A two-dimensional (2D) Janus layer has different physical properties than a 2D non-Janus layer. When a van der Waals (vdW) heterostructure contains a Janus layer, some unusual phenomena different from those in a heterostructure made of two non-Janus layers are expected. Here, we study electronic structures and carrier dynamics of a vdW heterostructure WSSe/GaN. Interesting band alignment and anomalous carrier dynamics have been found in the heterostructure. When the Janus layer WSSe is flipped, the band alignment of the heterostructure shifts from type I to type II. Moreover, the type-I heterostructure shows exotic carrier dynamics thoroughly different from those in a traditional type-I heterostructure. Its transfer time of electrons (78 ps) is 200 times that of holes (323 fs), and the recombination time of electrons and holes is 178 ns, which is much longer than that in the type-II heterostructure. By calculating electronic parameters and phonon modes in the heterostructures, we find that the variation of band alignment originates from built-in electric field and surface work functions of the heterostructure. The anomalous carrier dynamics is attributed by the common effects of built-in electric field and electron-hole coupling. Our work not only reveals exotic carrier dynamics in a vdW heterostructure containing a Janus layer but also may extend applications of 2D heterostructures to some fields such as optoelectronics, catalysis, etc.

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• Carrier dynamics
• Electronic structure
• 2-dimensional systems