Cross‐sectional scanning tunneling microscopy on semiconductor structures is evolving into a technique to analyze structural, chemical, and electronic properties on the atomic and nanometer scale in all spatial dimensions, in particular, in the lateral and in‐depth spatial dimensions of the structure. This technique has been used on the ultrahigh vacuum cleaved (110) plane of (001)‐grown AlGaAs/GaAs heterostructures. Measurements of (i) interface roughness, alloy fluctuations, and ordering; (ii) the variation of electronic properties over an interface as well as fluctuations within the alloy; and (iii) the distribution of individual dopant sites are reported on.