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

摘要

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 tradeoff between improved morphological quality and compositional homogeneity at low-T_G (～450-550℃) versus improved threading dislocation densities at high-T_G (～600-630℃), 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℃), while it rises to ～70meV for high-T_G grown films (T_G = 630℃) 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.