| Summary: | 博士 === 國立交通大學 === 光電工程系所 === 96 === 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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