The fast development of silicon photonics makes it feasible to construct optical interconnects that can replace electrical interconnects for chip-level data communications with low energy consumption and large bandwidth , . However, for photodetection in the silicon-based optical interconnect, it still needs to be integrated with another absorbing material, e.g. germanium or a III-V compound semiconductor , , leaving a big challenge for direct monolithic integration with the complementary metal-oxide-semiconductor (CMOS) technology and in achieving high bandwidth limited by the absorbing materials’ poor electrical properties. Graphene, a unique CMOS-compatible, two-dimensional (2D) material, provides great potential in the realization of high-performance optoelectronic devices , , , , , . In particular, significant efforts have been devoted to graphene photodetectors (PDs) , , , , , , , , , , . The distinct properties of graphene in terms of ultrahigh carrier mobility , , zero bandgap property, which enables wavelength-independent light absorption over a very wide spectral range , , , and tunable optoelectronic properties ,  enable the realization of graphene PDs with a large spectral bandwidth and high speed.
Graphene PDs rely on devices with broken inversion symmetry , , . For graphene-based integrated PDs, the inversion symmetry can be conveniently relaxed through an asymmetric positioning of the waveguide with respect to the graphene coverage, and several devices ,  have been reported. However, because of the modest light-matter interaction between the single-layer graphene (SLG) and the waveguide mode, the size of the devices has to be tens to hundreds of micrometers to achieve a reasonable responsivity, thus limiting high-speed operation. A small device, however, gives weak absorption of light and, thus, eventually low responsivity. This counteracting effect represents a big challenge for graphene-based PDs supporting both high responsivity and large bandwidth. The typical bandwidth of the graphene PD is ~76 GHz, however with a modest responsivity of 1 mA/W . Another promising scheme is to break the symmetry of the potential profile of the device by different metallic-induced doping  near the metal-graphene contact regions. In this scheme, an internal (built-in) electric field  is formed to separate the photogenerated carriers , . A milestone for high-speed photodetectors based on SLG has been demonstrated by the free-space top-illumination technique . However, the internal built-in electric field exists only in narrow regions of ~200 nm adjacent to the electrode/graphene interfaces , . The large distance of 1 μm between the two electrodes  limits the collection efficiency of photogenerated carriers and thus the responsivity. Moreover, the required graphene of very high quality typically relies on the exfoliation method , which restricts the potential for large-scale integration.
In order to improve the performance, plasmonic bowtie nanostructure has recently been employed to enhance the light-graphene interaction in a extremely localized area, resulting in larger bandwidth . Here, we report an ultra-compact, on-chip, high-speed graphene photodetector based on a plasmonic slot waveguide , , . The subwavelength confinement of the plasmonic mode gives rise to enhanced light-graphene interactions, and the narrow plasmonic slot of 120 nm enables short drift paths for the photogenerated carriers. A small integrated graphene photodetector with a graphene coverage length of 2.7 μm is demonstrated without any drop in response up to a frequency of 110 GHz and an intrinsic responsivity of 25 mA/W. Increasing the device size to 19 μm results in an increased intrinsic responsivity of 360 mA/W, equivalent to a high external quantum efficiency of 29%. These performances can further be improved significantly. With the extremely broad absorption band of graphene, our device shows great potential. Moreover, the use of graphene grown here by chemical vapor deposition (CVD) allows scalable fabrication, and we believe that our work greatly pushes the 2D material toward practical applications, e.g. in optical interconnects, high-speed optical communication, and so on.
The schematic of the proposed graphene-plasmon hybrid photodetector is shown in Figure 1A, where the light from a fiber is first coupled to a silicon waveguide through a grating coupler  and further to the plasmonic slot waveguide by a short tapered structure , . The plasmonic slot waveguide (see the cross-section of the device in Figure 1C) consists of two asymmetric metallic contacts of Au(90 nm)/Pd(5 nm) and Au(90 nm)/Ti(5 nm), resulting in different doping levels in the graphene . Photocurrent can be generated from contributions by photothermoelectric (PTE), photobolometric (PB), and photovoltaic (PV) effects. The proposed device, which follows the principle in , utilizes the PV current as the dominant contribution. The PTE effect is due to temperature gradient in the device and to the doping asymmetry. The generated photocurrent is proportional to (S2–S1)dT/dx, where S1 and S2 are the Seebeck coefficients for the two metal-graphene contacts. In the proposed device, though the different metallic doping introduces a difference in the Seebeck coefficients, the whole graphene layer is illuminated by light within the narrow plasmonic gap, resulting in a near-zero net temperature gradient and thus negligible PTE current. The PB response of the graphene photodetector can be observed only in biased devices. However, as we do not have a gating voltage, and the graphene is under low doping, this results in a negligible PB current . Thus the PV current is the only known dominant contribution. The potential difference indicated in Figure 1B gives rise to efficient separation of the photgenerated carriers and formation of the photocurrent. The performance with respect to responsivity and speed of the proposed photodetector really takes advantage of the plasmonic slot waveguide with the narrow gap g of 120 nm. First, the plasmonic slot waveguide provides subwavelength light confinement in the nanometer scale, as shown in Figure 1C, resulting in extremely strong graphene-light interaction and thus high responsivity. With the optimum geometry with a small gap and thin Au layer of the plasmonic slot waveguide, the single-layer graphene leads to extremely high light absorption of ~1 dB/μm (see the Supplementary Material), which is at least one order of magnitude higher than that of graphene-silicon waveguide photodetectors , . Second, the internal electric field mentioned above covers the whole plasmonic slot region of 120 nm. Thus, the photogenerated carriers can be effectively separated, leading to high responsivity. Furthermore, the narrow plasmonic slot gives short drift paths for carriers, resulting in ultrafast carrier transition through the photodetection region and thus high speed.
