| Summary: | 碩士 === 中原大學 === 電子工程研究所 === 94 === In recent years, the surface acoustic wave (SAW) devices have become an attractive research field for the application of high frequency filters as well as sensors. In order to obtain high frequency and low insertion loss devices, it is important to use piezoelectric thin films to fabricate layered structure SAW devices.
In this thesis, high quality epitaxial AlN thin films have been deposited on GaN/Sapphire substrates at low temperature of 300�aC using Helicon sputtering system. The layered structure SAW devices fabricated on AlN/GaN/Sapphire were investigated and demonstrated.
The characteristics of AlN/GaN/Sapphire and GaN/Sapphire and the effects of temperature and humidity are discussed. Superior SAW properties in terms of insertion loss and sidelobe rejection have been obtained for the SAW devices made on AlN/GaN/Sapphire, compared to those of the ones made on GaN/sapphire. The temperature coefficient of frequency(TCF) of GaN/Sapphire and AlN/GaN/Sapphire are -49.2ppm/℃ and -44.8ppm/℃, respectively. It was found that AlN layer can compensate the TCF of SAW devices of GaN/Sapphire substrates. In addition, the ratio of center frequency variation of AlN/GaN/Sapphire is 3.2ppm, which is much lower than that of GaN/Sapphire (61.1ppm) at relative humidity ranging from 40 to 80%. These results show that the characteristics can be improved by depositing AlN on GaN/Sapphire.
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