Ellipsometric measurements of the dielectric functions of solid solutions are given for the first time for the whole composition range at room temperature (293 K) between 0.75 eV and 5.7 eV. The energy and the broadening parameters for each critical transition are given as functions of the composition. The upward bowing of the energies is compared to theoretical estimates. The spin - orbit splittings in the valence band show a negligible bowing. Alloying leads to a small broadening which is negligible for the fundamental gap.

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The dielectric function sr + is, in Hgi_~Zn~Te is found, for the first time, at 293 K. It is deduced from spectroscopic ellipsometry measurements for the entire composition range and for photon energies ranging from 0.75 eV to 5.7 eV. A detailed analysis of the chemical treatments leading to the best surfaces which correspond to the highest values of s, at the E2 peak is presented. The oxydation of the surface after the last stripping is also studied. The pseudodielectric function is analysed for the critical point model. The variation, with x of the parameters describing the main critical transitions ED, ED + ho, El, El + hi and E2 are obtained. The results are discussed in relation to the band structure and to the properties of these compounds. RAsumA. La fonction didlectrique sr+ is, des solutions solides Hgi_~Zn~Te est donn6e, pour la premiAre fois, I 293 K. Ces r6sultats sent d6duits de mesures d'ellipsom6trie spectroscopique dans tout le domaine de composition et pour des 6nergies de photon allant de 0,75 eV I 5,7 eV. Les traitements chimiques permettant d'obtenir les meilleures surfaces correspondant aux plus fortes valeurs de s, au pic E2 sont analys6s en d6tail. De mime, l'oxydation de la surface est suivie par ellipsom6trie. La fonction pseudodidlectrique est analysde avec le modAle de points critiques. Les variations des paramAtres ddfinissant les principales transitions ED, ED + ho, El, El + hi et E2 sont donndes en fonction de la composition x. Ces rdsultats sont discutds en relation avec la structure de bande et les propridtds de ces solutions solides. 1.surfaces [3]. The fundamental gap Eo, at 0 K, varies strongly from -0.3 eV for HgTe to 2.4 eV for ZnTe with a high bowing parameter of at least 0.6 eV [4,5]. This bowing originates from