Piezoelectric coupling constant in epitaxial Mg-doped GaN and design of pentacene acoustic charge transfer devices
Xu, Xiaofeng (2006) Piezoelectric coupling constant in epitaxial Mg-doped GaN and design of pentacene acoustic charge transfer devices. Masters thesis, Iowa State University.
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Surface acoustic wave (SAW) filters were fabricated on an Mg-doped GaN epilayer using liftoff photolithography. The measurement of the time response showed that these devices had very weak output signals. The center frequencies were 136.09MHz, 114.48MHz and 98.64MHz for devices with wavelengths of 40μm, 48μm and 56μm, respectively. No change of the reflection coefficient was detectable using the available network analyzer for all devices around the center frequencies. The insertion loss for all devices was higher than 80dB. The upper bound of the electromechanical coupling coefficient (K2) of the Mg-doped GaN epilayer was calculated to be 1×10-4%. This is much less than other III-V materials, and so this material does not appear to be promising for SAW applications. A design of an acoustic charge transfer (ACT) device which has been previously proposed with pentacene as the charge transfer channel is discussed. Because the charge mobility of pentacene is very small, the minimum requirement of the SAW power per unit acoustic aperture was calculated to be very high. In order to reduce the required electrical drive power, a parabolic SAW beam compressor was used and the detailed design of the parabolic compressor is given. It is known that commercially available pentacene contains nonnegligible amounts of impurities so that its charge mobility will be low. Therefore, a temperature gradient furnace was set up to distill pentacene to improve the carrier mobility. The device is expected to be fabricated using standard liftoff photolithographic techniques. However, some problems were met when the pentacene charge transfer channel was fabricated. Solutions, such as heating the substrate during the deposition and growing some other material between pentacene and the substrate as an adhesive, are under investigation.
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