Epitaxial Growth of α‐Mercuric Iodide

Abstract

Mercuric iodide has been grown for the first time heteroepitaxially on a substrate. a-HgIz is deposited as a single crystalline layer which is azimuthally completely oriented. The orientation of the grown layer has been confirmed with texture X-ray diffractometry, reflection powder diffractometry and Atomic Force Microscopy.Quecksilberiodid wurde das erste Ma1 heteroepitaktisch auf einem Substrat gezuchtet. Die Abscheidung des a-HgIz erfolgt als einkristalline, azimutal vollstandig orientierte Schicht. Die Orientierung der gewachsenen Schicht wurde mit Hilfe der Texturrontgendiffraktometrie, der Reflexionspulverdiffraktometrie und der Kraftmikroskopie bestimmt.Mercuric iodide crystals are used for room temperature y-and X-ray detectors (JAMES et al.). Difficulties in the manufacturing of such detectors are caused by the high mechanical sensitivity of the crystals. Cutting, sawing and polishing introduces damage to the crystalline structure of the material ( VAN SCYOC et al.; KELLER et al.) and generates trapping levels which deteriorate the detector quality.One of our mercuric iodide studies is to investigate the possibility of epitaxial growth of a-HgI2 layers on foreign substrates. Oriented mercuric iodide films would enable manufacturing of larger and cheaper X-ray detectors in desired size and orientation.To the best of our knowledge, there have been no reports up to now on the epitaxial growth of this compound. However, several papers exist dealing with the oriented growth of P-HgI2 crystallites (MONIER; NEUHAUS), a yellow orthorhombic modification stable at temperature above 127 "C. Mercuric iodide is a paradigm of the Ostwald step rule and for that reason even at temperatures below 127°C yellow crystals can be formed (KLEBER et al. ; NEWKIRK). Unfortunately, neither the b-phase nor the polycrystalline a-phase produced in the / 3 -+ a transition can be used for applications.a-HgI2 crystallizes in the tetragonal space group P42/ nmc with lattice constants a = 0,4361 nm and c = 1,2450 nm (JEFREY, VLASSE). The structure is build up by I-Hg-I sheets and the chemical bond in the HgId-tetrahedra is mainly covalent with small ionic contributions (30%) (KOJIMA et al. ;PIECHOTKA, KALDIS 1986). Between the adjacent sheets, only weak van-der-Waals forces exist. It can be, therefore, expected that the oriented growth will be initiated at the iodine planes.A main goal was the search for a suitable substrate fitting to the tetragonal symmetry of the closed packed iodine planes perpendicular to the c-axis of a-HgIz. Different possible substrates, selected by crystallochemical criteria, are listed in Table 1.