Quantifying Bulk and Surface Recombination in CdSeTe Absorbers by Modeling Terahertz and Photoluminescence Decays

We demonstrate a technique for estimating critical recombination parameters Shockley-Read-Hall lifetime (tau SRH) and front and back surface recombination velocities (SF and SB) in Cd(Se)Te. We evaluated uniform single heterostructures of glass/Al2O3/CdSexTe1-x/air (x = 0 & 0.2) using time resolved terahertz spectroscopy (TRTS) and time resolved photoluminescence (TRPL) with variable excitation wavelengths and powers as well as front vs back-side illumination. The addition of TRTS to traditional TRPL enhances analysis by providing insight into the first nanoseconds after photoexcitaton when TRPL is dominated by redistribution of carriers. Data were fit using numerical simulations of the semiconductor and Poisson' equations with best-fit parameters determined by Bayesian inference. Our CdSe0.2Te0.8 films had tau SRH similar to 50 ns, compared with similar to 26 ns for CdTe. tau SRH values extracted from our model were approximately double those derived from multi-exponential fit of TRPL decays, which do not rigorously account for surface recombination. Se alloying also decreased SB from 2x105 to 6x103 cm/s. Upper limits of 104 and 103 cm/s were determined for SF for CdTe and CdSe0.2Te0.8, respectively. Precise determination of recombination velocity at the well-passivated Al2O3/ CdSexTe1-x interface was limited by the dominant bulk and/or back surface recombination in these films. This study was made on passivated heterostructures of absorber layers to for the primary purpose of evaluating new methods of parameter determination. Further development of the models toward full devices can lead to rapid correlation of changes in bulk lifetime and surface recombination rates with processing and could enable evaluation of effects of electric fields and other phenomena that cannot be captured using conventional approaches.