Letter Cite This: ACS Photonics 2018, 5, 2693−2700
Atomic Layer GaSe/MoS2 van der Waals Heterostructure Photodiodes with Low Noise and Large Dynamic Range Arnob Islam, Jaesung Lee, and Philip X.-L. Feng* Department of Electrical Engineering & Computer Science, Case School of Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States
ACS Photonics 2018.5:2693-2700. Downloaded from pubs.acs.org by UNIV OF TEXAS SW MEDICAL CTR on 10/09/18. For personal use only.
S Supporting Information *
ABSTRACT: We report on the demonstration of atomic layer van der Waals (vdW) heterostructure photodiodes operating in the visible regime, enabled by stacking single- to few-layer ntype molybdenum disulfide (MoS2) on top of few-layer p-type gallium selenide (GaSe) crystals. The atomic layer vdW photodiode exhibits an excellent photoresponsivity of ∼3A/W at the wavelength of 532 nm when symmetric few-layer graphene (FLG) contacts with low contact resistance are used. On the other hand, for a GaSe/MoS2 photodiode with asymmetric GaSe/FLG and MoS2/gold (Au) contacts, a very low noise equivalent power of NEP ∼ 10−14 W/ Hz is obtained due to dark current reduction, which demonstrates the feasibility of detecting sub-pW ( 102, owing to very low
i P y DR = 10 logjjj sat zzz k NEP {
(1)
where Psat is the optical power at which the photocurrent starts to deviate from linear characteristics and saturate upon increasing optical illumination, and NEP is noise-equivalent power. NEP can be expressed as5 NEP =
in 9
(2) 1/2
where in = (2eBIdark) is shot noise, which is a fundamental noise source for the measurable Idark. Figure 4c and f show the Iph versus P for devices #1 and #2, from which we are able to obtain Psat and calculate DR using eq 1. We obtain a DR of ∼70 dB for device #2, which is significantly better than values reported in the literature for 2D photodetectors.28,29 We obtain a very low NEP for device #2 under forward bias. At 2 V bias, we achieve NEP = 10−14 W/ Hz (Figure 5a). This indicates that, in principle, we will be able to detect sub-pWlevel optical illumination using this photodiode. We also calculate normalized photocurrent to dark current ratio (NPDR), which is defined as30 NPDR ≡
Iph /Idark P
=
1 NEP
2e Idark
(3)
We obtain an NPDR value on the order of 108 mW−1 for nW-level optical illumination. Thus, this photodiode provides an ideal platform for detecting ultra-low-level illumination. We compute the detectivity of the photodiodes by using the following equation:6 2697
DOI: 10.1021/acsphotonics.8b00318 ACS Photonics 2018, 5, 2693−2700
Letter
ACS Photonics
Figure 6. Photocurrent mapping and wavelength-dependent photoresponse. (a) Photocurrent mapping on device #3 with a 532 nm laser (VD = 1.8 V, VG = −10 V, P = 100 nW). (b) Comparison of responsivity for device #3 under 532 and 405 nm laser illumination (VD = 2 V, VG = −10 V), where the red circle indicates the position of the laser spot. Inset shows the optical microscopy image of device #3. Scale bar: 10 μm.
Table 1. Comparison of Device Performance with Other Atomically Thin Photodiodes in the Literature van der Waals heterostructure
thickness
9 (mA/W)
NEP (W/ Hz )
DR (dB)
response time (ms)
ref
p-MoS2/n-WSe2 p-black phosphorus/n-MoS2 p-WSe2/n-WSe2 p-GaSe/n-MoSe2 p-GaSe/n-MoS2 (CVD) p-GaSe/n-MoS2
1L/1L 11 nm/1L 1L 1L/1L 3L/1L 3L/1L 5−6L/1L ∼15 to 20L/3L
11 (@532 nm) 420 (@633 nm) 210 (@532 nm) 30 (White light) ∼60 (@300 nm) ∼50 (@532 nm) ∼3000 (@532 nm) ∼350 (@532 nm)
N.A. N.A. N.A. N.A. N.A. ∼10−14 ∼10−13 ∼10−12
N.A. N.A. N.A. N.A. N.A. 70 63 54
N.A. N.A. N.A. N.A. 80 50 (extrinsic) N.A. N.A.
