High‐Performance Flexible MoS2 Transistors Using Au/Cr/Al/Au as Source/Drain Electrodes

2D transition metal dichalcogenides such as molybdenum disulfide (MoS2) are promising candidates for flexible electronics because of their bandgap tunability, high carrier mobility, and mechanical flexibility. However, flexible MoS2 field effect transistors (FETs) are typically faced with high contact resistance which is identified as a critical limiting factor to their potential applications. The present work successfully addresses this challenge by using Al contacts without any annealing process. It is found that the contact resistance is strongly coupled to the Al thickness, increasing the Al thickness is beneficial to reduce the contact resistance. The observed variation in the device electrical characteristics can be associated with the formation of the natural aluminum oxide (AlxOy) film at the interface. An ultrathin Au insert layer can further advance the device performance. An optimal contact resistance of 2.03 kΩ and an on/off current ratio of 1.57 × 109 can be achieved in the flexible MoS2 FETs using Au/Cr/Al/Au as source/drain electrodes. Furthermore, no apparent degradation in the device properties is observed under systematic cyclic bending testing with bending radii of 18 and 15 mm. The successful integration of Au/Cr/Al/Au source/drain electrodes into MoS2 FETs promises its potential application in flexible electronic devices.


Introduction
Building flexible electronic systems is expected to meet the needs of emerging applications where mainstream silicon-based DOI: 10.1002/aelm.202300277electronic products cannot provide solutions, such as flexible circuits, [1,2] electronic paper, [3] electronic skin, [4] flexible wearable sensors, [5] flexible displays, [6] and flexible solar cells, [7] etc.A major challenge in realizing high-performance flexible electronic devices is to select suitable flexible, stretchable, and bendable semiconductor materials. [8][11] Typically, molybdenum disulfide (MoS 2 ), a member of 2D transition metal dichalcogenides (2D-TMDs), appears to be one of the most prevailing candidates due to its wide bandgap, high carrier mobility, high mechanical strength, and the possibility for large-scale synthesis. [12][22] Previous studies have demonstrated that lower work functions of the contact metals lead to smaller Schottky barrier heights (SBHs) and consequently higher carrier injection, resulting in eliminating contact resistance effects. [23,24]Namely, the choice of lower work function metal contacts is crucial in advancing flexible MoS 2 FET electrical performance.Fang et al. have performed a comprehensive ab initio study on the interfacial properties of monolayer MoS 2 contacted with metals, and the theoretical results have shown that the SBHs of the Al/MoS 2 system are much lower than that of the Au/MoS 2 system. [25]However, Shimazu et al. and Zhang et al. have investigated the electrical performance of MoS 2 FET with different metal contacts, and they found that the Au/MoS 2 system appears to be more advantageous as compared to the Al/MoS 2 system. [26,27]These inconsistent results puzzle us in using low work function metal Al as contacts for improving electrical performance, especially contact properties of flexible MoS 2 FET.Additionally, the impact of contact metal thickness on the performance of MoS 2 FETs has been rarely reported.Therefore, a thorough investigation of the Al as metal contacts for flexible MoS 2 FET is highly desirable.
In this contribution, to the best of our knowledge, for the first time, we report a comprehensive study of flexible MoS 2 FETs utilizing Al contacts and then provide guidance for optimizing the Al thickness of this system.On the basis of our experimental results, we find that contact formed at the metal-to-MoS 2 interface can be effectively modulated through Al thickness controlling, which leads to distinct contact resistance in the device.With an increase in Al thickness up to 50 nm, there is a noticeable enhancement in both the on-state current and field effect mobility, while the contact resistance experiences a significant decrease.In pursuit of optimizing the contacts to our MoS 2 , we also investigate the influence of the ultrathin Au interlayer inserted between the metal contact and the MoS 2 film on the device electrical per-formance.We relate the observed I-V characteristics to the interface chemistry which could be explained by differential oxidation of the Al electrode.Additionally, the performances of flexible MoS 2 FETs with different bending conditions are also characterized.These results reveal that low work function metal Al can be used to engineer high-efficiency electrical contacts for flexible MoS 2 FETs.

