Damage Accumulation and Annealing in Ion Irradiated Silicon
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DAMAGE ACCUMULATION AND ANNEALING IN ION IRRADIATED SILICON F. PRIOLO*, A. BATTAGLIA**, C. SPINELLA** and E. RIMINI* * Dipartimento di Fisica, 57 Corso Italia, 195129, Catania, Italy ** IMETEM-CNR, 57 Corso Italia, 195129, Catania, Italy
ABSTRACT The evolution of pre-existing damage structures in Si under high energy ion irradiation is discussed. Different initial morphologies are investigated: a sample partially pre-damaged with heavy ions and a sample partially pre-damaged with light ions are compared within them and with an undamaged single crystal. It is shown that ion irradiation can produce either damage accumulation, in the form of amorphous regions, or damage annealing depending on the pre-existing damage morphology, on the substrate temperature, and on the doping content in the irradiated layer. These data are discussed and interpreted on the basis of the existing models on ion induced amorphization and crystallization. INTRODUCTION The formation of an amorphous layer during ion implantation of crystalline Si represents a subject of both fundamental and technological importance which has interested materials scientists for more than twenty years [1-10]. However, in spite of the huge amount of experimental work performed in this area the mechanisms leading to the nucleation and growth of the amorphous phase are still unclear. Different models have been proposed in the past to explain amorphization. These models might be divided into two major categories postulating, respectively, homogeneous[2,3] and heterogeneous[4-6] nucleation. In the former case amorphous is thought to nucleate as a result of an interaction among defects, produced within different collision cascades, which live long enough to escape from the volume of generation and to distribute homogeneously throughout the irradiated region. In contrast, in the latter case, the amorphous phase is supposed to form heterogeneously within a single collision cascade. Structural observations on the damage produced by low-dose heavy ion implants in silicon are consistent with the formation'of an amorphous phase along a single ion track [7-10]. On the other hand studies on room temperature light and self-ion irradiation in Si support an heterogeneous nucleation model [3]. In particular it has been proposed that amorphization of Si can be envisioned as a critical-point phenomenon, in which, as soon as the amorphous phase is nucleated, the different type of defects greatly accelerate the transition [3,11]. As a matter of fact the amorphous regions act as a sink for the ion generated defects which accumulate there thus facilitating further growth of amorphization. To elucidate this effect in these last years the interaction of pre-existing damage structures with defects produced by high energy ion irradiation has been investigated. Several competing phenomena occur simultaneously. In the case of pre-existing continuous amorphous Si layers ion irradiation can result in either epitaxial recrystallization or layer-by-layer amorphization depending on ion energy, m
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