Fig. 1: Challenges in unipolar barriers and the concept of ultra-thin polar barriers (UTPB).

a Schematic illustration of ideal and non-ideal unipolar barriers. An ideal unipolar barrier completely blocks one type of carrier while allowing the other to flow freely. In contrast, a non-ideal barrier fails to block one carrier type completely and also impedes the flow of the other carrier. b Band diagrams of two common cases of non-ideal unipolar nBn structures. Case 1: type-II aligned barrier, where a small conduction band offset (ΔE_c) causes leakage of dark current despite a zero valence band offset (ΔE_v). Case 2: wide bandgap barrier, where a large ΔE_c is achieved, but the presence of ΔE_v hinders the flow of photogenerated carriers. c Band diagram illustrating the UTPB concept. A polar ultra-thin barrier, with a controllable polarization direction, depletes electrons in the n-type contact layer and facilitates the tunneling of photogenerated holes from the absorption layer, thereby effectively suppressing dark current and improving the collection of photogenerated carriers. d Design of the polarized water-intercalated WSe₂/H₂O/PdSe₂ structure as a UTPB detector. In this design, the n-type WSe₂ serves as the contact layer, the n-type narrow-bandgap PdSe₂ as the absorption layer, and the strongly polarized water layer acts as the barrier. e, f STEM characterizations of the WSe₂/H₂O/PdSe₂ heterostructure show that few-layer WSe₂ and thick PdSe₂ are used for better electrical control and optical absorption. The 0.75 nm water layer is uniformly distributed over a large area, with atomically sharp and clean interfaces. B, barrier; E_c, conduction band edge; E_v, valence band edge; ΔE_c, conduction band offset; ΔE_v, valence band offset; E_f, Fermi level; P, polarization electric field. STEM, scanning transmission electron microscopy. Scale bars: (e), 10 nm; top panel of (f), 20 nm; bottom panel of (f), 2 nm.