In certain problems of electrical transport through condensed matter, it is important to know the potential distribution with nanometer resolution, e.g., at interfaces (Schottky barriers) or pn junctions. Scanning tunneling potentiometry, a new application of scanning tunneling microscopy, is capable of providing this information. The tunnel current is used for simultaneously sensing probe-to-sample distance and local potential. We have used this technique for the localization of semiconductor junctions in cleaved MBE-grown AlGaAs-GaAs laser structures. Among other applications, the electron injection into the active GaAs layer of a heterostructure laser diode was readily observed. © 1986.