On the Optical Properties of Defect-Chalcopyrite Single Crystals: The Aluminium Compounds HgAl₂Se₄, CdAl₂Se₄ and ZnAl₂Se₄.

Using both infrared and Raman spectroscopy all of the optical phonon modes of the materials under investigation have been observed and assigned to the irreducible representation of the point group of the crystals. The infrared reflectivity spectra of HgAl₂Se₄ as well as of oriented CdAl₂Se₄ single crystals show five well separated optical phonon modes for both directions of polarization, behaviour which is predicted by group theory for A^IIB^III₂C^VI₄ compounds crystallizing in space group I . ZnAl₂Se₄ has been found to crystallize in space group I 2m with a statistical distribution of the vacancies and half the aluminium atoms at the sites 4d. In the infrared reflectivity spectra of various ZnAl₂Se₄ samples in comparison with HgAl₂Se₄ and CdAl₂Se₄, the low frequency mode of B symmetry is missing. The optical properties in the band gap region have been determined using polarized transmittance and reflectivity measurements and diffuse reflectivity measurements. The estimated band gap energies at room temperature are 2.20 eV, 3.05 eV and 3.30 eV for HgAl₂Se₄, CdAl₂Se₄ and ZnAl₂Se₄, respectively.