Steep‐Slope Gate‐Connected Atomic Threshold Switching Field‐Effect Transistor with MoS2 Channel and Its Application to Infrared Detectable Phototransistors

Abstract For next‐generation electronics and optoelectronics, 2D‐layered nanomaterial‐based field effect transistors (FETs) have garnered attention as promising candidates owing to their remarkable properties. However, their subthreshold swings (SS) cannot be lower than 60 mV/decade owing to the limitation of the thermionic carrier injection mechanism, and it remains a major challenge in 2D‐layered nanomaterial‐based transistors. Here, a gate‐connected MoS2 atomic threshold switching FET using a nitrogen‐doped HfO2‐based threshold switching (TS) device is developed. The proposed device achieves an extremely low SS of 11 mV/decade and a high on‐off ratio of ≈106 by maintaining a high on‐state drive current due to the steep switching of the TS device at the gate region. In particular, the proposed device can function as an infrared detectable phototransistor with excellent optical properties. The proposed device is expected to pave the way for the development of future 2D channel‐based electrical and optical transistors.


Introduction
2D-layered nanomaterials, such as graphene, [1][2][3][4][5][6][7] black phosphorus, [8][9][10][11][12][13] and transition metal dichalcogenides (TMDCs), [14][15][16][17][18][19][20][21][22][23] have received substantial attention in the fields of nanoelectronics and optoelectronics in the past few years. Recent studies on devices fabricated using 2D material for channel have revealed that they have excellent current on/off ratio, satisfactory charge carrier mobility, low subthreshold DOI: 10.1002/advs.202100208 swing (SS), and interesting optical properties.  Although conventional 2D-channel-based field-effect transistors (FETs) exhibit a low SS value, the SS cannot be lower than 60 mV/decade at room temperature owing to the limitation of the thermionic carrier injection mechanism. [24][25][26] The SS of the transistor needs to be as low as possible to maintain the total power consumption, and a low SS leads to a high switching speed and low power consumption per device. [26,27] To overcome the SS limit, several types of steep-slope 2D-channel devices have been proposed such as tunneling FET, [13,[28][29][30] negative capacitance FET, [24,25,31,32] and phase FET. [26,33,34] In particular, phase FETs are currently considered the most promising steep-slope device. The concept of phase FET was first implemented by Shukla et al. in 2015 using an insulator-tometal transition (IMT) material, VO 2 , integrated in series with the source of the conventional FET. [35] When external perturbations such as temperature, pressure, and electrical stimulus are applied to VO 2 , phase transition is induced, which causes an abrupt change in conductivity. The phase FET using VO 2 shows excellent properties such as an abrupt increase in current with applied voltage, short switching time, high on-current, low fabrication temperature, and high compatibility with conventional complementary metal-oxide-semiconductor technology. Owing to these advantages, in 2017, Grisafe et al. applied the phase FET concept with VO 2 in the MoS 2 -channel FET to further boost the 2D-channel device performance. [33] The suggested MoS 2channel phase FET has a low operation voltage and low SS, but it involves several problems, such as a high off-state leakage current and low thermal stability, according to the limitations of the IMT materials. In 2019, HfO 2 -based threshold switching (TS) device was suggested for MoS 2 -channel phase FETs instead of IMT materials, because of the low off-state leakage current characteristic of ≈1 pA and the superior thermal stability of ≈90°C. [34] However, these source-connected phase FETs decrease not only the off-state leakage current but also the on-state drive current, regardless of the material used in the TS device because the TS device is located in the main current flow path. Therefore, research on realizing steep switching devices while maintaining the superior characteristics of MoS 2 FET needs to be conducted. Achieving a low SS value of FETs in terms of the optical concept is also important because one of the main challenges in optics is reducing the total power consumption in optoelectronics. [36] The SS corresponds to the gating efficiency, such that reducing the SS makes it possible to ensure low voltage operability. [37] In addition, the photogating effect-based phototransistor, which operates the photogenerated carriers similar to the back-gate voltage in a FET, has a close relationship with gating efficiency because the optical operating mechanism is threshold voltage shifting. [38] Therefore, the improvement in gating efficiency by decreasing the SS can enhance the optical characteristics of photogating effect-based phototransistors. However, few studies have focused on lowering the SS value of phototransistors, and research on this aspect is essential for next-generation optoelectronics.
