C. R. Gorla

Rutgers, The State University of New Jersey, New Brunswick, NJ, United States

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Publications (17)25.12 Total impact

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    ABSTRACT: High-quality zinc oxide (ZnO) films were epitaxially grown on R-plane sapphire substrates by metalorganic chemical vapor deposition at temperatures in the range of 350°C to 600°C. In-situ nitrogen compensation doping was performed using NH3. Microstructural and optical properties of the films, as well as the N-doping effects, were studied. The metal-semiconductor-metal ultraviolet sensitive photodetectors were fabricated on N-doped epitaxial ZnO films. The detector showed fast photoresponse, with a rise time of 1 µs and a fall time of 1.5 µs. Low-frequency photoresponsivity, on the order of 400 A/W at 5 V bias, was obtained.
    Journal of Electronic Materials 04/2012; 29(1):69-74. · 1.64 Impact Factor
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    ABSTRACT: 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 Adler's matrix method, and the devices were simulated using the quasi-static approximation based on Green's function analysis.
    IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 10/2001; · 1.82 Impact Factor
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    ABSTRACT: 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/SiO <sub> 2 </sub> /Si system as ZnO and Si have positive temperature coefficients of delay, while SiO <sub> 2 </sub> has a negative one. In the present work, ZnO thin films were grown on SiO <sub> 2 </sub> /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/SiO <sub> 2 </sub> /Si system were investigated through modeling and computer simulation based on the transfer matrix method. Acoustic velocity dispersion data obtained from measurement on testing structures agree well with the dispersion curves predicted by computer simulation. The results show that the ZnO/SiO <sub> 2 </sub> /Si system is promising for fabricating low-loss SAW devices. © 2001 American Vacuum Society.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 08/2001; · 1.43 Impact Factor
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    ABSTRACT: 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.
    Journal of Crystal Growth 01/2001; 225(2):197-201. · 1.55 Impact Factor
  • C. R. Gorla, W. E. Mayo, S. Liang, Y. Lu
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    ABSTRACT: 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 (20) plane of ZnAl2O4 was parallel to the (101) plane of Al2O3. A twin (A2) of orientation A1, i.e. (20) ZnAl2O4//(101) Al2O3, and a closely related orientation (B) wherein the (20) ZnAl2O4 plane is parallel to the (20) ZnO plane (which is equivalent to a 5° clockwise rotation about the [2] 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 was observed to be influenced by the orientation of the grains in addition to the enhanced growth at grain boundaries. The growth rate was observed to follow a linear rate law during early stages (for grains with orientation A1), suggesting an interface-controlled reaction. The structures of the ZnO/Al2O3, ZnO/ZnAl2O4 and ZnAl2O4/Al2O3 interfaces were studied for grains with this orientation (A1). The 13.7% lattice mismatch between ZnO and ZnAl2O4 was relieved by a series of misfit dislocations spaced five to six (100) ZnO planes apart. Due to the small lattice misfit (2.1%) at the ZnAl2O4/Al2O3 interface, very few misfit dislocations were present. This interface was faceted and the sapphire surface had a series of single steps. It is expected that the reaction at the ZnAl2O4/ZnO interface is the rate-controlling step due to the necessity for a dislocation climb (of a large number of misfit dislocations) for movement of this interface. © 2000 American Institute of Physics.
    Journal of Applied Physics 04/2000; 87(8):3736-3743. · 2.21 Impact Factor
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    ABSTRACT: 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 (110) ZnO∥(012) Al2O3, and [0001] ZnO∥[011] 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 for 30 min, whereas a 150 nm layer formed when the film was annealed at 1000 °C for 150 min. To prevent this reaction from occurring, the maximum process temperature should be below 750 °C. The surface acoustic wave properties of the piezoelectric ZnO were evaluated by fabricating SAW devices on (110) ZnO/(012) Al2O3. An effective electromechanical coupling coefficient, keff2, of 6% was achieved for a 1.5 μm thick ZnO film, which is close to the value for bulk single-crystal ZnO. The photoluminescence spectra were obtained both at room temperature and at 11 K. The full width at half maximum of the 3.363 eV band edge emission photoluminescence peak measured at 11 K was 6 meV, which is close to that for single-crystal ZnO. We also evaluated the anisotropic absorption characteristics of the (110) ZnO film, which can be used for a high contrast ultraviolet light modulator. © 1999 American Institute of Physics.
    Journal of Applied Physics 02/1999; 85(5):2595-2602. · 2.21 Impact Factor
  • M. Wraback, H. Shen, S. Liang, C. R. Gorla, Y. Lu
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    ABSTRACT: 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. © 1999 American Institute of Physics.
    Applied Physics Letters 01/1999; 74(4):507-509. · 3.52 Impact Factor
