Laser probing of carrier diffusion dynamics

Abstract

A new diagnostic technique can be used to measure simultaneously the excess free‐carrier lifetime τ and the diffusion length L with spatial resolution in semiconductor samples. The technique uses pulses of above‐band‐edge light to generate carriers near the sample surface. The carrier density evolution into the bulk is measured by means of the free‐carrier absorption of an ir laser beam. Computer fitting the time variation of the absorption at any beam position to the carrier transport equation yields τ and L, enabling the diffusion coefficient D = L²/τ to be evaluated. Using 10⁴‐W/cm² injection light and a 3.39‐μm laser probe on a 1500‐Ω cm silicon sample yielded bulk values in good agreement with calculated and independently measured values.