Sungbong Park

Enhanced direct bandgap emission in germanium by micromechanical strain engineering

We propose a new class of optoelectronic devices in which the optical properties of the active material is enhanced by strain generated from micromechanical structures. As a concrete example, we modeled the emission efficiency of strained germanium supported by a cantilever-like platform. Our simulations indicate that net optical gain is obtainable even in indirect germanium under a substrate biaxial tensile strain of about 1.75% with an electron-hole injection concentration of 9 x 10(18) cm(-3) while direct bandgap germanium becomes available at a strain of 2%. A large wavelength tuning span of 300 nm in the mid-IR range also opens up the possibility of a tunable on-chip germanium biomedical light source.

Monolithic integration and synchronous operation of germanium photodetectors and silicon variable optical attenuators

We demonstrate the monolithic integration of germanium (Ge) p-i-n photodetector (PDs) with silicon (Si) variable optical attenuator (VOAs) based on submicrometer Si rib waveguide. A PD is connected to a VOA along the waveguide via a tap coupler. The PDs exhibit low dark current of ~60 nA and large responsivity of ~0.8 A/W at the reverse bias of 1 V at room temperature. These characteristics are uniform over the chip scale. The PDs generate photocurrents precisely with respect to DC optical power attenuated by the VOAs. Two devices work synchronously for modulated optical signals as well. 3-dB cut-off frequency of the VOA is ~100 MHz, while that of the PD is ~1 GHz. The synchronous response speed is limited by the VOA response speed. This is the first demonstration, to the best of our knowledge, of monolithic integration of Ge PDs with high-carrier-injection-based optical modulation devices based on Si.

Monolithic Integration of a Silica-Based Arrayed Waveguide Grating Filter and Silicon Variable Optical Attenuators Based on p–i–n Carrier-Injection Structure

We demonstrate a monolithic integration of a silica-based arrayed waveguide grating filter and silicon variable optical attenuators. The integrated device perform wavelength demultiplexing in 16 channels and provide high-speed power-level adjustment with a response of <15 ns in each channel.

Influence of carrier lifetime on performance of silicon p-i-n variable optical attenuators fabricated on submicrometer rib waveguides

We investigated influence of carrier lifetime on performance of silicon (Si) p-i-n variable optical attenuators (VOAs) on submicrometer Si rib waveguides. VOAs were fabricated with and without intentional implantation of lattice defects into their intrinsic region. Carrier lifetime was measured by pulse responses for normal incidence of picosecond laser pulse of 775 nm to the VOA, as approximately 1 ns and approximately 7 ns for the VOAs with and without defects, respectively. Carrier lifetime is determined by the sum of surface recombination and Auger recombination for VOAs without defects, while Schockley-Read-Hall recombination is dominant for the VOA with defects. As a result, attenuation efficiency (dB/mA) is 0.2-0.7 and 0.04-0.1, while 3-dB bandwidth is 40-100 MHz and over 200 MHz for the VOAs with and without defects, respectively. There is a trade-off relation between attenuation and response speed of the VOAs with respect to carrier lifetime i.e., attenuation efficiency is linearly proportional to the carrier lifetime, whereas response speed is inversely proportional to it.

All-silicon and in-line integration of variable optical attenuators and photodetectors based on submicrometer rib waveguides.

We demonstrate a monolithic integration of variable optical attenuators (VOAs) and photodetectors (PDs) based on submicrometer silicon (Si) rib waveguide with p-i-n diode structure for near infrared (NIR) light. To make the Si PD absorptive for NIR, we introduced lattice defects at the rib core by means of argon ion implantation. At reverse bias of 5 V, the PD exhibits dark current of approximately 1 nA, responsivity of approximately 65 mA/W at 1560-nm wavelength, and a 3-dB cut-off frequency of approximately 350 MHz, while the VOA shows approximately 100 MHz. The PD has an absorption coefficient as low as approximately 0.5 cm(-1), which is favorable for an in in-line PD configuration, where the PD absorbs a small portion of the optical power. For DC light, the PD precisely detects the optical power attenuated by the VOA with a detection range of over 40 dB. The 3-dB cut-off frequency of synchronous operation between the VOA and PD is approximately 50 MHz, which is limited by RF noise originating from the VOA drive current. Putting an isolation groove between the VOA and PD is effective for avoiding performance degradation in DC and RF operation.

Effect of Post-Growth Annealing on Morphology of Ge Mesa Selectively Grown on Si

Effect of the post-growth annealing on the morphology of a Ge mesa selectively grown on Si was studied from the viewpoint of near-infrared photodiode applications. By ultrahigh-vacuum chemical vapor deposition, Ge mesas were selectively grown at 600°C on Si (001) substrates partially covered with SiO2 masks. The as-grown Ge mesas showed trapezoidal cross-sections having a top (001) surface and {311} sidewall facets, as similar to previous reports. However, after the subsequent post-growth annealing at -800°C in the ultrahigh-vacuum chamber, the mesas were deformed into rounded shapes having a depression at the center and mounds near the edges. Such a deformation cannot be observed for the samples annealed once after cooled and exposed to the air. The residual hydrogen atoms on the Ge surface from the germane (GeH4) decomposition is regarded as a trigger to the observed morphological instability, while the final mesa shape is determined in order to minimize a sum of the surface and/or strain energies.

Si Photonics and Fiber to the Home

This paper describes Si photonics and the key application to fiber-to-the-home (FTTH). Mass production in terms of complementary metal-oxide-semiconductor technology is a prerequisite for Si photonics to provide a cost-effective solution, and FTTH should meet this requirement. We describe a design of the FTTH chip architecture with wavelength-division multiplexing based on ring-resonator demultiplexers and multiplexers. The grand challenge of Si photonics for this application is the production of Ge photodetectors and modulators.

Integration of optical devices based on Si, Ge and SiOx

Monolithic integration of various kinds of optical components on a silicon wafer is the key to making silicon photonics practical technology. Applying silicon (Si) photonics to telecommunications further requires low insertion loss and polarization independence. We demonstrate monolithic integration of Si variable optical attenuators germanium photodiodes, a SiOx-based arrayed waveguide grating filter, and a spot size converter that connects each component with low loss. Moreover, We also show a method for achieving low polarization dependent loss in a Si-VOA. Our results indicate the great potential of silicon photonics technology for optical devices in telecommunications.

Silicon photonic devices and their integration technology

Various photonic devices covering passive to active functions have been developed and monolithically-integrated on a silicon wire waveguide platform. Obstacles to practical applications are being eliminated by applying state-of-art fabrication technologies and unique device designs.

A monolithic integration of Ge photodiodes with Si variable optical attenuators

Ge p-i-n photodiodes integrated with Si variable optical attenuators exhibit low dark current of 60 nA and high responsivity of 0.85 A/W at −1 V. These Ge photodiodes have potential for monolithic integration with other Si photonic components.