非極性"a"平面氮化鎵族奈米結構之光學特性

Abstract

本論文中，我們研究兩種利用有機金屬氣相沉積法成長的非極性”a”平面奈米結構的樣品，前者為不同銦含量之氮化銦鎵/氮化鎵(InGaN/GaN)多重量子井(MQWs)，後者為含有氮化銦鎵/氮化鎵多重量子井結構的奈米柱(nanorods)。我們利用光激發螢光(PL)、螢光激發光(PLE)、以及時間解析的光激發螢光(TRPL)等光譜技術進行樣品的光學特性分析。 第一部份，我們控制不同的長晶溫度長出不同銦含量的氮化銦鎵/氮化鎵多重量子井，透過陰極激發光(CL)以及共焦顯微影像，得知我們的樣品發光具有侷域化(localized)的現象。從室溫光激發螢光實驗中，顯示了隨著銦含量的變化，樣品在銦含量為24%時得到最好的發光強度。同時，在螢光激發光PLE的實驗觀察到較大的史托克位移(Stoke’s shift)存在於銦含量較多的樣品中。再者，在非極性結構的極化發光(polarized light emission)特性量測中，我們發現發光極化率隨銦含量的增加而有減弱的情形。最後，時間解析的光激發螢光(TRPL)光譜顯示出銦含量增加造成較深的侷限深度(localization depth)，但由於過多的銦含量會造成大量的晶格品質缺陷，所以在銦含量24%時有最佳的載子捕捉及放光效率。 第二部份，我們比較氮化銦鎵/氮化鎵多重量子井及含有其結構的奈米柱之光學特性。實驗結果顯示做成奈米柱後，除了釋放部分材料內部的應力外，同時也因量子井結構裸露增加了樣品的出光面積及強化側向光散射現象，故有效提升樣品的發光強度。但側向出光的增加也造成其極化率有減少的現象。

In this thesis, we studied two kinds of nanostructures grown by metalorganic chemical vapor deposition. The samples were grown along nonpolar a-axis on r-plane sapphire. The photoluminescence (PL), photoluminescence excitation (PLE) and time-resolved photoluminescence (TRPL) were performed to investigate the optical properties of the grown samples. The first series are a-plane InGaN/GaN multiple quantum wells with different indium (In) compositions and the second series are a-plane nanorods with embedded InGaN/GaN multiple quantum wells (MQWs). In the first part, the characteristics of different In compositions of a-plane InGaN/GaN MQWs have been investigated. The con-focal image scan revealed the localized states dominated the emission of our samples. From room temperature PL measurement, we could observe the best luminescence efficiency with 24% In composition. Moreover, the un-shift emission peak from the power-dependent PL measurement indicated the absence of QCSE within our samples. The temperature-dependent PL and PLE exhibited more apparent localized effect and larger Stoke’s shift with increasing In compositions. The polarization-dependent PL shows that the degree of polarization decreased with increasing In composition, which can be attributed to the enhancement of zero-dimensional nature of the localizing centers. The TRPL results indicated that although the localization would be stronger as increasing In composition, large number of defects would be formed to worse material quality with high Indium composition (30%). In the second part, InGaN/GaN multiple quantum wells and nanorods with embedded InGaN/GaN multiple quantum wells were prepared by self-assembled Ni clusters and inductively coupled plasma-reactive ion etching and their optical properties were strudied. From PL measurement, the emission wavelength red-shift and smaller PL peak shift from 20 to 300 K compared to as-grown sample suggested the fact of strain relaxation of nanorods structures. The larger activation energy in nanorods sample revealed that there are higher potential barriers for carriers within nanorods structures. Furthermore, the shorter lifetime from TRPL measurement at low temperature showed that nanorods sample had better radiative recombination rate. Finally, we studied the in-plane polarization effect on nanorod and as-grown samples. The degree of polarization was reduced due to the strong sidewall scattering which enhanced the PL intensity simultaneously.