Silicon-plus photonics

Abstract

Silicon photonics has become very popular because of their compatibility with mature CMOS technologies. However, pure silicon is still very difficult to be utilized to obtain various photonic functional devices for large-scale photonic integration due to intrinsic properties. Silicon-plus photonics, which pluses other materials to break the limitation of silicon, is playing a very important role currently and in the future. In this paper, we give a review and discussion on the progresses of silicon-plus photonics, including the structures, devices and applications.

Key words： silicon-plus; hybrid; plsamonic; photodetector; modulator; graphene; III-V

Introduction

In the past decade, silicon photonics has become very popular because of their compatibility with mature CMOS technology, and great progresses have been achieved [ 1, 2]. However, it is still very difficult to realize various photonic functional devices for large-scale photonic integration by utilizing the material system of pure silicon due to the intrinsic properties of silicon material. An effective solution is to develop silicon-plus photonics by introducing other materials to overcome the disadvantages of silicon. People have already tried to put various materials on silicon, e.g., metals [ 3], III-V semiconductors [ 4, 5], group four materials (like germanium (Ge) [ 6], graphene [ 7– 10], etc.), polymer [ 11, 12], magnetic-optical materials [ 13], liquid-crystals [ 14], etc. In this way, silicon-plus photonics becomes very powerful and useful for various applications of optical communications as well as optical sensing in the wavelength range from visible light to mid-infrared light.

First, metal is a kind of material used widely for silicon photonics. Regularly metal is used to make electrodes/micro-heaters for the propagation of electrical signals or the supply of electrical power, which is indispensable for thermal-tunable/switchable silicon photonic integrated devices and active silicon photonic devices (including lasers, modulators and photodetectors). The process for making metal strips on silicon is easy, including sputtering and lift-off processes. Another important application of metal for silicon photonics is that one can realize nanoplasmonic waveguides on silicon so that the diffraction limit can be broken and thus ultra-compact silicon photonic devices can be realized [ 3]. This importance of shrinking the device footprints has been realized for a long time because the small footprint is helpful for achieving ultra-high integration density, ultra-high speed as well as ultra-low power consumption. In recent years, the combination of metal and silicon photonics has generated silicon hybrid plasmonics which is a very attractive new platform for photonic integration. Usually the metal absorption is not favorable for silicon photonics while the metal absorption loss can be possibly compensated by introducing some gain medium. On the other hand, the light absorption of metal can be utilized to detect light power by measuring the resistance of the metal strip illuminated by incident light.

Second, semiconductor materials (e.g., III-V, Ge) have been also introduced for silicon-plus photonics so that one can realize lasers, amplifier, photodetecors, electro-optical modulators. Note that there is usually a large lattice mismatch between these semiconductor materials (III-V, Ge) and silicon, one usually needs some special fabrication processes. For the combination of III-V and silicon, the most popular two technologies are the molecular bonding developed at University of California, Santa Barbara, and the adhesive bonding at Ghent University. In these ways, III-V/silicon lasers [ 4], modulators, and photodetectors [ 5] have been developed very successfully in the past decade. Meanwhile, people have also been making great efforts to directly grow different semiconductor materials on silicon by optimizing the processes. Fortunately, there are some great progresses reported recently. For Ge, big progress has been achieved for the process of the epitaxial growth on silicon in recent years [ 15– 18], e.g., using a two-step Ge epitaxial deposition. With the techniques of high-quality Ge growth on silicon, many Ge-on-Si active devices have been demonstrated, including high-speed optical modulators [ 16], metal-semiconductor-metal (MSM) photodetectors [ 17], and PIN photodetectors [ 18]. Low defect-density growth of GaAs [ 19] and InP [ 20] using selective-area growth on pre-patterned (001)-Si has been also demonstrated. By leveraging this technology, recently millimeter-long InP waveguide was directly grown on silicon, with superior material quality [ 21]. More recently, graphene has attracted lots of attention as one of the most attractive material to be working with silicon photonics because of its unique optoelectronic properties [ 22– 24]. With the hybrid platform of graphene-silicon, one can achieve compact optical modulators [ 7, 25] and high-speed photodetectors [ 8– 10]. The large nonlinearity coefficient of graphene also makes it very attractive for all-optical devices [ 26– 28].

