| Summary: | 碩士 === 國立交通大學 === 光電工程系所 === 97 === 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.
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