High Temperature Elastic Constant Prediction of Some Group III-Nitrides

Laser Ultrasound Investigations of AlScN(0001) and AlScN(11-20) Thin Films Prepared by Magnetron Sputter Epitaxy on Sapphire Substrates

The laser ultrasound (LU) technique has been used to determine dispersion curves for surface acoustic waves (SAW) propagating in AlScN/Al₂O₃ systems. Polar and non-polar Al_0.77Sc_0.23N thin films were prepared by magnetron sputter epitaxy on Al₂O₃ substrates and coated with a metal layer. SAW dispersion curves have been measured for various propagation directions on the surface. This is easily achieved in LU measurements since no additional surface structures need to be fabricated, which would be required if elastic properties are determined with the help of SAW resonators. Variation of the propagation direction allows for efficient use of the system's anisotropy when extracting information on elastic properties. This helps to overcome the complexity caused by a large number of elastic constants in the film material. An analysis of the sensitivity of the SAW phase velocities (with respect to the elastic moduli and their dependence on SAW propagation direction) reveals that the non-polar AlScN films are particularly well suited for the extraction of elastic film properties. Good agreement is found between experiment and theoretical predictions, validating LU as a non-destructive and fast technique for the determination of elastic constants of piezoelectric thin films.

An Experimental and Theoretical Study of Impact of Device Parameters on Performance of AlN/Sapphire-Based SAW Temperature Sensors

The impact of device parameters, including AlN film thickness (h_AlN), number of interdigital transducers (N_IDT), and acoustic propagation direction, on the performance of c-plane AlN/sapphire-based SAW temperature sensors with an acoustic wavelength (λ) of 8 μm, was investigated. The results showed that resonant frequency (f_r) decreased linearly, the quality factor (Q) decreased and the electromechanical coupling coefficient (Kt2) increased for all the sensors with temperature increasing from −50 to 250 °C. The temperature coefficients of frequency (TCFs) of sensors on AlN films with thicknesses of 0.8 and 1.2 μm were −65.57 and −62.49 ppm/°C, respectively, indicating that a reduction in h_AlN/λ favored the improvement of TCF. The acoustic propagation direction and N_IDT did not obviously impact the TCF of sensors, but they significantly influenced the Q and Kt2 of the sensors. At all temperatures measured, sensors along the a-direction exhibited higher f_r, Q and Kt2 than those along the m-direction, and sensors with N_IDT of 300 showed higher Q and Kt2 values than those with N_IDT of 100 and 180. Moreover, the elastic stiffness of AlN was extracted by fitting coupling of modes (COM) model simulation to the experimental results of sensors along different directions considering Euler transformation of material parameter-tensors. The higher f_r of the sensor along the a-direction than that along the m-direction can be attributed to its larger elastic stiffness c₁₁, c₂₂, c₄₄, and c₅₅ values.

Coherent-interface-induced strain in large lattice-mismatched materials: A new approach for modeling Raman shift

Strain engineering as one of the most powerful techniques for tuning optical and electronic properties of Ill-nitrides requires reliable methods for strain investigation. In this work, we reveal, that the linear model based on the experimental data limited to within a small range of biaxial strains (< 0.2%), which is widely used for the non-destructive Raman study of strain with nanometer-scale spatial resolution is not valid for the binary wurtzite-structure group-III nitrides GaN and AlN. Importantly, we found that the discrepancy between the experimental values of strain and those calculated via Raman spectroscopy increases as the strain in both GaN and AlN increases. Herein, a new model has been developed to describe the strain-induced Raman frequency shift in GaN and AlN for a wide range of biaxial strains (up to 2.5%). Finally, we proposed a new approach to correlate the Raman frequency shift and strain, which is based on the lattice coherency in the epitaxial layers of superlattice structures and can be used for a wide range of materials.

