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Surfaces, Interfaces, and Applications

Si-MoS2 Vertical Heterojunction for Photodetector with High Responsivity and Low Noise Equivalent Power Gwang Hyuk Shin, Junghoon Park, Khang June Lee, Geon-Beom Lee, Hyun Bae Jeon, Yang-Kyu Choi, Kyoungsik Yu, and Sung-Yool Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21629 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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ACS Applied Materials & Interfaces

Si-MoS2 Vertical Heterojunction for Photodetector with High Responsivity and Low Noise Equivalent Power

Gwang Hyuk Shin1,2,†, Junghoon Park1, †, Khang June Lee1,2, Geon-Beom Lee1, Hyun Bae Jeon1,2, Yang-Kyu Choi1, Kyoungsik Yu1* and Sung-Yool Choi1,2*

1School

of Electrical Engineering, KAIST, Daehakro, Yueseong-gu, Daejeon, 34141, Republic of Korea

2Graphene/2D

Materials Research Center, KAIST, Daehakro, Yueseong-gu, Daejeon, 34141, Republic of Korea

*Email: [email protected], [email protected]

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†These two authors contributed equally to this work

Keywords: MoS2, silicon, photodetector, p-n junction, heterojunction

Abstract

In this study, we propose the fabrication of a photodetector based on the heterostructure of p-type Si and n-type MoS2. Mechanically exfoliated MoS2 flakes are transferred on to a Si layer; the resulting Si-MoS2 p-n photodiode shows excellent performance with a responsivity (R) and detectivity (D*) of 76.1 A/W and 1012 Jones, respectively. In addition, the effect of the thickness of the depletion layer of the Si-MoS2 heterojunction on performance is investigated using the depletion layer model; based on the obtained results, we optimize the photoresponse of the device by varying the MoS2 thickness. Furthermore, low-frequency noise measurement is performed for the fabricated devices. The optimized device shows a low noise equivalent power (NEP) of 7.82 × 10-15 W Hz-1/2. Therefore, our proposed device could be utilized for various optoelectronic devices for low-light detection.

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Introduction Sensor networks have been extensively developed as a key technology for human-computer interaction interfaces such as the Internet of Things (IoT) and real-time personal health care systems;1-5 these technologies enable electronic devices to connect to each other and exchange large amounts of data, thus allowing the dissemination of valuable information to individuals.6, 7

However, to effectively implement these technologies in practice, it is necessary to develop

high-performance, low-power sensing systems. In particular, optoelectronic sensors, which satisfy these requirements, are indispensable components with wide applications in various fields, including communication, image sensing, biomedical diagnosis, environmental monitoring, automotive engineering, and manufacturing; therefore, several studies have been conducted on various photodetectors.8-10 In recent times, two-dimensional (2D) semiconductor materials, such as transition metal dichalcogenides (TMDs), have attracted significant attention as channel materials in optoelectronic devices because of their good electrical properties, tunable bandgap based on thickness, and high light absorption.11, 12 Among the TMDs, molybdenum disulfide (MoS2) has a direct bandgap of 1.8 eV and 1.3 eV for a monolayer or multilayer configuration, respectively.13 The light absorption of MoS2 is not only one order of magnitude higher than Si and GaAs, but also it has a broadband spectrum from the visible to near-infrared spectral regions (350–950 nm). 14, 15 Many studies have been conducted on photodetectors using MoS2 considering the abovementioned good optical properties; for example, Lopez-Sanchez et al. developed a monolayer MoS2 phototransistor with a high responsivity (R) of 880 A/W.16 In addition, the development of high-performance lateral MoS2 p-n junctions using a chemical 3



