The Effect of Self-Implantation on the Interdiffusion in Amorphous Si/Ge Multilayers
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THE EFFECT OF SELF-IMPLANTATION ON THE INTERDIFFUSION IN AMORPHOUS Si/Ge MULTILAYERS 1
B. Park , F. 1) 2)
SpaepenI,
J.M.
Poate
2
and D.C.
Jacobson
2
Division of Applied Sciences, Harvard University, AT&T Bell Laboratories, Murray Hill, NJ 07974
Cambridge,
MA 02138
ABSTRACT Artificial amorphous Si/Ge multilayers of equiatomic average composition with a repeat length around 60 A have been prepared by ion beam 29 sputtering. Implantation with Si led to a decrease in the intensity of the X-ray diffraction peaks arising from the composition modulation, which could be used for an accurate measurement of the implantation-induced mixing distance. Subsequent annealing showed no difference between the interdiffusivity in an implanted and unimplanted sample.
INTRODUCTION Amorphous silicon and germanium can be produced by a variety of methods, such as vapor deposition, sputtering, ion implantation and pulsed laser quenching. The structure of these materials was studied intensively in the early 1970's, and it was found that it could be modeled best by a continuous random network of covalent bonds [E]. In an ideal network each atom has exactly four nearest neighbors, as in the perfect diamond cubic crystal; the non-periodicity of the structure is the result of slight distortions of the bond angles away from the ideal tetrahedral angle. The real amorphous material, however, contains growth- or implantation-induced structural defects, such as broken bonds, which are expected to play an important role in atomic transport. Since most of these defects are not in (metastable) thermal equilibrium, their disappearance can give rise to relaxation effects, i.e. a continuous decrease in the rate of transport. Atomic diffusion in these materials has only recently become the object of study. The main difficulty here is that the measurements must be made at temperatures that are sufficiently low to prevent crystallization, so that the rate of diffusion is often quite small. Special techniques must therefore be used to measure these small diffusion lengths. Poate et al. [2] used Rutherford backscattering spectrometry to measure the broadening of the concentration profiles of impurities (Cu, Ag, Au) implanted in preamorphized Si. They found that these impurities diffused similarly to the slower component of the diffusion in crystalline Si. The most sensitive technique for diffusion measurements, which was developed by Hilliard and coworkers [3], is based on monitoring the decay of the composition modulation in an artificially created multilayer using X-ray diffraction. Since the technique allows measurements down to 2 10-23 cm /s and is non-destructive, it is especially well-suited for amorphous materials. It has been used very successfully to characterize both the diffusion and relaxation behavior in amorphous metals [3]. Prokes and Spaepen [4] and Janot et al. [5,6] used this diffraction technique to
measure the interdiffusivity in amorphous Si/Ge multilayers, which was found to be many orders of magnitude greater than in crysta
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