氮化鎵發光二極體P型金屬歐姆接觸特性及雷射剝離技術之研究

Abstract

本論文主要研究氮化鎵材料與元件製程相關之技術，其中包括P型歐姆接觸特性、發光二極體的P型歐姆接觸及利用準分子雷射剝離氮化鎵發光二極體技術。在P型歐姆接觸方面，我們發現藉由使用P型氮化鎵材料熱退火條件為氮氣環境中800℃加蓋(with cap)，再退火30分鐘，提高了霍爾濃度並改善了材料品質。將此再次熱退火條件結合P型歐姆接觸-Ni/Pd/Au金屬結構在氧氣環境550℃，5分鐘的金屬退火處理下，可得到良好的歐姆接觸並得到低特性電阻值約為3.9×10-4 Ω-cm2 ，由SIMS分析結果來看，低阻值的產生可能與退火過程中在氮化鎵介面產生跟NiO及Au-Pd-Ga合金有關。 在發光二極體P型歐姆接觸方面，我們以同樣Ni/Pd/Au金屬結構為P型歐姆接觸並調整各種不同的製程及熱退火條件，藉以建立氮化鎵發光二極體最佳操作性能的製程方式。比較結果發現以氮氣環境中800℃加蓋(with cap)，再退火15分鐘，並配合以Ti/Al/Ni/Au為n型金屬電極所製作之發光二極體可達順向導通電壓為3伏特，阻值為24.5Ω的最佳電性量測結果。 在雷射剝離技術研究上，我們成功地利用準分子雷射技術及二次轉移過程將氧化鋁基板去除，並將氮化鎵發光二極體黏合(bonding)到銅基板上。由I-V量測結果來看，在銅基板上的發光二極體其阻值比氧化鋁基板上的低。而由電激發光譜發現，經過雷射剝離技術的發光二極體，其波長有些微的紅移現象。

We study the process techniques of GaN material and GaN-based device including ohmic contact on p-GaN, p-type ohmic contact on GaN LED,and excimer laser lift-off of GaN-based LED. For p-GaN Ohmic contact, by re-annealing the in-situ p-GaN sample in the external furnace under 800℃ for 30 minutes with cap in N2-ambient, the hole carrier concentration was increased and the material quality was also improved. We obtained good p-type ohmic characteristic with a specific contact resistivity as low as 3.9×10-4 Ω-cm2 by using re-annealing samples combined with the Ni/Pd/Au metallization scheme followed by thermal treatment in oxygen ambient at 550℃ for 5 minutes. From the SIMS measurement, we found the low specific resistance maybe related to the formation of NiO and Au-Pd-Ga alloy on surface of GaN. For p-type ohmic contact on GaN-based LEDs, we applied the same p-type ohmic contact(Ni/Pd/Au) and re-annealing condition on GaN-based LED, and obtained an optimum process condition for LED sample by re-annealing the sample in the external furnace under 800℃ for 15 minutes with cap in N2-ambient. With the Ti/Al/Ni/Au metallization scheme as n-type contact, the LEDs showed low turn-on voltage of 3V and low resistance of 24.5Ω. In the development of excimer laser lift-off technique, we successfully separated GaN based LED from sapphire substrate using excimer laser and transferred on the Cu substrate by laser lift-off (LLO) technique and two-step transformation process. From the I-V curve, we found the LED on Cu substrate had smaller resistance than LED on Sapphire substrate. The LLO-LED electroluminescence(EL) spectrum showed the peak wavelength is slightly red-shifted from the LED on sapphire.