Temperature-Dependent Structural and Elastic Properties of Cubic and Hexagonal Sc_xAl_1-xN

The novel wide-bandgap semiconductor, Scandium Aluminum Nitride (Sc<i>_x</i>Al_1-<i>x</i>N) significantly enhances its piezoelectric properties compared to Aluminum Nitride (AlN). This unique increase in piezoelectric coefficient <i>d</i>₃₃(<i>x</i>) makes Sc<i>_x</i>Al_1-<i>x</i>N a highly favorable material for numerous applications, including improved piezo-acoustic devices, which can be utilized as high-frequency filters [1]. The optimization of the devices is impeded by the trade-off between piezoelectric response and the stiffness of the crystal. To address this, a detailed investigation of the anisotropic elastic, piezoelectric, and thermodynamic properties is necessary. However, the ternary alloy Sc<i>_x</i>Al_1-<i>x</i>N undergoes a solid-solid phase transition from hexagonal wurtzite (wz) to cubic rock salt (rs) structure as the Sc content <i>x</i> increases, which differs from other ternary nitrides [2]. The coefficient <i>d</i>₃₃(<i>x</i>) of wz-Sc<i>_x</i>Al_1-<i>x</i>N exhibits the highest values and the elastic properties undergo significant changes when the Sc content is proximate to this transition. The specific Sc:Al ratio, pressure and temperature conditions and seed layer selection (either AlN(0001) or ScN(111)) that induce this B4-B1 transition have yet been explored und will be discussed in this work. Accordingly, this study will discuss these conditions as they provide valuable insights to improve the efficiency of devices that rely on maximum piezoelectric responses. The physical properties of resonators are dependent on the operating temperature (<i>T</i>). The structural changes can be described utilizing lattice parameters <i>c</i>(<i>x, T</i>) and <i>a</i>(<i>x, T</i>), internal parameter <i>u</i>(<i>x, T</i>), and elastic tensor <i>C</i>_ij(<i>x, T</i>). In order to determine temperature-dependent structural properties of pulsed magnetron co-sputtered thin films, in-situ X-ray diffractometry measurements ranging from 300 K to 1000 K were conducted to determine the lattice parameters <i>c</i>(<i>x, T</i>) and <i>a</i>(<i>x, T</i>) for hexagonal and cubic Sc<i>_x</i>Al_1-<i>x</i>N(0001)/Si(111) thin films, respectively. The Debye model was utilized to determine the coefficients of linear thermal expansion α_aand α_cfor wz-Sc<i>_x</i>Al_1-<i>x</i>N along the〈1000〉and [0001] directions and α_a’ for rs-Sc<i>_x</i>Al_1-<i>x</i>N along the 100 directions. For the first time, the linear thermal expansion coefficient was experimentally determined for wz-Sc<i>_x</i>Al_1-<i>x</i>N (0.0&lt;<i>x</i>&lt;0.5) and rs-Sc<i>_x</i>Al_1-<i>x</i>N (0.5&lt;<i>x</i>&lt;1.0). This temperature-dependent structural information was used to compute additional structural specifications such as bond lengths and bond angles for random alloys of wz-Sc<i>_x</i>Al_1-<i>x</i>N and rs-Sc<i>_x</i>Al_1-<i>x</i>N. Scanning electron microscopy (SEM) was utilized to observe the effects of heating on surface morphology. To understand the elastic properties of wz- and rs-Sc<i>_x</i>Al_1-<i>x</i>N fully, various anisotropic elastic properties including shear modulus <i>G</i> and Young’s modulus <i>E</i> were calculated for multiple crystallographic planes, by employing simulated tensor quantities at ambient conditions. For the first time, the in-plane epitaxial relation between rs-Sc<i>_x</i>Al_1-<i>x</i>N(111) and Si(111) was determined, which is rs-Sc<i>_x</i>Al_1-<i>x</i>N[110] || Si[100]. Further thermodynamic properties such as specific heat, heat conductivity and Debye temperature were calculated. This newfound knowledge of temperature- and direction-dependent structural and elastic properties offers the potential for improved efficiency of piezo-acoustic devices.<br/><b>References</b><br/>[1] N. M. Feil, E. Mayer, A. Nair, B. Christian, A. Ding, C. Sun, S. Mihalic, M. Kessel, A. Zukauskaité, and O. Ambacher, J. Appl. Phys. <b>130</b>, 164501 (2021); doi: 10.1063/5.0055028.<br/>[2] O. Ambacher, S. Mihalic, E. Wade, M. Yassine, A. Yassine, N. Feil, and B. Christian, J. Appl. Phys. <b>132</b>, 175101 (2022); doi: 10.1063/5.0120141.