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Enhanced Quantum Efficiency in Vertical Mixed-thickness nReS2/p-Si Heterojunction Photodiodes Bablu Mukherjee, Amir Zulkefli, Ryoma Hayakawa, Yutaka Wakayama, and Shu Nakaharai ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.9b00580 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019
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Enhanced Quantum Efficiency in Vertical Mixed-thickness n-ReS2/pSi Heterojunction Photodiodes Bablu Mukherjee, *,† Amir Zulkefli, †,‡ Ryoma Hayakawa,† Yutaka Wakayama, †,‡ and Shu Nakaharai*,† International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan ‡ Department of Chemistry and Biochemistry, Faculty of Engineering, Kyushu University, 1-1 Namiki, Tsukuba 3050044, Japan †
ABSTRACT: We fabricated few-layer, multilayer and mixed-thickness rhenium disulfide (ReS2) based on a vertical van der Waals n–p junction for a photosensing applications. ReS2 flake deposition onto a p++Si substrate led to the formation of an n– p heterojunction with rectifying characteristics and good photosensing ability under reverse bias. A thin ReS2 layer with a Si heterojunction showed weak photosensing performance with a fast response, whereas a thick multilayer ReS2/Si showed an improvement in photocurrent but an overall degradation of the response time. To overcome the trade-off between responsivity and speed, a mixed-thickness ReS2/Si was fabricated. This heterojunction was found to exhibit the best photoresponse, with a short response time and high quantum efficiency. A high photoresponsivity (at 3 V) of ~33.47 A/W at a high-speed operation of 80 μs was recorded, making this one of the fastest reported transition metal dichalcogenide with silicon photodiodes with high responsivity. The heterointerface of Si with thickness-independent direct-bandgap ReS2 of mixed thickness enabled more gain related to photogenerated carrier trapping, resulting in the observed high photoresponsivity and fast (μs) response. This work demonstrates that a mixture of different thickness of ReS2-based n–p junction results in improved photoresponsivity and speed in optoelectronics and sensor applications.
KEYWORDS: 2D transition metal dichalcogenide, ReS2, Photodiode, Fast, Responsivity, Van der Waals heterostructure
Advances in two-dimensional (2D) materials and their van der Waals (vdW) interface with three-dimensional (3D) semiconductors, which dominates overall device performance, remain important to studying the fundamental physics of vertical heterostructures1,2,3. Twodimensional (2D) layered transition metal dichalcogenides (TMDCs) exhibit a wide range of electronic properties, including intrinsic metallic, semiconducting and insulating properties, and many TMDCs exhibit a direct-to-indirect bandgap transition depending on their number of layers. Thus, achieving a better optical response requires reducing the number of layers to a monolayer to access the direct bandgap. However, the optical absorption is reduced because of the one-atomic-layer thickness, which makes such a monolayer device inactive in actual applications. Here TMDCs with a direct bandgap that is independent of the number of layers play an important role in increasing optical absorption in multilayer-thick films and enhancing the optical response in device applications. Recently, MoS2– Si diode structures in devices with a 2D–3D interface have been extensively studied by different research groups4,5,6,7,8,9,10,11,12,13. In addition to the highly studied MoS2, other 2D materials14,15 including graphene–Si16,17,18 and WS2–Si19,20 heterostructures, have also been reported.
To realise optoelectrical device applications, a highquality n–p junction diode is essential to fabricating the structure. Thickness-independent direct-bandgap rhenium disulfide (ReS2) materials are useful for optics and optoelectronics due to their various properties21,22,23,24 which has motivated us to further study the uniform-layerthick and mixed-thickness ReS2-based devices. Due to a lack of interlayer registry and weak interlayer coupling, bulk ReS2 behaves as electronically and vibrationally decoupled monolayers stacked together22. Two-dimensional layered TMDCs and their interfaces are interesting for future optoelectronic device applications25,26,27,28,29. Conventional technology in n–p-junction-based photodetectors has shortcomings with respect to doping control, area selectivity, air stability and low damage. Furthermore, from a technology perspective, most of these doping processes are incompatible with CMOS manufacturing technology. To overcome these difficulties, the n-ReS2/p-Si heterostructure is very promising because it has substantial advantages in a device fabrication process and is compatible with CMOS technology. A few-layer ReS2 phototransistor can reach a maximum attainable photoresponsivity of 88.6 × 103 A/W30. We have recently demonstrated a multilayer ReS2 photodetector with a high responsivity of 4 A/W at a 50 ms response time and a low response time of 20 µs at 4 mA/W
