Gaurav Saraf

Integrated ZnO nanotips on GaN light emitting diodes for enhanced emission efficiency

Enhancement of light extraction from an integrated ZnO nanotips/GaN light emitting diode (LED) is demonstrated. The device is composed of a GaN LED with a Ga-doped ZnO (GZO) transparent conductive layer and ZnO nanotips grown on GZO for light extraction. The light output power of a ZnO nanotips/GZO/GaN LED exhibits 1.7 times enhancement, in comparison with a conventional Ni∕Au p-metal LED. The higher emission efficiency is attributed to the enhanced light transmission and scattering in the ZnO∕GaN multilayer.

Dye-sensitized solar cells using ZnO nanotips and Ga-doped ZnO films

Ga-doped ZnO (GZO) transparent conducting films and well-aligned ZnO nanotips were sequentially grown on a glass substrate using metal–organic chemical vapor deposition (MOCVD). The morphology control of ZnO from dense films to nanotips was realized through temperature-modulated growth. The ZnO nanotips/GZO structure was sensitized with dye N719 to form photoelectrochemical cells. It is found that the power conversion efficiency linearly increases with the length of ZnO nanotips. For the 1.0 cm2 dye-sensitized solar cell built from 4.8 µm ZnO nanotips, a peak incident photo-to-current conversion efficiency of 79% (at ~530 nm) and a power conversion efficiency of 0.77% under the illumination of one sun-simulated sunlight were achieved. UV light harvesting directly by ZnO was observed. The I–V characteristics of the cells were analyzed using a one-diode equivalent circuit model.

Ferromagnetism in Fe-implanted a-plane ZnO films

Fe ions of dose 5×1016cm−2 were implanted at 200keV into a-plane ZnO epitaxial films. The epitaxial quality of the postannealed samples was verified by x-ray diffraction ω-rocking curves and φ scans, whereas x-ray absorption spectroscopy identified the presence of both Fe2+ and Fe3+ ions, as well as changes in their relative concentration during postannealing. Superconducting quantum interference device measurements show that the as-implanted and postannealed films are ferromagnetic at room temperature. The saturation magnetization reduces during annealing possibly due to the decrease in the number of oxygen vacancies.

In-plane anisotropic strain in a-ZnO films grown on r-sapphire substrates

We deposited (112¯0) nonpolar a-plane ZnO (a-ZnO) films on (011¯2) r-sapphire substrates using metalorganic chemical vapor deposition. Unit cell deformation due to interfacial strain in films was determined by triple-axis x-ray diffraction. Due to low symmetry of a-plane, anisotropic strain is observed along the [0001] (c-axis) and [11¯00] (m-axis) in-plane axes. Out-of-plane strain along [112¯0] (a-axis) is tensile and relaxes for film thickness ⩾2μm. The in-plane m-axis is under tensile strain and c-axis is under compressive strain. Increase in film thickness increases in-plane strain anisotropy due to faster relaxation along the m-axis. Thermal and lattice mismatch strains are separated by curve fitting.

A ZnO Nanostructure-Based Quartz Crystal Microbalance Device for Biochemical Sensing

We report a ZnO-nanostructure-based quartz crystal microbalance (nano-QCM) device for biosensing applications. ZnO nanotips are directly grown on the sensing area of a conventional QCM by metalorganic chemical vapor deposition (MOCVD). Scanning electron microscopy (SEM) shows that the ZnO nanotips are dense and uniformly aligned along the normal to the substrate surface. By using superhydrophilic nano-ZnO surface, more than tenfold increase in mass loading sensitivity of the nano-QCM device is achieved over the conventional QCM. The ZnO nanotip arrays on the nano-QCM are functionalized. The selective immobilization and hybridization of DNA oligonucleotide molecules are confirmed by fluorescence microscopy of the nano-QCM sensing areas.

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.

DNA immobilization and SAW response in ZnO nanotips grown on LiNbO/sub 3/ substrates

DNA immobilization enhancement is demonstrated in a structure consisting of ZnO nanotips on 128/spl deg/ Y-cut LiNbO/sub 3/.The ZnO nanotips are grown by metal-organic chemical vapor deposition (MOCVD) on the top of a SiO/sub 2/ layer that is deposited and patterned on the LiNbO/sub 3/ SAW delay path. The effects of ZnO nanotips on the SAW response are investigated. X-ray diffraction and scanning electron microscopy are used to analyze the ZnO nanotips, which are of single crystalline quality, and they are uniformly aligned with their c-axis perpendicular to the substrate surface. The photoluminescence (PL) spectrum of the ZnO nanotips shows strong near bandedge transition with insignificant deep level emission, confirming their good optical property. DNA immobilization enhancement is experimentally validated by radioactive labeling tests and SAW response changes. The ZnO nanotips enhance the DNA immobilization by a factor of 200 compared to ZnO film with flat surface. DNA hybridization with complementary and noncomplementary second strand DNA oligonucleotides is used to study the selective binding of the structure. This device structure possesses the advantages of both traditional SAW sensors and ZnO nanostructures.

