Voltage-controlled tunable surface acoustic wave devices using multifunctional MgZnO/ZnO structures
Creators: name: Li, Rui
Publisher: name: No Publisher Supplied
Year: 2017

description: Surface acoustic wave (SAW) devices have been widely used for signal processing, frequency control, and sensing applications. It is also emerging as a promising technology for the miniaturized on-chip microfluidic pumping and biomolecule manipulation. Up to date, most SAW devices operate at fixed frequencies. The frequency tunability is highly demanded in advanced wireless communication and sensing systems for programmable and adaptive applications. The tunable SAW devices can extend the effective operating bandwidth, increase functionality and versatility, and reduce the architecture complexity.   The goal of this research is to demonstrate the voltage controlled tunable SAW devices using the multifunctional MgZnO- and ZnO-based multilayer structures. Two types of novel tunable SAW devices are designed and fabricated. The first device consists of the Ni-doped piezoelectric ZnO (NZO)/ n-type semiconductor ZnO/GaN/c-Al2O3 multilayer structure. The unique acoustic dispersion relationship in the ZnO/GaN heterostructure generates the multi-mode SAW responses, facilitating high-frequency operation. The acoustoelectric interaction in the piezoelectric-semiconductor structure is used to realize frequency tuning through modulation of free charges in the semiconductor n-ZnO layer and confined charges in the n-ZnO/GaN heterojunction. A dc bias voltage is applied to a Ti/Au gate layer deposited on the path of SAW delay line to control the electrical conductivity for tuning the acoustic velocity. For a device operating at 1.25 GHz, the maximum SAW velocity change of 0.9% is achieved within -25 V to 0 V, equivalent to the frequency change of 11.2 MHz. This voltage-controlled frequency tuning device has potential applications. The ZnO/GaN-based tunable SAW device with a buried interdigital transducer (IDT) is also designed and fabricated. The device operates at 1.35 GHz with frequency tunability of 0.9% at a low biasing voltage range of -12V ~ 0V and expanded linear tuning range. This voltage-controlled frequency tuning device has potential applications in resettable sensors, adaptive signal processing, and secure wireless communication.  To extend the multifunctional MgZnO- and ZnO-based tunable SAW technology into the low-cost applications, such as radio-frequency identification (RFID), another prototype of voltage controlled tunable SAW device is directly built on glass substrates using the multifunctional MgZnO/ZnO structure. The dual-input-voltage-controlled tunable SAW device is formed by the integration of a piezoelectric NZO–based SAW delay line with a semiconducting MgZnO (MZO)-based thin film transistor (TFT). The IDT electrodes are buried under the NZO layer. It is found that the interface between MZO channel and SiO2 gate dielectric layer of the TFT significantly impacts on SAW tuning performances. The MZO-SiO2 interface is engineered by inserting an ultrathin MgO layer as a barrier to stop Zn diffusion from MZO into SiO2. The integrated TFT-SAW device with interface modification enables SAW frequency tuning of 0.53% under solely Vgs control. With extra Vds control, the SAW frequency tunability is expanded its tuning range  (0.46% - 0.63%). This dual input voltage controlled frequency tuning device on glass technology is promising for low-cost and portable applications such as smart sensors and reconfigurable RFID tags.