Compositional and doping superlattices in III-V semiconductors

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Abstract
The unusual electrical and optical properties of three important types of semiconductor superlattices are reviewed. The artificial structures which were grown by molecular beam epitaxy (MBE) consist of a periodic sequence of ultrathin crystalline layers of alternating composition (Al x Ga1-x As/GaAs or Ga x In1-x As/GaAs y Sb1-y ) or of alternating doping (n-GaAs/p-GaAs). The electronic energy bands in these superlattives are split into quasi-two-dimensional subbands whose spacing and width can be tailored by appropriate choice of the design parameters of the structure. After a brief outline of the electronic properties of compositional superlattices, we discuss the unique properties of GaAs doping superlattices in detail. The space charge induced band edge modulation in doping superlattices leads to an indirect energy gap in real space. As a consequence, the electrons and holes are spatially separated and their recombination lifetimes are strongly enhanced, i.e. non-equilibrium free-carrier distributions can be metastable. This implies the unique possibility of tuning the carrier concentration in a given superlattice sample over a wide range. Tuning the carrier concentration, in turn, is associated with a strong variation of the effective energy gap, of the absorption coefficient, and of the two-dimensional subband structure. Experimental results on GaAs doping superlattices demonstrate the tunability of electron and hole conduction by selective electrodes and by photoexcitation, the optical tunability of the absorption coefficient, and the electrical and optical tunability of the luminescence. The two-dimensional subband structure and its tunability was observed by resonant Raman scattering and by Shubnikov-de-Haas measurements.