A promising material for nanomotors was found
Scientists of the Moscow Technical University have shown that films of hafnium oxide with non-standard geometry behave as good piezoelectrics. The result will help create new micro- and nanoscale electromechanical motors compatible with modern silicon technology. The research is published in the journal ACS Applied Materials & Interfaces.
Hafnium oxide films have long been used in processors as an element of transistors. Non-volatile memory devices – flash drives – are also made on the basis of these films. The material is able to “memorize” information because it has ferroelectric properties: polarization that can be changed by an external electric field. Also, hafnium oxide films are perfectly compatible with all semiconductor electronics.
All ferroelectrics are piezoelectrics. Films of hafnium oxide exhibit a reverse piezoelectric effect: mechanical deformations of the material can be created with the help of an electric field. Such materials can become the basis for various micro- and nanomotors that are controlled by electrical voltage: miniature movers, movable and adjustable microlenses, switches in an electrical circuit, and injectors for microdoses of medicine.
Ordinary flat films of hafnium oxide on hard silicon crystals are poor piezoelectrics. They weakly convert electrical voltage into mechanical voltage, almost 100 times worse than standard piezo materials.
In the laboratory of promising data storage concepts of the Moscow State Technical University, it was discovered that when films are “suspended” on thin flexible membranes of a specific geometry, it is possible to achieve a gigantic electromechanical response. At first, scientists showed that using the method of atomic layer deposition with hafnium oxide, it is possible to uniformly cover a three-dimensional object of any shape, that is, to create any film of three-dimensional configuration. Next, they experimentally verified that if the films are grown on a stepped flexible membrane, then in this configuration the hafnium oxide films behave as good piezoelectrics.
“Our result opens up broad prospects for finding new configurations of three-dimensional piezoelectric devices that will improve their performance. This opens up a new direction in nano- and microsystem engineering,” says Anastasia Chupryk, head of the Laboratory of Advanced Data Storage Concepts of the Moscow Institute of Technology.
Researchers have developed a membrane manufacturing technology for growing films of any configuration. The stepped flexible membrane was chosen to make the experiment clear and reliable: the geometry parameters can be changed in a controlled manner and the configuration can be accurately accounted for in the theoretical justification. Further, experimental studies of ferroelectric properties were conducted: they showed that growing the film on a flexible substrate does not spoil its properties. With the help of piezoelectric feedback microscopy, scientists have demonstrated piezoelectric properties – a giant electromechanical response of the material, which is 25 times greater than the response of a flat film.
To understand the reason why such an effect is observed, physicists conducted quantum mechanical calculations and numerical simulations. It was assumed that the magnitude of the effect can be influenced by residual mechanical stresses that are formed in the process of crystallization of the film with a flexible substrate. It turned out that residual mechanical stresses have a negligible effect.
“The effect we observe is a synergy of the film’s own ferroelectric properties and the flexibility of the membranes, in which areas sometimes sag, then bend and become more sensitive to the influence of the electric field,” comments Anastasia Chupryk.
The proposed approach allows for the development of new promising piezoelectric devices, including nanoactuators and nanoswitches, nanoinjectors, miniature tunable reflectors, and lenses.