High-resolution Synchrotron Radiation Photoemission Research Articles

Formation of germanene with free-standing lattice constant

Two-dimensional sheets of ultra-thin germanium with the lattice parameter of free-standing germanene, i.e., 0.41 nm, are formed by atomic segregation epitaxy on face-centered-cubic Ag0.9Al0.1(111) thin films grown on a Ge(111) template. Low-energy electron diffraction clearly shows a square-like (√13 × √13)R ± 46° superstructure, expressed as (4114) in Matrix form, whose 92° angle between the two vectors is clearly observed, as well as incommensurate (1.41 × 1.41)R ± 27° spots. Scanning tunneling microscopy (STM) images show the square-like superstructure and a defective honeycomb-like arrangement with this characteristic 0.41 nm lattice constant, corresponding to LEED patterns. Complimentary high-resolution synchrotron radiation photoemission spectroscopy measurements reveal the electronic structure of the honeycomb-like germanene sheet and further demonstrate that it overlays the surface of the Ag0.9Al0.1(111) alloy.

(Invited) Thin Epitaxial Single-Crystal Si on Sige Followed By in-Situ Deposition of High-k Dielectrics – Novel Gate Stacks for Achieving Extremely Low Dit and Highly Reliable SiGe MOS

Silicon-germanium (SiGe) is replacing silicon (Si) as the channel layer for the p-type metal-oxide-semiconductor (pMOS) in the aggressively scaled complementary MOS (CMOS) technology. Many research efforts involving SiGe gate stacks have resulted in the attainment of low interfacial trap densities (Dits). Si-passivated SiGe gate stacks are the most promising method to achieve both low Dit and high device reliability. However, regardless of the growth methods, most publications reported the pileup of Ge atoms on top of the grown Si even after extensive treatments. The subsequent process in forming gate stacks, including oxidation, inevitably caused the segregated Ge to form GeOx, leading to degradation of the interfacial quality of the gate stacks.We have used molecular beam epitaxy (MBE) to grow thin single-crystal Si of 0.79nm thickness (six MLs) epitaxially on epi-Si1-xGex(001) under ultra-high vacuum (UHV). Different from its counterpart of chemical vapor deposition (CVD), MBE enables epi-Si growth on Ge at growth temperatures below 300 °C. The measurements using reflection high-energy electron diffraction (RHEED), high-resolution synchrotron radiation X-ray diffraction (HR-XRD), and the scanning transmission electron microscopy with high-angle annular-dark-field (STEM-HAADF) have revealed high crystallinity of the epi-Si, the Si1-xGex layers and abrupt interfaces of the high-k/epi-Si/Si1-xGex layers. Moreover, our unique capability of epi growth/analysis enabled the study of the electronic surface structures of epi-Si/epi-Ge using in-situ high-resolution synchrotron radiation photoemission spectroscopy (SRPES) via a portable sample transfer station under UHV. We have used SRPES to demonstrate that the as-grown Si1−xGex(001)-2×1 surfaces with Ge content ranging from 0.1 to 0.9 are all terminated with buckled Ge–Ge dimers, and the surface electronic structure is similar to that of epi-Ge(001)-2×1. The direct deposition of epi-Si on as-grown SiGe removed the top buckled Ge-Ge dimers, which diffused into the epi-Si and some segregated to the top of the Si. As a result, the topmost layer beneath the epi-Si was 1 ML of Ge, and the interfaces of high-k/epi-Si/Si1−xGex are similar to those of the high-k/epi-Si/Ge. Again, the hetero-interfaces of our gate dielectrics are different from those grown using CVD.The well-controlled interfaces have enabled the achievement of low interfacial trap densities (Dit) of (3 - 6) × 1011 eV-1cm-2 in these high-k/epi-Si/Si1-xGex samples. The minimum Dit values remained at 3 × 1011 eV-1cm-2 regardless of the Ge content, demonstrating the effective passivation of the low-temperature deposited epi-Si. The limited Ge segregation during the low-temperature growth of epi-Si may attribute to the attainment of the low Dit values. We have benchmarked the minimum Dit (Dit, min) of our work with other state-of-the-art SiGe gate stacks. The Dit,min values from the other efforts increased with increasing Ge content, which may be caused by the GeOx formation that was difficult to control in the high-Ge-content (HGC) Si1-xGex.The effective charge sheet densities for the Si1-xGex gate stacks were extracted via examination of capacitance-voltage (C-V) hysteresis with decreasing stress voltage in the accumulation region of the MOS capacitors. We have attained very high acceleration factors of 8-12, indicating high reliability of the HfO2/epi-Si/Si1-xGex pMOS gate stacks.Furthermore, we have used a scavenging method, consisting of a cyclic process of an O2 exposure followed by a UHV annealing, to eliminate the segregated and diffused Ge in epi-Si before high-k deposition. Upon O2 exposure at room temperature, both the Si and Ge surface atoms are simultaneously oxidized to give rise to four Si charge states and Ge suboxides, respectively. The subsequent in-situ annealing at 500 °C under UHV moved the oxygen atom in the Ge suboxides to bond with the nearby Si atom. The annealing also caused the diffused Ge inside the epi-Si to segregate to the surface. The processes of O2 exposure followed by annealing were repeated three times resulting in an oxidized Si/Ge surface having only the four Si oxidized states without GeOx, but with a minimal amount of segregated Ge.

