Indirect Diffusion Mechanism of Boron Atoms in Crystalline and Amorphous Silicon
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1070-E05-01
Indirect Diffusion Mechanism of Boron Atoms in Crystalline and Amorphous Silicon Salvo Mirabella1, Davide De Salvador2,3, Enrico Napolitani2,3, Elena Bruno1, Giuliana Impellizzeri1, Gabriele Bisognin2,3, Emanuele Francesco Pecora1,4, Alberto Carnera2,3, and Francesco Priolo1,4 1 MATIS, CNR-INFM, Via Santa Sofia, 64, Catania, I-95123, Italy 2 Physics Department, University of Padova, Via F. Marzolo, 8, Padova, I-35131, Italy 3 MATIS, CNR-INFM, Via F. Marzolo, 8, Padova, I-35131, Italy 4 Physics and Astronomy Department, University of Catania, Via Santa Sofia, 64, Catania, I95123, Italy ABSTRACT The diffusion of B atoms in crystalline and amorphous Si has been experimentally investigated and modeled, evidencing the indirect mechanism of these mass transport phenomena. The migration of B occurs after interaction with self-interstitials in crystalline Si (cSi) or with dangling bonds in amorphous Si (a-Si). In the first case, an accurate experimental design and a proper modeling allowed to determine the microscopic diffusion parameters as the B-defect interaction rate, the reaction paths leading to the diffusing species and its migration length. Moreover, by changing the Fermi level position, B atoms are shown to interact preferentially with neutral or doubly positively charged self-interstitials. As far as the amorphous case is concerned, B diffusion is revealed to have a marked transient character and to depend on the B concentration itself. In particular, boron atoms can move after the interaction with dangling bonds whose density is transiently increased after ion implantation or permanently enhanced by the presence of boron atoms themselves. Unexpectedly, B diffusivity in a-Si is seen to be orders of magnitude above than in c-Si and to depend on the thermal history, i.e. the relaxation status of the amorphous phase. These data are presented and their implications discussed. INTRODUCTION Atomic diffusion in semiconductors is an intriguing, elementary process relevant for many applications in microelectronic device fabrication. Boron is the main p-type dopant in crystalline silicon (c-Si), and its migration in Si matrices represents a crucial issue for the continuous scaling-down of microelectronic devices. Today, B-doped c-Si can be prepared with unprecedented purity and accuracy, representing a proper ideal system for fundamental diffusion studies. In fact, B diffusion in c-Si has been widely investigated in the last 30 years, evidencing the indirect feature of B migration which can occur after interaction with a Si self-interstitial (I) [1-5]. A single diffusion event needs the mediation of an I which, through a kick-out reaction with substitutional B (BS), generates a BI complex (at a rate g) moving throughout the crystal for a free mean path (λ), until a kick-in reaction re-gives the immobile BS [2-5]. In such a picture, the macroscopic B diffusivity (D) comes out as D=gλ2, and if the overall migration process involves many single events (i.e., gt is well higher than 1, t being the diffusion time), a
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