奈米金粒鑲埋於鑽石(111)面之同質磊晶成長

Abstract

本論文主要探討以微波電漿化學氣相沉積(MPCVD)鑽石薄膜同質磊晶於鍍上金層之單晶鑽石(111)晶面的成長情形，本研究主要為分析鑽石薄膜的品質、金粒與鑽石薄膜的應力以及鑽石薄膜脆裂問題。本論文第一部分主要探討奈米金粒在單晶鑽石(111)上的形成，利用電子束蒸鍍(E-gun)在單晶鑽石上鍍金層，並在MPCVD系統內利用電漿使單晶鑽石上之金層形成奈米金粒，藉由不同的微波功率、不同甲烷濃度之電漿以及不同厚度的金層，得到金粒分佈較均勻的條件。第二部分則利用低甲烷濃度進行鑽石薄膜的成長，藉由不同的沉積時間，分別在未鍍金的單晶鑽石(111)上進行直接成長與分階段成長，觀察鑽石膜有無出現裂痕，進一步利用多次鍍金成長鑽石膜的方法以抑制裂痕產生。 首先利用真空退火處理，發現退火溫度越高，金粒在鑽石表面的分佈越均勻。由於鑽石膜的沉積皆在高濃度氫氣環境下進行，所以利用MPCVD，以增加微波功率的方式進行氫電漿處理、在固定功率的甲烷電漿下沉積鑽石膜二十分鐘，觀察金粒在各種電漿下的分佈情形。分析技術方面，使用掃描式電子顯微鏡(SEM)和原子力顯微鏡(AFM)觀察表面形貌和表面粗糙度，並配合X光能量散射光譜儀(EDS)進行元素分析。本實驗以功率800W 、壓力80 torr之甲烷濃度0.5%下的電漿處理，以厚度為20nm之金層，在鑽石(111)面上分佈最為均勻。 在鑽石薄膜成長部分，分為兩部份，第一部份為鍍金並直接成長鑽石薄膜，以相同的甲烷和氫氣混合比例、壓力、微波功率等參數，比較不同製程時間之鑽石薄膜的成長，配合AFM以及X光繞射探討鑽石薄膜的成長模式，並以TEM分析鑽石薄膜與奈米金粒間的界面情形，結果顯示金粒間的鑽石膜是以島狀鑽石成長，並以側向接合成長出鑽石薄膜，具有橫向覆蓋成長的磊晶行為(epitaxial lateral overgrowth，ELO)，幾無晶界存在，達到幾近完美的接合，顯現奈米金粒對(111)鑽石磊晶模式未造成重大改變；而金粒與鑽石膜呈現磊晶關係，兩者之間的界面存在著石墨層，是金粒形成的原因。第二部份是進行未鍍金(step growth)以及多次鍍金(multi-layer growth of gold)成長單晶鑽石，利用光學顯微鏡(OM)、拉曼光譜儀(Raman)對鑽石膜進行表面形貌與鍵結的分析。結果發現成長八小時，厚度為3.4 □m的鑽石膜在拉曼光譜中出現1324~1326cm-1的峰值，表示鑽石膜受到張應力，然而只有多次鍍金成長的鑽石膜，在相同製程時間下並沒有裂痕出現。在 X光繞射以及倒空間mapping (RSM)的分析結果，得到鑽石之單位晶胞的兩個方向之平面間距皆變大，而鑲埋在鑽石內的金粒則在兩個方向受到壓應力，導致其平面間距變小。最後利用TEM分析多次鍍金成長之鑽石膜，發現鑽石晶面接合處出現許多{111}面上的缺陷，如疊差、差排。差排密度隨著鍍金的次數增加而下降至1.41x108 cm-2，故可知金粒具有降低差排密度的功用，能有效地抑制(111)面磊晶鑽石膜的裂痕產生。

This thesis focuses on the study of growth of homoepitaxial diamond film with embedded gold nanoparticles (AuNPs) by microwave plasma chemical vapor deposition (MPCVD). The first part of this thesis deals with the preparation of gold nanoparticles on diamond (111) single crystal substrate. The effect of various plasma conditions on the distribution of gold nanoparticles on diamond (111) was explored by varying power and methane concentration in plasma. In the second part, the well fabricated AuNPs/ diamond (111) was used as the substrate for further growth of oriented diamonds by MPCVD. Finally, the results of multi-step growth of epitaxial diamond film with embedded AuNPs are presented. The gold film was deposited on ~ 2 mm sized diamond (111) single crystal substrate by electron beam evaporation. The as-deposited Au on diamond was then annealed in vacuum. The higher temperature results in more uniform distribution of AuNPs on diamond substrate. The distribution of AuNPs on diamond is also affected with the thickness of the deposited gold layer and the MPCVD conditions for homoepitaxial diamond film including hydrogen/methane concentration, microwave . The morphology and roughness after the plasma treatment were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results show that the as-deposited diamond on 20nm thickness of gold layer has more uniform distribution in 0.5% methane plasma at microwave power of 800 W. In the diamond growth process, oriented diamond films were deposited on diamond substrate covered with gold by the same process parameters including mixed ratio of CH4 and H2, pressure, power, etc. Comparing with the diamond film in the various time, we can set up the growth model of diamond characterized by AFM and X-ray diffraction (XRD). At the beginning, the growth of diamond islands appears between AuNPs, followed by lateral overgrowth with coalenscence when diamond islands covers the AuNPs. From cross-sectional transmission electron microscopy (TEM) observation, a graphite layer exists at the AuNPs/diamond interface. Secondly, diamond films obtained by step direct growth on diamond seed and by multi-layer growth of gold were characterized by optical microscopy (OM) and Raman spectroscopy. The results show that cracks appear after diamond film deposition for eight hours, while the film processed with multi- layer growth of gold shows no cracks. Raman spectra show the peak in the range of 1326-1332 cm-1, suggesting that diamond films are in tensile stress. XRD and reciprocal space mapping (RSM) were used to evaluate the effect of embedded AuNPs on cracking by determination of the d-spacings of diamond and embedded gold. The results show the out-of-plane and in-plane d-spacings of diamond become larger than the bulk values, whereas gold’s d-spacings become smaller. TEM reveals the distribution of embedded AuNPs and the microstructure of epitaxial lateral growth of diamond with formation of stacking faults and threading dislocations after the coalescence of islands in the step growth. After 4 step growth, the dislocation density can be reduced to 1.41x108 cm-2. It implies the embedded gold particles may restrain the formation of the cracks on diamond.