High-Performance Zinc Tin Oxide TFTs with Active Layers Deposited by Atomic Layer Deposition

New deposition techniques for amorphous oxide semiconductors compatible with silicon back end of line manufacturing are needed for 3D monolithic integration of thin-film electronics. Here, three atomic layer deposition (ALD) processes are compared for the fabrication of amorphous zinc tin oxide (ZTO) channels in bottom-gate, top-contact n-channel transistors. As-deposited ZTO films, made by ALD at 150-200 degrees C, exhibit semiconducting, enhancement-mode behavior with electron mobility as high as 13 cm(2)V(-1)s(-1), due to a low density of oxygen-related defects. ZTO deposited at 200 degrees C using a hybrid thermal-plasma ALD process with an optimal tin composition of 21%, post-annealed at 400 degrees C, shows excellent performance with a record high mobility of 22.1 cm(2)V(-1)s(-1)and a subthreshold slope of 0.29 V dec(-1). Increasing the deposition temperature and performing post-deposition anneals at 300-500 degrees C lead to an increased density of the X-ray amorphous ZTO film, improving its electrical properties. By optimizing the ZTO active layer thickness and using a high-kgate insulator (ALD Al2O3), the transistor switching voltage is lowered, enabling electrical compatibility with silicon integrated circuits. This work opens the possibility of monolithic integration of ALD ZTO-based thin-film electronics with silicon integrated circuits or onto large-area flexible substrates.