Exciton-impurity interactions in high-purity InP

Temperature-dependent photoluminescence measurements have been performed to study the linewidth of the lower and upper polariton branches of the free-exciton transition in four high-purity n-type InP epilayers with a concentration of neutral shallow (deep) donors in the range of 7.9×10¹³--2×10¹⁵cm^-3 [(0--1.45)×10¹⁴cm^-3]. A line-shape analysis of the photoluminescence spectra revealed that the emission from upper-branch polaritons broadens rapidly when the temperature increases, while that of lower-branch polaritons displays a much smaller broadening in the corresponding temperature range. Moreover, when the concentration of neutral deep-donor centers is higher than that of neutral shallow donors, the emission linewidth from lower-branch polaritons exhibits a striking narrowing in the temperature range of 10--30 K. In addition, we have observed that the onset of emission linewidth narrowing from lower-branch polaritons shifts to lower temperatures as the neutral shallow-donor concentration is increased but kept below that of the neutral deep donors. In contrast, when the shallow-donor concentration is higher than that of the deep-donor centers, the emission linewidth narrowing from lower-branch polaritons vanishes. That behavior is similar to that obtained in a sample free of deep-donor centers, which at low temperatures does not display an emission linewidth narrowing of the lower polariton branch. Taken all together, our results cannot be explained within the framework of the standard polariton transport model, but are nevertheless well reproduced by a phenomenological model which takes into account polariton scattering by bound excitons, ionized impurities, and phonons.