9 12 33 34 35 this work
D* = 9 × (AB)1/2 /in
(∼2.3 eV) laser, although we obtain lower absorption in GaSe, higher absorption in MoS2 occurs. In the ideal case, for GaSe/ MoS2 heterostructures, peak responsivity wavelength will be determined by peak responsivity of the material that has a larger number of layers. On the other hand, MoS2 has a direct band gap in its single-layer form, which can absorb more than few-layer GaSe (indirect band gap) does, although it is thinner compared to GaSe. On top of that, multilayer interference at the MoS2/GaSe/SiO2/Si stack can also play an important role in determining the absorption, and thus photocurrent, which will make the situation more complex. Therefore, in reality, peak responsivity of a heterostructure will be determined by the interplay of all these effects. In order to investigate this, we use a 405 nm laser (3.07 eV) to conduct photocurrent measurement and compare the photoresponse with 532 nm laser illumination for device #3 (1L MoS2 and 5−6L GaSe). We have found that using 405 nm laser illumination, we obtain 2−3 times higher responsivity at different incident power levels (Figure 6b). It is also worth mentioning that for device #3, the measured responsivity (∼3A/W) is significantly higher than those of previous devices, although its comparatively high Idark limits its DR (63 dB) and NEP (∼10−13W/ Hz ). The detailed photoresponse of this device is presented in Supporting Information, S3. In Table 1, we compare the performance of our photodiodes with other atomically thin photodiodes (homojunction and heterojunction) reported. For fair comparison, we only focus on the heterostructures where at least one of the constituting materials is atomically thin (i.e., one-layer to three-layer). We find that in terms of 9 our device has similar or better performance compared to that of existing vdW heterostructures. In other important aspects such as NEP, DR, and
(4)
where A is the effective area of the photodetector and B is bandwidth. The unit of detectivity is Jones (cmHz1/2/W). For device #2, the active area (A) of the device is ∼100 μm2. By operating device #2 in forward bias, we achieve the highest D* for the photodiode, which is on the order of 1010 Jones (Figure 5b). Furthermore, we also characterize the dynamic photoresponse of device #2. We find that the rise time (τr) of the device is ∼50 ms (Figure 5c,d). The response time of the device is not as fast as we expect from an ideal 2D vdW photodiode, owing to the fact that the measured response time of device #2 is not yet limited by the vdW heterostructure itself. The observed response time is first limited by the time constant of the external low-noise preamplifier. Next it will be limited by the RC time constant set by the device structure, as carriers are still trapped during the lateral transport from the heterojunction to the metal contacts. Response time can be improved first by improving measurement scheme and readout integration, and then by making truly vertical vdW heterostructure with only out-of-plane photogenerated carrier extraction considering faster carrier transit time. In order to spatially observe the photocurrent contribution from the different locations of the photodiode, e.g., bare MoS2 or GaSe, MoS2/GaSe, MoS2/FLG or GaSe/FLG, we perform photocurrent mapping with a 532 nm laser (∼1 μm spot size) on device #3. From the mapping results (Figure 6a), we have found that the maximum photocurrent contribution originates from the MoS2/GaSe heterostructure region. In the literature, on a SiO2/Si substrate, GaSe and MoS2 have peak responsivities around 3.3 and ∼2.2 eV light illumination, respectively.31,32 Therefore, using a 532 nm 2698
DOI: 10.1021/acsphotonics.8b00318 ACS Photonics 2018, 5, 2693−2700
Letter
ACS Photonics
photocurrent measurements are performed in moderate vacuum (10−20 mTorr) and at room temperature.
response time, this study has yielded excellent performance in metrics that have not been attained in previous studies. Recently there has been a study of a GaSe/MoS 2 heterostructure photodetector,35 which shows a responsivity of ∼60 mA/W and response time of ∼80 ms, comparable to those of device #2 in this work. However, the linear dynamic range of that device should be lower than that of device #2 in this work, due to the smaller Iph/Idark ratio.35
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b00318.
■
CONCLUSION In summary, we have demonstrated GaSe/MoS2 vdW heterostructure photodiodes and extensively analyzed their performance metrics and evaluated their figures-of-merit as photodetectors. We have attained vdW heterojunction photodiodes with ultralow noise (10−14 W/Hz1/2) and large dynamic range (DR ≈ 70 dB) for GaSe/MoS2 vdW heterostructures with asymmetric contacts. In addition, these atomic layer vdW photodiodes can also exhibit excellent responsivity (∼3A/W) by using FLG contacts with low contact resistance. The results also show that metal−semiconductor junctions can play a crucial role in suppressing dark current and regulating the photoresponse in order to realize high-performance vdW photodiodes. We believe that this study can serve as a great guideline for designing vdW photodiodes by taking into account the important trade-off between high responsivity and low noise or high DR, imposed by the choice of metal− semiconductor contacts. This study opens new opportunities for employing p−n material pairs from both TMDCs and group III monochalcogenides, to create 2D nanoelectronic, optoelectronic devices, and such devices on flexible substrates.
