Fast MoTe2 Waveguide Photodetector with High Sensitivity at

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Fast MoTe waveguide photodetector with high sensitivity at telecommunication wavelengths Ping Ma, Nikolaus Flöry, Yannick Salamin, Benedikt Baeuerle, Alexandros Emboras, Arne Josten, Takashi Taniguchi, Kenji Watanabe, Lukas Novotny, and Juerg Leuthold ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00068 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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Fast MoTe2 waveguide photodetector with high sensitivity at telecommunication wavelengths P. Ma1‡, N. Flöry2‡, Y. Salamin1, B. Baeuerle1, A. Emboras1, A. Josten1, T. Taniguchi3, K. Watanabe3, L. Novotny2, J. Leuthold1 1

Institute of Electromagnetic Fields (IEF), ETH Zurich, 8092 Zurich, Switzerland

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Photonics Laboratory, ETH Zurich, 8093 Zurich, Switzerland

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National Institute for Material Science, 1-1 Namiki, Tsukuba 305-0044, Japan

KEYWORDS: 2D materials, transition-metal dichalcogenide, photodetector, waveguide, optoelectronics, silicon photonics

ABSTRACT. Two-dimensional (2D) materials integrated with planar photonic elements enable a new class of electro-optical components and hold much promise for a novel 2D material integrated circuit technology. Here, we present a few-layer MoTe2 waveguide photodetector that is operable across the entire O-band of the near-infrared optical telecommunication spectral range. The device is realized in a two-terminal in-plane electrode configuration without applying external gating. It features a competitive photoresponsivity of 23 mA/W as well as a low dark current, both of which lead to a highly sensitive photodetection with a large dynamic range. Moreover, the design of the photodetector exhibits a fast photoresponse with a bandwidth approaching 1 GHz, outperforming prior TMDC-based photodetectors. Optical data experiments proved the operation of our device at

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data rates of 1 Gbit/s, revealing the applicability of integrating 2D materials for practical optical data communication applications.


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Over the last years, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDCs), and others like black phosphorus (BP) have revealed not only a great potential for 2D electronics1,2, but also for optoelectronics3,4,5 and data communication applications6,7,8. They possess unique and intriguing optoelectronic properties and therefore enable a new class of electro-optical (EO) devices, paving the way for a new 2D materials integrated circuits technology. One highly promising prospect of stacking and combining 2D materials9 is the co-integration with a large variety of material systems such as CMOS or silicon photonics10,11,12, as 2D materials can be deposited onto various substrates without requiring lattice matching4. Among various prototypical optoelectronic devices, tremendous efforts have been devoted to the development of photodetectors, a key component that converts light into electrical signals13. In the near-infrared (NIR) telecom spectral range, graphene photodetectors have been extensively studied and shown in some aspects competitive performance to their II-VI or III-V semiconductor counterparts, though still in their infant phase of development7,14,15,16,17. Yet, due to the lack of a bandgap, graphene photodetectors commonly suffer from a large dark current under bias, resulting in high shot noise and limited dynamic signal power range. The dark current can be kept low by limiting the applied bias voltage, however, this compromises the photoresponsivity. BP, a layered crystal of phosphorus, has been suggested as a promising alternative to graphene18,19, but BP itself possesses many challenges in terms of fabrication, such as its limited stability under ambient conditions20,21. Furthermore, since BP generally shows ambipolar behavior22, the reported device designs require a top gate to electrostatically tune the device characteristics in order to achieve a low dark current and high-speed operation. This complicates the fabrication process and induces extra losses by the conductive gate material. TMDCs are also attractive candidates for optoelectronic applications2,5, due to the particularly strong light-matter interaction that they exhibit amongst 2D materials23.


