Recent progress of silicon integrated light emitters and photodetectors for optical communication based on two-dimensional materials

Feng Li; Feng Li; Jiabao Zheng; Qi Yao; Qi Yao; Ya-Qing Bie; Ya-Qing Bie

doi:10.1364/OME.435902

1. Introduction

Simply reducing the technology nodes can hardly support Moore’s law [1]. Silicon photonics [2] are now being actively studied by many semiconductor foundries as well as by academic research groups because integrating optical and electronic components onto a single silicon chip not only provides a promising direction for high speed communication but also makes the best use of the existing semiconductor technology. As shown in Fig. 1, integrated active devices such as light emitters, modulators, and photodetectors usually work together with passive devices to enable on-chip optical communications. Information in the electrical domain can be converted to the optical domain and be transported through silicon waveguides or optical fibers to the receiver side, then be converted back to the electrical domain for further processing.

Fig. 1. Schematic of an optical communication system based on an emitter and a photodetector integrated on silicon waveguide. Infrared photons from the emitter can be modulated and coupled into the waveguide. After traveling along the waveguide, the optical signals can be detected by the photodetector at the end of the waveguide.

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Since silicon is an indirect semiconductor with poor emission and absorption efficiency in the telecommunication range, many pioneer works propose that two-dimensional(2D) materials with unique optical properties can be directly integrated with silicon photonics through van der Waals force [3] as excellent active devices. Such interface will be free from defects and usually compatible with the back end of the line (BEOL) process. In comparison, the long-sought integration of III-V semiconductors on silicon suffers from the interface lattice mismatch therefore large density of strain and defects.

Van der Waals material family includes lots of members with distinguished physical properties. 2D magnetic materials provide great potential for making atomic thin data storage [4–6] and nonreciprocal optical devices [7]. The superconductivities observed in TMDs and magic-angle twisted graphene bring the hope of nearly dissipation-less computing units [8, 9]. The exciton emission from direct band 2D semiconductors can be well controlled by an electrostatic gate [10]. Moreover, the atomic-thin characteristics make them attractive to achieve super-small volume and high-density integration. Thus, layered materials with extraordinary optical and electronic properties provide a unique platform for on-chip optical communications. For example, graphene, a Dirac semimetal, has a broadband optical response covering from ultraviolet, visible down to long-wavelength IR range [11, 12]. The carbon sp² bond of graphene is chemically stable and the absorption varies with carrier densities controlled by gate voltages. Beyond the gapless graphene, monolayer TMDs such as MoS₂, MoSe₂, MoTe₂ are found to be direct band semiconductors with unique spin-valley locking effects and they have been used to build lasers or high-sensitivity photodetectors. Later, 2D materials such as black phosphorus (BP), black arsenide, and indium selenide have been proved as anisotropic [13] direct band semiconductors [14, 15]. The tunable bandgap size with the thickness offers an important knob to control the optical response frequency. Multilayer artificial superlattices can be easily stacked together and create unconventional band structures which are particularly beneficial for optoelectronic devices in a broad wavelength range [16, 17].

Many reported studies on 2D light-emitters and photodetectors are fundamental and influential [10, 18–29]. The desire for advanced device performances such as strong and broadband emission, stable modes-control, high-speed or high-sensitivity photodetection, energy efficiency [30], and scalable integration designs triggers intensive discussion in this field. Previously, reviews have analyzed the compatibility of van der Waals materials with CMOS technology [31], the coupling theory of the integration system [32, 33], the choice of materials beyond graphene as integrated optoelectronic devices [34–41]. And some reviews discuss individual issues such as the size-energy efficiency challenges in nanolasers [42], the assessment criteria [43] of 2D lasers, the physical mechanisms of the excitonic lasing phenomenon [44], and the importance of hexagonal-boron nitride(h-BN) as the optical confinement for photonic integrated circuits.

Here we summarize the most recent designs of the silicon integrated light-emitters and photodetectors based on a few promising 2D materials. First, an inspection of the optical properties can help to select the most appropriate material candidates. Second, an analysis of the physical mechanisms is critical to locate the fundamental difficulties of making high-performance devices. Finally, by comparing performances of the most advanced integrated 2D devices, we also propose the next opportunities that make use of quantum materials, valley-photonics, hybrid and 3D integrations strategies to achieve energy-efficient and miniaturized on-chip optical communication.

2. Photonics integrated 2D emitters

Excitons in 2D semiconductors have shown unique properties such as thickness-dependent emission energy [45], spin-valley locking [46], interlayer coupling [47], etc. Since photons can hardly be confined within the atomic thin layers, external photonic cavities are indispensable for strong emission devices, such as lasers. Robust on-chip light sources depend on a few key factors, such as the intrinsic quantum efficiency, the defect level related to the material growth and fabrication process, the lifetime of excited carriers, the transparent condition, the emission rate, and the system loss, etc. The general lasing criteria [43, 48–50] have been used to evaluate the performance of lasers based on atomic thin gain materials. Firstly, we focus on the choice of 2D materials.

2.1 Material choice

The emission wavelength of 2D materials covers from visible to mid-infrared (e.g. TMDCs semiconductors ∼ 1-2 eV, and BP ∼ 0.3-2 eV [14, 15]). Here we briefly summarize the fundamental parameters of several 2D materials in Table 1 and Fig. 2.

Fig. 2. Lattice structures: a. monolayer 2H-MoTe₂, b. 2H-MoS₂, c. MoS₂/WSe₂ heterobilayer (top) and MoSe₂/WSe₂ heterobilayer (bottom), d. monolayer BP; e. graphene.

