Yasunori Tokuda

Giga-bit scale DRAM cell with new simple Ru/(Ba,Sr)TiO/sub 3//Ru stacked capacitors using X-ray lithography

We have fabricated experimental memory cell arrays with a unit cell size of 0.29 /spl mu/m/sup 2/ (0.38 /spl mu/m/spl times/0.76 /spl mu/m). The layout was designed for a half-pitch 8F/sup 2/ cell with 0.14-/spl mu/m process technology, which is promising for 1-gigabit DRAMs and beyond. We developed three advanced technologies for this fabrication. Firstly, synchrotron radiation (SR) X-ray lithography was used to replicate ultra-fine patterns instead of optical lithography. Secondly, we introduced a simple stacked capacitor composed of a metal-organic chemical vapor deposition (MOCVD) grown (Ba,Sr)TiO/sub 3/ (BST) high-dielectric-constant film sandwiched by Ru-metal electrodes. Thirdly, we developed advanced etching techniques for the fine pattern fabrication using improved ECR discharged plasmas, which give less microloading effects.

Remarkable Breakdown Voltage Enhancement in AlGaN Channel HEMTs

We demonstrated a remarkable breakdown voltage enhancement in a new high-electron-mobility transistor (HEMT) with a wider bandgap AlGaN channel layer. A Si ion implantation doping technique was utilized to achieve sufficiently low resistive source/drain contacts. The obtained maximum breakdown voltage was 1650 V with a gate-drain distance of 10 mum. This result is very promising for the further higher-power operation of high-frequency HEMTs.

Design and Characteristics of Guardring-Free Planar AlInAs Avalanche Photodiodes

This paper reports a guardring-free planar AlInAs avalanche photodiode (APD) for optical fiber communications. AlInAs APDs can achieve a larger gain-bandwidth product (GBP) with lower excess noise than commercial InP APDs, and the guardring-free planar structure enables these superior AlInAs APDs to have both a low dark current and high reliability for practical use. We present the structure, fabrication, designs, APD characteristics, and receiver sensitivity, systematically. The guardring-free planar structure and its peculiarities are described, and we show APD designs for 2.5-Gb/s and 10-Gb/s applications and their characteristics. A 0.2-mum -thick AlInAs multiplication layer achieves a GBP of 120 GHz and an excess noise factor of 2.9 at a multiplication factor of 10. Their dark currents are less than 20 nA and their lifetime is evaluated to be 25 million hours at 85degC. Lastly, we demonstrate that the guardring-free planar AlInAs APDs with a transimpedance amplifier achieve the remarkable sensitivity of -37.0 dBm at a bit error rate of 10-10 for 2.5-Gb/s signals and of -29.9 dBm at a bit error rate of 10-12 for 10-Gb/s signals. This performance indicates that the guardring-free planar AlInAs APDs have made great advances against commercial InP APDs and other AlInAs APDs.

Widely tunable SOA-integrated DBR laser with combination of sampled-grating and superstructure grating

A 30 nm tunable laser diode utilizing front SG-DBR and rear SSG-DBR was demonstrated for the first time. High output power over 45 mW as well as good wavelength control was achieved with SMSR greater than 40 dB.

Optical high-frequency pulse generation with wavelength tunability by using a DBR laser with vernier-type gratings

An optical high-frequency pulse generation method with wavelength tunability is presented by taking advantage of vernier-type gratings in a distributed Bragg reflector laser diode. Experimental results showed that sinusoidal optical pulses, whose lasing wavelengths were tunable, were created with repetition frequencies around 30 GHz. The widths of the radio frequency spectra, furthermore, were drastically reduced by means of mode locking with externally injected optical pulses.

A widely tunable SOA-integrated DBR laser by combination of sampled and superstructure gratings

We report on the development of the first 30 nm tunable laser diode utilizing a front sampled grating-distributed Bragg reflector (SG-DBR) and a rear superstructure grating DBR (SSG-DBR). A high output power greater than 18 mW was achieved with superior longitudinal mode stability, as well as an overall power variation of less than 3 dB for the well-defined laser. Furthermore, lasers integrated with a semiconductor optical amplifier (SOA) enabled the output power to be controlled independently both by the active current and the SOA current, as well as achieving a maximum output power of 45 mW. The side-mode suppression ratio was in excess of 40 dB for all the 50 GHz spaced wavelength channels.

Lasing mode analysis of self-pulsating DFB laser diodes for all-optical signal recovery at 40 Gbps

Three-electrode-type self-pulsating (SP) distributed feedback (DFB) laser diodes (LDs) were fabricated. Lasing mode selectivity and pulsating frequency of SP-DFB LDs were investigated experimentally and device parameter dependences were well explained by numerical simulations.

Widely wavelength tunable optical clock generation by use of injection locked DBR lasers with vernier gratings

We propose a versatile optical clock generation method with 10 nm wavelength tunability using widely tunable DBR lasers. Injection locking was experimentally demonstrated for 30 GHz sinusoidal pulses at 1545 and 1560 nm as well.