This invention relates to the use of MgxZn1-xO based materials and structures for acoustic devices, and more particularly, to the tailoring of the piezoelectric properties to achieve flexibility in the Surface Acoustic Wave (SAW) and Bulk Acoustic Wave (BAW) design and fabrication for applications in telecommunications and various sensors.
Currently, several technologies exist that provide modifying the piezoelectric properties in ZnO based multilayer structures. Theoretical analysis was reported for multilayers of two different materials (see E. K. Sittig xe2x80x9cTransmission Parameters of Thickness-Driven Piezoelectric Transducers Arranged in Multilayer Configurationsxe2x80x9d, IEEE Trans. SU, SU-14 (4), 167, October 1967), and for off-axis ZnO multilayers (see E. Akcakaya, E. L. Adler, G. W. Farnell, xe2x80x9cApodization of Multilayer Bulk-Wave Transducersxe2x80x9d, IEEE Trans. UFFC, 36(6), 628, November 1989 and E. Akcakaya, E. L. Adler, G. W. Farnell, xe2x80x9cAnisotropic Superlattice Transducers: Characteristics and Modelsxe2x80x9d, Proc. IEEE 1988 International Ultrasonics Symposium, 333, 1988). E. K. Sittig shows individual transducers consisting of multiplicity of piezoelectrically active layers electrically connected with conductive or non-conductive layers of different characteristics acoustic impedances. Akcakaya et al discloses calculating electromechanical characteristics of transducers consisting of multilayers of ZnO with alternating crystal orientation. BAW thin film resonators (TFRs) using sputter deposited off-axis ZnO multilayers with alternating crystal structure were demonstrated by J. S. Wang, K. M. Lakin, (xe2x80x9cSputtered C-axis Inclined ZnO Films for Shear Wave Resonatorsxe2x80x9d, Proc. IEEE 1982 International Ultrasonics Symposium, 480, 1982) and by B. Hadimioglu, L. J. La Comb, Jr., L. C. Goddard, B. T. Khuri-Yajub, C. F. Quate, E. L. Ginzton, (xe2x80x9cMultilayer ZnO Acoustic Transducers at Millimeter-Wave Frequenciesxe2x80x9d, Proc. IEEE 1987 International Ultrasonics Symposium, 717, 1987.) BAW TFRs using alternating multilayers of ZnO and non-piezoelectric materials were demonstrated by W. S. Hu, Z. G. Liu, R. X. Wu, Y. F. Chen, W. Ji, T. Yu, D. Feng, (xe2x80x9cPreparation of Piezoelectric-Coefficient Modulated Multilayer Film ZnO/Al2O3 and its Ultrahigh Frequency Responsexe2x80x9d, Appl. Phys. Lett., 71(4), p. 548, July 1997). Piezoelectric property tailoring in the ternary compound AlxGa1-xN, was demonstrated by C. Deger, E. Born, H. Angerer, O. Ambacher, M. Stutzmann, J. Hornsteiner, E. Riha, G. Fischerauer, (xe2x80x9cSound velocity of AlxGa1-xN thin films obtained by surface acoustic wave measurementsxe2x80x9d, Appl. Phys. Lett., 72(19), p. 2400, May 1998). Deger et al shows determining SAW and BAW velocities in AlxGa1-xN thin films by tailoring the piezoelectric properties of AlxGa1-xN films. Y. F. Chen, S. N. Zhu, Y. Y. Zhu, N. B. Ming, B. B. Jin, R. X. Wu, xe2x80x9cHigh-frequency Resonance in Acoustic Superlattice of Periodically Poled LiTaO3xe2x80x9d, Appl. Phys. Lett., 70(5), 592, February 1997 and H. Gnewuch, N. K. Zayer, C. N. Pannell, xe2x80x9cCrossed-Field Excitation of an Acoustic Superlattice with Matched Boundaries: Theory and Experimentxe2x80x9d, IEEE Trans, UFFC, 47(6), 1619, November 2000 describe a piezoelectric property tailoring method suitable only for those piezoelectric materials which are also ferroelectric materials facilitating construction of acoustic superlattice (ASL) and optic superlattice (OSL) devices.
