Calculation of Photocarrier Generation from Optical Absorption for Time-domain Simulation of Optoelectronic Devices

Photocarrier generation rate in optoelectronic materials is often calculated using the Poynting vector in the frequency domain. However, this approach is not accurate in time-domain simulations of photoconductive devices because the instantaneous Poynting vector does not distinguish between power flux densities of optical and low-frequency electromagnetic fields. The latter is generated by photocurrents and is not supposed to contribute to the photocarrier generation since the corresponding photon energy is smaller than the bandgap energy of the optoelectronic material. In this work, an optical absorption-based model is proposed to accurately calculate the generation rate in time-domain simulations. The proposed approach considers the material dispersion near the optical frequency corresponding to the bandgap energy of the optoelectronic material. The instantaneous optical absorption is calculated from the polarization current density associated with the dispersion model. Then, the optical absorption is used to calculate the generation rate. Numerical examples show that the proposed approach is more accurate than the Poynting vector-based method in calculating the instantaneous optical absorption. The proposed method is further validated against experimental results by modeling a photoconductive device. In the multiphysics simulation, the Poynting vector-based method overestimates the carrier generation rate and even generates divergent carrier densities when the low-frequency fields are strong, while the proposed method produces results that match with experimental measurements well.