Van der Waals (vdW) 2D/3D heterostructures are extensively studied for high-performance photodetector applications. Until now, the type of 2D materials has been the primary area of interest rather than the design of 3D semiconductors. In this study, high-speed broadband photodiodes (PDs) based on vdW p-WSe 2 /n-Ge heterojunctions are reported, and the performance compared with different n-Ge regions formed via the ion-implantation process. The fabricated PD with a typical long n-Ge region and low doping concentration responds to a broad spectral range from visible to infrared near 1550 nm with a response time of ≈3 µs and responsivity of 1.3 A W −1 . The inferior responsivity of PDs with short n-Ge regions can be improved as demonstrated by experimental results and process simulation. Density functional theory calculations are performed to estimate the variation of the energy band structures with the doping concentration of n-Ge. Fast photoresponse and efficient carrier separation across the heterojunction can be expected regardless of the n-Ge doping concentration. Based on the experimental results together with theoretical band structure and process simulation, it is shown that the heterojunction with an optimized n-Ge design is a promising high-speed broadband photodetector that can be implemented with complementary metal-oxidesemiconductor design and fabrication processes.

Recently, the inherent piezoelectric properties of the 2D transition-metal dichalcogenides (TMDs) tin monosulfide (SnS) and tin disulfide (SnS2) have attracted much attention. Thus the piezoelectricity of these materials has been theoretically and experimentally investigated for energy-harvesting devices. However, the piezoelectric output performance of the SnS2- or SnS-based 2D thin film piezoelectric nanogenerator (PENG) is still relatively low, and the fabrication process is not suitable for practical applications. Here we report the formation of the SnS2/SnS heterostructure thin film for the enhanced output performance of a PENG using atomic layer deposition (ALD). The piezoelectric response of the heterostructure thin film was increased by ∼40% compared with that of the SnS2 thin film, attributed to large band offset induced by the heterojunction formation. Consequently, the output voltage and current density of the heterostructure PENG were 60 mV and 11.4 nA/cm2 at 0.6% tensile strain, respectively. In addition, thickness-controllable large-area uniform thin-film deposition via ALD ensures that the reproducible output performance is achieved and that the output density can be lithographically adjusted depending on the applications. Therefore, the SnS2/SnS heterostructure PENG fabricated in this work can be employed to develop a flexible energy-harvesting device or an attachable self-powered sensor for monitoring pulse and human body movement.

Ge/MoS 2 van der Waals heterostructure enables bias-dependent selective detection of visible and near infrared.

In this work, a theoretical study on the electronic properties of the metal/Bi2O2Se interface is presented through the density functional theory calculation. Particularly, the effects of Cr, Pd, Pt, Au, and Bi on monolayer, bilayer, and trilayer Bi2O2Se are explored. Naturally created Se vacancies on the Bi2O2Se surface are also considered by constructing two interface structures: the metal/Bi2O2Se with the Se vacancies remaining or filled with metals. For the metal/monolayer Bi2O2Se, it is observed that the Bi2O2Se layer is fully metallized since Bi2O2Se strongly interacts with the metal. Meanwhile, for the metal/bilayer and trilayer Bi2O2Se, the Bi2O2Se layers remain semiconducting except for the layer right next to metal. Among the considered metals, regardless of whether the Se vacancies are filled with metal or not, the semiconducting layers in bilayer and trilayer Bi2O2Se form Ohmic contact with Bi. It is also found that filling the Se vacancies with the metals heavier than Se increases the interface distance between metal and Bi2O2Se, and hence results in the weak Fermi level pinning effect.