Fourier Transform Infrared Spectroscopy Measurements of Multi-phonon and Free-Carrier Absorption in ZnO

Fourier transform infrared (FTIR) measurements were carried out on thin films and bulk single crystals of ZnO over a wide temperature range to study the free-carrier and multi-phonon infrared absorptions and the effects of hydrogen incorporation on these properties. Aluminum-doped ZnO thin films were deposited on quartz substrates using atomic-layer deposition (ALD) and sol-gel methods. Hall-effect measurements showed that the ALD films have a resistivity of rho = 1.11 x 10(-3) abroken vertical bar cm, three orders of magnitude lower than sol-gel films (rho = 1.25 abroken vertical bar cm). This result is consistent with the significant difference in their free-carrier absorption as revealed by FTIR spectra obtained at room temperature. By reducing the temperature to 80 K, the free carriers were frozen out, and their absorption spectrum was suppressed. From the FTIR measurements on ZnO single crystals that were grown by the chemical vapor transport method, we identified a shoulder around 3350 cm(-1) and associated it with the presence of two or more hydrogen ions in a Zn vacancy. After reducing the hydrogen level in the crystal, the measurements revealed the multi-phonon absorption of ZnO in the range of 700-1200 cm(-1). This study shows that the multi-phonon absorption bands can be completely masked by the presence of a large concentration of hydrogen in the crystals.