High photoresponsivity and broadband photodetection with a band-engineered WSe₂/SnSe₂ heterostructure

Abstract

van der Waals (vdW) heterostructures formed by stacking different two-dimensional layered materials have been demonstrated as a promising platform for next-generation photonic and optoelectronic devices due to their tailorable band-engineering properties. Here, we report a high photoresponsivity and broadband photodetector based on a WSe₂/SnSe₂ heterostructure. By properly biasing the heterostructure, its band structure changes from near-broken band alignment to type-III band alignment which enables high photoresponsivity from visible to telecommunication wavelengths. The highest photoresponsivity and detectivity at 532 nm are ∼588 A W⁻¹ and 4.4 × 10¹⁰ Jones and those at 1550 nm are ∼80 A W⁻¹ and 1.4 × 10¹⁰ Jones, which are superior to those of the current state-of-the-art layered transition metal dichalcogenides based photodetectors under similar measurement conditions. Our work not only provides a new method for designing high-performance broadband photodetectors but also enables a deep understanding of the band engineering technology in the vdW heterostructures possible for other applications, such as modulators and lasers.

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Introduction

Recently, two-dimensional (2D) layered materials have received significant attention for photodetection applications due to their high optical absorption,^1–10 high carrier mobility and good flexibility.^11–16 For example, semiconducting transition metal dichalcogenides (TMDs, such as MoS₂, WSe₂, MoS₂ and MoSe₂) are considered as promising candidates for future photodetection applications.¹⁷ Indeed, photodetectors with TMDs have been demonstrated with high photoresponsivity (e.g., 800 A W⁻¹ at 561 nm with monolayer MoS₂).¹⁸ However, the previously reported operation bandwidth of a TMD-based photodetector is typically limited by the bandgap of the TMD material. For example, the bandgap of MoS₂ is between ∼1.2 eV (bulk) and ∼1.8 eV (monolayer).^19,20 Therefore, in principle, a MoS₂ based photodetector typically cannot effectively detect light with a photon energy smaller than 1.2 eV. Such an intrinsic bandgap limitation poses a great challenge for various photonic applications (e.g., telecommunications).

van der Waals (vdW) heterojunctions, formed by stacking different individual 2D layered materials, can offer a new dimension in breaking the abovementioned intrinsic bandgap barrier.^21–31 In general, three types of band alignments can be categorized when different 2D materials are stacked together,³² including type-I,³³ type-II,^34–38 and type-III^39–43 band alignments. Thus far, most of the reported vdW heterostructure photodetectors are based on the type-II band alignment to utilize the excellent electron–hole pair separation ability in heterostructures. Recently, type-III band alignment has been explored as a platform for tunneling field effect transistors (TFETs) due to its enhanced tunneling current density.^40,44–49 The 2D layered materials based type-III band alignment TFETs with improved responsivity and detectivity have been the focus of many recent studies in the visible wavelengths (e.g. 5.7 A W⁻¹ under 660 nm,⁵⁰ 244 A W⁻¹ under 550 nm,⁵¹ 180 A W⁻¹ under 405 nm ⁴⁷). However, the applications of the type-III band alignment TFETs in the field of broadband photodetection have rarely been reported. In this paper, we report a high photoresponsivity and broadband photodetector based on a WSe₂/SnSe₂ heterostructure. By applying a large negative gate voltage and a reverse bias voltage, the heterojunction band structure changes from type-II to type-III band alignment and a band-to-band tunneling phenomenon occurs. The increased band bending at the heterojunction results in the tunneling-assisted transition which possibly allows photocurrent generation with energy smaller than the bandgap of WSe₂ (e.g., 980 nm and 1550 nm) for broadband photodetection (e.g. photo-excited electron tunnel from WSe₂ to the conduction band of SnSe₂). Under the type-III band alignment conditions, our heterojunction device shows a high photoresponsivity up to ∼588 A W⁻¹ at a wavelength of 532 nm and ∼80 A W⁻¹ at a wavelength of 1550 nm. The corresponding photodetectivity reaches 4.4 × 10¹⁰ Jones and 1.4 × 10¹⁰ Jones for visible and telecommunication wavelengths, respectively.

