Doping Superlattices

Abstract

Doping (“NIPI”) superlattices represent a new type of artificial periodic semiconductor structures with tunable carrier concentration and tunable energy gap. The prototype structure, recently prepared by molecular beam epitaxy (MBE), consists of a periodic sequence of thin (5<d<300 nm) n(Si)- and p(Be)-doped GaAs layers. The space charge field of the ionized impurities varying in the direction of layer sequence produces a parallel periodic modulation of the energy bands which determines the unusual electrical and optical properties of the material. The 1-dimensional periodic potential induces a splitting of the CB and VB into subbands similar to those in the compositional superlattices. In contrast to the AlAs/GaAs superlattice, GaAs doping superlattices exhibit an indirect energy gap in real space, with the electrons and holes separated by half a superlattice period. Deviations from thermal equilibrium are thus quasi-stable, and excess-carrier lifetimes become very large (in the order of 10³ sec). We demonstrate the tunability of bipolar conductivity, absorption coefficient, electro-and photoluminescene, and subband spacing by external and by optical excitation in this new class of semiconductor materials.