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However, as their direct bandgaps (for monolayers) mainly lie in the visible range (1.65-2.2 eV), most of the demonstrated device concepts have concentrated on this part of the spectrum24. Only MoTe2 in its semiconducting 2H-phase has a layer-dependent bandgap that extends to the NIR25,26,27. Nevertheless, few studies have reported the material properties and the use of TMDCs in the NIR range27. In particular, reports about functional devices integrated with silicon photonics operating at the commonly used telecommunication wavelengths (e.g., 1310 nm and 1550 nm) are missing. In this letter, we report the realization of a fast few-layer MoTe2 waveguide photodetector operating at wavelengths of around 1310 nm. By making use of the thickness dependent bandgap of MoTe2 we achieve a competitive photoresponsivity across the entire O-band of the optical telecommunication spectral range. The few-layer MoTe2 photodetector is integrated into a guided-wave silicon photonics platform and realized in a two-terminal configuration that does not require external gating. This drastically reduces fabrication complexity and avoids parasitic absorption losses from conductive gate materials that would reduce the quantum efficiency. Moreover, our device design allows to scale up the photoresponsivity, as it can be built on a MoTe2 flake with a designed thickness and length so as to efficiently absorb the incident light. As a result of the low dark currents at nano-ampere level, our device realizes highly sensitive photodetection and a large dynamic range. By design, the light absorption path is along the waveguide propagation direction, perpendicular to the collection path of photo-generated carriers. Hence, our device has no trade-offs between bandwidth and quantum efficiency as surface-normal illuminated photodetectors do. As a consequence we observe a fast photoresponse with a 3-dB cutoff frequency exceeding 500 MHz, outperforming those of prior works of TMDCs photodetectors27,28. In addition, we performed data transmission experiments to prove the viability of our device and observed wide eye opening for 1 Gbit/s data streams.


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Figure 1. Few-layer MoTe2 waveguide photodetector. (a) Schematic illustration of a grapheneMoTe2-Au photodetector stacked onto a silicon waveguide which is buried in SiO2. The device is encapsulated by a large h-BN flake. (b) Cross-sectional view of the device. The spatial overlap of the waveguide mode and the few-layer MoTe2 facilitates photon absorption which leads to the generation of electron-hole pairs and gives rise to a photocurrent. (c) Optical microscope image of a fabricated

photodetector. A 2D material stack and two metal electrodes were placed on a planar waveguide. A laser light signal was coupled into the device via grating couplers, and fed into the active section of the device by silicon access waveguides. (d) High-magnification optical microscope image of the active section of the device, consisting of a few-layer MoTe2 flake with a graphene-MoTe2-Au contacting scheme. (e) Atomic force microscope (AFM) topography of the absorbing MoTe2 flake region highlighted in white in (d) before transfer onto the waveguide. The inset shows a line-cut across the MoTe2 flake, indicating a thickness of 19.5 nm.


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The structure of the device is illustrated conceptually in Figure 1a and 1b. The 2D layer stack consists of a few-layer MoTe2 flake as the light absorbing semiconducting material, contacted directly with a gold (Au) pad on one side, and a graphene layer as a spacer between MoTe2 and Au on the other side. In order to protect the active part of the device, a large and thick h-BN flake is placed on top of the photodetector. The MoTe2 flake is well aligned to the silicon waveguide underneath so that the guided light can be absorbed efficiently via evanescent coupling. The absorption of a MoTe2 flake with a certain thickness depends on both the layer thickness dependent material absorption25,26 (see Supporting Information, S1) and the overlap with the optical mode. Au pads and graphene are placed away far enough from the waveguide to avoid extrinsic losses and reduction of quantum efficiency. The photodetector was fabricated on a standard silicon-on-insulator (SOI) wafer with waveguides built by using the LOCal Oxidation of Silicon (LOCOS) technique29,30. Flakes of 2D materials were stacked and transferred employing a polymer-based pick-up technique31,32 (see Methods and Supporting Information, S2). Top-view microscope images of the fabricated device are shown in Figure 1c and 1d with an atomic force microscopy (AFM) scan of the MoTe2 flake after exfoliation prior to transfer in Figure 1e. Grating couplers are used to couple optical signal from external light sources and the MoTe2 photodetector is accessed by silicon passive waveguides. The presented device employs a MoTe2 flake with a thickness of 19.5 nm and length of 50 µm. The electrical behavior and photoresponse of the photodetector can be understood by Figure 2a-c, which illustrate the energy band diagrams of the photodiode (a) at thermal equilibrium, (b) with a positive bias voltage applied to graphene, and (c) with a positive bias applied to Au. The exfoliated MoTe2 and graphene flakes are considered to be lightly pdoped as experimentally verified33,34 (see Supporting Information, S3). The metal electrode made of Au possessing a medium work function (φAu~5.1 eV) aligns its Fermi level close to


