Thermal and low-energy ion doping of silicon(001) during molecular beam epitaxy: Dopant incorporation kinetics and mechanisms

Abstract

Thermal and accelerated-ion doping, with In and Sb, during Si(001) molecular-beam epitaxy were investigated as a function of growth temperature T$\sb{\rm s}$ (500-1050$\sp\circ$C), dopant energy E$\sb{\rm d}$ (thermal-500 eV), and Si deposition rate R$\sb{\rm Si}$ (0.18-4.7 $\mu$m h$\sp{-1}).$ Surface segregation during thermal doping led to severe profile broadening and low temperature-dependent incorporation probabilities $\sigma\sb{\rm d,th}.$ On the other hand, $\sigma\sb{\rm d}$+ for In-ions at energies E$\sb{\rm In}$+ $\ge$ 200 eV or Sb-ions accelerated by potentials V$\sb{\rm Sb}$+ $\ge$ 300 V was essentially unity up to T$\sb{\rm s}\sim850$-900$\sp\circ$C. At lower ion energies, $\sigma\sb{\rm d}$+ was temperature and energy dependent, but was still much higher than $\sigma\sb{\rm d,th}.$ Abrupt tailored depth profiles were easily obtained by controlling the ion current; layers $\delta$-doped with a 250 V Sb-ion beam were $\le$2 nm wide.

Concentration transient analysis (CTA) was developed in order to obtain segregation data from SIMS depth profiles of modulation-doped films. The surface-segregated layer, trapped in the film using programmed T$\sb{\rm s}$ changes, formed a concentration spike with an integrated area corresponding to the dopant surface coverage $\theta\sb{\rm d}.$ CTA measurements showed that Sb coevaporation led to $\theta\sb{\rm Sb}$ values as high as 0.9 ML at 675$\sp\circ$C, whereas segregation was insignificant, $\theta\sb{\rm Sb}\le4\times10\sp{-3}$ ML, in films doped with Sb-ions accelerated by 100 V. Effective Sb segregation energies $\rm \Delta G\sb{Sb},$ calculated using CTA data, were both T$\sb{\rm s}$ and R$\sb{\rm Si}$ dependent. Since the segregant supply in these experiments was at the surface, rather than in the bulk, the effective $\rm \Delta G\sb{Sb}$ values were related to segregation from near-surface sites which reach equilibrium with the surface during film growth.

The $\delta$-doping and surface segregation results were the basis for modifying the dopant incorporation model developed by our group. By accounting for film growth separately from the equations describing dopant populations in the lattice potential wells, diffusion becomes the only mechanism for changing concentration gradients. A minimum of four sites between the surface and the bulk were necessary to describe the temperature and growth-rate dependences of Sb incorporation and segregation. The energy parameters for the intermediate sites were obtained by fitting experimental Sb incorporation and segregation data.