1. Introduction

Two-dimensional (2D) materials with excellent electronic and optoelectronic properties have unique advantages and application prospects as photodetectors [1,2]. In Particular, they have a negligible thickness and can integrate with waveguides without increasing volume, which has attracted intense interest from photo circuits [3,4]. Now, waveguide-integrated 2D photodetector based on the photoconductivity effect of 2D materials (so-called resistor-type photodetector) has been intensely reported, which exhibits excellent photodetection performance [5,6]. For instance, Schuler et al. reported a waveguide-integrated graphene photodetector with responsivity exceeding 35 mA/W [7]. Ma et al. fabricated the waveguide-integrated MoTe₂ Photodetector with photoresponsivity of 23 mA/W [8]. Moreover, Youngblood et al. reported Waveguide-integrated black phosphorus photodetector with high responsivity up to 135 mA/W and low dark current [9]. However, 2D resistor-type photodetectors require additional external drivers, making packages inconvenient, and limiting their practical application in photonic circuits. There is an eagerness to explore the self-powered photodetector integrated with the waveguide.

A photovoltaic detector driven by light illumination provides an alternative for the self-powered operation [10,11]. This device depends on a p-n or Schottky junction, where photo-excited electrons or holes are unidirectionally transported and driven by the internal electric field. Up to now, free-space photovoltaic detectors based on 2D materials have been intensely reported. For instance, stacking of 2D materials constructs a van der Waal heterojunction with a p-n (or Schottky) junction [12]. Moreover, the local chemical doping of a 2D material causes the regional difference of the 2D material, resulting in p-n junction [13]. Besides, the defect-engineering carried out by focused ion irradiation is a mature and automated technology to directly write junctions on graphene-based heterostructure [14], which locally modifies electronic structures of 2D materials via defects generated by incident energetic Ga⁺ ions. However, the exploration of the photovoltaic 2D detectors is limited to the free space photodetector instead of the waveguide integration.

This work demonstrates a self-powered waveguide-integrated photodetector based on the defect-engineered WSe₂/G heterostructure. The waveguide is constructed with a silicon-on-insulator (SOI) wafer and a polymer strip. Defects induced by ion irradiation modify the WSe₂/G heterostructure to WSe₂/G-WS_2-0.18/G, generating a Schottky junction at the defect boundary. And WSe₂/G-WS_2-0.18/G is inserted between SOI and the polymer strip as the photovoltaic device. WSe₂/G-WS_2-0.18/G photodetector exhibits a broadband photoresponse ranging from 450 nm to 1550 nm with the on/off ratio of 10⁴ at 0 bias. And this waveguide-integrated photodetector can directly read the optical signal (100 kHz) in the waveguide via an oscilloscope without the signal amplification and the external power source. Our work explores the application potential of the photovoltaic 2D detectors as the waveguide-integrated photodetector, paving an avenue to realize the self-powered photodetector in photonic circuits.

2. Experimental details

Figure 1(a) shows a schematic diagram to prepare the waveguide-integrated photodetector. I: The substrate is an SOI wafer with a 3 µm thick SiO₂ BOX and a 340 nm thick silicon DEVICE layer(p-type Si). II: Two Au-electrodes as the drain (D) and source (S) are coated on the surface of SOI with a separation of 20 µm. III: Graphene and WSe₂ monolayer are transferred onto electrodes with WSe₂ on the top, constructing a van der Waals heterostructure (WSe₂/G). VI: The heterostructure is partially (right side) irradiated by the Ga⁺ ion beam, following the same experimental process in Ref. [14]. Ion irradiation generates Se-vacancies on the topic atomic layer of WSe₂ monolayer and fabricates a Schottky junction on the heterostructure. This work uses this partially irradiated heterostructure as the photodetector. Figure 1(b) displays this photodetector’s SEM (scanning electron microscope) image. V: The polymer (AZ-1518) photoresist is spin-coated onto the heterostructure surface at 4000 rpm, achieving a smooth coating. VI: Then, we use photolithography to fabricate a polymer strip between electrodes. The polymer strip and the SOI construct a ridged waveguide, which confines the light in the SOI and propagates along the polymer strip. Figure 1(c) shows the waveguide-integrated photodetector constructed by the ridged waveguide and the partially irradiated heterostructure.

