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Abstract
The thin film lithium niobate platform has shown its potential to support high-performance active and passive integrated photonic devices. Yet, due to the transparency of lithium niobate, it is not suitable for building a photodetector monolithically for conventional communication wavelengths. In this work, we demonstrate a high-speed photodetector on the thin film lithium niobate platform using hybrid integration of two-dimensional materials, i.e., black phosphorus. The black phosphorus and lithium niobate hybrid waveguide exhibit a high absorption coefficient of 1.56 dB/µm. The constructed metal-semiconductor-metal photodetector also presents a high responsivity of 2.64 A/W (at an input optical power of 25.1 µW). The 3-dB bandwidth of the device is as high as 1.97 GHz.
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1. Introduction
For several decades, integrated photonics have been considered as a scaled and low-cost solution for the rapid development of communication, computation, and quantum technologies [1–3]. Many material platforms have been developed and adopted for integrated photonics, such as silicon-on-insulator (SOI) [4], indium phosphide (InP) [5], silicon nitride (Si3N4) [6], and polymers [7,8]. With the great progress in fabrication process [9], thin film lithium niobate (TFLN) emerges recently due to excellent properties of the lithium niobate material, including broad transparency window, high stability, and high Pockels and nonlinear optical coefficients [10]. Therefore, a set of integrated optical devices has been developed on the TFLN platform, including high performance modulators [11–13], frequency comb generators [14,15], frequency shifters [16], and wavelength converters [17,18]. Yet, due to the intrinsic material properties of lithium niobate, integrations of a laser or photodetector (PD) would rely on hybrid integration of, e.g., III-V compound semiconductors [19–21], which, however, requires complicated fabrication processes for wafer bonding and III-V structures.
Two-dimensional (2D) materials, on the other hand, can be easily transferred onto other material platforms using the van der Waals force [22–24]. Their dangling-bond-free surface further facilitates this integration without affecting their properties. This type of hybrid integration brings up an alternative and flexible approach to realize integrated photonic devices on the TFLN platform, such as a PD. Black phosphorus (BP) is a 2D in-plane anisotropic material with an excellent carrier mobility and a narrow direct bandgap [25–28]. Formed by layered sp3-hybridized phosphorus atoms, BP has shown a considerable and anisotropic thermal conduction [29], carrier transport [30], and light absorption [31] along the Armchair and Zigzag polarization directions [32]. According to the number of layers or the applied electric field, the band gap of BP can be tuned from 2 eV to 0.3 eV, which provides broadband photo-detections from the UV to mid-IR wavelength range. Integrated BP PDs have been investigated on a variety of platforms, including SOI [33–36] and Si3N4 [37].
In this paper, we demonstrate, for the first time, an integrated high-speed BP PD on a TFLN waveguide operating at C-band. The lithium niobate and BP hybrid waveguide provides a large absorption coefficient of 1.56 dB/µm, which is higher than that of the III-V/TFLN integrated PD. The present device also exhibits a responsivity of 2.64 A/W (at an input optical power of 25.1 µW) and a large 3 dB bandwidth of 1.97 GHz (at a bias voltage of 1.6 V).
2. Device structure and fabrication
Figure 1(a) shows the schematic structure of the proposed hybrid BP-TFLN waveguide PD, which consists of a Mach-Zehnder interferometer (MZI) structure and two grating couplers for light input and output. This structure allows the absorption of the hybrid waveguide to be quantitatively measured [36]. However, it may cause an additional 3 dB insertion loss for the device, which should be counted when deriving the responsivities of the PD. The fabrication process of the device started with the TFLN structure patterning on a commercial x-cut lithium-niobate-on-insulator (LNOI) wafer with a Si substrate (NanoLN) using electron-beam lithography (50 keV, Raith Voyager) and Argon plasma dry etching. Then, BP flakes (HQ Graphene) were mechanically exfoliated onto the Si/SiO2 substrate using a Nitto tape inside a nitrogen filled glovebox. A polymer stamp was then used to pick up and stack the flakes at 60-90°C. We aligned the BP flake to one of the MZI arm waveguides and laminated it onto the target waveguide at a temperature of 160°C to obtain clean contact interfaces. The non-vertical sidewalls (typically 60°) of the LN waveguide, resulting from the etching [38, 39], avoid an abrupt change in the height of the BP flake and it from breaking as well. The chip was then immersed into chloroform to dissolve the polymer stamp, and the electrode patterns were defined again by electron beam lithography using the PMMA resist. 5 nm of titanium and 70 nm of gold were prepared with thermal evaporation and lift-off processes. Finally, 2 nm of alumina is deposited on top again using thermal evaporation to passivate the BP.
