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Optimization of nanowire array structure to enhance light extraction efficiency


By
Fatimah Alowa

July 26, 2017 - Posted in Discussion
Light extraction efficiency of conventional UV LED nanowires (NWs) remains very low, often valued below 10%, which greatly limits the performance of the LED and its external quantum efficacy. This can be attributed to many reasons, most important of which are the polarization properties of ultraviolet light. Unlike in visible light LEDs, the polarization of UV lights— emitted from high Al concentration AlGaN active regions—is largely transverse magnetic (TM), meaning that the electric field is parallel to the growth direction of the nanowires. This effect prohibits effect light extraction from the top surfaces of the NWs.

By optimizing the parameters of the LED nanowire array, such as the radius and spacing between nanowires, the LED structure can act like a photonic crystal; although in this case, the targeted wavelength is located outside the photonic ban-gap region. In other words, the structure acts as to redirect the trapped light into radiated modes by overcoming the effects of total internal reflection. Djavid et al. reported the following figure showing light extraction efficiency vs nanowire spacing and diameter.

Their results show that an optimum value of nanowire spacing and size does exist. The data shows that nanowires with smaller diameter cannot support coupled modes, so increasing the size of the NWs positively affect the light extraction efficiency. The diameter value, however, should not exceed a certain value such that the light will get trapped inside the individual nanowires. Likewise, the spacing between the nanowires has to be chosen to prohibit the effects of light trapping (when the spacing is very small), but also takes into account the reduction of the active region volume with larger spacing. Theoretically, light extraction efficiency values, as high as 72%, can be realized by optimizing the nanowire array structure alone.


References:

Djavid, Mehrdad, and Zetian Mi. "Enhancing the light extraction efficiency of AlGaN deep ultraviolet light emitting diodes by using nanowire structures." Applied Physics Letters 108.5 (2016): 051102.


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