少數層二硫化鉬之電性與熱電傳輸性質

Abstract

本實驗主旨為，藉由機械剝離法與製程技術製作少數層二硫化鉬熱電元件，量測在溫度變化區間80 K至600 K的電性與熱電效應，研究實驗中隨溫度變化所產生的電性與熱電現象，並探討電性與熱電傳輸特性。 本實驗二硫化鉬為n型半導體材料，經調控背向閘極偏壓，量測源極-汲極電流Isd得電流開關比約為107，且調控背向閘極偏壓量測熱電效應，席貝克係數 (S)約可由150 µV/K經調控至280 µV/K，並觀測發現S與Isd隨場效應變化相關。 經變溫量測電性與熱電結果，觀察到在低溫200 K下，電性與熱電量測結果皆符合Mott變程跳躍理論。並且在高溫300 K至460 K，電性量測結果符合熱活化傳輸機制，且在460 K附近發生相變化，電性傳輸特性由原本的半導體轉變成金屬特性，因傳輸特性為熱活化傳輸或金屬傳輸特性時，載子受熱電效應影響相似，故熱電量測結果在高溫300 K至600 K皆滿足金屬的S與T關係式。而實驗中因介面能態陷阱與通道載子作用，可觀測到電流和熱電遲滯現象。而由矽閘極電子與氧化層深井作用，也可觀測到高溫電流和熱電台階現象。 討論熱電效應中一項重要因子「熱電功率因子 (PF)」，藉由調控閘極偏壓可觀測到PF隨場效應變化，且PF也會隨溫度增加而上升，並對PF與二硫化鉬層數關係比較，發現熱電功率因子會隨層數影響而變化。最後比較本實驗二硫化鉬與其他文獻研究材料之PF，發現二硫化鉬相較於各材料的熱電功率因子較大，明確顯示本實驗二硫化鉬對於熱電元件研究的發展是具有潛力。

In recent years, many studies have concentrated on electrical and optoelectronic properties of few-layer MoS2. The thermoelectric properties are, however, still not well studied yet. In this study, we designed a pattern of current leads integrated with heater to investigate electrical and thermoelectric transport properties in few-layer MoS2 flakes in the temperature range from 80 to 600 K. The transport properties are different in different temperature regimes. We separated temperature behaviors into three different regimes for detailed discussions. At temperatures below 200 K, the temperature behavior of resistance (R) and Seebeck coefficient (S) can be described by the equations of R~exp⁡(〖(T_0/T)〗^(1/3)) and S~T^(1/3), respectively, indicating that both electrical and thermoelectric transport satisfies with the two-dimensional variable-range hopping (2D-VRH) transport. In the temperature range from 300 to 460 K, the electron transport behavior changes from 2D-VRH to thermally activated transport and the thermoelectric transport changes to linear behavior as described by the equation S~T^1. At temperatures above 460 K, the electron transport behavior changes from insulating to metallic behavior whereas the thermoelectric transport still follows the relation of S~T^1. Additionally, we observed hysteresis and step-like feature in current-voltage loops in both electrical and thermoelectric data. The thickness-dependent and gate-dependent thermoelectric power factor (PF=S^2 σ) are studied and a maximum PF of 1.7 mW/m·K2 is measured at VG= 60 V in single layer MoS2 flakes. Our research results help to understand the electrical and thermoelectric characteristics in MoS2, and to show a high PF value that has potential for future applications of thermoelectric devices.