二維二硫化鎢電晶體之研究

Abstract

本研究使用層離法（exfoliation）來獲取二維過渡金屬硫化物（Transition Metal Dichalcogenides, TMDs/ TMDCs）中的二硫化鎢（tungsten disulfide, WS2）作為電晶體之主動層，並與另一種絕緣二維材料六方晶氮化硼搭配，來探討不同的六方晶氮化硼與二硫化鎢晶體堆疊方式所產生之電性表現差異。與化學氣相沉積等長晶方式相比，透過層離法所獲得之晶體有著較少的缺陷，因而可以更好的研究此種材料本身的物理性質，以及其作為電晶體的電性表現。 本研究首先探討堆疊結構對於電晶體特性的影響，使用單層結構二硫化鎢之二維電晶體的次臨界擺幅為2.71 V/dec，開關電流比為~104，而其場效載子遷移率在常溫下可以達到 85.2 cm2V-1s-1，在5K的低溫之下更高達566.4 cm2V-1s-1，代表其極具潛力，可做為下一世代的半導體材料。引入六方晶氮化硼作為二硫化鎢下方之緩衝層的雙層堆疊結構，次臨界擺幅獲得改善，但場效載子遷移率與開關電流比卻下降，其原因可能是六方晶氮化硼緩衝層過厚，雖能減少介面缺陷，但等效絕緣層厚度也相對增加，導致閘極偏壓對於電晶體的控制力下降。在雙層結構之上，引入六方晶氮化硼作為二硫化鎢上方之封裝層的六方晶氮化硼/二硫化鎢/六方晶氮化硼三明治結構電晶體，則能有效阻絕環境中的水氣及氧氣對元件造成的影響，因此場效載子遷移率、次臨界擺幅與開關電流比均得到改善。接著針對引入六方晶氮化硼之雙層及三明治結構的電晶體進行了變溫分析，但在低溫情況下的電性表現與預期中不一致，包括載子遷移率及開關電流比均不增反減，顯示本研究的電晶體載子傳輸是由變程跳躍機制所主導，代表堆疊晶體製程中還有問題需要被解決，才能使得電晶體之載子傳輸機制變為band-like transport機制。 最後針對不同退火處理條件進行探討，將未經退火處理、氬氣退火處理以及氬氫混氣退火處理後之三明治結構二硫化鎢電晶體進行比較。結果顯示，所有退火處理條件均能改善電晶體之電性表現，但也會造成電晶體之開啟電壓（Von）與臨界電壓（Vth）偏移，這源自於二硫化鎢本身於退火處理後形成的硫空缺以及吸附於元件上之水氣與氧氣的移除，此兩種現象都等效於添加N-型摻雜物至二硫化鎢通道層中。本研究也對二硫化鎢的物理性質進行相關的探討，例如系統中之非線性非交互性傳輸效應（nonreciprocal transport effect, NRTE）。此現象可於非中心對稱系統中，二硫化鎢晶體結構正是屬於此類非中心對稱系統。倘若能加以改善接觸電阻的相關問題，便能將二維電晶體實用化之願景向前推進一步，也能對二維材料本身之諸多物理性質進行更深入的研究及探討。

In this thesis, the exfoliation method was employeed to prepare two-dimensional (2D) tungsten disulfide (WS2), which is one of the 2D transition metal dichalcogenides (TMDs or TMDCs) commonly used as the semiconductor layer for 2D field-effect transistors (FETs). The 2D materials obtained by the scotch tape exfoliation method have lower defect density and better crystal quality than those obtained by epitaxial methods such as the chemical vapor deposition (CVD) and solution fabrication. Thus, it’s easier to investigate the intrinsic properties via these exfoliated 2D materials. First, we invesgtiage the effect of devce structure on the performance of 2D FETs. The 2D WS2 FET with a single-layered structure has a subthreshold swing of 2.71 V/dec, on/off current ratio of ~ 104. Its field-effect mobility reaches 85.2 cm2V-1s-1 at room temperarure and 566.4 cm2V-1s-1 at 5 K, showing a great potential for next-generation semiconductor device applications. The Hexagonal-boron nitride (hBN) layer is then introduced as the buffer and/or encapsulation layer. The 2D WS2 FET with a double-layered strucurture, hBN underneath WS2, exhibits improved subthreshold swing but lower field-effect mobility and on/off current ratio. Although the insersion of hBN buffer layer can reduce the interface defect states between WS2 and SiO2, but the gate modulation becomes weaker since the effective thickness of the gate dielectric increases. Next, an additional hBN layer is introduced on top of the WS2 channel to form a sandwich strucurture to prevent the influence of moisture and oxygen in the atmosphere. The 2D FET with a sandwich structure indeed shows improved electrical performance in terms of the field-effect mobility, subthreshold swing and on/off current ratio. The temperature-dependent electrical characteristics of 2D WS2 FETs with the double-layered and sandwich structures are further investigated. Both the mobility and on/off ratio decrease as the temperature reduces, implying the carrier transport mechanism is dominated by Variable Range Hopping (VRH). To realize band-like transport in the 2D WS2 FETs with double-layered and sandwich structures, the contact resistance and stacking procedure must be optimized. Finally, the effect of thermal annealing is studied. The electrical performance of 2D WS2 FETs with a sandwich structure is improved after thermal annealing in Ar or Ar/H2 environment. Negative shifts of on voltage (Von) amd threshold voltages (Vth) are observed after the thermal annealing because of the formation of sulfur vacancies and removal of adsorbed mosture and oxygen, both acting as donors in WS2 FETs. Nevertheless, the contact resistance of the FET is still high compared with other researches. This issue must be resolved prior to further investigating the intrinsic physical properties of 2D WS2. Such as the non-reciprocal transport effect in the non-centrosymmetric WS2 system discussed in this thesis.