doping process was reported by Choi et al.17 However, these photodetectors require back gate voltages of high magnitude in order to enhance the photoresponsivity of the device; however, on the contrary, the operational voltage of such devices needs to be reduced to enable their use in practical applications. Photodetectors can be categorized as phototransistors and photodiodes.18 Considering electrical power consumption, the two-terminal photodiodes are more energy efficient than the three-terminal phototransistors; in addition, photodiodes are more suitable for sensing applications than phototransistors because of the former’s simple fabrication process. In particular, in a photodiode, excited electron-hole pairs are separated due to the built-in electric fields in the depletion region;18 therefore, it is essential to form high-quality p-n junctions that yield strong built-in electric fields.19, 20 Recently, the formation of mixed-dimensional van der Waals heterojunctions in 2D layered semiconductors with 3D Si structures have shown great potential for a wide range applications including solar cells21-23 and photodetectors 24-29 owing to the strong electric field generated in their p-n junctions. In particular, the development of several photodiodes based on MoS2/Si have been studied using different fabrication processes, such as the sputtering process24, thermal synthesis25, mechanically exfoliated method,26, 27 and chemical vapor deposition (CVD).28, 29 However, there is room for further improvement of the R, detectivity (D*), and noise power equivalent (NEP) by optimizing the thickness of the MoS2 layer in these photodiodes as well as the fabrication process. Therefore, to the best of our knowledge, for the first time, we report on the photoresponse and 1/f noise characteristics in photodiode developed using mechanically exfoliated multilayer MoS2/p-type Si heterojunctions by optimizing the thickness of the MoS2 layer at the nanoscale. The performance of the photodetector is greatly improved by controlling the thickness of the 4


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MoS2 and the absorbed power of the light at the depletion layer is systematically investigated. Among the various devices considered in our study, the one with the 48-nm thick MoS2 layer showed the best performance with an R, D*, and NEP of 76.1 A/W, 1.6 × 1012 Jones, and 7.82 × 10-15 A/Hz1/2, respectively, for external reverse bias, which is 10-fold improvement in R with suitable D* over other reports for Si-MoS2 photodiodes.[24-29] This result can be attributed to the low contact resistance in MoS2 using Ti/Au electrode, optimized fabrication process, and the quality of MoS2 flakes with relatively low-defect density compared to synthesized MoS2. In addition, this device exhibited a zero-bias operation (photovoltaic characteristic) with an open-circuit voltage (Voc) of 0.5 V and short-circuit current density (Jsc) of 161 mA/cm2. Thus, this p-n vertical heterojunction exhibited good photoresponse characteristics, which could contribute to the application of MoS2/Si heterojunctions in optoelectronic devices such as photodetectors and solar cells. Figure 1a illustrates the structure of the cross-sectional device. The device fabrication process is as follows. First, 20 μm × 20 μm square patterns were etched on a wafer using the photolithography process; this wafer consisted of a 90-nm thermally grown-Si dioxide layer on a low p-doped (≈ 1016 cm-3) Si substrate. The etching was performed by dipping the sample in a buffered oxide etcher (BOE) for 120 s to expose the Si surface. Next, mechanical exfoliation was performed for the other parts of the 90-nm SiO2/Si sample. These prepared MoS2 flakes were immediately transferred onto the patterned Si surface via a pick-up transfer method using an alignment system (see Figure S1, Supporting Information) to form the high-quality SiMoS2 interface. The electrodes that are connected to the Si and MoS2 layers were patterned using photolithography; then, Ti/Au (10 nm/40 nm) was deposited on these electrodes using a thermal evaporation system. Finally, the fabrication of the device was completed using a lift5



off process. Figure 1b shows the spatial map of the photocurrent at the Si-MoS2 heterojunction. Figure 1c represents the optical microscopic image of the device. The Si and multilayer MoS2 p-n vertical heterojunction is formed within the square-shaped region. We illuminated a focused laser beam on this Si-MoS2 p-n heterojunction region, and observed that the spot size had a diameter of 10 μm. The thickness of the MoS2 layer (approximately 48 nm) was confirmed using an atomic force microscope (AFM) as depicted in Figure 1d. In addition, the Raman spectrum in the p-n junction shows MoS2 peaks of the in-plane 𝐸12𝑔 and out-of-plane A1g vibrations as well as a peak for the crystalline Si optical phonon. Two dominant peaks for the MoS2 layer have the wave numbers of 386 and 412 cm-1, indicating a bulk MoS2 Raman mode; in contrast, the Si peak exists at the wave number of 521 cm-1 as shown in Figure 1e. A cross section of a transmission electron microscope (TEM) image of the Si-MoS2 heterojunction is shown in Figure 1f. The structural materials of the MoS2, Si and interfacial native oxide are confirmed by energy dispersive spectrum (EDS) mapping (see Figure S2, Supporting Information). We analyzed the photoresponse of our device using different wavelengths of light, including 405 nm, 520 nm, and 660 nm, under the same illumination power intensity of 500 nW, thus resulting in an estimated power density of 0.16 Wcm-2 as shown in Figure 2a. The I-V characteristics of the Si-MoS2 p-n heterojunction shows a rectifying diode behavior in the dark current state. The ideal factor (n) of the p-n heterojunction was obtained as n = 1.49 (see Figure S3, Supporting Information). Furthermore, in the reverse bias mode, the photo-excited carriers separately drift because of the enhanced electric field, resulting in a large increase in photocurrent. In contrast, the photocurrent was negligible for the forward bias because of the decrease in the size of the depletion layer. It was observed that the photoresponse of our photodetector was the highest at the wavelength of 660 nm. Therefore, we studied the I-V 6


