We have systematically investigated the impact of device size scaling on the light output, spectral shift, and self-heating of 400 nm InGaN light-emitting diodes (LEDs). Devices with diameters in the range 20–300 μm have been studied. It is shown that smaller LED pixels can deliver higher power densities (despite the lower absolute output powers) and sustain higher current densities. Investigations of the electroluminescence characteristics of differently sized pixels against current density reveal that the spectral shift is dominated by blueshift at the low current density level and then by redshift at the high current density level, owing to the competition between the bandgap shrinkage caused by self-heating and band-filling effects. The redshift of the emission wavelength with increasing current density is much faster and larger for the bigger pixels, suggesting that the self-heating effect is also size dependent. This is further confirmed by the junction-temperature rise measured by the established spectral shift method. It is shown that the junction-temperature rise in smaller pixels is slower, which in turn explains why the smaller redshift of the emission wavelength with current density is present in smaller pixels. The measured size-dependent junction temperature is in reasonable agreement with finite element method simulation results.

The mechanisms of size-dependent efficiency and efficiency droop of blue InGaN micro-pixel light emitting diodes (μLEDs) have been investigated experimentally and by simulation. Electrical characterisation confirms the improvement of current spreading for smaller μLEDs, which enables the achievement of the higher efficiency at high injection current densities. Owing to the higher ratio of sidewall perimeter to mesa area of smaller μLEDs, a lower efficiency was observed at a low injection current density, resulting from defect-related Shockley-Read-Hall non-radiative recombination. We demonstrate that such sidewall etch defects can be partially recovered by increased thermal annealing time, consequently improving the efficiency at low current densities.

Studying neuronal processes such as synaptic summation, dendritic physiology and neural network dynamics requires complex spatiotemporal control over neuronal activities. The recent development of neural photosensitization tools, such as channelrhodopsin-2 (ChR2), offers new opportunities for non-invasive, flexible and cell-specific neuronal stimulation. Previously, complex spatiotemporal control of photosensitized neurons has been limited by the lack of appropriate optical devices which can provide 2D stimulation with sufficient irradiance. Here we present a simple and powerful solution that is based on an array of high-power micro light-emitting diodes (micro-LEDs) that can generate arbitrary optical excitation patterns on a neuronal sample with micrometre and millisecond resolution. We first describe the design and fabrication of the system and characterize its capabilities. We then demonstrate its capacity to elicit precise electrophysiological responses in cultured and slice neurons expressing ChR2.

The high-frequency modulation of individual pixels in III-nitride-based micro-pixel light-emitting diode arrays, where each array consists of 16 × 16 individually addressable 72-μm-diameter pixels, are reported. The devices investigated have peak emission wavelengths at 370, 405, and 450 nm, respectively. The optical -3-dB modulation bandwidth of a typical pixel from the 450-nm-emitting device was found to be approximately 245 MHz. Data transmission at rates of up to 1 Gb/s is demonstrated from a single pixel emitting at 450 nm, using on-off keying nonreturn-to-zero modulation, with a bit-error ratio of less than 1 × 10^-10. Such devices have potential for free-space or fiber-coupled visible light communications.

Using a GaN-based light emitting diode (LED) epitaxial structure grown on Si, individually addressable 10 × 10 micro-pixelated LED (μLED) arrays with pixel diameters of 45 μm and peak emission at ∼470 nm have been demonstrated. The electrical and optical properties of these μLEDs were compared with those of broad-area LEDs fabricated from the same epistructure. The μLEDs can sustain a much higher current density, up to 6.6 kA/cm2, before thermal rollover. Also, the fabricated μLEDs show good pixel-to-pixel uniformity, which demonstrates potential for low-cost micro-displays. Furthermore, these μLEDs demonstrate a high electrical-to-optical modulation bandwidth of up to ∼270 MHz and are suitable for visible light communication at data transmission rate up to 400 Mbit/s. The electrical-to-optical modulation bandwidth of the μLEDs increases rapidly with injection currents less than ∼6 mA, temporarily saturates at injection currents of ∼6 to ∼35 mA, and gradually increases again with injection currents up to 110 mA. Carrier density dependent recombination processes are responsible for the bandwidth increase at low current, the resistance-capacitance product determines the modulation bandwidth in the saturation region, and self-heating, which changes series resistance of μLEDs, may cause a further bandwidth increase at high current.