High-performance InGaN µLEDs integrated onto flexible substrates

Abstract

Flexible optoelectronic systems, from wearable biomedical devices to conformal displays, demand light sources that combine high efficiency with mechanical compliance. Indium Gallium Nitride (InGaN) microLEDs (µLEDs) are a leading candidate, yet two constraints have limited their adoption on flexible platforms: lateral scaling into the few-micrometer regime often incurs severe efficiency losses, and high-quality III-nitride epitaxy requires high-temperature growth on rigid substrates. This thesis shows that the size-dependent efficiency degradation of scaled µLEDs is largely governed by process-induced surface recombination that can be systematically suppressed. A device-centric framework is developed to decouple electrical injection, internal radiative efficiency, photon extraction, and mechanical integration, enabling the dominant loss mechanisms to be isolated and addressed in a deliberate hierarchy. Optimized ohmic contacts to n-type Gallium Nitride (n-GaN) and p-type Gallium Nitride (p-GaN) establish a low-loss electrical baseline for interpreting size effects. Sidewall-induced Shockley–Read–Hall (SRH) recombination is identified as the primary scaling-limited loss and is quantified through a practical critical diameter, critical diameter (dcrit), separating bulk- and surface-dominated regimes. Combined chemical sidewall repair and polymeric encapsulation reduce the surface recombination velocity (vs) from∼ 3×10^3 cm s^−1 to below 10 cm s^−1, with model extrapolation predicting a shift in dcrit from 43 µm to below 0.1 µm; experimentally, no size-dependent efficiency penalty was observed over the measured diameter range of 6 to 17 µm. With surface losses suppressed, geometry-driven extraction strategies become viable: perforated device architectures introduce internal sidewalls to enhance light extraction without a recombination penalty. Fully optimized devices are then transferred from sapphire onto polyethylene naphthalate substrates (PEN) via a photonic-assisted process while preserving reverse leakage below 100 fA and wall-plug efficiency (WPE) (∼ 28%). Backside potassium hydroxide (KOH) texturing of the exposed n-polar Gallium Nitride (GaN) raises the peak WPE of 5 µm devices to∼ 31%, with less than 5% variation under bending at an 11 mm radius of curvature (ROC). Overall, the results show that surface and interface physics set the practical performance limits of scaled InGaN µLEDs. The framework developed here provides guidance for engineering efficient, mechanically compliant light sources for high-resolution displays and biophotonic systems.