The Role of Carbon and Point Defects in Silicon
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THE ROLE OF CARBON AND POINT DEFECTS IN SILICON
U. GUsele
Department of Mechanical Engineering and Materials Science School of Engineering, Duke University, Durham, NC
27706
ABSTRACT An overview of the behavior of intrinsic point defects in silicon and their interaction with carbon is given for temperatures above about 500"C. The diffusive mechanism of carbon in silicon, which involves silicon self-interstitials, is treated in some detail and compared with the diffusion mechanism of oxygen. The solubility of interstitial carbon is estimated. Co-precipitation of carbon and self-interstitials or oxygen are dealt with in terms of simple volume considerations. It is proposed that the contradicting results on the influence of intrinsic point defect supersaturations on oxygen nucleation and precipitation may possibly be explained in the frame-work of opposite effects depending on the carbon concentration. Finally the influence of carbon on the incorporation and diffusion of gold in silicon is discussed. INTRODUCTION Besides oxygen, carbon is the most widely discussed impurity in silicon, mainly because of its influence on oxygen precipitation. The present paper will concentrate on simple concepts of phenomena which involve some interaction between carbon and intrinsic point defects such as the diffusion of carbon or the formation of carbon-point-defect agglomerates. It is partly based on a more extensive report [1] on the effects of carbon, oxygen, and intrinsic point defects in carbon-rich silicon ribbon materials used for solar cells. In the beginning a short summary of our present-day knowledge of intrinsic point defects in silicon will be given as far as it is relevant for the subject of this paper. Wherever it is appropriate, properties of oxygen in silicon will be mentioned in order to explain differences or similarities of the behavior of carbon and oxygen in silicon. For additional and complementary information on the behavior of carbon in silicon the reader is referred to the excellent overview by Kolbesen and Muhlbauer [2,3] and to the appropriate articles in these proceedings. INTRINSIC POINT DEFECTS It is now generally accepted that both self-interstitials and vacancies are present in silicon and contribute to self- and impurity diffusion under thermal equilibrium conditions as well as under conditions of surface oxidation [4-6]. The material transport of silicon in silicon under thermal equilibrium conditions is governed by the uncorrelated self-diffusion coefficient [5,71 eq eq DSD = DICI + DVCV (1)
where DI is the diffusivity of self-interstitials, denoted as I, and Cleq their thermal equilibrium concentration. DV and cveq are the corresponding quantities for vacancies, denoted as V. By careful investigations of the diffusion behavior of gold in silicon DICieq has been determined and DVCveq has been estimated to be [8,9] Mat. Res. Soc. Symp. Proc. Vol. 59. c1986 Materials Research Society
420
eq DICI = 914 exp (-4.84 eV/kT) cm 2 s-I eq DVCV = 0.6 exp (-4.03 eV/kT) cm 2 s- 1
(2) ,
(3)
where k is
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