image: This is a representative D-V curve and the piezoresponse hysteresis loop of Zn0.975V0.025O films. view more
Credit: ©Science China Press
In recent years, with the growing concerns over environmental protection and human health, environment-friendly materials have received increasing attention, and for decades researchers have been fiercely studying lead-free piezoelectric materials with high piezoelectric properties. After more than 7-years of innovative research, Professor PAN Feng and his group from Key Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, presented a new type of environment-friendly piezoelectric material with giant piezoresponse and simple structure, small-ion-doped ZnO. They also provided a general rule describing the impact of doping on the piezoresponse of ZnO films. Specifically, if doped ions substitute at Zn2+ sites, doping ZnO with small/big ions can enhance/reduce the piezoresponse. This rule is a useful guide in fabricating enhanced piezoresponse in wurtzite materials and offers a new paradigm to seek environment-friendly piezoelectric materials with high piezoelectric properties. Their work, titled "Giant piezoresponse and promising application of environmental friendly small-ion-doped ZnO", was published in SCIENCE CHINA Technological Sciences.2012, Vol 55(2).
Piezoelectric materials are key materials for the fabrication of various transducers, pressure sensors and actuators, piezoelectric oscillator and actuators, transformer, surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices, which are widely used in the fields of information, energy, machinery, electronics, national defense, among others. Because of their excellent piezoelectric property, lead (Pb)-based piezoelectric materials is one of the most widely exploited and extensively used piezoelectric materials. However, Pb is highly toxic and its toxicity can be further enhanced due to its easy volatilization during processing. Thus, processing and use of Pb-based piezoelectric materials can contaminate environments and damage human health, thereby limiting their applications. With the rise in environmental awareness, lead-free piezoelectric materials have received greater attention, the prevailing trend being that environment-friendly lead-free piezoelectric materials will replace Pb-based piezoelectric materials.
As a piezoelectric material, ZnO has various advantages. Firstly, it has the strongest piezoelectric response among the tetrahedrally-bonded semiconductors. Secondly, it is structurally simple and easy to fabricate. Moreover, ZnO films are compatible with semiconductor processes, and therefore has been widely used as sensors and actuators in micro-electromechanical systems and as SAW and BAW devices in the field of communications. However, performance improvements in piezoelectric devices demand significant piezoelectric behavior and stronger piezoresponse; in that regard d33 is the important parameter for evaluating piezoelectric property in ZnO. For bulk ZnO, and for an oriented ZnO film, the piezoresponses are ~9.9 pC/N and ~12.4 pC/N, respectively, which are approximately one or two orders of magnitude lower compared with Pb-based piezoelectric materials. Many researchers have focused on pure ZnO films and attempted to improve its properties by optimizing the preparation conditions; the results though were ineffective.
Doping is a good method to improve the piezoelectric properties of Pb-based piezoelectric materials, and there are many successful examples of improvements in the properties of ZnO films by doping. Doping with Al and Ga can improve the quality and conductivity of ZnO films. Co-doping can induce room-temperature ferromagnetism in Co-doped ZnO films. For this reason, transition-metal doping of ZnO films has been investigated and piezoresponses quantified. It is found that Zn0.975V0.025O and Zn0.94Cr0.06O films possess maximum d33 values of ~170 and ~120 pC/N, respectively, which are about one order of magnitude larger than for pure ZnO films, but quite comparable with those of perovskite piezoelectrics. As shown in Figure 1, the displacement-applied voltage (D-V) curve of Zn0.975V0.025O is typically a butterfly-like loop. The corresponding piezoresponse hysteresis loop is switchable. From a macroscopic point of view, the giant piezoresponse in ZnO films is considered to be the emergence of switchable spontaneous polarization as well as to a relatively high permittivity. In contrast, 2 at.% Cu-doped ZnO films have a d33 of ~13.6 pC/N, and Fe- and Co-doped ZnO films with the same doping concentration have d33 values of ~6pC/N and ~11 pC/N, both smaller than for pure ZnO films (~12) pC/N. Through analyzing and calculating the X-ray absorption spectroscopy spectrum of the dopant, it was found that V5+ (radii of 0.59 Å), Cr3+ (radii of 0.63 Å), Cu2+ (radii of 0.72 Å), Fe2+ (radii of 0.76 Å), and Co2+ (radii of 0.79 Å) substitute for Zn2+ (radii of 0.74 Å) in Zn0.975V0.025O, Zn0.94Cr0.06O films Zn0.98Cu0.02O, Zn0.98Fe0.02O, and Zn0.98Co0.02O respectively. From a microscopic point of view, the piezoresponse in ZnO is mainly governed by the ease of noncollinear bonds along the polar c axis toward the direction of the applied field. Small-ion substitution, i.e. V5+ (0.59Å), Cr3+ (0.63 Å), and Cu2+ (0.72 Å), for Zn2+(0.74 Å) make the V-O1, Cr-O1, and Cu-O1 bonds rotate easier in the direction of the applied field and enhance the corresponding electromechanical responses. For 2 at. % Fe- and Co-doped ZnO films, the bigger ionic size in the Zn2+ site makes the rotation of the Fe-O1 and Co-O1 bonds difficult, thus decreasing the piezoresponse.
In ZnO:Mn films where Mn2+ (0.80Å) with a big radius gets substituted at the Zn2+ sites, the piezoresponse is only 8.2 pC/N, which is smaller for pure ZnO films. Substitution of Mn3+/Mn4+ (0.66Å/0.60Å), with small radius and greater positive charge, at Zn2+ site, yields an enhanced piezoresponse of up to 86 pC/N. Through investigating the relationship between Mn ionic size and the piezoresponse values, the ionic size and the chemical state of the dopant have been confirmed as the key factors in the piezoresponse of doped-ZnO films.
In ZnO:Fe films, Fe2+ (0.76 Å), with a big radius, also gets substituted at Zn2+ sites, the piezoresponse is only 7 pC/N. After annealing in O2, Fe2+ is oxidized to Fe3+ (0.64 Å) and the piezoresponse improved to 120 pC/N. The modified ZnO films with high piezoresponses can be used as promising environment-friendly piezoelectric materials. The substitution rule has provided a new way in seeking lead-free piezoelectric materials. Pure ZnO and V-doped ZnO films were used to fabricate SAW devices. Compared with devices based on pure ZnO films, the devices based on ZnO:V films with giant piezoresponses have smaller insertion losses and higher electromechanical coupling coefficients.
By enhancing the piezoresponse of ZnO and made comparable to that of Pb-based piezoelectric materials, the performances of available ZnO-based SAW devices were improved significantly. Additionally, the enhanced piezoelectricity can widely extend the applications range of ZnO. Because ZnO not only possesses high piezoelectric response, but also is an abundant raw material that is environment-friendly, it competes and stands to replace Pb-based piezoelectric materials as the promising piezoelectric materials.
This work was completed by many researchers from different institutes and universities. This work was supported by the National Hi-tech (R&D) Program of China, the National Natural Science Foundation of China and the National Basic Research Program of China.
See the article: Pan F, Luo J T, Yang Y C, et al. Giant piezoresponse and promising application of environmental friendly small-ion-doped ZnO. SCI CHINA Tech Sci, 2012, 55(2):1.