MoTe2

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Functional Nanostructured Materials (including low-D carbon)

Gate-tunable photodetection/voltaic device based on BP/MoTe2 heterostructure Yuan Xie, Enxiu Wu, Jing Zhang, Xiaodong Hu, Daihua Zhang, and Jing Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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ACS Applied Materials & Interfaces

Gate-tunable photodetection/voltaic device based on BP/MoTe2 heterostructure

Yuan Xie#, Enxiu Wu#, Jing Zhang, Xiaodong Hu, Daihua Zhang* and Jing Liu* State Key Laboratory of Precision Measurement Technology and Instruments, School of Precision Instruments and Opto-electronics Engineering, Tianjin University, NO. 92 Weijin Road, Tianjin, China, 300072

#These authors contribute equally to this manuscript. *Corresponding author email: D.Z. [email protected]; J.L. [email protected] ORCID iD: Yuan Xie: https://orcid.org/0000-0003-2740-231X

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Van der Waals heterostructures based on two-dimensional (2D) materials have attracted tremendous attentions for their potential applications in optoelectronic devices, such as solar cells and photodetectors. In addition, the widely tunable Fermi levels of these atomically thin 2D materials enable tuning the device performances/functions dynamically. Herein, we demonstrated a MoTe2/BP heterostructure, which can be dynamically tuned to be either p-n or p-p junction by gate modulation due to the compatible band structures and electrically tunable Fermi levels of MoTe2 and BP. Consequently, the electrostatic gating can further accurately control the photo-response of this heterostructure in terms of the polarity and the value of photo-responsivity. Besides, the heterostructure showed outstanding photodetection/voltaic performances. The optimum photo-responsivity, external quantum efficiency and response time as a photodetector were 0.2 A/W, 48.1% and 2 ms, respectively. Our study enhances the understanding of 2D heterostructures for designing gate tunable devices and reveals the promising potentials of these devices in future optoelectronic applications. Keywords: MoTe2; BP; heterostructure; gate modulation; photovoltaic inversion;


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Introduction Due to the excellent electronic and optoelectronic properties and the minimum amount of surface dangling bonds, a considerable amount of research interests has been focused on the heterostructures built by atomically thin two dimensional (2D) materials. Separate 2D flakes with distinct physical properties can be artificially stacked together to form heterostructures in a specially designed sequence, so that the created heterostructures would combine the physical properties of individual 2D materials for new functions.1–5 So far, numerous novel phenomenon and properties of 2D heterostructures have been investigated, such as ultrafast carrier transport, negative differential conductance, interlayer excitons and ballistic transport.6–10

2D heterostructures also show strong light-matter interactions that leads to large photon absorption and photocurrent generation, making them promising candidates for optoelectronic applications.6 In addition, owing to their ultrathin thickness and steep interfacial charge carrier gradient, the carrier density (and Fermi level) of each 2D material in the heterostructures may be tuned efficiently by the gate bias, offering a promissing way to design dynamically tunable devices.11–14 More importantly, the gate tunable behavior may realize different types of heterojunctions in a single device. Inspired by these advantages, a variety of 2D heterostructures comprised of p- and ntype 2D materials, such as BP/MoS2, WSe2/MoS2, GaTe/MoS2, MoTe2/MoS2, ReS2/ReSe2 and TaxWxSe2/MoS2 have been demonstrated with gate-tunable electrical and optoelectrical properties.15–20 However, due to the lack of careful design of the relative band offset of the junctions, these 2D heterojunctions show rather limited capability to modulate the electrical and optoelectrical properties. In addition, the Fermi levels of the p- and n-type materials in these junctions are tuned by the back gate voltage simultaneously, so that the built-in potential of the heterojunctions have negligible change when the back gate voltage varies. As a result, it is very difficult to modulate the polarity of the heterojunction and corresponding photodetection characteristics. Therefore, it is very critical to select paired 2D material with compatible band structures to build heterostructures to achieve large gate tunability. For instance, specially designed WSe2/GeSe heterostructure with tailored band alignment have been



