Ultrafast carrier dynamics in type-II ZnO-SnO heterostructure thin films

We investigate the carrier relaxation and charge transfer dynamics in type-II ZnO-SnO heterojunction thin films using wavelength-dependent femtosecond transient absorption measurements. Under SnO-selective excitation conditions, absorption signals related to ZnO are observed on a subpicosecond time scale, which indicates ultrafast electron transfer from SnO to ZnO. The spatial separation of electrons and holes across the ZnO-SnO interface leads to a long-lived carrier decay process with a lifetime of ∼4 ns, 2 times longer than resonant excitation of both ZnO and SnO in the heterostructures. Our results provide a framework for understanding the photophysics of tin oxide semiconductor heterostructures.