以石墨為接觸之二維二碲化鉬電晶體研究

Abstract

過渡金屬二硫族化物 ( transition metal dichalcogenides, TMDCs) 層與層之間透過弱凡得瓦力堆疊在一起，使得塊狀晶體可以沿著每個三層結構剝離成單獨的二維薄片，這些TMD薄片沒有懸鍵，因此具有化學穩定性和低載子散射等特性。在TMDCs 中，二碲化鉬 (Molybdenum Ditelluride, MoTe2) 有最小的能隙，有助於增強電子和電洞的載子注入。 本研究透過膠帶層離法 ( tape exfoliation ) 製備二碲化鉬、二硒化鎢 ( tungsten diselenide, WSe2 )、六方晶氮化硼 ( hexagonal boron nitride, hBN ) 及石墨 ( graphite )晶體，以石墨作為接觸電極，製作雙閘極單層結構之二碲化鉬電晶體和雙層結構之二碲化鉬/二硒化鎢電晶體。室溫下，單層結構之二碲化鉬電晶體呈現雙極特性，p型與n型開關電流比分別為 〖7.68×10〗^5 和 〖3.11×10〗^5 ；次臨界擺幅為0.82 V/dec和2.03 V/dec。於5 K的低溫下，p型與n型開關電流比分別提高至 〖9.94×10〗^6 和 〖9.61×10〗^5；次臨界擺幅下降為0.42 V/dec和1.66 V/dec，此元件於低溫下有較好的特性，其電晶體載子傳輸機制為band-like transport。而雙層結構之二碲化鉬/二硒化鎢電晶體室溫下p型與n型開關電流比分別為 〖6.28×10〗^5 和 〖1.77×10〗^6 ；次臨界擺幅為0.84 V/dec和2.21 V/dec。於5 K的低溫下，p型與n型開關電流比分別為 〖1.52×10〗^4 和 〖4.95×10〗^4；次臨界擺幅為1.74 V/dec和1.02 V/dec，此元件載子傳輸屬變程跳躍，於室溫下電性表現較佳。 相同材料會因為原本晶體品質狀況不同，或是製作元件過程中造成缺陷程度的差異，使主導的機制不同。石墨導電性佳，與二碲化鉬皆屬二維材料，相對於直接與三維金屬接觸，更能減少金屬誘導間隙態的產生，本研究以石墨作為接觸電極製作二碲化鉬之電晶體電子及電洞皆能注入，不須化學摻雜的情況下，透過調控閘極區域，就能在導電通道中選擇性地傳輸電子和電洞，可以減少電路中需要的電晶體數量，從而簡化電路設計。雙極性電晶體能夠在不同極性下工作，使得電路設計具有更高的靈活性和穩定性[1]。

Transition metal dichalcogenides (TMDCs) are layered materials held together via weak van der Waals forces. The Bulk crystal can be exfoliated to two-dimensional sheet. The TMDCs material is notable for smooth and no dangling bonds, chemical stability and low carrier scattering. Among TMDCs, molybdenum ditelluride (MoTe2) has the smallest bandgap, enhancing carrier injection for both electrons and holes. In this study, MoTe2, tungsten diselenide (WSe2), hexagonal boron nitride (hBN), and graphite crystals were prepared through the mechanical exfoliation method by the blue tape. The dual-gate MoTe2 transistor and MoTe2/WSe2 transistor were fabricated with graphite contact electrodes. At room temperature, the MoTe2 transistor exhibited ambipolar behavior. The on/off current ratios of p-channel and n-channel were 7.68×10⁵ and 3.11×10⁵, and the subthreshold swings were 0.82 V/dec and 2.03 V/dec, respectively. At 5 K, the on/off current ratios of p-channel and n-channel increased to 9.94×10⁶ and 9.61×10⁵, and the subthreshold swing decreased to 0.42 V/dec and 1.66 V/dec. This device showed better performance at low temperature, indicating the carrier transport mechanism is phonon scattering-dominated band-like transport. For the MoTe2/ WSe2 transistor, the on/off current ratio of p-channel and n-channel were 6.28×10⁵ and 1.77×10⁶, and the subthreshold swing were 0.84 V/dec and 2.21 V/dec, respectively at room temperature. At 5 K, the on/off current ratios of p-channel and n-channel are 1.52×104 and 4.95×104, and the subthreshold swings were 1.74 V/dec and 1.02 V/dec. In this device, the carrier transport mechanism is dominated by variable range hopping, which shows better electrical performance at room temperature. The dominant mechanism in similar materials may be different due to the quality of crystals and defects caused during the fabrication process. Graphite contacts have excellent conductivity and easily to exfoliate, which can reduce the formation of metal-induced gap states compared to three-dimensional metal contacts. In this study, MoTe2 transistors with graphite contacts allowed both electron and hole injection without chemical doping. By controlling the gate region, selective transport of electrons and holes in the channel could be achieved, decreasing the number of transistors in the circuit and simplifying circuit design. Ambipolar transistors can operate under different polarities, offering flexibility and stability in circuit design.