Abstract
Infrared spectroscopic ellipsometry (IRSE) over the wave-number range from 300 to 1200 is used to determine the anisotropic room-temperature optical properties of highly resistive, Si-doped n-type and Mg-doped p-type α-GaN. The approximately 1-μm-thick films were deposited on c-plane sapphire by molecular beam epitaxy without a buffer layer. The free-carrier concentrations are obtained from Hall measurements. The IRSE data are analyzed through model calculations of the infrared optical dielectric functions parallel (∥) and perpendicular (⊥) to the c axis of the α-GaN films. We obtain the thin-film phonon frequencies and broadening values and the optical mobility and effective-mass parameters for n- and p-type α-GaN. In agreement with Perlin et al. [Appl. Phys. Lett. 68, 1114 (1996)] we determine the effective electron masses as and For p-type GaN with hole concentration we find which agrees with recent theoretical studies of the Rashba-Sheka-Pikus parameters in wurtzite GaN. However, no substantial anisotropy of the effective hole mass is obtained to within 25%. The ellipsometry data also allow for derivation of the model quantities which are almost isotropic but may vary between 4.92 and 5.37 depending on whether the films are undoped or doped. In heavily-Si-doped n-type α-GaN we observe a thin carrier-depleted surface layer and additional infrared-active vibrational modes at 574, 746, and 851 . Raman measurements of the GaN films are also performed, and the results are compared to those obtained from the IRSE investigations.
- Received 28 December 1999
DOI:https://doi.org/10.1103/PhysRevB.62.7365
©2000 American Physical Society