Oxidation and hydrogenation of SiGe(0 0 1)-2 × 1 at room temperature and in situ annealing: A synchrotron radiation photoemission study

This study used high-resolution synchrotron radiation photoemission to investigate the epi SiGe(001)-2 × 1 surface that was exposed to atomic oxygen and hydrogen at room temperature, followed by in situ annealing at 500 °C. The SiGe(001)-2 × 1 surface that was grown by molecular beam epitaxy (MBE) was terminated with buckled Ge–Ge dimers, where the Ge dimers exhibited great similarity compared to those on epi Ge(001)-2 × 1. The exposure of atomic oxygen started from as low as 0.1 L up to 500 L, although the interfacial reaction practically stopped at 10 L of exposure. Atomic oxygen bonded only with the up-dimer atoms, while the atomic hydrogen passivated both dimerized atoms. Upon annealing, the GeOx oxides and GeHx hydrides disappeared completely, and the Ge region became similar to that of the clean surface. However, over half of the Ge–Ge dimers were removed from the surface after annealing in both interfaces. Heat caused the oxygen atoms in GeOx to move over to the Si region, thereby giving rise to 1+ to 4+ charge states of SiOx, which was similar to the case of the oxidation of c-Si(001)-2 × 1. The catalytic effect of Ge decreased the elevated temperature to, at most, 500 °C.

In-plane strain-free stanene on a Pd2Sn(111) surface alloy

Here we report the growth of laterally strain-free stanene, that is a two-dimensional (2D) honeycomb structure of tin atoms, on a Pd(111) crystal terminated by a ${\mathrm{Pd}}_{2}\mathrm{Sn}$ surface alloy. The atomic geometry and the electronic structure have been thoroughly investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), Auger electron spectroscopy, high-resolution synchrotron radiation photoemission spectroscopy, and advanced first principles calculations. The STM images clearly reveal the epitaxial growth of honeycomb stanene on the preformed ${\mathrm{Pd}}_{2}\mathrm{Sn}$ surface alloy. LEED patterns clearly show commensurate (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} spots, corresponding to a lattice constant of 0.47 nm, in perfect accord with the cell size of free-standing stanene. The measured very low buckling, within 20 pm, is derived from section profiles of atomic-scale STM images. This contrasts the significantly larger buckling of free-standing stanene, possibly due to a STM tip effect as well as a non-negligible interaction with the underlying surface alloy. The electronic structure exhibits a characteristic 2D band with parabolic dispersion, which is in good accordance with electronic structure calculations in density functional theory.

Photoemission Spectroscopy Study on Hydrogen Termination Effect on SiO2/Si Structure Fabricated Using H+-Implanted Si Substrate

Using proton (H+)-implanted silicon (Si) substrates, we clarified the effect of dangling bond termination by hydrogen on the interfacial strain in the silicon dioxide (SiO2)/Si system. The variations of the SiO2/Si interface structure caused by H+ implantation into a SiO2/Si sample and by hydrogen out-diffusion heat treatment were analyzed by high-resolution synchrotron radiation photoemission spectroscopy. We found that H+ implantation into the SiO2/Si sample [intentional generation of the interfacial dangling bonds] can increase the intensity of the strained-Si peaks in the Si 2p photoemission spectrum. In addition, our study revealed that the strained Si atom amount and dangling bond density are reduced by hydrogen out-diffusion heat treatment. These findings suggest that the increase/decrease in the dangling bond density by H atoms results in the increase/decrease in local strain field around a dangling bond, thereby changing the length of the Si–Si bonds beneath the SiO2/Si interface. Out-diffused hydrogen seems to play roles to not only reduce the dangling bond density but also relax the local strain at the SiO2/Si interface. The hydrogen termination effect is expected to have an advantage in structural stability in the SiO2/Si system as compared with the pure thermal termination effect.

