Trade-off between morphology, extended defects, and compositional fluctuation induced carrier localization in high In-content InGaN films

We elucidate the role of growth parameters (III/N flux ratio, temperature T_G) on the morphological and structural properties, as well as compositional homogeneity and carrier localization effects of high In-content (x(In)>0.75) In-polar InGaN films grown by plasma-assisted molecular beam epitaxy (PAMBE). Variations in III/N flux ratio evidence that higher excess of In yields higher threading dislocation densities as well as larger compositional inhomogeneity as measured by x-ray diffraction. Most interestingly, by variation of growth temperature T_G we find a significant trade-off between improved morphological quality and compositional homogeneity at low-T_G (∼450-550°C) versus improved threading dislocation densities at high-T_G (∼600-630°C), as exemplified for InGaN films with x(In)=0.9. The enhanced compositional homogeneity mediated by low-T_G growth is confirmed by systematic temperature-dependent photoluminescence (PL) spectroscopy data, such as lower PL peakwidths, >5× higher PL efficiency (less temperature-induced quenching) and a distinctly different temperature-dependent S-shape behavior of the PL peak energy. From these, we find that the carrier localization energy is as low as ∼20meV for low-T_G grown films (T_G=550°C), while it rises to ∼70meV for high-T_G grown films (T_G= 630°C) right below the onset of In-N dissociation. These findings point out that for the kinetically limited metal-rich PAMBE growth of high In-content InGaN a III/N flux ratio of ∼1 and low-to-intermediate T_G are required to realize optically more efficient materials.