Damage Formation in Semiconductors During Mev Ion Implantation
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DAMAGE FORMATION IN SEMICONDUCTORS DURING MeV ION IMPLANTATION t T. P. SJOREEN, 0. W. HOLLAND, M. K. EL-GHOR, AND C. W. WHITE Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 ABSTRACT Damage produced by 1.0-2.5 MeV self-ion and O-ion implantation into Si and Ge single crystals has been characterized by cross-sectional electron microscopy and ion channeling. In Si, it is observed that the damage morphology varies substantially along
the ion's track. Near the end-of-range of the ion, damage accumulation is very similar to that which occurs during medium- to low-energy implantation (i.e., damage increases
monotonically with dose until the lattice is made completely amorphous). In front of this end-of-range region, however, damage saturates at a very low level for moderate implantation fluences. A model based on homogeneous damage nucleation in Si is discussed. For Ge, damage accumulation is very different; a monotonic increase of damage with dose is observed over the entire range of the ion. Possible mechanisms responsible for the differences between Si and Ge are discussed. INTRODUCTION It is of interest to study the mechanisms responsible for the nucleation and growth of damage in semiconductors during ion irradiation. During irradiation, collisions between the ion and the atoms of the solid as the ion penetrates the lattice lead to a transfer of energy to the lattice atoms. These collisions lead to energetic knock-ons (i.e., atoms that are displaced from lattice sites) which can then produce their own sequence of displacement collisions. The collection of the scattering events initiated by the ions, as well as the energetic knock-ons, is referred to as the collision cascade of the ion. The role of the collision cascade in damage nucleation and growth remains controversial. One concept is that of heterogeneous nucleation in which stable damage is produced directly by a collision cascade. Early on, Brinkman ascribed a special importance to the cascade in damage nucleation. 1 He put forth the idea of a displacement cascade in which dislocations are nucleated near the end-of-range (EOR) of the ion as a result of spatial separation of interstitials and vacancies in the cascade. Another heterogeneous nucleation concept is that of the thermal spike.- In this model, the agitation of the lattice atoms within the volume of the collision cascade is considered to be represented by a local rise in the lattice temperature. In very dense cascades, where the energy deposited per unit path length is high, the temperature rise is thought to exceed the melting point of the solid. Such localized melting and rapid solidification has been used to account for the crystalline-to-amorphous transition observed in Si during heavy-ion irradiation.3 However, there are many aspects of damage nucleation and growth which do not lend themselves to this interpretation of the collision cascade's role. Damage growth in Si during ion irradiation has been shown to increase with dose rate for a constant fluence of self-ions. 4
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