Effect of Facile p-Doping on Electrical and Optoelectronic Characteristics of Ambipolar WSe2 Field-Effect Transistors
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We investigated the electrical and optoelectronic characteristics of ambipolar WSe2 field-effect transistors (FETs) via facile p-doping process during the thermal annealing in ambient. Through this annealing, the oxygen molecules were successfully doped into the WSe2 surface, which ensured higher p-type conductivity and the shift of the transfer curve to the positive gate voltage direction. Besides, considerably improved photoswitching response characteristics of ambipolar WSe2 FETs were achieved by the annealing in ambient. To explore the origin of the changes in electrical and optoelectronic properties, the analyses via X-ray photoelectron, Raman, and photoluminescence spectroscopies were performed. From these analyses, it turned out that WO3 layers formed by the annealing in ambient introduced p-doping to ambipolar WSe2 FETs, and disorders originated from the WO3/WSe2 interfaces acted as non-radiative recombination sites, leading to significantly improved photoswitching response time characteristics.
KeywordsWSe2 Ambipolar field-effect transistors p-doping Electrical characteristics Optoelectronic characteristics
Atomic force microscopy
Transition metal dichalcogenides;
X-ray photoelectron spectroscopy;
Two-dimensional (2D) materials have attracted considerable interest as promising candidates for next-generation electronics and optoelectronic devices [1, 2]. Although graphene is one of the most well-studied 2D materials, it lacks an intrinsic bandgap, restricting its wide application. Meanwhile, 2D transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, WS2, and WSe2, are advantageous in that they can be used as a channel material of field-effect transistors (FETs) due to their intrinsic bandgap properties, good carrier mobility, and high on/off ratio [2, 3]. Hence, TMDs have been widely used in various devices, such as transistors [4, 5, 6], sensors [7, 8, 9, 10], logic circuits , memory devices , field-emission devices , and photodetectors [14, 15]. In particular, FETs based on WSe2 have demonstrated great ambipolar characteristics such as high carrier mobilities, outstanding photoresponsive properties, excellent mechanical flexibility, and durability [16, 17, 18]. Nevertheless, doping WSe2 is required to further improve field-effect mobilities or contact properties which are essential in a variety of electronic applications [16, 19]. Among a lot of approaches for doping, thermal annealing in ambient to form WO3 layers on a WSe2 surface has been demonstrated to be a facile as well as an efficient p-type doping processes [20, 21, 22]. For example, Liu et al. thermally annealed WSe2 films in ambient without use of additional substances to dope the films in the p-type manner and improved the hole mobility to 83 cm2 V−1 s−1 with employing hexagonal boron nitride substrate . However, thorough studies on the optical and optoelectronic characteristics of WSe2 doped by WO3 are desired for the optoelectronic applications such as phototransistors, photodiodes, and light-emitting diodes [17, 18, 23, 24].
In this work, we explored the electrical, optical, and optoelectronic properties of ambipolar WSe2 FETs before and after thermal annealing in ambient. The oxidized layer (WO3) formed on a WSe2 surface during the annealing successfully introduced p-doping to the ambipolar WSe2 FETs, leading to a shift of the transfer curve to the positive gate voltage direction. Interestingly, long-lasting photoconductivity, which is a phenomenon of the conductance’s being retained after the light irradiation is turned off, disappeared after the annealing. Furthermore, we performed various experiments, such as X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, and Raman spectroscopy to investigate the origin of the changes in the electrical and photoswitching characteristics of the ambipolar WSe2 FETs.
WSe2 flakes were prepared by the micromechanical exfoliation method from a bulk WSe2 crystal, and were transferred to a 270-nm-thick SiO2 layer on a heavily doped p++ Si wafer (resistivity ~ 5 × 10−3 Ω cm) that was used as the back gate of the FET devices. The thickness of the WSe2 flakes was measured using an atomic force microscope (NX 10 AFM, Park Systems). To create electrode patterns, we spin-coated poly(methyl methacrylate) (PMMA) 495K (11% concentration in anisole) as an electron resist layer at 4000 rpm. After the spin-coating, the samples were baked on a hot plate at 180 °C for 90 s. We designed the electrode patterns using an electron-beam lithography instrument (JSM-6510, JEOL), and developed the patterns with a methyl isobutyl ketone/isopropyl alcohol (1:3) solution for 120 s. Finally, titanium metal (30-nm-thick) electrodes were deposited using an electron-beam evaporator (KVE-2004L, Korea Vacuum Tech).
Thermal annealing in ambient was performed on a hot plate at certain temperatures. Thermal annealing in vacuum was performed using a rapid thermal annealing system (KVR-4000, Korea Vacuum Tech) at 4.5 × 10−4 Torr and 200 °C for 1 h.
Photoluminescence and Raman spectroscopy measurements were performed using a confocal imaging system (XperRamn 200, Nanobase) with the incident laser wavelength of 532 nm. X-ray photoelectron spectroscopy measurements were performed using an electron energy analyzer (AXIS SUPRA, Kratos). The electrical characteristics of the devices were measured using a probe station (JANIS, ST-500) and a semiconductor parameter analyzer (Keithley 4200-SCS). Photoresponses of the devices were measured under laser (MDE4070V) illumination.
