氮化鋁鎵/氮化鎵之異質接面雙極性電晶體電流增益改善之研究

Investigation of Current Gain Improvement of AlGaN/GaN Heterojunction Bipolar Transistors

曾裕騰(Tseng, Yu Teng)

指導教授 ： 鄭克勇

國立清華大學/電機資訊學院/電子工程研究所/博士(2016年)

摘要

氮化鋁鎵/氮化鎵的單異質接面雙極性電晶體在不使用複雜的二次成長射極之下，在本論文中成功的發展出來，並且透過二次離子質譜儀之分析發現在780°C的電漿輔助分子束磊晶成長下，依然會有嚴重的鋁以及矽之同步擴散現象存在於氮化鋁鎵/氮化鎵的異質接面上，鋁跟矽之摻雜分佈改變了能帶的形狀導致氮化鋁鎵/氮化鎵共射極電流增益嚴重地被限制並使之低於0.8，一旦將未摻雜的氮化鎵間隔層放在基極-射極介面上可以有效的和緩鋁跟矽的同步擴散現象。在氮化鋁鎵/氮化鎵的單異質介接雙極性電晶體嵌入20奈米的未摻雜氮化鎵間隔層，可將共射極電流增益提高到2，其共射極電流增益高於沒有間隔層的結構2.5倍。在量測期間也發現在射極周圍的光激發現象，並且在論文中做了完整的討論。
透過分析及比較高解析X-光電子能譜，可以得知其峰值的位移量顯示了表面上產生了氮的空位缺陷，導致形成一層N型氮化鎵薄膜，進而增加了達成P型氮化鎵的低歐姆接觸阻值的困難性。此外在使用N型的歐姆接觸(鈦/鋁/鈦/金)在P型氮化鎵上又得到了較低之電阻值，再度證明了表面損壞的原因就是感應耦合電漿蝕刻在P型氮化鎵表面上產生了一層N型氮化鎵的薄膜。為了解決被感應耦合電漿蝕刻造成表面損壞的情形，本論文使用數位蝕刻的技術來平衡樣品表面氮與鎵的比例，同樣地透過高解析X-光電子能譜儀取得氮與鎵的比例，而得知在經過蝕刻過後的表面其比例從1降至0.706，但經由數位蝕刻的方式可使得氮與鎵的比例從0.706升至0.877，更可以降低其表面接觸的阻值達到23%之多。

關鍵字

氮化鋁鎵 ； 異質接面雙極性電晶體

並列摘要

AlGaN/GaN single heterojunction bipolar transistors (SHBTs) without using regrown emitter junction are demonstrated. Secondary ion mass spectroscopy analysis shows that a severe co-diffusion of Al and Si exists in AlGaN/GaN heterostructures grown at 780°C by plasma-assisted molecular beam epitaxy. The altered composition and doping profiles greatly degrade the common-emitter current gain of AlGaN/GaN HBTs to ≤ 0.8. A GaN spacer layer is inserted at the emitter-base junction to alleviate this problem. In an AlGaN/GaN HBT structure inserted with a 20 nm unintentionally doped GaN spacer layer, a current gain β about 2 is achieved. The current gain is improved about 2.5 times larger than the structure without the spacer layer. The light-emitting phenomenon is also demonstrated and investigated in this article.
Using high resolution X-ray photoelectron spectroscopy (XPS), the surface binding energy and composition ratio of Ga and N are determined. The XPS peak shift in dry etching processed p-type GaN represents the existence of an n-type thin layer on the surface originated from nitrogen vacancies. This extra surface layer makes the formation of a low contact resistance difficult. On the contrary, a lower contact resistance is obtained when an n-type metal contact (Ti/Ai/Ti/Au) stacks is used on the etched p-type GaN surface, which confirms that the source of surface damage is coming from surface nitrogen vacancies. In order to overcome the surface damage problem in the base region, a digital etching technique is developed. The mole fraction ratio of N/Ga measured by high resolution XPS decreases from 1 to 0.706 after the conventional base-mesa process. Applying the digital etching technique to the base surface processing, the N/Ga ratio increases from 0.706 to 0.877. Besides, the base contact resistance can be reduced about 23% than the sample treated with inductively coupled plasma etching.

並列關鍵字

AlGaN ； Heterojunction Bipolar Transistors

參考文獻

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