Fabrication and Characteristics Analysis of SAW Filter Using Al 0.36 Ga 0.64 N Thin Film on Sapphire Substrate

Transcription

1 phys. stat. sol. (c) 0, No. 1, (2002) Fabrication and Characteristics Analysis of SAW Filter Using Al 0.36 Ga 0.64 N Thin Film on Sapphire Substrate Cheol-Yeong Jang, Min-Jung Park, Sun-Yeol Ryu, Hyun-Chul Choi, Jung-Hee Lee, and Yong-Hyun Lee 1 ) School of Electronic and Electrical Engineering, Kyungpook National University, Daegu , Korea (Received July 22, 2002; accepted October 1, 2002) PACS: Dq The surface acoustic wave (SAW) characteristics of Al x Ga 1 x N thin films, such as velocity, electromechanical coupling coefficient k 2, temperature coefficient of frequency (TCF), and propagation loss, were investigated. Al x Ga 1 x N thin film was deposited on sapphire substrate by metal organic chemical vapour deposition and the Al mole fraction of x ¼ 0.36 was measured using Rutherford backscattering spectroscopy. SAW velocity of 5510 m/s and TCF of ppm/ C were measured when the value was and temperature ranged between 30 and 60 C. k 2 varied from 1.26% to 2.22%. The fabricated SAW filter exhibited good device performance with insertion loss of db and sidelobe attenuation of 20 db when the wavelength was 60 mm (l/4 ¼ 15 mm) and the centre frequency was MHz. Introduction Recently, surface acoustic wave (SAW) filters have been studied for applications in various mobile communication systems. Piezoelectric single crystals such as LiNbO 3, LiTaO 3, and ZnO, and AlN or ZnO thin films [1, 2] have been widely investigated to determine their SAW characteristics such as velocity, loss, electromechanical coupling coefficient (k 2 ), and temperature coefficient of frequency (TCF) [1 3]. In this paper, experimental results were obtained for a SAW filter fabricated on an AlGaN/sapphire substrate, which was prepared by metal organic chemical vapour deposition (MOCVD). Experimental The AlGaN piezoelectric thin film was deposited on 2-inch sapphire(0002) substrate using MOCVD at 1020 C with a TMGa flow rate of 40 mmol/ min, TMAl flow rate of 70 mmol/min, H 2 /NH 3 flow of 4/4 slpm, and growth pressure of 300 Torr. Prior to the epitaxial undoped GaN growth, an initial buffer layer was grown at 550 C with a TMGa flow rate Al electrode of 30 mmol/min. The thickness of the grown AlGaN samples was 0.6 mm and the sheet resistance was W/&. Figure 1 AlGaN layer shows a cross-sectional view of the grown sample structure. Buffer layer(gan) Sapphire substrate Fig. 1 (online colour). Cross-sectional view of the SAW filter 1 ) Corresponding author; Phone: ; Fax: ; yhlee@ee.knu.ac.kr # 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim /02/ $ 17.50þ.50/0

2 phys. stat. sol. (c) 0, No. 1 (2002) 249 Input IDT L Output IDT Fig. 2 (online colour). SAW IDT pattern W b a λ Ground electrode W: aperture, λ : wavelength b/a : metallizationlength, L : IDTcenter tocenter spaces The SAW interdigital transducer (IDT) was patterned by a lift-off process using Al metallization. The IDT pattern is shown schematically in Fig. 2 and the detailed specification is summarized Table 1. SAW velocities were calculated by measuring the centre frequency response S 21 using a network analyser (HP8753C). The electromechanical coupling coefficient k 2 was also calculated from conductance of frequency response S 11. Results and Discussion Analysis of Al/GaN thin film Figure 3 shows the Rutherford backscattering spectroscopy (RBS) result for the AlGaN/GaN thin film with an Al mole fraction of A smooth AlGaN surface is also essential for SAW devices. Figure 4 shows a typical smooth atomic force microscopy (AFM) image of a sample with root mean square roughness of nm. Characteristics of SAW filter The input and output IDTs with 82 split electrode finger pairs with a mm wavelength range were used to generate and detect the SAWs. The SAW velocities of the AlGaN/sapphire structure were measured by frequency response and are shown in Fig. 5. The SAW velocity was measured as a function of the value, which was varied from 0.06 to 0.29, where h is the thickness of the AlGaN thin film. From Fig. 5, it can be seen that the SAW velocity decreased almost linearly with increasing value. The electromechanical coupling coefficient is also an important factor for SAW devices. The coupling coefficient was calculated from the frequency response. Figure 6 Table 1 SAW IDT pattern specification wavelength, l (mm) 20, 40, 60 aperture, W (mm) 1800 IDT finger pairs 160, 82, 54 IDT centre to centre space, L (mm) 3650, 4100, 5000, 5900, 6800 metallization ratio 0.5