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responsivity, making it one of the fastest reported TMDC photodetectors31,32. These characteristics are attributable to the modulation of occupancies of intrinsic ReS2 and extrinsic ReS2/SiO2 interface trap states. The trade-off relation between responsivity and speed in the ReS2 phototransistor, where a high photoresponsivity corresponds to low-speed operation and a lower photoresponsivity corresponds to higher-speed operation. In the n-p photodiode the device response time depends on the diffusion time (for the photogenerated carriers outside depletion width; ~ in several microsecond), drift time (for the photogenerated carriers inside depletion width; ~ in several nanoseconds) and RC (resistance × capacitance) delay constant of the circuit. A reverse bias n–p photodiode further aids high-speed operation compared with the operation of a phototransistor by increasing the depletion width and reducing depletion capacitance (Cdepletion) at the interface across the heterojunction region for the photodetector and sensor applications where such substrate-related (oxide-related) interface trap states are absent. Vertical geometry in the n–p junction is advantageous because it facilitates the application of a strong electric field to separate the generated excitons in the thin TMDC materials to contribute to the current before the excitons recombine. Here, we utilise a direct-bandgap ntype semiconducting thin ReS2 material (bandgap ~1.4–1.5 eV) on top of a highly hole-doped Si substrate to create a vertical n–p diode configuration. Most of the available TMDCs do not exhibit a direct bandgap at few-layer thickness, whereas ReS2 has a direct bandgap irrespective of thickness, although its optical absorption increases with increasing thickness. In this paper, for the first time, we report the realisation of a ReS2/Si photodetector by overcoming the aforementioned difficulty by applying a simple process to transfer large-area ReS2 flake directly onto a highly holedoped (p++) patterned Si wafer. The few-layers ReS2/Si heterojunction shows good rectification characteristics such as n–p diode formation. Towards achieving a good photoresponse with a short response time, we studied three different n–p heterostructures. First, we studied a fewlayers-thick uniform ReS2/Si photodiode device, which exhibits a fast time response (rise time 0.01 s photocurrent vs. time under successive ON/OFF of incident light) under reverse-bias operation; however, less photocurrent is generated (photoresponsivity ~82 mA/W at 2 V, 532 nm laser) because of low optical absorption. Second, we studied multilayer-thick ReS2/Si devices, where the produced photocurrent was increased (photoresponsivity ~7.35 A/W at 2 V, 532 nm laser) because of the increase in optical absorption with increasing thickness; however, the rise and decay time response increased (more than 0.1 s). Thus, the device’s response slows because of the interlayer screening effect and greater ReS2 thickness, which is related to nonuniform optical absorption. Lastly, we studied mixedthickness ReS2/Si photodiodes, which provide the best sensing performance in terms of both photocurrent and response time, to overcome the trade-off relation between responsivity and speed compared with thin and multilayer ReS2/Si photodiodes. The evolution of the photocurrent vs. time under high-frequency modulation of an incident laser in the mixed-thickness ReS2/Si photodiode shows high
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photoresponsivity of ~33.47 A/W with a high speed of 80 μs. These observations indicate the potential of a vertical mixed-thickness ReS2/Si photodiode in reliable and highspeed photodetection applications. The heterointerface of Si with thickness-independent direct-bandgap ReS2 of mixed-thickness enables greater gain with high speed operation (shorter photoresponse time) under reverse bias. High photoresponsivity at high speed imparts the mixedthickness ReS2/Si devices with the best performance among all reported TMDC–Si photodiode structures. External quantum efficiency (EQE) spectra comparing three different thick ReS2/Si photodiode summarise the enhanced conversion efficiency over a broad wavelength region of the nonuniform mixed-thickness ReS2/Si n–p heterostructure device with high quantum efficiency, which can be further integrated into large-wafer-scale CMOS-compatible technology for optoelectronics and sensor applications.
