高性能二維材料光偵測器研究

Abstract

本論文採用兩階段成長方法，製備出大面積且均勻的多層二硫化鉬薄膜。首先，我們利用射頻濺鍍系統將非晶的二硫化鉬沉積於藍寶石基板上，並透過控制濺鍍時間來獲得不同厚度的薄膜。隨後，經過高溫硫化處理，形成不同層數的二硫化鉬薄膜，並透過穿透式電子顯微鏡精確判斷其層數，包括1層、3層、6層。石墨烯薄膜則使用低壓化學氣相沉積法製備，隨後透過轉印技術將石墨烯轉印至二硫化鉬上，形成 (graphene/MoS2) 的異質結構。與單獨二硫化鉬光偵測器相比，石墨烯作為載子傳輸層，二硫化鉬作為光吸收層，使得異質結構光偵測器的響應度 (responsivity) 提升超過106倍。此現象應來自石墨烯層中極短的載子傳輸時間與二硫化鉬層中相對較長的載子壽命，從而增加了 (graphene/MoS2) 光偵測器的光導增益。此外，我們發現隨著二硫化鉬層數的增加，元件的響應時間 (response time) 明顯縮短。為了進一步優化元件性能，我們透過重複轉印3層二硫化鉬的方式製備出多層結構，成功將響應時間從單層二硫化鉬的超過50毫秒大幅減少至6層二硫化鉬的低於10毫秒。同時仍保有極高的響應度，可以達到 776.8 A/W。這一結果表示，多層二硫化鉬中多餘的電子儲存有助於實現光吸收層中的電荷中和。另外，我們也使用了不同的二維材料作為光吸收層，並利用相同的結構製備出同樣基於石墨烯載子傳輸層的光偵測器，其中WS2元件在630 nm時響應度達到 1521.9 A/W，MoS2元件在660 nm時達到 6077.9 A/W，WSe2 元件在750 nm時具有 3977.8 A/W的光響應，此結果告訴我們不同的二維材料組合不僅可實現非常高的光導增益，且能覆蓋光偵測器於可見光區的偵測波段。同時我們利用了二維材料原子級厚度以及其容易堆疊的特性，本項工作先將二維材料轉印至厚度120 µm 的PET基板，再將石墨烯轉印至二維材料上，即可製作出可撓式的光偵測器，並分別在不同彎曲條件下進行量測。結果顯示即使在100 R的彎曲半徑下石墨烯/TMD可撓式光偵測器仍保持高於500 A/W的響應度，這些實驗結果證實，二維材料異質結構光偵測器不僅在性能表現上相當優異，即使製作於軟性基板時也展現出良好的穩定性與可靠性，未來有望應用於穿戴式裝置與可撓式電子產品。

In this thesis, wafer-scale and uniform multilayer molybdenum disulfide (MoS2) films were prepared using a two-step growth method. Amorphous MoS2 was first deposited onto sapphire substrates using RF sputtering, with thickness controlled by sputtering times. High-temperature sulfurization was used to form MoS2 films of 1-, 3-, and 6-layers, determined by transmission electron microscopy (TEM). Graphene films were prepared using low-pressure chemical vapor deposition (LPCVD), and monolayer graphene was transferred onto MoS2 to fabricate photodetectors. Compared with standalone MoS2 photodetectors, the heterostructure photodetectors demonstrated over six orders of magnitude responsivity enhancement, attributed to the ultra-fast carrier transit time in graphene and the longer carrier lifetime in MoS2, resulting in high photoconductive gain. In addition, it was found that the response time of the device significantly decreases with the increasing number of MoS2 layers. To further optimize device performance, this work fabricated multilayer structures by sequential transferring three layers of MoS2 films. This approach successfully reduced the response time from over 50 ms for monolayer MoS2 to less than 10 ms for the six-layer MoS2 device. The device also retained an exceptionally high responsivity, reaching up to 776.8 A/W. This result suggests that the excess electron storage in multilayer MoS2 contributes to charge neutrality within the light absorption layer during light on and off procedures. Moreover, this work also employed different 2D materials as the light absorption layers and fabricated photodetectors based on the same structure, utilizing graphene as the carrier transport layer. The WS2-based device exhibited a responsivity of 1521.9 A/W at 630 nm, while the MoS2 device reached 6077.9 A/W at 660 nm, and the WSe2 device showed a responsivity of 3977.8 A/W at 750 nm. These results indicate that different combinations of 2D materials can not only achieve exceptionally high photoconductive gain but also enable wavelength-tunable detections across the visible light range. Leveraging the atomic thicknesses and easy stackings of 2D materials, this work also transferred the 2D photo-absorption layers onto a 120 µm-thick PET substrate, followed by transferring graphene as the carrier transport layer on top, thereby fabricating flexible photodetectors. Measurements under various bending conditions revealed that even at a bending radius of 100 R, the graphene/TMDs flexible photodetectors maintained high responsivities > 500 A/W. These results demonstrate that 2D material-based heterostructure photodetectors not only exhibit outstanding optoelectronic performance but also show excellent stability and reliability when fabricated on flexible substrates, indicating their potential for applications in wearable and flexible electronic devices.