Polymer materials are also very attractive for silicon-plus photonics because the fabrication processes are easy and compatible with silicon photonics. Furthermore, polymer materials can be synthesized as designed to be nonlinear optical materials with high nonlinear optical coefficients (e.g., electrical-optical coefficients), which is useful for realizing all-optical devices, and electrical optical modulators [ 11, 12]. Polymer materials are also useful to compensate the serious temperature dependence of silicon photonic devices by utilizing the negative thermo-optical effect [ 29]. It is even possible to have gain medium based on polymer doped with some gain materials [ 30]. Therefore, the silicon-plus photonics with polymer is becoming more and more attractive.

In this paper, we give a review and discussion on the progresses of silicon-plus photonics, including the structures, devices and applications.

Silicon-plus photonics

Table 1 shows a summary of the materials developed for silicon-plus photonics in the past decade. The possible functionality and the available wavelength-range of these materials are also included in Table 1. In the following parts, we give an introduction for silicon-plus photonics with these materials.

Tab.1 Summary of materials used for silicon-plus photonics

silicon+ X material		function	visible sensing	near-infrared commun.	mid-infrared sensing
metal		heater	√	√	√
		hybrid plasmonics	√	√	√
		photothermal detector			√
III-V		laser, amplifier, photodetectors, modulators		√	√ (~2 mm)
II-VI		laser, amplifier, photodetectors, modulators			√
IV	Ge/Sn	lasers		√
	Ge	photodetectors, modulators
	graphene	photodetectors, modulators		√	√
polymer	EO* polymer	modulator		√
	strong nonlinear effect	all-optical		√
	polymer with dopants	emission, amplifier	√	√

Note: *EO, electro-optic

Silicon-plus photonics with metals

Metal has been used very widely for silicon photonics as electrodes/micro-heaters for the propagation of electrical signals or the supply of electrical power, which is indispensable for thermal-tunable/switchable silicon photonic integrated devices, and the active silicon photonic devices (including lasers, modulators and photodetectors). Figure 1 shows a thermal-optical switch with metal micro-heaters [ 31] and the array. The fabrication processes for making metal strips on silicon is easy, including the sputtering processes, and the lift-off processes.

Another important application for silicon photonics with metal is that one can realize nanoplasmonic waveguides [ 32– 35], which breaks the diffraction limit and thus enables ultra-compact silicon photonic devices. In recent years, the combination of metal and silicon photonics generates silicon hybrid plasmonics [ 35] which is a very attractive new platform for photonic integration. Figure 2(a) shows the cross section of a silicon hybrid nanoplasmonic waveguide, which consists of a silicon region, a metal layer and a low-index region between them. The low-index material could be SiO₂, Al₂O₃, SiN, or polymer when operating at near-infrared, which is the window for optical fiber communications. For the visible light range, which is useful for optical sensing, the high-index material could be Al₂O₃ or SiN, while the low-index material could be polymer, SiO₂, liquid, or gas. For the silicon hybrid plasmonic waveguides, the silicon core layer can be etched deeply or shallowly and the width of the metal strip does not have to be equal to that of the silicon core region. This makes the design of silicon hybrid plasmonic waveguides very flexible. As an example, when choosing h_{Si_rib} = h_Si = 300 nm for the silicon hybrid plasmonic waveguide, the calculated field distribution E_y(x, y) for the quasi-TM fundamental mode is shown in Fig. 2(b). It can be seen that the field at the 50 nm-SiO₂ nano-layer is enhanced greatly. Because of the ultra-strong optical confinement and relatively low propagation loss, silicon hybrid nanoplasmonic waveguide has been attracting lots of attention and various hybrid nanoplasmonic waveguides with some modifications have been proposed in the following years [ 36– 44].

