Phase diagram of the TlGaSe2-CuGaSe2 system

Abstract

Matiev, Khamkhoev, Evloev. INTRODUCTION TlGaSe 2 and CuGaSe 2 belong to the III-III-VI 2 and I-III-VI 2 families of semiconductors and are of considerable practical interest as attractive materials for optoelectronic applications [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16].TlGaSe 2 , a typical incomplete-valence semiconductor, has a specific crystal structure in which Tl + is in octahedral coordination, and Ga 3+ is in tetrahedral coordination [1][2][3].CuGaSe 2 has the chalcopyrite structure, in which Cu and Ga form an ordered sublattice. The unit cell of CuGaSe 2 contains eight atoms (2Cu, 2Ga, and 4Se) and has a c / a ratio close to 2 [11,12].In this paper, we report phase relations in the TlGaSe 2 -CuGaSe 2 system over the entire composition range.EXPERIMENTAL TlGaSe 2 -CuGaSe 2 alloys were prepared from presynthesized TlGaSe 2 and CuGaSe 2 . The starting chemicals used were 3N thallium, OSCh 11-4 copper, OSCh 17-4 selenium, and 3N gallium. To remove the oxide film and other contaminations, Cu was etched in 5% HNO 3 for 8-10 min and then washed with running distilled water. Thallium was distilled in vacuum.Ampules (thick-walled silica, 25-mm inner diameter) were cleaned in 40% HF for 5 min, rinsed with distilled water, and then calcined in vacuum at 1400 K. The inner wall of the ampules was graphitized to prevent reaction with the melt. TlGaSe 2 and CuGaSe 2 were synthesized by reacting stoichiometric elemental mixtures at 1150 and 1450 K, respectively, for 5 h at a residual pressure of 10 -3 Pa in two-zone furnaces. The melt was homogenized by vigorous stirring. The TlGaSe 2 and CuGaSe 2 samples thus prepared were then equilibrated by annealing for 240 h at 1000 and 900 K, respectively.The phase purity and homogeneity of TlGaSe 2 (dark cherry) and CuGaSe 2 (dark gray) were checked by differential thermal analysis (DTA) and x-ray diffraction (XRD).TlGaSe 2 -CuGaSe 2 samples (6 g) were prepared by a similar procedure.DTA was carried out at a heating rate of Ӎ 10° C/min using a programmed temperature controller [17], N-306 XY recorder, and high-sensitivity amplifier. The temperature was measured by PR-30/6 Pt/Pt-Rh thermocouples calibrated against the melting points of Bi, Pb, Se, Te, Sb, KCl, NaCl, Na 2 SO 4 , Ag, and Cu. The accuracy in temperature measurements was ± 5°ë . The samples (1 g) were sealed under vacuum in silica Stepanov vessels 5 mm in inner diameter. As a reference substance, we used calcined alumina.Electrical resistivity was measured using Shch 31 and V7-30 multirange voltmeters with an accuracy of 0.05 and 5%, respectively. Fine-particle samples for resistivity measurements were pressed into silica capillaries 10 mm in length and 2.7 mm in diameter. The ends of the capillaries were soldered with indium, to which copper leads were attached.Phase compositions were determined by XRD on a DRON-3 powder diffractometer (Ni-filtered Cu K α radiation, 40 kV, 20 mA, continuous scan rate of 1 ° (2 θ )/min, rotating sample). RESULTS AND DISCUSSIONThe T -x phase diagram of the TlGaSe 2 -...