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

摘要

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: τ_0 after annealing in darkness (i.e. complete boron-oxygen defect deactivation), τ_d after illumination at room-temperature (i.e. in the completely degraded state) and τ_(0p) 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. τ_0, τ_d as well as τ_(0p), 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-Ω cm Cz-Si material after permanent recovery we measure lifetimes of τ_(0p)（Δ_n = 1.5× 10~(15)cm~(-3)） = 210 μs without applying the RTA process and up to τ_(0p)（Δn = 1.5 × 10~(15)cm-(-3)） = 2020 μs 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 nontreated 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.