Imaging of InGaN inhomogeneities using visible apertureless near-field scanning optical microscope

The optical properties of epitaxially grown islands of InGaN are investigated with nanometer-scale spatial resolution using visible apertureless near-field scanning optical microscopy. Scattered light from the tip-sample system is modulated by cantilever oscillations and detected at the third harmonic of the oscillation frequency to distinguish the near-field signal from unwanted scattered background light. Scattered near-field measurements indicate that the as-grown InGaN islanded film may exhibit both inhomogeneous In composition and strain-induced changes that affect the optical signal at 633 and $532nm$⁠. Changes are observed in the optical contrast for large three-dimensional InGaN islands (hundreds of nanometers) of the same height. Near-field optical mapping of small grains on a finer scale reveals InGaN composition or strain-induced irregularities in features with heights of only $2nm$⁠, which exhibit different near-field signals at 633 and $532nm$ incident wavelengths. Optical signal contrast from topographic features as small as $30nm$ is detected.