Fig. 1. Structure of the proposed hybrid BP-TFLN waveguide PD. (a) Schematic illustration and cross-sectional view of the device. (b) Optical microscope picture of a finished device.
The TFLN ridge waveguide structure has a slab thickness and a ridge height of both 200 nm. The width of the waveguides in the MZI section is wr = 1.5 µm. The splitting ratio of the multimode interference (MMI) coupler is designed to be 1:1. The central coupling wavelength of the grating couplers is chosen at 1.55 µm for transverse electrical (TE) mode [39]. Two Ti/Au electrodes (thickness hm = 75 nm) were fabricated in contact with the BP on both sides of the TFLN ridge waveguide, which forms a metal-semiconductor-metal (MSM) junction. The metal electrodes for collecting photo-currents are designed with an asymmetric distribution at the opposite sides of the waveguide as shown in Fig. 1(a). This asymmetric design is used to promote the photocurrent by creating a potential difference [40,41]. The position of the signal electrode in the present design is located at 2.44 µm from the ridge waveguide (ls = 2.44 µm), while the ground electrode is 4.52 µm from the waveguide (lg = 4.52 µm). Finally, a thin film of Al2O3 was deposited on the device to passivate the device. This helps isolate the BP from the ambient air and improve the stability of the device [42]. A picture of the fabricated device is shown in Fig. 1(b). For this sample, the length of the BP flake is about 41.03 µm along the light propagation direction.
We then characterized the thickness and orientation of the BP flake using atomic force microscopy (AFM) and Raman spectroscopy. Figure 2(a) shows the 2D sketch of the BP-TFLN PD, as well as the AFM image of the active region. The thickness hb of the BP flake was measured to be about 39.88 nm by the topographic cross-sectional profile shown in Fig. 2(b). The thickness of the BP flake here was chosen with a good balance: sufficiently thick to ensure a great optical absorption and a high mechanical reliability, but sufficiently thin to reduce the dark current. To better illustrate the orientation of the BP flake, we define θ as the angle of the armchair direction of the BP from the z-axis of LN crystal, which is designed perpendicular to the TFLN waveguide. The angle-dependent Raman scattering intensity of A2g/A1g, with respect to θ, was further measured and fitted as shown in Fig. 2(c). The fitted curve indicates that the armchair direction has an angle of θ=13.6°. The Raman spectrum of the multilayer BP with the excitation laser polarization along the armchair direction was then measured as shown in Fig. 2(d). The results are consistent with previous studies with the A1g peak located at 368 cm-1, the B2g peak located at 446 cm-1, and the A1g peak located at 473 cm-1. Aside from those, no obvious defect-related peaks were observed, indicating that no structural defects were introduced into the BP material during the fabrication processes.
Fig. 2. (a) 2D sketch and AFM image of the BP-TFLN PD of the BP in the active region. (b) Topographic measurement along the red arrow in the AFM image in (a). (c) Measured angle-dependent Raman intensity (solid dots) of the A2g/A1g signal in the wafer plane, where the result is fitted (dash line) by a k·cos2θ function (k is a constant). A three-dimensional schematic structure of the multilayer BP is also shown. (d) Raman spectrum of the device with the excitation laser polarization along the armchair direction.
