Size-dependent optoelectrical properties of 365 nm ultraviolet light-emitting diodes

In this paper, the size-dependent optical and electrical properties of 365 nm InGaN/AlGaN ultraviolet micron-size light-emitting diodes (mu LEDs) on c-plane sapphire substrates is investigated. The series resistance of the mu LED increased from 20 Omega to 15 k Omega when the diameter of the device decreased from 150 to 3 mu m. The ideality factor increased from 4 to 4.6 over the same range of diameters due to the increase in the defect density for the smaller mu LEDs Moreover, electroluminescence characterization showed a fixed and red-spectral-shift emission for the mu LEDs with diameters smaller than 10 mu m and larger than 15 mu m, respectively. The redshift was due to band-gap narrowing in InGaN/AlGaN multi-quantum wells as a result of selfheating at higher current densities in the larger diameter mu LEDs. Due to an increase in the heat dissipation of devices with a high surface to volume ratio, the smaller diameter devices were found to have higher light extraction efficiency and no measurable emission spectrum shift.