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characteristics at the wavelength of 660 nm by varying the laser power intensity from 5 μW to 12 nW; these I-V characteristics are shown in Figure 2b. It can be seen from the figure that, as the light intensity increases, the photocurrent also increases. It is interesting to note that the photoresponse for our proposed device can be divided into two different regions; first, when the reverse bias of less than 1 V is applied, the magnitude of the photocurrent was relatively constant and proportional to the light intensity. However, for a reverse bias of over 1 V, the measured photocurrent increased as a function of the reverse bias voltage, eventually showing saturation characteristics from 12 nW to 5 μW. We measured the time-resolved photoresponse in our device at a reverse bias voltage of 1 V with an incident light intensity of 50 μW for 660 nm; the results are shown in Figure 2c. As can be seen from the figure, our proposed device shows good photodiode switching behavior depending on the laser On/Off state. The On/Off ratio of the photoresponse switching is over 10 with the dark current of 0.3 μA; furthermore, the presence of trap states with curve fitting of the decay (τdecay) time constant was investigated (see Figure S4, Supporting Information). In addition, our proposed device showed photovoltaic characteristics, as indicated in Figure 2d. The photovoltaic I-V curve is positioned at the 2nd quadrant because the forward bias region of our device exists on the negative voltage region. In particular, when a laser beam with the wavelength of 660 nm and intensity of 50 μW is illuminated on the p-n junction of the device, an open-circuit voltage of 0.5 V and shortcircuit current density of 161 mA/cm2 were observed; these obtained characteristics can be explained using an energy band diagram, which is shown in Figure 2e. Under the forward bias condition, there is a negligible depletion layer in the p-n junction and thus the number of photoexcited carriers is relatively small compared to the forward bias current; In contrast, under equilibrium states or low reverse bias voltage, the presence of a depletion layer is observed, thus photo-excited carriers are available. However, the native oxide layer on the Si surface as 7



well as the Schottky barrier near electrodes hinder the migration of the electron-hole pair, and act as a tunneling barrier. Consequently, the observed photocurrent appears constant below the reverse bias of 1 V. The presence of the tunneling barrier was confirmed by temperature dependent I-V measurements (see Figure S5, Supporting Information). In the reverse bias region, the width of the depletion layer increases, leading to a significant increase in the electric field in the junction. Thus, the photo-excited carriers can be more easily separated owing to the lowered tunneling barrier height. Furthermore, the traps in the interfacial layer between the Si and MoS2 layers capture electrons, thus increasing the lifetime of the holes.28 This trapping mechanism as well as the high-electric field in the p-n junction of our proposed photodetector can significantly increase photoresponse. Figure 3a shows the R and D* as a function of the light intensity. The R represents the gain in the electrical output per optical input; it is calculated using the following equation: 𝐼𝑝ℎ

𝑅 = 𝑃𝑖𝑛

(1)

where Iph and Pin indicate the photocurrent (Iph=Iilluminated – Idark) and illumination power, respectively. In particular, our proposed device reaches a maximum R of 76 AW-1 at a low illumination intensity of 12 nW and reverse voltage bias of 5 V. In contrast, the D* indicates the ability to measure weak optical signals distinguished from noise; it is given by the following equation based on the shot noise approximation: 𝐷∗ =

𝐴1/2𝑅 (2𝑞𝐼𝑑)1/2

(2)

where A, R, q, and Id represent the active area of the photodetector, responsivity, element charge, and dark current, respectively. The maximum D* for our device is measured as 1012 Jones. It is noted that these results for R and D* are higher than previous studies on similar photodetectors24-29 as well as the Si photodetector (see Figure S6, Supporting Information). 8


