探討平頭式氮化銦鎵與氮化鎵多重量子井包覆奈米柱核殼結構綠光發光二極體之成長與光電特性

Abstract

近年幾來氮化鎵奈米柱型發光二極體引起世界各地廣大的研究興趣，鑒因於其結構可有效增加主動層面積、降低成長的缺陷密度、增加光萃取率以及提供半極化面或是非極化面的量子井成長以抑制量子侷限史塔克效應等優點。從近年來的相關發表中均指出三維成長的氮化鎵奈米柱通常具有多個面向的量子井成長方向，乃由於其不同面向的長速、銦融入率皆不同，使得不同面向量子井光電特性皆不同。而奈米柱結構上由於氮化鎵沿c軸方向成長的長速較快，進而使得奈米柱頂部易形成較尖銳的幾何結構，造成在外加電場下不均勻的電場分布以及電致發光波長隨注入電流產生嚴重的藍移現象等等，以上諸多問題皆會使得奈米柱發光二極體在光電特性上難以控制而限制其應用。 因此，本論文中我們提出一新穎的奈米柱發光二極體結構，於奈米柱的頂部加上一層氮化矽之鈍化層，預期使用這種新型的基板於成長時可抑制極化面與半極化面量子井的成長，進一步增加非極化面量子井的成長面積，抑制量子侷限效應並提升元件之發光效率。此外，較均勻的量子井厚度以及銦含量分布亦被實現於此結構成長之奈米柱發光二極體。在電激發光特性上，於奈米柱頂部的氮化矽介電質材料可做為一電流阻擋層，避免尖端結構形成電場的聚集，進而改善目前奈米柱發光二極體電激發光波長隨電流嚴重藍移之關鍵問題。於本篇研究中，我們成功的製備出一平頭式奈米柱發光二極體，並與文獻上的各式發光二極體結構做比較與討論，我們相信它將扮演未來奈米柱發光二極體發展之重要角色。

In recent years, c-axis-oriented gallium nitride (GaN) based nanorods light-emitting diodes (LEDs) have attracted intensive attention. The reason why GaN nanorod LEDs is so popular can be attributed to the following advantages. First, the area of active regions will increase, while they were grown on three-dimensional (3D) structure nanorods template. Second, the defect density of MQWs grown on nanorods template can be decreased. Third, light extraction performance is greatly enhanced. Fourth, the growth of non-polar and semi-polar multiple quantum well can decrease the polarization-induced effect. In the epitaxial process of the GaN materials in LEDs, the growth rate of multi quantum wells and the indium (In) composition are various in different growth orientations, causing inhomogeneous thickness of the active regions and anisotropic indium distributions in multi quantum wells (MQWs). Furthermore, while re-growing III-nitride materials on the nanorods template, the gradually accumulated materials have a tendency to form pointed tips on the top of each nanorods. This implies a non-uniform electric field distribution will occur on the topmost of nanorod LED. As a result, EL emission wavelength will have a giant blue shift as the injected current increases. These characteristics may make the emission wavelength of nanorod LED deviated from the original design and limit its applications. To conquer the challenges, we propose a novel nanorod LED structure. In our newly designed nanorod LED, the top of nanorod is covered by a dielectric material nano-disk. During the nanorod LED growth, the passivation of dielectric nano-disk on the top can suppress the growth of polar and semi-polar MQWs, leading to a high proportion of non-polar active region. Therefore, the quantum confined stark effect in MQWs can be greatly reduced. Moreover, indium spatially distribution in MQWs may become uniform. On the other hand, with this dielectric material nano-disk, the electric field on the tip of nanorod LED is reduced substantially. Thus, the major of difficulty of the giant blue shift existing in the EL spectra can be avoided. In this work, we have successfully demonstrated that our tip-free core-shell nanorod LEDs indeed possess, the superior features as described above when compared with its conventional counterpart. It is believed that our study shown here will pave a key study for the advancement of high efficient LEDs with excellent performance.