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Table 1. Fundamental optical characteristics of a few 2D materials

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Emission of monolayer 2H-MoS₂, 2H-MoSe₂, 2H-WS₂, 2H-WSe₂, InSe [128, 129], and 1L/2L black phosphorus are in the range from 600∼985 nm (Table 1) at which silicon has a strong absorption [130]. Therefore, these materials cannot be integrated with silicon but with silicon nitride(Si₃N₄). Here we include the silicon nitride photonics just for a comparison. Interlayer exciton emission of van der Waals heterobilayers (e.g.MoSe₂/WS₂, MoSe₂/WSe₂, and MoS₂/WSe₂) is tunable on the order of tens of meV under the vertical external electrical field due to the quantum stark effect [86, 131]. Although graphene has been demonstrated as an electrically driven broadband emitter working from visible to infrared [23, 24] due to the radiation of ultrafast hot carriers [24]. This thermal radiation is physically different from the emission due to the exciton combination in TMDCs. Recently, the graphene emitter enhanced by a silicon photonic crystal cavity(PhCC) has also been demonstrated [120]. Among all 2D materials, 2H-MoTe₂, 3∼4 layer BP and MoS₂/WSe₂ heterostructures have the proper bandgap for silicon integration. As shown in Fig. 1, infrared photons can be injected and travel along the silicon waveguide with negligible absorption, then be absorbed by photodetectors at the end of the waveguide.

Compared to traditional III-V semiconductors, TMDCs have a lower photoluminescence quantum yield (<6% as shown in Table 1) probably due to the high level of defects [132]. Moreover, air-sensitive materials such as BP have to be encapsulated by dielectric layers like h-BN, Al₂O₃, or HfO₂ to avoid degradation. The encapsulation layers form a gap between integrated 2D materials and the waveguide and further reduce the coupling efficiency between emitter and waveguide. So far, material scientists have been working on developing wafer-scale single crystals 2D materials for industrial applications. But at the same time, preparing defect-free 2D materials with high quantum yield is another way of system optimization. Alternatively, if the 2D material is integrated with high-quality-factor resonators such as PhCC, distributed Bragg reflector, etc, the emission rate can be greatly enhanced. We will discuss the mechanisms of the recently reported 2D nanolasers in the next part.

2.2 Physical mechanisms and criteria of 2D nanolasers

2.2.1. Characteristics of 2D nanolasers

Nanolasers can be used as a robust intermittent light source on the PIC for optical communication [133]. Two-dimensional materials have been integrated with various kinds of high-Q resonators such as PhCC [71, 87, 101, 102], whispering gallery mode(WGM) cavity [62, 72, 73], distributed Bragg reflector(DBR) cavity [75, 88], and grating resonator [83]. The schematics of typical 2D nanolasers are summarized in Fig. 3 and the performances of corresponding lasers are listed in Table 2. Since the experimental demonstration of silicon cavity-based 2D lasers is still rare, as a comparison we include the silicon nitride, silicon oxide, and GaP cavities to illustrate the lasing mechanisms.

Fig. 3. Laser designs based on different gain materials and resonators. The lasing wavelength is ranging from visible (b, c, d, e, f, h), near-IR (a, i, j) to mid-IR(g). a. The monolayer MoTe₂ is integrated on a 1D silicon PhCC[101]. b. The nanolaser[71] is made of a monolayer WSe₂ on a GaP L3 PhCC. c. The PCSEL is made of a monolayer WS₂ on Si₃N₄ cavity operating at room temperature[60]. d. The WS₂-Si₃N₄ microdisk pulsed nanolaser[62]. HSQ was used as the protective layer and the optical confinement factor is enhanced by ∼30%, which significantly improves the exciton-photon interaction in the cavity. e. The MoS₂-SiO₂ sphere microdisk cavity[72] working at room temperature. f. The SiO₂ sphere cavity is placed on top of the monolayer MoS₂[73]. g. The mid-IR nanolaser with few-layer BP inside Si/SiO₂ DBRs[88]. h. The low-threshold VCSEL under room temperature using WS₂ embedded in the TiO₂/SiO₂ DBRs[75]. i and j are two nanolasers based on van der Waals heterostructures integrated with silicon PhCC[87] and Si₃N₄ grating resonator[83], respectively. Figures reproduced with permission: b[71] and d[62] from © 2015 NPG, a[101] and h[75] from © 2017 NPG, c[60] and j[83] from © 2019 NPG, e[72] from © 2015 ACS, f[73] from © 2018 ACS, g[88] from © 2020 Wiley-VCH, i[87] from © 2019 AAAS.

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Table 2. Performance list of laser in references^a,^b

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The traditional laser performance in a cavity can be described by the following rate equations. The first is the carrier rate equation, where N is the carrier density. The excited carriers can relax in several ways including the spontaneous emission(R_sp), nonradiative channels(R_nr), the leakage(R_l), and the stimulated radiation(R_st). Here η_i is the conversion efficiency of photons to carriers, V is the volume of active region and P is the excitation power:

(1)$$\frac{{dN}}{{dt}} = \frac{{{\eta _\textrm{i}}P}}{{hv \cdot V}} - ({R_{\textrm{sp}}} + {R_{\textrm{nr}}} + {R_\textrm{l}}\textrm{)} - {R_{\textrm{st}}}$$

(2)$$\frac{{d{N_\textrm{p}}}}{{dt}} = {\varGamma }({R_{\textrm{st}}} + {\beta _{\textrm{sp}}}{R_{\textrm{sp}}}) - \frac{{{N_\textrm{p}}}}{{{\tau _\textrm{p}}}}$$

The second is the photon rate equation. Here N_p is the photon density, and Γ is the optical confinement factor, which is the ratio of the active region volume V occupied by electrons or electron-hole pairs to the volume V_p occupied by photons. The Purcell effect (F_p: Purcell factor) enhanced spontaneous emission factor in a cavity β_sp =F_pβ₀ denotes the ratio of spontaneous emission coupled into lasing mode and β₀ is the spontaneous emission factor without the cavity. Here τ_p is the photon life time in a cavity, where the cavity quality factor Q=ω₀·τ_p and ω₀ is the cavity resonance angular frequency.

