Numerical simulations of two-photon absorption time-resolved photoluminescence to extract the bulk lifetime of semiconductors under varying surface recombination velocities

Abstract

We investigate the limitations of two-photon absorption time-resolved photoluminescence to measure the low-injection bulk lifetime of different semiconductor materials under varying surface recombination. The excitation source is assumed to be a sub-bandgap pulsed laser and the localized absorption and carrier generation was modeled using a focused TEM00 Gaussian beam under the assumption of diffraction-limited performance. The subsequent carrier kinetics were simulated by applying the finite-difference time-domain method to the continuity equation. Three typical semiconductor materials were modeled: direct bandgap low-mobility material (such as CZTS), direct bandgap high mobility (such as GaAs), and indirect bandgap high mobility (such as float-zone silicon). The extracted effective lifetime as a function of surface recombination velocity was compared to the bulk lifetime and the effective lifetime calculated using an analytical 1D approximation. For the direct bandgap materials, focusing inside the material yields an effective lifetime within a few percent of the bulk lifetime, regardless of the surface recombination velocity, while for excitation close to the surface it is up to 30% lower than the bulk lifetime at high surface recombination velocities (>104 cm/s). For the indirect bandgap material, the effective lifetime is dominated by the surface, making the bulk lifetime inaccessible, even at surface recombination velocities of 100 cm/s. Finally, we use the 1D approximation to find under what conditions the bulk lifetime can be extracted by this method and determine that both the bulk diffusion length and the product of the bulk lifetime and surface recombination velocity must be much less than twice the device thickness.