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| 研究生： |
林韋孝 Lin, Wei-Hsiao |
|---|---|
| 論文名稱： |
不同極性氮化鎵之特性及應用於光檢測器之研究 Study of various polarities GaN layers and their application of photodetector |
| 指導教授： |
張守進
Chang, Shoou-Jinn 劉冠廷 Liu, Kuan-Ting |
| 學位類別： |
碩士 Master |
| 系所名稱： |
工學院 - 奈米科技暨微系統工程研究所 Institute of Nanotechnology and Microsystems Engineering |
| 論文出版年： | 2008 |
| 畢業學年度： | 96 |
| 語文別： | 英文 |
| 論文頁數： | 80 |
| 中文關鍵詞： | 極性 、二氧化鈦 、有機金屬化學氣相沉積 |
| 外文關鍵詞： | MOCVD, polarity, TiO2 |
| 相關次數： | 點閱：119 下載：2 |
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在本論文中，我們利用有機金屬化學氣相沉積法 (metalorganic chemical vapor deposition, MOCVD) 在成長過程中藉由鋁插入層 (Al intermediate layer) 來成長不同極性氮化鎵薄膜於藍寶石基板上。由於鎵極性 (Ga-polarity) 氮化鎵薄膜的化學穩定性較氮極性 (N-polarity) 來的好，於是我們利用化學液體蝕刻來判斷氮化鎵薄膜的極性。利用原子力顯微鏡 (atomic force microscopy, AFM) 觀察出氮極性氮化鎵薄膜較鎵極性來的粗糙。掃描式電子顯微鏡 (scanning electron microscopy, SEM) 觀察出在經過化學液體蝕刻過的無鋁插入層氮化鎵薄膜表面上較鋁插入層之氮化鎵薄膜顯得更為平滑且凹洞也較少。同時，由X光繞射儀 (X-ray diffraction, XRD) 所測量到的半高寬 (full-width-half-maximum, FWHM) 顯示在經過化學蝕刻後的鋁插入層氮化鎵薄膜比無鋁插入層氮化鎵薄膜來得大。由以上的觀察可得知利用有機金屬化學氣相沉積在成長過程中插入鋁插入層成功地成長出氮極性氮化鎵薄膜。此外，拉曼 (Raman) 散射頻譜測得氮極性薄膜中存在伸張力 (tensile stress) ，而鎵極性則無任何應力存在。這個殘留於氮極性的應力是由於在氮極性氮化鎵薄膜當中有比較多的群集鎵空缺 (Ga vacancy clusters) 所導致。除此之外，霍爾量測 (Hall measurement) 測得氮極性氮化鎵薄膜之殘留電子濃度較鎵極性氮化鎵薄膜要來的小。
此外，我們也針對鎂 (Magnesium, Mg) 離子佈植於經由有機金屬化學氣相沉積法所成長出之氮極性與鎵極性之氮化鎵薄膜被有系統的分析。藉由X光繞射儀分析得到其鎂離子佈植掺雜後之氮極性氮化鎵薄膜，在經過合適的熱退火條件處理下其半高寬比鎵極性樣品來的小。光激發螢光譜 (photoluminescence, PL) 同時也量測出經由熱退火過之鎂離子佈值之後氮極性氮化鎵薄膜，由鎂受體束縛激子 (Mg-acceptor bound exciton) 所產生於光譜中的3.46eV強烈峰值相較於鎵極性氮化鎵薄膜來得大。由拉曼頻譜所測得之E2 聲子 (phonon) 的線寬 (linewidth) 顯示經由鎂離子佈植之後的氮極性氮化鎵薄膜較鎵極性來的好。霍爾量測指出經過不同熱退火條件之下鎂離子佈植後之鎵極性薄膜皆保持n型; 反之， 氮極性氮化鎵薄膜在合適熱退火條件下呈現p型傳導之特性。這個現象是由於氮極性氮化鎵薄膜當中存在較多群集鎵空缺導致鎂原子較容易置換進入氮化鎵薄膜中而成為受體以提高p型氮化鎵之傳導性。
對於不同極性氮化鎵薄膜應用於金半金紫外線光檢測器之分析，其氮極性之暗電流較鎵極性來得高，其原因歸咎於氮極性之蕭基能障 ( Schottky barrier height ) 比鎵極性來的低。然而,其氮極性之光電流與光響應度較鎵極性表現上來的好，其原因是由於粗糙的氮極性氮化鎵薄膜可以減少入射光子的全反射。除此之外,我們利用二氧化鈦奈米微粒 (TiO2 nano-particles) 應用於氮極性與鎵極性氮化鎵金半金光檢測器上，對於有黏著二氧化鈦奈米微粒之氮極性金半金光檢測器，其特性表現相對於無黏著二氧化鈦奈米微粒元件特性表現上來的好。這個現象是由於氮極性本身表面較粗糙之材料特性導致在二氧化鈦奈米微粒更易附著於表面，進而達到有效降低入射光的全反射，也同時增加光子的吸收。
In this thesis, the polarities of GaN layers grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrate by deposited Al intermediate layer process during growth. Wet chemical etching is applied to identify the polarity of as-grown GaN layers because the Ga-polarity GaN layers have the better chemical stability than N-polarity layers. Atomic force microscopy (AFM) imaging shows the N-polarity GaN layer is much rougher than Ga-polarity GaN layer. Scanning electron microscopy (SEM) imaging shows that the after chemical etching GaN layers without Al intermediate layer have a flat and lower pitted surface while the layer with Al intermediate layer is rougher as well as some defects. It is also found that we can achieve a larger X-ray diffraction (XRD) full-width-half-maximum (FWHM) for the GaN layers with Al