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the valence band of the semiconducting MoTe235, while the Fermi level of graphene lies


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closer to the valence band of MoTe2. After contacting, two Schottky junctions with a slight


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asymmetry are formed, which significantly suppress the injection of carriers into MoTe2 and


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give rise to a low dark current. The asymmetric contacting scheme leads to a built-in field


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Figure 2. Electrical characteristics and photoresponse of the MoTe2 photodetector. (a-c) Schematic band diagrams of the Au-MoTe2-graphene structure (a) in thermal equilibrium state; (b) with a positive bias voltage applied to graphene; (c) with a positive bias voltage applied to Au. The Fermilevels of graphene are tuned by applying bias voltages. Incident photons are absorbed, and the generated electron-hole pairs flow towards the electrodes and are collected there. (d) Measured current-voltage (I-V) characteristics of the photodetector without light (black line) and upon light incidence at various power intensities (red, 17.8 µW; blue 89 µW; green 178 µW). (e) Logarithmscale I-V curves without light (black line) and with light of intensity of 158 µW (red line).

even at zero bias and consequently promotes a zero-bias carrier separation. An applied bias


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voltage increases the potential drop in the device and leads to enhanced photodetection (schematically illustrated in Fig. 2b and 2c). In addition to the strong band bending in MoTe2, the Fermi level of graphene also gets tuned by the applied bias voltage between the two contacts, i.e. the lateral voltage36,37. Since the density of states of graphene varies with the shifts of its Fermi level, this further helps to efficiently extract photocarriers. The few-layer

Figure 3. Static performance characteristics of the MoTe2 photodetector. (a) Measured photocurrent as a function of the incident light power at a bias of -3 V. A photoresponsivity of 23 mA/W is obtained. (b) Photoresponsivity and derived normalized photocurrent-to-dark-current ratio (NPDR) as a function of applied bias voltages. (c) Photoresponsivity and derived external quantum efficiency (EQE) as well as (d) wavelength dependence of the photoresponse for two devices with a 19.5-nmthick (black hollow scatters) and a 50-nm-thick MoTe2 flake (red cross hollow scatters), respectively.

MoTe2 absorbs incident photons via indirect band-to-band transitions (see Supporting


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Information, S1), and the photo-excited electrons and holes are collected under both forward and reverse biases in an efficient manner. Current-voltage (I-V) curves were recorded in the dark and upon light incidence as shown in Figure 2d-e. A TE-polarized continuous-wave laser at a wavelength of 1310 nm was used as the light source and coupled into and out of the device via a pair of grating couplers. As visible in Figure 2e, the dark current is very low, only reaching 20 nA at -1 V bias, while a pronounced photoresponse is observed when light is coupled into the waveguide. The magnitude of the photocurrent increases with optical power, as more photons are absorbed, and subsequently more photo-generated carriers are generated and collected. A photoresponse at zero bias voltage was observed, which stems from the built-in field as a consequence of the asymmetric contact scheme in our device. Next, we evaluate the static performance of the photodetector with respect to the photoresponsivity, photosensitivity, and spectral response. Derived from the I-V curves, Fig. 3a shows the photocurrents as a function of the incident optical power, which was estimated by taking reference on a nearby passive silicon waveguide device with an identical waveguide and grating coupler design but without a photodetector stack. The linear dependence spans a wide range of optical powers, from -40 dBm to -10 dBm which reached the maximum power level of our laser source delivered to the device. We did not observe any sign of saturation, signifying the potential of an operation at even higher input optical power and consequently a larger dynamic power range. It is noteworthy that the minimal detectable power is more than 10 dBm lower than that of a typical graphene photodetector8, which is attributed to the use of a semiconductor with a bandgap instead of a semimetal. As shown in Fig. 3b, the photoresponsivity increases with the bias voltage, as more carriers can be extracted before recombination. The derived responsivity under 3 V bias is 23 mA/W at a wavelength of 1310 nm, which is comparable to both waveguide-integrated graphene8,15 and


