The physics and applications of n-i-p-i doping superlattices

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Abstract
The idea of fabricating “artificial” semiconductors with novel electronic properties by introducing a superlattice was put forward independently by a number of authors around 1970.1–8 Two different schemes were considered in order to introduce a periodic superlattice potential: the “compositional superlattice”, made by alternating between ultrathin layers of different semiconductor material, and the “doping superlattice” which, in the most general case, consists of a periodic sequence n- and p-doped layers with intrinsic regions in between (n-i-p-i crystals) grown C:.:m an otherwise uniform semiconductor. Although they had pro- posed both schemes in their first publication, Esaki and Tsu1 gave preference to compositional superlattices for their subsequent experimental investigations9 because of the unfavorable properties associated with dopants as free-carrier scattering centers and because of expected impurity diffusion.6 In 1972, Dohler performed the first detailed theoretical study of a doping superlattice. He investigated the electronic bandstructure and properties of an idealized n-i-p-i structure consisting of two-dimensional n- and p-doped semiconductor layers, sep- arated by intrinsic layers.7.8 The conclusion was not merely that quantum phenomena should be observable in such structures in spite of impurity scattering but that, in addition, these materials should exhibit very exotic electronic and optical properties. The most spectacular of these properties would be a tunable band gap and free carrier density and, resulting from that, tunable conductivity, luminescence spectra, absorption coefficient, and recombination lifetimes, the latter ones possibly reaching values far in excess of seconds. The term “tuning” expresses that the properties of a given sample can be varied optically or electrically within wide limits. The term “tailoring” is used for adjusting material parameters of superlattices by appropriate choice of “design parameters” (superlattice period, doping levels, etc.).