A. Koma

Atomic and electronic structures of MgO/Ag(0 0 1) heterointerface

Thin film growth of MgO on Ag(0 0 1) was studied using reflection high-energy electron diffraction, Auger electron spectroscopy, electron energy loss spectroscopy (EELS), and ultraviolet photoemission spectroscopy (UPS), as a model system of the oxide/metal interface. At a substrate temperature of 450 K, a single-domain MgO film ([1 0 0]film// [1 0 0]substrate) grew heteroepitaxially on Ag(0 0 1). The in-plane lattice constant of the film changed continuously from the value of the substrate to that of the film, as is the case with the heteroepitaxial growth of alkali halides. The density functional calculations were performed for model clusters in order to investigate the initial adsorption structure. MgO adsorbed preferentially with the O atom above the Ag atom, and the Mg atom on the hollow site. EELSresults showed that the band gap energy did not change for MgO thickness from 1 to 20 ML, indicating that the band gap energy of a 1 ML thick MgO film adjacent to a metal substrate was same as that of MgO surface. The UPSresults, on the other hand, showed an upward shift of the valence band with decreasing film thickness. These UPSresults could be explained by the image charge screening of a hole created by photoionization in the near presence of a metal. 2002 Elsevier Science B.V. All rights reserved.

Van der Waals type buffer layers: epitaxial growth of the large lattice mismatch system CdS/InSe/H–Si(111)

Abstract The lattice mismatch at the heterostructure CdS/Si(111) is close to 8%. To relieve the structural stress at this interface, an InSe van der Waals-type buffer layer was grown between the Si substrate and the CdS film. After the Si(111) substrate surface was passivated by hydrogen, the InSe-buffer layer was grown on top by molecular beam epitaxy with high crystalline quality. The lattice orientations of the InSe film are in registry with the Si substrate, as shown by reflection high energy electron diffraction. As seen in the atomic force microscope, the InSe films have roughly the same terrace width of 50–70 nm as the Si substrate due to the wafer miscut. By contrast the step edges of the InSe film are more regular than those of the silicon. The CdS-film was grown on the InSe. The crystal axis of the film is in full registry with the InSe substrate as monitored by reflection high energy electron diffraction. The film morphology is determined as simultaneous growth of multilayers observed by atomic force microscope.

Effects of anharmonicity of ionic bonds on the lattice distortion at the interface of alkali halide heterostructures

Abstract The initial stage of heteroepitaxial growth of alkali halide on other alkali halide was studied using reflection high energy electron diffraction, electron energy loss spectroscopy and Auger electron spectroscopy. The lattice distortion relaxed more rapidly for the system where the lattice constant of the film was larger than that of the substrate (i.e. positive misfit), than for the system where the lattice constant of the film was smaller than that of the substrate (i.e. negative misfit). Classical Monte Carlo calculations were performed for KCl films on various kinds of substrates to clarify the relation between the lattice distortion and the ionic potential. The rapid relaxation of lattice distortion for the positive misfit could be explained qualitatively in terms of anharmonicity in ionic bonds.

Heteroepitaxial growth of SrO on hydrogen-terminated Si(100) surface

Abstract Sharp interface structure of SrO film on hydrogen-terminated Si(100) is obtained by an alternate supply of Sr metal and O 2 gas. The hydrogen-terminated Si is chemically inactive, especially against oxidation. The method of alternate supply of Sr and O 2 gas makes it possible to cover Si surface with the first Sr layer for prevention of formation of Si–O bonding when O 2 gas is supplied to the substrate. Epitaxial SrO films are grown when the substrate temperature is maintained above 400°C. The orientation relationship between epitaxial SrO and Si is found to be (100)SrO//(100)Si and [001]SrO//[001]Si.

