A Comparison of the Diffusivity of As and Ge in Si at high Donor Concentrations
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A COMPARISON OF THE DIFFUSIVITY OF As and Ge in AT HIGH DONOR CONCENTRATIONS
Si
K. KYLLESBECH LARSEN, P. GAIDUK, and A. NYLANDSTED LARSEN Institute of Physics, University of Aarhus, DK-8000, Aarhus C, Denmark ABSTRACT The effect of high background doping (8x109 - 5x10 cm- 3) on the diffusion of Ge and As in Si has been studied. A strong enhancement is found for As for donor concentrations higher than -2xlO2° cm- , but not for Ge. These experimental findings are discussed within the percolation model. INTRODUCTION It has been previously demonstrated by us that the diffusion of ion implanted Sn, Sb, and As in Si single crystals show an anomalous behaviour at high donor concentrations guring rapid thermal annealing at temperatures of 1000 - 10502• [1-5). For donor concentrations higher than approximately 2x10 cm the effective diffusion coefficient D is found to be proportional to (n/n ) . where n and n are thsfsctual and intrinsic carrier concentrations at the dIffusion temperature, respectively, and where S has a value between -3 and -5 depending on the diffusing specie, i.e., significantly higher than the value of one expected for diffusion via singly charged negative vacancies or of two expected for diffusion via doubly charged negative vacancies. Moreover, in the case of Sb where M~ssbauer spectroscopy has been possible, the diffusivity enhancement has been found to correlate with the appearance of a new defect complex-Fontaining Sb: For the highest donor concentration (-5 x 10 2 cm ) about 70% of the implanted Sb was found in these complexes. The diffusion enhancement observed for Sb has been modelled within the percolation model by Mathiot and Pfister [6], and by Antoncik [7] using a system of reaction-diffusion equations in which the reaction constants depend on temperature and donor background concentration. The purpose of this paper is to compare the diffusivities of As and Ge at high donor concentrations and to try to model the results within the percolation model. A comparison between As and Ge is interesting because they have approximately the same covalent radii (1.18 A and 1.22 A) and the same fractional interstitial component of diffusion at high temperatures (0.3 and 0.3-0.4, respectively)[8,9] but are differently charged as substitutional impurities. EXPERIMENTAL The silicon samples were p-type (boron doped) 150 Qcm, floatzone refined monocrystals. The background donor concentration was formed by implantation of P to gifferent doses followed by rapid thermal annealing (RTA) at 1050 C. In this way flat carrier density profiles extending to depths of more 0 than3 3000 A with carrier concentrations between 9x10 and 5x10 cm could be obtained [3]. Arsenic or germanium were subsequently implanted at an energy of 80 keV to doses of either 4x10 or Mat. Res. Soc. Symp. Proc. Vol. 163.
1990 Materials Research Society
602
6x10"cm2 . Annealing and diffusion were then achieved by RTA in a duration of an argon ambient at a temperature of 10500 C for the temperature rise time of about 5s). Carrier 15s (includi
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