Micro light-emitting diodes (µLEDs), crucial for advanced displays and communication systems, face efficiency challenges due to sidewall defects. This study investigates the impact of various passivation layers, including SiO2, Al2O3, and HfO2, on AlGaInP-based 620 nm red µLEDs. We fabricated devices with two mesa sizes and demonstrated that atomic layer deposition (ALD) passivation, especially with HfO2, significantly enhances performance. HfO2 passivation shows improvement in external quantum efficiency (EQE) by up to 57.9% for a 13 × 20 µm2 device size and 43.9% for a 26 × 40 µm2 device size, compared to µLEDs with SiO2. Furthermore, it reduces surface recombination velocity (SRV) by 19.7% compared to SiO2. These findings highlight the potential of HfO2, via ALD, in advancing µLED technology by improving efficiency and performance through effective defect mitigation.