Using a universal definition of work function as an electronic chemical potential that determines equilibrium of electron transfer, the origins of work function are treated by dividing terms from bulk and surface. The general methodology of modifying work function is presented. The electronic chemical potential view of work function is extended to interfaces and the methodology of band alignment between electrodes and device materials is discussed.
This short review of scientific terms mainly deals with the electron energy level alignment at metal semiconductor interfaces. The energy level alignment relies on various elements such as the concept of thermal equilibrium, physics and chemistry of the materials comprising the interface and peculiar characteristics of the interface itself. In this review efforts have been devoted within the allotted space to include as many scientific terms as possible and to explain them as neatly as possible. The author, however, finds a difficulty to fill the big gap in the general concept and the historical background between two interfaces of metal with inorganic and organic semiconductors. This makes it impossible to universally explain the scientific terms, and in some cases separate explanations had to be made.
Metal-semiconductor, semiconductor-semiconductor and insulator-semiconductor interfaces are basic constituent elements of semiconductor devices. How energy bands align at these interfaces and how well carriers are controlled through these interfaces have been important research topics of surface science as well as basic design issues for semiconductor devices. This paper attempts to review the historical evolution of various models concerning the alignments of energy bands and related Fermi level pinning phenomena at inorganic crystalline semiconductor interfaces, mainly focusing on those of compound semiconductors. The topics discussed include the natural band line-up, surface state model, charge neutrality level, interfacial model, MIGS model, UDM, DIGS model, interface bond polarization model, IFIGS model and model solid theory. It is suggested as a result of reviewing that, by sufficiently removing bond disorder and defects, the natural band line-ups may become realizable after all with feasibility of their artificial modifications by polarization of interface chemical bonds. The strong Fermi level pinning at metal-semiconductor interfaces may be removed in nano-scale structures.
The method to determine the energy band alignments of high-k dielectrics to Si(100) is described, in which the energy band gap values and the valence band lineups are measured from the analyses of energy loss signals of core line and valence band spectra measured by X-ray photoelectron spectroscopy (XPS), respectively. Also, it is demonstrated how useful the measurement of the cut-of energy for photoemission is to evaluate the effective work function in metal/high-k dielectric. Total photoelectron yield spectroscopy (PYS) is highlighted as a powerful way to quantify energy distribution of electronic defect states in a dielectric or at the dielectric/Si interface in the energy region corresponding to the Si band gap.
Recent semiconductor devices require both introduction of many materials and aggressive scaling. As a result, present semiconductor devices contain various nano-scale interfaces, and a lot of unusual interface phenomena that cannot be explained by ordinary concepts of interfaces have been observed. In this paper, we have pointed out that ordinary concepts of interfaces cannot be applied directly to these interfaces, and propose two new concepts of “Generalize charge neutrality level (GCNL)” and “Fixing of Fermi level based on the generation of oxygen vacancies by interfacial reactions”.
The electronic structure at the interface formed between an organic film and a metal electrode plays a crucial role in the performance of electronic devices using organic semiconductors such as electroluminescent displays (light-emitting diodes), field-effect transistors, and photovoltaic cells. In particular, the energies of the highest-occupied and lowest-unoccupied molecular orbitals relative to the Fermi level of metals, i.e., the energy level alignment at organic/metal interfaces, are of fundamental importance in discussing the barrier heights for the charge injection and separation at the interface. However, it is generally not easy to discuss the interfacial electronic structure precisely since the interface energetics is usually modified due to various interface-specific phenomena. In this article, we introduce the recent progress in studies of the energy level alignment at organic/metal interfaces: (1) basic understanding, (2) interface dipole layer, and (3) interface states.
From the viewpoint of dielectrics physics, electrostatic phenomena occurring at material interfaces is briefly described. The importance of charge storage due to Maxwell-Wagner effect and electrostatic energies stored at the interfaces is emphasized for analyzing the interfacial electronic phenomena. Analyzing of pentacene field effect transistors (FETs) as a Maxwell-Wagner effect element showed that the derived current equation well describes the I-V characteristics of pentacene FETs. It is also shown that injected carriers from source electrode are the origin of carriers of OFETs and form a space charge field in organic FETs (OFETs). Furthermore, it is demonstrated that the electric field induced optical second harmonic generation (EFISHG) method is a potential way to probe static electric field distribution along the FET channel. The method also enables us to directly visualize carrier motion in the OFET channel.
Inverse photoemission spectroscopy (IPES) is the powerful technique to directly investigate the unoccupied electronic states of materials. In this technique, electrons are injected to samples, and then emitted photons from the samples are detected. In this article, we describe the outline of IPES including its principle, and apparatuses we have developed and constructed. As examples, we present experimental results of Ti 3p-3d resonant IPES of the Mott insulator YTiO3, which is an important material for understanding of the mechanism of metal-insulator transition due to electron correlation, and angle-resolved IPES of LaB6(001), which is used as an electron source. Finally, we report a spin polarized electron gun in progress.
We have studied structures and magnetic properties for sevral iron silicides grown on Si(111) 7×7 clean surfaces by solid phase epitaxy method, that is deposition of Fe on the surfaces at room (or low) temperature and subsequent annealing. We also have summarized almost all silicide phases depending on the deposition thickness and the annealing temperature as a schematic phase diagram. We found that some phases show ferromagnetic property arising from interface structures. For the ferromagnetism of the β-FeSi2 phase even at room temperature, the existence of a ferromagnetic silicide (close to Fe3Si) interface layer was pointed out, which leads to a hetero structure of β-FeSi2/Fe-rich silicides/Si(111). For the ferromagnetic property of 2×2-Fe phase at 40 K, the large spin polarization was proposed at the Fe atoms in the B5-type interface structure between the c-FeSi(111) ultra-thin film and the Si(111) substrate. This interface ferromagnetism will extend applications of the spintronic devices.
NIR-Photoluminescence (NIR-PL) spectroscopy is a powerful analytical technique to determine the electronic and molecular structures of semiconducting single-walled carbon nanotubes (SWCNTs). The origin of the observed PL peak is reliably assigned to SWCNTs with specific chiral indexes (n, m) because the emission and excitation spectra show characteristic peaks depending on the molecular structure of SWCNTs. The detailed composition of the bulk samples can be deduced from the PL intensities divided by the theoretically calculated PL yields. We also report band gap modification of SWCNTs by incorporating fullerene molecules (nanopeapods) based on the PL results. It is found that differences in the optical transition energies between unfilled semiconducting SWCNTs and the corresponding C60 nanopeapods strongly depend on the tube diameter and “2n + m” family types. The change in optical band gap is rationally explained by local strain of SWCNTs upon C60 insertion and hybridization of the electronic states between the encapsulated C60 and the outer SWCNTs.