Previously, Prof. JianJang Huang and his group from National Taiwan University have demonstrated high-brightness nanorod LED arrays using the chemical mechanical polishing (CMP) process. The optical output power is balanced between the effects of non-radiative recombination due to sidewall defects (which decrease light output efficiency) and mitigated QCSE (quantum confined stark effect) due to strain relaxation (which increases internal quantum efficiency). The exploration of low-temperature optical behaviours of nanorod LEDs will help identify the correlation between these two factors.

In their latest work, the researchers have studied the low-temperature EL (electroluminescent) spectra of InGaN/GaN nanorod arrays. The device exhibits a much higher optical output percentage increase when the temperature decreases, which is mainly attributed to increased carriers in the quantum wells for radiative recombination. Also, due to a better spatial overlap of electrons and holes in the quantum wells, the increased number of carriers can be more efficiently recombined in the nanorod device.

While the nanorod array shows nearly constant peak energy in the EL spectra at various injection currents at room temperature, a blue shift has been observed at 190K. The results suggest that with less non-radiative recombination and thus more carriers in the quantum wells, carrier screening and band filling still prevail in the partially strain relaxed nanorods. Moreover, when the temperature drops to 77K, the blue shift of planar devices disappears and the optical output power decreases because there are fewer carriers in the quantum wells for radiative recombination.

The observations indicate the effects of both strain relaxation and sidewall defects on the light output power of GaN nano-devices at low temperature. To follow this up, the researchers now plan to reduce the sidewall defects by decreasing the etching damage and by proper choice of passivation material.

Full details can be found in the journal Nanotechnology.