研究生: |
賴朝興 Lai, Chao-Hsing |
---|---|
論文名稱: |
以有機金屬化學氣相沉積法磊晶成長氮化鎵系列高電子遷移率電晶體於矽基板之研製 The fabrication of GaN-based HEMT on Silicon Substrate by MOCVD |
指導教授: |
蘇炎坤
Su, Yan-Kuin |
學位類別: |
碩士 Master |
系所名稱: |
電機資訊學院 - 微電子工程研究所 Institute of Microelectronics |
論文出版年: | 2014 |
畢業學年度: | 102 |
語文別: | 英文 |
論文頁數: | 73 |
中文關鍵詞: | 氮化鎵 、有機金屬化學氣相沉積 、高電子漂移率電晶體 、多層低溫氮化鋁 、氟離子 、閘極掘入 |
外文關鍵詞: | GaN, MOCVD, HEMT, multi-LT-AlN interlayers, fluorine ions, recessed gate |
相關次數: | 點閱:124 下載:1 |
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由於三五族氮化鎵系列材料具有高電子漂移率、高電子飽和速率、高熱傳導係數、寬能隙以及高崩潰電場,因此主要可以應用於高速、高功率以及高溫的電子元件,且目前市場上已大量被運用於製作高電子遷移率電晶體(HEMT)。但是氮化鎵基板本身相當昂貴,因此常使用藍寶石基板或者碳化矽基板來磊晶成長氮化鎵薄膜,相較於使用矽基板來磊晶成長,矽基板擁有高熱傳導係數、價格便宜以及可提供的基板尺寸可達到18英吋等優勢,故我們選擇矽基板磊晶成長氮化鎵系列高電子遷移率電晶體(HEMT)。然而矽基板與氮化鎵本身有著很大的晶格不匹配的問題,氮化鎵薄膜因此會伴隨著許多的裂痕和差排缺陷導致氮化鎵薄膜的品質不佳,嚴重地影響了元件特性。另一方面由於氮化鎵系列的HEMT元件為常開型元件,然而常關型的HEMT元件有著較低功率損耗、較高元件安全性以及將其使用在電路中的設計簡單等特性,可以更為被廣泛的應用在各種電子設備中,因此製作常關型的元件為近年來的趨勢。一般的常關型HEMT元件通常是建立於一個好的常開型HEMT元件之上,透過製程手段或者額外增加磊晶層的方式來達到元件常關的特性,因此我們首先需要製作一個好的常開型HEMT元件。
因此在本論文中,主要可分為兩個部分,第一個部分首先我們會使用有機金屬化學氣相沉積(MOCVD)去成長氮化鋁鎵/氮化鎵高電子遷移率電晶體結構,嘗試使用不同的磊晶參數以達到最佳化的氮化鋁鎵層,其中包含改變氮化鋁鎵層中鋁的含量以及厚度,當我們鋁的含量從15提升到25 %,元件特性可以獲得大幅度提升;此外,由於矽基板與氮化鎵之間很大的晶格不匹配問題,很難成長足夠厚且品質好的氮化鎵薄膜,因此我們採用多層低溫氮化鋁插入層(Multi-LT-AlN interlayer)去減少矽基板與氮化鎵薄膜的晶格不匹配,來成長更厚以及品質更佳的氮化鎵薄膜,進而提升元件的特性表現,當我們插入多層低溫氮化鋁插入層後,GaN薄膜厚度可以從0.5成長到2 μm,元件電流、導通電阻、轉導、漏電流以及崩潰電壓均可獲得提升以及改善。
在第二個部分中,我們將採用參數最佳的常開型HEMT元件透過增加氟離子植入以及閘極掘入的製程的方式,當我們採用單純氟離子植入時,臨界電壓雖然可有效偏移從-4偏移至-0.8 V,但是還是不足以成為常關型元件;因此我們改採用閘極掘入的方式,成功的讓臨界電壓從-4偏移至+0.3 V,但是其元件特性卻是不佳。最後我們改採用雙重製程的方式,成功的讓元件的臨界電壓-4偏移至+0.4 V,讓原本常開型HEMT元件轉變成常關型HEMT元件且其元件特性也能夠保持。
Because III-V groups of GaN has the property of high mobility, high saturation electric drift velocity, high thermal conductivity, wide band gap, and high breakdown field, the GaN is mainly suitable for high frequency applications, high power and high temperature device. Moreover, The GaN has been massively applied to fabricate high electron mobility transistor (HEMT) in the market. Because the GaN bulk substrate is very expansive, the substitute substrates to grow the GaN film are sapphire or silicon carbide substrate. Compared with using the sapphire or silicon carbide substrates, silicon substrate possesses many benefits including high thermal conductivity, low wafer price and large size wafer up to 18 inches. Hence we have used the silicon substrate to grow GaN-based HEMT. However, a large lattice mismatch between silicon substrate and GaN film leads to many cracks and dislocations, which will result in a low quality of GaN film and seriously affect the device performance.
The normally-off operation of HEMT possesses low power dissipation, high device safety and simple circuit configurations compared with the normally-on operation of HEMT, the normally-off operation can be widely applied in electron equipment. Therefore, the fabrication of normally-off operation has come to be the recent tendency. Generally speaking, a normally-off operation of HEMT is usually based on the normally-on operation of HEMT with additional process or cap layer to achieve the normally-off operation. Therefore, having a good normally-on HEMT is essential for the fabrication of normally-off HEMT.
This thesis is made up of two parts. In the first part, using the MOCVD grows the AlGaN/GaN HEMT structure. The Al composition and thickness of AlGaN layer are changed and optimized with different epitaxial conditions. When the Al composition is increase from 15 to 25 %, the DC performance can be effectively enhanced. Due to the lattice mismatch between GaN film and silicon substrate, the thicker and high quality GaN film can’t be realized. Therefore, we use the multi-LT-AlN interlayers to reduce the stress between GaN film and silicon substrate, which can grow a thicker and higher quality of GaN film. After the multi-LT-AlN interlayers is used in growing the GaN film from 0.5 to 2 μm, the DC performance is enhanced further.
In the second part, we use the optimum normally-on operation HEMT in the first part to fabricate normally-off operation HEMT with fluorine plasma treatment and recessed gate structure. The threshold voltage can be effectively shifted from -4 to -0.8 with fluorine plasma treatment, but it is not enough to be a normally-off poeration. Therefore, we try to apply the recess gate. Although the nomally-off operation can be achieved and the threshold voltage is shifted to +0.3 V, the DC perfromance is poor. Finally, the recess gate with fluorine plasma treatment is uesd. The threshold voltage of normally-on HEMT can be shifted toward +0.4 V. therefore, the normally-on operation HEMT is successfully become the normally-off operation HEMT and the performance is better.
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