Results and Discussion
Figure 1a shows the schematic architecture of our flexible MoS 2 FETs.The abridged general view of the fabrication process is given in Figure S1 (Supporting Information).Figure 1b presents a transparent PEN/ITO/Al 2 O 3 substrate that has undergone fabrication of MoS 2 FETs and Figure 1c displays a representative optical microscope image of the as-fabricated MoS 2 FETs.The thickness of the target MoS 2 samples was measured by a combination of optical contrast and Raman spectroscopy (see Figure S2, Supporting Information).The Raman spectrum, displayed in Figure 1d, obtained from the MoS 2 sample enclosed by the blue box in Figure 1c provides valuable insights into the vibrational modes and molecular properties of the MoS 2 film.We obtained two main vibrational modes, the E 1 2g peak located at 382.5 cm −1 represents the in-plane vibrational mode, while the A 1g peak at 405.8 cm −1 represents the out-of-plane vibrational mode.The Raman frequency difference between these two vibrational modes is ≈23.3 cm −1 , indicating three-layer MoS 2 . [28]The fundamental electrical performances of the flexible MoS 2 FETs with various metal contacts were investigated.Figure 1e shows the forward and reverse transfer characteristics for both linear (right axis) and semilogarithmic (left axis) scales of a typical MoS 2 FET using Au/Cr/Al-80/Au as source/drain electrodes (named Au/Cr/Al-80/Au-MoS 2 FET) under the drain voltage (V DS ) varying from 0.05 to 1 V.The fabricated Au/Cr/Al-80/Au-MoS 2 FETs demonstrate clear n-type semiconducting behavior, which is in agreement with the previous reports. [16,17,19] The key electrical parameters of the flexible MoS 2 FETs with various metal contacts were compared by exploring their forward transfer characteristics at V DS = 0.05 V, including on/off current ratio, mobility, contact resistance, hysteresis, and subthreshold slope.Figure 2 shows the extracted on-state current (I ON ), offstate current (I OFF ), and on/off current ratio of these devices with different contact electrodes.I ON is defined as I DS at V BG = 5 V and I OFF is defined as the minimum of I DS .As shown in Figure 2a, the statistical I ON of the flexible MoS 2 FETs showed a significant improvement when the thickness of Al was increased from 20 to 50 nm.However, as the thickness of Al was further increased, the I ON remained relatively stable, which is also reflected in the I ON map of Figure 2b.In contrast, the statistical I OFF of the flexible MoS 2 FETs remained relatively constant when the thickness of Al was increased from 20 to 50 nm, but started to decrease as the thickness of Al was further increased, as shown in Figure 2c,d.As a result, Figure 2e,f clearly demonstrate that the on/off current ratio of the flexible MoS 2 FETs increased with the increase in Al thickness.In addition, the use of the ultra-thin Au contacts at the bottom of the source-drain electrodes (bottom Au contacts) contributed to the enhancement of both I ON and I OFF of the flexible MoS 2 FETs, ultimately leading to an increase in the on/off current ratio.Furthermore, the experimental results indicate that the Au/Cr/Al-80/Au-MoS 2 FETs achieved the highest on/off current ratio of 1.57 × 10 9 (see Figure S4 and Table S1, Supporting Information).
Figure 3 shows the extracted field-effect mobility in the linear regime (μ FE ), intrinsic mobility (μ 0 ), and contact resistance (R SD ) of these devices with different contact electrodes.The μ FE was calculated using equation μ FE = g m L/(WC OX V DS ), where L, W, g m , and C OX are the channel length, width, transconductance, and gate capacitance respectively.And the μ 0 and R SD were evaluated using the Y-function method. [29,30]From Figure 3a,c, it can be seen that the trend of the mobility and the on-state current of the flexible MoS 2 FETs are basically consistent, as shown in the mobility map depicted in Figure 3b,d.The experimental results indicate that among all tested configurations, the Au/Cr/Al-80/Au-MoS 2 FETs achieved the highest μ FE of 38.3 cm 2 V −1 ·s −1 and the highest μ 0 of 94 cm 2 V −1 ·s −1 .The average μ FE of Cr/Al-20/Au MoS 2 FETs (1.4 cm 2 V −1 ·s −1 ) is relatively low due to high contact resistance.Increasing the thickness of aluminum from 20 to 50 nm results in a significant improvement in field-effect mobility.Subsequently, as the thickness of Al continues to increase, the improvement in field-effect mobility becomes more subtle.By increasing the thickness of Al layer and introducing an ultrathin Au interlayer, the average μ FE improved to 20.7 cm 2 V −1 ·s −1 , representing a proportional increase of ≈1478%.The intrinsic mobility shows a similar variation pattern to that of field-effect mobility.However, the increase in intrinsic mobility is less pronounced compared to that of field-effect mobility as the thickness of Al increases from 20 to 50 nm, mainly because the former is less affected by the contact resistance. [31]As is well known, contact