臺灣博碩士論文加值系統

以氮-三族材料為基礎的發光二極體現今已經被廣泛地應用於固態照明、顯示器、行動裝置上。不過，儘管此元件已經在商業上達到普及化，在技術上仍存在著許多要被克服的困難。研究如何改善元件的效率是很重要的課題，故本論文中主要的目的是藉由調降發光二極體中N型材料的載子遷移率，使得原本的非對稱性元件能近似於對稱性元件，並藉由此方法提升元件效率。
因為對稱性元件中P型區電場強度相對比起非對稱性元件來的小，造成載子複合區域的偏移離原本複合中心也相對來的小。另外元件也因載子有效複合效率的改善，進而提升整體輻射複合效率。這篇論文裡所使用的方法是採用兩種不同成分的氧化鋅材料來調降N型材料的遷移率；第一種是採用摻鈷的氧化鋅，另一種是不摻鈷的純氧化鋅。
將第一種摻鈷的氧化鋅應用於氮化鎵發光二極體元件，我們觀察出其效率與傳統元件相較之下並無顯著提升。對於摻鈷氧化鋅材料進行霍爾量測，由量測結果推測沒有使元件效率變好的原因為整體N型材料之遷移率並沒有因加了摻鈷氧化鋅而有明顯的調降，所以這一層在N電極下的摻鈷氧化鋅結構在這裡僅呈現串聯電阻的效果。然而，將第二種不摻鈷的純氧化鋅材料進行霍爾量測，可觀察到整體結構的載子遷移率有顯著調降。將此結構應用於氮化鎵發光二極體元件上，可發現不論是電特性或是光特性和傳統元件相比都有了改善。以不同厚度純氧化鋅，包含25奈米、50奈米及100奈米應用在元件製程上和傳統元件比較下外部量子效率分別提升9.19 %、0.33 %、8.98 %；另外，將元件進行濕式蝕刻的製程以改善漏電流的影響，我們量測到三種不同厚度的氧化鋅元件的效率和傳統元件相比分別增加17.02 %、4.26 %、16.27 %。從這裡我們得知進行濕式蝕刻改善漏電流情形將使得元件效率比起未進行蝕刻前的效率來的好。
適當厚度的純氧化鋅應用在發光二極體元件上可有效改善元件操作效率，因為該結構使N型材料的遷移率有著明顯的調降。而更重要的是，因為發光二極體的效率為內部量子效率和光取出率兩者的乘積，本研究效率的提升僅侷限於內部量子效率，若能集合氧化鋅奈米結構進一步改善發光區表面光取出率，這對發光二極體效率提升將會是有前瞻性的突破。

Light-emitting diodes (LED) based on group III–nitride semiconductors have generated much attention for applications, such as solid-state lighting, displays, and automobiles. Blue LED devices have been commercially available for many years though, there are still many obstacles waiting us to overcome. How to improve transfer efficiency of devices is a hot topic for study. In this thesis, the main purpose of the research is to reduce the mobility of n-GaN and therefore enhance the performance of LED device, making the performance of asymmetric devices more like symmetric devices.
For symmetric device, the electric field in the p-type region is relatively small, therefore leading to a small shift in recombination center. In addition, the total recombination rate is not seriously affected by drift-induced loss in symmetric devices. Based on above basis, we would like to develop a method to reduce the mobility of n-GaN by testing two kinds of ZnO materials on GaN-based LED devices. The first is ZnO with Co doping and the other is ZnO without Co doping.
Our results showed that efficiency of LED devices with Co doping ZnO is poorer than that of conventional LED devices. We further simulated the situation of devices fabrication by growing Co doping ZnO on sapphire substrate and on n-GaN/sapphire substrate and then measured the mobility of the simulated structure by using Hall instrument. We found that the reason causing low efficiency of LED devices with Co doping ZnO is that the mobility reduction magnitude is low (〈 10 %). The bulk of Co doping ZnO below the n-electrode may be regarded as series resistance which will weaken the performance of LED devices. In contrast to Co doping ZnO, however, the pure ZnO grown on n-GaN/sapphire substrate reduced the mobility of n-GaN significantly (at least 23.47 %). Using the pure ZnO to fabricate the LED devices, the electrical properties as well as optical properties were improved. Consequently, the efficiency of devices was enhanced as well. Compared with conventional LED devices, the efficiency of LED devices with ZnO thin film thickness 25nm, 50nm and 100 nm has improved by 9.19 %, 0.33 % and 8.98 %, respectively. Furthermore, after performing the process of etching, a way to deal with the effect of leakage current, the efficiency of LED devices with ZnO thin film thickness 25 nm, 50 nm and 100 nm has respectively improved by 17.02 %, 4.26 % and 16.27 % compared with conventional LED devices. From the results revealed, we realized that all the properties were superior to those of devices without the process of etching. So, we postulated that LED devices having pure ZnO grown on n-GaN with appropriate thickness may reduce the n-GaN mobility and therefore result in a device performance efficiency superior to conventional LED devices.
Our study is a progressive research for LED fabrication. Practically, the transfer efficiency is determined by the product of internal quantum efficiency (IQE) and light extraction efficiency (LEE). In the past, there have been countless laboratory researches focusing on how to enhance the LEE. However, little has been done on enhancing the IQE in the LED field. Our study has successfully increased the device performance by improving the IQE. By applying our IQE enhancing method coupled with previous LEE enhancing technologies, the transfer efficiency of LED can be greatly improved in the future.

