High performance and gate-controlled GeSe/HfS2 negative differential resistance device

Transition metal dichalcogenides (TMDs) have received significant attention owing to their thickness-dependent folded current–voltage (Ids–Vds) characteristics, which offer various threshold voltage values. Owing to these astonishing characteristics, TMDs based negative differential resistance (NDR) devices are preferred for the realization of multi-valued logic applications. In this study, an innovative and ground-breaking germanium selenide/hafnium disulfide (p-GeSe/n-HfS2) TMDs van der Waals heterostructure (vdWH) NDR device is designed. An extraordinary peak-to-valley current ratio (≈5.8) was estimated at room temperature and was used to explain the tunneling and diffusion currents by using the tunneling mechanism. In addition, the p-GeSe/n-HfS2 vdWH diode was used as a ternary inverter. The TMD vdWH diode, which can exhibit different band alignments, is a step forward on the road to developing high-performance multifunctional devices in electronics.


Introduction
Graphene (Gr), formed from a single layer of carbon atoms, is considered to be a suitable material for use in eld-effect transistors (FETs), photodetectors and sensors owing to its zero bandgap. [1][2][3][4] The zero bandgap in Gr has inspired researchers to develop a novel family of materials that possess a suitable bandgap and have analogous features. 5,6 Beyond the discovery of Gr, 7 the transition metal dichalcogenides (TMDs) are considered to possess remarkable high performance electronic and optoelectronic characteristics. 8 TMDs based devices are widely used in different applications, such as photovoltaics, data storage, diodes, and as chemical sensors. [9][10][11][12][13] Black phosphorus (BP) and molybdenum ditelluride (MoTe 2 ) have been widely used in high-performance devices because of their thickness-dependent bandgap and work function, large hole charge carrier densities, and unexpected mobility. Recently, both materials have attracted extraordinary attention owing to their unique optical characteristics. [14][15][16] On the other hand, the electronic and optical properties show deterioration because both TMDs are unstable and easily oxidized under ambient conditions. 17 In contrast, GeSe, an emerging and auspicious p-type transition metal dichalcogenide (TMD), with a unique structure, is considered to be an alternate to BP. The bandgap nature of GeSe changes from indirect (1.08 eV) to direct (1.7 eV) in the bulk compared to the monolayer, respectively. [18][19][20] It has signicant application potential in FETs, chemical sensors, and photovoltaic devices. 21 Other widely studied Hf-based layered TMDs, such as HfS 2 and HfSe 2 , in which the layers are linked by van der Waals forces, have been discovered. The HfS 2 has an octahedral coordinate structure with an indirect bandgap (1.1-1.2 eV) in the case of the monolayer. 22 It has broad applications in electronics and optoelectronics because of its narrow bandgap, large and tunable work function, and huge mobility. [23][24][25] It is clear from the literature that the device applications of HfS 2 are quite limited. Contact resistance also plays a vital role in the performance of the devices; a genuine issue is presented when the work function is not the same as that of the TMDs materials and metal contacts. 26,27 If there is contact resistance between the metal contact and the TMDs materials it is difficult to achieve a high performance in electronics and optoelectronics. By stacking the TMDs materials, a compact system is developed called the van der Waals heterostructures (vdWHs), which offers a novel platform to develop nano-devices.
The nano devices based on 2D-TMDs vdWHs have been efficiently used in FETs, 28 sensors, 29 data storage devices, 30 photodetectors, 31-34 integrated circuits, 35 energy storage, 36 ampliers, 37 inverters, 38 spin-eld effect transistors, 39 water splitting, 40 and diodes. 38,41 Heterostructures and homojunction type devices have been designed by the doping of TMDs materials (chemically and electrostatically), and Fermi-level pinning. These techniques are not suitable for use in a high-performance device. The performance of the devices decreased with the passing of time. Secondly, the performance of the TMDs based nano devices is also controlled by controlling the Schottky barrier height (f B ) of the metal-TMDs junction. 38,42 The f B is considered to be a key parameter to control the performance of nano devices.
