Piezoelectric film11/13/2023 ![]() This characteristic is important when the film has to be employed in heterostructures with an ultrathin top electrode, especially to improve the interface coupling properties. In spite of this, RF power has the advantage of producing stoichiometric films with a reduced roughness due to a well-controlled deposition rate. These free electrons move and screen the piezoelectric potential generated under mechanical actions, in turn reducing drastically its amplitude. ![]() These small grains can reduce the effective displacement if their polarizations are not fully aligned, and the presence of impurities at grain boundaries can generate a large number of free electrons inside their structure, reducing the final resistivity. ![]() Values are particularly low when radio frequency (RF) is employed, due to generally smaller grains. ![]() However, for sputter-deposited ZnO films, the typical d 33 values reported in the literature are of few pm/V, lower than those obtained in films grown with other techniques. As an example, sputter deposition allows obtaining good insulating films with controlled defects. Among them, PVD, and in particular magnetron sputter deposition, allow us to maintain a good control on the film growth conditions to obtain reproducible and controllable properties in c-axis-oriented ZnO films of tens to hundreds nm thickness. Several deposition methods have been exploited to obtain piezoelectric ZnO, such as the hydrothermal treatment, the sol-gel spin coating technique, the ultrasonic spray pyrolysis technique or physical vapor deposition (PVD) methods, such as magnetron sputtering, pulsed laser deposition or laser molecular beam epitaxy. Reducing the thickness, the piezoelectric response decreases, due to the reduction in grain size and crystal quality, making more difficult the exploitation of ZnO advantages in nanofabrication and nanodevices, where thinner oxide films are required. In these applications, the typical ZnO film thickness is in the range of micrometers, with a maximum reported piezo response of tens of pm/V for an oriented or composite film, and up to 240 pm/V for V-doped ZnO. This fact has pushed the interest for this material in the field of sensors, actuators, and nanogenerators, in spite of the fact that the d 33 coefficient is still about one order of magnitude lower than that of other common piezoelectric ceramic compounds. ZnO has the advantage that it can be easily grown along its c-axis and in the form of a variety of nanostructures. In the case of ZnO, this corresponds to the c-axis direction. Ceramic materials exhibit a macroscopic piezoelectric response when its crystalline structure has an axis that lacks inversion symmetry, with dipole moments aligned along the same direction. Zinc oxide (ZnO) is extensively studied and employed because of its many applications, e.g., in sensors, photovoltaics or in piezoelectric devices. By means of piezoelectric force microscope, we prove a piezoelectric response of the film in accordance with the literature, in spite of the low ZnO thickness and the reduced grain size, with a unipolar orientation and homogenous displacement when deposited on Ti electrode. The introduction of Ti or Pt as bottom electrode maintains a good surface and crystalline quality. Subsequent annealing at 773 K further improves the film quality. The results show an optimal reduction in surface roughness and at the same time a good crystalline quality when 75% O 2 is introduced in the sputtering gas and deposition is performed between room temperature and 573 K. The role of thermal treatments, as well as sputtering gas composition, is investigated by means of atomic force microscopy and x-ray diffraction. We optimize the fabrication of piezoelectric ZnO to reduce its surface roughness, improving the crystalline quality, taking into consideration the role of the metal electrode underneath. The piezoelectric response of ZnO thin films in heterostructure-based devices is strictly related to their structure and morphology.
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