High-Responsivity Graphene–Boron Nitride ... - ACS Publications

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High-Responsivity Graphene−Boron Nitride Photodetector and Autocorrelator in a Silicon Photonic Integrated Circuit Ren-Jye Shiue,*,† Yuanda Gao,‡ Yifei Wang,† Cheng Peng,† Alexander D. Robertson,‡ Dmitri K. Efetov,† Solomon Assefa,§ Frank H. L. Koppens,∥ James Hone,‡ and Dirk Englund*,†

Nano Lett. 2015.15:7288-7293. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 10/07/18. For personal use only.

†

Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ‡ Department of Mechanical Engineering, Columbia University, New York, New York 10027, United States § IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, United States ∥ ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels, Barcelona, Spain S Supporting Information *

ABSTRACT: Graphene and other two-dimensional (2D) materials have emerged as promising materials for broadband and ultrafast photodetection and optical modulation. These optoelectronic capabilities can augment complementary metal−oxide− semiconductor (CMOS) devices for high-speed and low-power optical interconnects. Here, we demonstrate an on-chip ultrafast photodetector based on a two-dimensional heterostructure consisting of high-quality graphene encapsulated in hexagonal boron nitride. Coupled to the optical mode of a silicon waveguide, this 2D heterostructure-based photodetector exhibits a maximum responsivity of 0.36 A/W and high-speed operation with a 3 dB cutoff at 42 GHz. From photocurrent measurements as a function of the top-gate and source-drain voltages, we conclude that the photoresponse is consistent with hot electron mediated effects. At moderate peak powers above 50 mW, we observe a saturating photocurrent consistent with the mechanisms of electron−phonon supercollision cooling. This nonlinear photoresponse enables optical on-chip autocorrelation measurements with picosecond-scale timing resolution and exceptionally low peak powers. KEYWORDS: graphene, photodetectors, optoelectronics, silicon photonics, autocorrelators, boron nitride

T

semiconductor materials, high optical powers are required in nonlinear applications including optical pulse characterization, sampling, and signal processing.8−10 Recently, the family of two-dimensional (2D) materials has emerged as an alternative optoelectronic platform with new functionalities,11,12 including broadband ultrafast photodetection,13−17 on-chip electro-optic modulation,18−21 light emission,22,23 saturable absorption,24 and parametric nonlinearities.25−27 Moreover, distinct 2D materials can be assembled nearly defect-free into entirely new types of 2D heterostructures,28,29 enabling optoelectronic

he past decades have seen concerted efforts to combine optical and electrical components into integrated optoelectronic circuits for a variety of applications, including optical interconnects,1−3 biochemical sensing,4 and all-optical signal processing.5 The leading photodetector architectures for photonic integrated circuits (PICs), are presently based on bulk semiconductor materials, predominantly Ge6 and InP.7 However, the integration of bulk semiconductor-based onchip detectors has faced important challenges, including in some cases front-end changes in complementary metal−oxide− semiconductor (CMOS) processing, a spectral response limited by the material’s bandgap, for example, up to the L telecommunication band for strained Ge, and an intrinsic speed limited by the material’s carrier mobility. In addition, because of the weak nonlinear optical response of these bulk © 2015 American Chemical Society

Received: June 15, 2015 Revised: September 12, 2015 Published: September 15, 2015 7288

DOI: 10.1021/acs.nanolett.5b02368 Nano Lett. 2015, 15, 7288−7293

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Figure 1. (a) Schematic of the hBN/SLG/hBN photodetector on a buried silicon waveguide. The lower-right inset shows the electric field energy density of the fundamental TE waveguide mode, obtained by finite-element simulation. (b) Optical microscope image (upper panel) and a scanning electron micrograph (lower panel) of the completed device. (c) Resistance of the graphene device as a function of gate-source voltage VGS. A resistance peak at VGS = 2 V indicates the charge neutrality point.

