Insight into the underlying mechanism behind the efficiency decrease for GaN LEDs as the chip size decreases

ZOGAN microLEDs, featuring a p-layer composed of both ZnO-based oxide and GaN-based nitride semiconductors, exhibit several unprecedented characteristics. Notably, they demonstrate significantly higher external quantum efficiency (EQE) than conventional GaN microLEDs, and maintain this efficiency regardless of chip size—a phenomenon not observed in typical GaN microLEDs. Additionally, the quantum wells (QWs) in ZOGAN LEDs are elongated under strain relaxation, which contrasts with the conventional understanding that the QWs contract upon the release of tensile stress from lattice mismatch. Our findings indicate that lattice mismatch is not the dominant factor in these effects. We propose a physical model that differs from the conventional surface-defect explanation, attributing the observed QW deformation and chip-size-independent EQE to two electric field mechanisms: electrons accumulated for the electron-hole imbalance within the QWs and surface effects from charged dangling bonds and defects at the mesa sidewalls. This model offers a coherent explanation for the unique performance of ZOGAN microLEDs and challenges prevailing assumptions in GaN LED physics.