Results and discussion

The optical image and schematic illustration of the WSe₂/SnSe₂ heterostructure device are shown in Fig. 1a. Few-layer p-type WSe₂ and n-type SnSe₂ flakes, mechanically exfoliated from their bulk materials, are transferred onto a 280 nm SiO₂/silicon substrate. An overlapping heterojunction region is formed by precisely controlling their locations. Pd/Au metal contacts (20/50 nm) are patterned onto the WSe₂ and SnSe₂ flakes to collect holes and electrons. The thicknesses of the WSe₂ and SnSe₂ few-layer flakes are determined by atomic force microscopy (AFM), giving the values of 6 nm and 15 nm, respectively (ESI Fig. S1†). Raman spectroscopy is employed to confirm the high quality of our device after the fabrication processes (ESI Fig. S2†).

Fig. 1 The WSe₂/SnSe₂ heterostructure device and its electronic properties. (a) Microscopy image (upper panel) and schematic illustration (bottom panel) of the heterojunction device. (b) I_d–V_g characteristics measured with variable V_d in the dark. Inset: |I_d|–V_g characteristics on the logarithmic scale. (c) I_d–V_d characteristics measured with variable V_g in the dark. Inset: |I_d|–V_d characteristics on the logarithmic scale. (d) The schematic band diagrams of the heterojunction. The scale bar is 10 μm.

Before testing the electrical characteristics of the WSe₂/SnSe₂ heterostructure, we also fabricated two individual WSe₂ and SnSe₂ based field effect transistors (FETs) to analyze the properties of WSe₂ and SnSe₂. WSe₂ and SnSe₂ FETs are fabricated on Si/SiO₂ substrates with Pd/Au (20/50 nm) as the drain and source electrodes. The transfer curves of the FETs (ESI Fig. S3†) show p-type characteristics for WSe₂ and n-type characteristics for SnSe₂, indicating that Pd can efficiently inject holes into WSe₂ and electrons into SnSe₂ when a negative gate voltage is applied.^41,52 Moreover, the nonlinear output results of the WSe₂ flake (ESI Fig. S3b†) indicate the existence of a Schottky barrier at the contact region, while the SnSe₂ results (ESI Fig. S3d†) indicate that an ohmic contact is formed between Pd and SnSe₂. Note that, since SnSe₂ is a degenerated n-doped semiconductor, the band alignment of SnSe₂ at the interface would be relatively unaffected by the V_g (see ESI Fig. S3c†).^35,43

Next, the electronic properties of the WSe₂/SnSe₂ heterojunction is studied. The device is characterized by applying a constant drain to source voltage (V_d) and a gate voltage (V_g) in the configuration shown in Fig. 1a. The device shows obvious p-type transport characteristics. The concentration and the type of the carriers can be tuned by V_g, at V_d = −2 V; the I_d–V_g result shows the gate-controlled current, with an on–off ratio of ∼10⁴ (see Fig. 1b). The reverse current (I_reverse, V_d < 0 V) at V_g < −30 V is the band-to-band tunneling (BTBT) current, whose magnitude increases as the gate voltage decreases. The magnitude of I_reverse at V_g > 40 V increases as the gate voltage increases due to the majority carrier drifting. The forward current (I_forward, V_d > 0 V) observed at V_g < −30 V can be attributed to the drifting of the minority carriers (see the discussion below). The magnitude of the reverse current increases with both positive and negative V_g and the maximum of the reverse current is 15 μA when V_d = −2 V and V_g = −70 V. The gate-dependent current–voltage (I_d–V_d) characteristics are shown in Fig. 1c, where two important features can be observed. First, owing to the ultrathin thickness nature of the heterojunction, the magnitude of both I_forward and I_reverse can be significantly controlled by modifying the gate voltage. Second, since the breakdown voltage (i.e. −0.01 V) is very much less than 4E_g/q, where E_g is the band gap of the semiconductors (1.62 eV of WSe₂ and 1 eV of SnSe₂) and q is the electron charge, the tunneling current (red region in the inset of Fig. 1c) can be attributed to Zener tunneling.⁵³

The qualitative band alignment of the WSe₂/SnSe₂ heterostructure at zero bias is sketched in Fig. 1d based on the reported electron affinity values in the literature.^54,55 Based on these values, a nearly broken band alignment is formed, where the Fermi level of WSe₂ locates at the edge of its valence band and that of SnSe₂ locates at the edge of its conduction band. The heterostructure resembles a p–n diode with a staggered band gap offset. The bottom of the conduction band and the top of the valence band of WSe₂ (SnSe₂) are at 3.6 eV (5.22 eV) and 5 eV (6 eV), so that the offsets of the conduction band and valence band are about 1.4 eV and 0.78 eV, respectively.

