Junichi Fujikata

Si nano-photodiode with a surface plasmon antenna

We developed a Si nano-photodiode with a surface plasmon (SP) antenna for a SiON waveguide-integrated structure. We showed that interfacial periodic nano-scale metal-semiconductor-metal Schottky electrodes function as an SP optical antenna and also as an optical coupler between a SiON waveguide and a very thin Si-absorption layer.

On-Chip Optical Interconnect

We describe a cost-effective and low-power-consumption approach for on-chip optical interconnection. This approach includes an investigation into architectures, devices, and materials. We have proposed and fabricated a bonded structure of an Si-based optical layer on a large-scale integration (LSI) chip. The fabricated optical layer contains Si nanophotodiodes for optical detectors, which are coupled with SiON waveguides using surface-plasmon antennas. Optical signals were introduced to the optical layer and distributed to the Si nanophotodiodes. The output signals from the photodiodes were sent electrically to the transimpedance-amplifier circuitries in the LSI. The signals from the photodiodes triggered of the circuitries at 5 GHz. Since electrooptical modulators consume the most power in on-chip optical interconnect systems and require a large footprint, they are critical to establish on-chip optical interconnection. Two approaches are investigated: 1) an architecture using a fewer number of modulators and 2) high electrooptical coefficient materials.

First demonstration of high density optical interconnects integrated with lasers, optical modulators, and photodetectors on single silicon substrate

Optical interconnects integrated with lasers, silicon optical modulators and germanium photodetectors on a single silicon substrate were demonstrated for the first time. A 5 Gbps line bit rate and 3.5 Tbps/cm2 transmission density were achieved.

Low-resistance tunnel magnetoresistive head

A tunnel magnetoresistive (TMR) head with a low resistance of about 30 /spl Omega/ and effective track width of 1.4 /spl mu/m was fabricated using an in situ natural oxidation (ISNO) technique. Its read-output was almost the same as that expected from test elements at the wafer level. We found no large difference in noise voltages between TMR head and GMR head when their resistance was about 30 /spl Omega/. A very low-resistivity TMR element with a resistance-area product of 14 /spl Omega//spl middot//spl mu/m/sup 2/ and a fairly high /spl Delta/R/R of 14% was also developed using ISNO. A signal-to-noise ratio consideration suggests that such low resistance is a key to TMR heads for high recording densities.

Thermal fluctuation aftereffect of exchange coupled films for spin valve devices

The annealing-time-dependent change in the unidirectional anisotropy field for exchange coupled films with FeMn, PdPtMn, NiMn, and NiO, which are generally used for spin valve, was investigated after thermal annealing below their blocking temperatures. It was shown that the distribution of local blocking temperature was similar for each exchange coupled film. The annealing-time-dependent change in the unidirectional anisotropy was explained by the use of thermal fluctuation aftereffect model, and the relaxation time for the anisotropy change was found to be much larger for FeMn than for PdPtMn, NiMn, and NiO, when normalizing annealing temperature by their blocking temperatures. From the viewpoint of device application, however, NiMn was proven to be the most promising antiferromagnet in terms of thermal stability because of its highest blocking temperature.

25 GHz operation of silicon optical modulator with projection MOS structure

We report a high-speed and compact silicon optical modulator based on the free carrier plasma dispersion in a silicon rib waveguide with a MOS (metal-oxide-semiconductor) junction structure. To achieve high-speed and high-efficiency performance, an improved structure of a very small rib waveguide including a projection MOS junction was studied. We demonstrated high speed of 25 GHz operation in case of 120-200 μm phase-shift length and high optical modulation efficiency of 0.5-0.67 Vcm for VπL by using a projection MOS junction structure. According to the carrier-density simulation, higher operation bandwidth up to 40 GHz can be realized.

