Effect of interface fields on the piezoelectric response of aluminum nitride thin films

Group III-Nitride wide bandgap semiconductors have attracted much attention in the optoelectronic and electronic research areas recently. III-Nitride semiconductor materials are attractive materials for use in optoelectronic and high speed electronics devices because they are direct bandgap semiconductors and the bandgap can be varied over a wide range. It has also been shown that the III-Nitride group of materials function exceptionally well in harsh environments. The piezoelectric properties of the III-Nitride material system have been studied and several of the III-Nitride compounds have been found to have non zero piezoelectric coefficients. This work shows that the observed piezoelectric coefficient of Aluminum Nitride (AlN) is directly related to the metal used as the topside contact. The data and preliminary analysis presented here indicate that AlN cannot be treated as an insulating material and must be treated as a semiconductor in order to model its piezoelectric behavior

Fabrication and Characterization of AlN-based, CMOS compatible Piezo-MEMS Devices

This paper details the development of high-quality, c-axis oriented AlN thin films up to 2 {\mu}m thick, using sputtering on platinum-coated SOI substrates for use in piezoelectric MEMS. Our comprehensive studies illustrate how important growth parameters such as the base Pt electrode quality, deposition temperature, power, and pressure, can influence film quality. With careful adjustment of these parameters, we managed to manipulate residual stresses (from compressive -1.2 GPa to tensile 230 MPa), and attain a high level of orientation in the AlN thin films, evidenced by < 5deg FWHM X-Ray diffraction peak widths. We also report on film surface quality regarding roughness, as assessed by atomic force microscopy, and grain size, as determined through scanning electron microscopy. Having attained the desired film quality, we proceeded to a fabrication process to create piezoelectric micromachined ultrasound transducers (PMUTs) with the AlN on SOI material stack, using deep reactive ion etching (DRIE). Initial evaluations of the vibrational behavior of the created devices, as observed through Laser Doppler Vibrometry, hint at the potential of these optimized AlN thin films for MEMS transducer development

A Lateral Field Excited Thin Film Bulk Acoustic Wave Sensor

Medical and environmental needs have served as a catalyst for the development of sensors that can probe the molecular level and below. This study addresses the practicality of highly sensitive aluminum nitride (AlN) thin film bulk acoustic wave resonators (FBARs) as sensors from theoretical and experimental points of view. Theoretically, COMSOL Multiphysics simulations predict that lateral field excitation of AlN produces an electric field perpendicular to the c-axis, with the electrical energy density being concentrated in the active area of the sensor. An analysis of the piezoelectrically stiffened Christoffel equation shows that the shear mode can be excited by an applied electric field in the x − y plane. Several thin films were deposited on various substrates such as borosilicate glass, silicon, sapphire, and fused silica using RF reactive magnetron sputtering and e-beam evaporation. To characterize film structure and composition, x- ray diffraction and x-ray photoelectron spectroscopy were used. An Agilent network analyzer was used to assess the performance of the sensor in air and water. In the most successful case, c-axis AlN films with a FWHM of 1.5 degrees were fabricated with quality factors between 33-36 in air and water. The magnitude of the admittance did not change appreciably when the film was exposed to water, indicating a shear mode was excited. Overall, a building block to a realizable AlN sensor was established

Measurement and ab initio Investigation of Structural, Electronic, Optical, and Mechanical Properties of Sputtered Aluminum Nitride Thin Films

We report our results on highly textured aluminum nitride (AlN) thin films deposited on glass substrates, oriented along the c-axis, using DC-magnetron sputtering technique for different values of back pressure. The structural, electronic, optical, piezoelectric, dielectric, and elastic properties of sputtered AlN thin films are measured and characterized. In particular, X-ray powder diffraction (XRD) technique shows that AlN thin films exhibit a hexagonal structure. Moreover, we employed ab initio simulations of AlN using the Vienna Ab Initio Simulation Package (VASP) to investigate the structural and the electronic properties of hexagonal AlN structures. The experimental lattice parameters of the as-prepared thin films agree well with those calculated using the total energy minimization approach. The optical parameters of AlN thin films, such as transmittance and refractive index, were measured using UV–vis measurements. Our measurements of refractive index, n, of AlN thin films yield a value of 2.2. Furthermore, the elastic, piezoelectric, and dielectric tensors of AlN crystal are calculated using VASP. The dynamical Born effective charge tensor is reported for all atoms in the unit cell of AlN. Interestingly, ab initio simulations indicate that AlN has a static dielectric constant approximately equal to 4.68, which is in good agreement with the reported experimental value. In addition, the clamped-ion piezoelectric tensor is calculated. The diagonal components of the piezoelectric tensor are found to be e33=1.79 C/m2 and e31=−0.80 C/m2. The large values of the piezoelectric coefficients show that a polar AlN crystal exhibits a strong microwave piezoelectric effect. Additionally, the components of the elastic moduli tensor are calculated. The extraordinary electronic, optical, piezoelectric, and elastic properties make AlN thin films potential candidates for several optoelectronic, elastic, dielectric, and piezoelectric applications