Microscopic Views of Atomic and Molecular Oxygen Bonding with epi Ge(001)-2 × 1 Studied by High-Resolution Synchrotron Radiation Photoemission.

In this paper, we investigate the embryonic stage of oxidation of an epi Ge(001)-2 × 1 by atomic oxygen and molecular O2 via synchrotron radiation photoemission. The topmost buckled surface with the up- and down-dimer atoms, and the first subsurface layer behaves distinctly from the bulk by exhibiting surface core-level shifts in the Ge 3d core-level spectrum. The O2 molecules become dissociated upon reaching the epi Ge(001)-2 × 1 surface. One of the O atoms removes the up-dimer atom and the other bonds with the underneath Ge atom in the subsurface layer. Atomic oxygen preferentially adsorbed on the epi Ge(001)-2 ×1 in between the up-dimer atoms and the underneath subsurface atoms, without affecting the down-dimer atoms. The electronic environment of the O-affiliated Ge up-dimer atoms becomes similar to that of the down-dimer atoms. They both exhibit an enrichment in charge, where the subsurface of the Ge layer is maintained in a charge-deficient state. The dipole moment that was originally generated in the buckled reconstruction no longer exists, thereby resulting in a decrease in the ionization potential. The down-dimer Ge atoms and the back-bonded subsurface atoms remain inert to atomic O and molecular O2, which might account for the low reliability in the Ge-related metal-oxide-semiconductor (MOS) devices.

Germanene Epitaxial Growth by Segregation through Ag(111) Thin Films on Ge(111).

Large-scale two-dimensional sheets of graphene-like germanium, namely, germanene, have been epitaxially prepared on Ag(111) thin films grown on Ge(111), using a segregation method, differing from molecular beam epitaxy used in previous reports. From the scanning tunneling microscopy (STM) images, the surface is completely covered with an atom-thin layer showing a highly ordered long-range superstructure in wide scale. Two types of protrusions, named hexagon and line, form a (7√7 × 7√7) R19.1° supercell with respect to Ag(111), with a very large periodicity of 5.35 nm. Auger electron spectroscopy and high-resolution synchrotron radiation photoemission spectroscopy demonstrate that Ge atoms are segregated on the Ag(111) surface as an overlayer. Low-energy electron diffraction clearly shows incommensurate "(1.35 × 1.35)" R30° spots, corresponding to a lattice constant of 0.39 nm, in perfect accord with close-up STM images, which clearly reveal an internal honeycomb arrangement with corresponding parameter and low buckling within 0.01 nm. As this 0.39 nm value is in good agreement with the theoretical lattice constant of free-standing germanene, conclusively, the segregated Ge atoms with trivalent bonding in honeycomb configuration form a characteristic two-dimensional germanene-like structure.

Direct Formation of C-C Double-Bonded Structural Motifs by On-Surface Dehalogenative Homocoupling of gem-Dibromomethyl Molecules.

Conductive polymers are of great importance in a variety of chemistry-related disciplines and applications. The recently developed bottom-up on-surface synthesis strategy provides us with opportunities for the fabrication of various nanostructures in a flexible and facile manner, which could be investigated by high-resolution microscopic techniques in real space. Herein, we designed and synthesized molecular precursors functionalized with benzal gem-dibromomethyl groups. A combination of scanning tunneling microscopy, noncontact atomic force microscopy, high-resolution synchrotron radiation photoemission spectroscopy, and density functional theory calculations demonstrated that it is feasible to achieve the direct formation of C-C double-bonded structural motifs via on-surface dehalogenative homocoupling reactions on the Au(111) surface. Correspondingly, we convert the sp3-hybridized state to an sp2-hybridized state of carbon atoms, i. e., from an alkyl group to an alkenyl one. Moreover, by such a bottom-up strategy, we have successfully fabricated poly(phenylenevinylene) chains on the surface, which is anticipated to inspire further studies toward understanding the nature of conductive polymers at the atomic scale.

