Impact of AlN Seed Layer on Microstructure and Piezoelectric Properties of Y_xAl_1−xN (x = 15%) Thin Films

Abstract

Alloying transition metals into aluminum nitride (AlN) has surged over the past decade to increase the piezoelectric performance of AlN for microelectromechanical systems (MEMS) applications. So far, the highest piezoelectric coefficients have been achieved by alloying scandium into AlN. But, most recently published studies have theoretically predicted by ab‐initio calculations that yttrium can also be a promising and less expensive alternative to scandium. This paper focuses on the impact of an AlN seed layer on the growth and piezoelectric properties of sputter‐deposited, polycrystalline Y0.15Al0.85N thin films on a silicon substrate. For the first time, the increase in piezoelectric performance of YxAl1−xN (x = 15%) thin films with a d33 of ≈7.85 pC N−1 due to this seed layer approach is presented, thus reaching in good agreement theoretical predictions. Furthermore, for the first time, the crystalline stability of Y0.15Al0.85N layers in a pure oxygen environment up to 1200 °C is reported, demonstrating a high oxygen resistance up to 800 °C even in this harsh environment. Detailed thin film analysis is done with various techniques such as X‐ray diffraction, atomic force microscopy, nano‐indentation, and high‐resolution transmission electron microscopy and correlated with piezoelectric coefficient measurements.

AlYN Thin Films with High Y Content: Microstructure and Performance

“…Similar behavior can be found in the neighboring material systems such as Al 1− x Sc x N. [ 42 ] Judging by the fact that the out‐of‐plane strain is proportional to the Y content, yet showing the opposite outcome, we conjecture that the factor responsible for it has to be connected to the sputtering of Y target as all other deposition parameters remained fixed. As reported previously, [ 20–22 ] Y target can be considered as a source of oxygen, the presence of which is confirmed by the EDX analysis of our films (Figure S2, Supporting Information). Furthermore, the concentration of this impurity scales up with the sputter power, leading to the staggering 5% in the case of Al 0.72 Y 0.28 N. The harmful effect of O impurities in AlN films was studied revealing the lattice distortions, [ 43,44 ] enhanced thermal resistance, [ 45,46 ] and formation of midgap defect states.…”

“…Macroscopic biaxial stress in nitride layers arises from lattice mismatch and differences in the coefficients of thermal expansion of nitride and Pt layers. [ 32 ] Thus, the application of the seed layer helps to reduce the effect of the disparaging parameters on the AlYN layer, [ 22 ] since those of AlYN and AlN are much closer, even considering the expansion of unit cell with increasing Y concentration. In addition to the biaxial stress, we observe the presence of hydrostatic strain, exhibited by the c lattice constant values being smaller than the simulated ones.…”

“…Although higher than the one of pure AlN, [ 36 ] the piezoelectric performance of the alloy films does not improve with higher Y content, as the d 33 coefficient remains constant at the value of ≈5 pm V −1 . In their previous works, Schlögl et al and Pandit et al claimed higher d 33 values 7.79 pm V −1 [ 21 ] and 7.85 pC N −1 , [ 22 ] respectively, despite the lower amount of yttrium‐based solute. This discrepancy may arise due to the use of another method of the d 33 determination based on the direct piezoelectric effect.…”

“…[ 21 ] In order to amend the influence of ill‐defined surfaces, an intermediate AlN film of 50 nm was introduced as a seed layer, facilitating a higher value of d 33 = 7.85 pC N −1 for the case of x = 0.15. [ 22 ] The AlYN film grown on the AlN seed layer exhibited smaller grains (34 nm vs. 134 nm for the film without the seed), oriented with (0002) and () planes parallel to the film surface, which may be responsible for the fractional enhancement of the piezocoefficient. Apart from the technical difficulties of AlYN alloying, there is also a controversy in the simulation results, which predicts the piezoelectric performance of the alloys as a function of solute fraction.…”

Metal‐Organic Chemical Vapor Deposition of Aluminum Yttrium Nitride

“…[36][37][38][39] The growth of AlYN has been demonstrated only by two groups so far: Linköping University [29,40] and TU Wien. [22,41] In both cases, the sputtering technique was used to grow the samples at temperatures below 1000 °C, and the maximum concentration of Y in crystalline wurtzite AlYN layers was below 22% and 15%, respectively. Sublimation or sputtering was used to grow pure YN samples, [42,43] while unsuccessful attempts to incorporate Y by metal-organic chemical vapor deposition (MOCVD) were made only by Koleske et al to deposit GaYN.…”