resistance is the key and primary factor responsible for the alterations in the electrical properties of the flexible MoS 2 FETs with various metal contacts.As shown in Figure 3e,f, it is evident that an increase in the thickness of aluminum from 20 to 50 nm leads to a significant reduction in contact resistance.The average R SD of Cr/Al-20/Au MoS 2 FETs (706.80 kΩ) decreased by 98.3% to 11.73 kΩ for the Cr/Al-50/Au MoS 2 FETs.As the thickness of aluminum continues to increase to 80 nm, the contact resistance remains mostly unchanged.Additionally, the implementation of ultrathin bottom Au contacts results in a slight improvement in contact resistance.Experimental results also suggest that Au/Cr/Al-80/Au-MoS 2 FETs demonstrated a remarkably low contact resistance of 2.03 kΩ, which is the lowest value observed among all tested configurations.Additionally, an assessment of the electrical performances (including on/off current ratio, R SD , and μ FE ) of flexible MoS 2 FETs with different source/drain electrodes was conducted, and the results are summarized in Table 1. [14,32,33,34,35,36,37]By employing the appropriate thickness and electrode stack structure, Al contacts can be implemented to maximize the advantageous electrical properties of flexible MoS 2 FETs (i.e., on/off current raion and R SD ).The extracted hysteresis and subthreshold slope (SS) of these devices with different contact electrodes was shown in Figure S5 (Supporting Information).
The previous discussion on the electrical parameters of the flexible MoS 2 FETs featuring various metal contacts indicates that the electrical performance of these devices is enhanced when the Al thickness is above 50 nm.Moreover, the electrical performance of Au/Cr/Al-80/Au-MoS 2 FETs is at the best level among all tested configurations.Without considering the Cr adhesion layer, we observed that varying the thickness of top Au contacts had almost no effect on the electrical performance of the flexible MoS 2 FETs.As a result, it can be inferred that the device electrical performance is mainly affected by Al thickness and ultra-thin bottom Au contacts.And the enhancement of device electrical performance is primarily attributed to the reduction of contact resistance, which leads to increased mobility and I ON .It is important to consider that Al contacts are prone to oxidation in the air, which may explain the suboptimal performance of Cr/Al-20/Au-MoS 2 FETs.The process of oxidation can lead to partial oxidation of the 20 nm-thick Al contacts and the formation of a natural aluminum oxide (Al x O y ) film at the interface, resulting in a thinner Al layer.This thinning may necessitate the replacement of the Al contacts with Au contacts, resulting in an elevated work function of the source-drain electrode. [23,38,39]Additionally, the degree of oxidation of Al contacts is influenced by their thickness, with thicker Al contacts exhibiting higher rates of oxidation and thicker initial oxide layers. [40]This results in a slight increase in the contact resistance and a slight decrease in the I OFF of Cr/Al-80/Au-MoS 2 FETs compared to Cr/Al-50/Au-MoS 2 FETs.The energy band diagrams of the flexible MoS 2 FETs with various contact, as depicted in Figure 4, suggest that the natural oxide layer introduces a tunneling barrier, which affects the contact resistance and leads to variations in the overall electrical performance of the FET. [41]o evaluate the flexible MoS 2 FETs under mechanical strains, bending tests with two different curvature radii (R) were conducted.A schematic drawing of the bending mechanism is shown in Figure 5a, inset is the measurement setup of the bending devices.The flexible MoS 2 FETs were placed on the bending molds with curvature radii of 18 and 15 mm.The strain value () can be determined using  = t/(2R+t), where t represents the thickness of the PEN substrate (125 μm). [42]The plot in Figure 5b,c depicts the transfer and output characteristics of a typical Au/Cr/Al-80/Au MoS 2 FET under different strains.The results show that I ON increases slightly as the bending strain becomes larger.This trend was observed in other tested Au/Cr/Al-80/Au MoS 2 FETs as well (refer to Figure S6, Supporting Information).The increase in I ON can be explained by the reduction in the resistance of the MoS 2 flake and the Schottky barrier heights at the source and drain. [43]The transfer characteristics of the Au/Cr/Al-80/Au MoS 2 FET were studied as a function of bending cycles, as shown in Figure 5d.The device showed no degradation in its transfer characteristics under 0.05 V bias voltage at a bending radius of 15 mm for up to 500 bending cycles.The performance of the device was further investigated in Figure 5e, which shows I ON and I OFF as a function of bending cycles.The results indicate that the Au/Cr/Al-80/Au MoS 2 FETs exhibit slight performance variations with bending strains, implying that these devices are robust under mechanical deformation conditions.