Hence, the 2D-TMDs vdWHs multifunctional devices with extraordinary performances, such as negative differential resistance (NDR) diode type devices and a high value of peak-tovalley current ratio, have yet to be realized. In this research, we fabricated advanced and unique GeSe/HfS 2 vdWHs for multiple applications, such as NDR diode type devices and broadband photo-detecting. Each material (p-GeSe and n-HfS 2 ) was characterized using different metal contacts to estimate the low resistance electrode and high mobility. The high peak-to-valley current ratio values were estimated at room temperature and explain the tunneling and diffusion currents using the tunneling mechanism. These high gate-modulated NDR characteristics, with an extraordinary peak-to-valley current, represent an outstanding potential in electronics, which are likely to be essential when developing highly efficient multi-valued logic applications.

Device fabrication
Nano-akes of the TMDs material (p-GeSe and n-HfS 2 ) were obtained by mechanical exfoliation. The Scotch tape method was used to exfoliate the materials. The dry transfer method was used to fabricate the NDR devices. A compound microscope was used to observe and estimate the thickness of the p-GeSe akes. Aer selection of the GeSe ake, a polydimethylsiloxane (PDMS) stamp was used to make the n-HfS 2 ake a suitable size and it was then placed onto a p-GeSe ake with a micro-aligner stage. The electrode pattern was designed, and a thermal evaporation system was used for the metal contacts of (Pd/Au, Ni/Au, Cr/Au, and Ag/Au: 6/60 nm) and (Sc/Au, Al/Au, Ti/Au, Pt/Au: 6/60 nm) for p-GeSe and n-HfS 2 , respectively. For the li-off process, acetone was used.
All of the measurements (electrical) were performed in a vacuum box with a Keithley-2400 and Keithley-6485.

Results and discussion
The p-type GeSe was exfoliated and moved onto the substrate (p-Si/SiO 2 ¼ 300 nm) with the help of a micro-aligner attached to the camera of a microscope. The n-type HfS 2 was transferred onto the GeSe to form the vdWHs. A schematic diagram is shown Fig. 1a. The optical image of the nal device is shown in Fig. 1b. Raman spectroscopy was used for the identication of materials. Fig. 1c shows the Raman spectra of each material (GeSe and HfS 2 ) and the heterostructure. Raman spectroscopy was used to determine the nature of the materials. Fig. 1c shows the Raman spectra of GeSe, HfS 2, and GeSe/HfS 2 . The peaks that appear at 151.8 and 191 cm À1 in the case of GeSe match those observed in the previously published literature. 19,43 In the case of n-HfS 2 , the Raman peaks appeared at 260 and 337 cm À1 and the Raman modes are consistent with previous reports. 44,45 Atomic force microscopy (AFM) was used to determine the thickness of the p-GeSe and n-HfS 2 akes, and these were found to lie in the range of 16 and 28 nm, respectively. The AFM image with height proles is shown in Fig. S1a and b (ESI †). The electrode Pd/Au and Sc/Au is used for p-GeSe and HfS 2 , respectively, to form the low resistance contacts. The types of charge carriers in p-GeSe and n-HfS 2 were conrmed by the transfer curve at a constant V ds ¼ 0.5 V ( Fig. S2a and b †). The V bg was swept between AE40 V and measured the output across the source to the drain.
First, we measured the p-GeSe and n-HfS 2 eld-effect transistors with dissimilar metal electrodes and then performed electrical measurements. To estimate the electrical performance, the charge carrier mobility (m FE ) of p-GeSe and n-HfS 2 was measured. The m FE of the device was extracted by using the following relationship: 46 In which, L and W represent the length and width of the channel respectively, dI ds dV bg provides the slopes, and C bg is the gate capacitance. In GeSe FET, the estimated m FE were observed to be Pd ¼ 90.7, Ni ¼ 74.5, Cr ¼ 51.4, and Ag ¼ 43.5 cm 2 V À1 s À1 . 47 Secondly, the conceived m FE in the case of n-HfS 2 with Sc, Al, Ti, and Pt were found to be 75, 61.6, 42.5, and 36 cm 2 V À1 s À1 (Fig. 1d). In the case of the ohmic contacts (Pd-GeSe/Sc-HfS 2 ), a large hole/electron mobility and lower value of the threshold voltage were obtained because of the high/low work function, respectively. The Pd/Sc metals with a high/low work function doped the GeSe/HfS 2 TMDs materials and improved the charge carrier densities in the channel of the device. Furthermore, the current-voltage (I ds -V ds ) characteristics were estimated using various contacts to determine the linear and non-linear behavior of the electrodes. Fig. S3 † displays the I ds -V ds curves of the GeSe and HfS 2 with various metal contacts. The metal contact (Pd), which has a high work function, shows a linear behavior (ohmic) with GeSe. Other metals show non-linear curves because of the difference in the work function. The linear behavior of the I ds -V ds