Optical and hybrid properties of the ZnSe/InSe/Si heterojunction

Abstract By molecular beam epitaxy, a thin (≈200 nm) ZnSe film was grown on a p -type Si substrate covered with an InSe buffer layer. The InSe buffer is used to bypass the huge lattice mismatch of 4.4% between ZnSe and Si in order to ensure the growth of a stress-free optically smooth ZnSe film. Reflection, photoluminescence and photocurrent properties at 300 K demonstrated that the thin ZnSe film grown represents a high quality etalon with the bandgap of cubic bulk ZnSe. Furthermore, it is shown that the ZnSe/InSe/Si heterojunction exhibits rectification and a nearly constant intrinsic photoconductivity between 500 and 900 nm. This appreciable behavior makes the ZnSe/InSe/Si heterojunction very attractive for applications to sophisticated light detectors.

Scanning tunneling microscope images of locally modulated structures in layered materials, MoS2(0001) and MoSe2(0001), induced by impurity atoms

Abstract The effect of impurity atoms on the surface electronic structure of semiconducting transition metal dichalcogenides, MoS2 and MoSe2, has been investigated by a scanning tunneling microscope (STM) and by its spectroscopic mode, scanning tunneling spectroscopy. The following impurity atoms have been studied: (1) alkali metal atoms deposited by using alkali dispensers, (2) impurity atoms unintentionally doped during the synthesizing process, and (3) a donor dopant of Re. STM images show locally modulated structures with bright area surrounded by dark area in nm scale at negative sample bias, and their global features depend on the bias polarity. The center region of the structure observed at negative sample bias is flat or caved in regardless of a kind of the impurity atom. It is concluded that the locally modulated STM image arises from the interaction between the electron donated by the impurity and electrons in the d-orbitals of neighboring Mo atoms. This interaction extends by about one Mo–Mo distance, which gives the image of 0.7 nm in diameter.

Surface phonons of LiBr/Si(100) epitaxial layers by high resolution electron energy loss spectroscopy

Abstract Surface and interface optical phonons (F-K mode) of LiBr epitaxial films with various thickness grown on Si(100) have been investigated by high resolution electron energy loss spectroscopy (HREELS). In addition to single excitations of surface and interface phonons, their multiple excitations and annihilations have also been observed. The dependence of surface phonon energy with film thickness are compared with dielectric theory calculations. It has been found that phonon energies deviate more than the predictions of dielectric approximation with the decrease in the film thickness down to few nm, indicating the necessity of microscopic treatment in this thickness range.

Accelerated photopolymerization and increased mobility in C60 field-effect transistors studied by ultraviolet photoelectron spectroscopy

Ultraviolet photoelectron spectroscopy (UPS) of C60 thin-film field-effect transistors was measured with biasing gate voltages. A time-dependent change in the electronic structure of the C60 film was observed during the UPS measurement, which has never been observed in a C60 film grown on a conductive substrate. The change was attributed to the accelerated polymerization of C60 by comparing the UPS with that of the photopolymerized C60. The polymerization was associated with the increase in the field-effect electron mobility. This result indicates that mobile carriers produce reactive radicals in organic semiconductors.

Electron spectroscopy of chemically synthesized ZnS clusters

Thin films of ionic compounds of ZnS clusters were measured by electron energy loss spectroscopy (EELS) and ultraviolet photoelectron spectroscopy (UPS). A size effect was observed in the valence plasmon energy measured by EELS from which the coherence length of the plasmon excitation can be estimated. The difference between the lowest excitations observed in UPS and EELS can be explained by the final state charging effect of a single cluster ion in UPS, which strongly depends upon the nominal charges of the clusters.

Initial oxidation process of Mg films characterized by AES, EELS, and UPS

Initial oxidation process of Mg films was investigated by Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS) and ultraviolet photoelectron spectroscopy (UPS). A polycrystalline Mg film was exposed to oxygen at room temperature. An inspection of EELS reveals that the initial oxidation process could be classified according to the curve profile in the low energy loss region. The oxide formation is initiated after oxygen exposure of 3 L (Langmuir).