Role of Lithium in the Formation of Exciton Luminescence of Zinc Oxide
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Journal of Applied Spectroscopy, Vol. 87, No. 5, November, 2020 (Russian Original Vol. 87, No. 5, September–October, 2020)
ROLE OF LITHIUM IN THE FORMATION OF EXCITON LUMINESCENCE OF ZINC OXIDE V. A. Nikitenko,* S. M. Kokin, S. G. Stoyukhin, and S. V. Mukhin
UDC 535.37:(546.47+546.34)
A detailed interpretation of exciton lines caused by the presence of a lithium impurity in ZnO crystals (T = 4.2 K) was carried out. The 368.5-nm line is caused by radiative recombination of excitons bound on neutral donor states Lii×. The 371.2-nm line is caused by the same process but occurring simultaneously with the transition of the donor to the excited state. The 368.2-nm line occurs when excitons bound on ionized Lii+ states emit. The 369.7-nm line | + | | is the emission of excitons bound on acceptor complexes of the form ( Zni+ LiZn ), ( InZn LiZn ), ( Lii+ LiZn ), etc. The × measurements also made it possible to calculate the ionization energy of shallow donors Lii as Ed = 0.033 eV. Thus, a method for exciton spectroscopy of lithium states affecting the optical and electrophysical characteristics of zinc oxide has been developed. Keywords: zinc oxide, luminescence, exciton, single crystals. Introduction. Zinc oxide has emerged as an especially promising material for fabricating semiconductor sources of UV and visible radiation, including lasers, because of the ability to produce high electron and impurity hole conductivity and the existence of a broad direct band gap (3.2–3.4 eV) and a large free-exciton binding energy (~60 meV) [1–8]. The use of this material in polariton lasers is a separate important thrust [9]. Exciton spectroscopy of ZnO crystals is the dominant technique for identifying point defects and total translational disruptions in the crystal structure and for evaluating the ability to generate various types of laser radiation [1–4, 7–12]. Previously, experimental results on sensitization of UV exciton luminescence of ZnO were reported [2]. The present work shows that lithium (Li) plays an important role in forming the excitonic properties of ZnO and presents a model for the effects of various charge states and the formation of associates of this amphoteric impurity on the exciton structure of ZnO emission spectra. The modeling was based on results for two-electron transitions involving bound excitons in ZnO–Li single crystals and an analysis of their emission spectra under various excitation conditions of the crystals (T = 4.2 K). Such studies have been carried out before [1, 4, 7, 8, 11] although they addressed mainly exciton luminescence associated with shallow donors of Group III elements. Exciton luminescence associated with impurity atoms of alkali metals in ZnO is insufficiently studied so that analyses of it are glaringly contradictory [1, 3, 4, 8, 11]. Experimental. ZnO single crystals prepared by a hydrothermal method and doped during growth with Li at concentrations 1017–1018 cm–3 and single crystals synthesized from the gas phase at the Institute of Semiconductor Physics, Siberian Branch, Russian
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