Theoretical and experimental comparison of contactless lifetime measurement methods for thick silicon samples

Abstract

Simulations of time dependent carrier profiles for thick as-grown silicon samples (e.g. ingots) were computed for the excitation conditions of two different transient photoconductance lifetime measurement methods. The simulations were performed using a partial differential equation system that allows computing also non-steady state conditions. The specific effective lifetimes for different measurement conditions can be extracted and compared. Simulation results and measurement results for μPCD (microwave detected photoconductivity decay), a non-steady state method and MDP (microwave detected photoconductivity), which operates typically with a steady state photo generation, were simulated and measured. It was found that the effective lifetimes measured at thick samples with each method may differ strongly. This discrepancy can be attributed to the different penetration depths of the laser light and microwave, but first and foremost to a varying light pulse length and its influence on the developing carrier profile. Altogether the MDP measurements or methods with a steady state photo generation in general are less prone to the surface impact and accordingly better suited for investigating the bulk properties of silicon samples.