3. Photo-detection performances
The optical properties of the present BP-TFLN PD were first simulated. Figure 3(a) shows the electric-field profile of the fundamental quasi-TE mode in the hybrid waveguide at 1550 nm wavelength. The absorption coefficient of the waveguide mode in the BP flake is simulated to be αBP = 1.56 dB/µm using a finite-difference time domain algorithm (Lumerical FDTD), when the thickness of the BP flake is 39.88 nm. Considering its length of 41.03 µm, the calculated light absorption of the present PD is nearly 100% assuming that other losses, such as interface scattering and absorption from electrodes, are negligible according to our simulations. To verify this simulation result, we measured the transmission spectra of the MZI structure before and after the transfer of the BP flake as shown in Fig. 3(b). The input 3 dB coupler of the MZI splits the input light into two arms equally. Before transferring the BP, the light power in the two arms reaching the output 3 dB coupler was also equal, which, in hence, gives a large ER for the MZI responses (> 23 dB). After transferring the BP, the ER decreased to less than 1 dB as shown in Fig. 3(b), due to high light absorption in the arm with BP. We can then deduct this absorption with the resulted difference in ERs as 99.66%. This indicates that the BP flake indeed presents a high optical absorption for the waveguide mode, which complies to the simulation results.
Fig. 3. (a) Simulated optical mode field profile in the device. (b) Spectra of the MZI device before and after the BP flake transfer.
We then characterized the static optical responses of the device using a source meter (Keithley 2450) and a tunable laser (81640A, Agilent). The light from the tunable laser was transmitted to the device under test (DUT) through a polarization control (PC), while the source meter supplying the bias voltage. The detected current from the DUT was directly read from the source meter. A temperature control (TC) was used to control the temperature of the DUT at 25°C. Figure 4 shows the current-voltage (IV) responses of dark currents and photo-currents at different input optical powers (counted in the BP-TFLN waveguide arm of the MZI structure) measured from fabricated BP-TFLN PDs operating at 1525 nm. The results show large dark currents with linearly increases with the bias voltages, which might be resulted from the low electrical resistivity of the BP flake. Both the dark currents and photo currents present a slight deviation from the linear IV characteristic, due to the existence of Schottky barriers between the BP layer and the Au electrodes [43]. In Fig. 4, two devices were actually measured, one for the device presented in Fig. 2 and the other with a similar BP thickness and length but a different armchair direction θ. One can find that the two devices show a similar dark current characteristic, but a different photo-current one, which might be resulted from the different BP orientations.
Fig. 4. (a) Measured IV curves with no input optical power, i.e., dark current Idark. (b) Measured IV curves for photo-currents at different input optical powers. The dot/triangle represents the device with BP armchair direction θ=13.6°/122.5°.
As shown in Fig. 5, the measured photo-currents exhibits a monotonic increase with respect of the input optical power. However, a slight saturation of the photocurrent can also be observed when the input laser power increases as shown in Fig. 5(a), which could attribute to the degradation of the Schottky barriers [43]. At zero bias shown in the inset of Fig. 5(a), the photo-generated electron-hole pairs can be separated due to the potential difference from the asymmetric electrode design. The responsivity R of the fabricated PD was further calculated at different bias voltages as shown in Fig. 5(b). The highest responsivity of 2.64 A/W is achieved at an optical power of 25.1 µW with 2 V bias voltage. The internal quantum efficiency (IQE) ηi can be obtained using ηi = Rhc/(eλ), where h is the Planck constant, c is the speed of light, e is the electron charge, and λ is the wavelength. The deducted IQEs of the device decrease from 211.3% to 57.4% (2 V bias voltage) and 106.9% to 26.5% (1 V bias voltage) when the light power increases from 25.1 µW to 251 µW, respectively. This result indicates that internal gain mechanism is indeed presented in the device, such as photoconductive gain [44]. To further identify the wavelength dependence of the device, its responsivity was further swept from 1520 nm to 1580 nm wavelengths at 25.4 µW optical power and 1 V bias voltage. As shown in Fig. 5(c), the BP-TFLN PD exhibits a flat responsivity, which indicates a large working wavelength range for the present device. Since the device exhibits a large dark current, its detectivity can be presented as D = RA1/2/(2qIdark)1/2. At 2 V bias, D can be derived as 1.49 × 108 Jones.
