A Complete Two-Dimensional Avalanche Photodiode Based on MoTe2−WS2−MoTe2 Heterojunctions With Ultralow Dark Current

Two-dimensional (2D)-material-based photodetectors have recently received great attention due to their potentials in developing ultrathin and highly compact devices. Avalanche photodiodes (APDs) are widely used in a variety of fields such as optical communications and bioimaging due to their fast responses and high sensitivities. However, conventional APDs based on bulk materials are limited by their relatively high dark current. One solution to tackle this issue is by employing nanomaterials and nanostructures as the active layers for APDs. In this study, we proposed and fabricated an atomically-thick APD based on heterojunctions formed by 2D transition metal dichalcogenides (TMDs). A typical device structure was formed by stacking a semiconducting monolayer WS2 onto two metallic few-layer MoTe2 flakes. Due to the Schottky barrier formed between the TMD layers and their atomic thicknesses, the dark current of the APD is greatly reduced down to 93 pA. In addition, the APD can operate through a broad spectral range from visible to near-infrared region, with a responsivity of 6.02 A/W, an external quantum eﬃciency of 1,406%, and an avalanche gain of 587. We believe that the 2D APD demonstrated here provides a feasible approach for developing all-2D optoelectronic devices with simultaneous high-sensitivity and low noise.

Two-dimensional (2D)-material-based photodetectors have recently received great attention due to their potentials in developing ultrathin and highly compact devices. Avalanche photodiodes (APDs) are widely used in a variety of fields such as optical communications and bioimaging due to their fast responses and high sensitivities. However, conventional APDs based on bulk materials are limited by their relatively high dark current. One solution to tackle this issue is by employing nanomaterials and nanostructures as the active layers for APDs. In this study, we proposed and fabricated an atomically-thick APD based on heterojunctions formed by 2D transition metal dichalcogenides (TMDs). A typical device structure was formed by stacking a semiconducting monolayer WS 2 onto two metallic few-layer MoTe 2 flakes. Due to the Schottky barrier formed between the TMD layers and their atomic thicknesses, the dark current of the APD is greatly reduced down to 93 pA. In addition, the APD can operate through a broad spectral range from visible to near-infrared region, with a responsivity of 6.02 A/W, an external quantum efficiency of 1,406%, and an avalanche gain of 587. We believe that the 2D APD demonstrated here provides a feasible approach for developing all-2D optoelectronic devices with simultaneous high-sensitivity and low noise.
Keywords: MoTe2, WS2, avalanche photodiodes, transition metal dichalcogenides, heterojunctions 2D atomic crystals have attracted extensive interests during the past decade due to their excellent electrical, optical, and optoelectronic properties. Echo with their ultrathin structure, a broad range of potential applications can be envisioned, including high-performance 2D transistors (Yoon et al., 2013;Liu et al., 2015;Choi et al., 2016;Chuang et al., 2016;Liu et al., 2019;Zheng et al., 2020), photodetectors (Mittendorff et al., 2013;Cheng et al., 2014;Mudd et al., 2015;Tao et al., 2015;Wang et al., 2015;Shim et al., 2016;Vu et al., 2017;Yu et al., 2017;Zeng et al., 2019;Wang et al., 2020), ultracompact and flexible light-emitting diodes (Withers et al., 2015;Wang et al., 2017;Shang et al., 2018), sensors (Burman et al., 2016;Shokri and Salami, 2016;Guo et al., 2017), to name but a few. In particular, 2D transition metal dichalcogneides (TMDs) are usually semiconductors with exotic characteristics which can open up new avenues for both of fundamental research and practical applications. For example, monolayer WS 2 is shown to be a direct bandgap semiconductor with strong room-temperature exciton binding energy. It has been widely employed as quantum emitters for studying of strong light−matter interactions in different types of micro-and nanocavities (Wen et al., 2017). In addition, monolayer WS 2 exhibits strong absorption across the visible spectral range, as well as excellent chemical stability (Bernardi et al., 2013;Zhang et al., 2013). These allow it to act as active layers for photodetectors. The few-layer MoTe 2 is reported to be a type-II Weyl semimetal, which can be employed as electrodes in the 2D devices for its high carrier mobility and good electric conduvtivity (Zhang et al., 2014;Beams et al., 2016;Chen et al., 2016). Furthermore, one can also explore new physical phenomena associated with the semimetal phase in the few-layer MoTe 2 (Keum et al., 2015).
