Influence of Size and Current Density on the Optoelectrical Properties of Green III-Nitride Micro-Light-Emitting Diodes

Abstract

High efficiency III-nitride light-emitting diodes (LEDs) have been applied to solid-state lighting. A luminous efficacy (about 150 lm/W) of InGaN/GaN-based LEDs is much higher than those of other self-emissive devices such as organic LEDs (OLEDs). As mobile electronics scale down, display technologies must also move to smaller form factors and ultra-high resolutions while maintaining high efficiencies to extend battery lifetime. III-nitride LEDs are an ideal candidate for achieving these goals. Current technologies including liquid crystal displays (LCDs) and organic light emitting diode (OLED) suffer from low efficiencies and brightness. For the realization of ultrahigh resolutions, around 10 μm or smaller light-emitting pixel sizes are required. In this regard, the InGaN/GaN μLED-based displays have emerged as a promising technology in terms of high brightness, contrast, luminous efficacies, resolution, compactness, operation under harsh conditions, and long lifetime, which cannot be satisfied by conventional LCD and OLED-based display technologies. Several groups have fabricated single-color μLED-based arrays with pixel dimensions as small as 12 μm. This self-emissive technology can be used in applications where high resolution, brightness, and efficiency are necessary such as smartphones, smartwatches, head-mounted and near-eye displays. In addition, μLEDs have exhibited improved thermal management, enhanced light extraction, and operation at higher current densities over their broad-area counterparts. Despite these benefits, surface recombination at small dimensions has been noted as a potential source for reducing device efficiencies. We have investigated the effect of chip size on the forward voltage, light output, emission image and spectral shift of InGaN based green LEDs (518 nm) as a function of current density. The size of the chips was varied from 10 to 100 μm. The smaller chips exhibited higher light output power although the lower output powers were obtained at the higher current densities. Furthermore, the smallest devices showed the better current spreading at higher current densities. It was observed that green μLEDs underwent spectral shift more pronouncedly when dc drive current increased than when chip size increased. It was observed that variation of dc drive current more pronouncedly affected the spectral shift of green μLEDs compared with change of chip size. The relation between the electroluminescence characteristics and current density of different-size devices exhibited that the devices experienced more dominantly blue shift than red shift. However, at the same current density, the peak of the devices was not shifted when the device size changed. Dependence of size and current density on the spectral shift of green μLEDs will be described and discussed in terms of band-filling and stress relaxation effects.