Solar Energy Materials and Solar Cells, Vol.131, 51-57, 2014
Lifetimes exceeding 1 ms in 1-Omega cm boron-doped Cz-silicon
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: tau(0) after annealing in darkness (i.e. complete boron-oxygen defect deactivation), tau(d) after illumination at room-temperature (i.e. in the completely degraded state) and tau(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. tau(0), tau(d) as well as tau(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-Omega cm Cz-Si material after permanent recovery we measure lifetimes of tau(0p)(Delta n=1.5 x 10(15)cm(-3))=210 mu s without applying the RTA process and up to tau(0p)(Delta n=1.5 x 10(15)cm(-3))=2020 mu s 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. (C) 2014 Elsevier B.V. All rights reserved.
Keywords:Silicon;Boron-oxygen defect;Carrier lifetime;Permanent recovery;Rapid thermal annealing (RTA)