Here, we demonstrate a gate-connected MoS 2 atomic threshold switching FET (ATS-FET) using a nitrogen-doped HfO 2 (HfO x :N)-based TS device for a next-generation steep-slope 2Dchannel device. The on-state drive current of a MoS 2 ATS-FET is maintained owing to the gate-connected structure and the SS of the device is reduced through the Ag conductive filament formed and ruptured in the HfO x :N layer. In particular, the proposed device can detect the infrared range light owing to the special properties of the Ge substrate. Therefore, the proposed ATS-FET can also be used as an infrared detectable phototransistor based on the photogating effect. Compared with the conventional MoS 2 phototransistor, the optical characteristics of the proposed device are enhanced by the combined action of the photogating effect and the low SS. It was also successfully confirmed that the responsivity and detectivity for the incident infrared light were increased.

Device Scheme and Typical Characteristics
A schematic of the gate-connected MoS 2 ATS-FET is shown in Figure 1a. The proposed ATS-FET was formed by connecting the MoS 2 FET gate electrode in series with the HfO x :N-based TS device. First, the MoS 2 FET was fabricated using p-type Ge (p-Ge) (N a = 1 × 10 16 cm −3 ), which is used for the back-side gate electrode. A 50-nm-thick silicon dioxide (SiO 2 ) layer was deposited on the p-Ge substrate via plasma-enhanced chemical vapor deposition to form a gate oxide. Then, the MoS 2 flake as a channel was transferred onto the SiO 2 /p-Ge substrate using the polydimethylsiloxane-based mechanical exfoliation method. A 40-nm-thick Ti layer was deposited as a source/drain (S/D) contact metal. The S/D contact metal was fabricated in parallel with a spacing of 10 µm, as shown in the top-view optical microscope image of Figure 1b. As shown in Figure S1, Supporting Information, two conventional peaks (E 1 2g and A 1g ) of the MoS 2 were clearly revealed at 382.2 and 407.6 cm −1 by Raman spectroscopy, indicating that the MoS 2 is present as a multi-layer. [39] The thickness of the MoS 2 was measured to be 11 nm by using atomic force microscopy (AFM), as shown in Figure S2, Supporting Information, which indicates that MoS 2 has ≈15 layers. Figure 1c shows the drain current-gate voltage (I D -V G ) characteristics of the MoS 2 FET, which exhibited n-type transfer characteristics with a high on-state current of ≈10 −6 A and a high current on/off ratio of ≈10 6 , similar to typical multi-layered MoS 2 FETs. [40] The HfO x :N-based TS device with an Ag/HfO x :N/Pt/Ti structure, which is essential for constructing the ATS-FET, was fabricated on a 90-nm-thick SiO 2 /Si substrate. Ag (100 nm), Pt (15 nm), and Ti (10 nm) metal layers were deposited via electronbeam evaporation. A 50-nm-thick HfO x :N interlayer was deposited using radio frequency sputtering with a HfO 2 ceramic target under a N 2 gas flow of 2 sccm. Through the top-view optical microscope image and cross-sectional view transmission electron microscopy image, the TS device was confirmed that the structure of that was completed, as shown in Figure S3, Supporting Information. Figure 1d shows the typical I-V characteristics and the operation mechanism of the HfO x :N-based TS device under the direct current sweep. The TS device shows an abrupt resistance switching during forward and backward voltage sweeping, which can be explained by the formation and dissolution of the Ag filament in the HfO 2 electrolyte. [41][42][43][44] When applying a forward sweep from 0 to 1 V, an Ag filament is formed between the two electrodes and the TS device changes from the off-state to the on-state. In contrast, when applying a backward sweep from 1 to 0 V, the device is turned off because the Ag filament is spontaneously ruptured to minimize the interfacial