Study of Nitride-Based Vertical-Cavity Surface Emitting Lasers with Hybrid Mirrors
在本研究中，我們發展了具混合式反射鏡共振腔之氮化鎵光激發面射型雷射的製作方法且研究了其雷射特性。此雷射乃由一個光程約五倍波長的共振腔、一組鋁化鎵/氮化鎵之布拉格反射鏡與一組氧化物反射鏡所組成。其雷射之臨界激發能量約為2.6 mJ/cm2而其所輻射波長為456.2 nm 且半高寬僅僅0.2 nm.此外，其極化程度與發散角皆相當優異，可分別達到84%與7.6度。更特別的是，此雷射的自發輻射耦合率高達0.02，是一般邊射型雷射的1000倍之高，這代表著其效率被微共振腔大幅提高。我們還發現了多個雷射點與穩定模態情況，這情況與共振腔與增益介質之不均勻有著極大關係。
我們也進一步建立電激發面射型雷射的製程並將此製程應用於光激發面射型雷射結構上製作出一個具有非長高品質因子的微共振腔發光二極體。我們在製程中使用了高透明度的氧化錫銦做為透明電極來減少吸收。此元件之發光波長落於463.2 nm，於10微安時，其半高可達至0.52 nm，即數值約895的品質因子。這表示著已相當接近雷射操作的可能。
最後，我們更進一步發展了光激發的光子晶體面射型雷射，我們製作了不同週期 (190 nm - 300 nm)的光子晶體且皆達到了大面積雷射操作，雷射波長落於395 nm到425 nm。此種雷射具有的極化與發散角分別約為53%與小於10度。由對照粗略理論的計算，我們發現雷射乃發生於光子晶體能帶的邊緣交集處，也就是滿足布拉格條件處，且越大的光子晶體週期會對應到具較高的歸一頻率的能帶邊緣，這可充份提供設計此種雷射的線索，而此實驗成果亦充份顯示此種雷射有相當大潛力應用於高功率大面積單模雷射。|
Over past few years, ntride-based materials have been widely used in several optoelectronic devices, such as light emitting diodes, laser diodes, and photo-detectors. These devices have highly potential in the applications such as flat panel display, competing storage technologies, automobiles, general lighting, and biotechnology, and so on. Therefore, nitride wide-bandgap devices have attracted lots of attention. In this study, we have demonstrated the fabrication of the optically-pumped nitride-based vertical cavity surface emitting laser (VCSEL) with hybrid mirrors and investigated characteristics of this kind of laser. The nitride-based VCSEL was formed by a five-lamda (λ) micro-cavity sandwiched by hybrid DBR mirrors, consisting of AlN/GaN DBR and Ta2O5/SiO2 DBR. The laser action was observed under the optical pumping at room temperature with a threshold pumping energy density of about 2.6 mJ/cm2. The GaN VCSEL emits 456 nm blue wavelength with a linewidth of 0.2 nm and the laser beam shows a large degree of polarization of about 84%, a high characteristic temperature to be about 244 K, and a small divergence angle to be about 7.6o. The coupling efficiency of spontaneous emission (β) of our VCSEL was fitted to be a value as high as 0.02, which is three order of magnitude higher than that of the typical edge emitting semiconductor lasers (normally about 10-5), indicating the enhancement of the spontaneous emission into a lasing mode by the high quality factor microcavity effect in the VCSEL structure. Furthermore, we found the multiple laser spots and stable mode behaviors of the nitride-based VCSEL. These two phenomena are believed to be related with the inhomogeneous gain and cavity. We also have established the fabrication process for nitride-based VCSELs and used the process to complete a current-injected high-Q micro-cavity light emitting diode (MCLED) based on the structure of our optically-pumped VCSEL. We used high-transparency indium-tin-oxide as our transparent contact to decrease cavity absorption. The MCLED showed a very narrow linewidth of 0.52 nm equivalent to a cavity Q value of 895 at a driving current of 10 mA and a dominant emission peak wavelength at 465.3 nm. The MCLED also showed an invariant emission peak wavelength with varying current. The results in this report should be promising for developing GaN-based VCSELs. Finally, we have further developed a novel nitride-based 2-D photonic crystal surface emitting laser (PCSEL) and investigated characteristics of this laser device. The structure of this device composed of a 5-λ cavity, an AlN/GaN DBR, and a triangular-lattice photonic crystal with a diameter of 50 μm. The lattice constants (a) of photonic crystals were ranged from 190 nm to 300 nm with a fixed ratio of radius of hole and lattice constant being 0.28. All these devices show a similar threshold pumping energy densities to be about 3.5mJ/cm2. These nitride-based 2-D PCSELs emit violet wavelengths ranging from 395nm to 425nm with a linewidth of about 0.11 nm, and has a degree of polarization and a divergence angle of the laser emission to be about 53% and smaller than 10o, respectively. The laser emission was observed to occur over a large area nearly equal to the whole area of photonic crystal. We also found that normalized frequency of each laser emission from photonic crystal devices can exactly correspond to the points of Brillouin-zone boundary, Γ、M、K points. Furthermore, the device with a larger lattice constant of PC would lase at the PC band edge with a larger normalized frequency. This observation could be a direction for designing this kind of laser device. These results suggest PCSEL could have strong competitiveness for the application of high power and single mode lasers.
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