The transfer characteristics of the heterostructure can be understood through band diagrams (Fig. 2a–c). When there is no gate voltage (i.e. V_g = 0 V), as shown in Fig. 2(a), the high Schottky barrier at the drain contact and the high difference of the conduction/valence band discontinuity restrain the current flow, leading to the zero dark current. Under these conditions, the device can be seen as a Schottky diode in series with a p–n junction and a resistor. When a negative gate voltage is applied (i.e. V_g < −30 V) as shown in ESI Fig. S3b,† the doping level of WSe₂ is increased and a near-ohmic contact is formed at the drain contact (the band bending direction at the drain contact shifts downwards). Thus, the model of the device under these conditions can be described as two resistors in series with a p⁺n junction. Therefore, when a reverse bias is applied (Fig. 2b), the valence band maximum of WSe₂ shifts above the conduction band minimum of SnSe₂, and the type of the band structure changes from near broken band alignment to broken band alignment (type-III). Thus, the electrons in the valence band of WSe₂ can tunnel into the conduction band of the SnSe₂ flake. This tunneling current can then be increased by increasing the reverse bias voltage, see Fig. 1c, due to the enlarged overlapping energy level between the valence band of WSe₂ and the conduction band of SnSe₂. However, when a forward bias is applied (Fig. 2c), the band alignment changes from type-III to type-II, the barrier height at the heterojunction is reduced, and the majority carriers (electrons in SnSe₂ and holes in WSe₂) can overcome the interface barrier and drift to their counterparts, leading to a high forward current. The carrier transportation under the forward bias conditions follows the mechanism as described for the traditional p–n diode, where the forward current increases exponentially with forward bias.⁴⁰ To further analyze the transfer characteristics of the heterostructure, scanning photocurrent mapping at λ = 532 nm under a confocal optical microscope (objective 100×, NA = 0.75, P_laser = 50 μW (corresponding to a power intensity of 8.5 × 10³ W cm⁻²)) was performed, as shown in Fig. 2d–f. The spot size (∼0.6 μm² at 532 nm) of the incident laser is much smaller than the size (∼50 μm²) of the heterojunction. When there is no bias (i.e. V_g = V_d = 0 V), as mentioned above, the heterostructure resembles a p–n junction. Thus, when there is an external light excitation, electron–hole pairs will be generated and separated efficiently at the heterojunction (Fig. 2d) where the photo-generated electrons move to the SnSe₂ flake while the holes move to the WSe₂ flake, due to the built-in electric field.⁵⁶ The part of the heterojunction near the drain contact shows a stronger photocurrent, which has also been observed in ref. 57, and can be attributed to the lower sheet resistance of this region compared to those of other parts. Note that the photocurrent mapping results only indicate the region where the dominant photocurrent is generated. A small photocurrent (e.g., ∼1 nA in Fig. 2d) can be observed in the drain part of the device. Under reverse bias with a negative V_g, the I_d is governed by the BTBT, where carriers from the valence band of the WSe₂ tunnel move to the conduction band of SnSe₂, as mentioned above (Fig. 2b). Since the reversed V_d enhances the electric field in the heterojunction area, Fig. 2e exhibits a larger I_ph (∼50 times larger than that in Fig. 2d) throughout the entire heterojunction. In contrast, while the device is at forward bias with a negative gate voltage, Pd can efficiently inject holes into WSe₂, which can overcome the thermionic barrier and reach the source contact. In addition, the band bending at the heterojunction is released, leading to small I_ph generation in the heterojunction (Fig. 2f). Due to the fact that the direction of the built-in field at the drain part is the same as that of the external field, around I_ph = 1 μA is generated at the drain contact and the direction is towards the drain part.

Fig. 2 Operation principle of our heterojunction device. (a–c) Band diagram of heterojunction and Schottky junctions, where (a) V_g = V_d = 0 V, (b) V_g = −70 V, V_d = −1 V and (c) V_g = −70 V, V_d = 1 V. The red/purple arrows indicate the carrier movement direction. The magenta arrows indicate the separation process of electron–hole pairs. The pink rectangular regions indicate the position of the strongest built-in electric field. (d–f) Photocurrent mapping of the heterojunction under different bias conditions at 532 nm. P_laser = 50 μW (8.5 × 10³ W cm⁻²). The dashed lines outline the flakes and heterojunction. The scale bar is 10 μm.

