Bipolar Nb₃Cl₈ Field Effect Transistors

Abstract

Field effect transistors based on few-layered van der Waals transition metal halide (TMH) Nb₃Cl₈ are studied in this work. Few-layered Nb₃Cl₈ exhibits typical N-type semiconducting behavior controlled by a Si gate, with the electrical signal enhancing as the thickness increases from 4.21 nm to 16.7 nm. Moreover, we find that the tunability of few-layered Nb₃Cl₈ FETs’ electrical transport properties can be significantly augmented through the use of an ionic liquid gate (or electrical double layer, EDL). This enhancement leads to a substantial increase in the on–off ratio by approximately a factor of ${10}^2$, with the transfer curve modulated into a bipolar fashion. The emergence of such bipolar tunable characteristics in Nb₃Cl₈ FETs serves to enrich the electronic properties within the transition metal halide family, positioning Nb₃Cl₈ as a promising candidate for diverse applications spanning transistors, logic circuits, neuromorphic computing and spintronics.

1. Introduction

Inspired by the exfoliation of graphene, the low-dimensional materials especially the two-dimensional (2D) van der Waals (vdW) materials have gained great attention due to their natural atomically thin structure, large surface-to-volume ratio, diverse properties, high carrier mobility, mechanical flexibility, heterostructure engineering capabilities, and environmental stability, promoting to unveil emerging physical phenomena, such as quantum Hall effect [1], fractional quantum Hall effect [2], strong correlation properties [3], catalytic performance [4], optoelectronic performance [5], high thermal conductivity [6], etc. These remarkable attributes make them highly promising for various applications in electronics, optoelectronics, energy storage, sensors, and beyond [7,8,9,10,11,12]. Since then, 2D semiconductors, such as transition metal dichalcogenides (TMDC) [13,14,15] and transition metal halogens (TMHs) [16,17], dominated by interlayer weak van der Waals interaction have been extensively investigated. Compared to TMDC materials, TMH materials exhibit more attractive advantages due to their richer magnetic and topological properties [18,19].

Nb₃Cl₈, as a member of vdW TMHs, is believed to be mechanically exfoliated, with an interlayer cleavage energy of 0.18 ${\mathrm{Jm}}^{-2}$ [20]. Recent study has confirmed the existence of a breathing Kagome lattice in Nb₃Cl₈, which could open a band gap at the Dirac point due to the spatial inversion symmetry being broken [21]. Due to Nb₃Cl₈’s structural characteristics, it has also been predicted to manifest quantum spin liquid owing to magnetic frustration [22], yielding a topological flat band (TFB) because of its inherent mirror symmetry. The TFB is then confirmed by using the angle-resolved-photon-energy-spectroscopy (ARPES) with a band gap around 1.12 eV [21]. The magnetic ground state of single layer Nb₃Cl₈ has also been reported recently [21]. Nb₃Cl₈, a material that can be mechanical exfoliated down to a single layer, possesses a suitable bandgap as a semiconductor, holds the promise for the low-dimensional material field-effect transistors (FETs), and exhibits strong electrostatic control. With its stability at room temperature, non-toxic nature, and other characteristics, Nb₃Cl₈ is considered a highly promising electronic system for studying electrical transport performance [20]. Recently, attempts have been devoted to probe the electrical properties of few-layered Nb₃Cl₈, which is encapsulated by hexagonal-boron nitride (h-BN) [23] and O₂ adatoms [24], respectively. However, the reported on–off ratio of Nb₃Cl₈ based FETs is around ${10}^0$, which greatly limits its application in electronic devices. Therefore, it is imperative and necessary to investigate effective tuning methods to enhance the on–off ratio and enrich the electrical transport characteristics of Nb₃Cl₈-based FETs.

In this work, we employ the mechanical exfoliation method to obtain high-quality few-layered Nb₃Cl₈ nanoflakes and fabricate FETs based on few-layered Nb₃Cl₈ with different thicknesses. Systematic studies are conducted on the electrical properties for few-layered Nb₃Cl₈ FETs equipped with Si and EDL gates, respectively. Our results show a N-type doping and on–off ratio of ${10}^0$ with Si gate FETs, while the devices exhibit bipolar doping effects and on–off ratio over ${10}^2$ with EDL gates. Our work could shed light on the future investigation on nano-electronic devices based on Nb₃Cl₈.