than the graphene detector.36 The ability for high responsivity photodetection and ultrafast optical autocorrelation in one small-footprint device opens the door to new applications and device concepts in integrated optics. Figure 1a illustrates the device architecture. Silicon waveguides were fabricated in an SOI wafer with a 220 nm silicon membrane on a 3 μm thick SiO2 layer. This step used shallow trench isolation (STI) commonly used in CMOS processing. A waveguide width of 520 nm supports a single transverse-electric (TE) mode shown in the simulated mode in Figure 1a (lower right panel). The chip was planarized by backfilling with a thick SiO2 layer, followed by chemical mechanical polishing to reach the top silicon surface. We then transferred the multilayered hBN/SLG/hBN stack onto the photonic chip using van der Waals (vdW) assembly.28 The graphene channel spans 40 μm of the waveguide. We implemented one-dimensional edge contacts to the encapsulated graphene layer using a series of etching and metallization steps.28 A polymer electrolyte (poly(ethylene oxide) and LiClO4) layer was spin-coated on the entire chip to gate the graphene device.37 Figure 1b shows the completed structure. The drain electrode is positioned only 200 nm from the waveguide to induce a pn junction near the optical mode.38 Confocal scanning beam excitation measurements confirm a strong photoresponse region near this metal/graphene interface, as demonstrated previously39 and shown for this particular device in Figure S1 in the Supporting Information. Measurements of the device resistance as a function of gate voltage, shown in Figure 1c, indicate a charge neutrality point (CNP) at a gatesource voltage of VGS = 2 V and a minimum device resistance of 77 Ω. From similar hBN/SLG/hBN stacks assembled on SiO2

properties and device concepts that were not feasible using bulk semiconductors. Here, we introduce a 2D heterostructure consisting of singlelayer graphene (SLG) encapsulated by hexagonal boron nitride (hBN) that enables both high-responsivity photodetection and ultrafast pulse metrology in a compact waveguide-integrated design. The mobility of BN-encapsulated graphene can reach up to 80 000 cm2/(V s) at room temperature, exceeding that of traditional semiconductors such as Ge by 1−3 orders of magnitude. We use one-dimensional contacts to the hBN/ SLG/hBN stack combined with a wide channel design to achieve a device resistance as low as ∼77 Ω. We measure operating speeds exceeding 40 GHz (3 dB cutoff) and a recordhigh detection responsivity exceeding 350 mA/W, approaching responsivities of on-chip Si−Ge photodetectors.6 We fabricated this 2D heterostructure in a back-end-of-the-line (BEOL) step on a silicon-on-oxide (SOI) PIC, which was fabricated in a CMOS-compatible process. The strong electron−electron interaction and weak electron−phonon coupling of the SLG30−32 result in a nonlinear photoresponse above ∼50 mW of peak input power, enabling direct autocorrelation characterization of ultrafast pulses. Unlike traditional free-space autocorrelators, optical nonlinear time-lens techniques,33 and frequency-resolved optical gating (FROG),34,35 this on-chip 2D heterostructure-based autocorrelator does not require parametric nonlinear effects, passive dispersive optics, or separate photodetectors. We measured a minimum timing resolution of 3 ps with a minimum required peak power of 67 mW, comparable to autocorrelators based on two-photon absorption (TPA) of silicon or III−V compound semiconductors, though TPA-based autocorrelators must be orders of magnitude longer 7289

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Figure 2. (a) Measured and (b) calculated responsivity as a function of gate-source VGS and drain-source VDS voltages. (c) and (d) Electron temperature profile of the SLG under c.w. optical excitation from the waveguide (top panel). Bottom panel: The Seebeck coefficient profiles of the SLG at different VGS and VDS that are indicated as the colored dots in (b).

photothermoelectric (PTE) effect.30,41 The photocurrent reaches a maximum of 0.36 A/W at VGS = 2 V and VDS = 1.2 V; this represents a more than 3-fold improvement over previous waveguide-integrated graphene photodetectors.38−40 To explain the measured 6-fold pattern in the responsivity map, we consider the PTE effect for the metal/graphene junction that overlaps with the waveguide. The evanescent field from the waveguide creates photoexcited carriers in the SLG, which rapidly thermalize, increasing the local electron temperature. Figure 2c (top panel) shows the numerically solved local electron temperature Tel across the source and drain contacts of the detector (Supporting Information). The photoexcited hotelectrons diffuse and create a potential gradient ΔV = −S(x)∇Tel(x), where ∇Tel(x) is the temperature gradient of the hot electrons and S(x) is the Seebeck coefficient. The total photocurrent collected between the source and drain contacts is given by

