We present time-resolved luminescence measurements from n(+)n(-)n(+)-GaAs homostructures and from an AlGaAs/GaAs heterostructure using a scanning tunneling microscope for excitation, From the transient decay of the luminescence intensity, the minority carrier lifetime in the semiconductor can be determined. The longest lifetime measured is 200 ns for a GaAs homostructure e. We also demonstrate that the presented technique can be used for the study of recombination at heterointerfaces.