energy. Figure 2 shows the electrical properties of gate-connected MoS 2 ATS-FET, which was formed by connecting the MoS 2 FET gate electrode in series with the HfO x :N-based TS device. Figure 2a shows the I D -V G characteristics of the MoS 2 FETs with and without the TS device in the gate region at V G from 0.5 to −3.5 V with a step of 0.05 V with constant V D = 0.5 V. The threshold voltage (V TH ) of the gate-connected MoS 2 ATS-FET is shifted toward the left side as compared with the MoS 2 FET from −2.19 to −2.60 V (the V TH was defined for V G when I D = 10 −11 A). The higher |V TH | of the gate-connected MoS 2 ATS-FET indicates that the applied gate voltage is not used entirely to turn off the transistor. In the case of drain current for both devices, the onand off-state currents are ≈10 −6 and 10 −12 A, respectively, and the on/off switching ratio is ≈10 6 . The reason for the same onand off-state currents in both devices is that the TS device is connected to the gate rather than the channel through which current flows directly, and therefore it does not interfere with the flow of the drive current. In addition, a sharp rise in the drain current is observed while maintaining the on-and off-state current values.  Figure 2c shows the distribution of SS min SS in 10 devices to confirm the device-to-device variation. As a result, the value and variation of SS min SS of ATS-FET tend to decrease compared to that of MoS 2 FET and it shows that our proposed device has good reliability. The I D -V G characteristics and SS min SS values for various V G steps are shown in Figure S4, Supporting Information. It was confirmed that as the V G step became finer, more data points appeared on the SS slope, and the SS min SS values are maintained at a very low value compared to the conventional MoS 2 FET. Furthermore, as a result of the I D -V D measurement ( Figure S5, Supporting Information), it was confirmed that a large gap was formed between the drain current values according to the gate step due to the low SS of the ATS-FET.

Electrical Properties of Gate-Connected MoS 2 ATS-FET
To explain the modulation of the electrical properties of the gate-connected MoS 2 ATS-FET, the schematic I D -V G curves and circuits are illustrated in Figure 3. Figure 3a shows a schematic illustration of the transfer I D -V G curve with and without the TS device at the gate region. As shown in Figure 3a, the gate-connected MoS 2 ATS-FET shows a left shift of V TH and a decrease of SS compared to the MoS 2 FET, when sweep V G is in the negative direction. These modulations can be explained by the equivalent circuit and voltage distribution for the device, as shown in Figure 3b. When low V G is applied, most of the voltage drops occur on the TS device owing to the high resistance, as shown on the left side of Figure 3b. Therefore, the applied gate voltage of the MoS 2 FET (V G,FET ) remains low and the MoS 2 channel remains in the onstate. For this reason, V TH of the gate-connected MoS 2 ATS-FET was shifted toward the left side. As V G increases in the negative direction, the voltage applied to the TS device (V G,TS ) increases, eventually forming an Ag filament between the two electrodes to turn on the TS device, as shown on the right side of Figure 3b. Then, the TS device changes to a low resistance state, which is makes the extremely low V G,TS drops, and V G,FET changes steeply to a value approaching V G . As a result of the sudden change in www.advancedsciencenews.com www.advancedscience.com  V G,FET , the SS of the gate-connected MoS 2 ATS-FET decreases significantly as compared with that of the MoS 2 FET. According to the above-mentioned, the main factor determining V TH and SS of the gate-connected MoS 2 ATS-FET is the formation and dissolution of the filament of the TS device in the gate region.