 

Fig. 1. (a) Schematic for the preparation of the waveguide-integrated photodetector; SEM images (top view) of the photodetector (b) and the waveguide-integrated photodetector (c).

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3. Results and discussion

3.1 Photodetector

Figure 2(a) illustrates the 3D schematic diagram of the photodetector prepared by partial irradiation. We measure the elemental composition of the pristine and irradiated heterostructure by the XPS (X-ray photoelectron spectroscopy) spectra in Fig. 2(b). The irradiation process does not change the atomic ratio of W (tungsten) and C (carbon), indicating the W and C atomic layers are well preserved. While, the Se (selenium) content slightly decreases after the irradiation, and the atomic ratio of Se:W is 1.8 instead of 2 in the pristine heterostructure. It demonstrates that incident Ga⁺ ions sputter Se atoms from the top atomic layer of the WSe₂ monolayer, leaving Se-vacancies. For convenience, we call the irradiated heterostructure WSe_2-xx/G. As shown in Fig. 2(c), Se-vacancies disturb the Raman spectra of the WSe_2-0.18, weakening the Raman peak located at 250 cm⁻¹. Figure 2(c) also displays the Raman mapping at 250 cm⁻¹, where the intensity of the 250 cm⁻¹ peak is uniform in the irradiated region, and there is a clear boundary between the irradiated and pristine ones. It means the Ga⁺ irradiation can well control the distribution of Se-vacancies. Figure 2(d) shows the AFM and KPFM images of the partially irradiated heterostructure. The thickness of the heterostructure has little variation, whereas the irradiation reduces the surface potential of the heterostructure from 5.15 eV (WSe₂/G) to 5.00 eV (WSe_2-0.18/G).

 

Fig. 2. (a) 3D schematic diagram of the photodetector; (b) XPS spectra of the pristine and irradiated heterostructures; (c) Top image is the Raman mapping located at 250 cm⁻¹, and the bottom is the Raman spectra of WSe₂/G (black line) and WSe_2-0.18/G (red line); AFM (d) and KPFM (e) images of the partially irradiated heterostructure.

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At the interface of WSe₂/G and WSe_2-0.18/G, the surface potential difference (0.15 eV) causes the migration of charges in the heterostructure to create electrostatic shielding, resulting in an internal electric field (IEF). Figure 3(a) shows the energy band diagram of WSe_2-0.18/G-WSe₂/G. The contact of WSe_2-0.18/G and WSe₂/G is the metal-graphene contact [14], where the trace of the Dirac point is bent to screen the potential difference at the interface. The IEF makes the current in the heterostructure unidirectionally pass through the interface. The |I_ds|-V_DS curve of the WSe_2-0.18/G-WSe₂/G confirms the rectification character induced by the IEF. Figure 3(b) shows that the rectification ratio is 10 in the dark (black line).

 

Fig. 3. (a) Corresponding energy band diagram of WSe₂/G-WS_2-0.18/G; (b) |I_ds|-V_DS curve at the wavelength of 808 nm; (c) Photoswitching characteristic of different wavelength; (d) Responsivity of broadband wavelength.

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Optoelectronic characteristics of the WSe_2-0.18/G-WSe₂/G interface are studied using continuous-wave (CW) laser sources. Figure. 3(b) shows the |I_ds|-V_DS curve at the wavelength of 808 nm. With the power density (P_laser) increased from 0.05 mW/cm² to 0.6 mW/cm², the short-circuit current and the open-circuit voltage are 117.6 nA and 0.1 V, respectively. Not limited to 808 nm, this WSe_2-0.18/G-WSe₂/G photodetector exhibits a broadband optoelectronic response. Figure 3(c) displays its photoswitching characteristic with the 0 V bias. To evaluate the photo-detecting performance, we calculate the responsivity (R) following the equation R = (I_ph-I_d)/P_laserS (I_d is the dark current and S is the effective illuminated area). Figure 3(d) shows that the value of R reaches up to 100 mA/W at the wavelength of 405 nm and then gradually declines to 1.8 mA/W with the wavelength changed to 1550 nm. As one can see, the WSe_2-0.18/G-WSe₂/G photodetector exhibits a broadband optoelectronic response ranging from 450 nm to 1550 nm, which attributes to the broad absorption band of graphene and the vacancy-rich tungsten selenide [15,16]. Moreover, the WSe_2-0.18/G-WSe₂/G photodetector has excellent photovoltaic function and does not need to be driven by an external source.

