Comparison of Electrical and Photoelectrical Properties of ReS2 Field

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Functional Nanostructured Materials (including low-D carbon)

Comparison of electrical and photoelectrical properties of ReS field-effect transistors on different dielectric substrates 2

Ghazanfar Nazir, Malik Abdul Rehman, Muhammad Farooq Khan, Ghulam Dastgeer, Sikandar Aftab, Amir Muhammad Afzal, Yongho Seo, and Jonghwa Eom ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06728 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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ACS Applied Materials & Interfaces

Comparison of electrical and photoelectrical properties of ReS2 field-effect transistors on different dielectric substrates Ghazanfar Nazir1, Malik Abdul Rehman2, Muhammad Farooq Khan1, Ghulam Dastgeer1, Sikandar Aftab1, Amir Muhammad Afzal1, Yongho Seo2, Jonghwa Eom1* 1

Department of Physics & Astronomy and Graphene Research Institute, Sejong University,

Seoul 05006, Korea 2

Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul

05006, Korea

KEYWORDS: ReS2, Schottky barrier height, h-BN, trap density of state, photodetector

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ABSTRACT: As one of the newly discovered transition metal dichalcogenides (TMDs), rhenium disulfide (ReS2) has been investigated mostly because of its unique characteristics such as direct bandgap nature even in bulk form, which is not prominent in other TMDs (e.g., MoS2, WSe2, etc.). However, this material possesses a low mobility and on/off ratio, which restricts its usage in high-speed and fast switching applications. Low mobilities or on/off ratios can also be caused by substrate scattering as well as environmental effects. In this study, we used few-layer ReS2 (FL-ReS2) as a channel material to investigate the substrate-dependent mobility, current on/off ratio, Schottky barrier height (SBH), and trap density of states of different dielectric substrates. The h-BN/FL-ReS2/h-BN structure was observed to exhibit a high mobility of 45 cm2 V–1 s–1, current on/off ratio of about 107, the lowest SBH of about 12 mV at a zero back-gate voltage (Vbg), and a low trap density of states of about 5×1013 cm–3. These quantities are reasonably superior compared to the FL-ReS2 devices on SiO2 substrates. We also observed a nearly fivefold improvement in photoresponsivity and EQE values for the FL-ReS2 devices on hBN substrates. We believe that the photonic characteristics of TMDs can be improved by using h-BN as substrates and capping layers.


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INTRODUCTION Regardless of all its advantages as a significant material because of its atomically thin layered nature, graphene1-3 is barely suitable for application to field-effect transistors (FETs) as switching materials because of its zero bandgap4. The lack of current applications involving graphene has motivated researchers to seek other materials that have comparable characteristics and possess appropriate bandgaps. The advent of transition metal dichalcogenides (TMDs)5-8 with appropriate bandgaps addressed this issue. As one of the recently discovered materials, rhenium disulfide9-18 (ReS2) possesses many profitable properties because of its weak interlayer coupling19 and distinct distorted 1T structure20. Irrespective of the layer thickness, ReS2 possesses a direct bandgap, which is 1.5 eV for monolayer and 1.58 eV for bulk materials21-22. Thus, a high probability of light absorption and emission can be expected for both monolayer and bulk ReS2. However, the low mobility of ReS2 makes this material less competent compared to other TMDs16, 22-24. A comparative study of mobility of ReS2 has been added in Table S1 of Supplementary Information. The deterioration in mobility occurs because of several factors, for instance, the scattering of carriers due to substrate roughness25-26, interfacial charged impurities27, substrate phonons27, adsorbents on the surface28, electron– phonon coupling, and adsorption of oxygen and water molecules during the fabrication process. Furthermore, the low mobility of these devices is attributed to contact resistance (RC) caused by the Schottky barrier at the metal–ReS2 interface29. To preserve the intrinsic properties of the channel material (ReS2), a simple but proficient approach involves the use of hexagonal boron nitride (h-BN) film as a dielectric substrate, which has been successfully corroborated for graphene as well as other TMD FETs30-31. Its large bandgap, inert nature, absent dangling bonds, low density of charged impurities, and flat surface offers the ultimate platform32 as a dielectric substrate and tunnel barrier, which are crucial 3 ACS Paragon Plus Environment


elements in two-dimensional (2D) optoelectronic devices. Furthermore, the reduction of charge impurities due to the h-BN flat surface without dangling bonds boosts the field-effect mobilities of 2D materials considerably. Numerous electro-optical studies have been carried out based on the favorable impact of the h-BN substrate on the carrier transport of TMDs. For example, the photodegradation of TMDs has been mitigated by encapsulation with h-BN layers. In such devices, a single layer of h-BN was proved to be inefficient to protect complete photodegradation; thus, three layers of h-BN were utilized33. A metal–insulator transition (MIT) mechanism based on percolation theory for a MoS2/h-BN heterostructure was also reported34. In addition, the spin transport in fully h-BN encapsulated graphene was explored to reveal h-BN as an effective tunnel barrier because of its clean and pinhole-free nature35. Aside from the substrate-induced charge scattering effects, there is another fundamental characteristic called the Schottky barrier height (SBH) that exists at the metal–semiconductor interface and exhibits a significant RC, which governs the transport properties of TMDs. Low RC and improved interface quality are prerequisites for better electronic devices. The value of SBH has been determined by the difference in the work function of the metal and electron affinity of the semiconductor36. It has commonly been proposed that the magnitude of the SBH can be substantially reduced by choosing the work function of the deposited metal to be almost identical to the electron affinity of the semiconductor. However, the reduction of the SBH by choosing hBN as the dielectric substrate has not been sufficiently explored. Here, we report a comparative study of a few-layer ReS2 (FL-ReS2) FET on different dielectric substrates. We demonstrate that ReS2 FETs encapsulated by h-BN films are efficient compared to ReS2 FETs on a SiO2 substrate regarding electro-optical properties. We observe a nearly tenfold higher mobility in h-BN/FL-ReS2/h-BN devices compared to ordinary ReS2 FETs