Avalanche photodiodes (APDs), which consist of p−n junctions or Schottky junctions, are able to convert the incoming photons into charge carriers undergoing cascade amplifications upon reverse bias (Johnson, 1965;Anderson and McMurtry, 1966;Aull et al., 2002). When operating in the avalanche regime, APDs can exhibit high internal gain for photodetection of ultrahigh sensitivity, making them widely used in optical communications (Liu et al., 1992;Ferraro et al., 2015), single-photon detection (Huntington et al., 2007;Faramarzpour et al., 2008;Ren et al., 2011), andbioimaging (Vo-Dinh et al., 2010;Wohnhaas et al., 2013). However, conventional APDs based on bulk materials are usually limited by their relatively high dark current. This issue can be alleviated by reducing the volume of the devices, which is typically accompanied by sacrifice of device responsivity and quantum efficiency. During the past decade, due to their atomic thicknesses, strong optical absorption, and high photocurrent conversion efficiencies, 2D crystals have been demonstrated to provide a new avenue for developing high-performance photodetectors with low dark current and high responsivity (Table 1).
Generally, in 2D photodetectors bulk metals such as titanium and gold are utilized as electrodes. A limitation of these electrodes is that acquiring defect-free interface between metal and 2D crystals without Fermi level pinning or carrier scattering is still a challenge, because defects and dangling bonds in these metal electrodes will introduce plentiful interfacial states (Gong et al., 2014;Kim et al., 2017). In contrast, the passivated and danglingbond-free surfaces of 2D crystals can make them integrate layerby-layer to form heterojunctions bonded through van der Waals (vdW) force. Accordingly, devices free of interface defects can be formed through stacking different types of 2D crystals. Specifically, all-2D photodetectors have been developed by respectively adopting the 2D crystals as electrodes and active layers (Britnell et al., 2013;Luo et al., 2015;Massicotte et al., 2016;Tan et al., 2016;Padilha et al., 2017;Tan et al., 2017;Wei et al., 2017;Zhang et al., 2017;Yao and Yang, 2018;Wei et al., 2019;Yang et al., 2019;Wu et al., 2020). Although these all-2D devices exhibit excellent photodetection performances, their roomtemperature dark currents are still relatively high (in the range of ∼0.1 nA-1 μA, Table 1). Such high dark currents will deteriorate the signal-to-noise (S/N) ratio of the photodetectors and consequently limit the device applications in sensing of low photon flows.
In this study, we present an all-2D APD composed of a monolayer WS 2 and two few-layer MoTe 2 flakes (MoTe 2 -WS 2 -MoTe 2 heterostructure). The metallic MoTe 2 flakes act as electrodes. A Schottky barrier can thereafter be formed at the interface between the semiconducting WS 2 and metallic MoTe 2 . Upon reverse bias, the APD is demonstrated with a responsivity (R) of 6.02 A/W, an external quantum efficiency (EQE) of 1,406%, and an avalanche gain (AG) of 587. In particular, due to the Schottky barrier, a dark current as low as 93 pA can be obtained. The R and EQE is at the forefront of the all-2D photodetectors, while the dark current is even better than many state-of-the-art commercial APDs. In addition, wavelength scanned measurements further indicated that the APD operated in a broad spectral range from 400 to 700 nm. Our results therefore demonstrate a facile approach for design and fabrication of room-temperature all-2D photodetectors with simultaneous high-sensitivity and low noise.  25

Materials
Monolayer WS 2 was purchased from 6Carbon Technology company in China. Few-layer MoTe 2 was grown on the silicon substrate covered with 300-nm-thick SiO 2 layer according to the process reported previously .

Device Fabrications
The MoTe 2 -WS 2 -MoTe 2 heterostructure was fabricated using a wet-transfer method (Wen et al., 2017). Specifically, Polystyrene (PS) was first coated on the surface of the substrate covered with monolayer WS 2 flakes. Then, the substrate covered with PS film was put into water so that the PS film pasted with WS 2 was stripped off from the substrate. Subsequently, the WS 2 adhered onto the PS film was transferred onto two few-layer MoTe 2 flakes separated with a micrometer-scale gap. After incubation at 110°C for 60 s, the sample was immersed into toluene solution for 2 hours to dissolve the PS film, whereby heterojunctions were formed between the WS 2 and two MoTe 2 flakes.

