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John O | February 2019

Tweaking component ratios creates electrically-different layers for transparent transistors

By Josh Perry, Editor


Researchers at the King Abdullah University of Science and Technology (KAUST) in Thuwal, Saudi Arabia created transparent thin-film transistors from a single hafnium-zinc oxide (HZO) composite by altering the ratio of metal oxides in each of the transistor layers.


Tweaking component ratios can produce conducting (left), insulating (center) and semiconducting HZO materials (right). (KAUST)


According to a report from KAUST, “Thin-film transistors generally comprise electrode, dielectric and channel layers that are deposited on a substrate from various conducting, insulating and semiconducting materials. They also require different reactors and thin-film deposition equipment.”


HZO can be tuned from conducting to semiconducting to insulating by changing the zinc-oxide/hafnium-oxide precursor ratio, which means that the entire transistor can be built from a single binary oxide in a reaction chamber.


Previous attempts at creating these types of transistors required high-deposition temperatures that made the inadequate for real-world applications, but researchers believe this new process is scalable and could be used in high-volume production.


“The all-HZO transistors exhibit excellent electrical properties on glass and plastics, demonstrating their potential for high-resolution transparent and flexible displays,” the article continued. “They also show outstanding performance when incorporated in circuits, such as inverters and ring oscillators, suggesting their viability and scalability.”


The research was recently published in Small. The abstract stated:


“A novel process is developed in which thin film transistors (TFTs) comprising one binary oxide for all transistor layers (gate, source/drain, semiconductor channel, and dielectric) are fabricated in a single deposition system at low temperature.


“By simply changing the flow ratio of two chemical precursors, C8H24HfN4 and (C2H5)2Zn, in an atomic layer deposition system, the electronic properties of the binary oxide (Hf xZn1−xO2−δ or HZO) are tuned from conducting, to semiconducting, to insulating.


“Furthermore, by carefully optimizing the properties of the various transistor HZO layers, all?HZO thin film transistors are achieved with excellent performance on both glass and plastic substrates. Specifically, the optimized all?HZO TFTs show a saturation mobility of ≈17.9 cm2 V−1 s−1, low subthreshold swing of ≈480 mV dec−1, high Ion/Ioff ratio of >109, and excellent gate bias stability at elevated temperatures. In addition, all?HZO inverters with high DC voltage gain (≈470), and all?HZO ring oscillators with low stage delay (≈408 ns) and high oscillation frequency of 245 kHz are demonstrated.


“This approach presents a novel, simple, high performance, and cost?effective process for the fabrication of indium?free transparent electronics.”

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