RESULTS AND DISCUSSION Details of the patterning-template substrate, thin-film sputtering conditions, preparation of large-area ReS2 flake and the transfer process are given in the Sample Preparation and Measurements Techniques section. Large-area, thin, uniform ReS2 films (Figure S1) and ReS2 films with different thicknesses were prepared using an Au-mediated transfer technique33. This structure is easily fabricated because the bottom-most layer of few-layer ReS2 will directly contact the drain as well as the Si surface. This device structure can lead to future 2D-layer print technology to prepare vertical n–p heterostructures in a full pre-patterned wafer. An illustration of few-layer and mixed-thickness ReS2/Si devices is shown in Figure 1a. The crystalline quality was confirmed by Raman spectroscopy (Figure 1b), which indicates few-layer (~5 nm) thickness and high chemical purity of the flake material, as indicated by the Raman spectrum matching that of highly crystalline ReS2. The Raman spectrum displays at least five modes in the 100–250 cm-1 range from the black circle region of the ReS2/Si sample. These Raman modes are 138.3, 144.3, 151.5, 162.2, and 212.3 cm-1, while the reference Si has a Raman peak at 520.3 cm-1. These Raman spectra refer to the thickness of a few layered (~ 5 nm) thick ReS2, which is similar to a 4–layer ReS2 sample previously reported34. The two most distinguished Raman peaks are observed at 162.2 and 212.3 cm-1, corresponding to the Eg mode, which is in-plane vibrations of the Re atoms in ReS2, and the Ag-like mode, corresponding to the out-of-plane vibrations of Re atoms, respectively, along with other labeled peaks. These spectra positions were consistent with the Raman spectrum of ReS2 previously reported22. The fabricated ReS2/Si heterostructure device is shown in the optical image (inset in Figure 1b). Uniform few-layer-thick ReS2 with a thickness of 5–6 nm was used to form the heterostructure (Figure S2). The electrical characteristics, the source–drain current (IDS) versus the source–drain voltage (VDS) under dark conditions, are highly asymmetric, where a negative bias at the n-ReS2 region corresponds to a forward bias in the vertical n–p heterostructure diode configuration. The heterostructure device Au-ReS2-Si has dominated the n-p diode characteristics originated from ReS2-Si junction since the Au-ReS2-Au device structure shows almost linear I-V characteristics (Figure S3). The reverse-bias condition in the n–p diode produces less current than the forward-bias condition because of the built-in barrier voltage at the interface. The template substrate with a Au/Cr/Al2O3/Si stack configuration was characterised. The
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electrical quality of the insulating Al2O3 layer indicates the absence of fixed oxide trap charges (zero hysteresis in the sweep current-bias) and a low leakage current (Figure S4). Under white-light illumination, the reverse-bias region produces a high photocurrent (Iphoto − Idark) compared with the forward-bias region because of the band alignment between the ReS2 and the Si interface of the formed depletion region and its sensing ability (Figure 2a). Iphoto (Iph) and Idark represent the total current with and without light illumination, respectively. The forward-bias characteristics under dark and light illumination were fitted with the standard n–p diode forwardbias equation (1),
(
𝐼𝐷𝑆 = 𝐼0 𝑒
𝑞𝑉𝐷𝑆
𝜂𝑘𝐵𝑇
)
― 1 (1)
where I0, q, kB and T represent the reverse-bias saturation current, electron charge, Boltzmann constant and absolute temperature of the junction, respectively. The nonideal parameter, η, which provides (Figure 2b) a very high value of 10 and 10.8 under dark and white-light illumination conditions, respectively, indicates the presence of high-density interface defect states and trap states in the ReS2/Si n–p junction, which dominates the electrical characteristics in the device. Both the Si and the ReS2 act as active optical absorbing layers, and we observed that defect trap states dominated the photoresponse. The power-dependent photosensing performance of the device under white-light and green-light illumination is shown in Figure 2(c, d), respectively, which indicates that white-light illumination produces a higher photocurrent than green-light
and blue-light illumination. The photocurrent–time (Iph–t) plot corresponding to a fixed bias (2 V) under successive ON and OFF states of different light illumination shows a fast response (~0.01 s; limited by the measurement instrument) of the device (Figure 3a). The Iph–t plot corresponding to zero bias shows a fast rise and fall response (Figure 3b). The small builtin electric field at the interface helps separate photogenerated carriers under zero-bias operation. The photogeneration mechanism can be further discussed on the basis of the powerlaw plots and the schematic band diagram (Figure 3(c, d)). Under the photosensing configuration (at a reverse bias), the interface will have many defect and trap states, as shown in Figure 3d. These defect and trap states play a vital role under specific-wavelength excitation (i.e. spectral peaks at 550 and 450 nm) as compared with wideband excitation, which is responsible for the high power-law coefficients (0.89 and 0.76) compared with that under white-light illumination (0.53). Bulk traps states in ReS2—the interface states and defect traps states—strongly influence the effective trap density under white-light illumination. This phenomenon produces high photocurrent and a low power-law coefficient. We also observed large hysteresis (Figure S5) in the IDS–VDS sweep curves, which further supports the existence of high-density interface and defect trap states. Under light illumination, photogenerated electron–hole pairs across the depletion region at the reverse-bias configuration separate, where holes drift towards Si under the built-in field across depletion region and electrons pass through trap and interface states, with some of them reaching the electrode through the thin ReS2 channel. This process results in short rise and decay response times on the order of 0.01 s.
Figure 1. (a) Schematic of a uniform few-layer-thick and mixed-thickness ReS2/Si heterostructure diode. (b) Raman spectra of the few-layer ReS2 sample from two different locations, as indicated by black and red circles corresponding to ReS2/Si and ReS2/Au substrates, respectively. The inset shows an optical image of the fabricated device, where ReS2 flake area is highlighted with a black dashed line. The scale bar is 10 µm.
