以化學機械研磨法製作氮化鎵奈米柱發光二極體及其在變溫環境下之電激發光特性

Abstract

近年來，氮化鎵及其奈米結構由於其良好的材料特性，受到各大研究團隊之研究與注目。但製作高發光強度且低漏電流特性之氮化鎵奈米結構發光二極體，仍受到材料與製程方面上的諸多限制，如缺乏快速且低成本之奈米結構製作方法，以及難以製作在奈米尺度下之並聯電極，使其難以運用在產業應用上。為了解決上述問題，有團隊利用鎳金屬奈米遮罩(nickel nanomask)、傾斜式銦錫氧化物沉積(ITO grown by oblique-angle deposition)等方法製作奈米結構之發光二極體，然而其漏電流過大以及發光效率低等缺點，依然有待解決。 在本篇論文中，我們發展一種嶄新且具有實用價值之奈米小球微影術，並以電漿輔助化學氣相沉積成長二氧化矽作為奈米柱側壁絕緣層，再以化學機械研磨法製作金屬接觸層，來製作高效率且低生產成本之奈米柱結構發光二極體。 利用本文所提出之方法，可有效解決過去奈米結構發光二極體在漏電流過大，且製作成本高之問題。在電性上，我們的奈米柱結構在-5V下僅有4.77nA的漏電流，理想因子(ideal factor)為7.35，其發光強度在注入電流密度為32A/cm2 (20mA)時有高達6807mW/cm2的表現。與其他團隊所提出之奈米結構製程方法相較，我們所發展之化學機械研磨法可得到最高之光強度輸出以及最小之逆向漏電流值。這結果顯示我們所提出之奈米小球微影術與化學機械研磨法可提供一種低成本的方式來製作高效能氮化鎵發光二極體，且此方法十分具有發展性與產業上之利用價值。 本文並提出了藉由抑制量子侷限史塔克效應(quantum confined Stark effect)，來提高發光二極體內部量子效率之奈米柱結構。我們比較了平面結構與奈米柱結構之發光二極體，在溫度以及電流不同下之電激發光頻譜特性。我們發現奈米柱結構之發光二極體在室溫下隨著電流增加，其光子能量近乎常數，而平面結構之發光二極體則有藍移之現象。此結果顯示藉由奈米球微影術製作之奈米柱結構，可有效抑制因氮化鎵晶格常數不匹配之應力所造成之量子侷限史塔克效應，增加電子與電洞波函數之間的空間重疊比例，並進而有效增加氮化鎵發光二極體之內部量子效率及其發光效率。

Due to the superior optical and electrical characteristics, GaN and related materials attract great interests as the new short wavelength lighting source. However, GaN based nanostructures still have some limitations, such as the lack of low cost nanostructure processes and the difficulties of parallel metal evaporation on tips of nanorods without short circuit. In order to achieve GaN based nanorod structures, some groups used nickel nanomasks or ITO grown by oblique-angle deposition to approach. However, they still suffer from large leakage current and low efficiency. In this thesis, we demonstrated a novel practical approach to fabricate a p-i-n nanorod light emitting diode (LED) arrays using nanosphere lithography for nanorod formation, PECVD (plasma enhanced chemical vapor deposition) grown SiO2 layer for sidewall passivation, and chemical mechanical polishing for uniform nanorod contact. Our nano-device demonstrates a reverse leakage current 4.77nA at -5V, an ideality factor 7.35, and an optical output intensity 6807mW/cm2 at the injection current density 32A/cm2 (20mA). Comparing with results shown by other groups, our work shows the best output power and the least reverse leakage current among relative researches. It showed that our methods have great industrial applicability for low-cost manufacturing high efficient GaN-based LEDs. Furthermore, we demonstrated the nanorod structures can mitigate quantum confined Stark effect, and improve the internal quantum efficiency. We compared the temperature and current dependent electroluminescence (EL) of blue planar and nanorod LEDs over a wide temperature range. We found that the photon energy of nanorod LEDs as the current level increased at room temperature are nearly constant. It reveals that our nanorod structures can effectively mitigate the strain induced quantum confined Stark effect, enlarge the overlap of electron and hole wavefunctions, and then effectively improve the internal quantum efficiency of GaN light emitting diodes.