■
Estimation of thickness of GaSe and MoS2 flakes by using Raman spectroscopy and gate tuning of the photodiode (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Arnob Islam: 0000-0002-3931-1506 Jaesung Lee: 0000-0003-0492-2478 Philip X.-L. Feng: 0000-0002-1083-2391 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We acknowledge the support from a National Science Foundation CAREER Award (ECCS #1454570). Part of the device fabrication was performed at the Cornell Nanoscale Science and Technology Facility (CNF), a member of the National Nanotechnology Infrastructure Network (NNIN), supported by the National Science Foundation (ECCS #0335765).
■
METHODS We fabricate vdW heterostructures by using completely dry, wet-chemical-free transfer techniques. In this process, we exfoliate GaSe and MoS2 flakes from their bulk crystals onto polydimethylsiloxane (PDMS) stamps and choose candidate flakes under an optical microscope. After that, we stack the chosen flakes on prepatterned substrates with electrodes (Au) together, to form vdW heterostructures by using proper alignment with micromanipulators in a transfer stage under an optical microscope. After fabricating the devices, we thermally anneal them at 250 °C for 1 h under constant N2 flow in moderate vacuum (∼20 mTorr).36 For Raman and photoluminescence (PL) measurements, a 532 nm diode laser is focused on the sample using a 50× microscope objective and scattered light (for Raman), and photoluminescence from the sample is collected in backscattering geometry and is then guided to a spectrometer (Horiba iHR550). Grating settings of 2400 and 1200 g mm−1 are used for Raman and PL, respectively. The signals are recorded using a liquid-nitrogen-cooled CCD. Spot size of the laser is optimized to be ∼1 μm. Laser power on the sample is kept at ∼200 μW during Raman and PL measurements. For low-noise electrical measurements of the fabricated vdW diodes, a Keithley-4200 SCS semiconductor parameter analyzer is employed. A 532 nm diode laser with ∼1 μm spot size is used for optical illumination. For photocurrent measurements, we use a low-noise current preamplifier (SR570) with a data acquisition card, which is controlled by a custom-made LabView program. For dynamic photoresponse measurement, we use a chopper to modulate the laser and record the photoresponse by using an oscilloscope. All
■
REFERENCES
(1) Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Neto, A. H. C. 2D materials and van der Waals heterostructures. Science 2016, 353, aac9439. (2) Geim, A. K.; Grigorieva, I. V. Van der Waals heterostructures. Nature 2013, 499, 419−425. (3) Wang, X.; Xia, F. Van der Waals heterostructures: Stacked 2D materials shed light. Nat. Mater. 2015, 14, 264−265. (4) Xia, W.; Dai, L.; Yu, P.; Tong, X.; Song, W.; Zhang, G.; Wang, Z. Recent progress in van der Waals heterojunctions. Nanoscale 2017, 9, 4324−4365. (5) Britnell, L.; Ribeiro, R. M.; Eckmann, A.; Jalil, R.; Belle, B. D.; Mishchenko, A.; Kim, Y. J.; Gorbachev, R. V.; Georgiou, T.; Morozov, S. V.; Grigorenko, A. N. Strong light-matter interactions in heterostructures of atomically thin films. Science 2013, 340, 1311− 1314. (6) Buscema, M.; Island, J. O.