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on the SiO2/Si substrate. We also observe high Ion/Ioff ratios and low SBHs for the encapsulated ReS2 devices. The observation of a low carrier trap density in h-BN/FL-ReS2/h-BN FETs is attributed to the clean surface of h-BN and environmental protection from oxygen and water molecules. We also examine the photodetector characteristics of FL-ReS2/SiO2 and FL-ReS2/hBN to compare their response under light. High photocurrents are observed in the FL-ReS2/h-BN devices compared to FL-ReS2/SiO2. A rapid rise/decay time is observed for the FL-ReS2/h-BN devices with improved responsivity and external quantum efficiency (EQE). Therefore, our experimental approach offers an extended study on dielectric dependent electro-optical characteristics for FL-ReS2 material that could be effectively applied on other materials as well.

EXPERIMENTAL SECTION Device fabrication Here, we fabricated two kinds of devices. For FL-ReS2/SiO2 devices, FL-ReS2 flakes were mechanically exfoliated on 300-nm-thick SiO2 on a degenerately doped Si substrate with the help of adhesive tape. After searching for a desired ReS2 flake through an optical microscope, photolithography was performed to form large patterns around the flake. Metallization of the large pattern was performed by Cr/Au deposition at a thickness of 5/30 nm, after which a lift-off process was carried out in acetone. After then, small metallic contacts were constructed to define the ReS2 channel region by e-beam lithography with Cr/Au (8/60 nm) deposition and subsequent lift-off in acetone. Finally, an annealing process was carried out in tube furnace at 200 ˚C with Ar/H2 (97.5% / 2.5%) gas flow for 4 hours. For FL-ReS2/h-BN devices, an h-BN flake was first mechanically exfoliated on 300-nmthick SiO2 on a degenerately doped Si substrate. Then, photolithography was carried out to 5 ACS Paragon Plus Environment


define a large pattern around the h-BN flake. The large pattern was filled with 5/30-nm-thick Cr/Au deposition as for the previous devices. The lift-off process was completed in acetone. In the subsequent process, FL-ReS2 flakes were placed on PDMS stamps and then dry-transferred37 on a pre-exfoliated h-BN flake inside the large pattern with the help of a micromanipulator and optical microscope. After transfer, these samples were then baked on a hot plate at 150 oC in the air for few minutes to reduce wrinkles or air bubbles. No photo-resist (like PMMA) involves in this fabrication process, so the organic residues is negligible on FL-ReS2 and h-BN surfaces. Subsequently an annealing process has been applied during the fabrication of the h-BN/FLReS2/h-BN devices. We annealed the devices after transfer of FL-ReS2 on pre-exfoliated bottom h-BN flake to remove the air bubbles. The annealing was done at 200 oC in with forming gas (Ar/H2 = 97.5%/2.5%) flow for 4 hours inside a tube furnace. Finally, source/drain contact electrodes were fabricated on FL-ReS2 using e-beam lithography with 8/60-nm-thick Cr/Au deposition and a lift-off process in acetone. For h-BN/FL-ReS2/h-BN devices, the top h-BN flake was further dry-transferred to the FL-ReS2/h-BN devices. We annealed the device again after transfer of top h-BN.

Optical and electrical measurement Raman spectroscopy (micro-Raman, Renishaw) was examined for both FL-ReS2/SiO2 and FL-ReS2/h-BN devices. For Raman spectroscopy, a laser wavelength of 514 nm was used with a low power equal to 511 µW to avoid structural deformation by the heating effect of the laser. In Raman spectroscopy, the laser spot size was 0.7 µm. To examine the thickness of ReS2 flakes, atomic force microscopy (AFM) was carried out in tapping mode under ambient conditions (n-Tracer, NanoFocus). For electrical transport measurements, we used a Keithley


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2400 sourcemeter and Keithley 6485K picoammeter for the devices at room temperature under a vacuum of 10–3 Torr. Some of the devices were then placed in another vacuum under pressures of 10–4–10–5 Torr to perform further measurements at low temperature.

RESULTS AND DISCUSSION Figure 1(a) presents a schematic of the h-BN/FL-ReS2/h-BN device. Figures 1(b–e) describe in detail the step-by-step procedure using optical microscope images. First, the h-BN flake was exfoliated on top of the 300-nm SiO2/Si substrate. AFM was used to estimate the thickness of each layer. The corresponding AFM images and height profiles are presented in Fig. S1. Raman spectroscopy was carried out for the FL-ReS2/SiO2 and FL-ReS2/h-BN devices and the results are presented in Fig. S2. We measured the Raman spectra in the range of 100–400 cm–1, where nearly 11 modes are observed. These Raman peaks are due to the low crystal symmetry with the fundamental vibrational modes (A1g, E2g, E1g) coupled to each other and to acoustic phonons. We can observe three prominent peaks at 153, 163, and 213 cm–1, which correspond to the E2g (in-plane) and A1g (out-of-plane) vibrational modes21. It is already known that ReS2 does not exhibit a corresponding change in the Raman peak locations with the layer thickness from the monolayer to the bulk21. The Raman vibrational modes of ReS2 are insensitive because of weak interlayer coupling, which distinguishes this material from other TMDs, of which the Raman peak positions depend on their thicknesses. Because of this property of ReS2, it remains a direct bandgap material even in bulk, contrary to other TMDs. However, in our case, we observe a change in the Raman peak positions of FL-ReS2 on different dielectric substrates (Fig. S2). It has been noted that the E2g and A1g peak positions do not change with the change in the substrate. However, there is slight change observed at about 280 cm–1 for the FL-



