Characteristics of a type-II n-MoS2/p-Ge van der Waals heterojunction

A few-atomic-layer molybdenum disulfide (MoS2) film on Si/SiO2 substrates grown by metal-organic chemical vapor deposition was investigated. The few-atomic-layer MoS2 film was subsequently transferred onto a (100) p-Ge substrate to build a van der Waals n-p heterojunction. The as-grown few-atomic-layer MoS2 film and the MoS2/Ge heterostructure were characterized atomic force microscopy, spectroscopic ellipsometry, high-resolution scanning transmission electron microscopy, Raman spectroscopy analyses, photoluminescence (PL) measurements at room temperature (RT, 300 K), and type-II band alignment of the heterostructure determined by ultraviolet photoelectron spectroscopy. The RT-PL measurements showed dominant peaks at 1.96 and 1.8 eV for the as-grown MoS2 and red-shifted PL peaks for that transferred onto Ge. We examined the electrical characteristics of the few-atomic-layer MoS2 by forming a type-II band alignment van der Waals heterojunction with a highly doped p-Ge. The heterojunction solar cell exhibited an open-circuit voltage of 0.15 V and a short-circuit current density of 45.26 μA/cm2. The external quantum efficiency measurements showed a spectral response up to approximately 500 nm owing to the absorption by the few-atomic-layer MoS2 film.