High performance broadband bismuth telluride tetradymite topological insulator photodiode

A small bulk gap and the presence of Dirac electrons due to conductive surface states make tetradymite topological insulators promising candidates for optoelectronic devices. In this work, we demonstrate a highly responsive Bi₂Te₃-Si heterostructure photodiode. The thermally evaporated Bi₂Te₃ film, exhibiting a nanocrystalline nature, shows p-type doping behavior due to bismuth vacancies. As a result of the work function difference between Bi₂Te₃ and p-type Si, charge transfer occurs and a Schottky barrier is formed. Using the thermionic emission model, the barrier height (Φ_B) is extracted to be ∼0.405 eV. For minimizing the effect of extrinsic defects, the photodiodes were capped with graphene or Si₃N₄. Since graphene acts as an efficient photoexcited carrier collector, the graphene capped device outperforms the Si₃N₄ capped device. The higher quality Bi₂Te₃ nanocrystalline film of the Si₃N₄ capped photodiode contributes to a one-order-of-magnitude improvement in responsivity at 1550 nm wavelength, as compared to the graphene capped photodiode. The Si₃N₄ capped photodiode shows photoresponse even at zero bias for 1550 nm wavelength. Built-in potential due to charge transfer at the interface of Bi₂Te₃ and Si capped with a graphene electrode exhibits the highest responsivity (8.9 A W^-1). Broadband photodetection is observed in both types of photodiodes.