Amir Muhammad Afzal*a,
Muhammad Zahir Iqbal
b,
Muhammad Waqas Iqbala,
Thamer Alomayric,
Ghulam Dastgeerd,
Yasir Javed
e,
Naveed Akhter Shadf,
Rajwali Khang,
M. Munir Sajidf,
R. Neffatihi,
Tasawar Abbasa and
Qudrat Ullah Khan
j
aDepartment of Physics, Riphah International University, 13 Raiwind Road, Lahore, Pakistan. E-mail: amirafzal461@gmail.com
bNanotechnology Research Laboratory, Faculty of Engineering Sciences, GIK Institute of Engineering Sciences and Technology, Topi 23640, Khyber Pakhtunkhwa, Pakistan
cDepartment of Physics, Faculty of Applied Science, Umm-Al-Qura University, 21955, Makkah, Saudi Arabia
dDepartment of Physics & Astronomy, Graphene Research Institute–Texas Photonics Center International Research Center (GRI–TPC IRC), Sejong University, Seoul 05006, Korea
eDepartment of Physics, University of Agriculture, Faisalabad, 38000, Pakistan
fDepartment of Physics, GC University, Faisalabad, 38000, Pakistan
gDepartment of Physics, University of Lakki Marwat, Lakki Marwat, KPK, Pakistan
hDepartment of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, Saudi Arabia
iLaboratoire de Physique de la Matière Condensée, Département de Physique, Faculté des Sciences de Tunis, Université Tunis El Manar, Campus Universitaire, 1060 Tunis, Tunisia
jGreater Bay Area Institute of Precision Medicine (Guangzhou), Fudan University, Nansha District, Guangzhou, Guangdong 511458, P. R. China
First published on 5th January 2022
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.
Black phosphorus (BP) and molybdenum ditelluride (MoTe2) 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.14–16 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.18–20 It has significant application potential in FETs, chemical sensors, and photovoltaic devices.21
Other widely studied Hf-based layered TMDs, such as HfS2 and HfSe2, in which the layers are linked by van der Waals forces, have been discovered. The HfS2 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.23–25 It is clear from the literature that the device applications of HfS2 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 amplifiers,37 inverters,38 spin-field 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 (ϕB) of the metal–TMDs junction.38,42 The ϕB 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-to-valley current ratio, have yet to be realized. In this research, we fabricated advanced and unique GeSe/HfS2 vdWHs for multiple applications, such as NDR diode type devices and broadband photo-detecting. Each material (p-GeSe and n-HfS2) 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.
All of the measurements (electrical) were performed in a vacuum box with a Keithley-2400 and Keithley-6485.
Atomic force microscopy (AFM) was used to determine the thickness of the p-GeSe and n-HfS2 flakes, and these were found to lie in the range of 16 and 28 nm, respectively. The AFM image with height profiles is shown in Fig. S1a and b (ESI†). The electrode Pd/Au and Sc/Au is used for p-GeSe and HfS2, respectively, to form the low resistance contacts. The types of charge carriers in p-GeSe and n-HfS2 were confirmed by the transfer curve at a constant Vds = 0.5 V (Fig. S2a and b†). The Vbg was swept between ±40 V and measured the output across the source to the drain.