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ReS2/h-BN device. This weak Ag-like peak exhibits substrate-dependent behavior. Therefore, the observed redshift in the Ag-like mode could be due to electron doping in FL-ReS2 from the h-BN substrate that leads to an increase in the work function of ReS2, which consequently reduces the SBH38-39.

Substrate-dependent electrical properties The mobility and Ion/Ioff ratio were measured to study the electrical performance of FLReS2 on different substrates. To estimate these characteristics, the transfer characteristics (Ids– Vbg) at Vds = 0.5 V for FL-ReS2 on various dielectric substrates were studied, as shown in Figs. 2(a–c) on linear and semi-logarithmic scales. The field-effect mobility (μ ) can be calculated by the following relation:

μ =

(

)

.

Where L is the channel length, W is the channel width, (

(1)

) is the slope of the transfer

characteristics, and C is the gate capacitance (~115 aF/µm2) for the Si/SiO2 substrate. It is evident from Figs. 2(a–c) that the mobility of FL-ReS2 depends on the choice of dielectric substrate. Nearly a sevenfold enhancement in mobility is observed in the FL-ReS2/h-BN device as compared to the FL-ReS2/SiO2 device. The enlarged mobility in FL-ReS2/h-BN devices is attributed to the h-BN substrate with a low surface roughness, charge impurity scattering, and surface optical phonon scattering relative to the SiO2 substrates28, 38. In contrast, nearly tenfold elevated mobility values are observed in the h-BN/FL-ReS2/h-BN structure compared to the FLReS2/SiO2 structure. The top h-BN layer in the h-BN/FL-ReS2/h-BN devices acts as a capping layer to shield the FL-ReS2 channel from environmental adsorbates such as oxygen, water


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molecules, and hydrocarbons. We have also compared our electrical properties (mobility, on/off ratio etc.) with recently reported studies on ReS2 with other dielectric substrates as shown in Table S2 of Supplementary Information. To have a quantitative idea about the charge impurities, hysteresis in the transfer characteristics was examined. A graph of the normalized current as a function of Vbg is presented in Fig. S3(a), where the reference current was chosen using Ids at Vbg = 40 V. While a strong hysteresis is found in FL-ReS2/SiO2, almost no hysteresis is found in the h-BN/FL-ReS2/h-BN structure as observe in h-BN encapsulated MoS2 in a previous report40. A semi-log plot is presented in Fig. S3(b) for FL-ReS2 on different dielectric substrates. Moreover, the h-BN/FLReS2/h-BN device has fewer charge impurities from the substrate defects or environment, so it exhibits an enhanced mobility. To operate devices in digital applications, it should have an output Ion/Ioff ratio of at least 104. We estimated the Ion/Ioff ratio for FL-ReS2 on different substrates in the transfer curves of Figs. 2(a–c). The FL-ReS2/SiO2 device exhibits an Ion/Ioff ratio of about 105, which is better than other TMDs on SiO2 substrates16,

41-43

. However, the FL-

ReS2/h-BN and h-BN/FL-ReS2/h-BN devices exhibit an Ion/Ioff ratio of about 107, which is 100 times higher than that of FL-ReS2/SiO2 because of the high mobilities. The transfer curves presented in Figs. 2(a–c) were obtained at 200 K, where we observe a MIT for the FL-ReS2/hBN and h-BN/FL-ReS2/h-BN devices. The MIT can be confirmed by the transfer curves at various temperatures in Figs. 2(d) and S5. However, we do not observe an MIT signal in the case of the FL-ReS2/SiO2 transfer curves in Fig. S4(c). The mobility of FL-ReS2/SiO2 increases monotonically with temperature, as shown in Fig. S4(d). Figure 2(d) illustrates the temperature-dependent transfer curves (Ids–Vbg) for FL-ReS2 on different substrates. The transfer curves were chosen in the range of 200–300 K for all devices to