Characterizations
Raman and photoluminescence (PL) spectra were measured using a micro-Raman spectrometer (inVia Reflex, Renishaw). The excitation laser with a wavelength of 532 nm was focused onto the samples through a ×50 objective (numerical aperture 0.80). The diameter of the focusing spot is ∼1 μm. The thicknesses of the WS 2 and MoTe 2 flakes were measured using an atomic force microscope (AFM, NTEGRA Spectra, NT-MDT). The photocurrents were measured by a sourcemeter (Keithley 2636B, Tektronix), with incidence wavelengths of 405, 532, 633, 785, and 1,064 nm. In addition, a supercontinuum laser source with an output wavelength range of 400-2400 nm (Fianium, SC400-4-PP) was employed to measure the photocurrent spectrum. The noise power spectra of the device under different bias voltages were collected using a semiconductor characterization system (FS-Pro ™ , Hongkong Base For Information Technology).

RESULTS AND DISCUSSION
The configuration of the all-2D APD device is schematically shown in Figure 1A, which consists of a monolayer WS 2 overlaid onto two few-layer MoTe 2 flakes. The metallic MoTe 2 flakes act as electrodes in the APD. In this way two heterojunctions were formed at the overlapped regions between the WS 2 and MoTe 2 . Afterwards, titanium and gold layers were consecutively deposited on the MoTe 2 flakes as electrodes for electrical readout. Optical microscope image of the APD is displayed in Figure 1B. The channel length (i.e., separation between the two MoTe 2 flakes) of the device is measured as 2 μm. The thickness of the WS 2 flake is 1.0 nm, and those of the two MoTe 2 flakes are 4.3 and 4.5 nm, respectively (Supplementary Figure S1, Supporting Information). The monolayer nature of the WS 2 can be further confirmed by Raman spectroscopy characterizations ( Figure 1C). Two strong peaks are observed at 352 cm −1 and 418 cm −1 , corresponding to the 2LA(M) and A 1g (Γ) modes of WS 2 , respectively. The peak intensity ratio of 2LA(M)/A 1g (Γ) can be determined as 6.6, which is a typical feature of monolayer WS 2 (Cong et al., 2014;Xu et al., 2015). For Raman spectrum collected from the MoTe 2 region, a small peak at 188.91 cm −1 can be observed, which is the B g mode of MoTe 2 in 1T′ phase Naylor et al., 2016;Chen et al., 2017). 1T′-MoTe 2 is a semimetal and a good candidate of 2D electrode material because of its low resistance and high carrier mobility (Zhang et al., 2014;Beams et al., 2016;Chen et al., 2016). Due to its strong exciton transition at room temperature, the pristine monolayer WS 2 exhibits a strong PL peak at 614 nm ( Figure 1D). In contrast, because of its semimetal nature, negligible PL signal can be observed in the 1T′-MoTe 2 region. It is noted that in the heterostructure regions both the Raman and PL signals from WS 2 are reduced ( Figures 1C,D). Such a phenomenon suggests intimate contact between the monolayer WS 2 and few-layer MoTe 2 . Once the intimate contact is formed, the WS 2 excitons or lattice vibrations will transfer their energies to the MoTe 2 underneath through electromagnetic coupling. Subsequently, the lattice vibration energy or exciton energy will be dissipated by the impurities, defects, and free electrons in the semimetal layer. As a result, the Raman and PL signals of the WS 2 will be quenched.