From Eq.(2), the photon generation rate can be written as:

(3)$${(\frac{{d{N_\textrm{p}}}}{{dt}})_{\textrm{gen}}} = {\varGamma }({R_{\textrm{st}}} + {\beta _{\textrm{sp}}}{R_{\textrm{sp}}})$$

Assuming that the gain per unit length of the material is g, in dt time, the increased photons number in the lasing mode with a group velocity v_g can also be written as:

(4)$${(d{N_\textrm{p}})_{\textrm{gen}}} = {\varGamma }{v_\textrm{g}} \cdot dt \cdot g \cdot {N_\textrm{p}}$$

Considering the steady-state of Eq.(2)($\frac{{d{N_\textrm{p}}}}{{dt}}\textrm{ = }0$), and Eq.(3) and Eq.(4), the laser gain at the laser threshold is:

(5)$${g_{\textrm{th}}} = \frac{1}{{{\varGamma }{v_\textrm{g}}{\tau _\textrm{p}}}}\textrm{ = }\frac{{{\omega _0}}}{{{\varGamma }{v_\textrm{g}}Q}}$$

The traditional laser gain calculation assumes the spontaneous emission is much smaller than the stimulated emission thus the β_sp term is neglected for the threshold calculation. And for the same material, we can assume that the gain required at the threshold is inverse proportional to the confinement factor Γ and the quality factor Q. However, for microcavities with large Purcell factors, the spontaneous emission cannot be neglected in the lasing analysis, which has been reported for PhCC and WGM cavities. It is challenging to directly compare the performances of lasers with different materials and cavities. Even for the same materials, PhCC and WGM mostly confine the emission in-plane with relatively small Γ and large quality factor Q while the DBR cavities confine in the out of plane direction with relatively large Γ and smaller quality factor Q. The detailed laser performances need to be carefully examined with consideration of material properties, the mode distribution, etc. and a fair comparison requires delicate experimental design.

Photonic crystal cavity is a type of resonator that allows small volume and high-quality factor and has been used for ultra-low threshold nanolasers. In 2017, Y. Li et al. [101] demonstrated a room-temperature silicon nanobeam cavity nanolaser(Fig. 3(a)) based on monolayer MoTe₂ and the lasing wavelength is located at 1132nm. The first integrated nanolaser (Fig. 3(b) based on monolayer WSe₂ and GaP L3 PhCC [71] was reported in 2015 by S. Wu et al. However, the single-mode operation feature of PhCC limits its applications such as multi-mode multiplex, wavelength division multiplexing. Therefore, cavities based on Fabry-Perot or whisper gallery mode were studied for multi-mode applications.

A microdisk/microsphere in which the resonant light travels around the surface forms a WGM cavity. The quality factor is in the range of 10³∼10⁸ and mode volume is larger than (λ_c/n)³. In 2015, the first 2D-material-integrated laser based on WGM cavity is demonstrated by Y. Ye et al. [62] where a monolayer WS₂ flake is sandwiched between a hydrogen silsesquioxane(HSQ) and a Si₃N₄ disk. The HSQ is used to enhance the optic confinement factor as shown in Fig. 3(d). However, the lasing threshold is up to the order of MW/cm² due to the very weak coupling between the WS₂ and the optical modes in the Si₃N₄ disk. Recent reports [72, 73] have decreased the threshold down to several kW/cm² using a SiO₂ sphere and a disk(see Fig. 3(e), (f)). Although the lasing threshold of the SiO₂ sphere is lower, the relatively complicated fabrication process makes it unsuitable for massive and large-scale integrations.

The DBR based Fabry-Perot cavity confines light in the vertical direction and can be used as the resonator for the vertical cavity surface-emission laser (VCSEL). The gain medium is encapsulated between the top and the bottom DBRs and is kept away from oxygen and water vapor in the air. Compared to the PhCC and WGM cavities where the gain medium is placed at the surface of resonators, the one-dimensional DBR cavity has a better optic confinement factor. In 2017, a monolayer WS₂ flake inside the SiO₂/TiO₂ DBR cavity [75] (Fig. 3(h)) showed a lasing behavior, and the linewidth narrows by a factor of 2 with the increasing pump power(Fig. 4(a) bottom panel). Later, Y. Zhang et al. [88] demonstrated a mid-IR laser using 100 nm BP as the gain medium in a Si/SiO₂ DBR cavity under 1520 nm pulse laser excitation (Fig. 3(g)).

Fig. 4. Power dependences of different 2D nanolasers and clear superlinear profiles of the emission output are observed as the pump power exceeds the threshold. a. top panel: the emission intensity of monolayer WS₂/DBR cavity laser at different excitation power. bottom panel: the linewidth narrows by a factor of 1.5 at the threshold[75]. b. top panel: the linewidth narrows by a factor of 6.6 with the increased pumping power for the BP/DBR cavity laser; inset: the polarization dependence of the lasing mode(solid) and the spontaneous emission (open dots). bottom panel: the output intensity versus the pump power[88]. c. Lasing behavior of interlayer exciton in MoSe₂/WSe₂ heterostructure integrated on Si₃N₄ grating cavity. Left: the output intensity (red dots) at different pump power [83]; right: linewidth (blue dots) narrows by a factor of 4 at the threshold. d. Lasing behavior of the monolayer WS₂ based PCSEL. Left: the output intensity (blue dots) at different pump power. Right: the most abrupt decrease of linewidth by a factor of 200 as the pump power exceeds the threshold[60]. Figures reproduced with permission: a[75] from © 2017 NPG, b[88] from © 2020 Wiley-VCH, c[83] and d[60] from © 2019 NPG.

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Conventionally, the output laser light is coherent spectrally and temporally with a clear power threshold and a reduction of linewidth [48]. However, when the gain material is integrated with a microresonator (e.g. PhCC), some conventional lasing features such as the specific polarization, linewidth reduction, may come from not only the enhanced stimulated emission but also the amplified spontaneous emission(ASE) because of the Purcell effect in a high-Q resonator. The ASE shares similar properties and might be mistakenly identified as a lasing behavior. The strict criteria for microlasers are called “quantum threshold condition”, which requires the mean photo number in an emission mode equals to 1, and the linewidth narrows by a factor of 2 at the threshold [43]. Alternatively, the correlation characteristics can be used to identify the laser behavior. It requires that the equal-time second-order coherence g²(0) approaches to unity when the pump level exceeds to quantum threshold with the increased emission, which can be measured by the Hanbury Brown-Twiss experiment [134]. But no reports measure the g²(0) because the ultra-low threshold power of 2D lasers makes it very challenging.