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The R significantly increases as the magnitude of the reverse bias voltage increases, as can be seen in Figure 3b; this can be explained considering that the photocurrent at a reverse bias voltage below 1 V passes through the SiOx native oxide interfacial layer via tunneling, whereas it goes over the native oxide barrier at the reverse bias over 1 V. Similarly, the photocurrents as a function of the light intensity based on the applied bias are shown in Figures 3c and 3d. When a reverse bias voltage of 1 V is applied, the photocurrent is linearly proportional to the light intensity; in particular, the curve can be fitted to that of a power law with an exponent value of 0.89, as shown in Figure 3c. However, the photocurrents are not linearly related to the light intensity when a reverse bias of 2, 3, 4, or 5 V are applied, as shown in Figure 3d; these non-linear characteristics can be attributed to the fact that the photocurrent increases significantly even for low light intensity because of traps in the native SiOx interfacial layer. As shown in the Figure 2e, when the reverse bias is applied over 2 V, photocarriers transport through the native SiOx interfacial layer by the drift and diffusion, and are captured at the traps in the interfacial layer. This effect prolongs the lifetime of the carriers, hence the photoresponse significantly increases at the weak light intensity. However, as the light intensity increases, the amount of the captured carriers can be saturated. For these reasons, the non-linear characteristics are originated from the traps in the SiOx interfacial layer. In our study, we calculated the thickness of the depletion layer for the Si-MoS2 p-n heterojunction in order to obtain further insight into the performance of the photodetector based on the film thickness. The electric field, potential, and depletion layer thickness of the Si-MoS2 p-n heterojunction were calculated using the depletion layer model (see Figure S7, Supporting Information). Figure 4a shows the space charge profile for the step junction. Based on this profile, we can obtain the Poisson’s equation as follows:

𝑑𝐸 𝑑𝑥

9


=―

𝑞𝑁𝑎 ε𝑆𝑖

𝑞𝑁𝑑

𝑜𝑟 ε𝑀𝑜𝑆

2

(3)


where ρ, Na, Nd, ε𝑆𝑖, and ε𝑀𝑜𝑆2 are the charge density, doping concentration of Si and MoS2, and permittivities of Si and MoS2, respectively. Using this equation (3), the electric field of the p-n junction was calculated; this is shown in Figure 4b. 𝐸(𝑥) = ―

𝑞𝑁𝑎 ε𝑆𝑖

𝑞𝑁𝑑

(𝑥𝑝 ― 𝑥) or ― ε𝑀𝑜𝑆 (𝑥 ― 𝑥𝑛) (0 ≤ x ≤ x𝑝 , x𝑛 ≤ x ≤ 0) (4) 2

where 𝑥𝑝 and 𝑥𝑛 are the depletion layer thickness of the Si and MoS2, respectively. The electric field should be continuous; thus, equating with the boundary condition yields the width of the depletion layers in the p-n junction. Furthermore, we can calculate the built-in potential as well as obtain the energy band diagram by integrating the electric field component as shown in Figures 4c and 4d; Our obtained result included the total depletion layer (W = 𝑥𝑝 + 𝑥𝑛) thickness as 334 nm with the values of 𝑥𝑝 and 𝑥𝑛 as 292 nm and 42 nm, respectively. To study the difference in performance of our photodetector with varying MoS2 layer thicknesses, we fabricated our proposed device with MoS2 thickness of 20, 48, 69, and 92 nm (see Figure S8, Supporting Information). Figures 5a and 5b show the changes in R and D* with varying MoS2 thickness; these results show that the highest performance among the fabricated devices is realized when the MoS2 thickness is 48 nm. This can be explained as follows; consider Figure 5c: when the MoS2 thickness is lesser than the MoS2 depletion layer (𝑥𝑛= 42 nm), the width of W is relatively thin, thus light absorption is low because the photocarriers are generated in this region (see Figure S9, Supporting Information); however, when the MoS2 layer is thicker than 𝑥𝑛, the incident light is attenuated from the MoS2 surface and the intensity of the absorbed light is reduced at W. Finally, when the MoS2 thickness is comparable to that of 𝑥𝑛, the light absorption efficiency is relatively high; therefore, the photoresponse can be optimized.

