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Measurements have been made of the complex refractive index of thin films of several of the rare earth elements in the photon energy range from 0,33 - 3.0eV, using a conventional ellipsometric method. The observations were carried out at various temperatures between 4.2 and 300ºK corresponding to the different magnetically ordered phases of the elements neodyraium, gadolinium, terbium, dysprosium, holmium, erbium and ytterbium. In the elements which show helical ordering, no evidence was found for absorption at the gaps corresponding to the superzone boundaries. Instead the results suggest that magnetic ordering of any type is seen in a general way through direct transitions between the exchange split levels of the band structure. The exchange energies estimated this way are in reasonable agreement with theoretical values. Suggested locations are given for the regions in k space corresponding to these "magnetic transitions". The Drude contribution to the optical conductivity has been obtained and the variation of σ(w)(_w=o) for the different elements is in good agreement with static conductivity measurements although the magnitude of the optical constant is in general larger by about 15%.This is presumably due to the effects of the joint anisotropy of the electron relaxation times and velocities at the Fermi surface.

The main purpose of the research reported was to study zinc selenide crystals with the object of developing red and yellow light emitting devices. Zinc sulphide and zinc sulpho-selenide mixed crystals were also studied to extend the colour range to green and blue. Most of the electroluminescent devices were of the Schottky barrier type and were prepared on chemically etched crystal surfaces. Electroluminescence (EL) was always observed when such devices were reverse biased. ‘Forward bias EL was only observed in diodes which contained a relatively thick (~200 A) semi-insulating layer under the Schottky contact. Almost all ZnSe diodes free of intentionally added luminescent centres emitted a yellow-orange band (self-activated) when biased in the reverse or forward directions. The optimum brightness (e.g. 800 Ft-L with a conversion power efficiency of 4 x 10 (^-3)) in the yellow region of the spectrum was obtained with reverse biased ZnSe: Mn diodes. The characteristic manganese emission in EL occurred at 5785 A, but was usually found to be swamped and broadened by the onset of self-activated emission which lies in the same region of the spectrum. A good red emission at 6400 A was obtained from reverse biased ZnSe: Mn, Cu, Cl diodes with a brightness of 200 Ft-L and a conversion power efficiency of 1.5 x 10 (^-3) Free exciton and pair emission in the blue have also been observed in undoped forward biased ZnSe diodes. These emissions have been studied in the temperature range from 20 - 360 K. Excitons became bound to neutral donors, or acceptors at temperatures below 65 K. The pair emission observed at low temperatures was associated with donor and acceptor levels with ionization energies around 26 meV and 122 meV respectively.