The detailed photoresponse behavior of our device under a λ = 532 nm laser is also studied (Fig. 3). Note that, all the measurements are carried out by locating the laser spot at the heterojunction where the largest photocurrent is generated (Fig. 2e). The time-dependent I_ph under pulsed light illumination (λ = 532 nm, 10 μW) is firstly tested at V_g = 0 V and −70 V with different bias voltages, respectively, shown in Fig. 3a. The steep rise and fall edges indicate the fast response of the device. The device shows the photodiode-like behavior, where the I_ph increases significantly when it is at reverse bias compared with that of forward bias both at V_g = 0 V and −70 V. This can be explained by the fact that the increased band bending under the reverse bias conditions results in the efficient separation of the photo-generated electron–hole pairs, which coincides with our photocurrent mapping results (Fig. 2e and f). Since the forward bias results in a much lower I_ph compared with that from reverse bias, in the following discussion we will only focus on the I_ph under the condition of the reverse bias voltage.

Fig. 3 Photoresponse of the WSe₂/SnSe₂ heterojunction device at 532 nm. (a) Photo-switching characteristics at different V_d. I_ph–V_d characterization of the WSe₂/SnSe₂ heterojunction at (b) V_g = 0 V and (c) V_g = −70 V. (d) I_ph dependence of the V_g and P_laser. (e) R and EQE dependence of V_g. (f) R measured with different input power intensities.

Fig. 3b and c illustrate the I_ph–V_d curves of the heterostructure with fixed gate voltages (V_g = 0 V and V_g = −70 V, respectively) under the λ = 532 nm laser with the laser power varying from 4 nW to 37 μW. The magnitude of the I_ph strongly depends on the V_d, due to the increased carrier drift velocity and the increased charge carrier separation efficiency, suggesting that a higher photocurrent can be readily achieved by applying a larger reversed V_d. With the increase in the P_laser, the I_ph at reverse bias increases and approaches 8 μA at V_d = −3 V and V_g = −70 V (Fig. 3c). Moreover, since the larger reversed V_d and V_g will lead to the larger electric field at the heterojunction region, the efficiency of the photo-excited carrier separation increases, resulting in a ∼62-fold increase of I_ph (P_laser = 37 μW, V_d = −3 V) at V_g = −70 V compared with that at V_g = 0 V. The I_ph as a function of V_g and P_laser is plotted in Fig. 3d. Clearly, the I_ph increases with the decreasing of the gate voltage and a higher incident laser power. For example, when P_laser = 1 μW, the photocurrent is remarkable (∼2 μA) under the condition of V_g < −40 V, confirming that a smaller V_g leads to a larger photocurrent.

To better understand the photoresponse properties of our device, several important figures of merit for photodetectors are studied, including the photoresponsivity (R = I_ph/P_laser), detectivity (D = R × s^1/2/(2 × q × I_dark)^1/2) and external quantum efficiency (EQE = (I_ph/q)/(P_laser/hυ)), where q is the electron charge, s is the illumination area, I_dark is the current when the laser is off and hυ is the photon energy.¹⁷ The gate-dependent R and EQE are shown in Fig. 3e. With a decrease in the V_g, the largest R and EQE could reach 588 A W⁻¹ and 1367%, respectively, benefitting from the increased band bending which has been mentioned above. An EQE larger than 100% has been previously observed, which is might due to the photo-excited carriers being recirculated many times before reaching the electrodes resulting from the long carrier lifetime.^50,58 The highest D is estimated to be 4.4 × 10¹⁰ Jones. The corresponding R as a function of the incident power intensity is summarized in Fig. 3f. It is clearly observed that the R linearly changes with the P_laser in the log scale. By fitting the experimental data with the equation of R ∼ P^β, the term β is obtained at −0.98 for 532 nm, indicating the superior photocurrent capability and excellent separation efficiency of photo-induced charge carriers.

