Interfacial Doping Effects in Fluoropolymer-Tungsten Diselenide Composites Providing High-Performance P-Type Transistors

Abstract

In this study, we investigated the p-doping effects of a fluoropolymer, Cytop, on tungsten diselenides (WSe₂). The hole current of the Cytop–WSe₂ field-effect transistor (FET) was boosted by the C–F bonds of Cytop having a strong dipole moment, enabling increased hole accumulation. Analysis of the observed p-doping effects using atomic force microscopy (AFM) and Raman spectroscopy shed light on the doping mechanism. Moreover, Cytop reduces the electrical instability by preventing the adsorption of ambient molecules on the WSe₂ surface. Annealing Cytop deposited on WSe₂ eliminated the possible impurities associated with adsorbates (i.e., moisture and oxygen) that act as traps on the surface of WSe₂. After thermal annealing, the Cytop–WSe₂ FET afforded higher p-type conductivity and reduced hysteresis. The combination of the Cytop–WSe₂ FET with annealing provides a promising method for obtaining high-performance WSe₂ p-type transistors.

1. Introduction

Transition-metal dichalcogenides (TMDs) are used as channel materials that can overcome the limitations of existing silicon devices, with controllable bandgaps, atomically thin 2D structures, and compact metal and chalcogen lattice structures [1,2,3]. The ability to control the channel thickness at the atomic level can improve gate control over the channel barrier and reduce short-channel effects, an issue inherent in silicon. Tungsten diselenide (WSe₂) is a TMD material, the ambipolar transport characteristics [4] of which can be adjusted by choosing a suitable contact metal [5,6,7] and the number of layers [8,9], thereby affording excellent optical properties with high quantum efficiency [10].

However, the WSe₂ transistor itself has intrinsic internal defects due to Fermi level pinning with the contact metal electrode, and impurities caused by the device fabrication processes. Furthermore, because the Fermi level of WSe₂ is close to the middle of the bandgap [11], it is difficult to inject holes or electron carriers between the contact metal and WSe₂. These issues limit the effective carrier mobility and lead to poor process yields and non-uniform properties [12]. To address these issues, an appropriate doping method must be used to control the electrical properties of WSe₂.

However, an appropriate method for doping TMDs, including WSe₂, is still lacking. Conventional doping techniques such as ion implantation are not compatible with TMDs, as these processes cause significant damage to the crystal structure of TMDs. One approach for fabricating doped TMDs is by replacing the transition metals or chalcogen atoms with other atoms. Another approach involves doping with gas. When TMDs are exposed to NO₂ or K gas, their electrical properties can be adjusted depending on the exposure concentration or time [13,14]. However, these methods involve complex processes and are limited in that the doping effect cannot be stably maintained for a sufficiently long time.

In this study, we present a simple p-doping technique in which Cytop is spin-coated on top of WSe₂, forming a fluoropolymer–WSe₂ composite (F-WSe₂ devices), in which the electrical polarity of multilayer WSe₂ can be successfully controlled. By annealing at temperatures (T_A) of 100, 200, and 300 °C, the carrier mobility is enhanced 20-fold, affording a maximum mobility of 112 cm² V⁻¹s⁻¹. Compared to the pristine WSe₂ devices, the F-WSe₂ device shows a significant improvement in the on current (≈6 × higher) and off current (≈9 × 10⁻⁴ × lower) at T_A = 200 °C, where the device exhibits high-performance metrics: μ_hole = 85 cm² V⁻¹s⁻¹ and I_on/I_off = 1.08 × 10⁶. Raman spectroscopy and atomic force microscopy (AFM) are also employed to account for the observed p-doping effects in the F-WSe₂ devices. The high annealing temperature causes molecular aggregation in Cytop, providing a strong p-type doping effect due to the higher density of the C–F dipole domains [15,16]. Furthermore, Cytop prevents the penetration of moisture and other contaminants into the F-WSe₂ devices [17]. As a result, the observed p-doping effect is effectively maintained with negligible changes. The high-performance characteristics of the p-doped WSe₂ transistor are maintained for 25 days under ambient atmosphere (only 4% variation in V_th and 19% variation in μ_hole due to air-exposure effects).