Large area planar stanene epitaxially grown on Ag(1 1 1)

Artificial post-graphene elemental 2D materials have received much attention recently. Especially, stanene, the tin analogue of graphene, is expected to be a robust 2D topological insulator, even above room temperature. We have grown epitaxial 2D stanene on a Ag(1 1 1) single crystal template and determined its crystalline structure synergetically by scanning tunneling microscopy, high-resolution synchrotron radiation photoemission spectroscopy, and advanced first principles calculations. From the STM images, we show that stanene forms a nearly planar structure in large domains. A detailed core-level spectroscopy analysis as well as DFT calculations reveal that the stanene sheet lays over an ordered 2D Ag2Sn surface alloy, but not directly on a bulk-terminated Ag(1 1 1) surface. The electronic structure exhibits a characteristic 2D band with parabolic dispersion due to the non-negligible interaction with the underlying surface alloy.

Temperature effect on the electronic structure of Nb:SrTiO3 (100) surface**Project supported by the Funds from the Chinese Academy of Sciences (Grant No. 1G2009312311750101) and the National Natural Science Foundation of China (Grant No. 11375228).

The effect of temperature on the electronic structure of Nb-doped SrTiO3 (100) surface is investigated by high-resolution synchrotron radiation photoemission spectroscopy. According to the x-ray photoemission spectroscopy (XPS) results, at an annealing temperature of less than 700 °C, the adsorbed carbon and hydroxyl on the STO surface could be removed, to expose the fresh intrinsic surface with a constant ratio of Ti/O. It is obvious that the STO would be doped by Ca+ impurities of bulks and O vacancies in the surface after annealing at 920 °C for one hour.

Efficiency enhancement caused by using LiF to change electronic structures in polymer photovoltaics

The enhancement of device performance achieved by inserting a LiF (lithium fluoride) interfacial layer between the active layer and aluminum (Al) cathode was investigated. Energy band diagrams and interfacial chemical reactions were examined using high-resolution synchrotron-radiation photoemission spectroscopy. If only a thin layer of Al is deposited on the P3HT:PCBM/LiF, the highest occupied molecular orbital level of the active layer does not change. This indicates that shifts in the energy band are only strongly influenced by the insertion of LiF, not the subsequent deposition of an Al layer. We found that the molecular energy band shifts occurred at the interface because of the presence of LiF increases the open-circuit voltage. Furthermore, the thin layer of LiF reduces the work function and improves contact with the cathode, thereby reducing the series resistance and improving the short circuit current density. Thus, without the influence of the Al layer, inserting LiF improves the device performance. The level of performance differs substantially from that observed at the organic layer/LiF/Al interfaces in organic light-emitting devices.

Surface atoms core-level shifts in single crystal GaAs surfaces: Interactions with trimethylaluminum and water prepared by atomic layer deposition

The surface As/Ga atoms 3d core-level spectra of the atomically clean GaAs(111)A-2×2, GaAs(001)-2×4, and GaAs(001)-4×6 surfaces were firstly presented using high-resolution synchrotron radiation photoemission as a probe. The technique clearly explicates behaviors of the As atoms in different surface reconstruction. For GaAs(111)A-2×2, the surface As sits in the same plane as the Ga atoms. As to the GaAs(001)-2×4 surface, the As–As dimers dominate the surface layer, while for GaAs(001)-4×6, the As existed in two forms, the As in the As–Ga dimer and the 3-fold coordinated As. Next, we present a microscopic view of in situ atomic layer deposition (ALD) of Al2O3 on GaAs taking the two (001) surfaces as examples. The precursors were trimethylaluminum (TMA) and water. TMA could exist in either a chemisorbed or physisorbed form, depending on the charge environment of the associated surface atoms. The subsequent water purge resulted in both adsorbed forms being etched off or transformed the physisorbed TMA into the As-O-Al(CH3)2 configuration. We found that the ALD process rendered the precursors partially and selectively in forming bonds with the surface atoms without disturbing the atoms in the subsurface layer. Upon annealing, the interfacial bonding was dominated with the AsAl as well as GaO bonds.