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    ABSTRACT: 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.
    Materials Science in Semiconductor Processing 01/1999; · 1.34 Impact Factor
  • Journal of Electronic Materials 11/1998; 27(11). · 1.64 Impact Factor
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    ABSTRACT: There has been increased interest in high quality ZnO films for use in a diverse range of applications such as in high frequency surface acoustic wave filters, buffer layers for GaN growth, transparent and conductive electrodes, and solid state lasers. In the present paper, ZnO films were epitaxially grown on R-plane sapphire substrates by metalorganic chemical vapor deposition at temperatures in the range 350–450C. X-ray diffraction and electron microscopy results indicate that the ZnO films are epitaxially grown on ( $$01\bar 12$$ ) Al2O3 surface with the ( $$11\bar 20$$ ) plane parallel to the surface. Cross-sectional high resolution-transmission electron microscopy imaging of the as-grown film shows that the interface is semi-coherent and atomically sharp, with misfit dislocations relieving the misfit strain between ZnO and sapphire. In order to check the thermal stability of the as-grown ZnO films, annealing in an O2+N2 ambience at 850C for 30 min was performed. The annealed films showed improved crystallinity. At the same time, limited reaction between ZnO and sapphire occurred, resulting in the formation of a 15–20 nm thick spinel layer at the interface.
    Journal of Electronic Materials 11/1998; 27(11). · 1.64 Impact Factor
  • Journal of Electronic Materials 01/1998; 27(8):1005-1005. · 1.64 Impact Factor
  • Journal of Electronic Materials 01/1998; 27. · 1.64 Impact Factor
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    ABSTRACT: There has been increasing interest in high quality piezoelectric ZnO thin films. ZnO has a high coupling coefficient, which makes it promising for high frequency, low loss SAW devices when the film is deposited on top of a high-velocity substrate such as diamond or sapphire. It is well known that for these devices performance is critically dependent on the quality of ZnO films. We report the epitaxial growth of (1120) ZnO thin films on R-plane sapphire substrates using MOCVD technique. The films' crystallinity and orientations were analyzed using X-ray diffractions. The smooth surface morphology and sharp interface properties were revealed by high-resolution cross-sectional TEM. Solid-electrode test transducers are used to characterize the surface wave velocity, the coupling coefficient and the electrode reflectivity. 10 μm. wavelength IDTs have been demonstrated to operate at 420 MHz and 560 MHz, with operating frequency being a function of film thickness. These correspond to SAW velocities of 4200 m/s and 5600 m/s, respectively. Coupling coefficients up to 6% and electrode reflectivity up to 8% per wavelength have been obtained
    Ultrasonics Symposium, 1997. Proceedings., 1997 IEEE; 11/1997
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    ABSTRACT: We have studied the formation of Si nanoparticles in a SiH <sub> 4 </sub> –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 deposition occurs on transmission electron microscopy grids placed in the reactor for collecting the particles. We have also studied the formation of Ge particles (50–200 nm) in a GeH <sub> 4 </sub> –Ar plasma generated in the same reactor. These particles are crystalline with a spherical morphology. © 1997 American Vacuum Society.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 06/1997; · 1.43 Impact Factor
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    ABSTRACT: 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<sup>-4</sup> 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 flat panel displays, the thermal stability of the Ga-doped ZnO films have been tested by a dc biased heater. The feasibility of film processing was also investigated through patterning and wet chemical etching. © 1997 American Vacuum Society.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 06/1997; · 1.43 Impact Factor
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    ABSTRACT: The white electroluminescence (EL) demonstrated from Si nanocrystals in a wider bandgap amorphous oxide matrix based structure has exciting opportunities in electroptic applications as well as novel LEDs. In this report, we review the electroluminescent properties of the devices for rapid thermally annealed samples at anneal temperatures ranging from 875°C to 1025°C. Depending upon the anneal conditions the EL spectra has shown two distinct spectral features; a strong emission peak at 380nm with a width of 50nm, and a broader features centered above 800nm,. Further, the I-V characteristics and corresponding EL spectra have been measured for sample temperatures ranging from 317K to 240K. In addition, Raman scattering estimated the mean particle sizes of the Si nanocrystals of 6.5nm and 8nm as well as provide insight to the nature of the amorphous matrix. The novel light emission from our devices demonstrates an exciting opportunity for Si nanocrystal (and nanocrystals in general) technology in a wide variety of applications.
    MRS Online Proceeding Library 01/1996; 405:253-258.
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    ABSTRACT: 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. 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 s, respectively.