see the clearer trend of the MIT near 200 K. The complete set of temperature-dependent transfer curves have been plotted in Figs. S4(c) and S5(a,b) for the FL-ReS2/SiO2, FL-ReS2/h-BN, and hBN/FL-ReS2/h-BN devices, respectively, at Vds = 0.5 V, with a demonstration of the MIT at about 200 K. However, the MIT is absent in FL-ReS2/SiO2, as shown Fig. S4(c). It has been illustrated in the literature that gate-induced carrier transport in the thin layer close to the surface of MoS2 (TMDs) may be affected by charge impurities and different kinds of disorders44 from the substrate and therefore diminish the possibility of attaining a MIT. In contrast, high-k dielectric materials, e.g., h-BN, can provide an impurity-free substrate that enhances the chance of MIT observation by overcoming the scattering mechanisms. Radisavljevic et al. investigated the MIT in monolayer MoS2 by using the top and bottom gate to tune the carrier doping concentration electrostatically inside the channel region of MoS245. Based on their temperaturedependent mobility measurement, they successfully interpreted the MIT at about 200 K. In our measurement [Fig. 2(e)], we also observed an MIT signal at about 200 K for the FL-ReS2/h-BN and h-BN/FL-ReS2/h-BN devices, so our data are consistent with the previous report45. From Fig. 2(e), the mobility decreases as the temperature decreases from 200 to 25 K. This behavior is consistent with the mobility limited by scattering from charged impurities46, but with the increase in temperature above 200 K, the decrease in mobility is due to electron–phonon scattering, which is a leading phenomenon at higher temperatures47. It has been proposed recently that ReS2 possesses a 1T` structure vulnerable to intrinsic band inversion between chalcogenide p- and metallic d- bands. Therefore, its bandgap can easily be tuned by the application of an external electric field and can thus lead to a phase transition. This useful modification leads us to the observation of a MIT48.


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The temperature-dependent mobilities for FL-ReS2 on different dielectric substrates are shown in Fig. 2(e). The mobility of the FL-ReS2/SiO2 device monotonically decreases as the temperature is lowered from room temperature to 25 K. However, the mobilities of the FLReS2/h-BN and h-BN/FL-ReS2/h-BN devices exhibit maxima at about 200 K, which is indirect evidence of an MIT. The decreasing mobility dependence below 200 K can be understood by considering the hopping transport of carriers between trap states in the FL-ReS2 channel based on multiple trap-and-release models explained in a previous report10.

Dielectric-sensitive SBH The Schottky barrier is the potential energy barrier for electrons formed at metal– semiconductor interfaces. One of its primary characteristics is the SBH, denoted by Φ . The value of Φ is predicted by the Schottky–Mott rule, which states that ΦB is proportional to the difference in the metal work function and semiconductor electron affinity. However, most semiconductors do not follow this rule because of the formation of metal-induced gap states49-50, which fills with electrons and consequently pins the center of the bandgap near the Fermi level. This effect is called Fermi-level pinning51-52. Therefore, the suitable selection of metals and semiconductors is a crucial tool to realize a low SBH value that will boost the device electrooptical performance. Here, we chose a Cr contact, of which the work function Φ ~ 4.5 eV53. We deposited Cr (~8 nm) together with 60 nm of gold as the protecting layer. We believe that our metal contacts are exclusively made of Cr owing to their direct adhesion with the semiconductor interface. The electron affinity for ReS2 is χ ~ 4.35 eV13, 54. Thus, ΦB can now be calculated by the following relation:

Φ = Φ − χ . 11 ACS Paragon Plus Environment

(2)


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Therefore, SBH (Φ ) is approximated as 150 meV. To extract the actual magnitude of Φ , we applied the SBH formula, which is based on 2D thermionic emission theory:

I = A∗#$ S T

() . # exp -− 2Φ /0 1

−

3

45

(3)

where A∗#$ is the 2D-equivalent Richardson constant = 4πqm∗ k # ⁄h( , =∗ is the effective mass, h is Plank’s constant, S is the area of the Schottky junction, q is the charge of the electron, k is Boltzmann’s constant, Vds the drain–source voltage, and n is the ideality factor whose value was estimated as about 3.2 for the FL-ReS2/SiO2 devices and about 1.4 for the FL-ReS2/h-BN and hBN/FL-ReS2/h-BN. The large ideality factor value in FL-ReS2/SiO2 clearly indicates a large density of trap states inside the channel, which could enhance the recombination of charge carriers and make the device diverge from the ideal diode behavior, of which the ideality factor should be about unity. However, our devices on h-BN exhibit an almost ideal diode behavior (n = 1.4), which verifies our hypothesis of the defect-free nature of h-BN that allows less recombination and boosts the channel carrier transport. Based on the above theory, an Arrhenius plot, i.e., ln (Ids/T3/2) versus 1000/T at different Vds, was formed for FL-ReS2/SiO2 (Fig. 3(a)), FL-ReS2/h-BN (Fig. 3(c)), and h-BN/FL-ReS2/h-BN (Fig. 3(e)). By fitting the data linearly to .

each Vds, we acquire slopes with S = − 2Φ − >>>/ 0

3

4. After plotting the slopes versus Vds,

the values of Φ were determined by extrapolating a straight line to the slope intercept for FLReS2/SiO2 (Fig. 3(b)), FL-ReS2/h-BN (Fig. 3(d)), and h-BN/FL-ReS2/h-BN (Fig. 3(f)). As our devices involve back-to-back geometry composed of two Schottky junctions, the current cannot flow fully in the high-forward-bias regime. Consequently, Eq. 3 is applicable only to the low-voltage-bias regime, as seen in Figs. 3(b), (d), and (f). The substrate-sensitive ΦB as a function of Vbg is presented in Fig. 3(g). An exponential decay of ΦB can be seen clearly as Vbg


