The structural and electrical properties of c-plane and a-plane InN thin films, as a function of thickness, are investigated and compared in this work. All samples were grown under same conditions on c- and r-plane Al₂O₃ substrates with RF-MBE, using GaN buffer layers. The epitaxial growth of a-plane InN proceeded through the nucleation, growth and coalescence of 3D islands, resulting to increased surface roughness with increasing film thickness. Step-flow growth mode was maintained during the growth of c-plane films. The rms roughness determined for AFM 2 x 2 μm² scans was 0.3 nm and 5.9 nm for 500 nm thick c- and a-plane films, respectively. However, the a-plane InN films did not exhibit any structural or surface morphology anisotropy, which is typical for a-plane GaN.  The lowest to date XRD (11-20) rocking curve (RC) FWHM of 1440 arcsec is reported for a-plane InN. The sheet electron density (N_s) and mobility were determined by single magnetic field Hall-effect measurements and exhibited a similar dependence on thickness for both c- and a-plane InN films. The measured N_s exhibited a minimum of ~ 2 x 10¹⁴ cm^-2 for 100 nm thickness. It increased linearly with increasing thickness above 100 nm but it also increased with decreasing thickness below 100 nm. The analysis of the Hall-effect measurements, by considering the contribution of two conducting layers, indicates a similar accumulation of low mobility electrons with N_s > 10¹⁴ cm^-2 at the films’ surface/interfacial region for both the a- and c-plane InN films. The actual electron mobility of the InN bulk increases with thickness for both orientations as a result of improving structural quality.

Acknowledgement: Work supported under MRTN-CT-2004-005583 (PARSEM) and the Greek State Scholarships Foundation (IKY).

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