NIR self-powered photodetection and gate tunable rectification behavior in 2D GeSe/MoSe2 heterojunction diode

Two-dimensional (2D) heterostructure with atomically sharp interface holds promise for future electronics and optoelectronics because of their multi-functionalities. Here we demonstrate gate-tunable rectifying behavior and self-powered photovoltaic characteristics of novel p-GeSe/n-MoSe2 van der waal heterojunction (vdW HJ). A substantial increase in rectification behavior was observed when the devices were subjected to gate bias. The highest rectification of ~ 1 × 104 was obtained at Vg = − 40 V. Remarkable rectification behavior of the p-n diode is solely attributed to the sharp interface between metal and GeSe/MoSe2. The device exhibits a high photoresponse towards NIR (850 nm). A high photoresponsivity of 465 mAW−1, an excellent EQE of 670%, a fast rise time of 180 ms, and a decay time of 360 ms were obtained. Furthermore, the diode exhibits detectivity (D) of 7.3 × 109 Jones, the normalized photocurrent to the dark current ratio (NPDR) of 1.9 × 1010 W−1, and the noise equivalent power (NEP) of 1.22 × 10–13 WHz−1/2. The strong light-matter interaction stipulates that the GeSe/MoSe2 diode may open new realms in multi-functional electronics and optoelectronics applications.


Scientific Reports
| (2021) 11:3688 | https://doi.org/10.1038/s41598-021-83187-z www.nature.com/scientificreports/ consideration will still be given to matching the band alignment for electron or hole transfers between bulk materials for the development of photodetector creation. By combing two semiconductors, the study of heterojunctions with the same lattice structures has become a hot topic in semiconductor technology. Recently, scientists have paid a great deal of attention to fabricating heterojunctions on graphene-like materials or heterojunctions by combining two semiconductors/semimetals, such as graphene-h-BN 7 , graphene-MoS 2 8,9 , and graphene-MoSe 2 [10][11][12] . On the other hand, some heterojunctions are designed based on TMDs materials, which include [2D/2D] structures via either mechanical exfoliation or vapor deposition methods that include MoSe 2 /WSe 2 heterojunction 13 , MoSe 2 /WS 2 heterostructures 14 , MoS 2 /black phosphorus heterojunction 15 , p-type GaSe/n-type MoSe 2 16 , [2D/1D] black phosphorus-zinc oxide nanomaterial heterojunction 17,18 , n-2D/p-oxide, and p-2D/n-oxide structures 17,[19][20][21] , such as vertical MoSe 2 -MoO x 22 . These novel semiconducting [2D/2D] TMDs/TMDs are now a primary focus of many researchers. Several TMDs based materials are under fabrication process to explore new physics and the next-generation photonic/ optoelectronic devices. In the present study, we demonstrated the p-GeSe/n-MoSe 2 heterojunction self-powered photodiodes. We observed good gate-tunable rectification characteristics of the p-GeSe/n-MoSe 2 heterojunction p-n diode. The high ratification ratio of 1.4 × 10 4 is obtained at V bg = − 40 V. The photovoltaic behavior of the p-GeSe/n-MoSe 2 heterojunction p-n diode at zero bias was investigated under various intensities of (53.3, 98.5, 123, and 139 mW/cm 2 ) with NIR (850 nm) incident photons. The high responsivity (R = 465 mAW −1 ), the detectivity D of 7.3 × 10 9 Jones, the normalized photocurrent to dark current ratio NPDR of 1.9 × 10 10 W −1 , the noise equivalent power NEP of 1.22 × 10 -13 WHz −1/2 , and the external quantum efficiency EQE of 670% were observed with a fast response time of 180 ms.The strong light-matter interaction in the device explicitly suggests that the p-GeSe/n-MoSe 2 heterojunction p-n diode is a promising candidate for optoelectronics technologies.

