Amorphization of Silicon by Ion Irradiation: The Role of the Divacancy
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AMORPHIZATION OF SILICON BY ION IRRADIATION: THE ROLE OF THE DIVACANCY R.G.
ELLIMAN* J.
LINNROS** AND W.L.
BROWN
AT&T Bell Laboratories, Murray Hill, NJ 07974, U.S.A. *CSIRO Materials Science and Technology, Clayton 3168, Australia. **Institute of Microelectronics, P.O.B. I084, Kista S-16421, Sweden ABSTRACT Fixed fluence ion irradiation of silicon is shown to produce either defected crystal or amorphous silicon depending on the ion flux employed. The amorphous threshold flux, defined as the minimum flux required to generate a continuous amorphous layer for a fixed fluence irradiation, is measured as a function of irradiation temperature. This critical flux for amorphization is shown to satisfy an Arrhenius expression with a unique activation energy of -1.2eV, which corresponds to the migration/dissociation energy of the silicon divacancy. These observations lead to the conclusion that the stability of the silicon divacancy controls the competition between defect production and dynamic defect annealing, and hence the crystalline to amorphous phase transformation. INTRODUCTION Despite the fact that ion beam induced amorphization of silicon has been studied for almost two decades there is still considerable debate and discussion about details of the amorphization mechanism. Discrete amorphous zones have been observed in irradiated samples 11,21 and these are thought to result either directly from the collapse of an energetic collision cascade 131 or indirectly from the local accumulation of simple defects followed by the collapse
of the defect laden crystal
14,51.
A continuous
amorphous layer is then envisaged to result from the accumulation and eventual overlap of these amorphous zones. Since both the size of an amorphous zone and the density of simple defects are reduced by dynamical defect annealing during subsequent ion irradiation the formation of a continuous amorphous layer will depend not only on the ion fluence but also on the substrate temperature and the ion flux [3,6-91. These dynamical effects have been incorporated in models of the amorphization process. In a recent study [10,111 we were able to show that amorphous silicon layers could be induced to crystallize epitaxially or grow in thickness during ion irradiation at elevated temperatures, depending on the ion flux employed for the irradiation. This has important implications for models of the amorphization process since it predicts that amorphous zones, formed directly by ion collisions or from the collapse of the crystalline phase, can grow as well as shrink during subsequent ion irradiation. The ion flux may therefore play a more significant role than previously anticipated. In the present study it is shown that the ion flux can play a critical role in the amorphization process, determining whether or not a continuous layer is formed during a fixed fluence irradiation at a given temperature. The critical flux is also shown to have a temperature dependence which is consistent with the view that annealing of the silicon divacancy is a limitin
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