3.2 Waveguide-integrated photodetector

Figure 4(a) illuminates the structure of the waveguide-integrated photodetector. Based on this structure, we use FDTD MODE solution to calculate the propagation mode of the waveguide at the wavelength of 1550 nm. As shown in Fig. 4(b), the SOI confines most of the light intensity inside the Si layer with a thickness of 0.34 µm, and the polymer layer with a width of 2 µm further constrains the light propagation along the polymer strip, which constructs a ridged waveguide. Please note, that thickness (width) of the Si layer (polymer strip) is specially selected to ensure the single-mode propagation at 1550 nm.

 

Fig. 4. (a) 3D structure diagram of the waveguide-integrated photodetector; Calculated (b) and measured (c) field distribution of the propagation mode in the polymer-SOI waveguide; (d) |I_ds|-V_DS of the waveguide-integrated photodetector with and without light illumination; (e) on/off switching of the Photoswitching characterization of the WSe₂/G-WS_2-0.18/G in different laser power.

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Through the end-coupling technique, we couple a CW laser at the wavelength of 1550 nm with the power of 50 nW into the waveguide. The measured propagation mode in Fig. 4(c) agrees with the calculated one in Fig. 4(b). The WSe_2-0.18/G-WSe₂/G photodetector (dashed lines) inserted between the polymer and Si layer has a good overlap with the light mode, indicating a well optical absorption of the photodetector. And the propagation loss induced by WSe_2-0.18/G-WSe₂/G is measured to be 3.06 dB/mm. Figure 4(d) depicts the |I_ds|-V_DS curve of the waveguide-integrated photodetector, exhibiting a prominent photoelectronic response. Figure. 4(e) shows the on/off switch ratio of 10⁴ under 0 bias, displaying good repeatability.

We use the experimental setup in Fig. 5(a) to characterize the speed performance of this waveguide-integrated photodetector. The high-frequency optical signal is provided by a 1550 nm polarization-maintaining fiber laser, which is modulated by an Electro-optic modulator (iXblue MX-LN-10) and an Arbitrary Function Generator. The WSe_2-0.18/G-WSe₂/G photodetector converts the optical signal into a photogenerated electromotive force and reads the data out through an oscilloscope. Figure 5 shows 20 kHz (Fig. 5(b)) and 100 kHz (Fig. 5(c)) signals. The dots in the figure are the original data, and the pink curve is the fitting data curve. Particularly worth mentioning is amplifiers are not required during testing. The signal can be extracted directly from the device through the probe and read on the oscilloscope. This further reduces power consumption. And when encapsulating, it only needs to design a simple voltage amplifier circuit that can be put into use, which can further reduce the space needed for on-chip integration.

 

Fig. 5. (a) Schematic diagram of main experimental equipment; the high-resolution photocurrent-time curve at the wavelength of 1550 nm with 20kHz (b) and 100kHz (c).

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4. Conclusion

A self-powered waveguide-integrated photodetector is fabricated based on the defect-engineered WSe₂/G heterostructure. Ion beam locally irradiates the WSe₂/G heterostructure and sputters Se atoms from the WSe₂ monolayer, leaving Se-vacancies. The boundary of the irradiated region forms a Schottky junction exhibiting the photovoltaic function. Integrated with a ridged waveguide, this photodetector exhibits a broadband photoresponse ranging from 450 to 1550nm and shows a high on/off ratio of 10⁴. Besides, it can directly read the optical signal (100 kHz at 1550 nm) in the waveguide without an external source or amplification. Our work demonstrates that the 2D photovoltaic detector is a promising waveguide-integrated element for self-powered detecting, which caters to photonic circuits’ high integration and low energy consumption requirements.