不同厚度之非極性"a"平面氮化銦鎵多重量子井結構之光學特性

Abstract

本論文中，我們利用有機金屬氣相沉積磊晶技術成長不同厚度之非極性"a"平面氮化銦鎵/氮化鎵多重量子井作為研究對象。我們利用光激發螢光(PL)、螢光激發(PLE)光譜、以及時間解析的光激發螢光(TRPL)光譜技術進行樣品的光學特性分析。 在室溫PL實驗中，顯示了非極性氮化銦鎵/氮化鎵多重量子井的內建電場並不影響其位能，且其發光強度隨著量子井厚度由3奈米增加到12奈米而減弱。除此之外，變溫PL實驗顯示了一個較明顯的載子侷限效應(localization effect)存在於量子井較厚的樣品中，同時它們在螢光激發光(PLE)實驗中也顯示出較大的史托克位移(Stokes shift)，我們將這樣的現象歸因於銦含量分佈不均勻以及晶格品質缺陷所造成。成長非極性結構的另一優勢在於極化發光(polarized light emission)的價值，我們所測得的發光極化率大約在60%左右。 另外，時間解析的光激發螢光(TRPL)光譜顯示出量子井厚度差異確實導致載子輻射復合速度不同，在最薄3奈米的樣品中有最佳的載子捕陷以及放光效率，這和不同厚度樣品在成長過程中所導致的銦含量不均程度及侷限深度差異有很大關係。 最後，利用陰極激發光(CL)和共焦顯微影像，進一步證實了樣品發光的均勻性以及強度隨著量子井厚度增加而逐漸退化變差的現象。

In this thesis, a-plane InGaN/GaN multiple quantum wells with different widths from 3 to 12 nm were grown on r-plane sapphire by metal organic chemical vapor deposition for investigation. We utilized several methods including photoluminescence (PL), photoluminescence excitation (PLE), and time-resolved photoluminescence (TRPL) to investigate the optical characteristics of our samples. The PL experiments revealed that the PL emission intensity would decrease with increasing the well width of the samples from 3 to 12 nm. Meanwhile, the power dependent PL measurement indicated no apparent emission peak shift for all samples due to no built-in electric field in a-plane MQWs. Moreover, the temperature dependent PL reveals that a more apparent localization effect exist in the sample with a thicker well width which is most likely due to more serious indium fluctuations or worse crystalline quality in the MQWs. In PLE experiment, a larger Stokes shift and a broaden PLE spectrum are observed when the well width becomes wider. In-plane polarization effect of a-plane InGaN/GaN MQWs emission was also investigated, which pointed out an average polarization degree ρ from our samples is about 60%. The TRPL experiments were carried out to further compare the differences of carrier dynamics between all our samples. Low temperature TRPL indicated that the shorter radiative lifetime could be obtained in the samples with thinner well width. The decay time corresponding to different energies of PL spectra at 9 K were analyzed, which indicated that the decay time on the lower energy side of the PL peaks was longer than that on the higher energy side due to the inhomogeneously broadened localization effect in our samples. In the temperature dependent TRPL, shorter τloc and smaller dτloc/dT prove that more effective capturing of excitons due to larger Eloc and shorter radiative lifetime of localized excitons are presented in thinner well width samples. Eventually, the results of cathodoluminescence and micro-PL scanning images further verified the more uniform and stronger luminescence intensity distribution observed for the samples with thinner quantum wells, indicating that the epitaxial condition and In fluctuation during the quantum well growth shall dominate the optical characteristics of the nonpolar a-plane MQWs. These results should provide a useful guidance for fabrication of light emitting devices using a-plane InGaN/GaN MQW structures.