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A. Photocurrent generation with two-dimensional van der Waals semiconductors. Chem. Soc. Rev. 2015, 44, 3691−3718. (7) Fazio, D. D.; Goykhman, I.; Yoon, D.; Bruna, M.; Eiden, A.; Milana, S.; Sassi, U.; Barbone, M.; Dumcenco, D.; Marinov, K.; Kis, A.; Ferrari, A. C. High responsivity, large-area graphene/MoS2 flexible photodetectors. ACS Nano 2016, 10, 8252−8262. (8) Tsai, D. S.; Liu, K. K.; Lien, D. H.; Tsai, M. L.; Kang, C. F.; Lin, C. A.; Li, L. J.; He, J. H. Few-layer MoS2 with high broadband photogain and fast optical switching for use in harsh environments. ACS Nano 2013, 7, 3905−3911. (9) Lee, C.-H.; Lee, G.-H.; Zande, A. M. V. D.; Chen, W.; Li, Y.; Han, M.; Cui, X.; Arefe, G.; Nuckolls, C.; Heinz, T. F.; Guo, J.; Hone, J.; Kim, P. Atomically thin p−n junctions with van der Waals heterointerfaces. Nat. Nanotechnol. 2014, 9, 676−681. 2699
DOI: 10.1021/acsphotonics.8b00318 ACS Photonics 2018, 5, 2693−2700
Letter
ACS Photonics (10) Hong, X.; Kim, J.; Shi, S.-F.; Zhang, Y.; Jin, C.; Sun, Y.; Tongay, S.; Wu, J.; Zhang, Y.; Wang, F. Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures. Nat. Nanotechnol. 2014, 9, 682− 686. (11) Zhang, K.; Zhang, T.; Cheng, G.; Li, T.; Wang, S.; Wei, W.; Zhou, X.; Yu, W.; Sun, Y.; Wang, P.; Zhang, D.; Zeng, C.; Wang, X.; Hu, W.; Fan, H. J.; Shen, G.; Chen, X.; Duan, X.; Chang, K.; Dai, N. Interlayer transition and infrared photodetection in atomically thin type-II MoTe2/MoS2 van der Waals heterostructures. ACS Nano 2016, 10, 3852−3858. (12) Deng, Y.; Luo, Z.; Conrad, N. J.; Liu, H.; Gong, Y.; Najmaei, S.; Ajayan, P. M.; Lou, J.; Xu, X.; Ye, P. D. Black phosphorus−monolayer MoS2 van der Waals heterojunction p−n diode. ACS Nano 2014, 8, 8292−8299. (13) Ganatra, R.; Zhang, Q. Few-layer MoS2: a promising layered semiconductor. ACS Nano 2014, 8, 4074−4099. (14) Wang, F.; Wang, Z.; Xu, K.; Wang, F.; Wang, Q.; Huang, Y.; Yin, L.; He, J. Tunable GaTe-MoS2 van der Waals p−n Junctions with novel optoelectronic performance. Nano Lett. 2015, 15, 7558−7566. (15) Zhou, X.; Cheng, J.; Zhou, Y.; Cao, T.; Hong, H.; Liao, Z.; Wu, S.; Peng, H.; Liu, K.; Yu, D. Strong second-harmonic generation in atomic layered GaSe. J. Am. Chem. Soc. 2015, 137, 7994−7997. (16) Late, D. J.; Liu, B.; Luo, J.; Yan, A.; Matte, H. S. S. R.; Grayson, M.; Rao, C. N. R.; Dravid, V. P. GaS and GaSe ultrathin layer transistors. Adv. Mater. 2012, 24, 3549−3554. (17) Hu, P.; Wen, Z.; Wang, L.; Tan, P.; Xiao, K. Synthesis of fewlayer GaSe nanosheets for high performance photodetectors. ACS Nano 2012, 6, 5988−5994. (18) Kang, J.; Tongay, S.; Zhou, J.; Li, J.; Wu, J. Band offsets and heterostructures of two-dimensional semiconductors. Appl. Phys. Lett. 2013, 102, 012111. (19) Aziza, Z. B.; Henck, H.; Pierucci, D.; Silly, M. G.; Lhuillier, E.; Patriarche, G.; Sirotti, F.; Eddrief, M.; Ouerghi, A. Van der Waals epitaxy of GaSe/Graphene heterostructure: electronic and interfacial properties. ACS Nano 2016, 10, 9679−9686. (20) Yuan, X.; Tang, L.; Liu, S.; Wang, P.; Chen, Z.; Zhang, C.; Liu, Y.; Wang, W.; Zou, Y.; Liu, C.; Guo, N.; Zou, J.; Zhou, P.; Hu, W.; Xiu, F. Arrayed van der Waals vertical heterostructures based on 2D GaSe grown by molecular beam epitaxy. Nano Lett. 2015, 15, 3571− 3577. (21) Du, Y.; Yang, L.; Zhang, J.; Liu, H.; Majumdar, K.; Kirsch, P. D.; Ye, P. D. MoS2 field-effect transistors with graphene/metal heterocontacts. IEEE Electron Device Lett. 2014, 35, 599−601. (22) Wang, T.; Li, J.; Zhao, Q.; Yin, Z.; Zhang, Y.; Chen, B.; Xie, Y.; Jie, W. High-quality GaSe single crystal grown by the Bridgman method. Materials 2018, 11, 186. (23) Zhang, D.; Jia, T.; Dong, R.; Chen, D. Temperature-dependent photoluminescence emission from unstrained and strained GaSe nanosheets. Materials 2017, 10, 1282. (24) Li, X.; Lin, M.