化學氣相沉積之二硫化鉬表面及界面特性研究:由成長控制至潛在應用

Abstract

本研究論文主要探討二維層狀材料二硫化鉬的成長機制、表面與界面特性及相關應用。首先，二硫化鉬以兩段式化學氣相沉積的方式成長於氧化矽(SiO2-Si)基板上，之後主要以光電子能譜研究其表面特性。結果顯示當二硫化鉬的厚度由十層遞減為三層時，殼層價電子之束縛能會向高能量偏移約0.2 eV，其原因可歸因於材料費米能階隨厚度的變化。由原子力顯微圖譜與價帶光電子能譜可發現，實驗所得的二硫化鉬屬多晶態的n型半導體且於價帶能量最高處前端有一缺陷能帶，此缺陷能帶主要是由材料表面的硫空缺所造成，進而形成類施體表面態而導致表面能帶往低能量彎曲。此外在較厚的二硫化鉬中，其塊材內的缺陷也可能形成類受體的缺陷能階而降低材料費米能階，故費米能階的變化來自於表面態及塊材缺陷的複合影響。當沉積奈米金於二硫化鉬表面，其價電子束縛能降低約0.3 eV，顯示高功函數金屬的p型摻雜特性及其與二硫化鉬間有電荷交換行為，且隱含表面修飾能調變二硫化鉬費米能階及其觸媒方面的應用潛力。當進行電化學催化產氫測試，其反應起始電位隨材料厚度漸少而降低，且阻抗分析也顯示較薄的二硫化鉬具有更為優異的電荷交換效率，進一步驗證了費米能階與材料厚度的關係。 二硫化鉬晶體的異向性及材料邊緣與基平面特性的差異促進了其廣泛的應用。本研究同時藉由熱力學分析及實驗驗證進行不同二硫化鉬前驅物的硫化速率調控，且同時控制二硫化鉬的晶體排列方向。結果顯示在相同硫化條件下，三氧化鉬的硫化速率遠大於鉬金屬，原因主要來自於結晶結構的不同，造成反應機制及活化能的差異，進一步於硫化氫硫化過程中引入氫氣可擴大硫化速率的差異。此外，拉曼圖譜及X光吸收圖譜均顯示三氧化鉬傾向形成平行基板排列的二硫化鉬，而鉬金屬則傾向形成垂直基板排列的二硫化鉬，此晶體排列方向的差異也是前驅物晶體結構及反應機制不同的結果。因此同時使用兩種前驅物即可得結合兩種異向排列之二硫化鉬，其中垂直排列的二硫化鉬能提供優異的電子傳導率而成為金屬電極及水平排列二硫化鉬之間的歐姆接觸橋樑，進而能應用於電子元件上。另外，沉積三氧化鉬於鉬金屬上也發現其能作為硫化犧牲層，而抑制鉬金屬的進一步硫化，使其能應用於以銅鋅錫硫為吸收層的薄膜太陽能電池。 為了製備一個以二硫化鉬為基礎的垂直異相結構元件，本研究選擇氧化鋅作為與其搭配的材料，因為氧化鋅為三維六方晶型半導體而二硫化鉬則為二維六方晶型半導體，兩材料間的晶格不匹配程度只有約2.8%，且兩者界面的能帶位置能形成n-n+異相接面而有更廣泛的應用。元件中氧化鋅採用原子層沉積的方式成長於二氧化鉬表面，上電極則以微影製程控制白金或金電極尺寸，並量測元件之電阻切換特性。電性量測結果顯示氧化鋅與垂直排列二硫化鉬之界面並不穩定，其電流電位曲線隨掃描圈數增加而改變，推測原因為二硫化鉬之晶體邊緣具高活性，易於高電流密度下與氧化鋅進行化學反應而部分氧化，進而增加元件電阻；反之，具雙重晶向排列的二硫化鉬有相當穩定的電流電位曲線，因在界面處的二硫化鉬為水平排列，其基平面有較高的化學惰性。當使用白金為上電極時，其電阻切換特性為雙極性，且具有良好的循環掃描穩定性與電阻持久性。由電流電位曲線適配及變溫電性量測結果，可推論其電子傳輸機制由空間電荷限制電流所主導，而氧空缺則扮演離子傳輸及形成導電通道切換電阻的角色。此外，於反向偏壓出現微分負電阻及重置電位主導設定電位之特性，可歸因於二硫化鉬與氧化鋅界面存在硫空缺及氧原子間之交互作用。二硫化鉬光電子能譜進一步驗證了界面態的存在，而穿透式電子顯微影像與電子色散X光譜則顯示氧空缺的存在及氧原子分布的梯度，最後對元件完整的電阻切換機制有詳盡的探討。 藉由兩段式化學氣相沉積法，二硫化鉬能成功成長於二氧化矽、鉬玻璃及石墨烯紙等基板上，其中石墨烯紙的基板造成二硫化鉬具有拉伸應力。本研究發現此種基板引發之應力會影響原子層沉積製程中水分子於二硫化鉬表面的吸附，進而影響氧化鋅的成核過程，推論原因為二硫化鉬的壓電特性形成一垂直基板的電場，其方向與水分子之偶極矩方向相反，阻礙了水分子於二硫化鉬基平面的凡得瓦吸附。此外石墨烯紙的超輕薄特性亦使其不易固定於真空系統中進行材料成長，使用有機物如真空膠帶、銀膠或金屬夾均無法成功成長氧化鋅。本研究應用靜電力與表面臭氧處理，同時解決石墨烯紙基板的固定問題及二硫化鉬表面輕水性的改善，最後能成功於具拉伸應力之二硫化鉬表面成長均勻的氧化鋅薄膜。