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Figure 2. (a) IDS–VDS characteristics of the fabricated n–p junction with and without white-light illumination. (b) Forward-bias electrical characteristics were fitted with the forward-bias diode equation. (c, d) Reverse-bias electrical characteristics of the diode under different intensities of incident white light and 550 nm light illumination, respectively. The inset in Figure 2(d) shows a schematic of the photocurrent measurements during global irradiation of light without focusing through the lens.
Figure 3. (a, b) Photocurrent–time (I–t) characteristics at fixed-bias operation at 2 V and 0 V, respectively, under multiple ON and OFF cycles of incident light. (c) Power dependence of the current at a reverse bias of 2 V under different light powers and wavelengths of illumination. (d) Schematic band diagram of ReS2/Si n–p junction diode under reverse-bias operation. Bulk trap states, interface trap states and defect levels are shown by green dashed lines.
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Figure 4. Fixed wavelength, 532 nm laser illumination: (a) IDS–VDS characteristics under dark conditions and different laser intensities. The inset shows a schematic of the device under laser illumination. Reverse-bias operation: (b) IDS–VDS characteristics at different temperatures and under laser (fixed intensity: 6.4 mW/cm2) illumination. (c) I–t response at two different temperatures. (d) The enlarged portions of I–t in the range 30.7– 40.8 s, showing the fast response of the device.
Fixed laser wavelength of 532 nm with different light intensities produce good photocurrent in reverse bias in comparison with that in forward bias condition (Figure 4a), which is much higher than the photocurrent value obtained using green light filter. The current in the n–p diode increases with increasing temperature, as a rise in temperature creates more electron–hole pairs. Minority carriers increase greatly with increasing temperature, resulting in an increase in the reverse-bias current in the I–V graph and in the background current of the I–t graph, as shown in Figure 4b and Figure 4c, respectively. On the other side, as the diode barrier voltage decreases with increasing temperature, more photogenerated carriers flow in the circuit, resulting in an increase in photocurrent with a rise in temperature (Figure 4c). Both room-temperature and high-temperature operation show a short response time under light illumination. A zoomed-in image of the response time reveals that a short response time of 0.01 s is achieved, as shown in Figure 4d. This temperaturedependency study of the diode further enhances the possibility of the device being used at high temperatures while maintaining good and fast photoresponse. In addition, it confirms that the minority carriers play a major role in the photoresponse of the vertical n–p heterostructure.
at wavelength λ and the effective active area of the flake, respectively. To the quantitative evaluation of the external photoresponsivity of different heterostructure devices, we use device active area (s) as the area of the flake, which has vander-Waals interfaces with the Si to form the n-p junction. The photoresponsivity calculated for a 532 nm laser wavelength and a fixed bias of 2 V is ~82 mA/W, which is comparable to the photoresponsivity of MoS2/Si9 and graphene/Si17 photodetectors. However, through our further planned experiments, we clearly found that the photoresponsivity could be increased by engineering the thickness of the ReS2 flake. In addition, the effects of a high density of interface defects and traps at the ReS2 and Si interfaces on the overall device performance cannot be neglected.
We calculated the photoresponsivity of the device, Rλ, which is the figure-of-merit of photodetectors, using the formula (2)
We studied two types of heterojunction devices with multilayer-thick ~90 nm (Figure S7) ReS2 flake and a mixture of different layers of few-atom-thick ReS2 ranging from ~4 nm to ~15 nm (Figure S8). The multilayer-thick ReS2 flake shows good photocurrent generation in reverse bias (Figure 6a) under white- and green-light illumination. The device response was further characterised under illumination with a green laser (532 nm). Time response (I–t) graphs of the multilayer-
𝑅𝜆 =
𝐼𝑝ℎ𝑜𝑡𝑜 ― 𝐼𝑑𝑎𝑟𝑘 𝑖𝑖𝑛(𝜆) × 𝑆
(2)
and compared the value with those reported for other photodiodes. Here, iin(λ) and S are the incident-light intensity
We investigated the rectification characteristics of the n–p heterojunction diode (Figure 5). Sinusoidal waves with varying frequency (10–100 Hz) were used as the input signal, where the output across the load resistance showed pulsed DC outputs. Another frequency response of the device is shown in the S.I. (Figure S6). No phase shift is noticeable, and attenuation occurs at the outputs in the frequency range, which further verifies that the diode can rectify electronic device applications at low-frequency operation.
