Prof. Lu's primary research area is ZnO, which is a multifunctional material possessing unique electrical, optical, piezoelectric, and mechanical properties. Semiconductor ZnO has a wide and direct energy bandgap (~3.3eV). ZnO can be grown at low temperatures, in contrast with the other wide bandgap materials, such as GaN and SiC. Its ternary compounds, formed by alloying ZnO with CdO and MgO, permit bandgap tuning from ~2.8 eV to ~4 eV. Doped ZnO ternaries can be made magnetic or ferroelectric, extending its applications beyond the traditional semiconductor confines. The ZnO research covers material growth, characterization, and devices. The device research covers electronic (diodes, FETs), optoelectronic (detectors, modulators) and piezoelectric (BAW,SAW) devices.

ZnO-based Research:

Prof. Lu's other research includes III-V (GaAs, InP) and III-Nitrides, passive microwave devices (integrated inductors and baluns), and piezoelectric devices (BAW, programmable SAW).

I. MOCVD Material Growth
        We have grown high quality epitaxial ZnO thin films on R-plane sapphire by MOCVD at low temperature (350^oC - 500^oC). The epitaxial relationship between ZnO films and R-sapphire substrates was identified using X-ray and electron diffraction techniques. An X-ray rocking curve full width half maximum (FWHM) of 0.25^o was achieved. Figure 1 shows the X-ray F-scan for a ZnO film grown on R-plane sapphire. The FWHM of the band edge emission photoluminescence peak measured at 11^oK is 6meV, which is close to the 3 meV FWHM of single-crystal bulk ZnO measured at 4.2^oK, and better than 8.9 meV for MBE grown ZnO on GaN/SiC measured at 4.2^oK. The ZnO-sapphire interface is atomically sharp and semicoherent, as evaluated by high resolution TEM. The effects of annealing on the ZnO/R-Al₂O₃ structure and its thermal stability have been studied in detail. We reported that while the crystal quality of the film improved during the annealing, ZnO reacted with Al₂O₃ forming a spinel (ZnAl₂O₄) layer at the interface. Below 800^oC the ZnO/R-Al₂O₃ interface was stable, and above this temperature a ZnAl₂O₄ spinel layer formed. The kinetics of the solid state reaction between ZnO and Al₂O₃ were evaluated. It was found that the reaction was interface-controlled.
        We have investigated the growth mechanism of ZnO on Si substrates, with and without a SiO₂ buffer layer on the Si surface. A thin layer of SiO₂ (~50nm) forms on the Si surface prior to ZnO growth. The growth conditions were optimized to obtain smooth films with preferred c-axis orientation for SAW device applications.
        Our new thrust in material research is the growth of Mg_xZn_1-xO, and Mg_xZn_1-xO/ZnO heterostructures, as well as ZnO based nanoscale materials.

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II. Device Research
The ZnO based device research has two main thrusts, optoelectronic devices and piezoelectric devices.

1. Electronic and Optoelectronic Device Research

        A. ZnO MSM UV Photoconductive and Schottky Detectors
        Prof. Lu's group demonstrated the first high speed ZnO MSM UV photoconductive detector. The detectors were fabricated on N-doped ZnO epitaxially grown on R-Al₂O₃. The devices show high photo responsitivity in the order of 400A/W at 5V bias. The photoresponse speed (~1ms) is much faster than all published results (tens ms to ms) due to the high material quality of the epitaxial ZnO films.
        More recently, we demonstrated the first ZnO MSM Schottky UV photodetectors. At a reverse bias of 1V, the circular Schottky photodiode exhibits a leakage current approximately five orders of magnitude smaller than that of its photoconductive counterpart we reported in. The photoresponsivity of the ZnO Schottky MSM UV detector is 1.5A/W, and the leakage current is about 1nA at 5V bias. The detector showed a fast photoresponse component with a rise time of 12ns, and a fall time of 50ns.
        This work is now being extended to smaller wavelengths, using Mg_xZn_1-xO to extend the bandgap from 3.32eV to 4.0eV.

