Surface acoustic wave resonator from thick MOVPE-grown layers of GaN(0001) on sapphire
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Surface acoustic wave resonator from thick MOVPE-grown layers of GaN(0001) on sapphire Sverre V. Pettersen1, Thomas Tybell1, Arne Rønnekleiv1, Stig Rooth2, Veit Schwegler3, and Jostein K. Grepstad1 1 Department of Physical Electronics, Norwegian University of Science and Technology, O.S. Bragstads Plass 2A, N-7491 Trondheim, NORWAY 2 Alcatel Space Norway AS, Knudsrødveien 7, N-3190 Horten, NORWAY 3 Department of Optoelectronics, University of Ulm, Albert Einstein Allee 45, D-89081 Ulm, GERMANY. ABSTRACT We report on fabrication and measurement of a surface acoustic wave resonator prepared on ~10µm thick GaN(0001) films. The films were grown by metal-organic vapor phase epitaxy on a c-plane sapphire substrate. The surface morphology of the films were examined with scanning electron and atomic force microscopy. A metallic bilayer of Al/Ti was subsequently evaporated on the nitride film surface. Definition of the resonator interdigital transducers, designed for a wavelength of λ=7.76µm, was accomplished with standard UV lithography and lift-off. S-parameter measurements showed a resonator center frequency f0=495MHz at room temperature, corresponding to a surface acoustic wave velocity of 3844m/s. The insertion loss at center frequency was measured at 8.2dB, and the loaded Q-factor was estimated at 2200. Finally, measurements of the resonator center frequency for temperatures in the range 25-155˚C showed a temperature coefficient of -18ppm/˚C. The intrinsic GaN SAW velocity and electromechanical coupling coefficient were estimated at vSAW=3831m/s and Κ2=1.8±0.4 ⋅10-3. INTRODUCTION The wide bandgap semiconductor gallium nitride has frequently made it to the headlines in the semiconductor science community during the last decade, mainly because of successful applications in short wavelength optical emitters such as blue laser diodes and light emitting diodes spanning the entire optical spectrum [1]. Moreover, the piezoelectric properties of the group III-nitrides render this material system an interesting option for surface acoustic wave (SAW) devices [2]. The GaN/sapphire system is of particular interest from an engineering point of view, with the sapphire substrate featuring a high and anisotropic SAW velocity, ranging from 5548m/s in the [ 1120 ] direction to 5687m/s in the [ 1 1 00 ] direction [3]. This provides rich opportunities for engineering the propagation properties of the SAW device by controlling the GaN film thickness and the orientation of the SAW device with respect to the in-plane crystalline axes of the sapphire substrate, as well as the SAW wavelength [3-5]. In order to take full advantage of the engineering potential of the GaN/sapphire system, it is important to establish the intrinsic GaN SAW propagation properties. An important engineering challenge in this regard is to grow thick epitaxial films with semi-insulating conduction properties, essential to SAW applications [6]. Recent advances in materials growth now render this a feasible endeavor. In the present paper, we have investigated th
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