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thick ReS2/Si heterojunction device characteristics were fitted using the rise and fall equations (equations (3) and (4), respectively). The rise and decay of the photocurrent can be expressed in the form of equations (3) and (4), respectively35:
( (𝑒
𝐼 = 𝐼0 + 𝐴1 1 ― 𝑒 𝐼 = 𝐼0 + 𝐴2
𝑡0 ― 𝑡 𝑠1
)
𝑡0 ― 𝑡 𝑠2
)
(3)
(4)
where I0 is the dark current, t0 is the initial time, A1 and A2 are the amplitudes of the photocurrent and s1 and s2 are the response times of the rise and decay curves, respectively. A green laser produces a photocurrent-response time of ~0.1 s with Rλ ≈ 7.35 A/W at 2 V, whereas a long response time is observed during the laser OFF state. During the OFF state, the photocurrent decreases rapidly (~0.1 s), followed by a slow response (~19 s), which could be due to the multilayer thickness of the ReS2 flake. The interlayer coupling and screening effect could be responsible for this behaviour, where nonuniform optical absorption occurs across the thick ReS2 flake. The nonuniform photogenerated carriers on the top surface could have a slow response to reach the bottom layer
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collected by the bottom-layer electrical contacts. A low photocurrent and long response times of ~25.3 s and ~31.4 s corresponding to the ON and OFF states of the laser, respectively, are observed under forward bias (Figure 6b). This long response time could be due to numerous factors and complex processes related to photogenerated carriers migrating across different layers and to nonuniformity of carrier generation and trapping. However, we achieved higher photoresponsivity (although both have the same mechanism of trapping) in thick multilayer ReS2/Si devices than in thin fewlayer ReS2/Si devices because the greater optical absorption of the thick devices generates more photogenerated carriers across the junction of depletion width of the ReS2/Si heterojunctions and generates additional trapping sites because of the increase of available bulk traps. Here, the separation mechanism of photogenerated carriers is similar to that in thin few-layer ReS2/Si devices; however, a remarkable slow decay response is observed because of the thick ReS2 channel, where trapped photogenerated electrons across various layers are responsible for the slow response.
Figure 5. Rectification characteristics: (a) schematic circuit diagram for the rectification diode performance. The output voltage was measured across the load resistance (RL). (b, c) and (d, e) Input signal and normalised output signal at 10 Hz and 60 Hz, respectively.
Figure 6. Uniform thick multilayer ReS2/Si n–p photodiode characteristics: (a) Reverse IDS–VDS curves were recorded under dark conditions and under global irradiation of green light (26.8 ± 0.1 mW/cm2) and white light (104 ± 0.1 mW/cm2). The inset shows an optical image of the device. (b) Rising and decaying photocurrent characteristics upon periodic irradiation with a 532 nm laser beam at fixed forward-bias (+2 V) and reverse-bias (−2 V) conditions.
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Figure 7. Nonuniform few-layer-thick ReS2/Si photodiode: (a) Reverse-bias characteristics under dark, green-light (26.8 ± 0.1 mW/cm2) and whitelight (104 ± 0.1 mW/cm2) illumination. The inset shows an optical image of the device. (b) IDS–VDS characteristics with and without 532 nm laser irradiation. (c) I–t (fixed 2.5 V bias) at 532 nm laser irradiation with different intensities and also multiple OFF and ON states of illumination. (d) Comparison of calculated EQE among four ReS2/Si photodiode structures with different thicknesses at a fixed wavelength of 532 nm.
The mixed-thickness of few-atomic-layer-thick ReS2/Si heterojunction device photoresponse properties under whiteand green-light illumination is shown in Figure 7a. The device response under 532 nm laser irradiation (I–V and I–t) is shown in Figure 7b and 7c, respectively. Reverse-bias operation (at 2.5 V) of the device results in the highest responsivity with a fast response time (both rise and fall times of ~0.01 s). The device response under white-light illumination and its powerlaw dependency (I ∝ P0.46) is shown in Figure S9. A smaller exponent indicates a larger effective trap density and high photoconductive gain related to the predominance of photogenerated carrier trapping. The mixed-thickness of fewlayer-thick nonuniform ReS2/Si heterojunction device is the most efficient among the three different device types for producing high photocurrent and short response times. A further device characterisation technique is the measurement of the EQE, which is calculated from the measured responsivity (Equation 5): 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝ℎ𝑜𝑡𝑜𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 𝑐𝑎𝑟𝑟𝑖𝑒𝑟𝑠
ℎ𝑐𝑅𝜆
𝐸𝑄𝐸 = = 𝑞𝜆 (5) 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 where q, h, λ and c are the electron charge, Planck’s constant, the wavelength and the speed of light, respectively. The mixedthickness ReS2/Si heterostructure device shows the highest EQE value and the fastest operation (Figure 7d) among the investigated devices. The EQE of the mixed-thickness ReS2/Si
device was 9808 %, with a 120 µs response time at a fixed 3 V bias and 532 nm wavelength. To quantify the fast response of the mixed-thickness ReS2/Si heterojunction device, we measured the photocurrent–time response of the device under a mechanically chopped continuous-wave 532 nm laser with different frequencies (Figure S10). High frequency (210 Hz to 6.1 kHz) photocurrent measurements (Figure 8a–d) show a rapid rise and decay response (limited by the instrument setting of a 10 μs interval time) of the device at each frequency. The rise time decreases with increasing frequency at frequencies up to the measurement setup limit of 10 μs. The change in current in each time interval of 10 μs is greater than 100 nA during the rise, which suggests that the device response would be faster if it could be tested in a faster measurement system. Higherfrequency photocurrent measurements show that the rise time of the mixed-thickness ReS2/Si heterostructures is 80–120 μs. High achievable photoresponsivities (at 3 V) of ~42.08 A/W and ~33.47 A/W at high operating speeds of 120 μs and 80 μs were recorded, respectively. We compared the photoresponsivity and response time of the mixed-thickness ReS2/Si photodiode with those reported for a Si-based n–p photodiode (Table S1), which revealed that it exhibits greater efficiency than other reported photodetectors.
