Free carrier absorption and lattice vibrational modes in bulk ZnO

The electronic properties of semiconductors are highly dependent on carrier scattering mechanisms determined by crystalline structure, band structure, and defects in the material. Experimental characteristics of lattice vibrational modes and free carrier absorption in single-crystal ZnO samples obtained from different sources are presented in this work to provide a further understanding of carrier scattering processes pertaining to electronic properties. Infrared absorption measurements indicate strong absorption peaks due to a combination of optical and nonpolar phonon modes in the 9-13 mu m spectral region. The Raman spectra obtained for these samples similarly reveal the presence of these phonon modes. Infrared absorption measurements also demonstrate free carrier absorption in the 3-9 mu m spectral region for higher conductivity samples, where a lambda(m) dependence is observed with m = 2.7-3, indicating both longitudinal optical phonon scattering and ionized impurity scattering. From these results, we show that infrared absorption can be used as a routine nondestructive technique to determine the material characteristics and quality of bulk ZnO.