Applied Science and Convergence Technology 2022; 31(5): 107-109
Published online September 30, 2022
Copyright © The Korean Vacuum Society.
aDepartment of Electrical Engineering and Smart Grid Research Center, Jeonbuk National University, Jeonju 54896, Republic of Korea
bDepartment of Materials Science and Engineering, University of Seoul, Seoul 02504, Republic of Korea
Photodetectors that can detect light over a broad spectral range have attracted significant attention. Recently, molybdenum disulfide (MoS2), a known transition metal dichalcogenide, is considered a good photodetector material. In this study, MoS2 film was prepared by metal-organic chemical vapor deposition (MOCVD) on an SiO2/Si substrate. The effect of Mo vacancies on the structural, electrical, and optical properties of MoS2 films was analyzed. Results show that the synthesized MoS2-based phototransistor exhibits a photoresponsivity as high as 125 A/W at 0.02 mW power density of the 850 nm laser at room temperature (300 K). The Mo vacancies were confirmed in the MoS2 bilayer through XPS measurements. MOCVD-grown bilayer MoS2-based phototransistors with Mo vacancies allowed the detection of a wider wavelength range of 400–1150 nm. Thus, introducing Mo vacancies improved the optoelectronic properties of MoS2 phototransistors.
Keywords: MoS2, Phototransistor, Photoresponsivity, Mo vacancy, 2D Transition metal dichalcogenides
Photodetectors based on two-dimensional (2D) materials, such as graphene, have attracted significant attention in recent years. Many advantages of 2D-based photodetectors have been reported, including high photoresponsivity, fast photoresponse, wideband detection, and sensitive photodetection, owing to their unique electronic and optoelectronic properties. Graphene with intrinsic zero-bandgap energy exhibits a large amount of dark current and a low absorption coefficient, which leads to a very low photoresponsivity [1,2]. Unlike graphene, transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) are suitable for photodetectors due to their bandgap of 1.0–2.0 eV, which covers the visible to the near-infrared range . Moreover, the band structures of MoS2 and other TMDs depend on the thickness of the film material . Because the geometrical, electronic, magnetic, and optical properties are strongly influenced by molybdenum (Mo) and sulfur (S) vacancies, the MoS2 bandgap can be engineered by vacancies, as well . Generally, the detection range of a photodetector is determined by the bandgap of the semiconductor used. Defect-induced MoS2 allows to detect a longer wavelength (i.e., mid- and far-infrared) owing to the narrow forbidden gap of MoS2 [6,7]. In this study, MoS2-based phototransistors with Molybdenum (Mo) vacancies were synthesized using metal-organic chemical vapor deposition (MOVCD) on SiO2/Si substrate and its optoelectronic properties were measured and analyzed.
Bilayer MoS2 films were deposited on a
Figure 1(a) shows the Raman spectra of the bilayer MoS2 films deposited by MOCVD. The two dominant Raman peaks are observed at an in-plane E vibrational mode (383.52 cm−1) and an out-of-plane A vibrational mode (404.44 cm−1). The frequency difference between A and E can be used to confirm the thickness of the MoS2 films. The difference between the E and A modes was approximately 21 cm−1, corresponding to bilayer MoS2 . XPS measurements revealed the elemental composition and chemical state of the bilayer MoS2 films. The three peaks that are shown in Fig. 1(b) are observed at 226.18, 228.68, and 231.98 eV corresponding to S 2
Figures 2(a) and 2(b) show the optical microscopy (OM) and schematic images of the bilayer MoS2-based phototransistor fabricated by MOCVD, respectively. To measure the electrical properties, Ti/Au electrodes were deposited on both sides as drain and source. The width and length of the MoS2 channel were 1,500 and 85 µm, respectively. The effective area of the MoS2 photodetector was 8.46 × 10−4 cm2. The
Figures 4(a)–(d) show the calculated R at 300 K and 80 K, which is illuminated by light in the wavelength range from 400–1,300 nm. The photoresponse occurs owing to the generation of electron-hole pairs when the device is exposed to incident light having higher energy than the bandgap of MoS2. Therefore, the cutoff wavelength of the wavelength-dependent photoresponsivity graph is related to the bandgap of MoS2 [9,10]. Photodetectors fabricated using bilayer MoS2 show a photoresponse beyond the cutoff wavelength of ~780 nm, corresponding to a photon energy of ~1.6 eV . Meanwhile, our MoS2- based phototransistor detected up to 1150 nm, and the photoresponsivity obtained at a drain voltage of 30 V reached 125 A/W at 850 nm at room temperature. The reason that our bilayer MoS2-based phototransistor can detect a wider wavelength than typical bilayer MoS2 devices is the presence of Mo vacancies, as confirmed by XPS measurements. Further study including transmission electron microscope, Rutherford backscattering spectrometry, and deep level transient spectroscopy analysis will be necessary to fully understand the effect of Mo vacancy on optoelectronic properties of MoS2. As the Mo vacancy exists, several defect energy levels emerged in the forbidden gap region [6,11]. In Figs. 4(b) and 4(d), we observed a blue shift in the cutoff wavelength corresponding to the change in bandgap when the temperature was decreased. This is caused by the thermal expansion of the crystal lattice in many semiconductors . As the temperature decreases, defect levels can be easily identified because thermal motion is suppressed . The peak located at 1,050 nm in Fig. 4(d) is related to the Mo vacancy defect level. To verify the change in the photoresponsivity of the MoS2-based phototransistor with the drain bias, we applied a drain voltage of 1–30 V. When the drain voltage was increased, the photoresponsivity at a wavelength of 850 nm increased from 0.5−125 A/W at room temperature (300 K), and from 9–1,453 A/W at low temperature (80 K). This result was due to the larger drain voltage at which the photogenerated carriers can be easily transferred to the electrode or obstruct photogenerated carriers from recombination . In addition, as the temperature decreased, the photoresponsivity increased owing to the increase in the carrier lifetime and mobility at low temperatures .
In Fig. 5, the transfer characteristic curve (
A bilayer MoS2-based phototransistor was fabricated using MOCVD on a SiO2/Si substrate to measure its optoelectronic properties. The Mo vacancies were confirmed by investigating the atomic percentages of Mo and S in the bilayer MoS2 film by XPS measurements. The MoS2-based phototransistor produced can detect a relatively wide wavelength of 400–1,150 nm at room temperature. We extracted a photoresponsivity of 125 A/W at 850 nm without applying a gate voltage and improved the mobility up to 1.43 × 10−3 cm2/V⋅s by illumination under white light. When forbidden gap tuning is developed, such as adjusting the ratio of vacancies, MoS2 will be suitable for optoelectronic devices for use in detection in the infrared range.
This work was supported by the National Research Foundation of Korea (NRF-2021R1C1C1006147).
The authors declare no conflicts of interest.