Optical Properties of Gate-Connected MoS 2 ATS-FET
In general, MoS 2 FETs have been widely studied as optoelectronic devices. [16,18,20,40,45] Similarly, the proposed gate-connected MoS 2 ATS-FET is also capable of photodetection. To assess the photodetection performance, the infrared light ( = 1550 nm) was incident perpendicularly and uniformly onto the channel region of the device shown in Figure 4a. The I D -V G characteristics of the gate-connected MoS 2 ATS-FET with and without irradiation of 1550 nm infrared light are presented in Figure 4b. With irradiation, the ATS-FET exhibited a negative V TH shift from −2.60 to −2.95 V (inset in Figure 4b) while maintaining a low SS of 11.1 mV/decade. In addition, rise time of 51.1 ms and decay time of 27.3 ms were measured as shown in Figure S6, Supporting Information, and the rise and decay time values were extracted between 10% and 90% of the increasing and decreasing drain current. The principle of threshold shift under incident infrared light can be explained using a band diagram and a schematic of www.advancedsciencenews.com www.advancedscience.com the proposed device as shown in Figure 4c,d. Figure 4c shows a band diagram of the channel/gate oxide/gate electrode of the MoS 2 FET. Under thermal equilibrium conditions, initial band bending is caused by the difference in work functions between MoS 2 ( MoS 2 = 4.2 eV) and Ge ( Ge = 4.516 eV) as shown in the band diagram. When infrared light is incident onto the device, the MoS 2 and SiO 2 layers hardly absorb the light and act as transparent windows owing to their wide bandgaps. Therefore, the incident light can reach Ge and is absorbed owing to the narrow bandgap of Ge (E g = 0.66 eV). The absorbed light creates electron-hole pairs in the Ge region, and the generated electrons accumulate at the SiO 2 /Ge interface because of the initial band bending. The accumulated electrons can cause two phenomena. One is decrease of the n-type doping in the MoS 2 channel, and the other is inhibition of the Ag + ion migration in the HfO x :N layer, as shown in Figure 4d. Decrease of the n-type doping in the MoS 2 channel causes right shifting of the transfer curve, and inhibition of the Ag + ion migration causes left shifting of the transfer curve. In the case of the gate-connected MoS 2 ATS-FET, the switch operation is determined by the turn on and off of the TS device. Therefore, the transfer curve is shifted to the left side, as shown in Figure 4b. This phenomenon, in which the threshold voltage is shifted through the charge generated by incident light, is called the photogating effect. [38,46,47] To further evaluate the performance of the phototransistor, we extract the light to dark current ratio, photocurrent, responsivity, external quantum efficiency (EQE), and detectivity from the I D -V G curves. As shown in Figure 5a, an ultrahigh light-to-dark current ratio can be obtained in the threshold shift region because the threshold shift and the significantly steep SS operate simultaneously in the proposed device, and the maximum value of the factor is 4.7 × 10 4 at V G = −2.9 V and V D = 0.5 V. The photocurrent ( I photo = I light − I dark ) shows a rapid change based on threshold voltage due to the threshold shift according to the incident light, and maximum values as high as 3.58 × 10 −7 A are achieved as shown in Figure 5b. Because the responsivity (R) and the EQE, as shown in Figure S7a and S7b, Supporting Information, are parameters related to I photo , they exhibit the same trend of change as I photo . R indicates the ratio of the generated photocurrent and effective incident optical power (R = I photo /P eff ), and P eff is calculated as P eff = P(A device /A spot ), where P is the actual output laser power and A is the area of the device and laser spot, and the value of P eff is 1.69 × 10 −7 W. EQE is the ratio of the number of photoexcited charge carriers to the number of incident photons on the device from the outside; it can be expressed as EQE = hcR/ e, where h is Planck constant, c is the speed of light, is the wavelength of incident light, and e is