Shuichi Ishida

Nonlocal quantum transport in narrow multibranched electron wave guide of GaAs-AlGaAs

Abstract Quantum transport of electrons through a narrow multibranched electron wave guide is investigated. The electron wave guides are fabricated from high and low mobility GaAsAlGaAs wafers. The background average resistance revealed quite different behavior in both samples. It is found in a high mobility (ballistic) sample that the resistance is negative and with applying magnetic field it approaches to zero at B∼0.2T, where the diameter of the cyclotron orbit is comparable to the channel width W∼0.3μm. In a low mobility (diffusive) sample, the average resistance is positive and non-zero in the entire field range. The positive resistance of the diffusive sample is comparable to a starting film resistivity, while the negative resistance originates from a bend of the current carrying channel and is attributed to the ballistic electron transmission.

Galvanomagnetic Properties of n-Type InSb at Low Temperatures : II. Magnetic Field-Induced Metal-Nonmetal Transition

Experimental evidence for the magnetic field-induced metal-nonmetal transition in n–InSb has been obtained, under the magnetic fields up to 13.3 kOe, by systematically investigating the Hall coefficient and the transverse magnetoresistivity for the samples containing 2×10 14 –4×10 15 cm -3 of donors with nearly fixed compensation ratio K ∼0.5 at liquid helium temperatures. The donor concentration dependence of the critical magnetic field strength at the transition satisfactorily agrees with the theoretical prediction on the impurity band. The observed minimum metallic conductivity proves practically insensitive to magnetic field and increases with increasing density of localized centers, in accord with Motts concept of the metal-nonmetal transition in doped semiconductors.

Galvanomagnetic Properties of n-Type InSb at Low Temperatures. I. Localization of Carriers and Metallic Impurity Conduction under Zero and Weak Magnetic Fields

The Hall coefficient and resistivity, under zero and weak magnetic fields, of n-InSb containing 1×10 13 –2×10 15 cm -3 of excess donors have been systematically investigated at liquid helium temperatures. By increasing compensating acceptors with a nearly fixed donor concentration ( N d ∼2×10 14 cm -3 , it is observed that the localization of the Hall carriers, or the metal-nonmetal transition, occurs at the excess donor concentration of ∼6×10 13 cm -3 (compensation ratio K ∼0.7) at 1.3 K. In the metallic range, the impurity concentration has been changed with a nearly fixed compensation ratio ( K ∼0.5), and it is confirmed that the impurity conduction (of metallic type) is recognized in the excess donor concentration range up to ∼4×10 14 cm -3 .

Nonlocal voltage fluctuations in a quasi ballistic electron waveguide

Abstract The nonlocal voltage fluctuations in a high mobility quasi ballistic electron waveguide are measured as a function of magnetic field. Although voltage probes are 1.2μm apart from the closest current path, nonlocal fluctuations are observed below 0.2T where the diameter of a cyclotron orbit is larger than the channel width. The behaviors are quite different from those in a diffusive sample and are considered to originate from a ballistic nature of the transport.

Overshoot of four-terminal magnetoresistance at GaAsAlGaAs narrow wire junctions

Abstract The magnetic field and temperature dependences of a four-terminal negative resistance are studied. The magnetic field dependence shows an overshooting behavior. The negative resistance decays with increasing magnetic fields, and exceeds the background resistance. Further increase of the field returns the overshooting contribution to zero. A possible explanation of this behavior is given by making use of the Landauer-Buttiker resistance formula. At low temperatures, the magnitude of the negative resistance is almost independent of temperature. This is also explained by the above formulation.

Galvanomagnetic Properties of n-Type InSb at Low Temperatures. III. Transport in the Band Tailing and Hopping between Donors in the Magnetic Freeze-Out Regime

The temperature dependence of the Hall coefficient and transeverse magnetoconductivity of n-InSb containing 2–5×10 14 cm -3 donors has been systematically investigated in the magnetic freeze-out regime. Anomalous behavior in the low temperature conduction band transport seperated from the impurity conduction is presented. Evidence is observed for the existence of mobility edge in the conduction band tail states. At high temperatures transport is predominated there by trap-limited mobility, while at lower temperatures by hopping between localized states. Hall mobility at the mobility edge well agrees with Friedmans theory. As for dependence of the impurity conduction on the temperature, magnetic field and donor concentration, a good agreement has been obtained between our experimental result and a recent theoretical prediction on the hopping conduction.

Temperature Dependence of the Inelastic Scattering Time in Metallic n-GaAs

Temperature dependence of the inelastic scattering time τ e in metallic n -GaAs ( n = N D - N A =7.83 ×10 16 cm -3 ) is determined experimentally in the wide temperature range of 50 mK–8 K by two methods: One is from the H 2 [ H : magnetic field] dependence of the magnetoconductivity Δ σ in weak fields, where Δ σ=σ( H , T )-σ(0, T ) is proportional to τ e 3/2 H 2 [ T : temperature]. The other is from the temperature dependence of Δ σ in strong fields, which is given by \( \varDelta \sigma=(e^{2}/2 \pi^{2}\hbar)[0.605(eH/c \hbar)^{1/2}-(D \tau _{\varepsilon})^{1/2}]\), where D is the diffusion constant. We find that the two methods give the T -1 temperature dependence of τ e . This result is in agreement with the recent theory of τ e ∝ T -1 in 3-dimensional system calculated by Isawa.

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.

Fabrication and transport characteristics of semiconductor wire and ring structures

We have fabricated narrow wires and small rings from GaAs/AlGaAs double heterostructures by using electron beam lithography and dry etching techniques. We describe the nature of universal conductance fluctuations in a single wire and the magnetoresistance oscillations with a period of‐h/e in a ring structure. It is experimentally shown that (i) from a single wire experiment, electrons with energies differing by the correlation energy Ecorr, have quite different magnetoresistance patterns, and (ii) from a ring experiment, the large aspect ratio (diameter/width) is important to obtain well‐defined oscillations.

Effects of the Anderson Localization on Magnetoconductivity in Metallic n-GaAs

Experimental results of magnetoconductivity in metallic n -GaAs with carrier concentration n = N D - N A =8.74 ×10 16 cm -3 have been analyzed with the theory of the Anderson localization. It was found that the coefficient of H 1/2 due to the localization agrees quantitatively with the theoretical prediction by Kawabata. On the other hand, we found that the value of g 3 due to the Coulomb interaction was nearly 8 times larger than the calculated value. These results obtained in n -GaAs are markedly different from previous results obtained in n -InSb.