2. Materials and Methods

2.1. Device Fabrication and Measurements

Multilayer WSe₂ flakes were mechanically exfoliated from bulk WSe₂ (SPI crystals) using Scotch tape. The exfoliated WSe₂ flakes were transferred onto a Si substrate with 300 nm-thick thermal SiO₂. To remove the chemical residue that remained after the transfer process, the Si substrates were immersed in acetone, rinsed with isopropyl alcohol, and dried. On a SiO₂/Si substrate with the WSe₂ flakes transferred, the source and drain electrodes were patterned using photolithography and the lift-off method. An e-beam evaporator was used to deposit 20 nm titanium and 100 nm gold. The p-doped Si was used as a back gate by applying silver paste. The fabricated WSe₂ transistor was doped with the fluorinated polymer Cytop (solution:solvent = 1:10) for 60 s at 3000 rpm by a general spin-coating process. The annealing process was then performed at 100, 200, and 300 °C for 30 min. The thickness of the spin-coated Cytop was approximately 14 nm.

2.2. Film Analysis

Surface images and line profiles of the Cytop-doped WSe₂ transistors annealed at different temperatures (i.e., T_A = 100, 200, and 300 °C) were acquired using AFM (XE7 Atomic Force Microscope, Park Systems, Korea) in non-contact mode. The dependence of the Raman spectra of the WSe₂ film on the annealing temperature was analyzed using an ALPHA300 (WITec Co., Germany) instrument with laser excitation at 532 nm, which has a resolution of about 1.1 cm⁻¹ at 1800 lines mm⁻¹ grating. The 532 nm laser line is often used to investigate the doping behaviors of TMDs due to the resonance Raman scattering by this wavelength [18,19,20,21]. The power of the excitation was 1 mW to minimize the heating effect [18].

2.3. Air-Stability Characterization

The F-WSe₂ device was doped with Cytop and annealed at 200 °C in a clean room with a relative humidity (RH) of 25% at room temperature (25 °C). Transfer curves were acquired every five days up to the 25th day. All electrical characteristics were measured in air.

2.4. Extraction of Parameters to Evaluate Electrical Performance

Hole mobility (μ_hole) is one of the parameters for evaluating the performance of transistors. The mobility of pristine WSe₂ and F-WSe₂ (T_A = 100, 200, and 300 °C) is extracted from the drain current plots of the transfer curves at V_ds = −1 V, using the Equation (1) for the linear region.

$${\mu }_{hole}=\frac{\partial \left|{\mathrm{I}}_{\mathrm{ds}}\right|}{\partial \left|V_{gs}\right|}\cdot \frac{\mathrm{L}}{\mathrm{w}}\frac{1}{{\mathrm{C}}_{\mathrm{ox}}\left|V_{ds}\right| }$$

(1)

where C_ox is the back-gate capacitance of the SiO₂, and L and W are the WSe₂ channel length and width, respectively.

The on/off current ratio is calculated as I_on/I_off, which is the ratio of I_off, which is the drain current in the off-state region under the threshold voltage (V_th) value, and I_on, which is the drain current in the on state at V_gs = −40 V.

3. Results and Discussion

3.1. F-WSe₂ Electronic Devices

Figure 1a indicates a schematic of the F-WSe₂ device. Cytop was spin-coated on the WSe₂ device in the pristine state and annealed at various annealing temperatures (T_A = 100, 200, and 300 °C). The inset of Figure 1a shows the chemical structure of Cytop. The C–F bond possesses a dipole moment, which accumulated holes and depleted electrons, enabling p-doping enhancement in the WSe₂ channel interface [22].

Figure 1b presents an optical microscope (OM) image showing the top-view of the F-WSe₂ device. Owing to the two-dimensional bonded structure of WSe₂ inorganic materials, these materials are greatly affected by the Cytop doping effect without any effects of variations such as grain boundary traps. As shown in the OM image, the doping process was performed by spin-coating Cytop without damaging or physically affecting the crystallinity of WSe₂. WSe₂ flakes were used as the channel for the transistor, and a device was manufactured with a channel length of 20 μm, a width of 17.37 μm, and a thickness of 43.61 nm (Figure S1).

