S. Takaoka

Transport Properties in Band-Tails of High Mobility Poly-Si TFTs

Electrical properties of high-mobility polycrystalline silicon thin film transistors (poly-Si TFTs) are measured over the wide temperature range from 300 K to 1.5 K in order to clarify the role of band-tail states in the grain boundaries (GBs). Two distinct regimes of weak and strong localization of electrical transport have been identified below and above the carrier trap state density N st 1.0 x 10 12 cm -2 in the GBs for high-mobility poly-Si TFTs. Poly-Si TFTs with a smaller carrier trap state density N st ( 6 x 10 11 cm -2 ) show metallic behavior where the mobility increases as temperature decreases. Poly-Si TFTs with larger N st ( 1.5 x 10 12 cm -2 ) show semiconducting behavior of mobility. At low temperatures, on the former poly-Si TFT the resistance changes due to the weak-localization effect and on the latter poly-Si TFT due to the variable-range hopping (VRH) in the regime of strong localization. In both regimes, negative magnetoresistances have been observed and interpreted by the quantum interference effect. All of these features are attributed to the band-tail states decaying exponentially from the band-edge, based on the observations of irregular GB structure in the poly-Si film from transmission electron microscope images and the roughness at the SiO 2 /poly-Si interface seen on atomic force microscope images.

Far-infrared magneto-absorption of double-layer two-dimensional electron system in tilted magnetic fields

Abstract Far-infrared magneto-absorption of double-layer two-dimensional electron system has been measured by Fourier transform spectrometer in tilted magnetic fields. We have measured the dependence of cyclotron mass ( m C ) on the parallel ( B ∥ ) and perpendicular ( B ⊥ ) magnetic fields systematically. The obtained B ∥ dependence of m C is complicated and varies with B ⊥ , which cannot be explained by semi-classical theory.

Magnetocapacitance Investigation of Quantum Hall Effect and Edge States

The quantum Hall effect for the GaAs/AlGaAs heterostrcture is investigated by an ac capacitance measurement between the two-dimensional electron system (2DES) and the gate on GaAs/AlGaAs. The capacitance minima at the quantum Hall plateaus are mainly determined not by the 2DES area under the gate but by the edge length of 2DES. There exists the high conductive region due to the edge states along the 2DES boundary, when the bulk conductivity σxx is small enough at low temperatures and high magnetic fields. From the temperature and frequency dependence of the capacitance minima, it is found that the measured capacitance consists of the contribution from the edge states and that of the bulk state, which is treated as a distributed circuit of a resistive plate with the conductivity σxx. The evaluated width of edge states from the capacitance is much larger than the magnetic length and the cyclotron radius expected from the one-electron picture. This wide width of edge states can be explained by the compressible-incompressible strip model, in which the screening effect is taken into account. Further the bulk conductivity of less than 10-12 S (S=1/Ω) is measured by the capacitance of the Corbino geometry sample, where the edge states are absent and the capacitance is determined by only σxx in this geometry. The localization of the bulk state is investigated by the obtained σxx.

Investigation of Hall resistivity in antidot lattices with respect to commensurability oscillations

In investigation of commensurability oscillations in antidot lattices, we find a relation between the resistivity ρ x x and the differentiated Hall resistivity dρ x y / d B , where B is the strength of the magnetic field. All peaks of ρ x x exactly correspond to those of dρ x y / d B . This coincidence is direct evidence that the extended Drude model for the oscillations cannot fully explain the magnetoresistance experimentally observed in antidot lattices.

Capacitance between adjacent edge channels at quantum Hall plateaux

Abstract We have measured the capacitance between adjacent edge channels (C M ) in the GaAs/AlGaAs two dimensional electron gas under the quantum Hall plateau condition to investigate the structure of edge channels. The value of C M is determined by the energy relaxation length of adjacent edge channels and the ratio of edge channel width to the separation between them. The separation lengths between the adjacent edge channels at ν=2 and 4 have been estimated from the information of both C M and the total width of edge channels.

Mechanism of commensurability oscillations in anisotropic antidot lattice

Abstract An experimental study of commensurability oscillations in anisotropic antidot lattice is performed. A fundamental peak of the resistivity tensor is determined by a peak of the conductivity tensor perpendicular to the current flow direction in the case of smaller diagonal component of conductivity than the off-diagonal one. Moreover, by using the model of electron diffusion along the antidot array, the peak of conductivity is explained. At the peak of conductivity, the electron can diffuse along the antidot array by skipping over the antidots, and the conductivity is enhanced.

Electron localization in band-tail transport of high-mobility poly-Si TFTs

Abstract The results of low-temperature transport properties on high-mobility poly-Si thin film transistors (TFTs) have been examined by measurement of the resistance as a function of temperature, magnetoresistance (MR) and Hall effect. It has been shown that the transition between the two-dimensional (2D) weak and strong localization (WL and SL) occurs around the sheet resistance R = π h /e 2 (⋍ 13 kΩ), where ( 1 2 )k F l ∼ 1 , in the band-tail states, caused by potential fluctuations in the inversion layer of TFTs. In the barely SL regime, R at low temperatures is found to obey the Efros-Shklovskii variable-range hopping (VRH) law in the presence of the Coulomb gap in the density of states. In both regimes of WL and SL, a negative MR has been observed and interpreted by the quantum interference effect in each regime.

Potential modulation of a dot array system induced by pulsed photoexcitation

In the far-infrared response of dot structures, a shift of the resonance peak of electrons in a dot can be clearly observed under pulsed photoexcitation. The peak shift is interpreted by a potential modulation due to photoexcitation. In a dot array fabricated on a double heterostructure, the potentials of vertically aligned two dots are also modulated. The modulation is different from each other. This potential modulation gives the possibility that the vertical coupling between two dots can be controlled by photoexcitation.

Magnetocapacitance in quantum Hall regime with external dc current

Abstract We have investigated the magnetocapacitance between gates and two dimensional electron system in a GaAs/AlGaAs heterostructure with an external dc current at quantum Hall (QH) plateau regime. With increasing dc current, the minimum values of capacitance at QH plateaux increase and the range of magnetic fields showing the capacitance minima decreases. The electrons excited by the external dc current enhance the σ XX which causes the breakdown of the QH effect. In a certain magnetic field and current direction, the capacitance dip at QH plateau splits into two valleys with dc current. These results are considered to be due to the change of effective gate voltage caused by the current. The breakdown current of the QH effect evaluated by the resistance measurement is several times larger than that obtained by the dc current dependence of the magnetocapacitance, since the capacitance is very sensitive to the residual σ XX . The vanishing of the capacitance minima may be the precursor of QH breakdown.

Breakdown of quantum hall effect in two dimensional electron system with antidot arrays

Abstract We have investigated the influence of antidot arrays on the breakdown of quantum Hall effect (QHE) in GaAs/AlGaAs heterostructures. In the samples without antidots, the value of the critical current remarkably decrease with increasing electron densities by illumination. In the samples with antidot arrays, the critical current still keeps its high value at relatively high electron density after illumination. These results indicate that the antidots can act as artificial potential fluctuations and modify the current distribution in QHE regime.