Schottky junctions can be formed at the two heterojunctions between the semiconducting WS 2 and semimetal MoTe 2 layer. Figure 2A shows the dependence of current on bias voltage in a representative device. Specifically, the current increases along with the increase of bias voltage and saturates at 10.4 V. Afterwards, the current increases notably when the bias is further increased. Such bias voltage dependence is typical of Schottky diodes. Due to the Schottky barrier, the device exhibits a dark current as low as 93 pA under a bias voltage of 59 V. Such a dark current is much lower than many photodetectors based on 2D materials and even lower than typical commercial APDs ( Table 1). The Schottky barriers at the heterojunctions can trigger electron avalanche effect upon applying a large electrical field across the device. The current at 10.4 V (26 pA) is defined as the saturation current I sat 59 . Bias voltages above 10.4 V will accelerate the charge carriers passing through the heterojunction, giving rise to ionization collisions of the lattice and generation of additional charge carriers, i.e., occurrence of electron avalanche effect. To further demonstrate the avalanche effect, a charge carrier multiplication factor, M, is defined as M I/I sat , with I the current above 10.4 V. When the avalanche effect occurs, parameter M will follow the behavior (Miller, 1957), where n represents ionization rate, V b is a fitting parameter.
Equation (1) can be rewritten as, suggesting a linear dependence of ln 1 − 1 M on ln(V). As shown in Figure 2B, for bias voltage above 10.4 V, ln 1 − 1 M varies linearly against ln(V), which is a direct indicator of avalanche carrier multiplication. By fitting the experimental data using Eq.
(2), the n and V b are determined as 7.7 and 17.6 V. It is noted that the n in our MoTe 2 -WS 2 -MoTe 2 is about 6 times that of a previous report (Lei et al., 2015a), where Schottky junction was formed between a layered InSe and metal electrode (n 1.3). The larger n and smaller V b means that the avalanche effect is easier to be initiated in the MoTe 2 -WS 2 -MoTe 2 heterostructure. Moreover, once the avalanche effect is triggered, more electrons will be generated due to the larger M under a certain bias voltage. We ascribe these merits to the atomic thickness of our device, where a much larger electric field can be induced across the Schottky junction under a moderate bias.
The avalanche effect and small dark currents of the MoTe 2 -WS 2 -MoTe 2 heterostructure can greatly benefit photodetection. Figure 2C illustrates the operation principle of the heterostructure APD. Electron−hole pairs will be generated in both of the 2D layers and Schottky junctions upon light illumination. When a bias voltage is applied to initiate the avalanche effect, carrier multiplications will occur, giving rise to a rapid increase of the photocurrent with the applied bias. Additionally, a larger S/N ratio will be obtained as well. Figure 2D shows the current responses of the device measured in the dark and under 532-nm laser excitations of different intensities (green: 0.13 W/cm 2 ; cyan: 0.38 W/cm 2 ; blue: 0.64 W/cm 2 ). The dark current remains below 100 pA even when the bias voltage is above 60 V. In contrast, the photocurrent increases slowly with applied bias smaller than 10.4 V, then dramatically grows when the bias voltage become larger due to the avalanche effect. Moreover, the photocurrent increases against illumination intensity. By plotting the photocurrent as a function of the laser intensity, linear dependences can be observed at two typical bias voltages (30 and 40 V, Figure 2E) for laser intensity upto 0.64/cm 2 . R, EQE, and AG are three important parameters evaluating the performance of an APD. Specifically, R, EQE, and AG can be calculated according to the following formulae (Yu et al., 2013;Long et al., 2019), where I ph , I dark , and P in represent photocurrent, dark current, and incidence light intensity, respectively. Parameter h is the Planck constant (h 6.62607015 × 10 −34 J s), c is the speed of light (c 2.99792458 × 108 m/s), e is quantity of a unit electric charge (e 1.602176634 × 10 −19 C), λ is wavelength of incidence light. Parameters I ph0 and I dark0 are the photocurrent and dark current before the occurrence of avalanche effect, respectively. In our analyses, I ph0 and I dark0 were taken at the bias of 10.4 V. We evaluate the R, EQE, and AG of the heterojunction device with an illumination intensity of 0.64 W/cm 2 . As shown in Figure 2D, the AG increases distinctly as a function of the bias voltage. At a bias of 59 V, the R and EQE are calculated to be 6.02 A/W and 1,406%, respectively, corresponding to an AG of 587. Table 1 summarizes the performances of various 2D photodetectors, including APD and non-APD types, as well as typical APDs that are commercially available. It is seen that although the R of the MoTe 2 -WS 2 -MoTe 2 heterojunction device is moderate, its EQE and AG are among the best ones. In particular, the AG of the heterojunction device is much better than the listed commercial APDs can exhibit. Moreover, the dark current of our device is merely 93 pA at a bias of 59 V, which is at the lowest level among both of the 2D and commercial photodetectors. The normalized photocurrent-to-dark