Artificial Semiconductor Superlattices
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ARTIFICIAL SEMICONDUCTOR SUPERLATTICES G.H. Dohler Max-Planck-Institut f•r Festk6rperforschung, 7000 Stuttgart 80, F.R.G.
Heisenbergstrasse
I,
Artificial superlattices are multilayer structures consisting of a onedimensional sequence of ultrathin layers of different semiconductors (compositional or hetero junction superlattices) or layers of the same material, modulated by periodic n- and p-doping (doping or p-n junction superlattices). The electronic properties of these new materials differ qualitatively from those of their components and can be tailored within wide limits by appropriate choice of the design parameters (i.e. superlattice period, alloy composition, and/or doping concentration). Different electronic phases (insulating, semiconducting, semimetallic, metallic, e.g.) may be obtained in this way. The doping superlattices, in particular, exhibit a unique feature. Their electronic properties cannot only be tailored but, in addition, they are tunable over a wide range in a given specimen. This unusual property is a consequence of a spatial separation between electrons and holes, which makes large deviations from thermal equilibrium metastable. Thus, transformations between different electronic phases such as from conducting to nonconducting or from absorbing to amplifying can be induced by weak external optical or electrical signals. During the past ten years compositional superlattices and, more recently, doping superlattices have been grown, mostly by molecular beam epitaxy (MBE) [I,2] , but also my metal-organic chemical vapor deposition (MOCVD) [3] and by liquid-phase epitaxy [4] (LPE). The electronic properties have been investigated extensively. The experiments have confirmed most of the theoretical predictions and have proven semiconductor superlattices as an ideal model substance for the demonstration of basic quantum mechanics and for the study of many-body and local field effects, and, in particular of two-dimensional quantum phenomena. Apart from that they also represent a very promising basic material for novel electronic and optoelectronic devices. A number of reviews on the subject of artificial semiconductor superlatrices have been prepared recently. For readers not specialized in the field of semiconductor physics we refer to Ref. 5. References 6 and 7 provide reviews of compositional and doping superlattices. Ref. 8 is a review paper emphasizing the electronic metastability of doping superlattices. References 9 and IO are recent general reviews on doping superlattices. Finally, two books, covering the subject of artificial superlattices in much detail will be published very soon [11,12]. i. 2. 3. 4. 5. 6. 7. 8. 9.
A.Y. Cho and J.R. Arthur, Progr. in Solid-State Chem. IO, 157 (1975) K. Ploog, in: Crystals, Growth, Properties and Applications, Vol. III (Springer, Heidelberg, 1980), p. 75 N. Holonyak, R.M. Kolbas, R.D. Dupuis, and P.D. Dapkus, IEEE J. of Quantum Electronics QE-16, 170 (1980) P. Zwicknagel, J. of Appl. Physics, in press G.H. D•hler, Scientific American, to be published Oct
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