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Fabry-Perot Cavity-Enhanced Optical Absorption in Ultrasensitive Tunable Photodiodes Based on Hybrid 2D Materials Qixing Wang, Jun Guo, Zijing Ding, Dianyu Qi, Jizhou Jiang, Zhuo Wang, Wei Chen, Yuanjiang Xiang, Wenjing Zhang, and Andrew Thye Shen Wee Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03579 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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Fabry-Perot Cavity-Enhanced Optical Absorption in Ultrasensitive Tunable Photodiodes Based on Hybrid 2D Materials Qixing Wang†, Jun Guo∥, Zijing Ding †,‡, Dianyu Qi †∥, Jizhou Jiang †,∥,Zhuo Wang†, Wei Chen †,‡,§

†

, Yuanjiang Xiang∥, Wenjing Zhang*∥ and Andrew T. S. Wee *†,‡

Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore ‡

Centre for Advanced 2D Materials, National University of Singapore, Block S14, 6 Science Drive 2, Singapore 117546, Singapore §

Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore

∥SZU-NUS

Collaborative Innovation Center for Optoelectronic Science & Technology, Key

Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China Corresponding authors: Andrew T. S. Wee (Email: [email protected], Tel: +65 66013757) and Wenjing Zhang (Email: [email protected], Tel: +86 13509610680).

1 Environment ACS Paragon Plus

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ABSTRACT: Monolayer two-dimensional (2D) transition metal dichalcogenides (TMDs) show interesting optical and electrical properties because of their direct bandgap. However, the low absorption of atomically thin TMDs limits their applications. Here, we reported enhanced absorption and optoelectronic properties of monolayer molybdenum disulfide (MoS2) by using an asymmetric Fabry-Perot cavity. The cavity is based on a hybrid structure of MoS2/ hexagonal boron nitride (BN)/Au/SiO2 realized through layer-by-layer vertical stacking. Photoluminescence (PL) intensity of monolayer MoS2 is enhanced over two orders of magnitude. Theoretical calculations show that the strong absorption of MoS2 comes from photonic localization on the top of the micro cavity at optimal BN spacer thickness. The n/n+ MoS2 homojunction photodiode incorporating this asymmetric Fabry-Perot cavity exhibits excellent current rectifying behavior with an ideality factor of 1 and an ultrasensitive and gate tunable external photo gain and specific detectivity. Our work offers an effective method to achieve uniform enhanced light absorption by monolayer TMDs, which has promising applications for highly sensitive optoelectronic devices.

KEYWORDS: Transition metal dichalcogenides (TMDs), MoS2, BN, Fabry-Perot cavity, absorption, photodiode


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Monolayer 2D TMDs are an emerging class of materials suitable for electronic and optoelectronic applications because of their large direct bandgap1, 2. The large choice of TMD bandgaps and the predicted type Ⅱ bandgap alignment nature3, 4 offer myriad possibilities for fabricating vertical and lateral hybrid structures for photovoltaic5-7, photodetecting2 and light emitting8, 9 applications. Another wide bandgap 2D material, hexagonal BN, which lacks charge traps, dangling bonds and has an atomically smooth surface, is widely used as a substrate for layered TMDs and graphene to improve their carrier mobility10, 11 and optical properties12-15. The intrinsically low absorption (< 7% for MoS2 at 532 nm) of monolayer TMDs16-19 limits their industrial application in optoelectronics. Many efforts have been devoted to enhancing their optical absorption, such as integrating TMDs with plasmonic nanostructures20-23 and Fabry-Perot cavities24-26 to enhance local optical field intensity. Although plasmonic structure can increase TMDs absorption, the enhancement position is limited at the edge or corner of the noble metal nanostructure due to the localization of the optical field. As a result, the overall absorption enhancement is low and uniform absorption cannot be achieved. In contrast, Fabry-Perot cavities have been demonstrated to uniformly enhance the absorption of TMDs, but the theoretical absorption is still low (about 22% at 532 nm)25. And this structure has been reported to enhance the PL of MoS226. However, there have been no reports of using such structure into optoelectronic devices.

