We have used cross-sectional atomic force microscopy in combination with dopant-selective etching to study structural details in InGaAsP/InP-based laser diodes. The different etch rates of p-type, n-type, and semi-insulating InP allowed us to relate topographic features in atomic force images of etched lasers to changes in the electrical characteristics of the semiconducting material. This allowed us to follow, e.g., the interdiffusion of Fe and Zn across the interface between p-InP:Zn and semi-insulating InP:Fe. A nonlinearity in the light-output power response suggests that this interdiffusion has given rise to leakage currents in the structure. In addition, electron-beam induced-current images imply a widening of the depletion layer at this interface. Effects due to Zn diffusion across the interface between p-InP:Zn and n-lnP:Si could also be observed. Unexpected traces of an unavoidable contamination at surfaces exposed to the atmosphere before regrowth was found in both atomic force microscopy images and cross-sectional transmission electron microscopy images of the same laser type. The results illustrate how the atomic force microscopy in combination with selective etching can serve as a simple and convenient tool for detailed structural studies of InGaAsP/InP-based lasers.