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increases in the case of the FL-ReS2/SiO2 device, but there is not much variation for FL-ReS2/hBN or h-BN/FL-ReS2/h-BN. In the case of FL-ReS2/SiO2, there exist many charge impurities and surface traps on the SiO2 substrate. These impurities mostly serve as p-type dopants because of the presence of oxygen and dangling bonds. At low Vbg, many electrons are captured by these impurities and the Fermi level of FL-ReS2 shifts toward the valence band, which in turn increases the work function. This leads to a significant rise in ΦB values. On the contrary, h-BN is a flat, inert, and clean substrate with negligible dangling bonds that provide an ideal substrate for overlaying a semiconducting layer (FL-ReS2). Hence, carriers can easily flow in the semiconducting channel in FL-ReS2/h-BN with a large mobility. It has also been recently reported that the h-BN surface can dope n-carriers to TMDs38, and hence a shift in the Fermi level toward the conduction band is expected. Consequently, it reduces the work function of the FL-ReS2, which explains the low ΦB values observed in the FL-ReS2/h-BN devices. Another subject that should be discussed is the fact that the adhesion of oxygen and water molecules along with hydrocarbons55 can immensely degrade the quality of TMDs. To understand the environmental effects on the electrical properties of ReS2, we took an FLReS2/SiO2 FET with a top h-BN as a capping layer. An optical image of that device is presented in Fig. S6(a). The transfer curves in Fig. S6(b) represent the Vds-dependent transport mechanism. It is interesting to note that the mobility of the FL-ReS2/SiO2 device (Fig. S4) is about 5 cm2 V–1 s–1, whereas the mobility of the (top h-BN)/FL-ReS2/SiO2 device (Fig. S6(a)) is about 10 cm2 V–1 s–1 at room temperature with Vds = 0.5 V. It is clear that environmental effects play a vital role in the electrical transport of the present experiment. These environmental adsorbates can potentially dope not only the semiconducting channel but also affect the interface between the metal/semiconductor junction. We can observe a lower ΦB value after encapsulating FL-ReS2



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with h-BN, as shown in Fig. 3(g). A nearly sevenfold lower ΦB is found in h-BN/FL-ReS2/h-BN compared to FL-ReS2/SiO2.

Dielectric-sensitive trap density (Nt) To observe the metal contact quality, Ids–Vds curves were measured for FL-ReS2/SiO2, FL-ReS2/h-BN, and h-BN/FL-ReS2/h-BN devices. The Ids–Vds curve for FL-ReS2/SiO2 does not vary linearly, suggesting a high Schottky barrier at the (Cr/Au)–ReS2 interface due to impurityinduced gap states in FL-ReS2. However, in the case of the FL-ReS2/h-BN and h-BN/FL-ReS2/hBN structures, Ids–Vds exhibits almost ohmic contact characteristics originating from the low SBH. Figure 4(a) presents the Ids–Vds curve measured at Vbg = 10 V for FL-ReS2 on different substrates. The current (Ids) is the highest in the case of the h-BN/FL-ReS2/h-BN structure because of the desirable ohmic characteristics in the device. The density of trap states (Nt) plays a vital role in the transport mechanism of TMD FETs56-57. The trap density can be determined quantitatively from the temperature-dependent log (Ids)–log (Vds) characteristics. The trap states slowly fill up by injected charge carriers and the conductivity of the device becomes independent of temperature at some value of Vds. The I–V characteristics at various values of temperature cross each other at a certain Vds, as seen in Figs. 4(b–d). This critical value of Vds is Vc and can be expressed by the following relation58:

V@ =

.AB C #DE DF

(4)

where Nt is the density of trap states, L is the channel length of the device (= 6.8 µm), GH is the permittivity of free space (= 8.85×10–12 F/m), and GI is the dielectric constant for SiO2 (= 3.9). We deduce Vc by extrapolating the I–V curves in the logarithmic scale at various temperatures 14 ACS Paragon Plus Environment

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(T = 200, 170, 140, 120, 100 K) with Vbg = 10 V for the FL-ReS2/SiO2, FL-ReS2/h-BN, and hBN/FL-ReS2/h-BN devices. These I–V characteristic lines cross each other at the critical value of Vds at a fixed Vbg. The extracted Vc and Nt are plotted as a function of Vbg for FL-ReS2 on different substrates in Fig. 4(e). The lowest Nt is found in the h-BN/FL-ReS2/h-BN device. We presume that these traps emerge from the bulk nature of the ReS2 crystal instead of the interface trap states59. It is observed that Vc decreases with Vbg because the density of trap states is reduced as free charge carriers fill the trap states.

Photoresponse of FL-ReS2 We have compared the optoelectronic properties of FL-ReS2/SiO2 and FL-ReS2/h-BN to investigate the substrate-dependent optoelectronic characteristics. Figure 5(a) presents the photocurrent as a function of time for the FL-ReS2/SiO2 and FL-ReS2/h-BN substrates when deep ultraviolet (DUV) light illumination is switched on and off. The devices were illuminated by DUV light with an intensity of 11 mW/cm–2 and wavelength of 220 nm. The photocurrent increases in the case of FL-ReS2/h-BN compared to FL-ReS2/SiO2. This rise in photocurrent for FL-ReS2/h-BN is attributed to the fact that the flat and inert surface of h-BN minimizes the scattering effects and reduces electron–hole recombination in the FL-ReS2 channel. The devices were measured in vacuum to remove any external degradation of oxygen or water molecules on the ReS2 surface. The photoresponse was measured at Vbg = 0 V to eliminate any conceivable gate contribution. However, the source–drain voltage was maintained at Vds = 1 V to provide drift to the charge carriers across the channel in both FL-ReS2/SiO2 and FL-ReS2/h-BN devices. The FL-ReS2/SiO2 device has a channel length (L = 6.8 µm) and width (W = 3.3 µm) with effective channel area (A = 22 µm2), whereas the FL-ReS2/h-BN device has the channel length



