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.

A broadband, self-biased photodiode based on antimony telluride (Sb2Te3) nanocrystals/silicon heterostructures.

Low bulk band gaps and conductive surface electronic states of tetradymite topological insulators (TTI) make them potential candidates for next generation ultra-broadband photodevices. Here, we demonstrate a broadband and self-biased photodiode based on a Sb2Te3-Si heterostructure. A low-cost thermal evaporation technique was employed to fabricate the photodiode. The self-biased nature of the photodiode was due to the built-in potential at the Sb2Te3-Si interface. Upon characterizing the Sb2Te3 nanocrystalline film via AFM, SEM, EDX, and XPS it was found that the film exhibited p-type behavior due to antimony vacancies or antisites. The fabricated photodiode showed an excellent rectification ratio of 3388 with n-Si confirming a robust Schottky barrier at the interface and a well-defined photocurrent upon illumination. Due to the p-type behavior of the Sb2Te3 nanocrystalline film, a rectification ratio of only 0.38 was observed with p-Si. The barrier at the interface also increases the carrier lifetimes, thereby eliminating one of the biggest drawbacks of ultrafast carrier recombination times in TTI as a photodetection material. Moreover, the photodiode exhibited excellent Ion/Ioff of three orders of magnitude under the self-biased conditions, and photocurrents ranging from 520 nm to 980 nm wavelengths were observed.

Two-dimensional layered semiconductor/graphene heterostructures for solar photovoltaic applications.

Schottky barriers formed by graphene (monolayer, bilayer, and multilayer) on 2D layered semiconductor tungsten disulfide (WS2) nanosheets are explored for solar energy harvesting. The characteristics of the graphene-WS2 Schottky junction vary significantly with the number of graphene layers on WS2, resulting in differences in solar cell performance. Compared with monolayer or stacked bilayer graphene, multilayer graphene helps in achieving improved solar cell performance due to superior electrical conductivity. The all-layered-material Schottky barrier solar cell employing WS2 as a photoactive semiconductor exhibits efficient photon absorption in the visible spectral range, yielding 3.3% photoelectric conversion efficiency with multilayer graphene as the Schottky contact. Carrier transport at the graphene/WS2 interface and the interfacial recombination process in the Schottky barrier solar cells are examined.