The investigation of polarization effects in the fundamental reflectivity spectra of non-cubic crystals can provide an additional information for the assignment of optical transitions (1). Related orthorhombic crystals Sb S and Sb Se are isomorphic and have layer structure, which lead to an anisotropy of electrical conductivity and edge absorption (2, 3). The energy spectrum and dipole selection rules were studied by group-theoretical methods (4).

and Sb,Te, 349 phys. stat. sol. 80) 343 (1968) Subject classification: 20.1; 13.1; 22.7 Institute of Applied Physics, Academy of Sciences of the Moldavian SSR, Kishinev Reflectivity Spectra of the Rhombohedra1 Crystals Bi,Te,, Bi,Se,, and Sb,Te, over the Range from 0.7 to 12.6 eV BY V. V. SOBOLEV, S. D. SHUTOV, Yu. V. POPOV, and S. N. SHESTATSKII The reflectivity spectra (polarisation E 1 C) of the homologous group of rhombohedra1 crystals Bi,Te,, Bi,Sg, and Sb,Te, have been investigated in the range 0.7 to 12.6 eV. The complex structure of the two reflectivity main maxima, lying in the range 1 to 6 and 6 to 11 eV, respectively, and its similarity in all three crystals have been established. The energy variation of the reflectivity peaks in dependence of the atomic composition shows the anomalous decrease of the interband gaps in Sb,Te, respective to Bi,T%. The results obtained are discussed in terms of published bismuth telluride band structure. KpncTannoB Bi,Tq, Bi,Se,, Sb,Te, B HHTepBane 0,7 no 12,5 3B B n o n~p~3 a q~u E l c.

Institute of Applied Physics, Academy of Sciences of the Moldavian SSR, Kishinev Ultraviolet Electroreflectance of Cuprous Oxide BY S. N. SHESTATSKXI and V. V. SOBOLEV During the last years the reflectivity and absorption spectra of Cu 0 have been 2 investigated with great intensity in the wide region of the fundamental absorption (1 to 5). Two communications concerning electroreflectance (ER) (6) and electroabsorption (7) of this material appeared quite recently. In our work (6) the first 3 electroreflectance study of cuprous oxide crystals etched by concentrated HNO was made in the region of blue-green and blue exciton series at T = 293 K. 0

Academy of Sciences of the Moldavian SSR, Kishinev Electroreflectance Spectriun of Cuprous Oxide Crystals S. N. SHESTATSKll and V.V. SOBOLEV Cu 0 crystals ha\e an important place in investigations of semiconductors 2 (1, 2). In the region of the long-wavelength fundamental absorption edge of this compound four exciton s e r i e s a r e known due to the doublet structure of the highest valence band and thelowestconduction hand at the centre of the Brillouin zone. According to (1, 2) at T = 77 K the most intensive exciton absorption lines occur a t A = = 6125 61 (yellow series) and at I? = 5496 61 (green series) and the most intensive

The electric field effect on the yellow exciton series of Cu20 has been investigated by Gross et al. (1, 2) and by Grun and Nikitine (3,4) and interpreted as the quadratic exciton Stark effect. The differential electroabsorption (EA) method developed in the last years facilitates quantitative investigations of this effect which are very difficult to be performed by the photographic method used in papers (1 to 4).In paper (5) a complicated EA structure in the region of the yellow exciton series of CuzO was observed at ac fields of 5.4 to 8.5 kV/cm which is due to the shift of "principal" exciton lines (2P to 5P) and to the appearance of ''new" lines (35, 3D, 45, and 5s).In the present work a wider range of modulating fields 0.3 to 22 kV/cm was used for the purpose of measuring the field dependence of EA peaks, the detection of EA structure in the region of the 28 line and the splitting of the 3P peak expected at high fields. The measurements were performed on Cu20 single crystals 20 to 220 pm thick at T = 77.3 K. It should be noted that the use of very low (0.3 kV/cm) modulating fields made it possible to detect at T = 77.3 OK all details of the EA fine structure observed recently in the paper (5) at T = 4.2. K. The automatical recording of the relative transmission change A J/J at double frequency 2f (f is the modulating frequency) was performed by a registration apparature involving a grating monochromator optical resolution about 0.5 meV and a synchronous detection unit.