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The total absorbed power (Pa) by the MoS2 film can be calculated using the following equation: 𝑃𝑎 = ―0.5𝜔|𝐸2𝑖𝑛|Im(ε𝑀𝑜𝑆2) (5) where Ein and ω refer to the incident light electric field and frequency, respectively. The imaginary part of the MoS2’s permittivity (ε𝑀𝑜𝑆2) is considered for the calculation of total absorption.30 We conducted the simulation of the light absorption efficiency at the depletion layer using finite-difference time-domain software package (FDTD, Lumerical). Considering the absorption at the depletion region (including MoS2 and Si), the absorption reaches the peak value at the thickness of ~ 42 nm as shown in Figure 6a. Also, we have determined the absorption spectra of broad thicknesses and wavelengths to observe the overall behavior of the fabricated device as shown in Figure 6b. Moreover, in order to emphasize that the MoS2 thickness of ~ 42 nm for carrier generations, the cross-sectional absorptions as a function of the space by varying the thickness of the MoS2 (d = 24, 30, 36, 42, 48, 54, 60, 66, 72, 78 nm) are provided. As can be seen from Figure 6c, thicker MoS2 layer leads to smaller absorption at the depletion region of MoS2 and higher loss at the surface. However, at the optimum thickness, incident light is absorbed at the targeted region where the carriers are generated. We measured the noise characteristics of our proposed device under dark current conditions; the obtained results are shown in the Figure 7a. The specific detectivity of the device is calculated from this noise spectral density. (see Figure S10, Supporting Information). In particular, the proposed device with a MoS2 thickness of 48 nm exhibited the lowest noise characteristic. For these devices, the 1/f noise characteristic is observed, which indicates the occurrence of the charge trapping/de-trapping or mobility fluctuation phenomenon. In the p-n junction diode, the magnitude of 1/f noise can be described by the Hooge’s model rather than the McWhorter’s model.31 We can utilize the Hooge equation as follows: 11



𝑆𝐼(𝑓) =

𝛼𝐻 𝑓𝑁

𝐼2 (6)

where 𝑆𝐼(𝑓), I, 𝛼𝐻, f, and N are the noise power spectrum, current, Hooge parameter and the number N of carriers engaging in the conduction process, respectively. Typically, the Hooge parameter in the semiconductor has the value of 2 × 10-3.31 Figure 7b shows the noise power density S and the normalized carrier number N/I2 as a function of the MoS2 thickness in the proposed devices at the reverse bias of 2 V and the frequency of 10 Hz. We can extract the normalized carrier number N/I2 from the Hooge equation. As the value of N/I2 increases, the noise power density decreases. The device with the MoS2 thickness of 48 nm represents the lowest noise power density because the relative carrier number depending on the magnitude of the current is the largest. This value of N/I2 can be influenced by the traps, which can capture the electrons or holes. The traps exist in the interfacial SiOx layer between the Si and MoS2 films as well as between the surface and inside of the MoS2 film. It is considered that thin MoS2 film devices are dominantly affected by the traps on the MoS2 surface, such as photoresist residue and adsorbed oxygen molecules, whereas thick MoS2 film devices are influenced by traps inside the MoS2 film such as sulfur vacancies and bulk traps.32-34 (see Figure S11, Supporting Information). In the device with the MoS2 thickness of 48 nm, we extracted the value for the NEP was 7.82 × 10-15 W Hz-1/2 at a reverse bias of 4 V and frequency of 10 Hz (see Figure S12, Supporting Information), which is lower than the available state-of-the-art Si avalanche photodiode, which has an NEP of 3 × 10-14 W Hz-1/2.35 In this study, we fabricated a Si-MoS2 photodetector and optimized its performance by varying the MoS2 thickness; in particular, the device with the MoS2 thickness of 48 nm shows the best performance with the R and D* of 76.1 A/W and 1012 Jones, respectively. The performances of various photodiodes based on Si-MoS2 heterojunctions are summarized in 12


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Table 1. The resulting high R of this 48-nm-MoS2-thickness photodetector originated from the Si-MoS2 heterojunction owing to the built-in electric field and traps in the native SiOx interfacial layer between the Si and MoS2. In addition, an NEP of 7.82 × 10-15 W Hz-1/2 was obtained for this optimized device, which is lower than the state-of-the-art commercial Si photodetector. Therefore, our proposed photodetector could be utilized for imaging sensors in cellphone cameras for low-light photography as well as for photodetectors in pulse oximeters for low-light detection. Furthermore, with large-area MoS2 film fabrication technologies, such as CVD, the proposed Si-MoS2 device could also be applied to flexible optoelectronic devices for high responsivity, but at low cost.

