| 摘要: | 單層六方氮化硼（hBN）是一種具有sp2結構的單原子層二維材料。其大帶隙、無懸空鍵、穩定的物化特性使hBN成為極佳的絕緣體和基板。化學氣相沉積法已廣泛用於二維材料製造，具有低成本、晶圓級生產等特性。然而，hBN晶格的三重對稱性具有多種的晶相，晶體聚合時可形成晶界缺陷，從而降低hBN的品質。因此，產生大晶粒尺寸和晶相一致的hBN，以降低薄膜上的晶界缺陷是重要的。為達到目的，我們須了解hBN的生長機制，以控制hBN的生長方式。在這篇論文，我們使用低壓化學氣相沉積法在銅箔上控制生長具有大晶粒尺寸的hBN。
首先，hBN晶相很大程度上取決於銅箔的晶相。因此，銅的預退火對於產生一致的晶相和均勻的催化效果是必要的。通過純氫退火可有效還原氧化層並使銅重新排列成<111>晶相，然後我們通過掃描電子顯微鏡分析hBN晶體旋轉角的分佈，可控制產生大面積單晶相的hBN。接下來，我們研究不同氫氣流率下hBN在銅上的成核和生長動力學。其成核密度和晶粒生長是BN-吸附原子擴散和蝕刻解吸速率之間競爭的結果。在低氫氣流率下，生長以成核為主。在高氫氣流率下，晶粒生長在初始生長階段為主，並能產生晶粒尺寸超過 25um的hBN。我們透過hBN的SEM成像和圖像分析，以及利用Johnson-Mehl-Avrami-Kolmogorov (JMAK)模型來描述hBN的成核和生長動力學。接著，我們發現在高溫下成核和晶粒生長的傾向會反轉，其可能是銅表面形態轉變的結果，在提高生長溫度的時銅表面變得更加起伏並且具有更多的銅原子台階。導致hBN的成核活化能降低。我們透過AFM驗證銅表面形態的轉變。
此外，我們發現由於氫會接在hBN邊緣，導致多層hBN會在高氫氣流率下形成。為了抑制多層hBN的形成。我們添加氮氣可以降低氫末端效應，並進一步研究多層hBN在不同N2:H2氣體比例下的生長行為，其中包括多層hBN在銅表面上的多寡、多層與單層hBN的面積比、第一層與多層hBN之間的扭曲角。
;Monolayer hexagonal boron nitride (hBN) is an atomically thin two-dimensional material with sp2 structure. Large band gap, without dangling bonds, and stable physical and chemical properties make hBN an extraordinary van der Waals stacking layers for insulator and supported material. Chemical vapor deposition which has been widely used in 2D material fabrication involve the properties such as low cost, wafer-scaled production. However, threefold symmetry of hBN lattice lead to various orientation in crystalline coalescence which may form grain boundaries defects and then reduce the quality of hBN. Thus, it is important to produce large grain size and aligned orientation hBN to lower the grain boundary defect on the overall film. To achieve the purpose, it is necessary to understand the growth mechanism of hBN for controlling the quality of hBN. In our work, we perform strategies to grow fully-covered hBN with large grain size on copper foil by chemical vapor deposition in low pressure.
First, the orientation of the hBN lattice strongly depends on the orientation of substrate, copper. Thus, copper pre-annealing process is necessary to produce uniform orientation and catalytic effect substrate. Pure hydrogen is confirmed to efficiently reduce copper oxide that promote copper realigned. Then we find out orientations of copper relative to the distribution of different hBN lattice rotation angles by scanning electron microscopy and produce large area single orientation hBN.
Next, we study the synthesis physicochemical mechanism underlying the nucleation and growth kinetics of hBN on copper with different hydrogen flow rate. Its nucleation density and grain growth are the result of competition between the diffusion of the BN-adatom species and etched desorption rate. In low hydrogen flow rate, the growth is overall dominated by nucleation. In high hydrogen flow rate, grain growth dominates in the initial growth stage and produces hBN grain size over 25um. Through SEM imaging of hBN and image analysis, Johnson-Mehl-Avrami-Kolmogorov (JMAK) model of phase transformation is used to describe nucleation and growth kinetics of hBN. Moreover, we found that these phenomena vary in high temperature that the tendency of nucleation and grain growth activation energies are totally reversed. It is result from the transform of the copper surface morphology that becomes more waviness while increasing the growth temperature. The waviness could be the piling up of copper atomic steps. That lead to the lowest nucleation activation energy for hBN. Copper surface morphology transformation is verified by AFM. Furthermore, we found that multilayers hBN would form under high hydrogen flow rate due to H-terminated hBN edges. Then we investigate the hBN growth behavior including density of multilayer, area ratio of multilayer to monolayer, twist angles between first layer and multilayer under different N2:H2 gas ratio. Inducing nitrogen gas may reduce H-terminated effect to suppress the formation of multilayer hBN. |