Waveguide-integrated Si nano-photodiode with surface-plasmon antenna and its application to on-chip optical clock distribution

We developed a waveguide-integrated Si nano-photodiode (PD) with a surface plasmon (SP) antenna for on-chip optical clock distribution. The interfacial periodic nano-scale metal-semiconductor-metal Schottky electrodes were shown to function as an SP optical antenna and also as an optical coupler between a SiON waveguide and a very thin Si-absorption layer. Furthermore, a very high speed response of 17 ps as well as enhanced photoresponsivity was achieved for a 10-mum coupling length. By using this technology, we fabricated a prototype of a large-scale-integration (LSI) on-chip optical clock system and demonstrated 5 GHz of optical clock circuit operation connected with a 4-branching H-tree structure.

First Demonstration of Athermal Silicon Optical Interposers With Quantum Dot Lasers Operating up to 125 °C

We previously proposed a photonics-electronics convergence system to solve bandwidth bottleneck problems among large-scale integrations (LSIs) and demonstrated a high bandwidth density with silicon optical interposers at room temperature. For practical applications, the interposers should be usable under high-temperature conditions or rapid temperature changes so that they can cope with the heat generated by the mounted LSIs. We designed and fabricated athermal silicon optical interposers integrated with temperature-insensitive components on a silicon substrate. An arrayed laser diode (LD) chip was a flip-chip bonded to the substrate. Each LD had multiple quantum dot layers with a 1.3-μm lasing wavelength. The output power was higher than 10 mW per channel up to 100 °C. Silicon optical modulator and germanium photodetector (PD) arrays were monolithically integrated on the substrate. The modulators were structured as symmetric Mach-Zehnder interferometers, which were inherently insensitive to temperature and wavelength. The phase shifters composed of p-i-n diodes were stable against temperature change under a constant bias-current condition. The PD photocurrent was also temperature insensitive, and the photo-to-dark current ratio was higher than 30 dB up to 100 °C. We achieved error-free data links at 20 Gbps and a high bandwidth density of 19 Tbps/cm2 operating up to 125 °C without adjusting the LDs, modulators, or PDs. The interposers are tolerant of the heat generated by the mounted LSIs and usable over the extended industrial temperature range without complex monitoring or feedback controls. The bandwidth density is sufficient for the needs of the late 2010s.

Si Waveguide-Integrated Metal–Semiconductor–Metal and p–i–n-Type Ge Photodiodes Using Si-Capping Layer

We studied Si waveguide-integrated metal–semiconductor–metal (MSM) and p–i–n-type Ge photodiodes (Ge-PDs), using a Si-capping layer. As for an MSM Ge-PD, the Schottky barrier height was increased up to 0.44 V by applying a 8–20 nm Si-capping layer, and a very low dark current density of approximately 0.4 nA/µm2 was achieved with a high responsivity of 0.8 A/W. In addition, a small electrode spacing of 1 µm realized high-speed photodetection of 20 Gbps. As for a p–i–n-type Ge-PD, by applying a 10–20 nm Si capping layer, the contact resistance between a metal electrode of Ti/TiN/Al and n+-Si capping layer was successfully reduced to 1×10-5 Ωcm2. A 45 GHz bandwidth was obtained with a low dark current density of 0.8 nA/µm2. Moreover, a more than 20 GHz bandwidth was achieved with zero-bias voltage. In the case of zero-bias voltage operation, a 3 dB bandwidth was a little affected by input power, which would originate from the photocarrier screening effect on the built-in electric field.

Large optical transmission through a single subwavelength hole associated with a sharp-apex grating

The effect of grating shapes on optical transmission in a bulls eye structure (a single subwavelength hole surrounded by a concentric grating in a metal) is discussed. Finite-difference time-domain calculations predict that a sharp-apex shape gives as high a reflective structure to the propagating surface plasmon polaritons (SPPs) as does a rectangular shape. Fabricated samples with a sharp-apex grating actually show large optical transmission (a factor of 400 greater than that of samples with a single hole) even when the number of corrugations is three. This result indicates that a sharp-apex grating acts as an effective SPP reflector to confine the energy around the hole, resulting in high optical throughput.