AlN, ZnO thin films and AlN/ZnO or ZnO/AlN multilayer structures deposited by PLD for surface acoustic wave applications

Abstract

Phone: þ33 587 502 331 (0002)-oriented AlN and ZnO thin films are deposited on C-sapphire substrates by pulsed laser deposition at 700 8C. Multilayered structures AlN/ZnO and ZnO/AlN are also grown on C-sapphire and investigated by X-ray diffraction, scanning electron microscopy and transmission measurements. Ellipsometry analysis is employed to investigate the stack of layers and the optical properties of thin films. This non-destructive analysis identified an intermediate layer between AlN and ZnO thin films for AlN/ZnO multilayer which didn't change the crystalline structure or piezoelectric properties of the multilayer. Surface acoustic wave velocities are respectively 5060 m s À1 for AlN and 5350 m s À1 for ZnO. In fact, investigations of surface acoustic wave (SAW) devices revealed a similar resonant frequency for both multilayers and in agreement with those measured for ZnO or AlN thin films.

Surface Acoustic Wave Sensor with Pd/ZnO Bilayer Structure for Room Temperature Hydrogen Detection

“…The acoustoelectric effect in the case of a single sensitive layer leads to a relative change of wave propagation velocity Δν given by where Δf is the frequency shift, f 0 the central frequency of the SAW device, ν 0 the unperturbed SAW velocity, K the electromechanical coefficient, C s = ε 0 + ε p is the sum of the permittivity of the region above the film and the substrate, and σ s is the surface conductivity of the sensing film [ 11 , 18 ]. In the case of bilayers, important additional parameters which determine velocity changes are the electrical conductivities of both layers and their thicknesses, since both film layers influence the SAW propagation through interaction between their mobile charges and the travelling electron potential φ of the SAW [ 17 ].…”

“…The sensitive layers of the SAW sensors were deposited onto ST-X quartz substrates by pulsed laser deposition (PLD). PLD was used because it presents a series of advantages in thin film deposition in general [ 17 , 26 ], and in particular has been proven to allow good control of sensitive film porosity, which is essential for SAW gas sensors [ 20 ]. The beam of a Nd:YVO 4 laser is focused onto the surface of the target, leading to ablation of the target material and deposition onto a substrate placed at a distance of 35 mm in front of the target and parallel to its surface ( Figure 1 ).…”

“…By using bilayer films, the absolute value of the multilayer film conductivity can be adjusted to operate in a region where the acoustoelectric effect is efficient. Various material combinations such as Pd/WO 3 , ZnO/GaN, AlN/ZnO, or ZnO/SiO 2 have been explored for SAW gas sensors based on this effect [ 9 , 16 , 17 , 18 ]. In the case of hydrogen sensing, an efficient acoustoelectric effect is desirable, since the mass effect for hydrogen is smaller than for heavier gases [ 9 ].…”

Stability studies of ZnO and AlN thin film acoustic wave devices in acid and alkali harsh environments

“…The velocities of Rayleigh wave and Sezawa modes were calculated to be 2566.1 m s À1 and 5111.2 m s À1 for l of 12 mm and to be 2679 m s À1 and 5305 m s À1 for l of 20 mm, respectively. The Rayleigh phase velocity of the pure ZnO is $2500 m s À1 , 18 whereas that for the Rayleigh wave in glass is 3200 m s À1 , 19 which is larger than that of ZnO lm. When the wavelength is decreased, more energy is conned within the ZnO lm, thus resulting in a lower velocity of wave propagation, and leading to a lower velocity of the layered structure.…”

Sub-limit detection in SAW sensors by FFT spectral analysis of frequency time instability

“…This perturbation can occur through mechanical or electrical effects, mainly through changes in mass density or electrical conductivity (Devkota et al, 2017). Various materials have been used for the sensitive layers of SAW sensors, such as ZnO, AlN, WO 3 , Pd or SnO 2 (Mujahid and Dickert, 2017;Dutheil et al, 2015). In sensing, one of the most important issues is the minimum amount of analyte that can still be distinguished, the so-called limit of detection (LOD).…”