Tomoyuki Kikutani

Fabrication of buried metal dot structure in split-gate wire by scanning tunneling microscope

Fabrication of a new class of quantum structure which has a buried nickel (Ni) dot in a split-gate quantum wire using a scanning tunneling microscope (STM) is described. In order to fabricate small structures at the desired wire surface position, we employ a combined STM/scanning electron microscope (SEM) system in high vacuum. The fabrication methods are those based on simple electrical evaporation with a tungsten (W) tip. On the free surface far from the split-gate electrodes, the structure produced after applying a single voltage pulse is a small mesa (150 nm diameter, 20 nm high). However near the gates, large holes (150 nm diameter at half-depth, 85 nm deep) are created. Such large holes act as the pinpoint antidot for the two-dimensional electron gas (2DEG) lying at a depth of 60 nm from the wafer surface. As a metallic material, we adopted a Ni. For burying Ni into the hole, we moved the Ni-coated W tip to the hole bottom by observing the SEM image and created a Ni dot in the hole by applying a single voltage pulse.

Spontaneous spin-splitting observed in resonant tunneling diode with narrow band-gap asymmetric quantum well

Abstract This paper deals with the experiments to observe the spontaneous spin-splitting (SSS) in the resonant tunneling diode (RTD), which was specially designed to have a narrow band gap as well as an asymmetric potential well. In current–voltage ( I – V ) characteristics, there observed a current peak doubly splitted (denoted as P 1–1 and P 1–2 ) at low bias field. The difference of the derivative peak height of P 1–1 between those with and without the parallel magnetic field (d I /d V ( B )−d I /d V ( B =0)), which was applied along the easy axis of the emitter electrode, increased with increasing field strength, while the quantity of P 1–2 stayed almost zero. The increase of the difference of the derivative peak height of P 1–1 was found to saturate almost at the coercive field of the electrode. It was also found that those variations of the derivative peak height were observed only in the case of electron injection via the ferromagnetic (NiFe) emitter electrode. The amount of splitting ( ∼15 meV ) observed was almost the same as those estimated from magnetoresistance measurements. These results suggest that the observed splitted peak is intimately related to the SSS expected at the center well in our unique RTDs.

Large and anisotropic zero-field spin-splittings in InxGa1−xAs/InyAl1−yAs (x,y>0.6) heterojunctions

Abstract Large and in-plane anisotropic zero-field spin-splittings are found in two-dimensional electron gas (2DEG) formed at In x Ga 1− x As/In y Al 1− y As ( x , y >0.6) heterojunctions. Low-temperature magnetoresistance (MR) measurements were carried out in van der Pauw, Hall bar and quantum wire field effect transistor (QWR-FET) samples. Maximum spin–orbit coupling constant α zero of 78 (×10 −12 eVm ) was attained at 1.5 K in the Hall bar sample with the 〈−1 1 0〉 direction. Also, an in-plane anisotropy of almost twice as well as a gate-voltage-dependent change of α zero are confirmed in QWR-FET samples with 〈−1 1 0〉 and 〈1 1 0〉 directions. Those results suggest the importance of interface contribution to the zero-field spin splitting, which might be enhanced in our unique heterojunctions.

High field magnetoresistance and ESR measurements on Ni stripes on GaAs substrate

Ni nanowires fabricated on a GaAs substrate, which have width narrower than 500 nm and thickness 50 nm, showed an antiferromagnetic-like domain structure. Many Ni nanowires fabricated on GaAs using the electron beam lithography technique have been studied by electron spin resonance (ESR). Splitting of ESR signals in the frequency-field diagram was observed around 57 GHz, although only a ferromagnetic resonance mode was observed for other frequencies. This mode is suggested to be an antiferromagnetic resonance related to the domain structure on the Ni nanowires.

Possible large zero-field spin-splittings in InxGa1−xAs/InyA11−yAs (x,y=0.75) heterojunctions

Abstract Zero-field spin-splittings are estimated from low-temperature magnetoresistances in two-dimensional electron gas (2DEG) in In x Ga 1−x As / In y A 1 1−y As (x,y=0.75) van der Pauw, Hall-bar and quantum wire field effect transistor (QWR-FET) samples. Maximum spin–orbit coupling constant α zero of 78 (×10 −12 eVm) was obtained in the Hall bar sample with 〈−1 1 0〉 direction, which has a sheet electron density and a mobility at 1.5 K of 1.1×10 12 /cm 2 and 5.54×10 5 cm 2 / Vs . In-plane anisotropies of mobility as well as of α zero are confirmed in QWR-FET samples with 〈1 1 0〉 and 〈1 1 0〉 directions. If those results are considered together with the fact that a part of α zero was able to be changed by the gate-voltage, interface effect contributing to the zero-field splitting might play an important role in this heterojunction.

