Identifying and overcoming the interface originating c-axis instability in highly Sc enhanced AlN for piezoelectric micro-electromechanical systems

Abstract

Enhancing the piezoelectric activity of AlN by partially substituting Al with Sc to form Al1–xScxN is a promising approach to improve the performance of piezoelectric micro-electromechanical systems. Here, we present evidence of an instability in the morphology of Al1–xScxN, which originates at, or close to, the substrate/Al1–xScxN interface and becomes more pronounced as the Sc content is increased. Based on Transmission electron microscopy, piezoresponse force microscopy, X-ray diffraction, and SEM analysis, it is identified to be the incipient formation of (100) oriented grains. Approaches to successfully reestablish exclusive c-axis orientation up to x = 0.43 are revealed, with electrode pre-treatment and cathode-substrate distance found to exert significant influence. This allows us to present first measurements of the transversal thin film piezoelectric coefficient e31,f and dielectric loss tangent tan δ beyond x = 0.3.

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

“…The results qualitatively agree with previous calculations of the elastic properties [35,37,62,63] and piezoelectric properties [35,37], while simultaneously extending the range of alloy space examined, which is necessary to accurately determine the decoupled nature of the responses. The results agree with the observed softening [22,64] and enhanced piezoelectric strain values [31,[65][66][67] of compositions studied in the literature. It should be noted that many methods are used in these measurements and reliable methods are just now being developed [63,64,68].…”

“…Recent work has focused on incorporation of these alloys into devices such as energy harvesters [16], ultrasonic transducers [17], and primarily in acoustic resonators [18][19][20][21][22][23][24][25], enabling alternative operation modes [26][27][28]. From a deployment perspective, the continued work on processing conditions highlights the difficulty of achieving high-quality materials with x > 0.2, where misoriented grains degrade the macroscopic piezoelectric response [29][30][31][32]. In parallel with the experimental work, the theory community progressed from the previously mentioned firstprinciples polymorph energies [3] to first-principles mixing energies [4,33], lattice calculations [34], property calculations [35][36][37], and the effects of cation ordering on performance [38].…”

Surface Morphology and Microstructure of Pulsed DC Magnetron Sputtered Piezoelectric AlN and AlScN Thin Films

“…Based on the planar SEM (Figure (a)), large triangular shape grains are randomly distributed on the surface and based on cross‐sectional analysis (Figure (b)) they start forming in the early stages of the growth. Literature suggests that these abnormal grains might not be c ‐axis oriented and described these structures as misoriented grains. Further investigations of their structure as well as influences on piezoelectric properties are discussed below.…”

“…Incorporating Sc into AlN leads to forming a metastable material and this additionally affects the crystalline quality as well as the microstructure . However, to this day, there is only one publication on misoriented grains formed in AlScN and the correlations between growth conditions and density of misoriented grains in AlScN should be investigated in more depth.…”

Abnormal Grain Growth in AlScN Thin Films Induced by Complexion Formation at Crystallite Interfaces

“…The observed transition with increasing Sc content from a metastable wurtzite (piezoelectric) alloy to rocksalt structure (non‐piezoelectric) is not abrupt and well‐defined and depends on process conditions, as reported by Akiyama et al The decay of the piezoelectric response in the broad transition zone is either due to a phase mixture or due to a loss of polar order, e.g., misaligned grains in the microstructure of the polycrystalline wurtzite phase. In this article, we address the problem of frequently encountered, “abnormally” oriented grains (AOG), which heavily disturb material properties and device performance. The phenomenology is assessed by electron microscopy techniques, and mechanisms for their creation and proliferation are given.…”