2.2 Experimental results
Figure 2A shows the fabricated graphene-plasmonic photodetector, where the dashed lines represent the graphene coverage boundary. The Raman spectrum shown in Figure 2B illustrates moderate degradation after the wet-transfer process, which is given in Section 4.2. Graphene-plasmonic hybrid photodetectors with different graphene coverage lengths were fabricated, and the cut-back method shows absorption coefficient of 0.8 dB/μm in the detection region, as presented in Figure 2C.
Being a key parameter, the intrinsic responsivity Rph of the graphene-plasmonic waveguide photodetector with respect to the power in the plasmonic waveguide was characterized with a light power of −4 dBm (400 μW) in the plasmonic waveguide. The photocurrent was measured by switching on and off the light, and the current difference was recorded. Measurements were performed at different bias voltages VB (Au/Ti electrode relative to Au/Pd electrode) for devices with different graphene-coverage lengths L (=2.7, 3.5, 9.0, 19 μm), as shown in Figure 3A. The potential profile at different bias voltages and the corresponding output electrical signals, due to the injection of an optical pulse into the chip, are also presented in Figure 3B and C, respectively. At zero bias voltage, an intrinsic responsivity of ~1.3 mA/W is observed for the device with the graphene coverage length of 19 μm (see the inset of Figure 3A). It is due to the asymmetric potential profile between the two electrodes, which is also addressed in . The zero responsivity measured at the negative bias around −0.1 V indicates a flat potential profile, as presented in Figure 3B, and no change in the optical signal is observed (see the second subplot in Figure 3C). Further increasing the negative bias voltage results in an increased responsivity, and a clear electrical pulse is obtained at −0.8 V (Figure 3C). Increasing the positive bias voltage also causes an increase in the responsivity and a clear output electrical pulse with opposite polarity compared to the negative biasing case. A bias voltage beyond 1.5 V leads to a significant increase in responsivity and much larger amplitude for the electrical pulse. Moreover, as the graphene coverage length L increases, an improvement in the responsivity is also observed, as shown in the inset of Figure 3A, since a larger fraction of optical power is absorbed, thereby leading to a higher photocurrent. As presented in Figure 3A, the smallest device with 2.7 μm graphene coverage gives a responsivity of ~26 mA/W at the bias voltage of 2 V. The quantum yield is represented by the external quantum efficiency (EQE), which defined by EQE=Rph×ħω/e, where ħω is the photon energy (ħ being the Planck constant and ω the frequency of light), and e is the electronic charge. Thus, the corresponding EQE of 2% is obtained. For the device with 19 μm graphene coverage, the highest responsivity of 360 mA/W is obtained with the bias voltage of 2.2 V, corresponding to a high EQE of 29%.
