Improved light extraction and thermal management in thin-film GaAs LEDs using hemispherical ZnSe lenses

Abstract

High-index optical elements enable improved photon outcoupling from high-refractive index III–V emitters, but their feasibility on electrically driven GaAs light-emitting diodes (LEDs) has been limited by the stringent requirement of near-optical contact between the element and the LED. Here, we demonstrate a practical and reversible integration of a ZnSe hemispherical lens with a thin-film GaAs LED using a spring-preloaded tip/tilt fixture that enables alignment and in situ optical contact monitoring. Integrating the ZnSe hemisphere consistently increases the emitted optical power and external quantum efficiency by a factor of 4.5–5.5 relative to emission into air, while a thin, moderately convex layer of oil used as a reference yields an intermediate enhancement. In addition to improved extraction, the hemisphere also strongly suppresses the current-induced spectral redshift, consistent with reduced self-heating; a bandgap-only estimate indicates that the air-emitting LED operates approximately 10 K hotter than the LED coupled to the hemisphere at 10 mA. Simulations describing the effect of the residual air gap thickness on the enhancement show a rapid degradation of the extraction benefits for gap thicknesses already starting from around 20 nm. These results show that a high-index external hemisphere can simultaneously enhance light extraction and improve thermal stabilization, addressing two coupled limitations in high efficiency GaAs LEDs and thermophotonic devices.