Optical technology is poised to revolutionize short-reach interconnects. The leading candidate technology is silicon photonics, and the workhorse of such an interconnect is the optical modulator. Modulators have been improved dramatically in recent years, with a notable increase in bandwidth from the megahertz to the multigigahertz regime in just over half a decade. However, the demands of optical interconnects are significant, and many questions remain unanswered as to whether silicon can meet the required performance metrics. Minimizing metrics such as the device footprint and energy requirement per bit, while also maximizing bandwidth and modulation depth, is non-trivial. All of this must be achieved within an acceptable thermal tolerance and optical spectral width using CMOS-compatible fabrication processes. This Review discusses the techniques that have been (and will continue to be) used to implement silicon optical modulators, as well as providing an outlook for these devices and the candidate solutions of the future.

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Silicon optical modulators

The past decade has seen rapid progress in research into high-performance Ge-on-Si photodetectors. Owing to their excellent optoelectronic properties, which include high responsivity from visible to near-infrared wavelengths, high bandwidths and compatibility with silicon complementary metal–oxide–semiconductor circuits, these devices can be monolithically integrated with silicon-based read-out circuits for applications such as high-performance photonic data links and infrared imaging at low cost and low power consumption. This Review summarizes the major developments in Ge-on-Si photodetectors, including epitaxial growth and strain engineering, free-space and waveguide-integrated devices, as well as recent progress in Ge-on-Si avalanche photodetectors. Owing to their excellent optoelectronic properties, Ge-on-Si photodetector can be monolithically integrated with silicon-based read-out circuits for applications such as high-performance photonic data links and low-cost infrared imaging at low power consumption. This Review covers the major developments in Ge-on-Si photodetectors, including epitaxial growth and strain engineering, free-space and waveguide-integrated devices, as well as recent progress in Ge-on-Si avalanche photodetectors.

High-performance Ge-on-Si photodetectors

Most photovoltaic (solar) cells are made from crystalline silicon (c-Si), which has an indirect band gap. This gives rise to weak absorption of one-third of usable solar photons. Therefore, improved light trapping schemes are needed, particularly for c-Si thin film solar cells. Here, a photonic crystal-based light-trapping approach is analyzed and compared to previous approaches. For a solar cell made of a 2 µm thin film of c-Si and a 6 bilayer distributed Bragg reflector (DBR) in the back, power generation can be enhanced by a relative amount of 24.0% by adding a 1D grating, 26.3% by replacing the DBR with a six-period triangular photonic crystal made of air holes in silicon, 31.3% by a DBR plus 2D grating, and 26.5% by replacing it with an eight-period inverse opal photonic crystal.

Improving thin-film crystalline silicon solar cell efficiencies with photonic crystals

As silicon-based electronics approach the limit of improvements to performance and capacity through dimensional scaling, attention in the semiconductor field has turned to graphene, a single layer of carbon atoms arranged in a honeycomb lattice. Its high mobility of charge carriers (electrons and holes) could lead to its use in the next generation of high-performance devices. Graphene is unlikely to replace silicon completely, however, because of the poor on/off current ratio resulting from its zero bandgap. But it could be used to improve silicon-based devices, in particular in high-speed electronics and optical modulators.

A role for graphene in silicon-based semiconductor devices

We use a rigorous electromagnetic approach to develop a light-trapping theory, which reveals that the standard limit developed by Yablonovitch can be substantially surpassed in nanophotonic regimes, opening new avenues for highly efficient solar cells

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Fundamental limit of nanophotonic light-trapping in solar cells

An efficient light-trapping scheme is developed for solar cells that can enhance the optical path length by several orders of magnitude using a textured photonic crystal as a backside reflector. It comprises a reflection grating etched on the backside of the substrate and a one-dimensional photonic crystal deposited on the grating. Top-contacted crystalline Si solar cells integrated with the textured photonic crystal back reflector were designed and fabricated. External quantum efficiency was significantly improved between the wavelengths of 1000 and 1200nm (enhancement up to 135 times), and the overall power conversion efficiency was considerably increased.

Efficiency enhancement in Si solar cells by textured photonic crystal back reflector

We present thermally reconfigurable multiplexing devices based on silicon microring resonators with low tuning power and low thermal crosstalk. Micro-heaters on top of the rings are employed to tune the resonant wavelengths through the thermo-optic effect of silicon. We achieve a low tuning power of 21 mW per free spectral range for a single ring by exploiting thermal isolation trenches close to the ring waveguides. Negligible thermal crosstalk is demonstrated for rings spaced by 15 µm, enabling compact multiplexing devices. The tuning time constant is demonstrated to be less than 10 µs.

Low power and compact reconfigurable multiplexing devices based on silicon microring resonators

We present a wavelength-tunable, compact, high speed and low power silicon microring modulator. With a ring radius of 5 microm, we demonstrate a modulator with a high speed of 12.5 Gbps and a driving voltage of 3 V to achieve approximately 6 dB extinction ratio in high speed measurement. More importantly, tunability of the resonant wavelength is accomplished by means of a microheater on top of the ring, with an efficiency of 2.4 mW/nm (2.4 mW is needed to tune the resonant wavelength by 1 nm). This device aims to solve the narrow bandwidth problem of silicon microcavity modulators and increase the data bandwidth in optical interconnect systems.

Wavelength-tunable silicon microring modulator.

We experimentally demonstrate the optical transmission at 1550 nm of the fundamental slot modes (quasi-TM modes) in horizontal single and multiple slot waveguides and ring resonators consisting of deposited amorphous silicon and silicon dioxide. We demonstrate that the horizontal multiple slot configuration provides enhanced optical confinement in low index slot regions compared to a horizontal single slot structure with the same total SiO2 layer thickness by comparing their thermo-optic coefficients for the horizontal slot ring resonators. We show in these early structures that horizontal slot waveguides have low propagation loss of 6~7 dB/cm. The waveguide loss is mainly due to a-Si material absorption. The addition of a-Si/SiO2 interfaces does not introduce significant scattering loss in a horizontal multiple slot waveguide compared to a horizontal single slot waveguide.

Horizontal single and multiple slot waveguides: optical transmission at λ=1550 nm

We demonstrate a compact, high speed Ge photodetector efficiently butt-coupled with a large cross-section silicon-on-insulate (SOI) waveguide in which the Ge p-i-n junction is placed in the horizontal direction to enable very high speed operation. The demonstrated photodetector has an active area of only 0.8×10 μm2, greater than 32 GHz optical bandwidth, and a responsivity of 1.1 A/W at a wavelength of 1550 nm. Very importantly the device can readily be integrated with high performance wavelength-division-multiplexing filters based on large cross-section SOI waveguide to form monolithic integrated silicon photonics receivers for multichannel terabit data transmission applications.

Ning-Ning Feng

Papers

Efficiency enhancement in Si solar cells by textured photonic crystal back reflector

Low power and compact reconfigurable multiplexing devices based on silicon microring resonators

Wavelength-tunable silicon microring modulator.

Horizontal single and multiple slot waveguides: optical transmission at λ=1550 nm

High-speed Ge photodetector monolithically integrated with large cross-section silicon-on-insulator waveguide