Effect of Externally-Imposed Radial Strain on the Piezoelectric Response of MOCVD-Grown Gallium Nitride
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Effect of Externally-Imposed Radial Strain on the Piezoelectric Response of MOCVD-Grown Gallium Nitride Jennifer A. Himes, James R. Willis, and Daniel A. Gulino Department of Chemical Engineering and Condensed Matter and Surface Science Program Ohio University Athens, OH 45701
ABSTRACT The large piezoelectric constants of GaN suggest possible application of GaN-based materials in piezoelectric sensors, among other areas. GaN’s wide band gap implies that these sensors will fare well over a broad temperature range and/or in a harsh environment. In this work, films of gallium nitride approximately 0.75 micron thick and grown by MOCVD were subject to an externally-imposed radial stress condition. Deposition was performed in a commercial MOCVD reactor (CVD, Inc.) at 1323K using trimethylgallium and ammonia as the chemical precursors. The substrate was one-inch diameter silicon (111). After deposition, titanium dots were deposited in various locations, including the wafer center, by evaporation. Stress was applied to the film/substrate system using a modified micrometer head (Mitutoyo) mounted to an Ionic Systems Basic Stressgauge (model 30285). Stress levels were calculated based on the magnitude of the imposed deflection as read from the micrometer head display, and the piezoelectric response at any particular dot with respect to the center dot was measured by measuring the voltage difference using a digital multimeter (Keithley 175). The micrometer head impinged on the center dot and served as one electrical contact point. Effective piezoelectric coefficients were measured as a function of imposed radial stress. Applied stresses in the range of 1 to 5 GPa resulted in effective piezoelectric coefficients ranging from –0.6 to –2.0 x 10-5 C/m2.
G11.58.1
INTRODUCTION With a wide, direct band gap and desirable physical properties, gallium nitride is a promising material for the fabrication of optoelectronic devices. The large piezoelectric constants of GaN indicate possible application of GaN-based materials in piezoelectric sensors, among other areas. The wide band gap indicates that these sensors will fare well in a broad temperature range and/or harsh environments.1 From piezoelectric thin films devices can be made to convert electrical impulses to mechanical energy or vice versa. The latter is termed the “direct” piezoelectric effect, and the former is termed the “inverse” piezoelectric effect. Sensors made from n-type GaN have been shown to be more sensitive than those made from silicon or silicon carbide. GaN sensors can be used, for example, for time-dependent measurement of vibration, sudden changes in acceleration, force, pressure, etc. in a wide frequency range above about 1 Hz.1 In other applications, piezoelectric effects have made ceramic materials an important part of the emerging technologies of smart materials and structures. Smart structures are those that incorporate actuators and sensors that are integrated into the structure and have structural functionality. The sensors and actuators are used to inf
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