Enhanced photoresponsivity and sensitivity in Ag/Bi₂Te₃ heterostructure films for visible light photodetectors

Bilayer thin-film structures have potential applications in many photodetectors. The intermixing of the layers by thermal annealing is an excellent way to tune the optoelectronic behaviours of such heterostructures. The main aim of this investigation is to enhance the photoresponse and photodetection by Ag diffusion into Bi₂Te₃ films via heat treatment. Ag/Bi₂Te₃ heterostructure films are formed by thermal evaporation and examined by cross-sectional FESEM, which revealed intermixing by annealing. XRD data, structural properties and surface morphology are observed using FESEM imaging. HRTEM and SAED confirmed the phases identified by XRD. The presence of elements in the annealed and unannealed states is verified by EDX. The optoelectronic properties that change upon annealing are investigated through UV-visible measurements. The increased absorbance, due to reduced transmittance, resulted in a decreased band gap. The band gap decreased from 0.875 eV (bilayer) to 0.793 eV (intermixed layer) upon annealing. The refractive index increased from 3.513 to 3.614, an increase of 0.101. There is an enhancement in the value of the nonlinear refractive index and third-order nonlinearity susceptibility. A decrease in contact angle indicates a transition from hydrophobic to hydrophilic nature. The photosensitivity increased from 9.37% (unannealed) to 33.19% (annealed), and the responsivity increased from 17.725 × 10^-2 to 62.096 × 10^-2 A W^-1 after annealing at 250 °C. The photocurrent in the mA range showed enhanced photoconductivity upon annealing. The obtained results from the study are suitable for advanced visible-light photodetectors and future optoelectronic devices with high energy efficiency and sensitivity.