current ratio (NPDR) can be further calculated as NPDR R/I dark 6.47 × 10 10 W −1 , which is better than most of the 2D photodetectors can provide ( Table 1). These results suggest that our 2D APD can provide an excellent S/N ratio and favor the detection of low-level signals. Figure 2F shows a typical 2D photocurrent map of the MoTe 2 -WS 2 -MoTe 2 heterojunction APD. To avoid electrical breakdown of the device, a relatively small voltage of 25 V is applied to reversely bias the left Schottky junction. According to Figure 2A, such a bias can already trigger the avalanche effect. In addition, to induce strong enough photocurrent for the 2D mapping, a relatively large incidence intensity of 305.73 W/ cm 2 was used. As shown in Figure 2F, photocurrent was visible near the left Schottky junction (reversely bias) and the channel region. However, negligible photocurrent can be found at the right Schottky junction which is forward biased. These observations can be understood by considering that in the avalanche regime, the photocurrent is proportional the magnitude of collision ionization. The photo-generated Frontiers in Materials | www.frontiersin.org July 2021 | Volume 8 | Article 736180 electrons in the reversely-biased Schottky junction and regions nearby will experience a longer acceleration path, which will therefore undergo more collision events. These additional collisions will generate more electrons, giving rise to stronger photocurrents. Figure 3A illustrates the switching characteristic of the broadband photoresponse of MoTe 2 -WS 2 -MoTe 2 heterojunction APD under a bias voltage of 50 V at 532-nm excitation. The three ON/OFF cycles are similar with each other, suggesting that photodetection performance of our APD is repeatable. Response time (RT), which is another important parameter characterizing a photodetector, can be deduced from one typical cycle. As shown in Figure 3B, the RTs are revealed as 1,260 and 475 ms for the laser-on and-off processes, respectively. As an APD, the RTs of our heterojunction photodetector is ordinary among the 2D photodetectors ( Table 1). The origin of such long RTs can be possibly due to introduction of carrier trapping centers during the device manufacturing processes. It is known that impurities and defects will be generated at the interface of the heterojunction by stacking different 2D materials via wet-transfer method (Rooney et al., 2017). These impurities and defects will act as trapping states for electrons and holes, which will increase the photocurrent gain but largely compromise the RT of the photodetectors (Hu et al., 2012;Lopez-Sanchez et al., 2013;Lei et al., 2015a;Lei et al., 2015b). To fasten the device responses, improvement of the device manufacturing processes is required.
We further study the broadband photocurrent performance of the MoTe 2 -WS 2 -MoTe 2 APD by illuminating the device at different wavelengths. To that end, the bias voltage was fixed at 30 V, the laser spot was focused onto the center of the left Schottky junction shown in Figure 2F. The incidence wavelength was selected by placing a specific narrowband optical filter in front of the exit of the supercontinuum laser. Representative switching behaviors of the APD illuminated at 480, 516, 532, 580, 610, and 633 nm are shown in Figure 3C. The device exhibits similar ON/OFF behaviors and same dark currents at different excitation wavelengths. Moreover, two clear photocurrent maxima were observed at 516 and 610 nm, respectively. This can be seen more clearly by plotting the R against illumination wavelength, i.e., the photocurrent spectrum ( Figure 3D). The peak at 610 nm should correspond to the exciton-A transition in the monolayer WS 2 , as corroborated with the PL spectrum shown in Figure 1D. The peak at 516 nm with a larger R can be ascribed to exciton-B in monolayer WS 2 . The separation between the two photocurrent maxima is 0.37 eV, which is consistent with the splitting energy of the valence band minimum in WS 2 arising from the spin-orbit coupling at K (K′) valley (Zeng et al., 2013;Zhao et al., 2013;Zhu et al., 2015). Previous studies have demonstrated that the recombination rate of exciton-B is Frontiers in Materials | www.frontiersin.org July 2021 | Volume 8 | Article 736180 6 much smaller than that of exciton-A, giving rise to a much lower PL quantum yield of exciton-B (Zeng et al., 2013;Zhao et al., 2013;Zhu et al., 2015). Accordingly, more photo-generated carriers will undergo avalanche multiplications under optical excitation associated with exciton-B, leading to a stronger photocurrent response. Another important observation is that the photocurrent response can extend to wavelengths larger than 614 nm, i.e., the exciton transition energy of WS 2 . Such an effect can be ascribed to photo-generated electron transmitting over the Schottky barrier from the Fermi energy of the metallic MoTe 2 59 . On the basis of the photocurrent spectrum shown in Figure 3D, the Schottky barrier height can be extracted as 1.93 eV (Supplementary Figure S2, Supporting Information), corresponding to a wavelength of 642 nm. This low Schottky barrier height guarantees photocurrent response of the APD under optical excitation with energy smaller than the exciton transition energy of the monolayer WS 2 .
