Current Issue

Molybdenum disulfide (MoS₂) and tungsten disulfide are typical two-dimensional (2D) transition metal dichalcogenide materials that have attracted significant attention for use in high-mobility semiconductor devices [1,2] and gas sensors. This is owing to their stable characteristics and sufficient bandgap that is suitable for device applications. Moreover, their high surface-to-volume ratio enables highly sensitive gas detection [3]. Recently, an atomically thin 2D transition metal dichalcogenide MoS₂ film formed by self-limiting layer synthesis was reported as a sensing material [4] and then was used as a flexible gas sensor [5].

However, studies on wafer-scale formation of MoS₂ and its application in low-power gas sensors are limited. Wafer-scale formation of a MoS₂ layer is necessary for high-throughput and low-cost fabrication of gas sensors. Moreover, room-temperature operation or low-power sensing of a target gas is a key element of sensors for Internet of Things (IoT) and Internet of Small Things (IoST) applications that require battery operation.

In this study, wafer-scale formation of a MoS₂ layer using the metal-organic chemical vapor deposition (MOCVD) method was examined and its application as an NH₃ sensor with room-temperature operation was proven. The fabricated MoS₂ sensor showed good sensitivity to 10 ppm NH₃ gas concentration.

The MOCVD method was used to deposit MoS₂ on four-inch wafers using M^oCl₅ precursor and H₂S gas. The diameter of the chamber was slightly greater than 5 in, which was sufficient to deposit a MoS₂ film on four-inch wafers. The chamber was divided into three zones based on the temperature (Fig. 1), and the process temperature of each zone was independent. Thus, high-quality MoS₂ could be deposited in Zone 2. The M^oCl₅ precursor and H₂S gas flowed from Zone 1 to Zone 3, and MoS₂ was mainly formed in Zone 2 having a temperature of approximately 720°C. The process pressure of the MOCVD chamber was approximately 1 torr and the process time was 10 min.

Fig. 1. 
The 5-inch MOCVD chamber composed of three zones of temperature control for deposition of a MoS₂ layer on fourinch wafers. The reactant gases flow from Zone 1 to Zone 3.

Fig. 2 shows a plain view of a fabricated two-layer MoS₂ film with triangular flakes, thereby exhibiting a typical crystalline MoS₂ structure. The size of a flake is approximately 250 nm, which is sufficient for gas sensing operations. The distribution of the flakes is quite uniform, thereby indicating that the two-layer MoS₂ film formed on the four-inch wafer is uniform.

Fig. 2. 
A plain view of a field emission scanning electron microscopy image of a fabricated MoS₂ film. The typical triangular shape of the crystalline MoS₂ film is observed.

The formation and uniformity of a two-layer MoS₂ film on a four-inch wafer were examined using Raman spectroscopy. The results are shown in Fig. 3. Fig. 3 (a) shows the direction of the gas flow along the wafer, and the measurement points of the Raman analysis are shown in Fig. 3 (b). A four-inch wafer is placed horizontally in Zone 2 of the chamber (refer to Fig. 1). The differences between the locations of the two peak points in the Raman spectra of all five points are approximately 21.731–22.274 cm⁻¹, which indicates uniform formation of a two-layer MoS₂ film on the four-inch wafer, as shown in Fig. 3 (b). The similarity in the magnitudes of the Raman peaks also proves uniform formation of a two-layer MoS₂ film on the entire four-inch wafer using the MOCVD method.

Fig. 3. 
Raman spectroscopy analysis of the fabricated MoS₂ layer. (a) The five measurement points on a four-inch wafer, and (b) the Raman spectroscopy results for all five points. The spectra at all 5 points show two-layer MoS₂ characteristics

The formation of a two-layer MoS₂ film was also confirmed by transmission electron microscopy (TEM) analysis, and the result is shown in Fig. 4. A two-layer MoS₂ film is noticeable in the cross-sectional TEM image, and the thickness of one layer is approximately 0.63 nm, which is in good agreement with previous reports [6,7]. Hence, a two-layer MoS₂ film is uniformly formed on the examined four-inch wafer.

Fig. 4. 
A cross-sectional TEM image showing uniform formation of a two-layer MoS₂ film.

