Improved Crystal Growth of Tl6SeI4 for γ‑Ray Detection Material by Oxide Impurity Removal

• Published in: Crystal growth & design 2017-11, Vol.17 (11), p.6096-6104
• Main Authors: Lin, Wenwen, Kontsevoi, Oleg Y, Liu, Zhifu, Das, Sanjib, He, Yihui, Stoumpos, Constantinos C, McCall, Kyle M, Malliakas, Christos D, Wessels, Bruce W, Kanatzidis, Mercouri G
• Format: Article
• Language: English
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/acs.cgd.7b01208

Tl6SeI4 is a promising wide-bandgap semiconductor for room-temperature high-energy photon detection. Because of the air-sensitive Tl precursor or Tl-based binary precursors used in the synthesis, this material can contain deleterious Tl oxide impurities. These impurities lead to problems during syntheses and crystal growth including glass attack, tube rupture, and parasitic nucleation, which subsequently deteriorate detector performance. In this work, we present a facile way to chemically reduce Tl oxides and eliminate oxygen impurities in Tl6SeI4 by adding high-purity graphite powder during synthesis. The addition of carbon leads to reduction of the residual Tl oxides and formation of CO2 and CO. The resistivity and hard radiation detection performance for 122 keV γ-rays of Tl6SeI4 single crystals were significantly improved. The improvement in the crystallinity was also confirmed by a narrower near-band-edge emission band in the photoluminescence spectra. We confirmed that the reaction between Tl oxide and graphite occurs, and propose a mechanism which is highly effective in substantially reducing oxide impurities from Tl-containing precursors. First-principles density functional theory calculations reveal that the presence of interstitial oxygen atoms (Oint) leads to the formation of a deep level located near the middle of the gap, which can act as carrier traps detrimental to detector performance. The calculations also indicate that graphite addition is safe for detector performance because all carbon-induced defects have high formation energy and are not likely to appear in lattice.