-W.; Puretzky, A. A.; Idrobo, J. C.; Ma, C.; Chi, M.; Yoon, M.; Rouleau, C. M.; Kravchenko, I. I.; Geohegan, D. B.; Xiao, K. Controlled vapor phase growth of single crystalline, twodimensional GaSe crystals with high photoresponse. Sci. Rep. 2014, 4, 5496. (25) Nourbakhsh, A.; Zubair, A.; Dresselhaus, M. S.; Palacios, T. Transport properties of a MoS2/WSe2 heterojunction transistor and its potential for application. Nano Lett. 2016, 16, 1359−1366. (26) Jariwala, D.; Howell, S. L.; Chen, K. S.; Kang, J.; Sangwan, V. K.; Filippone, S. A.; Turrisi, R.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. Hybrid, gate-tunable, van der Waals p−n heterojunctions from pentacene and MoS2. Nano Lett. 2016, 16, 497−503. (27) Abderrahmane, A.; Jung, P. G.; Kim, N. H.; Ko, P. J.; Sandhu, A. Gate-tunable optoelectronic properties of a nano-layered GaSe photodetector. Opt. Mater. Express 2017, 7, 587−592. (28) Sanctis, A. D.; Jones, G. F.; Wehenkel, D. J.; Bezares, F.; Koppens, F. H. L.; Craciun, M. F.; Russo, S. Extraordinary linear dynamic range in laser-Defined functionalized graphene photodetectors. Sci. Adv. 2017, 3, e1602617.
(29) Chowdhury, R. K.; Maiti, R.; Ghorai, A.; Midya, A.; Ray, S. K. Novel silicon compatible p-WS2 2D/3D heterojunction devices exhibiting broadband photoresponse and superior detectivity. Nanoscale 2016, 8, 13429−13436. (30) Islam, A.; Feng, P. X.-L. Effects of asymmetric Schottky contacts on photoresponse in tungsten diselenide (WSe2) phototransistor. J. Appl. Phys. 2017, 122, 085704. (31) Lei, S.; Ge, L.; Liu, Z.; Najmaei, S.; Shi, G.; You, G.; Lou, J.; Vajtai, R.; Ajayan, P. M. Synthesis and photoresponse of large GaSe atomic layers. Nano Lett. 2013, 13, 2777−2781. (32) Dhyani, V.; Das, S. High-speed scalable silicon-MoS2 PN heterojunction photodetectors. Sci. Rep. 2017, 7, 44243. (33) Baugher, B. W. H.; Churchill, H. O. H.; Yang, Y.; JarilloHerrero, P. Optoelectronic devices based on electrically tunable p−n diodes in a monolayer dichalcogenide. Nat. Nanotechnol. 2014, 9, 262−267. (34) Li, X.; Lin, M.-W.; Lin, J.; Huang, B.; Puretzky, A. A.; Ma, C.; Wang, K.; Zhou, W.; Pantelides, S. T.; Chi, M.; Kravchenko, I.; Fowlkes, J.; Rouleau, C. M.; Geohegan, D. B.; Xiao, K. Twodimensional GaSe/MoSe2 misfit bilayer heterojunctions by van der Waals epitaxy. Sci. Adv. 2016, 2, e1501882. (35) Zhou, N.; Wang, R.; Zhou, X.; Song, H.; Xiong, X.; Ding, Y.; Lü, J.; Gan, L.; Zhai, T. P-GaSe/N-MoS2 Vertical heterostructures synthesized by van der Waals epitaxy for photoresponse modulation. Small 2018, 14, 1702731. (36) Yang, R.; Zheng, X.; Wang, Z.; Miller, C. J.; Feng, P. X.-L. Multilayer MoS2 transistors enabled by a facile dry-transfer technique and thermal annealing. J. Vac. Sci. Technol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom. 2014, 32, 061203.
2700
DOI: 10.1021/acsphotonics.8b00318 ACS Photonics 2018, 5, 2693−2700
