Hiroyuki Wado

Correlation between thermal hysteresis width and broadening of metal–insulator transition in Cr- and Nb-doped VO2 films

We investigated the effects of Cr and Nb doping on the metal–insulator transition (MIT) of single-oriented VO2 films deposited on Al2O3 substrates. The MIT temperature (TMI) increased with trivalent Cr doping, whereas it decreased with pentavalent Nb doping. The temperature coefficient of resistance (TCR) and the thermal hysteresis width of the MIT (ΔTMI) were suppressed by Cr and Nb doping, and Nb doping was about twice as effective as Cr doping for decreasing TCR and ΔTMI. We found that the maximum TCR and ΔTMI for the doped VO2 films have a correlation with the lattice constant irrespective of doping elements, suggesting that the lattice deformation caused by metal-ion doping is involved in the decreases in TCR and ΔTMI. There is also an apparent correlation between the maximum TCR and ΔTMI for the doped VO2 films. The present findings suggest that the combination of metal-ion doping with other techniques such as strain control is required to achieve nonhysteretic MIT with a large TCR in VO2 films.

High temperature coefficient of resistance of low-temperature-grown VO2 films on TiO2-buffered SiO2/Si (100) substrates

The introduction of a TiO2 buffer layer significantly improved the temperature coefficient of resistance (TCR), a measure of the sharpness of the metal–insulator transition, for films of VO2 grown on SiO2/Si (100) substrates at growth temperatures below 670 K. X-ray diffraction and Raman scattering measurements revealed that polycrystalline VO2 films were formed on the TiO2-buffered substrates at low temperatures below 600 K, whereas amorphous films were formed at these temperatures on SiO2/Si (100) substrates without a TiO2 buffer layer. Electron microscopy studies confirmed that the TiO2 buffer layer enhanced the grain growth of VO2 films at low growth temperatures. The VO2 films grown at 600 K on TiO2-buffered substrates showed a large TCR of more than 80%/K as a result of the improved crystallinity and grain size of the VO2 films. Our results provide an effective approach toward the integration of VO2-based devices onto Si platforms at process temperatures below 670 K.

Flat grating lens utilizing widely variable transmission-phase via guided-modes

We experimentally demonstrate a polarization-independent flat grating lens in the near-infrared region. The grating lens consists of ridges in the square lattice arrangement, and the ridge dimensions are gradually changed to distribute a phase map with focusing ability. It is well known that guided modes in gratings offer unity-reflection at a resonance, and therefore the transmission phase is widely varied around the resonance. We employ such transmission phase behavior and show that high transmittance is obtained in each unit cell for wide variation range of the transmission phase at the operation wavelength by sharpening the resonance. This enables us to accomplish a highly efficient transmissive grating lens.

High-Power Vertical-Cavity Surface-Emitting Laser under a Short Pulsed Operation

We report on the lasing characteristics of a single vertical-cavity surface-emitting laser (VCSEL) with InGaAs/GaAs multiple-quantum-wells under high-power pulsed operations. External quantum efficiency was improved by decreasing carrier overflow in the active region, and it was found that increasing the number of quantum wells suppressed carrier overflow. The highest peak pulsed power of over 12.5 W at an injection current of 20 A was achieved by a single VCSEL with five InGaAs quantum wells (QWs) in the active region.

SOI-based Si/SiO2 high-mesa waveguides for a compact infrared sensing system

High-mesa waveguides have been fabricated for a compact infrared sensing system, as they have a benefit of having optical evanescent field outside of their solid waveguides and thus this contributes to the sensing of gas or liquid in a compact area. Fabricated semiconductor on insulator (SOI)-based Si/SiO2 high-mesa waveguides, by using neutral loop discharge (NLD) plasma etching technique, showed extremely low propagation loss compared to those fabricated by using conventional reactive ion etching (RIE) technique. Moreover, we also demonstrate actual sensing for liquid methanol. By using 6 mm SOI-based Si/SiO2 high-mesa waveguide with waveguide width of 0.7 micro-meters, we could successfully obtain sufficient infrared absorption for the first time, therefore, this proved that the proposed waveguide had an optical field outside of the waveguide.

Spin torque diode effect of the magnetic tunnel junction with MnGa free layer

We fabricated a magnetic tunnel junction (MTJ) using an MgO barrier and MnGa electrode, which shows large magnetic anisotropy, and we investigated the spin torque diode effect in the MTJ. The magnetoresistance ratio increased up to approximately 40% by inserting thin FeB/Fe layers at the MnGa/MgO interface. The obtained diode effect was as high as 70 GHz, which is thought to occur due to the coupled precession acoustic mode in the MnGa/Fe-B multi-layer. Numerical simulation suggests that a very high frequency diode signal of more than 150 GHz can be expected in the optical mode precession.We fabricated a magnetic tunnel junction (MTJ) using an MgO barrier and MnGa electrode, which shows large magnetic anisotropy, and we investigated the spin torque diode effect in the MTJ. The magnetoresistance ratio increased up to approximately 40% by inserting thin FeB/Fe layers at the MnGa/MgO interface. The obtained diode effect was as high as 70 GHz, which is thought to occur due to the coupled precession acoustic mode in the MnGa/Fe-B multi-layer. Numerical simulation suggests that a very high frequency diode signal of more than 150 GHz can be expected in the optical mode precession.

Multi-Gas Sensor by Infrared Spectrometer

In order to detect many types of gases (CO2, NOx, SOx, C2H5OH) in the automotive cabin by infrared absorption sensor, we developed a novel micro electro mechanical systems (MEMS) based Fabry–Perot spectrometer with an ultra wide wavelength range (3.20–8.40 μm) compared to previously reported spectrometers (typically 2.80–5.80 μm). The wavelength range of a Fabry–Perot spectrometer is known to increase by increasing the ratio of the refractive indices of the multilayer mirrors. Thus, a novel mirror structure was proposed replacing the low refractive index layer of SiO2 (nL = 1.44) with “air (nL = 1.00)” for a wider wavelength range. To fabricate the proposed structure, the internal stress of the four ultra-thin polycrystalline silicon films (ca. 320 nm) was controlled tensile by the deposition temperature. A gas sensor was fabricated using our developed spectrometer. It was found that the sensor detected CO2 and C2H5OH successfully.

High-power vertical-cavity surface-emitting laser under pulsed operation

We have investigated InGaAs multiple-quantum-wells vertical-cavity surface-emitting laser (VCSEL) under high-power pulsed operation. The peak pulsed power of over 12.5 W has been achieved with the single VCSEL device which has five InGaAs quantum wells.