Locally Gated SnS2/hBN Thin Film Transistors with a Broadband Photoresponse

Next-generation flexible and transparent electronics demand newer materials with superior characteristics. Tin dichalcogenides, Sn(S,Se)2, are layered crystal materials that show promise for implementation in flexible electronics and optoelectronics. They have band gap energies that are dependent on their atomic layer number and selenium content. A variety of studies has focused in particular on tin disulfide (SnS2) channel transistors with conventional silicon substrates. However, the effort of interchanging the gate dielectric by utilizing high-quality hexagonal boron nitride (hBN) still remains. In this work, the hBN coupled SnS2 thin film transistors are demonstrated with bottom-gated device configuration. The electrical transport characteristics of the SnS2 channel transistor present a high current on/off ratio, reaching as high as 105 and a ten-fold enhancement in subthreshold swing compared to a high-κ dielectric covered device. We also demonstrate the spectral photoresponsivity from ultraviolet to infrared in a multi-layered SnS2 phototransistor. The device architecture is suitable to promote diverse studied on flexible and transparent thin film transistors for further applications.


Results
schematically illustrates the device geometry of the proposed SnS 2 /hBN transistors, where atomically flat hBN acted as the gate dielectric 18 and a multi-layered SnS 2 nanosheet was used as the carrier transport layer. Constructing the bottom encapsulation of hBN is beneficial because the SnS 2 layer is far from the underlying potential fluctuation (SiO 2 substrate). Our recent investigation showed that the interface trap sites located at the 2D/SiO 2 interface could represent more than 10 12 states/cm 2 eV 19 . Because contamination should be avoided during the fabrication process, we used a polymer-incorporated Scotch-tape residual-free technique to minimize the use of chemical solvents. In contrast to the wet transfer method described in other reports, this technique has the ability to control the interface trap state D IT down to the 10 10 states/cm 2 eV range for a suspended 2D channel structure 19 . In this work, we fabricated more than five SnS 2 devices with typical S/D dimensions: a channel length/ width (L/W) ratio of 0.3 and a gate lead with a width of 5 μm. Optical images of the step-by-step preparation of the hetero-structured device are depicted in Fig. 1b. The as-made devices were subsequently characterized via atomic force microscope (AFM) analysis to quantify the thickness of the hBN dielectric and the SnS 2 , as illustrated in the left inset of Fig. 1c. The AFM cross-sectional profiles labeled line A (black) and line B (red) indicate a clear overlap between the channel area and the local gate, as illustrated in Fig. 1c. For the material characterization, the Raman spectra of the as-exfoliated SnS 2 exhibit two non-degenerated scattering modes, with the out-of-plane A 1g mode located at 320.6 cm −1 and the weak in-plane E g peak located at 213.5 cm −1 (see the Supplementary, Fig. S1) under room temperature (see Fig. 2a, left). The data are in agreement with those of previous works 20,21 . On the other hand, in-plane mode, E 2g of hBN is displayed in Fig. 2a right in consistent with other literature (The full Raman spectra of the heterojunction area can be seen in Fig. 2b) 22 .
Next, we proceed to examine the SnS 2 channel TFTs and the influence of the hBN on the SS performance. Figure 3a shows the drain current I DS behavior of the devices as a function of the gate voltage V G under a constant  S2a). This value is one order of magnitude smaller than that of a high-κ (Al 2 O 3 ) covered SnS 2 transistor (in a top-gated configuration) 10 . It is well known that SS = ln(10)(k B T/e)(1 + η) and η = (C D + C IT )/C BN , where k B is the Boltzmann constant, T is the absolute temperature, e is the elemental charge, C D is the depletion capacitance, C IT = e 2 D IT is the interface trap capacitance, and C BN is the bottom-gate capacitance 19 . Thus, our devices demonstrated a factor η ≈ 8. Despite implications that high-quality SnS 2 /hBN contact is indicated, the research has not yet fully explained how the interfacial quality is correlated with the electronic characteristics, especially for a gate stack. Nonetheless, we attribute such SS enhancement to the highly coupled interface with negligible chemical residues. Hysteretic effect in I DS -V G characteristics often reflects the quality of channel/dielectric junction. The interfacial quality was further confirmed by the forward and backward direction sweeping of I DS − V G transfer curves which results a negligible hysteresis by amount of <200 mV as displayed in Fig. S2b. Consistent results were observed in all of the samples with current switching ratios in the 10 4 to 10 5 range and n-type conduction, as described in the literature 9,11,13 . The transconductance g M defined by dI DS /dV G displays a maximum value of ~0.12 μS at V DS = 0.7 V, as displayed in the inset of Fig. 3b. An important figure of merit of the transistor field-effect mobility μ FE is determined by the relationship nm is the thickness of the hBN layer and ε BN = 3-4 is the dielectric constant of the hBN) 23 . As a result, μ FE is calculated to be 0.1-0.5 cm 2 /Vs, comparable to those of single-layered MoS 2 (0.1-10 cm 2 /Vs range) 24 . It should be note that different growth method commonly influences the electrical properties of SnS 2 crystal. Song et al. 10 , De et al. 11 and Ahn et al. 33 reported the mobility of approximately 1-2 cm 2 /Vs from SnS 2 grown by vapor transport technique. However, the SnS 2 solid crystal prepared by vertical Bridgman technique have showed poor mobility around 0.1 cm 2 /Vs 13 . Beside contact engineering and dielectric interface improvement, the material's mobility seems influenced by growth method of SnS 2 .