Experimentation
We prepared all the p-GeSe and the n-MoSe 2 atomically thin flakes by peeling them from their parent bulk crystals using a scotch tape mechanical exfoliation technique, which is similar to the technique that is employed for the exfoliation of graphene 23,24 , and we transferred it onto a Si/SiO2 (300 nm) substrate using a transparent poly (dimethylsiloxane) (PDMS) stamp using an aligned dry transfer 25,26 . The multilayer p-GeSe and the n-MoSe 2 flakes were identified using an optical microscope, and the multilayer n-MoSe 2 was directly stacked on the top of the p-GeSe flake. Raman spectroscopy and atomic force microscopy (AFM) were also conducted. Electron beam lithography was used for the metal deposition of palladium/gold (Pd/Au:10/20 nm) and (Cr/Au:10/20 nm) onto the p-GeSe and the n-MoSe2, respectively. The lift-off processes were conducted to form electrodes on the multilayer p-GeSe and n-MoSe2 flakes. The electrical characterization at room temperature were exhibited using a Keithley 4200A-SCS parameter analyzer. The photovoltaic characteristics of the p-GeSe/n-MoSe 2 heterojunction photodetector was performed using a continuous wave laser beam from a diode NIR laser (850 nm) that was directly illuminated onto the device.

Result and discussions
A schematic illustration of the demonstrated p-GeSe/n-MoSe 2 heterostructure device is depicted in Fig. 1a. The optical image of the p-GeSe/n-MoSe 2 heterostructure is shown in Fig. 1b. The atomically thin flakes of the p-GeSe and the n-MoSe 2 were peeled from their parent bulk crystals using a scotch tape mechanical exfoliation technique, which was transferred on a 300 nm SiO 2 /Si substrate 27 . A few-layers of the n-MoSe 2 were directly stacked on the top of the p-GeSe nanoflake by precisely determining their locations and an overlapping heterojunction region was formed. To clean the surface and optimize the charge carrier, the flakes of the p-GeSe and the n-MoSe 2 were annealed at 200 °C for 1 h in an argon environment 28 . The Pd/Au (10/20 nm) and the Cr/Au (10/20 nm) optimum metal electrodes were then designed onto the p-GeSe and the n-MoSe2 flakes. Figure 1c-f shows the thickness of the p-GeSe (n-MoSe 2 ), which is ~ 8 nm (~ 6 nm), and their height profiles that were measured by the atomic force microscopy (AFM) analysis. Raman spectroscopy was used to confirm the material p-type GeSe and the n-type MoSe 2 shown in Figure S1(a). The p-GeSe/n-MoSe 2 heterostructure device was electrically characterized at room temperature by applying drain to source voltage (V ds ) and electrostatic backgate voltage (V g ). To validate the doping nature of the material GeSe and MoSe 2, the back-gate voltage (V g ) was swept from − 40 to + 40 V at constant V ds = 2 V, and the transfer characteristics revealed that the material GeSe (MoSe 2 ) exhibited a p-type (n-type) nature with an ON/OFF ratio of 2.42 × 10 3 (1.28 × 10 3 ), which is depicted in Fig. 2a, b. The semiconductor material and the metal interface could exhibit either ohmic behavior or rectifying behavior depending on the semiconductor and the metals working function values. Hence, we used optimum metal contacts for Pd (Φ ~ 5.6 eV) 29 Cr (Φ ~ 4.5 eV) p-GeSe(n-MoSe 2 ) 30,31 . Figure 2c shows ohmic behavior with a high work function of the Pd. To induce ohmic behavior between the metal and the n-MoSe 2 , we used the low work function of the Cr depicted in Fig. 2d. The field-effect carrier mobility ( µ FE ) of the p-GeSe and the n-MoSe 2 was calculated using the following equation 32-34 . where W is the channel width, L is the channel length, C bg is the gate capacitance (~ 115 aF/µm 2 ) for the SiO 2 substrate, and ( dI ds dV bg ) is the slope of the transfer curve. The mobilities of the p-GeSe and the n-MoSe 2 were estimated to be 110 cm 2 V −1 s −1 and 85 cm 2 V −1 s −1 , respectively.
