Lifetimes exceeding 1 ms in 1-Ω cm boron-doped Cz-silicon

Abstract

We perform carrier lifetime investigations on oxygen-rich boron-doped Czochralski-grown silicon (Cz-Si) wafers. As a characteristic feature of oxygen-rich boron-doped silicon materials, their lifetime is generally limited by boron–oxygen-related defects, intensifying their recombination-active properties under illumination or, more generally speaking, minority-carrier injection. In this study, we examine the following characteristic lifetime values of boron-doped Cz-Si: τ0 after annealing in darkness (i.e. complete boron–oxygen defect deactivation), τd after illumination at room-temperature (i.e. in the completely degraded state) and τ0p after illumination at elevated temperature (i.e. after ‘permanent recovery’). We show that the permanent recovery process can be strongly influenced by a rapid thermal annealing (RTA) step performed in a conventional belt-firing furnace in advance of the permanent recovery process. We show that all measured lifetimes, i.e. τ0, τd as well as τ0p, are strongly influenced by the RTA process. We observe a strong increase of the lifetime after permanent recovery, depending critically on the RTA process parameters. On 1-Ωcm Cz-Si material after permanent recovery we measure lifetimes of τ0p(Δn=1.5×1015cm−3)=210µs without applying the RTA process and up to τ0p(Δn=1.5×1015cm−3)=2020µs using optimized RTA conditions. Apart from the very high lifetimes achieved, the applied RTA process step also strongly influences the kinetics of the permanent recovery process. The recovery process is accelerated by almost two orders of magnitude, compared to a non-treated sample, which proves the industrial relevance of the process. We discuss the results within a recently proposed defect model which ascribes the observed dependence of the kinetics of the recovery process to the presence of boron nano-precipitates and their interaction with free interstitial boron atoms.