| 摘要: | 自從石墨烯(Graphene)發現以來，其特殊的二維結構伴隨的物理性質，一直受到很多矚目，如超高的載子遷移率(≥200000 cm2V-1s-1)，但其沒有能帶間隙的本質，限制了它在電子元件上的應用，於是科學界便開始尋找其他具二維特性的材料，二維過渡金屬二硫屬化物(Two-dimensional transition metal dichalcogenides, 2D TMDs)便是其中一種，它具有無懸浮鍵的表面以及適當大小的能帶間隙，使其作為電子及光電元件有十分大的潛力。
本篇著重於二碲化鎢(WTe2)及二碲化鉬(MoTe2)兩種TMDs材料的合成，目前科學界常用於TMDs合成的方式有二，其一為剝除法，為直接從結晶良好的塊材上剝除到基板上，以此方式較難以得到大面積且均勻的薄膜，二為化學氣相沉積，為利用氣相反應的方式讓氣體原子反應沉積到基板上，其缺點在於必須利用硫/碲化的製程來修補試片表面缺陷，此製程在控制上不易且碲具有毒性，在此我們選擇以物理氣相沉積的方式合成薄膜，藉由三步驟的方式合成二碲化鎢及二碲化鉬薄膜，首先由濺鍍的方式沉積薄膜，接著利用電子束蒸鍍系統及電漿輔助化學氣相沉積系統，沉積二氧化矽覆蓋層，而後進行退火處理，增加試片的結晶性。
藉由拉曼光譜儀、X射線光電子能譜、穿透式電子顯微鏡等分析儀器我們可確認薄膜的品質，由分析結果可得知，以物理氣相沉積的方式製作出的薄膜，與化學氣相沉積及剝除法所得的薄膜具相似的拉曼訊號，證明了以此方式也可以得到結晶良好的薄膜，同時我們也針對二碲化鎢及二碲化鉬的濺鍍溫度以及退火溫度進行討論，此兩種材料皆對於製程溫度十分敏感，形成具良好結晶相的薄膜並不容易。
我們以濺鍍、覆蓋層沉積、退火的三步驟製程，成功在二氧化矽及藍寶石基板上合成出1T’相之二碲化鎢、1T’相之二碲化鉬、2H相之二碲化鉬之薄膜，避免了難以控制的碲化製程，未來也可將此製程套用於其他TMDs材料，為合成TMDs薄膜提供了一條新的途徑。
;Since the discovery of graphene, various two-dimensional (2D) materials have been explored. Graphene has a two-dimensional structure and unique physical properties, like high carrier mobility (≥200000 cm2V-1s-1). However, graphene is a gapless material, which limits its applications in electronic devices. Other 2D materials have been explored. Two-dimensional transition metal dichalcogenides (2D TMDs) is one of 2D materials. The absence of dangling bonds, and finite and distinctive band gaps, making it a good candidate for electronics and optoelectronics.
In experiment, we focused on the synthesis of tungsten ditelluride (WTe2) and molybdenum ditelluride (MoTe2). There are two common methods to obtain TMDs thin films. First is mechanical exfoliation, which is hard to create a large and uniform thin film.
This method is most used in research. Second is chemical vapor deposition (CVD), which needs tellurization to achieve high quality thin film. However, tellurization process is hard to control, and tellurium is harmful for health. We develop a three-step process. Comprising of physical vapor deposition, capping layer deposition and annealing process. First, we used ultra-high vacuum sputter system (UHV-sputter system) to deposit WTe2 or MoTe2 thin film. Second, we deposited a silicon oxide capping layer, to prevent oxidation of thin film oxidize or sublimate during the following annealing. Finally, the annealing process will improve thin film quality and crystallinity.
Various analysis methods, including Raman spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy were used to characterize the thin film quality. The different phases of thin films like 1T’-WTe2, 1T’-MoTe2 and 2H-MoTe2 were confirmed by Raman analysis. Our thin films show similar properties as compared with CVD and exfoliation methods. Moreover, The thin film quality is very sensitive to sputtering temperature and annealing temperature. Without proper time and temperature, it’s hard to synthesize thin film successfully.
We developed a new process to synthesize WTe2 and MoTe2 thin films. The PVD method without tellurization process is safe and easily controlled. This process can also expand to other TMDs materials. Our research shows the potential of PVD process for TMDs thin film in the future. |