Previously all reports show the superlinear output power dependence and a drop of linewidth at the threshold with the same polarization of the resonant mode. However, only four lasers [60, 75, 83, 88] (Fig. 3 (c), (g), (h), and (j)) are found with a clear narrowing of linewidth and the characterizations of the linewidth are shown in Fig. 4. Among them, there is of no discussion about the quantum threshold condition for the DBR cavity with WS₂ [60]and mid-IR BP laser [88]. And Ge. X. et al. [60] estimated that at the threshold, the photon generation rate is still smaller than the cavity loss rate thus they concluded that WS₂ based photonic crystal surface-emitting laser(PCSEL) does not reach the quantum threshold condition although the linewidth narrowing is the most obvious as shown in Fig. 4(d).

The only report that verifies the mean photon number is from Eunice Y. Paik et al. [83] as shown in Fig. 3(j). They made use of the interlayer excitons in the MoSe₂/WSe₂ heterobilayers. Intralayer excitons are excited by a pump laser in the WSe₂ layer. Some electrons transfer to the lower MoSe₂ conduction band on a timescale of 10-100 fs, while others recombine as intralayer excitons with lifetimes of 1-10 ps. The type-II semiconductor heterobilayers form a three-level system and the population reverse can be formed in the interlayer bandgap in good agreement with the density required for the transparency condition. The time coherence and the abrupt increase of the spatial coherence across the laser threshold are all demonstrated simultaneously for the first time in this research field.

2.2.2. Opportunities for the next studies of on-chip 2D nanolasers

The recent progress of 2D materials-based nanolasers brings great promises for the small footprint and integrated on-chip light sources. However, the scientific and engineering challenges remain for forging 2D nanolasers into efficient devices for optical communication. Firstly, the electrically driven laser based on 2D materials has yet to be demonstrated although light-emitting diodes(LEDs) have been integrated with the PhCC [70]. As shown in Fig. 5(a), interlayer excitons in heterobilayers can be electrically tuned by the quantum-confined Stark effect in the heterostructures. Therefore, the emission wavelength can be possibly controlled by the vertical electric field, as discovered in WSe₂ bilayer [131] and MoS₂/WSe₂ [86]heterobilayers (See Fig. 5 (b) and (c)). Secondly, for on-chip applications, the laser light usually needs to be coupled with a waveguide to enable in-plane propagation. An on-chip microlaser made of monolayer WSe₂ and a Si₃N₄ ring resonator [67] has been demonstrated coupling to a Si₃N₄ waveguide. Such configuration can be extended to the telecommunication range by integrating with silicon photonics. It is also possible to guide vertical emissions from DBR cavities into the waveguides using grating couplers for integrated photonics. Finally, experimental verifications of time/spatial coherence and the quantum threshold will help understand the mechanism of nanolasers.

Fig. 5. Electrical tunability of interlayer exciton due to the quantum-confined Stark effect. a. The schematic of interlayer excitons in h-BN encapsulated van der Waals heterostructure. b. Contour plot of the WSe₂ bilayer interlayer exciton emission intensity as a function of photon energy and gate voltage difference ΔV[131] between the top and bottom layers. c. Evolution of the infrared interlayer exciton emission in the MoS₂/WSe₂ heterostructure[86] at 20 K. The emission energy varies with the applied electric field E and the tunability is ∼ 80 meV. Figures reproduced with permissions: b[131] from © 2018 ACS, c[86] from © 2019 APS.

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2.3 LEDs and thermal emitters

Although individual LEDs based on TMDCs [10, 21, 28], black phosphours [20], 2D heterojunctions [18, 25, 27–29] and graphene [23, 24] have been intensively studied, silicon integrated electrical driven emitters are still rare. In 2017, Y.Q. Bie et.al. reported that the MoTe₂ based LEDs [100] can be integrated with a silicon photonic crystal waveguide. In addition to the excitonic emission, the thermionic emission of hot electrons in graphene has also been amplified by a silicon photonic crystal cavity [120]. The electron temperature is raised by the flow of electrical current and the emission is similar to blackbody radiation. Beyond the silicon photonics, GaP cavities [70] and Nb₂O₅/SiO₂ DBR cavities [74] have been used to integrate with h-BN encapsulated WSe₂ LED as electrical driven light sources. Similar to photoluminescence, the quantum efficiency of electrical luminescence is relatively low due to the intrinsic defects in 2D materials. In the future, we would expect that the efficiency of electroluminescence could be improved by advanced material synthesis technology. We also believe that placing the electrical electrodes at the right place without affecting the performance of photonic devices also needs to be considered for the complicated electrical devices, thus we provide a discussion of 3D photonics designs in Section 4.4.

3. Integrated 2D photodetectors

Photodetectors are essential for many applications such as imaging, tracking, and optical communication, etc. They can be placed at the destination of the route for the on-chip optical communication as shown in Fig. 1. The performances of the integrated photodetectors can be characterized by a few figures of merits, such as the responsivity, the linear dynamic range, the quantum efficiency, the specific detectivity, and the gain-bandwidth. The responsivity measures the electrical output per optical input and is usually expressed in units of either amperes or volts per watt of incident optical power. A photodetector fulfills the condition of linearity only for a limited range of the input signal level. At a lower level of input power, the output of the detector is dominated by noise; at a higher level of input, the detector’s output no longer increases proportionally to its input signal. The linear range depends on the type of photodetectors. Quantum efficiency is the incident photon to converted electron ratio and can be calculated from responsivity R as R·E_photon/e, where E_photon is the photon energy. The detectivity is the inverse of the noise-equivalent power(NEP). The signal-noise ratio(SNR) can be calculated as the readout electrical power divided by the NEP of the detector. If the NEP is 10⁻¹² W/Hz^0.5, it means the photodetector can detect a signal of one picowatt power with an SNR of one after one-half second of averaging. The NEP can be calculated as the power spectral density without any optical input divided by the responsivity, and this means for the fixed noise power spectral density, a larger responsivity gives a better SNR. Generally, we hope our detector can have a large gain-bandwidth which means for a frequency-modulated signal, the photodetector can follow and give a response at the same frequency with enough SNR, like 3dB. The high sensitivity (NEP) and large bandwidth photodetectors are the critical characteristics because the lowest cost of energy per bit and the highest communication speed is the lifeblood for the on-chip optical communications.

