氮化鎵/氮化銦鎵之發光二極體因為有效率下降的問題，所以導致氮化鎵這種發光二極體在發展上受到限制，而效率下降的因素有許多可能的因素，而也有許多可能都還在被討論當中，而一般來說，會把效率下降的機制分成內部耗損(Inter Losses)和載子溢漏(Carrier Leakage)，而造成內部耗損的原因又可以分成以下幾種，歐傑複合(Auger Recombination)、作用區體積減少(Reduced Effective Volume)、載子離域(Carrier Delocalization)。另外造成載子溢漏的原因又可分成以下幾種，極化電荷(Polarization Charges)、少量電洞注入(Poor hole injection)、非對稱的載子傳輸特性(Asymmetry)、電流壅塞(Current Crowding)、電子逃脫量子井(Electron Overfly)、缺陷輔助的穿隧(Defect-assisted tunneling)和輻射複合的飽和(Radiative Recombination Saturation)。而其中非對稱的載子傳輸特性這一點，一般而言電子的遷移率遠高於電動的遷移率，導致電子有很高的機率流過多重量子井，無法在作用區有效的輻射複合進一步造成下率的下降，而一般文獻中所提到防止載子溢漏的方法是在多重量子井上方多加一層電流阻擋層(Electron-Blocking Layer)在P型氮化鎵之前提高能障防止電子溢流過作用區來改善元件。而我們是藉由研究LED製程，包括電流擴散層、蝕刻條件、電極與半導體的接觸研究及LED結構的改良，來改善氮化鎵藍光發光二極體的光電特性。

Abstract
Study of improving optical and electrical property of GaN based blue light emitting diode

Author: Jhen-Yu Yang
Adviser: Jung-Chun-Andrew Huang
Department of physics, National Cheng Kung University

SUMMARY

In order to improve the efficiency of GaN based LED, we investigate procedure of making LED, including improve current spread layer, etching condition, contact between electrode and GaN, and LED structure. In current spread layer, we use Ni/Au bilayer contact with p-type GaN, and we find that after annealing at 450 Celsius it has better current spread and better uniformity of light output. In etching condition, we use Inductive Couple Plasma Etcher(ICP-Etcher) to etching our LED epitaxial wafer, and we find that as Cl2/Ar gas flow rates were fixed at 10/25 sccm, the etched surface roughness has the lowest value of 1.45 nm at constant ICP/bias power = 300/100W and 5mTorr chamber pressure for n-type GaN. We use Cr/Au bilayer as electrode ,and the electrode contact with n-type GaN is omhic contact. In LED structure, we use different thickness of ZnO deposit on n-type GaN to improve electrical and optical property, and find out the optimal thickness of ZnO. After we find out the optimal thickness, we change working pressure when we deposit ZnO, and then find out the best working pressure. Finally we try to figure out what mechanism cause efficiency improvement. We use hall effect to measure carrier concentration and the result show that disparity in carrier concentration are influence efficiency of LED.

Key words: Efficiency Droop, GaN based LED, Zinc Oxide, ICP etching, Ohmic contact

INTRODUCTION

Because GaN/InGaN based light emitting diode have efficiency droop problem, it limit the development of this kind of LED. There are many possible mechanism cause efficiency droop, some of them are also in debate. In general, we can classification these mechanism into two main part, “Internal Losses” and “Carrier Leakage”. There are also some reason cause internal losses, including “Auger Recombination”, “Reduced Effective Volume”, “Carrier Delocalization”. Furthermore, carrier leakage can also cause by some reason, including “Polarization Charges”, “Poor hole injection”, “Asymmetry”, “Current Crowding”, “Electron Overfly”, “Defect-assisted tunneling”, “Radiative Recombination Saturation”. One of these reason “Asymmetry”, in general, electron has much higher mobility than hole, it’s cause electron overflow from quantum well. Therefore, electron can’t recombination with hole effectively, and cause efficiency droop. A comment solution for this in other literature is add electron blocking layer above quantum well to increase barrier to avoid electron overflow from the active region. In our research, we improve optical and electrical property of our GaN based blue LED device by investigate procedure of making LED, including improve current spread layer, etching condition, contact between electrode and GaN, and LED structure.

MATERIALS AND METHODS

In the procedure of making LED we know the first thing we need to do is etching LED epitaxial wafer. Because of that we need know what is the best etching parameter, we test the etching parameter and analysis by scanning electron microscope (SEM) and atomic force microscope (AFM) and Alpha-Step Profilometer. We use SEM to see the surface morphology and use AFM to see the surface roughness and use Alpha-Step Profilometer to calculate etching rate after ICP etching. The second step of our LED procedure is deposit transparent conductive layer (TCL), and we use e-beam evaporator to evaporate Ni/Au bilayer. After that we use annealing technique to improve the current spread. The third step of our LED procedure is deposit ZnO layer, and we use sputter to deposit different thickness of ZnO. The fourth step of our LED procedure is deposit electrode, and we use e-beam evaporator to evaporate Cr/Au bilayer. Finally,we use KEITHLEY 2400 and integrating sphere to measure electrical and optical property. The carrier concentration of n-type GaN and n-type GaN with ZnO are measure by Van Der Pauw method.

RESULTS AND DISCUSSION

We enhance the efficiency about 15.01% compare to the standard LED by sputter ZnO 25 nm at working pressure 5E-3 mbar. This is the best enhancement in our results. In other condition we also have enhancement except deposit ZnO 100 nm. We measure the carrier concentration of n-type GaN and n-type GaN with ZnO, we find that the best enhancement of LED have better carrier concentration symmetry to p-type GaN. We think this symmetry cause the efficiency improvement. The worst efficiency is LED with 100 nm ZnO, and we think this is because the sheet resistance is increase, in addition this condition have thickest ZnO film cause the total series resistance increase cause bad electrical property.

CONCLUSION

In etching condition, we find best parameter to etch our epitaxial wafer, and use SEM to see surface morphology. In SEM picture we see the surface is more smooth than others etching condition, and has less cavity than others. In transparent conductive layer, we find the best annealing temperature is 450 Celsius, and it has best current spread and it has uniform light output. In zinc oxide layer, we find the best thickness of zinc oxide, and we find the best working pressure when we deposit zinc oxide. At this condition, LED has best enhancement about 15.01%, and this is because p-type GaN and n-type GaN with ZnO has better carrier concentration symmetry. In electrode layer, we find our Cr/Au is ohmic contact to n-GaN, and if try anneal it, it will become to Schottky contact, and if the negative electrode has ZnO layer it will always ohmic contact whatever we anneal it or not.

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