Improved frequency response of the 3.5 GHz SMR devices using Ga-doped AlN thin films as the piezoelectric layers

Abstract

In this paper, the aluminum nitride (AlN) and gallium-doped aluminum nitride (AlGaN) films are adopted to fabricate the 3.5 GHz solidly mounted resonator (SMR) devices. A reactive radio frequency (RF) magnetron sputtering system and a co-puttering subsystem were used to deposit AlN and AlGaN thin films as piezoelectric layers. The sputtering parameters were fixed as sputtering pressure of 35 mTorr and sputtering power of 200 W for AlN piezoelectric layers. For the co-sputtering parameters, the sputtering pressure was set 10 mTorr, and the sputtering powers of Al and GaN targets were 175 W and 25 W, respectively, to deposit AlGaN piezoelectric layers. The X-ray diffraction (XRD) patterns showed that as compared with the 2[Formula: see text] values of the diffraction peaks of AlN thin films, those of AlGaN thin films shifted to lower values. Even the diffraction intensity of (002) peak of AlGaN thin film was higher than that of AlN film, both AlN and AlGaN thin films presented the strongly [Formula: see text]-axis orientated crystallization. The energy dispersive X-ray spectrometer (EDS) analysis showed that the content of Ga element in the AlGaN thin film, ([Formula: see text]), was about 19%. SEM images showed that the obtained AlN and AlGaN thin films exhibited uniform grain sizes and smooth surfaces. Finally, both the frequency responses of the fabricated SMR devices were measured using a network analyzer and the difference in their efficiencies were well compared. The frequency response of the fabricated SMR device with [Formula: see text][Formula: see text]N thin film showed the center frequency of 3.51 GHz, the return loss [Formula: see text] of −10.466 dB, quality factor [Formula: see text] of 170.952, electromechanical coupling coefficient [Formula: see text] of 4.026%, and FoM value of 6.838, respectively. The results show that the frequency response of SMR device has been improved by Ga doped in AlN thin film, although the [Formula: see text] value is still less than that of obtained using single crystalline AlGaN thin film.