Inorganic Materials, Vol. 40, No. 6, 2004, pp. 563–566. Translated from Neorganicheskie Materialy, Vol. 40, No. 6, 2004, pp. 656–659. Original Russian Text Copyright © 2004 by Atroshchenko, Voronkin, Galkin, Lalayants, Rybalka, Ryzhikov, Fedorov.
Effect of Tellurium Doping on the Structural Perfection of ZnSe L. V. Atroshchenko, E. F. Voronkin, S. N. Galkin, A. I. Lalayants, I. A. Rybalka, V. D. Ryzhikov, and A. G. Fedorov Institute of Single Crystals, National Academy of Sciences of Ukraine, pr. Lenina 60, Kharkov, 61001 Ukraine e-mail:
[email protected] Received June 11, 2003
Abstract—Isovalent doping with Te is shown to have an advantageous effect on the structural perfection of ZnSe crystals, causing the wurtzite–sphalerite phase transition to reach completion. The optimum Te content of ZnSe crystals is 0.3–0.6 wt %. According to x-ray diffraction results, ZnSe1 – xTex crystals doped with less than 0.3 wt % Te contain twins and stacking faults. Doping with ≥0.6 wt % Te leads to tetragonal distortions of the ZnSe lattice.
INTRODUCTION Te-doped ZnSe (ZnSe1 – xTex) crystals are used as high-efficiency scintillators for x-ray and gamma-ray detection in the range 30–200 keV and for α- and β-particle detection owing to their high energy conversion efficiency and radiation hardness and the high thermal stability of their performance characteristics. Data on the influence of Te doping on the structural perfection of ZnSe crystals are however missing, and the Te content of the melt has not yet been optimized in terms of the structural perfection of growing crystals. The crystal growth of II–VI compounds and their solid solutions presents a number of problems: the tendency toward nonstoichiometry, high melting points, segregation and chemical inhomogeneity, phase transformations, and crystallization in two polymorphs. It is well known that ZnSe may crystallize in both the wurtzite (W) and sphalerite (S) structures, depending on the preparation conditions [1]. Since most of the physicochemical properties of II–VI crystals depend on their structural perfection and defect chemistry, the study of their defect structure is of current interest. The objective of this work was to investigate the structural perfection of melt-grown ZnSe1 – xTex crystals in comparison with undoped ZnSe using x-ray diffraction (XRD) and optical microscopy. EXPERIMENTAL ZnSe1 – xTex crystals were grown in graphite crucibles by the Bridgman–Stockbarger method at an Ar pressure pÄr = 0.5 to 3 MPa in a positive-pressure vertical furnace. The melt was heated to ∆í = 30–200 K above the melting point, and the solidification rate was varied from 1 to 10 mm/h.
As the growth charges, we used powder ZnSe–ZnTe solid solutions containing 0.5 to 3 wt % Te. The Te content of the grown crystals was determined by x-ray fluorescence analysis (VRA-30 instrument) with a minimum detection limit of 0.01 wt % and relative uncertainty no higher than 5%. Local elemental compositions were determined by electron probe x-ray microanalysis on a JSM-820 scanning electron microscope equipped with a Link AN10/85S energy dispersive x-ray analysis system. The analytical accuracy was ±0.3–0.5 wt %. The highest Te content of the crystals was 7 wt %. The presence of twins and mosaic blocks was inferred from splittings of XRD peaks in reciprocalspace scans along 〈111〉. XRD patterns of 1-mm3 samples cut from different parts of crystals were taken using a double-crystal spectrometer (Cu K α1 radiation, 400 reflection from a Si monochromator). The presence of stacking faults in ZnSe1 – xTex was ascertained by powder XRD, from systematic shifts of certain reflections. To avoid distortions of diffraction line profiles, XRD patterns were taken with CuKβ radiation. Microstructural defects were examined by polarized-light and phase-contrast optical microscopy. RESULTS AND DISCUSSION It is well known that, at room temperature, meltgrown ZnSe crystals have the sphalerite structure but contain stacking faults because the wurtzite-to-sphalerite (W S) phase transition does not reach completion. In crystals grown in ultrapure graphite crucibles, the phase transition occurs at 1420°C during heating and at 1410°C during cooling [2]. All atoms in the
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Fig. 1. Twin lamellae revealed by chemical etching on the (110) surface of ZnSe and ZnSe1 – xTex (