demonstrated with giant gate tunability and excellent optoelectronic characteristics.21 This mainly attributes to that the Fermi level of WSe2 can be modulated across the entire band gap while the Fermi level of GeSe remains relatively stational under different back gate bias. Nonetheless, large negative gate voltage (-60V) is required for the WSe2/GeSe heterojunction to change from p-n junction to p-p junction, which is caused by the large gap of WSe2 and GeSe. In this work, we reported a MoTe2/BP heterostructure with gate tunable rectification behavior and photo-response, which may be ideally useful for developing devices with adaptive responses. MoTe2 has a bandgap of 0.9~1.1 eV, which is quite close to that of Si and much smaller than those of other 2D materials such as WSe2 and MoS2.22 More importantly, the carrier polarity of MoTe2 could be easily modulated to be either p- or n-type because of the weak Fermi level pinning at the metal/ MoTe2 interface.23 On the other hand, bulk BP shows a ~0.3 eV direct bandgap and hole mobility up to 1000 cm2 V-1s-1.24 As expected, the MoTe2/BP heterostructure presented excellent performance as a photodetector and photovoltaic cell. We measured the responsivity, external quantum efficiency (EQE) and response time of the heterostructure as a photodetector, which were 0.2 A/W, 48.1% and 2 ms, respectively. Besides, the moderate bandgaps and weak Fermi level pinning of MoTe2 and BP were both favorable for the heterostructure to be tuned by the gate bias in terms of the electrical and optoelectrical characteristics. Specifically, the heterostructure can be tuned to be either p-p or p-n junction by electrical gating, leading to the rectification ratio and photocurrent showing completely opposite polarities. A table (Table S1) is listed in the Supplementary Information to compare the figure-of-merits of the MoTe2/BP heterojunction with previously reported 2D material heterojunctions. It suggests the great potential of the MoTe2/BP heterostructure for adaptive electronic and optoelectronic applications. Results and Discussions The MoTe2/BP heterostructures were fabricated by sequentially stacking the flakes of boron nitride (BN), BP and MoTe2 on the SiO2/Si (n + doped) substrate (the thickness of SiO2 layer was 285 nm). A heterojunction was then formed at the overlapping region.


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Contact patterns were defined by electron-beam lithography followed by deposition of 10/30 nm Cr/Au. The schematic and scanning electron microscopy (SEM) image of the device are shown in Fig. 1 (a) and (b), respectively. The thicknesses of MoTe2 and BP flakes were 10 nm and 9 nm, respectively, as shown in Figure S1. Figure 1 (c) shows the band alignments of MoTe2 and BP, in which MoTe2 (BP) had bandgap and electron affinity of ~0.9 eV (~0.4 eV) and 3.8 eV (4.1 eV), respectively.22,25 From a structural point of view, the entire device can be treated as a MoTe2/BP heterojunction, a MoTe2 field effect transistor (FET) and a BP FET on the BN layer. The Raman spectra (Figure S2) confirmed the structure of the MoTe2/BP. The characteristic peaks of MoTe2 and BP were observed at 169, 232 and 287 cm-1 (for MoTe2), 359, 438 and 465 cm-1 (for BP), respectively. Figure 1(d) and (e) present the transfer characteristics of MoTe2 and BP FETs, respectively. The MoTe2 FET exhibits ambipolar transfer characteristics with the on-off ratio up to 103. The ambipolar characteristic indicates that the Fermi level of MoTe2 can be tuned from valence band edge to conduction band edge as Vgs changes. Moreover, the ambipolar transfer characteristic of MoTe2 was enhanced on BN as compared with those directly fabricated on the SiO2 substrate (Figure S3), which may attribute to the hole doping of MoTe2 from the SiO2 interlayer. 26,27 Therefore, in order to optimize the gate modulation, we used BN as an ideal substrate to preserving the intrinsic properties of 2D materials. On the other hand, the BP FET exhibited unipolar p-type transfer property with an on-off ratio of ~102, which indicates that the Fermi level of BP lies around the valence band edge as Vgs changes. These results were consistent with previous reports.23,28 Besides, the linear output characteristics of both MoTe2 and BP FETs indicate that the contacts between metal and MoTe2/BP were Ohmic contacts (Figure S4).



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Figure 1. (a) Schematic illustration of a MoTe2/BP heterostructure device on a BN/Si/ SiO2 substrate. (b) SEM image of the fabricated device, where D1 and D2 (S1 and S2) denote the metal in contact with BP (MoTe2). Scale bar is 5 µm. (c) Energy band profiles of MoTe2 and BP. (d) Transfer curve of MoTe2 measured between S1 and S2 at Vds=1 V. (e) Transfer curve of BP measured between D1 and D2 at Vds=1 V.

