Optical properties of HgS and HgS : Co 2+ crystals
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Ki-Su Yu Department of Physics, Wonkwang University, hi 570-749, Republic of Korea
Jae-Eun Kim and Hae Yong Park Department of Physics, Korea Advanced Institute of Science and Technology, Taejon 305-701, Republic of Korea
Wha-Tek Kim Solid State Physics Laboratory, Department of Physics, Chonnam National University, Kwangju 500-757, Republic of Korea (Received 7 June 1991; accepted 31 July 1991)
Melt-grown crystals of HgS and HgS: Co2+ were used to measure their optical absorption spectra. The optical energy band gaps of these crystals were 2.030 eV and 1.870 eV at room temperature, respectively. The impurity optical absorption peaks of Co2+ were observed at 4030, 5988, 12285, 12672, and 12905 cm"1. These peaks can be attributed to the electronic transitions between the split energy levels of Co2+ ion located at the Td symmetry site, where the crystal field, Racah, and spin-orbit coupling parameters were given by Dq — 403, B — All, and A = —155 cm"1, respectively.
I. INTRODUCTION Mercury sulflde (HgS) has two different crystal structures, hexagonal ( a - H g S , cinnabar) and cubic (/3-HgS, metacinnabar). Red a - H g S is stable at room temperature and transforms to black /3-HgS at 344 "C, 1 and its energy band gap is 2.10 eV at 300 K.2 Since a - H g S has relatively high photoelectric sensitivity in the visible spectrum range, it shows promise for optoelectronic applications. Even though studies on both bulk crystal3"5 and thin film6"8 samples of a - H g S have been reported to a great extent, there has been no report on the electrical and optical properties of this material when it is doped with impurities of 3d transition elements. In this paper, we report the results of the optical properties of ct-HgS : Co2+ crystal as one of a series of studies investigating the electrical and optical properties of a;-HgS doped with 3d elements. The optical energy gap for this crystal is determined from the optical absorption spectra of this compound. In addition, the impurity optical absorption peaks are explained by the origin of impurity optical absorption due to cobalt. II. EXPERIMENTAL PROCEDURE 2+
The ampoule for HgS and HgS : Co crystals was prepared by use of a binary compound of HgS (99.999%) and CoS (99.9%) which was inserted into a quartz tube and sealed under a vacuum of 10~6 Torr. This ampoule was placed into a furnace and heated at 650 °C for 350 h. And then HgS and HgS : Co2+ bulk crystals with average J. Mater. Res., Vol. 6, No. 12, Dec 1991
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dimensions of 2 x 3 x 1.5 mm were obtained using the slow cooling method (2 °C/h), for which the amount of Co2+ impurity was about 2 mole%. X-ray diffraction analysis revealed that HgS and HgS: Co2+ crystals were hexagonal in structure and their lattice parameters were a — 4.142 A and c = 9.489 A for HgS and a = 4.136 A and c = 9.476 A for HgS:Co 2 + . The optical absorption spectra of these compounds were measured at room temperature with a UV-VIS-NIR spectrophotometer (Hitachi U-3400) and the optical absorptio
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