Yicheng Lu

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...

ZnO Schottky barriers and Ohmic contacts

ZnO has emerged as a promising candidate for optoelectronic and microelectronic applications, whose development requires greater understanding and control of their electronic contacts. The rapid pace of ZnO research over the past decade has yielded considerable new information on the nature of ZnO interfaces with metals. Work on ZnO contacts over the past decade has now been carried out on high quality material, nearly free from complicating factors such as impurities, morphological and native point defects. Based on the high quality bulk and thin film crystals now available, ZnO exhibits a range of systematic interface electronic structure that can be understood at the atomic scale. Here we provide a comprehensive review of Schottky barrier and ohmic contacts including work extending over the past half century. For Schottky barriers, these results span the nature of ZnO surface charge transfer, the roles of surface cleaning, crystal quality, chemical interactions, and defect formation. For ohmic contacts...

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.

Thermal stability of W ohmic contacts to n‐type GaN

W was found to produce low specific contact resistance (ρc∼8.0×10−5 Ω cm2) ohmic contacts to n+‐GaN (n=1.5×1019 cm−3) with limited reaction between the metal and semiconductor up to 1000 °C. The formation of the β–W2N and W–N interfacial phases were deemed responsible for the electrical integrity observed at these annealing temperatures. No Ga out‐diffusion was observed on the surface of thin (500 A) W contacts even after 1000 °C, 1 min anneals. Thus, W appears to be a stable contact to n+‐GaN for high temperature applications.

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.

Dye sensitized solar cells using well-aligned zinc oxide nanotip arrays

Well-aligned zinc oxide (ZnO) nanotip arrays were grown on fluorinated tin oxide coated glass using metal organic chemical vapor deposition. Dye (N719) sensitized photoelectrochemical cells comprising of ZnO nanotip arrays were fabricated and characterized. It is found that the power conversion efficiency of the cells increases with the length of the ZnO nanotips. The cells with 3.2μm ZnO nanotip array exhibited an incident photon-to-current conversion efficiency of 21.1% (at 550nm) and a power conversion efficiency of 0.55% under 1 sun irradiance. Light harvesting in ZnO nanotips also contributes to the photocurrent in the UV range.

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.

High contrast, ultrafast optically addressed ultraviolet light modulator based upon optical anisotropy in ZnO films grown on R-plane sapphire

An optically addressed ultraviolet light modulator has been demonstrated which exploits the optical anisotropy in a ZnO film epitaxially grown on (0112) sapphire. This device achieves both high contrast and high speed by exploiting the anisotropic bleaching of the anisotropic absorption and concomitant ultrafast polarization rotation near the lowest exciton resonances produced by femtosecond ultraviolet pulses. The resultant modulation is characterized by a contrast ratio of 70:1, corresponding to a dynamic polarization rotation of 12°, and it decays to a quasiequilibrium value within 100 ps.

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.

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.