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 (ΦB) 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.