As shown in Fig. 6, we categorized typical photodetectors by the device geometries, then their performances are listed in Table 3. Generally, there are a few types of photodetectors, such as photoconductive style, heterojunction, p-n junction, etc. Among all 2D materials, direct band semiconductors (such as MoTe₂, multilayer BP) and semi-metals(graphene) can be used for photodetectors because the excited carriers in the conduction band increase the conductivity effectively. The narrow band semiconductors can absorb photons with energies higher than the bandgap for allowed states, then the excited carriers will relax to the ground state via electron-phonon or electron-electron interaction. The relaxation timescale of the absorber set a fundamental bandwidth limitation for the photodetector. For example, the ultrafast hot carrier dynamics of the Dirac fermion limit the speed of graphene photodetectors up to 500 GHz [135]. The speed of BP-based photodetector [90] is about three orders of magnitude slower than that of similar graphene photodetectors [121, 122] because the ultrafast relaxation dynamics of hot carriers in graphene don’t exist in BP. However, the photodetectors based on gapless graphene suffer from the large dark current which sacrifices the detectivity of the tiny on-chip optical signal, while 2D semiconductors (e.g. TMDCs, BP) take the benefits from the finite bandgaps. Moreover, because the absorption of the thin layer materials is small due to the limited amount of matter in the vertical direction (e.g. graphene only absorbs ∼ 2.3% of normal incident light), an increase of the absorption rate is critical to guarantee high responsivity for room-temperature applications of 2D photodetectors.

Fig. 6. On-chip integrated photodetectors based on 2D materials. a, b: waveguide integrated photoconductive PDs based on multilayer BP (a[91], b[93]), which operated at 1550 nm[91] and mid-IR range[93], respectively. c. waveguide integrated photodetectors with vertical carrier channel in vertical graphene/MoTe₂ junction[104]. d∼f: integrated photodetectors utilizing metal/graphene/MoTe₂ junctions(d[103]) Graphene/Si junctions (e[112] and f[113]). the low dark current in d is suppressed by the barrier between graphene and MoTe₂. g∼i: In plane p-n junction integrated with silicon slot waveguide(g[114]), photonic crystal defect waveguide(h [100], i[116]). j. silicon ring resonator integrated photodetector based on photo-thermoelectric(PTE) effect of graphene, which shows the highest responsivity among PTE graphene photodetectors[118]. k. silicon ring resonator integrated photodetector based on 60 nm MoTe₂ [105], the photodetection of 1550 nm in MoTe₂ is originated from the strain-induced bandgap reduction of MoTe₂. l[121] and m[122] is two plasmonic enhanced photodetectors whereas the strong electric field is formed near Au antenna(l) and hundreds of nanometer wide slot(m, the scale bar is 600 nm). Figures reproduced with permissions: a[91] from © 2015 NPG, b[93] and l[121] from © 2019 ACS, c [104] from © 2020 NPG, d[103] and i[116] from ©2018 ACS, e[112] from © 2013 NPG, f[113] and g[114] from © 2016 ACS, h[100] from © 2017 NPG, j[118] from ©2021 NPG, k[105] from © 2020 NPG, m[122] from © 2020 De Gruyter.

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Table 3. Performance list of on-chip photodetectors based on 2D material^a

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Here we will discuss strategies that help to increase the absorption of 2D materials, then the device geometries that affect the bandwidth. Finally, we also notice that solving the contact issue is essential to improve the responsivity of devices.

3.1 Enhancing the light-matter interaction in 2D materials

We find three typical methods for enhancing light-matter interaction. Firstly, if the layered material is placed on a waveguide [12, 136, 137] the photons will be absorbed on the surface of the waveguide along the propagation direction instead of the vertical direction. As long as the 2D materials flake size is not the limitation, the absorption stops only if no photon is confined in the waveguide. Secondly, absorption in 2D materials can be enhanced by a high-Q dielectric microcavity (e.g. PhCC [125, 138], ring resonator [118], etc.) as shown in Fig. 6 (j), (k). Thirdly, excitons absorption can be significantly enhanced by the surface plasma polaritons [139] at the metal-material interface(e.g. air gap in Au film [140, 141]). As shown in Fig. 6 (l), (m), the graphene channel [90] is 600 nm wide, and the gap (∼100 nm) between two bowtie-shape antennas confines a strong electric field. For both dielectric cavities and plasmon cavities, the absorption is enhanced because of the increased transition rate. Although the power consumption may increase due to the impressive metal absorption loss in the plasmon cavities and the enhancement only occurs at the resonance modes, there is a trade-off between the enhanced absorption and the power consumption. The ohmic loss in metals doesn’t exist in dielectric cavities, although the enhancement factor at the hot spot in metallic structures can be a few orders higher than dielectric cavities. Since the enhanced absorption rate improves the responsivity of photodetectors, these methods will be further explored for on-chip photodetectors.

3.2 Vertical junctions with short photocarrier channel

Usually, a shorter width of the channel allows a faster carrier transition. For in-plane Schottky or p-n junction photodetectors, the depletion width is fixed once the barrier height and the doping levels of the materials are settled. Even for the high doping p-n junctions (10¹⁹ cm^-3), the depletion width at zero bias is in the order of 100 nm. The bandwidth of these photodetectors is determined by the capacitance of the p-n junction. It is the inverse of the time needed for charge carriers to cross the depletion region. Although the depletion region is smaller for the high doping case, the dopants reduce the optical generation of carriers which also affects the responsivity. However, for vertical junctions based on 2D materials, the width is limited not by the semiconductor doping but by the thickness of the material. Since the thickness of 2D devices can be in the nanometer scale and such a vertical junction allows efficient in-plane photon absorption, such a geometry guarantees a large gain-bandwidth. As shown in Fig. 6(c), Nikolaus Flöry et al. [104] placed a gold electrode/multilayer MoTe₂/graphene stack on top of a silicon waveguide and made a vertical heterojunction photodetector. The graphene is used as both the bottom transparent electrode and the active material. The MoTe₂ multilayers work as the absorption material in the telecom O-band (1,260–1,360 nm). Comparing with their previous lateral MoTe₂/graphene photodetector on waveguide (see Table 3 [103], Fig. 6(d)), the bandwidth increased from 0.5 GHz to 46 GHz. And there is no significant change of the area-normalized photo-dark-current ratio, which indicates the unchanged photodetection mechanism in both vertical and lateral channel MoTe₂/graphene photodetectors.

