Band Structure and Optical Properties of Tetrahedrally Coordinated Cu‐ and Ag‐Halides

Abstract

With the exception of the edge excitions, which have been investigated since more than 40 years [5], there has been for about three decades only very limited experimental information about the valence states of the Cu-and Ag-halides. It was only very recently that photoemission experiments [6 to 121 allowed to get definite conclusions about the position of the d-levels, and to derive approximate partial p and d densities of states curves in the valence band region IS].At this state of knowledge it seems to be useful to review in the present paper all experimental and theoretical investigations of the valence and conduction band states in CuC1, CuBr, CuI, and AgI and to check the many single information on their consistency within one unifying interpretation. I n many cases details, concerning mainly the energtic position of structures, have been determined from the published spectra for this purpose. We show that for the first time a semiempirical band structure model can be given, which interprets all available data within the limits of their experimental or methodical errors.After a short discussion of the crystallization properties of the compounds studied (Section 1.2), the relevant symmetry properties and model band structures are presented in Section 2.1. Section 2.2 gives a survey of the available numerical band structure calculations. The experimental investigations of the edge excitonic states and their theoretical interpretation are discussed in Section 2.3. The more deeper lying valence and the more higher lying conduction band states are treated in Section 2.4. The semiempirical band model, obtained from combination of photoelectron data with results of X-ray emission, X-ray absorption, and ultraviolet absorption experiments is presented in Section 2.5.Not included systematically in the present article are investigations of more complex properties as, e.g. nonlinear optical susceptibilities, piezooptical and electrooptical coefficients, biexciton production and annihilation [ 131, and Raman scattering. Papers from these fields have been considered only in very few cases where they yielded additional information on the band structures discussed here. Crystal structures and phase transitionsThe elements of group IV, bound purely covalent, crystallize in the diamond structure (space group 0;). The binary 111-V and 11-VI compounds, which mostly have the zincblende lattice (Ti) or the wurtzite lattice (C&) under normal conditions, exhibit increasing values of the Phillips ionicity ft [la] with increasing distance within the periodic table and the I-VII compounds are found a t the strongly ionic end of the series of tetrahedrally coordinated crystals. Values of f i for the Cu-and Ag-halides are as follows: CuI (ft = 0.692), CuBr (0.735), CuCl (0.746), CuF (0.766), AgI (0.770), AgBr (0.850), AgCl (0.856), and AgF (0.894). Thus, whereas the Cu-halides and AgI still form the zincblende or the wurtzite lattice, in AgBr, AgC1, and AgF the ionicity is already so large that the rocksalt structure (0:) is favoured ...

Room-temperature ultraviolet luminescence from γ-CuCl grown on near lattice-matched silicon

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