Electromechanical Coupling Coefficient for Surface Acoustic Waves in GaN-on-Sapphire

Abstract

The electromechanical coupling coefficient for surface acoustic waves propagating in GaN-on-sapphire structure has been evaluated using in situ measurements of the SAW attenuation during the evaporation of a metal film on the GaN surface. The extracted values for samples with GaN layer thicknesses of 2.2-2.4 mm were in the range of 0.06-0.09% at SAW frequencies of 200-300 MHz.Introduction Surface acoustic waves (SAWs) have found important applications in material research, signal processing, and telecommunications areas. There is a considerable interest in SAW investigations in thin-film structures based on GaN family of semiconductors. The key parameter for SAW devices is the electromechanical coupling coefficient, K 2 . The efficiency of the SAW interdigital transducer (IDT) is characterized by its radiation conductance G a ðf 0 Þ ¼ 8K 2 f 0 C T N at center frequency f 0 , where C T is the IDT static capacitance and N is the number of the IDT electrode pairs. The electromechanical coupling coefficient for SAWs can be expressed as K 2 ¼ 2DV=V, where DV is the magnitude of the SAW velocity change that occurs when the free surface of the piezoelectric is shorted by a thin highly conducting metal film, and V is the unperturbed SAW velocity. The K 2 values for SAWs in GaN-on-sapphire structures have been evaluated from transducer performance analysis and extracted from the velocity measurements by several authors [1][2][3][4]. However, there is a considerable disagreement in the results, and further investigations are necessary. For this purpose, we have chosen the technique, different from those mentioned above. We report on the evaluation of the electromechanical coupling coefficient for GaNon-sapphire structures using in situ measurements of the SAW attenuation during the evaporation of a metal film on the surface of the piezoelectric [5,6].

Surface acoustic waves in semi-insulating Fe-doped GaN films grown by hydride vapor phase epitaxy

“…[6][7][8][9][10][11][12][13] In these studies, the thickness and the resistivity of the GaN films were assumed to be key factors to affect the surface acoustic modes and the electromechanical coupling coefficients. 9,10,[14][15][16] The emergence of various acoustic modes is dependent on the ratio between the GaN film thickness and the SAW wavelength. 6,8 The electromechanical coupling coefficient of GaN layers with different thickness and different resistivities were reported in a large range from 0.015% to 4.3%, and it was pointed out that the high resistivity of GaN layer might be preferable for improving the performances of SAW devices.…”

“…In this work, Fe-doped GaN epitaxial films with wurtzite structure were grown on (0001) sapphire substrate, and the crystal orientation GaN is parallel to sapphire [11][12][13][14][15][16][17][18][19][20]. Two Fe-doped GaN samples of different thickness were employed in our experiment, and their thickness are respectively 33 lm (sample 1) and 15lm (sample 2).…”

“…Each delay line consists of two symmetric IDTs, and each IDT includes 50 pairs of electrode finger. The propagation of the SAW is made to along the or [11][12][13][14][15][16][17][18][19][20] direction of GaN. The transmission and reflection parameters of delay lines were measured by vector network analyzer (Agilent E5071C).…”

Side-gate effects on transfer characteristics in GaN-based transversal filters

“…2,3 Focusing on the high piezoelectricity, several authors have investigated transversal filters fabricated on ͑Al͒GaN layers on sapphire [4][5][6][7][8][9][10] or SiC 11 substrates, and discussed fundamental properties of surface acoustic wave ͑SAW͒ in these systems, such as dispersion of SAW velocity, 5,7,9,11 temperature coefficient of frequency, 6,7,10 and effective electromechanical coupling constant K eff 2 . 4,6,8 It has been reported that the device characteristics of GaN-based transversal filters are sensitive to ultraviolet illumination. 12,13 Recently, Grajal et al 14 showed that the insertion loss of transversal filters fabricated on AlGaN/ GaN heterostructures changes when dc bias voltages are applied to their interdigital transducers ͑IDTs͒.…”

SAW Filters Composed of Interdigital Schottky and Ohmic Contacts on AlGaN/GaN Heterostructures

“…D UE to their excellent electrical and thermal properties as well as marked piezoelectricity [1], group-III nitrides have been widely applied to not only high-frequency highpower electron devices [2], [3] but also surface acoustic wave (SAW) devices: The fundamental properties of SAWs in (Al)GaN layers-such as the electromechanical coupling coefficient [4], [5], dispersion of the SAW velocity [6]- [8], response to ultraviolet illumination [9], and interaction between photogenerated carriers and SAWs [10]-were investigated. The present authors previously discussed the characteristics of SAW devices fabricated on GaN layers exposed by dry etching [11], side-gate effects [12], and excitation of SAWs using n + -GaN-based interdigital transducers (IDTs) [13].…”