The gate-tunable electrical characteristics were also investigated. Figure 3a exhibits the gate dependent output characteristics of the p-GeSe/n-MoSe 2 heterostructure diode, and Fig. 3b shows the same output curves in a corresponding logarithmic plot. It revealed that the rectifying behavior of the device is tuned by the electrostatic www.nature.com/scientificreports/ gate-voltage. The forward bias rectifying current increases as the gate voltages (V g ) increased from − V g to + V g , the electrons are attracted to the interface between GeSe and SiO 2 to form accumulation layer results the Fermi level of GeSe moves towards the conduction band and lowering potential barrier height results in decreasing rectification current attributed to electrostatic doping of electrons. Moreover, we investigated the rectification ratio, which is defined as the ratio of the forward current over the reverse current, I f /I r , up to 1.4 × 10 4 at V g = − 40 V. We found that at a positive gate voltage of V g = + 40 V, both the reverse and forward currents increase concurrently, which suppress the rectification as a result. In anticipation of the negative gate voltage, the reverse current is constrained to increase the rectification in the p-GeSe/n-MoSe 2 heterostructure diode, which is depicted in Fig. 3c. Additionally, we estimated the ideality factor to confirm the performance of the rectifying behavior of the p-GeSe/n-MoSe 2 heterojunction diode using the thermionic emission theory 4,35 .
where I S is the reverse bias saturation current,n is the ideality factor, q is the elementary charge, T is the temperature, and K B is Boltzmann's constant. After the interpretation above, the equation becomes  Figure 3d illustrates the ideality factor function of the gate voltage, the ideality factor of 1.1 is obtained at V g = − 40 V, which is close to the ideal diode value (η = 1). The relative degrading of the gate tunable ideality factor is attributed to the surface carrier recombination at the interface of the p-GeSe/n-MoSe 2 diode, which results in a decrease in the electric field 21 . The variation in the electron affinity and the bandgap between the monolayers generates an atomically sharp hetero-interface, and the interface band alignment of the p-GeSe/n-MoSe 2 heterostructure is predicted to be a type II band alignment, which is shown in Fig. 4. Furthermore, we investigated the self-powered photovoltaic characteristics of a p-GeSe/n-MoSe 2 heterostructure device. The self-powered photodetectors are devices that can separate photoexcited carriers by the built-in electrical field at the junctions without any external power source. On this principle, the p-n junctions can be established for the photovoltaics 36,37 . We used an NIR (850 nm) laser with various illumination power intensities (53.3, 98.5, 123, and 139 mW/cm 2 ) to measure the photocurrent generated from the photodiode that was based on the p-GeSe/n-MoSe 2 heterojunction. A strong photoresponse was observed in the p-GeSe/n-MoSe 2 junction region, which showed that a continuous charge separation occurred at the junction. Figure 5a presents the I ds -V ds curves of the p-GeSe/n-MoSe 2 heterojunction in dark and under photon irradiation with wavelength of 850 nm at zero bias with a constant gate voltage (V g = 0). The I ds -V ds curves are shifted down under the irradiation of light, which revealed that the device can be developed for self-powered photovoltaic energy conversion under the action of open-circuit voltage (V oc ). We investigated an open-circuit voltage (V oc ) of 0.349 V and a shortcircuit current (I sc ) of 14.5 nA for the 139 mW cm −2 light intensity. The external quantum efficiency (EQE) was investigated by using the following formula.