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Figure 8. (a) High-frequency response of mixed-thickness ReS2/Si heterostructure at 3 V bias under pulsed 532 nm laser irradiation (intensity 7.2 mW/cm2). Photocurrent vs. time at laser modulation frequencies of (a) 210 Hz, (b) 1 kHz, (c) 4.1 kHz and (d) 6.1 kHz. The time interval between two data points was 10 μs.
Good photoresponses have been reported in lateral multilayer/monolayer MoS2 heterojunctions because of the built-in field at the heterojunction, which can separate and collect the photogenerated electron–hole pairs at the junction36,37. To better understand the working mechanism of the mixed-thickness ReS2/Si heterostructure, we plotted (Figure 9a, b) equilibrium band diagrams of multilayer ReS2 with Si and thin ReS2, respectively. Due to the very high hole doping concentration (~ 5 × 1018 /cm3) in Si, the depletion width in the Si region is quite small (~ few nm) as compare with the depletion region in ReS2 at the 2V reverse bias. Apart from an increase in the optical absorption in the mixedthickness ReS2 compared with the uniform-thickness samples as shown in Figure S11, the interface edge states plays an essential role in improving the photoresponsivity and response time of the samples (both rise and decay). An exciting property of ReS2 is its direct bandgap irrespective of its thickness; thus, the observation of the highest photoresponsivity and highest speed in the mixed-thickness ReS2/Si photodiode is recorded. For a uniformly thick ReS2/Si heterostructure, only the depletion region across n-p heterojunction interface (Figure 9a) becomes active towards separate photogenerated electron–hole pairs under a vertical electric field in a photodiode; nonuniform optical absorption will occur in thick multilayer ReS2/Si, making the device photocurrent–time response slow. Photogenerated carriers can easily separate across the depletion region in few-layer ReS2/Si heterostructures, where photogenerated holes and electrons drift towards p-Si and n-ReS2, respectively, which makes fast time-response in the device. However, the photocurrent magnitude is low because optical absorption is poor. As the photoresponsivity is low for few-layer ReS2/Si, it is not suitable for applications that require fast operation (photoresponsivity
decreases with decreasing response time). In a mixedthickness ReS2/Si heterostructure, all of the interfaces—fewlayer ReS2/Si, multilayer ReS2/Si and few-layer/multilayer ReS2—actively contribute in photocurrent generation. Edge states in the 2D crystal provide higher charge scattering, including an increase in the carriers’ trapping sites. Herein, edge states at the interface between two different thicknesses ReS2 play an important role, where photogenerated electrons can be trapped in the edge states. Next, the mechanism of the photocurrent generation at the lateral heterointerface of the few-layer/multilayer ReS2 with Si heterostructures will be discussed. Photogenerated holes can separate and transfer through the valence band of multilayer-thick ReS2 to Si (Figure 9b), whereas photogenerated electrons drift toward the electrode and some are trapped in the bulk defect states of ReS2 and/or at interface edge states. Thus, along with the carriers’ separation across the depletion region in Si and few-layer ReS2 junction, the trapping mechanism at the edge states is the key to achieving the higher photoresponse. On the other side, gain related to bulk traps at ReS2 and Si, gain related to traps at the interface between ReS2 and Si and photoconduction gain of ReS2 and Si occurs in all of the devices. By contrast, the heterointerface of Si with thickness-independent directbandgap ReS2 of mixed thickness allows gain related to photogenerated carrier trapping at the interface between different layers of ReS2 to dominate, which is one of the reasons to exhibit high photoresponsivity. The light illumination induces a complex trapping process, recombination and a complex process of carrier generation where multiple interfaces with different materials coexist in mixed-thickness devices. Such devices require further study via frequencylocked measurements to elucidate the exact contribution of the different interfaces to device performance.
