Pulsed Ion Beam Induced Crystallization and Amorphization of Silicon
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PULSED ION BEAM INDUCED CRYSTALLIZATION AND AMORPHIZATION OF SILICON JAN LINNROS*, W. L. BROWN AND R. G. ELLIMAN** AT&T Bell Laboratories, Murray Hill, NJ 07974 USA *Present address: Institute of Microelectronics, Box 1084 164 21 Kista, Sweden "**Presentaddress: CSIRO Materials Science and Technology, Locked Mail Bag 33, Clayton Victoria 3168, Australia ABSTRACT The ion-bombardment-induced movement of an amorphous/crystalline interface in silicon has been studied under pulsed beam conditions. Irradiations were performed with a 1.5 MeV Xe beam at temperatures of 200-300 C which induced a planar motion of the interface, either to epitaxially crystallize the amorphous material or to planar amorphize the crystalline material. It is found that at a fixed peak and average beam current the result of pulsed irradiation can vary from amorphization at low pulse repetition frequency to crystallization at high pulse repetition frequency. The frequency which characterizes this change in behavior is virtually temperature independent but strongly dependent on peak beam flux. INTRODUCTION Amorphous layers in silicon recrystallize by solid phase epitaxy at temperatures above 450 C [1,2]. At lower temperatures this process proceeds immeasurably slowly. However, ion irradiation can induce solid phase epitaxial crystallization at temperatures as low as 150 C, see Refs. 3,4 and references
therein. In this case the extent of regrowth is proportional to ion dose and the interface velocity is correspondingly proportional to ion dose rate. Unlike thermally induced regrowth, ion induced epitaxy exhibits a weak temperature dependence characterized by an activation energy of only 0.2-0.3 eV. The rate of regrowth has been shown to be proportional to the energy deposited by the ions in atomic collision processes at the amorphous/crystalline interface. These results have led to an interpretation in which the crystallization is due to the interaction of beam produced point defects with the interface. Recently, it has been found that by changing the bombardment conditions the motion of the interface can be reversed, resulting in planar, layer by layer amorphization [5,6]. The competition between crystallization and amorphization is controlled experimentally by the dose rate and temperature, as illustrated in Figure 1. These irradiation conditions can be adjusted such that a dynamical equilibrium is established between crystallization and amorphization. At this balance point the crystalline/amorphous interface remains at a fixed depth. The dose rate required for this balance is a function of temperature which can be expressed as an Arrhenius relationship with an activation energy of -1.2 eV [7]. Generally, dose rate controls the production rate of defects and temperature controls the concurrent annealing rate. The observed Arrhenius dependence may, therefore, be attributed to a specific bombardment produced defect controlling the interface motion. The derived activation energy and a scaling law for different ions [7] suggest that this defect may b
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