The Competition between Ion Beam Induced Epitaxial Crystallization and Amorphization in Silicon: The Role of the Divacan
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THE COMPETITION BETWEEN ION BEAM INDUCED EPITAXIAL CRYSTALLIZATION AND AMORPHIZATION IN SILICON: THE ROLE OF THE DIVACANCY
J. LINNROS*, R. G. ELLIMAN** AND W. L. BROWN* * AT&T Bell Laboratories, Murray Hill, NJ 07974 ** CSIRO Chemical Physics, Clayton 3168, Victoria, Australia
ABSTRACT The transition from ion induced epitaxial crystallization to planar amorphization of a preexisting amorphous layer in silicon has been investigated. The conditions for dynamic equilibrium at the transition were determined for different ion species as a function of dose rate and temperature. The critical dose rate for equilibrium varies exponentially with 1/T, exhibiting an activation energy of -1.2 eV. Furthermore, for different ions, the critical dose rate is inversely proportional to the square of the linear displacement density created by individual ions. This second order defect production process and the activation energy, which is characteristic of divacancy dissociation, suggest that the accumulation of divacancies at the amorphous/crystalline interface controls the balance between crystallization and amorphization.
INTRODUCTION Amorphous layers in crystalline silicon can be epitaxially crystallized during ion irradiation at much lower temperatures (150-400°C) than those required for conventional thermal regrowth (T > 450 C) [1-9]. This ion induced crystallization is mediated by defects created at or near the interface by the incident ions. The crystallization rate is weakly temperature dependent with an activation energy of 0.2-0.3 eV [5,71 suggesting that the migration of single point defects is the limiting mechanism. Recently, a reversal of the crystallization process has been found in which the amorphous layer grows in a planar manner at the expense of the crystal [10,11]. This transition occurs at high ion dose rates and/or low substrate temperatures. The present study [12] reports on the condition for dynamical equilibrium between crystallization and amorphization in terms of a relation between dose rate and temperature for different ion species.
EXPERIMENT AND INTERPRETATION Amorphous layers were produced by 29 Si ion implantation (70 keV, 6 X 1014 ions/cm 2 at LN 2 temperature) in (100) silicon, followed by a thermal anneal at 500 C for 100 min. This procedure results in a 750 A thick amorphous surface layer with a well-defined amorphous/crystalline interface. Subsequent ion irradiations with 1.5 MeV Xe, Kr, Ar or Ne ion beams were performed with the samples mounted on a heated target block. Beam heating effects were avoided by using small beam defining apertures to reduce the total beam current. Doses were chosen to promote a few hundred A of interface movement which was subsequently measured by Rutherford backscattering and channeling of 2 MeV He ions in a glancing exit angle geometry (exit angle -6.5° ). Mat. Res. Soc. Symp. Proc. Vol. 74. ' 1987 Materials Research Society
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