Analysis of SAW properties in ZnO/Al/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ structures

Piezoelectric thin films on high acoustic velocity nonpiezoelectric substrates, such as ZnO, AlN, or GaN deposited on diamond or sapphire substrates, are attractive for high frequency and low-loss surface acoustic wave devices. In this work, ZnO films are deposited on Al/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ (0 /spl les/ x /spl les/ 1) substrates using the radio frequency (RF) sputtering technique. In comparison with a single Al/sub x/Ga/sub 1-x/N layer deposited on c-Al/sub 2/O/sub 3/ with the same total film thickness, a ZnO/Al/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ multilayer structure provides several advantages, including higher order wave modes with higher velocity and larger electromechanical coupling coefficient (K/sup 2/). The surface acoustic wave (SAW) velocities and coupling coefficients of the ZnO/Al/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ structure are tailored as a function of the Al mole percentage in Al/sub x/Ga/sub 1-x/N films, and as a function of the ZnO (h/sub 1/) to Al/sub x/Ga/sub 1-x/N (h/sub 2/) thickness ratio. It is found that a wide thickness-frequency product (hf) region in which coupling is close to its maximum value, K/sub max//sup 2/, can be obtained. The K/sub max//sup 2/ of the second order wave mode (h/sub 1/ = h/sub 2/) is estimated to be 4.3% for ZnO/GaN/c-Al/sub 2/O/sub 3/, and 3.8% for ZnO/AlN/c-Al/sub 2/O/sub 3/. The bandwidth of second and third order wave modes, in which the coupling coefficient is within /spl plusmn/0.3% of K/sub max//sup 2/, is calculated to be 820 hf for ZnO/GaN/c-Al/sub 2/O/sub 3/, and 3620 hf for ZnO/AlN/c-Al/sub 2/O/sub 3/. Thus, the hf region in which the coupling coefficient is close to the maximum value broadens with increasing Al content, while K/sub max//sup 2/ decreases slightly. When the thickness ratio of AlN to ZnO increases, the K/sub max//sup 2/ and hf bandwidth of the second and third higher wave modes increases. The SAW test devices are fabricated and tested. The theoretical and experimental results of velocity dispersion in the ZnO/Al/sub x/Ga/sub 1-x/N/c-Al/sub 2/O/sub 3/ structures are found to be well matched.

Structural anisotropy in a-MgxZn1−xO (0≤x≤0.33) films on r-sapphire

The a-plane MgxZn1−xO (0≤x≤0.25) films were grown on r-plane (011¯2) sapphire substrates using metal-organic chemical vapor deposition. Growth was done at temperatures from 520 °C, with a typical growth rate of ∼500 nm/h. Film thickness was varied by changes in deposition time, while Mg/Zn metal-organic flow was varied for changes in Mg composition. The a-plane films are dense and uniform. Strain anisotropy in the films was characterized by synchrotron x-ray diffraction. In-plane strain anisotropy increases with an increase in ZnO film thickness and reduces with an increase in Mg composition of MgxZn1−xO. The surface of the films characterized by atomic force microscopy showed rippled morphology with needles running along the surface. The morphology anisotropy in the films increases with an increase in ZnO film thickness and reduces with Mg composition of MgxZn1−xO films. The morphology anisotropy with ZnO film thickness and Mg composition of MgxZn1−xO is correlated with respective strain anisotropy varia...

a-plane MgxZn1−xO films deposited on r-sapphire and its surface acoustic wave characteristics

Piezoelectric zinc oxide (ZnO) and its ternary alloy magnesium zinc oxide (MgxZn1−xO) films are deposited on r-plane (011¯2) sapphire (Al2O3) substrates using the hybrid deposition technology by combining metal-organic chemical vapor deposition (MOCVD) and sputtering. An ultra-thin ZnO buffer is first grown on r-Al2O3 by MOCVD technique, followed by a thick piezoelectric MgxZn1−xO (0⩽x⩽0.3) film deposited using RF sputtering. The sputtering targets are made by mixing ZnO and MgO powders in appropriate composition ratio, and nickel oxide (NiO) powder (2wt%) is added as the compensation dopant to achieve piezoelectricity. The as-deposited MgxZn1−xO films have a-plane (112¯0) orientation in a wurtzite crystal structure. The crystallinity of the films is further improved by annealing at 600–700°C in oxygen ambient. It is found that a ZnO thin buffer layer and post-deposition annealing process significantly improve the film’s piezoelectric properties. The c-axis of the MgxZn1−xO film lies in the plane of the s...