Monolithically Integrated Tunable ZnO SAW Chip

本文主要研究了氧化锌单片集成可调谐声表面波(MITSAW)芯片的材料生长、器件设计和制造及其应用。这种新型芯片集声、光、电处理于一体的材料系统。它利用表面声波(SAW)和二维电子气(2DEG)之间的可调谐声电和声光相互作用，形成了一种具有独特电学、光学、声学和力学性质的多功能材料。压电型氧化锌的高机电耦合系数，加上蓝宝石(Al_2O_3)的低声损耗和高速度，为高频和低损耗射频应用提供了条件。将氧化锌和氧化镁合金化形成了三元化合物镁锌锌氧化物，它允许从3.32 eV到4 eV的带隙调谐。具有二维EG的氧化锌/镁锌氧化物异质结可与声表面波集成，形成独特的声速调谐机制。2DEG与横向电场相互作用，产生欧姆损耗，从而衰减和减慢表面声波。这个机制被用来调节声速。由ZnO/R-Al_2O_3系统提供的高耦合系数允许速度调节高达1%。MITSAW芯片结合了宽禁带(~3.3 eV)半导体氧化锌和透明导电氧化锌电极的光学特性，可用于紫外光信号处理。所提出的MITSAW由利用MOCVD在R面蓝宝石(R-Al_2O_3)衬底上生长的ZnO/Mg_xZn_(1-x)O量子阱结构组成。选择了R面蓝宝石作为衬底来代替常用的C面衬底，因为这种衬底在氧化锌薄膜中提供了面内各向异性，通过将器件平行于氧化锌薄膜的c轴对准，激发了瑞利型表面声波，而当器件垂直于c轴对准时，则激发了Love类表面声波。瑞利波模式适用于气态环境传感，而Love波模式不含垂直波成分，适用于液体环境传感。氧化锌MITSAW芯片还提供了声光双模传感机制。同样，平行于c轴和垂直于c轴的光学特性也不同，这使得高对比度调制器等新型光学器件得以制造。氧化锌MITSAW芯片技术的成功开发将为行业提供最先进的多功能芯片技术。它不仅将改进现有的器件，而且将从根本上开发许多重要应用领域的新方法，包括可调/自适应通信系统、新型多模可调化学和生化传感器以及延迟线和多路复用器等光信号处理器。

This proposed research focuses on the materials growth, device design and fabrication, andapplications of a ZnO monolithically integrated tunable surface acoustic wave (MITSAW) chip. Thenovel chip integrates acoustic, optical and electrical process' in one material system. It uses tunableacousto-electric and acousto-optic interaction between surface acoustic waves (SAW) and a twodimensional electron gas (2DEG) in a ZnO/MgxZn1-xO quantum well.ZnO is a multifunctional material possessing unique electrical, optical, acoustical, and mechanicalproperties. The high electromechanical coupling coefficients of piezoelectric ZnO in conjunction with thelow acoustic loss and high velocity of sapphire (Al2O3) offers high frequency and low loss RFapplications. Alloying ZnO with MgO forms the ternary compound MgxZn1-xO, which permits band-gaptuning from 3.32 eV to 4 eV. ZnO/MgxZn1-xO heterostructures with 2DEG can be integrated with SAW tocreate a unique acoustic velocity tuning mechanism. The 2DEG interacts with the lateral electric fieldresulting in ohmic loss, which attenuates and slows the surface acoustic wave. This mechanism is used totune the acoustic velocity. The high coupling coefficients offered by the ZnO/R-Al2O3 systems allowsvelocity 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 usedfor UV optical signal processing. The proposed MITSAW consists of a ZnO/MgxZn1-xO quantum wellstructure grown on a R-plane sapphire (R-Al2O3) substrate using MOCVD. R-plane sapphire is choseninstead 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 areexcited, while Love type SAWs are excited when the devices are aligned perpendicular to the c-axis. TheRayleigh wave mode is suitable for gaseous environment sensing, while the Love wave mode, which hasno vertical wave component, is ideal for liquid environment sensing. The ZnO MITSAW chip also offersan acoustic-optical dual mode sensing mechanism. Likewise, the optical properties parallel andperpendicular 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 withstate-of-the-art new multifunctional chip technologies. It will not only improve the existing devices butalso develop fundamentally new approaches to many important application areas, includingtunable/adaptive communication systems, novel multi-mode tunable chemical and biochemical sensors,and optical signal processors such as delay lines and multiplexers.