Contributions of vacancies and self-interstitials to self-diffusion in silicon under thermal equilibrium and nonequilibrium conditions

Abstract

Since many years, the contribution of vacancies ($V$) and self-interstitials ($I$) to silicon (Si) self-diffusion is a matter of debate. Native defects and their interaction among themselves and with foreign atoms influence the processes taking place during device fabrication, starting with the growth of Si single crystals and ending with doping of nanosized electronic devices. Considering this relevance, it is remarkable that present data about the properties of native point defects in Si are still limited and controversial. This work reports experiments on self-diffusion in Si for temperatures between 650${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C and 960${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C to verify recent results of Shimizu et al. [Phys. Rev. Lett. 98, 095901 (2007)] that give rise to inconsistencies in $V$-mediated self- and dopant diffusion. Two different structures of isotopically controlled epitaxial layers of Si are used for the diffusion study. One structure consisting of 20 bilayers of ${}^{29}$Si/${}^{28}$Si was grown by molecular beam epitaxy (MBE). The other structure with a ${}^{28}$Si layer sandwiched between natural Si was grown by means of chemical vapor deposition. Self-diffusion in (${}^{29}$Si/${}^{28}$Si)${}_{20}$ multilayers (ML) was analyzed by means of secondary ion mass spectrometry (SIMS) and neutron reflectometry, whereas self-diffusion in ${}^{\mathrm{nat}}$Si/${}^{28}$Si/${}^{\mathrm{nat}}$Si sandwich (SW) structures was measured with SIMS only. Analysis of the experimental profiles reveals an enhanced self-diffusion in ML compared to SW structures. The enhanced diffusion is ascribed to the dissolution of $V$- and $I$-related defect clusters grown-in during MBE. On the other hand, self-diffusion in the SW structures accurately confirms the data of Shimizu et al. that are considered to represent data for thermal equilibrium conditions. The temperature dependence of self-diffusion is described by $V$- and $I$-mediated contributions with temperature-dependent thermodynamic properties of $V$. This interpretation can solve the inconsistency between self- and dopant diffusion in Si, but further experiments are required to verify this concept.