Monolithically Integrated Tunable ZnO SAW Chip

This proposed research focuses on the materials growth, device design and fabrication, and applications of a ZnO monolithically integrated tunable surface acoustic wave (MITSAW) chip. The novel chip integrates acoustic, optical and electrical process' in one material system. It uses tunable acousto-electric and acousto-optic interaction between surface acoustic waves (SAW) and a two dimensional electron gas (2DEG) in a ZnO/MgxZn1-xO quantum well. ZnO is a multifunctional material possessing unique electrical, optical, acoustical, and mechanical properties. The high electromechanical coupling coefficients of piezoelectric ZnO in conjunction with the low acoustic loss and high velocity of sapphire (Al2O3) offers high frequency and low loss RF applications. Alloying ZnO with MgO forms the ternary compound MgxZn1-xO, which permits band-gap tuning from 3.32 eV to 4 eV. ZnO/MgxZn1-xO heterostructures with 2DEG can be integrated with SAW to create a unique acoustic velocity tuning mechanism. The 2DEG interacts with the lateral electric field resulting in ohmic loss, which attenuates and slows the surface acoustic wave. This mechanism is used to tune the acoustic velocity. The high coupling coefficients offered by the ZnO/R-Al2O3 systems allows velocity tuning up to 1%. Combined with the optical characteristics of the wide and direct band gap (~3.3eV) semiconductor ZnO and transparent conductive ZnO electrodes, the MITSAW chip can be used for UV optical signal processing. The proposed MITSAW consists of a ZnO/MgxZn1-xO quantum well structure grown on a R-plane sapphire (R-Al2O3) substrate using MOCVD. R-plane sapphire is chosen instead of the popular C-plane substrate, as this substrate provides in-plane anisotropy in the ZnO layer. By aligning the device parallel to the c-axis of the ZnO film, Rayleigh type surface acoustic waves are excited, while Love type SAWs are excited when the devices are aligned perpendicular to the c-axis. The Rayleigh wave mode is suitable for gaseous environment sensing, while the Love wave mode, which has no vertical wave component, is ideal for liquid environment sensing. The ZnO MITSAW chip also offers an acoustic-optical dual mode sensing mechanism. Likewise, the optical properties parallel and perpendicular to the c-axis are different, allowing novel optical devices, such as high contrast modulators, to be fabricated. The successful development of the ZnO MITSAW chip technology will provide industry with state-of-the-art new multifunctional chip technologies. It will not only improve the existing devices but also develop fundamentally new approaches to many important application areas, including tunable/adaptive communication systems, novel multi-mode tunable chemical and biochemical sensors, and optical signal processors such as delay lines and multiplexers.