Fig.1 Thermal-optical switch with metal micro-heaters and the array [31]

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Fig.2 Cross section of a silicon hybrid nanoplasmonic waveguide with a metal cap; (b) calculated field distribution for the quasi-TM fundamental mode when w_co = 200 nm and h_slot = 50 nm [35]

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Usually the metal absorption is not favorable for silicon photonics while the metal absorption loss can be possibly compensated by introducing some gain medium [ 37]. On the other hand, the light absorption of metal can be utilized to detect light power by utilizing the thermal-resistance effect of metal (heated by the incident optical power), as shown in Fig. 3 [ 45]. An electrical wheatstone bridge can be applied to accurately measure the resistance so that the resistance change of the metal strip is with very high sensitivity. Particularly, for silicon hybrid plasmonic waveguides, the nano-scale confinement of light enhances the thermal-resistance effect due to the metal absorption so that improved performances can be achieved in comparison with the traditional long-range surface plasmonic waveguides.

Fig.3 Configuration of photodetector based on a silicon hybrid plasmonic waveguide (HPW). Inset shows the cross-section of silicon HPW [45]. (a) Schematic configuration; (b) optical mode field distribution; (c) temperature distribution due to the light absorption of metal

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Silicon-plus photonics with III-V semiconductors

Adhesive bonding

Adhesive bonding is a classical technique to bring together two substrates of different materials [ 46]. Due to the soft and adhesive nature of the agent (usually polymers) applied between the two substrates, micro-structures (such as roughness) and unmatched material properties (such as thermal expansion coefficient) are largely compensated. As compared to other bonding techniques, adhesive bonding is more reliable and can provide a better yield. However, the drawback of this type of bonding comes from the bonding agent. Such an insulating polymer layer will prevent electrical contact. Although optical coupling, on the other hand, does not need a direct contact, the extra thickness presented by the polymer will also dramatically decrease the coupling strength of the photonic structures on the two substrates. By carefully choosing the polymer agent and optimizing the bonding procedure, a divinylsiloxane-bis-benzocyclobutene (DVS-BCB) layer of ~50 nm thickness has been successfully applied in bonding of IIIV epi-layers on top of a patterned silicon-on-insulator (SOI) wafer [ 47], as shown in Fig. 4(a). This enables the hybrid photonic integration of passive Si circuits with active functionalities provided by the IIIV materials using adhesive bonding technique.

Fig.4 (a) Cross section of a Si/IIIV hybrid integrated sample using the adhesive bonding technique; (b) schemetic structure of the present active section; (c) tip end of a fabricated adiabatic taper; (d) fabricated chip where two 6-channel transceivers are present. The total size is 3 mm × 0.65 mm [48]

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Recently, we have developed a wavelength-multiplexed duplex transceiver module on a silicon chip using this type of bonding technology. A dual-functional hybrid-integrated active section based on the electro-absorption (EA) effect of an IIIV quantum-well structure is used for both modulation and detection [ 48], as shown in Fig. 4(b). Multilayer adiabatic tapers are used for light coupling between the passive access silicon waveguides and the active section [ 49]. Since the IIIV structure here is mostly straight, wet-etching technique is used to fabricate it. The undercut behavior of the wet-etching also facilitates to achieve an ultra-sharp tip-end of the taper, as shown in Fig. 4(c), which ensures low transmission losses of the whole structure. To expand the communication capacity, an arrayed-waveguide-grating (AWG) is also prepared in the module. There are six wavelength-channels with a spacing of 1.6 nm multiplexed or demultiplexed in the transmitter or receiver, respectively. On each channel, there is one hybrid-integrated active section. One fabricated chip is shown in Fig. 4(d). Since the modulation and detection both rely on the EA effect and eventually the same device structure, duplex link can therefore be realized. 30 Gb/s data transmission and receiving for each wavelength channel can be measured, as shown in Fig. 5. An aggregate data rate of 180 Gb/s is provided for the present transceiver module.

Fig.5 Eye patterns of 2⁷−1 on-return-to-zero pseudo random bit sequence for (a) modulation and (b) detection of one EA section [48]

