InGaN based LEDs are known to be very efficient in the blue range. However, although InGaN can theoretically cover all visible range, quantum efficiency drops when emission wavelength emission is increased due to quantum confined Stark effect. Furthermore, indium incorporation is hindered by compressive strain induced by lattice mismatch between InGaN and GaN. To tackle the lattice mismatch problem, a full InGaN structure on a relaxed InGaN substrate is proposed.
The structure consisting of five InxGa1-xN / InyGa1-yN multi quantum wells on top of an InyGa1-yN buffer layer is grown by MOVPE on an InGaNOs substrate from Soitec company. Three InGaNOs substrates of lattice parameters of 3.190, 3.200 and 3.205 Ångströms were co-loaded in order to compare their ability to incorporate indium for the same growth conditions. For reference, a sample grown on GaN template will allow us to compare the wavelength red-shift resulting of the use of InGaNOs template.
The samples were characterized by photoluminescence at room temperature using 375 nm and 405 nm laser diodes. It is shown that long wavelengths can be reached thanks to the use of InGaNOs substrates. For same active region growth conditions as reference sample, a red-shift up to 65 nm (from 445 to 510 nm) is observed, demonstrating InGaNOs potential for easier In incorporation. Using different growth conditions, wavelengths up to 600 nm have been reached. First internal quantum efficiency measurements demonstrate a good quality material. InGaNOs seems promising for emission in the “green gap”and beyond.