Here, we incorporated the asymmetric Fabry-Perot cavity into a

photodiode device which shows excellent photodiode behavior. In this letter, we report an asymmetric Fabry-Perot cavity based on a MoS2/BN/Au/SiO2 hybrid structure achieved through layer by layer stacking. Monolayer MoS2 on top of this hybrid structure exhibits enhanced PL intensity of over two orders of magnitude. Theoretical calculations show that the electric field intensity is localized on the monolayer MoS2 at optimal


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BN spacer thickness, and 65.7% absorption at 532 nm is achieved for monolayer MoS2 at this optimal thickness. The photodiode based on this asymmetric Fabry-Perot cavity reveals a clear current rectifying behavior, with ultrasensitive and gate tunable photoresponse. The external photo gain and specific detectivity of the diode can be tuned up to 5.8×104 and 2.6×1010 Jones, respectively. This asymmetric Fabry-Perot cavity is therefore promising for 2D TMD-based optoelectronic device applications. Our devices were fabricated on Si/SiO2(300 nm) substrates. First, gold (Au) patterns were prepared with electron-beam lithography (EBL) and Ti/Au (5nm/50nm) films were deposited by thermal evaporation, followed by lift off in acetone and IPA. Hexagonal BN was next exfoliated on polydimethylsiloxane (PDMS) and then transferred onto the gold electrode. Monolayer MoS2 was also exfoliated, characterized by optical microscopy and Raman spectroscopy, and transferred by PDMS onto the analysis area comprising SiO2, Au/SiO2, BN/SiO2 and BN/Au/SiO2 vertically stacked hybrid structures (See Methods). The optical microscope image and schematic illustration of the structure are shown in Figure 1a and Figure 1b respectively. This structure enables us to directly compare the PL and Raman spectra on four different regions using the same piece of exfoliated monolayer MoS2. We have fabricated over twenty devices with different BN thicknesses. The data presented in Figure 1 are from the same device with BN thickness of 34.9 nm (device 1). Both PL and Raman spectra shown here were measured at room temperature using a continuous wave (CW) 532 nm laser with spot diameter of 0.5 µm. Figure 1c shows the PL spectra of monolayer layer MoS2 on different substrates. The PL spectra here reveal typical MoS2 A exciton PL peaks at around 656 nm and B exciton peaks at around 610 nm. The MoS2 PL intensity on BN/Au/SiO2 substrate is enhanced over 125 times compared to that on SiO2 (see also Supporting Information, Figure S1). In comparison, the


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enhancement factors (EF) are around 3 and 4 respectively for MoS2 on BN/SiO2 and Au/SiO2 substrates with the reference MoS2 on SiO2. In addition to the enhanced PL intensity, the shapes of the spectra are also changed. The MoS2 PL full width at half maximum (FWHM) are 27.6 nm (SiO2), 16.7 nm (Au), 15.7 nm (BN/SiO2), 15.2 nm (BN/Au/SiO2). The MoS2 PL peaks are much sharper and narrower on Au/SiO2, BN/SiO2 and BN/Au/SiO2 than that on SiO2, consistent with a previous report12. This can be explained as less doping from charge trap centers on substrate and enhanced radiative decay rates of exciton transition12. The Raman spectra for MoS2 on different substrates are shown in Figure 1d. They show two intrinsic Raman modes of MoS2, E2g1 at around 385 cm-1 and A1g at around 404 cm-1. The 19 cm-1 separation of these two modes indicates the monolayer nature of the exfoliated MoS227. In addition to PL intensity enhancement, the Raman mode intensity is also highest for MoS2 on BN/Au/SiO2. The Raman intensities of MoS2 on BN/SiO2 and Au/SiO2 are quenched, in good agreement with the report of Buscema et al12. An image representation of the PL and Raman mode intensity distributions are presented in Figure 1e and Figure 1f respectively. The image in Figure 1e was obtained by integrating the PL spectra of MoS2 in the range of 630-690 nm. The integrated Raman E2g1 mode is shown in Figure 1f. Both figures reveal uniform PL and Raman mode intensities on every substrate. To clarify the underlying mechanism of the PL and Raman intensity enhancements for MoS2 on BN/Au/SiO2, an asymmetric Fabry-Perot cavity model is used24-26, 28. The top MoS2 layer and the bottom gold surface act as reflective cavity mirrors. When incident light penetrates the top MoS2 layer, it will undergo multiple internal reflections. By optimizing the BN spacer thickness, constructive resonance is achieved, which leads to photon localization at the MoS2 layer. Next, the BN thickness dependent Raman and PL enhancement are studied. Figure 2a and Figure 2b