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Direct bonding

In addition to adhesive bonding, molecule bonding is a mainstream approach in the category of direct wafer bonding. Most of SOI wafers in Si photonics research and commercialization are made by this bonding technique. It relies on chemical reactions, namely polymerization reactions, to take place at the bonding interface to form strong covalent bond [ 50]. Equations (1) and (2) below show the polymerization reactions in widely used Si-to-Si, Si-to-SiO₂ or SiO₂-to-SiO₂ bonding, where two surfaces need to be passivated with a high density of polar hydroxyl groups (-OH) [ 51]. They are bridging bonds between the mating surfaces to enable spontaneous bonding at room temperature. A thermal anneal process is usually necessary to speed up the polymerization process to form strong Si-O-Si bonds and also drive the gas products (H₂O and H₂) out of the bonding interface [ 51]. Annealing process at more than 900°C is typical for the Si- or SiO₂-based bonding. To avoid excessive strain due to mismatch in thermal expansion coefficient between Si and IIIV materials, annealing temperature over 400°C is prohibited in IIIV-to-Si direct bonding where an element III or V in IIIV material will replace a Si element in Eq. (1). The same polymerization reactions still take place in Si-to-IIIV molecule bonding. But special processes like plasma surface treatment and forming outgassing channels in Si or SOI surface prior to bonding are necessarily developed to enable strong, high-yield InP-to-Si direct wafer bonding at a low annealing temperature of 300°C [ 52]. A smooth (<1 nm RMS surface roughness), flat and clean surface is critical to the direct bonding quality, which can be readily achieved in modern semiconductor fabrication facilities. The interface oxide as a product of Eq. (2) is in the order of 10 nm, so proximity of Si and IIIV materials allows strong optical coupling between two parties.

(1) \mathrm{S}\mathrm{i}-\mathrm{O}\mathrm{H}+\mathrm{S}\mathrm{i}-\mathrm{O}\mathrm{H}\to \mathrm{S}\mathrm{i}-\mathrm{O}-\mathrm{S}\mathrm{i}+\mathrm{H}\mathrm{O}\mathrm{H}\left(\mathrm{g}\right),

(2) \mathrm{S}\mathrm{i}+\mathrm{2}{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}\to {{\displaystyle \mathrm{S}\mathrm{i}\mathrm{O}}}_{\mathrm{2}}+\mathrm{2}{\mathrm{O}}_{\mathrm{2}}\left(\mathrm{g}\right).

Recently developed hybrid IIIV-on-Si platform is a promising application of III-V-to-Si direct bonding technique [ 4]. Similar to the devices based on adhesive bonding technique discussed above, thin IIIV epitaxial layers are bonded onto SOI substrate to provide optical gain and interact with Si waveguide underneath [ 4]. Compact hybrid Si microring lasers fabricated on this platform are attractive candidates for low-power consumption transmitter/transceiver applications in data communication [ 53]. However, for almost all compact SOI-based photonic devices whose operation results in noticeable current flow in the semiconductor materials, such as carrier-injection mode Si microring modulators, hybrid or heterogeneous Si microring/micro-disk lasers, device heating can prevent devices from reaching high performance. The root cause is the thick (>800 nm typically) buried oxide (BOX) layer in the SOI wafers whose thermal conductivity is around 100× worse than that of the Si. So when current flows through the semiconductors, the finite serial resistance leads to significant joule heating which cannot be dissipated quickly due to compact footprint and thermal block by the BOX layer [ 54].

A recent novel design is to etch some trenches outside the device region but through the BOX layer to reach the Si substrate [ 55, 56]. Then highly thermal conductive materials (e.g., Au or Gu) can fill up those trenches to act as thermal shunts, so the heat from the device can be “grounded” to the Si substrate quickly. Figure 6(a) shows the 3D schematic of such a hybrid Si microring laser with integrated thermal shunts. Both p-type and n-type electrodes (Fig. 6(b)) are connected with shunts surrounding the devices for efficient heat dissipation [ 56]. A drastic device performance improvement is demonstrated by a record-high 105°C continuous-wave (CW) operation in a 50 mm in diameter device in Fig. 6(c), 35°C higher than that of the identical device in the same chip but without thermal shunt [ 56]. Better thermal management also greatly enhances the dynamic performance. Direct modulation bandwidth was extended from 3 to 5.5 GHz at 25 mA injection current and a decent eye diagram at 12.5 Gbps with 7.9 dB extinction ratio was obtained under 35 mA injection current (Fig. 6(d)) [ 57]. Those performance improvement paves the way to deploy those lasers in a harsh environment like data centers where temperate can fluctuate between 30°C and 85°C.

