| Summary: | 博士 === 國立臺灣大學 === 光電工程學研究所 === 100 === Several nanostructures including current diverting structure, nanorod sidewall reflectors, and photonic crystals are utilized to improve the performances of the GaN-based light-emitting diodes (LEDs). Solutions to some bottlenecks such as low extraction efficiency, high junction temperature and emission directionality control are addressed for the application of LEDs in general lighting and display technology, etc.
Current diverting structure is a relatively high resistivity region fabricated by ion implantation to lower the effective p-type GaN doping concentration. This structure can be regarded as a cold zone since less electrical and optical energies flow through it due to current and carrier re-distribution. The corresponding lower temperature region provides a path for heat dissipation, resulting in lower junction temperature of the LED device. The junction temperature is analyzed using forward voltage method and infrared thermal imaging system. Devices with the ion-implanted cold zone demonstrate lower junction temperature as compared with the conventional one (39°C vs. 60.6°C at 100 mA in forward voltage method). Lower junction temperature leads to less performance degradation, which is demonstrated by 35-mA higher power saturation current and 86.8% output power enhancement factor at 300 mA.
Nanorod sidewall reflectors are fabricated using nanosphere lithography (NSL) at the periphery of light-emitting mesa in order to interact with laterally propagated light. NSL includes spin-coating and dry-etching processes, which can be easily integrated into LED fabrication. Various fill factors of the nanorod arrays can be achieved by adjusting the concentration of silica nanoparticle. Output power enhancement and emission pattern are fill factor dependent, where the device with the most sparse nanorod arrangement exhibits the highest output power enhancement (30.43%) because of higher coupling efficiency. On the other hand, the device with the densest nanorod arrangement shows the most concentrated radiation profile because of more compact distribution which can redirect the laterally propagated light to be collected in the surface normal direction. Moreover, a special design where the light-emitting mesa is covered with thick metal electrode is utilized to improve the polarization ratio. The p/s ratio as high as 1.88 in the surface normal direction can be achieved for the lateral emission interacted with nanorod arrays.
Photonic crystals (PhCs) are defined by using electron-beam lithography as further research of the quasi-periodic nanorod arrays structure. First the deeply etched nanohole PhC structure is fabricated surrounding the light-emitting mesa area for higher coupling efficiency of the lower order modes inside GaN material. The output power and beam shaping capability are both dependent on the parameters of PhC structure including the pitch and the diameter of the pattern. Furthermore, we combine the nanohole PhC sidewall structure with the shallow PhC structure on the surface of the light-emitting mesa as surface texturing for the extraction of higher order modes. Output power enhancement factors of the devices with nanohole PhC sidewall reflectors, shallow PhC surface texturing, and both structures are 31.4%, 40.2% and 56.4%, respectively. The surface texturing contributes to the extraction efficiency enhancement while the directionality improvement is related to nanohole sidewall reflectors, which redirect the lateral emission for the collection in the surface normal direction.
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