3.2. Electrical Characteristics

For quantitative comparison, the same device was characterized before and after the Cytop doping process to investigate the dependence of the change in the WSe₂ electrical properties. After plotting the transfer curves (I_ds–V_gs) and output curves (I_ds–V_ds) of the pristine WSe₂ device described previously, F-WSe₂ devices annealed at T_A = 100, 200, and 300 °C for 30 min were sequentially analyzed.

Figure 2a presents a comparison of the transfer curves (I_ds–V_gs) of the pristine and F-WSe₂ devices. The pristine device exhibits the typical transfer characteristics of ambipolar charge transport (V-shaped curve). The on current of the pristine WSe₂ device before doping was 2.30 × 10⁻⁶ A, and the off current was 1.26 × 10⁻⁸ A. When T_A = 100 °C, the on current was similar to that obtained with the pristine sample, but the off current declined significantly to 8.46 × 10⁻¹² A. For the sample annealed at T_A = 200 °C, the on current was 8.52 × 10⁻⁶ A. It is thought that the p-type current increased significantly at negative bias due to hole carrier transmission through Cytop, and the n-type current and electron carrier transmission decreased at positive bias. In addition, when the T_A was increased after doping, the threshold voltage (V_th) gradually shifted to the positive direction, indicating that the Cytop molecules acted strongly as p-type dopants [23,24]. This phenomenon may be related to the electric dipole moment of the C–F bond at the end groups of the Cytop-encapsulating molecule [25,26,27]. At T_A = 300 °C, the on current was 4.10 × 10⁻⁵ A. Compared to the pristine WSe₂ device, the on current (≈6× higher) and off current (≈9 × 10⁻⁴ × lower) were significantly improved for the device annealed at T_A = 200 °C. The high-temperature annealing (T_A = 300 °C) enabled the pristine WSe₂ film to be highly doped and caused metallic-like behavior, which is a result consistent with the previous report [23]. The metallic-like behavior of the pristine WSe₂ device at T_A = 300 °C still suffered from a significantly low on/off current ratio (~70), high off-current (3.15 × 10⁻⁶ A), and hysteresis (Figure S2). In the output curve at T_A = 300 ℃, we observed metallic-like behavior. Owing to the highly shifted V_th, the only linear region appeared in the operating voltage −40 V < V_g < 40 V, exhibiting no saturation characteristics.

The output characteristics (I_ds–V_ds) of the pristine WSe₂ device and the F-WSe₂ device according to the annealing temperature (T_A) are shown in Figure 2b–e. For the samples annealed at T_A = 100 and 200 °C after doping with Cytop, the devices showed clear unipolar p-type behavior (Figure 2c,d). In addition, compared to pristine WSe₂, the negative I_ds of F-WSe₂ became more negative as the T_A increased. This is because a dipole moment is induced on Cytop, and the channel conductance increases as the width of the Schottky barrier decreases. Based on the above investigations, we concluded that the optimal annealing temperature was T_A = 200 °C, which exhibited the optimized on/off current ratio and carrier mobility as high as μ_hole = 85 cm² V⁻¹s⁻¹ and I_on/I_off = 1.08 × 10⁶.

To statistically evaluate the variation in the effect of Cytop doping on the WSe₂ device, ten pristine WSe₂ devices were characterized, and the F-WSe₂ devices were annealed at T_A = 100, 200, and 300 °C, respectively, after Cytop doping. Figure 3a clearly shows that the higher the T_A, the stronger the Cytop doping effect, resulting in a higher μ_hole.

Figure 3b–d shows a histogram of the device number against the current level (on–off current, on/off ratio). The histograms were divided into increments of 10. In the I_on/I_off histogram (Figure 3b), I_on/I_off increased significantly at T_A = 100 and 200 °C after doping. However, at T_A = 300 °C, the Cytop doping effect was strong, leading to rapid p-doping, and the on/off ratio (I_on/I_off) was lower than those at T_A = 100 and 200 °C. This is because the on and off currents both increased owing to excessive p-doping. This trend was confirmed by the histograms of I_on and I_off for each of the ten devices (Figure 3c,d). At T_A = 100 and 200 °C, the devices exhibited an off current and improved on current compared to the pristine device. The transfer curves for the ten devices are shown in Figure S3.