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are comparisons between calculated electric field intensity and experimental Raman and PL enhancement factors at different BN thicknesses respectively (see Supporting Information S1for detailed explanation). The electric field intensity calculation is based on the transfer matrix method (see Methods). We can see that the electric field intensity calculation broadly matches the experimental Raman and PL enhancement at different BN thickness. The large fluctuation in PL enhancement factors may be due to imperfections in the fabrication process, where contaminations in the gap between MoS2 and the substrate may contribute an effect. Figure 2c is the calculated electric field distribution cross section of the Fabry-Perot cavity. Monolayer MoS2 is placed at the z=0 nm plane, and the two white dashed lines display the boundaries of MoS2, BN and Au. The 532 nm light is incident in the z direction. It is clear from Figure 2c that the strongest electric field intensity is localized at the MoS2 layer for 30 nm thick BN. Due to the local enhancement of the electric field inside monolayer MoS2, the absorption of the pump laser (532 nm) is greatly enhanced. Figure 2d represents the calculated absorption as a function of BN thickness for monolayer MoS2 on the BN/Au/SiO2 hybrid structure. The maximum absorption of the pump laser is about 65.7% for 30.2 nm thick BN, which is about 12.9 times higher than the calculated absorption of monolayer MoS2 on SiO2 (5.1%). Such a large absorption by monolayer MoS2 is the highest reported using a Fabry-Perot cavity, to the best of our knowledge. In order to have a clear understanding of the absorption enhancement effect of the FabryParot cavity structure, we present calculated and experimental absorption spectra of MoS2 on BN/Au/SiO2, SiO2 substrate at the BN thickness of 50 nm (Figure 2e and Figure 2f). There are two absorption peaks, which correspond to the A and B excitons absorption respectively. Both experimental and calculated spectra show significant absorption enhancement for MoS2 on


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BN/Au/SiO2 compared to that on SiO2. The experimental data show weaker absorption compared to the theoretical calculation. This may be explained from the experimental measurement method used. The experimental absorption spectra of MoS2 were measured using confocal differential reflectance method, where absorption = ∆R/R17, 23 (See Methods). In this method, the back scattering signal intensity is measured, and there will be inevitable scattering losses. This will lead to lower experimental absorption compared to calculated prediction, which is observed in a previous report29. To explore a device application that uses the enhanced absorption of MoS2 on the Fabry-Perot cavity structure, a prototype photocurrent device was designed as shown in the schematic in Figure 3a. This device consists of one piece of monolayer MoS2 on top of BN/SiO2 and BN/Au/SiO2. The BN thickness used is 50.5 nm. The photocurrents in these two regions were measured when each area was exposed to 532 nm laser (0.1064 mW, spot diameter 2 µm) (Figure 3b). The source drain voltage is 1 V, and back gate voltage is 0 V. The photocurrent of MoS2 on BN/Au/SiO2 (38.7 nA) is about 14 times higher than that MoS2 on BN/SiO2 (2.8 nA). Such a large photocurrent enhancement confirms that the enhanced light absorption of monolayer MoS2 on BN/Au/SiO2 can be converted to a photocurrent. Next, a photodiode incorporating the Fabry-Perot cavity was fabricated to demonstrate that the strong absorption of the BN/Au/SiO2 hybrid structure can be used in a real application (Figure 4a). In the first step, the source-drain electrode and back gate electrode were fabricated using EBL and thermal evaporation respectively. A layer of BN is exfoliated and transferred onto the back gate electrode using the PDMS dry transfer method to act as a back gate dielectric. Finally, monolayer MoS2 was transferred on top. The optical image of the device is shown in Figure 4b. To improve the contact between the top MoS2 layer and source-drain electrode, high


