Ling Liao

Low‐loss polycrystalline silicon waveguides for silicon photonics

Photonic integrated circuits in silicon require waveguiding through a material compatible with silicon very large scale integrated circuit technology. Polycrystalline silicon (poly‐Si), with a high index of refraction compared to SiO2 and air, is an ideal candidate for use in silicon optical interconnect technology. In spite of its advantages, the biggest hurdle to overcome in this technology is that losses of 350 dB/cm have been measured in as‐deposited bulk poly‐Si structures, as against 1 dB/cm losses measured in waveguides fabricated in crystalline silicon. We report methods for reducing scattering and absorption, which are the main sources of losses in this system. To reduce surface scattering losses we fabricate waveguides in smooth recrystallized amorphous silicon and chemomechanically polished poly‐Si, both of which reduce losses by about 40 dB/cm. Atomic force microscopy and spectrophotometry studies are used to monitor surface roughness, which was reduced from an rms value of 19–20 nm down to ab...

Integrated silicon photonics for optical networks [Invited]

Feature Issue on Nanoscale Integrated Photonics for Optical Networks Fiber optic communication is well established today in long-haul, metro, and some data communication segments. Optical technologies continue to penetrate more into the network owing to the increase in bandwidth demands; however, they still suffer from too expensive solutions. Silicon photonics is a new technology developing integrated photonic devices and circuits based on the unique silicon material that has already revolutionized the face of our planet through the microelectronics industry. This paper reviews silicon photonics technology at Intel, showing how using the same mature, low-cost silicon CMOS technology we develop many of the building blocks required in current and future optical networks. After introducing the silicon photonics motivation for networks, we discuss the various devices--waveguides, modulators, Raman amplifiers and lasers, photodetectors, optical interconnects, and photonic crystals--from the points of view of applications, principle of operation, process development, and performance results.

High-Speed Silicon Modulator for Future VLSI Interconnect

We demonstrate a silicon optical modulator capable of transmitting data at a bit rate up to 40 Gbps. Such a high-speed modulator enables integrated silicon photonic chips for future high data streams VLSI interconnect applications.

Si 0.5 Ge 0.5 relaxed buffer photodetectors and low-loss polycrystalline silicon waveguides for integrated optical interconnects at λ=1.3 μm

Silicon based photonic circuits are an attractive option for future generations of microprocessors, if standard VLSI electronics can be coupled with on chip optical interconnects and photodetectors for information transfer and clock distribution. A silicon, VLSI compatible, integrated waveguide-photodetector technology for operation at (lambda) equals 1.3 micrometers is presented. Functionality at 1.3 micrometers permits the use of Si/SiO2 waveguides and offers compatibility with short-haul silica fiber optic systems. These waveguides have a large index contrast ((Delta) n equals 2) thus offering superior optical confinement in strip waveguides with dimensions as small as 0.5 micrometers by 0.2 micrometers . The strong confinement and these small dimensions allow high interconnect line densities without cross-talk or RC delay concerns. We measure optical losses in polysilicon waveguides as low as 13 dB/cm at (lambda) equals 1.3 micrometers using an optical cutback technique. A completely relaxed Si0.5Ge0.5 buffer with low threading dislocation density (approximately 106 cm-2) is used as an epitaxial template for a P-I-N photodetector. The relaxed buffer is grown at 815 degree(s)C with ultra high vacuum chemical vapor deposition using a composition graded layer technique with a grading rate of 10% Ge/micrometers . We measure carrier collection efficiencies of 50% and responsivity of 3 mA/W. A beam propagation model is used to determine an effective absorption length less than 2 micrometers in photodetectors butt- coupled to polySi waveguides.

Erbium-doped silicon light-emitting devices

Research in erbium-doped silicon (Si:Er) is discussed in light of our effort to improve the luminescence performance of our LEDs and to demonstrate an integration scheme for a microphotonic clock distribution system. Excitation from Si:Er can occur int ow ways: (1) direct excitation of an Er ion by high energy electrons or (2) energy transfer from an injected electron-hole pair to an Er ion in the lattice. In an LED the first excitation mechanism corresponds to operation in reverse bias, and the latter corresponds to operation in forward bias. We have studied the forward bias case, and we use an energy pathway model to describe the excitation and de-excitation processes. The competing, nonradiative processes against excitation and spontaneous emission are discussed. Maximization of light output can be approached in three ways: (1) decreasing the number of nonradiative energy pathways, (2) enhancing the probability of the radiative pathway, or (3) simply increasing the concentration of active Er sties. We report specific methods that address these issues, and we discuss more device structures that can be used as emitters, optical waveguides, and optical switches in a fully integrated microphotonic system.

Polysilicon Waveguides for Silicon Photonics

Photonic integrated circuits in silicon require waveguiding through a material compatible with silicon VLSI technology. Polysilicon (polySi), with a high index of refraction compared to SiO 2 and air, is an ideal candidate for use in silicon optical interconnect technology. Inspite of its advantages, the biggest hurdle to this technology is that losses of 350dB/cm have been measured in as-deposited polySi waveguide structures, as against ldB/cm losses measured in waveguides fabricated in crystalline silicon. We report methods for reducing scattering and absorption, which are the main sources of losses in this system. To reduce surface scattering losses we fabricate waveguides in smooth recrystallized amorphous silicon and Chemo-Mechanically Polished (CMP) polySi, both of which reduce losses by about 40dB/cm to 15dB/cm. Atomic Force Microscopy (AFM) and spectrophotometry studies are used to monitor surface roughness which has been reduced from an RMS roughness value of 19–20nm down to about 4–6nm. Bulk absorption/scattering losses can depend on size, structure, and quality of grains and grain boundaries which we investigate by means of Transmission Electron Microscopy (TEM). Although the lowest temperature deposition has twice as large a grain size as the highest temperature deposition, the losses appear to not be greatly dependent on grain size in the 0.1pm to 0.4pm range. Additionally, absorption/scattering at dangling bonds is investigated before and after a low temperature Electron-Cyclotron Resonance (ECR) hydrogenation step. After hydrogenation, we obtain the lowest reported polySi loss values at λ = 1.54μm of about 15dB/cm.