Experimental Section In our study, the electrical measurements were conducted using a semiconductor parameter analyzer (4200 SCS, Keithley Instruments) and probe station (MS-TECH). All measurements were performed in ambient conditions. Temperature dependence tests were performed using the same instruments; in addition, low-temperature ambient conditions were realized using liquid nitrogen. For noise measurements, a spectrum analyzer (89419A, HP) and batterypowered current amplifier (SR830, Stanford Research System) were utilized.

Supporting Information Supporting Information is available online from the author. Transfer method; TEM EDS images; ideality factor; time-resolved photocurrent response; temperature dependent I-V curve; comparison table; depletion layer thickness calculation; MoS2 Thickness dependent I-V curve; I-V curve of MoS2 thickness of 2 nm; specific detectivity

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of the device; explanation of the traps; the noise power density spectrum and photocurrent as a function of the light intensity; Thickness-dependent UV-Vis absorption. Conflicts of interest. The authors declare no competing financial interest. Acknowledgement. This research was supported by grants from the Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF‐2016M3D1A1900035), and the Global Frontier Research Center for Advanced Soft Electronics (Grant No. 2011-0031640).

Notes and references (1) Wang, Z. L., Toward Self-Powered Sensor Networks. Nano Today, 2010, 5, 512-514. (2) Wang, Z. L., Self-Powered Nanosensors and Nanosystems. Adv. Mater., 2012, 24, 280-285. (3) Ruh, D.; Reith, P.; Sherman, S.; Theodor, M.; Ruhhammer, J.; Seifert, A.; Zappe, H., Stretchable Optoelectronic Circuits Embedded in a Polymer Network. Adv. Mater., 2014, 26, 1706-1710. (4) Russew, M. M.; Hecht, S., Photoswitches: from Molecules to Materials. Adv. Mater., 2010, 22, 3348-3360. (5) Rogers, J. A.; Someya, T.; Huang, Y., Materials and Mechanics for Stretchable Electronics. Science, 2010, 327, 1603-1607. (6) Bao, Z.; Chen, X., Flexible and Stretchable Devices. Adv. Mater., 2016, 28, 4177-4179. (7) Zhan, Y.; Mei, Y.; Zheng, L., Materials Capability and Device Performance in Flexible Electronics for the Internet of Things. J. Mater. Chem. C, 2014, 2, 1220-1232. (8) Peng, M.; Liu, Y.; Yu, A.; Zhang, Y.; Liu, C.; Liu, J.; Wu, W.; Zhang, K.; Shi, X.; Kou, J.; Zhai, J.; Wang, Z. L., Flexible Self-Powered GaN Ultraviolet Photoswitch with PiezoPhototronic Effect Enhanced On/Off Ratio. ACS Nano, 2016, 10, 1572-1579. (9) Goossens, S.; Navickaite, G.; Monasterio, C.; Gupta, S.; Piqueras, J. J.; Pérez, R.; Burwell, G.; Nikitskiy, I.; Lasanta, T.; Galán, T.; Puma, E.; Centeno, A.; Pesquera, A.; Zurutuza, A.; Konstantatos, G.; Koppens, F., Broadband Image Sensor Array based on Graphene–CMOS Integration. Nature Photonics, 2017, 11, 366-371. (10) Yoo, J.; Jeong, S.; Kim, S.; Je, J. H., A Stretchable Nanowire UV-Vis-NIR Photodetector with High Performance. Adv. Mater., 2015, 27, 1712-1717. (11) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S., Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol., 2012, 7, 699-712. (12) Huang, X.; Zeng, Z.; Zhang, H., Metal Dichalcogenide Nanosheets: Preparation, Properties and Applications. Chem. Soc. Rev., 2013, 42, 1934-1946. 14


