C. R. Gorla

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.

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.

Transparent and conductive Ga-doped ZnO films grown by low pressure metal organic chemical vapor deposition

Highly transparent conductive Ga-doped zinc oxide (ZnO:Ga) has been deposited on 3 in.×4 in. Corning 7059 glass and other substrates using a high speed rotating disk reactor low pressure metal organic chemical vapor deposition system. Diethylzinc, oxygen, and triethylgallium were used as precursors. The films exhibit low resistivity, ∼2.6×10−4 Ω cm, high optical transparency (>85%) in the visible range, good adhesion, and are highly stable. The film properties were correlated with the growth conditions, including flow rate, temperature, pressure, and doping concentrations. The microstructural properties of the films, such as surface and interface morphology, crystallinity, and composition were studied using scanning electron microscopy, x-ray diffraction, and secondary ion mass spectroscopy. The resistivity and transmittance of the films were investigated by four-point probe measurements, photoluminescence spectroscopy, and optical absorption spectroscopy. In order to meet the needs for application to fla...

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.

Structure and interface-controlled growth kinetics of ZnAl2O4 formed at the (112̄0) ZnO/(011̄2) Al2O3 interface

The solid state reaction between metalorganic chemical vapor deposition grown epitaxial ZnO films and the R-plane sapphire substrate after annealing at 1000 °C for various times in an O2/N2 atmosphere was studied in detail. Multiple epitaxial relationships between the reaction product (ZnAl2O4) and the reactants were observed, as determined by cross-sectional transmission electron microscopy. In the dominant epitaxial relationship (A1), the (220) plane of ZnAl2O4 was parallel to the (1101) plane of Al2O3. A twin (A2) of orientation A1, i.e. (220) ZnAl2O4//(1011) Al2O3, and a closely related orientation (B) wherein the (220) ZnAl2O4 plane is parallel to the (1210) ZnO plane (which is equivalent to a 5° clockwise rotation about the [112] ZnAl2O4 or [0001] ZnO zone axis relative to A2), were also observed. Enhanced growth was observed at grain boundaries. It was necessary to measure the spinel growth rate from grains with the same orientation far away from grain boundaries because the growth rate wa...

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.

Silicon and germanium nanoparticle formation in an inductively coupled plasma reactor

We have studied the formation of Si nanoparticles in a SiH4–Ar plasma discharge generated in a helical resonator type inductively coupled plasma reactor. It is observed that Si particles vary in sizes from 5 to 15 nm under different conditions. The particles were mostly spherical and made up of a crystalline core with a 1–2 nm thick amorphous shell. The size distribution was narrow for particles formed at a pressure of 200 mTorr, plasma power of 400 W and silane flow rate of 20 sccm (+980 sccm Ar). The effect of a dc bias applied to the particle collecting grids has also been studied. It is found that a negative bias (−25 to −100 V) applied to the grids used for particle collection results in a large increase in the number of Si nanoparticles collected, while a positive bias does not change the collection efficiency considerably, suggesting that the particles are positively charged. Under very low flow rates and under high plasma powers, the Si particle density decreases considerably and a film like depos...

Two-step metalorganic chemical vapor deposition growth of piezoelectric ZnO thin film on SiO2/Si substrate

Piezoelectric ZnO thin films deposited on semiconductor substrates are used for surface and bulk acoustic wave and devices, which offer advantages such as low power consumption, circuit miniaturization, and cost reduction by integration with microwave monolithic integrated circuit technology. Furthermore, temperature compensated surface acoustic wave (SAW) devices, which are attractive for both communication and sensor technologies, may be achieved in the ZnO/SiO2/Si system as ZnO and Si have positive temperature coefficients of delay, while SiO2 has a negative one. In the present work, ZnO thin films were grown on SiO2/Si substrates by metalorganic chemical vapor deposition. The structural properties of the films were investigated using x-ray diffraction, scanning electron microscopy, and scanning probe microscopy. A two-step growth process was developed to obtain ZnO films with both good crystalline quality and surface morphology. The SAW properties of the ZnO/SiO2/Si system were investigated through mo...

High-Gain, High-Speed ZnO MSM Ultraviolet Photodetectors

High quality zinc oxide (ZnO) films were epitaxially grown on R-plane sapphire substrates by metalorganic chemical vapor deposition at temperatures in the range 350--600 C. In-situ nitrogen compensation doping was performed using NH{sub 3}. The metal-semiconductor-metal ultraviolet-sensitive photodetectors were fabricated on nitrogen-compensation-doped epitaxial ZnO films. The photoresponsivity of these devices exhibits a linear dependence upon bias voltage up to 10 V, with a photoresponsivity of 400 A/W at 5 V. The rise and fall times are 1 and 1.5 {micro}s, respectively.