In this study, the growth mechanism, surface/interface properties, and potential applications of MoS2 are demonstrated. The thickness-dependent surface states of MoS2 thin films grown by the CVD process on the SiO2-Si substrates are investigated by XPS. Both the core levels and valance band edges of MoS2 shift downward ~0.2 eV as the film thickness increases from 3 to 10 layers, which can be ascribed to the variations of Fermi level. Grainy features observed from the AFM topographies, and defect states illustrated at the valance band spectra indicate the influences of both surface states and bulk defects, which lead to the variations of Fermi level in n–type MoS2 with thickness. When Au contacts with our MoS2 thin films, the binding energy reduces due to the hole-doping characteristics of Au, and easy charge transfer from the surface defect sites of MoS2. HER performance also exhibits that the easy charge transfer and the decrease in reaction barrier at the thin MoS2 surface. The anisotropic crystal structure and different properties between edge and basal plane of MoS2 have given rise to versatile applications. Here, we are able to manipulate the orientations of MoS2 by controlling the sulfurization kinetics in both MoO3 and Mo precursors. Thermodynamic information and SEM observations indicate that MoO3 has much higher sulfurization rate than Mo metal in H2S. Introducing H2 with H2S is able to enlarge the rate difference between these two precursors. Raman and XAS studies further reveal the orientation evolution of MoS2 is related to the MoO3/Mo ratio in the precursor, so that MoO3-derived MoS2 is terrace-terminated whereas Mo-derived one is edge-terminated. The differences in the orientation of MoS2 and sulfurization rate between MoO3 and Mo metal are attributed to the crystal structures of the Mo precursors and the reaction routes with H2S. The formation of terrace-terminated MoS2 on the Mo surface is also expected to suppress sulfurization of the bottom Mo metal. The hybrid hexagonal material of 2D-MoS2 and 3D-ZnO is utilized as a memristor. The microstructure of this hybrid material was analyzed by Raman, XRD, and HRTEM. ZnO grown by atomic-layer deposition shows c-axis preferred orientation on terrace-terminated MoS¬2 and stable I-V behavior at the ZnO/MoS2 interface, while edge-terminated MoS2 results in randomly oriented ZnO and unstable I-V characteristics, which could arise from the chemical reaction at the interface. The device with dual-oriented MoS2 on Mo metal is demonstrated by employing both MoO3 and Mo precursors. The MoS2/ZnO interface plays an important role for the resistive switching behaviors. Good retention and endurance of the device is achieved, which could be attributed to the formation of dual-oriented MoS2 on the Mo back contact that offers a stable interface with ZnO. The negative differential resistance (NDR) characteristics and voltage-dependent HRS are observed at the reverse bias, which could be related to the surface properties of MoS2. The shift in binding energy after the formation of MoS2/ZnO n-n+ heterojunction indicates the generation of interface states. Temperature-dependent I-V measurement illustrates that carrier transport mainly follows the trapping/detrapping controlled SCL process accompanying with ionic transportation. The complete switching mechanism is also proposed. This work demonstrates that the combination of oxide with TMD could be a potential configuration for electronic applications.