intermediate layer than that without Al intermediate layer. These observations indicate that N-polarity GaN layer can be successfully grown by deposited Al intermediate layer process during growth in MOCVD. Moreover, the result of Raman scattering shows that the tensile stress is present in as-grown N-polarity layer while free stress is found in Ga-polarity layer. The residual stress has an effect on vacancy formation to a degree: Ga vacancy clusters are promotes in tensile stress. In addition, Hall measurement shows a lower carrier residual concentration for the N-polarity GaN compared with Ga-polarity layers.
Then, magnesium (Mg) implantation characteristics in N- and Ga-polarity GaN layers grown by MOCVD have been systematically investigated. The lattice polarity of GaN layers on sapphire substrate is studied by deposited Al intermediate layer process during the growth. It is found that we can observe a smaller XRD FWHM for the Mg implanted N-polarity GaN layer after a proper post-implantation annealing treatment than that Ga-polarity one. Photoluminescence (PL) analysis also exhibits the annealed implantation N-polarity GaN layers have a strong neutral Mg-acceptor bound exciton line at 3.46 eV than that Ga-polarity one. The Raman spectrum E2 phonon linewidth illustrates that high temperature annealing is found to result in the nearly full recovery of the crystalline quality of Mg-implanted N-polarity GaN. Hall measurement results further indicate that the p-type conductivity can be successfully achieved for Mg implanted N-polarity GaN regardless of the Mg implanted Ga-polarity GaN still remain n-type conductivity under the same implantation and post-implantation annealing conditions. This phenomenon could be attributed to the enhanced Mg atoms substitution since N-polarity layer with more Ga vacancy clusters and improve the p-type conductivity.
For N- and Ga-polarity metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors (PDs), it is found the dark current of N-polarity is higher than that of Ga-polarity. This is attributed to Schottky barrier height of N-polarity GaN being lower than Ga-polarity one. Therefore, the results of photocurrent and responsivity for N-polarity GaN enhancement can be attributed to that a rough N-polarity metal-semiconductor-metal (MSM) UV sensors surface can result in a reduction of photon path length for light incident. Besides, the N-polarity GaN MSM PDs with a titanium dioxide (TiO2) nano-particles roughened surface can effectively improve the performance than N-polarity or Ga-polarity regardless of TiO2 nano-particles coating. This phenomenon could be due to that naturally N-polarity GaN surface is rougher to residue the coated TiO2 nano-particles and then effectively reduce total reflection of incident light, moreover increasing the light absorption.
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