Excitation-Dependent Carrier lifetime and Diffusion Length in Bulk CdTe Determined by Time-Resolved Optical Pump-Probe Techniques

We applied time-resolved pump-probe spectroscopy based on free carrier absorption and light diffraction on a transient grating for direct measurements of the carrier lifetime and diffusion coefficient D in high-resistivity single crystal CdTe (codoped with In and Er). The bulk carrier lifetime τ decreased from 670 ± 50 ns to 60 ± 10 ns with increase of excess carrier density N from 10¹⁶ to 5 × 10¹⁸ cm^-3 due to the excitation-dependent radiative recombination rate. In this N range, the carrier diffusion length dropped from 14 μm to 6 μm due to lifetime decrease. Modeling of in-depth (axial) and in-plane (lateral) carrier diffusion provided the value of surface recombination velocity S = 6 × 10⁵ cm/s for the untreated surface. At even higher excitations, in the 10¹⁹-3 × 10²⁰ cm^-3 density range, D increase from 5 to 20 cm²/s due to carrier degeneracy was observed.