Piezoelectric Response in WO3-x Thin Films by Aluminum Clustering

We report piezoelectric response of d 33 = 35 ± 5 pm V − 1 on aluminum doped tungsten trioxide thin lms (Al-WO 3 − x ), prepared by RF-sputtering and post annealing treatment in air atmosphere. Using XPS characterization indicate a stoichiometry of WO 2.7 and Raman a distorted octahedral tungsten vibration mode of monoclinic WO 3 at 236.9 cm − 1 , 691 cm − 1 and 803 cm − 1 corresponding to O-W-O chemical bonds. The grazing incidence X-ray diffraction revealed a non-centrosymmetric monoclinic (P2 1 /c) and tetragonal (P4/nmm) mixed phases of WO 3 − x with islands of piezoelectric domains as observed by atomic force microscope, additionally atom probe tomography revealed diffusion of aluminum ions from Al 2 O 3 substrate.


Introduction
Outstanding applications in the eld of Internet of Things (IoT) [1]- [3] and micro-electromechanical systems (MEMS) [4,5] have increased the demand for environmentally friendly, low cost, and reliable materials to fabricate several types of sensors, actuators and power components [6][7][8][9]. This has expanded the interest on properties such as electrochromism and piezoelectricity in various materials [10][11][12]. Piezoelectric materials have gained importance in modern day technology due their wide range of applications, from positioners in electronic microscopes, crystal oscillators, sound and vibration sensors, and energy harvesting devices [13][14][15][16][17][18][19]. Piezoelectricity occurs mainly in non-centrosymmetric structures [20]- [23], which refers to those lacking an inversion point, i.e., a point or atom position at speci c coordinates inside the Bravais unit cell, with respect to a crystallographic plane; in the spatial distribution this causes uneven distribution of electronic states and when a mechanical force is applied part of those accumulated electron charges are released, causing a piezoelectric response, as described extensively in the literature [21], [24]- [28]. Several materials can exhibit piezoelectricity to some degree, this includes ceramic materials [20], [29], polymers [23,24] and semiconductors [31], with lead zirconate titanate, also known as PZT, is one of most common piezoelectric ceramics as reported in the literature, with remarkable piezoelectric response derived from interaction at morphotropic phase boundary (MPB) [25]. Sustainable environmental concerns and also that it is an insulator [27,28] has caused an intensi ed on-going research worldwide in search for lead-free conducting piezoelectric materials [27,29,30], and previous investigations in the materials eld indicate that piezoelectric semiconductors are potential candidates due to intrinsic piezoelectricity and conductivity [36]. Also, some binary compounds such as ZnO and AlN, have been considered attractive piezoelectric materials and currently are under extensive research [25,32], some reports indicates addition of ZnO or WO 3 as dopants in PZT or in BaTiO 3 -SrTiO 3 ceramics can induce piezo-response [33][34][35][36]. Recently, Chen et al. successfully used ZnO-WO 3 − x nanorods for piezoelectric-photoelectrochemical water splitting due to intrinsic ZnO piezoelectricity and the fact that WO 3 − x charge carrier concentration can be tuned in function of oxygen vacancies [42]. Corby et al. found that vacancy concentration of 2% for stoichiometric oxides meaning oxygen concentration, can maximize photocurrent in water splitting performance [43], similar to the results reported by Soltani et al. [44]. Kim et al. reported piezoelectric response in an oxygen-de cient WO 2.96 lm with a d 33 coe cient of 7.9 pm/V, attributed to the non-centrosymmetric structures within lm thickness, that corresponds to monoclinic and tetragonal phases [45]. In this communication, we are reporting piezoelectric response for aluminum doped WO 3 thin lms annealed at 400°C, along with extensive characterization by grazing incidence X-ray diffraction (GIXRD) and atom probe tomography (APT).

Results And Discussion
Piezoelectric response by PFM WO 3 thin lms were deposited on sapphire with a resulting thickness of 225 nm and subsequently annealed at various temperatures, see experimental methods section. A piezoelectric response was found in the lm annealed at 400°C, determined by characterization using piezo force microscopy technique in dual AC resonance tracking (DART) mode, as described extensively in [22, 26-28, 46, 47]. The surface topography of this lm is shown in Fig. 1a, and Fig. 1b) and 1c) corresponds to the piezo force microscopy signal phase before and after measurements, revealing a local hysteresis loops; hysteresis loops corresponding to red circles, where piezo response domains appear as yellow, white and violet colored regions show polarization direction piezoelectric domains, as described by Kholkin et al. [27]. The The measured d 33 coe cient is assume to occur due to non-centrosymmetric phases in combination with potential oxygen vacancies and aluminum doping induced a different stoichiometric composition of WO 3 − x lms, mainly for those processed at 400°C in agreement with reports as found in the literature [38, 48-57]. And con rmed by XPS measurements (Supplemental material) which reveals a stoichiometry of WO 2.7 in the surface of the lm annealed at 400°C and in agreement with grazing incidence x-ray diffraction (GIXRD) and atom probe tomography (APT) as presented in this communication.
Crystallographic structure as determined by GIXRD As presented in Fig. 2, an evolution of crystallinity occurred on the lms from room temperature to annealing process at 400°C and 550 ºC corresponding to polycrystalline amorphous structure, re ections at 23.1° corresponds to (001), (021) and (121)  we were able to determine that aluminum is diffused creating changes on the electronic states mainly on island form over lm and mixed monoclinic and tetragonal (α-WO 3 − x and γ-WO 3 − x ) specially when is processed at 400°C, in agreement with Ahart et al. [25] and Ibrahim et al.
[68] who explained in detail that mixed phases can produce a morphotropic phase boundary (MPB).