2. Methods

2.1. Manufacture and Testing of FETs

We employ mechanical exfoliation to obtain few-layered Nb₃Cl₈ from high-quality bulk Nb₃Cl₈. Nb₃Cl₈ nanoflakes are then deposited on SiO₂ (300 nm)/Si⁺⁺ substrate. Morphology and thickness of few-layered Nb₃Cl₈ are characterized by optical microscopy and atomic force microscopy (AFM) [25]. Then, we further study its stability upon heating and organic solvent compatibility (acetone and isopropanol) for following fabrications. It reveals that heating could cause degradation of Nb₃Cl₈. Therefore, we avoid using any heating step for the whole fabrication process, such as the BN-encapsulated process. Next, we fabricate the contact electrode of the Ti5 nm/Au50 nm film by using the electron beam lithography and electron beam evaporation. Finally, we use a semiconductor analyzer (Keysight B1500a) to measure the field effect curves and I-V curves of the as-prepared Nb₃Cl₈ devices.

2.2. Ionic Liquid Gating Method

We use the ionic liquid N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis (trifluoromethylsulfonyl) imide (DEME-TFSI) as a gate on the Nb₃Cl₈ surface to create an EDL. It exhibits great modulation compared with conventional solid dielectric materials [26,27]. A droplet of DEME-TFSI is deposited to cover both the device channel and the gate pads, after which the entire area is covered with a thin glass lid for observation under the microscope [28].

2.3. Raman Spectra

Raman spectra are acquired for both bulk and few-layered Nb₃Cl₈ using a 532 nm excitation source. The experiments to investigate the layer-dependent Raman behavior are conducted using a confocal microscope Raman spectrometer (LabRAM HR Evolution, Horiba Scientific, Raman detector, Lille, France) in a backscattering geometry. The emitted Raman signals are collected and dispersed by a 1800 gr/mm grating, with a 532 nm laser serving as the excitation source. The laser beam was focused onto exfoliated Nb₃Cl₈ flakes using a 100× objective (NA = 0.9, WD = 0.21 mm), resulting in a spot size of approximately 1 $\mathsf{\mu }$m. Each Raman spectrum was obtained by averaging ten measurements, with an integration time of 2 s for each spectrum.

3. Results and Discussion

Figure 1a,b show the ball-and-stick schematic diagram of the Nb₃Cl₈ at the top and side views. The green and blue balls represent Nb and Cl atoms, respectively. The structure belongs to the space group P$\overline{3m}$1. The gray dash lines identify the trimer clusters combined by Nb atoms. The short distance between Nb-Nb is d₁ = 2.8 Å and the long one is d₂ = 3.9 Å. The large trimer clusters and the small ones alternative arrangement forms the breathing Kagome lattice. The optical microscopy image of a typical exfoliated multi-layered Nb₃Cl₈ is shown in Figure 1c, where we can see clearly the optical contrast between different layers. AFM scanning helps us to confirm the height difference between layers, where the thinnest part is detected to be about 1.3 nm with about two layers. Therefore, the 2D van der Waals nature is confirmed by both optical microscopy and AFM.

I-V curves of a typical thin-layer Nb₃Cl₈ device with gate voltage varying from −50 to 50 V are shown in Figure 2a, while its optical microscopy image is shown in the inset. Non-linear I-V curves are obtained by varying the drain voltage in between ±1 V, which means a non-ohmic contact is formed between Nb₃Cl₈ and electrodes. By increasing the gate voltage gradually from −50 to +50 V, the absolute value of $I_{ds}$ is increasing. Fixing the $V_{ds}$ constantly at +1 V, by changing Si gate voltage from −50 to +50 V gradually, the field-effect curves are obtained and shown in Figure 2b. Combing transfer curves shown in Figure 1a,b, the intrinsic N-type semiconductor of thin-layer Nb₃Cl₈ device can be confirmed, which is consistent with the previous work [24]. Compared with the transfer curves for devices at different thickness in Figure 2b, we can find that increasing thickness of Nb₃Cl₈ could play a positive role to give rise to $I_{ds}$. This phenomenon could be attributed to an improved contact and/or a channel conductivity enhancement. The field-effect curves exhibit similar trends for each thickness devices, and the on-off ratio is approximately consistent. This indicates that the electrical modulation of the material remains relatively stable across the range of thicknesses from 4.21 to 16.7 nm tested in this experiment.

