載子遮蔽效應與壓電效應對氮化鎵發光二極體之光性影響研究

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研究生：	林宜瑾 Yi-chin Lin
論文名稱：	載子遮蔽效應與壓電效應對氮化鎵發光二極體之光性影響研究 Carrier screening and piezoelectric effects on optical performance of GaN-based LED
指導教授：	劉正毓 Cheng-yi Liu
口試委員:
學位類別：	博士 Doctor
系所名稱：	工學院 - 化學工程與材料工程學系 Department of Chemical & Materials Engineering
論文出版年：	2015
畢業學年度：	103
語文別：	英文
論文頁數：	82
中文關鍵詞：	發光二極體、氮化鎵、壓電場、載子遮蔽電場、熱膨脹係數
外文關鍵詞：	Light emitting diode, GaN, piezoelectric field, carrier screening field, CTE mismatch
相關次數：	點閱：10 下載：0
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發光二極體中的氮化鎵晶體具有壓電特性，產生之壓電場會使得主動層(多重量子井)之能階傾斜，此現象稱為量子侷限史塔克效應。能階傾斜造成載子侷限能力變差、載子在空間上分離(造成複合效率下降)。而載子空間分離會產生一載子遮蔽電場，其電場方向與壓電場相反，故可以減緩量子井之能階傾斜。本研究為探討壓電場與載子遮蔽電場之間的關係，及其對於發光二極體之光性的影響。藉由調變固晶製程之固晶溫度，我們發現高溫固晶可以釋放愈多氮化鎵磊晶薄膜中的壓應力，由這個發現，我們可以改變氮化鎵磊晶薄膜中的殘留壓應力，進而研究壓電場(由殘留應力造成)與載子遮蔽電場之間的關係。本論文首先提出:氮化鎵磊晶薄膜與藍寶石基板之間的熱膨脹係數不匹配造成之壓電場主導著氮化銦鎵多重量子井的能階傾斜。然而，減少熱膨脹係數造成之壓電場強度對發光二極體之光性並無太大的影響。這是因為載子遮蔽電場會抵消掉壓電場，而且我們認知到載子遮蔽電場的強度與壓電場造成之能階傾斜程度以及載子濃度有關。故進一步發展出載子遮蔽電場為熱膨脹係數造成之壓電場與外加電場的函數，之間的關係可表示為: 。本研究之兩個重大結論為:(1)熱膨脹係數造成之壓電場主導多重量子井的能階傾斜；(2)載子遮蔽電場是所有內部電場中影響發光二極體之發光特性最嚴重。

GaN crystal has the piezoelectric characteristic, the producing piezoelectric field would cause the energy level tilting in the InGaN MQWs, which is named as quantum-confinement Stark effect. Energy level tilting causes some disadvantages, i.e., the carrier confinement ability decay, the carrier spatial separation (reducing carrier recombination). The carrier spatial separation also causes a carrier screening effect with a direction opposite to the piezoelectric field, which could ease the quantum-confinement Stark effect. To explore the relationship between the piezoelectric field and the carrier screening effect, and their effects on the optical performance of the GaN-based LEDs. We adjust the process temperature of the die-attachment process, to vary the residual compressive stress in the GaN epilayer. We note that the higher temperature of the die-attachment process, the more compressive stress of the GaN LEDs can be released. By this concept, the residual compressive stress in the GaN epilayer could be controlled. In this Ph. D. study, we first propose that the CTE mismatch (between the GaN epilayer and the sapphire substrate) causes a CTE-induced piezoelectric field, which dominates the energy levels distortion in the InGaN MQWs of the GaN-based LEDs. However, the decreasing in the CTE-induced piezoelectric field does not have a significant effect on the optical performance of the GaN-based LEDs. One of the major reasons could be that the carrier screening effect could compensate the CTE-induced piezoelectric field. And, we realize that the magnitude of the carrier screening field depends on the piezoelectric field induced energy level tilting and the carrier concentration. Thus, the carrier screening field is a function of the CTE-induced piezoelectric field and applied electric field, which can be described as . Two major conclusions are: (1) the CTE-induced piezoelectric field dominates the initial energy level tilting in the InGaN MQWs; (2) the carrier screening effect is the major internal electric fields that influences the optical property of the GaN-based LEDs.

Table of Contents
中文摘要    I
Abstract    II
Table of Contents    IV
List of Figures    VI
Chapter 1 Introduction    1
1.1 Introduction of GaN-based compounds    1
1.2 Introduction of GaN-based LEDs    4
1.3 Mechanism of carrier recombination    7
1.4 Introduction of initial electrical field in GaN-based LEDs    9
1.5 Introduction of the carrier screening effect    14
Chapter 2 Motivation    16
Chapter 3 Decouple the residual stresses induced by lattice mismatch and CTE mismatch in GaN-based LED    18
3.1 Influence factors on the stress behavior of the GaN epilayer    18
3.1-1 Experimental procedures    18
3.1-2 Methodology of biaxial stress analysis in GaN    18
3.1-3 Stress evolution of GaN epilayer from room temperature to epitaxial temperature    19
3.1-4 Comparison of measured and theoretic stress values    21
3.2 Threading dislocations formation to relieve the LM-induced stress    26
3.2-1 Experimental procedures    26
3.2-2 Stress relaxation by forming threading dislocations    26
3.3 Relation between dislocation density and residual stress in GaN    29
3.4 Summary    30
Chapter 4 CTE-induced piezoelectric field in GaN epilayer of GaN-based LEDs    31
4.1 Determine the value of piezoelectric field in MQWs    31
4.1-1 Direct piezoelectric effect    31
4.1-2 Flat-band condition    32
4.2 Effect of CTE-induced piezoelectric field on the distortion of energy levels in MQWs    35
4.2-1 Experimental procedures    35
4.2-2 Stress levels of the GaN-based LEDs varing with LED die-attachment process    36
4.2-3 Influence factors on the energy levels distortion in InGaN MQWs.    37
4.3 CTE-induced piezoelectric effect on L-I-V curves of the GaN LEDs.    42
4.4 Summary    44
Chapter 5 Carrier screening effect dominates the optical properties of GaN-based LEDs.    45
5.1 Influence factors of carrier screening effect.    45
5.1-1 Net electric field in InGaN MQWs    45
5-1-2 Carrier screening field in InGaN MQWs    46
5.2 Verification the effect of carrier screening field by simulated band diagram.    52
5.3 Carrier screening effect is a function of CTE-induced piezoelectric field and applied electric field.    59
5.4 Summary    61
Chapter 6 Conclusions    62
References    64
                                

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