We further measured the bandwidth of these graphene-plasmonic photodetectors, as presented in Figure 4A–D. The experimental setup can be found in the Supplementary Material. All measurements were carried out with the bias voltage of 1.6 V to get a good signal-to-noise ratio. At low frequency (below 40 GHz), we used a vector network analyzer (VNA, 40 GHz bandwidth) to measure the current (optical) frequency response Rop. No response drop is found within the 40 GHz bandwidth (the blue lines in Figue 4A–D), indicating a large detection bandwidth. Then, the devices were further characterized by the alternative method of fast Fourier transform (FFT) of the impulse response of the detectors. Within the 40 GHz bandwidth, the frequency response obtained by the impulse response method (see the red solid red lines) agrees quite well with that of the VNA method mentioned above. With the aid of the impulse response method, we obtain the measurement system optical bandwidth of at least 85 GHz for the smallest device with the graphene coverage length of 2.7 μm. Increasing the device size results in a broader impulse response (see the Supplementary Material) and thus a smaller bandwidth. The device with a graphene coverage length of 19 μm exhibits the measurement system’s optical bandwidth of at least 75 GHz. All the impulse response measurements for the devices can be found inthe Supplementary Material. Note that such systematic bandwidth is significantly influenced by the bandwidth of the oscilloscope, the RF cable, and the bias Tee. Thus, we have employed the third method (FB+ESA) utilizing frequency beating (FB) between two coherent light fields to measure the RF power in a wideband electrical spectral analyzer (ESA, 110 GHz bandwidth). As presented in Figure 4A–D, for the frequency near 40 GHz, the response obtained by the FB+ESA method (triangular marks) overlaps quite well with that obtained by the VNA method (blue curves). Within 110 GHz, no drop in detection response is observed for the device with a graphene coverage length of 2.7 μm, indicating a bandwidth >110 GHz. A longer device leads to a slight drop in the detection response at high frequencies (~100 GHz). The small frequency dip at ~46 GHz is attributed to the impedance mismatch between the electrode pads and the RF probe. The device with a graphene coverage length of 19 μm (Figure 4D) exhibits a 1.5-dB optical bandwidth of 110 GHz. With the increase of the graphene coverage length, the responsivity is significantly improved, as demonstrated in Figure 3A, while the current frequency response Rop (shown by the red triangular marks) at the high frequency slightly drops, indicating a tradeoff between responsivity and bandwidth.
The detector with graphene coverage length of 19 μm was further used to receive 10, 20, and 40 Gbit/s return-to-zero (RZ) optical signals with a pseudo-random binary sequence (PRBS) length of 231–1, which is amplified by an erbium-doped fiber amplifier (EDFA) and injected into the chip. The output electrical signal from the graphene photodetector was electrically amplified (40 GHz bandwidth electrical amplifier) and fed to the high-speed oscilloscope (70 GHz bandwidth) to record the eye diagram. Clear and open eye diagrams were obtained for all signal (10, 20, and 40 Gbit/s), as shown in Figure 4E–G, indicating no pattern effect of the detector and proving the feasibility of using such graphene photodetectors in realistic optical communication applications.
3 Conclusion and discussion
In summary, we have demonstrated an on-chip, ultra-high-bandwidth photodetector based on a single-layer CVD graphene and plasmonic slot waveguide hybrid structure. The narrow plasmonic slot waveguide not only enhances light-graphene interactions but also enables the effective separation of the photogenerated carriers, leading to high responsivity and large bandwidth. An optical bandwidth larger than 110 GHz with large responsivity of 360 mA/W has been achieved. With narrower plasmonic gap as well as thinner Au layer, higher absorption by graphene is expected, thus promising even higher responsivity, as analyzed in the Supplementary Material. Furthermore, tuning the Fermi level by the top gate  would further improve the absorption of graphene and thus the responsivity. Moreover, the use of CVD-grown graphene enables large-scale integration.
It should be noted that the narrow plasmonic slot of 120 nm results in an ultrafast transit of carriers to the electrodes. The transit-time-limited bandwidth of the photodetector is given by ft=3.5/2πttr , where ttr is the transit time through the photodetection region. The carrier velocity can be estimated by a typical fitting function presented in . The saturation carrier velocity ranges from 1×107 cm/s  to 5.5×107 cm/s . Here we consider a relatively conservative carrier velocity of 1.4×107 cm/s. At zero bias voltage, the difference in Fermi level between the palladium- and titanium-doped graphenes is 0.05 V , resulting in the built-in electric field of ~0.4 V/μm. With a typical carrier mobility of 4×103 cm/Vs for CVD graphene on SiO2 and a fitting parameter γ=3, a corresponding carrier velocity of 1.1×107 cm/s is obtained, which means that it takes only 1.1 ps for the carriers to transit through the 120-nm plasmonic slot gap. Thus, a single transit-time-limited bandwidth of 520 GHz is expected. The capacitance of the photodetector can be estimated by taking a similar photodetector of plasmonic slot with amorphous germanium  for reference, where the capacitance was measured to be 5 fF with a 20-μm-long plasmonic slot waveguide of 100-nm-thick Au and 160 nm gap. Thus a capacitance of 0.83 and 6.7 fF is estimated for our device with graphene coverage lengths of 2.5 and 19 μm, respectively. The resistance is composed of the serier resistances by Ti-graphene and Pd-graphene contacts and a loading resistance of 50 Ω. A conservative contact resistivity of 1000 Ωμm  is used, corresponding to a total contact resistance of 800 and 100 Ω for the devices with graphene coverage lengths of 2.5 and 19 μm, respectively, resulting in bandwidths of 150 and 220 GHz, which are both smaller than the transit-time-limted bandwidth. Actually, one could find a slight frequency response drop around 100 GHz when the graphene coverage length changes from 2.5 to 19 μm, as shown in Figure 4, which is consistant with the analysis above. Further optimizing the contact resisance and Au thickness would help increase the bandwidth.