Piezoelectric ZnO thin films have been used for multilayer SAW and BAW devices due to the high electromechanical coupling coefficients (see F. Moeller, T. Vandahl, D. C. Malocha, N. Schwesinger, W. Buff, xe2x80x9cProperties of thick ZnO layers on oxidized siliconxe2x80x9d, Proc. 1994 IEEE Int. Ultrasonics Symp., pp. 403-406; Kim, Hunt, Hickernell, Higgins, Jen, xe2x80x9cZnO Films on {011}-Cut less than 110 greater than -Propagating GaAs Substrates for Surface Acoustic Wave Device Applicationsxe2x80x9d, IEEE Trans. Ultrasonics, Ferroelectrics and Frequency Control, v. 42, no3, pp. 351-361, May 1995; H. Ieki, H. Tanaka, J. Koike, T. Nishikawa, xe2x80x9cMicrowave Low Insertion Loss SAW Filter by Using ZnO/Sapphire Substrate with Ni Dopantxe2x80x9d, 1996 IEEE MTT-S Digest, pp. 409-412; and H. Nakahata, H. Kitabayashi, S. Fujii, K. Higaki, K. Tanabe, Y. Seki, S. Shikata, xe2x80x9cFabrication of 2.5 GHz Retiming Filter with SiO2/ZnO/Diamond Structurexe2x80x9d, Proc. 1996 IEEE Int. Ultrasonics Symp., pp. 285-288). Recently, the ternary compound magnesium zinc oxide (MgxZn1-xO), formed by alloying ZnO with MgO, has attracted increasing interest for UV optoelectronic applications (see A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, xe2x80x9cMgxZn1-xO as a II-VI widegap semiconductor alloyxe2x80x9d, Appl. Phys. Lett., vol. 72, n. 19, pp. 2466-2468, May 11, 1998; A. K. Sharma, J. Narayan, J. F. Muth, C. W. Teng, C. Jin, A. Kvit, R. M. Kolbas, O. W. Holland, xe2x80x9cOptical and structural properties of epitaxial MgxZn1-xO alloysxe2x80x9d, Appl. Phys. Lett., vol. 75, n.21, pp. 3327-3329; and T. Minemoto, T. Negami, S. Nishiwaki, H. Takakura, Y. Hamakawa, xe2x80x9cPreparation of Zn1-xMgxO films by radio frequency magnetron sputteringxe2x80x9d, Thin Solid Films, vo. 372, pp. 173-176, Sep. 1, 2000). Its energy bandgap can be extended from 3.3 eV (ZnO) to 4.05 eV (Mg0.35Zn0 65O). Although the solid solubility limit of MgO in ZnO is less than 5% in equilibrium conditions, a higher range of Mg composition can be achieved using non-equilibrium growth methods.
Currently, the ZnO film thickness and the dimensions of the devices (such as SAW filters) were the only parameters available for modification, limiting the design flexibility, as well as the processing latitude. It would be useful to provide a SAW or a BAW device in which their characteristics can be tuned using other parameters.
The present invention provides a method of controlling piezoelectric properties in various acoustic devices. The method involves using MgxZn1-xO film as a new piezoelectric material and adjusting Mg mole percent in the MgxZn1-xO film to tailor piezoelectric properties in the MgxZn1-xO film. Similarly, the method further involves using MgxZn1-xO/ZnO as a new piezoelectric multilayer structure and adjusting Mg mole percent in the MgxZn1-xO to tailor piezoelectric properties in the acoustic devices. Thus, the piezoelectric properties in ZnO based devices can be tailored by using MgxZn1-xO/ZnO multiplayer structures as well as by using MgxZn1-xO single layer with controlled Mg composition.
In addition to being piezoelectric, both ZnO and MgxZn1-xO are wide-bandgap semiconductors. Thus piezoelectric and semiconductor devices can be integrated on the same material system. This leads to new classes of devices with integrated features and tunability.