High Transparent and Low Resistance Molybdenum-doped Indium Oxide Ohmic Contact to p-GaN

摘要: 在發光二極體(LED)元件中，電極和半導體之間的毆姆接觸好壞是影響元件效能的重要因素。到目前為止，n型氮化鎵上的毆姆接觸已經可以做到很好的程度了，然而，對p型氮化鎵而言，因為很難成長出高摻雜的p型氮化鎵 (雜質濃度高於1018 cm-3)，以及找不到任何金屬的功函數高於p型氮化鎵的功函數 (趨近於7.5eV)，以至於非常不容易達到好的毆姆接觸，進而限制了發光二極體的操作電壓，數年來一直是許多研究團隊想克服的瓶頸。因為金屬電極有其缺點，因而逐漸發展出以透明導電的氧化物電極作為p型氮化鎵上的電極。在工業上已廣泛應用多年的p型氮化鎵電極–Ni/Au/ITO，因為需要用到兩道製程 (利用電子束蒸鍍機蒸鍍Ni/Au之後，隨即送入濺鍍機沉積ITO)，造成製程上的不方便，因此我們想要尋求一種新的透明導電電極，直接鍍在p型氮化鎵上，便可達到很好的毆姆接觸效果。 在本篇論文，我們研究了一種具有高穿透率，且可以在p型氮化鎵上形成很低的毆姆接觸阻抗的透明導電材料–氧化銦鉬 (IMO)。我們嘗試了各種不同的表面清潔方式，以及不同的濺鍍條件，從中找出最適合的條件。我們發現此種材料在氮氣底下退火後，可以形成歐姆接觸，並且有很低的特徵接觸阻抗。為了更近一步了解此種材料和p型氮化鎵接面的變化，我們嘗試了不同的分析方法，如AFM，SIMS，和XPS。根據分析的結果，我們解釋了氧化銦鉬 (IMO)在p型氮化鎵上形成歐姆接觸的原因。最後，我們利用此種材料作為p型氮化鎵上的電極，製造出一個LED，並量測此LED的操作電壓。我們得到了一個很低的操作電壓，此外，此種材料在波長400 nm時，有高達80%以上的透光率，因此我們可以說，氧化銦鉬 (IMO)是一種極具潛力的透明導電材料，可應用於LED上的p型氮化鎵電極。Abstract: Ohmic contacts between electrodes and semiconductors play a crucial role in performance of light emitting diodes (LEDs). Up to now, very low resistance ohmic contacts on n-type GaN have been consistently preformed. But for p-type GaN ohmic contacts, it has been difficult to achieve low resistance contacts due to both the difficulty of growing heavily doped p-GaN (>1018 cm-3), and the absence of metal with a work function larger than that of p-GaN(∼7.5eV), which limits the operating voltage of the devices. Owing to the shortcomings of metal schemes, transparent conducting oxide (TCO) materials substitute for metal schemes as p-electrodes. The Ni/Au/ITO scheme is widely used on p-GaN in industry. However using such electrode requires two processes during the deposition. Therefore it is essential to find another TCO material that could form an ohmic contact on p-GaN with only one single layer. In this thesis, we have investigated a promising high transparent and low resistance molybdenum-doped indium oxide (IMO) ohmic contact on p-GaN. We try different surface treatments and different sputtering conditions to find the most suitable process. We find this IMO contact film exhibits ohmic contact with low contact resistance after annealing under nitrogen ambient. For further investigation of the interface between IMO layer and p-GaN, a series of surface analyses such as atomic force microscopy (AFM), secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) are performed. Based on these results, we explain the mechanisms of the IMO ohmic contact on p-GaN. Finally, with such high quality ohmic contacts, we fabricate the LEDs with annealed IMO ohmic contacts showing a low forward voltage at injection current of 20mA. And the light transmittance of IMO layer (3500 Å) on quartz is above 80% at 400nm. Therefore IMO contact scheme is a potential candidate for application to the p-electrode in GaN-based LED.Chapter 1 Introduction 1.1 The History of GaN……..…………………………………………1 1.2 Metal-Semiconductor contact………..………………………………6 1.3 Ohmic contact on GaN……..…………………………………..…......8 1.3.1 n-type ohmic contact…………………………………......…10 1.3.2 p-type ohmic contact………………………………….......…..12 1.4 Motivation……………………………………………………...…....15 1.4.1 Transparent Conducting Oxide (TCO)………………...……...16 1.4.2 Molybdenum-doped indium oxide (IMO).................................19 References...................................................................................................20 Chapter 2 Experimental 2.1 Introduction………………………………………………........……27 2.1.1 Transmission Line Method (TLM)……………………...……28 2.1.2 Circular Transmission Line Method (CTLM)………........…..30 2.2 Surface treatment…………………………………………...…….....35 2.3 Photolithography……………………………………………........…38 2.4 RF Sputtering…………………………………………………....….40 2.5 Lift-off…………………………………………………………...….42 2.6 Rpid Thermal Annealing (RTA)…………….…………..…...…...…42 2.7 I-V measurement……………………………………………..……...44 2.8 Analysis method...…………………………………………..………44 2.8.1 Atomic Force Microscope (AFM)…….………………..…….44 2.8.2 Secondary Ion Mass Spectrometry (SIMS)…….…...………..46 2.8.3 X-ray photoelectron spectroscopy (XPS)….…………...…….46 2.9 LED fabrication…………………………………………………......47 References……...…………………………………………………………50 Chapter 3 Experimental Results of IMO ohmic contacts and IMO LEDs 3.1 Resistivity and CTLM data…………………………………...…….51 3.1.1 Different sputtering conditions….....……………………..…..51 3.1.2 Different surface treatment…………………………………...54 3.1.3 Different annealing temperature……………………………...64 3.2 AFM data............................................................................................67 3.3 SIMS data...........................................................................................69 3.4 XPS data.............................................................................................69 3.5 Transmittance data…………………………………………………..73 3.6 LED data…………………………………………………………….77 Chapter 4 Discussion 4.1 Surface treatment……………………………………………...….…83 4.2 Anneal treatment…….…………………………………………..….87 4.3 AFM……….………………………………………………………..88 4.4 SIMS…….……………………………………………………….….88 4.5 XPS………………………………………………………………89 4.6 Transmittance……………………………………………………92 4.7 LED………………………………………………………………92 References………………………………………………………………95 Ch5 Summary and Future Work 5.1 Summary…………………...……………………………………….98 5.2 Future Work…………………………………………………………9