高效能可見光及紫外光發光二極體及其奈米結構之研究

Investigation of high performance ultra-violet and visible nanostructure light emitting diodes

柯閔詠(Min-Yung Ke)

指導教授 ： 黃建璋

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.00822

摘要

近年來，氧化鋅相關材料及氮化鎵奈米結構由於其良好的材料特性，被廣泛的運用於短波長發光元件。但製作高純度之近紫外光氧化鋅同質接面發光二極體仍受到材料方面的諸多限制，如缺乏高穩定度的p型氧化鋅及大量的本質缺陷存在於氧化鋅內部，導致其相關元件之發光頻譜往往會伴隨著顯著的缺陷放光並降低近紫外波段之發光效率。為了解決上述之問題，p型氮化鎵由於與氧化鋅具有相同之晶體結構及相似之晶格常數，已被大量的運用於氧化鋅發光二極體之製作。而更有研究團隊利用原子層沉積鍍膜(atomic layer deposition)、脈衝雷射蒸鍍技術(pulse laser deposition)及有機金屬氣相沉積 (metal-organic chemical vapor deposition)等磊晶技術成長高品質氧化鋅薄膜，克服氧化鋅發光二極體缺陷發光之問題，但其高成本及耗時等缺點，依然有待解決。
本文中將會透過加入適當設計之二氧化矽電流阻擋層以及載子侷限結構，並使用射頻濺鍍製作一大面積低成本且高純度近紫外光之ｎ型氧化鋅/p型氮化鎵異質接面發光二極體。
　　利用射頻濺鍍成長之傳統氧化鋅異質pn接面發光二極體，其發光頻譜為400至800nm之寬頻光，是由於氮化鎵及氧化鋅內部之本質缺陷所造成；相反的，加入一二氧化矽電流阻擋層後，電子和電洞得以被侷限在氧化鋅/二氧化矽及二氧化矽/氮化鎵接面，使其發光頻譜峰值為394nm並大幅降低缺陷發光。
我們更進一步在n型氧化鋅及p型氮化鎵介面插入二氧化矽/氧化鋅/二氧化矽載子侷限結構，使載子能夠更有效注入及侷限於氧化鋅發光層，達到377nm氧化鋅近紫外波長放光。
本文亦提出了具光指向性之奈米結構發光二極體元件。首先我們利用奈米球微影術製作氮化鎵奈米柱發光二極體，從電致發光頻譜上可發現其峰值在不同電流注入下近乎常數，這是由於奈米柱結構的製作降低氮化鎵薄膜中之壓電效應，並有效的釋放量子井之應力。同時，我們亦透過光強度對角度變化的量測可知，奈米柱發光二極體具有高度的增益及指向性，顯示著具有奈米柱結構之發光元件具有光波導之特性。
　　綜合氧化鋅及氮化鎵奈米柱發光二極體之實驗結果，我們欲製作一具高光指向性之紫外光氧化鋅奈米柱發光二極體。藉由水熱法成長氧化鋅奈米柱於p型氮化鎵上，並在其界面加入一氧化鎂電子阻擋層，透過此結構之設計，電子有效的被阻擋於氧化鋅側並與電洞結合，達到390nm近紫外放光，在光強度對角度變化量測方面,此元件也顯示出高度之光指向性且發散角為30度，顯示具奈米柱結構之氧化鋅發光元件具有成為高指向性UV發光元之潛力。

關鍵字

發光二極體 ； 氧化鋅 ； 異質接面 ； 載子侷限結構 ； 奈米小球 ； 奈米柱 ； 光指向性

並列摘要

Due to some superior optical and electrical characteristics, ZnO related materials and GaN nanostructure LEDs attract great interests as the new short wavelength lighting source. But ZnO materials still have some limitations like the lack of stable p-ZnO and large amounts of intrinsic defects which cause the difficulty in realizing high purity UV/blue ZnO homojunction LEDs. In order to achieve high purity UV light emission ZnO LEDs, GZO/p-GaN, GZO/p-GaN with a SiO2 layer and GZO/p-GaN with a sandwich structure heterojunction LEDs were designed for optimizing light emission properties.
From EL spectra, a broadband emission from 400nm to 800nm, sharp peak emission at 394nm and excitonic emission at 377nm can be observed from LEDs with different structure, respectively. Broadband emissions of LEDs without a SiO2 layer due to transition in the GaN along with defects in the ZnO layers. When LEDs with a SiO2 layer, light emission attributed to recombination of accumulated carriers between n-ZnO/SiO2 and p-GaN/SiO2 junctions. We also characterized samples with the light emitting ZnO layer sandwiched between two SiO2 thin films. Carriers can effective injection and confinement in the sandwich structure, result in a strong excitonic emission at the wavelength 377nm.
. We further investigate light output behaviors of GaN nanorod LEDs. GaN nanorod LEDs were fabricated by nanosphere lithography. The electroluminescence peak wavelengths of the nanorod LEDs nearly remain as constant for an injection current level between 25mA and 100mA, which indicates that the quantum confined stark effect is suppressed in the nanorod devices. The radiation profiles of the nanorod LEDs show the high directionality of emission light and waveguiding effect. The result is associated with the vertical guiding effect along the nanorod cylinder and the Bragg scattering of photons extracted from the sidewall by the rest of the rods.
Finally, we integrated the experimental results of ZnO based and GaN based nanorod LEDs, high purity UV/blue light emission ZnO nanorod LED can be achieved.
We grown ZnO nanorod by aqueous solution method on p-GaN surface with a MgO electron blocking layer. A sharp excitonic emission peak at 390nm can be observed of LEDs with MgO layer. This result indicates electrons can be blocked at MgO/ZnO interface effectively and holes can easily pass through the p-GaN/MgO interface due to the different band offset between ZnO/MgO and p-GaN/MgO. ZnO LEDs with nanorods also can act as a waveguide. A narrower radiation profile with divergence angle 30o and light emission can be redirected into the surface normal direction. From the results, ZnO LEDs with nanorods have great potential to become new UV light sources with high light emission directionality.

並列關鍵字

light emitting diodes ； ZnO ； heterojunction ； carrier confinement structure ； nanosphere ； nanorod ； light emission directionality

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