1. W.C. Roentgen: Ann. Phys. 45, 737 (1914)
2. W. Ackerman: Ann. Phys. 46, 197 (1916)
3. H.P. Beerman: Am. Ceram. Soc. Bull. 46, 737 (1967)
4. E.H. Putley: In Semiconductors and Semimetals, Vol. 5, ed. by R.K. Williardson, A.C. Beer (Academic, New York 1970) p. 259
5. H. Kawai: Jpn. J. Appl. Phys. 8, 975 (1969)