Studies of the Properties of Vacancies and Their Clusters in CoSi 2 by Positron Lifetime Spectroscopy
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STUDIES OF THE PROPERTIES OF VACANCIES AND THEIR CLUSTERS IN CoSi2 BY POSITRON LIFETIME SPECTROSCOPY Y. ITO, Y. SHIRAI, Y. YAMADA and M. YAMAGUCHI Department of Metal Science and Technology, Kyoto Univ., Kyoto, Japan
ABSTRACT Properties of thermal vacancies in CoSi2 and defects in electron irradiated and proton irradiated CoSi2 have been studied by positron lifetime spectroscopy. It has been found that thermal vacancies in CoSi 2 are easily quenched in. The effective formation enthalpy of vacancies has been estimated to be about 1.1 eV. The recovery of electron irradiated CoSi 2 occurs in two stages; the first stage around 310 K attributed to the migration of vacancies forming secondary defects and the final stage between 670 and 700 K, where the secondary defects dissolve. Since the positron lifetime at the secondary defects is smaller than that at vacancies, the secondary defects must be collapsed vacancy clusters, such as faulted dislocation loops. In the recovery of proton irradiated CoSi2 , microvoids, which are annealed out above 820 K, as well as collapsed vacancy clusters are also formed. The three dimensional vacancy clustering is probably due to the existence of implanted hydrogen atoms. INTRODUCTION Transition metal silicides have recently received much attention in Si device applications 1 Among them, COSi2 can be used as Shottkey barriers, ohmic contacts and gate electrodes because of its low electrical resistivity and high temperature stability. On the other hand, CoSi 2 is expected to be used as new high temperature structural materials because of its low density and excellent oxidation resistance 2-4. However, properties of point defects in CoSi2, which affect the electrical and mechanical properties of the material, have scarecely been studied. We have studied vacancies and their clusters in CoSi2 by positron lifetime spectroscopy. Positrons are sensitively trapped by vacancy-type defects and positron lifetime depends on defect species 5. In this paper, we report the recovery of electron irradiated and proton irradiated CoSi2 and estimate the effective formation enthalpy of vacancies in CoSi2. EXPERIMENTAL PROCEDURE A CoSi2 rod was produced by melting pure cobalt (99.9 %) and silicon (99.9999 %) in a plasma arc-furnace. The CoSi2 rod was remelted in an ASGAL optical floating zone furnace to grow a single crystal. Specimens of 9 mm x 12 mm x 0.8 mm were cut from the single crystal. The specimens were fully annealed for 6 hours at 1273 K in evacuated quartz capsules and chemically polished. Quenched-in defects were introduced by water- or air-cooling from high temperatures above 610 K. After each cooling positron lifetime measurements were performed at 100 K in a cryostat. Electron irradiation was carried out using a Van de Graff type accelerator at the Department of Nuclear Engineering, Kyoto University. Specimens were mounted in a cryostat and irradiated with 1.0 MeV electrons below 250 K to a dose of 2.5 x 1018 e-/cm 2 . Proton irradiation was carried out with 2.0 MeV protons below 100 K to a dos
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