Nuri William Emanetoglu

Structural, optical, and surface acoustic wave properties of epitaxial ZnO films grown on (011̄2) sapphire by metalorganic chemical vapor deposition

High-quality ZnO films are receiving increased interest for use in low-loss high-frequency surface acoustic wave (SAW) devices, acousto-optic and optical modulators, as buffer layers for III-nitride growth, and as the active material in ultraviolet solid state lasers. In this work, high quality epitaxial ZnO films were grown on R-plane sapphire substrates by metalorganic chemical vapor deposition. The structural, piezoelectric, and optical properties of the ZnO films on R sapphire have been investigated. The epitaxial relationship between ZnO and R-Al2O3 was found to be (1120) ZnO∥(0112) Al2O3, and [0001] ZnO∥[0111] Al2O3. The interface between as-grown ZnO and R sapphire was atomically sharp and semicoherent, as evaluated by transmission electron microscopy. On annealing the films at temperatures above 850 °C, a solid state reaction occurred between ZnO and Al2O3, resulting in the formation of ZnAl2O4 (spinel) at the interface. A 15–20 nm spinel layer formed when the ZnO film was annealed at 850 °C fo...

Epitaxial ZnO piezoelectric thin films for saw filters

ZnO is a wide bandgap semiconductor material with high piezoelectric coupling coefficients. It can be used for making low-loss surface acoustic wave (SAW) filters operating at high frequency. We report MOCVD growth of epitaxial ZnO thin films on R-plane sapphire substrates. The crystallinity and orientation of the films, as well as the epitaxial relationship between the ZnO films and the R-plane Al2O3 substrate were studied using X-ray diffraction techniques. An atomically sharp interface structure was revealed by high-resolution TEM. Surface morphology was investigated using field emission SEM. SAW filters with 10 and 16 μm wavelength were fabricated. Low insertion loss and high piezoelectric coupling coefficient, up to 6%, were achieved. The acoustic velocities range from 4050 to 5800 m/s, varying as a function of ZnO film thickness.

Schottky diode with Ag on (112̄0) epitaxial ZnO film

Silver Schottky contacts were fabricated on (1120) n-ZnO epilayers, which were grown on R-plane sapphire substrates by metalorganic chemical-vapor deposition. The flatband barrier height was determined to be 0.89 and 0.92 eV by current–voltage and capacitance–voltage measurements, respectively. The ideality factor was found to be 1.33.

Surface acoustic wave ultraviolet photodetectors using epitaxial ZnO multilayers grown on r-plane sapphire

A surface acoustic wave (SAW) ultraviolet (UV) photodetector is made of a zinc oxide (ZnO) based epitaxial multilayer structure on an r-plane sapphire (r-Al2O3) substrate. Piezoelectric and semiconducting ZnO layers are used for SAW excitation and photodetection, respectively. A thin Mg0.2Zn0.8O layer grown between the two ZnO layers isolates the semiconducting layer from the piezoelectric one. In contrast to previously reported SAW UV detectors on GaN and LiNbO3, the Sezawa SAW mode in the ZnO∕r-Al2O3 system is used for its high acoustic velocity and large maximum effective piezoelectric coupling constant. The interaction of the SAW with the photogenerated carriers in the semiconducting ZnO layer results in a phase shift and an insertion loss change, as functions of light wavelength and power. The ZnO SAW UV detector can be used as a passive zero-power remote wireless UV sensor.

Selective MOCVD growth of ZnO nanotips

ZnO is a wide bandgap semiconductor with a direct bandgap of 3.32eV at room temperature. It is a candidate material for ultraviolet LED and laser. ZnO has an exciton binding energy of 60 meV, much higher than that of GaN. It is found to be significantly more radiation hard than Si, GaAs, and GaN, which is critical against wearing out during field emission. Furthermore, ZnO can also be made as transparent and highly conductive, or piezoelectric. ZnO nanotips can be grown at relatively low temperatures, giving ZnO a unique advantage over the other nanostructures of wide bandgap semiconductors, such as GaN and SiC. In the present work, we report the selective growth of ZnO nanotips on various substrates using metalorganic chemical vapor deposition. ZnO nanotips grown on various substrates are single crystalline, n-type conductive and show good optical properties. The average size of the base of the nanotips is 40 nm. The room temperature photoluminescence peak is very intense and sharp with a full-width-half-maximum of 120 meV. These nanotips have potential applications in field emission devices, near-field microscopy, and UV photonics.

Control of morphology and orientation of ZnO thin films grown on SiO2/Si substrates

ZnO is a wide bandgap semiconductor possessing unique electrical, mechanical, and optical properties. Piezoelectric ZnO film has a high electro-mechanical coupling coefficient, which makes it a promising material for high frequency and low loss surface acoustic wave (SAW) devices in RF/microwave applications. High quality piezoelectric ZnO films grown on Si substrates also pave the way for integration of SAW devices with Si IC technology. In this work ZnO films are grown on SiO2/Si substrates by metal–organic chemical vapor deposition. The growth process is optimized to obtain highly oriented ZnO films with a smooth surface morphology. The structural properties of the films are investigated using X-ray diffraction, electron microscopy, and scanning probe microscopy. To obtain ZnO films with both good crystallinity and smooth surfaces, we have developed a two-step growth technique. A high temperature (450–500°C) buffer layer is initially deposited, which provides a highly crystalline template for the subsequent growth of a low temperature (300–330°C) layer. High quality ZnO thin films have been achieved, which are needed for fabrication of low-loss SAW devices.