3 250 Cheol-Yeong Jang et al.: Fabrication and Characteristics Analysis of SAW Filter RBS measurement(algan/gan/buffer layer ) TMAl/TMGa : 70/40 µmol NH 3 /H 2 : 4/4 µmol Al composition : 35 % (XRD : 36 %) Fig. 3 (online colour). RBS data of Al 0.36 Ga 0.64 N thin film shows the coupling coefficient as a function of value. The following equations are used for calculating k 2 : k 2 ¼ 1 G 0 ð1þ 8N 2 f c C s k 2 ¼ p 4N G 0 ; ð2þ B 0 Fig. 4 (online colour). AFM image of AlGaN thin film

4 phys. stat. sol. (c) 0, No. 1 (2002) 251 velocity (m/s) Al 0.36Ga 0.64N/sapphire Fig. 5 Fig. 6 Fig. 5. SAW velocity as a function of Fig. 6. k 2 as a function of k Al 0.36Ga 0.64/sapphire Insertion Loss (db) Input to output space length Fig. 7 Fig. 8 Fig. 7. Insertion loss as a function of IDT space length Fig. 8. Propagation loss as a function of propagation loss (db/l) Al 0.36Ga 0.64N/sapphire Fig. 9. Frequency response S 21 (l ¼ 60 mm)

5 252 Cheol-Yeong Jang et al.: Fabrication and Characteristics Analysis of SAW Filter where N is the IDT electrode finger pair number, f c is the centre frequency, and G 0, C s, and B 0 are the radiation conductance, capacitance and susceptibility of the centre frequency response [4]. The measured TCF was ppm/ C with the temperature in the range 30 to 60 C when the value was To estimate propagation loss and insertion loss an HP8753C network analyser was used. Figure 7 shows the insertion loss as a function of input to output IDT space width. Figure 8 shows the propagation loss as a function of value, which increased with increasing. The measured propagation loss ranged from to db/l. The frequency response S 21 is shown in Fig. 9. The centre frequency is MHz, the bandwidth is MHz, and the Q factor is , which are well matched to the designed value of 90 MHz for the centre frequency and 1.5 MHz band width. Conclusion The characteristics of a SAW filter using an AlGaN/sapphire structure were investigated. The SAW velocity varied from 5510 to 4987 m/s. The electromechanical coupling coefficient k 2 varied from 2.22% to 1.26% with changing value. The propagation loss varied from to db/l as the value varied from to Acknowledgement This work was supported by Grant No. ( ) from the Basic Research Program of the Korea Science and Engineering Foundation. References [1] N. W. Emanetoglu, G. Patounakis, S. Liang et al., IEEE Trans. Ultrason. Ferroelec. Freq. Contr. 48, 1389 (2001). [2] T. Shibata, Y. Hori, K. Asai et al., IEEE Ultrason. Symp (pp ). [3] J. Tsutsumi, O. Ikata, and Y. Satoh, IEEE Ultrason. Symp (pp ). [4] G. D. O Clock, Jr. and M. T. Duffy, Appl. Phys. Lett. 37, 55 (1973).