鑽石薄膜的石墨化對場發射特性的影響之研究

The study on the effect of graphitization process on the electron field emission properties of diamond films

鄧光佑(Kuang-Yau Teng)

指導教授 ： 林諭男

淡江大學/理學院/物理學系博士班/博士(2012年)

https://doi.org/10.6846/TKU.2012.00850

摘要

在這篇論文中，我們使用3個主題來證明鑽石薄膜的石墨化可提高電子場發射(Electron Field Emission，EFE)特性。（i）高能量離子輻照，（ii）超音波-偏壓輔助成長 (Ultrasonication-Bias Enhanced Growth，U-BEG）之過程和（iii）氮氣電漿的微波電漿輔助化學汽相沈積法(Microwave Plasma Enhanced Chemical Vapor Deposition，MPECVD）之過程。
（i）在高能量離子照射效應的過程中，我們觀察到2.245 GeV的金離子照射對MCD （Microcrystalline Diamond）鑽石薄膜上的晶粒結構有巨大的改變，但是在UNCD （Ultra-nanocrystalline Diamond）薄膜上的微結構的影響就不太明顯。改變的程度隨著照射離子數的增加而改變，其臨界離子數約8.4×1013ions/cm2。對於MCD薄膜，由於金離子的照射可使其平均晶粒尺寸變小。一些晶粒仍保持原貌而只有結構性的缺陷產生。一些晶粒被完全擊碎成超小顆的晶粒並伴隨著非結晶碳的產生。這些過程會使薄膜的電子場發特性變差。離子照射薄膜再退火之後，可使缺陷變少、非結晶碳再結晶和使被擊碎的鑽石晶粒再重新成長。薄膜經由退火過程可產生奈米石墨相的形成，並導致電子場發射特性變佳。兩相對照下，UNCD薄膜在高能量（2.245 GeV）金離子照射下會產生局部高溫使晶界上的非結晶碳再結晶成奈米石墨相聚集並使電子場發射特性變佳。退火作用的過程可進一步提高再結晶過程和電子場發射特性。
（ii）在U-BEG過程中，會有類似大晶粒的鑽石存在，而這些大晶粒鑽石基本上都是屬於鑽石晶粒的聚集，而鑽石會聚集在一起主要是因為許多單顆的球狀晶粒的奈米晶鑽石顆粒所聚集起來的。當負偏壓(Negative Bias Voltage)越大鑽石晶粒尺寸越小。然而，由穿透式電子顯微鏡(Transmission Electron Microscopy，TEM)的觀察發現電子場發射特性變佳的主要因素是在U-BEG過程中成長鑽石薄膜會沿著晶界處產生石墨相。
（iii）在氮氣電漿中(CH4/N2)成長鑽石薄膜，其顆粒狀結構有明顯的改變，是由等軸的幾何結構變成針狀結構。像針狀的鑽石晶粒的長寬比增加是以基板溫度為首要條件，在700℃成長薄膜時可達到最大長寬比，然後更高的基板溫度時長寬比例會下降。薄膜的導電性也隨著基板溫度的增加而增加在700℃成長薄膜時可達到最大。UNCD薄膜的電子場發射特性也是如此。然而，由TEM的觀察發現電子場發射特性變佳的主要因素是在700℃成長UNCD薄膜時其針狀的鑽石晶粒周圍有石墨相的形成。
由以上三個例子證明，在鑽石薄膜中使電子場發射特性變佳的真正因素是鑽石晶粒間有石墨相的形成。這樣的認知，可得知如何經由改變其顆粒結構，使電子場發射特性變佳。

關鍵字

電子場發射

並列摘要

In this thesis, we used 3 examples to demonstrate the effect of graphitization process on enhancing the electron field emission (EFE) properties of diamond films, viz. (i) high energy ion irradiation, (ii) ultrasonication-bias enhanced growth (U-BEG) process and (iii) N2-plasma MPECVD process.
(i) In the high energy ion irradiation effect process, we observed that irradiation of 2.245 GeV Au-ions imposed significant modification on the granular structure of MCD diamond films but induced less marked influence on the microstructure of UNCD films. The extent of modification increased with the fluence of the irradiated ions. The critical fluence is around 8.4 x 1013 ions/cm2. For MCD films, the average grain size was reduced due to Au ion irradiation. Some of the grains remained intake and only structural defects were induced. Some of the grains were completely disintegrated to ultra-small grains in accompanied with the presence of amorphous carbons. These processes degraded the EFE properties of the films. Post-annealing the ion irradiated films healed the defects, recrystallized the amorphous carbons and induced the re-growth of the disintegrated diamond grains. The post-annealing process induced the formation of nano-graphite phase and resulted in the enhancement on the EFE properties for the films. In contrast, for the UNCD films, the high fluence energetic (2.245 GeV) Au ion irradiation induced the local heating that crystallized the grain boundary a-C phase into nano-graphite clusters and enhanced the EFE properties for the films.Post-annealing process further enhanced the re-crystalization process and improved the EFE properties.
(ii) In the U-BEG (Ultrasonication-Bias Enhanced Growth) process, the granular structure was changed from faceted large grains microstructure to roundish nano-grain granular structure. The extent of size reduction for the diamond grains increased with the magnitude of negative voltage applied. However, TEM examination revealed that the prime factor enhancing the EFE properties for the diamond films grown by U-BEG process in the induction of graphitic phase along the grain boundaries of the films.
(iii) In the diamond films grown by N2-plasma (CH4/N2), the granular structure was altered markedly from equi-axed geometry to acicular one. The aspect ratio of the needle-like diamond grains increase with substrate temperature first, reaching the largest on for the films grown at 700℃, and then decreased for higher substrate temperature. The conductivity of the films also increased with the substrate temperature and is largest for the films grown at 700℃. So does the EFE properties for the UNCD films. However, TEM investigation revealed the authentic factor, resulting in superior EFE properties for the 700℃-grown UNCD films is the formation of graphitic phase encasing the needle-like diamond grains.
All the three cases show that the formation of graphitic phase among the diamond grains is the genuine factor that enhanced the EFE properties for the diamond films. Such an understanding sheds a light on how to enhance the EFE properties of diamond films via the modification on their granular structure.

並列關鍵字

EFE

參考文獻

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