Shiro Hidaka

Realization of In0.75Ga0.25As two-dimensional electron gas bilayer system for spintronics devices based on Rashba spin-orbit interaction

Narrow gap InGaAs two-dimensional electron gas (2DEG) bilayer samples are fabricated and confirmed to have good electronic qualities as well as strong Rashba-type spin-orbit interactions (SOIs). The 2DEG systems are realized by molecular beam epitaxy in the form of wide quantum wells (QWs) with thicknesses tQW ∼ 40–120 nm modulation doped in both the upper and lower InAlAs barriers. From the Hall measurements, the overall mobility values of μe ∼15 m2/V s are found for the total sheet electron density of ns ∼8 × 1011/cm2, although the ns is distributed asymmetrically as about 1:3 in the upper and lower 2DEGs, respectively. Careful low temperature magneto-resistance analysis gives large SO coupling constants of α ∼20 × 10−12 eV m as well as expected electron effective masses of m*/m0 ∼0.033-0.042 for each bilayer 2DEG spin sub-band. Moreover, the enhancement of α with decrease of tQW is found. The corresponding self-consistent calculation, which suggests the interaction between the bilayer 2DEGs, is carried...

Effects of cationic growth conditions of molecular beam epitaxy on ferromagnetic properties of Mn-doped ZnSnAs2 thin films

The effects of cationic growth parameters of molecular beam epitaxy on the crystalline and magnetic properties of Mn-doped ZnSnAs2, (Zn,Sn,Mn)As2, thin films were investigated in order to control ferromagnetism at room temperature. The combination of cationic beam equivalent pressures of Zn, Sn, and Mn atoms was varied at an optimal substrate temperature of 320 °C. Clear hysteresis loops were observed for the samples at 300 K, indicating ferromagnetism at room temperature. We found that the change in Zn flux does not significantly affect the composition of (Zn,Sn,Mn)As2 thin films and ferromagnetic properties and that ferromagnetic properties definitely depend on the amounts of Mn and Sn fluxes. X-ray photoelectron spectroscopy analysis revealed that the sum of the amounts of Mn2+ and Mn4+, which may contribute to the ferromagnetism in (Zn,Sn,Mn)As2 thin films, was approximately 60–70% of the total Mn amount with respect to ferromagnetic (Zn,Sn,Mn)As2 samples. This result indicates that the ferromagnetic properties could be controlled in multilayered (Zn,Sn,Mn)As2 by changing the combination of Zn, Sn, and Mn fluxes.

High-In-content InGaAs quantum point contacts fabricated using focused ion beam system equipped with N2 gas field ion source

Quantum point contacts (QPCs) in high-In-content InGaAs modulation-doped heterostructures fabricated using a focused ion beam (FIB) system equipped with a N2 gas field ion source (GFIS) are demonstrated. The minimum physical size of the fabricated QPCs in this study is ?30 nm, which is smaller than the typical physical size of QPCs (>50 nm) obtained by electron beam lithography and etching techniques. In addition, the fabricated QPCs are characterized electrically at low temperatures with magnetic fields. Since some of them show conductance quantization behaviors, the results indicate that the GFIS-FIB process is promising for quantum device fabrication.

Antiferromagnetic-ferromagnetic phase transition in (Zn,Sn,Mn)As2 epitaxial thin films

The magnetization of (Zn,Sn,Mn)As2 thin films epitaxially grown on InP(001) substrates exhibited an anomalous temperature dependence, increasing slightly with temperature, compared with estimates based on the mean-field theory (MFT). Assuming that ferromagnetic (FM) and antiferromagnetic (AF) phases coexist at low temperature, these anomalous magnetic properties can be well explained by an AF-FM transition through a paramagnetic phase. The Neel temperature TN of the AF phase is estimated from the threshold of the difference curve between the experimental and theoretical data, assuming the magnetization of the FM phase, which follows the MFT. The estimated TN rapidly increases by ∼50 K in the range of Mn concentrations where the hexagonal MnAs (h-MnAs) phase precipitates. This can be explained by the superparamagnetic behavior of the precipitated h-MnAs nanoclusters, which possess a high blocking temperature.