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(L = 6.8 µm) and width (W = 3.8 µm) with effective channel area (A = 26 µm2). However, the FL-ReS2 flakes on different dielectric substrates (SiO2 and h-BN) have same thickness (6 nm). Therefore, the small difference in channel width does not affect the electro-optical characteristics much. We believe that the different characteristics between FL-ReS2/SiO2 and FL-ReS2/h-BN observed in our experiments are mainly from the substrate effect. To show the advantage of the FL-ReS2/h-BN devices over the FL-ReS2/SiO2 devices, their photo response (rise and decay time) was estimated. These parameters were calculated using the original built-in fitting equations:

JKL (M) = JNOIP + RS JKL (M) = JNOIP + RS

T) TUVWX

(5-a)

YT) TZX[\]

(5-b)

Equation (5-a) was used to estimate the rise time for both FL-ReS2/h-BN and FL-ReS2/SiO2 devices, whereas Eq. (5-b) was used to estimate the decay time for both devices. The factor J^_ (M) is the photocurrent, J `/ is the dark current in the absence of light illumination, M is the time for switching on/off the DUV light, and A is the scaling constant. The estimated values of the rise and decay time for the FL-ReS2/h-BN device is 3.5 and 16 s, respectively, which are much better than those for the FL-ReS2/SiO2 device, 6 and 21 s, respectively. It is evident that the FL-ReS2/h-BN photodetector is efficient compared to the FL-ReS2/SiO2 photodetector.

The

basic

phenomena

of

photodetectors

are

the

charge

carrier

generation/recombination processes that depend on the photodetector response to light. The photocurrent is sensitive to the dielectric substrate choice. As the SiO2 substrate is not sufficiently flat and has more defects compared to h-BN, the defect states capture electron/hole pairs that are produced by light. This is the reason that FL-ReS2/SiO2 devices


Page 17 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


are less responsive to light compared to FL-ReS2/h-BN devices. FL-ReS2/SiO2 devices take more time to generate photocurrent because of the trapping of carriers in SiO2 defect sites15. In contrast, FL-ReS2/h-BN devices are efficient and respond quicker than FL-ReS2/SiO2 devices because of the flat, inert, and defect-free nature of the h-BN substrate. In addition to the response time, photodetector performance is also evaluated by two other factors such as responsivity (Rλ) and EQE. To estimate the photoresponsivity defined as the photocurrent produced per unit power of incident light on the effective device area of the photodetector, we used the following equation:60

Rb =

∆ de ^f

(6)

where ∆IPh is the generated photocurrent and is defined as ∆IPh = IPh – Idark, P is the incident light intensity equal to 11 mW cm–2, and A is the effective area equal to 22 µm2 for FL-ReS2/SiO2 and 26 µm2 for FL-ReS2/h-BN. The responsivity value for the FL-ReS2/SiO2 device is 13 A/W and that for the FL-ReS2/h-BN device is 60 A/W, which is nearly five times higher. The EQE is defined as the number of photo-induced carriers per incident photon and can be given by61

EQE =

_@ij b

(7)

where c is the speed of light, Rλ is the photoresponsivity, and λ is the incident light wavelength. The EQE is highly dependent on the wavelength of incident light. For the same wavelength, the EQE depends only on the photoresponsivity because all other remaining factors are constant. The observed EQE values are 0.73% and 3.4% for the FL-ReS2/SiO2 and FL-ReS2/h-BN device, respectively. The increase in EQE for the FL-ReS2/h-BN device is attributed to the enhanced photocurrent because of the large generation of electron/hole pairs without capture by defects in the dielectric substrates. Therefore, our FL-ReS2/h-BN devices are advantageous for some 17 ACS Paragon Plus Environment


previously reported devices. A comparison of the electrical and photoelectrical properties of our FL-ReS2 devices with previously reported devices are presented in Table.1.

CONCLUSIONS In summary, we have successfully fabricated FL-ReS2 devices on different dielectric substrates and have conducted a comparison based on their electrical and photoelectrical response. The nearly tenfold higher mobility and high on/off ratio in h-BN/FL-ReS2/h-BN devices compared to FL-ReS2/SiO2 devices are attributed to the lower SBH and absence of defects in the channel region. The results are further validated by calculating the density of trap states, which show the smallest value in the h-BN/FL-ReS2/h-BN devices. In addition, we have compared the photoresponse of FL-ReS2 devices on SiO2 and h-BN substrates. The fast photoresponse for the FL-ReS2/h-BN devices are due to the flat, inert, and defect-free nature of h-BN, which does not allow the carriers to be trapped in the defects sites. The photoresponsivity and EQE were estimated and found to be nearly five times higher for the FL-ReS2/h-BN devices compared to their counterpart FL-ReS2/SiO2 devices. The electrical and photoelectrical characteristics of the TMDs can be improved by employing h-BN as the substrate and capping layer, and the improvement in characteristics is significant.

ASSOCIATED CONTENT Supporting Information. AFM images and corresponding height profiles for bottom h-BN, FL-ReS2, and top h-BN, respectively, in h-BN/FL-ReS2/h-BN device presented in Fig. 1. Raman analysis for FL18 ACS Paragon Plus Environment

Page 18 of 35

Page 19 of 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ReS2 on different dielectric substrates. Normalized transfer curves for FL-ReS2/SiO2, FLReS2/h-BN, and h-BN/FL-ReS2/h-BN devices. Ids–Vbg curves and mobility for different temperatures at fixed Vds = 0.5 V for FL-ReS2/SiO2 device. Transfer characteristics of hBN/FL-ReS2/SiO2 device. Ids–Vds curves at different Vbg at room temperature for the hBN/FL-ReS2/SiO2 device.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Funding Sources This research was supported by the Priority Research Center Program (2010-0020207) and the Basic Science Research Program (2016R1D1A1A09917762) through the National Research Foundation of Korea funded by the Ministry of Education.



