2014 Nous avons constaté que des monocristaux de séléniure de zinc, bien qu'étant de structure cubique, présentent une faible biréfringence de type biaxial. L'étude de cette biréfringence accidentelle selon plusieurs directions cristallographiques a permis de préciser la position de l'ellipsoïde des indices dans le cristal et de déterminer l'angle des axes optiques ainsi que le plan dans lequel ils se trouvent. Abstract. 2014 In spite of their cubic structure, ZnSe single crystals show a weak biaxial birefringence. The study of light propagation along several crystallographic directions determines the orientation of the index ellipsoïd with respect to the crystal axes. Moreover the technique allows the measurement of the angle of the optic axes and the determination of the plane in which they move when the incident wavelength changes.
ZnSe single crystals grown in vapour phase technique with sphalerite structure are cut according to faces (110), (110), (001) in order to study the coefficient γ41 of the electrooptic tensor. These samples exhibit, without any electric field applied, a parasitic birefringence linked to the plane crystal defects. This ZnSe is biaxial and presents a great dispersion of index as well as a variation of the position of the optic axes between the absorption threshold and 6000 Å, but the axis [110] remains always a principal axis for the dielectric tensor.
Measurements of light absorption on ZnSe single crystals, conducted from 80 K to room temperature, show that the forbidden band gap decreases with increasing temperature because of the electron–phonon interaction. It is established for temperatures ranging from 140 to 320 K that longitudinal optic (LO) and acoustic (A) phonons operate simultaneously and exclusively so that [Formula: see text]. The first term, resulting from the Franz–Keldysh effect applied to the mean square field produced by LO phonons, provides the value of this field. It reaches 105 V cm−1 at room temperature.
In order to explain anomalous photovoltage which appears when ZnSe single crystals containing a high density of twins are irradiated, an elementary calculation is developed of the field produced by a stacking fault, normally to the fault plane. The ions are assumed to be point‐like charges and it is found that the crystal displacement from one side to the other of the fault plane gives a [111] field of about 105 V/cm in the middle of the fault.
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