Fig. 5. (a) Measured photo-currents curves and (b) calculated PD responsivities at different bias voltages for the device shown in Fig. 2. The insets give corresponding zoom-in views at 0 V bias voltage. (c) Wavelength dependence of the PD responsivity at 25.4 µW input optical power and 1 V bias voltage.
The high-speed performance of the integrated BP-TFLN waveguide PD was measured by a lightwave element analyzer system (LEA, 6433 L, Ceyear) with a setup shown in Fig. 6. The light produced from the LEA was first connected to an optical attenuator (OA) to control the input light power. Then, the light was split into two parts: one (10%) to a commercial PD to monitor the light power and another (90%) to the device under test (DUT) through a polarization control (PC). The detected electrical signal from the DUT was collected by a microwave probe (67A, GGB) and returned to the LEA. The source meter provides a bias voltage to the chip through a bias tee. A temperature control (TC) was used to monitor and maintain the temperature of the DUT at a constant value.
Figure 7(a) shows the normalized optical-electrical (OE) responses and 3 dB bandwidths at different bias voltages. The 3 dB bandwidth presents a linear increase at a low bias voltage (< 1.6 V) which corresponds to the decrease of the carrier transition time. The highest 3 dB bandwidth of 1.97 GHz was obtained. At this value, the bandwidth of the present device is limited by the electric RC response of the electrodes according to the electrical model of the device structure. To investigate the influence of input laser power to the bandwidth, we then measured OE responses when the input optical power changes from 22.4 µW to 224 µW, and the results are shown in Fig. 7(b). Obviously, the bandwidth of device is insensitive to the input power.
Fig. 7. OE responses and corresponding 3 dB bandwidths at (a) different bias voltages and (b) different input laser powers.
The present BP-TFLN waveguide PD shows its advantages at absorption coefficient and high-speed detection compared to other platforms. Therefore, Table 1 gives a comprehensive comparison for the present device and several waveguide integrated representative 2D material PDs using MSM electrodes. When the thickness of BP decreases, the light absorption coefficient reduces, which requires longer device length to maintain a high responsivity. However, long length of the BP poses difficulties on fabrication processes, and narrows the 3 dB bandwidth as well. Thus, several approaches to increase the absorption coefficient, such as integrating with graphene [36] or using a plasmonic waveguide [46], are introduced. The TFLN waveguide in the present device provides a relatively low refractive index waveguide, as compared to SOI, and enhances the absorption from the integrated BP flake. The reduced device length also helps to achieve the highest bandwidth as demonstrated here.
Table 1. Performances of some waveguide integrated MSM PDs
4. Conclusion
In this paper, we have proposed and demonstrated an integrated waveguide PD operating at C-band on the TFLN platform using 2D BP material. The BP-TFLN hybrid waveguide structure shows a high absorption coefficient of 1.56 dB/µm theoretically. In experiments, a high responsivity of 2.64 A/W (at an input optical power of 25.1 µW) and a 3 dB bandwidth up to 1.97 GHz have been measured in a fabricated device of about 40-µm long detection length. The present study verifies the possibility of a waveguide PD on TFLN by the hybrid integration of 2D materials. The bandwidth of the device can be further increased by reducing the distance between the electrodes, and its dark current can be reduced by decreasing the thickness of the BP flake or using a hetero-junction made of multilayered 2D materials [37].
Funding
National Key Research and Development Program of China (2019YFB2205303); National Natural Science Foundation of China (62135012, 62105107, 61961146003, 62205054); Leading Innovative and Entrepreneur Team Introduction Program of Zhejiang (2021R01001); Natural Science Foundation of Zhejiang Province (LD22F040004); Basic and Applied Basic Research Foundation of Guangdong Province (2021A1515012215); Natural Science Foundation of Jiangsu Province (BK20210207); Fundamental Research Funds for the Central Universities (2021QNA5001, 2242021R10015).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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