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▼ [14p-A33-13] Carrier transport properties of MoS2 field-effect transistors produced by multi-step chemical vapor deposition method
Keywords:Transition metal dichalcogenides (TMDs), Molybdenum disulfide (MoS2), Field-effect transistor (FET)
A main purpose of this study is to explore carrier transport properties of field-effect transistors (FET), which are formed by an original multi-step CVD method. This process is consisted of four steps: MoO3 thermal evaporation, annealing, sulfurization and post-annealing. This method enables us to grow patterned MoS2 thin films simply by using shadow masks in MoO3 thermal evaporation step. Here, we observed a wide range of electronic properties (ambipolar and n-type) by changing post-annealing temperature (TPA). Importantly, high TPA of 1000 ºC yielded a high electron mobility of 13.1 cm2/Vs.
Figure 1(a) indicates the drain current-gate voltage (Id-Vg) transfer characteristics at a fixed drain voltage (Vd) of 5 V. The FET, formed at TPA = 600 ºC, has a top contact-top gate configuration. The inset shows an optical microscope image of the FET. In general, the MoS2 transistors have an n-type property. However, the transistor exhibited ambipolar operation; both electron and hole transports were observed in response to the gate bias voltages. This ambipolar operation was observed in transistors below TPA = 1000 ºC. Meanwhile, the transistor operation transited from ambipolar to n-type transport at a TPA of 1000 ºC as shown in Figure 1(b). The inset in Fig. 1(b) presents the corresponding well-modulated drain current by gate voltage. To get insight into the detailed mechanism, we examined the compositions of thin films using X-ray photoelectron spectroscopy (XPS). The S/Mo ratios depending on TPA are plotted in Figure 1(c). There were large amounts of excess sulfur (ca. 20 %) up to 800 ºC. Although the TPA at 900 °C caused a large decrease in the S/Mo ratio, a TPA of 1000 ºC was needed to produce stoichiometric MoS2 film. These results demonstrate that excess sulfurs are responsible for the ambipolar operation by acting as acceptors that generate holes. Moreover, high TPA of 1000 ºC had another distinct effect, i.e., it improved the crystallinity of the MoS2 films. The electron mobility consequently reached 13.1 cm2/Vs.
Figure 1(a) indicates the drain current-gate voltage (Id-Vg) transfer characteristics at a fixed drain voltage (Vd) of 5 V. The FET, formed at TPA = 600 ºC, has a top contact-top gate configuration. The inset shows an optical microscope image of the FET. In general, the MoS2 transistors have an n-type property. However, the transistor exhibited ambipolar operation; both electron and hole transports were observed in response to the gate bias voltages. This ambipolar operation was observed in transistors below TPA = 1000 ºC. Meanwhile, the transistor operation transited from ambipolar to n-type transport at a TPA of 1000 ºC as shown in Figure 1(b). The inset in Fig. 1(b) presents the corresponding well-modulated drain current by gate voltage. To get insight into the detailed mechanism, we examined the compositions of thin films using X-ray photoelectron spectroscopy (XPS). The S/Mo ratios depending on TPA are plotted in Figure 1(c). There were large amounts of excess sulfur (ca. 20 %) up to 800 ºC. Although the TPA at 900 °C caused a large decrease in the S/Mo ratio, a TPA of 1000 ºC was needed to produce stoichiometric MoS2 film. These results demonstrate that excess sulfurs are responsible for the ambipolar operation by acting as acceptors that generate holes. Moreover, high TPA of 1000 ºC had another distinct effect, i.e., it improved the crystallinity of the MoS2 films. The electron mobility consequently reached 13.1 cm2/Vs.