Large-Area Pulsed Laser Deposited Molybdenum Diselenide Heterojunction Photodiodes

Two-dimensional (2D) semiconductors, such as transition-metal dichalcogenides (TMDs), have attracted immense interest due to their excellent electronic and optical properties. The combination of single and multilayered 2D TMDs coupled with either Si or II-VI semiconductors can result in robust and reliable photodetectors. In this paper, we report the deposition process of MoSe₂-layered films using pulsed laser deposition (PLD) over areas of 20 cm² with a tunable band gap. Raman and X-ray diffraction indicates crystalline and highly oriented layered MoSe₂. X-ray photoelectron spectroscopy shows Mo and Se present in the first few layers of the film. Rutherford backscattering demonstrates the effect of O and C on the surface and film/substrate interface of the deposited films. Ultraviolet-visible spectroscopy, Kelvin probe, photoelectron spectroscopy, and electrical measurements are used to investigate the band diagram and electrical property dependence as a function of MoSe₂ layers/thickness. As the MoSe₂ thickness increases from 3.5 to 11.4 nm, the band gap decreases from 1.98 to 1.75 eV, the work function increases from 4.52 to 4.72 eV, the ionization energy increases from 5.71 to 5.77 eV, the sheet resistance decreases from 541 to 56.0 kΩ, the contact resistance decreases from 187 to 54.6 Ω·cm², and the transfer length increases from 2.27 to 61.9 nm. Transmission electron microscopy (TEM) cross-sectional images demonstrate the layered structure of the MoSe₂ with an average interlayer spacing of 0.68 nm. The fabricated MoSe₂-Si photodiodes demonstrate a current on/off ratio of ∼2 × 10⁴ orders of magnification and photocurrent generation with a 22.5 ns rise time and a 190.8 ns decay time, respectively.