3.3 Contact issue of 2D materials

The contact resistance and contact capacitance also have an impact on the responsivity and the bandwidth of photodetectors. The excited carriers need to be collected by the electrodes to form photocurrent. The contact resistance will decrease the photocurrent, thus reduces the responsivity. The contact capacitance would affect the bandwidth. The lifetime ${\tau _{total}}$ of excited carriers in a photodetector can be written as follows:

(6)$$\frac{1}{{{\tau _{\textrm{total}}}}} = \frac{1}{{{\tau _{\textrm{contact}}} + {\tau _{\textrm{drift}}}}} + \frac{1}{{{\tau _\textrm{r}}}}$$

(7)$${\tau _{\textrm{drift}}} = \frac{d}{{2\mu E}}$$

where τ_contact is the transient time of carriers from the channel edge to the electrodes, μ nd E is the carrier mobility and electric field intensity, respectively. The carrier transition time in the channel materials τ_drift is shown in Eq.(2) where d is the material thickness and τ_r is the material-related carrier recombination time. Choosing a metal with lower contact resistance/capacitance also improves the responsivity/bandwidth. Recently, P.C. Shen et al. [142] realized the ultralow contact resistance between bismuth and monolayer p-MoS₂ where metal-induced gap states and gap-state pinning are reduced and it is worth trying with bismuth for new photodetectors.

As a summary, we conclude that a sensitive high-speed photodetector will benefit from an ultrashort channel length, well-controlled contact electrodes as well as with possible resonators. We look forward to advanced technologies which can fully unveil the potential of 2D photodetectors.

4. Prospective of the on-chip integration design

Besides supercomputers, integrated silicon photonics have shown their value for applications in autonomous vehicles, construction site mapping, drones, and robots, etc. The integration of photonic and electronic systems provides a compact low cost and high-performance solution compatible with the current semiconductor foundry. In this part, we will analyze potential efforts and look for new opportunities for 2D materials on photonic chips.

4.1 2D materials and heterostructures with advanced properties

Firstly, besides the materials listed in Table 1, Weyl semimetals (WTe₂, T_d-MoTe₂, TaIrTe₄, etc.) [143] and topological insulators (1T’-MoTe₂) [144] have all been demonstrated with broadband responses. Integrating with optical cavities or tuning the twisted angle of 2D materials can enhance the emission or absorption at a certain frequency range, then improves the detection or emission efficiency. These materials integrated with silicon/silicon nitride photonic circuits can work as novel communication devices in a broadband frequency range for on-chip sensing. In addition, Weyl semimetals are specifically sensitive to the orbital angular momentum of light [145].

Secondly, we can fine-tune the band alignments using all kinds of heterojunctions and look for the best match for ultrasensitive photon detection. For example, Y. Chen et.al. reported the unipolar barrier photodetectors based on WS₂/h-BN/PdSe₂ and BP/MoS₂/graphene, which successfully reduces the leak dark current from majority carriers. The designs depend directly on the choice of materials with desired band structures [146]. The on-chip communication version of such high-efficiency photodetectors is still missing.

Finally, the quality and robustness of 2D materials are key factors for device applications. Therefore, the advanced growth technology is the foundation of the semiconductor industry. Recently, many amazing works have reported the growth of large-scale single-crystal 2D materials [147–149]. Wafer-scale single-crystal monolayer graphene and h-BN are grown mostly on copper [147, 148] films. Centimeter scale monolayer MoS₂ is reported grown on Au (111) surface [150]. However, most of the device applications require a transfer of 2D materials away from the metal films. Luckily, X. Xu et.al. reported a seeded 2D epitaxy method of growing a few-layer 2H-MoTe₂ on top of an amorphous glass surface [149], which can drastically simplify the device fabrication process and reduce possible damages during the material transfer. In general, defects level TMDs is still in the order of 10¹³ cm^-2 [132], which is a general consideration for real applications because of the robustness requirement. We believe that materials scientists are making great efforts in improving the quality and we are looking forward to embracing their success.

4.2 Making use of the spin-valley locking properties

The spin-polarized exciton emission from TMDs provides a tuning knob on the light sources and photodetectors. While wavelength-division multiplexing devices make use of the wavelength to improve the signal capacity, the embedded spin-valley locking provides an extra degree of freedom for on-chip optical communications. Recently, the separation of photon emissions from valley-polarized excitons has been reported in various structures including plasmonic nanostructures [151–154], photonic crystal waveguides [155, 156], conventional optical waveguides [157, 158], and topological systems [159]. Recently, W. Liu et.al. demonstrated [159] that the helical topological exciton-polaritons can be observed at up to 200 K by combing a monolayer WS₂ and a nontrivial hexagonal silicon nitride photonic crystal. The nontrivial photonic crystal consists of two areas with different topologies. The scatter-free chiral propagation is verified at the boundary where polaritons with opposite helicities are transported to opposite directions. However, these designs can only passively control the propagation like optical routers. For TMDs, it is possible to control the injection of spin current to realize polarized exciton emission. Similarly, photodetectors can read optical orbital/spin angular momentum of light [145] via the Valley Hall effect. Since the previously reported emission wavelength is away from the telecommunication range, to this extent, we believe that the light-matter interaction of the spin-polarized exciton may provide new opportunities for the future success of silicon-integrated emitters and photodetectors.

4.3 Hetero-integration of different photonic devices

2D materials have shown many extraordinary physical properties. However, it is worth noticing other rival materials. The commercial optical transceiver based on III-V devices hybrid wafer bonded on silicon substrates can easily have 100 Gbit·s^-1 bandwidth but the wafer bonding method is unfriendly for the small foot-print and large scale integration requirement. For reliable room-temperature operation, energy-efficient, and economic integrated photonics circuits, traditional CMOS foundries are working on the epitaxial growth of III-V on Si, which is a challenging but appealing platform for photonic integration. Integration technologies based on III-V quantum dots [160], Er-doped silicon [161], as well as the van der Waals materials, are also being intensively explored for next-generation optical communications. The competition between different systems may cause the death of some technologies forever or bring a symbiosis of some others. In this aspect, 2D material devices are physically friendly for being integrated with other 2D/3D systems [162, 163]. As far as we know, lithium niobate has excellent electro-optic modulation properties originating from the Pockels effect, and hybrid silicon/lithium niobate Mach–Zehnder modulators have been recently demonstrated with high performance beyond 100 Gbit·s⁻¹ [164]. It is possible to combine an Er-doped silicon emitter, a lithium niobate modulator, and a MoTe₂ based photodetector to form a hetero-integrated optical transceiver.

