Aluminum nitride (AlN) films have been explored for sensor and actuator applications, but the resultant piezoelectric coefficient is still too low to make the films more competitive with more commonly used piezoelectric materials such as lead zirconate titanate (PZT). While AlN does have the disadvantage of a lower piezoelectric response, it does have the ability to maintain its piezoelectric properties above 400°C, something that is not possible with other piezoelectric materials. It is desirable to achieve a larger piezoelectric response for AlN in order to facilitate the integration of nitride based devices into existing technologies but conventional methods of improving the response by growing higher quality film only result in slight improvements in the piezoelectric response. A method of improving the d33 piezoelectric coefficient beyond any values found in literature may be possible by exploring methods of improving PZT films.Rare earth doping has been reported to improve the piezoelectric properties of PZT resulting in significant increases in the piezoelectric coefficient. Research has been conducted using rare earth dopants to improve upon the optical properties of AlN, but the impact on piezoelectric effect has never been considered.Thin, 250-1000 nm, AlN:Er films have been reactively sputtered using erbium (Er)/aluminum alloyed targets to explore any improvement in piezoelectric properties of the AlN:Er films as compared to AlN films. AlN films with 0.5 and 1.5% Er concentrations have been found to have piezoelectric coefficients that are larger than comparable ‘Er-free’ AlN films. AlN films with only 0.5% Er quantities were found to increase the d33 coefficient compared to a similar AlN film depending on the thickness of the film. This increase results in d33,f values greater than 7pm/V which is larger than most values found in literature. By increasing the Er content to 1.5%, values of d33,f were found to be as large as 15 pm/V. This enhanced piezoelectric response is still lower than that of PZT, but can be used to create superior actuator devices than that of typical AlN films.

Articles you may be interested inImpact of annealing temperature on the mechanical and electrical properties of sputtered aluminum nitride thin films J. Appl. Phys. 117, 065303 (2015); 10.1063/1.4907208Temperature dependence of the transverse piezoelectric coefficient of thin films and aging effects Stress and piezoelectric properties of aluminum nitride thin films deposited onto metal electrodes by pulsed direct current reactive sputtering

The electrical behavior of aluminum nitride ͑AlN͒ thin film structures consisting of alternating AlN and platinum ͑Pt͒ layers has been studied. Typical single layer AlN thin films are insulating due to the wide bandgap properties of the material, but stacked AlN-Pt structures can be conductive. Conductivity studies of the structures indicate regions of semiconductor behavior as well as regions where tunneling occurs. The thickness of the AlN layers, as well as the number of AlN-Pt interfaces in the structures, is found to impact the conduction and tunneling mechanism. Fowler-Nordheim theory and plots were used to determine trends in the electrical behavior and it was found that the field enhancement factor depends on the total thickness of the AlN layers, while the conduction mechanism, tunneling, or multistep hopping between midbandgap states, depends on the number of interfaces as well.

A Rapid Thermal Annealing (RTA) system was used to anneal sputtered and MOVPEgrown Aluminum Nitride (AlN) thin films at temperatures up to 1000°C in ambient and controlled environments. According to Energy Dispersive X-Ray Analysis (EDAX), the films annealed in an ambient environment rapidly oxidize after five minutes at 1000°C. Below 1000°C the films oxidized linearly as a function of annealing temperature which is consistent with what has been reported in literature [1]. Laser Doppler Vibrometry (LDV) was used to measure the piezoelectric coefficient, d 33 , of these films. Films annealed in an ambient environment had a weak piezoelectric response indicating that oxidation on the surface of the film reduces the value of d 33 . A high temperature furnace has been built that is capable of taking in-situ measurements of the piezoelectric response of AlN films. In-situ d 33 measurements are recorded up to 300°C for both sputtered and MOVPE-grown AlN thin films. The measured piezoelectric response appears to increase with temperature up to 300°C possibly due to stress in the film.

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