Experimental and theoretical characterization of the surface acoustic wave propagation properties of GaN epitaxial layer
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Jin Yong Kim Film Characterization and Properties Group Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8522
Tae Kun Lee Department of Materials Science and Engineering, Seoul National University of Technology, Seoul 131-743, Korea
Young Jin Kim Department of Materials Science and Engineering, Kyonggi University, Suwon 442-760, Korea (Received 2 May 2002; accepted 12 February 2003)
Surface acoustic wave (SAW) propagation properties of gallium nitride (GaN) epitaxial layers on sapphire were theoretically and experimentally characterized. GaN thin films were grown on a c-plane sapphire substrate using a metalorganic chemical vapor deposition system. The experimental characterization of SAW propagation properties was performed with a linear array of interdigital transducer structures, while SAW velocities were calculated by matrix methods. Experimentally, we found pseudo-SAW and high-velocity pseudo-SAW modes in the GaN/sapphire structure, which had a good agreement with calculated velocities.
I. INTRODUCTION
With recent advances in mobile telecommunication systems, the demands for higher frequency communication devices are increasing daily, and many efforts have been given to the development of high-frequency surface acoustic wave (SAW) devices for gigahertz bands. Higher-frequency SAW devices can be more easily realized by adapting new high-SAW-velocity materials: high velocity nonpiezoelectric substrates combined with piezoelectric thin films. Electronics performance and size requirements place increasing demands on downsizing and monolithic electronic components, which are of great advantage especially to mobile telecommunication devices. The monolithic integration of SAW devices with active semiconductor devices could be realized by using a layered structure, such as ZnO and AlN on Si, SiC, diamond, and sapphire substrates.1–3 ZnO and AlN have been widely used for the piezoelectric materials for a layered structure SAW filter. GaN is a newly introduced piezoelectric material for SAW devices. Since GaN is a semiconductor material with piezoelectric properties and its production skills are established up to the level of satisfying commercial needs, it can be practically used in hybrid optoelectronic devices for various important applications, such as J. Mater. Res., Vol. 18, No. 5, May 2003
acousto-optic and piezoelectric devices. In addition, epitaxially grown GaN layer with a perfectly flat surface can prevent acoustic wave scattering on surface. Analysis of SAW propagation characteristics in the GaN/sapphire layer structure is important for practical applications. However, there are few reports on SAW properties of GaN. To study the dispersive SAW velocity, theoretical calculation and numerical analysis were also carried out by using the reported elastic constants.4–11 We compared the calculated dispersive velocities with the experimentally measured ones and derived the elastic constants by fitting the velocity dispersion.
II. NUMERICAL CALCULATIONS
To inte
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