Effect of Thermal Annealing in Ammonia on the Properties of InGaN Nanowires with Different Indium Concentrations

The utility of an annealing procedure in ammonia ambient\nis investigated\nfor improving the optical characteristics of In<sub><i>x</i></sub>Ga<sub>1–<i>x</i></sub>N nanowires (0.07 ≤ <i>x</i> ≤ 0.42) grown on c-Al<sub>2</sub>O<sub>3</sub> using\na halide chemical vapor deposition method. Morphological studies using\nscanning electron microscopy confirm that the nanowire morphology\nis retained after annealing in ammonia at temperatures up to 800 °C.\nHowever, significant indium etching and composition inhomogeneities\nare observed for higher indium composition nanowires (<i>x</i> = 0.28, 0.42), as measured by energy-dispersive X-ray spectroscopy\nand <i>Z</i>-contrast scanning transmission electron microscopy.\nStructural analyses, using X-ray diffraction and high-resolution transmission\nelectron microscopy, indicate that this is a result of the greater\nthermal instability of higher indium composition nanowires. The effect\nof these structural changes on the optical quality of InGaN nanowires\nis examined using steady-state and time-resolved photoluminescence\nmeasurements. Annealing in ammonia enhances the integrated photoluminescence\nintensity of In<sub><i>x</i></sub>Ga<sub>1–<i>x</i></sub>N nanowires by up to a factor of 4.11 ± 0.03\n(for <i>x</i> = 0.42) by increasing the rate of radiative\nrecombination. Fitting of photoluminescence decay curves to a Kohlrausch\nstretched exponential indicates that this increase is directly related\nto a larger distribution of recombination rates from composition inhomogeneities\ncaused by annealing. The results demonstrate the role of thermal instability\non the improved optical properties of InGaN nanowires annealed in\nammonia.