Thermal Droop Effects in AlGaN Ultraviolet-C Light-Emitting Diodes

Al-rich AlGaN based light-emitting diodes (LEDs) operating in the ultraviolet-C (UV-C) spectral wavelength (<280 nm) are important for various critical applications. However, to date, the efficiency of UV-C LEDs remains significantly low in these challenging wavelengths. Here, we have demonstrated that, unlike several established reasons, thermal effects (including self-heating and thermal droop) could cause severe efficiency degradation in UV-C LEDs. Infrared thermal imaging was utilized to measure the accurate internal temperature of epitaxially grown and fabricated UV-C LED heterostructure on nanopatterned sapphire substrate (NPSS). The temperature-dependent measurements show reduced light output and efficiency when operating at increasing ambient temperatures up to 85 °C. We have compared UV-C LED chips of different sizes to correlate these findings with the device area. For a 250 × 500 μm2 device, the peak light output power (LOP) was ∼29.4 mW at 340.5 A/cm2, while the peak LOP for a 500 × 500 μm2 device was ∼45 mW at 270.3 A/cm2. Though the maximum achievable efficiency was significantly limited by thermal droop, the external quantum efficiency in the smaller device was measured to be 0.8% higher because of improved light extraction efficiency. Thermal imaging-based experiments and heat diffusion equation-based numerical analysis indicate that smaller devices can sustain higher temperatures until they reach thermal droop.