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Figure 9. (a) The band diagram of the multilayer ReS2/Si heterojunction and (b) the multilayer ReS2–few-layer ReS2 heterojunction in an equilibrium state.
CONCLUSIONS In summary, we demonstrated a vertical n–p ReS2/Si van der Waals junction diode for photosensing under incident power and the role of a different layer thickness of ReS2 to modulate device performance. We found that the quantum efficiency could be further enhanced by selecting mixed-thickness of the thickness-independent direct-bandgap ReS2 flake. We studied three different heterostructures composed of different and mixed-thickness ReS2 flakes. The mixed-thickness ReS2/Si exhibited the best photoresponse, with a fast response and high quantum efficiency. Furthermore, the evolution of photocurrent with time was studied under different highfrequency laser modulation. A high achievable photoresponsivity (at 3 V) of ~33.47 A/W at high-speed operation of 80 μs was recorded, indicating that the mixedthickness ReS2/Si photodiode is faster and exhibits greater responsivity than other reported TMDC/Si-based photodiodes. Our findings suggest that the heterostructure with a mixedthickness of ReS2 exhibits high quantum efficiency with a high photoresponsivity and fast response on the order of microseconds, thus overcoming the trade-off relation between responsivity and speed encountered with thin and multilayer ReS2/Si photodiodes. A model based on gain related to carrier trapping dominating at the edge states in the interface between ReS2 layers of different thickness explains the observed high responsivity. By contrast, the separation of photogenerated carriers across the interface between Si and thin ReS2 enables a fast response. Given both the high responsivity and the fast response, this TMDC/Si-based photodiode is among the fastest reported thus far. Through this work, we have developed a guiding principle for improving the performance of ReS2/Si n– p vertical heterostructure photodetectors, which can be further integrated into large-wafer-scale CMOS-compatible technology for applications in optoelectronics and sensors.
METHODS Sample Preparation and Measurements Techniques: A highly hole-doped (resistivity ~0.001 Ω·cm) 4-inch silicon wafer (p++Si substrate) was patterned using standard photolithography techniques to open square-shaped (1 mm × 1 mm) windows in a resist for depositing an Al2O3 layer of ~80 nm thickness, followed by deposition of Cr/Au (5 nm/50 nm),
which was used as a bottom template in the device fabrication process. ReS2 flake was transferred via the wet-transfer method onto a pre-fabricated patterned (p++) Si substrate so that some part of the ReS2 would directly contact the Si surface to form a ReS2/Si heterojunction. Vertical ReS2/Si n–p diodes were fabricated in three steps. Firstly, a square-shape-patterned Au/Cr/Al2O3 stack was fabricated on a Si substrate. Secondly, Au-mediated ReS2 films were fabricated on top of the PMMA/PAA/SiO2/Si substrate to obtain large-area uniform flake. Thirdly, the wet-transfer technique was used to transfer the flake in the patterned substrate to form a diode heterostructure. These three steps are detailed as follows: Standard photolithography techniques were used to pattern the highly p-doped Si wafer. A sputtering unit (Shibaura Mechatronics Group) was used to deposit an insulating Al2O3 film (80 nm thickness, base pressure: 9.4 × 10−5 Pa, Ar flow: 20 SCCM, Actual pressure: 0.311 Pa, RF power: 200 W) followed by metallisation by depositing Cr/Au (Cr: 5 nm, Au: 50 nm, Ar flow: 20 SCCM, Act pressure: 0.307 Pa, DC power: 50 W, current: 0.125 A, voltage: 393 V). A SiO2 (90 nm)/Si substrate was spin coated (3000 rpm, 60 s) with a water-soluble layer of polyacrylic acid (PAA), followed by heating on a hotplate for 5 min at 110°C. The second layer of PMMA-A6 was spin coated (3000 rpm, 60 s), followed by heating on a hotplate at 150°C for 5 min. ReS2 crystal (HQ Graphene) was mechanically exfoliated on thermal tape (Nitto Denko, model NO319Y-4LSC). Au (100 nm thick) was sputtered directly onto the exfoliated ReS2/thermal tape. Fresh thermal tape was used to exfoliate Au/ReS2 flake. Then, Au-mediated exfoliated ReS2 flakes were pasted onto the PMMA/PAA/SiO2/Si substrate using thermal tape at 100°C. The flake/PMMA was detached from the wafer under a water bath followed by transfer to a glass slide sample holder. The target flake was aligned under a microscope to the junction between Si and the Au/Cr/Al2O3/Si stack. Contact was made, followed by a slow increase of the substrate temperature to 150°C in 5 min. The temperature of the substrate was lowered to room temperature, and the glass slide was detached from the sample holder. After the flake was transferred to the desired position, the sample was heated in acetone on a hotplate at 60°C for 5–10 min and IPA was used to remove the PMMA. The