After the formation of a two-layer MoS₂ film, a semiconductor process was used to fabricate a gas sensor. First, photolithography was performed to form a photoresist on a MoS₂ film. Then, a SF6-based plasma etch was applied to etch the MoS₂ film outside the photoresist. After the removal of the photoresist, photolithography was again performed for the lift-off process to form finger-type patterns. Ti/Au (10/100 nm) was then deposited using an e-beam evaporator, and the deposited photoresist was lifted off. The current–voltage (I-V) characteristics of the fabricated sensor and its sensitivity to 10 ppm of NH₃ gas were measured using a gas sensing system.

The components of a MoS₂ film in the sensor area were analyzed using TEM-energy dispersive X-ray spectroscopy (EDS), and the result is shown in Fig. 5. Fig. 5 shows the presence of Mo, S, Si, and O components, and Table 1 summarizes their atomic ratios. Owing to the large analysis area, the atomic ratios are inadequate, and the formation of the MoS₂ layer is clear.

Fig. 5. 
TEM EDS analysis of a MoS₂ film.

A micrograph of an array of fabricated sensors after sawing the wafer is shown in Fig. 6. It shows that the MoS₂ film is only formed beneath the comb-type metal lines and that the array of sensors is fabricated uniformly. The slight distortion of the sensor structure shown in Fig. 6 is only due to the nonlinear characteristics of the micrograph.

The I–V characteristics of the fabricated sensors are shown in Fig. 7. The sensors in Columns 5 and 6 show little deviation in the currents, and the current level is approximately 2 nA at an applied voltage of 1 V. The current shows a slight nonlinear dependence on the applied voltage, which indicates two current components: Ohmic and Schottky currents. However, the Ohmic current component is much greater than the Schottky current component.

Fig. 6. 
A micrograph of an array of fabricated sensors with a two-layer MoS₂ film after sawing of the wafer. The array of gas sensors is formed uniformly.

Fig. 7. 
Current–voltage (I–V) characteristics of the fabricated gas sensors: (a) sensors in Column 5 and (b) sensors in Column 6 in the sensor array shown in Fig. 6.

The responsivity of the sensors to 10 ppm NH₃ gas was examined, and the results are shown in Fig. 8. A diced sample was connected to a TO46 package using wire bonding. The TO46 package was inserted into a small chamber, where 3 V was applied to the sensors through the TO46 package using a power supply that measured the current of the sensors when gases were applied sequentially. Air, 10 ppm NH₃ gas, and air gas were sequentially applied to the chamber for 3600 s using an MFC system to check the sensitivity of the sensors. The fabricated sensors showed good responsivity to 10 ppm NH₃ gas, although they did not have a heater. The responsivity or sensitivity of the sensors is defined as the ratio of the current when NH₃ gas is applied to the current immediately before NH₃ gas injection and is calculated as approximately 6–6.5, as shown in Fig. 8. Moreover, it exhibits good recovery without heater operation. The variation in the sensor current increases as the concentration of NH₃ gas increases from 1 ppm to 10 ppm. However, the linearity of the sensitivity is not critical because the main application of the developed sensors is to detect NH₃ gas with ultra-low power consumption. Hence, the developed sensors are adequate for IoT and IoST appliances that require battery operation or low-power operation and only detect specific gases such as NH₃, as shown above.

Fig. 8. 
Responsivity of the fabricated gas sensors to 10 ppm NH₃ gas. (a) Current change from about 1 nA to 7 nA, and (b) from 2 nA to 12.5 nA by 10 ppm HH3 gas.

Wafer-scale formation of a two-layer MoS₂ film and its application as an ultra-low-power gas sensor to detect 10 ppm NH₃ gas were suggested and implemented. A two-layer MoS₂ film was uniformly formed on a four-inch wafer, as confirmed using Raman spectroscopy and TEM analysis. The size of a MoS₂ flake was approximately 250 nm, which was sufficient for sensing 10 ppm NH₃ gas at room temperature. Because the operating current of the gas sensor is in the nanoampere range, the power consumption of the sensor operation is below 10 nW, which enables its operation for longer than one year, even with a low-capacity battery. Hence, the developed sensor is highly desirable for IoT and IoST applications.