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vacuum (10-5 mbar) post-annealing at 250 oC for half hour was used. Ohmic contact was achieved for the device fabricated by this method (Supporting information, Figure S2). The achievement of the Ohmic contact was attributed to the phase transition of monolayer MoS2 on gold film after annealing30. The mechanism of the tunable Id-Vds behavior is displayed in the inset of Figure 4c. The Fermi level (Ef) of the monolayer MoS2 on BN in regime II (n+ MoS2) is modulated by the gate electrode, however, the Ef of the monolayer MoS2 in regime I (n MoS2) is inert as there is no BN coupling, and remains close to the conduction band (Ec). Thus a lateral n MoS2/n+ MoS2 homojunction was formed when a positive back gate voltage is applied, which resulted in gate tunable rectifying behavior. Figure 4c presents the Id-Vds output characteristic plots of the device. Clear current rectification behavior is observed, with a high density current passing through the device only when the drain is positively biased. The observed current rectification clearly demonstrates an nn+ diode is formed within the lateral n MoS2/n+ MoS2 homojunction. The ultrathin nature of the homojunction makes gate tunability of the diode characteristics possible. The output characteristics (Id-Vds) of the diode under different back gate voltages reveal that the output current (Id) strengthens with increasing positive back gate voltage (Vg). By applying higher positive back gate voltage, the carrier density in the diode is increased leading to decreased diode resistance, which makes it more difficult for drain to source current to saturate. The diode characteristics can also be determined from its ideality factor. Figure 4d shows |Id|Vds curve in semi-logarithmic scale. The fitted diode ideality factor (n) is 1.0 at Vg=5V as shown in the red line of Figure 4d, which indicates excellent diode behavior for the lateral n-n+ diode. The ideality factor of 1 also suggests the total diode current is diffusion current rather than defect


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mediated space-charge recombination current31, 32. In other words, the interface is very clean between n and n+ MoS2. TABLE 1. Comparison of photo gain and specific detectivity of various reported devices materials GaN AlGaN InAs nanowire ZnO nanowire CdS Nanoribbon PbS quantum dot PbSe–TiO2–Graphene Hybrids BP flake MoS2

photo gain 108 102 104 1.4 106 5.8×104

detectivity (Jones) 7.245×109 1×1014 2.6×1011 1.8×1013 3×1013 3×1013 2.6×1010

ref 33 34 29 35 36 37 38 39 this work

To study the photoresponse of the diode, semi-logarithmic plots of Id as a function of Vds in dark (solid lines) and illumination (dash dotted lines) are displayed (Figure 4e). We can see that the output current increases greatly when the laser is incident on the photodiode. Figure 4f presents the tunability of the external photo gain and specific detectivity of the diode by the back gate. The external photo gain is a critical parameter for evaluating the sensitivity of phototransistors, which is defined as the number of collected electrons per unit time due to the number of absorbed photons per unit time. The gain can be expressed as2, 36, 40

G=

τ ttran

=

I pho Pabs

×

hν e

where τ is carrier life time, ttran is the transit time for photoexcited carriers from source to drain, I pho is the photocurrent, Pabs is absorbed light power by the effective device area, hν is the incident photon energy, and e is the electron charge. As can be seen from Figure 4f (black line), the external photo gain can be tuned up to 5.8×104 when high positive back gate voltage is applied, which is comparable to that in several other reported devices (See TABLE 1). The


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saturation behavior of the photo gain at high positive back gate voltage is due to the saturation of the carrier drift velocity. Another typical figure of merit to evaluate the performance of a detector is the specific detectivity D * , which can be given as37, 41