3.3. Variation of Cytop Microstructure and Chemical Composition with Annealing Temperature

Atomic force microscopy (AFM) and Raman spectroscopy measurements were performed to analyze the effect of Cytop doping. To further investigate the morphological properties of Cytop depending on the annealing temperature, the surface film properties were characterized using AFM, as shown in Figure 4. Thus, it can be seen that the particle size increased with the annealing temperature. The samples annealed at T_A = 100 °C did not show well-defined domains (Figure 4a). At T_A = 200 °C, the domains gradually became visible (Figure 4b). At T_A = 300 °C, the domain size increased and the domains became clear (Figure 4c). In other words, with increasing annealing temperature, the surface changed clearly in terms of the size and shape of the domains [28]. These results indicate that the C–F bond of Cytop increases the overlap of the molecular dipole moment, as fewer impurities remain at higher temperatures.

Raman spectroscopy is a non-destructive tool used to investigate doping effects in 2D materials [29]. As evidence of the p-doping effect by Cytop, the blueshift of the Raman peak after Cytop formation on the WSe₂ appears to be related to the p-doping phenomenon, which is a result consistent with previous p-doping effects on TMDs [22,30]. To be specific, n-doping leads to softening and a decrease in the strength of the A_1g phonon, while p-doping causes a blueshift and an increase in the intensity of the A_1g phonon [31,32,33,34]. Figure 5a shows the Raman spectra of F-WSe₂ and pristine WSe₂ as a function of the T_A. The A_1g peak clearly gained intensity for the sample treated at T_A = 300 °C. These results indicated that F-WSe₂ was p-doped. For further investigation, the degree of shift of the peaks of the A_1g and E¹_2g modes was investigated (Figure 5b). The peak of the A_1g mode appeared at 255.05 cm⁻¹ for pristine WSe₂ and at 256.31 cm⁻¹ for F-WSe₂ (T_A = 300 °C), representing a blueshift of 1.26 cm⁻¹. Similarly, the peak of the E¹_2g mode was also blueshifted by 1.25 cm⁻¹. Based on the above results, we conclude that the peak corresponding to the A_1g phonon shows a blueshift and increased intensity in the case of F-WSe₂ compared to the pristine sample. This trend is more pronounced with increasing T_A, indicating that higher levels of p-doping in WSe₂ can be achieved at higher T_A.

3.4. Doping Mechanism

The Fermi level of undoped WSe₂ is close to the middle of the bandgap; thus, the channel of the transistor forms a large hole barrier with the contact metal (Figure 6a). Therefore, the hole injection is restricted. On the other hand, when WSe₂ is coated with Cytop, the C–F bond, which generates an electric dipole near the WSe₂ interface, causes greater hole accumulation. The generated electrical dipole can lead to hole accumulation by creating a negative pole on the WSe₂ channel surface (Figure 6b). As holes accumulate in the WSe₂ channel, the Fermi level decreases [35]. As a result, the hole injection barrier with the contact metal is also reduced, and the hole mobility can increase at the same negative voltage bias as before doping.

The higher the thermal annealing temperature, the larger the movement of the polymer chains of Cytop, and the fluorine atoms can be rearranged on the WSe₂ surface [22]. Thus, depending on the thermal annealing temperature, more dipole moments can be induced as the C–F bonds are more aligned on the WSe₂ surface, resulting in a large downward shift of the Fermi level, and the p-doping effect thus becomes stronger. As T_A increased, impurities (e.g., moisture) on the WSe₂ surface decreased and the C–F bonds in Cytop became more densely aligned to the WSe₂ surface, resulting in no hysteresis and highly p-doped characteristics. In conclusion, evaluation of the electrical characteristics and doping mechanism of transistors indicates that the degree of induction of the dipole on Cytop varies depending on the annealing temperature; thus, the charge transport of the channel and the degree of the p-doping effect can be controlled.