substrates, which also included a Hall-bar geometry that was not possible for us to include on the waveguide-integrated device, the mobility of graphene is in the range of 40 000− 60 000 cm2/V·s at carrier densities between 2−4 × 1012 cm−2.28 We characterized the transmission of the completed device in the telecommunications band at 1550 nm using lensed fibers for edge-coupling to SiN mode converters at the input and output of the ∼4 mm-long Si waveguide, as reported previously.39 The hBN/SLG/hBN detector stack is located at the center of the waveguide, ∼2 mm from either edge facet. Depositing the stack increased the waveguide transmission loss from 11 to 13.2 dB. Attributing this 2.2 dB excess loss to the 40 μm long graphene−waveguide overlap, we calculate an absorption coefficient of 0.055 dB/μm. This coefficient is slightly higher than the absorption of 0.043 dB/μm that we estimated independently from a finite element simulation of the waveguide evanescent field coupling to the SLG39 (note that the hBN is transparent in the telecom band since it has a bandgap of 5.2 eV). We attribute the slightly higher loss (0.48 dB) of the experimental device to additional scattering and reflection losses at the waveguide-detector stack interface. We observed negligible transmission loss from the electrolyte. Figure 2a presents responsivity measurements as a function of VGS and VDS when the detector is illuminated via the waveguide at 25 μW continuous-wave (c.w.) laser inside the waveguide. Here the responsivity is defined as the ratio of the short-circuit photocurrent (Supporting Information) to the optical power in the waveguide, R = Iph/Pin. Unlike previous waveguide-integrated graphene photodetectors, which included only drain-source voltage control,38−40 the additional top electrolyte-gate allows us to independently tune the graphene Fermi level and electric field across the waveguide mode. Scanning VGS and VDS produces a 6-fold pattern in the photocurrent, which qualitatively matches the behavior of the

IPh = G

∫0

L

S(x)∇Tel(x)dx

(1)

where G is the conductance of graphene and L the is length of the graphene channel. From the Mott formula,30,42−44 S can be expressed as S(μ) = −

π 2kB2T 1 dσ 3e σ dμ′

(2)

where kB is the Boltzmann constant, T is the lattice temperature, e is the electron charge, σ is the conductivity of graphene, and μ is the chemical potential. Equation 2 indicates that S depends strongly on the carrier density and the chemical potential of graphene, which can be tuned by VGS and VDS in our device. Figure 2c and d show examples of the spatially resolved S(x) using parameters obtained from the measured conductance and the capacitance of the electrolyte (Supporting 7290

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Figure 3. (a) High-speed response of the graphene photodetector. The red dashed line shows the fitting to the experiment results with a RC lowpass filter model. The extracted 3 dB cutoff frequency is at 42 GHz. (b) Photocurrent as a function of input optical power. (c) Spectral response of the normalized photocurrent for c.w. laser excitation.

Figure 4. (a) Photocurrent of the on-chip graphene detector under pulsed (250 fs) excitation. The black curve shows a fitting of the experimental . (b) Autocorrelation traces of the graphene-based autocorrelator with different input peak power of the laser data (red dots) with IPhoto ∝ P0.47±0.003 in pulses. (c) Response time (τ) of the graphene autocorrelator at different input excitation power. (d) Autocorrelation trace measured with low input average power of 1.4 μW, corresponding to a peak power of 67 mW.

Information). By integrating S(x) and ∇Tel along the source and drain contacts using eq 1, we can therefore calculate the photocurrent with respect to VGS and VDS, producing the photocurrent mapping in Figure 2b, which shows a 6-fold pattern qualitatively similar to the measured gate- and biasdependent responsivity. From Figure 2b−d, the photoresponse is maximized when the integral of eq 1 is maximized at specific VGS and VDS voltages. Therefore, tuning VGS and VDS in our device allows us to optimize the responsivity compared with previous works without VGS tuning.38−40 It is also important to consider photovoltaic (PV) effects that generate photocurrent at the graphene p−n junction. However, the PV effect only produces a single sign reversal with respect to VGS and VDS and does not agree to the measured 6-fold pattern in the experiment.30,43,44 We therefore estimate the PV effect to be small compared with the PTE, which is confirmed by theoretical approximations presented in the Supporting Information. To test the dynamic response of the hBN/SLG/hBN detector, we coupled two narrowband (1 MHz) laser sources with a detuning frequency Δf ranging from 1 to 50 GHz into the waveguide. The interference of these two laser fields produces an intensity in the detector that oscillates at frequency Δf. Figure 3a displays the measured power at Δf, obtained on an electrical spectrum analyzer (maximum frequency 50 GHz) via high-speed RF probes (Cascade FPC-GS-150). This measurement indicates a 3 dB cutoff frequency at 42 GHz, matching the highest reported graphene photodetector speeds.16 We observed this ultrafast photoresponse even under zero drain-source bias, which distinguishes graphene

