Enhancement of photodetection based on perovskite/mos 2 transistor. hybrid thin film

Transcription

1 Vol. 38, No. 3 Journal of Semiconductors March 2017 Enhancement of photodetection based on perovskite/mos 2 transistor hybrid thin film Fengjing Liu 1; 2, Jiawei Wang 2, Liang Wang 2, Xiaoyong Cai 2, Chao Jiang 2;, 1; and Gongtang Wang 1 School of Physics and Electronics, Shandong Normal University, Jinan , China 2 CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing , China Abstract: Perovskite/MoS 2 hybrid thin film transistor photodetectors consist of few-layered MoS 2 and CH 3 NH 3 PbI 3 film with various thickness prepared by two-step vacuum deposition. By implementing perovskite CH 3 NH 3 PbI 3 film onto the MoS 2 flake, the perovskite/mos 2 hybrid photodetector exhibited a photoresponsivity of 10 4 A/W and fast response time of about 40 ms. Improvement of photodetection performance is attributed to the balance between light absorption in the perovskite layer and an effective transfer of photogenerated carriers from perovskite entering the MoS 2 channel. This work may provide guidance to develop high-performance hybrid structure optoelectronic devices. Key words: perovskite; MoS 2 ; photodetector DOI: / /38/3/ EEACC: Introduction Recently, transition metal dichalcogenide (TMD) twodimensional (2D) materials have received enormous attention Œ1 5. Among these TMD materials, MoS 2 has been widely investigated as field-effect transistors (FETs) Œ6, sensors, and phototransistors because of its excellent electrical and optical properties. Photodetectors based on monolayer MoS 2 Œ7 have exhibited excellent photoresponse up to 880 A/W. On the contrary, multilayer MoS 2 Œ8 has shown lower photoresponsivity ~100 ma/w owing to low absorption coefficient of its indirect bandgap, which may restrain its application for high performance photodetectors. To increase the photo generation rate, stacking MoS 2 with other materials having high optical absorption coefficient has been applied recently Œ9. For instance, Jinsu Pak et al. Œ10 introduced surface treatment with copper phthalocyanine and Kufer et al. Œ11 adopted PbS quantum dots stacked onto MoS 2. Both approaches have been demonstrated to be useful methods for enhancement of the photodetection of MoS 2 field effect devices. On the other hand, methylammonium lead halide perovskites (e.g. CH 3 NH 3 PbX 3, X D I, Br, Cl) have achieved very high power conversion efficiency larger than 20.1% in perovskite solar cells due to their superior optical-electric property, such as large light absorption and long electron-hole diffusion lengths Œ In addition, perovskite material has also been used as an active material in photodetectors such as perovskite/graphene Œ15; 16 and perovskite/ws 2 heterostructures Œ17. The combination of MoS 2, which features great electrical properties, and CH 3 NH 3 PbI 3 perovskite, is the key point to realize a high-efficiency photodetector. In this study, we paid much attention to optimize the formation conditions for perovskite/mos 2 hybrid structure. In particular, a two-step, all-vacuum perovskite formation procedure guaranteed the complete chemical reaction to obtain a stoichiometric as-prepared perovskite/mos 2, and a photodetector based on an optimized device structure exhibited a high photoresponsivity of 10 4 A/W and fast response time of about 40 ms. 2. Experiments A heavily n-doped Si wafer and thermally grown 300 nm thick SiO 2 layer were employed as the gate electrode and dielectric, respectively. Few-layer MoS 2 film was produced from a bulk crystal by mechanical cleavage and subsequently transferred onto the substrate. The thickness of the MoS 2 flake and its uniformity were confirmed by atomic force microscope (AFM) with a tapping mode and optical microscopy, respectively. Electron beam lithography (EBL) was utilized to make patterns for the source/drain electrodes by conventional lithography using PMMA resist and then Cr/Au (10 nm/50 nm) was deposited by thermal evaporation. After finishing the MoS 2 FETs device fabrication, all devices were annealed at 180 ı C for 180 min in Ar to improve contacts and remove organic residues. A 75 m aperture grid was used to confine perovskite deposition within a minimal region. Two-step vacuum deposition process of perovskite layers was adopted. Three thicknesses of 50, 100 and 200 nm PbI 2 were deposited with precise control of thickness through vacuum thermal deposition (Auto-306, Edward Co.). The deposition rate was calibrated and monitored by using a quartz oscillator. Then different PbI 2 thickness layers reacted with CH 3 NH 3 I vapor in a tube furnace * Project supported by the National Natural Science Foundation of China (Nos , ) and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA ). Corresponding author. Jiangch@nanoctr.cn, wanggt@sdnu.edu.cn Received 7 November 2016, revised manuscript received 30 November Chinese Institute of Electronics

