氮化鎵面射型雷射與極激子在多模氮化鎵微共振腔內色散的光學特性研究

Abstract

由於,以氮化鎵所混合而成的半導體基材具有從0.7到6.2電子伏特的寬廣能隙和其直接能隙的特性，使其發光波長範圍能從紅外光到紫外光並且發光效率也比較高。因其以上特性，使得氮化鎵基材發光元件可廣泛應用，故其成為世界上各研發團體的重要研究課題之ㄧ。在此研究論文中，我們製作氮化鎵面射型雷射(VCSEL)，並研究分析其主要的特性。 於此，我們提出含有上下兩高反射鏡介電質之氮化鎵面射型雷射結構。並利用雷射剝離技術並沉積成長介電質反射鏡，製作出一含有氮化鎵與氮化銦鎵(GaN/InGaN)多量子井(MQW)的面射型結構。在室溫下以一雷射(波長為355 nm)為激發光源，在光激發操作下，研究其發光特性。此氮化鎵面射型雷射的Q係數(quality factor)為1000，雷射波長為 412 nm，雷射頻譜之半高寬為 0.26 nm。從實驗中，我們可以得到雷射頻譜之極化率為 79.4 %和其發散角為5度。此氮化鎵面射型雷射於室溫下之臨界條件為 784 nJ且其特徵溫度為 130 K。 我們利用 Hakki-Paoli 方法和量測不同溫度下的頻譜，並計算求得此面射型雷射的增益特性。在 80 K臨界條件時，光增益為 2.2×103 cm-1。在不同操作溫度下，我們發現溫度越低，光增益隨著注入的載子數目增加而變大的速率越快。線寬增加係數 (linewidth enhancement factor) 在室溫下為 4.3，其值隨著操作溫度下降而降低，直至溫度為 80 K時，其也降至0.6了。利用微螢光激發 (micro-PL)方式量測發現在雷射發光孔徑中有螢光強度不均勻的現象，並對其發光強度不同的位置各自量測並計算其增益的特性。發現，螢光強度較強的位置得到的增益值比在較弱位置的大，並由其增益的頻譜圖觀察到，波長從 400 nm到 420 nm的增益斜率比較大，而波長從 420 nm到 445 nm的增益斜率趨於平緩。 另外，在實驗過程中觀察到一個特別的現象，從波長 470 nm到 370 nm的範圍中，發現其模態間距 (mode spacing) 有漸漸變小的趨勢。故我們利用材料本身的折射率色散和極激子 (polariton) 的色散去對其共振模態作曲線的吻合程度，發現極激子的色散曲線比較吻合。

Due to the inherent advantages of GaN-based compound materials, such as wide band gap varying from 0.7 to 6.2eV、direct band gap characteristics…, etc. Which all make them possible to emit emission wavelength ranging from red to ultra-violet with higher illumination. Therefore, GaN-based semiconductors have recently attracted much interest owing to their applications in optoelectronics devices. In this study, the fabrication of GaN-based vertical cavity surface emitting lasers (VCSELs) is analyzed and characterized. We proposed a GaN-based VCSEL structure which consists of InGaN/GaN MQWs and two dielectric DBRs with high reflectivity. We investigated the laser emission characteristics of the GaN-based VCSEL under optically pumping operation at room temperature. The quality factor of VCSEL is 1000, indicating a good interfacial layer quality of the structure. The laser emits emission wavelength at 412 nm with a linewidth of 0.26 nm. The measurement results reveal the linewidth reduction, degree of polarization of 79.4%, and the divergent angle of 5°. The laser has a threshold pumping energy of 784 nJ at room temperature and the characteristic temperature is 130K. Meanwhile, we used Hakki-Paoli method and the measured photoluminescence spectrum to estimate the temperature dependent optical gain and linewidth enhancement factor of the VCSELs. At 80 K, the optical gain of 2.2×103 cm-1 was estimated at the threshold condition with a carrier density of 6.8×1019 cm-3 by pulse laser. We found that the gain increases more rapidly as a function of the injected carrier density at lower temperature by two difference pumping sources. The α-factor at 300 K was estimated to be 4.3 and decreased to as low as 0.6 at 80 K. Micro-PL intensity mapping indicated that there exists nonuniform PL emission intensity over the VCSEL aperture. The gain values of the highest PL intensity are larger than the ones of lower PL intensity. We obtained the sharp slope of gain spectrum from 400 nm to 420 nm, while the slope of the gain spectrum ranging from 420 nm to 445 nm is smooth. The frequency spacing between adjacent PL peaks decreases by almost a factor of five from 470 nm to 370 nm. We use the intrinsic material index dispersion and polariton dispersion to fit the experimental data, it shows that the latter fitting curve is much better than the former one.