In this work, the devices were characterized around 1540 nm. In terms of optical absorption bandwidth, research has shown that graphene exhibits ultrawide band absorption from the terahertz to the visible range . Ultra-broad-band photodetection has also been demonstrated from 800 nm up to 20 μm . The only restriction of the optical wavelength bandwidth is the inverse taper coupling bandwidth from the silicon waveguide to the plasmonic slot waveguide. Nevertheless, previous works have shown that such coupling bandwidth is at least 100 nm , . This suggests that our device could work over a broad wavelength range. The devices demonstrated here are fully CMOS-compatible and can easily be integrated with the silicon platform, and the use of CVD-grown graphene shows a promising way to achieving multifunctional integrated graphene devices for optical interconnects.
4.1 Fabrication process
The device was fabricated on a commercial silicon-on-insulator sample with a top silicon layer of 250 nm and a buried oxide layer of 3 μm. The top silicon layer was first thinned down to 100 nm by dry etching (STS advanced-silicon-etching machine, ASE) in order to obtain good coupling efficiency with the plasmonic slot waveguide. The grating couplers and silicon waveguides were patterned by electron beam lithography (EBL, E-Beam Writer JBX-9500FSZ), and fully etched by the ASE dry-etching process. After that, the graphene layer was wet-transferred and patterned by standard ultraviolet (UV) lithography (Aligner: MA6-2) and oxygen (O2) plasma etching. Then, the Au/Pd (90 nm/3 nm)-graphene contact was patterned by a second EBL process, followed by metal deposition and lift-off. Finally, the Au/Ti (90 nm/3 nm)-graphene contact was patterned by a third EBL step, followed by metal deposition and lift-off.
4.2 Graphene wet-transfer process
SLG grown by CVD on a Cu foil (GRAPHENE SUPERMARKET) was transferred onto the devices by a wet-chemistry method. The SLG sheet was transferred by the following steps. First, a layer of photoresist (AZ5200 series) was spin-coated (3500 rpm for 1 min) onto the SLG, and the photoresist/SLG stack was then released by etching the underlying Cu foil in a Fe(NO3)3 solution (17 wt%) at room temperature. The stack floating in the solution was then washed by deionized (DI) water a couple of times and transferred to the target device, followed by drying at room temperature for at least 24 h. Afterward, the photoresist on the SLG was dissolved in acetone at room temperature. Finally, the graphene device was cleaned by ethanol and DI water and then dried before further processing.
5 Supplementary Materials
Impulse response of the fabricated photodetector.
Simulation of graphene absorption.
The authors would like to thank Vitaliy Zhurbenko for the help in calibrating the RF cables and Bias Tee. They thank Prof. Fengnian Xia and Dr. Peter David Girouard for constructive discussion. This work was supported by the Center for Silicon Photonics for Optical Communication (SPOC, DNRF123, Funder Id: http://dx.doi.org/10.13039/501100001732) and the Center for Nanostructured Graphene (CNG, DNRF103, Funder Id: http://dx.doi.org/10.13039/501100001732), both sponsored by the Danish National Research Foundation; the QUANPIC project sponsored by VILLUM FONDEN (ref 00025298, Funder Id: http://dx.doi.org/10.13039/100008398); the Independent Research Fund Denmark (Project No. 9041-00333B, Funder Id: http://dx.doi.org/10.13039/501100011958), and the mid-chip project sponsored by VILLUM FONDEN (No. 13367, Funder Id: http://dx.doi.org/10.13039/100008398). N.A.M. is a VILLUM Investigator supported by VILLUM FONDEN (No. 16498, Funder Id: http://dx.doi.org/10.13039/100008398) and X.Z. is supported by VILLUM Experiment (No. 17400, Funder Id: http://dx.doi.org/10.13039/100008398).
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The online version of this article offers supplementary material (https://doi.org/10.1515/nanoph-2019-0167).
About the article
Published Online: 2019-09-21
Published in Print: 2020-02-25
Conflict of interest: The authors declare no conflict of interests. A recent post in arxiv by Z. Ma et al. has shown that significant enhancement of responsivity can be achieved when further decreasing the plasmonic gap down to 15 nm. Their data further supports the use of the proposed plasmonic slot waveguide for high performance graphene photodetectors.
Citation Information: Nanophotonics, Volume 9, Issue 2, Pages 317–325, ISSN (Online) 2192-8614, DOI: https://doi.org/10.1515/nanoph-2019-0167.
©2020 Yunhong Ding et al., published by De Gruyter, Berlin/Boston. This work is licensed under the Creative Commons Attribution 4.0 Public License. BY 4.0