To establish efficient carrier injection from outside (e.g., S/D metal), which is needed to enhance the device performance, the contact issues have been preliminarily investigated for the MoS 2 system, and several novel approaches have been suggested 25 . So far, the ohmic contact formation for SnS 2 materials is still unclear. Nevertheless, a linear increment in I DS can be observed for different V G bias conditions, suggesting an ohmic-like contact at the nickel/SnS 2 junction at a small V DS bias range, as depicted in the inset of Fig. 3c. However, an ambiguous result emerges when the V DS is extended to a few voltages (quasi-linear region, before current saturation): a slightly nonlinear dependence of I DS is found (indicated by the black circle) in I DS -V DS output characteristics of the SnS 2 device, as shown in Fig. 3b. We attribute this nonlinear behavior to the rise of the Schottky barrier height, eφ SB because a contact mismatch occurs between the high work function, W F of nickel metal (W F = 5.2 eV) 26 and the electron affinity, eχ s of SnS 2 (eχ s = 5.0 eV) 27 . Owing to the similarity in crystal structure and chalcogenide compound compared with MoS 2 layered material, similar consequences could be expected for other 2D systems. To better address this point, we measured the temperature-dependent I-V characteristics as temperature varied from 300 to 410 K ( Supplementary Information, Fig. S3). Carrier transport across a metalsemiconductor barrier involves a quantum mechanical tunneling and a thermionic-emission process, so that the devices measured at high temperature regime allowed suppression of the tunneling current contribution 25 . At a high temperature regime, an expression similar to the Arrhenius equation and also known as thermally activated transport model can be derived as g DS = g 0 exp(−E A /k B T), where g DS = dI DS /dV DS is the conductance, and g 0 is the fitting parameter 19,25 . The conductance g DS fitted with this equation is depicted in Fig. S3b (see Supplementary  Information). The activation energy E A as a function of V G acquired from Fig. S3b is illustrated in Fig. 3c. In this plot, we can determine a 135 meV of eφ SB for Ni/SnS 2 contact by evaluation of the starting point of deviation from the linear response by following Radisavljevic and Kis 28 . Such Schottky barrier determination is based on activation energy measurement. The details of the evaluation method of φ SB can be found in other literatures 28,29 as well as our previous pulications 19,25 . A measured E A = 0.18 eV at zero V G generally indicates the position of the impurity donor level with respect to the conduction band of SnS 2 , and the activation energy is close to the value of 0.14 eV reported by Pan et al. and the value of 0.13 eV reported by De et al. 9,11 . Figure 4a shows the I-V transfer curves with (photon energy of 2.48 eV, green line) and without (dark state, black line) monochromatic light illumination at V DS = 0.1 V. The current under illumination I ILL , defined as I ILL = I PH + I DA (I PH and I DA are the photocurrent and dark current, respectively), exhibits a dramatic I DS increment of the SnS 2 phototransistor in both the on and off states of the device, whereas the incident light with an intensity of 23.5 μW has about a 30-fold influence in the off-state and 2-fold in the on-state. With light illumination on different states of the device (on-and off-states), the devices exhibit different photocurrent response. Lowering the Schottky barrier (on-state), an additional photocurrent excited by photo-induced band-to-band transition contributes to the drain current. Raising the Schottky barrier (off-state) restricts the dark current, resulting in a more pronounced photocurrent extraction. Therefore, we carefully conclude that photo-excited carrier transport primarily dominates over the thermionic and tunneling current, which is in agreement with other publications 30,31 . We found that the effective transconductance g ' M of the SnS 2 channel under light illumination showed clear increasement compared to the dark state, as depicted in the inset of Fig. 4a. The SnS 2 phototransistor was further exposed to different monochromatic lights ranging in λ from 500 to 1000 nm, representing the series photo-induced I-V transfer properties at the on-state of the device with V DS = 0.1 V, as displayed in Fig. 4b. Electron-hole pair generation by optical means usually requires an incident photon energy close to the band gap of the multi-layered SnS 2 . Interestingly, the device weakly responds to light with a long λ (such as 600 nm, corresponding to 2.07 eV), implying an extrinsic type of the phototransistor with a defect-assisted energy level introduced. This effect can be explained by the defect-level involvement of the band gap of SnS 2 ; the transition between the defect-level and the conduction/valence band edge can contribute to I PH 29 . Photoresponsivity, R PH is an important metric of the phototransistor and is estimated by I PH /P L , where P L is the optical power (see Fig. S4) and the broad spectral response is shown in Fig. 4c. In this photoresponsivity calculations, calibration of device's active area is excluded. The device performance exhibits an R PH of 0.47-0.65 mA/W at the visible light range and is reduced to 0.33 mA/W at infrared due to the weak light absorption with an applied gating of 7 V. The measured R PH as function of V G is given in Fig. S4a. The responsivity of SnS 2 /hBN devices is lower than that of MoS 2 based phototransistor (over 343 A/W) 30 , but it is higher than that in a SnS 2 nanosheet photodetector reported by Tao 35 . Therefore, we believe that extrinsic type of device with wide spectral response is probably due to sulfur vacancy induced deep states near bottom of conduction band.