Page 20 of 35

Top h-BN

(a)

(b)

Bottom h-BN 10 µm

ReS2

(d)

(c)

10 µm

(e)

10 µm

10 µm

Figure 1. Device structure. (a) Schematic of h-BN/FL-ReS2/h-BN device on highly degenerated p-doped Si with SiO2 (300 nm) layer. Device fabrication scheme: (b) h-BN flake on Si/SiO2 substrate inside large deposited Cr/Au pattern, (c) transfer of FL-ReS2 flake on bottom h-BN, (d) contact fabrication by e-beam lithography followed by Cr/Au deposition, and (e) final transfer of top h-BN flake on bottom ReS2/h-BN flake.


Page 21 of 35

(b)

-5

10

1.0

FL-ReS2/SiO2 ~1 05

0.4

off

10

/I on

I

2

10

10

10

-9

-11

10

-13

(e) 100 -1 -1 2

µ = 36 cm /Vs

15

9 6 3

300K 250K 200K @ FL-ReS2/SiO2 300K 250K 200K @ FL-ReS2/h-BN

0 -60 -40 -20 0

x2

20 40 60

Vbg (V)

FL-ReS2/h-BN h-BN/FL-ReS2/h-BN

50 25 0 0

/20

300K 250K 200K @ h-BN/FL-ReS2/h-BN

FL-ReS2/SiO2

75

0

-75 -50 -25 0 25 50 75 Vbg (V)

12

-75 -50 -25 0 25 50 75 Vbg (V)

µ (cm V s )

10

h-BN/FL-ReS2/h-BN

Ids (µA)

10

-7

3

-13

(d)

10

6 2

0.0

15 12 9 7 6 I /I ~ 10 on off 3 2 µ = 43 cm /Vs 0

Ion/Ioff ~ 10

-9

10

-75 -50 -25 0 25 50 75 Vbg (V)

-5

9 7

10

(c) Ids (A)

10

-11

0.2

µ ~ 5 cm /Vs

-13

12

-7

Ids (µA)

Ids (A)

-9

0.6

15

FL-ReS2/h-BN

50 100 150 200 250 300 T (K)


Ids (µA)

10

-11

10

0.8

-7

10

-5

Ids (A)

(a)

Ids (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Figure 2. Transport properties of FL-ReS2 under different dielectric environments. Transfer characteristics (Ids–Vbg) of (a) FL-ReS2/SiO2, (b) FL-ReS2/h-BN, and (c) h-BN/FLReS2/h-BN at 200 K. (d) Transfer characteristics (Ids–Vbg) of FL-ReS2 on different substrates at temperatures ranging from 200 to 300 K to observe an MIT. The complete transfer characteristics covering the full temperature range from 25 to 300 K has been presented in Figs. S4(c) and S5(a,b) for FL-ReS2/SiO2, FL-ReS2/h-BN, and h-BN/FL-ReS2/h-BN, respectively. The FL-ReS2/SiO2 transfer curves should be divided by 20 and the FL-ReS2/h-BN transfer curves should be multiplied by two to obtain the original data. (e) Temperature dependent mobility of FL-ReS2 on different dielectric substrates. All experiments were performed at Vds = 0.5 V with same channel length of 6.8 µm and channel width of 3.3 µm for FL-ReS2/SiO2 and 3.8 µm for FL-ReS2/h-BN respectively.


Page 22 of 35

Page 23 of 35

(b)

-24

FL-ReS2/SiO2

-27

Vds = 1 V

-30 -33 Vds = 0.1 V

-36 4

-18

6 8 10 1000/T (1/K)

0.8

0.2 0.4 0.6 0.8 1.0 Vds (V)

FL-ReS2/h-BN

-24 -27 6 7 8 9 10 11 1000/T (1/K)

(f)

-18

0.6

Slope (K)

h-BN/FL-ReS2/h-BN

-21 -24

FL-ReS2/h-BN ΦB ~ 21 mV

0.4 0.2 0.0 0.0

-30

0.30 0.24

0.3

0.6 0.9 Vds (V)

1.2

h-BN/FL-ReS2/h-BN ΦB ~ 12 mV

0.18 0.12

-27 5

6

7 8 9 10 11 1000/T (1/K)

(g)

200

0.06 0.0

0.3

FL-ReS2/SiO2 FL-ReS2/h-BN

150 ΦB (mV)

3/2

1.0

(d)

5

FL-ReS2/SiO2 ΦΒ ~ 92 mV

0.6

-21

(e)

1.2

12

Slope (K)

3/2

ln (Ids/T )

(c)

Slope (K)

3/2

ln (Ids/T )

(a)

ln (Ids/T )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


h-BN/FL-ReS2/h-BN

100 50 0 0

10

20 30 Vbg (V)

40


0.6 0.9 Vds (V)

1.2


Figure 3. Substrate-sensitive SBH. Arrhenius plot ln (Ids/T3/2) versus 1000/T at different drainto-source voltages from Vds = 0.1 V (purple) to 1 V (brown) of (a) FL-ReS2/SiO2, (c) FL-ReS2/hBN, and (e) h-BN/FL-ReS2/h-BN. Slope achieved from Arrhenius plot as a function of Vds for (b) FL-ReS2/SiO2, (d) Vds for FL-ReS2/h-BN, and (f) h-BN/FL-ReS2/h-BN. (g) SBH as a function of Vbg for different substrates. Note: Channel length is 6.8 µm and channel width of 3.3 µm for FL-ReS2/SiO2 and 3.8 µm for FL-ReS2/h-BN respectively.