Electronic Supplementary Material

Supplementary material (Table S1: Values of bulk phonon deformation potentials and elastic constants for GaN and AlN from each reference used in Table 1, Fig. S1: Lattice parameters of SL layers using Eq. (8), and Fig. S2: Raman mapping using Eq. (7)) is available in the online version of this article at 10.1007/s12274-021-3855-4.

The Behavior of Gold Metallized AlN/Si- and AlN/Glass-Based SAW Structures as Temperature Sensors

Thin AlN piezoelectric layers have been deposited on high resistivity Si and glass substrates by reactive RF magnetron sputtering, in order to manufacture one-port gigahertz operating surface acoustic wave (SAW)-type resonators to be used as temperature sensors. The growth morphology surface topography, crystallographic structure, and crystalline quality of the AlN layers have been analyzed. Advanced nanolithographic techniques have been used to manufacture structures having interdigitated transducers with fingers and finger interdigit spacing width in the range of 250-170 nm. High resonance frequency ensures the increase of the sensitivity, but also of its normalized value, the temperature coefficient of frequency (TCF). The resonance frequency shift versus temperature has been measured in the -267°C-+150°C temperature range, using a cryostat setup adapted for on wafer microwave measurements up to 50 GHz. The sensitivity and the TCF were determined in the 25 °C-150 °C temperature range.

Optimization of AIN Composite Structure Based Surface Acoustic Wave Device for Potential Sensing at Extremely High Temperature

A surface acoustic wave (SAW) device with an aluminum nitride (AlN) composite structure of Al2O3/IDTs/AlN/Metal/Si was proposed for sensing at extreme high-temperature in this work. Optimization allowing determination of optimal design parameters for SAW devices was conducted using the typical coupling of modes (COM) model. The SAW propagation characteristics in the layered structure were investigated theoretically by employing the finite element method (FEM). Multiple acoustic-wave modes that occurred in the AlN composite structure was analyzed, and the corresponding suppression of spurious mode was proposed. The COM simulation parameters corresponding to the effective acoustic-wave mode were extracted, and the optimized parameters of the one-port SAW resonator with a high-quality factor were determined.

Structural, elastic constant, and vibrational properties of wurtzite gallium nitride: a first-principles approach

Perdew-Wang proposed generalized gradient approximation (GGA) is used in conjunction with ultrasoft pseudopotential to investigate the structural, elastic constant, and vibrational properties of wurtzite GaN. The equilibrium lattice parameters, axial ratio, internal parameter, bulk modulus, and its pressure derivative are calculated. The effect of pressure on equilibrium lattice parameters, axial ratio, internal parameter (u), relative volume, and bond lengths parallel and perpendicular to the c-axis are discussed. At 52 GPa, the relative volume change is observed to be 17.8%, with an abrupt change in bond length. The calculated elastic constants are used to calculate the shear wave speeds in the [100] and [001] planes. The finite displacement method is employed to calculate phonon frequencies and the phonon density of states. The first- and second-order pressure derivative and volume dependent Gruneisen parameter (γ(j)) of zone-center phonon frequencies are discussed. These phonon calculations calculated at theoretical lattice constants agree well with existing literature.

Atomistic modeling of III-V nitrides: modified embedded-atom method interatomic potentials for GaN, InN and Ga(1-x)In(x)N

Modified embedded-atom method (MEAM) interatomic potentials for the Ga-N and In-N binary and Ga-In-N ternary systems have been developed based on the previously developed potentials for Ga, In and N. The potentials can describe various physical properties (structural, elastic and defect properties) of both zinc-blende and wurtzite-type GaN and InN as well as those of constituent elements, in good agreement with experimental data or high-level calculations. The potential can also describe the structural behavior of Ga(1-x)In(x)N ternary nitrides reasonably well. The applicability of the potentials to atomistic investigations of atomic/nanoscale structural evolution in Ga(1-x)In(x)N multi-component nitrides during the deposition of constituent element atoms is discussed.