2 J. Semicond. 2017, 38(3) Fengjing Liu et al. Fig. 1. (Color online) (a) Schematic of the perovskite/mos2 photodetector. The molecular structures of the MoS2 and perovskite are also shown. (b) Optical images of a fabricated MoS2 FET device and perovskite/mos2 hybrid photodetector, with perovskite stacking on the MoS2 FET. The AFM image of the MoS2 FET channel is also shown. (c) X-ray diffraction (XRD) spectra of the CH3 NH3 PbI3 perovskite on SiO2. (d) SEM picture of CH3 NH3 PbI3 deposited on MoS2. at 120 ıc for 30, 60 and 120 min, respectively. Crystallinity of the perovskite film was evaluated using an X-ray diffraction (XRD) system with 2 ranging from 10 to 50. The morphology of perovskite deposited on MoS2 was also characterized by the scanning electron microscope (SEM) (Zeiss Merlin). under a dark ambient environment using UV/vis spectrometer Lambda 650 (PerkinElmer). The electrical properties of the MoS2 FET devices before and after they were stacked with perovskite layers were measured at room temperature using a Keithley 4200 semiconductor analyzer in a vacuum TTP4 probe station (Lake Shore). The optoelectronic properties were measured using a 450 nm laser, and the light intensity was varied by a continuous attenuator. The steady-state photoluminescence (PL) of the samples was measured using a Princeton Instruments Acton SpectraPro SP2500 apparatus. Transmittance measurement was performed Fig. 1(a) presents the schematics of perovskite/mos2 hybrid structure and the molecular structures of the MoS2 and CH3 NH3 PbI3. The optical images of MoS2 FET fabricated on a SiO2 /Si substrate are shown in Fig. 1(b). Thickness of the MoS2 flake channel is found to be 4.5 nm, as indicated in Fig. 1(b) which corresponds to 7 layers of the MoS2 filmœ18. The XRD pattern is shown in Fig. 1(c) with sharp and strong diffusion peaks, suggesting good purity and crystallinity of the perovskite films. Fig. 1(d) presents an SEM image of the top 3. Results and discussion

3 Fig. 2. (Color online) (a) I ds V g curves of the MoS 2 devices without (black) and with (red) 100 nm perovskite deposition measured at a fixed V ds D 2 V. (b) I ds V g curves measured at a fixed V ds D 2 V under dark and illuminated conditions (wavelength D 450 nm) at different laser intensities. Insert is the partial enlarged drawing from V g D 5 V to V g D 10 V. (c) Photocurrent of four devices with different perovskite thickness measured at V g D 5 V and V ds D 2 V. (d) Photoresponsivity of four devices as a function of incident laser power. Table 1. Summary of the thickness and electrical properties of MoS 2 FETs. Device PbI 2 thickness Mobility Responsivity (nm) (cm 2 V 1 s 1 / (A/W) # # # # surfaces of perovskite film deposited on MoS 2, which reveals the perovskite film is composed of small uniform particles with full coverage of the substrate. Fig. 2(a) shows the transfer characteristics of the MoS 2 FETs stacked with various thicknesses of perovskite layers. The FET exhibited n-type behavior for a pristine MoS 2 FET and the mobility (/ of the MoS 2 FET was around 20 cm 2 V 1 s 1, which was estimated from the linear region in the I d V g curve with the Eq. (1) D di d L ; (1) dv g W C i V d where L is the channel length, W is the channel width and C i is the capacitance between the channel and the back gate per unit area C i D " 0 " r /t: Here, " 0 is the vacuum permittivity " r is the relative permittivity, and t is the thickness of SiO 2 layer (300 nm). Comparing with the pristine MoS 2 FET, hybrid perovskite/mos 2 FET exhibited similar mobility, but the threshold voltage shifted from 22 to 7 V, which can be explained by the effects of trapping and dielectric screening coming from the perovskite layer on MoS 2. A large density of trap states in perovskite films has been observed in several studies ΠWhen a perovskite layer is stacked onto MoS 2, the electrons in MoS 2 can be captured by traps at the interface of the perovskite layer, and this results in a reduction of electron concentration in MoS 2 channel leading to a shift of threshold voltage. What is more, the dielectric environment of MoS 2 channel can be influenced when perovskite stacking onto the MoS 2, which will also influence the MoS 2 FET electrical properties. We studied the photo-response characteristic for the perovskite/mos 2 hybrid device under different light intensity of a 450 nm laser. The results are shown in Fig. 2(b). We find that the photocurrent increases monotonically with the increasing of light intensity. To survey the influence of perovskite thickness on photodectection properties, we fabricate three different kinds of devices with different perovskite thickness. For eliminating the influence of different MoS 2 property, MoS 2 FET devices with almost the same thickness and mobilities were selected. The power-dependent photocurrents (I ph D I illuminated