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(31) Kleinpenning, T. G. M., 1/ f Noise in p‐n Junction Diodes. Journal of Vacuum Science & Technology A, 1985, 3, 176-182. (32) Sangwan, V. K.; Arnold, H. N.; Jariwala, D.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C., Low-Frequency Electronic Noise in Single-Layer MoS2 Transistors. Nano Lett., 2013, 13, 4351-4355. (33) Xie, X.; Sarkar, D.; Liu, W.; Kang, J.; Marinov, O.; Deen, M. J.; Banerjee, K., LowFrequency Noise in Bilayer MoS2 Transistor. ACS Nano, 2014, 8, 5633-5640. (34) Kim, C.-K.; Yu, C. H.; Hur, J.; Bae, H.; Jeon, S.-B.; Park, H.; Kim, Y. M.; Choi, K. C.; Choi, Y.-K.; Choi, S.-Y., Abnormal Electrical Characteristics of Multi-layered MoS2FETs Attributed to Bulk Traps. 2D Materials, 2016, 3, 015007. (35) Itzler, M. A.; Krainak, M. A.; Campbell, J. C.; Sun, X.; Yang, G.; Lu, W., Comparison of Linear-Mode Avalanche Photodiode Lidar Receivers for Use at One-Micron Wavelength. 2010, 7681, 76810Y.

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Figure 1. (a) Schematic diagram of the cross-sectional structure of the proposed device. (b) Spatial map of the photocurrent is obtained by illuminating a focus laser beam with the power of 5 μW and the wavelength of 660 nm. (c) Optical microscope image of the Si-MoS2 p-n heterojunction. (d) AFM topography result of the p-n heterojunction. (e) Raman spectrum of the p-n heterojunction. (f) Cross sectional TEM image of the Si-MoS2 heterojunction, where the scale bar is 10 nm in length.

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Figure 2. (a) I-V characteristics of the Si-MoS2 p-n heterojunction photodetector with a laser beam of different wavelengths at a light intensity of 500 nW. (b) I-V characteristics of the proposed device for different light intensities at a wavelength of 660 nm. (c) Time-resolved photoresponse of the proposed device. (d) Photovoltaic characteristics of the proposed device. (e) Energy band diagram of the proposed device showing the operation mechanism.

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Figure 3. (a) R and D* of the proposed Si-MoS2 photodetector for different light intensities with an R of 76.1 A/W and D* of 1012. (b) Responsivity of the proposed device for different bias voltages. Photocurrent of the proposed device for different light intensities at the bias voltage of (c) 1 V and (d) 2, 3, 4, and 5 V showing linear and non-linear characteristics, respectively.

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Figure 4. (a) Space charge profile of the Si-MoS2 heterojunction. (b) Electric field of the heterojunction obtained using the Poisson’s equation. (c) Electric potential of the heterojunction obtained by integrating the electric field component. (d) Energy band diagram of the heterojunction.

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Figure 5. (a) R and (b) D* of the proposed device for different MoS2 thicknesses. (c) Schematic diagram of the depletion layer in the Si-MoS2 heterojunction for different MoS2 thicknesses.

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Figure 6. Simulation results of the absorptance at the depletion layer depending on the MoS2 thickness at the wavelength of 660 nm (a), and the absorbed power as a function of the MoS2 thickness and the wavelength (b), and the cross-sectional image as a function of the space by varying the thickness of the MoS2 (d = 24, 30, 36, 42, 48, 54, 60, 66, 72, 78 nm) (c). A white scale bar in (c) is 20 μm. Colour maps indicate the intensity of the absorbed power. The thickness-dependent UV-Vis absorption data from the experiment is shown in Figure S13.

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Figure 7. (a) Noise power density S as a function of the frequency in the Si-MoS2 photodetectors under dark current conditions for different MoS2 thicknesses. (b) S and the normalized carrier number as a function of the MoS2 thickness in the proposed device at the reverse bias of 2 V and the frequency of 10 Hz.

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Structure

Si-MoS2 diode

Page 24 of 25

R (A/W)

D* (Jones)

NEP (W/Hz0.5)

Reference

76.1

1.6 × 1012

7.82 × 10-15

This work

0.3

1013

-

24

11.9

2.1 × 1010

-

25

0.21

-

-

26

7.2

109

-

27

8.75

1.4 × 1012

-

28

9

1014

-

29

Table 1. Performance comparison of several photodiodes based on Si-MoS2 heterojunctions.

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