| 摘要: | 六角硼氮化物（Hexagonal boron nitride）近年成為石墨烯和其他二維材料 應用中的關鍵組成部分。六角硼氮化物對石墨烯是一個良好的基板，因為其表面沒 有電荷陷阱，是一種能帶隙為 6.8 電子伏特的絕緣體，並且其層狀結構類似於石墨烯。六角硼氮化物是增強二維材料電子性能的良好基板，因為其具有較小的基板 效應。許多研究使用 h-BN 作為基板進行場效電晶體（FET）研究，並顯著改善了電子性能。
六角硼氮化物的結構是由 B 原子和 N 原子交替排列形成的蜂窩結構，遵循六角晶格形成的規律。在銅基板上使用化學氣相沉積（CVD）方法製備 h-BN，而 h-BN 的形狀是三角形，因為 N 原子的生長速度比 B 原子慢。在工業上，通常使用氨硼烷作為前驅體，因其具有無毒和不易燃的特性。由於固態前驅體的存在，溫度對氨硼烷的影響非常重要，不同的溫度和時間將導致氨硼烷的分解程度不同。
在 CVD 過程中，h-BN 的層數高度依賴於基板。銅、金、鎳和鎳鐵都可以作為 h-BN 的合成基板。硼和氮的溶解度取決於不同的金屬或合金。在工業上，通過控 制硼和氮的擴散速率，可以合成單層 h-BN。對於銅來說，硼的溶解度低，氮不溶 解，h-BN 在低源蒸發下往往形成單層。對於鎳鐵合金，硼和氮的溶解度高，h-BN 往往形成多層。通過使用不同溫度、不同前驅體重量和載氣流速的源蒸發，可以控 制 h-BN 的成核密度。
通過增加氫氣流速，可以合成多層 h-BN，因為氫氣對其進行了端基處理效應。 我們發現氫終止了三角形的 h-BN 邊緣，源頭將有更多機會通過單層形成成核並生 長多層 h-BN。多層 h-BN 的堆疊角度為 0°、30°和60°。由於 B 和 N 是交替排列形成蜂窩結構，其旋轉對稱性為 60°。
通過控制前驅體的蒸發或在反應中添加氮氣，可以合成單層 h-BN，以增加氮的鋸齒邊生長速度。然而，一些研究使用密度泛函理論（DFT）計算發現，在高氮環境下，h-BN 會產生類似硼空位的缺陷。缺陷區域將導致 h-BN 的電子性質和光物理性質變化。缺陷 h-BN 將具有光致發光峰，其波長由缺陷類型決定，並且廣泛應用於量子通信領域的單光子發射器研究中。使用角度分辨光電子能譜測量 h-BN 的價帶，可以觀察到缺陷引起的電子性質變化，並發現 h-BN 在與氮氣相互作用時呈 p 型行為。由於在缺陷區域周圍將形成三個電子空穴，圍繞硼空位。在我們的研究中，我們發現添加氮氣可以增加生長速度，使整塊銅基板完全被單層 h-BN 覆蓋。 這些結果顯示了增加樣品產量並減少不需要的多層 h-BN 的概念。
在這篇論文中，我們通過控制氫氣和氮氣的流速以及源頭溫度來合成多層和 單層 h-BN，觀察多層的生長。使用角度分辨光電子能譜（ARPES）檢測了非氮、高 氮和低氮流量合成的 h-BN 的電子性質。使用電子背散射（EBSD）觀察了銅晶體的 退火重組結構。;Hexagonal boron nitride is emerging as a key component in the application of graphene and other two-dimensional layered material. Hexagonal boron nitride is a good substrate for graphene because of no charge trap on the surface, insulator with bandgap 6.8ev, and the layered structure like graphene. Hexagonal boron nitride is good substrate for the 2D material to enhance the electronic property due to less substrate effect. Many researches use h-BN as substrate to do the FET and improve the electronic property obviously.
The structure of h-BN is B atom and N atom alternately arranged to form a honeycomb structure with the law of hexagonal lattice formation. Usually on Cu substrate, using CVD synthesized h-BN and the shape of h-BN is triangle because the N atom side will grow slower than B atom. In industry, they usually used ammonia borane as precursor because of its non-toxic and non-flammable properties. Because of the property of solid state precursor, the temperature influence in the ammonia borane is significance, and different heating time cause the ammonia borane decomposition changing.
In the CVD process, the number of layers of h-BN highly depended on the substrate. Copper, gold, Nickle, and Ni-Fe can be the synthesis substrate for h-BN. The solubility of boron and nitrogen depended on the different metal or alloy. In the industry, the V monolayer h-BN can be synthesized by controlling the diffusion rate of boron and nitrogen. For copper, low solubility for boron and insolubility of nitrogen, the h-BN tends to form the monolayer in LPCVD with low source evaporation. For Ni-Fe alloy, the high solubility of boron and nitrogen, h-BN tended to form the multilayer. The h-BN nucleation density can control by the source evaporation by using different temperature or different source weight and flow rate of carrier gas.
The multilayer h-BN can be synthesized by increasing hydrogen flow rate because of hydrogen termination effect. We found that the hydrogen terminated the triangle shape h-BN edge and source will have more chance through under the monolayer to nucleate and grow the multilayer h-BN. The multilayer stacking angle is 0°, 30°, and 60°. Because the B and N are alternatively arranged to form honeycomb structure, the rotation symmetry is 60°.
The monolayer h-BN can be synthesized by controlling the precursor evaporation or adding nitrogen gas into the reaction to increase the growth speed of the nitrogen zigzag edge. However, there are some researchers used DFT to calculate the h-BN will have point defect like boron vacancy under the high nitrogen environment. The defect region will cause the electronic property of h-BN became different, and also the photophysical property. The defect h-BN will have photoluminescence peaks and the wavelength are VI decided by the type of defect, and it is widely used on researches of single photon emitter source in the quantum communication region. The defect caused electronic property changed can be observed by using angle resolved photoemission energy spectrum to measure the valence band of h-BN and found h-BN p-type behavior with nitrogen gas interacting. Because of in the defect region, there will form three electron holes around boron vacancy. In our research, we found that with nitrogen gas adding the growth speed increasing, and the whole piece copper substrate is monolayer fully covered. These result shows the concept to increase the sample yield on decrease the unwanted multilayer h BN.
In this thesis, we synthesized the multilayer and monolayer h-BN by controlling the flow rate of hydrogen and nitrogen and source temperature to observe the multilayer growing. ARPES checked the non-nitrogen, high nitrogen, and low nitrogen synthetization h-BN electronic property. The copper crystalline used EBSD to observe the annealing reconstruction structure of the sample. |
