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Near-field scanning photoluminescence microscopy of InGaN/GaN quantum structures

Nanoluminescence spectroscopy and imaging techniques are becoming popular to investigate optical properties of semiconductor nanomaterials. Conventional micro-photoluminescence (PL) techniques are affected by diffraction phenomenon, which limits the lateral resolution to approximately 0.6 μm and thus, they cannot provide information of luminescence features with dimension below the classical diffraction limit. This limitation can be overcome by near-field scanning optical microscopy (NSOM) where it is possible to achieve spatial resolution of the order of 50 - 100 nm. InGaN based material has attracted great interest since it plays a key role in the group III-Nitride optoelectronic devices, such as high-brightness blue/green light emitting diodes and laser diodes. In order to retrieve information on the spatial inhomogeneities of the emission patterns in InGaN based materials, we have carried out NSOM-PL measurements on InGaN/GaN multi-quantum wells (MQWs) and InGaN quantum dots (QDs) grown by metal organic chemical vapor deposition (MOCVD). The near-field PL intensity from these samples is found to be spatially inhomogeneous on a sub-micron scale. In the NSOM-PL intensity images, bright island-like features are observed. After deconvolution with the spatial resolution of the NSOM, the size of these features is estimated to be in the range of 100 to 200 nm. The spatially resolved improved optical emission from these InGaN/GaN quantum structures is associated with strain-induced clusters formed at the interface of the multi-layers.