Chemical distribution by atom probe tomography
In order to investigate chemical volume distribution a series of atom probe tomography characterizations were completed, which is an abrasive technique used to obtain time of ight mass spectrometry from events occurred due to laser pulse ionic evaporation at high-vacuum as described in the literature [69], for all APT measurements a well-de ned interface between WO 3 lm and Al 2 O 3 substrate was revealed. From mass spectrum it was possible to achieve chemical composition distribution mainly at the WO 3 lm thickness (0-400nm) and traces of aluminum, oxygen and tungsten was found as shown in Fig. 3. Tungsten concentration remains around 27% during annealing process and oxygen concentration is about 70% at 400 ºC with strong traces of aluminum (~ 3%) which forms clusters, as shown in Fig. 3a; it is our believe this clusters are formed during annealing process by ionic diffusion from sapphire substrate due to voids occurred by oxygen vacancies which allowed aluminum ions to undergo onto WO 3 [56], [65], which is in agreement with mixed phases as encountered by grazing incidence x-ray diffraction.
For sample processed at 500°C lower concentration (> 1%) of aluminum ions is found and corresponds mainly to α-WO 3 − x as shown in Fig. 3b and no traces of grain boundaries was found for all samples.

Conclusions
We report a piezoelectric response with d 33 = 35 ± 5 p/V for Al-WO 3 − x in thin lms processed at 400°C.
The grazing incidence x-ray and atom probe indicates that piezo-response effect is caused by aluminum diffusion creating mixed phase between γ-WO 3 − x (monoclinic) and α-WO 3 − x (tetragonal) corresponding to a non-centrosymmetric. And we found that at elevated annealing temperatures (> 400°C) lm recrystallizes in α-WO 3 − x followed by γ-WO 3 − x causing a reorder to piezoelectric domains, as con rmed by atom probe tomography where no clustering of aluminum was encountered for sample processed at 500°C.

RF magnetron sputtering
The tungsten trioxide (WO 3 ) thin lms were deposited by radio frequency magnetron sputtering technique using a 99.99% pure WO 3 disk as target and Al 2 O 3 as substrate. The base pressure was set up to 1×10 -6 Torr before allowing Ar into the chamber as plasma source. The deposition rate was 1 Å/s, at a working pressure of 3 mTorr and 225 W of RF power. The wafer was cut into several samples for subsequent annealing process at temperatures of 300 ˚C, 400 ˚C, 500 ˚C and 550 ˚C, for 45 min with a 15 min ramp down, in air. A lm thickness of ~220 nm was measured for the as-deposited lm using pro lometry.

Piezo Force Microscopy (PFM)
Domain imaging, switching and piezoelectric hysteresis loops were investigated by piezoresponse force microscopy (PFM) using the Dual AC Resonance Tracking (DART) mode, in a commercial Atomic Force Microscope (AFM) model In nity 3D Asylum Research with two internal lock-ins ampli ers. The PFM was operated in vertical mode with an AC voltage amplitude of 5 V pk-pk and at a drive frequency of 398 kHz far below the resonance of the cantilever, applied between the bottom electrode and the Pt/Ir conductive tip during imaging PFM. To achieve the measurement of local polarization (hysteresis loops) a voltage of -10 to 10 V pk-pk was applied using the spectroscopy PFM mode.
Grazing Incidence X-Ray Diffraction (XRD) Crystallographic structure was obtained with the aid of Panalytical Empyrean system, with Cu Kα radiation source (λ=1.54 Å) at an operating accelerating voltage of 40 kV and an emission current of 30 mA.
Scanning angle was varied from 20˚ to 80˚ with a step size of 0.05˚.

Atom Probe Tomography (APT)
Three-dimensional chemical distribution for W, O and Al was obtained with a Cameca® LEAP 4000X high-resolution system, equipped with a UV laser (λ ∼ 355 nm). All measurements were taken at a set temperature of 50 K with an evaporation rate of 0.2 and a laser frequency of 100 kHz. The laser beam was set to 20 pJ/V, and all data were reconstructed from SEM images using the Cameca IVAS© 3.6.14 package. Additional samples were prepared by directly coating the micro tip coupons provided by the CAMECA with the multilayer sputtering system.   is the PFM Signal Phase before and after measurements the local hysteresis loops. d) Amplitude, e) phase, and f) piezo response (d33) versus AC Applied bias voltage of WO3 thin lm annealed at 400°C.

Figure 2
Grazing incidence X-ray diffraction pattern for all as-deposited WO3 thin lms over sapphire. GIXRD measurements were performed in samples annealed at 300 °C, 400 °C, 500 °C, 550 °C, and roomtemperature as reference. The sample at 300 ˚C presents an amorphous structure, while 400 ˚C shows a mixture of cubic Al, monoclinic and tetragonal WO3 phases. Sample at 500 ˚C presents a mixture of monoclinic WO3 and cubic Al-WOx. Finally, the sample annealed at 550 ˚C consists of tetragonal WO3 and cubic Al-WOx phase. (It is possible to observe the evolution of crystallographic structures between 2θ = 32° and 46° as indicated by orange dotted box).

Figure 3
A series of atom probe tomography with corresponding concentration pro les for 400 °C and 500 °C WO3-x thin lms as-deposited over Al2O3 substrates. a) Sample annealed at 400˚C where it is possible to observe some Al clusters (grey color indicated by the arrow). b) Sample annealed at 500˚C with no traces of aluminum clusters. The composition pro les were created with a bin width of 2.5 nm and background corrected with IVAS 3.6.14 obtained along the long axis of the images corresponding to lm growth direction.