Air Force

Fabricating ZnO thin film transistors with pulsed laser deposition

The combination of transparency, high mobility, and room-temperature processing makes the ZnO-TFT a very promising low-cost optoelectronic device for the next generation of invisible and flexible electronics

Materials

The combination of transparency, high mobility, and room-temperature processing makes the ZnO-TFT a very promising low-cost optoelectronic device for the next generation of invisible and flexible electronics. Wide bandgap semiconductors based on Zinc oxides are good candidates for the channel material in transparent thin film transistors (TTFT) because of their simultaneous high electron mobility and optical transparency properties.
Historically, TFTs have been made from amorphous silicon (a-Si) films. Transistors made from a-Si, however, suffer from a number of deficiencies including low electron mobility, light sensitivity operation, limited switching ratios, and poor threshold voltage uniformity. These deficiencies make a-Si TFTs unsuitable for state-of-the-art display circuit applications that demand higher switching speeds and accuracy. Zinc oxide is an ideal TFT semiconductor for these applications, having a higher mobility than a-Si. Additionally, ZnO thin film transistors are more easily fabricated and less expensive to produce than indium tin oxide TFTs.

The AFRL team has developed nanocrystalline ZnO films prepared by pulsed laser deposition (PLD) to fabricate ultra short gate length first thin film transistors that operate at microwave frequencies. The team has developed a process that utilizes multiple film layers to protect the transistor gate while above layers are patterned, processed, and etch. Through standard lithography techniques, this novel method bypasses the use of physical masks and allows for ultra short gate length transistors. AFRL has also perfected the application of zinc oxide films onto various surfaces using PLD, which employs a UV laser beam to first remove zinc oxide nanocrystals from a source and then deposit them as a thin film on the desired surface. The ZnO films have reduced parasitic capacitance, high field effect mobilities in excess of 100 cm2N.s2 and microwave cutoff frequencies of fmax=10GHz.

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