1. Ploog, K., D�hler, G.H.: Adv. Phys.32, 285 (1983)
2. D�hler, G.H.: Two-dimensional systems: physics and new devices. In: Springer Series in Solid-State Sciences. In: Bauer, G., Kuchar, F., Heinrich, H. (eds.) Vol. 67, p. 270. Berlin, Heidelberg, New York: Springer; IEEE J. Quant. Electron. QE22, 1682 (1986)
3. Horikoshi, Y., Ploog, K.: Appl. Phys. A37, 47 (1985)
4. D�hler, G.H.: Superlatt. Microstruct.1, 427 (1985)
5. First experimental results are given in: Jantsch, W., Lischka, K., Eisenbeiss, A., Pichler, P., Clemens, H., Bauer, G.: App. Phys. Lett., (in press)