Fig.6 (a) 3D schematic of a hybrid Si microring lasers with integrated thermal shunts; (b) top-view scanning electron microscope (SEM) image of such a device where ring laser and bus waveguide are highlighted in red and blue, respectively; (c) temperature-dependent LI characteristic and (d) measured eye diagram at 12.5 Gbps [56,57]

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Growth

While bonding technology based approaches provide a viable path toward a full on-chip optical link that incorporates lasers, amplifiers, modulators, photodiodes, etc., the circuit integration density and the mass-production capacity are limited by the non-standard bonding process, which also increases the overall cost due to the inefficient use of III-V wafers. Therefore, for high volume, large integration density applications such as inter- or even intra-chip optical interconnects (promising for resolving the interconnect bottleneck), epitaxial growth of III-V semiconductors on silicon is more preferable. One could also fully enjoy the economies of scale offered by processing in advanced CMOS foundries.

Initiated more than three decades before, to grow high quality III-Vs on silicon, scientists have to overcome considerable hurdles. III-V semiconductors are intrinsically very different from silicon, in terms of lattice constant (ε_InP/Si = 8.06%), thermal expansion coefficient and the interface polarity of the materials. Direct growth will results in high density of crystalline defects, including misfit and threading dislocations, twins, stacking faults and anti-phase boundaries, which considerably degrade the material quality for optoelectronics [ 58]. A few solutions have been proposed. By adding phosphorus, one may overcome the lattice-mismatch issue and grow high-quality GaP-related materials on (001)-Si substrates, and pulsed lasing has been achieved [ 59], while it is still challenging to move to the communication wavelength range. On the other hand, GaSb based room temperature laser has been integrated on silicon, regardless its highly mismatched lattice constant with silicon [ 60]. Besides the growth of planar layer of III-Vs on silicon, the direct integration of dimension-reduced materials on silicon, such as nanowires [ 61, 62] and quantum dots [ 63, 64], have also been extensively explored. Although it is relatively straightforward to grow high quality III-V nanowires on silicon, and room temperature lasing has been widely demonstrated, the precise control of laser position, the way of electrical injection, and the coupling of light with silicon waveguides are still very challenging. In addition, quantum dots (QDs) lasers on silicon have been realized with good thermal stability and high output power. However, a thick buffer layer is still required to accommodate the lattice mismatch, therefore the way of efficient light coupling into silicon waveguides requires considerable efforts in the future.

Localized growth of III-Vs on pre-patterned silicon wafer is another promising path toward large volume, highly dense integration of III-V optoelectronics on silicon. Thanks to the renewed interest of the electronics industry in using high-mobility compound semiconductors in next-generation CMOS [ 65], low defect-density growth of GaAs [ 19] and InP [ 20] using selective-area growth on pre-patterned (001)-Si has been demonstrated. By leveraging this technology, recently, it has been demonstrated that millimeter-long InP waveguide can be directly grown on silicon, with superior material quality (see the inset of Fig. 7(a)).

Fig.7 (a) Schematics of an array of InP-on-Si distributed feedback laser (DFB) lasers. Inset: a transmission electron microscope (TEM) image of the grown InP-on-Si waveguide along the longitudinal direction; (b) light in-light out curves of an array of DFB lasers. Inset: photoluminescence image of an array of lasers under large area pump condition [21]

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Based on this technology, optically pumped InP-on-Si distributed feedback laser (DFB) with robust single mode operation has been realized at room temperature (see the schematics of the laser in Fig. 7(a)) [ 21]. The well-understood in-plane laser configuration facilitates the co-integration of this DFB laser with silicon photonic and electronic circuits in the future, with the possibility of electrical injection. In addition, thanks to the technique of selective growth and the wafer-scale processing capability, it is straightforward to scale this technology to large volume and high integration density scenes. An array of DFB lasers with well-controlled emission wavelengths has been demonstrated. Relatively uniform lasing behavior of an array of 10 DFB lasers have also been presented (see Fig. 7(b)), illustrating the uniformity of the III–V material and devices and showing its promise for high capacity wavelength division multiplexing (WDM) applications.

