電性能提升之凡德瓦異質結構場效電晶體

Abstract

過渡金屬二硫化族化物，特別是二硫化鉬（MoS2），被視為極具微縮潛力、能延續半導體摩爾定律的新興材料；然而，過高的接觸電阻、低載子遷移率與易被傳統摻雜方式破壞的問題，使過渡金屬二硫化族化物電晶體難以被廣泛應用。對此本研究提出凡得瓦異質結構電晶體，由二硫化鉬與二硫化鎢堆疊，作為下世代的先進元件結構。本研究將量測不同堆疊次序的異質結構下(如二硫化鎢/二硫化鉬與二硫化鉬/二硫化鎢)光學性質與電學性質，並進行充分的機制與原理探討。 首先，我們進行二硫化鎢/二硫化鉬與二硫化鉬/二硫化鎢兩種異質結構的光致發光光譜分析，從兩者的光學性質探討異質介面的電荷轉移機制。接著完成異質結構電晶體，詳細探討其電學特性，並發現其二硫化鎢/二硫化鉬場效電晶體對於電特性的改善效果有限，然而倒置異質結構堆疊次序的二硫化鉬/二硫化鎢電晶體，則相較於純二硫化鉬電晶體大幅提升了汲極電流（約兩倍）與場效載子遷移率（從43.3至62.4 cm2V-1s-1）。此外，本研究也透過光致發光分析、Y函數法（Y-function method）、遲滯分析與變溫電性量測加以驗證其顯著的提升機制來自於電荷轉移機制、通道凡得瓦自主封裝與接觸介面蕭特基能障下降（從120至52meV）。最後，製作出二硫化鎢/二硫化鉬/二硫化鎢雙重異質結構場效電晶體，藉由兩個異質結構的結合，除可以於室溫下巨幅提升場效載子遷移率至102.5cm2V-1s-1外，並於30K的溫度下可達204.3cm2V-1s-1，此低溫下實現的極高載子遷移率說明庫倫散射被抑制，雙重異質結構電晶體的電特性得以進一步增強。

Transition metal dichalcogenides (TMDCs), especially MoS2, have been considered as highly scalable electronic materials to extend Moore’s law. Nevertheless, high contact resistance, low mobility and lattice distortion caused by traditional substitutional doping still impose serious difficulties on TMDC electronics. In this research, we propose novel Van der Waals heterostructure field-effect transistors (FETs) made by MoS2 and WS2 as next-generation device architecture. The heterostructures with different stacking order, WS2/MoS2 (WS2 first) and MoS2/WS2 (MoS2 first), are investigated by optical and electrical measurements. Firstly, we conduct the photoluminescence study of both WS2/MoS2 and MoS2/WS2 heterostructure to reveal an evidence of charge-transfer mechanism. Secondly, the heterostructures are made into devices and discussed in detail. The WS2/MoS2 heterostructure FET shows limited electrical improvement whereas the reverse stacking, MoS2/WS2 heterostructure FET, exhibits large improvement in drain current (~2x) and field-effect mobility (from 43.3 to 62.4 cm2V-1s-1), compared to single MoS2 FET. Such significant enhancement is mainly due to the charge-transfer effect, the Van der Waals self-encapsulation in the device channel and the Schottky barrier height reduction at the contact interface (from 120 to 52 meV), which are confirmed by the photoluminescence analysis, Y-function method, hysteresis analysis and variable temperature electrical measurement. Finally, WS2/MoS2/WS2 double heterostructures FETs were also fabricated. The combination of two heterostructures can further boost the mobility up to 102.5 cm2V-1s-1 at room temperature and 204.3 cm2V-1s-1 at 30 K. The much higher mobility realized at the lower temperature indicated the suppression of Coulomb scattering and thus the electrical performance of the double heterostructures FET can be further enhanced.