To investigate the electrical properties of this heterostructure, we performed measurements between electrodes D1 (in contact with BP) and S1 (in contact with MoTe2). Figure 2(a) shows the transfer characteristics of the MoTe2/BP heterojunction. The Ids was strongly modulated as Vgs swept from -60 V to 60V. The non-monotonic


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transfer curve was a clear signature of ambipolar transport behavior, indicating that the concentration of both holes and electrons can be modulated by electrostatic gating. Moreover, the polarity of the heterostructure was reversed from p-dominated type to ndominated type when Vds changed from -1V to +1V. A further investigation on the output curves of the heterostructure revealed an interesting phenomenon that the heterostructure exhibited inverted rectification characteristics under different back gate bias, which can be hardly implemented by conventional diodes. As shown in Fig. 2(b) and (c), the output curves of the heterostructure exhibited completely different rectification property under different gate bias. When Vgs changed from 0V to 60V, the heterostructure exhibited strong current rectification property with the forward current (IF) much larger than the reverse current (IR). In contrast, when Vgs changed from -20V to -60V as shown in Fig. 2(c), the IR increased rapidly and became larger than IF. Figure 2(d) shows the current rectification ratio (IF / IR) calculated by dividing IF at Vds=+1 V by IR at Vds=-1 V, which kept at around 102 as Vgs changed from 0 V to 60 V and then decreased to 10-1 as Vgs changed from -20 V to -60 V. The inset of Fig. 2(d) depicts the extracted IF and IR at Vds = ± 1 V as a function of Vgs from Fig. 2 (b) and (c), which are consistent with the transfer curves of the heterostructure shown in Fig. 2 (a). We also observed similar phenomena in other five BN/MoTe2/BP devices. As shown in Figure S5, the maximum rectification ratio range (blue rectangles) is from 10-1 to 102 and the minimum range (red rectangles) is from 10-1 to 101.

Since the contacts between BP (MoTe2) and metal electrode were nearly Ohmiccontacts, we attributed the inversion of the rectification behavior mainly from the BP/MoTe2 interface, at which the depletion region and built-in electrical field were formed. As previously mentioned, the Fermi level of MoTe2 can be tuned from valence band edge to conduction band edge while the Fermi level of BP moves slightly around valence band edge under different Vgs. When a large negative Vgs (around -60V) is applied, the Fermi level of MoTe2 is tuned near the valence band, and therefore, the heterostructure becomes a p-p junction as plotted in Fig. 2(e) with the built-in electrical field pointing from BP to MoTe2. As a result, when a positive/negative bias is applied on the BP, the external electrical field strengthens/overcomes the built-in electrical field,



resulting in smaller IF than IR and the rectification ratio smaller than one. Further decreasing the negative Vgs (to around -20V), the Fermi level of MoTe2 moves to the middle of the bandgap, under which case the MoTe2 is intrinsic and the heterostructure becomes a p-i junction. The consequence is the increased resistance of the junction which suppresses the current flow through it. As the Vgs continues increasing to a positive value, the Fermi level of MoTe2 is tuned to the conduction band edge, so that the MoTe2 exhibits n-type behavior. Thus, the heterostructure becomes a typical p-n diode as illustrated in Fig. 2(f) with the built-in electrical field pointing from MoTe2 to BP. When a positive/negative bias is applied on BP, the external electrical field overcomes/strengthens the built-in electrical field, resulting in larger IF than IR and rectification ratio greater than one. As Vgs increases positively, the drain current rapidly increases and the rectification ratio reaches a maximum value at Vgs = 10 V. As Vgs increases from 10V to 60V, the rectification ratio decreases slightly, which can be attributed to the change of the sheet resistance of the MoTe2. The total resistance of the device is the sum of the junction resistance at the interface of MoTe2 and BP, and the sheet resistances of MoTe2 and BP, respectively (the contact resistance can be neglected as compared to the sheet resistance of MoTe2 and BP). Since MoTe2 has a much larger resistance than BP (see in Fig. 1(d) and (e)), the resistance of the heterojunction is mainly affected by the resistance of MoTe2 as it is forward biased. When Vgs increases from 10V to 60V, the |IF| increases slightly due to the saturated n-doping of MoTe2, while the |IR| increases rapidly, causing the slight decrease of the rectification ratio (Figure S6). These experimental results indicate that the MoTe2/BP heterostructure can be externally tuned to either p-n or p-p junction by the gate bias, which further shows reversible rectification behavior. The entire device can be treated as a simple model, which consists of three main parts: MoTe2 FET, MoTe2/BP junction and BP FET. The MoTe2 and BP FETs are two gate-tunable resistors that would not change the direction of rectification, while the junction comprises two inverted diodes connected in parallel which changes the direction of rectification as Vgs changes (Figure S7). Although the polarity tunable diode has been achieved by dual-gated WSe2 devices29,30, such homojunctions require two separated gate bias to control over the built-in voltage, which complicates the device structures. In present MoTe2/BP heterojunction, a global