Analysis of SAW properties of epitaxial ZnO films grown on R-Al/sub 2/O/sub 3/ substrates

ZnO thin films with a high piezoelectric coupling coefficient are widely used for high frequency and low loss surface acoustic wave (SAW) devices when the film is deposited on top of a high acoustic velocity substrate, such as diamond or sapphire. The performance of these devices is critically dependent on the quality of the ZnO films as well as of the interface between ZnO and the substrate. In this paper, we report the studies on piezoelectric properties of epitaxial (112~0) ZnO thin films grown on R-plane sapphire substrates using metal organic chemical vapor deposition (MOCVD) technique. The c-axis of the ZnO film is in-plane. The ZnO/R-Al/sub 2/O/sub 3/ interface is atomically sharp. SAW delay lines, aligned parallel to the c-axis, were used to characterize the surface wave velocity, coupling coefficient, and temperature coefficient of frequency as functions of film thickness to wavelength ratio (h//spl lambda/). The acoustic wave properties of the material system were calculated using Adlers matrix method, and the devices were simulated using the quasi-static approximation based on Greens function analysis.

Characteristics of Mg/sub x/Zn/sub 1-x/O thin film bulk acoustic wave devices

Piezoelectric thin film zinc oxide (ZnO) and its ternary alloy magnesium zinc oxide (Mg/sub x/Zn/sub 1-x/O) have broad applications in transducers, resonators, and filters. In this work, we present a new bulk acoustic wave (BAW) structure consisting of Al/Mg/sub x/Zn/sub 1-x/O/n/sup +/-ZnO/r-sapphire, where Al and n/sup +/ type ZnO serve as the top and bottom electrode, respectively. The epitaxial Mg/sub x/Zn/sub 1-x/O films have the same epitaxial relationships with the substrate as ZnO on r-Al/sub 2/O/sub 3/, resulting in the c-axis of the Mg/sub x/Zn/sub 1-x/O being in the growth plane. This relationship promotes shear bulk wave propagation that affords sensing in liquid phase media without the dampening effects found in longitudinal wave mode BAW devices. The BAW velocity and electromechanical coupling coefficient of Mg/sub x/Zn/sub 1-x/O can be tailored by varying the Mg composition, which provides an alternative and complementary method to adjust the BAW characteristics by changing the piezoelectric film thickness. This provides flexibility to design the operating frequencies of thin film bulk acoustic wave devices. Frequency responses of devices with two acoustic wave modes propagating in the specified structure are analyzed using a transmission line model. Measured results show good agreement with simulation.

Mg/sub x/Zn/sub 1-x/O: a new piezoelectric material

Piezoelectric ZnO thin films have been successfully used for multilayer surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices. Magnesium zinc oxide (Mg/sub x/Zn/sub 1-x/O) is a new piezoelectric material, which is formed by alloying ZnO and MgO. Mg/sub x/Zn/sub 1-x/O allows for flexibility in thin film SAW device design, as its piezoelectric properties can be tailored by controlling the Mg composition, as well as by using Mg/sub x/Zn/sub 1-x/O/ZnO multilayer structures. We report the metal-organic chemical vapor deposition (MOCVD) growth, structural characterization and SAW evaluation of piezoelectric Mg/sub x/Zn/sub 1-x/O (x<0.35) thin films grown on (011~2) r-plane sapphire substrates. The primary axis of symmetry, the c-axis, lies on the Mg/sub x/Zn/sub 1-x/O growth plane, resulting in the in-plane anisotropy of piezoelectric properties. SAW test devices for Rayleigh and Love wave modes, propagating parallel and perpendicular to the c-axis, were designed and fabricated. Their SAW properties, including velocity dispersion and piezoelectric coupling, were characterized. It has been found that the acoustic velocity increases, whereas the piezoelectric coupling decreases with increasing Mg composition in piezoelectric Mg/sub x/Zn/sub 1-x/O films.

Voltage tunable surface acoustic wave phase shifter using semiconducting/piezoelectric ZnO dual layers grown on r-Al2O3

A voltage tunable surface acoustic wave phase shifter is made of a ZnO based semiconducting/piezoelectric dual-layer structure on an r-Al2O3 substrate by using a hybrid growth technology. Piezoelectric and semiconducting ZnO layers are used for acoustic wave excitation and n-type conducting channel, while SiO2 serves as the gate insulator. The acoustic velocity of the device is tuned by changing the n-channel conductance with a dc bias. The in-plane anisotropy of the piezoelectric ZnO∕r-Al2O3 structure enables multimode operations. Sezawa and Love wave modes are used to get high effective coupling coefficients, which result in large phase shifts.