To fabricate native and monolithic full color micro-displays with a pixel pitch below 10 µm, the three primary colors should be achieved with the InGaN alloy. The prerequisite is to get an efficient red emission with thin InxGa1-xN quantum well (QW) width and an In content of 35%. However, the In content is limited to 25% when grown on GaN. A full InGaN structure combined with different types of relaxed InGaN pseudo-substrates are used to reduce the strain in the active zone. Red electroluminescence was obtained until 650 nm. Homogeneous red emitting InGaN based QWs were also demonstrated.
A full InGaN structure is designed on the partially relaxed InGaN pseudo-substrate fabricated by Soitec (InGaNOS). By combining its in-plane lattice parameter and the growth conditions of the active zone, it permits to cover the whole visible range with thin quantum well width, thus by increasing the InN mole fraction in the quantum wells. An InGaN/GaN superlattice based buffer layer and a new InGaNOS substrate with a low V shaped defect density helps to get a better crystalline quality. It leads to an internal quantum efficiency higher than 10% at 640 nm. 10 µm circular micro-light emitting diodes show a red electroluminescence with a central emission wavelength of 625 nm. An external quantum efficiency of 0.14% at 8 A/cm² at 625 nm is also demonstrated. The light extraction efficiency is estimated to be below 4%, mainly due to the emission from backside through the buried oxide and the sapphire substrate.
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