site of potassium was determined by using Xe titration and Xe photoelectron spectroscopy. Potassium atoms are preferentially adsorbed between the rows of dimers of the 2\ifmmode\times\else\texttimes\fi{}1 reconstruction (cave-valley sites). Controversy exists about the degree of ionicity of the K-Si bond, and the way the surface is metallized after K adsorption. Using work-function change, thermal desorption, and ultraviolet photoemission measurements, we determined that the potassium overlayer has metallic character above 0.5-monolayer (ML) coverage. We have studied by means of photoemission of adsorbed xenon the way the surface potential is affected by K adsorption. The charge transfer from K to Si gives rise to a reduction of the local work function at sites close to K atoms. In addition, a long-range reduction affects the whole surface from coverages of 0.2 ML on.

Surface and interface properties for the Cu/W(110) system and their effect on oxygen adsorption.

The results of angle-resolved ultraviolet photoemission measurements and electronic-structure calculations are presented for Cu adsorbed on the W(110) surface. Data as a function of Cu coverage suggest the existence of sets of surface and interface states resulting from the Cu-W interaction. Calculations corroborate this suggestion and identify the surface and interface character and orbital symmetries of the states. Near the zone boundary along the \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}-H\ifmmode\bar\else\textasciimacron\fi{} symmetry line, these states lie in gaps in the projected bulk band structure for the W(110) surface and are localized either within the Cu overlayer (surface states) or shared between the Cu and first W layer (interface states). Reasonable agreement is obtained between experiment and theory. We also performed low-energy electron diffraction and measured the behavior of the surface and interface states with ${\mathrm{O}}_{2}$ coverage. We conclude that the enhancement of the ${\mathrm{O}}_{2}$ dissociative sticking coefficient for the 1-monolayer Cu film results from an increase in the precursor-state accommodation coupled with the ability of Cu to displace easily giving ${\mathrm{O}}_{2}$ continued access to the more active W surface.

Role of dopants in the electronic structure of hydrogenated amorphous silicon

The experimental results of boron and phosphorus doping effects are reviewed for plasma CVD hydrogenated amorphous silicon (a-Si:H) films prepared under different conditions. The results analysed comprise (a) transport properties, (b) changes in the optical properties (optical absorption coefficient, refractive index and dielectric constant of the material) (c) role of hydrogen from infrared absorption hydrogen measurements, and (d) possible defect states from ESR measurements. These results are augmented with the coordinated X-ray and ultraviolet photoemission measurements, to derive the hydrogen/dopant-induced changes in the electronic structures of a-Si:H. Finally, based on these results some possible structural configurations for these dopants in a-Si:H under different conditions are envisaged.

Electronic and chemical properties of hydrogen exposed GaAs(100) and (110) surfaces studied by photoemission

Various aspects of the physics of the interaction of GaAs surfaces with H2 discharges are shown in this paper. Ultraviolet photoemission measurements have been done on (110) cleaved surfaces. Comparison of the core-level spectra with electronic structure calculations (molecular orbital model) shows that some of the Ga atoms on the hydrogenated surface have two dangling bonds, instead of one on the clean relaxed surface. This indicates that Ga–As bonds on the (110) surface have been broken by exposure to the hydrogen in our discharge conditions. In parallel, some new surface states in the valence-band region are detected which we have tentatively attributed to the formation of As–H bonds. On p-type (110) cleaved surfaces, the Fermi level is pinned near valence-band maximum (VBM) +0.4 eV after hydrogenation. In a second part of the paper, we describe x-ray photoemission spectroscopy results on the plasma treatments of GaAs(100) wafers in H2/AsH3 (deoxidation, Fermi level position as a function of the surface stoichiometry, etc.).

Photoemission study of single-crystalline(La1−xSrx)2CuO4

Ultraviolet and x-ray photoemission measurements have been performed on single-crystal ${({\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x})}_{2}\mathrm{Cu}{\mathrm{O}}_{4}$ with $x=0.0 \mathrm{and} 0.04$. Considerable differences are found in the photoemission spectra when compared with the results for sintered samples: a single-peak structure for the $\mathrm{O} 1s$ level and the absence of the peak at 9 eV of unknown origin reported for sintered samples, etc. Comparison is also made with band-structure calculations.

Electronic structure and properties of sputtered Ta-Cu films.