Page 24 of 35

Page 25 of 35

(b)

225

FL-ReS2/SiO2

Ids (nA)

150

FL-ReS2/h-BN

log Ids (A)

(a)

h-BN/FL-ReS2/h-BN

75 0 -75

x10 -150 -1.0 -0.5 0.0 0.5 Vds (V)

1.0

(d)

-3

VC = 42 V -8

10

200K 170K 140K 120K 100K

-10

10

0.1

-4

10 -6 10 -7 10 -8 10

Vc ~ 6V 170K 150K 130K 110K

1 log Vds (V)

1 10 log Vds (V)

10

h-BN/FL-ReS2/h-BN

-5

10

Vc = 4 V

-6

10

170K 150K 130K 110K

-7

10

-8

10

10

0.1

1

15

FL-ReS 2 /SiO 2 FL-ReS 2 /h-BN h-BN/FL-ReS 2/h-BN

x1

0

FL-ReS 2 /SiO 2

12 9

FL-ReS 2 /h-BN h-BN/FL-ReS 2/h-BN

6

-3

3

x3

13

Vc (V)

60

20

10

log Vds (V)

80

40

100

-3

10

-5

(e)

-6

-12

10 FL-ReS2/h-BN -4 10

0.1

FL-ReS2/SiO2

10

10

log Ids (A)

log Ids (A)

(c)

-4

10

Nt (x10 cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

0 0

10

20 30 V bg (V)

40

Figure 4. Dielectric substrate-dependent trap density: (a) Output characteristic (Ids–Vds) curves at fixed Vbg = 10 V for FL-ReS2/SiO2, FL-ReS2/h-BN, and h-BN/FL-ReS2/h-BN devices 25 ACS Paragon Plus Environment


for Cr/Au metal contacts. A factor of 10 should be multiplied to obtain the original values for the FL-ReS2/h-BN and h-BN/FL-ReS2/h-BN devices. Temperature-dependent output characteristics of (b) FL-ReS2/SiO2, (c) FL-ReS2/h-BN, and (d) h-BN/FL-ReS2/h-BN devices at Vbg = 10 V. (e) Critical voltage (Vc) versus Vbg for different substrates. The Vc for the FL-ReS2/SiO2 substrate should be multiplied by three to obtain the exact values. The trap density (Nt) is also calculated from Vc for different dielectric substrates. The trap density for the FL-ReS2/SiO2 device should be multiplied by 10 to obtain the original values. The channel length of the device was 6.8 µm.


Page 26 of 35

Page 27 of 35

(a)

20 Rλ = 13 Α/W EQE = 0.73 %

10

0.0

s

0

300 450 600 750 900 Time (s)

200

1s ~2

FL-ReS2/SiO2

30 τ rise

~6

y

IPh (nA)

0.4 0.2

FL-ReS2/SiO2

40

τ d eca

IPh (µA)

0.6

(b) 50

FL-ReS2/h-BN

240 280 Time (s)

320

(c) ~ 3.5 s τr

0.32

Rλ = 60 Α/W

6s

0.40

~1

0.48

ise

0.56

FL-ReS2/h-BN

ay

IPh (µA)

0.64

τ dec

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EQE = 3.4 %

480 520 560 600 Time (s) Figure 5. (a) Dielectric substrate-dependent photocurrent response for FL-ReS2 FET. Temporal photocurrent response for (b) FL-ReS2/SiO2 and (c) FL-ReS2/h-BN devices. All measurements were performed in vacuum at room temperature. The FL-ReS2 flakes have the same channel length (= 6.8 µm) with width (3.3 µm) on SiO2 and (3.8 µm) on h-BN substrates.



Page 28 of 35

Table 1. Comparative study of our photodetector with other reported 2D material-based photodetectors.

Photodetectors (Material)

Rλ (AW–1)

EQE (%)

Graphene

1 × 10–3

6–16

62

Graphene

6.1 × 10–3

1500

63

Black phosphorus (11.5 nm) (100 nm)

135 × 10–3 657 × 10–3

10 50

64

Monolayer MoS2

7.5 × 10–3

-

65

Multilayer-MoS2

9.0 × 10–5

-

66

Monolayer WSe2

3.7

860

67

Tri-layer WSe2

7

40

68

Multilayer ReS2

16

3168

24

ReS2 bi-layer film

4 × 10–3

0.99

15

FL-ReS2 (< 10 nm) FL-ReS2/SiO2 FL-ReS2/h-BN

13 60

0.73 3.4

Reference

This work


Page 29 of 35

Table of contents (TOC)

100

200


75


150

h-BN/FL-ReS2/h-BN

ΦB (mV)

2

-1 -1

µ (cm V s )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50 25

h-BN/FL-ReS2/h-BN

100 50 0

0 0

50 100 150 200 250 300 T (K)

0


10

20 30 Vbg (V)

40


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