Telecommunication wavelength detection is an important issue for various applications. Here, we test our heterojunction device at telecommunication wavelengths. The spatially resolved photocurrent mapping with a λ = 1550 nm laser (V_d = −1 V, V_g = −70 V and P_laser = 10 μW) reveals pronounced photocurrent generation in the overlapped region (Fig. 4a). Fig. 4b and c depict the gate-dependent photoresponse of the device under 980 nm and 1550 nm lasers, where I_ph shows the same tendency as that at 532 nm. However, a threshold for the near-infrared photon detection can be observed. For example, under the bias conditions, V_d = −2 V and P_laser = 10 μW, for λ = 980 nm, the photocurrent is not detectable when the V_g is larger than −60 V. As for the incident light at λ = 1550 nm, under the same bias conditions, this threshold voltage shifts to V_g = −70 V. This fascinating phenomenon can be attributed to the tunneling-assisted transition in the band-bending heterojunction: an extremely negative gate voltage and a high reverse bias voltage increase the built-in electric field at the heterojunction, and the edges of the conduction band and valence band are tilted along the direction of the electric field. The electron and hole wave functions have a tail which extends into the band gap, enhancing the e–h interactions with sub-bandgap photons.⁵⁹ Thus, under such a high built-in electric field, our device has the potential to allow the photo-excited carrier generation with a photon energy smaller than the intrinsic band-gap of WSe₂ and transfer to the conduction band of SnSe₂.^53,60 The magnitude of the photocurrent under the 980 nm laser is around 2.6 times larger than that generated under the 1550 nm laser because the photon energy of 980 nm is higher than that of 1550 nm. Note that there is no photoresponse of our device when it is forward biased even when the device is exposed to an intense light. Fig. 4d and e show the I_ph–V_d curves of the heterostructure with a fixed gate voltage (V_g = −70 V) under the λ = 980 nm and 1550 nm lasers with the laser power varying from 6 nW to 400 μW and 17 nW to 400 μW, respectively. The magnitude of the I_ph shows strong dependence on the reverse bias voltage. This results from the larger built-in electric field at the larger reverse bias voltage. Fig. 4f depicts the R of the device under λ = 980 and 1550 nm as a function of the laser intensity. The maximum R of our device (according to the fitting results) is 396 A W⁻¹ at 980 nm (P_laser = 100 mW cm⁻²) and is 80 A W⁻¹ at 1550 nm (P_laser = 100 mW cm⁻²). The R at the visible wavelength and telecommunication wavelength range is superior to those of the previously reported 2D materials and their heterostructure based photodetectors (see ESI Fig. S4†). The highest detectivity and EQE at these two wavelengths are 4.4 × 10¹⁰ Jones, 500% and 1.4 × 10¹⁰ Jones, 64%, respectively. The time-dependent I_ph under pulsed light illumination (at 1550 nm) is determined (ESI Fig. S5†). The rise time and the decay time are 16 and 45 ms, respectively. We believe that the speed of the device can be further improved by improving the crystal quality.

Fig. 4 Photoresponse of the WSe₂/SnSe₂ heterojunction device at 980 nm and 1550 nm. (a) Photocurrent mapping of the heterojunction under different bias conditions at 1550 nm. P_laser = 10 μW (2 × 10⁵ mW cm⁻²). I_ph dependence of the V_g and P_laser at (b) 980 nm and (c) 1550 nm. I_ph–V_d characterization of the WSe₂/SnSe₂ heterojunction at (d) 980 nm and (e) 1550 nm. (f) R at different input power intensities. The scale bar is 10 μm.

Conclusions

In summary, we report a high-sensitivity and broadband few-layer WSe₂/SnSe₂ heterostructure based photodetector. The band-to-band tunneling mechanism allows the proposed device to detect light from visible to telecommunication wavelengths. The highest responsivity and detectivity at 532 nm are 588 A W⁻¹ and 4.4 × 10¹⁰ Jones and those at 1550 nm are 80 A W⁻¹ and 1.4 × 10¹⁰ Jones, which outperform the state-of-art 2D materials based photodetectors. Our work shows an attractive platform for practical applications of utilizing few-layer TMD based heterojunctions for high-performance optoelectronic devices (such as modulators^61,62 and lasers).^63–68

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge funding from the Academy of Finland (Grant No. 276376, 284548, 295777, 304666, 312551, 314810, and 320167), the Academy of Finland Centre of Excellence program (project 312297), the Academy of Finland Flagship program (project 320167), Business Finland (OPEC, A-photonics), the European Union's Seventh Framework Program (Grant No. 631610), the European Union's Horizon 2020 research and innovation programme (Grant No. 820423, S2QUIP), Aalto Centre of Quantum Engineering, China Scholarship Council, and the provision of technical facilities of the Micronova, Nanofabrication Centre of Aalto University.

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Footnote

† Electronic supplementary information (ESI) available: (1) AFM image of a heterostructure, (2) Raman spectra of the heterostructure, (3) electrical characteristics of WSe₂ and SnSe₂ based FETs, (4) photoresponsivity comparison of this work and previous results, (5) photo-switching characteristics of the device. See DOI: 10.1039/c8nr09248f

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