        B. ZnO UV Optical Modulators
        In collaboration with US Army Research Lab, we have demonstrated the first optically addressed normal incidence UV high contrast modulator which exploits the in-plane optical anisotropy in epitaxial ZnO grown on R-plane sapphire. An ultra-fast dynamic polarization rotation of ~12^o was achieved, and the modulator exhibits a high contrast ratio of 70:1.
        This work is now being extended to electro-optic modulators.

        C. ZnO Schottky Diode
        Prof. Lu's group demonstrated the first ZnO Schottky diodes on R-plane sapphire, which were used in MSM photodetectors. We are currently optimizing the ZnO Schottky diode, which will lead to the development of the ZnO MESFET.
        This work is being extended to Mg_xZn_1-xO.

        D. ZnO MESFET and ZnO/Mg_xZn_1-xO based HFET
        Prof. Lu's group is currently in the process of developing ZnO MESFETs, and heterojunction FETs based on the ZnO/Mg_xZn_1-xO heterostructure.

2. Piezoelectric Device Research
        The ZnO/R-Al₂O₃ material system is particularly attractive for piezoelectric devices, due to the anisotropic properties (c-axis of ZnO being in-plane), high electromechanical coupling, high acoustic velocities, and low acoustic attenuation. We have demonstrated ZnO thin film based SAW devices with very high piezoelectric coupling coefficients, up to 6%, and velocities up to 5700 m/s in the ZnO/R-Al₂O₃ material system.
        We have also studied the ZnO/SiO₂/Si system for temperature compensated SAW filters integrated with ICs. The work includes theoretical modeling and computer simulation to determine the proper film thickness' for desired SAW device properties. The same material system is being investigated for thin film resonator (TFR) devices.
        The work on piezoelectric devices is being extended to the monolithically integrated tunable SAW chip technology, presented below.

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III. XYZ-On-A-Chip
        This research is sponsored by the National Science Foundation, ECS-0088549 "Monolithically Integrated Tunable SAW Chip". The work 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/Mg_xZn_1-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 Mg_xZn_1-xO, which permits band-gap tuning from 3.32 eV to 4 eV. ZnO/Mg_xZn_1-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/Mg_xZn_1-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.

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III-V Materials
        In collaboration with Army Research labs, we invented a unique bonding technology to generate controllable in-plane uniaxial strain in a multiple quantum well (MQW) structure by using substrates possessing direction-dependent thermal expansion. We demonstrated a normal incidence MQW light modulator with the highest contrast ratio (5000:1) at room temperature operation.

III-Nitrides
        We studied the energy bandgap of GaN by thermomodulation spectroscopy. A theoretical model was established to explain the spectrum by considering the modulation of dielectric constant and GaN epilayer thickness. Varshni coefficients of the energy gap were determined. The GaN broadening parameter and its temperature dependence were reported for the first time. Thermal annealing effects on the hydrogen passivation of MOCVD grown Mg-doped p-type GaN were studied. The photoluminescence peaks corresponding to the exciton bound to Mg-H complex and the exciton bound to Mg acceptors were identified. An effective annealing process was established to release hydrogen from the film and to activate Mg acceptors. In collaboration with industries, the blue LED and UV detectors have been demonstrated.

Passive Microwave Devices
        In collaboration with Bell Labs, we have demonstrated integrated RF inductors on Si MCM substrates. A Q factor of 30 has been achieved for an inductor of 4 nH operating at 1-2 GHz. This team has also demonstrated Si based monolithic RF spiral transmission line baluns that operate at the frequency range from 1.2 to 3.5 GHz for wireless communications.

Piezoelectric Devices
        Research in piezoelectric devices includes surface transverse wave resonator based gravimetric sensor, investigation of the properties of dilithium tetraborate piezoelectric resonators and transducers, and more recently programmable SAW filters. Several types of programmable SAW filters, including IDT selection and electrode programmable designs, have been successfully investigated.
 

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