Microcharacterization of electroluminescent diodes with the scanning electron microscope (SEM)

A complete material characterization of electroluminescent diodes necessarily requires a high spatial resolution because of the micron-sized thickness of the different epitaxial layers. A modified arrangement of a scanning electron microscope (SEM) has proven to be an extremely useful tool for obtaining information on the various parameters of each of these layers. It will be shown that the analysis of the electron beam induced voltages (EBIV) and currents (EBIC) allows not only the location of the junction itself, but also the detection of all other built-in barriers. Measurements of the EBIC permit separation of barriers less than 1 µm apart; even barriers in a direction oposite to that of the junction are detected in this manner. The peak value of EBIC is largely independent of the barrier height, but it is sensitive to variations in the concentration of the components of the lattice. Because of the exponential decrease of EBIC with the distance between primary electron beam and barrier, the minority carrier diffusion length can be obtained for all different barriers. The height of barriers, 2.5 µm separated and with the same direction as the p-n junction, can be determined quantitatively by the maximum of EBIV. By measurement of the cathodoluminescence (CL) signal regions of different recombination probabilities for radiative transitions (and therefore different luminescence yields) can be distinguished. Thus inhomogeneities of the structure can be detected. By spectral CL analysis the spatial variation of the width of the bandgap will be shown.