where λ is the incident light wavelength, h is the planks constant, and c is the velocity of light. We obtained a value for EQE of 670% in the p-GeSe/n-MoSe 2 diode. The power intensity-dependent EQE is depicted in Fig. 5b. www.nature.com/scientificreports/ Additionally, we also characterized the transient photoresponse of the device. The dynamic photoresponse rise and fall time of the p-GeSe/n-MoSe 2 diode was observed under an NIR laser light irradiation with a wavelength (λ) of 850 nm at various power intensities, which is shown in Fig. 5c. The rise time is the τ r , the time it takes by the device to reach 90% from 10% and the fall time is τ f , the time it takes by the device to decay from 90 to 10% 36,38,39 . We found a rise time of 180 ms and a fall time of 360 ms, which are shown in Fig. 5d. The response time of the device is not as fast as we expected, which may be due to the charge carrier trapping and the longer charge dissociation time [40][41][42] . Moreover, in order to evaluate the performance of the device, several important figures of merits were calculated. For example, responsivity (R), detectivity (D), the normalized photocurrent to a dark current ratio (NPDR), and the noise equivalent power (NEP) with variation of incident light power intensities were calculated. The  www.nature.com/scientificreports/ responsivity ( R = J p /P in ), where J p is the photocurrent density and P in is input power per area, and the detectiv- where q is the elementary charge and J d is the dark current density, are significant facets of the photo detector 36,38,39 , which is shown in Fig. 6a. The greater value of responsivity is attributed to the higher photocurrent 43 . Similarly, the device that has a lower dark current provides a higher detectivity. Thus, the greater values of both R and D are important aspects of an efficient photodetector 37,43,44 . We obtained a high responsivity of R = 465 mAW −1 and detectivity of D = 7.3 × 10 9 Jones.  www.nature.com/scientificreports/ Figure 6b shows the intensity-dependent normalized photocurrent to dark current ratio. The NPDR = R/I d , where R is the responsivity, I d is the dark current, and the noise equivalent power (NEP = 1/(NPDR 2q/I d ).We investigated the values of NPDR of 1.9 × 10 10 W −1 and NEP of 1.22 × 10 -13 WHz −1/2 under the power intensity of 139 mW cm −2 . The NEP revealed that the photodetector, which is based on the p-GeSe/n-MoSe 2 heterostructure, has the capability of detecting power as low as 10 -13 . Additionally, we characterized the spectral responsivity of the p-GeSe/n-MoSe2 heterojunction. The device was subjected to constant illuminating power of 53 mW cm -2 with wavelength ranging from 220 to 850 nm. Figure S1c shows a sharp increase of the spectral response on the short wavelength side is reasonably due to more photon energy absorbed by the device, attributed to more electrons and holes generation under larger photons energy. Table S1 in supplementary information shows the comparative investigated photoresponse and sensitivity of our device based on p-GeSe/n-MoSe 2 heterojunction, which is much higher than the previously reported values. The strong light-matter interaction in the device explicitly suggests that the p-GeSe/n-MoSe 2 heterojunction p-n diode is a promising candidate for optoelectronics technologies.

Conclusions
In summary, we demonstrate a p-GeSe/MoSe 2 based multifunctional HJ p-n diode. The diode explicitly exhibits gate tunable high rectification of ~ 1 × 10 4 at negative gate bias (Vg = − 40 V). The introduction of the ohmic contacts reveals that the rectification behavior of a p-n diode is solely attributed to the sharp interface between metal contacts and GeSe/MoSe 2 . Our device shows high photoresponse at an NIR (850 nm). The high responsivity of 465 mAW −1 , the excellent EQE (670%), the fast rise time of 180 ms, and the decay time of 360 ms were obtained. The device also shows detectivity D of 7.3 × 10 9 Jones, a normalized photocurrent to dark current ratio NPDR of 1.9 × 10 10 W −1 , and a noise equivalent power NEP of 1.22 × 10 -13 WHz -1/2 .The NEP revealed that the photodetector, which is based on the p-GeSe/n-MoSe 2 heterostructure, has the capability of detecting power as low as 10 -13 . These results suggest that p-GeSe/MoSe 2 based multifunctional heterojunction p-n diode may have great potential for electronics and optoelectronics applications as high-performance self-powered photodetectors.