Pyroelectric Effect in Wurtzite Gallium Nitride
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(a) Departmentof ElectricalEngineering,University of Virginia, Charlottesville,VA 229032442, USA, [email protected] (b) A. F. loffe Physico-TechnicalInstitute, 194021 St. Petersburg,Russia (c) APA Optics, Inc., 2950 N. E. 84th Lane, Blaine, MN 55449, USA
ABSTRACT We report on the measurements of the pyroeffect in wurtzite n-type GaN films deposited over basal plane sapphire substrates. The voltage drop between the contacts was measured while the sample was subjected to uniform heating. Our results show that the pyroelectric effect in GaN can be partially attributed to the secondary pyroelectricity, caused by the development of strain in the material due to thermal expansion.
INTRODUCTION Large piezoelectric constants of GaN and AIN [1] point out possible applications of GaN - based materials in piezoelectric sensors. Due to a wide band gap, these sensors are expected to operate in a broad temperature range and/or in a harsh environment. Recently, we reported [2] on the measurements of the piezoresistive effect in n-type wurtzite GaN films, which confirm these expectations. Our study showed that these samples had a strong dynamic response to an
applied force. The gauge factor (GF) of these structures was measured at room temperature for both longitudinal and transverse configurations. The dynamic effect was related to a strong piezoeffect in GaN. The maximum dynamic GF observed was - 130 (approximately four times larger than for SiC [3]). In this paper, we present the results of our experimental and theoretical studies of the pyroelectric effects in wurtzite GaN films in a temperature range from 20 OC to 80 OC. We also propose simple equation describing the pyroeffect under realistic condition of the pyroelectric charge relaxation. This equation relates the pyroelectric coefficients to piezoelectric properties of GaN. Our theory is in good agreement with our experimental data and can be applied to analyze the pyroelectric effect in many different types of materials.
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EXPERIMENT
GaN layers (3 - 5 gtm thick) were deposited over basal plane sapphire substrates, slightly off-axis. The sample geometry is shown in Fig. 1. Except for the center stripe, the GaN film has been completely etched down to the sapphire substrate (see Fig. 1). The standard Hall measurements showed that the electron concentration was close to 5x10 16 cm- 3 and the Hall mobility was on the order of 350 cm 2 /V-s. A typical sample resistance was close to 2 kQ.
75 Mat. Res. Soc. Symp. Proc. Vol. 423 (01996 Materials Research Society
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The voltage drop between the contacts 1 and 2 was measured while the sample was subjected to uniform heating. In these experiments, we abruptly changed the ambient temperature and monitored the sample temperature (using a SmS based thermoresistor) and voltage responses as functions of time. The experimental results will be described in Section III, where they will be interpreted using our theory. We conducted hot thermal shock experiments in the range +20 oC - +80 OC oil as a thermal bath.
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