基於石墨接觸之扭轉二維二碲化鉬場效電晶體研究

Abstract

在過渡金屬二硫族化物 ( transition metal dichalcogenides, TMDCs ) 中，單層二碲化鉬 ( Molybdenum Ditelluride, MoTe2 ) 材料具備直接能隙 ( direct bandgap ) 、強自旋軌道耦合 ( spin-orbit coupling )，以及破缺反演對稱性 ( broken inversion symmetry )，因此二碲化鉬是極具潛力的二維材料。 本研究透過膠帶層離法 ( tape exfoliation ) 製備二碲化鉬、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，製作扭轉二碲化鉬場效電晶體。本研究著重探討電晶體室溫與變溫電性變化，觀察電荷傳輸性質。 首先，針對交錯式與共平面頂閘扭轉二碲化鉬電晶體的室溫與變溫電性進行比較，發現室溫下的交錯式頂閘電晶體展現更優越的電性表現，其輸出特性曲線皆呈現良好的歐姆接觸，其轉換特性曲線呈現雙極性，電子與電洞遷移率為232.4 cm2V-1s-1 和 245.1 cm2V-1s-1；開關電流比為1.9×10^5 和 1.8×10^5。在低溫5 K時，電子與電洞遷移率提升至238.5 cm2V-1s-1 和 302.5 cm2V-1s-1；開關電流比為8.5×10^5 和 1.4×10^6。 交錯式頂閘電晶體在高溫區 ( 約100~295 K ) 屬於受聲子散射 ( phonon scattering ) 影響的帶傳輸 ( band-like transport ) 趨勢，載子遷移率呈現隨溫度降低而上升趨勢。在低溫區 ( 約5~50 K )，載子熱能不足以跨越局域位能起伏，電導率 σ(T) 對溫度呈現莫特變程跳躍 ( Mott VRH ) 傳輸特徵，以局域態間的跳躍 ( hopping ) 或穿隧 ( tunneling ) 為主。 最後，再針對不同接觸電極厚度的交錯式頂閘扭轉二碲化鉬電晶體進行室溫電性分析，實驗中發現接觸電極較厚且經過後退火處理的樣品，其n型端導通電流變低，而其p型端導通電流略微提升。推測此現象可能與樣品潔淨度及石墨接觸電極厚度相關。RIE製程中殘留的雜質可能形成電荷陷阱或散射中心，影響載子傳輸並降低電晶體效能[1]。此外，較厚的石墨接觸電極因機械剛性較高，於乾式轉移後不易完全貼合MoTe2，導致有效接觸面積降低，並可能改變界面電荷轉移與介面態分佈，進而影響注入能障與接觸電阻。因此，接觸電極厚度存在最佳化條件，而非越厚越好。

Among transition metal dichalcogenides (TMDCs), monolayer molybdenum ditelluride (MoTe2) possesses a direct bandgap, strong spin-orbit coupling, and broken inversion symmetry, making it a highly promising two-dimensional material. In this study, MoTe2, hexagonal boron nitride (hBN), and graphite crystals were prepared via the tape exfoliation method. Graphite was employed as the contact electrode to fabricate twisted MoTe2 field-effect transistors (FETs). This work focuses on investigating the electrical characteristics at room temperature and under variable temperatures to elucidate charge transport properties. First, the room-temperature and temperature-dependent electrical properties of staggered-gate and coplanar top-gated twisted MoTe2 FETs were compared. The staggered-gate top-gated devices exhibited superior electrical performance at room temperature. Their output characteristics demonstrated good ohmic contact, while the transfer characteristics showed clear ambipolar behavior. The electron and hole mobilities reached 232.4 cm²V⁻¹s⁻¹ and 245.1 cm²V⁻¹s⁻¹, respectively, with on/off current ratios of 1.9×105 and 1.8×105. At low temperature (5 K), the electron and hole mobilities increased to 238.5 cm²V⁻¹s⁻¹ and 302.5 cm²V⁻¹s⁻¹, while the on/off current ratios improved to 8.5×105 and 1.4×106, respectively. The temperature-dependent carrier transport mechanism in staggered-gate top-gated twisted MoTe2 FETs is not governed by a single mechanism but varies across different temperature regimes. In the high-temperature region (approximately 100~295 K), carrier transport is primarily limited by phonon scattering. As temperature decreases, phonon scattering is suppressed, leading to an increase in carrier mobility, consistent with band-like transport behavior. In the low-temperature region (approximately 5~50 K), carrier thermal energy is insufficient to overcome local potential fluctuations, resulting in a sharp decrease in conductivity with decreasing temperature. The conductivity σ(T) exhibits characteristics of Mott variable-range hopping (VRH), where transport is dominated by hopping or tunneling between localized states. Finally, room-temperature electrical characteristics of staggered-gate top-gated twisted MoTe2 FETs with different contact electrode thicknesses were analyzed. Devices with thicker graphite contact electrodes subjected to post-annealing exhibited reduced n-type on-state current, while a slight enhancement in p-type on-state current was observed. This behavior is likely related to sample cleanliness and the thickness of the graphite contacts. Residual contaminants from the reactive ion etching (RIE) process may act as charge traps or scattering centers, thereby degrading carrier transport and device performance[1]. Moreover, thicker graphite electrodes possess higher mechanical rigidity and may not fully conform to MoTe2 after dry transfer, reducing the effective contact area and altering interfacial charge transfer and interface state distribution. These effects can influence the injection barrier and contact resistance. Therefore, an optimal contact electrode thickness exists, rather than thicker electrodes necessarily yielding better performance.