4 Fig. 3. (a) UV vis spectra of three different perovskite thickness devices. (b) Steady-state photoluminescence (PL) spectra of device 2, device 3, device 4. The black line is the PL from perovskite area; the red line is from perovskite/mos 2 area. Fig. 4. (a) Stability test of photoswitching characteristics of perovskite/mos 2 photodetector at V gs D 5 V, V ds D 2 V, P light D 5:9 mw/cm 2. (b) The photoresponse curves of the device and the fitted lines (red) rising and (blue) decaying response curves. I dark / are presented in Fig. 2(c) for all devices with different perovskite thickness. While all photocurrent increases with the intensity increase of incident light power, the 100 nm thickness device shows the highest I ph in the whole measurement region. Photoresponsivity (R/ is an important factor to identify the performance of photodetectors and is defined as follows: R D I ph P in A ; (2) where I ph is the photocurrent, P in is the incident irradiation power, A is the active area. Fig. 3(d) shows the photoresponsivity of the devices operating under a source drain bias of 2 V and V g D 5 V under various illumination intensities. It is obvious that the photoresponsivities of the perovskite/mos 2 hybrid devices are much better than pristine MoS 2 device. Among four samples, the hybrid sample with 100 nm perovskite shows the highest photoresponsivity of 10 4 A/W. We suppose two key factors may influence the photoresponsivity of perovskite/mos 2 hybrid devices, e.g. light absorption and vertical transport of photogenerated carriers in perovskite

5 layer. When incident light is weak, light is mainly absorbed by the top surface of perovskite film, the photogenerated carriers will recombine during the transition from perovskite to MoS 2 channel. The thicker perovskite film is, the more recombination occurs. However, when incident light is strong enough, the total perovskite film can absorb light efficiently, namely, the thicker perovskite film, the more photogenerated carriers. Considering these two factors, 100 nm perovskite sample (device #3) shows the highest photoresponsivity under low light intensity. On the contrary, 200 nm perovskite sample (device #4) shows larger photoresponsivity under high light intensity as evidenced in Fig. 2(d). The properties of these perovskite/mos 2 devices were also summarized in Table 1. To further investigate the influence of perovskite thickness on photodetector characteristic, we performed UV vis and photoluminescence (PL) analyses on perovskite/mos 2 devices. Fig. 3(a) shows the UV vis absorption spectra of the three different thickness samples deposited by the same processing procedure other than the deposition duration. From the UV vis absorption spectra, we find the absorbance of device 4 with 200 nm perovskite film is slightly higher than device 3 with 100 nm and much higher than device 2 with 50 nm perovskite films. To study the efficiency of charge transport from different thickness perovskite to MoS 2 film, we have conducted steady-state PL measurements. As illustrated in Fig. 3(b), the thinner perovskite film, the more efficient PL quenching shows that a larger proportion of photogenerated carriers are transferred from the perovskite in thinner samples to the MoS 2 layer Œ22. Although the thicker samples can generate more electron-hole pairs, the photo-generated carriers may recombine before they reach the interface of MoS 2 and perovskite, and have no contribution to photocurrent. For our experimental results, the hybrid device with 100 nm perovskite exhibited the best photoresponsivity. Finally, we performed photo response measurements. Fig. 4(a) shows the time-dependent photoresponse properties of 100 nm perovskite device. The photoswitching characteristic of the 100 nm device was investigated under illumination with 450 nm laser wavelengths. The photodetectors exhibited good on-off switching, indicating the excellent reproducibility of the photodetectors. The temporal photoresponse curves were shown in Fig. 4(b). We extracted rising and decay time based on the following relationship: I D I max.1 e t r /; (3) I D I max e t d ; (4) where I is photocurrent, I max is a maximum photocurrent, t is time, and r and d are rising and decaying time, respectively. The results reveal a rise time of about 40 ms and a decay time of about 50 ms. 4. Conclusion In summary, we have fabricated perovskite/mos 2 hybrid FET photodetectors by utilizing two-step vacuum deposition of perovskite onto MoS 2. By optimizing the thickness of perovskite layers, we achieve a photoresponsivity of 10 4 A/W and fast response time of about 40 ms. Absorption spectroscopy and the PL analysis indicated a balance between light absorption and effective transfer of photogenerated carriers from perovskite to the MoS 2 channel is the key factor to influence the performance of hybrid photodetectors. References [1] Sangwan V K, Jariwala D, Kim I S, et al. Gate-tunable memristive phenomena mediated by grain boundaries in single-layer MoS 2. Nat Nanotech, 2015, 10(5): 403 [2] Lopez-Sanchez O, Lembke D, Kayci M, et al. Ultrasensitive photodetectors based on monolayer MoS 2. Nat Nanotech, 2013, 8(7): 497 [3] Gong Y J, Lin J H, Wang X L, et al. Vertical and in-plane heterostructures from WS 2 /MoS 2 monolayers. 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