Growth and properties of new artificial doping superlattices in GaAs

Abstract

We present a new type of an artificial periodic semiconductor structure with tunable carrier concentrations and tunable energy gap which has been prepared by molecular beam epitaxy (MBE). The structure consists of a periodic sequence of thin (50 < d < 3000 ) n(Si)- and p(Be)-doped GaAs layers separated possibly by thin intrinsic (i-) layers called a “NIPI” structure. The space charge field of the ionised impurities varying in the direction of layer sequence produces a periodic ‘parallel’ 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 conduction and valence bands into sub-bands similar to those in the well-known compositional superlattices. In contrast to the GaAs/AlxGa1−xAs superlattice, however, GaAs doping superlattices exhibit an ‘indirect energy gap in real space’, with the electron and hole states spatially separated by half a period. Deviations from thermal equilibrium are thus quasi-stable, and excess-carrier lifetimes become very large. In this new class of semiconductor materials we demonstrate the tunability of conductivity, absorption coefficient, and luminescence by external voltage and by optical excitation.