Another key parameter is the detectivity, D * which is the reciprocal of the noise equivalent power, given by D * = R PH A 1/2 /(2eI DA ) 1/2 . Here, A is the device effective area. The calculated D * value showed a typical range of 1.4 × 10 6 to 5.1 × 10 6 Jones at V DS = 0.1 V and V G = 7 V. Furthermore, the external quantum efficiency, EQE is measure of the ratio of the number of carriers produced by the number of photons. The EQE can be converted from R PH by employing EQE = R PH hc/λe, here h and c are Plank constant and speed of light, respectively. We observed approximately 0.1% of EQE at visible light range.

Discussion
We fabricated SnS 2 /hBN heterostructured devices and characterized the devices by electrical and optical measurements techniques. The interfacial behavior between the SnS 2 and hBN layered crystal is discussed. The locally placed gate separated by an hBN insulating layer presented an efficient modulation of the channel conductance with a current on/off ratio of up to ~10 5 . The insertion of an ultra-flat dielectric layer allowed the device to exhibit SS values as low as 585 mV/decade. The detailed temperature-dependent electrical transport measurements led to the determination of eφ SB = 135 meV at the nickel/SnS 2 interface. Moreover, we demonstrated the extrinsic type of the SnS 2 -based phototransistor with a wide range of light response and a high photoresponsivity of approximately 0.7 mA/W.

Methods
The bottom-gated SnS 2 devices were fabricated on a thermally oxidized n + -type silicon substrate in which a 90-nm-thick SiO 2 insulating layer offered electrical isolation from the back-gate, as well as optical detectability for the ultrathin nanosheet via optical contrast. The bottom electrode that served as both the optical indicator and the gate terminal of the transistor was pre-defined onto a silicon substrate by utilizing a standard photolithography process. We used SnS 2 and hBN bulk solids obtained from the 2D semiconductor Inc. to generate nano-flakes using a Scotch-tape mechanical exfoliation method. We employed a technique developed in our previous work called "dry transfer," based on a polydimethylsiloxane framework, to transfer the desired hBN flake onto the pre-patterned gold bottom-gate 2,19,25 . Subsequently, we deposited a piece of SnS 2 on top of the hBN layer using the same technique. The SnS 2 channel conductivity was monitored via metallization of the source/drain electrodes using a thermal evaporator system under a deposition rate of 5 Å/s to form a nickel/gold metal stack. An AFM (Park Systems, XE-100) operated under noncontact mode with a Nanosensor AR5-NCH tip was employed to characterize the topographic images of the devices. Raman signals were collected via commercially available confocal Raman spectroscopy (WiTec, alpha 300) with the excitation laser line of λ = 488 nm in ambient conditions. The electrical transport properties of the SnS 2 /hBN devices were obtained with a semiconductor parameter analyzer (Hewlett Packard, 4156 A) in a vacuum cryostat (ASK, 700 K) under a pressure of 10 −3 Torr. The photo-induced I-V measurements were conducted similarly under ambient conditions. To probe the photocurrent measurements, light wavelength λ spectra ranging from 300 to 1000 nm were generated by a system that consisted of a 300 W Xenon Arc lamp, a power supply (Newport, 69911), and an automated 1/8 m monochromator (Newport, 74004) with double grating. The excitation light intensity was recorded through a silicon photodiode detector (Newport, 918D-UV-OD3) mounted optical power meter (Newport, 1918-C). The collected power and irradiance data as function of photon wavelength is given in Fig. S4(b). During the photoresponse characterization, the device (active area, ~10 −7 cm 2 ) was illuminated by a monochromatic light guided by fused silica fiber optic bundle (Newport, 77577) with typical 3 mm in diameter uniform beam.