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back gate is sufficient to implement the effective tunability on the rectification behavior of the heterostructure, which simplifies the device fabrication.

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Figure 2. (a) Transfer curve of MoTe2/BP junction measured between D1 and S1 at Vds= +1 V (red curves) and Vds=-1 V (blue curves). The solid curves are in linear scale and the dashed curves are in semi-log scale. (b) Output curves of MoTe2/BP heterostructure under different Vgs ranging from -20V to 60V in semi-log scales. (c) Output characteristics of MoTe2/BP heterostructure under different Vgs ranging from -60V to - 20V. (d) Current rectification ratio as a function of Vgs at Vds = ± 1V in semi-log scale. Inset is the current at Vds = ± 1V with respect to Vgs. (e) Band alignment of MoTe2-BP p-p heterojunction. (f) Band alignment of MoTe2-BP p-n heterojunction.

In Fig. 3, we further explored the photoresponse of the MoTe2/BP heterostructure. Figure 3(a) and (b) are the dynamic photocurrent of the heterojunction under 532 nm laser illumination with various incident power and zero junction bias. In both cases, the



absolute values of the photocurrent increased as the incident power. However, when the heterostructure formed p-n junction at Vgs= 30 V (as shown in Fig. 3(a)), the photocurrent was negative, while it became positive when the heterostructure formed p-p junction at Vgs=-50 V (as shown in Fig. 3(b)). The observed opposite photoresponse of p-n and p-p junctions is consistent with the inversible rectification behavior of the heterostructure, which is attributed to the opposite direction of the built-in electrical field. The responsivity of the p-n junction was calculated to be up to 0.2 A/W under the light illumination power of 2.4 mW/cm2. The external quantum efficiency (EQE) reached 48.1%, which was calculated by the formula 𝐸𝑄𝐸 = 𝐼𝑝ℎ/𝑃𝑖𝑛(ℎ𝑐 𝑞𝜆), where h, c, q and 𝜆 are Plank’s constant, the speed of light, electron charge and the wavelength of laser, respectively. We also measured the response and recovery time of the device to light illumination (Figure S8), which was as fast as 2 ms and 4 ms, respectively. Figure 3 (c) and (d) depict the output curves of the p-n (at Vgs= 30 V) and p-p junctions (at Vgs=-50 V) under different incident power. In accordance with the dynamic photocurrent, the short circuit current (Isc) observed in the output curves of the heterostructure were negative or opposite when the heterostructure formed p-n or p-p junctions, respectively, under different gate bias. The corresponding open circuit voltage (Voc) became positive or negative as the gate bias was positive or negative, respectively. With different incident powers, the Isc and Voc ranged from ±100 pA and ±50 mV, respectively. The excellent photovoltaic characteristics are attributed to the efficient separation of the photon-excited electron-hole pairs, indicating the strong built-in field formed between MoTe2 and BP. Figure 3(e) and (f) illustrate the band structures of the p-p and p-n junctions under light illumination. When laser irradiates on the device, a large number of electron-hole pairs are generated. For p-n junction, the direction of the built-in electrical field is from MoTe2 to BP (see Fig. 3(f)), and thus, the photon excited electrons/holes move towards MoTe2/BP, respectively, resulting in a negative Isc and a positive Voc. In contrast, for p-p junction, since the direction of the built-in electrical field is from BP to MoTe2 (see Fig. 3(e)), the photon excited electrons/holes move towards BP/MoTe2, respectively, leading to a positive Isc and a negative Voc. These experiment results indicate that the polarity of the photoresponse of the MoTe2/BP heterostructure can be modulated by external gate voltage.