While tremendous progress has been made on epitaxial growth of IIIV lasers on silicon, comprehensive reliability test on those monolithic lasers are still missing, mostly due to the fact that this research topic is still in its infancy. While threading dislocation can be mostly suppressed, the further evolution of other defects, such as twins, stacking faults and misfits at the IIIV/Si interface, under harsh operation conditions are still not fully studied. Tricks such as using defect filtering layers and QD based active layer can mitigate such problems. Recently, it is very encouraging to see the demonstration of electrically pumped IIIV quantum dot lasers on silicon with an extrapolated mean time to failure of over 100158 h.

Silicon-plus photonics with Ge

Ge is a type of group IV material and is likely to be compatible with silicon intrinsically. In recent years, great efforts had been made for the process of direct epitaxial growth of Ge on silicon and significant processes have been achieved. CMOS-compatible epitaxial growth of Ge has been realized with high quality. Therefore, Ge is considered as one of the most attractive candidates to realize active devices on silicon for optical fiber communications. For example, Ge-on-Si waveguide electro-absorption modulator with electro-optic bandwidth substantially beyond 50 GHz has demonstrated recently with a fully integrated silicon photonics platform on 200 mm silicon-on-insulator wafers [ 16]

It is also well known that Ge has absorption coefficient of ~10000 cm⁻¹ at 1.31 mm and ~5000 cm⁻¹ at 1.55 mm [ 66], and the absorption range can be even extended up to 1.6 mm by utilizing bandgap shrinkage related to tensile strain [ 6], which makes it useful for realizing photodetectors. With the Ge/Si platform, one can realize normal-incidence photodetectors for free-space or fiber-optic coupling [ 67], as well as waveguide-type photodetectors for on-chip integration [ 68]. Basically waveguide-type Ge/Si photodetectors are usually preferred because the carriers are collected in a direction perpendicular to the light propagation (absorption) direction [ 69]. In this way, the light absorption length is decoupled from the carrier collection path and consequently one can realize high-speed waveguide-type photodetectors with almost 100% quantum efficiency.

As Si is an ideal multiplication material because of the low k<0.1 (defined as the ratio of the ionization rate of one carrier type to the other) [ 70], one should realize that the Ge-on-Si platform is very attractive for realizing avalanche photodiodes (APDs) with a separated absorption-charge-multiplication (SACM) structure. The first CMOS-compatible Ge/Si SACM APD was demonstrated by Kang et al. with a gain-bandwidth product (GBP) as high as 340 GHz [ 67]. Currently Ge/Si APDs have been very competitive in high-speed optical communication systems with higher GBPs and better sensitivity compared to traditional group III-V APDs [ 69]. Figure 8(a) shows waveguide-type Ge/Si APDs [ 71– 73], which have two popular methods of coupling: evanescent coupling and butt-coupling, as shown in Figs. 8(b) and 8(c), respectively. For the evanescently-coupled case, light guided in the silicon optical waveguide will be coupled evanescently to the Ge layer and be absorbed [ 74]. It is evident that butt-coupling is more efficient than the evanescent coupling because light directly enters the Ge absorption layer, and thus the photodiode length can be shortened to achieve higher bandwidth without compromising the responsivity. However the fabrication of the butt-coupled APD is complicated. The Ge/Si system is also an appealing candidate to realize high-efficiency fast photodiodes with large saturated output power for high-power applications, such as microwave photonics, because of the high thermal conductivities of Si and Ge, the durability and low cost of the substrate material [ 75]. High performance Ge/Si uni-traveling carrier (UTC) photodiodes have been realized recently [ 76].

Fig.8 (a) Microscope picture for waveguide-type Ge/Si SACM APDs; (b) an evanescently-coupled Ge/Si APD; (c) a butt-coupled Ge/Si APD [71]