Results and Discussion
Figure 4a, b shows that photoswitching characteristics became improved at VGS = 5 V (VGS > Vn↔p) by the thermal annealing, which is in agreement with the results in Fig. 3. This behavior also can be explained by the promoted recombination processes at the induced recombination sites between WSe2 and WO3 interface. The PL result demonstrated the existence of non-radiative recombination sites at WO3/WSe2, which will be discussed afterward. At VGS = − 15 V (VGS ~ Vn↔p), we could not observe the distinct change after the thermal annealing due to the highly rapid photoswitching characteristics (Fig. 4c, d). This rapid photoswitching behavior originates from the location of EF in the middle of WSe2 bandgap, which suppresses the additional charge injection after turning off the irradiation (see the section 4 in Additional file 1 for detail). For the case of VGS = − 90 V (Fig. 4e, f), τdecay and τlong were maintained and shortened, respectively, although the current after the annealing was much higher than that before the annealing (more than 20 times). Importantly, there is a trade-off between the photo-induced current and decay time constants in phototransistors, because the trapped photogenerated minority carriers can produce an additional electric field, thereby leading to the increased channel current and demanding continuous charge injection even after the irradiation is turned off [32, 33]. In this regard, the preservation of τdecay and shortened τlong in spite of the significantly increased photo-induced current signifies the improved photoswitching characteristics by the annealing in ambient as shown in Fig. 4e, f. Regarding τrise, the location of EF moves to the valence band by p-doping, which causes non-charge neutrality to become stronger due to the decreased hole trap sites where the photogenerated holes can occupy (Additional file 1: Figure S6a). Due to the strong non-charge neutrality, under the irradiation, the more charges are injected for satisfying the charge neutrality. And, photogenerated carriers will undergo more scattering with free carriers while passing through the channel to contribute to the photocurrent, so that τrise time can become longer. For that reason, the τrise becomes longer at VGS = − 90 V after thermal annealing as shown in Fig. 4e, f (see the section 4 in Additional file 1 for more detail).
PL spectroscopy analysis was conducted for two different monolayer WSe2 flakes (labeled as sample 1 and sample 2) as shown in Fig. 6c. The inset of Fig. 6c corresponds to an optical image of sample 1. Each WSe2 flakes were annealed for 30 min and 60 min at 250 °C in ambient. The optical and PL mapping images of the other monolayer WSe2 flake (labeled as sample 2) are provided in the Additional file 1: Figure S7. As the annealing time increased, the optical bandgaps of the WSe2 became wider. The optical bandgap was extracted from the photon energy of the maximum intensity in PL spectrum because that corresponds to the resonance fluorescence originating from the bandgap. While the optical bandgap of the sample 1 was measured as ~ 1.60 eV before the annealing corresponding to the bandgap of monolayer WSe2 , the bandgap value changed to ~ 1.61 eV after the annealing for 60 min. Although the increase (~ 10 meV) of the optical bandgap is slight, this phenomenon can be explained by the formation of the WSe2-WO3 in-plane heterojunctions and the dielectric screening effect. Since WO3 has a larger bandgap of 2.75 eV compared to WSe2 (1.60 eV for a monolayer) , the optical bandgap of the monolayer WSe2 flakes increased through the annealing in ambient. Furthermore, the formation of WO3 on WSe2 can generate a stronger dielectric screening effect due to the larger dielectric constant of WO3 (~ 90) compared to that of WSe2 (~ 22) [41, 42]. Consequently, the stronger dielectric screening effect leads to the diminished exciton binding energy and slightly increased the optical bandgap during the thermal annealing .
Interestingly, in perspective of the PL intensity, it obviously decreased as the annealing time increased as shown in Fig. 6d. The PL quenching behavior of monolayer WSe2 can be easily observed in PL mapping images integrating the PL intensity in peak region, as increasing annealing time (inset of Fig. 6d). A similar phenomenon was observed in the MoS2 treated by oxygen plasma . These results can be explained as follows. Since WO3 has an indirect bandgap , the band structure of WSe2 may be partially changed to that with an indirect bandgap, which leads to reduced PL intensity. Additionally, the lattice mismatch between the WSe2 and WO3 structures provides traps and recombination sites in the bandgap of WSe2 that can affect the electrical and optical characteristics of the WSe2. For instance, disorder, defects, and sulfur vacancies can produce shallow or deep trap sites in the MoS2 layers, giving rise to the recombination process [31, 45]. Therefore, as the annealing time increased, disorder and the defects originating from the lattice mismatch of the WSe2-WO3 structure lead to non-radiative (Shockley-Read-Hall) recombination , and to reduced PL intensity. Collectively, the experimental results of the XPS, Raman, and PL spectroscopies demonstrate the formation of WO3 on the WSe2 surface by the annealing in ambient, and those are in agreement well with recent researches on the oxidation of 2D materials [20, 46]. Besides, from the analysis of PL spectroscopy, it was supported that non-radiative recombination sites induced by WO3 layer could contribute to the improved photoswitching characteristics by promoting the recombination processes.
In summary, we fabricated ambipolar WSe2 FETs and studied the electrical properties and photoswitching responses before and after thermal annealing in ambient. We observed that the WSe2 FETs were successfully doped in the p-type manner and that the photoswitching responses became considerably faster after the ambient thermal annealing. The XPS, Raman, and PL studies demonstrated that the WO3 layer formed on the WSe2 surface can play the roles of a p-doping layer and non-radiative recombination sites to promote faster photoswitching behavior. This study provides a deeper understanding of effects on electrical and optoelectronic characteristics of ambipolar WSe2 FETs by the facile p-doping process via the thermal annealing in ambient.
TL and JP supervised the experiments and characterization. JS (Junseok Seo) designed and carried out the experiments. KC and WL helped to analyze the results of the electrical and optoelectronic characterization. JS (Jiwon Shin), J-KK, and JK helped to fabricate WSe2 FETs. JS (Junseok Seo), JP, and TL contributed to writing and editing the manuscript, and all authors contributed to the data analysis and discussion of the results. All authors read and approved the final manuscript.
The authors appreciate the financial support of the National Creative Research Laboratory program (Grant No. 2012026372) through the National Research Foundation of Korea funded by the Korean Ministry of Science and ICT.
The authors declare that they have no competing interests.
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