Enabling portable light-field displays project full 3D holograms out of tiny chips built using standard semiconductor processing equipment has been the goal of optoelectronics startup, Ostendo Technologies. To deliver the right level of resolution, the individual voxels and micro-optics of such a projector chip has to be shrunk so the whole chip may fit slim smartphones.

Hence, monolithically integrated full colour LED arrays play a big role, where each individual LED can be driven to output just any colour including a full white mix. This small feat in the world of LED design was achieved by researchers at Ostendo Technologies.

LED structure

In an article, "Growth of monolithic full-colour GaN-based LED with intermediate carrier blocking layers", lead author Dr. Hussein S. El-Ghoroury who also happens to be Ostendo Technologies' Founder and CEO shares a novel tricolour InGaN-based LED design obtained through a common metal-organic chemical deposition (MOCVD) process.

His team relied on specially designed intermediate carrier blocking layers (ICBLs) to control the carrier injection distribution across the active regions of multiple quantum wells stacked one upon another, guiding the majority of carriers into the designed quantum wells so they would recombine and generate light at the QW's specific wavelengths depending on the current-densities running throughout the device.

The monolithic InGaN-based LED they designed is able to achieve three primary colours of light from one device at selected current densities, starting at 650nm and then decreasing to 460nm or lower as the injection current increases.

 
monolithic tricolour InGaN-based LED diagram (cr) Figure 1: Simplified cross-section diagram of a monolithic tricolour multi-layer InGaN-based LED structure.  

The epitaxial structures of these monolithic LEDs were grown on c-plane (0001) sapphire substrates and in the MQW active region, a variety of AlGaN-based alloy layers were incorporated to control carrier distribution as well as improving material quality. The ICBL between the blue and green wells was composed of 10nm Al0.07Ga0.93N sandwiched between 5nm GaN layers. The ICBL between the green and red wells was composed of 10nm Al0.20Ga0.80N sandwiched between 5nm GaN layers.

Colour change

 
full colour emissions monolithic LED (cr) Figure 2: Images of full colour emissions under different injection currents (a) to (f).  

Under varying injection currents the light emission changed from red (650nm) to green (530nm) and then to blue (460nm) at 15, 200 and 400mA, respectively.

 
photoluminescence emissions tricolour LED wafer diagram (cr) Figure 3: Photoluminescence emissions (red, green and blue) from a tricolour LED wafer.  

At low currents (around 5mA), the device first emits a red light and then the colour shifts to amber, yellow, green and blue as currents are increased. All colours can be combined and mixed by using different combinations of current pulse intensity and width, write the researchers in their paper and Ostendo Technologies is now busy refining such colour mixing techniques to achieve other colours, including white light, with correlated colour temperature across adjacent different pixels.

First published on EDN Europe.