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sample was then heated under vacuum at 130°C for 30 min to enhance the heterojunction connection. All electrical measurements were performed in the standard two-probe measurement configuration. The current–voltage (I–V), current–time (I–t) and all electrical characteristics (temperature dependency, C–V, C–f) of the device were measured using an Agilent 2636A and a semiconductor device analyser (Agilent B1500A) source-measurement unit. Rectification diode characteristics were recorded with a digital oscilloscope (Tektronix TBS1052B (50 MHz, 2 ch, USB)) and a Keysight (Agilent) 33220A function generator. The devices were tested in a high-vacuum chamber (5 × 10−3 Pa) in a Lakeshore probe station. The photoresponse of the photodetectors was measured using a continuous-wave laser beam from a diode laser (532 nm, diode-pumped solid-state DPSS laser) and a xenon lamp (Asahi Spectra Co. Ltd., MAX 303). An optical chopper system (New Focus Newport 3501 optical chopper) and chopper wheel (MC1F2 and MC1F60) were used to produce laser pulses with various frequencies as high as 6 kHz. A power metre (Ophir Optics, PD300) was used to measure light intensity. The laser beam could directly irradiate the device through a transparent glass window of the Lakeshore vacuum chamber. An atomic force microscope (Olympus/SHIMADZU, Nano search microscope, model OLS3500/SFT-3500, dynamic scanning probe) and a Raman microscope (Nanophoton, model Ramanplus, 532 nm laser, with ×100 -0.9 NA objective lens and 1200 lines/mm grating) were used for the thickness measurement and sample characterisation.
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ACKNOWLEDGMENT This research was supported by the World Premier International Center (WPI) for Materials Nanoarchitectonics (MANA) of the National Institute for Materials Science (NIMS), Tsukuba, Japan with a Grant-in-Aid for Scientific Research (JSPS KAKENHI Grant No./Project/Area No.17F17360). The authors acknowledge staff members of MANA and the Namiki Foundry at NIMS for their support in sample preparation. A part of this study was supported by NIMS Nanofabrication Platform and NIMS Molecule & Material Synthesis Platform in Nanotechnology Platform Project sponsored by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.
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Bablu Mukherjee: 0000-0002-5625-5948 Amir Zulkefli: 0000-0001-7013-9962 Ryoma Hayakawa: 0000-0002-1442-8230 Yutaka Wakayama: 0000-0002-0801-8884 Shu Nakaharai: 0000-0002-6329-3942
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ASSOCIATED CONTENT Supporting Information Figure S1: Optical image and surface topography of Aumediated large area thin layer ReS2. Figure S2, S7 & S8: Surface morphology and thickness determination of few-layer, multilayer and mixed-thickness ReS2, respectively. Figure S3: Sweep I-V characteristics of Au-ReS2-Au device structure. Figure S4: Electrical characteristics of patterned insulating Al2O3 layer. Figure S5: Sweep IDS-VDS of the vertical heterojunction diode. Figure S6: Rectification characteristics of n-p ReS2/Si vertical diode. Figure S9: Power dependent photocurrent and power law plot of mixed-layer ReS2/Si photodiode. Figure S10: Schematic of the pulsed photocurrent-time measurement setup. Figure S11: (1Reflection) spectra of ReS2 flakes. Table S1: Comparison of the presented work with reported 2D material-based photodiodes.
AUTHOR INFORMATION Corresponding Authors * Email:
[email protected] (B.M.) * Email:
[email protected] (S.N.)
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For Table of Contents Use Only Enhanced Quantum Efficiency in Vertical Mixed-thickness n-ReS2/p-Si Heterojunction Photodiodes Bablu Mukherjee, *,† Amir Zulkefli, †,‡ Ryoma Hayakawa,† Yutaka Wakayama, †,‡ and Shu Nakaharai*,† International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan ‡ Department of Chemistry and Biochemistry, Faculty of Engineering, Kyushu University, 1-1 Namiki, Tsukuba 3050044, Japan †
Table of Contents Significance: The van der Waals n-p junction diode is the most ubiquitous and fundamental building block of modern electronics, with various applications including sensors, detectors, photovoltaics, and light emitters. 2D transition metal dichalcogenides (TMDCs) aids various properties and functionalities, which allows forming van der Walls heterojunction of n-p junction with other 2D or 3D semiconductors. Here we have formed vertical n-p heterojunction diode using physical contact between direct bandgap few layer ReS2 and highly hole doped Si surface. We have demonstrated that n-p heterojunction of mixed-thickness of ReS2 and Si results very high photoresponsivity with fast time-response to enabling practical application in advanced electronic and optoelectronic technologies.
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