Atsuo Yagi

Metal–insulator transition and spin degree of freedom in Silicon 2D electron systems

Abstract Magnetotransport in 2DESs formed in Si-MOSFETs and Si/SiGe quantum wells at low temperatures is reported. Metallic temperature dependence of resistivity is observed for the n-Si/SiGe sample even in a parallel magnetic field of 9 T, where the spins of electrons are expected to be polarized completely. Correlation between the spin polarization and minima in the diagonal resistivity observed by rotating the samples for various total strength of the magnetic field is also investigated.

Effects of Tailing of Density of State on the Mobility of Si-MOSFETs at Low Temperatures?A Proposal for the Method of Characterization of Si-SiO2 Interfaces?

An attempt is made to explain the electron mobility of silicon MOSFETs in low electron desities at low temperatures by taking account of the phenomenological change in the electrostatic screening due to smearing of the two-dimensional subband tail. The results of application of this approach to various samples are satisfactory for characterizing the electrical properties of Si-SiO2 interfaces in silicon MOS devices.

Phase boundaries between quantum hall metal and hall insulator in Si-MOSFET's and many-body enhancement of valley- and Zeeman-splitting

We have experimentally studied metal-insulator transitions in a two-dimensional electron system in Si-MOSFETs in a magnetic field and derived a phase diagram on a ν (Landau level filling factor) vs. disorder (resistivity in the absence of magnetic field) plane. Structures of stabilized phase boundaries at ν=1 and ν=2 are discussed based on many-body enhancement of valley- and Zeeman-splitting, respectively.

Collapse of the quantum Hall state by floating-up of extended-state bands with increasing disorder

We measured the diagonal and Hall conductivities across the transition between the integer quantum Hall effect state and the insulating state in a two-dimensional electron system formed in Si-MOSFETs. Sample dependence of the transition indicates that the insulating state is caused by Anderson localization due to disorder, not by a formation of an electron solid.

Effects of Silicon-Nitride Films on the Electronic Properties of Si–SiO2 Interfaces

The electronic properties of Si–SiO2 interface states in Metal-Nitride-Oxide-Silicon (MNOS) with thick oxide films are investigated. It is shown that a large number of acceptor-like interface states are created when MNOS is annealed at high temperatures, and that subsequent low temperature hydrogen annealing reduces the number of these states. We found that the energy distribution of the interface states is independent of their density. The possible cause of these interface states may be thermal stress induced at the interface.