Ambipolar diffusion coefficient in molecular-beam-epitaxy-grown silicon layers

The noncontact transient grating technique under constant electron-hole pumping to the density 4×1019 cm−3 was used to characterize the ambipolar diffusion coefficient Da in Si:Al, Si:In, and Si:Sb molecular-beam-epitaxy-grown layers. Da was found to be almost constant at a value of ≊8 cm2/s up to an equilibrium carrier density of 2×1019 cm−3 in the layer and was independent of the doping type. At higher doping density, evidence for a sharp increase in Da was observed. For example, Da increased to a value of 20 cm2/s at a doping density of about 1020 cm−3. The Da behavior is in reasonable agreement with results of the high-density ambipolar diffusion theory of Young and van Driel and is incompatible with majority-carriers diffusion coefficients according to the formula Da = 2DnDp/(Dn + Dp). An explanation for this behavior is given.