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BP photodetectors19. To quantify the sensitivity of a guided-wave photodetector the normalized photocurrent-to-dark-current ratio (NPDR) is an important metric38. As a result of the very low dark current while taking into account the extracted photoresponsivity, the NPDR of our device is remarkably high. As presented in Figure 3b, our device achieves a NPDR of 200-400 mW-1 in forward bias and up to 600 mW-1 in reverse bias, which is several orders of magnitude higher than those of e.g. graphene photodetectors15,7. These values are also exceeding the reported NPDR for gated BP devices19 and are on par with typical Gebased p-i-n photodetectors39. To gain further insight into the performance limit, another photodetector using a thicker MoTe2 layer (50 nm thick and 35 µm long) was fabricated and tested. As shown in Figure 3c, it offers an even higher responsivity of 0.4 A/W at -3 V bias voltage, which is approaching the performance of sophisticated non-avalanche waveguideintegrated II-IV40 and III-V41 semiconductor devices. The larger responsivity is attributed to the enhanced absorption of a thicker MoTe2 flake as well as the stronger optical mode confinement to the MoTe2 flake. Although some surface normal illuminated TMDC photodetectors have demonstrated high responsivities, these devices are normally operated in a photogating mode, yielding a slow photoresponse, typically in the order of milli-seconds42. The extracted external quantum efficiencies (EQE) of our devices are 1.5% and 35% for the thin and thick-layer devices biased at -3 V, respectively. These values underline the high efficiency of the photodetection mechanism in our devices, considering that by evanescent coupling only a fraction of the light is absorbed (see Supporting Information, S4). The wavelength dependent photoresponse is plotted in Figure 3d, measured by using a tunable laser operating around 1310 nm. The responsivity of a thin-layer device gradually decreases from ~50 mA/W to one fifth with increasing wavelength, as the photon energy falls below the bandgap, consistent with the material characterization (see Supporting Information, S1). On


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the contrary, the thick-layer device shows a rather flat responsivity over almost the entire Oband, indicating that the indirect bandgap for a layer of such thickness is even smaller.

Figure 4. Dynamic performance characteristics of the MoTe2 photodetector. The relative radio frequency (r.f.) signal response as a function of the light intensity modulation frequency. Experimental test data (black scatters) and a fitting curve (red dashed line) are plotted. A 3-dB cut-off frequency of ca. 500 MHz is observed. The inset shows the measurement setup. (b) Detected eye diagram of a 1 Gbit/s On-Off keying random bit sequence normalized by the symbol duration T.

To characterize the dynamic performance of the photodetector, the normalized frequency response was measured. For this purpose, an amplified 1300 nm continuous-wave laser was first modulated by a commercial optical intensity modulator (MOD) that was driven by radio frequency (RF) signals from an electrical synthesizer, and then fed into the device. Also, a bias voltage of -3 V was applied. The RF power of the demodulated electrical signal of the device was examined by an electrical spectrum analyzer (ESA). As shown in Figure 4a, a constant frequency response was observed until the 3-dB cut-off frequency that exceeds 500 MHz. Because the photodetector was tested with a 50 Ohm load system, we believe its speed is fundamentally limited by the transit time of carriers rather than RC-limited. This is supported by our studies on the mobility of the MoTe2 flake (see Supporting Information,


