Extended-Perimeter GaAs/InGaP Light-Emitting Diodes for Electroluminescent Cooling

Electroluminescent cooling (ELC) occurs when the amount of optical energy produced by electroluminescence is greater than the injected electrical energy. To satisfy the first law of thermodynamics, this difference is supplied by the absorption of thermal energy from the crystal lattice, resulting in cooling. In contrast to conventional cooling technologies, such as mechanical compressor and thermoelectric cooling, ELC could open the pathway towards efficient, compact and pollution-free refrigerators with access to cryogenic temperatures. Light-emitting diodes (LEDs) based on the III-V semiconductor GaAs are a promising candidate for demonstrating ELC. However, exceptionally high internal quantum efficiency (IQE) designs are paramount to achieve this goal. A significant loss mechanism preventing unity IQE in GaAs-based devices is non-radiative surface recombination at the perimeter sidewall. To address this issue, an unconventional LED design will be presented in which the distance from the current injection point to the device’s perimeter is extended while maintaining a constant front contact grid size. This effectively moves the perimeter beyond the lateral spread of current at the operating current density (10¹ – 10² A/cm²). In addition, a theoretical model on current spreading in extended perimeter LEDs is developed to guide the device design and aid in the interpretation of current-voltage characteristics. Upon extending the perimeter, we have observed an order of magnitude decrease in the non-radiative recombination saturation current density, which is ascribed to a reduction in perimeter recombination due to the lower perimeter-to-surface area ratio. From devices fabricated with varying sizes and perimeter extensions, the optimal design is found to be a 450×450 μm² LED with 250 μm extension from contact to perimeter. This ultimately results in a 19% relative increase in external quantum efficiency, compared to a typical tight perimeter extension of 25 μm. Therefore, with extended perimeter LEDs, we can achieve a significant decrease in perimeter recombination. This is enabled by lowering the perimeter-to-surface area ratio, while minimizing the influence on the size of the active light-emitting diode area, thereby more easily allowing for high current densities. These findings aid in the advancement of electroluminescent cooling in LEDs and could prove useful in other dedicated semiconductor devices where perimeter recombination is limiting.