Computationally efficient multiphysics electro-opto-thermal drift-diffusion models for short pulse and continuous wave excitation applied to MUTC photodetectors

Abstract

We present two models for analyzing the dynamics of photodetectors. One model applies when the photodetector is illuminated by a train of optical pulses whose duration is short compared to the impulse response; the other model applies when the optical excitation is effectively continuous. The first major contribution of this work is the identification of a quasiadiabatic heating regime governing the device response to subpicosecond to picosecond-scale optical pulse excitation. We demonstrate theoretically that due to the finite optical-to-acoustic phonon relaxation time and the minimal thermal diffusion length during the pulse, lattice heating occurs on a timescale that is significantly faster than macroscopic heat transport. Hence, the heat equation can be mathematically decoupled into a local, diffusionless energy deposition phase and a subsequent diffusive cooling phase, enabling the computationally efficient study of long pulse trains without sacrificing accuracy. The second major contribution of this work is the introduction of a quasi-one-dimensional electro-thermal drift-diffusion model derived by exploiting the symmetry and Gaussian distribution of the laser beam's optical mode. By evaluating the radial diffusion operator exactly at the optical axis, we transform the three-dimensional lateral heat spreading into a linear relaxation term, enabling accurate simulation of the device geometry without the computational cost of a multidimensional mesh. Both models are then applied to high speed modified uni-traveling-carrier (MUTC) photodetectors to quantify self-heating dynamics and the resulting impact on quantum efficiency.