Photoresponse and electronic switching in n-Si/p-FeSe heterojunctions designed for visible-light communication

Herein, 200 nm-thick p-FeSe films were deposited onto n-Si substrates to fabricate heterojunction photodiodes for visible-light communication applications. Structural and morphological analyses confirmed the preferential growth of tetragonal FeSe, with an average grain size of 301 nm. Dark and illuminated current-voltage measurements revealed switching behavior with strong rectification, where the asymmetry ratio exceeded 10³ at 300 K and 10⁷ at 30 K under dark conditions. The devices exhibited a current responsivity of 1.50 A/W and an external quantum efficiency of up to 300%. In addition, the noise-equivalent power was in the picowatt range, while the specific detectivity reached 10¹⁰ Jones, demonstrating high sensitivity to weak optical signals with markedly reduced noise. Moreover, temperature-dependent dark and illuminated measurements performed over the 30-320 K range showed that the photocurrent increased as the temperature decreased, while preserving the linear recombination mechanism down to 30 K. This behavior suggests that the photoresponse is governed by charge exchange between trap states, which are localized defect-related energy levels in the band gap, and sensitizing centers, which stabilize the photocurrent at low temperatures. These results demonstrate that n-Si/p-FeSe heterojunction photodiodes are promising candidates for visible-light communication and electronic switching applications, even at cryogenic temperatures.