Determine the origins of the narrow two-photon photocurrent peaks

Determine whether the narrow two-photon absorption photoresponsivity peaks 1–5 in lateral CuScP₂S₆ devices originate from excitonic transitions, direct band-to-band transitions, one-photon absorption involving strongly bound excitons, or deep-level defect-assisted transitions, and delineate the nature of any underlying exciton states.

Background

The two-photon absorption photoresponsivity spectrum of lateral CuScP₂S₆ devices contains five narrow peaks at photon energies of approximately 1.15, 1.23, 1.31, 1.40, and 1.49 eV. Their near-even energy spacing and decreasing intensity suggest a sequence of phonon sidebands associated with an exciton, but the correspondence of several peaks with higher-energy direct interband transitions leaves their interpretation ambiguous.

The authors specifically note that the available measurements cannot conclusively identify the origin of peaks 1–5. They indicate that computational calculations, gate-voltage-dependent measurements, and polarization-resolved optical measurements are needed to test the excitonic hypothesis and determine the nature of the states responsible for the observed photocurrent features.