( A∆f )1/2 D = (cm Hz1/ 2 W -1 or Jones) NEP i NEP = n (W ) R *

Where A is the effective device area, ∆f is the electrical bandwidth, NEP is the noise equivalent power, in is the noise current, and R is the responsivity. It is clear from Figure 4f that specific detectivity can be tuned by the back gate voltage. The specific detectivity increases with the back gate voltage, and saturates at high voltage. The calculated specific detectivity is 2.6×1010 Jones at Vg=5 V, which is comparable to several previous reports for other materials (See TABLE 1). The overall performance of our device locates at the average level among the devices in TABLE1, and both high external photo gain and specific detectivity are achieved. The high external photo gain and specific detectivity of our photodiode is attributed to the large surface-to-volume ratio of monolayer MoS2 and the atomically smooth surface of BN. The monolayer nature of MoS2 also makes the photodiode much easier to be tuned by the back gate voltage. In summary, we fabricated a MoS2/BN/Au/SiO2 Fabry-Perot cavity through layer-by-layer stacking. Both PL and Raman intensities of MoS2 in this hybrid structure are greatly enhanced. This is attributed to light localization at the monolayer MoS2 and thus increased absorption by MoS2. A photodiode based on this Fabry-Perot cavity structure is designed and tested, and it shows nearly perfect diode behavior, with high and tunable external photo gain and specific


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detectivity. Our work offers an excellent method to achieve uniform and enhanced light absorption by monolayer TMDs, which can be incorporated into highly sensitive optoelectronic devices.


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Figure 1. Enhanced PL and Raman intensities of monolayer MoS2 on BN/Au/SiO2 substrate. (a) Optical micrograph of monolayer MoS2 on different substrates. (b) Schematic to illustrate the process of sample preparation. First, Ti / Au (5nm/50 nm) film (yellow) was thermally evaporated on SiO2 (300 nm) substrate (purple). Then, BN (light blue) was exfoliated and transferred using PDMS on to the Au film and SiO2 substrate. Third, monolayer MoS2 (blue) was exfoliated and transferred using PDMS on the area with SiO2, Au/SiO2, BN/SiO2 and BN/Au/SiO2. (c) Semi-logarithmic PL spectra of the monolayer MoS2 on different substrates. (d) Raman spectra of the monolayer MoS2 on different substrates. (e), (f) The A peak logarithmic intensity mapping image of PL spectra and E2g1 peak linear intensity mapping image of Raman spectra, respectively, for the monolayer MoS2 on different substrates. The unit of the color bars is CCD counts. The mapping area corresponds to the dash dotted square in (a), and the width of the mapping area is 12 µm. The integration times for both PL and Raman mapping are 0.5 second. The scale bar (white) is 3 µm.


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Figure 2. Simulation of electric field intensity and absorption for the monolayer MoS2 on BN/Au/SiO2 substrate. (a) Experimental enhancement factor (EF) of Raman E2g1 peak intensity (black squares) and calculated biquadrate of electric field intensity (E4) (red line) as a function of BN thickness for the monolayer MoS2 on BN/Au/SiO2 substrate. The laser wavelength is 532nm. EF of Raman intensity is obtained by normalizing the E2g1 peak intensity of MoS2 on BN/Au/SiO2 to that on SiO2. (b) Experimental logarithmic EF of PL intensity (black squares) and calculated quadratic of electric field intensity (E2) (red line) as a function of BN thickness for the monolayer MoS2 on BN/Au/SiO2 substrate. The laser wavelength is 532nm. EF of PL intensity is obtained by normalizing the PL A peak intensity of MoS2 on BN/Au/SiO2 to that on SiO2. The error bars are standard deviations of three data points. (c) Electric field intensity profile (black curve and the color image) across the monolayer MoS2 (0.64nm), BN (30nm), and Au film (50nm). The position of the interface for air/MoS2 is set to z=0, and the two white dash lines show the interfaces of MoS2/BN and BN/Au.(d) The calculated absorption of MoS2 at 532 nm as a function of BN thickness for the MoS2 on BN/Au/SiO2. (e) The calculated absorption spectra for the monolayer MoS2 suspended (black), on SiO2 (red) and BN/Au/SiO2 (blue). The


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BN thickness is 50 nm. (f) The experimental absorption spectra for the monolayer MoS2 on SiO2 (red) and BN/Au/SiO2 (blue). The thicknesses of BN, Au film and SiO2 are set to 50 nm, 50 nm and 300 nm, respectively, for the simulation with the transfer matrix method.

