1. Chang, L.L., Esaki, L., Howard, W.E., Ludeke, R. and Schul, G., J. Vac. Sci. Technol. 10, 655 (1973)
2. Petroff, P.M., Gossard, A.C., Wiegmann, W. and Savage, A., J. Crystal Growth 44, 5 (1978)
3. For a recent review see: Ploog, K., in Crystals-Growth, Properties, and Applications (Edited by H.C. Freyhardt), p. 73, Berlin (1980)
4. Holonyak, N., Kolbas, R.M., Dupuid, R.D. and Dapkus, P.D., IEEE J. of Quantum Electronics QE-16, 170 (1980)
5. Kolbas, R.M., Holonyak, N., Jr., Vojak, B.A., Hess, K., Altarelli, M., Dupuis, R.D. and Dapkus, P.D., Solid State Commun., 31, 1033 (1979)