Parametric numerical study of the modulation transfer function in small-pitch InGaAs/InP infrared arrays with refractive microlenses.

We report on the modulation transfer function (MTF) in short-wave infrared indium gallium arsenide (InGaAs) on indium phosphide (InP) planar photodetector arrays. Our two-dimensional numerical method consists of optical simulations using the finite-difference time domain method and drift-diffusion simulations using the finite-element method. This parametric study investigates MTF dependence on pitch, the addition of refractive microlenses, the thickness of the InGaAs absorber, and the doping concentration of the InGaAs absorber. A focus is placed on the connection between the lateral diffusion of photogenerated holes in InGaAs and the MTF. It is found that the MTF of small-pitch arrays exhibit sub-ideal behavior due to pixel cross-talk resulting from a long minority carrier diffusion length. By incorporating monolithic microlenses with the InP substrate, the MTF response is improved for all pitches investigated, particularly for spatial frequencies near the respective cutoff frequencies. We also find a strong dependence of the MTF on the thickness and doping concentration in the absorbing region. Trends in dark current and quantum efficiency are reported.