Origins of nanoscale emission inhomogeneities of high content red emitting InGaN/InGaN quantum wells

The origin of the nanoscale emission inhomogeneities of red emitting InGaN/InGaN quantum wells (QWs) grown directly on a GaN template and on an InGaN on sapphire (InGaNOS) substrate is investigated. InGaNOS is a partly relaxed InGaN pseudo-substrate fabricated by Soitec. As the latter approach provides an interesting optical internal quantum efficiency of 6.5% at 624 nm at 290 K, a deeper study, at the microstructure level, was conducted. The emission inhomogeneities on InGaNOS were highlighted by cathodoluminescence wavelength mappings where three areas were chosen: one emitting at a shorter wavelength, i.e., 588 nm, and two at a longer wavelength, i.e., 607 and 611 nm. Specimens from these zones were extracted by focused ion beam milling to perform cross-sectional characterization techniques. High-angle annular dark field scanning transmission electron microscopy images demonstrated that, while red emitting areas present homogeneous QWs, shorter wavelength areas exhibit non-uniform QWs, in terms of thickness and In composition. Complementary deformation mappings in the growth direction obtained by geometrical phase analysis show that longer emission wavelengths are originating from homogeneous QWs with an InN mole fraction evaluated at 39.0 ± 1.5%. This result demonstrates the possibility of achieving red emission with a coherent (In,Ga)N alloy when using an adapted substrate. A comparison of identical QWs grown on a GaN template is also given.