Hetero-integration is a broad and complex topic and requires future efforts in material growth, device fabrication as well as integrated system design. A matured commercial system will include not only the aforementioned devices but also optical isolators [165], routers [166], phase shifters [167], cavities, and waveguides [168, 169]. We hope the 2D materials that are compatible with the CMOS technologies will be used in the fabrication of such a compact optical platform.

4.4 Proposals of 3D silicon photonics with 2D material active devices

As the technology node of field-effect transistor continues to scale down to sub-ten-nanometers, the traditional CMOS technology is working on 3D integration to increase transistor density and to overcome the interconnect latency and physical limitation on transistor feature-size [170]. 3D silicon photonics architecture is also needed to overcome the difficulties in sophisticated co-integration electronic design tools, the processing skills, and the huge mismatch in footprint among the CMOS devices and photonic blocks [171]. The 2D material fabrication process is proved to be compatible with BEOL as well as monolithic 3D integration [171, 172]. Here we propose a tiny transceiver based on 3D integration of silicon photonics with the 2D light emitter, photodetectors, and logic gate controllers as shown in Fig. 7. The electrical signal can also be transmitted to control the light emission. The optical signal can also be converted by the photodetector to an electronic gating signal. The electronic and photonic devices layers are separated into different layers to solve the incompatibility of processing technology. We believe compact 3D photonic designs could help to improve energy efficiency and to reduce the footprint mismatch of photonic blocks for optical communication.

Fig. 7. 3D integration of 2D material-based devices and photonic circuits. The top layer is the electrical wire layer and the bottom is the photonics layer. The 3D integration allows a compact integration of electrical controls with 2D active devices and passive optical parts.

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5. Conclusion

In this review, we have summarized various designs and characteristics of the silicon integrated light emitters and photodetectors: direct band 2D materials or hetero-bilayers with long lifetime excitons are the best fit for nanolasers; the sensitivity of photodetectors can be optimized by increasing the effective absorption rate while large bandwidth photodetectors require short photocarrier channel length and excellent contacts. Future devices can make use of novel properties such as the non-trivial topology and the spin-valley locking effects etc. For large-scale on-chip applications, atomic thin devices will need to address challenges such as high-quality material growth, synergic working with other components on silicon imposers. We also propose that the 3D silicon photonics with 2D active devices can improve energy efficiency and reduce the footprint mismatch of photonic blocks and inspire applications in autonomous vehicles, construction site mapping, drones, and robots.

Funding

National Key Research and Development Program of China (nos. 2020YFA0309300, nos.2019YFA0210203); National Natural Science Foundation of China (No.61974167); Natural Science Foundation of Guangdong Province (No.2021A1515011787); Guangzhou Basic and Applied Basic Research Foundation (No.202102020353); the Guangdong Program (No. 2019QN01X113).

Disclosures

The authors declare no conflicts of interest.

Data availability

No data were generated or analyzed in the presented research.

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Material	λ_emission (nm)	Bandwidth (nm)	Lifetime (ns)	Quantum yield(%)	Lattice	Integration Substrate
MoS₂	652 [51]	∼131 [51]	0.09 [54]	0.4 [51]	2H	Si [59]
MoSe₂	792 [52]	∼167 [52]	1 [55]	1.67 [55]		Si₃N₄ [60–69]
WS₂	623 [53]	∼76 [53]	0.8 [56]	6 [56]		GaP [57, 70, 71]
WSe₂	752 [52]	∼146 [52]	1.3 [57]	0.18 [58]		SiO₂ [72–75]
InSe	985 [76]	∼150 [76]	1 [77]	—	Rhombohedrl	SiO₂ [78]
MoSe₂/WS₂	810 [79]	∼100 [79]	93 [80]	<0.1	Heterobilayer	—
MoSe₂/WSe₂	899 [81]	∼100 [82]	∼2 [82, 83]	<0.1	Heterobilayer	Si₃N₄ [83] GaP [84] h-BN [85]
MoS₂/WSe₂	1240 [86]	∼100 [86]	∼1 [87]	<0.1	Heterobilayer	Si [87]
1L BP	708 [15]	—	—	—	Zinc blende	Si [88–95]
2L BP	961 [15]
3L BP	1268 [15]
4L BP	1441 [15]
MoTe₂	1147 [96, 97]	∼100 [96, 97]	0.002 [98, 99]	<0.1	2H	Si [100–105]
Graphene	600∼1600 [23, 24]	∼1000	∼1 [23, 24]	—	Hexagonal	Si₃N₄ [106–109]
						Si [110–126]
						Chalcogenide [127]

Gain medium	Integration	λ_exc (nm)	λ_lasing (nm)	Q	β_sp	F_p	Γ(%)	Threshold (W/cm²)
1L MoTe₂ [101]	Si PhCC	633(CW)	1132	5603	0.1	76	0.0145	6.6
MoS₂/WSe₂ [87]	Si PhCC	740(CW)	1128	525	0.17	/	/	1.72×10³
1L WS₂ [60]	Si₃N₄ PhCC	450(CW)	637	2500	/	/	0.092	97
1L WS₂ [62]	Si₃N₄ microdisk	473(Pulse)	612.2	2604	0.5	17	0.11	5∼8×10⁶
MoSe₂/WSe₂ [83]	Si₃N₄ grating	729(Pulse)	917	630	0.046	2.35	2.08	1.4
1L WSe₂ [71]	GaP PhCC	632(CW)	739.7	2465	0.19	37	/	1
4L MoS₂ [72]	SiO₂ microdisk/sphere	514(CW)	600∼800	2600∼3300	0.68	8	0.15	7×10³
1L MoS₂ [73]	SiO₂ sphere	532(CW)	663.6	582	0.22	/	0.15	380
1L WS₂ [75]	TiO₂/SiO₂ DBR	532(CW)	636.6	570	0.77	3.4	/	0.44
100 nm BP [88]	Si/SiO₂ DBR	1520(Pulse)	3611	516	0.01	15	2	1.3

