S. Muthukumar

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.

Ga-doped ZnO single-crystal nanotips grown on fused silica by metalorganic chemical vapor deposition

In situ Ga-doped ZnO nanotips were grown on amorphous fused silica substrates using metalorganic chemical vapor deposition. Structural, optical, and electrical properties of as-grown ZnO nanotips are investigated. Despite the amorphous nature of fused silica substrates, Ga-doped ZnO nanotips are found to be single crystalline and oriented along the c-axis. Photoluminescence (PL) spectra of Ga-doped ZnO nanotips are dominated by near-band-edge emission with negligible deep-level emission. The increase in PL intensity from Ga doping has been attributed to the increase of Ga donor-related impurity emission. Current–voltage characteristics of the ZnO nanotips are measured by conductive-tip atomic force microscopy, which shows the conductivity enhancement due to Ga doping.

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.

Growth and structural analysis of metalorganic chemical vapor deposited (112̄0)MgxZn1−xO(0<x<0.33) films on (011̄2) R-plane Al2O3 substrates

MgxZn1−xO (0<x<0.33) thin films were grown on R-plane (0112) sapphire substrate by metalorganic chemical vapor deposition. It was found that a thin ZnO buffer layer with a minimum thickness of ∼50 A is needed to achieve wurtzite-type MgxZn1−xO films on R-plane sapphire. The x-ray Δω(1120) rocking curve and Δ2θ(1120) full width at half maximum for Mg0.18Zn0.82O film were measured to be 0.275° and 0.18°, respectively, indicating strong mosaicity and strain in the films. In-plane reflections show the lower lattice mismatch along the c axis of the MgxZn1−xO films on R-plane sapphire. Optical transmission spectra indicate the good quality of the films.

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.

Analysis of BAW responses in ZnO multi-layer structures using transmission line method

With the advent of epitaxial semiconductor growth technology, piezoelectric multilayer materials became available for broad application. Piezoelectric ZnO has large electromechanical coupling coefficients making it a promising candidate for multilayer thin film resonant filter devices. A family of methodologies has been developed to explain the analogous behavior of such multi-layer structures in terms of solutions to the acoustic wave differential equation. In the work encompassed by this paper, the model using transmission line representations to simulate the resonant behaviors in BAW devices is demonstrated. The model shows the electrical admittance response as a function of frequency.

Selective growth of ZnO nanotips using MOCVD

ZnO is a wide bandgap semiconductor having a direct bandgap of 3.32 eV at room temperature. It has an exciton binding energy of 60 meV. 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. The ZnO nanotips can be grown at relatively low temperatures, giving ZnO a unique advantage over other wide bandgap semiconductors, such as GaN and SiC. In the present work, we report the selective growth of ZnO nanotips on various substrates using MOCVD. The ZnO nanotips are single crystalline, n-type conductive and show good optical properties. These nanotips have potential applications in field emission devices and UV photonics.