Magnetoelectric Effect in AIN/CoFE Bi-Layer Thin Film Composites

The present work is aimed at fabricating bi-layer aluminum nitride (AlN)/cobalt iron (CoFe) magnetoelectric (ME) thin films using reactive rf/dc magnetron sputtering. A systematic study on structural, morphological, piezoelectric, magnetic and magnetoelectric properties is undertaken. Except for AlN and CoFe, no other phases were detected with the layer thicknesses measured at160 and 130 nm, respectively. The rms roughness measured was around 2.096 nm for AlN and 1.806 nm for CoFe. The bi-layer thin film exhibited both good piezoelectricity and ferromagnetism, as well as ME effect. A 52% change observed in the piezoelectric signal, measured using magnetic field assisted piezoresponse force microscopy, can be ascribed to the existence of a stress-mediated magnetoelectric coupling between AlN and CoFe

Magnetron Sputter Deposition of Nanostructured AlN Thin Films

Aluminum nitride (AlN) is a material of growing interest for power electronics, fabrication of sensors, micro-electromechanical systems, and piezoelectric generators. For the latter, the formation of nanowire arrays or nanostructured films is one of the emerging research directions. In the current work, nanostructured AlN films manufactured with normal and glancing anglemagnetron sputter depositions have been investigated with scanning and transmission electron microscopy, X-ray diffraction, atomic force microscopy, and optical spectroscopy. Growth of the nanostructures was realized utilizing metal seed particles (Ag, Au, and Al), allowing the control of the nucleation and following growth of AlN. It was demonstrated how variations of seed particlematerial and size can be used to tune the parameters of nanostructures and morphology of the AlN films. Using normal angle deposition allowed the growth of bud-shaped structures, which consisted of pillars/lamellae with wurtzite-like crystalline structures. Deposition at a glancing angle of 85° led to a film of individual nanostructures located near each other and tilted at an angle of 33° relative to the surface normal. Such films maintained a high degree of wurtzite-like crystallinity but had a more open structure and higher roughness than the nanostructured films grown at normal incidence deposition. The developed production strategies and recipes for controlling parameters of nanostructured films pave the way for the formation of matrices to be used in piezoelectricapplications

Nitrogen flow rate dependent atomic coordination, phonon vibration and surface analysis of DC Magnetron sputtered Nitrogen rich-AlN thin films

In this work, the effect on crystallite orientation, surface morphology, fractal geometry, structural coordination and electronic environment of DC magnetron sputtered AlN films were investigated. X-ray diffraction results disclosed that the c-axis orientation of AlN films increased with the preferred wurtzite hexagonal structure above 17% N2 flow. X-ray reflectivity data confirmed AlN film density increased with increasing N2 flow and was found to be 3.18g/cm3 for 40% N2. The transition of electrons from N 1s to 2p states hybridized with Al 3p states because of {\pi}* resonance was obtained from X-ray absorption spectroscopy of the N K-edge. The semi-empirical coordination geometry of nitrogen atoms has been studied by deconvolution of N K-edge. The surface composition of AlN films at 40% N2 consists of 32.08, 51.94 and 15.97at.% Al, N and O respectively. Blue-shifting of A1(LO) and E1(LO) modes in the Raman spectra at phonon energies 800 and 1051cm-1 respectively was most likely due to the presence of oxygen bonds in the AlN films

Deposition and characterisation of c-axis oriented AlScN thin films via microwave plasma-assisted reactive HiPIMS

In this work, we demonstrate that highly oriented c-axis aluminium scandium nitride (AlScN) piezoelectric thin films can be deposited via microwave plasma-assisted reactive high power impulse magnetron sputtering (MAR-HiPIMS), without the necessity of substrate heating. A combination of in situ plasma diagnostics, i.e. time-of-flight mass spectrometry (ToF-MS), modified quartz crystal microbalance (m-QCM), and magnetic field measurements allowed to optimise the deposition conditions, in turn maximising the nitrogen supply and ionic flux at the substrate region, while maintaining stable discharge conditions. The AlScN thin films synthesised in this study were deposited as chemically gradient coatings with varying levels of scandium doping, and were characterised using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). Obtaining highly textured films was made possible with the addition of microwave plasma to the optimised HiPIMS discharge, where the wurtzite AlScN films (with up to 20 at. % Sc) exhibited a stronger texture in the (0002) orientation compared to films prepared without microwave plasma. Additionally, the use of a microwave plasma led to a significant decrease in oxygen content in the films and increase in nitrogen content, ensuring stoichiometric compositions. Based on the results mentioned above, it is expected that the AlScN thin films fabricated via MAR-HiPIMS would exhibit a strong piezoelectric response