from typical semiconductor high-speed photodiodes.6 The detected photocurrent at VDS = 0 (Figure 3b) linearly reduces with input power, indicating vanishing dark current. The responsivity at VDS = 0 reaches 78 mA/W. The internal quantum efficiency (IQE) of the detector, defined as the number of generated charge carriers normalized by the absorbed photon number, can be expressed as IQE = 1.24/ λ[μm] × R/Ag, where R is the responsivity of the detector, λ is the input laser wavelength, and Ag is the absorption of graphene. Accounting for the fact that the graphene absorbs ∼33−40% (1.7−2.2 dB) of the optical power in the waveguide at 1550 nm, we can thus estimate an IQE of 16−19% at VDS = 0 and a maximum IQE of 72−87% at maximum responsivity (0.36 A/W). As shown in Figure 3c, the spectral response of the detector under c.w. laser excitation varies uniformly in the spectral range from 1510 to 1570 nm. We next tested the detector using 250 fs pulses at 1800 nm, produced by an optical parametric oscillator pumped by a Ti:sapphire oscillator with 78 MHz repetition rate. As shown in Figure 4a, the photocurrent shows a nonlinear photoresponse . This approximately square that is fit well by IPh ∝ P0.47±0.003 in root dependence in photocurrent with pump power suggests a supercollision-dominated cooling mechanism, for which IPh ∝ Pin .32 Using this nonlinear photoresponse, we characterized our photodetector’s ultimate speed limitations, which are dictated by the internal carrier dynamics. Figure 4b plots photocurrent traces as a function of the time delay Δt between pairs of 250 fs laser pulses for a range of incident powers. These traces show a clear dip at Δt = 0 with a width that corresponds 7291

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to the carrier relaxation time, that is, the time needed for the graphene detector to return to equilibrium. The half-width at half-maximum response times τ are plotted in Figure 4c for a sweep of input powers. The minimum response time is approximately ∼3 ps. The slight increase of the response time at higher input power is unclear to us at this point and requires future studies. The picosecond-scale nonlinear photocurrent from the photodetector can be used for on-chip autocorrelation measurements for, for example, ultrafast optical sampling. We tested the autocorrelator with an average power of only 1.4 μW (peak power of 67 mW) in the waveguide. The autocorrelation trace (Figure 4d) shows a clear dip with a minimum resolution down to 3 ps. With optimized coupling efficiency (>80%) from optical fibers to waveguides,45 our on-chip autocorrelator would require a lower peak power than free-space autocorrelators based on parametric frequency conversion46 and is comparable to TPA-based semiconductor autocorrelators with waveguide integration, though these require more than 10 mm of waveguide length36,47,48 compared with the 40 μm length of the graphene device. We noticed that the femtosecond-scale heating of the hot-electrons in graphene in a recent report could in principle improve the timing resolution of the on-chip autocorrelator to sub-50 fs.17 It is also important to note that this graphene-based autocorrelator supports a broad spectral range that is identical to the spectral response of the graphene detector, which in this work is from 1500 to 1800 nm. In summary, we have demonstrated a compact, high-speed, and high-responsivity graphene photodetector integrated into a SOI waveguide. The detector reaches a responsivity of 0.36 A/ W and a 3 dB cutoff frequency of 42 GHz. The measured photocurrent of the detector as a function of VGS and VDS can be modeled by the PTE effect at the p−n junction near the metal/graphene interface. The fabrication processes presented here show the potential of 2D heterostructure-based photodetector architectures integrated with CMOS-compatible silicon PICs. We expect that the detector responsivity can be further improved by increasing the IQE17,30 and raising the absorption using bi- or trilayer graphene instead of SLG. Under pulsed excitation, the nonlinear photocurrent response enables the characterization of ultrafast optical pulses and presents new on-chip functionalities for autocorrelation and ultrafast optical sampling measurements. The peak power required for autocorrelation measurements is as low as 67 mW with a timing resolution of 3 ps, though substantially shorter timing resolution may be possible based on the femtosecond heating of hot electrons in graphene. 17 The fully integrated autocorrelator and detector opens the door to compact photonic integrated circuits for ultrafast measurements, modelocking, and other high-speed optoelectronic applications.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

R.-J.S., Y.G., and Y.W. contributed equally to this work Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Xuetao Gan for helpful discussion on the experiment scheme and data analysis. Financial support was provided by the Office of Naval Research (Awards N00014-131-0662 and N00014-14-1-0349). Device fabrication was partly carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences (contract no. DE-AC02- 98CH10886). R.-J.S. was supported in part by the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award no. DESC0001088. Y.F.W. was supported in part by Tsinghua Xuetang Talents Program.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b02368. Electrical measurement setup, spatial photocurrent mapping of the graphene detector, modeling of PTE effects, experimental setup of the autocorrelation measurement and the characterization of TPA effects in silicon. (PDF) 7292

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