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As an indirect material, Ge has been used successfully for excellent photodetectors and modulators. It is also desired to overcome the limit of the indirect material and develop a Ge-on-silicon laser source, so that monolithic integration is enabled for realizing lab-on-a-chip as well as photonic network-on-chip. Note that the energy gap (0.8 eV) from the top of the valence band to the momentum-aligned G valley is close to the actual bandgap (0.66 eV). It is possible to compensate this small energy difference to form a laser gain medium by introducing heavy n-type doping as well as some tensile strain, as proposed in Ref. [ 1 9]. The first optically [ 20] and electrically pumped Ge-on-Si laser action in was reported by using Ge with a phosphorous doping level of ~1 × 10¹⁹ and 4 × 10¹⁹ cm⁻³, respectively [ 77]. However, pump–probe measurements of similarly doped and strained material did not show evidence for net gain [ 22] and, in spite of numerous attempts, researchers have failed to substantiate the above results. Ge bandstructure can be also modified towards a direct-bandgap semiconductor when there is some tensile strain, which is usually enabled by introducing micromechanically stressed Ge membranes [ 9] or Si₃N₄ stressor layers [ 23]. A stressor-free technique was also presented to enable>5.7% [ 24] uniaxial tensile strain in Ge µ-bridges via selective wet under-etching of a prestressed layer [ 10]. An alternative technique to achieve a direct-bandgap material is to decrease the energy of the Г-valley to below that of the L-valley, by incorporating sufficient Sn atoms into a Ge lattice to form a kind of GeSn alloys [ 78– 80]. The major challenge is the low (<1%) equilibrium solubility of Sn in Ge [ 81] and the large lattice mismatch of ~15% between them [80]. Partially and fully relaxed GeSn layers on Si [ 82] and on lattice-matched InGaAs ternary alloy [ 83] have been reported. Recently, Wirths et al. reported lasing in a direct-bandgap GeSn material and observed a threshold in emitted intensity and strong linewidth narrowing when T≤90 K [ 84]. This kind of direct-bandgap group IV materials may pave a way towards the CMOS-compatible monolithic integration on silicon.

Silicon-plus photonics with graphene

Graphene is well-known as a two-dimensional sheet of carbon atoms arrayed in a honeycomb structure, and has many unique optoelectronic properties [ 23], such as carrier mobility as high as 200000 cm²·V⁻¹·s⁻¹ at room temperature [ 85], broadband absorption of πα = ~2.3% per layer for vertically incident light, giant Kerr constants and other large optical nonlinearities [ 30]. These properties make graphene very attractive for many devices and applications, like photodetectors [ 8– 10, 86], modulators [ 7], and saturable absorbers [ 87].

To enhance the light-matter interactions, an interesting approach is putting a graphene sheet on the top of an optical waveguide to form a kind of hybrid optical waveguide, so that the light-matter interaction is extended as long as the hybrid optical waveguide and on-chip graphene devices can be realized, as shown in Fig. 9. Particularly, this graphene-silicon platform is very useful for realizing active photonic integrated devices on silicon such as graphene electro-absorption modulators [ 7], waveguide-type graphene photodetectors [ 8– 10], and some all-optical devices [ 87]. For example, in Ref. [ 28] a local and non-local optically induced transparency (OIT) effect in graphene-silicon hybrid photonic integrated circuits was demonstrated for the first time, which enables all-optical modulation over a broad wavelength range (see Figs. 10(a)–10(b)). This OIT effect is different from the saturated absorption effect in graphene reported previously [ 87]. For the saturated absorption effect, the graphene absorption was locally reduced at the position where the graphene sheet is illuminated by light, with an optical power density of up to 0.5~0.7 × 10⁶ W/cm² from a high-power femto-second laser [ 87]. In contrast, for the OIT effect, a low-power continuous-wave laser is used and the power density to produce the OIT effect is extremely low, only ~2 W/cm², which can be achieved with carriers being generated in the silicon layer and injected into the graphene layer. This kind of OIT effect enables low power, all-optical, broadband control and sensing, modulation and switching locally and non-locally.

Fig.9 SEM image of graphene-silicon hybrid nanophotonic wires [28]

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Fig.10 (a) Three-dimensional schematic illustration of a graphene-silicon hybrid nanophotonic wire; (b) dynamic responses of the output power for TE- and TM- polarization modes with a modulated optical pump locally [28]

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Graphene has very high intrinsic thermal conductivity and thus it is also potential to play an important role in the thermal managements for silicon photonics as heat spreaders, flexible heaters, etc. The graphene transparent flexible heat conductor is used to deliver heat from non-local metal heaters to a part of the nanophotonic integrated devices, as shown in Fig. 11(a)–11(b), which shows the thermally tuning silicon Mach-Zehnder interferometer (MZI) with a graphene heat conductor [ 88]. Because of the single-atom thickness and the transparency of graphene, the proposed graphene heat conductor can be utilized in almost any substrate or platform, such as silicon and silica. Furthermore, the excellent flexibility of graphene makes it available for complex surfaces with non-planar nanostructures.