Integration type	Structure	λ_detect (nm)	Responsivity	NEP (pW/Hz^0.5)	Device Speed(GHz)
Si PhC waveguide/SLG [116]	inplane p-n junction	1550	170 mA/W	-	18
Si PhC waveguide/2L MoTe₂ [100]	inplane p-n junction	1160	5 mA/W	-	0.15
Si slot waveguide/SLG [114]	inplane p-n junction	1560	76 mA/W	-	15
Si slot waveguide/SLG [115]	inplane p-n junction	1550	273 mA/W	-	-
Si waveguide/SLG [121]	bowtie-shape antenna	1550	500 mA/W	-	110
Si waveguide/SLG [122]	slotted gold film	1550	360 mA/W	-	110
Si waveguide/SLG [113]	graphene heterojunction	1550	370 mA/W	-	-
Si waveguide/SLG [117]	metal/graphene junction	2000	20 mA/W	-	20
Si waveguide/SLG [117]	metal/graphene junction	1550	400 mA/W	-	40
Si waveguide/60 nm MoTe₂ [105]	ring resonator	1550	500 mA/W	90	3.5×10⁻²
Si waveguide/SLG [118]	ring resonator	1555	90 V/W	-	12
Si waveguide/11.5 nm BP [91]	photoconductive	1550	125 mA/W	-	3
Si waveguide/20 nm BP [90]	plasmon-enhanced	1550	6250 mA/W	-	0.15
Si waveguide/40 nm BP [93]	photoconductive	4000	2×10⁴ mA/W	120	-
Si waveguide/SLG/11 nm MoTe₂ [104]	vertical channel	1265	150 mA/W	-	46
Si waveguide/SLG/19.5 nm MoTe₂ [103]	Schottky junction	1310	23 mA/W	-	0.5
Si waveguide/Graphene [112]	Schottky junction	1550	0.1 mA/W	-	2×10⁻³
Si F-P cavity/SLG [126]	F-P cavity	1550	20 mA/W	3407	0.12
DBR F-P cavity 2L Graphene [124]	F-P cavity	864	21 mA/W	-	-

Material	λ_emission (nm)	Bandwidth (nm)	Lifetime (ns)	Quantum yield(%)	Lattice	Integration Substrate
MoS₂	652 [51]	∼131 [51]	0.09 [54]	0.4 [51]	2H	Si [59]
MoSe₂	792 [52]	∼167 [52]	1 [55]	1.67 [55]		Si₃N₄ [60–69]
WS₂	623 [53]	∼76 [53]	0.8 [56]	6 [56]		GaP [57, 70, 71]
WSe₂	752 [52]	∼146 [52]	1.3 [57]	0.18 [58]		SiO₂ [72–75]
InSe	985 [76]	∼150 [76]	1 [77]	—	Rhombohedrl	SiO₂ [78]
MoSe₂/WS₂	810 [79]	∼100 [79]	93 [80]	<0.1	Heterobilayer	—
MoSe₂/WSe₂	899 [81]	∼100 [82]	∼2 [82, 83]	<0.1	Heterobilayer	Si₃N₄ [83] GaP [84] h-BN [85]
MoS₂/WSe₂	1240 [86]	∼100 [86]	∼1 [87]	<0.1	Heterobilayer	Si [87]
1L BP	708 [15]	—	—	—	Zinc blende	Si [88–95]
2L BP	961 [15]
3L BP	1268 [15]
4L BP	1441 [15]
MoTe₂	1147 [96, 97]	∼100 [96, 97]	0.002 [98, 99]	<0.1	2H	Si [100–105]
Graphene	600∼1600 [23, 24]	∼1000	∼1 [23, 24]	—	Hexagonal	Si₃N₄ [106–109]
						Si [110–126]
						Chalcogenide [127]

Gain medium	Integration	λ_exc (nm)	λ_lasing (nm)	Q	β_sp	F_p	Γ(%)	Threshold (W/cm²)
1L MoTe₂ [101]	Si PhCC	633(CW)	1132	5603	0.1	76	0.0145	6.6
MoS₂/WSe₂ [87]	Si PhCC	740(CW)	1128	525	0.17	/	/	1.72×10³
1L WS₂ [60]	Si₃N₄ PhCC	450(CW)	637	2500	/	/	0.092	97
1L WS₂ [62]	Si₃N₄ microdisk	473(Pulse)	612.2	2604	0.5	17	0.11	5∼8×10⁶
MoSe₂/WSe₂ [83]	Si₃N₄ grating	729(Pulse)	917	630	0.046	2.35	2.08	1.4
1L WSe₂ [71]	GaP PhCC	632(CW)	739.7	2465	0.19	37	/	1
4L MoS₂ [72]	SiO₂ microdisk/sphere	514(CW)	600∼800	2600∼3300	0.68	8	0.15	7×10³
1L MoS₂ [73]	SiO₂ sphere	532(CW)	663.6	582	0.22	/	0.15	380
1L WS₂ [75]	TiO₂/SiO₂ DBR	532(CW)	636.6	570	0.77	3.4	/	0.44
100 nm BP [88]	Si/SiO₂ DBR	1520(Pulse)	3611	516	0.01	15	2	1.3

Recent progress of silicon integrated light emitters and photodetectors for optical communication based on two-dimensional materials

Abstract

1. Introduction

2. Photonics integrated 2D emitters

2.1 Material choice

2.2 Physical mechanisms and criteria of 2D nanolasers

2.2.1. Characteristics of 2D nanolasers

2.2.2. Opportunities for the next studies of on-chip 2D nanolasers

2.3 LEDs and thermal emitters

3. Integrated 2D photodetectors

3.1 Enhancing the light-matter interaction in 2D materials

3.2 Vertical junctions with short photocarrier channel

3.3 Contact issue of 2D materials

4. Prospective of the on-chip integration design

4.1 2D materials and heterostructures with advanced properties

4.2 Making use of the spin-valley locking properties

4.3 Hetero-integration of different photonic devices

4.4 Proposals of 3D silicon photonics with 2D material active devices

5. Conclusion

Funding

Disclosures

Data availability

References

Data availability

Cited By

Figures (7)

Tables (3)

Equations (7)

Optical Materials Express