Atom-to-atom interactions for atomic layer deposition of trimethylaluminum on Ga-rich GaAs(001)-4 × 6 and As-rich GaAs(001)-2 × 4 surfaces: a synchrotron radiation photoemission study

High-resolution synchrotron radiation photoemission was employed to study the effects of atomic-layer-deposited trimethylaluminum (TMA) and water on Ga-rich GaAs(001)-4 × 6 and As-rich GaAs(001)-2 × 4 surfaces. No high charge states were found in either As 3d or Ga 3d core-level spectra before and after the deposition of the precursors. TMA adsorption does not disrupt the GaAs surface structure. For the (4 × 6) surface, the TMA precursor existed in both chemisorbed and physisorbed forms. In the former, TMA has lost a methyl group and is bonded to the As of the As-Ga dimer. Upon water purge, the dimethylaluminum-As group was etched off, allowing the now exposed Ga to bond with oxygen. Water also changed the physisorbed TMA into the As-O-Al(CH3)2 configuration. This configuration was also found in 1 cycle of TMA and water exposure of the (2 × 4) surface, but with a greater strength, accounting for the high interface defect density in the mid-gap region.

Band alignment of ZnTe/GaAs heterointerface investigated by synchrotron radiation photoemission spectroscopy

ZnTe films have been grown on GaAs substrate by metalorganic vapor phase epitaxy. High-resolution synchrotron radiation photoemission spectroscopy measurements are used to determine the valence band offset of ZnTe/GaAs heterojunctions. Based on the binding energies of Zn 3d and Ga 3d core levels and valence band maximum values, the valence band offset has been determined to be 0.19 eV for ZnTe/GaAs heterojunction. The heterojunction shows type I band configuration with a conduction band offset of 0.65 eV. The accurate determination of the band alignment of ZnTe/GaAs heterojunction facilitates the design of optical and electronic devices based on ZnTe/GaAs structure.

Growth mechanism of atomic layer deposited Al2O3 on GaAs(001)-4 × 6 surface with trimethylaluminum and water as precursors

A microscopic view of in situ atomic layer deposition Al2O3 on clean n-GaAs (001)-4 × 6 surfaces probed by high-resolution synchrotron radiation photoemission is presented. The precursors of trimethylaluminum (TMA) and water partially and selectively bond with the surface atoms without disturbing the atoms in the subsurface layer. The first-cycle TMA acts differently on the surface As atoms; namely, TMA is dissociative on As in the As-Ga dimer but is physisorbed on As that is 3-fold Ga coordinated. Water drastically alters the TMA-covered surface to etch off the dissociated TMA with As, giving Ga–O bonding for the later deposition of Al2O3 and at the same time to transform the configuration of the physisorbed TMA to bond strongly with As. Approximately six cycles of purges (TMA + water) are required before the interaction at the interface is complete. In comparison, the e-beam deposition of Al2O3 on GaAs greatly disturbs the III-V surface so that a high As 4+ charge state appears and the surface Ga atoms become clustered.

In situ study of the atomic layer deposition of HfO2 on Si

The authors investigated in situ the initial stages of the atomic layer deposition (ALD) growth of HfO2 on Si(001)/SiO2 substrates by using tetrakis-di-methyl-amino-Hf and H2O as precursors. The surface morphology and the chemical and electronic properties of HfO2 ultrathin films were studied after each ALD cycle by surface-sensitive techniques. Atomic force microscopy image analysis was performed by analyzing the height–height correlation function (HHCF), the root mean square surface roughness, and the surface fractal dimension, as function of the number of ALD cycles. Parameters directly related to HHCF, e.g., surface width, correlation length, local slope, and roughness exponent, were calculated and used for determination of scaling exponents. A complex behavior of all parameters up to the eighth ALD cycle was evidenced. High-resolution synchrotron radiation photoemission spectroscopy was applied to characterize the chemical nature of Si/SiO2/HfO2 interface. Changes arising in the Si 2p, O 1 s, and Hf 4f core level lines after each ALD cycle up to the complete formation of two layers of HfO2 were observed. The thickness of the growing HfO2 layer was calculated to estimate the growth per cycle to approximately 0.1 nm/cycle. By means of ultraviolet photoemission spectroscopy, variations of valence band maximum and secondary electron cutoff after each ALD cycle were observed and the presence of an interfacial dipole was pointed out. Finally, the loss function onset of electron energy loss spectroscopy changed during ALD because of bandgap variations from SiO2 to HfO2. By combining all experimental results a new and fully comprehensive growth model of ALD during the initial stages was developed.

