Ultrafast carrier dynamics of amorphous zinc tin oxide graded thin films

We report the fabrication of amorphous zinc tin oxide (ZTO) thin films using rf magnetron sputtering and rapid postannealing. A vertically graded composition alloy (Zn1-xSnxO) is constructed in a single amorphous layer, which exhibits strong visible light absorption. The ultrafast carrier dynamics of the ternary zinc tin oxide film is investigated by femtosecond transient absorption (TA) spectroscopy, which shows that the photocarriers in ZTO films have nearly identical relaxation dynamics under an ultraviolet or visible pump, indicating that the visible light absorption is induced by band gap reduction rather than defect state in the ZTO film. Our results provide insights into understanding the photophysical mechanism of amorphous ZTO films and suggest that this gradient film is a promising candidate for solar cell and photodetector devices.