curves validate the ohmic behavior because of the low potential barrier between the junction of the metal-TMDs. Furthermore, the I ds -V ds characteristics were also measured with different electrodes (Sc, Al, Ti, and Pt) to nd the most suitable electrode for highperformance n-HfS 2 devices. Primarily, the linear tendency of the I ds -V ds characteristics depends on the difference in the work function between the metal and TMDs materials and potential barrier between them. The energy band diagram of the GeSe with different metals before and aer contact between the metals and the GeSe material is shown in Fig. S4. † The values of the work functions, electron affinities and bandgaps of GeSe and n-HfS 2 were taken from previously reported studies. 41,[48][49][50][51][52][53][54][55][56] Temperature-dependent electrical measurements were performed to obtain the Schottky barrier height (f B ). Then, we extracted the f B between the metals with TMDs by using the standard current model for thermionic emission, given as: In which, A is the area of the junction, A* is the Richardson constant, q is the elementary charge (1.6 Â 10 À19 C), k B is the Boltzmann's constant (1.38 Â 10 À23 m 2 kg s À2 K À1 ), and T is the temperature, respectively. Fig. S5 Fig. 2a shows the current-voltage values (I ds -V ds ) are measured at zero back gate voltages (V bg ). The device demonstrates a high value for the peak-to-valley current ratio (PVCR ¼ 5.4) between 0.5 V and 1.1 V, which is larger than the previously reported literature values. 57,58 The highest and lowest values of the current are called the peak current (I peak ) and the valley current (I valley ). Fig. 2b shows the gate-dependent I ds -V ds characteristics, which indicates that the peak current (I peak ) is successfully modulated using the back gate voltage (V bg ). When the V bg decreases, the I peak also decreases. When the V bg is tuned from 40 V to À40 V, the Fermi level (E f ) of GeSe moves downward because of the accumulation of charge carriers (holes). 59 As a result, the energy band bending of GeSe is increased. The E f of HfS 2 is hardly tuned by V bg because of the thick GeSe. The shiing of the energy band in the downward direction in GeSe behaves like a potential well at the interface of the heterojunction, which decreases the I peak . It is difficult to escape the charge carriers from the potential well because of the strong connement of the electron carriers in the potential well. Therefore, the value of PVCR for the heterojunction was tuned to between 3.83 and 5.87 A/A using the V bg (À40 V to 40 V) (Fig. 2c). To check the stability of the I peak to I valley , ten consecutive I ds -V ds sweeps were measured. The estimated values show stable peak-and valley-current values (Fig. 2d). 60 For a graphical explanation of the NDR mechanism, a band energy diagram is shown in Fig. 3. Fig. 3a shows the band energy diagram of the GeSe/HfS 2 TMDs vdWH NDR device before and aer contact at a zero biased voltage. The E f of the GeSe and HfS 2 are aligned aer contact at the same level. A type-III bandgap (broken bandgap) was developed between the GeSe and HfS 2 because the valence band of the GeSe exists above the conduction band of HfS 2 . The current mechanism can be explained based on the diffusion and tunneling current by using the band energy diagram. Fig. 3b shows the energy band diagram of the GeSe/HfS 2 TMDs vdWH NDR device under a different bias voltage. At a negative bias voltage (V ds < 0 V), the tunneling current becomes prominent over the diffusion current. During the negative region, the charge carriers (electron) tunnel from GeSe (lled valence band states) to HfS 2 (empty conduction band states), which amplies the current. The tunneling current also becomes prominent between 0 and 0.5 V. In this region, the charge carriers (electrons) tunnel from the conduction band of HfS 2 to the empty valence states of GeSe, increasing the current. The current is unceasingly enhanced in anticipation of the Fermi level of HfS 2 , which is aligned with the uppermost valence band energy of the GeSe. The overlapping of the lled states in HfS 2 with unoccupied states in GeSe gives rise to the maximum tunneling current (I peak ). In the range of 0.5 V < V ds < 1 V, the magnitude of the current is decreased because of the reduction in the degree of overlap between the lled and empty states. Therefore, the tunneling current is decreased as the V bias is increased, which results from the NDR behavior in the heterojunction device. At a higher bias voltage (V ds > 0.5 V), the diffusion current predominantly contributes to the NDR behavior of the GeSe/ HfS 2 TMDs vdWH NDR device. In this region, the electrons are capable of diffusing from HfS 2 to GeSe by attenuating the potential well, which again increases the current in the device.