Figure 3. Comparison of photocurrents for the monolayer MoS2 on different substrates. (a) Schematic diagram of the device. The CW laser is 532 nm, 110 µW, and spot diameter 2 µm. Excitation position 1 (green solid arrow) is the area with MoS2/BN/Au/SiO2, and excitation position 2 (green dashed arrow) is the area with MoS2/BN/SiO2. (b) Id-time curves for MoS2 on BN/SiO2 and BN/Au/SiO2 substrates.


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Figure 4. Ultrasensitive tunable photodiodes. (a), (b) Schematic cross-section and optical micrograph of the device respectively. The Fermi level of monolayer MoS2 on BN in regime II is modulated by the gate electrode, however, the Fermi level of the MoS2 monolayer in regime I is inert as there is no BN coupling, and keeps close to the conduction band. Figure 4b inset: schematic of the laser modulation. The photoresponse is modulated by the laser. (c) Id-Vds characteristics of the diode show that the current under forward bias increases greatly with the applied gate voltage, while the current under backward bias almost keeps constant. Insets: Schematic band diagrams of the device in forward and backward biases at applied positive gate voltages. Ec: conduction band minimum, Ev: valence band maximum, Ef: Fermi level. (d) Fitting the ideality factor n=1.0 of the diode at Vg=5V. (e) Semi-logarithmic plots of Id as a function of Vds in dark (solid lines) and illumination (dash dotted lines). The black, red and blue curves are corresponding to Vg=-1.25, 0 and 5V, respectively. (f) The external photo gain (black squares) and specific detectivity (red dots) of the diode can be tuned up to 5.8×104 and 2.6×1010 Jones, respectively.


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Methods. Fabrication and characterization of the devices. The patterns of Au substrates were first written by using FEI EBL followed by thermal evaporating Ti / Au (5nm/50 nm) on SiO2 (300 nm) substrate. The BN was exfoliated from bulk crystal (HQ Graphene) using PDMS stamps and then transferred on Au and SiO2 using precise transfer method. BN of different thicknesses were first selected from optical contrast using optical microscopy and then confirmed using AFM (Bruker). MoS2 (SPI) was also exfoliated from bulk crystal using PDMS stamps. Their thicknesses were first estimated from optical contrast and then confirmed by using Raman spectra. Large area monolayer MoS2 was then transferred on the area with SiO2, Au/SiO2, BN/SiO2 and BN/Au/SiO2 by using precise transfer method. Both PL and Raman spectra were collected using commercial WITec Raman system using 532 nm laser. The absorption spectra of MoS2 were measured using confocal differential reflectance method with WITec Raman system, where absorption = ∆R/R17,

23

, and R is back scattering signal intensity. Electrical

characterizations were carried out using an Agilent 2912A source measure unit at room temperature, and 515 nm laser (laser power 0.124 mw, spot size 2mm) was used. Calculations. The electric field intensity and absorption were calculated using transfer matrix

method. The BN index of refraction used is 2.1, SiO2 index of refraction is 1.46. The index of refraction of MoS2 comes from the published paper25. The absorption spectrum was calculated based on the published model25. After calculating the absorption spectrum at optimal BN thickness, the 65.7% absorption of MoS2 at 532 nm was obtained from the absorption spectrum.


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ASSOCIATED CONTENT Supporting Information. The normalized PL spectra and further electrical characterization. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Email: [email protected]. *Email: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We acknowledge support by NUS research scholarship (WQX), A-STAR 2D Pharos grant (SERC 1527000012). This work is financially supported by the National Natural Science Foundation of China (51472164), the 1000 Talents Program for Young Scientists of China, Shenzhen Peacock Plan (KQTD2016053112042971), the Educational Commission of Guangdong Province (2015KGJHZ006, 2016KCXTD006), the Science and Technology Planning Project of Guangdong Province (2016B050501005), the Natural Science Foundation of SZU (000050). REFERENCES


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