Ab-initio Calculations for Indium Diffusion in Silicon

Abstract

As CMOS devices are scaled down to nanometer regime, it is even more stringent to control the impurity profiles at the front-end process. Especially, ion implantation for ultra shallow junction in nano-CMOS technology requires a new alternative material considering low diffusivity coefficient, activation energy and implant statistics. Recently, indium has been employed in the fabrication of retrograde p-tub and halo region for nchannel in CMOS. In this work, we investigated the atomic-scale characterizations of indium diffusion in silicon by ab-initio calculations. We performed VASP (Vienna Abinitio Simulation Package) calculations for defect structures in a cubic super-cell, containing 216 silicon atoms with a single indium atom. The energy landscape, provided by VASP for Si: In, indicates that the lowest-energy structure (Ins + Sii Td ) consists of indium sitting on a substitutional site stabilizing a silicon self-interstitial in a nearby tetrahedral position. And the second lowest-energy structure is Ini Td , the interstitial indium in the tetrahedral position (Fig. 1). The lowest-energy configuration of indium is equal to the structure of the boron lowest-energy (B s + Sii Td ). However, the second lowest energy configuration of boron has a different structure (Bi Hx ) from the structure of indium. Though boron has the same number of valence electrons to indium, atomic radius of indium is bigger than that of boron. Therefore, it can be expected that the diffusion path of indium may differ from the diffusion path of boron. Ab-initio study in this work enabled us to quantum-mechanically perform electronic structure relaxation and get its total energy at the local minimum. We could figure out the atomistic configuration during indium diffusion in silicon through our ab-initio calculations, which consists of steps of searching for saddle points from a minimum and reaction pathway between two stable states using by TST (Transition State Theory) [1, 2]. After finding the transition states, the energy barrier for the diffusing particle was obtained through calculating the exact total energy at the transition state.