| Near-field scanning optical microscopy (NSOM) has pushed the limits of optical resolution by two orders of magnitude when compared to classical optical microscopy. This impressive development has been motivated by a continuously increasing need for characterization of nanometer-size, mesoscopic structures in various fields of science and technology. In NSOM, the spatial resolution is not limited by diffraction, making it possible to resolve details as small as 8 A. NSOM also offers other advantages over non-optical microscopic techniques due to its ability to employ all the wealth of contrast mechanisms available in optical methods. We have developed an NSOM instrument that operates in the super-resolution range of 50--150 nm and apply it to investigate InGaN quantum wells (QW) and epitaxial layers---active media in nitride-based short- wavelength light emitting devices.;With the main direction of the nitride research aimed at realization of a blue laser source containing InGaN QWs as a gain medium, investigation of optical and structural properties of InGaN QWs is important in the effort to advance the laser performance. Nitride semiconductor heterostructures still have large densities of extended defects due to fundamental difficulties associated with the epitaxial growth of these materials. Also, InxGa 1-xN ternaries exhibit significant spatial inhomogeneity of indium concentrations (indium clustering) that can result in deep localization of electronic states, raising the question about the role of such localized states in the formation of optical gain in InGaN QW lasers. We address both the defect issue and the issue of localized electronic states by applying NSOM to investigate optical processes in nitride semiconductor heterostructures with high spatial resolution. The implications of the obtained results are discussed in terms of their applicability for optimization of InGaN QW blue lasers. |
