| Summary: | 碩士 === 國立臺灣科技大學 === 電子工程系 === 102 === It is difficult to grow high-quality epitaxial DBRs for GaN-based vertical-cavity surface-emitting lasers (VCSELs) resulting in VCSEL materials of low gain and high loss. Therefore we replaced the general silicon-oxide/silicon-nitride insulating layers with Si diffusion-defined confinement structure to enhance the current confinement. Si diffusion in p-GaN was used to convert p-GaN into n-GaN that formed a 3D current-blocking structure. Hopefully the VCSELs with this structure will have good performance in terms of threshold current and optical output power.
The experimental results demonstrated that Si diffusion-defined confinement structure effectively confined the current path to increase the gain of the diode and the probability of stimulated emission. From the emission spectra at various current levels, we observed the full width at half maximum (FWHM) of the primary emission peak was reduced to below 2nm, and there were two or three resonant-cavity modes competing for the gain. The largest side mode suppression ratio obtained was 5. And we found almost all the primary peak wavelengths were located at the edge of the stop band of the bottom DBR.
Unfortunately we didn’t observe lasing while operating diodes up to 20mA in current. But the far-field pattern exhibited a half power angle of about 30°, which was much smaller than the 60° of the Lambertian radiation pattern observed in LEDs. Moreover, from angle-resolved emission spectra, we observed off-axial resonant modes of which the peak wavelengths blue-shifted regularly with increasing angle. These characteristics indicated these devices have strong resonant cavity effect.
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