Conclusion
In summary, we have investigated the influence of source/drain electrodes on the electrical properties of MoS 2 FETs, specifically focusing on the role of Al contacts in flexible MoS 2 FETs.Our study demonstrates the successful fabrication of high-performance flexible MoS 2 FETs with Au/Cr/Al/Au contacts, achieving an optimal contact resistance of 2.03 kΩ and an on/off current ratio of 1.57 × 10 9 .We found that the contact resistance strongly depends on the thick-

Experimental Section
Materials and Device Fabrication: Indium tin oxide (ITO) coated polyethylene naphthalate (PEN) was purchased from Peccell Technologies Inc. (Japan), in which the electrical resistivity of the top ITO layer was ≈15 Ω cm −1 .The fabrication processes of the flexible MoS 2 FETs were as follows.First, thirty-nanometer Al 2 O 3 films were deposited on ITO/PEN substrates in a commercial atomic layer deposition (ALD) system (Picsun Ltd.) at room temperature using Trimethylaluminum (TMA)  and O 2 plasma. [44]One cycle of plasma-enhanced ALD deposition consisted of 0.1 s of TMA pulse, followed by 10 s of N 2 purge, then 8 s of O 2 plasma pulse, again followed by 10 s of N 2 purge.The TMA was maintained at 18 °C and the generator power of O 2 plasma was fixed at 2500 W at an O 2 flow rate of 150 sccm for stable vapor pressure and dose.Subsequently, MoS 2 flakes (2-6 layers) were mechanically exfoliated from commercial synthetic MoS 2 crystals (XFNano) and transferred onto the flexible substrate.Then, source/drain regions were defined by the common electron beam lithography (EBL) patterning technique.For this purpose, polymethyl methacrylate (PMMA) was used as a photoresistor and it was spin-coated at 4000 r min −1 for 1 min.Then the substrate was heated at 170 °C for 3.5 min.Finally, various metal contacts were formed by thermal deposition and lift-off.The study thermal evaporates Cr/Al/Au contact with various thicknesses of metal Al (20, 50, 80 nm) and Au/Cr/Al/Au contact with ultrathin Au interlayer (3 nm) to examine the effects of metal thickness and interlayer on the device performance.A thin adhesion layer of Cr (5 nm) was inserted between Al and substrates to enhance device yield.Additional details of source/drain electrodes are provided in Table S2 (Supporting Information).
Materials and Device Characterization: Raman characterization of the MoS 2 flake was performed with a Horiba Xplora Raman spectrometer with 532 nm laser excitation.Electrical characterization of the devices was carried out by using an Agilent B1500A parametric analyzer in ambient environments.To ensure the reproducibility and reliability of the results, multiple devices were used for each contact configuration, as detailed below.For the Cr/Al-20/Au MoS 2 FETs, a total of ten samples were meticulously prepared and characterized.Similarly, five individual samples were used for the Cr/Al-50/Au MoS 2 FETs.Moreover, eight samples were utilized for the Cr/Al-80/Au MoS 2 FETs, and ten samples were studied for the Au/Cr/Al-80/Au MoS 2 FETs.The electrical parameters were extracted from each of the aforementioned samples and have been sequentially labeled from one to ten for ease of reference during data analysis and presentation.