Non-linear I-V behavior in Figure 2a indicates that there may exist a high contact resistance between the Nb₃Cl₈ thin layer and the Ti/Au electrodes. This resistance could impede the Si gate’s modulation to the sample’s intrinsic carriers. Therefore, we measure the resistance between the two middle electrodes using both two-probe and four-probe methods. As shown in Figure 3a, the two-probe measurement shows a non-linear I-V curve and a changing resistance with different $V_{ds}$ due to the Schottky barrier between the electrode and the Nb₃Cl₈ channel. The red curve in Figure 3a shows that the total resistance ($R_{tot}$ = d$V_{ds}$/d$I_{ds}$) is about 25 M$\Omega$, which is composed of the Nb₃Cl₈ channel resistance ($R_{ch}$) and the contact resistance ($R_c$). Then, we select a safe input current range from −20 to 20 nA for the four-probe measurement, preventing an excessive voltage causing the device breakdown. A linear V-I curve and a stable resistance in Figure 3b shows that the channel intrinsic resistance ($R_c$ = d$V_{23}$/d$I_{14}$) of Nb₃Cl₈ is about 14 M$\Omega$. The contact resistance ($R_c$) is then calculated to be about 10 M$\Omega$, which is of the same order as the channel resistance. Therefore, resistance obtained by the four-probe method compared to the two-probe method (Figure 3c) demonstrates a more accurate field effect of the Nb₃Cl₈ flake.

Since the limited tuning effect of the solid Si gate, we then turn to a stronger tuning method by using the ionic liquid to create a non-volatile but hygroscopic EDL [29]. A field effect transistor with 16.7 nm Nb₃Cl₈ is prepared here for ionic liquid tuning. The transistor’s optical image and schematic diagram are shown in Figure 4a,b. The transfer curves under ionic liquid with different $V_{ds}$ are shown in Figure 4c, which exhibits a stronger tuning effect compared with solid Si gate shown in Figure 2b. Without external doping, the Nb₃Cl₈ exhibits an intrinsic N-type semiconductor, which is consistent with the results shown in Figure 2b. Tuned by the ionic liquid, Nb₃Cl₈ shows bipolar semiconductor characters, where the dominated carrier type could be changed by varying the voltage applied on the ionic liquid. Take the transfer curve at $V_{ds}$ = 0.1 V, for example, when the gate voltage applied on the ionic liquid increases from −0.7 to 1.5 V, negative carriers (electrons) accumulates in Nb₃Cl₈ channel and $I_{ds}$ increases responsively, showing a N-type field effect. When the gate voltage decreases from −0.7 to −1.5 V, positive carriers (holes) take in charge, showing a P-type field effect. Besides the bipolar FET characters, the on–off ratio could be further enlarged by a factor of ${10}^2$ compared with the solid Si gated FET, which conforms again the strong doping ability of EDL. The corresponding $I_{ds}$ could be further increased by increasing the $V_{ds}$, which can be observed obviously in Figure 4c. The bipolar characters and enhanced on–off ratio under EDL are further conformed by checking additional FETs with varying thickness (12.4 and 6.08 nm) of Nb₃Cl₈.

4. Conclusions

By using the mechanical exfoliation method, the high-quality few-layered TMH material Nb₃Cl₈ is acquired. Systemically electronic properties are studied by fabricating the Nb₃Cl₈-based FETs. By applying the solid Si gate and ionic liquid doping methods, the intrinsic N-type semiconductor characters are discovered. Furthermore, the implementation of the ionic liquid results in the formation of an electric double layer at the surface of the Nb₃Cl₈ channel, inducing the bipolar characteristics in Nb₃Cl₈-based FETs. This enhancement is evidenced by the significant increase in the on–off ratio, rising from approximately ${10}^0$ with Si gating to around ${10}^2$ with ionic liquid tuning. The effectiveness of our ionic liquid in achieving this tuning effect was further verified by examining Nb₃Cl₈ FETs with varying thicknesses. Considering the reported electronic performances and light absorption properties, our findings present new possibilities for future electronic studies involving Nb₃Cl₈. Moreover, these results may provide valuable insights that can be extrapolated to other TMH materials.