Fig.11 A thermally tuning MZI with a non-local traditional metal heater and a graphene transparent flexible heat conductor [88]

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Figure 12 shows an extremely small thermally tuning silicon micro-disk resonator (R = 5 mm) with a graphene heat conductor. The graphene heat conductor covers a part of the micro-disk to reduce excess loss, as shown in the inset of Fig. 12. From the measured dynamic spectral responses, it can be seen that the resonant wavelength of the micro-disk has a redshift of ~10 nm when the heating power varies from 0 to 38 mW. Therefore, the proposed transparent flexible graphene heat conductor is very convenient to achieve non-local heating for thermally tuning photonic integrated devices.

Fig.12 A thermally tuning silicon micro-disk with a graphene transparent flexible heat conductor [88]

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Silicon-plus photonics with polymer

Polymer materials can be synthesized as designed and thus have become a very important part for optoelectronic devices operating in visible light as well as near-infrared light. Furthermore, the fabrication processes for polymer photonic integrated circuits is very convenient and low-loss polymer optical waveguides have been developed [ 89]. As a result, polymer materials have been used widely for passive linear photonic integrated circuits. Particularly, for silicon-plus photonics, polymer materials also have some unique properties to be very useful [ 90].

First, polymer materials usually have a negative thermo-optical coefficient, which is comparable with that of silicon material. Therefore, it is promising to combine polymer materials with silicon photonic integrated circuits so that the significant temperature sensitivity of silicon photonic integrated circuits (PICs) can be compensated [ 91– 93]. Second, one can design some polymer materials with high nonlinear optical coefficients, which helps realize high efficient nonlinear photonic devices on silicon. For example, when using electrical-optical polymer to fill in the nano-slot region of silicon nano-slot waveguides or metal nano-slot waveguides, one can have efficient electrical-optical modulation, which is useful for realizing high-speed optical modulation on silicon [ 94, 95]. A 70 GHz all-plasmonic Mach-Zehnder modulator on silicon was demonstrated to enable high-speed optical communication at the microscale of ~10 mm. This dramatic size reduction helps achieve an ultra-low power consumption of 25 fJ/bit up to the highest speeds [ 97]. All-optical signal processing devices on silicon can also be realized by introducing some polymer materials with high nonlinear optical coefficients [ 11, 12]. It is even possible to have polymer materials with gain by doping some gain materials (like quantum dots, dye, etc.), which is very attractive to realize silicon-based optical amplifiers and even light emitters. In Ref. [ 96], Korn et al. demonstrated pulsed lasing at room temperature with peak output powers of up to 1.1 W at a wavelength of 1310 nm by combining standard SOI waveguides with dye-doped organic cladding materials. A challenge for silicon-plus photonics with polymer is that polymer usually has rather low temperature stability, which might be an issue for the fabrication process and the practice use. People are trying to develop the polymer materials with improved temperature stability. Recently, a 64 GBd operation of a silicon-organic hybrid modulator at elevated temperature (~80°C) were demonstrated [ 98]. It is expected to have more exciting results on polymer materials for silicon photonics in the near future.

Conclusions

In this paper, we have given a discussion on silicon-plus photonics. Silicon-plus photonics makes silicon photonics more powerful by introducing other materials to complement the disadvantages of silicon. Currently, researchers have reported lots of materials, including metals, semiconductors, two-dimensional materials, polymer materials, etc., for working together with silicon photonics. All these materials help silicon realize various functionality elements, including lasers, photodetectors, modulators, amplifier, tunable/switchable devices, as well as the passive linear photonic devices. These achievements make it possible to realize large-scale photonic integrated circuits on silicon in the near future.

Acknowledgement

This work was supported partially by the National Natural Science Foundation of China (Grant Nos. 61422510, 11374263 and 61431166001), the Doctoral Fund of Ministry of Education of China (No. 20120101110094), the Fundamental Research Funds for the Central Universities.

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