My first publication (1952) dealt with the interpretation of the double reversal of the sign of the Hall effect in crystalline Te. With Epstein and Lark-Horovitz, I measured the changes of the resistivity and the Hall effect of Te at the melting point and in the liquid state in 1957. We discovered that molten Te is still a semiconductor because its chain structure remains intact up to 80 deg. C above the melting point. Meeting Stan Ovshinsky in 1963 started more intense and continuing electrical, optical and calorimetric studies of chalcogenide alloy films. We developed with M. H. Cohen the first band model (CFO model) for understanding the mobility gap and mobility edges and the basic optoelectronic properties known at that time. The switching and memory phenomena discovered by Stan were of course foremost on the research agenda, we studied the radiation hardness of the devices and tried to understand the physics of switching. I derived a general expression for the thermopower valid in semiconducting glasses as well as in crystals. With my student H.-Y. Wei, I studied photovoltaic effects and space charges at metal-chalcogenide contacts. My student Marc Kastner pointed out the special role played by the non-bonding lone-pair p-electrons of Group V elements in chalcogenides. The lone-pair band becomes the valence band. This manifests itself in the negative sign of the pressure coefficient of the optical gap of lone-pair semiconductors,i.e., chalcogenide glasses. By reducing the gap of amorphous As2Te3 with pressure we (with N. Sakai)made the material metallic and observed superconductivity near 3 K in the amorphous state. M. Kastner and D. Adler and myself developed the valence alternation model to explain the negative correlation energy of the native defects in chalcogenides and the effect of doping in chalcogenide and pnictide glasses. The negative correlation energy explained the surprising absence of paramagnetic centers in these materials discovered by my student S. Agarwal. My student P. J. Gaczi studied the anisotropy of the g-factor of photo-induced paramagnetic centers in sulphide glasses and showed that the centers are indeed valence alternation centers. With my students R. E. Johanson and A. Vomvas I showed that the photoconductivity per absorbed photons of amorphous and vitreous semiconductors has an universal behaviour at low temperatures because it is due to energy-loss hopping following the theory of B. I. Shklovskii, H. Fritzsche and S. D. Baranovskii. Beginning 1992 I began explaining the origin of the reversible and irreversible photostructural changes in chalcogenide glasses as well as the photo-induced optical anisotropies. My theory predicts that even unpolarized light induces optical anisotropies. Subsequent experiments verified this prediction. The theory also explained light-induced diffusion, photopolymerization and giant densification, as well as photo-induced fluidity. One can also understand why some additives inhibit photostructural changes. These studies are summarized in Chapter 10 in "Insulating and Semiconducting Glasses" edited by P. Boolchand (World Scientific Publ. 2000). In my Ovshinsky Award Lecture I tried to answer the question why chalcogenides are ideal materials for Ovshinsky's Ovonic threshold and memory devices and presented an improved theory of switching.

Prof. Hellmut Fritzsche




Prof. Hellmut Fritzsche

and

Prof. Mihai Popescu, from the National Institute of Materials Physics, Bucharest, Romania

Prof. Radu Grigorovici received the Stanford R. Ovshinsky Prize for Lifetime Achievements in Amorphous Materials

Back to top