Electronic structures of Ga2O3(Gd2O3) gate dielectric on n-Ge(001) as grown and after CF4 plasma treatment: A synchrotron-radiation photoemission study

The interfacial electronic structure of Ga2O3(Gd2O3) (GGO) on n-Ge(001) is determined using high-resolution synchrotron radiation photoemission. The excitation photon energy was specifically chosen to observe the interaction at the GGO/Ge interface (hv = 463 eV) as well as the possible diffusion of Ge up to the GGO surface (hν = 120 eV). The Ge 3d core-level spectra were fit to extract the contributing components. Photoemission measurements were done for four samples, as deposited, N2 annealed, CF4 plasma treated, and the combined CF4 plasma treated and N2 annealed. No surface passivation was employed prior to the dielectric deposition. SRPES data clearly showed that the elemental Ge in the as-deposited sample was effectively kept in the wafer. Prevention of Ge diffusion was attributed to formation of a thin germanatelike oxide layer. Other than contributions from bulk Ge, an analytical fit to the Ge 3d cores gives two components that are associated with bonding to Gd2O3 (GdGe*) and to Ga2O3 (GaGe*), which had chemical shifts of 3.46 and 1.80 eV, respectively. We hereby label them as MGe*, where M stands for either Gd2O3 or Ga2O3. Area occupations of the GdGe* and GaGe* oxides are ∼87% and ∼10%, respectively. A CF4 plasma treatment disturbs the film itself as well as the interfacial oxide so that the GGO surface begins to show both elemental Ge and Ga. Nevertheless, the follow-up N2 annealing produces the GdGe*+GaGe* layer with characteristics similar to those at the GGO/Ge interface. Both GdGe* and GaGe* states in the CN-treated sample show simultaneously a smaller chemical shift by 0.31 ± 0.02 eV than those in the as-deposited sample. The treatments also induce upward band bending on both the high κ and the Ge sides, which causes the valence band offset at the GGO/Ge interface to be 2.95 eV.

Interfacial reactions at Al/LiF and LiF/Al

High-resolution synchrotron radiation photoemission spectroscopy was used to investigate the chemical properties of Al–LiF interfaces. An electronic state appeared at the Al/LiF interface with a binding energy 4.8 eV higher than that of the metallic Al 2p core level, but the state was hardly found to be present at the LiF/Al interface. This indicates that intensive chemical reaction could occur at the Al/LiF interface, while the reaction occurring at the LiF/Al interface would be weak. This result explains well the unsymmetrical electron injection from different sides of the symmetrical device of indium-tin-oxide\Al\LiF\tris(8-hydroxyquinoline) aluminum\LiF\Al showing unsymmetrical current-voltage characteristics.

Interfacial electronic structure of magnesium deposited on Ag-covered tris(8-hydroxyquinolato) aluminum film: A synchrotron radiation photoemission study

The deposition of magnesium on tris(8-hydroxyquinolato) aluminum (Alq3) precovered with thin silver was investigated with high resolution synchrotron radiation photoemission. The noble-metal deposit first occupies the vicinity of the pyridyl ring. The appearance of a Fermi level suggests that the incorporated Ag behaves as a metallic cluster, consistent with the results of the Ag 3d cores, which show a nonzero Doniach-Sunjic singularity index of 0.061. Upon Mg adsorption on the Ag∕Alq3 surface, a MgAg mixture begins to develop such that the Ag 4d valence band becomes narrowed and its centroid shifts towards higher binding energy. A phase transformation is thus observed at the interface, from a Ag-rich phase to a Mg-rich phase. The product of the MgAg mixture prevents subsequent Mg atoms from penetrating into the Alq3 film, which grows as a metal on top of the MgAg∕Alq3 interface.

Energy structures and chemical reactions at the Al∕LiF∕Alq3 interfaces studied by synchrotron-radiation photoemission spectroscopy

The chemical properties and energy levels of Al∕LiF∕Alq3 were investigated via high-resolution synchrotron-radiation photoemission spectroscopy. No clear chemical reaction was found with LiF deposited on Alq3. The core-level spectra show that Li+ ion and Alq3 anion are created only after Al is deposited on LiF∕Alq3 surfaces. Combined with the increase of the electron concentrations indicated by the Fermi-level position in valence-band spectra, the results provide direct evidence of the proposed chemical reaction, 3LiF+Al+3Alq3→AlF3+3Li+Alq3−, which leads to the excellent electron injection efficiency in Al∕LiF∕Alq3.