Ultrafast charge recombination in undoped amorphous hydrogenated silicon

Nonradiative bimolecular recombination of photocarriers in room-temperature $a-\mathrm{S}\mathrm{i}:\mathrm{H}$ films was studied as a function of the photocarrier density and excitation energy, using ultrafast laser spectroscopy over the range of 150 fs to 4 ns. It is demonstrated that for excitation with photons whose energy exceeds the optical gap of $a-\mathrm{S}\mathrm{i}:\mathrm{H}$, 1.7 eV, the time evolution of transient absorbance (detected at 1.5–1.6 eV) depends only on the initial density $N_{\mathrm{ex}}$ of the photocarriers $(>\mathrm{}{10}^{18}{\mathrm{cm}}^{\mathrm{-}3}).\mathrm{}$ For $N_{\mathrm{ex}}\sim 5\times {10}^{18}$ to $2\times {10}^{20}{\mathrm{cm}}^{\mathrm{-}3},$ the bimolecular decay of the photocarriers is bimodal: In the first few picoseconds the decay kinetics are controlled by recombination and trapping of free carriers, whereas at later delay times the kinetics are controlled by intraband migration of the trapped charges and their recombination with thermally emitted free carriers. An analysis of the decay kinetics in the framework of the multiple-trapping model (which included mesoscopic nonuniformity of the photocarrier generation) gave the rate constants of $2.3\times {10}^{-8}$ and $6\times {10}^{-9}{\mathrm{cm}}^{\mathrm{-}3}/\mathrm{s}$ for the fast and slow components of the recombination kinetics, respectively. This bimodality accounts for a considerable spread of bimolecular rate constants found in the literature. Our results seem to be incompatible with the previously suggested models of Auger process in undoped $a-\mathrm{S}\mathrm{i}:\mathrm{H}$. This work emphasizes the need for further theoretical understanding of the recombination mechanisms involved. We argue that in the low-density regime, the direct bimolecular recombination of the excess charges can be a “rare event” that causes the Staebler-Wronsky effect.