First, we measured the p-GeSe and n-HfS2 field-effect transistors with dissimilar metal electrodes and then performed electrical measurements. To estimate the electrical performance, the charge carrier mobility (μFE) of p-GeSe and n-HfS2 was measured. The μFE of the device was extracted by using the following relationship:46
![]() | (1) |
In which, L and W represent the length and width of the channel respectively, provides the slopes, and Cbg is the gate capacitance. In GeSe FET, the estimated μFE were observed to be Pd = 90.7, Ni = 74.5, Cr = 51.4, and Ag = 43.5 cm2 V−1 s−1.47 Secondly, the conceived μFE in the case of n-HfS2 with Sc, Al, Ti, and Pt were found to be 75, 61.6, 42.5, and 36 cm2 V−1 s−1 (Fig. 1d). In the case of the ohmic contacts (Pd–GeSe/Sc–HfS2), 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/HfS2 TMDs materials and improved the charge carrier densities in the channel of the device. Furthermore, the current–voltage (Ids–Vds) characteristics were estimated using various contacts to determine the linear and non-linear behavior of the electrodes. Fig. S3† displays the Ids–Vds curves of the GeSe and HfS2 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 Ids–Vds curves validate the ohmic behavior because of the low potential barrier between the junction of the metal–TMDs. Furthermore, the Ids–Vds characteristics were also measured with different electrodes (Sc, Al, Ti, and Pt) to find the most suitable electrode for high-performance n-HfS2 devices. Primarily, the linear tendency of the Ids–Vds 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 after 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-HfS2 were taken from previously reported studies.41,48–56
Temperature-dependent electrical measurements were performed to obtain the Schottky barrier height (ϕB). Then, we extracted the ϕB between the metals with TMDs by using the standard current model for thermionic emission, given as:
![]() | (2) |
As a result, the energy band bending of GeSe is increased. The Ef of HfS2 is hardly tuned by Vbg because of the thick GeSe. The shifting of the energy band in the downward direction in GeSe behaves like a potential well at the interface of the heterojunction, which decreases the Ipeak. It is difficult to escape the charge carriers from the potential well because of the strong confinement 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 Vbg (−40 V to 40 V) (Fig. 2c). To check the stability of the Ipeak to Ivalley, ten consecutive Ids–Vds 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/HfS2 TMDs vdWH NDR device before and after contact at a zero biased voltage. The Efof the GeSe and HfS2 are aligned after contact at the same level. A type-III bandgap (broken bandgap) was developed between the GeSe and HfS2 because the valence band of the GeSe exists above the conduction band of HfS2. 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/HfS2 TMDs vdWH NDR device under a different bias voltage. At a negative bias voltage (Vds < 0 V), the tunneling current becomes prominent over the diffusion current. During the negative region, the charge carriers (electron) tunnel from GeSe (filled valence band states) to HfS2 (empty conduction band states), which amplifies 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 HfS2 to the empty valence states of GeSe, increasing the current. The current is unceasingly enhanced in anticipation of the Fermi level of HfS2, which is aligned with the uppermost valence band energy of the GeSe. The overlapping of the filled states in HfS2 with unoccupied states in GeSe gives rise to the maximum tunneling current (Ipeak). In the range of 0.5 V < Vds < 1 V, the magnitude of the current is decreased because of the reduction in the degree of overlap between the filled and empty states. Therefore, the tunneling current is decreased as the Vbias is increased, which results from the NDR behavior in the heterojunction device. At a higher bias voltage (Vds > 0.5 V), the diffusion current predominantly contributes to the NDR behavior of the GeSe/HfS2 TMDs vdWH NDR device. In this region, the electrons are capable of diffusing from HfS2 to GeSe by attenuating the potential well, which again increases the current in the device.
Furthermore, to check the effect of temperature, the Ids–Vds measurements were performed at different temperatures from 50–300 K (Fig. 4a). We observed that the Ipeak is improved, whereas the Ivalley 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 shifted towards positive values. To confirm the highest values (Table S1†) of the peak and valley current, the analytic NDR device model was used. The Ids–Vds 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
![]() | (3) |
![]() | (4) |
In which α, Ev − GeSe, Ec − HfS2, DOSGeSe(E), DOSHfS2(E), fGeSe(E), and fHfS2(E) represents the screening factor, the highest valence band energy in GeSe, the lowest conduction band energy in HfS2, the density of states and the Fermi–Dirac distribution functions of GeSe and HfS2, 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 Ids–Vds curves estimated using the analytical model.
Basically, most of the Ipeak is primarily occupied by tunnel currents. The Ipeak looks to be allied with the density of states in the CB and VB of the HfS2 and GeSe. Both the current values (Ipeak and Ivalley) show different behaviors because the tunneling current is increased and the diffusion current is decreased as the temperature decreases. The parasitic series resistance (Rs) was also determined using the analytic model to enable accurate analysis. The Rs 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/HfS2 TMDs vdWH. In most thin-film 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 p-channel material. The entire resistance (R) in GeSe could be controlled by Vbg. The source and back gate electrodes were used as a supply and input voltage, respectively. The electrode on HfS2 is connected to the ground. The output voltage is measured across the middle-shared electrode. Fig. 5d shows the change in VOUT as a function of VIN at fixed VDD = 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< VInput < 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 VDD. 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/HfS2 TMDs provides an opportunity to develop multi-valued logic applications.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra07276e |
This journal is © The Royal Society of Chemistry 2022 |