Measurement of the Effective Piezoelectric Constant of Nitride Thin Films and Heterostructures Using Scanning Force Micr
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Measurement of the Effective Piezoelectric Constant of Nitride Thin Films and Heterostructures Using Scanning Force Microscopy B. J. Rodriguez1, D-J. Kim2, A. I. Kingon2, R. J. Nemanich1 Department of Physics1 and Department of Materials Science and Engineering2, North Carolina State University, Box 8202, Raleigh, NC, 27695-8202, USA ABSTRACT Piezoelectric properties of wurtzite AlN and GaN/AlN are investigated using scanning force microscopy (SFM). The magnitude of the effective longitudinal piezoelectric constant d33 of AlN and GaN/AlN thin films are measured and reported, and the d33 coefficients of these films are verified using an interferometric technique. Simultaneous imaging of the topography, and of the phase and magnitude of the piezoelectric strain is performed. Using a GaN film with patterned polarities, we demonstrate that polarity can be inferred from the phase image of the piezoelectric strain. We report d33=3±1 pm/V for AlN/SiC and 2±1 pm/V for GaN/AlN/SiC. Films grown by organo-metallic vapor phase epitaxy (OMVPE) on SiC, sputtered AlN films and films grown by molecular beam epitaxy (MBE) are characterized and compared. INTRODUCTION The piezoelectric properties of III-V nitrides are important for new heterostructure devices. As such, measurement of the piezoelectric properties of nitride thin films and heterostructures with nanometer scale resolution is of considerable interest for determining how defects and patterned structures affect the piezoelectric response (piezoresponse). Spontaneous and piezoelectric polarization-induced electric fields have been used to achieve a two dimensional electron gas (2DEG) at the interface of AlGaN/GaN heterostructures with sheet concentrations as high as 2x1013cm-2 in unintentionally doped high electron mobility transistors (HEMTs) [1]. In their wurtzite phase, InN, GaN and AlN are pyroelectric materials with spontaneous polarization along the [000 1 ] direction (PSP = -0.042, -0.034, and -0.090 C/m2) for InN, GaN and AlN, respectively [2]. Unlike ferroelectric materials, the orientation of this polarization cannot be changed by the application of an electric field. In GaN, the [0001] axis points from the Ga atom to the nearest neighbor N atom (cation to anion) in the positive z direction, and from Nface to Ga-face, since Ga will be on the top position of the {0001} bilayer. The orientation of spontaneous polarization is defined by convention such that the positive direction is also from cation to anion along the crystallographic c-axis. Since the sign of the calculated spontaneous polarization of the III-V nitrides is negative, this indicates that the spontaneous polarization points from the Ga-face towards the N-face in GaN (in the [000 1 ] direction). While spontaneous polarization is independent of strain, piezoelectric polarization is strain-induced [3]. The total polarization of a nitride layer is the sum of the spontaneous and the piezoelectric polarizations. In the case of a relaxed layer, the total polarization is equal to the spontaneous polarization. I
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