Carrier Concentration and Mobility in GaN Epilayers on Sapphire Substrate Studied by Infrared Reflection Spectroscopy

We report the measurement of carrier concentration and mobility of metalorganic chemical vapor deposited GaN thin films on the sapphire substrate by an infrared reflection technique. By fitting with the experimental data we obtain all the parameters of the lattice vibration oscillators and of the plasmon. From the plasmon frequency and the damping constant we have derived the carrier concentration and the electron mobility. The concentration agrees with the Hall data very well while the mobility values are smaller than that of the Hall measurement by a factor of about 0.5. We attribute such mobility lowering to the increase of scattering for the electrons coupling with the incident photons.

GaN layers are grown on sapphire substrate by electron cyclotron resonance molecular beam epitaxy (ECR-MBE) using an ECR plasma cell with ion removal magnets on the cell top for the nitrogen source. The efficiency of the ion removal magnets in this ECR plasma cell is 99%. High-quality GaN layers are obtained. In particular, (2×2) and (4×4) RHEED (reflection high-energy electron diffraction) patterns are observed during GaN growth and during cooling after growth, respectively, indicating a flat and smooth surface of GaN. These results show the superiority of the ion-removed ECR plasma cell.


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