Mark Beals

High performance, waveguide integrated Ge photodetectors

Photonic systems based on complementary metal oxide semiconductor (CMOS) technology require the integration of passive and active photonic devices. The integration of waveguides and photodetector is one of the most important technologies. We report a Ge p-i-n photodetector that is monolithically integrated with silicon oxynitride and silicon nitride waveguides. All processes and materials are CMOS compatible and can be implemented in the current integrated circuit process technology. The small size of the devices results in low absolute dark current. The waveguidecoupled Ge devices show high efficiency (~90%) over a wide range of wavelengths well beyond the direct band gap of Ge, resulting in a responsivity of 1.08 A/W for 1550 nm light. The device speed of 7.2 GHz at 1V reverse bias is strongly affected by the capacitance of the probe pads. The high-performance of the devices at low voltage ( </= 1V) facilitates the integration with CMOS circuits.

Impedance matching vertical optical waveguide couplers for dense high index contrast circuits

We designed and demonstrated a compact, high-index contrast (HIC) vertical waveguide coupler for TE single mode operation with the lowest coupling loss of 0.20 dB +/- 0.05 dB at 1550 nm. Our vertical coupler consists of a pair of vertically overlapping inverse taper structures made of SOI and amorphous silicon. The vertical coupler can suppress power oscillation observed in regular directional couplers and guarantees vertical optical impedance matching with great tolerance for fabrication and refractive index variations of the waveguide materials. The coupler furthermore shows excellent broadband coupling efficiencies between 1460 nm and 1570 nm.

Demonstration of a Fourth-Order Pole-Zero Optical Filter Integrated Using CMOS Processes

We demonstrate a compact fully tunable narrowband fourth-order pole-zero optical filter that is fabricated in a silicon complementary-metal-oxide-semiconductor foundry. The filter is implemented using silicon on oxide channel waveguides and consists of a Mach-Zehnder interferometer with two ring resonator all-pass filters (APFs) on each arm. The filter architecture is based on the sum and difference of the APFs responses. The ring resonators introduce a nonlinear phase response in each arm that allows carving narrow frequency bands out of a broad spectrum. In this paper, we demonstrate a 3-dB filter bandwidth of 1.0 GHz with a stopband rejection of better than 25 dB. The filter free spectral range is 16.5 GHz. Thermooptic phase shifters are used to tune the filter. As silicon has a large thermooptic coefficient compared to silica, the demonstrated filter requires a low tuning power of less than 300 mW. In addition, this filter is compact with dimensions 25 times smaller than the same filter would be if it were made using standard silica on silicon waveguides with a 0.8% step index contrast

Process flow innovations for photonic device integration in CMOS

Multilevel thin film processing, global planarization and advanced photolithography enables the ability to integrate complimentary materials and process sequences required for high index contrast photonic components all within a single CMOS process flow. Developing high performance photonic components that can be integrated with electronic circuits at a high level of functionality in silicon CMOS is one of the basic objectives of the EPIC program sponsored by the Microsystems Technology Office (MTO) of DARPA. Our research team consisting of members from: BAE Systems, Alcatel-Lucent, Massachusetts Institute of Technology, Cornell University and Applied Wave Research reports on the latest developments of the technology to fabricate an application specific, electronic-photonic integrated circuit (AS_EPIC). Now in its second phase of the EPIC program, the team has designed, developed and integrated fourth order optical tunable filters, both silicon ring resonator and germanium electro-absorption modulators and germanium pin diode photodetectors using silicon waveguides within a full 150nm CMOS process flow for a broadband RF channelizer application. This presentation will review the latest advances of the passive and active photonic devices developed and the processes used for monolithic integration with CMOS processing. Examples include multilevel waveguides for optical interconnect and germanium epitaxy for active photonic devices such as p-i-n photodiodes and modulators.

Transparent amorphous silicon channel waveguides with silicon nitride intercladding layer

We have experimentally demonstrated single mode amorphous silicon channel waveguides with low optical transmission loss of 2.7±0.4 dB/cm for TE mode in the 1550 nm range. This result was achieved by using hydrogen passivation of a-Si dangling bonds and a thin, low loss silicon nitride intercladding layer prepared by plasma enhanced chemical vapor deposition between the waveguide core and the oxide cladding layer. The silicon nitride intercladding layer reduces waveguide sidewall roughness scattering and preserves the hydrogen passivation.

Low-Loss Amorphous Silicon Channel Waveguides for Integrated Photonics

Amorphous silicon (a-Si), single-mode, channel waveguides were fabricated and measured with transmission losses as low as 6.5 dB/cm for the TE mode and 4.5 dB/cm for the TM mode. Variations in the PECVD a-Si deposition conditions yielded a-Si materials with bulk losses <1 dB/cm

Waveguide-Integrated Ge p-i-n Photodetectors on SOI Platform

We demonstrate a fully CMOS processed Ge p-i-n photodetector integrated with a Si waveguide on a SOI platform with a high responsivity of 1.0 A/W at lambda=1520 nm, and a 3 dB bandwidth of >4.5 GHz measured at lambda=1550 nm

Ultralow energy, integrated GeSi electroabsorption modulators on SOI

We report a waveguide-integrated, gigahertz GeSi electroabsorption modulator on SOI with a 10 dB extinction ratio at 1540 nm, a ultralow energy consumption of 50 fJ/bit, and an operation spectrum range of 1539-1553 nm.

Tunable Narrowband Optical Filter in CMOS

We demonstrate a compact, fully tunable, narrowband (1GHz) 4thorder pole/zero optical filter that is fabricated in a silicon complementary metal oxide semiconductor foundry.

Waveguide Integrated Ge p-i-n Photodetectors on a Silicon-on-Insulator Platform

We present selectively grown Ge p-i-n photodetectors coupled to high index contrast Si(core)/SiO2(cladding) waveguides on a silicon-on-insulator (SOI) platform. Two coupling schemes, namely butt-coupling and vertical coupling, were demonstrated in this study. With the butt-coupling scheme we have achieved a high responsivity of 1.0 A/W at 1520 nm and a 3 dB bandwidth greater than 4.5 GHz at 1550 nm. With the vertical coupling scheme, where the light couples from a Si waveguide evanescently to the Ge detector on top of it, a responsivity of 0.22 A/W and a 3 dB bandwidth of ~1.5 GHz have been demonstrated at 1550 nm. The devices were fabricated on a standard 180 nm industrial complementary metal oxide semiconductor production (CMOS) line, and can be integrated with CMOS circuitry for electronic and photonic integrated circuits