Figure 1 .
Figure 1.a) The schematic depiction of the flexible MoS 2 FET used in this study.b) The photograph of the transparent PEN/ITO/Al 2 O 3 substrate that has undergone fabrication of MoS 2 FETs.c) The optical microscopy image of a typical MoS 2 FET.Scale bar: 20 μm.d) The Raman spectrum of the MoS 2 sample within the blue box in (c).e) Transfer characteristics of a typical Au/Cr/Al-80/Au-MoS 2 FET under different V DS (varying from 0.05 to 1 V).f) Output characteristics of the Au/Cr/Al-80/Au-MoS 2 FET under different V BG (varying from −5 to 5 V), demonstrating a good Ohmic contact.
Figure 1f displays the output characteristics of the Au/Cr/Al-80/Au-MoS 2 FET under the back-gate voltage (V BG ) varying from −5 to 5 V. Drain current (I DS ) shows a good linear relationship with V DS , which demonstrates a good Ohmic contact between few-layer MoS 2 channel and Au/Cr/Al-80/Au electrodes.The typical transfer characteristics and output characteristics of the Cr/Al/Au-MoS 2 FETs with various Al thicknesses (named Cr/Al-20/Au-MoS 2 FETs, Cr/Al-50/Au-MoS 2 FETs and Cr/Al-80/Au-MoS 2 FETs) are provided in the Figure S3 (Supporting Information).

Figure 2 .
Figure 2. a) The statistical I ON of the flexible MoS 2 FETs with different metal contacts.b) I ON map for each MoS 2 FET location.c) The statistical I OFF of the flexible MoS 2 FETs with different metal contacts.d) I OFF map for each MoS 2 FET location.e) The statistical on/off current ratio of the flexible MoS 2 FETs with different metal contacts.f) On/off current ratio map for each MoS 2 FET location.Au-80 refers to the Au/Cr/Al-80/Au MoS 2 FET.

Figure 3 .
Figure 3. a) The statistical μ FE of MoS 2 FETs with different metal contacts.b) μ FE map for each MoS 2 FET location.c) The statistical μ 0 of MoS 2 FETs with different metal contacts.d) μ 0 map for each MoS 2 FET location.e) The statistical R SD of MoS 2 FETs with different metal contacts.f) R SD map for each MoS 2 FET location.Au-80 refers to the Au/Cr/Al-80/Au MoS 2 FET.
ness of the Al layer, with increasing Al thickness leading to a reduction in contact resistance.This observation is attributed to the formation of a natural aluminum oxide (Al x O y ) film at the interface, which affects the contact resistance and overall electrical performance of the flexible MoS 2 FETs.The implementation of ultra-thin bottom Au contacts results in a slight improvement in contact resistance, which could further enhance device performance.Importantly, our study highlights the mechanical robustness of the fabricated flexible MoS 2 FETs, which show no significant degradation during systematic cyclic bending testing, indicating their potential for use in flexible electronics applications.Overall, our findings provide valuable in-sights into reducing the contact resistance of MoS 2 FETs, which could aid in both fundamental investigations and device design.

Figure 4 .
Figure 4. Energy band diagrams in V BG > 0 and V BG < 0 for the flexible MoS 2 FETs with various contact.Al x O y forms on the Al contacts surface naturally.

Figure 5 .
Figure 5. a) Schematic representation of the bending apparatus, inset is the measurement setup of the bending devices.b) Comparison of transfer characteristics of an Au/Cr/Al-80/Au MoS 2 FET at different bending radii with V DS = 0.05 V. c) Comparison of output characteristics of the Au/Cr/Al-80/Au MoS 2 FET at different bending radii with V BG = 0 V. d) The transfer characteristics of the Au/Cr/Al-80/Au MoS 2 FET on a flexible substrate up to 500 bending cycles.e) I ON and I OFF as a of bending cycles.

Table 1 .
On/off current ratio, contact resistance (R SD ), and field-effect mobility (μ FE ) of flexible MoS 2 FETs with various source/drain electrodes.