Erbium Alloyed Aluminum Nitride Films for Piezoelectric Applications

Abstract

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.

Implications of heterostructural alloying for enhanced piezoelectric performance of (Al,Sc)N

“…This study provides a theorydriven understanding of the fundamental materials science concepts underlying this behavior in addition to experimental realization of predicted property enhancements. Additionally, it provides an opportunity to revisit the results of earlier studies on Al 1−x Sc x N as well as other Al 1−x R x N such as R = vanadium [39], tantalum [39,40], erbium [41], or chromium [42] in a different context, providing fundamental insights to guide future studies of these and other heterostructural alloy systems.…”

Synthesis and Calculations of Wurtzite Al1−xGdxN Heterostructural Alloys

“…[26][27][28][29][30] While it seems unlikely from an ionic radius perspective that any rare earth cations would be able to incorporate into wurtzite AlN, recent work has showed that Er 3+ and Yb 3+ , much larger cations than Sc 3+ , were substituted successfully at x = 0.015 and up to x ≈ 0.15, respectively. 31,32 In GaN, Yb 3+ can incorporate up to x ≈ 0.3, consistent with the larger radius of Ga 3+ (∼0.47 Å). 33 Incorporating small amounts of Gd 3+ (∼0.94 Å) into wurtzite AlN and GaN has been investigated primarily for optoelectronic applications such as cathodoluminescence and fieldemission devices, 34 given the sharp ultraviolet emission of Gd 3+ at approximately 320 nm and the AlN host lattice as an established optoelectronic material.…”

Synthesis and Calculations of Wurtzite Al1–xGdxN Heterostructural Alloys

“…26−30 While it seems unlikely from an ionic radius perspective that any rare earth cations would be able to incorporate into wurtzite AlN, recent work has shown that Er 3+ and Yb 3+ , much larger cations than Sc 3+ , were substituted successfully at x = 0.015 and up to x ≈ 0.15, respectively. 31,33 In GaN, Yb 3+ can incorporate up to x ≈ 0.3, consistent with the larger radius of Ga 3+ (∼0.47 Å). 32 Incorporating small amounts of Gd 3+ (∼0.94 Å) into wurtzite AlN and GaN has been investigated primarily for optoelectronic applications such as cathodoluminescence and field-emission devices, 34 given the sharp ultraviolet emission of Gd 3+ at approximately 320 nm and the AlN host lattice as an established optoelectronic material.…”

“…After significant effort, Sc 3+ has been substituted into the wurtzite AlN crystal structure at compositions of up to x ≈ 0.43 (i.e., Al 0.57 Sc 0.43 N) before phase separation to the rocksalt ScN end-member crystal structure occurs. ,,− Recent work has shown that the solubility of Sc 3+ depends both on ionic size and on ionicity, with Sc–N bonds more ionic than Al–N bonds; however, this ionicity effect is difficult to deconvolute from other effects. Other transition-metal cations such as Cr, Zr, Hf, Y, Ta, and Ni have also been investigated as alloys with AlN, and their properties have been studied. − While it seems unlikely from an ionic radius perspective that any rare earth cations would be able to incorporate into wurtzite AlN, recent work has shown that Er 3+ and Yb 3+ , much larger cations than Sc 3+ , were substituted successfully at x = 0.015 and up to x ≈ 0.15, respectively. , In GaN, Yb 3+ can incorporate up to x ≈ 0.3, consistent with the larger radius of Ga 3+ (∼0.47 Å) …”