Ultrafast Photovoltage Polarity Switching in Cd₃As₂ Nanobelt Field Effect Transistors

Topological devices have been extensively studied for their suppressed backscattering, versatile spin texture, strong spin-orbit coupling, and symmetry-protected states. An accessible topological Dirac semimetal, Cd₃As₂, is very attractive in pushing next generation photodetectors as it exhibits these characteristics as well as remarkably high carrier mobilities, chemical stability, and broadband detection.



Previous work has also demonstrated THz photodetection in Cd₃As₂ devices. However, the exact mechanism remains unclear, whether the ultrafast photodetection originates from the photo-thermoelectric or photovoltaic effect. We performed a systematic study as a function of fluence, temperature, and back-gate voltage to elucidate this behavior. We show complex interplay between photogenerated electrons and holes at the interface through spatiotemporally-resolved photovoltage microscopy. We observed strong photovoltage modulation at the picosecond timescale and its sign can be flipped at relatively low fluences of 10^-5 J/cm². We further show that the photovoltage flipping can be understood by ultrafast light modulation of contact band bending via device modeling.