Kiyohiko Toyama

Strain dependence of hole effective mass and scattering mechanism in strained Ge channel structures

Strain dependence of hole effective mass (m∗) in the strained Ge channel was systematically studied, and monotonic m∗ reduction by more than 20% was clearly observed when the strain increased from 0.8% up to 2.8%. The scattering mechanism, which strongly depended on the modulation-doping structure as well as strains, was also investigated based on the Dingle ratio evaluation, and the interface roughness scattering was found to be effectively suppressed by adopting the inverted structure even for the largely strained channels.

Magnetotransport properties of Ge channels with extremely high compressive strain

Ge channel structures with extremely high compressive strain up to 2.8% were fabricated and their magnetotransport properties were evaluated. It was found that at the same hole density the sample with the higher strain showed the lower hole effective mass and that the compressive strain effectively reduced the effective mass. The Dingle ratios obtained were very high (>5) for all samples, indicating that remote impurity scattering was a dominant scattering mechanism rather than high angle scatterings caused by degradation of the channel layers. This result strongly suggests that Ge channels with extremely high strain are very promising for high performance complementary-metal-oxide-semiconductor applications.

Electronic transport properties of the Ising quantum Hall ferromagnet in a Si quantum well.

Magnetotransport properties are investigated for a high mobility Si two-dimensional electron system in the vicinity of a Landau level crossing point. At low temperatures, the resistance peak having a strong anisotropy shows large hysteresis which is attributed to Ising quantum Hall ferromagnetism. The peak is split into two peaks in the paramagnetic regime. A mean field calculation for the peak positions indicates that electron scattering is strong when the pseudospin is partially polarized. We also study the current-voltage characteristics which exhibit a wide voltage plateau.

On Effects of Gate Bias on Hole Effective Mass and Mobility in Strained-Ge Channel Structures

The effects of gate bias on hole effective mass (m*) and Hall mobility were studied in strained-Ge channel modulation-doped structures. Shubnikov–de Haas oscillations were analyzed with and without the bias and a significant m* increase from 0.15 to 0.22 m0 was observed with the increase in the carrier density due to the strong nonparabolicity of the valence band. This is a clear demonstration that modification of carrier density via gating considerably affects m*, which may have critical effects on device properties. The gate bias dependence of Hall mobility was also investigated and the dominant scattering mechanism was clarified in various temperature and carrier density regions.

Well-width dependence of valley splitting in Si/SiGe quantum wells

The valley splitting in Si two-dimensional electron systems is studied using Si/SiGe single quantum wells (QWs) with different well widths. The energy gaps for 4 and 5.3 nm QWs, obtained from the temperature dependence of the longitudinal resistivity at the Landau level filling factor ν=1, are much larger than those for 10 and 20 nm QWs. This is consistent with the well-width dependence of the bare valley splitting estimated from the comparison with the Zeeman splitting in the Shubnikov–de Haas oscillations.The valley splitting in Si two-dimensional electron systems is studied using Si/SiGe single quantum wells (QWs) with different well widths. The energy gaps for 4 and 5.3 nm QWs, obtained from the temperature dependence of the longitudinal resistivity at the Landau level filling factor ν=1, are much larger than those for 10 and 20 nm QWs. This is consistent with the well-width dependence of the bare valley splitting estimated from the comparison with the Zeeman splitting in the Shubnikov–de Haas oscillations.

Insulating phases induced by crossing of partially filled Landau levels in a Si quantum well

We study magnetotransport in a high-mobility Si two-dimensional electron system by in situ tilting of the sample relative to the magnetic field. A pronounced dip in the longitudinal resistivity is observed during the Landau-level crossing process for noninteger filling factors. Together with a Hall resistivity change which exhibits the particle-hole symmetry, this indicates that electrons or holes in the relevant Landau levels become localized at the coincidence where the pseudospin-unpolarized state is expected to be stable.

Magnetoresistance and spin polarization in the insulating regime of a Si two-dimensional electron system

We have studied the magnetoresistance in a high-mobility Si inversion layer down to low electron concentrations at which the longitudinal resistivity

Effects of increased compressive strain on hole effective mass and scattering mechanisms in strained Ge channels

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Landau level crossing and pseudospin phase transitions in Si quantum wells

has an activated temperature dependence. The angle of the magnetic field was controlled so as to study the orbital effect proportional to the perpendicular component

Hole density and strain dependencies of hole effective mass in compressively strained Ge channel structures

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