Effect of fluorine implantation dose on boron transient enhanced diffusion and boron thermal diffusion in Si/sub 1-x/Ge/sub x/

Abstract

This paper studies how boron transient enhanced diffusion (TED) and boron thermal diffusion in Si/sub 1-x/Ge/sub x/ are influenced by a high-energy fluorine implant at a dose in the range 5 /spl times/ 10/sup 14/ cm/sup -2/ to 1 /spl times/ 10/sup 16/ cm/sup -2/. Secondary ion mass spectroscopy (SIMS) profiles of boron marker layers are presented for different fluorine doses and compared with fluorine SIMS profiles and transmission electron microscopy (TEM) micrographs to establish the conditions under which boron diffusion is suppressed. The SIMS profiles show that boron thermal diffusion is reduced above a critical F/sup +/ dose of 7 - 9 /spl times/ 10/sup 14/ cm/sup -2/, whereas boron TED is suppressed at all doses. Fitting of the measured boron profiles gives suppressions of boron TED diffusion coefficients by factors of 6.8, 10.6, and 12.9 and of boron thermal diffusion coefficient by factors of 1.9, 2.5, and 3.5 for F/sup +/ implantation doses of 9 /spl times/ 10/sup 14/, 1.4 /spl times/ 10/sup 15/, and 2.3 /spl times/ 10/sup 15/ cm/sup -2/ respectively. The reduction of boron thermal diffusion above the critical fluorine dose correlates with the appearance of a shallow fluorine peak on the SIMS profile in the vicinity of the boron marker layer, which is attributed to vacancy-fluorine clusters. This reduction of boron thermal diffusion is explained by the effect of the clusters in suppressing the interstitial concentration in the Si/sub 1-x/Ge/sub x/ layer. The suppression of boron TED correlates with a deep fluorine peak around the range of the fluorine implant and TEM micrographs show that this peak is due to a band of dislocation loops. This suppression of boron TED is explained by the retention of interstitials in the dislocation loops, which suppresses their backflow to the surface. The fluorine SIMS profiles show that the fluorine concentration in the Si/sub 1-x/Ge/sub x/ layer increases with increasing germanium concentration and that the fluorine concentration in the Si/sub 1-x/Ge/sub x/ layer after anneal is much higher than after implant. This indicates that fluorine is transported into the Si/sub 1-x/Ge/sub x/ layer from the adjacent silicon, and is explained by the lower formation energy for vacancies in Ge than in Si. This accumulation of fluorine in the Si/sub 1-x/Ge/sub x/ layer during anneal is advantageous for devices like SiGe heterojunction bipolar transistors, where the boron must be kept within the Si/sub 1-x/Ge/sub x/ layer.