Although the piezoelectric film is a single crystal film or a preferred polycrystalline film, but in which the atomic accumulation is not as tight and orderly as in the crystal, the dielectric constant value of the piezoelectric film is different from that of the crystal. In addition, there is a large residual internal stress in the film and the cause of the measurement, and the dielectric constant value of the film is different from the corresponding numerical value of the crystal.
It has been shown that the dielectric constant of piezoelectric film is not only related to the direction of crystal, but also to the test conditions. The dielectric constant of piezoelectric film has a considerable dispersion, except for the internal stress and the test conditions, the composition of the film deviates from the difference between the stoichiometric ratio and the thickness of the film. It is generally believed that the dielectric constant of the film becomes smaller with the thickness thinning. In addition, the dielectric constant of piezoelectric films will change obviously with the change of temperature and frequency.
From reducing dielectric loss and relaxation frequency of piezoelectric thin films, it is hoped that it has high resistivity, at least ρv≥108ω·cm. AlN Thin Film resistivity 2x1014~1x1015ω·cm, much higher than 108ω·cm, so in this respect, AlN is very excellent film. In addition, the conductivity of AlN piezoelectric films obeys the change of temperature.
The piezoelectric effect of the crystals are not symmetrical centers, so the electron mobility is also anisotropic, the conductivity is also different. The conductivity of AlN piezoelectric film along the c-axis direction is different from that of the vertical C-axis. Piezoelectric material is a functional material to realize the mutual conversion between mechanical energy and electric energy, and its development has a long history. Since the 1880s from the Curie brothers in quartz crystals found the piezoelectric effect, piezoelectric materials began to attract widespread attention, along with the research in-depth, has emerged a large number of piezoelectric materials, such as piezoelectric functional ceramic materials, piezoelectric films, piezoelectric composite materials. These materials have a very wide range of uses, and play an important role in the power, magnetism, sound, light, heat, humidity, gas, force and other functional conversion devices.
Because the breakdown strength of the dielectric is the intensity parameter, and there are all kinds of defects in the thin film, so the breakdown strength of the piezoelectric film has a considerable dispersion. The breakdown theory of the dielectric, for intact film, the breakdown strength should be increased with the decrease of film thickness. But in fact, because the film contains a lot of defects, the thickness of the more hours of the impact of the more significant, so in the thickness to a certain value, the film's breakdown strength is drastically smaller. For the film breakdown strength, in addition to the film itself, there are also in the test electrode edge effect. Because the thicker the film, the more uneven the electric field on the edge of the electrode, the breakdown strength decreases with the increase of film thickness.
In addition to the above several factors, the dielectric film breakdown strength is also in compliance with the membrane structure. For piezoelectric films, the breakdown field strength is also in compliance with the electric field direction, that is, it is also anisotropic in the breakdown field strength. Because of the grain boundary of polycrystalline Films, the breakdown strength of the polycrystalline film is lower than that of amorphous film. For similar reasons, the breakdown strength of the preferred piezoelectric film in the direction of grain orientation is lower than that of the vertical.
As with other dielectric films, the breakdown strength of piezoelectric films is also in compliance with external factors such as voltage waveforms, frequencies, temperatures and electrodes. Because the breakdown strength of piezoelectric film is related to a variety of factors, so for the same kind of film, the breakdown strength of the paper reported in the relevant literatures is often inconsistent, even the difference is larger, for example, the breakdown strength of ZnO film is 0.01~0.4mv/cm, AlN film is 0. $number 0mv/cm.