使用二硒化鎢電晶體之反向器開發

Abstract

隨著閘極尺寸不斷微縮，摩爾定律將因為短通道效應達到其物理極限，二為材料因它原子級薄的厚度，被提出可以用來降低短通道效應，作為次世代電晶體的通道材料。在本篇論文中，基於二硒化鎢(tungsten diselenide, WSe2)優異的雙極性(ambipolar)載子遷移率以及大氣下的化學穩定，我們提出使用二硒化鎢實現基本邏輯元件反向器(inverter)的方法。 首先，我們使用鈀(palladium)作為p型二硒化鎢場效電晶體(field effect transistors, FETs)的源極/汲極接觸金屬，並且展現出歐姆接觸的特性。應用氧氣環境的退火(annealing)步驟，可以改善p型二硒化鎢場效電晶體的汲極電流。藉由傳輸線分析法(transmission line measurement)，可以萃取出接觸電阻率(contact resistance)，以及片電阻率(sheet resistance)。 其次，我們使用低功函數金屬作為n型二硒化鎢場效電晶體之源極/汲極接觸金屬。在200°C退火程序後，鈦接觸之元件呈現n型的特性，更進一步展現出依厚度而定(thickness-dependent)的電性表現。使用修改過的金屬沉積程序，可以讓銦接觸元件呈現固有的n型特性。隨後，我們使用鎳作為n型二硒化鎢場效電晶體之源極/汲極接觸金屬，藉由鎳-二硒化鎢介面的場致發射(field emission)，呈現出準歐姆接觸(quasi-Ohmic contact)。 我們在單一的二硒化鎢元件上整合鎳與鈀。使用非對稱接觸金屬之二硒化鎢整流器，在不同的閘極偏壓下呈現出雙極性的特性。除此之外，我們在非對稱接觸金屬之間增加一個內部節點，使得原本的整流器變成使用閘極調變接觸電阻率(gate-tunable contact resistance)之反向器。我們也整合n型與p型二硒化鎢電晶體，做成互補式反向器。最後，兩種反向器都在存放於大氣環境五十天後依然呈現良好的大氣穩定性(air stability)。

With the down-scaling gate length, Moore’s law reaches its physical bottleneck due to the short channel effect. To reduce the short channel effect, 2D materials are proposed to be the channel materials of the next generation transistors because of its atomically thin body. In this thesis, we propose a method to fulfill the fundamental logic device, inverter, with tungsten diselenide(WSe2) with its excellent ambipolar mobility and chemical stability in the air. First, we use palladium as the source/drain contact metal for p-type WSe2 ¬field effect transistors and demonstrate the Ohmic contact characteristics. The annealing process in oxygen environment is applied to improve the drain current of p-type WSe2 ¬FETs. Also, the contact resistance and the sheet resistance values are extracted from the transmission line measurement. Second, we try to use low work function metals as the source/drain metal for n-type WSe2 FETs. Titanium-contacted devices show n-type characteristics after the 200°C annealing process. Furthermore, the thickness-dependent electrical characteristics are unveiled. Indium-contacted devices show naturally n-type characteristics with adapted metal deposition process. Subsequently, nickel is applied as the source/drain metal for n-type WSe2 FETs. It shows quasi-Ohmic contact via field emission of the nickel-WSe2 junction. We integrate the nickel and palladium on the on a single WSe2 device. A WSe2 rectifier is implemented with asymmetric contact metals and shows the ambipolar properties under different gate biases. Moreover, we add an internal node between the asymmetric contact metals and make the rectifier turn into an inverter with gate-tunable contact resistance. We also integrate the n-type and p-type WSe2 FETs into the complementary inverters. Last, both kinds of inverters demonstrate good air stability after the fifty-day storage in the air. Based on the results above, we obtain a method for fabricating both n-type and p-type WSe2 FETs and the physical mechanisms. We are able to implement the inverters with intrinsic WSe2, which perform ambient-environmental stability, as well.