Determination of recombination coefficients for nanocrystalline silicon embedded in hydrogenated amorphous silicon

Abstract

The spectroscopic pump-probe reflectance method was used to investigate recombination dynamics in samples of nano crystalline silicon embedded in a matrix of hydrogenated amorphous silicon. We found that the dynamics can be described by a rate equation including linear and quadratic terms corresponding to recombination processes associated with impurities and impurityassisted Auger ionisation, respectively. We determined the values of the recombination coefficients using the initial concentrations method. We report the coefficients of 1.5×10 11 sec −1 and 1.1 × 10 −10 cm 3 sec −1 for the impurity-assisted recombination and Auger ionization, respectively. © On the other hand, it has been determined that parasitic effects such as free-carrier absorption and fast non-radiative recombination occur in nanocrystalline Si embedded in a matrix material; these effects severely limit the device efficiency [6]. It has been shown that the recombination and relaxation processes, especially at high carrier concentration, are enhanced and occur at a faster rate [7][8][9] than in bulk silicon [11, 12]. Further, the main recombination process in highly doped or injected bulk silicon, with a carrier concentration of 10 18 cm −3 or higher, was quantified and demonstrated to be of Auger type [13, 14]. It has also been demonstrated that the recombination time in silicon has a predominant quadratic dependence on the carrier concentration and proceeds with an Auger coefficient of approximately 10 −31 cm 6 s −1 [13]. For comparison, the carrier population decay in amorphous hydrogenated silicon is a more complex bimodal process consisting of fast and slow components of different origins. The former corresponds to electron-hole recombination and trapping, while the latter is related to intraband migration of the trapped charges and their recombination with thermally emitted free carriers, with corresponding rate constants of 2.3× 10 −8 and 6× 10 −9 cm 3 s −1 [15], respectively.In this work we attempted to quantify the recombination coefficients in samples comprising nanocrystalline silicon embedded in a hydrogenated amorphous silicon matrix. Specifically, we investigated the carrier population decay dynamics using femtosecond (fs) pump-probe spectroscopic reflectance measurements. The decay process dynamics for the initial photo-excited carrier concentration were investigated over a timespan of 20 ps in the range of initial concentration between 2× 10 19 and 9 × 10 19 cm −3 . We found that, in this regime and timespan, the decay is best described by a kinetic rate model involving linear and quadratic terms corresponding to the recombination of a free carrier with a deep-level traps, and band-to-band recombination simultaneous with neutral impurity ionisation (Auger ionisation), respectively. To quantify the related coefficients we analysed the carrier decay curves by use of the initial carrier concentration method which is widely used in chemistry to determine reaction rate [16, 17]. We found that the carrierimpurity recombination occ...

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