Influence of MBE growth parameters on film properties of ZnSnAs 2 :Mn thin films on InP substrates

We investigated the influence of growth parameters on the film properties of multinary ZnSnAs2 thin films with and without Mn doping grown by molecular beam epitaxy (MBE) on InP substrates. As growth parameters, the substrate temperature and the beam equivalent pressure (BEP) were varied. The most stoichiometric ZnSnAs2 thin film was obtained when the substrate temperature was 340 °C. We found that the high-quality Mn-doped ZnSnAs2 (ZnSnAs2:Mn) thin films can be realized by decreasing the beam flux of Zn source.

Impact of film thickness on crystalline and magnetic properties in Mn-doped ZnSnAs 2 thin films

We investigated the effect of film thickness on structural and magnetic properties of ZnSnAs2:Mn thin films. Samples with 80, 170 and 250 nm values of thickness were epitaxially grown on semi-insulating InP(001) substrates, followed by ZnSnAs2 buffer layers using molecular beam epitaxy (MBE). The lattice constants and crystal structures were determined by X-ray diffraction (XRD). The reciprocal lattice mapping revealed that the 80-nm-thick sample could be completely lattice-matched with InP (001) substrates. Ferromagnetic hysteresis was clearly observed at 300 K for the 170 nm and 250 nm samples.

Anomalous temperature dependence of magnetic properties in Mn-doped ZnSnAs 2 epitaxial thin films

We have observed anomalous temperature dependence of magnetization of ZnSnAs₂:Mn epitaxial thin films grown on InP(001) substrates compared with the values estimated by the mean-field theory using S = 5/2, as a function of temperature: The ZnSnAs₂:Mn epitaxial thin films exhibits slightly increasing magnetization with rising temperature. From the first-principles calculation, when Mn substitutes for neighboring Zn sites in ZnSnAs₂, MnZ_n-MnZ_n pairs result, which prefer antiferromagnetic (AFM) ordering. On the other hand, MnZ_n-MnS_n and MnS_n-MnS_n pairs can introduce both holes and spins, leading to ferromagnetic (FM) ordering. Thus, assuming that the magnetic property of ferromagnetic phase follows the mean-field theory, it can be explained by coexistence of FM and AFM phases.

Growth and characterization of (Zn, Sn, Ga)As 2 thin films grown on GaAs(001) substrate by molecular beam epitaxy

We fabricated (Zn, Sn, Ga)As 2 thin films grown on semi-insulating GaAs(001) substrate by molecular beam epitaxy and characterized their structural, compositional and electrical properties in terms of conduction-type control. The X-ray diffraction peaks of (Zn, Sn, Ga)As 2 films which shift toward lower angle compared with that of (Zn, Sn)As 2 film indicate that Ga incorporating in (Zn, Sn)As 2 decrease their lattice constants. Compositional analysis by electron-probe micro analyzer (EPMA) indicated that the composition ratios of Zn to all the cation sites are smaller than that of Sn. Hall effect measurements of these (Zn, Sn, Ga)As 2 thin films showed n-type conductivity. These results suggest that Ga atoms are substituted for Zn atoms more than Sn atoms and change their conduction type from p-type to n-type.

Characterization of spin-orbit coupling in gated wire structures using Al2O3/In0.75Ga0.25As/In0.75Al0.25As inverted heterojunctions

Gated parallel wire structures obtained from inverted-modulation-doped heterojunctions made of high-In-content metamorphic InGaAs/InAlAs were investigated. The narrowest wire width was found to be ∼190 nm made using electron beam lithography and reactive ion etching. Magneto-transport was measured at low temperatures. Weak anti-localization and suppression with applied negative gate voltages were observed in low-mobility wide wires (1360 nm), which were considered for a two-dimensional system. The dependence on the gate voltage of spin-orbit coupling parameters was also obtained by fitting. The parameters decreased as the negative gate voltages increased. The trend might originate not from the electron system at the InGaAs/InAlAs interface but from the other electron system accumulated at the Al2O3/InGaAs interface, which can also contribute to conductivity. In high-mobility narrow wires (190 nm), which are close to a one-dimensional system, weak anti-localization peaks were still observed, indicating str...

Verification of electrical spin injection into InGaAs two-dimensional electron gas from CoFe electrode by four-terminal non-local geometry

We performed electrical spin injection into In0.75Ga0.25As two-dimensional electron gases from Co0.8Fe0.2 electrodes by four-terminal non-local spin-valve (NLSV) measurement. We observed clear SV signals in NL resistance at 1.5 K. From the electrode spacing dependence of the signals, we estimated spin diffusion length and spin polarization to be ∼5.1 μm and ∼5.7 %, respectively. These are larger than those reported in similar systems.