Furthermore, to check the effect of temperature, the I ds -V ds measurements were performed at different temperatures from 50-300 K (Fig. 4a). We observed that the I peak is improved, whereas the I valley declined with the falling temperature ( Fig. 4b  and c). The value of the peak-to-valley current was enhanced from 5.8 to 10 A/A with the temperature (Fig. 4d). The values of the peak voltage and valley voltage were shied towards positive values. To conrm the highest values (Table S1 †) of the peak and valley current, the analytic NDR device model was used. The I ds -V ds characteristics were also measured using this model and the peak-to-valley ratio was estimated. The total current (tunneling and diffusion current) is calculated by using the following equation: 52  In which a, E v À GeSe, E c À HfS 2 , DOS GeSe (E), DOS HfS 2 (E), f GeSe (E), and f HfS 2 (E) represents the screening factor, the highest valence band energy in GeSe, the lowest conduction band energy in HfS 2 , the density of states and the Fermi-Dirac distribution functions of GeSe and HfS 2 , respectively. Fig. 4e and f show the measured and calculated values, which demonstrate the consistency of the results. The measured value is obtained from the experimental data. The calculated value is conceived from the theoretical I ds -V ds curves estimated using the analytical model.
Basically, most of the I peak is primarily occupied by tunnel currents. The I peak looks to be allied with the density of states in the CB and VB of the HfS 2 and GeSe. Both the current values (I peak and I valley ) show different behaviors because the tunneling current is increased and the diffusion current is decreased as the temperature decreases. The parasitic series resistance (R s ) was also determined using the analytic model to enable accurate analysis. The R s is the contact resistance between the contact and TMD materials. The metal-semiconductor (MS) junction resistance is enhanced at a lower temperature because of the decline in the n-type charge carriers, which leads to an enhanced depletion width at the MS junction. Therefore, a high voltage value is required to operate the NDR device at a lower temperature. 61,62 We measured multiple devices of the same thickness to check the self-consistency (Fig. S6 †). The effect of the thickness is also measured, as shown in Fig. S8. † In the case of a few layers, the device shows a linear behavior because of direct tunneling.
Finally, an innovative ternary inverter has been designed, which is the elementary building block of multivalued logic applications. This ternary inverter is fabricated by using the GeSe/HfS 2 TMDs vdWH. In most thin-lm transistors (TFT), the BP was used as p-channel materials that oxidize in the ambient environment and decrease the performance of the devices. 52 To overcome this critical issue, we used p-GeSe as a pchannel material. The entire resistance (R) in GeSe could be controlled by V bg . The source and back gate electrodes were used as a supply and input voltage, respectively. The electrode on HfS 2 is connected to the ground. The output voltage is measured across the middle-shared electrode. Fig. 5d shows the change in V OUT as a function of V IN at xed V DD ¼ 2 V from 5 to 25 V. We observed three distinct states that are mentioned as state-2, state-1, and state-0. At state 2, the output voltage is greater than 1.75 V, which appears at 4.9 V< V Input < 7.9 V. In state-1 and 0 the output voltage is around 0.85 and 0.22 V, respectively (Fig. 5d). Load line circuit analysis was performed to explain the working of the inverter. The point of intersection between the two curves indicates the point of operation of the circuit (Fig. 5c and d).
During the low voltage range, the load resistor offers a path with a small resistance between the source and drain. In this state, the input voltage is higher than the threshold voltage and a high value of the output voltage is obtained, which is almost equal to the V DD . When a high voltage is applied at the input terminal, the GeSe TFT is turned off and the resistance decreases between the output terminal and the ground. At an intermediate voltage, the device works as an NDR device. The ternary device based on GeSe/HfS 2 TMDs provides an opportunity to develop multi-valued logic applications.

Conclusion
In this work, we designed a novel and innovative p-GeSe/n-HfS 2 TMDs vdWH diode type NDR device. An astonishing peak-tovalley current ratio (z5.8) was conceived at 300 K. The NDR device was also measured at different back-gate voltages and the NDR behavior was successfully modulated. The performance of the NDR device was also measured at different temperatures. From an application point of view, the p-GeSe/n-HfS 2 vdWH) diode was used as a ternary inverter.

Conflicts of interest
The authors declare no conicts of interest.