With the knowledge of dark current, AG, RT, and M, the S/N ratio can be readily calculated according to (Lei et al., 2015a), where BW is the bandwidth. In a specific calculation, BW is set as inverse of the RT (Lei et al., 2015a). Under an illumination intensity of 0.64 W/cm 2 and a bias voltage of 59 V, the S/N ratio is calculated as 71 dB, which is 10 times larger than that of a 2D APD (60 dB) consisted of layered InSe and bulk metal electrode (Lei et al., 2015a). The enhanced S/N ratio in our device is attributed to its ultralow dark current even in the avalanche regime.
Finally, we characterized the current noise density of the MoTe 2 -WS 2 -MoTe 2 APD, whereby the noise equivalent power (NEP) and normalized detectivity (D*) can be calculated. Figure 4 shows the low-frequency noise power spectra of the device under different applied bias voltages. The noise increases against the bias voltage. In addition, all of the four noise power spectra can be well fitted using the equationS n (f ) K ⎛ ⎝ I β dark f α ⎞ ⎠ (Chang et al., 2011), where Sn(f) is the spectral density of the noise power, K is a constant, and α and β are two fitting parameters. The fitting results suggest that the 1/f noise prevails at low frequencies (1-100 Hz) for our APD. The 1/f noise usually exists in 2D photodetectors (Balandin, 2013;Na et al., 2014), which is induced by the disorder or defects (Clément et al., 2010). By optimizing the fabrication processes, especially the stacking of the 2D crystals, it is expected that the 1/f noise can be further reduced.
By integrating S n (f) within a given bandwidth B (usually B is set as 1 Hz), the total noise current power can be expressed as (Chang et al., 2011), The NEP and D* can thereafter be obtained as (Chang et al., 2011), where A is the effective APD area used to normalized the noise, which is 294 μm 2 according to the triangular WS 2 region shown in Figure 1B. Parameter B is the test bandwidth which is set as 100 kHz. Therefore, for a bias voltage of 40 V, the corresponding NEP and D* are calculated as 7.49 × 10 -11 W/Hz 0.5 and 7.24 × 10 9 Jones, respectively.

CONCLUSION
In summary, we successfully demonstrate an all-2D APD structure with ultralow dark current. By stacking a monolayer semiconducting WS 2 onto two few-layer semimetal MoTe 2 flakes, two back-to-back Schottky barriers were formed at the two heterojunctions between the WS 2 and MoTe 2 . Due to the double Schottky barriers and good crystallinity of the 2D crystals, the fabricated device structure can exhibit excellent electrical avalanche effect. When operating in the avalanche regime, the heterojunction structure can act as an APD with improved photodetection performances and a remarkably low dark current. The EQE of our APD is 1,406%, with an AG of 587 and dark current as low as 93 pA. Moreover, due to the small Schottky barrier height, the 2D APD can operate in a broadband spectrum range from 400 to 700 nm. Further optimization of the APD performances is possible. For example, by selecting 2D crystals with favorable energy band structures and alignments, it is possible to establish proper Schottky barriers to further minimize the dark currents and expand the operation wavelength ranges. Additionally, by improving the processing techniques of the 2D stacked heterostructures, one can reduce the RT and current noise. We therefore believe that the results obtain in the current study can pave the way for design and fabrication of miniaturized all-2D optoelectronic devices with supreme performances.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

AUTHOR CONTRIBUTIONS
HC and SD conceived the study and supervised the project. TO fabricated the all-2D APD device and characterized the device performances. XW and SL helped prepare the samples and characterizations. TO, XW, SL, HC, and SD analyzed the data and discussed the results. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.