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S3), as well as by other reports18,43,44. The speed of the photodetector can be further advanced by improving carrier mobilities44 and by reducing the gap distance between electrodes (currently impeded by the fabrication challenges of the alignment of graphene) with which the bandwidth scales quadratically. Alternatively, the in-plane electrical design can be reconfigured into a vertical structure, where the drift path for carriers is tremendously reduced, yielding a fast operation beyond 100 GHz36. To verify the usability of the studied photodetector for optical communication applications, we performed a data experiment using a second device with an identical design but a shorter length of 42 µm which results in a lower responsivity. The optical carrier was first amplified by an O-Band optical amplifier, encoded by a MOD with a random bit sequences at a line rate of 1 Gbit/s, and fed into the device via a grating coupler. A bias-tee was used to apply a bias voltage to the device. The generated electrical signal was read out with a microwave probe, amplified with a 50 Ω RF power amplifier and finally detected by a digital storage oscilloscope. The impedance mismatch between the photodetector and electrical power amplifier, in combination with the noise figure of the amplifier reduced the signal-to-noise ratio (SNR). Ideally, one could use a trans-impedance amplifier to efficiently amplify the RF signal. Still, as can be seen in Figure 4b, a clear and open eye is visible with a bit-error-ratios (BERs) of 3.5 × 10ିଷ at 1 Gbit/s, after applying a timing recovery, an adaptive least mean square (LMS) equalizer and hard symbol decision. In summary, we experimentally demonstrate a high-speed waveguide photodetector employing few-layer MoTe2 operating in the O-band telecommunication spectral range. A competitive and scalable photoresponse of 23 mA/W, high sensitivity with NPDR of 600 mW-1 and a large dynamic range have been achieved. Our devices operate at a very low dark current at nano-ampere level without the need of any external gating, therefore enabling a more practical and easy integration with silicon photonics and future 2D electronic integrated


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circuits. Also, a high frequency response exceeding 500 MHz is demonstrated, outperforming prior TMDC-based devices. Data stream experiments revealed wide eye opening for data rates of up to 1 Gbit/s. These results present another step forward towards the integration of TMDCs with existing silicon and CMOS technology and highlight the potentials of 2D materials for optical communication applications. Given the tremendous interest and effort in 2D material research even better performance and device functionalities can be foreseen by further developments and different design approaches.

MATERIALS AND METHODS Device fabrication. The photodetector was fabricated on a standard silicon-on-insulator (SOI) wafer. The buried silicon waveguide with a dimension of an effective width w=400 nm and height h=220 nm was first built by using the LOCal Oxidation of Silicon (LOCOS) technique (see Supporting Information, S2). A 5 nm thick dielectric SiN insulating layer was then deposited by atomic layer deposition. Next to the waveguide, bottom electrical contact pads made of 5 nm Ti and 50 nm Au were defined by electron-beam lithography and subsequently produced by means of a lift-off process. The spacing between Au pads was measured to be 650 nm. Crystals of MoTe2, graphene and h-BN were exfoliated onto silicon wafers using a scotch-tape cleavage technique. Flakes were identified in an optical microscope and characterized with an atomic force microscope (AFM) in tapping mode. Transfer of flakes was carried out using a dry polymer-based pick-up technique employing a stamp of polydimethylsiloxane (PDMS) and polypropylene carbonate (PPC). The final stack of 2D materials was aligned onto the waveguide chips with the micromechanical stage of a SUSS MJB4 mask aligner.

ASSOCIATED CONTENT


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Supporting Information S1. Layer-dependent transmission spectra of MoTe2 flakes. S2. Fabrication of photonic waveguides by LOCOS process. S3. Mobility and transit time estimation. S4. Limits to the responsivity and internal quantum efficiency. S5. Comparison with prior-art photodetectors made by 2D and related materials. S6. Discussion of the dark current and noise sources

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] *[email protected]

Author Contributions P.M. and N.F. conceived the concept, designed and fabricated the devices, designed and performed the experiments. Y.S. contributed to the experiments. B.B. and A.J. contributed to the data experiments and analysis. A.E. contributed to the device fabrication. T.T. and K.W. synthesized the h-BN crystals. P.M., N.F., L.N., and J.L. analyzed the data, and co-wrote the manuscript, with support from all authors. ‡These authors contributed equally.

FUNDING SOURCES This research was supported by the NCCR-QSIT program (grant no.51NF40-160591) and the Swiss National Science Foundation (grant no. 200021_165841). K.W. and T.T. acknowledge


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support from the Elemental Strategy Initiative conducted by the MEXT, Japan and JSPS KAKENHI Grant Numbers JP15K21722.

ACKNOWLEDGMENT This work was carried out partially at the Binnig and Rohrer Nanotechnology Center and in the FIRST Center for Micro- and Nanotechnology at ETH Zurich. We are grateful to B. Cheng and A. Messner for the help with the focused ion beam (FIB)-SEM inspection and temperature-dependent measurements, respectively.

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Fast MoTe2 Waveguide Photodetector with High Sensitivity at

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