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Figure 3. (a) Dynamic photocurrent response under different illumination power at Vgs= 30 V. (b) Dynamic photocurrent response under different illumination power at Vgs= -50 V. (c) Ids-Vds curves measured in dark and at various incident power (Vgs=30 V). Inset is the open voltage (Voc) and short current (Isc) at different power. (d) Ids-Vds curves measured in dark and at various incident power (Vgs=-50 V). Inset is the Voc and Isc at different power. (e) Band alignment of MoTe2-BP p-n heterojunction under illumination. (f) Band alignment of MoTe2-BP p-p heterojunction under illumination.

In Fig. 4, we further studied the dependence of the device photocurrent on gate bias. As shown in Fig. 4(a), the Isc (Voc) of the heterostructure changes from positive (negative)



to negative (positive) as the back gate voltage sweeps from -60 V to 60 V. The reversed photoresponse was attributed to the reversion of the direction of the built-in electrical field in the MoTe2/BP heterostructure as gate bias increased. Figure 4(b) shows the change of rectification ratio and photocurrent as a function of the back gate bias. Within the p-p junction regime (back gate voltage ranged from -60V to -20V), the photocurrent was positive but decreased as the back gate voltage increased, which indicated that the built-in electrical field decreased as the gate bias increased from -60 V to -20 V. The rectification ratio also decreased in this region as gate bias increased. Within the p-n junction regime (back gate voltage ranged from -20V to 60V), the photocurrent became negative as the direction of the built-in electrical field inversed. The photocurrent increased as the gate bias increased, indicating the increased built-in electrical field, and thus, increased rectification of ratio. These results indicate that the photoresponse of the heterostructure can be actively controlled and gradually tuned by the electrical gate, which can be a promising adaptive photodetector/voltaic device.

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Figure 4. (a) Ids-Vds curves measured at various back gate bias under 532 nm light illumination with incident power of 2.4 mW/cm2. (b) Rectification ratio and photocurrent as a function of Vgs. Conclusion In summary, we demonstrated a MoTe2/BP heterostructure showing promising gate tunable properties. The heterostructure can be tuned to be either p-p or p-n junction by


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electrostatic gating. As a result, the direction of both rectification and photovoltaic characteristics of the device can be effectively tuned by gate bias. Such interesting behaviors were well explained by the gate-tunable band alignment of MoTe2 and BP. Furthermore, we observed excellent optical properties including high responsivity and EQE with fast response time of the device, which were induced by the large built-in electric field formed between MoTe2 and BP. Our study may provide a better understanding of 2D heterostructures for designing novel functional devices.

Experimental Section Device Fabrication: Mechanically exfoliated BN flakes were transferred onto a 285 nm SiO2/n+ doped Si substrate. Then, MoTe2 and BP flakes were mechanically exfoliated from bulk materials and transferred onto the SiO2/Si substrate covered with BN. Electrodes were patterned by electron beam lithography (EBL) with positive Ebeam resist. 10/30 nm Cr/Au was deposited by E-beam evaporation after exposure and development, which was followed by a standard lift-off process to complete the fabrication.

Characterizations: We used commercial Raman spectrometer (Renishaw, Inc.) to obtain the Raman spectroscopy with a 532 nm laser source. The AFM images were taken with Bruker Dimension Icon. The device measurements were performed using an Agilent B1500A semiconductor parameter analyzer in ambient air.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. AFM images of device. (S1); Raman spectrum of the MoTe2/BP heterostructure. (S2); Transfer characteristics of the MoTe2 on SiO2 and BN substrates. (S3); Output characteristic curves of MoTe2 and BP transistor. (S4); Rectification ratio as a function of back gate bias of different samples. (S5); Transfer characteristics of MoTe2 transistor at Vds = 1V and Vds = -1V. (S6); Equivalent electrical circuit model of the MoTe2/BP heterostructure. (S7); Dynamic photoresponse of the MoTe2/BP heterostructure. (S8); Table S1: Review of gate-tunable optoelectronic properties based on 2D material heterojunction.



Conflicts of interest There are no conflicts to declare.

Acknowledgments This work is supported by National Key R&D Program of China (2018YFA0307200), National Science Foundation of China (No. 21405109) and Seed Foundation of State Key Laboratory of Precision Measurement Technology and Instruments, China (No. Pilt1710).

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