the electron charge. R and EQE increased steeply at the subthreshold region and reached the highest values of 2.1 AW −1 and 169.0%, respectively. Detectivity ( D* = R(A device /2eI dark ) 0.5 ), one of the most important parameters for photodetectors, is defined as the signal-to-noise ratio of the photodetector normalized by the device area. Because of the ultrahigh light-to-dark current ratio in the threshold shift region, D* was measured to have a considerably high value of 2.7 × 10 12 cmHz 0.5 W −1 as shown in Figure 5c. All parameters mentioned above have incomparable selectivity for the applied gate voltage owing to the extremely low SS and threshold shift. The optical performance of the gate-connected MoS 2 ATS-FET is dramatically enhanced, as compared with that of the conventional MoS 2 FET ( Figure S8, Supporting Information), which is without the TS device. A comparison of the critical optical performance indicators is presented in Table S1, Supporting Information. These optical characteristics prove that the ultra-low SS of the gate-connected MoS 2 ATS-FET can be beneficial when a gateconnected MoS 2 ATS-FET is used as a photodetector. Compared to recently reported advanced phototransistors, as shown in Table 1, the gate-connected MoS 2 ATS-FET can not only operate in the infrared region, but also exhibit a significantly high photo-todark current ratio, responsivity, and detectivity. These excellent optical performance characteristics of the gate-connected MoS 2 ATS-FETs indicate that the proposed device has significant potential for application in optoelectronics.

Conclusion
We developed a gate-connected MoS 2 ATS-FET using a HfO x :Nbased TS device to overcome the SS limit in 2D channel-based transistors. The proposed ATS-FET has an extremely low SS of 11.1 eV/decade with a range of abrupt current transitions close to 10 4 . The operation mechanism was successfully investigated based on electrical characteristics and circuits, and the main factor determining V TH and SS is the formation and dissolution of the Ag filament in the TS device. In addition, because the TS device used to obtain a low SS is connected to the gate and does not lower the drive current, the ATS-FET has a high on-state drive current of ≈10 −6 A and a high on-off ratio of ≈10 6 . Moreover, CdTe-MoS 2 200-1700 3 × 10 4 3.7 × 10 −2 6.1 × 10 10 [ 54] HgTe quantum dot PT ≈1500-2500 10 >1 >10 11 [ 55] Si nanomembrane 1550 10 2 7 × 10 −3 [56] GeSn/Ge 1550 2.3 × 10 2 99 3 × 10 11 [ 57] GeSn heterojunction 1550 6.8 × 10 −1 [58] the gate-connected MoS 2 ATS-FET can be used as an infrared detectable phototransistor owing to the detection of infrared light through a Ge gate electrode, which has a narrow bandgap. When the infrared light is incident onto the device, the threshold voltage is shifted from −2.6 to −2.95 V, while maintaining a low SS. Owing to the shifting of the threshold voltage and the maintained low SS, the proposed device achieves significantly high performance as an optical device, featuring characteristics such as an ultrahigh light-to-dark current ratio (4.7 × 10 4 ) and high-selective detectivity (2.7 × 10 12 cm Hz 0.5 W −1 ). Owing to these advantages, gate-connected MoS 2 ATS-FETs are considered promising candidates for next-generation 2D channel-based electrical and optical devices that require high switching speed and low power consumption.

Experimental Section
Characterization of MoS 2 Flakes: To confirm the existence and measure the thickness of MoS 2 flakes, Raman Fourier transform infrared spectroscopy (LabRam ARAMIS IR2, Horiba Jobin Yvon) and AFM (XE-100, Park systems) were performed. The Raman spectroscopy was performed with an excitation wavelength of 532 nm and a spatial resolution of 1 µm. AFM was performed with lateral and vertical resolutions of 2-3 and 0.1 Å, respectively.
Measurement of Electrical and Optical Characteristics: The electrical properties of the fabricated devices were measured using a Keithley 4200-SCS with laser irradiation at 1550 nm wavelength and without laser irradiation.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.