3.5. F-WSe₂ Device Performance Based on Doping Effect

Figure 7a shows the negative bias stress (NBS) data for the pristine WSe₂ device and F-WSe₂ device. The F-WSe₂ device was annealed for 30 min at T_A = 200 °C after coating Cytop on the pristine WSe₂ device. By performing the bias stress test, the decay of the electrical properties over time can be monitored to evaluate the trapping effect in the device. This effect was visualized by applying V_gs = −40 V and V_ds = −1 V for 700 s. The current level in the pristine sample decreased continuously, whereas the current level remained almost constant after Cytop doping. The large amount of fluorine contained in Cytop can diffuse into the WSe₂ channel during annealing at 200 °C, and thus the stability of the transistor can be greatly improved through fluorine diffusion [36,37,38]. In addition, the Cytop polymer has low polarity and no OH group, which shows that it can efficiently remove trapping sites due to moisture [39,40]. As a result, the WSe₂ transistor subjected to Cytop doping and subsequent annealing maintained a more stable state under the gate bias stress.

As shown in Figure 7b, the device was stored under ambient atmosphere to evaluate the effect of Cytop on the lifetime (or long-term stability) of the F-WSe₂ device (T_A = 200 °C). The electrical properties of the device were measured for 25 d in air. The on current increased slightly over time. The observed incremental enhancement of the on current resulted from a slight oxygen doping when the F-WSe₂ was exposed to air, which resulted in an incremental enhancement of mobility as a function of the air exposure time, which is consistent with the previously reported paper [27].

As shown in Figure 7c, the field-effect mobility increased by 6.6 cm²/V⋅s with increasing air exposure time. As shown in Figure 7d, the threshold voltage remained almost constant. Thus, the increase in mobility and I_ds during aging may be due to W oxidation-induced p-doping by oxygen atoms [41].

The performance of the doped WSe₂ transistor did not deteriorate as the time of exposure to air increased. For a given sample, the passivation effect of Cytop was consistently observed (Figure S4). To investigate the Cytop encapsulation effect, we compared the transfer curve measurements in a vacuum (<2 × 10⁻³ torr) and air for samples annealed with the pristine WSe₂ and F-WSe₂ at 200 °C, respectively (Figure S5). The pristine WSe₂ exhibited a variation in the transfer curve depending on the atmospheric environment (i.e., air and vacuum). This means that the pristine WSe₂ device was vulnerable to air, as shown in Figure S5a [42]. In contrast, the transfer curve of F-WSe₂ in Figure S5b maintained its characteristics unchanged by the air exposure effect, which is consistent with the previously reported studies using Cytop [27,38]. The hydrophobic surface energy of Cytop can protect the device from water and oxygen molecules, which can cause unwanted leakage effects through the 2D semiconductor surface [43,44]. The Cytop coating protects the surface of the WSe₂ channel, and it was confirmed that the p-doping performance could be maintained for a long time by the Cytop encapsulation.

4. Conclusions

A simple and stable p-doping technique was proposed by coating WSe₂ with Cytop, an amorphous fluorinated polymer. This p-doping phenomenon is characterized by a positive V_th shift and on/off current ratio, as well as improved hole mobility, resulting from the effect of the Cytop C–F bond dipole on the WSe₂ surface. At a higher annealing temperature, more hole carriers were induced, as the C–F bonds were aligned at higher T_A. This in turn induced stronger hole accumulation in WSe₂ and lowered the Fermi level of WSe₂. The observed p-doping mechanism was explained by analysis of the variation in the AFM images and Raman spectra according to the annealing temperature. Furthermore, because the hydrophobicity of Cytop protects the surface of WSe₂ from the surrounding environment, it was confirmed that the p-doping effect of Cytop was maintained for a long time even in air, with a small change in device performance compared to the initial doping state. The combination of p-doping and WSe₂ with Cytop provides a new and promising solution for obtaining high-performance p-FETs with TMD semiconductors.