Abstract(in Chinese) I
Abstract(in English) III
Acknowledgment V
Contents VII
Table Captions X
Figure Captions XI
Chapter 1 Introduction 1
1.1 Development of Light Emitting Diodes 1
1.2 GaN-based Light emitting diodes 2
1.3 Organization of this thesis 2
Chapter 2 Motivation and related theories 5
2.1 Related theories 5
2.1.1 Physics of LED devices 5
2.1.2 Shift of recombination location 6
2.1.3 Drift-induced leakage current 7
2.2 Motivation 11
2.2.1 Reduce n-GaN mobility to arise IQE 11
2.2.2 Method of reducing n-GaN mobility 11
2.3 Summary 12
Chapter 3 Conventional LED and ZnO growth methods 16
3.1 Process of conventional LED devices 16
3.2 Properties of conventional LED devices 19
3.3 Analysis of growing ZnO methods 21
3.4 Experiment of etching ZnO methods 23
3.4.1 Dry etch 23
3.4.2 Wet etch 24
3.4.3 Conclusion 24
3.5 Summary 25
Chapter 4 ZnO overlapped on P-GaN and n-GaN LEDs 35
4.1 Process of LEDs with ZnO overlapped on p-GaN and n-GaN 35
4.2 Properties of LEDs with ZnO overlapped on P and N-GaN 38
4.3 Discussion 39
4.4 Summary 40
Chapter 5 ZnO overlapped on n-GaN LEDs 44
5.1 LED devices with Co doping ZnO 44
5.1.1 Process of LED devices with Co doping ZnO 44
5.1.2 Problem of LED devices with Co doping ZnO 47
5.1.3 Properties of LED devices with Co doping ZnO 49
5.1.4 Summary 50
5.2 Hall measurement of ZnO on n-GaN 51
5.2.1 Analysis of Co doping ZnO on n-GaN 51
5.2.2 Analysis of pure ZnO on n-GaN 52
5.2.3 Summary 54
5.3 LED devices without Co doping ZnO 55
5.3.1 Process of LED devices without Co doping ZnO 55
5.3.2 Properties of LED devices without Co doping ZnO 58
5.3.3 Summary 61
5.4 Discussion 63
Chapter 6 Conclusion and future work 83
6.1 Conclusion 83
6.2 Future work 84
Reference 85

Reference for Chapter 1
[1.1] Mark P. Mills, The LED Illumination Revolution, Forbes. from http://www.forbes.com/2008/02/27/incandescent-led-cfl-pf-gur u_in_mm
_0227energy_inl.html , February 27, 2008
[1.2] “LEDs and OLEDs, ,Timeline of LED Inventors and Developments, from http://www.edisontechcenter.org/LED.html, 2013
[1.3] Holonyak Nick; Bevacqua, S. F. Coherent (Visible) Light Emission from Ga(As1−x Px) Junctions. Applied Physics Letters, 1 (4): 82, December, 1962.
[1.4] Schubert, E. Fred. Light-Emitting Diodes. Cambridge University Press. ISBN 0-8194-3956-8, 2003
[1.5] Sang-I Park, Yujie Xiong, Rak-Hwan Kim, “Printed Assemblies of Inorganic Light-Emitting Diodes for Deformable and Semitransparent DisplaysScience, Vol. 325, No. 5943, pp. 977–981, August, 2009.
[1.6] H. Morkoç, S. Strite, “Large‐band‐gap SiC, III‐V nitride, and II‐VI ZnSe‐based semiconductor device technologies Applied Physics. 76, 1363, September, 1994
[1.7] Yu-Tsung Lu,Improvement in light output efficiency of InGaN/GaN blue LED by current blocking layer and surface passivation layer, (2003), Unpublished master’s thesis, Department of Electrical Engineering, National Cheng Kung University, Taiwan.

Reference for Chapter 2
[2.1] Hardwigg, Schematic diagrams of Light Emitting Diodes, from https://upload.wikimedia.org/wikipedia/commons/d/d7/ PnJunction
-LED-E.svg, April 22, 2011.
[2.2] S. M. SZE, “Semiconductor Devices Physics and Technology, 2nd, Chapter4, pp. 28, 2001
[2.3] E. F. Schubert, Light-Emitting Diodes, 2nd ed., Cambridge Univ. Press,
New York, 2006.
[2.4] David S. Meyaard, Guan-Bo Lin, “Asymmetry of carrier transport leading to efficiency droop in GaInN based light-emitting diodes , Applied Physics Letters, 99, 251115, December 23, 2011.
[2.5] Guan-Bo Lin, David Meyaard, Analytic model for the efficiency droop in semiconductors with asymmetric carrier-transport properties based on drift-induced reduction of injection efficiency, Applied Physics Letters, 100, 161106, April 19, 2012
[2.6] Industrial Technology Research Institute, MLED Meeting, 2014/1/9, Unpublished PowerPoint file

Reference for Chapter 3
[3.1] K. Ip, M.E. Overberg, K.W. Baik, “ICP dry etching of ZnO and effects of hydrogen, Solid-State Electronics, 47 , pp. 2289–2294, December 2002.
[3.2] Han-Ki Kim, J.W. Bae “Inductively-coupled-plasma reactive ion etching of ZnO using BCl3-based plasmas and effect of the plasma treatment on Ti/Au ohmic contacts to ZnO “, Thin Solid Films, 447 –448, pp. 90–94, 2004