Films of ${\mathrm{Ta}}_{\mathrm{x}}$${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ have been produced in a co-sputtering system with dc- and rf-sputtering guns and a substrate attached to a rapidly rotating, water-cooled table. Samples were produced in the composition range 0.1\ensuremath{\le}x\ensuremath{\le}0.9. X-ray diffraction and electrical resistivity data between 4.2 and 300 K have been obtained. Diffraction measurements show that for 0.6\ensuremath{\le}x\ensuremath{\le}0.9 the samples are glassy. The room-temperature resistivities follow the Mooij correlation with a zero temperature coefficient of resistivity at a resistivity of 35 \ensuremath{\mu}\ensuremath{\Omega} cm. The resistivity-versus-temperature data for ${\mathrm{Ta}}_{0.90}$${\mathrm{Cu}}_{0.10}$ are compared to the extended Ziman-Faber theory and a localization theory by Kaveh and Mott. Of these two theories, the Kaveh-Mott theory best describes the experimental results. Ultraviolet and x-ray photoemission measurements have been made as a function of composition. The position of the Cu 3d levels appears to be correlated with the amorphous or crystalline character of the structure. A linear muffin-tin-orbital calculation of the electronic structure of amorphous ${\mathrm{Ta}}_{0.59}$${\mathrm{Cu}}_{0.41}$ has been performed. The calculation, based on a periodically extended amorphous cluster of 39 atoms, gave an electronic density of states in good agreement with photoemission results.

An angle-resolved photoemission study of the reaction of CH 3SH and (CH 3S) 2 with Cu(111) and Ni(100)

Angle-resolved ultraviolet photoemission measurements using He I radiation indicate that both CH3SH and (CH3S)2 adsorb molecularly on both Cu(111) and Ni(100) at low temperature (≈110 K), and react to produce a common surface species on heating to room temperature. This surface species is also formed in all cases by room temperature exposure, and is identified as methyl mercaptide, CH2S-. The photoemission spectrum of this species is compatible with C3v 01 symmetry, indicating that the S-C bond lies sufficiently close to perpendicular to the surfaces for 01 there to be no substantial loss of level degeneracy. A comparison of the reaction temperatures and reactivities of the two faces to the two adsorbates is discussed, particularly by comparison with the more widely studied formation of methoxy on similar surfaces.

Electronic structure, photoemission, and magnetism inGd2Co andEr2Co glasses

The electronic and magnetic structures of amorphous ${\mathrm{Gd}}_{2}$Co and ${\mathrm{Er}}_{2}$Co are studied by ultraviolet and x-ray photoemission measurements and band calculations. The spectra for both glasses show a band of Co 3d levels centered at a binding energy ${E}_{B}$ of about -1.0 eV and peaks arising from 4f levels at ${E}_{B}$=-8.4 eV and ${E}_{B}$apeq2-5--9 eV for Gd and Er, respectively. Magnetization data have shown that a-${\mathrm{Gd}}_{2}$Co possesses a ferrimagnetic structure with a small Co moment. The linear-muffin-tin-orbital method is used to perform self-consistent, spin-polarized calculations in the semirelativistic approximation for ${\mathrm{Gd}}_{2}$Co. Because of the complexity of the glassy structure, the calculations are performed for body-centered-tetragonal and cubic fluorite structures, and the sensitivity to local environment effects studied. The calculations are in a good overall agreement with the photoemission and magnetic-moment results.

Rare gas titration studies of Si(111) surfaces

Ultraviolet photoemission (UPS) measurements of the coverage-dependent electron binding energies of adsorbed argon and xenon atoms on Si(111) surfaces are used to obtain surface structural information. We distinguish three submonolayer types of adsorption sites on Si(111)-7×7 which fill sequentially and whose coverage relative to a full monolayer are determined with UPS: an initial site (∼4%), a majority site (∼76%), and another minority site (∼20%) which fills last. These results exclude most structural models for the 7×7 but are consistent with recent vacuum tunneling micrographs and the structure proposed by Binnig and Rohrer. The sensitivity of rare gas UPS binding energies to other structural differences induced by impurities, laser annealing, and sputtering are described.

Study of Electronic Structures of Ge(111)7 ×7-Sn and Ge(111) “2 ×8” Surfaces by Angle-Resolved UPS

Angle-resolved ultraviolet photoemission measurements have shown that the surface electronic structure of the Ge(111)7 ×7-Sn surface is essentially the same as that of the Si(111)7 ×7 surface. Metallic surface states of the Ge(111)7 ×7-Sn surface reveal a slight dispersion of an approximate 2 ×2 periodicity. The so-called dangling- and the back-bond surface states of the Ge(111)“2 ×8” surface also show a slight dispersion. The dangling- and the back-bond surface states of the Ge(111)7 ×7-Sn surface lie at nearly the same binding energies as those for the Ge(111)“2 ×8” surface rather than those for the Si(111)7 ×7 surface.