Critical layer thickness study in In0.75Ga0.25As/In0.5Al0.5As pseudomorphic resonant tunneling diode structure grown on GaAs substrates

Abstract We have studied critical layer thickness (CLT) in an In 0.75 Ga 0.25 As resonant tunneling diode structure grown on a GaAs substrate via an InAlAs step-graded buffer (SGB) with two types of inverse step(IS)-SGB. We have observed red or blue shift in photoluminescence spectra and CLT change depending on SGB condition. This change reflects from the difference of residual strain InAlAs SGBs adopted here.

Fabrication of a Split-Gate Quantum Wire Having a Ferromagnetic Dot

We have fabricated split-gate quantum wires having a buried ferromagnetic dot, by successively utilizing electron-beam (EB) and two-step scanning tunneling microscope (STM) fabrication. For STM fabrication, we used an STM/scanning electron microscope (SEM) combined with a system operated in high vacuum. The fabrication method is a kind of electrical evaporation with a tungsten (W) tip (top curvature is less than 50 nm). In the first step, a W tip was brought between the split-gate, and then a hole was fabricated by applying a pulse voltage between the W tip and the sample surface. In the second step, a W tip coated with nickel (Ni) was brought near the fabricated hole. Then by applying a pulse voltage between the Ni-coated W tip and sample surface, electrically evaporated Ni from the tip is buried into the hole. In a preliminary measurement at 0.3 K, we obtained the following unique transport properties. In a 4-terminal conductance (G 4t ) as a function of gate voltage (V g ), we observed a clear kink (an abrupt change of dG 4t /dV g and step structures) before full pinch-off of the wire. In both regions of G 4t , that is, when V gk < V g (before the kink appears) and when V g < V gk (after the kink appears) (V gk is the gate voltage at which the kink appears), some step structures are seen. The step difference (ΔG 4t ) is, however, different between the two regions. That is, ΔG 4t = 2 - 4 x (2e 2 /h) before the kink appears, while ΔG 4t = (1/8) - (1/4) x (2e 2 /h) after the kink appears.

Cross-Sectional Transmission Electron Microscope Observation of Small Structures Made by Field-Induced Scanning Tunneling Microscope Fabrication

We observed cross-sectional transmission electron microscope images of small structures on a GaAs substrate made by the scanning tunneling microscope (STM) field-induced fabrication method. A cross-sectional image of a GaAs dot, fabricated by applying a voltage pulse to a W tip, was 400 nm wide and had a highly symmetric double ditch structure. The inside of the dot consisted of GaAs polycrystal and the boundary was clearly limited by specific crystal planes. If the fabrication mechanism is considered to be field-induced evaporation in the STM regime, the anisotropy would have arisen due to a difference in work function between each plane. We also observed a Ni dot fabricated using a Ni-coated tip. The dot was a spherical with about a 110 nm diameter and it consisted of Ni polycrystal. Using the tip, we could obtain only one or two Ni dots, suggesting it behaves like a solid source rather than a liquid ion source.

Normal-super-normal junction fabricated in a split-gate wire

The fabrication and low temperature transport properties of a new class of quantum structures which have a buried superconductor dot in a split-gate wire are described. To create such a structure, we employed a combined scanning electron microscope (SEM)/scanning tunneling microscope (STM) and performed a two-step fabrication process using a tip-induced field and thermal evaporation. We chose indium as a superconductor material and the dot size at the two dimensional electron gas (2DEG) plane was about 100 nm. Such a structure is expected to form a normal (2DEG)-super (In dot)-normal (2DEG) double tunneling junction when the wire is properly squeezed. In transport experiments at 0.3 K, different conductance step behavior and different nonlinear current-voltage characteristics were observed in the systems with and without an In dot.

Coulomb blockade and dot size in split-gate wire with a small mesa or a hole made by scanning tunneling microscopy

Abstract Coulomb blockade effects and related dot sizes of a two-dimensional electron gas (2DEG) are investigated in split-gate quantum wires with a mesa or a hole made by scanning tunneling microscopy (STM). From the dependencies on front (split)- and back-gate voltages, the smallest size of the 2DEG dot under the mesa is found to be limited by the mesa size itself giving stable Coulomb gaps. In a wire with a hole, Coulomb oscillations modulated by random telegraph signal (RTS) oscillation were observed. This suggests the coexistence of a “dot” and electron traps both probably created via STM fabrication. The “dot” was found to be larger than in the mesa case and to be unstable due to the interaction between the “dot” and traps.