Two-dimensional (2 D) electron systems in compound semiconductors are markedly important not only for studies of low-dimensional electron physics but also for application in recent information-oriented society. To investigate behavior of such 2 D electrons in nanometer-scale regions, the local density of states (LDOS) was characterized at the clean surfaces in ultra-high vacuum using a low-temperature scanning tunneling microscope (LT-STM). This LT-STM was equipped with a chamber for molecular beam epitaxy (MBE) of III-V compound semiconductors. At the InAs(111)A surfaces, LDOS standing waves were clearly imaged around the defects. The wave features were confirmed to originate from the Friedel oscillation of naturally formed 2 D states in the surface electron accumulation layer. Also, the electronic states of highly Si-doped InGaAs surface quantum wells (QWs), grown on the GaAs(111)A substrates, were characterized. The LDOS spectra revealed the formation of artificial 2 D subbands quantized in the QWs. The effects related to phase coherence in the QWs are discussed. It is shown that nanoprobing the clean surfaces is very useful for microscopic understanding of quantum mechanical phenomena aiming at advanced electronic devices for future communication technologies.
Theoretical studies on surface electrical conduction on nanometer scales are presented. First the mechanism of the observation of surface states in scanning tunneling microscopy is discussed, where it is essential to take account of the current flowing parallel to surfaces. Second the electrical conductance in double-tip scanning tunneling microscopy is discussed. The double-tip conductance of group-IV semiconductor (111)2×1surfaces is calculated, where it is found that the buckling of π-bonded chains induces asymmetry between valence and conduction bands and reduces the anisotropy of conduction. Finally the electrical resistance of monatomic steps of the Si(111)√3×√3-Ag surface is calculated. The calculated value of conductance reproduces well the experimental values. The transmission of Bloch states is discussed using generalized phase and amplitude of waves including evanescent waves. The resistance arises from both the difference in Bloch wave numbers and the discontinuity of the logarithmic derivatives of the periodic part of Bloch waves.
Hydrogen is adsorbed on metal surfaces either molecularly or atomically. Due to the light mass, hydrogen has a large zero-point vibrational energy and an extended feature of its nuclear wavefunction. The zero-point motion affects the hydrogen adsorption site and the wavefunction is possibly delocalized over the surface. Because of the nuclear spin, on the other hand, molecular hydrogen is classified into ortho and para species, that have different rotational quantum numbers. The ortho and para species have different adsorption energy and undergo ortho-para conversion through magnetic interaction with metal surfaces.
X-ray absorption spectroscopy study of hydrogen bonded system is presented. The electronic structure of water is strongly influenced by the local hydrogen-bonded configurations. At the surface and bulk defects of ice, there are electronic states localized on broken (or dangling) hydrogen bonds. Broken hydrogen-bonded configurations and electronic states in liquid water are also shown.
Novel functions and properties of transition-metal oxides such as high-temperature superconductivity and colossal magneto-resistance effect have attracted much attention. In these compounds, strong Coulomb interaction between electrons plays a significant role and it is believed that characterizations of local electronic states are important to make clear the mechanisms of the functions. We have performed scanning tunneling microscopy/spectroscopy to investigate local electronic states of two model compounds, Ca2-xNaxCuO2Cl2 and Sr3Ru2O7, which are a high-temperature superconductor and an itinerant metamagnet, respectively. In Ca2-xNaxCuO2Cl2, we discovered a checkerboard-like modulation of the local density of states (LDOS) in the so-called pseudo-gap state which is adjacent to the superconducting phase. In Sr3Ru2O7, we succeeded in observing the magnetic field effects on LDOS across the metamagnetic transition as well as the novel surface superstructure. These observations provide useful information to understand the effects of electron correlation in transitionmetal oxides.
Electronic devices utilizing semi-conducting properties have been used in most of information communication equipments. The advances in micro-structuring and integration technology enable the function enhancement of the semiconductor devices. However, the development will soon be pushed to its limitation. Therefore, the nano-electronic devices on the basis of new principles have been investigated in recent years. We developed a new type of nanodevice, atomic switch, that is realized by local control of solid-state electrochemical reaction using a mixed ionic and electronic conductor. The atomic switch works by the formation and annihilation of an atomic bridge between the electrodes. Here, we introduce its operation principle, basic research development, and unique functionality of the atomic switch. Furthermore, the development for the practical application of the device is described.
Quantum phenomena of carbon nanotube are overviewed. Optical and transport properties of carbon nanotubes can be understood in terms of electron-photon and electron-phonon interaction. We will discuss the relevance of these interactions to the quantum properties.
The applicability of scanning probe microscopy (SPM) in the time dependent dielectric breakdown (TDDB) characteristics of silicon oxide has been demonstrated. Our study demonstrates that cumulative failure rates by the Weibull plot of TDDB measurement for a 9.5 nm-thick oxide were found to be straight lines and shape parameters were not based on bias voltages, but were almost fixed. These results indicate that failure mode within the limits of these voltage conditions has not changed and the SPM method is applicable to the evaluation of TDDB characteristics of silicon oxides. We also applied this method to oxides on a silicon substrate surface with and without damage by plasma etching. The life expectancy in a real working voltage domain was searched for, and it presumed that a difference arose in these.