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 [3]. Moreover, the band structures of MoS₂ and other TMDs depend on the thickness of the film material [4]. 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 [5]. 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 p-doped silicon substrate with a 300 nm thick SiO₂ layer, using MOCVD with a reactor pressure of 10 Torr and a growth temperature of 400 °C. After cleaning the substrate with isopropyl alcohol, acetone, and deionized water, it was placed in the vacuum chamber. Molybdenum hexacarbonyl and hydrogen sulfide were used as the sources of molybdenum and sulfur, respectively. X-ray photoelectron spectroscopy (XPS) (Nexsa XPS system, Thermo Fisher Scientific) was used to measure the binding energies of Mo and S in the bilayer MoS₂ films. XPS was performed with a monochromatic Al Kα source under high vacuum (2.0 × 10⁻⁷ Torr). Raman spectra were recorded using a Raman microscope (ARAMIS, Horiba Jobin Yvon) equipped with a 514 nm laser that induces excitation with a spot size of 0.7 µm and a laser power of 8 mW. An MoS₂-based phototransistor was fabricated by depositing Ti/Au (5/50 nm) electrodes on an MoS₂ film on an SiO₂/Si substrate using an ebeam evaporator. All optoelectrical measurements were performed under vacuum conditions (10⁻³ Torr) using a four-point probe station (M6VC, MS-tech) with a commercial parameter analyzer (2636 B, Keithley Inc.). A monochromator (CS130, Newport) was used to select the wavelength using a xenon lamp (300 W) as the light source. An optical power meter (1936-R, Newport), an Si detector (10UVSI, ICC), and a Ge detector (71SI00422, Newport) were used to measure the light power density. Si and Ge detectors were used to cover the wavelength range from 400 to 1,100 nm and from 800 to 1,700 nm, respectively.

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 [8]. 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 2s, Mo 3d_5/2, and Mo 3d_3/2 respectively, which are the characteristic peaks of MoS₂. Figure 1(c) shows the two peaks corresponding to S 2p_3/2 and S 2p_1/2 at 161.58 and 162.88 eV, respectively. In addition, we can investigate the atomic percentages of Mo and S using XPS. The atomic ratio of Mo to S derived from XPS was approximately 2.42.

Figure 1. (a) Raman spectra of MOCVD grown bilayer MoS₂, (b) Mo 3d-scan, and (c) S 2p-scan of bilayer MoS₂ measured by XPS.

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 I_ds–V_ds curves of the bilayer MoS₂-based phototransistor at room temperature (300 K) and low temperature (80 K) are shown in Figs. 3(a) and 3(b). The curves are nonlinear, indicating Schottky contact between Ti/Au (5/50 nm) and MoS₂, and appear in the dark and under illumination at 900 nm. The photocurrent, I_ph (I_ph = I_illumination – I_dark), was measured under near-infrared (NIR) light (900 nm). Photoresponsivity (R) is an important parameter that indicates the performance of a MoS₂-based phototransistor and can be calculated using the following equation:

Figure 2. (a) Optical image and (b) Schematic image of bilayer MoS₂-based phototransistor.

Figure 3. I_ds – V_ds characteristics of bilayer MoS₂-based phototransistor at (a) room temperature (300 K) and (b) low temperature (80 K).

where P_light is the light power density and A is the effective area of the MoS₂ channel.

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 [4]. 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 [12]. As the temperature decreases, defect levels can be easily identified because thermal motion is suppressed [13]. 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 [14]. In addition, as the temperature decreased, the photoresponsivity increased owing to the increase in the carrier lifetime and mobility at low temperatures [15].

Figure 4. Calculated wavelength-dependent photoresponsivity of the bilayer MoS₂-based phototransistor with power measured using the Si detector at (a) room temperature (300 K) and (c) low temperature (80 K), and using the Ge detector at (b) 300 K and (d) 80 K.

In Fig. 5, the transfer characteristic curve (I_ds-V_g) of the bilayer MoS₂-based phototransistor under 5.1 mW/cm² white light power density with fixed V_ds = 10 V exhibited typical n-type semiconductor transfer properties. As the back gate (V_g) increased, the contact barrier decreased, and the injection and collection of carriers were facilitated, thereby increasing the photocurrent [16]. Moreover, the mobility (µ) of the illuminated MoS₂-based phototransistor improved from 3.99 × 10⁻⁴ to 1.43 × 10⁻³ cm²/V⋅s, which was calculated using the following equation:

Figure 5. Electrical transport characteristics of back-gate MoS₂-based phototransistor in dark and illuminated conditions.

where g_m is the transconductance, L_channel = 50 µm is the channel length, W_channel = 1,400 µm is the width, and C_g is the gate capacitance per unit area.

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.