氮化鎵微共振腔發光元件研究

Abstract

過去幾十年來，由於氮化鎵材料具有直接能隙與强鍵結力等優點，因此吸引許多學界與業界的目光並且廣泛的製作成各種光電元件，如發光二極體、雷射二極體與光偵測器等，並且應用於日常生活中的照明、光儲存、平面顯示及生物科技中。本論文旨在設計並製作具混合式布拉格反射鏡和電流侷限氮化鋁之電激發氮化鎵微型共振腔發光元件。 我們分別利用240nm厚度的ITO製作出在77K和利用30nmsputter所沈積的ITO在300K下，連續波電流操作氮化鎵面射型雷射，並且同時亦有在室溫操作下的使用雙介電材質的布拉格反射鏡之面射型雷射，而這些雷射元件能有如此重大的改善是因為使用了厚度約為50 nm的透明導電層和提昇磊晶材料品質的氮化鎵基板。然而為了要達到這種雙介電材質的面射型雷射需要相當複雜的技術，其中包含了雷射剝離、化學機械式研磨以及基板接合等技術。 製作室溫操作氮化鎵面射型雷射，側向光學侷限以及透明導電層將會是面射型雷射製作的重點。因為目前在氮化鎵面射型雷射當中仍有較高的光學損耗，這會增加元件效能的控制難度。本論文將針對透明導電層，即是氧化銦錫，進行討論，其中包含了氧化銦錫的設計，製作以及特性量測。另外為了得到更好的側向光學侷限效果，我們也製作利用嵌入氧化鋁之微共振腔發光元件，此氧化鋁在此結構當中不僅具有光學侷限效果，同時亦達到電流侷限效果。也因為加入此氧化鋁層，可以將原本微共振腔中光路徑上的透明導電層移除，而同時電流亦能夠有效的均勻注入，也降低了微共振腔中的光損耗。從頻譜量測結果得知，製作嵌入式氧化鋁之微共振腔，具有相當窄的線寬0.52 nm，其發光波長為440 nm，因此換算為共振腔品質因子為846，並且量測到的共振腔模態間距為0.7 nm，這和理論計算數值相同，這些結果都顯示嵌入式氧化鋁的確有側向光學侷限之效果。另外其發光波長幾乎不會隨著波長變化而變化，這將具有應用於溫度感測元件上的潛力。未來若能再進而改善底部布拉格反射鏡的磊晶品質將能實現低閥值操作電流的氮化鎵面射型雷射以及氮化鎵極化子雷射。

For several decades, GaN-based material has attracted much attention of academia and industry and widely used in several optoelectronic devices due to its wide direct bandgap and strong binding energy, such as light emitting diodes, laser diodes, and photon-detectors which can be applied in lighting, optical storage, display, and biotechnology. The thesis is focus on the design and fabrication of the electrically pumped GaN-based microcavity light emitting devices with hybrid distributed Bragg reflectors and an AlN current blocking layer. We have recently demonstrated the continuous wave (cw) current injection of GaN-based VCSEL with hybrid mirrors at 77K with 240 nm ITO and 300 K with 30 nm sputtered ITO. Meanwhile, the room temperature operation of GaN-based VCSEL devices was reported using optical cavities sandwiched by double dielectric DBRs. The major improvements of these devices to achieve room temperature operation are by using a thinner transparent conducting layer of about 50 nm to improve the current spreading and by using the GaN substrate to ensure the good crystal quality of active layers. However, to form VCSELs with double dielectric DBRs required complex fabrication process, such as laser lift-off or elaborated polishing and bonding process. Despite of demonstration of room temperature current injected GaN-based VCSELs, the lateral optical confinement and the transparent conductive ITO film was still a lack in these VCSEL structures, resulting in higher optical loss and difficulty in controlling the quality of output beams. About ITO characteristics improvement, the study consists of the design, fabrication, and characteristics of the ITO films. In order to improve the optical confinement of these devices, we investigated a microcavity light emitting device (MCLED) with a buried AlN current aperture, which can also be used as a lateral optical confinement layer. Since a pre-defined current aperture with a small diameter is introduced in the MCLED structure, the transparent current spreading layer can be omitted from the optical path. The current still can be injected effectively in the current aperture and the optical loss introducing by the transparent current spreading layer could be neglected. The emission from the MCLED with a buried AlN layer shows a very narrow linewidth of 0.52 nm, corresponding to a cavity Q-value of 846, and a dominant emission peak wavelength at 440 nm. The measured cavity mode spacing is approximately 0.7 nm, which is consistent with the estimated value, demonstrating the effect of lateral optical confinement provided by the AlN layer. In addition, the emission peak wavelength as a function of the current is almost invariant with an increasing injection current indicating potential temperature-sensitive applications. Further optimization of bottom DBR growth and crystal quality in this structure would promise to realize low threshold GaN-based VCSELs or GaN-based polariton lasers.