Spectroscopic imaging of InGaN epilayers

We review several spectroscopic imaging techniques, with progressively higher spatial resolution, applied to InGaN epilayers and quantum wells (QW). The techniques discussed are: photoluminescence (PL) mapping, confocal laser scanning spectroscopy, and cathodoluminescence imaging. PL mapping (via point-by-point PL spectroscopy) is necessary when samples show macroscopic inhomogeneity. Confocal microscopy uses a diffraction-limited laser spot to address a sample in a raster scan. Here we report, for the first time, observations, on a microscopic scale, of spectroscopic fine structure in InGaN layers of moderate indium content. Cathodoluminescence (CL) imaging provides a vast amount of information on thin semiconductor layers that are well-matched to the penetration profile of electron beams of modest energies (from ≈1 to 60 keV). Panchromatic images provide useful information when compared with corresponding structural images produced by electron scattering in the sanning electron microscope (SEM). Application of image processing techniques to such images reveal striking correspondences on a size-scale below one micron.