The Al0.05Ga0.95As/Al0.35Ga0.65As single‐ and multiple‐quantum‐well structures with well widths less than 20 A˚ have been successfully fabricated by liquid‐phase epitaxy using low‐temperature, two‐phase, and dummy wafer methods. The transmission electron microscope cross‐section technique is applied to study the thickness variation of quantum well and the transition width at the heterojunction interface. An interesting phenomenon observed during liquid‐phase epitaxy growth is that the interface of Al0.05Ga0.95As grown on Al0.35Ga0.65As is rather flat and sharp whereas the opposite growth sequence gives rise to a wavy interface. It is believed that the flat Al0.05Ga0.95As/Al0.35Ga0.65As interface is due to a tendency to deposit the epilayer when the Al0.05Ga0.95As solution is in contact with the Al0.35Ga0.65As substrate and the wavy Al0.35Ga0.65As/ Al0.05Ga0.95As interface is due to a tendency to dissolve the substrate when the Al0.35Ga0.65As solution is in contact with the Al0.05Ga0.95As substrate. The transition width is less than 10 A˚ at the Al0.05Ga0.95As/Al0.35Ga0.65As interface but is about 30–50 A˚ at the inverted Al0.35Ga0.65As/ Al0.06Ga0.95As interface. These values are about the same as those reported in the recent literature. The peak energy of the photoluminescence spectrum reveals the quantum size effect.