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Hafnia (HfO2)


HiTUS deposited hafnia makes an excellent dielectric coating. The characteristic that distinguishes hafnia from other dielectric materials such as alumina and silica is its high dielectric constant. Coupled with good breakdown characteristics, a wide energy band gap and a high deposition rate PQL’s hafnia has been effectively used in areas such as touch screen mobile phone technology, thin film transistors (TFTs) and thin film electroluminescent (TFEL) devices.


A reactive deposition process is used to sputter deposit PQL’s hafnia coatings from a hafnium target. Using oxygen as the reactive gas and argon as the sputtering gas, the deposition process can be optimised to obtain thin films with maximum transparency through the visible waveband. The high optical band gap of approximately 5.5 eV yields coatings exhibiting loss-free transmission down to less than 250 nm. The power applied to the target controls the deposition rate. For a target power density of just 6.4 Wcm-2 the deposition rate for hafnia thin films grown on the S500 is approximately 20 nm/min. HiTUS deposited hafnia coatings can be sputter grown at room temperature making them suitable for use in the field of flexible plastic electronics. By varying the process pressure during deposition, these films can also be optimised for minimal stress.


In TFTs, the desire for increased drive current has led to progressively thinner films of silica being used for the gate dielectric. However, this has also led to unacceptably high gate leakage currents. To reduce this leakage but continue to improve the drive current, manufacturers of TFTs now use high-k dielectrics such as hafnia. With a dielectric constant of between 20 and 25 thick hafnia layers may be used whilst still increasing the gate dielectric capacitance. Publications written in collaboration with Cambridge University on TFTs sputter deposited using PQL’s hafnia can be found here:

Download "Stability of thin film transistors incorporating a zinc oxide or indium zinc oxide channel deposited by a high rate sputtering process"

Download "Reactive Deposition of Channel and Insulating Layers for TFT Applications at Ambient Temperatures Using a Remote High Density Plasma" - correction to this paper, please note the hafnia breakdown should be 3 MV/cm.

Our hafnia coatings have also been effectively used in the fabrication of thin film electroluminescent (EL) light emitters. These inorganic solid state devices rely on the impact excitation of a doped wide band gap semiconductor (e.g. ZnS:Mn) for the emission of light. This excitation is caused by the injection of hot electrons from the interface between the doped semiconductor and a dielectric. We have used many different dielectric coatings including alumina, tantala, hafnia, yttria and aluminium nitride but the high dielectric constant associated with the hafnia yields devices with the lowest threshold voltage. The low temperature deposition process of the hafnia and all the other materials used in the structure has allowed us to fabricate working, flexible EL devices that are transparent in the absence of an applied electric field. More information on these EL devices can be found in the following publications.

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