Enhanced microLED efficiency via strategic pGaN contact geometries

Abstract

Micro light-emitting diode (microLED) structures were modeled and validated with fabricated devices to investigate p-type GaN (pGaN) contact size dependence on power output efficiency. Two schemes were investigated: a constant 10 μm diameter pGaN contact and varying microLED sizes and a constant 10 μm diameter microLED with varying contact sizes. Modeled devices show a 17% improvement in output power by increasing the microLED die size. Fabricated devices followed the same trend with a 70% improvement in power output. Modeled microLED devices of a constant size and varying inner contact sizes show optimized power output at different current densities for various contact sizes. In particular, lower current densities show optimized output for smaller pGaN contacts and trend towards larger contacts for higher current densities in a balance between undesirable efficiency losses at high-current injection and preventing surface recombination losses. We show that for all device geometries, it is preferential to shrink the pGaN contact to maximize efficiency by suppressing surface recombination losses and further improvements should be carefully considered to optimize efficiency for a desired operational brightness.

Fig. 1.

(a) Material, thickness, and doping concentration of the LED epitaxial stack grown on dimple-patterned sapphire. (b) Schematic cross-section of the fabricated devices. (c) Devices examined had a constant 10 $\mu$m pGaN contact diameter and variable pixel diameter ranging from 10 to 20 $\mu$m. (d) Inversely from c, devices examined had a constant 10 $\mu$m microLED diameter and a pGaN contact diameter ranging from 4 to 10 $\mu$m.

Fig. 2.

Modeled device results for a constant 10$\mu$m pGaN contact diameter and varying microLED pixel diameters as depicted in Fig. 1(c). The grey arrows point in the trend of larger microLED sizes. (a) Output power versus current density. (b) IQE versus current density. (c) Surface recombination current versus current density. (d) Ratio of surface recombination current to total operational current versus current density.

Fig. 3.

Measured results for fabricated microLEDs with a 10 $\mu$m pGaN contact diameter and varying microLED pixel diameters as depicted in Fig. 1(c). (a) A cross-sectional schematic of the device and measurement setup. Light was measured from the bottom of the microLED through the sapphire substrate. (b) Spectral profile and microscopy image of a microLED device-under-test. (c) Microscopy image depicting a fabricated device. (d) Current density versus voltage curves. (e) Measured output power versus current density. (f) Qualitative EQE measurements versus current density.

Fig. 4.

Modeled device results for microLEDs with a 10 $\mu$m diameter and varying pGaN contact diameters as depicted in Fig. 1(d). (a) IQE versus current density. (b) Surface recombination current versus current density. (c) Ratio of surface recombination current to total operational current versus current density. (d) Normalized power output versus pGaN contact diameter plotted for several current densities.

Fig. 5.

Power density graphics for microLEDs with a 10 $\mu$m diameter. (a) Each row corresponds to the labeled pGaN contact diameter and each column corresponds to a different operational current density where the area is calculated from the total microLED area independent of contact size. (b) Plots on the bottom represent cross-sectional line probes of power density across the center of the microLED for the column’s operational current density. All pGaN contact diameters are plotted in each graph.