Superlattices show quantum effects

Recent advances in controlling the epitaxial growth of semiconductor hetero‐structures have made possible the observation of man‐made quantum‐size effects in such structures. By sandwiching a layer of gallium arsenide about 100 Å thick between confining layers of aluminum–gallium arsenide, one creates a potential well about 300 milli‐electron volts deep in which confined, discrete energy levels can be observed for electrons above the conduction band edge. By expanding this sandwich to hundreds of GaAs layers, with interspersed ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{As}$ layers of comparable thickness, one has created a superlattice, with an artificial periodicity one or two orders of magnitude longer than the atomic spacing, superposed on the natural periodicity of the crystal (see PHYSICS TODAY, August 1975, page 17). This results in a splitting of the conduction bands into mini‐bands corresponding to the mini‐Brillouin zones of the longer periodicity. When such a superlattice is doped with donor impurities, a pseudo‐two‐dimensional electron gas will be confined in each GaAs layer.