1.Collaborative Innovation Center of Suzhou Nano Science and Technology, School of Physics Science and Technology, Soochow University, Suzhou 215006, China 2.Key Laboratory of Thin Films of Jiangsu Province, Soochow University, Suzhou 215006, China 3.Analysis and Testing Center, Soochow University, Suzhou 215006, China 4.College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11975163), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China, and the Postgraduate Research & Practice Innovation Program of Jiangsu Province, China (Grant No. KYCX20_2649)
Received Date:30 October 2020
Accepted Date:18 January 2021
Available Online:20 April 2021
Published Online:05 May 2021
Abstract:A variety of carbon-based thin films are prepared by self-developed helicon wave plasma chemical vapor deposition (HMHX, HWP-CVD) through changing the parameters of plasma discharge. The Ar/CH4 plasma discharge is diagnosed in situ by Langmuir probe, emission spectroscopy and mass spectrometry. The carbon thin films are characterized by scanning electron microscopy (SEM) and Raman spectroscopy (Raman). The results show that under the given parameters, the plasma discharge modes are all helicon wave discharge modes. Under a given CH4 flow rate, the energy distribution in the plasma is enough to dissociate the methane molecules and form carbon free radicals. The preparation of different carbon-based films is realized by adjusting the CH4 fluence. The research result shows that when the plasma is rich in CH and H radicals, it is suitable for growing diamond-like carbon films. When the plasma is rich in C2 radicals and less H, it is favorable for growing vertical graphene nanosheets. According to the results of plasma diagnosis and material characterization, the decomposition mechanism of methane molecules under the action of Ar helicon wave plasma (HWP) is proposed, and the growth model of carbon-based materials is established, the feasibility of Ar/CH4-HWP in the preparation of carbon-based nanomaterials is verified, which provides a reference for preparing the carbon-based materials by HWP-CVD technology. Keywords:optical emission spectroscope/ mass spectroscopy/ carbon nanomaterials/ helicon wave plasma
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3.结果与讨论发射光谱可以确定等离子体的物质组成. 图2(a)是纯氩、CH4流量为85和145 sccm时螺旋波放电产生等离子体的发射光谱图, 图2(b)—(d)分别对应纯氩放电、CH4流量为85和145 sccm时螺旋波放电图. 纯氩气放电时等离子体气氛中含有大量的Ar I和Ar II离子[17]. 甲烷流量增加后等离子体气氛具有不同含量的活性基团. 发射谱线波长在350—850 nm范围内含有CH基团(431.2 nm, $ {\rm{A}}^{2}\Delta \to {\rm{X}}^{2}\Pi $), C2基团(516.5 nm, Swan $ {\rm{D}}^{3}{\Pi }_{\rm{g}} \to $$ {\rm{A}}^{3}{\Pi }_{\text{μ}} $), H Balmer (656.2 nm, Hα, n = 3→2; 486.1 nm, Hβ, n = 4→2; 434.5 nm, Hγ, n = 5→2)和激发态氩原子Ar* (751, 764, 812 nm). 发射谱线强度的变化定性反映了粒子浓度变化情况[18]. 因此根据发射光谱强度的变化分析了CH4流量变化对等离子体中活性基团浓度的影响. 图3是CH, C2, H Balmer, Ar*发射谱线强度随着CH4流量增加的变化情况. 激发态的H和Ar数密度随CH4流量增加降低, 而激发态的C2数密度随CH4流量增加而增加, 激发态CH数密度在CH4流量为105 sccm时达到峰值, 随后降低. 可以发现在CH4流量小于105 sccm时等离子体中是富含H自由基和CH活性基团, CH4流量大于105 sccm时等离子体中富含C2活性基团, 而H自由基减少. 根据文献报道, CHx是类金刚石薄膜形成的主要前驱体[3], Cheng和Teii[4]提出垂直石墨烯纳米片(VGs)的生长主要基团是C2以及不饱和自由基. H原子在DLC和VGs成膜过程中对非晶相的刻蚀起着重要作用. 氩离子在促进CH4解离以及对样品表面刻蚀起到一定的作用. 随着CH4浓度的增加, 等离子体的化学性质在衬底附近发生变化, 这将影响碳基薄膜的成核密度和质量[19]. 图 2 (a)纯氩、CH4流量为85和145 sccm时的发射谱线图; (b)?(d)对应纯氩气、CH4流量为85和145 sccm时的放电照片图 Figure2. (a) OES of pure argon, methane with the flow rate of 85 and 145 sccm; (b)?(d) discharge photos of pure argon, methane with the flow rate of 85 and 145 sccm.
图 3 CH, C2, H Balmer和Ar*的发射强度与CH4流量的关系 Figure3. Emission intensity of CH, C2, H Balmer and Ar* as a function of methane flow rate.