The properties of GaAs/AlGaAs quantum‐well heterostructures have been investigated using transmission electron microscopy (TEM) together with photoluminescence. Modifications to the reactor are described which allow abrupt interfaces (∼5 A˚) and strong optical emission to be obtained up to 300 K from single layers <40 A˚ thick. TEM and Raman scattering have been used to characterize heterostructures, which has resulted in an optimized gas handling system for precise control of reactant fluxes in a low‐pressure apparatus. The effects of pressure transients, gas‐flow recirculation, and residual aluminum incorporation in thin GaAs films grown over AlGaAs are presented.