Research field introduction
As the existing electronic device technology based on the characteristics of current silicon semiconductors approaches the fundamental physical limit, there is a strong demand for a new concept technology that can overcome this. While the conventional electronic device technology uses only the charge characteristics of electrons, the spintronics is a technology to overcome the limit by using both quantum mechanical spin phenomena as well as charge. The spintronic device has characteristics such as high speed and ultra low power consumption as well as nonvolatility which is an intrinsic characteristic of spin, and has attracted attention as a next generation device. We are devoting much effort to research the core technology for implementing these spintronic devices.
Representative details of techniques include the spin information storage element technology, the spin information processing element technology and the spin communication element technology. Through these studies, we would like to develop a new concept spintronic device.
Gate controlled spin hall transistor
- - Spin-orbit coupling is a phenomenon in which spins and charges are converted, which is the most important physical phenomenon for the development of next-generation spin electronic devices.
- - We fabricate spin electronic devices that simultaneously use spin Hall effect and Rashba effect.
- - We show that the spin signal can be converted into the electric signal and the electric signal can be converted to the spin signal. The magnetization direction of the injection can be controlled by the gate voltage.
The control of spin orbit coupling
- - The spin-orbit coupling constant along the spin crystal direction was measured.
- - The Rashba field can be much larger than the Dresselhaus field in this system if the Rashba parameter can be gated and the Dresselhaus parameter can be observed to be constant with respect to the gate voltage.
- - We have shown that the effective Rashba parameter increases when the Rashba field and the external field are parallel and decreases when it is antiparallel, which shows that the external field interferes with the Rasbha effect.
Measurement of Dzyaloshinskii-Moriya Interaction (DMI) using electrically induced spin-wave
- - In the presence of DMI of the magnetic material, the frequency of the spin wave changes along the traveling direction of the spin wave, and the DMI of the magnetic material can be measured using this.
- - Two antennas for injection and detection of the spin wave were formed on the thin film. A microwave source was connected to one antenna to generate a spin wave in the magnetic thin film and a spin wave passed from the antenna was detected with the other antenna located on the other side.
- - The DMI constant was determined from the actual measured frequency difference. In the Pt/Co(20)/MgO, MgO/Co(20)/Pt thin film, the DMI constant is proportional to the reciprocal of the thickness and it tells us that DMI is caused by the interface.
Analysis of the spin behavior using ultrafast laser
- - Utilizing a ultrafast laser measurement device, we studied for extremely high-speed spin phenomena generated from spintronic devices and found out the principle.
- - demagnetization-driven spin generation is caused by the electron-magnon coupling, and the spin generation rate is proportional to the time change of magnetization.
- - We showed that the spin current generated by ultrafast demagnetization and spin-dependent Seeback effect may cause precession in ferromagnets.
Silicon/III-V semiconductor spin hybrid devices using epi-transfer technology
- - 2DEG is indispensable for the implementation of the spin transistor using the Rashba effect. Factors controlling the spin-orbit coupling inside the 2DEG were observed according to the change of the energy band structure.
- - The InAs 2DEG structure epitaxially grown on the InP substrate for vacuum thin film deposition was separated from the substrate using a wafer bonding technique and transferred onto a silicon substrate. The junction interface sturcture, the electrical properties including electron mobility and the gate dependence of the spin-orbit coupling were observed.
- - By observing the spin injection / detection in the n-InGaAs channel transferred onto the silicon substrate at room temperature through the non-local spin valve signal, we presented the possibility of a silicon / III-V compound semiconductor spin device ahead of the rest of the world.
2D nanoflake synthesis and characterization to control spin-orbit coupling strength
- - Nb-doped MoS2 crystal was synthesized and its characteristics were measured.
- - We confirmed that the charge type of Nb-doped MoS2 is a hole and we observed that the hole concentration of Nb-doped MoS2 was almost constant at 2 to 3 × 1019/cm3 regardless of the temperature. It shows that Nb is excessively doped in MoS2 and charge collision due to ion impurities has a greater influence on mobility than phonon collision.
- - For Nb-doped MoS2 devices application, p-n homo-junction devices were fabricated. Because MoS2 doped with Nb is excessively pinged to p-type and intrinsic MoS2 shows n-type characteristics, a rectification phenomenon appears depending on the gate voltage and it is possible to apply to n-type and p-type MoS2 by contacting the same metal to device.